Radiation Detection and Interdiction at U.s. Borders

Radiation Detection and Interdiction at U.S. Borders

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Radiation Detection and Interdiction at U.S. Borders Edited by Richard T. Kouzes Joseph C. McDonald Denis M. Strachan Sonya M. Bowyer

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1 Oxford University Press, Inc., publishes works that further Oxford University’s objective of excellence in research, scholarship, and education. Oxford New York Auckland Cape Town Dar es Salaam Hong Kong Karachi Kuala Lumpur Madrid Melbourne Mexico City Nairobi New Delhi Shanghai Taipei Toronto With offices in Argentina Austria Brazil Chile Czech Republic France Greece Guatemala Hungary Italy Japan Poland Portugal Singapore South Korea Switzerland Thailand Turkey Ukraine Vietnam

Copyright © 2011 by Oxford University Press Published by Oxford University Press, Inc. 198 Madison Avenue, New York, New York 10016 www.oup.com Oxford is a registered trademark of Oxford University Press All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior permission of Oxford University Press. Library of Congress Cataloging-in-Publication Data Radiation detection and interdiction at U.S. borders / edited by Richard T. Kouzes ... [et al.]. p. cm. ISBN 978-0-19-975450-2 (hardcover : alk. paper) 1. U.S. Customs and Border Protection. Radiation Portal Monitor Project. 2. Nuclear terrorism—United States—Prevention. 3. Radiation—Measurement. 4. Terrorism—Government policy—United States. I. Kouzes, Richard. II. Title. HV6433.86.R23 2011 363.325’564—dc22 2010046709 1 3 5 7 9 8 6 4 2 Printed in the United States of America on acid-free paper

DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor Battelle Memorial Institute, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or Battelle Memorial Institute. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

{ contents } List of Illustrations Preface Acknowledgments Introduction Acronyms and Abbreviations Units

xi xix xxi xxiii xxvii xxix

1 Overview of Radiation Interdiction 1.1 Radiation Portal Monitor Project History 4 1.1.1 Interdiction Goals and Objectives 5 1.1.2 Radiation Portal Monitor Project Mission 6 1.1.3 Initial Radiation Portal Monitor Project Activities 7 1.1.4 The Beginning of Radiation Portal Monitor Deployment 1.1.5 Deployment Advances 11 1.1.6 Moving Toward Project Completion 13 1.2 Detecting Threats 14 1.2.1 Threats 15 1.2.2 Example Incidents 16 1.2.3 Instrumentation to Counter the Threat 17 1.2.4 Specifications and Standards 19 1.2.5 The Multilayer Defense 20 1.3 The Necessity of Interdiction 21 1.4 References 24

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2 Radiation Sources 2.1 Physics and Statistics of Radiation Sources 26 2.1.1 Gamma-Ray Interactions with Matter 27 2.1.2 Neutron Interactions with Matter 29 2.1.3 Neutron and Gamma-Ray Spectroscopy 31 2.1.4 Statistics 33 2.2 Background Radiation Sources 35 2.2.1 Cosmic Background 36 2.2.2 Earth–Terrestrial Background 46 2.2.3 Construction Materials 48 2.2.4 Weather-Related Variations 49 2.3 Naturally Occurring Radioactive Materials 57 2.3.1 Radioactive Sources of Concern and Common Legitimate Sources 2.3.2 Photon Emission Spectra from Cargo 61 2.4 Scope and Impact of Medical Radioisotopes 64

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Contents 2.4.1 Medical Radionuclide Use 65 2.4.2 Medical Radionuclide Survey Results 67 2.4.3 Medical Radionuclide Decay Properties 68 2.4.4 Detector Response Calculation Configurations 69 2.4.5 Detector Response Modeling Method 72 2.4.6 Detector Response Modeling Results 72 2.4.7 Expected Occurrence of Radionuclide Alarm Events 74 2.5 Industrial Radiation Sources and Special Nuclear Materials 77 2.5.1 Common Industrial Radiation Sources 78 2.5.2 Special Nuclear Materials 80 2.6 Electromagnetic Interference Effects 81 2.6.1 Sources of Radio Frequency Interference 83 2.6.2 Radio Frequency Interference 84 2.6.3 Electromagnetic Pulse Effects 85 2.6.4 Summary 86 2.7 References 86

3 U.S. Customs and Border Protection Radiation Interdiction Approach 3.1 Radiation Detection Mechanisms 91 3.1.1 Gamma-Ray Detection Mechanisms 92 3.1.2 Neutron Detection Mechanisms 96 3.2 Interdiction Options 97 3.2.1 Radiation Detection for Interdiction 98 3.2.2 Instrumentation Options 98 3.2.3 General Instrument Requirements 99 3.2.4 Options Considered for Scanning at Mail and Express Consignment Courier Facilities 101 3.2.5 Options Considered for Scanning at Land Border and Rail Crossings 101 3.2.6 Options Considered for Airport Cargo Scanning 102 3.2.7 Options Considered for Seaport Scanning 102 3.2.8 Radiation Portal Monitor Specifications 104 3.3 Instruments and Capabilities 106 3.3.1 Detection Technologies 106 3.3.2 Radiation Portal and Area Monitors 110 3.4 Imaging Systems 135 3.4.1 Nonionizing Radiation Technologies for Imaging and Identification 137 3.4.2 Ionizing Radiation Imaging Technologies 139 3.4.3 Future of Cargo Scanning 154 3.4.4 Future of Scanning People 155 3.5 Active Interrogation Techniques 155 3.5.1 Interrogation Techniques 156 3.5.2 Signature Detection 159 3.5.3 Active Interrogation Requirements 159 3.6 References 161

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4 Enhancing the Effectiveness of Radiation Portal Monitor Systems

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4.1 Modeling and Simulation 163 4.1.1 Photon Detection Efficiency 165 4.1.2 Specific Detector Simulations 168 4.1.3 Unshielded-Source Results 171 4.1.4 Spectral Distributions 172 4.1.5 Vehicle Modeling 175 4.1.6 Model Results for Simulated Drive-Through Scenarios 178 4.1.7 Observations 180 4.2 Intelligent Algorithms for Plastic Scintillator Gamma-Ray Detectors: Energy Windowing 181 4.2.1 Thresholds and Nuisance Alarms 182 4.2.2 Description of Naturally Occurring Radioactive Material and Special Nuclear Material Signatures 184 4.2.3 Algorithms for Radiation Detection 185 4.2.4 Summary of Energy Windowing Studies 203 4.3 Other Intelligent Algorithms for Plastic Scintillator Gamma-Ray Detectors 203 4.3.1 Absolute Threshold Algorithm 203 4.3.2 Cross-Talk Suppression in Multilane Deployments 204 4.3.3 Vehicle Speed and Detector Measurement Time 204 4.3.4 Tracking Algorithms for Background Suppression from Vehicles 205 4.3.5 Spatial Distribution of Naturally Occurring Radioactive Material Versus Point Sources 206 4.3.6 Spatial Distributions for Passenger (Noncommercial)Vehicles 206 4.3.7 Spatial Optimization 207 4.4 Baseline Suppression 218 4.4.1 Vehicle Profiles 220 4.4.2 Observations on Baseline Suppression 223 4.4.3 Baseline Suppression for Energy Window Ratios 224 4.4.4 Summary 226 4.5 Spectroscopic Portal Monitors (SPMs) 226 4.5.1 Drivers and Requirements for Deploying Spectroscopic Portal Monitor Systems 228 4.5.2 Prototype Spectroscopic Portal System 229 4.5.3 Specification for Spectroscopic Portal Monitors 232 4.5.4 Comparison of Thallium-Doped Sodium Iodide and High-Purity Germanium Detector Materials 234 4.5.5 Advanced Spectroscopic Portal (ASP) Program 235 4.5.6 Deployment Strategy 236 4.6 Human Factors in Radiation Portal Monitoring Systems 237 4.6.1 Human Role in Radiation Portal Monitor Security Decision Making 237 4.6.2 System Trust 239 4.6.3 False and Nuisance Alarms 240 4.6.4 Situational Awareness 242

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Contents 4.6.5 Applications to Radiation Portal Monitor Systems: The Likelihood Display Concept 243 4.6.6 Distinguishing between Illicit Material and Naturally Occurring Radioactive Material: Human Factors Applications 244 4.6.7 Human Factors Impact 246 4.7 References 246

5 Radiation Portal Monitor Project Deployment Summary 5.1 5.2 5.3 5.4

5.5

5.6

5.7

5.8

5.9

Introduction 249 Deployment Approach 251 Deployment Process Flow 251 Northern and Southern Land Borders 252 5.4.1 Primary Scanning 252 5.4.2 Secondary Scanning 256 5.4.3 Ancillary Equipment 257 Seaports 258 5.5.1 Primary Scanning 259 5.5.2 Secondary Scanning 261 5.5.3 Ancillary Equipment 261 International Mail/Express Consignment Courier Facilities 5.6.1 Primary Scanning 263 5.6.2 Secondary Scanning 266 5.6.3 Ancillary Equipment 268 International Airports 268 5.7.1 Primary Scanning 269 5.7.2 Secondary Scanning 269 5.7.3 Ancillary Equipment 269 Rail Crossings 270 5.8.1 Primary Scanning 270 5.8.2 Secondary Scanning 271 5.8.3 Ancillary Equipment 272 References 272

6 Operational Considerations for Radiation Interdiction 6.1 6.2 6.3 6.4 6.5

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263

273

Overview of Operations for Radiation Interdiction 273 Operational Impacts of Gamma-Ray Alarms 276 Operational Impact of Neutron Alarms 281 National Integration of Radiation Portal Monitor Data 283 References 285

7 Related Work 7.1 Testing, Evaluation, and Standards 287 7.2 International Atomic Energy Agency Activities 291 7.3 Second Line of Defense Program 294 7.3.1 Second Line of Defense Core Program 295 7.3.2 Second Line of Defense Megaports Initiative 296 7.3.3 United States Interagency Relationships 297

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7.4 Department of Defense Programs 297 7.4.1 Joint Service Installation Pilot Project and Unconventional Nuclear Warfare Defense 298 7.4.2 Installation Protection Program “Guardian” 299 7.5 U.S. Department of Homeland Security’s Science and Technology, and Domestic Nuclear Detection Office Efforts 301 7.6 References 304

8 The Future for Interdiction of Radiological and Nuclear Threats at Borders 8.1 Detection Technologies 307 8.1.1 Signatures 307 8.1.2 Detectors 308 8.1.3 Passive Detection 309 8.2 Alarm Algorithms 309 8.3 Signal Processing and Alarm Criteria 310 8.4 Radioactive Isotope Identification 311 8.5 Vehicle Geometry Recording 312 8.6 Identification and Tracking Subsystems 312 8.7 Smaller Radiation Detection Systems 312 8.8 Imaging and Other Active Probes 313 8.8.1 Imaging 313 8.8.2 Interrogation 314 8.8.3 Interrogation and Detection with Imaging 314 8.9 Data Handling and System Control 315 8.10 Automatic Triage with Smart Alerts to Remote Centers 8.11 Data Fusion 316 8.12 Communication Standards 316 8.13 Modularization (Both Hardware and Software) 317 8.14 Multithreat Interdiction Technology Integration 317 8.15 Remote State-of-Health Monitoring 317 8.16 Control 317 8.17 Instruments for the Port of the Future 318 8.18 Away from U.S. Ports of Entry 318 8.18.1 Advance Scanning and Container Security 318 8.18.2 Small-Boat Scanning 318 8.18.3 Automation Aids 319 8.19 Summary 319 8.20 References 320

Contributors Index

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{ list of illustrations } Figures 1.1 1.2

The 307 U.S. ports of entry representing 621 crossings Primary portal at Fort Street Cargo Facility, Detroit, Michigan, showing the first installed radiation portal monitor system commissioned in October 2002 1.3 Examples of Soviet nuclear weapons displayed in a Russian museum 1.4 Soviet 90Sr orphaned source recovered in the former Soviet Republic of Georgia 1.5 Orphaned radioactive well logging sources 1.6 An example of the total gamma-ray counting rates in counts per second from a typical scintillating plastic used in vehicle radiation portal monitor systems showing the mean distribution of 5,000 cargo vehicles with and without naturally occurring radioactive material plus background, the distribution of background alone, and a simulated test source 2.1 Graph of the Z of the absorber and the energy of the gamma ray 2.2 Neutron elastic scattering cross sections for three common elements 2.3 Neutron spectrum from a 252Cf spontaneous fission source 2.4 Energy spectrum produced by gamma rays from 60Co and displayed by a multichannel analyzer 2.5 Illustrative example of Gaussian distribution 2.6 Forbush decrease as seen with the four neutron monitors over the course of 13 days in late October and early November 2003 2.7 Process of primary cosmic particles entering the upper atmosphere and producing secondary particles as they strike air molecules along the way 2.8 Relationship between magnetic rigidity and the minimum particle energy necessary for a cosmic particle to reach the surface of the Earth excluding absorption effects in the atmosphere 2.9 Lines of magnetic rigidity cutoff that relate to the minimum energies that can be observed at locations shown in Figure 2.10 2.10 Map illustrating the location of some of the 52 cosmic-ray neutron monitors used to examine the relationship between cosmic background and the background measured at the radiation portal monitor locations

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11 15 17 17

19 28 30 32 33 34 37

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List of Illustrations

2.11 The neutron count rate measured at the Newark Neutron Monitor in Newark, Delaware, with the neutron count rate measured at four radiation portal monitor locations shown as the lower four traces 44 2.12 The neutron count rate at the Newark Neutron Monitor against the total gamma-ray background count at four radiation portal monitor locations during a major solar flare in late October 2003 44 2.13 The neutron count rate measured at the Newark Neutron Monitor in Newark, Delaware, with the neutron count rate measured at four radiation portal monitor locations shown as the lower four traces 45 2.14 Maps based on the U.S. Department of Energy National Uranium Resource Evaluation program and extracted from the U.S. Geological Survey in Digital Data Series DDS 9. The top map shows the abundance of potassium (40K). The middle map shows the abundance of thorium 47 (232Th). The lower map shows the abundance of uranium (238U). 2.15 Map illustrating the locations of weather stations around the conterminous United States and the surrounding border regions that were available for examining the relationship between weather and the background measured at the radiation portal monitor locations 50 2.16 Average diurnal response of total gamma-ray, low-energy gamma-ray, high-energy gamma-ray, and neutron background as a function of time of day 55 2.17 High-purity germanium spectrum of marble tile 62 2.18 High-purity germanium spectrum of typical kitty litter 63 2.19 High-purity germanium spectrum of typical snow and ice melt salt, dominated by 40K 63 201 2.20 Gamma-ray spectrum emitted by a patient who had a Tl stress test weeks earlier 65 2.21 Energy distribution of dominant gammas emitted from selected medical radioisotopes 70 2.22 Front-view scale drawing of Lane 1 71 2.23 Top view of Lane 1 showing optical zones of detection 71 2.24 Low-energy responses of Lane 1 panels to 99mTc for the back-to-back configuration 73 2.25 Time in days for medical radioisotopes to decay below the alarm threshold 75 2.26 Photograph of two types of industrial radiography sources assemblies 79 60 2.27 Photograph of Co source that may have been used for cancer radiation therapy or other applications mentioned in the text 79 2.28 Replicas of the “Little Boy” and “Fat Man” atomic bombs 81 2.29 Radio frequency energy spectrum taken in New York City 82 3.1 Schematic diagram showing basic components of photomultiplierbased scintillation detector 92 3.2 Mass attenuation, transfer, and absorption coefficients for NaI 94

List of Illustrations

3.3 3.4

3.5

3.6 3.7 3.8 3.9 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17 3.18 3.19 3.20 3.21 3.22 3.23 3.24 3.25 3.26 3.27 3.28 3.29 3.30 3.31 3.32 3.33

Comparison of three gamma-ray spectra of natural background radiation in Southeastern Washington using three types of detectors The mass attenuation and mass energy-absorption coefficients, μ/ρ and μen/ρ, respectively, as a function of gamma-ray energy for CsI from XCOM tables Neutron total absorption cross sections for the 6Li(n,α)3H, 10B(n, α)7Li, and 3He(n,p)3H interactions from the Korea Atomic Energy Research Institute Cross-Section Plotter Polyvinyl toluene plastic scintillator with photomultiplier tubes attached at the right Measured spectra from polyvinyl toluene plastic scintillator detector Measured spectra from NaI(Tl) detector Two large-volume NaI(Tl) detectors with photomultiplier tubes attached and enclosed in metal shielding Layout of radiation sensor panel components within the environmental enclosure Alarm view scan, supervisory computer shown at a test facility Radiation portal monitor concept for cargo portals with existing U.S. Customs and Border Protection kiosks Basic architecture of a radiation portal monitor system with limited ancillary equipment Inside an NaI(Tl)-based prototype radiation sensor panel developed by Pacific Northwest National Laboratory Mobile radiation portal monitors in two-sided configuration Concept for a rail radiation portal monitor system Remotely operated radiation portal monitor Artist’s image of a straddle carrier portal concept Prototype mobile straddle carrier radiation portal monitor Concept for a portable source identification device Example personal radiation detector: Radiation Pager® Example radioisotope identifier device Bank of monitors for viewing area surveillance system imagery Auto dialer Modular booth Gate arm deployment Representative inductive loop presence sensor Lane speaker box and booth master unit for wireless intercom Optical character recognition reconciliation tool interface shown at a test facility Optical break-beam presence sensors typically used on radiation portal monitors Example programmable logic controller module Rail identification system components Strobe/siren unit installed at mail/ECCF deployment

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97 107 108 108 109 111 113 114 114 116 117 118 119 120 122 122 124 124 126 126 127 128 128 129 132 132 133 133 134

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List of Illustrations

3.34 Visual identification system images displayed at a test facility 3.35 Visual identification system camera and lighting attached to a radiation portal monitor 3.36 The Acoustic Inspection Device is a handheld gun that transmits ultrasonic pulses and detects return echoes to determine the contents of sealed containers 3.37 An example of the PNNL radio frequency imaging system installed at a security gate 3.38 Nonintrusive weapons detection using millimeter waves reveals hidden objects of metal and plastic 3.39 Transmission of a radiographic image using a 3.8 MV linear accelerator 3.40 Schematic of imaging system incorporating simultaneous backscatter and transmission of X-ray images from a single source 3.41 Backscatter image of stowaways in a cargo container 3.42 Computed tomographic image of a suitcase taken with an airport luggage scanner 3.43 The X-ray spectrum is hardened after passing through filters of aluminum, copper, and tin 3.44 A transmission image with a 3.5-MeV beam of a 0.46-mdiameter object with alternating lead and aluminum spokes 3.45 Mass attenuation coefficients for photons in various materials 3.46 Isometric drawing of a relocatable vehicle and cargo inspection system for truck inspection at a border 3.47 Sectional view of the relocatable vehicle and cargo inspection system truck inspection station 3.48 Two examples of a large-footprint system employing a high-energy linac 3.49 Example of a high-resolution backscatter X-ray image of automobiles 3.50 Transmission image of a truckload of durians, a spiny Southeast Asian fruit 3.51 A backscatter X-ray image of the same truckload of durians, showing an anomaly 3.52 A backscatter X-ray imager for checking people for contraband 3.53 Image from a scan with a BodySearch “Z-Backscatter™” scanner 3.54 Image taken with a transmission system during trials (plastic knife on the hip); this individual received 0.25 μSv 3.55 A typical accelerator to produce a neutron beam 3.56 A typical electron accelerator used to produce bremsstrahlung photons for imaging; in this case, a pallet of plywood 3.57 Large panels of 6Li glass fiber thermal neutron detectors 4.1 Calculated intrinsic detection efficiency for PVT and NaI(Tl) 4.2 Calculated absolute detection efficiency for polyvinyl toluene and NaI(Tl)

135 136

137 138 139 140 142 143 143 144 144 145 148 148 150 151 152 152 153 154 155 157 158 160 165 168

List of Illustrations

4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13

4.14

4.15

4.16 4.17 4.18 4.19 4.20

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Measured 133Ba spectrum from Na(Tl) and PVT detectors 171 Calculated photon flux and spectra for polyvinyl toluene and NaI(Tl) for 241Am 172 Calculated photon flux and spectra for polyvinyl toluene and NaI(Tl) for 133Ba 173 Calculated photon flux and spectra for polyvinyl toluene and NaI(Tl) for 232Th 174 A photo of a 20 ft (6 m) “dry van” trailer and the three-dimensional screen model representation of the trailer vehicle 176 A three-dimensional rendering of the cargo as a “cutaway” of the trailer shown in Figure 4.7 176 Model-to-data ratio for 133Ba source locations within intermodal cargo container without cargo 177 Source locations used with the trailer model for validation 178 Simulated drive-through profile showing dense cargo suppression of background 179 Simulated drive-through profile for naturally occurring radioactive material cargo 180 Example of the total gamma-ray counting rates in counts-per-second from a typical polyvinyl toluene used in vehicle radiation portal monitor systems showing the distribution of cargo vehicles with or without naturally occurring radioactive material plus background, the distribution of background alone, and a simulated test source. The vehicle distribution is seen to be downshifted relative to the background distribution, showing the effect of shadow shielding 183 Spectra from polyvinyl toluene for (A) naturally occurring radioactive material radiation and background, and (B) highly enriched uranium, weapons grade plutonium, and background to illustrate the differences in the spectra at low energies 186 Count rates in counts per second per energy window in the three energy bins for background, highly enriched uranium, weapons-grade plutonium, fertilizer, and tile obtained by summing the spectra in Figure 4.3 192 The ratio of counts in the (A) low- and (B) medium-energy windows to the counts in the high-energy window 193 Typical naturally occurring radioactive material alarm vehicle profile from a cargo radiation portal monitor 195 Typical nonradioactive cargo vehicle profile showing background suppression due to shadow shielding 196 Total count profiles for each of 700 vehicles passing through a 4-panel radiation portal monitor 197 The channel-by-channel ratio of counts in a net source spectrum to counts from a background spectrum for spectra from 57Co, highly enriched uranium, 133Ba, weapons-grade plutonium, and depleted uranium 198

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List of Illustrations

4.21 Detection probability versus number of injected counts for 57Co 4.22 Detection probability versus number of injected counts for 133Ba 4.23 Detection probability versus number of injected counts for depleted uranium 4.24 Results from a single measurement with 2 kg (4.4 lb) of depleted uranium walked through a radiation portal monitor, displayed as the standard deviations above or below zero for four delta statistics versus sample observation time 4.25 Schematic of radiation portal monitor background count rate during a commercial vehicle passage, illustrating the dense-load background suppression effect 4.26 Schematic of count rate of two commercial vehicle passages, illustrating the difference in the profiles of a uniform source and a point-like source 4.27 Radiation portal monitor geometry 4.28 Effect of limiting position on m values 4.29 Comparison of spatial profile model 4.30 Example of a measured temporal profile 4.31 Comparison of simulated and measured spatial profile 4.32 Effect of source strength on optimal limiting position, which is the distance over which the signal is integrated 4.33 Optimal sum intervals 4.34 Signal-to-noise ratio for constant sources 4.35 Figure of merit for point source sensitivity 4.36 Comparison of (a) spatial profiles and (b) MDAs for varying d values 4.37 Ensemble plots from 979 vehicles showing vehicle percent baseline suppression profiles for all four radiation portal monitor panels at Site (A), with all narrow lanes pooled 4.38 Typical vehicle percent baseline suppression profiles 4.39 Profiles of percent ratio suppression based on vehicle energy window ratios vis à vis background energy window ratios for the narrow lanes at Site (A) 4.40 Spectroscopic portal monitor prototype utilizing four NaI(Tl) logs 4.41 The measured spectral shape of a 228Th source as a function of lead-shielding thickness 4.42 Background and alarming stationary vehicle spectra for an NaI(Tl) portal 4.43 Drive-by spectra from 60Co and UO2 cargo 4.44 Timeline for the introduction of advanced spectroscopic portal systems into the U.S. Customs and Border Protection environment 4.45 General model of human element in radiation portal monitor security systems

201 201 202

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207 208 210 211 213 213 214 215 216 217 218

221 222

225 229 231 232 233 236 238

List of Illustrations

4.46 Two general classes of human-mediated inspection systems 4.47 Two-state radiation portal monitor alarm system involves four postalarm steps 4.48 Likelihood alarm radiation portal monitor system provides more information to officers through green, yellow, and red indications 5.1 Radiation portal monitors in privately owned vehicle lanes 5.2 Standard four-panel cargo radiation portal monitors 5.3 Eight-panel wide cargo radiation portal monitor at secondary 5.4 Dual-use radiation portal monitor 5.5 Cantilever cargo portal 5.6 Secondary and bus portal 5.7 Ship at dock at a seaport terminal 5.8 Radiation portal monitors at a seaport terminal 5.9 Seaport terminal exit gate 5.10 Primary and secondary truck exit gate portals 5.11 Small conveyer belt radiation portal monitor configuration in an international mail facility 5.12 Typical radiation sensor panel configuration for a doorway in an international mail facility 5.13 Tug portal for scanning packages and mail while in transit to an international express consignment courier facility 5.14 Cart-mounted, portable radiation portal monitor system for international mail/express consignment courier facility 5.15 Truck portal at an international mail facility 5.16 Airport tug radiation portal monitor prototype at test bed facility 5.17 Rail radiation portal monitor prototype 6.1 A record of the gamma-ray signal observed for the passage of vehicles through a radiation portal monitor over a period of about 6 hours 6.2 Record of the observed gamma-ray signal from a number of vehicles passing through a radiation portal monitor 6.3 Shipments of smoke detectors can cause radiation portal monitor alarms due to the presence of 241Am sources in most units 6.4 Radiation source used as a density gauge for liquids flowing through the vertical cylinder 6.5 Depleted uranium is used in a number of commercial applications, including shipping shields for strong commercial radiation sources, military munitions and armor, and airplane counterweights 6.6 An example of a gauge used to measure concrete dryness and soil density; such gauges can contain both gamma and neutron sources 6.7 Nuclear fuel assembly containing thousands of fuel pellets 6.8 Neutron spike event induced by a cosmic ray as seen at channel 31 6.9 Radiation portal monitor response to a vehicle containing a neutron and gamma-ray source; the center curve is the neutron response

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239 243

244 254 254 255 255 256 257 258 259 260 262 264 265 266 267 267 269 271 278 278 279 280

280 281 282 283 283

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Tables 2.1 2.2

2.3 2.4

2.5

2.6 2.7 2.8 2.9 3.1

3.2 4.1 4.2 4.3 4.4

4.5 5.1 7.1 7.2 7.3 7.4

Average Correlation Between Radiation Portal Monitor Locations and Similar Cosmic Ray Observatory Locations 46 Average Correlation of the Low-Energy, High-Energy, and Neutron Background from Nine Radiation Portal Monitor Locations with Nine Weather Parameters 49 Average Correlation between Seasonal Cycle and the Measured Radiation Portal Monitor Background 56 Naturally Occurring Radioactive Material and Technologically Enhanced Naturally Occurring Radioactive Material Activity in Bq/kg for Various Materials and Foods 60 Other Cargo Items Known to Contain Significant Levels of Naturally Occurring Radioactive Material and Technologically Enhanced Naturally Occurring Radioactive Material 61 Approximate Relative Percentage of Alarming Cargo Loads Containing the Listed Radioactive Materials Observed at Some Border Crossings 62 Summary of Survey Results for Medical Radionuclide Use 67 Intrinsic Decay Properties of Medical Isotopes 69 Frequencies of Interest 83 Absolute Gamma-Ray Detection Efficiency for a Radiation Portal Monitor Detector with the Sources 2 m (6.5 ft) from the Detector Face 106 Ancillary Equipment Applications 125 Photon Emission Data for Model Sources Used in MCNP Simulations; Dominant Gamma Rays Are Shown in the Shaded Boxes 169 Power Values for Various Source Shield Configurations 212 Vehicle Profile Summary Statistics for a Single Vehicle from Four Portal Panels at Site (A) as Shown in Figure 4.38 223 Absolute Detection Efficiency Comparison for a PVT-Based System Versus a NaI(Tl) System Consisting of Four 10 cm × 10 cm × 40 cm (4 in. × 4 in. × 16 in.) Crystals 230 Detection Efficiencies for Radionuclides 234 Ancillary Equipment Available for Deployment with Radiation Portal Monitor Systems 250 Description of Scope and Purpose of American National Standards Institute Standards for Homeland Security Applications 290 Personal Radiation Detectors 292 Vehicle Portal Monitors 293 Some Properties of Scintillator Materials 294

{ preface } U.S. Customs and Border Protection (CBP) is the agency within the U.S. Department of Homeland Security with primary responsibility for interdiction of terrorist threats at U.S. borders. Events in recent years have resulted in a dramatic change to CBP’s scope of responsibility. In early 2002, as part of its response to the events of September 11, 2001, and the heightened threat of nuclear or radiological terrorism, CBP created the Radiation Portal Monitor Project (RPMP). Through the deployment of sophisticated radiation detection technology at U.S. ports of entry, the objective of the RPMP is to significantly enhance CBP’s ability to scan for illicit trafficking of radiological threats at our nation’s legal ports of entry (POEs). In this book, we provide a wide range of information from multiple authors on applicable radiation detection and interdiction methods, and reports on the approach taken in the first several years of the project by CBP, and on its behalf, by the Pacific Northwest National Laboratory RPMP team in deploying equipment and establishing operational procedures to effectively interdict nuclear and other radioactive material threats. While concentrating on the efforts of the RPMP, this information is placed into the context of broader security efforts taking place around the world. The terrorist threat is an international concern and efforts to prevent terrorist acts are found within many sectors of the U.S. government, other governments around the world, and international organizations. Only through a coordinated multinational effort can we hope to counter threats from those who would seek to harm others through nuclear or radiological means. Through its technical and deployment efforts, the RPMP has generated a large body of scientific work. This document describes most of the project and significant technical results of the first several years of this effort, including introduction to the radiation interdiction problem, description of technical approaches and possible enhancements, discussion of lessons learned to date from technology deployments, overview of related work, and future considerations of radiation interdiction for border security. We have chosen to focus this document on the time frame from the inception of the RPMP in January 2002 through September 2005, the first several years of the project. In October 2005, the RPMP became a joint program that was comanaged by CBP and the newly created Domestic Nuclear Detection Office. We felt that it would be beneficial to take the opportunity provided by this natural transition to document many of the technical and operational aspects developed and observed in this first stage of the project. Therefore, this document primarily covers the time frame prior to the RPMP becoming a joint program in October

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2005. Because of the sensitive nature of any such U.S. Department of Homeland Security program, we have been careful to share only certain information and data so that this book may be made more widely available. This book is not intended to represent a broad consensus on all methods and approaches for interdiction of radiological materials. Rather, it represents the expansive knowledge base of the RPMP team and the approach taken by this project. The RPMP balanced several factors in defining a viable and effective plan for interdiction of radioactive materials at POEs. Characteristic of any large effort of national importance, there have been varied opinions expressed about the best approach to protect our POEs. We have always considered such a debate to be invaluable to vetting and defining the RPMP. This book is primarily the product of a small group of authors at Pacific Northwest National Laboratory, but it represents the body of work done by many individuals at the laboratory and CBP, as well as many external contractors. We, the editors, are privileged to represent this collaborative work. Richard T. Kouzes Joseph C. McDonald Denis M. Strachan Sonya M. Bowyer Editors January 2007

{ acknowledgments } The technical work described herein was produced by the individual contributions of many staff members at Pacific Northwest National Laboratory who have worked on the Radiation Portal Monitor Project. Our gratitude goes to each individual whose dedication and time have contributed to the progress of the project. Of special note are the early project leaders who established the foundation of the project: Randy Hansen, James Hartley, Richard Kouzes, John Schmidt, Robert Thompson, and Ray Warner. We wish to thank Hope Matthews and Lynn Roeder for their assistance in preparing this book. We also want to express our appreciation to the leaders at U.S. Customs and Border Protection who directed and supported the efforts described here over the first several years of the project. Of particular note is Mr. John Pennella, who initiated the Radiation Portal Monitor Project; Mr. Christopher Milowic, who oversaw the project at its beginning; Ms. Sharon Sharp-Harrison, who dealt with the dayto-day activities of the project since its inception; Mr. Ira Reese, a constant supporter of the project through his role as director of Laboratories and Scientific Services; and Mr. Todd Hoffman, the Office of Field Operations sponsor of the project. Our thanks to Tracy Mustin for her assistance. This work was sponsored by the U.S. Department of Homeland Security’s U.S. Customs and Border Protection. The Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.

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{ introduction } Events on September 11, 2001, changed the way Americans think about threats and defense. New defense approaches to reduce terrorist threats must now include scanning of cargo and passenger transportation modes for terrorist weapons and components. The radiological threats of most concern include acquired or stolen weapons, improvised nuclear devices, special nuclear material for weapons construction (including plutonium and highly enriched uranium), and material or assemblies for radiological dispersal devices (also known as “dirty bombs”). All of these radiological threats produce gamma ray or photon radiation, while plutonium, unique in its role as part of a weapon of mass destruction, also emits neutron radiation. What follows is an outline of the topics in this document. Chapter 1 focuses on the motivation for radiation interdiction at borders. Radiological scanning instrumentation is being broadly deployed at U.S. and foreign borders to search for nuclear and radiological contraband with passive and active sensing techniques. Although very few threatening radioactive cargos are expected, it is necessary to survey every vehicle and conveyance entering the country so these rare radionuclide threat items can be intercepted. This is a different requirement than that used for narcotics interdiction where the consequences of something escaping detection are less consequential. At the same time, such activity for the interdiction of radiological threats cannot impact the flow of commerce. Following the tragic events in 2001, the U.S. government emphasized the need for the U.S. Customs and Border Protection (CBP) to quickly and effectively deploy interdiction systems to combat the increasing threat of terrorists who might attempt to smuggle radioactive material into the U.S. through its ports of entry. To fulfill this requirement, the Radiation Portal Monitor Project (RPMP) was established in January 2002 at Pacific Northwest National Laboratory (PNNL). The primary objective of the RPMP is to quickly and effectively deploy radiation portal monitor (RPM) systems, as funding permits, at all U.S. ports of entry sites (i.e., international mail, express consignment courier facilities, land and rail border crossings, seaport terminals, and international airport terminals) that are selected by CBP on a prioritized basis. The design and operation of radiation detection systems for the interdiction of radiological materials at U.S. borders must be appropriately matched to the detection of a specified set of threats. In the case of

 A threat is a circumstance or event that could lead to the creation and/or exploitation of a flaw in a system.

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Introduction

RPM systems, this requirement for an operationally viable approach amounts to having a specified target for a detectable quantity of radioactive material under a specified geometry. Chapter 2 discusses sources of radiation and possible interferences for radiation detection. An introduction to the physics and statistics of ionizing radiation is covered with the purpose of introducing the terms used throughout this book. After this introduction, four sources of ionizing radiation are discussed: • Ubiquitous background radiation from soil, construction materials, and cosmic rays • Naturally occurring radioactive materials • Medical sources of radiation • Industrial and special nuclear materials

This is followed by a discussion of nonionizing radiation used for telecommunication that can interfere with the operation of an ionizing radiation detection device. Chapter 3 discusses interdiction of targeted materials, a difficult task intensified by the need for increasingly sophisticated technology. In this section, interdiction technologies are discussed, along with new developments, especially in areas such as active interrogation techniques and imaging. After a brief review of radiation detection basics, the operational realities under which detection must be performed at ports of entry are provided. This is followed by a discussion of current instrumentation and capabilities, and by information about more advanced instrumentation and techniques being developed. Chapter 4 discusses enhancing the effectiveness of RPM systems. Plastic detector–based RPM systems have many advantages, which includes their simplicity and low cost. However, they also have significant limitations under normal operation. For example, the use of gross counts to trigger alarms can lead to burdensome naturally occurring radioactive material–related alarms in some operating environments. Efforts to enhance the effectiveness of RPM systems are discussed, principally via changes to the software analysis of the count rate data. These changes allow more information to be gleaned from the collected data, and in combination with gross counts, allow better discrimination between normal commerce and materials of concern. An important element of this discussion is the use of modeling and simulation to better understand how count rate data are collected, how background count rates are suppressed by such phenomena as shadow shielding, and what suppression means to the count rate alarm levels. In addition, information is presented about how the data might be manipulated to gain more discrimination. The chapter then discusses the future role of spectroscopic detectors as RPMs. Lastly, the discussion focuses on the role of humans in making RPM systems more effective tools for detecting materials of concern. Chapter 5 discusses RPM deployments. The RPMP was initiated to provide CBP with technical advice on the interdiction of radiological and nuclear materials,

Introduction

xxv

and it grew into a deployment project to specify, procure, and deploy equipment targeted at the interdiction of illicit radioactive materials at U.S. ports of entry. The project thus hinged upon the fundamental technologies and tools capable of detecting radioactive materials. Chapter 6 discusses operational considerations for interdiction. Radiation interdiction equipment is deployed into the complex and busy operational environment of border crossings. Customs and Border Protection officers are responsible for enforcing hundreds of laws, and their new major role of protection against terrorism has added to their workload. This section considers some of the specific operational problems encountered. Border crossings were operated long before radiation detection devices were needed. Therefore, the installation of radiation detectors required that the devices fit within the physical confines of the existing facilities, and, once installed, have a minimal impact on facility operations. At border crossings, the impact on operations is minimized by incorporating the radiation scanning program into the existing standard operating procedures as much as possible. Chapter 7 discusses activities related to the interdiction of nuclear and radiological material that are underway around the world. Some of these activities include standards and testing that are important aspects of any instrumentation planned for field deployment. For equipment that is deployed for radiation interdiction purposes, standards provide vendors with the minimum requirements that the equipment must meet. Significant efforts are made both domestically and internationally in developing instrument standards for border security equipment and testing equipment of all types against these standards. For the last decade, the U.S. Department of Energy has worked to interdict illicit materials around the world, notably under the Second Line of Defense Program. The U.S. Department of Defense has also had an active program for force protection against radioactive threats, most notably the Guardian Project. Within the U.S. Department of Homeland Security, other major efforts (beyond the RPMP) largely relate to future requirements for interdiction. In the international arena, several nations have undertaken programs similar to those in the United States, and the International Atomic Energy Agency has been an important leader internationally. Chapter 8 discusses the future for interdiction of radiological and nuclear threats at borders. Radiation detection systems currently deployed at ports of entry and the additional systems being deployed are tailored in various ways to serve specific roles. These systems have been designed with precise dimensions and packaging and include newly specified user interfaces, electronics, alarm algorithms, and communication capabilities. Nonetheless, these systems rely on principles that had previously been demonstrated in field applications. In most—if not all—cases there is no presumption that current instruments are the most effective, efficient, or economical instruments that can ultimately be obtained. Continuing research and development activities are essential to ensure that U.S. ports of entry

xxvi

Introduction

are equipped with increasingly capable, yet affordable, radiation detection technology that remains highly effective against a dynamic terrorist threat. This book mainly covers the first several years of the RPMP from its inception by CBP in January 2002 to the time it became a joint program comanaged by CBP and the Domestic Nuclear Detection Office in October 2005. Since October 2005, the RPMP has continued to improve and press forward in effectively providing the protection needed at our nation’s ports of entry.

{ acronyms and abbreviations } BF ANSI ASP CBP CBRNE DB DHS DNDO DoD DOE DT DTRA DU ECCF EMP FOM FWHM HEU HPGe HSARPA IAEA IEC IGY IMCC IND IPP JPM JSIPP LSS MCNP MDA NaI(Tl) NIS NNSA NORM NRC NRF NURE OCR PB PBS

percent branching fraction American National Standards Institute Advanced Spectroscopic Portal U.S. Customs and Border Protection chemical, biological, radiological, nuclear, and explosive driver bottom Department of Homeland Security Domestic Nuclear Detection Office U.S. Department of Defense U.S. Department of Energy driver top Defense Threat Reduction Agency depleted uranium express consignment courier facility electromagnetic pulse figure of merit full width at half maximum highly enriched uranium high-purity germanium Homeland Security Advanced Research Projects Agency International Atomic Energy Agency International Electrotechnical Commission International Geophysical Year intermodal cargo container improvised nuclear device Installation Protection Program Joint Program Manager Joint Service Installation Pilot Project Laboratories and Scientific Services Monte Carlo N-Particle Minimum detectable amount thallium-doped sodium iodide National Integration System National Nuclear Security Administration naturally occurring radioactive material U.S. Nuclear Regulatory Commission nuclear resonance fluorescence National Uranium Resource Evaluation optical character recognition passenger bottom percent baseline suppression

xxviii

PLC PNNL POE POV PRD PRIDE PRS PSID PT PVT Rad/Nuc RDD RDT&E RFI RIID RO-RPM RPM RPMP RSP SAIC SLD SNM SPM TENORM UNSCEAR UNWD VACIS VIS WGPu WMD

Acronyms and Abbreviations

programmable logic controller Pacific Northwest National Laboratory port of entry privately owned vehicle personal radiation detectors Port Radiation Inspection, Detection, and Evaluation percent (window) ratio suppression portable source identification device passenger top polyvinyl toluene radiological/nuclear radiation dispersal device research, development, testing, and evaluation radio frequency interference radiation isotope identifier device remotely operated radiation portal monitor radiation portal monitor Radiation Portal Monitor Project radiation sensor panel Science Applications International Corporation Second Line of Defense (within DOE) special nuclear material spectroscopic portal monitor technologically enhanced NORM United Nations Scientific Committee on the Effects of Atomic Radiation Unconventional Nuclear Warfare Defense Vehicle and Cargo Inspection System Visual Identification System weapons grade plutonium weapons of mass destruction or disruption

{ units } per mm-Hg μR/hr μrem μSv Bq Ci cm cps cps/ng eV ft GB GeV GV in kCi keV kg MBq MV MeV MHz mph mrem m/s mSv Pa pC Sv V/m ZeV

percent change per millimeter of mercury microroentgen per hour microrem microsievert becquerel curie centimeters counts per second counts per second per nanogram electron volt foot gigabyte giga-electron-volt gigavolt inch kilocurie kilo-electron volt kilogram megabecquerel megavolt megaelectron volt megahertz miles per hour millirem meters per second milliseivert pascal picocoulomb seivert volts per meter zepta electron volts

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Radiation Detection and Interdiction at U.S. Borders

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{1}

Overview of Radiation Interdiction Richard Kouzes

September 11, 2001, changed the way Americans think about threats to the homeland and national defense. Terrorists are more concerned with instilling fear than destroying or deterring the use of military capability. The psychological and economic disruption caused by a terrorist attack does not necessarily require the spatial precision or timing of sophisticated military delivery systems. Consequently, the means by which an adversary’s weapon might reach its target has expanded from conventional military systems, such as missiles, to include common means of transport such as pedestrians, cars, trains, planes, and ships. New approaches to reduce the terrorist threat by these means must now include scanning of cargo and passenger for potential weapons.

terrorist weapons: two types of weapons Weapons of mass destruction: Nuclear, biological, or chemical weapons that cause catastrophic loss of life and/or property. Weapons of mass disruption: Any weapon that causes catastrophic consequences to a way of life, either through economic, social, or health impacts.

Radiological threats of most concern include acquired or stolen nuclear weapons such as improvised nuclear devices (INDs), special nuclear material (SNM) for weapons construction (including plutonium and highly enriched uranium [HEU]), and material or assemblies for radiological dispersal devices (RDDs), also known as dirty bombs. All these threats are comprised of radioactive materials that produce a gamma radiation signature. Some—most importantly plutonium— also emit neutron radiation. Detection of these threats is critical. Radiological scanning instrumentation is undergoing broad deployment globally with passive and active–sensing techniques to search for nuclear, radiological, and other contraband. Active techniques are defined here to include X-ray or gamma-ray radiography, and neutron or gamma-ray-induced signatures for the detection of explosives and SNM. Passive detection techniques include gamma-ray or neutron signature detection for radiological materials.



Radiation Detection and Interdiction at U.S. Borders

While very few conveyances are expected to contain threatening radioactive material, it is still necessary to scan all vehicles, cargo, mail, and packages at ports of entry (POEs) because of the very serious consequences of illicit nuclear materials being smuggled into the country. This is in stark contrast to the scanning process for drugs or other contraband where one can have effective enforcement while only surveying a statistical sample of vehicles, especially when intelligent targeting can increase detection probabilities. Because many ports of entry process a huge volume of cargo, scanning must be performed within a limited time frame to reduce the impact on the flow of legitimate trade and travel. If this efficiency is not achieved, and legitimate trade and travel is delayed, terrorists will have succeeded in achieving negative economic impact from reduced trade. To reliably intercept threats, a layered approach to defense must be taken in which each layer is comprised of a unique combination of technology along with human capabilities. As an example, intelligence information may lead to targeting certain vessels or cargo, and then passive scanning methods (such as radiation portal monitors [RPMs] and handheld or belt-worn radiation detectors) can be used for radiological interdiction. Radiography systems can also be used to look within cargo containers or packages to scan for hidden areas or suspect commodities, while acoustic techniques can be used to scan bulk liquid carriers for hidden material or structures. With available technology, vehicle surveys for radioactive materials have been implemented in an operationally acceptable manner at U.S. ports of entry (Kouzes 2004, 2005). Highly trained U.S. Customs and Border Protection (CBP) officers evaluate the attitude and behavior of people passing through control points and evaluate the input from all detection mechanisms to arrive at a probability-of-threat decision. The highly complementary nature of technology and human skill combined in this example results in effective detection of radiological and nuclear threats. This chapter provides an introduction to the port-of-entry radiation interdiction scenario, the history leading up to technologies deployed for interdiction, and some of the policy concerns that affect technical solutions for interdicting nuclear and radiological materials of concern.

1.1 Radiation Portal Monitor Project History Megan Lerchen, Richard Kouzes, and Robert Thompson Following the events of September 11, 2001, the U.S. government emphasized the need for CBP to quickly and effectively deploy radiation interdiction systems to  A POE is a legal entry point across an international border and may consist of multiple physical sites or crossings.  In 2001, the U.S. Customs Service was part of the Department of Treasury. With the creation of the Department of Homeland Security, the name was changed to U.S. Customs and Border Protection.

Overview of Radiation Interdiction

5

combat the increasing threat of terrorists penetrating the United States at ports of entry. Deployment of these systems required specialized science and technology support to detect and identify radioactive materials and components associated with the development and delivery of nuclear weapons and other radiological contraband. The Applied Technology Division at CBP, as part of its Radiation Monitoring Support Program, required a broad range of technical support in identifying and understanding the fundamental nature of the nuclear interdiction mission, evaluating and testing commercially available threat interdiction technologies, and deploying a nationwide network of radiological and nuclear material interdiction systems. To fulfill this requirement, the Radiation Portal Monitor Project (RPMP) was established in January 2002 at Pacific Northwest National Laboratory (PNNL). The term “RPM” refers to the primary radiation detection technology and configuration used in the project.

1.1.1 interdiction goals and objectives Customs and Border Protection deploys and operates the best available technology to (1) prevent smuggling of nuclear and radiological materials into the United States consistent with its strategic plan to prevent entry of illicit materials by terrorists at U.S. ports of entry, (2) balance legitimate trade and travel with security, and (3) modernize technology. To further strengthen its prevention capabilities, CBP developed near-term, midterm, and long-term strategic goals that address technology gaps and limitations with currently deployed detection technologies. The CBP continues to identify emerging technologies and plans to use both passive detection and active interrogation methods. Working in complex border environments, CBP effectively and efficiently scans all passengers, vehicles, and cargo, while maintaining the steady flow of legitimate trade and travel. Secondary scanning is conducted, when needed, to resolve primary scanning alarms and investigate suspect items. Customs and Border Protection applies a layered defense of radiation sensors, each with specific capabilities and defined applications, consisting of RPMs, radionuclide identifiers, and personal radiation detectors (PRDs). The intent of terrorists, criminal organizations, and rogue nations to cause damage to people and property, and to disrupt the economy of the United States, is widely known from well–documented prior attempts. Failure to deter and interdict radiological and nuclear materials and weapons entering the United States could result in serious loss of life, disruption of commerce, and the destruction and/or

Herein, whether a time before or after the creation of Department of Homeland Security is being referred to, the name CBP is used.  PNNL is a U.S. Department of Energy multiprogram national laboratory operated by Battelle Memorial Institute.



Radiation Detection and Interdiction at U.S. Borders

“U.S. Customs and Border Protection is addressing the terrorist threat -hours a day. We have a multi-layered approach that encompasses working with our foreign counterparts, employing intelligence, technology, advanced information in the field and the most professional workforce worldwide. We are aware of the terrorist threat and are evolving hourly to face it and keep America safe.” Robert C. Bonner, CBP Commissioner September , 

contamination of property beyond any event previously experienced (Reichmuth 2005). To minimize this threat, CBP develops and deploys highly integrated countermeasure systems that increase the ability to detect, identify, and deter the import of illicit radiological materials and improvised or stolen nuclear devices. These systems serve to complement other CBP targeting and data systems used for passenger and cargo scanning under such measures as the Container Security Initiative and the Customs-Trade Partnership Against Terrorism. Radiological and nuclear interdiction activities directed by CBP also work in harmony with international efforts, such as the U.S. Department of Energy (DOE) Second Line of Defense (SLD) and Megaports Initiative programs (see Chapter 7).

1.1.2 radiation portal monitor project mission The RPMP provides scientific and technical expertise on radiation detection and materials to support the CBP mission of radioactive materials interdiction at U.S. land, sea, and air ports of entry. This is accomplished through the expertise of PNNL staff in developing, identifying, testing, and deploying radiation detection tools that meet international detection standards, have minimal impact to the legitimate flow of trade and travel, are cost effective, and are (or can be made) commercially available. Critical responsibilities of CBP include detecting, identifying, and interdicting illicit radiological and nuclear materials at U.S. ports of entry. These responsibilities are closely aligned with the overall mission of the Department of Homeland Security (DHS) to strengthen border security against terrorist threats. The Science and Technology Directorate at DHS oversees research conducted to improve general detection and interdiction capabilities, while the Domestic Nuclear Detection Office (DNDO), created at DHS in 2005, now conducts research and development of advanced detection systems, and tests and procures these systems for scanning and interrogating vehicles and containerized cargo for radiological and nuclear threats. Together, these DHS organizations share the overall accountability for developing, deploying, and operating effective technologies to detect and identify smuggled threat materials at international mail facilities and express consignment courier facilities (ECCFs), land and rail border crossings, seaport and international airport terminals, and areas between the ports of entry. These organizations also

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7

have the broader responsibility to coordinate detection and interdiction activities with other agencies, both domestically and internationally. One of the primary objectives of the RPMP is to deploy RPM systems quickly and effectively at all sites selected by CBP on a prioritized basis. These systems must provide effective interdiction capabilities for radiological items associated with weapons of mass destruction (WMDs) and other radiological threats. If other scanning technologies become available that can accomplish CBP objectives more effectively, efficiently, or economically, they may replace the RPM systems in accordance with CBP direction. Solutions to scanning problems at a particular site must address several factors, including current and future traffic volumes, site layout, type of cargo, radiation detection equipment performance, and defined threats. These and other factors may change over time. The second objective of the RPMP is to provide the scientific and technical expertise needed to design and support the deployment of RPM systems, and to interpret the acquired data. This includes specific expertise in radiation detection systems and their field application, operational simulation modeling, computational analysis, and evaluation of the transport phenomena associated with radioactive materials.

1.1.3 initial radiation portal monitor project activities In late autumn 2001, CBP was faced with an increased need to understand the threats and vulnerabilities posed by potential materials and components associated with the development and delivery of nuclear weapons and other radiological contraband. This extended to the problem of scanning inbound traffic for elevated radiation signatures—a potential indicator of a WMD or RDD—and identifying tools that could be used in the field for routine scanning. To meet this need, starting from initial tasking in January 2002, the CBP Applied Technology Division eventually enlisted PNNL for its expertise in radiation detection physics. Thus, the staff at PNNL began to provide broad technical support to identify and understand the fundamental nature of the nuclear and radiological materials interdiction mission, evaluate and test commercially available threat interdiction technologies, and deploy a nationwide network of nuclear and radiological materials interdiction systems. Customs and Border Protection requested consultation in these areas because PNNL staff had been providing science and technology support to CPB since the early 1990s in areas such as WMD interdiction considerations, WMD interdiction training to field operations and international inspectors, and nonintrusive inspection system evaluation. Consequently, as part of the CBP Radiation Monitoring Support Program, the RPMP was established. Initially consulted by CBP to focus on determining the radiological threat, PNNL staff was also asked to identify functional requirements for RPMs and suitable commercially available technologies that could be immediately deployed. A PNNL project manager and a small team of scientists and engineers advised CBP



Radiation Detection and Interdiction at U.S. Borders

on radiation interdiction. These early activities led to an understanding of CBP project needs, followed by instituting the RPMP to support these needs. Potential nuclear and radiological materials that could be used in nuclear weapons or RDDs were identified as plutonium, HEU, and several alpha-, beta-, and gamma-emitting radioisotopes. Various detection technologies were identified as capable of sensing these materials, including commercial devices ranging from handheld instruments to radiation portal systems for scanning vehicles. Efforts toward specifying a primary detection technology soon focused on RPMs capable of scanning cargo and entire vehicles. The conclusion of these efforts was a recommendation that CBP deploy RPMs as the main tool for interdiction of radiological and nuclear materials at border crossings. Portal monitors used to detect SNM and other radioactive sources in vehicles had been studied for decades, including extensive work conducted at the Los Alamos National Laboratory (Fehlau 1986, 1987; Fehlau et al. 1983) in New Mexico dating back to the 1970s. Unfortunately, commercially available systems were not ideally suited to the needs of CBP for scanning incoming cargo or traffic. In 2002, available commercial portal monitors were designed for one of two purposes: to prevent SNM from being smuggled or accidentally transported out of SNM facilities, or to detect radioactive sources entering steel scrap-reprocessing facilities. In the SNM case, the photon emissions are relatively low energy. In the steel scrap case, the photon emissions are relatively high-energy gamma rays but could be substantially shielded in a truck bed full of steel scrap. The portal monitors required by CBP needed capabilities similar to those used for both SNM and steel scrap applications. In addition, the potential for another type of threat—RDD material that only emits bremsstrahlung continuum gamma rays (such as Sr or Y)—also needed to be addressed. The PNNL staff analyzed portal monitor detector capabilities under various threat scenarios and conditions based on these broad radiation interdiction needs. Because the requirement to secure U.S. borders must be balanced by the need to maintain the flow of legitimate trade and travel, operational and cost constraints at ports of entry are generally different from those at steel scrap facilities and SNM facilities. To acknowledge this, an analysis of the measurement time required to confidently detect each source material in a variety of shielding configurations was useful in choosing technologies for deployment. For example, the specific technology solution for scanning intermodal containers at a seaport terminal could be markedly different from the solution for scanning mail and packages at an international mail facility. The staff identified effective technologies and devices that could be relatively quickly configured to a variety of deployment sites.

1.1.4 the beginning of radiation portal monitor deployment In July 2002, the director of CBP’s Applied Technology Division requested that PNNL expand its services to include deployment of radiation detection systems at

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9

CBP ports of entry. Because the timeline for RPM deployment remained immediate and urgent, initial efforts focused on fielding RPM systems as quickly as possible. This urgency drove the early, rapid installation of temporary RPM systems without the benefit of complete infrastructure modifications needed to support permanent systems. It soon became evident that such an approach did not yield the benefits initially desired (i.e., rapid, low-cost deployments). The time to deploy a permanent system was found to be only somewhat longer than a temporary system deployment, and the cost savings of performing the permanent installation rather than a temporary deployment followed by a permanent deployment could, therefore, not be justified. Thus, the conceptual approach of temporary installations was quickly abandoned in favor of permanent installations. In evaluating its initial approach, the RPMP recognized that RPM deployments would not be a small effort. Therefore, the RPMP received its first significant funding increase, with the intent that RPM deployments begin immediately. Thus, over six short months, the RPMP grew from a relatively small advisory role into a significant, deployment-driven national effort. With the new infusion of funding, the RPMP began efforts in earnest to place RPMs at U.S. borders, resulting in large demands on staffing and contracting. The change from providing advice to deploying RPM systems represented a significant change in project focus; PNNL was now being appointed to execute its own advice given to CBP. To ensure success in placing RPMs at the ports of entry specified by CBP, PNNL produced a schedule and budget to be spent over several years of deployment. In August 2002, the RPMP plans included deployment of RPM systems at selected international mail facilities and ECCFs, land border crossings, seaport terminals, international airports, and rail crossings at CBP ports of entry. Figure 1.1 shows the U.S. ports of entry, including where the RPMP has deployed equipment. By September 2002, numerous tests had been completed on portal monitor systems purchased from several vendors, and a specification was completed for the purchase of RPM systems for deployment (Stromswold et al. 2005). The DOE Radiation Detection Panel performed a review of the RPMP deployment plans. The result was a recommendation from DOE to the U.S. Customs Service that existing commercial equipment met the needs for border scanning and should be deployed immediately. As a result, a brief specification for procurement of commercial off–the-shelf equipment was written and released. This procurement resulted in an award to Ludlum Measurements, Inc., for the first large-scale deployment of RPMs. While awaiting delivery of equipment on this contract, initial deployments began with equipment that had been purchased in spring 2002 from various manufacturers for testing. These deployment activities focused on

ECCF deployments did not include United Parcel Service™and FedExs® because these companies had stated to Congress that they would perform their own deployments; subsequently, CBP agreed with this arrangement.  This panel is a collection of experts assembled by DOE from the national laboratories that advise on matters of radiation detection. 



Radiation Detection and Interdiction at U.S. Borders

Mail/ECCF Land border Maritime Air cargo

figure 1.1 The  U.S. ports of entry representing  crossings ( data). Over , vehicles, , aircraft, and  ships pass through these ports every day.

international mail facilities, ECCFs, and northern land border crossings. By the fall quarter of 2002, the RPMP began its initial sequence of successful deployments at Detroit, Michigan.

The First Deployment: Fort Street Cargo Facility, Detroit, Michigan The RPMP was directed to conduct its first RPM deployment at the Fort Street Cargo Facility in Detroit, Michigan. This port is the busiest commercial land crossing in the nation, averaging approximately 8,000 to 12,000 trucks per day. To add to this complexity, the only access to the port is over the privately owned Ambassador Bridge. The RPMP was requested to fully implement the RPM system without impeding the traffic volume or flow of revenue-generating traffic over the bridge. In late August 2002, the RPMP team was requested by CBP to design and install the first-ever operational portal system at a U.S. legal border crossing and, subsequently, train local CBP officers and supervisors on how to use the system. The aggressive schedule called for the portal system to be fully operational by midOctober 2002. This effort required bringing together multiple stakeholders, including DOE Headquarters, local and Headquarters U.S. Customs Service officials, local and Headquarters Government Services Administration officials, Detroit International Bridge Company management, vendors, craftsmen, subcontractors, and PNNL staff, to reach agreement on an acceptable design, acquiring and assembling all necessary equipment, installing the equipment, supporting CBP in developing its operational procedures, and training CBP officers on how to use the system, all in the span of about 30 days.

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figure 1.2 Primary portal at Fort Street Cargo Facility, Detroit, Michigan, showing the first installed radiation portal monitor system commissioned in October .

The portal system became operational just before midnight on October 17, 2002, meeting the aggressive schedule established by CBP. Great strides have been made in the RPMP since this early, expedited installation, but many of the founding principles established through this first deployment effort remain. Figure 1.2 shows the first RPM deployment at the Fort Street Cargo facility in Detroit.

1.1.5 deployment advances In the ensuing years, the strategy to deploy RPM systems was refined, and much progress was made and still continues. As more was learned and needs were better understood, additional ancillary equipment was identified to better enable CBP operations. Although initially viewed as a large, but straightforward project, the RPMP eventually became more attuned to the adage, “when you’ve seen one port, you’ve seen one port.” During this period, while the basic RPM system was well defined, the equipment toolset available for RPM deployments expanded to meet individual port needs for efficient operations.



Radiation Detection and Interdiction at U.S. Borders

In 2003, a memorandum of understanding between the newly formed DHS and DOE established that DHS work could be performed at PNNL on equal footing with DOE work. In July 2003, the project scope was expanded as a result of identifying additional sites requiring RPM systems. The revised scope, schedule, and cost baselines included implementing some additionally capable systems including thallium-doped sodium iodide [NaI(Tl)]-based spectroscopic portal monitor (SPM) systems. The revised scope also included establishing a detailed deployment schedule based on the current, best knowledge of the time frame when required funding could be provided. Deployment of the relatively small number of SPM systems added a substantial cost to the RPMP, with the total required funding estimated to be about $1.3 billion. This funding supported a scheduled completion of RPM system deployments and final project closeout in 2009. The full RPMP specification for procurement of RPMs was released in spring 2003 and resulted in a contract being placed for systems with Scientific Applications International Corporation (SAIC), which had purchased ExploraniumTM, a manufacturer of radiation portal monitors. The SAIC RPM systems were first deployed in 2004. The RPMP shares several common goals with the SLD program, including the Megaports Initiative within DOE (see Chapter 7). The RPMP also cooperates closely with two other organizations—the American National Standards Institute (ANSI) and the International Atomic Energy Agency (IAEA). The IAEA has been deploying radiation detection equipment under the Safeguards Program since the 1990s; as part of this effort, a specification was developed for radiation detection equipment to be used at border crossings. The SLD program and several DOE national laboratories had active participation in the IAEA standards development effort. The RPMP became similarly involved with the IAEA standards development effort in 2003, assisting with the development of the equipment standards and bringing that information back to similar U.S. efforts (IAEA 2005). When DHS was formed in 2003, part of the Science and Technology organization included a standards effort and produced standards for border security equipment (as well as other types of equipment) under the auspices of ANSI. The RPMP played a very active role in the writing of ANSI standards for border security equipment that were first released in 2004, and it continues with the development of new standards. Much of the work performed by the RPMP in writing specifications preceded the ANSI work, and these RPMP documents contributed significantly to the products produced by ANSI. The RPMP staff learned quickly that deployments required more than just standard equipment. While land border deployments were the most consistent, with

 The RPMP has undergone a number of changes in scope, schedule, and priority that have produced multiple changes in budget estimates. As this document covers the period only up to October 1, 2005, the figures and schedules given here are not current for the project.  Exploranium™ is a registered trademark of Science Applications International Corporation.

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similar requirements for infrastructure at existing CBP installations, international mail and ECCF deployments were each unique and required smaller RPM models for installation at conveyor belts, as well as the use of area mode monitoring. Rail systems were larger and required the integration of complex, industry-specific railcar identification systems. Seaport deployments introduced the use of relocatable systems and required installing infrastructure where CBP had no prior permanent physical presence. Seaports also introduced the requirement for the RPMP to develop the concepts of remotely operated RPM (RO-RPM) systems that integrate multiple technologies, straddle-carrier based systems, and mobile RPM systems. Since the beginning of the RPMP, improvements were sought in the radioisotope identifier device (RIID) used for identification in secondary processing. The RPMP performed experimental work to test and define the capabilities of various RIID replacement devices. An outgrowth of this effort were specifications for several new devices for given applications. These included specifications for a portable source identification device (PSID), which called for a large NaI(Tl) detector to be mounted on a small truck. This PSID was intended for use in scanning cargo containers, especially at seaport terminals and rail crossings. The CBP staff modified some of the mechanical capabilities and renamed this device the mobile radiation identification system. The mobile radiation identification system has not been deployed. Another very significant technology that was considered to meet multiple drivers was the SPM system based on NaI(Tl) detectors (Kouzes et al. 2005; Milbrath et al. 2005). This technology was intended to address the need for improved sensitivity to potential threats in high-volume scanning situations, the need for improved identification in secondary processing, the operational need for primary identification on rail lines, and the need for mobile identification capability with better sensitivity than a handheld RIID. With high-level government recognition of the need for such systems, development of the SPM specification began in September 2003. Within the newly created DHS, responsibility for this system fell under the Science and Technology organization, which elected to issue a broad area announcement that incorporated many of the requirements spelled out in the CBP specification. This bid process, now transitioned to DNDO, should lead to systems for deployment in 2007.

1.1.6 moving toward project completion The RPMP continues to make significant positive impacts on the national security of the United States. As the project matures and takes on greater diversity of deployment configurations, the RPM systems—including supporting subsystems—have also diversified to meet individual site needs. Despite this increased complexity, the increasing number of RPM systems in operation at ports of entry has led to a recognized need for standardization and the use of robust, cost-effective, and

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Radiation Detection and Interdiction at U.S. Borders

low-maintenance equipment. To the extent possible, RPM systems are based on standardized reference designs for each site type (vector) but are tailored to meet individual site needs and constraints. In fiscal year 2006, RPMP funding was moved from CBP to the new DNDO organization. Continuous progress has been made so that by July 2008, RPMs were routinely scanning 100% of international mail and express courier packages, 92% of cargo and 82% of privately owned vehicles (POVs) transported across northern land border crossings, 100% of cargo and 95% of POVs transported across southern land border crossings, and 98% of containerized seaport cargo, by volume.

1.2 Detecting Threats Richard Kouzes The design and operation of radiation detection systems for the interdiction of radiological materials must be appropriately matched to threats that are to be detected. A threat is a circumstance or event that could lead to the creation and/or exploitation of a flaw in a system. A threat has an associated radiological object referred to as a threat object. Since radiological and nuclear threats and threat objects span a great spectrum, it is necessary to reduce the number of threat objects to a manageable and highly relevant set that can be more effectively used to design, test, and constrain deployment and operating parameters. While operating characteristics and parameters, including thresholds, for the instrumentation are typically adjustable, the capability of equipment is limited on the low end by its inherent sensitivity, the environment in which it is used, and the implemented operational procedures. Measurement parameters are often set to detect a specified threat but may also be set to be less sensitive to accommodate individualized operating limitations at specific locations. Such factors that affect the detection capability of a deployed system create the need for a policy and approach to systematically and consistently set system operations across a broad range of deployed venues. In the case of RPM systems, this requirement for an operationally viable approach amounts to having a specified target for a detectable quantity of radioactive material under a specified geometry. Given these constraints, the size of the neutron and gamma-ray detection elements, and other components of the RPM system, can be designed. The size of the detector systems then fixes the inherent sensitivity (absolute efficiency) of the system. The systems must then be deployed at an existing site and still retain their sensitivity to the threats given this inherent sensitivity. The ANSI standard N42.35 (ANSI 2006a) embodies this approach, setting minimum standards for RPM equipment for border security purposes (see Chapter 7). This standard sets requirements for the response of RPM systems to a variety of different sources by stating amounts of specific sources that must produce alarms in specified geometries.

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1.2.1 threats For border security applications, the threats of most concern for detection are conventional nuclear WMDs originating from a nation state, INDs, such as might be built by sophisticated and well-funded terrorists, SNM that could be used for making nuclear weapons, and other radioactive materials that might be used for an RDD and/or weapons of mass disruption built by terrorists organizations or individuals. Detection equipment may also be used to detect other types of illicit trafficking, such as contaminated products or non-manifested items, but these are not specifically targeted. For non-border applications, the targeted threats might be different; for example, detection at major public event, such as at a public stadium, might focus only on RDD materials, as that threat seems more probable for such a venue than does material for making a nuclear weapon. Decisions by policy makers about the targeted threat can have a large bearing on the cost and complexity of detection equipment. Figure 1.3 shows a photograph of three Soviet nuclear weapons. Of greatest concern would be an attempt to smuggle the smallest of these examples, the artillery shell weapon in the foreground. For SNM, an upper bound on the minimum quantity that would need to be detected would be those amounts designated by DOE as sufficient for creating a nuclear explosive device: 25 kg of U or 4 kg of Pu (CG-SMG-2 2003). These quantities are similar to those designated by the IAEA as significant quantities: 25 kg of HEU or 8 kg of plutonium (IAEA 2009). Because these materials could be transported in multiple shipments of smaller amounts, and shielding could be used, the deployed radiation detection equipment must be

figure 1.3 Examples of Soviet nuclear weapons displayed in a Russian museum.

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Radiation Detection and Interdiction at U.S. Borders

sensitive to much smaller masses of material than these quantities. Radioactive materials that might be used for an RDD would be reasonably assumed to have larger radiation signatures than the targeted SNM quantities. Exceptions to this might be that the material may be shielded, or it may be used by a terrorist who simply wants to make a “statement” by exploding a small amount of radioactive material resulting in little consequence other than the disruption of daily activities.

1.2.2 example incidents The IAEA maintains a database of incidents of illicit trafficking of radiological sources (IAEA 2004). This database includes 18 incidents involving the smuggling of weapons-grade nuclear materials between 1993 and 2004. Some examples of radiological incidents include the following: • : Police at the Munich airport intercepted a suitcase from Moscow, Russia, containing approximately one-half kilogram of nuclear reactor fuel ( g of weapons-grade plutonium). • : In the Czech Republic, Prague police seized . kg of HEU from a former Russian nuclear institute worker. • : In the Chechen Republic, Chechen rebels buried a small amount of  Cs in a Moscow park and notified a Russian television crew. • : Radioactive material with explosives and an antitamper device were found near a railway east of Grozny, Russia. • : Three woodsmen in the former Soviet Republic of Georgia suffered radiation burns after handling the ~ kCi Sr core from an abandoned radiothermal generator, seen in Figure .. Such electrical generators are widely used to power remote locations, such as lighthouses. • : Russian customs officials said they had uncovered some  attempts to smuggle nuclear or radiological materials. • : Litvinenko was poisoned with ~ mCi of Po in London and died on November , . As many as , people were exposed in  countries, with  having an increased cancer risk from an exposure of over  mSv.

Another notable incident occurred in 1987 in Goiania, Brazil, when a container of Cs was scavenged from a defunct medical clinic. This was not a terrorist act, but simply the mishandling of an “orphaned” source. By the time the 1.4 kCi source was identified 11 days later, it had been tracked up to 100 miles away, killed 4 people from radiation doses of 4 to 6 Sv, burned 28 people, and contaminated 249 people, including children that had played with the glowing powder. Over 110,000 people requested medical screening, and the cleanup cost was about $20 million, with 275 truckloads of waste left in a large repository requiring long-term protection. An RDD incident might produce a similar result. Figure 1.5 shows an example of orphaned U.S. radiological sources found in a field under a water bucket.

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figure 1.4 Soviet Sr orphaned source recovered in the former Soviet Republic of Georgia.

figure 1.5 Orphaned radioactive well logging sources.

1.2.3 instrumentation to counter the threat Whenever radiation detection equipment is to be deployed, a policy decision is made about the detection requirements for threats, which drives the detector design and thus the inherent sensitivity of the instrument. Once the equipment is procured and deployed, another decision must be made regarding where to set the alarm threshold; that is, the level of radiation at which the instrument will generate

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Radiation Detection and Interdiction at U.S. Borders

an alarm. In 2002, DOE provided CBP with a targeted threat definition to be used in deploying radiation detection equipment at ports of entry. The alarm algorithm used in an RPM system is complex, so only a simple example of a gross-count instrument is presented here. Consideration of more complex algorithms will be discussed in later chapters. One approach to operating detection equipment is simply to set the threshold at the lowest value such that an acceptable rate of “false” alarms occurs. False alarms are defined as instrument alarms produced by statistical fluctuations in the background radiation or by some sort of malfunction of the instrument itself. These false alarms are unrelated to the additional alarms resulting from actual radioactive material in commerce that are actually true alarms. The acceptable false alarm rate must be determined operationally for each location, but it is typically less than 1 in 10,000 measurements. This approach utilizes the maximum capability of any specific deployed instrument but may cause significant operational problems by requiring CBP officers to resolve both false and true alarms. Designing RPM systems for this approach leaves no “excess capacity” of detection should there be a desire to detect a smaller source. This approach, operating on the edge of capability for an instrument, also means that a different threat is targeted at each deployment site, since the signal of interest rides on top of a site-dependent background. Variable site deployment and environmental factors impact the signal. An alternate approach to determine the threshold is to use measurements from a “targeted” radioactive source value that is directly related to the threat. Because a series of radiation level measurements (e.g., counts per second or dose rate) tends to produce a normal distribution, the threshold might be determined by setting it at the peak of the distribution from a series of measurements in a standard geometry from one specific source. This implies that the instrument should alarm about 50% of the time when that source is again presented in the same geometry relative to the detector. To detect the source 100% of the time, the threshold would need to be set lower than this value. This “targeted” source approach is commonly used. Generally, one specific radioisotope and source strength is used in a fixed, stationary position. The source strength is determined from the threat and is used as a surrogate for the threat. Results of static measurements from a given radioactive source for a deployed system can be scaled to the response to a targeted quantity of that source, as well as apply other correction factors. Once “calibrated” with the radioactive source, the hundreds of RPM operating parameters are set to detect the threat with a specified probability of detection. Figure 1.6 shows an example of the total gamma-ray counting rates, in counts per second, from a typical plastic scintillation detector used in a vehicle RPM system. The dash-dotted curve plot shows an unnormalized distribution of background count rate alone. This skewed-Gaussian-like distribution shows a width due to statistical variations, weather, and other effects from the surrounding

S gna probab ty d str but ons

Overview of Radiation Interdiction

Background suppresion

19

Background Vehicles Test source

NORM

1000

2000

3000 4000 Counts/second

5000

6000

figure 1.6 An example of the total gamma-ray counting rates in counts per second from a typical scintillating plastic used in vehicle radiation portal monitor systems showing the mean distribution of , cargo vehicles with and without naturally occurring radioactive material plus background (dashed curve), the distribution of background alone (dash-dotted curve), and a simulated test source (solid curve). The normalization is arbitrary.

environment. The tail toward higher energy is due to increases in background based on these factors, especially weather effects and the movement of cargo in the surrounding environment (but not directly within the RPM). The dashed curve distribution is the response to measurements of about 5,400 cargo vehicles with and without naturally occurring radioactive material (NORM), plus the observed background count rate. This vehicle distribution is also a skewed-Gaussian-like curve with an extended high count-rate tail from various NORM-bearing cargos (see Section 2.3 of Chapter 2 for a full discussion of NORM). The peak of this curve is downshifted relative to the background distribution, showing the effect of shadow shielding of the environmental background radiation by the vehicle (discussed in Chapter 4). The solid curve is from a simulated test source, which might, for example, represent a targeted threat source driven through an RPM. A set of repeated measurements generates a narrower Gaussian-like distribution with a centroid at the average count rate. A gross-count threshold set at the peak of this distribution would, on average, generate an alarm 50% of the time when this source was exposed to the RPM under similar conditions. A threshold set at a lower count rate than the peak could alarm on every observation of this source. Typically, a 99.9% probability of detection, or better, is desired.

1.2.4 specifications and standards The specification developed at PNNL for the procurement of RPM systems (Stromswold et al., 2005) to be deployed at U.S. borders used a similar approach to

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Radiation Detection and Interdiction at U.S. Borders

that discussed in the last section. It specified that RPM systems must have minimum count rates in response to a variety of sources of different energies in a specific geometry. This requirement translated into a minimum area and thickness for the detector systems. Absolute detection efficiency was given in net count rate above background per microcurie of source activity. The ANSI N42.35 (ANSI 2006a) is the RPM standard that specifies the systems that must generate alarms in response to certain prescribed sources when the threshold is set for a given false alarm rate. The intent of this ANSI standard is to provide minimum requirements and allow the comparison of systems against an absolute; it is not directly tied to any specific threat definition. It is common practice for procurements to require that systems meet the ANSI standard, as well as additional, more stringent, radiological constraints. The RPM specification developed at the IAEA is similar to the ANSI standard in setting minimum detection requirements (IAEA 2005). This latter document provides a set of technical specifications that can be used in design, testing, qualifying, and purchasing radiation-monitoring equipment for border crossings. Because of continual advances in the field of border radiation-monitoring equipment, the specifications represent a consensus on the minimum detection requirements presently achievable. This specification is based on work undertaken through multiple meetings of the IAEA-coordinated research project entitled “Improvement of Technical Measures to Detect and Respond to Illicit Trafficking of Nuclear and other Radioactive Material.”

1.2.5 the multilayer defense The goal of deploying radiation detection equipment is to interdict specified threats. Several factors may impact the realization of this goal, including a general requirement for noninterference with the legitimate flow of trade and travel. This means that the equipment needs to be automated so that it only requires action (such as stop and search) for a small percentage of scanned commerce. Because of the presence of natural and man-made radioactive materials in normal commerce, an operational limitation may be imposed that requires either an increase in personnel resources or a reduction in operational sensitivity. The potential presence of shielding of a threat must also be considered, since shielding can have a significant effect on the observed signal from a source. Radiation detection instruments, like RPMs, are just one component of a multilayer defense against illicit trafficking. The first layer is located at the potential source point for a threat, such as facilities in the former Soviet Union that store large quantities of plutonium and HEU. Starting in the 1990s (as a result of the  Coordinated IAEA research project, “Improvement of Technical Measures to Detect and Respond to Illicit Trafficking of Nuclear and Other Radioactive Materials,” Consultants’ Meeting in Vienna, Austria, March 17–21, 2003.

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Nunn-Lugar Cooperative Threat Reduction Program), the United States established several projects to help secure radioactive source material in the former Soviet Union. For example, the Material Protection, Control and Accountability Program within the U.S. National Nuclear Security Administration helped establish improved physical security and accounting practices at many of the former Soviet Union’s weapons facilities. At the same time, radiation detection equipment was being deployed as the second line of defense along the border of the former Soviet Union by the SLD program to interdict radioactive materials leaving these countries (see Section 7.3 of Chapter 7 for a full discussion of SLD). Another recent program beyond the SLD is the effort to scan cargo at foreign seaports before it disembarks for U.S. ports. Once commerce arrives at the U.S. border, it is scanned for radioactivity and subjected to a wide range of additional examinations, including X-ray imaging, evaluation of manifests and other paperwork, and the scrutiny of highly trained personnel. The combination of these defense layers provides protection and deterrence greater than individual components. Additional material related to threats and detection can be found in the collection of papers presented at the 2004 Health Physics Society Summer School (Brodsky and Johnson 2004).

1.3 The Necessity of Interdiction Joseph McDonald Transportation and commerce go hand in hand, and it is necessary to ensure that any measures taken to uncover unlawful or dangerous materials do not severely impact the normal, legitimate flow of vehicles transporting goods and passengers. It is difficult to achieve an optimum balance between the actions necessary to prevent the transport of illicit materials and the free movement of legal materials. Being more restrictive in prevention necessarily impacts the efficiency of the movement of legitimate material. The legal responsibility for detecting and interdicting illicit materials rests with CBP and other law enforcement agencies, and their goal is to prevent the smuggling of radioactive and nuclear material and a variety of other illicit items into the Unites States. Therefore, officials from these agencies collaborate with their counterparts in other countries to detect and prevent the movement of illicit materials before they appear at the U.S. border. Sophisticated detection devices have been developed to survey for the presence of even minute amounts of radioactive material; these methods are discussed in 

See http://lugar.senate.gov/nunnlugar/. The term “first line of defense” is applied to the security surrounding a location where nuclear material is stored. Thus, the “second line of defense” is a perimeter farther away (such as at a country’s border) that is the next ring of protection against material being removed from a country. 

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Radiation Detection and Interdiction at U.S. Borders

the following paragraphs. However, the deployment of advanced technology is not the complete solution to the complex problem of effectively uncovering illicit materials among vast quantities of legally transported goods. The interdiction system is comprised of regulatory requirements that control materials of concern. Radioactive materials fall under the jurisdiction of national agencies such as the U.S. Nuclear Regulatory Commission or DOE, who have the responsibility for controlling the possession and use of a wide variety of radioactive materials. The Department of Transportation controls the shipment of radioactive materials. The Nuclear Regulatory Commission, DOE, and Department of Transportation also have law enforcement powers enabling them to take action against the improper use or transport of radioactive materials. The process of registration or licensing of materials also aids in controlling illicit transport of radionuclides. Periodic inspections of facilities with licensed radioactive materials and requirements to report the loss of control of those materials further increases the safety and security of these materials. In addition, one of the most effective weapons in the campaign to limit the trafficking of illicit radioactive materials is the deployment of well-trained and experienced customs officers, police, and other law enforcement personnel. Successful interdiction is aided by radiation detection equipment, but another invaluable factor in that success is the intuition of the experienced agent who perceives illegal activity that a machine cannot. The illicit trafficking of radioactive materials has been a concern of the United States for a number of years. One of the events that highlighted the importance of radiation interdiction was the breakup of the Soviet Union. International concerns increased when the transport of radioactive materials was uncovered in countries, such as Austria, that have been pathways from Eastern to Western countries for quite some time. Radiation detection instruments were placed at strategically located border crossings in several European countries, and U.S. safeguard activities increased as well. Given the security at the U.S. sites that manufactured or stored nuclear weapons, it was determined that the likelihood of a stolen U.S. weapon was small, but finite. The task of detecting and interdicting illicit radioactive materials is simple to describe, but its implementation is daunting because of the inherent scale of the problem. Radiation detectors had to be developed that had the capability for scanning all vehicles and passengers that could be transporting illicit radioactive material. This scanning would also have to take place without substantially affecting the legitimate flow of trade or travel. A later section on “Interdiction Options” describes the details of radiation detection, identification, and the subsequent actions after a radiation source has been found (see Section 3.2 in Chapter 3). The approach to prevent the trafficking in illicit radioactive materials is similar to what would be used to prevent crimes in other areas. Law enforcement officials have long recognized that crimes can take place when there are opportunities, means, motives, and criminals to carry out the acts. Therefore, it is to be expected

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that law enforcement personnel would work toward limiting the opportunities to obtain radioactive materials. As mentioned earlier, administrative controls, licensing, inspections, and required reporting from those legally using radiation sources are effective methods for controlling the misuse, loss, or theft of sources that could result in those sources finding their way to terrorists. Physical control, barriers, locks, and tags also serve to restrict the means that criminals might use to obtain illicit radioactive sources. In the case of ordinary criminals, it may be possible to make theft of radioactive materials so difficult and dangerous that any monetary rewards for stealing a source might not be worth the risk. Reducing or removing the motive in the case of terrorists may be considerably more difficult if the motivation is political, philosophical, or religious. Finding and arresting criminals or terrorists before they are able to complete their plots is probably the most difficult task, and there cannot be any hope for success in this area without significant investigation by intelligence agencies. Obviously, such intelligence is difficult to obtain in a timely manner. Officials charged with the task of interdicting illicit radioactive materials face a difficult problem because of the inherent scope of the problem. More than 330 thousand vehicles, 57 thousand containers, approximately 2,500 aircraft, and nearly 600 ships pass through U.S. ports of entry each day. There are more than 600 border crossings to protect, and scanning for illicit radioactive material on this massive scale requires a large number of searches conducted in part with radiation detection and identification equipment. In order not to delay the flow of legitimate trade and travel, rapid scanning is necessary; however, less time taken to scan vehicles or passengers means less time is available for counting with radiation detectors. Shorter counting times often limit the effectiveness of the radiation detection and identification equipment. Radiation detection equipment, being quite sensitive to low levels of radioactivity, will also detect NORM present in common commercial items such as roof tiles, porcelain fixtures, camera lenses, welding rods, and even cat litter. Many of these items are manufactured from materials that contain natural uranium, thorium, or other elements whose isotopes can be radioactive. Radioactive materials are also found in radiopharmaceuticals used to diagnose and treat various diseases. If a patient has recently received a nuclear medicine treatment, it is likely that person will trigger an alarm when passing through a radiation detector at a port of entry. These events, which are referred to as innocent or nuisance alarms, must be addressed, and decisions are made many times daily as to whether a radiation alarm has resulted from a naturally radioactive item, a patient with radioactive material in his or her body, a man-made legitimate source, or from an illicit radioactive source. Customs officers have experience in dealing with the types of decisions that must be made when a person is suspected of transporting an illegal substance or when a package or container might contain an illicit shipment. Determining whether the capsules a person is carrying in a prescription bottle are legally

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Radiation Detection and Interdiction at U.S. Borders

obtained medicines or illegal drugs may be as difficult, or in some cases more difficult, than determining whether a radiation alarm is innocent. Customs officers have an array of tools to help them with such decisions, and radiation detection and identification equipment represent some of these tools. But, as mentioned earlier, the most effective weapon against smugglers and potential terrorists attempting to cross U.S. borders is the well-trained and experienced officer performing an evaluation. Ultimately, how effective this combination of technological and human interdiction factors can be is an open question. The actions taken after a radioactive source has been detected are as important, or in some ways more important, than the steps taken beforehand. If an illicit radioactive source is found, it may represent a radiation hazard to customs officer as well as others in the immediate vicinity. If the material is part of an RDD, explosives may be present and the hazard is thereby increased. The port of entry at which a source is found may have to be temporarily shut down until the hazard is addressed. This potentiality must be considered in planning for emergencies. If a radioactive source such as an RDD has been detected, there is also a possibility that the device might be detonated, which would make the port of entry area a radioactively contaminated zone. After dealing with the initial impact of such an occurrence, it would be necessary to decontaminate the area by removing, or otherwise dealing with, the dispersed radioactive material. If the radioactive material is a type that decays rapidly, it might be possible to quarantine the area until that radioactivity has decayed to innocuous levels. Preparing for the possibility of events, such as the detonation of an RDD, is an essential element of an emergency preparedness plan for federal, state, and local officials. Emergency exercises or disaster drills have been and are continuing to be carried out to prepare for expected threats from illicit radioactive materials. Resources and guidance documents are available from agencies such as the National Council on Radiation Protection and Measurements (NCRP 2001), the National Memorial Institute for the Prevention of Terrorism (MIPT 2006), and the American National Standards Institute Homeland Security Standards Panel (ANSI 2006b).

1.4 References ANSI. a. Evaluation and Performance of Radiation Detection Portal Monitors for Use in Homeland Security. ANSI N., American National Institute of Standards, Washington, DC. ANSI. b. The American National Standards Institute Homeland Security Standards Panel. Accessed April  at www.ansi.org (last updated November ). Brodsky A and RH Johnson Jr. (eds.). . Public Protection from Nuclear, Chemical, and Biological Terrorism. ISBN ---, The  Health Physics Society Summer School, Medical Physics Publishing, Madison, WI.

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CG-SMG-. . Joint CBP/DOE, Classification Guide for Nuclear Smuggling Information, U.S. Department of Energy, Office of Security, Washington, DC. Fehlau PE. . An Applications Guide to Pedestrian SNM Monitors. LA--MS, Los Alamos National Laboratory, Los Alamos, NM. Fehlau PE. . An Applications Guide to Vehicle SNM Monitors. LA--MS RP, Los Alamos National Laboratory, Los Alamos, NM. Fehlau PE, C Garcia, RA Payne, and ER Shunk. . Vehicle Monitors for Domestic Perimeter Safeguards. LA--MS, Los Alamos National Laboratory, Los Alamos, NM. IAEA. . IAEA Illicit Trafficking Database. Office of Nuclear Security, International Atomic Energy Agency, Vienna, Austria. Accessed on October , , at http:// www.iaea.org/NewsCenter/Features/RadSources/PDF/itdb_.pdf. IAEA. . Technical/Functional Specifications for Border Radiation Monitoring Equipment. IAEA-SVS-X, International Atomic Energy Agency, Vienna, Austria. IAEA. . IAEA Annual Report . International Atomic Energy Agency, Vienna, Austria. Kouzes RT. . Radiation Detection and Interdiction for Public Protection from Terrorism. In Public Protection from Nuclear, Chemical, and Biological Terrorism, eds. A Brodsky and J Johnson, R.H. Madison, WI: Medical Physics Publishing. pp. –. Kouzes RT. . Detecting Illicit Nuclear Materials. American Scientist (): –. Kouzes RT, JH Ely, BD Milbrath, JE Schweppe, ER Siciliano, DC Stromswold. . Spectroscopic And Non-Spectroscopic Radiation Portal Applications To Border Security. IEEE Transactions on Nuclear Science NSS San Juan Conference Record N- -. Milbrath BD, DC Stromswold, J Darkoch, J Ely, RR Hansen, RT Kouzes, RC Runkle, WA Sliger, JE Smart, DL Stephens, LC Todd, and ML Woodring. . Field Tests of a NaI(Tl)-Based Vehicle Portal Monitor at Border Crossings. PNNL-SA-, Pacific Northwest National Laboratory, Richland, WA. MIPT. . The MIPT Responder Knowledge Base Website. Accessed April , , at http://www.rkb.mipt.org (last updated November ). NCRP. . Management of Terrorist Events Involving Radioactive Material. NCRP Report , National Council on Radiation Protection, Bethesda, MD. Reichmuth B, S Short, and T Wood. . Economic Consequences of a Rad/Nuc Attack. Presented at the  IEEE–DHS R&D Conference, Boston, Massachusetts, April –, . Stromswold DC, JH Ely, RT Kouzes, JE Schweppe, and BS Carlisle. . Performance Specifications for Portal Monitors (Rev .). Pacific Northwest National Laboratory, Richland, WA.

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Radiation Sources

In this section, sources of radiation and possible interferences with threat detection are discussed. The radiation emanating from these sources is both ionizing and nonionizing. Ionizing radiation can impart enough energy to release an electron from an atom, thus creating an ion. Nonionizing radiation has less energy and can take the form of visible, ultraviolet and infrared light, radio-frequency waves, or microwaves. First, an introduction to the physics and statistics of ionizing radiation is provided so that terms used throughout this book are introduced. This section, while basic for some, provides information for others who are less familiar with the terminology and science of radiation and statistics associated with radiological processes and detection. After this introduction, four sources of ionizing radiation are discussed: the ubiquitous background radiation from soil, construction materials, and cosmic rays; naturally occurring radioactive materials (NORM); medical sources of radiation (radiopharmaceuticals); and industrial and special nuclear materials. This is followed by a discussion of nonionizing radiation used for telecommunication that can interfere with the operation of an ionizing radiation detection device. The sources of interference fall into two broad categories—electromagnetic and radio frequency-generating devices. Of the sources of ionizing radiation, the special nuclear materials and a number of specific radionuclides are the radioactive materials of most concern at border crossings. Their detection is the primary focus of the remainder of this document.

2.1 Physics and Statistics of Radiation Sources Joseph McDonald There are hundreds of radioactive elements with unstable nuclei from which gamma rays, charged particles, and neutrons are emitted. A specific radioactive isotope of an element is referred to as a radionuclide. Three representative radionuclides that emit gamma rays are Am, Cs, and Co. These radionuclides are used for legitimate purposes, but they may also be used for illicit purposes. If any of these three radionuclides were to be dispersed by an RDD, they would

Radiation Sources

27

remain hazardous for a long period because of their long half-lives. The half-life of a radionuclide is the time required for one half of the initial number of nuclei to disintegrate. This results in the radioactivity decreasing by one half. The halflife of Am is 432 years, and it decays with the emission of both gamma rays and alpha particles. Two primary gamma rays are emitted with energies of 59.5 kilo-electron volts (keV) and 26.3 keV, while the alpha particles have energies of about 5 MeV. This radionuclide is used in various commercially available devices such as smoke detectors, thickness gauges, and medical diagnostic instruments. A second radionuclide that also has several legitimate applications is Cs, which has a half-life of 30.2 years and emits a gamma ray with an energy of 662 keV. This isotope had been used for cancer radiation therapy; however, the 662 keV gamma rays have a limited ability to penetrate the body, and therefore, this isotope is generally no longer used for external-beam radiation therapy. It has been used as an implant source for brachytherapy where the source is placed near the tumor, and it has been used to reduce the restenosis or reclosure of coronary arteries that have undergone balloon angioplasty. In addition to medical uses, Cs is used in industrial sources for radiography, in various industrial gauges, and for food irradiation to sterilize pathogens, such as E. coli. A radionuclide that emits fairly high-energy gamma rays is Co, which has a half-life of 5.27 years and emits two gamma rays with energies of 1.17 and 1.33 megaelectron volts (MeV). These penetrating gamma rays have been and are still being used in external–beam radiation therapy. There are several other uses for Co in radiation research, but its relatively short half-life means the source must be replaced fairly often. In addition to medical applications, Co is used in industrial sources for radiography and for food irradiation. Neutron sources are rarely encountered but may include Cf or Am-Be sources that are used in moisture gauges and devices for examining oil wells. In addition, cosmic rays produce a background of neutrons that varies at different geographical locations, is dependent upon weather conditions, and changes with the solar activity cycle (see Section 2.2). Several radionuclides have been identified as being potential candidates for use in an RDD; among them are Co, Sr, Cs, Ir, Pu, and Am. As mentioned earlier, some of these sources emit gamma rays with energies high enough to travel long distances in air and penetrate into the human body, causing damage. Both  Pu and Am emit alpha particles that can produce neutrons when they interact with certain materials.

2.1.1 gamma-ray interactions with matter When gamma-ray photons interact with materials, they deposit energy by means of several mechanisms that depend on their energy and the characteristics of the



Radiation Detection and Interdiction at U.S. Borders

material in which the energy is absorbed (See Section 3.1 of Chapter 3). These mechanisms include the following • • • • •

Photoelectric effect Rayleigh (coherent) scattering Compton effect Pair production Photonuclear interactions

These mechanisms are dependent on the atomic number, Z, of the element acting as an absorber and are also dependent upon the energy, E, of the gamma rays. As can be seen in Figure 2.1, for a low Z absorber like aluminum, the photoelectric effect is the dominant means of absorption for low-energy gamma-ray photons, while Compton scattering is important at intermediate energies, and pair production becomes important at high energies. The symbols, hn, refer to the photon energy given here in units of MeV. As shown in Equation (2.1), the cross section, s, for these various effects is a measure of the probability, P, of an interaction per unit fluence. Gamma-ray fluence (F) is the number of photons incident per unit area.

s=

P F

(2.1)

The photoelectric cross section is strongly dependent on the Z of the absorber— it is proportional to Z. The incoming gamma ray interacts with an electron of an

120

Z of absorber

100 80 60

Photoelectric effect dominant

Compton effect dominant

Pair production dominant

40 20 0 0.01

0.1

1 10 Photon energy hν, in MeV

100

figure 2.1 Graph of the Z of the absorber and the energy of the gamma ray. The two curves indicate the energy and Z for which the respective effects, photoelectric-Compton or Compton pair production, are equal.

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29

atom and vanishes (all its energy is absorbed), producing a free electron with an energy that is the difference between the energy of the incoming gamma ray and the binding energy of the original electron. The binding energy is the energy needed to release the electron from the atom. The result of this removal of an electron, such as those from the inner shells of a high-Z atom, is that another electron may fill that vacancy and generate an X-ray that is characteristic of that atom. The Compton effect is a scattering interaction where the incoming gamma-ray photon transfers some of its energy to an electron and both continue traveling along different trajectories. The Compton scattering cross section is proportional to Z, and the energy deposited by the incoming gamma ray is shared with the electron after the interaction. Rayleigh scattering is the result of a less important interaction in which the incident gamma ray collides with a tightly bound electron and the gamma ray is scattered without an energy loss. Compton scattering is most prevalent for the gamma rays from radionuclides that may be encountered in the search for illicit radioactive materials, with an impact on the source spectrum and on the detection process. When the energy of the gamma ray exceeds 1.02 MeV, it can interact with the nucleus of an atom in which all of its energy is converted to mass and kinetic energy of the two resulting particles that are an electron and a positron (the antiparticle of the electron). Above 1.02 MeV, the pair production cross section increases approximately as Z. Soon after the creation of the electron–positron pair, these two particles can interact and annihilate each other, resulting in the creation of two gamma rays traveling in opposite directions, each with an energy of 0.51 MeV. Another process, relatively unimportant in the present context, is the photonuclear effect. It occurs when a gamma ray interacts with the nucleus of an atom that subsequently emits neutrons or protons. This effect occurs in many materials when the photon energies exceed 10 MeV, but the cross sections for the photonuclear effect are only a few percent of those for the three major cross sections (photoelectric effect, Compton scattering, and pair production).

2.1.2 neutron interactions with matter Processes that occur in neutron interactions are also basically similar in nature to those for gamma rays. For instance, the processes can be classified as either scattering or absorption interactions with cross sections being defined for neutron interactions. Nuclear cross sections are generally expressed in barns, which are equal to 10– m. A barn is a unit of area, and it corresponds to the probability of interaction mentioned above and in Section 3.1 of Chapter 3. The smaller the area, or cross section, the lower the likelihood is of an interaction, and vice versa. The values of neutron cross sections vary over a wide range depending on the material in which the neutron is traveling and the energy of the neutron.

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Radiation Detection and Interdiction at U.S. Borders

One of the neutron interactions with a large cross section is elastic scattering. This interaction is similar to billiard balls colliding. The energy transferred to a nucleus and the angles of recoil of the neutron and nucleus are given by expressions similar to those for colliding billiard balls. As illustrated in Figure 2.2, the elastic scattering cross section for hydrogen, H, is almost a factor of 10 larger than the corresponding cross section for C over most of the energy range shown. In an elastic scattering event, the collision is most effective in the transfer of energy if the mass of the atom is very close to the mass of the neutron. This occurs in the case of the hydrogen atom, whose nucleus is a proton, because the masses of the neutron and proton are nearly equal. Materials containing a large amount of hydrogen (such as water, paraffin, and polyethylene) are very efficient at scattering, slowing down, and absorbing neutrons. In such materials, the neutron collision results in energy loss until it eventually has approximately the same thermal energy as the material itself. Neutrons with energies equivalent to the thermal energy of a material at room temperature are referred to as thermal neutrons; their energy is approximately 0.025 eV at 20°C (68°F). There are additional neutron-scattering interactions that are not as simple as billiard ball collisions; they are known as inelastic or nonelastic interactions, depending on the type of particles or energy resulting from the collision. These interactions take place when the incoming neutron enters the nucleus of the

103

Cross sect on - barns

102

1H

16O

12C

101

1

10−1 10−8

10−7

10−6

10−5 10−4 10−3 Neutron energy - MeV

10−2

10−1

1

10 20

figure 2.2 Neutron elastic scattering cross sections for three common elements. Above a neutron energy of about 0.1 MeV, the O cross section becomes complex due to the presence of sharp increases or decreases (resonances) in the cross section (data from KAERI 2006).

Radiation Sources

31

atom and forms a compound nucleus with added energy. This nucleus can return to its ground state energy by emitting neutrons, other nuclear particles, and possibly gamma rays. An interaction where the incident neutron enters the nucleus and then is reemitted is known as inelastic scattering. When the neutron is absorbed and another lower-energy neutron and perhaps an alpha particle or proton is emitted, this process is known as a nonelastic scattering interaction. When a neutron has lost most of its energy through elastic or inelastic scattering interactions and is at thermal energy, it can easily be captured by a nucleus with the resulting emission of a gamma ray. This can be written symbolically as  H(n,γ)H. The thermal neutron, n, incident on a hydrogen nucleus (a proton), H, is captured (with a cross section of tens of barns) to form a deuterium atom, H, that consists of a neutron and a proton. The process is called radiative capture, and a gamma ray with an energy of 2.223 MeV is emitted. Low-energy and thermal neutrons can initiate the process of fission in heavy nuclei, such as U or Pu. This interaction results in the splitting of the nucleus into two roughly equal parts along with the emission of additional neutrons, charged particles, and a large amount of energy in the form of gamma rays and other photons. High-energy neutron interactions are generally more complex and can produce fragments of nuclei, multiple neutron, charged particle, and gamma-ray emissions. These interactions are not normally produced by radionuclides that are of concern at border crossings. However, cosmic-ray interactions in the Earth’s atmosphere represent a background radiation that can be detected with RPMs. The neutrons produced by cosmic-ray interactions can have energies high enough to cause spallation, or the breaking off of parts of nuclei. These processes can, in turn, result in additional radiations that may be detected. In some interactions, subatomic particles such as pi and mu mesons are released.

2.1.3 neutron and gamma-ray spectroscopy 2.1.3.1 Neutron Spectra Radionuclide-based sources of neutrons emit broad spectra that have few features. These spectra are normally characterized by their average energies. An example of a neutron energy spectrum is shown in Figure 2.3. The quantity BE∙E in the figure is proportional to the number of neutrons incident per unit area, per unit time. Narrow neutron spectra that are approximately monoenergetic are not produced from radionuclide sources. These narrow spectra can be produced in accelerator laboratories and will not normally be encountered by RPM detectors. The energy spectra of radionuclide-based neutron sources are usually determined with organic scintillation detectors (Section 3.1 of Chapter 3). Organic liquids and He gas-filled proportional counters are also used for neutron spectroscopy.



Radiation Detection and Interdiction at U.S. Borders 0.6

BEE (s−1)

0.5 0.4 0.3 0.2 0.1 0 0.01

0.1

1 Energy (MeV)

10

figure 2.3 Neutron spectrum from a 252Cf spontaneous fission source. The average energy of this spectrum is approximately 2.2 MeV, and the maximum energy is approximately 20 MeV (data from ISO 2001).

2.1.3.2 Gamma-Ray Spectra Radionuclide sources emitting gamma rays are fairly common, and these sources emit gamma-ray photons that are nearly monoenergetic. The interactions of gamma rays with detector materials, such as NaI(Tl), produce light pulses that are proportional to the energy deposited and can be detected with a photomultiplier tube (Chapter 3, Section 3.1). The electronic pulses produced from the photomultiplier can then be counted with a multichannel analyzer. These pulses result in the production of narrow distributions of counts, known as photopeaks. The NaI(Tl) detector has to be large enough so that the gamma-ray photon can deposit its full energy in the detector. As mentioned above, gamma-ray interactions are dependent on the atomic number, Z, of the detector material. Since NaI has a relatively large, effective Z, gamma rays having energies up to a few MeV deposit their full energy in a detector having dimensions of about 100 mm (3.9 in.). An energy spectrum produced by the gamma rays emitted from Co is shown in Figure 2.4. The two main emissions at 1.17 and 1.33 MeV are shown as photopeaks. Additional features of spectra are described in ORTEC® (2006). Figure 2.1 shows the energy regions where the three predominant gamma-ray interactions occur. These interactions are dependent on the Z of the material in which the gamma-ray deposits its energy. At low energies, the photoelectric effect is dominant and the peak (referred to as a photopeak) of such a low-energy photon interaction in NaI(Tl) results in a very narrow distribution. But, as the energy of the gamma-ray increases to the point where Compton scattering is predominant, there is a range of energy depositions that can occur depending on the scattering angle and the number and energy of photons that escape the crystal. These interactions give rise to a range of pulses of varying heights that make up the region up to about channel 400 in Figure 2.4; this is called the Compton continuum. Compton scattering of gamma-ray photons by electrons results in a scattered electron with a maximum energy dictated by the mechanics of the “billiard



ORTEC is a registered trademark of AMETEK Advanced Measurement Technology, Inc.

Radiation Sources

33

350 1.17 MeV 300

Count/channe

1.33 MeV

Compton edge for 1.33 MeV

250 Backscatter

200 150 100 50 0 0

80

160

240 320 Channel

400

480

540

640

figure 2.4 Energy spectrum produced by gamma rays from Co and displayed by a multichannel analyzer.

ball”–like scattering of the electrons and the photons (Knoll 1989). This maximum energy is known as the “Compton edge” and is shown in Figure 2.4 at approximately channel number 390. Ionizing photons are also produced in continuous spectra called “bremsstrahlung.” These photons are usually referred to as X-rays, and are generated when electrons bombard heavy metals such as tungsten in X-ray tubes. Bremsstrahlung, a German word, means braking radiation, and it describes the deceleration of the electrons as they transfer their energy to tungsten atoms. Characteristic X-rays appear as line spectra and are produced by transitions of atomic electrons (see Section 3.1, Chapter 3).

2.1.4 statistics If large numbers of 1-second measurements of the radiation emitted from a radionuclide source are taken and the number of detected counts is plotted versus the value of the radiation intensity, a distribution is obtained that is similar to a Gaussian or normal distribution (Figure 2.5). The measured values are plotted as differences from the mean value, so the most probable value for the measurement occurs at the midpoint (zero on the horizontal axis). The distribution takes the form of a bell-shaped curve with its maximum at the value of the quantity of radiation that was measured most often. This value is also the mean or average value for such a distribution. As more and more readings are taken, the curve becomes narrower and taller, still approximately centered on the mean or average value of the quantity of radiation. The width of the distribution is proportional to a quantity known as the standard deviation, σ.

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Radiation Detection and Interdiction at U.S. Borders

0.4

0.3

0.2

0.1

0 −4

−3

−2

−1

0

1

2

3

figure 2.5 Illustrative example of Gaussian distribution.

4

The mean or average value, x , can be calculated with Equation (2.2): N

x=

∑x i =1

N

i

=

x1 x 2 + .... x N N

(2.2)

The standard deviation, σ, is given in Equation (2.3): 1 N ∑ (xi − x )2 N i =1

s=

(2.3)

From this equation, it can be seen that s is proportional to the difference between measurements and the mean value x ; therefore, s is a measure of the width of the distribution. It is often found that one quantity, such as the measured amount of radiation, shows a dependence on another quantity, such as the amount of material between the source and the radiation detector. As an example, the intensity of cosmic rays at sea level is dependent on the barometric pressure because the pressure is proportional to the number of air molecules that are above the point of measurement. More particles mean more absorption of the cosmic rays. For two variables, x and y, a quantity known as the correlation coefficient, r, can be defined: N

r=

∑ (x

x )( y

i

y)

i =1

N

∑x i =1

N

i

x

∑( y i =1

y )2

(2.4)

Radiation Sources

35

The correlation coefficient approaches one if x is strongly dependent on y, and approaches zero if there is no connection. Gamma rays and neutrons are emitted as a result of nuclear interactions, and radionuclides with short half-lives emit photons or particles more frequently than those with long half-lives. A radionuclide such as Na has a half-life of approximately 15 hours and emits large numbers of 2.75 MeV gamma-ray photons. The count distribution registered by a gamma-ray detector exposed to this source would be one where the number of gamma rays detected as a function of their energy forms an approximately Gaussian distribution about a mean value. Gaussian distributions are normally found for measurements in which there are a large number of random events. The neutrons emitted by the spontaneous fission that occurs in a Pu source occur far less frequently than the gamma rays from Na because the half-life of  Pu, at 88 years, is much longer than the half-life of Na (t/ = 15 h). When events occur relatively infrequently, their distribution may be characterized by a different type of distribution called a Poisson distribution. In a Poisson distribution, the standard deviation is given by the square root of the number of counts in the distribution as seen in Equation (2.5):

s Poisson = n

(2.5)

where n is the number of counts measured. Differences in the types of distributions of counts for neutron and gamma rays detected can be useful in the analysis of these radiations. For instance, detection circuits often calculate the standard deviation of electronic pulses from a detector that has been irradiated with gamma rays arising from natural background radiation on the assumption they follow a Poisson distribution. When a source is detected, an increase in the number of pulses is registered, and that increase can be expressed in terms of the standard deviation of the background. The difference between the background distribution of pulse counts and the measured amount from an illicit radioactive source may be expressed as a multiple of standard deviations, or sigmas, as discussed in Chapter 4. It is important to have analysis techniques to examine the pulse count distributions because the detection of an illicit radioactive source often depends on identifying a small increase in counts as compared with counts from natural background radiation.

2.2 Background Radiation Sources Paul Keller Sources of background radiation include cosmic sources, terrestrial sources, and man-made sources. Cosmic sources can vary with the solar cycle and solar

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Radiation Detection and Interdiction at U.S. Borders

activity, such as solar flares. The measured cosmic background at the surface of the Earth is influenced by latitude, barometric pressure (including altitude), solar activity, diurnal cycle, and weather. Terrestrial background sources can vary spatially because of minerals in the soil and temporally by changes in the weather. Cosmic radiation was first discovered by Victor Hess in 1912 when he took a gold leaf electroscope radiation detector aboard a balloon and flew it to an altitude of 5300 m (17,389 ft). As the balloon climbed, he noted an increase in radiation and deduced that the radiation was coming from outside the Earth; therefore, he dubbed it cosmic radiation. Since then, cosmic radiation has been found to have two major components: galactic and solar. As their names suggest, galactic indicates sources other than the sun, and solar indicates radiation from the sun.

2.2.1 cosmic background The energy of cosmic particles ranges from MeV to 1 ZeV (10 eV). The highest energy particles likely originate from outside our galaxy, and with their high momentum, they are not deviated by the galactic magnetic field. Cosmic particles of this high energy are very rare and hit the Earth at a rate of one particle per square kilometer per century and preserve information about the direction of their origin. Lower-energy particles are much more prevalent and hit the surface of the Earth at a rate of many particles per square centimeter per second. Galactic cosmic radiation is fairly constant, so solar activity has a significant impact on the fluctuation in cosmic radiation that reaches the surface of the Earth and thus the measured background seen at RPM locations. There is a clear variation in cosmic radiation throughout the 11-year solar cycle, the yearly cycle, and even a noticeable variation in the diurnal cycle. All of these variations highlight the strong influence the sun has on cosmic radiation measured at the Earth. Solar activity influences the cosmic radiation at the surface of the Earth in two ways. First, solar activity contributes to cosmic radiation. Protons emitting from the sun are accelerated by solar flares and make their way to the Earth. High-energy neutrons are produced from nuclear reactions in the solar atmosphere during solar flares and also make their way to the Earth. Second, the propagation of cosmic radiation is affected by shock waves that originate on the sun and influence the interplanetary magnetic fields. Solar cosmic radiation intensity is a function of various solar activities including sunspots and heliospheric structure (Hess 1912). Rapid changes in cosmic-ray intensity are usually marked by a Forbush decrease, a term applied to the decrease in cosmic-radiation intensity immediately following a coronal mass ejection (Yanchukovsky and Philimonov 1999). During a coronal mass ejection, not only are a large number of charged particles ejected from the sun, but also the local magnetic field is greatly increased as these particles travel outward from the sun. This magnetic field sweeps away many of the charged particles that form cosmic radiation.

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The net effect is a drop in the cosmic radiation arriving at the Earth. This decrease during solar activity was first described by Scott E. Forbush in 1937 (Forbush 1937). Figure 2.6 illustrates the recorded neutron flux at four locations on the surface of the Earth during a Forbush decrease in late October 2003. This event began with an × 17.2 magnitude solar flare that started on the sun at 09:51 Greenwich Mean Time on October 28, 2003. Within 20 hours, the pulse of energy from the flare arrived at the Earth, producing a rapid change in the magnetic field that resulted in nearly a 20% drop in cosmic particle flux recorded at the surface of the Earth. Cosmic radiation incident upon the upper atmosphere primarily consists of charged particles with energies typically above 300 MeV. Lower-energy cosmic particles usually do not penetrate the upper atmosphere. Primary cosmic particles are about 90% protons, but other charged particles with masses up to iron nuclei are common with traces of heavier element nuclei, possibly including U (Cronin 1999). Charged particles, such as protons, take a more indirect path from the sun to the Earth than do neutrons because of interplanetary magnetic fields. Cosmic radiation also consists of about 0.1% gamma radiation. Gamma radiation is important in determining the origin of a cosmic ray burst as it is deviated by neither the magnetic field of the Earth nor the interplanetary magnetic field. Upon arrival at the Earth’s atmosphere, the primary cosmic radiation particles undergo nuclear interactions through collisions with atmospheric nuclei to produce secondary particles. Some of these secondary particles reach the surface of

6000

Count rate (counts/m nute)

5600 5200 4800 4400 4000 3600

6-Nov

5-Nov

4-Nov

3-Nov

2-Nov

1-Nov

31-Oct

30-Oct

29-Oct

28-Oct

27-Oct

26-Oct

25-Oct

2800

24-Oct

3200

Start of each date (UT)

figure 2.6 Forbush decrease as seen with the four neutron monitors (from top to bottom: Oulu, Finland; Rome, Italy; Hermanus, South Africa; and Haleakala, United States) over the course of 13 days in late October and early November 2003.



Radiation Detection and Interdiction at U.S. Borders Primary cosmic particle π° π−

π±

γ

γ

e+

π

μ± N

N

p

n p

n N n p n

n N n

p

p n

P

Positron Electron Gamma ray Pion Muon High energy nucleon Disintegration product Nuclear disintegration

n

P

n

n

e±

Meson components

p n p

n

μ-

e−

Electromagnetic components

N

P

e+ e− γ π μ N, P n, p

n n

Secondary particles and emissions

pn p

Nucleonic components

figure 2.7 Process of primary cosmic particles entering the upper atmosphere and producing secondary particles as they strike air molecules along the way.

the Earth and contribute to the background radiation. Figure 2.7 illustrates this collision process. The number of secondary particles reaching the surface of the Earth is a function of five parameters: latitude, weather on the Earth, solar activity, diurnal cycle, and barometric pressure (including altitude effects). Because primary cosmic particles interact with the atmosphere, longer paths through the atmosphere reduce the flux reaching the ground, resulting in lower background radiation levels because of the reduction in the flux reaching the ground. Usually, only some energy is imparted to each secondary particle during collision and multiple collisions occur. This results in a cosmic ray shower of lowerenergy particles arriving at the surface of the Earth than are found in the outer atmosphere. Cosmic radiation is highly dependent on elevation with higher backgrounds at higher elevations. The most common radionuclide produced by cosmic radiation is C. Other radionuclides generated include H, Na, and Be. In addition, some cosmic radiation-induced neutrons reach the surface of the Earth. The amount of neutron background at the surface of the Earth is dependent upon atmospheric pressure; this is because pressure directly affects the interaction length for cosmic particles reaching the surface of the Earth. The important background components for RPMs resulting from cosmic rays are part of the low-energy gammaray flux observed in the gamma-ray detector arising from decay gamma rays from radionuclides produced by the cosmic rays, and the majority of the neutron background observed in the neutron detector from secondary neutrons. Arthur Compton first demonstrated that cosmic-ray intensity is dependent on magnetic latitude (Compton 1933). This is caused by the magnetic field of the Earth that creates complex trajectories for incoming particles and results in a

Radiation Sources

39

sorting of the particles by geography. This results in a variation in the portion of the cosmic-ray spectrum that reaches the Earth’s atmosphere. The spectrum is controlled by the geomagnetic cutoff that varies from a minimum at the magnetic poles, which is theoretically zero and where particles can easily penetrate, to a vertical geomagnetic rigidity cutoff of up to 17 GV at the magnetic equator. Geomagnetic rigidity is a relative measure of the ability for a cosmic particle to penetrate the magnetic field of the Earth and is the particle momentum per unit charge. It is the minimum electric potential, usually given in GV, that a charged particle arriving at the top of the Earth’s atmosphere must have to create a particle cascade that can reach sea level at that location. The rigidity, R, of a charged particle is given by Equation (2.6): R=

A 2 E Q

(2.6)

M0 E

where E = the kinetic energy of the particle given in GeV/nucleon A = mass number of the particle, Q = charge of the particle, and M0 = the atomic mass unit (0.9315016 GeV/c2). The relationship between the minimum particle energy per nucleon and the magnetic rigidity is shown in Figure 2.8 where the minimum particle energy in GeV is seen to be very similar to the magnetic rigidity in GV. The lines of magnetic rigidity cutoff around the Earth are illustrated in Figure 2.9. 20

M n mum part c e energy per nuc eon (GeV) E

18 16 14 12 10 8 6 4 2 0 0

2

4

6

8

10

12

14

16

18

20

Magnetic rigidity (GV). R

figure 2.8 Relationship between magnetic rigidity and the minimum particle energy necessary for a cosmic particle to reach the surface of the Earth excluding absorption effects in the atmosphere.

90°N

60°N

30°N

0°

30°S

60°S

90°S 180°W

150°W

120°W

90°W

60°W

30°W

0°

30°E

60°E

90°E

120°E

150°E

180°E

figure 2.9 Lines of magnetic rigidity cutoff that relate to the minimum energies that can be observed at locations shown in Figure 2.10. Magnetic rigidity cutoff lines are labeled as GV.

Radiation Sources

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In high-latitude regions, the geomagnetic cutoff is low, so most particles present in the outer atmosphere produce secondary particles that make it to the surface of the Earth. For these high-latitude regions, the number of particles reaching the surface of the Earth is a function of the amount of air through which they must travel. In the polar regions at sea level, detected cosmic particles have an energy of at least 430 MeV. At the South Pole, which has an altitude of 2820 m (8600 ft) above sea level, the minimum detectable energy is about 300 MeV. Higher altitudes with the same magnetic rigidity will have a lower minimum detectable energy. From midlatitudes to the equatorial region, the minimum detectable energy is controlled fully by the geomagnetic cutoff and is independent of the altitude. At these locations, the intensity (count rate) is controlled at altitude through atmospheric absorption of the secondary particles generated in the outer regions of the atmosphere. Therefore, high-latitude neutron monitors are placed around the Earth to measure anisotropies in cosmic radiation and mid- to lowlatitude neutron monitors are placed to measure cosmic-ray densities and anisotropy components that exhibit anomalous behavior before the arrival of an interplanetary disturbance at the Earth (Belov et al. 2003).

2.2.1.1 Radiation Portal Monitor Locations in Cosmic Background Study Thirteen RPM locations were used to study the connection between cosmic background and measured background. These locations were spread across the entire United States and gave information on the variation of background radiation levels with location and elevation. Data were available over a range of 6 to 30 months for this study and include three noticeable solar events. The analysis results of cosmic effects at specific locations were used to form some general conclusions about cosmic radiation effects at all U.S. locations.

2.2.1.2 Cosmic Radiation Data Several external sources were used in this study to find the relationship between RPM location background and cosmic radiation. Cosmic-radiation data in the form of neutron flux measurements were obtained from 52 ground-based observatories around the Earth, some of which are shown in Figure 2.10. These groundbased observatories record neutrons arriving at the surface of the Earth and are operated by a variety of academic and research institutions where cosmic and solar activities are studied. There are two main types of ground-based neutron monitors used by the cosmic-ray research community, commonly called International Geophysical Year (IGY) monitors and NM64 monitors. The IGY monitors were the first series of neutron monitors to be developed for extensive cosmic ray research (Simpson 1957). These monitors came into use during a famous solar flare, IGY 23.02.1956, in 1957. They are small counters with low statistical accuracy, but at the time, the IGY monitor was one of the better instruments for detecting low-energy secondary neutrons that did not suffer ionization losses. Over time, most of the IGY neutron

90°N

60°N

30°N

0°

30°S

60°S

90°S 180°W

150°W

120°W

90°W

60°W

30°W

0°

30°E

60°E

90°E

120°E

150°E

180°E

figure 2.10 Map illustrating the location of some of the 52 cosmic-ray neutron monitors used to examine the relationship between cosmic background and the background measured at the radiation portal monitor locations.

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43

monitors were replaced with NM-64 neutron monitors, although a few IGY monitors still exist. These ground-based neutron monitors detect secondary particles produced in the energy range of 500 MeV to 20 GeV and have a higher sensitivity at the lower part of that energy range (~500 MeV to 4 GeV). An improved monitor with a new counter and geometry was developed in the 1960s that offered higher statistical accuracy than the IGY monitor. This neutron monitor was developed for the International Quiet Sun Years of 1964–65, since greater counting capacity was required (Carmichael 1968). These monitors consist of boron trifluoride (BF), proportional counters surrounded by polyethylene, and lead. The boron is enriched in B to 90% of the total boron or greater; B has a greater cross section for neutrons than B. They are often designated as xx-NM-64 where xx is the number of tubes in the monitor. Secondary cosmic particles interact with the lead surrounding the counters, resulting in nuclear disintegrations of charged fragments and neutrons. These monitors are used to detect particles with energies in the range of 10 MeV to several GeV. The disintegration process from the interaction of neutrons produces a multiplication effect with more secondary fragments generated than incident particles. Finally, the neutrons are moderated by the polyethylene and then counted with BF proportional counters, which are efficient thermal neutron detectors. Many of the neutron observatories worldwide archive neutron counts at a rate of one per minute or one every 5 minutes, while others only archive at a rate of one per hour. A few are experimenting with recording counting rates on an interval of seconds. For this study, most analyses were done at the hourly rate and a few at the minute sampling rate. These observatories often provide the uncorrected rates, the atmospheric pressure, and the pressure-corrected count rates.

2.2.1.3 Major Solar Events The largest solar event during the 2-year span of data used in this study (2003 and 2004) occurred on October 29, 2003, with a 22% decrease in neutron background count rate at the surface of the Earth. Figure 2.6 shows this 13-day event as seen with four of the neutron monitors. Figure 2.11 shows this same event as seen at four RPM neutron monitors along the northern border and the Newark Neutron Monitor. This figure clearly shows that the Forbush decrease is not only visible in the cosmic-ray observatory data but also in the measured neutron background at all four RPM locations. Many of the minor peaks are coincident as well, though the RPM neutron data often contains more noise than the data from a cosmic-ray observatory. Figure 2.12 shows the gamma-ray background at these same four RPM locations during the solar flare and the Forbush decrease is clearly seen. There is much less similarity between plots and the measured neutron count rate at the Newark Neutron Monitor. However, there is a reduction in the gamma count fluctuation during the Forbush decrease, indicating a possible connection between some gamma-ray background and cosmic sources.

6.5 6

Average count rate

5.5 5 4.5 4 3.5 3 2.5 2 6-Nov

5-Nov

4-Nov

3-Nov

2-Nov

1-Nov

31-Oct

30-Oct

29-Oct

28-Oct

27-Oct

26-Oct

25-Oct

24-Oct

1.5

Start of each day (universal time)

figure 2.11 The neutron count rate measured at the Newark Neutron Monitor in Newark, Delaware (top solid trace), with the neutron count rate measured at four radiation portal monitor locations shown as the lower four traces.

3500 3300

Averaged count rate

3100 2900 2700 2500 2300 2100 1900 1700

Start of each day (UT)

figure 2.12 The neutron count rate at the Newark Neutron Monitor against the total gamma-ray background count at four radiation portal monitor locations during a major solar flare in late October 2003. The top solid trace shows the neutron count rate; the other four traces are gamma count rates at the RPM locations.

6-Nov

5-Nov

4-Nov

3-Nov

2-Nov

1-Nov

31-Oct

30-Oct

29-Oct

28-Oct

27-Oct

26-Oct

25-Oct

24-Oct

1500

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45

Averaged count rate

6.5 5.5 4.5 3.5 2.5

1-Aug

31-Ju

30-Ju

29-Ju

28-Ju

27-Ju

26-Ju

25-Ju

24-Ju

23-Ju

22-Ju

21-Ju

20-Ju

1.5

Start of each day (UT)

figure 2.13 The neutron count rate measured at the Newark Neutron Monitor in Newark, Delaware (shown as the top solid trace), with the neutron count rate measured at four radiation portal monitor locations shown as the lower four traces (two solid and two dashed).

The second large solar flare event occurred on July 26, 2004, and caused a 10% drop in neutron background. Figure 2.13 shows this event as seen at the Newark Neutron Monitor and four RPM locations. This graph shows hourly counts covering the time period around the second largest solar flare and the subsequent Forbush decrease that occurred during this study. For each RPM location, the average count over an hour is used. This figure again shows that neutron background measured at the RPM locations is tied directly to the cosmic background. Some RPMs experience a greater decrease in count rate than others because of the variation in the RPM location.

2.2.1.4 Correlations Many of the cosmic radiation observatories record data at 1- to 5-minute intervals, while others record it at an hourly interval. They all supply data on an hourly count as well. The RPM data are generally recorded once a second, though there are often many gaps. For the correlation studies, all data were correlated on an hourly basis. For the RPM data, this involved summing up the counts over an hour and reweighting the hourly count for missing data. When most or all of the data are missing for a specific hour from either an RPM or a cosmic-ray observatory, that hour is not included in the correlation calculation. Table 2.1 shows the average correlations of the 13 RPM locations with 16 of the cosmic-ray observatory neutron monitors. A value of 1.0 would indicate perfect correlation, and lower values indicate poorer correlations. A correlation coefficient above 0.45 is considered a “strong” correlation, between 0.3 and 0.45 “good”, between 0.2 and 0.3 “moderate”, between 0.1 and 0.2 “weak”, and below

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Radiation Detection and Interdiction at U.S. Borders

table 2.1 Average correlation between radiation portal monitor locations and similar cosmic-ray observatory locations Correlation with Cosmic Neutronsa

RPM Background

Neutron Low-energy gamma High-energy gamma Total gamma a

0.423 0.271 0.316 0.288

Correlation between cosmic neutrons and RPM neutron and gamma-ray background is shown.

0.1 insignificant. Chance has a correlation of zero. Correlation pairs were chosen between the RPM location and the cosmic ray observatory locations to approximately match the elevation and magnetic rigidity. Although this table shows average results, a relationship clearly exists between the cosmic background and both neutron and gamma-ray background recorded at the RPM locations. Stronger correlations exist between specific panels at RPM locations and specific cosmic-ray observatories.

2.2.2 earth–terrestrial background Another major source of background radiation is terrestrial. Terrestrial background primarily comes from U, Th, and their progeny, and from K that occurs in rock and soil. Much of the actual gamma-radiation background measured at and just above the surface comes from radon gas transported to the surface through pores and cracks in the soil and rocks. Radon gas is the progeny of U and Th. For example, U decays to Pb through Rn in the U decay series. Uranium-235 decays to Pb through Rn in the actinium decay series. Thorium-232 decays to  Pb through Rn in the Th decay series. In the 1970s and 1980s, the DOE conducted an aerial radiometric survey of the conterminous United States as part of the National Uranium Resource Evaluation (DOE-NURE) program (DOE 1985). The survey data from this program have been integrated into contour maps of equivalent U, Th, K, and total gamma-radioactivity exposure for the conterminous United States. Figure 2.14 (located at the front of this book) shows four U.S. Geological Survey maps based on NURE data (Phillips 1993). The maps show concentrations of K,  Th, and U. The hottest (pinkish purple) zones (>57 picocoulomb [pC]/kg⋅s or > 8 μR/hr) show regions of the highest background gamma radiation and are generally from mineral deposits in the soil containing U, Th, and K. These regions include the Sierra Nevada mountains and parts of east-central California that have large deposits of granite, and southern Nevada and parts of Utah that have mountain ranges high in both granite and volcanic rock and basins filled with alluvium shed from the mountain ranges. Sporadic regions of the Rocky Mountains and nearby

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3.8

K (%)

Potassium concentrations

0

21.6

eTh (ppm)

Thorium concentrations

0

5.4

eU (ppm)

Uranium concentrations

0

figure 2.14 Maps based on the U.S. Department of Energy National Uranium Resource Evaluation (NURE) program (DOE 1985) and extracted from the U.S. Geological Survey in Digital Data Series DDS-9 (Phillips 1993). The top map shows the abundance of potassium (K). The middle map shows the abundance of thorium (Th). The lower map shows the abundance of uranium (U).

mountain ranges within Colorado, Wyoming, and Montana have many deposits of granite and metamorphic rock that contain U and are part of this highest background zone. Other high background zones include southern Arizona, West Texas, and central and southern Idaho. The next hottest (red) zones (ca 57 pC/kg⋅s or 8 μR/hr) include regions of New Hampshire and Maine as well as the Black Hills of South Dakota. These regions are



Radiation Detection and Interdiction at U.S. Borders

predominately composed of granite and metamorphic rock that are high in radioactivity but surrounded by less radioactive sedimentary rock. Another red zone is the area surrounding Reading, Pennsylvania, which is composed of metamorphic rock high in U. The orange zones (ca 50 pC/kg⋅s or 7 μR/hr) include the Appalachian Mountains that are composed of granite with elevated amounts of U and Th, especially within its fault zones along black shale and soil that contain moderate to high levels of U. The yellow zones (ca 42 pC/kg⋅s or 6 μR/hr) include the Ohio shale found in northwest Ohio and northeast Indiana that contains U from a narrow outcrop and that was spread over this region by glacial action. Other U-bearing black shale (Chattanooga and New Albany shales) containing this general radioactivity level is found in Kentucky, Indiana, and other parts of Ohio. The green zones (28–36 pC/kg⋅s or 4–5 μR/hr) cover wide regions of the United States and include deposits of glacial Lake Agassiz in North Dakota that have some radioactive clay and silt, and the Mississippi River region that has sands containing U-bearing glauconite. The blue zones represent fairly low-exposure areas (ca 21 pC/kg⋅s or 3 μR/ hr) and include the southern states, various parts of Texas, and the Nebraska Sand Hills. These zones are composed of light quartz sand devoid of much Th and U. The violet zones represent very low-exposure areas (<14 pC/kg⋅s or 2 μR/hr) and include areas high in basalt that is low in U. These areas include much of the region between the Cascades and Pacific coast in the Pacific Northwest and parts of the Columbia Plateau in eastern Oregon. This zone also includes Pleistocene glacial deposits in Michigan, parts of Wisconsin, and the northern half of Minnesota. The coastal regions of the Carolinas and Georgia, and the entire state of Florida are in this zone, which are composed of unconsolidated sands, silts, and clays devoid of K, Th, and U. The black zones represent water for example, Great Lakes, Salt Lake, and so on, and areas of missing data. Because water absorbs gamma radiation, practically no gamma radiation is detected over large bodies of water.

2.2.3 construction materials The presence of radioactive elements in construction materials is highly variable. The aggregate for concrete and road construction is one major source of radioactive materials in construction. In some areas, such as Denver, Colorado, some residences needed to be fitted with active ventilation to prevent the build-up of radon levels in the basements (Cohen 1991; Marcinowski et al. 1994; Nero et al. 1986, Porstendorfer et al. 1994). Some sources of aggregate can no longer be used in the construction of residences and office buildings. In 2003, the American Society for Testing and Materials developed protocols for the installation of radon monitors in buildings (ASTM 2003).

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table 2.2 Average correlation of the low-energy, high-energy, and neutron background from nine radiation portal monitor locations with nine weather parameters Weather Parameter

Temperature Dew point Humidity Pressure Visibility Wind direction Wind speed Gust speed Precipitation

Low-Energy Gamma

0.55 0.57 0.13 –0.15 –0.15 0.11 –0.15 –0.09 0.15

High-Energy Gamma

–0.18 –0.09 0.28 –0.27 –0.23 0.10 0.04 0.07 0.26

Neutrons

0.21 0.20 –0.03 –0.44 –0.01 0.05 0.08 0.07 0.03

These same construction materials show up as NORM during transport across borders. The impact of construction materials in the local infrastructure at ports upon the RPM background can be significant but is highly variable. Attempts are made to limit such effects during RPM installation as much as possible, but the local environment is usually predetermined when the RPM systems are installed.

2.2.4 weather-related variations Several weather parameters correlate with the measured background radiation as shown in Table 2.2, in which the correlation between background and various weather parameters is shown. A correlation coefficient above 0.45 is considered a “strong” correlation; between 0.3 and 0.45 “good”, between 0.2 and 0.3 “moderate”, between 0.1 and 0.2 “weak”, and below 0.1 insignificant. The data analysis shows a strong positive correlation between low-energy gamma-ray background and temperature and dew point, a weak negative correlation with barometric pressure, visibility, and wind speed, and a weak positive correlation with precipitation and wind direction. There is a moderate positive correlation between high-energy gamma-ray background and humidity, and a moderate negative correlation with precipitation and visibility. There is a weak negative correlation to temperature and a weak positive correlation to wind direction. There is a good-to-strong positive correlation between neutron background and barometric pressure and a moderate positive correlation to temperature and dew point.

2.2.4.1 Radiation Portal Monitor Locations in Weather Variability Study Nine RPM deployment sites were analyzed to study the effects of weather on the measured background. These sites are located across the northern United States border and include sites on both the East and West Coasts. These locations provide a wide variation in weather conditions and elevations. Data from the various RPM locations were available over a range of 6 to 18 months and therefore represent all seasons.



Radiation Detection and Interdiction at U.S. Borders

2.2.4.2 Weather Data For each RPM location, data from three neighboring weather stations were used in the analyses. On average, the nearest weather stations were within 13 km (8 mi) of the RPM location, and the average elevation difference was 12 m (39 ft) between the RPM and the weather station. For one RPM location, the nearest weather station was over 40 km (25 mi) away. Figure 2.15 illustrates the distribution of weather stations around the conterminous United States and the border and coastal regions of neighboring countries. These stations are listed with the World Meteorological Organization and are generally operated by the U.S. National Weather Service, Environment Canada, and Servicio Meteorológico Nacional in México. Stations not operated by these organizations follow their own practices and those of the World Meteorological Organization. The data were acquired through the Weather Underground web portal, which provides archived weather data (Wunder 2006). All the U.S. stations in this study operated 24 hours a day, while several of the Canadian stations did not operate overnight. For most stations, weather data include the following: • • • • • • •

temperature dew point humidity atmospheric pressure visibility wind direction wind speed and wind gust speed

50°N

45°N

40°N

35°N 500 km 30°N

25°N 125°W

115°W

105°W

95°W

85°W

75°W

65°W

figure 2.15 Map illustrating the locations of weather stations around the conterminous United States and the surrounding border regions that were available for examining the relationship between weather and the background measured at the radiation portal monitor locations.

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• precipitation • visual observations • time and date of the recording.

Some stations were missing specific kinds of data, such as pressure and visibility. Additional data were available from private weather stations but were not included here. Most weather data are recorded once per hour, while precipitation is sometimes recorded as sparsely as once every 4 hours. The background data from the RPMs are generally recorded once per second. To correlate these data streams, the weather data for a given RPM data record were linearly interpolated from the two nearest weather readings as long as the weather readings were within 2 hours of the RPM reading. The RPM data outside this range were ignored in the correlation calculations. Also, no extrapolation of weather data outside of a recorded range was performed.

2.2.4.3 Temperature The analyses show that temperature correlated well with the background readings. For all RPM locations, low-energy gamma-ray background showed a strong positive correlation with temperature (i.e., as temperature increased, the background increased) with an average correlation coefficient of 0.55. For high-energy gammaray background, there was weak negative correlation with temperature, but it varied by location for a resulting average correlation of –0.18. For some locations the correlation was positive; at others, it was negative (i.e., the background decreased for increased temperature). This correlation between background count rate and temperature could be a combination of temperature–induced changes in the background, as well as temperature effects on the equipment. Dueñas et al. (1997) showed that the greatest influence on the release of radon (Rn) from soil in Malaga, Spain, were humidity and soil thermal gradient. This could be one explanation of the strong positive correlation between temperature and gamma-ray background. Another researcher noted a 55% increase in the radon exhalation rate for a soil sample when the soil temperature was increased from 5°C to 50°C (41°F to 122°F). While this temperature change is outside normal conditions, it still indicates that radon exhalation can be temperature induced and likely contributes to the gamma-ray background detected by the RPMs. There is a weak positive dependence of RPM instrument background count rate with an increase in temperature primarily due to phototube noise. Test results showed that the background-subtracted gross count rate remained constant over the temperature range of –20°C to +40°C (–4°F to +104°F) with sources emitting gamma rays of energy 662 keV to 2614 keV. However, it decreased by 25% at +55°C (131°F) for most of the sources tested (Stromswold and Rohrer 2006).



Radiation Detection and Interdiction at U.S. Borders

2.2.4.4 Barometric Pressure The analysis shows a strong negative correlation between neutron background and barometric pressure. The average correlation coefficient is –0.44. As discussed above, the cosmic-ray research community has found a similar correlation between background neutrons and barometric pressure (Yanchukovsky et al. 2001). Barometric pressure is a measure of the mass of an imaginary column of air situated above a barometer. The mass directly relates to the number of molecules in a unit volume of air. Therefore, a higher barometric pressure provides more interaction between cosmic particles and air molecules, and therefore more absorption. This results in a lower count during higher barometric pressure and a higher count with a lower barometric pressure. Often cosmic-ray data are pressure corrected with a standard coefficient of 0.95% per mm-Hg to include the correlation between the time changes of the barometric coefficient and the modulation of the nucleonic intensity. This shows that cosmic sources are a significant part of the fluctuation in neutron background measured at the RPM locations. The analysis shows a weak negative correlation between low-energy gamma-ray background and barometric pressure, and a moderate negative correlation with high-energy gamma-ray background. In the environmental monitoring community, it is generally accepted that radon levels increase shortly after arrival of a low–pressure air mass and decrease shortly after the onset of a highpressure air mass. Low pressure allows radon to come out of the soil at a more rapid rate than normal. Clements and Wilkening (1974) and Schery et al. (1982) have described the relationship between barometric pressure and radon transport. Kasztovszky et al. (2000) found a strong negative correlation between radon concentrations and barometric pressure. These explanations of the negative relationship between gamma-ray background and barometric pressure indicate that pressure–induced radon exhalation is one of the sources of gamma-ray background.

2.2.4.5 Precipitation The analyses of individual locations show a moderate correlation between highenergy gamma-ray background and precipitation at most RPM deployment sites with a weak correlation at the remaining sites. The overall correlation coefficient for all locations was 0.26. This correlation is likely due to deposition of radon progeny from rain, snow, or other types of precipitation. Additionally, deposition of manmade nuclear materials and cosmic-induced radionuclides likely contributes to this effect. Deposition of short-lived radon progeny by precipitation is a well-known phenomenon in the environmental monitoring community (Fujitaka et al. 1992; Klemic 1996). These progeny attach to aerosols in the air and can be washed out of the atmosphere. The background level at the ground rises quickly as the decay products are deposited by precipitation. A downpour can potentially produce a

Radiation Sources

53

very dramatic increase in normal background levels. However, since the radon decay products are short lived, background levels return to normal within a few hours after the rain stops. This phenomenon of increased gamma-ray background with rainfall has been observed and documented on the network of radiation monitors within the Neighborhood Environmental Watch Network (NEWNET 2005) operated by Los Alamos National Laboratory. Another study has shown that radon exhalation from samples of concrete, alum shale, and alum shale–bearing soil increases with moisture content with some samples showing up to a 20-fold increase in radon exhalation (Miles and Algar 1988). Stranden et al. (1984) showed that radon exhalation from shale, soil, and concrete rises rapidly with moisture until the material is saturated. Above that point, radon exhalation actually decreases. In moist soil, a radon atom that enters water-filled pore space has a good chance of decaying in the water, resulting in a reduced transport of radon to the air and a decreased radon background. Therefore, a simple correlation probably does not show the full relationship between precipitation and gamma-ray background. Precipitation also brings down gamma emitters from the atmosphere. A likely contributing factor to this relationship is the Cs and Co from man-made sources and Be from cosmic-radiation interaction with the atmosphere found in rainwater (DEFRA 2000).

2.2.4.6 Humidity The correlation results show a moderate positive correlation between highenergy gamma-ray background and humidity with the average correlation coefficient of 0.28. The analyses also show a moderate positive correlation with precipitation as mentioned above. It is likely that because humidity increases with precipitation, part of the relationship between humidity and high-energy gamma-ray background is explained by precipitation.

2.2.4.7 Visibility Humidity, precipitation, and visibility are all products of atmospheric moisture. The correlation results show a strong connection between these three. Visibility is reduced with precipitation and high humidity. When humidity is 100% (i.e., dew point and temperature are equal), then fog forms and visibility drops. The correlation results also showed a moderate negative relationship between high-energy gamma-ray background and visibility with an average correlation coefficient of –0.23. With visibility and precipitation being negatively correlated, this relationship is likely related to the positive correlation of gamma-ray background with precipitation.

2.2.4.8 Wind Data analysis showed a weak-to-moderate negative correlation between wind speed and low-energy gamma-ray background at all but one RPM deployment



Radiation Detection and Interdiction at U.S. Borders

site. Kovach (1945) found that exhalation of radon (Rn) from soil increased with wind speed because the local air pressure is lowered. This is the opposite of the observed correlation coefficient. However, Crozier (1969) describes a more complex relationship between wind speed and radon exhalation rates; in his model, wind creates both positive and negative changes in local air pressure. This change is controlled by large- and small-scale topographical structures. On the upwind side of obstacles, the pressure is high, which results in a decreased radon exhalation rate, and thus a lowering of radon-induced background with wind speed. On the downwind side, the pressure is low, which produces an increased radon exhalation rate. A simpler explanation is that wind increases air mixing to reduce the buildup of radon gas. Therefore, it is likely that one of these latter explanations is the cause of the negative correlation between low-energy gamma-ray background and wind speed. Three of the nine RPM deployment sites showed a weak positive correlation between neutron background and wind speed. This effect has been noted in the cosmic-ray community as well, and some cosmic-ray neutron monitors include a wind correction on the neutron detectors (Malan and Moraal 2002). This positive correlation is caused by a relationship between wind speed and measured barometric pressure. In windy conditions, the barometric pressure alone is not a measure of atmospheric thickness.

2.2.4.9 Thunderstorms Aglietta et al. (1999) have reported a variation in cosmic-ray counts during thunderstorms in the upper atmosphere. The authors conclude that strong atmospheric electric fields associated with thunderstorms have an effect on the propagation of secondary cosmic-ray particles (Aglietta et al. 1999; Inan 2005). Others speculate that X-rays are produced during high-altitude lightning discharges and have named this phenomenon Terrestrial Gamma Flashes. The study reported in this document did not take into account thunderstorms because the initial weather data sets did not contain thunderstorm information.

2.2.4.10 Diurnal Cycles Figure 2.16 shows that the diurnal variation in the background is between 1% and 2% of the total value, which is small when compared with seasonal and weather influences. Figure 2.16 shows that low-energy gamma-ray background reaches a minimum around 1200 and maximum around 2100 in the late evening. The diurnal relationship with high-energy background peaks in the early morning around 0600 and reaches its minimum in the late afternoon and early evening between 1500 and 2000. Gale and Peaple (1958) and Fisenne (1980) found that the diurnal changes in radon concentration were directly related to air mixing. Their studies showed that radon activity typically peaks in the early morning and reaches

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its low in mid-to-late afternoon. This corresponds well to the diurnal cycle in background count rates observed here. A temperature inversion occurs in the early morning when the air at the ground level is colder than at higher levels. This inversion results in very little air mixing, causing an increased build up of radon. In the late afternoon, the inversion disappears and there is better air mixing. This results in lower concentrations of radon. Figure 2.16 also illustrates there is a smaller diurnal relationship for neutron background when compared with gamma-ray background. There is a consistent pattern of lower counts in the morning around 0900 and higher counts in the late afternoon to evening between 1600 and 1800. For certain locations in the early morning, there is a minor dip around 0200 and a minor peak around 0300. A diurnal fluctuation in cosmic-ray intensity has been well known for at least 50 years (Kane 1955). This fluctuation is from charged particles in the ionosphere interacting with charged cosmic particles. As solar radiation warms the ionosphere, the ionosphere becomes more ionized and changes the way it interacts with cosmic particles.

High energy gamma Norma ized count rate (%)

Norma ized count rate (%)

Low energy gamma 102% 101% 100% 99% 98%

102% 101% 100% 99% 98% 0 00 22 00 20 00 18 00 16 00 14 00 12 00 10 00 8 00 6 00 4 00 2 00 0 00

0 00 22 00 20 00 18 00 16 00 14 00 12 00 10 00 8 00 6 00 4 00 2 00 0 00 Time of day

Time of day Neutron Norma ized count rate (%)

Norma ized count rate (%)

Total gamma 102% 101% 100% 99% 98%

101% 100% 99% 98% 0 00 22 00 20 00 18 00 16 00 14 00 12 00 10 00 8 00 6 00 4 00 2 00 0 00

0 00 22 00 20 00 18 00 16 00 14 00 12 00 10 00 8 00 6 00 4 00 2 00 0 00 Time of day

102%

Time of day

figure 2.16 Average diurnal response of total gamma-ray, low-energy gamma-ray, high-energy gamma-ray, and neutron background as a function of time of day. These curves are averaged over all panels, locations, and days.



Radiation Detection and Interdiction at U.S. Borders

table 2.3 Average correlation between seasonal cycle (day within year) and the measured radiation portal monitor background RPM Background

Correlation with Seasonal Cycle

Neutron Low-energy gamma ray High-energy gamma ray Total gamma ray

0.259 0.588 0.413 0.530

2.2.4.11 Seasonal Variation For low-energy gamma-ray background, Table 2.3 shows a strong correlation between the seasonal cycle and the low-energy gamma-ray background with an average correlation coefficient of 0.588. Low-energy gamma-ray background generally follows the seasonal cycle with high values in summer peaking in July and low values in winter in January and February. Because temperature is cyclical on a yearly basis and follows this same trend, temperature is likely the major component of the relationship between low-energy gamma-ray background and the seasonal cycle. For high-energy gamma-ray background, Table 2.3 shows a good correlation between seasonal cycle and high-energy gamma-ray background with an average correlation coefficient of 0.413. For some RPM locations, there were higher backgrounds in the autumn and winter around October through December. For others a general trend was not readily discernible, though a drop in summer is evident. This rise in late autumn and winter is likely related to increased precipitation at these times, and the reduction in summer is likely due to reduced precipitation. It was shown above that increased precipitation causes increased high-energy gamma-ray background due to radionuclides from radon progeny, man-made sources, and cosmic-induced sources being washed out of the atmosphere. Also, this time period is marked by lower barometric pressure and more changes in pressure. It was shown earlier that high-energy gamma-ray background increased with lower pressure and pressure differentials. The correlation between neutron background and seasonal cycle was less than observed for gamma-ray background. The average correlation coefficient was 0.259. Most RPM locations showed little relationship between neutron background and season except a couple that showed a drop in the late autumn and early winter. This could be due to lower temperatures in the winter, since there is a moderate positive correlation between neutron background and temperature shown earlier.

2.2.4.12 Summary of Weather Effects on Background A strong relationship exists between temperature, dew point, and seasonal cycle with low-energy gamma-ray background. Correlation coefficients at most RPM locations were strong for these parameters. In part, this is from the exhalation of

Radiation Sources

57

radon and its progeny from the soil. There is also a relationship between low-energy gamma ray and humidity, barometric pressure, visibility, precipitation, wind speed, and the diurnal cycle. The relationship to precipitation, visibility, and humidity is due in part to radon progeny, man-made isotopes, and cosmic radiation–induced nuclides washing out of the atmosphere with precipitation. High-energy gamma-ray background has a moderate relationship with humidity, visibility, precipitation, and the seasonal cycle. Correlation coefficients are positive with humidity and precipitation and negative with visibility. This relationship is in part from radon progeny, man-made isotopes, and cosmic ray–induced radionuclides washing out of the atmosphere. High-energy gamma-ray background has a weak negative relationship to barometric pressure, partly because of the increased radon exhalation at lower pressure. A strong relationship exists between neutron background and barometric air pressure. Barometric pressure is a measure of the amount of air over a location. This directly relates to the amount of cosmic–induced neutrons that make it to the surface of the Earth. Correlation coefficients at most border crossings and terminals were negative between pressure and neutron background. This negative relationship indicates that background levels tend to go down when pressure goes up and vice versa. This also supports cosmic radiation as a source of some of the neutron background. Concentration of radon and its progeny fluctuate with the time of day and season. This is primarily caused by weather and produces small diurnal and seasonal changes on the background gamma data.

2.3 Naturally Occurring Radioactive Materials Richard Kouzes, James Ely, Joseph McDonald, and Edward Siciliano United States ports of entry average more than 300,000 vehicles per day, approximately 2,500 aircraft, and nearly 600 ships at more than 600 border sites. To avoid impeding the flow of legitimate trade and travel, scanning for illicit radioactive materials must be quickly conducted. However, because of the importance of this task, it must also be done accurately. Balancing these two opposing requirements is difficult, and it is made even more so by the presence of nonthreatening radioactive materials, such as medical radionuclides that may be present in patients that have been recently treated (Kouzes and Siciliano 2005), and NORM (Kouzes et al. 2006) being legally transported. Commercial shipments of materials such as glazed-ceramic products, abrasives, road salt, and kitty litter contain low concentrations of naturally occurring radionuclides such as K, Th, and U, and their daughters. Although low in concentration, a large volume of these radionuclides in a typical cargo container often triggers alarms from radiation detectors that must be investigated when they occur at border crossings.



Radiation Detection and Interdiction at U.S. Borders

To carry out more efficient radiation detection and isotopic identification at border crossings, it is necessary to anticipate and recognize the types of cargo that contain NORM so that such “nuisance” alarms can be quickly resolved. For any given port of entry, predicting the expected nuisance alarm rate from NORM is difficult, and experience has shown there is a wide range in the rate of such alarms. Clearly, any reduction in time spent by CBP officers in determining whether an alarm is a threat will help make their search for actual illicit radioactive materials more efficient.

2.3.1 radioactive sources of concern and common legitimate sources While it is true that nearly all cargo contains small amounts of natural radioactive materials, even large volumes of cargo rarely contain enough radioactivity to require a hazardous-cargo transportation label. The presence of NORM is a daily part of legitimate commerce and is neither a threat to national security nor to the health and safety of border inspection personnel or others involved in commerce (Gesell and Lowder 1980; ISO 2002; NCRP 1988; RIN 2003; UNSCEAR 2001). Indeed, most NORM is unregulated by any government agency. When raw materials are processed, filtered, refined, or selectively concentrated in any way, the concentration of the radioactive material may be increased in some products, by-products, or waste products, resulting in technologically enhanced NORM (TENORM). The isotopes present in NORM or TENORM cargo that are most likely to trigger alarms in RPMs at border crossings include the U series of radionuclides, the Th series of radionuclides, and naturally occurring K (Kouzes et al. 2003). Each of these isotopes is found in small concentrations virtually anywhere on Earth. Uranium, for example, is found at the parts-per-million level in most natural materials. Moreover, these isotopes have extremely long half-lives. The two radioactive “series” begin with radionuclides with long half-lives of 4.5 × 10 y for U, and 14 × 10 y for Th, and, in the case of natural K, K has a half-life of 1.3 × 10 y. The U and Th series each contain a sequence of radioactive decay products, some of which emit radiation that can trigger an alarm in a RPM. An important decay product in the U series that is often mentioned separately is  Ra. Several other radionuclides appear as TENORM on occasion: for example,  Lu, which is commonly found in television screens and is about 50 times more radioactive than K. For cargo being scanned by RPMs at border crossings, the radioactive materials of concern include Pu, U and U, Np, Am, and a number of other radioactive sources that could be used for an RDD (Kouzes 2004). Cargo that frequently contains NORM or TENORM—especially in bulk quantities—includes the following:

Radiation Sources • agricultural products • ceramic glazed materials (including antique pottery and dental ceramics) • camera lenses • polishing compounds and abrasives • propane tanks • kitty litter • road salt

59 • • • • • •

ore rock vulcanized rubber television and computer monitors medical isotopes smoke detectors.

Each of these items contains varying levels of K, and U- and Th-chain daughters. In addition to NORM and TENORM sources, a number of other man-made sources of radiation are found in legal commerce. These sources include medical radionuclides and specialized commercial products that use radioactive materials in the function of their products. A large number of radionuclides are used in medical diagnosis and treatment; the most common of these are mTc, I, I, Tl (which contains significant Tl), Ga, In, and Pd (Kouzes and Siciliano 2005). Because of the frequency of alarms triggered by medical radionuclides, this topic is covered separately in Section 2.4. Examples of specialized commercial radiation sources other than those used for medical diagnostics are those used for radiation therapy, weld examination, liquid-level gauges, and, in some older gauges and instruments, Ra-containing paints used to illuminate gauge pointers and numbers. These specialized commercial radionuclides include Am, Ba, Cf, Cs, Co, Co, Ir, and Ra. A few commercial neutron sources that are used for soil and concrete moisture measurements and the examination of oil and gas wells consist of mixtures, such as AmBe,  PoBe, PuBe, and RaBe. In addition, slightly radioactive depleted U is used for the construction of shipping casks, military armor, and aircraft counterweights. Shipments of large quantities of UO or UF are associated with the nuclear power industry and can produce significant numbers of neutrons. For the most part, these additional nonthreatening sources of radiation are distinguished from NORM and TENORM sources by their higher concentration and smaller volume. Thus, they are typically localized and usually easy to find. Table 2.4 lists the activity for several radionuclides in various materials and selected food items that may appear in commerce. Note that some values have a wide range, depending upon where the material originated. Although bananas and Brazil nuts are relatively well known, several other fruits and vegetables contain significant NORM (mostlyK) that is more concentrated in dried fruits and vegetables. Fresh fruit, like bananas, is usually packed at low density to protect it from shipping damage, and thus the cargo may not emit significant radiation to trigger an alarm. All values listed in Table 2.4 should be considered highly unpredictable because the radionuclides found in these products depend on many uncontrolled variables, such as where the item was grown and what fertilizer was used. Table 2.5 lists additional cargo items that are known to contain significant levels of NORM or TENORM.



Radiation Detection and Interdiction at U.S. Borders

table 2.4 Naturally occurring radioactive material and technologically enhanced naturally occurring radioactive material activity in Bq/kg for various materials and foods Approximate Activity (Bq/kg) of Isotopes in Substancesa

Substance 40

226

300–2000 10,300 130 3900 14 210 130 44–1060 150–500 5200 2000–4000 40–8000 2100 600–10,000 7800 200–300 8060 170 780 40–200 965 40–70 2300 – 110 40–1000 210 7800 1000 10,300 130

K

Adobe Alum shales (Sweden) Banana Basalt Beer Brazil nuts Carrots Coal ash Concrete Diorite Feldspar Fertilizers Gabbro Granite Granodiorite Kitty litter Light salt Lima bean (raw) Limestone Marble Marble tile Monazite sand Peridotite Phosphates Red meat Sandstone Scotch-Brite® padsb Shales Slate U.S. soils White potatoes

238

232

20–90 2200 0.037 37 – 37–259 0.0222–0.074 22–220 40 51 40–100 20–1000 33 30–500 99 – – 0.074–0.185 15 20–30 – 30–1000 18 5–800 – 70 –

20–90 2200 – 37 – – – 56–440 40 51 40–100 230–2300 33 30–500 99 21–136 – – 15 20–30 63 30–1000 18 150–2300 – 70 350

23–200 6.1 – 37 – – – 44–280 40 24 70–200 20–30 20 40–70 73 18–43 – – 5.3 20 220 50–3000 12 5–170 – 70 310

40 70 160 0.0925

40 70 140 –

4.5 70 130 –

Ra

U

Th

a

Note that the values of activities can be highly variable (Beck et al. 1980; Gesell and Lowder 1980; ISO 2002; NCRP 1988; RIN 2003; UNSCEAR 2001).

b

Scotch-Brite is a registered trademark of the 3M Company.

Because of the possibility that some cargo may not produce alarms at border crossings, it is useful to compare the expected radioactive materials with those actually found to trigger alarms at U.S. border crossings. Table 2.6 provides information on radioactive materials observed at three border crossings. Kitty litter and medical radiopharmaceuticals account for about half of the observed “innocent” materials causing nuisance alarms, followed by

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61

table 2.5 Other cargo items known to contain significant levels of naturally occurring radioactive material and technologically enhanced naturally occurring radioactive material Isotopea

Item 226

Aircraft counterweights Camera lenses and other optical glass Cloisonné jewelry Colored ceramic glazes Dental ceramics Firebrick Fluorescent lamp starters Gas lantern mantles Gyroscope rotors Magnesium-thorium alloys (aircraft parts) Manufactured anhydride (by-product gypsum) Oilfield scrap metal; natural gas containers and pipes Phosphogypsum Polishing powder Specialty glass (colored) Strike plates for flint-lock muskets Thoriated tungsten welding electrodes (welding rods) Tundish nozzles used in steel making and smelting Zirconium sand a

Ra

238

U

232

Th

x x x x

x x x x

x x x x x x

x

x

x x

x x

x x

x x x

The column marks (x) indicate the presence of the three isotopes 226Ra, 238U, and 232Th.

abrasives and refractory materials, at Crossing A on the northern border. Crossing B, also on the northern border, has a different distribution of alarms, though kitty liter is still a significant source of alarms. Alarm information from the southern border, as in Crossing C, indicates that ceramics are a dominant NORM source of alarms, as well as manufactured goods like television sets (also dominated by the presence of K).

2.3.2 photon emission spectra from cargo The simple detection of radiation does not provide enough information to identify the isotopic source of that radiation and therefore to determine whether the radioactive source is innocent or illicit. Additional information is needed in the form of a photon emission spectrum that can be analyzed to determine the nature of the radioactive material. However, this is not an easy measurement to perform in the few tens of seconds available to CBP officers while they check a vehicle. This identification activity is handled during a secondary scanning process where a handheld isotope identifier instrument may be used. Some examples of the photon spectra emitted by unshielded NORM that produce alarms at border crossings are shown in Figure 2.17, Figure 2.18, and Figure 2.19.



Radiation Detection and Interdiction at U.S. Borders

table 2.6 Approximate relative percentage of alarming cargo loads containing the listed radioactive materials observed at some border crossings Source Material

Location A % of Identified Alarms

Kitty litter Medical (In, I, Tc, Tl) Abrasives/scouring pads Refractory material Mica Fertilizer/potash Granite/marble slabs Ceramics/tile/toilets Trucks/cars Aluminum Earth Bentonite Salt Other metal Televisions Gas tankers Smoke detectors Other

Location B % of Identified Alarms

34 16 14 8 5 5 4 4 2 – – – – – – – – 8

Location C % of Identified Alarms

25 – 5 – – 13 – 9 – 15 11 5 5 3 – – – 9

– – – – – – 10 28 – – – – – – 27 13 4 18

Note: No information is available for entries indicated by a “–.”

Energy (keV) 2000

2500

208T

40K

214Bi

214Bi

214Bi

1500

56596 66a marble tile

214Bi

212Pb

Counts

104

208T 214Bi

214Pb

214Pb

105

228Ac

106

1000 228 228Ac Ac 228Ac

500

103 102 101 100 10−1 0

1024

2048

3072

4095 5120 Channel

6144

7168

8192

figure 2.17 High-purity germanium spectrum of marble tile. Uranium chain, Th chain, and K are all prominent.

Radiation Sources

63 Energy (keV) 500

1000

1500

2000

2500

214Bi

214Bi

208T

40K 214Bi

228Ac

228Ac

214Bi

103 Counts

228Ac

214Bi 208T

214Pb

104

56596 67a Super clay kitty litter by Arm & Hammer 228Ac 214Pb

212Pb

105

102

101 100 10−1 0

1024

2048

3072

4095 Channel

5120

6144

7168

8192

figure 2.18 High-purity germanium spectrum of typical kitty litter. Uranium chain, Th chain, and K are all prominent.

Energy (keV) 1000 1500

500

2000

2500

40K 1481

105

104

56595 73A PAX Snow & ice melt

Counts

103

102

101 100 10−1 0

1024

2048

3072

4095 Channel

5120

6144

7168

8192

figure 2.19 High-purity germanium spectrum of typical snow and ice melt salt, dominated by K (K Activity of 2.55 Bq/g).



Radiation Detection and Interdiction at U.S. Borders

The spectra were measured using a high-purity germanium (HPGe) gammaray detector and associated electronics for long counting periods to show the detail of the gamma-ray lines observed. The U- and Th-series chains, along with  K, are visible in Figure 2.17 and Figure 2.18 for marble tile and kitty litter, respectively. Figure 2.19 is dominated by the K peak in the salt used for melting snow and ice. It is evident from the photon energy spectra of these common radionuclides that an RPM at a border crossing may be confronted by a wide range of photon energies that complicate the process of separating an innocent radioactive source from one of concern. The dominance of NORM, TENORM, and medical radionuclide–related alarms at border crossings presents an operational challenge because of the time and staff required to resolve and release these nuisance alarms.

2.4 Scope and Impact of Medical Radioisotopes Richard Kouzes and Edward Siciliano Presently, most polyvinyl toluene (PVT)-based RPMs do not discriminate radionuclides that pose a threat from those that do not, such as medical radionuclides (Kouzes et al. 2006; Kouzes and Siciliano 2005). A large number of radionuclides used in medical diagnosis and treatment emit gamma-ray radiation that is easy to detect many hours, or even days, after they are administered. The most common of these medical radionuclides are isotopes of technetium (mTc), iodine (I), thallium (Tl), gallium (Ga), and indium (In) (Frost and Sullivan Healthcare Group 2002). Patients treated with medical radiopharmaceuticals containing these radionuclides can produce an alarm in their lane and one or more adjacent lanes of a multilane RPM deployment depending on the energy of the emitted gamma rays. The operational impact of outpatients containing radionuclides can be significant, as every alarm produced at a border crossing must be investigated at a secondary onsite scanning location. The dominant operational impact from radiopharmaceuticals is nuisance alarms caused by cross talk between multiple lanes that alarm simultaneously from one medical patient. These cross-talk alarms require resolution and can be an operational burden, particularly at high-volume border crossings, where traffic is literally brought to a halt until the vehicle causing the alarms is identified. In this section, three objectives are discussed. The first is to provide a realistic and reasonably complete estimate of the types, dosages, and numbers of radionuclides used for medical procedures in the United States. The second objective is to use this information as source input to Monte Carlo N-Particle (MCNP) (Briesmeister 2000) calculations that simulate the photon detection capabilities of RPM systems. The third objective is to combine the medical isotope data with the

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65

MCNP simulation results to estimate the impact on radiation scanning at border crossings.

2.4.1 medical radionuclide use Each day in the United States, over 39,000 medical procedures involving radiopharmaceuticals are performed (Frost and Sullivan Healthcare Group 2002). The overwhelming majority of these procedures are diagnostic in nature and performed on an outpatient basis (i.e., patients leave the treatment facility less than 24 hours after receiving the initial dose). Patients commonly cross borders to receive such treatments. Therefore, within a short time period after treatment, most of these patients—traveling either as pedestrians or as passengers in vehicles—cause alarms if they pass in the vicinity of radiation detectors when crossing the border. Figure 2.20 shows a photon spectrum emitted from a patient who had been administered the medical radionuclide Tl during a stress test weeks earlier to evaluate cardiac performance. While shipments of medical isotopes to hospitals are regulated and transported with hazardous-cargo labels declaring their activity, patients that have received radionuclides generally travel freely without documentation. Their radioactivity can be quite large, similar to that of GBq (tens of millicuries) sources. Photon energy peaks at 167 keV and 135 keV are from Tl, and significant thallium K X-rays are present at 70–85 keV. The peak at 440 keV clearly indicates the presence of a Tl radionuclide impurity. Thallium-202 is less than 1% of the activity at the time of administration but becomes relatively more prominent as the 3.04-day Tl decays away faster than the 12.23-day Tl. For comparison, materials of concern like U and plutonium emit low-energy gamma rays predominantly below 500 keV.

100000 10000 100

Counts

1000 10 1 0

100

200

300 Energy

400

500 (keV)

figure 2.20 Gamma-ray spectrum emitted by a patient who had a Tl stress test weeks earlier.



Radiation Detection and Interdiction at U.S. Borders

To estimate the frequency of occurrence and determine the signatures of these nuisance radiopharmaceutical-based alarms, a survey was performed of commonly used medical radionuclides. The categories of data sought were the total number of such procedures, the specific types of procedures, the specific radioisotopes used in these procedures, and the dosage administered per procedure. The results presented do not represent an exhaustive search and thus should be considered an estimate. Data reported here were obtained from a single example of the medical community, and the business and finance communities. Because these professions have been compiling information for different purposes, data from all three sources were deemed necessary to cover the range of our objectives. To attain a single, comprehensive, and consistent set of data, the information from these sources was compiled, assessed (outliers removed), and then merged. The resulting information was used as a database for estimating the number and characteristic signature of medical incidents that may be detected with RPM systems. The two primary sources of data for this survey were anonymous hospital records, and a market report on U.S. radiopharmaceuticals. These data provide a reasonably accurate and conservative initial estimate for the number of expected alarms, and the typical dosages are reasonably accurate so that their detection with existing RPMs can be estimated. Medical radiopharmaceuticals commonly detected in people crossing borders are listed in Table 2.7. The most prevalent medical radionuclide is mTc. Because these data are 4 to 5 years old, they likely represent an underestimate of usage. During the past 8 years, there has been an annual increase of about 5% in the total number of radiopharmaceutical diagnostic procedures. Moreover, given the regulatory hurdles to new radiopharmaceuticals, patents, and hospital equipment infrastructure, the dominance of mTc will probably continue for the foreseeable future. For this data set, a total of 2797 exams were performed on 2,694 patients of whom 39.2% were male, 58.2% female, and 2.4% pediatric. The majority of these exams (91%) were administered to outpatients. Consistent with the national average, about 90% of the total procedures involved mTc (Table 2.7). Data from the business and finance community were taken from U.S. Radiopharmaceuticals Markets (Frost and Sullivan Healthcare Group 2002) and are summarized also in Table 2.7. These data cover the 11-year period from 1998 through 2008; data from 2001 are used as the base year for extrapolation into the years beyond 2002. Because of the financial perspective of this report, data from this source are organized according to the dominant market sectors, with emphasis on marketplace drivers and projected revenues. Medical procedure types define the market sectors, where the major division of the overall market is made between diagnostic and therapeutic procedures. According to Frost and Sullivan Healthcare Group (2002), a total of 14.2 million diagnostic procedures were compared with approximately 0.20 million therapeutic procedures performed in the United States during 2001. The total number of diagnostic procedures consists of

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table 2.7 Summary of survey results for medical radionuclide use Radionuclide Half-Life

Organ

Estimated Percent of 2001 Dosages at Local Hospital (2797 Total)

Percent of Procedures Administered in U.S.A. During 2001 (14,390,000 Total) a

Annual Procedures in U.S.A. Per 10,000 Population for 2001

14

Kidneys, bladder, stomach Bladder, liver, kidneys, marrow Marrow, liver Brain, heart, bladder

1.07

1.89

9.7

0.61

1.07

5.5

0.25 1.43 (estimated) 0.11 0.86 0.00

0.35 2.02

1.8 10.3

8.54 0.89 0.04 0.04 87.06

0.19 0.03 Insufficient data 0.34 1.40 0.39 0.39 91.51

1.0 0.2 Insufficient data 1.7 7.2 2.0 2.0 468

0.36 0.00

0.42 Insufficient data

0.00

Insufficient data

2.2 Insufficient data Insufficient data

C

57

51

5730 yr

Co

271.8 day

Cr F

27.7 day 109.8 min

18

67

Ga I 125 I

78 hr 13.3 hr 59.41 day

131

I In 153 Sm 89 Sr 99m Tc

8 day 67.32 hr 46.27 hr 50.5 day 6 hr

201

Tl Xe

72.91 hr 5 day

Y

64.1 hr

123

111

133

90

Intestines, marrow Thyroid, kidneys Thyroid, spleen, lungs Thyroid, kidneys Liver, marrow, spleen Skeleton, marrow Skeleton, marrow Heart, liver, spleen, marrow, kidneys, bladder, stomach, bones Heart, lungs Lungs Skeleton, marrow, liver, testes

a

Frost and Sullivan Healthcare Group (2002).

three main categories: 0.29 million positron emission tomography scans, 5.83 million cardiology, and 8.08 million other.

2.4.2 medical radionuclide survey results Results from the Frost and Sullivan Healthcare Group survey show that in more than 75 medical procedures using more than 45 different commercially available products, only 17 isotopes were used as radioactive agents (Frost and Sullivan Healthcare Group 2002). These isotopes are listed in Table 2.7, along with their percentage use nationally and at a local hospital. The national data were used to calculate the number of procedures per 10,000 people per year, based on a U.S. population of 281.4 million from the year 2000 census. As seen in Table 2.7, mTc is the dominant radionuclide (13.16 million) in radiopharmaceutical procedures out of the 14.39 million procedures administered



Radiation Detection and Interdiction at U.S. Borders

in 2001. At the other extreme of usage are the isotopes I, Rb, Xe, and Y. Although identified in the survey, these radionuclides apparently are rarely used and, because of insufficient data, are omitted from further consideration in this study. Because Cs is used for therapeutic implants that are removed after a procedure, it is not listed in the table. Thus, only 12 of the 15 radionuclides listed in Table 2.7 are considered further in this study.

2.4.3 medical radionuclide decay properties Radioactive parameters relevant for determining radiation detector response to emitted gamma-ray photons are listed in Table 2.8. These parameters include the half-life, the photon energies (greater than 30 keV) and relative intensities as percent branching fraction (%BF) of photons emitted per decay, and the administered activities (in both mCi and MBq). The last column gives the number of photons emitted per second, obtained by multiplying the activities by the total number of photons per decay. The values in this last column are valid only for the initial activities. For example, to calculate the number of photons emitted per second after 24 hours following a mTc procedure, the value listed in the last column should be multiplied by 0.063. Several radionuclides listed in Table 2.7 were eliminated from further consideration (Table 2.8) in this study because they do not directly produce decay photons. Isotopes C, Sr, and Y decay through electron emission and therefore emit no direct gamma-ray photons. The interaction of these electrons with matter produces secondary bremsstrahlung photons that have a spectrum of energies, which are detectable but not evaluated in this study. The isotope F decays by positron emission, which interacts with matter-producing pairs of 511 keV photons upon positron–electron annihilation. This process was also not evaluated. The 11 medical isotopes shown in Table 2.8 were used as sources for the MCNP calculations performed in this study. As shown in Table 2.8, each of the isotopes has a unique set of photons produced by its decay. To gain insight into these distinct photon signatures and the response of the RPM systems to them, a semilogarithmic scatter plot of their %BF versus decay energy is shown in Figure 2.21. Also shown in this figure are two vertical lines at 25 keV and 250 keV indicating the energy boundaries for which detector responses are evaluated in this study. A distinctive feature seen in Figure 2.21 is that most of the medical isotopes considered in this study emit photons below 250 keV. Thus, the RPM system response to these low energies (Low E) is the main area of interest. Because the isotopes Cr, Ga, and I emit photons with energies above 250 keV, there should  Too recent for inclusion in Table 2– is the use of 131Cs in brachytherapy (www.isoray.com). This isotope decays by electron capture. Thus, the emission is of too low an energy to be of further consideration here.

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table 2.8 Intrinsic decay properties of medical isotopes Isotope HalfLife

Emitted Photon Energies (> 30 keV) Percent Branching Fractions (> 1.0%)

57

271

keV

d

%BF keV

51

Co

Cr

27.7 d

67

Ga

78.3 h

123

I

13.2 h

125

131

I

I

60.1 d 8.04 d

111

In

67.3 h

153

Sm

46.7 h

99m

Tc

6.01 h

201

Tl

73.1

Xe

85.54 320.0

136.4

–

–

–

–

10.69

–

–

–

–

–

–

–

–

–

–

–

–

–

–

93.31 184.5

208.9

300.2

393.5

0.0006 0.022

3.56 × 104

0.075

2.78

3.34 × 105

10.00

370

5.15 × 108

0.300

11.1

1.89 × 107

%BF keV

91.27

%BF keV

2.95

37.00 20.45

2.33

16.60

4.64

30.98

31.88 158.9

–

–

–

12.43

2.71

83.30

–

–

–

30.98 20.09

31.88 4.38

35.49 6.66

– –

– –

– –

N/A

N/A

N/A

636.9

722.8

–

41.80

1547

1.59 × 109

3.225

119

2.65 × 108

140.0

5180

5.47 × 109

16.91

626

5.85 × 108

20.00 740

1.07 × 109

10.00

3.18 × 108

%BF keV

9.83

%B keV

80.18 284.3

%BF keV

2.62

6.06

22.98

26.08

%BF keV

24.15

11.57

40.90

41.54

47.00

48.50

17.96

32.36

9.42

2.85

5.32

28.30

–

–

–

–

–

–

–

–

82.78 135.2

167.4

%BF keV %BF keV

140.4

364.4 81.25

7.27

26.80 171.2 2.40 90.24

87.20

–

–

68.89

70.82

80.12

1.80 245.3 94.00

– – –

69.67 103.1

%BF keV

26.89

45.67

16.08

4.39

2.67

5.25

30.63

30.97

34.97

36.01

80.99

–

d

%BF

14.06

25.98

7.10

1.74

36.97

–

h 133

122.0

Activity Activity Number of in mCi in MBq Photons Emitted Per Second

9.43 370

also be some RPM response in this high-energy (High E) region. However, these radionuclides have a larger fraction of photons below 250 keV than above, so the RPM response to Low E is important, as discussed below. Another feature seen in Figure 2.21 is the influence of the photons below 25 keV. This low-energy cutoff effect was studied by comparing the 5 keV–250 keV RPM responses to the 25 keV– 250 keV responses. The fractional number of photons shown in this graph helps to explain the fractional differences seen in the results for these two Low E responses.

2.4.4 detector response calculation configurations A computer model was constructed to simulate pairs of RPM panels, with one pair per lane. Only results for one traffic lane (Lane 1) are reported here. A twolane study is reported in Kouzes et al. (2006) where lane-to-lane cross talk alarms are discussed. Each pair of units has two identical towers facing each other, where



Radiation Detection and Interdiction at U.S. Borders Medical decay gamma rays with E > 5 keV and %BF > 1% 57Co

51Cr

67Ga

123I

131I

111In

153Sm

99Tc

201Tl

100 Percent branch ng fract on

90 80 70 60 50 40 30 20 10 0 1

10

25 Decay energy (keV)

100

250

1000

figure 2.21 Energy distribution of dominant gammas emitted from selected medical radioisotopes. The vertical axis is the percent branching fraction (%BF).

each tower consists of a steel support in which two detector enclosures are mounted. A schematic of the model RPM is shown in Figure 2.22. As indicated, the model coordinates are defined such that this view is in the y–z vertical plane, where the z direction is vertically upward. The key components shown in this figure are the four detector enclosures (shaded rectangles) and the steel supports to which the four enclosures are attached. For reference to these four locations in Lane 1, the notation 1Pb, 1Pt, 1Db, and 1Dt is introduced (P for passenger, D for driver, t for top, b for bottom). For the purposes of this calculation, a lane curb-to-curb width of 4.3 m (14 ft) is used. Also shown in Figure 2.22 is the y–z position of the sources (S). To roughly simulate the presence of a passenger who contains a medical isotope after having a heart or thyroid treatment, the sources in this study are taken as uniform spheres with 0.01 m radii, and with the y–z position of the centers fixed to give a 0.76 m (30 in.) off-center lane shift toward the 1Pb detector and a 1.5 m (60 in.) height from the pavement. This side-to-side and height position was chosen to be within the range of passenger vehicle profiles for subcompacts to large pickup trucks, indicated by the dashed rectangles (Ramsey and Sleeper 1994). Note that the asymmetric location of the sources should cause an asymmetric response in the detectors dictated approximately by the distances from the source to panel centers. By this reasoning, the count rates in the Lane 1 panels are expected to satisfy the inequality: 1Pb > 1Pt > 1Db > 1Dt. The steel-support structure acts as a blinder, or partial shield, for the RPM panels. The blinder gives rise to detection zones, A, B, C, D, and E, as illustrated in Figure 2.23.

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Lane 1 passenger top (1Pt)

Lane 1 driver top (1Dt)

236cm Lane 1 passenger bottom (1Pb)

Lane center line

Lane 1 driver bottom (1Db)

206cm

S

155cm

Curb

Curb 4.27m Tower 1P

z

Tower 1D

y

figure 2.22 Front-view scale drawing of Lane 1 (S = Source).

1P Traffic flow Zone E

Zone D S Zone A

S

S

S

Zone C

Zone B

y x 1D

figure 2.23 Top view of Lane 1 showing optical zones of detection (S = Source).



Radiation Detection and Interdiction at U.S. Borders

2.4.5 detector response modeling method A series of MCNP (Briesmeister 2000) calculations was performed with the RPM model. The relative positions, surface geometries, and composition of the plasticpanel photon detectors were specified as input, as were all other components (e.g., aluminum doors, lead shielding, etc.) that could influence the photon responses. Additional input required for the MCNP calculations included the specific source type, its spatial distribution, and its position with respect to the gamma-ray and neutron detectors. The gamma-ray emission energies and %BF listed in Table 2.8 were used to specify the source types for the nine medical radionuclides evaluated in this modeling study. The third source position coordinate (x) was varied in 0.61 m (2 ft) increments from 8.6 m (28 ft) to 0 m (0 ft) (centerline of the portal panels). Thus, a total of 135 calculations were performed with 15 x values for 9 isotopes. The numerical output for these calculations was obtained from the MCNP photon pulse height output option (referred to as tally 8) taken separately for each of the plastic panels in the model. With 1 keV energy bins, this option provided an energy distribution of pulses created from the energy deposited by each photon entering the plastic. These counts-per-photon values were converted into countsper-second (cps) values with the photon-per-second factors for each isotope shown in Table 2.8. No additional reduction or rescaling of the output was performed. Thus, the cps results reported here represent a detector efficiency of 100% and therefore overestimate the actual detector responses that can be as low as 2% for photons below 25 keV. The models used to compute the detected signal include not only a large amount of detail on the physical systems but also simplifications that can affect the results. One simplification is the omission of shielding effects from the vehicle. Because a vehicle’s undercarriage, engine, and drive train provide a significant shielding effect, omission of shielding from these components leads to an overestimate of the counts seen in the lower detector of the RPM, but not in the upper detector. Another simplification is the discrete-step approximation to evaluating a moving source. In these calculations, the effects of vehicle motion were approximated with a static source over a series of equally spaced positions and equal simulation times. In reality, the movement of the vehicle through a portal limits the time available for detection and therefore could increase the statistical uncertainty.

2.4.6 detector response modeling results The complete set of results from these calculations was converted to figures representing the system response versus source position (distance from portal centerline in Lane 1). To simulate the data provided by the actual detectors, the energy

Radiation Sources

73

distribution output from MCNP is summed over two energy regions: the Low E from 5 keV to 250 keV, and the High E from 250 keV to 3.0 MeV. To illustrate some of the features from the results, Figure 2.24 shows the Low E region response to m Tc for the four panels in Lane 1. The asymmetric behavior of the responses seen in Figure 2.24 is characteristic of all Lane 1 results and is caused by the steel supports. Notable is the “left–right” asymmetry between the driver side (dashed lines) and passenger side (solid lines) detectors. For x > 7.5 m (24.5 ft), the source is beyond the optical field of view of both the 1D and 1P detectors, and their responses are small and essentially equal. Crossing this boundary, the 1D responses change slope, while the 1P responses do not. Recall that the source is closer to the 1P detectors, but for x > 3.7 m (12.3 ft) it is beyond the partial-panel field of view for that side. Thus, for those values of x, the 1D response is significantly larger than the 1P response. Crossing this second boundary, the source becomes exposed to the 1P detectors, and their responses rapidly increase, overtaking the 1D responses. For these small values of x, the closer proximity of the source to the 1Pb panel starts to become important and increases the difference between the top and bottom responses on each side. Note that at x = 0, the 1Pt value is just slightly greater than the 1Db value. This behavior is observed in all cases, reflecting the equality in source-to-panel distances for these two panels. The qualitative behavior discussed in the example for mTc is characteristic of the other isotopes evaluated in this study. The only significant difference among them is the scale of their values. Thus, the ratio of maximum values for any two

Lane-1 low-energy component response to 99mTc 1D bottom

1D top

1P bottom

1P top

Counts per second (<250 keV)

10000000

1000000

100000

10000 0.0

1.0

2.0

3.0 4.0 5.0 6.0 7.0 Source position in lane-1 (m)

8.0

figure 2.24 Low-energy responses of Lane-1 panels to mTc for the back-to-back configuration. The source is closest to the 1P-bottom panel detectors.

9.0

10.0



Radiation Detection and Interdiction at U.S. Borders

panels within the same lane should be approximately the same for all isotopes. The quantitative results for Lane 1 include the following: (1Db/1Pb) = 0.287 to 0.292 for the left–right ratio of bottom panels (1Dt/1Pt) = 0.549 to 0.597 for the left–right ratio of top panels (1Dt/1Db) = 0.627 to 0.661 for the up–down ratio on the driver side (1Pt/1Pb) = 0.301 to 0.349 for the up–down ratio on the passenger side The narrow range of values for some of the left–right and up–down ratios suggests that responses from individual panels or sums of panels within the same lane may provide useful information in determining the source location. Studies show that clear differences between the Lane 1 and Lane 2 ratios may be helpful in identifying the source lane from the cross talk lane in a multilane alarm situation (Kouzes et al. 2006).

2.4.7 expected occurrence of radionuclide alarm events The fraction of border crossings for which medical isotopes may be expected to cause alarm events in the RPM systems can be estimated from this model. This is done in two steps. First, a reasonably conservative value for a detection threshold is assumed and compared to the numerical results for the 11 evaluated radionuclides (Table 2.8). In this step, it is determined whether any of the isotopes at their maximum (initially administered) activities can be “seen” by the RPM systems. Second, for those radionuclides whose maximum values exceed the detection threshold, the number of days is determined after which the count rate would fall below the threshold. By combining those results with the number of procedures per day (Table 2.7), an estimate for the fractional occurrence of medically caused alarm events is obtained. To estimate if any of the individual panel responses obtained in this study would be detectable above the background radiation, an arbitrary value of 10% above background was assumed to be detectable. Applying the threshold to the maximum RPM response values leads to several observations. First, most of the values are much greater than this threshold, so without further considerations, this result indicates that 8 of the 11 medical radionuclides evaluated should be detectable when initially administered. Second, because most of the maximum responses exceed the threshold by many orders of magnitude, reducing the computed values by factors of 50 to 100 to account for less than 100% signal conversion efficiency would change the detection status of only one radionuclide (Cr). Values for the remaining seven radionuclides would still be much greater than the threshold and, thus, still easily seen by all panels. Results show that for the assumed threshold value, some of the medical radionuclides should be detected



Actual threshold values depend upon many factors.

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75

by the RPM systems in both the primary lane and the neighboring lane as far away as 9.1 m (30 ft) or more. Because at least 8 of the 11 medical radionuclides evaluated in this study would be detectable when initially administered, to estimate the number of medical events expected at border crossing per day, it is necessary to determine how many days after the initial procedure these cases would remain above the detection threshold. For this evaluation, Equation (2.7) is used: f ⎞ ⎛ Max cps ⎞ ⎛ half-life t = ln ⎜ ⎟ ⎝ Threshold ⎠ ⎝ ln( ) ⎠

(2.7)

where the Maxcps values are the upper-limit net signal values, Threshold is the net count alarm level, and the half-life values are derived from Table 2.8. Results for the eight isotopes above threshold are shown in Figure 2.25, where the number of days above threshold ranges from 3.4 days for a mTc procedure to over 115 days for a typical I procedure. Results show a ~12% cross talk rate (Kouzes et al. 2006). Therefore, cross talk to the neighboring lanes would be observed for the first 2 days after a mTc radiopharmaceutical treatment. The results shown in Figure 2.25 together with the number of procedures listed in Table 2.7 can be used to roughly estimate the number of above-background events per unit population. The 2001 U.S. population was about 281 million, and it is assumed that a random sample of this population crosses the border every day. It is unlikely that the population of medically treated patients will actually randomly cross the border on the day of treatment. Because mTc treatments are the most common (13.2 million/year), this value can be used to estimate the number

Number of days above 350 cps threshold

Se ected med ca sotopes

201Tl 99mTc 153Sm 111In 131I 123I 67Ga 51Cr

0

10

20

30

40 50 60 70 80 Days post-administration

90

100

110

figure 2.25 Time in days for medical radioisotopes to decay below the alarm threshold.

120



Radiation Detection and Interdiction at U.S. Borders

of alarms per day. A patient treated with mTc is expected to be above the detection threshold for about 3 days. An estimate of the fraction of people crossing the border (f) that will create a medical alarm each day is provided in Equation (2.8): f = (13.2/281) ⋅ (3/365) = 3.9 ⋅ 10–4 = 1/2581

(2.8)

From this analysis, one alarm would arise from about every 2,600 people crossing the U.S. border, if those crossing the border are drawn randomly from the general population. Assuming a typical vehicle load of two passengers, this would lead to about one medical source related alarm per 1,300 vehicles. Because of large demographic differences, a conclusion about alarm rates at the border based upon these assumptions must be qualified. Because most of the procedures are stress test related, the age and health of the population carrying a burden related to such tests might be typical of the reasonably affluent part of the population that has medical insurance. It might be reasonable to expect that at northern border crossings near urban areas, the observed alarm rates could be similar to the predictions, as the flow of people across the border might be a reasonable random sample. Rural areas might be expected to be farther from medical facilities, with a resulting reduction in observed alarms. The prediction of alarm rate is generally consistent with observed rates from border crossing data, although there is a large variation from site to site from about 1 in 500 to 1 in 2,000 vehicles. An important caveat of this analysis is that only nuclear half-lives and emission spectra were used, and not in vivo effective half-lives and spectra that include biological half-life effects. In vivo half-lives may well be less for some radionuclides because of natural elimination processes, and thus the observable lifetimes may in actuality be less than those derived here. Estimating border crossings from only U.S. data is obviously incomplete. In an attempt to obtain some information on medical isotope usage in Canada and Mexico, a United Nations Scientific Committee on the Effects of Atomic Radiation report (UNSCEAR 2001) was examined. Annex D, Section III of this reference, entitled “Diagnostic Administration of Radiopharmaceuticals,” lists the numbers of diagnostic procedures per 1,000 people averaged from 1991–1996 for some specific isotopes, and the average activities of these isotopes. These data are from 40 countries and unfortunately are sparse (nonzero entries for mTc and I only) and outdated compared with the detailed 2001–2002 U.S. data. For comparison with the U.S. results, the most useful data from the UNSCEAR report are the total diagnostic numbers per 1,000 people for Canada and Mexico. These values are 64.6 and 1.06, respectively. The comparable number for the U.S. is 51.1. The UNSCEAR data also shows mTc as the isotope used in essentially all of the  POV occupancy is about 2.1 persons on the northern border and about 2.2 persons on the southern border.

Radiation Sources

77

procedures listed for these countries, with the range of dosages being 70 MBq to 925 MBq. The comparable number for the U.S. is 626 MBq. One interpretation of these results is that, per person, Canadians represent a smaller, more affluent population with public health care and, thus, may be able to obtain slightly more diagnostic procedures than the U.S. population. Similarly for Mexico, the value of 1/50 of the U.S. possibly reflects the larger, less affluent population with less access to medical facilities providing these services. Also, because the UNSCEAR data are dated, they probably do not reflect the emergent trend of U.S. citizens traveling to Canada and Mexico for less expensive or noninsured radioisotope treatments. Although limited in quality and scope, the UNSCEAR data appear consistent with the 2001 U.S. data and help support the use of the 2001 U.S. data as a reasonably good first approximation for estimating the number of medical incidents expected at border crossings. These data also could imply that the alarm rate on the northern U.S. border will tend to follow the rate predicted above, while the alarm rate on the southern U.S. border may be substantially lower. The operational impact of medical-related nuisance alarms is important because of the disruption in operations when a “hot” patient causes multiple alarms at a border crossing. Each alarm requires secondary processing, so multiple alarms produce an operational burden. There is no clear path for reducing this burden. This study represents the first step in determining how medical-related nuisance alarms might be handled by appropriate instrumentation by looking at the potential number of these alarms and how they arise. While the impact of radiopharmaceutical-related alarms may not be eliminated, minimizing their impact is the subject of ongoing work.

2.5 Industrial Radiation Sources and Special Nuclear Materials Joseph McDonald The detection of illicit radioactive sources and radioactive materials is made difficult by the fact there are several lawful uses of radioactive substances. Devices as common as smoke detectors are present in nearly every home in the United States and contain a small source of Am that emits gamma rays and alpha particles. There is a long list of devices and materials that emit various types of radiation at a low intensity. However, there are other legitimately used radiation sources that emit copious amounts of gamma rays or even neutrons. These sources may be used for industrial or medical purposes and are always stored in containers constructed of lead or other materials that absorb radiation. Many radiation sources are man-made radionuclides, which are radioactive atoms, generated by neutron bombardment of elements in a nuclear reactor. Others may be produced by refinement of slightly radioactive materials that are concentrated to the point where



Radiation Detection and Interdiction at U.S. Borders

their radioactivity per unit mass is much higher. Before these radioactive materials can be sold, purchased, or used, they are controlled by governmental agencies such as the NRC (U.S. Nuclear Regulatory Commission). Material production must be reported and recorded, the user must apply for a license to use the material, and there must be a plan for eventual disposal. Another class of radioactive substances exists that is not directly under the control of the NRC. Many materials and devices contain NORM (such as U-bearing clay) that may be used in the production of roofing tiles. The U isotopes that may be present in the clay have not undergone any processing such as irradiation in a reactor. The U is a minor component of the clay and is present at such a low level that the roofing tiles are not considered to be a radiation risk requiring warning labels that indicate the presence of a radioactive source. Other radioactive materials go into the production of a large number of industrial products, and a long list of items and the radionuclides contained is provided in Section 2.3 on NORM.

2.5.1 common industrial radiation sources At the beginning of the 20th century, numerous discoveries relating to radiation and radioactivity were made. Marie and Pierre Curie were notable for their discovery of the properties of Ra, and in 1903, they were awarded half the Nobel Prize; the other half was awarded to Henri Becquerel for his discovery of spontaneous radioactivity. Among the various radionuclides the Curies extracted from pitchblende ore, the source of U, were new elements that were to be named polonium (Po) and Ra. It was not long after the discovery of Ra that the radioactive properties of the Ra isotope Ra were put to use in several areas. The most significant use of Ra was for medical treatments of cancer and other diseases. The energetic gamma rays emitted by Ra could penetrate tissue and irradiate and shrink tumors. Treatments with this radionuclide have long since stopped because more effective radiation treatments have been developed. In years past, Ra was also used for medical radiography. Radiographs, or “X-rays,” as they are commonly known, were easily produced with the highly penetrating Ra gamma rays. But, the high cost and difficulty in the use of Ra led to its replacement by other sources of radiation. Industrial radiography now makes use of other radionuclides, such as Ir, that emit 320 keV gamma rays. Iridium-192 sources are used to obtain radiographs of pipeline welds, boilers, and aircraft parts to determine whether there are any voids or defects in these dense metal parts. Radiation is used extensively in the manufacture of many products. Radioactive sources have been used to dry ink on packages, cure composite materials such as carbon-epoxy composite airplane parts, serve as fluid level and thickness gauges, enhance the wear characteristics of automobile tires, sterilize medical products and disposable bandages, and in a homeland security application to inactivate

Radiation Sources

79

anthrax spores that might be placed in mail. The gamma ray–emitting sources  Cs and Co have been used for cancer radiation therapy and to disinfest vegetables and spices. They have also been used to create mutations in wheat and other grains to make them more resistant to drought and disease. Some prepared meat products are sterilized by the radiation from either Cs or Co to reduce the danger from E. coli contamination. Because of the widespread industrial uses of radiation, there are large numbers of radioactive sources in the United States. The DOE and the NRC track the locations of nearly all radioactive sources; however, there are sources that make their way out of regulatory control either by accident or as a result of deliberate action. These devices have come to be known as orphan sources, and they can represent a potential problem if they are found by terrorists or unwittingly by those unfamiliar with radioactivity or radiation sources. Two equally dangerous situations can occur if a radioactive source is out of regulatory control. First, it might not be recognized as being a hazardous device by a person who is not familiar with the warnings that may appear on such sources or on their containers. It is even possible that the person picking up a source may be

figure 2.26 Photograph of two types of industrial radiography sources assemblies. They are normally deployed with a long cable or chain attached to the connections at the right-hand side of the photo. (Photo credit: Oak Ridge Associated Universities, Tennessee.)

figure 2.27 Photograph of Co source that may have been used for cancer radiation therapy. (Photo credit: Oak Ridge Associated Universities, Tennessee.)



Radiation Detection and Interdiction at U.S. Borders

illiterate. The source then poses a serious hazard to the person handling it and to anyone who comes near that person while the source is in their possession. Second, the source might be clearly recognized as a hazard by a terrorist who then uses the source as a weapon. Photos of some common industrial radiation sources are shown in Figure 2.26 and Figure 2.27.

2.5.2 special nuclear materials Special nuclear material is defined in the Atomic Energy Act of 1954 (USNREG 2006) as the fissionable radionuclides including Pu, U, or U enriched in the isotope  U, which is referred to as HEU. These isotopes undergo fission, a process whereby nuclei of the material break apart with the release of numerous gamma rays, charged particles, neutrons, and a large quantity of energy that eventually appears as heat in the surrounding material. The fission process can be maintained at a safe level by controlling the amount of fissionable material or the population of slow neutrons that can lead to additional fissions and a chain reaction. Although SNM is no longer produced for use in nuclear weapons in the United States, the government still has a large number of nuclear weapons and large amounts of SNM that is guarded at DOE and military sites. Measures are in place to account for the material and to verify that locks and seals on entrances to SNM storage areas are intact, and trained guards are authorized to take the necessary measures to prevent the theft of this material. Radiation detectors also surround SNM storage facilities to alert security personnel of any potential unauthorized movement of SNM. Other significant deterrent measures are in place that make theft or diversion of U.S. SNM a very remote possibility. However, it is a matter of concern as to whether the SNM in other countries is as well controlled and guarded. Several countries have received help from the U.S. to upgrade their control of SNM, as discussed in Section 7.2. A potential threat regarding SNM is the possibility that a terrorist might obtain fissionable material and attempt to construct an IND. Although it is not as likely, it is within the realm of possibility that a military nuclear weapon constructed by one of the countries possessing such weapons could be stolen and used for blackmail or for an actual detonation. Significant efforts have been made by the United States and other countries to generate intelligence regarding the possibility of such events, and radiation detection and identification systems are in place at many locations where nuclear smuggling is thought to be likely. Modern nuclear weapons have evolved into many forms, but the most wellknown and earliest nuclear weapons were large and relatively unsophisticated.

 Controlled fission is a useful process that results in heat used to produce steam for electrical generation in nuclear power reactors. There are currently 104 operating power reactors in the United States, producing nearly 20% of the nation’s electricity, and over 400 reactors worldwide. This number is likely to grow substantially over the next decades.

Radiation Sources

81

figure 2.28 Replicas of the “Little Boy” and “Fat Man” atomic bombs (left and right, respectively). (Photo credit: Los Alamos National Laboratory, New Mexico.)

These two weapons, developed at the end of World War II by the United States, were known by the code names of “Little Boy” and “Fat Man,” perhaps because of their shapes. Little Boy was a bomb in which U was used, and Pu was used in the Fat Man bomb. Photographs of replicas of these two bombs are shown in Figure 2.28. A complete description of the techniques used to measure and characterize SNM is beyond the scope of this document. An extensive discussion of the analysis of nuclear materials—specifically SNM—can be found in the Passive Nondestructive Assay of Nuclear Materials report (Reilly et al. 1991).

2.6 Electromagnetic Interference Effects John Leonowich Just as radiation detection instrumentation must be tested for effects of environmental changes, they must also be tested for the possibility that electromagnetic radiation may interfere with their correct operation. The ANSI standards covering instrumentation used for Homeland Security purposes (ANSI 2006a, 2006b, 2006c, 2006d) require that RPMs perform under a range of temperature, humidity, electromagnetic, and vibrational conditions that might arise in a deployment situation. For electromagnetic susceptibility, ANSI N42.35 (2006d) requires that an RPM should not be affected by radio frequency fields over the frequency range of 20 MHz to 2500 MHz at an intensity of 10 V/m (ANSI 2006d).



Radiation Detection and Interdiction at U.S. Borders

There has been a significant rise in the use of electronically controlled devices outside the clinical and laboratory environment, coupled with a tremendous rise in the use of the electromagnetic spectrum in the radio frequency region (3 kHz to 300 GHz). These devices are often used in homes, workplaces, and aircraft and attached to patients or implanted in their bodies and are potentially susceptible to interference at field levels well below human exposure standards (IEEE 2006). In addition, portable wireless communications equipment, such as cellular phones, handheld transceivers, and vehicle-mounted transceivers, is a potential source of interference. The number of land mobile transmitters in the United States alone currently exceeds 25 million, and personal communications systems are burgeoning throughout the world. To an ever-increasing extent, wireless communications equipment, for example, those used by first responders, is likely to be used in close proximity to detection equipment, and thus this equipment must not be susceptible to this type of interference. Much detection equipment is commercial off-theshelf technology, which may not be hardened against radio frequency interference (RFI) to the extent that equipment constructed to military specification may be. There is increasing evidence, both anecdotal (first responders at national security events in the United States) and measured, that some detection equipment is susceptible to interference, particularly to digital cellular communications. Radio frequency interference has the potential to cause “false positive” readings, and possibly (though not demonstrated) “false negatives.” These problems can cause field personnel to become uncomfortable with the use of the detection equipment because of its unreliability.

Frequency: 431.95 MHz Value: 0.0002238% (FCC OCC) 1000 100 10

Exposure [%]

1 0.1 0.01 0.001 0.0001 0.00001 0.000001

100 Isotropic result

200

300

400 500 Frequency [MHz]

600

700

figure 2.29 Radio frequency energy spectrum taken in New York City. (Photo credit: L-3 Communications, Narda Microwave-East, New York.)

Radiation Sources

83

table 2.9 Frequencies of interest (adapted from Strauss et al. ) Frequency (MHz)

Uses

10–6–300

Electromagnetic pulse from improvised nuclear device or assembled components AM radio ISM (semiconductor manufacturing, radio frequency heat sealers) ISM (radio frequency heat sealers) ISM (semiconductor manufacturing) FM radio Navigation (VOR and ILS localizer) TV channels 7–13 Instrument landing system glide slope TV channels 14–46 Integrated digital enhanced network AMP, CDMA, and TDMA (cell phones) ISM band for commercial electronics (including COST CBRNE detectors using wireless communications) DME, TCAS Nonlethal weapon (postulated) a Global positioning system at 1227.5 Global positioning system at 1575.42 Personal communications services phones ISM band for commercial electronics (Wi-Fi and microwave ovens) Ultrawide bandb other frequencies are also possible

0.58–1.6 13.56 27.12 40.68 88–108 108–118 174–216 329–335 470–668 806–821 824–849 902–928 960–1215 1000–100,000 1215–1240 1565–1590 1850–1910 2400–2484 3100–10,600 a

A nonlethal weapon is meant to destroy or incapacitate electronic systems. Most details of these systems are classified; however, it is known that several countries have been developing these systems for many years. There is also a recently declassified U.S. military system called “Active Denial Technology” that can control crowds by causing surface heating of the skin, which is uncomfortable but reversible. This technology operates around 94 GHz.

b

Ultrawide band is a fairly new modality that greatly differs from conventional radio frequency signals, which have a very narrow bandwidth. It is unique in that it achieves wireless communications without the use of a sine wave radio frequency carrier. Instead, it uses modulated high-frequency low-energy pulses of less than one nanosecond in duration. Because its bandwidth is larger than its central frequency, it can therefore be considered a “packet” containing a number of frequencies. Ultrawide band technology has many applications, including wireless communications, collision avoidance systems for vehicles, and classified military applications. Detection systems have not been tested for susceptibility to ultrawide band.

AMP: Analog mobile phone

TCAS: Traffic alert and collision avoidance system

CDMA: Code division multiple access

TDMA: Time division multiple access

DME: Distance measuring equipment

VOR: Very high-frequency multiple range system

ISM: Industrial, scientific, and medical

Wi-Fi: Wireless fidelity

2.6.1 sources of radio frequency interference We are surrounded by an ever-increasing amount of complex radio frequency energy from many sources. Figure 2.29 shows a measured radio frequency spectrum from 100 to 800 MHz taken in New York City. Note the number of emitters shown and the complexity of the spectrum. The last comprehensive survey of the U.S. radio frequency energy environment was performed in 1978 for the

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Radiation Detection and Interdiction at U.S. Borders

U.S. Environmental Protection Agency and is clearly outdated (Tell and Mantiply 1978). Table 2.9 shows a list of current sources that potentially could interfere with radiation detection equipment. Most of these sources did not exist in 1978. Cellular communications are expected to continue to use higher frequencies above 2450 MHz in the future, which can only exacerbate this problem. Of course, there are many emitters above 2450 MHz, including military and civilian radar and satellite communications. Because of the higher frequencies, as well as the narrower beam widths of these systems, they are less prone to produce RFI in detection equipment. However, testing at these higher frequencies has not been reported in the literature for medical and radiation detection equipment.

2.6.2 radio frequency interference Devices can operate inaccurately and erratically because of interference from various emitters of radio frequency energy. Radio frequency interference specifications for radiation detection equipment are not yet specifically covered by any international standard, although national standards exist (ANSI 2006a–2006d). However, because the use of this equipment and its potential failure may lead to life-threatening situations or false conclusions on possible threats, the present International Electrotechnical Commission standard for medical devices (IEC 1993) may be a useful starting point to establish credible limits for radiation detection equipment. This standard sets a minimum immunity level of 3 V/m in the 26–1000 MHz frequency range. For non-life-supporting devices, testing is required only at the specific frequencies of 27.12, 40.68, and 915 MHz. Technology exists to protect, or “harden,” most medical devices from radio frequency fields that are much more intense than the 3 V/m specified in present RFI standards. Most of these techniques, including shielding, grounding, and filtering, are inexpensive if they are incorporated into the initial design of the electronics system. Military equipment is often hardened to withstand fields up to 200 V/m. Many factors affect the severity of RFI in electronic devices, including: (1) the coupling between a source of interference and the device, (2) the frequency of the radio frequency carrier, (3) the modulation imposed on the fields from each source, and (4) the distance between the radio frequency source and the susceptible device. Effects of coupling occur primarily when the susceptible device is near the source. Capacitive coupling occurs in a region near the source where the electric field is dominant, for example, the tip of a dipole antenna. While coupling is a critical factor for RFI under near-field conditions, in the far field it is the carrier frequency that is crucial to the introduction of radio frequency into a device. Generally, the frequencies with the greatest ability to induce RFI are wavelengths that are comparable to the maximum dimension of a device housing, or to the length of the external cables and leads connected to the device (COMAR 1998).

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Modulation also affects the degree of interference for a given set of exposure conditions; amplitude modulation (including pulsed radio frequency) is usually the most significant for RFI. The amplitude-modulated radio frequency carrier can be detected at the semiconductor junctions in the device; significant interference occurs if the modulating frequencies are within the physiological pass band of the device. Digital mobile communications systems often use pulsed amplitude modulation, a type of modulation that can enhance the potential for RFI. For example, cellular telephones based on some digital technologies generate peak powers of up to 8 watts and are modulated at 2 to 217 pulses per second. This range spans the physiological frequencies of the human body, from about 0.5 Hz to several hundred Hz, that are monitored by many medical devices. This is often termed the “physiological pass band.” While this has little relevance directly to radiation detection equipment, it has been shown that portable electronic devices are more susceptible to digital signals that exhibit complex modulations (Strauss et al. 2006).

2.6.3 electromagnetic pulse effects Electromagnetic pulse (EMP) was a serious concern during the Cold War. Following the explosion of a nuclear weapon, an intense amount of electromagnetic radiation is released, including in the subradio frequency to radio frequency bands. The frequency spectrum may extend from below 1 Hz to above 300 MHz. The high-altitude EMP produced in an exoatmospheric nuclear explosion is the form of EMP commonly of most interest because of the large area covered in a single explosion. This high–intensity EMP can disrupt or damage critical electronic facilities and infrastructure over an area as large as the continental United States unless protective measures are taken in the facilities. The military has spent billions of dollars on “hardening” their systems against EMP. A recent Congressional report made the following assessment regarding EMP: …EMP is one of a small number of threats that can hold our society at risk of catastrophic consequences … It has the capability to produce significant damage … to the ability of the United States … to project influence and military power… (Foster et al. ) The Commission found that the cost to retrofit systems (i.e., to harden them) would be prohibitive, but protection included in design of a system would add only 1% to 3% to the total cost of the system. Recent review of potential detection systems found only one radiation detector that explicitly mentioned EMP hardening. Electromagnetic pulse effects can also be generated by other means than the detonation of a nuclear weapon. It is possible to develop a device that would disrupt infrastructure over a limited area with readily available components. The device could be designed to fit in a 226 kg (500 lb) bomb casing. Therefore, it is

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Radiation Detection and Interdiction at U.S. Borders

critical that detection equipment be assessed to determine whether it is capable of withstanding such a limited attack.

2.6.4 summary Radio frequency interference and EMPs are potentially serious and have not been adequately addressed in the design and deployment of radiation detection systems. These effects are real and well understood. There are engineering solutions that can, at some cost, mitigate the problem. It is recommended that current standards developed to assess performance of radiation detection equipment be revised to realistically test equipment to potential electromagnetic threats. Additionally, first responders and other emergency management personnel should be made aware of these potential effects so that they can recognize and deal with problems correctly.

2.7 References Aglietta M, B Alessandro, P Antonioli, F Arneodo, L Bergamasco, M Bertaina, M Brunetti, C Castagnoli, A Castellina, D Cattani, S Cecchini, A Chiavassa, G Cini, B D’Ettorre Piazzoli, G Di Sciascio, W Fulgione, P Galeotti, M Galli, PL Ghia, G Giovannini, M Iacovacci, G Mannocchi, C Morello, G Navarra, A Pagliarin, O Saavedra, GC Trinchero, P Vallania, S Vernetto, and C Vigorito. . Gamma-rays and ionizing component during thunderstorms at Gran Sasso. In th International Cosmic Ray Conference, Vol. , pp. –, Salt Lake City, UT. American Institute of Physics, College Park, Md. Eds., Brenda L. Dingus, David B. Kieda, and Michael H. Salamon ANSI. a. American National Standard for Portable Radiation Detection Instrumentation for Homeland Security. ANSI N., American Nuclear Standards Institute, Washington, DC. ANSI. b. American National Standard Performance Criteria for Alarming Personnel Radiation Detectors. ANSI N., American Nuclear Standards Institute, Washington, DC. ANSI. c. American National Standard Performance Criteria for Hand-Held Instruments for the Detection and Identification of Radionuclides. ANSI N., American Nuclear Standards Institute, Washington, DC. ANSI. d. American National Standard for Evaluation and Performance of Radiation Detection Portal Monitors for Use in Homeland Security. ANSI N., American Nuclear Standards Institute, Washington, DC. ASTM. . Standard Practice for Installing Radon Mitigation Systems in Existing Low-Rise Residential Buildings. E- ASTM International, West Conshohocken, PA. Beck HL, CV Gogolak, KM Miller, and WM Lowder. . Perturbations on the natural radiation environment due to the utilization of coal as an energy source. In The Natural Radiation Environment III, Vol. , pp. –, eds. TF Gesell and WM Lowder Washington, DC: U.S. Department of Energy.

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Belov AV, JW Bieber, EA Eroshenko, P Evenson, R Pyle, and VG Yanke. . Cosmic ray anisotropy before and during the passage of major solar wind disturbances. Advances in Space Research, :–. Briesmeister JF (Ed.). . MCNP™: A General Monte Carlo N-Particle Transport Code. LA––M, Los Alamos National Laboratory, Los Alamos, New Mexico. Carmichael H. . Cosmic ray measurements. Annals of the IQSY :–. Clements WE and MH Wilkening. . Atmospheric-pressure effects on Rn- transport across earth-air interface. Journal of Geophysical Research, ():–. Cohen BL. . Variation of radon levels in United-States homes correlated with house characteristics, location, and socioeconomic-factors. Health Physics ():–. COMAR. . IEEE Committee on Man and Radiation: Radio frequency interference with medical devices. Institute of Electrical and Electronics Engineers Engineering in Medicine and Biology Magazine ():–. Compton AH. . A geographic study of cosmic rays. Physical Review :. Cronin JW. . Cosmic rays: The most energetic particles in the universe. Reviews of Modern Physics ():S–S. Crozier WD. . Direct measurement of radon- (thoron) exhalation from ground. Journal of Geophysical Research :–. DEFRA. . Monitoring of Radioactivity in Air and Rainwater in the UK: Annual Results Report . DEFRA/RAS/., Department of the Environment, Food, and Rural Affairs (DEFRA), London. DOE. . A Technical History of the NURE HSSR Program. GJBX-(), prepared by Information Systems Programs, Energy Resources Institute, Norman, Oklahoma, for the U.S. Department of Energy, Grand Junction, Colorado. Dueñas C, MC Fernández, J Carretero, E Liger, and M Pérez. . Release of Rn- from some soils. Annales Geophysicae-Atmospheres Hydrospheres and Space Sciences ():–. Fisenne IM. . Radon- Measurements at Chester. EML-, Environmental Measurement Laboratory, New York. Forbush SE. . On the effects in cosmic-ray intensity observed during the recent magnetic storm. Physical Review :–. Foster JS Jr, E Gjelde, WR Graham, RJ Hermann, HM Kluepfel, RL Lawson, GK Soper, LL Wood, and JB Woodard. . Report of the Commission to Assess the Threat to the United States from Electromagnetic Pulse (EMP) Attack, Volume : Executive Report. House Armed Services Committee, Washington, DC. Frost and Sullivan Healthcare Group. . U.S. Radiopharmaceutical Markets. Accessed November , , at http://www.frost.com/prod/servlet/report-toc.pag?ctxixpLink= FcmCtx&searchQuery=U.S.+Radiopharmaceutical+Markets&repid=A---- &bdata=aHRcDovLddymcmzdCjbvcJjaCjYXRhbGnLXNlYXJjaCkbzx dWVyeVRleHQVSTLitSYWRpbBoYXJtYWNldXRpYFsKhcmtldHNAfkBTZWF yYggUmVzdWxcB%BQDEyOTgMjIMjEyNTQ%D&ctxixpLabel=FcmCtx (last updated February ). Fujitaka K, M Matsumoto, K Kaiho, and S Abe. . Effect of rain interval on wet deposition of radon daughters. Radiation Protection Dosimetry (–):–. Gale HJ and LHJ Peaple. . A study of radon content of ground-level air at Harwell. International Journal of Air Pollution :–.

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Gesell TF and WM Lowder (Eds.). . Natural Radiation Environnent III, Vol. , pp. – . U.S. Department of Energy, Washington, DC. Hess VF. . Observations in low level radiation during seven free balloon flights. Physikalische Zeitschrift, :. International Electrotechnical Commission (IEC). . Medical Electrical Equipment, Part . General Requirements for Safety, IEC Standard --. IEC, Geneva, Switzerland. IEEE. . Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields,  kHz to  GHz. IEEE C.-, Institute of Electrical and Electronics Engineers, New York. Inan U. . Atmospheric science - Gamma rays made on Earth. Science, (): –. ISO. . Reference Neutron Radiations - Part : Characteristics and Methods of Produc tion. ISO –:, International Organization for Standardization, Geneva, Switzerland. ISO. . Draft International Standard. Monitoring for Inadvertent Movement and Illicit Trafficking of Radioactive Material. ISO/TC/SC ISO/DIS , International Organization for Standardization, Geneva, Switzerland. KAERI. . Table of Nuclides. Korea Atomic Energy Research Institute, Taejon, South Korea. Accessed November , , at http://atom.kaeri.re.kr/ Kane RP. . Recurrence phenomenon in the -hour variation of cosmic-ray intensity. Physical Review, :–. Kasztovszky Z, L Sajó-Bohus, and B Fazekas. . Parametric changes of radon (Rn-) concentration in ground water in northeastern Hungary. Journal of Environmental Radioactivity, ():–. Klemic G. . Environmental Radiation Monitoring in the Context of Regulations on Dose Limits to the Public. Proceedings of  Conference of the International Radiation Protection Association, pp. –. Austrian Association for Radiation Protection, Vienna. Knoll GF. . Radiation detection and measurement. nd ed., John Wiley and Sons, New York. Kouzes RT. . Radiation detection and interdiction for public protection from terrorism. In Public Protection from Nuclear, Chemical, and Biological Terrorism, eds. A Brodsky and J Johnson. Madison, WI: R.H. Medical Physics Publishing. pp. – Kouzes RT, JH Ely, JC Evans, WK Hensley, E Lepel, JC McDonald, JE Schweppe, E Siciliano, D Strom, and ML Woodring. . Naturally occurring radioactive materials in cargo at U.S. borders. Packaging, Transport, Storage & Security of Radioactive Material ():–. Kouzes RT, JH Ely, BD Geelhood, RR Hansen, EA Lepel, JE Schweppe, L Siciliano, DJ Stron, and RA Warner. . Naturally occurring radioactive materials and medical isotopes at border crossings. In Nuclear Science Symposium Conference Record, Vol. , pp. –. Portland, OR: Institute of Electrical and Electronics Engineers. Kouzes RT and ER Siciliano. . The response of radiation portal monitors to medical radionuclides at border crossings. Radiation Measurements, ():–. Kovach EM. . Meteorological influences upon the radon content of soil–gas. Transactions of the American Geophysical Union, ():–. Malan D and H Moraal. . The effect of wind on pressure correction of the SANAE neutron monitor counting rate. South African Journal of Science (–):–.

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Marcinowski F, RM Lucas, and WM Yeager. . National and regional distributions of airborne radon concentrations in United-States homes. Health Physics, ():–. Miles JCH and RA Algar. . Variations in radon- concentrations. Journal of Radiological Protection :–. NCRP. . Radiation Exposure of the U.S. Population from Consumer Products and Miscellaneous Sources. NCRP No. , National Council on Radiation Protection and Measurements, Bethesda, MD. Nero AV, MB Schwehr, WW Nazaroff, and KL Revzan. . Distribution of airborne Rn- concentrations in United-States homes. Science ():–. NEWNET. . Neighborhood Environmental Watch Network (NEWNET). Los Alamos National Laboratory, Los Alamos, NM. Accessed November , , at http://newnet. lanl.gov (last updated June , ). ORTEC. . AN Experiments in Nuclear Science Lab Manual. Accessed November , , at http://www.ortec-online.com/application-notes/an/an-content.htm (last updated October , ). Phillips JD. . National Geophysical Data Grids: Gamma-Ray, Gravity, Magnetic, and Topographic Data for the Conterminous United States. DDS-, U.S. Geological Survey, Denver, CO. Porstendorfer J, G Butterweck, and A Reineking. . Daily variation of the radon concentration indoors and outdoors and the influence of meteorological parameters. Health Physics ():–. Ramsey CG and HR Sleeper. . Architectural graphic standards, th ed. J. Wiley & Sons, New York. Reilly D, N Ensslin, H Smith, and S Kreiner. . Passive Nondestructive Assay of Nuclear Materials. LA-UR-–, Los Alamos National Laboratory, Los Alamos, NM. RIN. . Radiation Information Network (RIN) - Radioactivity in nature. Idaho State University, Pocatello, ID. Accessed February , at http://www.physics.isu.edu/ radinf/natural.htm Schery SD, DH Geaddert, and MH Wilkening. . Transport of radon from fractured rock. Journal of Geophysical Research (NB):–. Simpson JA. . Cosmic-radiation neutron intensity monitor. Annals of the IGY : –. Stranden E, AK Kolstad, and B Lind. . The influence of moisture and temperature on radon exhalation. Radiation Protection Dosimetry :–. Strauss B, MG Morgan, J Apt, and DD Stancil. . Unsafe at any airspeed? Cell phones and other electronics are more of a risk than you think. In IEEE Spectrum Vol. , pp. –. New York: Institute of Electrical and Electronics Engineers. Stromswold DC and JS Rohrer. . Temperature Effects on Spectra from PVT Plastic Detectors. PNNL- Rev. , Pacific Northwest National Laboratory, Richland, WA. Tell RA and ED Mantiply. . Population Exposure to VHF and UHF Broadcast Radiation in the United States, Las Vegas, Nevada. USEPA ORP/EAD –, U.S. Environmental Protection Agency, Washington, DC. UNSCEAR. . Sources and Effects of Ionizing Radiation: UNSCEAR  Report to the General Assembly with Scientific Annexes. Vol. I: Sources. United Nations, New York.

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USNREG. . Nuclear Regulatory Legislation: th Congress (Volume , No. , Rev. , nd Session, and Volume , No. , st Session). NUREG-, U.S. Government Printing Office, Washington, DC. Wunder. . The Weather Underground, Inc.,Ann Arbor, MI. Accessed February , at http://www.wunderground.com Yanchukovsky VL and GY Philimonov. . Forbush-decreases in cosmic rays for March and October,  for data of spectrograph on the basis of neutron monitor. Proceedings of the th International Cosmic Ray Conference, D Kieda, M Salamon, and B Dingus (eds.), pp. –. Hamburg, Germany. Yanchukovsky VL, GY Philimonov, and RZ Zhisamov. . The monitoring of atmospheric pressure variations by neutron monitor data. Proceedings of th International Cosmic Ray Conference, Hamburg, Germany, pp. –.

{3}

U.S. Customs and Border Protection Radiation Interdiction Approach

Interdiction of radiological materials is a daunting task, made even more difficult by increasingly sophisticated threats and technology. In this section, the radiation interdiction approach is discussed, along with new developments in active interrogation techniques and imaging. After a brief review of radiation detection basics, a discussion of the reality under which radiation detection must be performed at ports of entry is provided. This is followed by a discussion of current instrumentation and capabilities, and then by information about more advanced instrumentation and techniques.

3.1 Radiation Detection Mechanisms Joseph McDonald The types of radiation that are expected to be emitted from illicit radioactive sources are primarily gamma rays and neutrons. Both types of radiation are uncharged and are indirectly ionizing (because they transfer energy to a surrounding medium by means of secondary charged particles generated by interactions within the medium). Unlike alpha and beta particles, gamma rays and neutrons do not have finite ranges in matter. As they pass through a medium, their absorption is proportional to the thickness of the medium. At relatively low energies, these radiations are attenuated strongly in materials such as lead and water. However, they can travel for many meters in air. A gamma-ray beam with an initial intensity I after passing through a thickness of an absorber, x, will have a reduced intensity I, given by the expression: I

I 0 e −mx

(3.1)

where μ is a linear absorption coefficient. Relatively low-energy gamma rays and neutrons are likely to be emitted by illicit radioactive sources. The energies of these sources are given in terms of

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Radiation Detection and Interdiction at U.S. Borders

electron volts (eV) that may range from about 10 keV to approximately 3 MeV for gamma rays, and from 0.025 keV to approximately 10 MeV for neutrons. Detectors for gamma rays and neutrons with these energies have been developed for radiation protection purposes in nuclear power plants and for basic physics experiments. Many of these detectors can be used for homeland security purposes (McDonald et al. 2004). However, some aspects of detecting illicit materials have required the development of new detectors or the refinement and adaptation of existing detectors.

3.1.1 gamma-ray detection mechanisms Many organic (containing carbon) and inorganic (not containing carbon) compounds are used as scintillators for radiation detection and measurement because they fluoresce, emitting light, when struck by radiation. The intensity of the light emitted may be quite low, therefore a sensitive detector such as a photomultiplier tube is used. A schematic diagram illustrating the basic components of a photomultiplier-based scintillation detector is shown in Figure 3.1. Among the earliest scintillating materials used in combination with photomultiplier detection are organic compounds, such as naphthalene (CH) and anthracene (CH). However, these are low-atomic number (Z) materials and as such have low stopping power for gamma-ray photons. The most frequently used inorganic compound for detecting and measuring gamma rays is sodium iodide (NaI) activated with 1% Tl because of its high-effective atomic number Z from the iodine content. The high-effective Z of NaI increases its gamma-ray absorption. Many other high-Z materials have been successfully employed as inorganic scintillators, including thallium-doped cesium iodide, CsI(Tl), and bismuth germinate, BiGeO. More recently, LaCl:Ce+ and LaBr:Ce+ (van Loef et al. 2001) have been investigated. The material LiI(Eu) can also be used for gamma-ray detection,

Photocathode

Electrons

Anode

Electrical connectors

Scintillator Incident photon

Light photon

Focusing electrode

Dynode Photomultiplier tube (PMT)

figure 3.1 Schematic diagram showing basic components of a photomultiplier-based scintillation detector.

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but it is more often used to detect low-energy neutrons because of its Li content. As described in the following paragraphs, Li strongly absorbs low-energy neutrons. Elements such as thallium or europium, when added in low concentrations to the material used to grow a scintillator crystal, create the activator sites that allow the production of visible photons and thus create one of the properties needed for an effective inorganic scintillator. Gamma-ray interactions in inorganic scintillators produce secondary electrons that may then have enough energy to populate activator (lattice) sites in the inorganic crystal, whose energies lie between the valence band and the conduction band (Knoll 2010). The sites in the crystal formerly occupied by electrons are referred to as holes. Subsequent recombination of electrons and holes can give rise to low-energy photons that can be detected by the photomultiplier. It is desirable that scintillation light pulses be large and that the light intensity be proportional to the incoming photon energy. The matrix material in which the scintillator compound is dissolved must also be transparent to the wavelengths of light produced so that the pulses can be detected with a photomultiplier or photodiode attached to an outside surface of the material. It is highly desirable to have pulses with short rise and decay times so that high pulse rates can be counted without pulse pile-up. This occurs when electronic pulses generated by the photomultiplier take place at such a high rate that the signal-processing electronics cannot resolve them. Gamma rays interact with matter through the photoelectric effect, Compton scattering, and pair production (see Chapter 2, Section 2.1.1). All of these processes lead to the deposition of energy that can be detected in a solid, liquid, or gas from the optical or electronic signals produced in those substances. Only the lowestenergy gamma rays usually deposit all of their energy in a single interaction; most undergo multiple interactions to deposit their full energy before exiting the scintillating medium. The more energy deposited in the scintillating medium, the more likely a spectrum can be obtained that allows identification of the specific isotope from which the gamma ray was emitted. Figure 3.2 shows the gamma-ray absorption coefficients for NaI. These three coefficients are very similar in magnitude to each other. The mass attenuation (μ/ρ) coefficient is used to determine the average energy loss for photons of a given energy in a material of density (ρ) as a function of distance traveled through the absorber. The other two coefficients are generally used for radiation dosimetry applications. The mass transfer coefficient (μtr/ρ) is used to determine the part of this energy loss transferred to charged particles. The mass absorption coefficient (μen/ρ) removes from the mass transfer coefficient that part of the energy lost due to bremsstrahlung (x-ray) photons. Sodium iodide [NaI(Tl)], as mentioned earlier, has a high probability of absorbing low-energy gamma-ray photons as indicated by its large absorption coefficient at low energies. Many organic detectors, such as plastic scintillators, indicate the presence of gamma radiation without providing much information about the full gamma-ray



Radiation Detection and Interdiction at U.S. Borders 10000

Mass coefficient, cm2/g

1000 μ/ρ μtr /ρ μen /ρ

100 10 0.1 0.1 0.01 0.001

0.01

0.1 1 Photon energy, MeV

10

100

figure 3.2 Mass attenuation (μ/ρ), transfer (μtr/ρ), and absorption (μen/ρ) coefficients for NaI(Tl). (Data from S.M. Seltzer, National Institute of Standards and Technology, Maryland.)

energy because the densities and atomic numbers of the materials making up the detectors are low, and gamma rays are less likely to be strongly absorbed. When the full energy of the gamma ray is deposited, as is often the case for high-purity germanium (HPGe) or large NaI(Tl) crystals the optical, and subsequently produced, electronic pulses may allow identification of the radiological material that was the source of the gamma rays. However, if the gamma-ray energy is above 10 MeV it may also pass through a solid-state scintillator without depositing its full energy. The energy resolution obtained for detectors capable of full-energy resolution, which results in peaks for specific gamma-ray photon energies, commonly varies from about 8% full width at half maximum (FWHM) of the photon energy peak for NaI(Tl) detectors to 0.1% FWHM for HPGe detectors. This higher resolution for HPGe can be seen in Figure 3.3. The difficulty in identifying a radionuclide with some types of scintillators found in the instruments presently used to search for and detect radioactive material is illustrated in Figure 3.3. As would be expected, an HPGe detector provides a spectrum that clearly shows the separated gamma-ray photon energy peaks characteristic of the emitting radionuclides. Both the NaI(Tl) and the large plastic scintillator have poorer resolution, making it more difficult to identify the radionuclides detected. Many of the gamma rays emitted from illicit radioactive materials occur at energies below about 1 MeV where the radiation absorption coefficients for the detector materials change rapidly. Because of the radiation absorption properties of most inorganic scintillators, their response as a function of incident gammaray energy will vary significantly. For example, Figure 3.4 shows the mass attenuation and mass energy-absorption coefficients as a function of energy for CsI, a

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Counts in 1,000,000 sec

10000000 Ge 110% 1000000 Plastic 24''×33''×2'' 100000 10000 Nal(Tl) 3''×3'' 1000 100 0

500

1000

1500 2000 Energy (keV)

2500

3000

figure 3.3 Comparison of three gamma-ray spectra of natural background radiation in Southeastern Washington using three types of detectors.

104

μ /ρ or μen /ρ, cm2/g

103

μ/ρ μen /ρ

102 101 100 10−1 10−2 10−3

10−2

10−1

100

101

102

Photon energy, MeV

figure 3.4 The mass attenuation and mass energy-absorption coefficients, μ/ρ and μen/ρ, respectively, as a function of gamma-ray energy for CsI. Data from XCOM tables (Hubbell 1982, Seltzer 1993, Berger et al. 2005).

detector material often used in personal radiation detectors (PRDs) (Hubbell 1982, Seltzer 1993). It can be seen that large changes occur at low energies as the photoelectric absorption coefficient decreases rapidly. At higher energies, the Compton interaction coefficient begins to increase, becoming dominant above about 0.5 MeV. The response as a function of incident gamma-ray energy for CsI(Tl) detectors will generally peak at approximately 60 keV and fall off at lower energies because of the attenuation in the detector material. Therefore, the sensitivity of the scintillation detector-based instruments with CsI(Tl) is relatively large for gamma rays such as those emitted by Am and Pu isotopes.

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Radiation Detection and Interdiction at U.S. Borders

3.1.2 neutron detection mechanisms Neutrons interact with materials largely through elastic scattering with the protons in materials. As they pass through the material, the neutrons slowly lose energy through multiple collisions, mainly with protons. The number of neutrons incident per unit area is known as the fluence. As discussed in Chapter 2, Section 2.1.2, and shown in Equation (3.2), the cross section, σ [in units of m or barns (10– m)] (See Chapter 2, Section 2.1), is defined as the probability, P, of an interaction per unit fluence, f:

s=

P f

(3.2)

Neutrons may be detected by the effects of recoiling protons resulting from collisions within the detector material or from the gamma rays and charged particles that may be released after the neutron is captured by a nucleus. The detectors used in neutron instruments are often more sensitive to very low-energy neutrons. These detectors are generally surrounded by several centimeters of a hydrogenous moderator, such as polyethylene, that has a high proton density, which reduce, or moderate, the energy of incident neutrons to low energies. A detector can then be mounted at the center of the polyethylene moderator in order to more effectively detect neutrons whose initial energies would have carried them through the detector with low probability of detection. A commonly used detector type is the He-filled gas proportional counter, which can be obtained in large sizes that may be meters long, centimeters in diameter, and filled with pressurized He gas. Their sensitivity can be high enough to detect individual neutrons. Unfortunately, He availability has become very limited since 2008 (Kouzes 2009). Glass-fiber detectors that are doped with Li are also used for neutron detection where physical ruggedness or flexibility in shape is important. These detectors can also be made in large sizes. The total cross sections for three important neutron interactions used in detectors are shown in Figure 3.5. Some detector materials, such as LiI and He gas, have large low-energy neutron cross sections that vary inversely with the neutron energy. For detection purposes, the moderation of fast neutrons with a hydrogenous material, such as polyethylene, makes it possible to take advantage of the large thermal-neutron absorption cross sections in the reactions Li(n,α)3H (Q = 4.78 MeV; σth = 940 b) B(n,α) Li (Q = 2.31 MeV (93%) or 2.79 MeV (7%); σth = 3837 b) 3 He(n,p)3H (Q = 0.764 MeV; σth = 5333 b), 6

10

7

(3.3)

where Q is the energy released in the reaction (Audi and Wapstra 1995), and σth is the thermal-neutron absorption cross section of the reaction (Mughabghab

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105

Cross section (b)

104

3He(n,ρ)3H

103 10B(n,α)7Li

102 101 6Li(n,α)3H

100 10−1 10−9 10−8 10−7 10−6 10−5 10−4 10−3 10−2 10−1 100 101 Energy (MeV)

figure 3.5 Neutron total absorption cross sections for the Li(n,α)H, B(n,α)Li, and  He(n,p)H interactions. Data from the Korea Atomic Energy Research Institute CrossSection Plotter (KAERI 2006).

et al. 1981). The high sensitivity of the detectors to slow neutrons resulting from these large reaction cross sections is advantageous because the low-energy neutrons are more likely to be emitted by illicit radioactive sources. Neutron area-survey instruments used for radiation protection purposes may also consist of a polyethylene moderator enclosing a slow neutron detector such as a He gas-filled counter. But, the large moderator often found in radiation protection instruments makes them heavy and difficult to use. Small personal radiation detectors (PRD) that are carried in a pocket or on a belt clip rely on the human body to moderate and reduce the energy of neutrons so that they can be more effectively detected. The development of lighter and more efficient neutron detectors is a subject of significant interest, and a number of programs are underway at national laboratories, universities, and industrial research laboratories to develop improved neutron detection instruments.

3.2 Interdiction Options Richard Kouzes Conceptually, there are many options available for the detection of radiation in materials crossing international borders. All radiation can be detected, but at varying impacts to federal funding requirements and to the flow of commerce across U.S. borders. The optimum choices are ones in which the detection is maximized while the fiscal and logistical impacts are minimized. In this section, the general needs for radiation detection for interdiction purposes are discussed, followed by the options available for the type and configuration

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of the equipment that affect the optimum detection at various ports of entry (land, rail, sea, or air).

3.2.1 radiation detection for interdiction Nuclear and other radioactive materials produce various types of radiation (alpha, beta, gamma, and neutron) depending on their nature. The level of radiation emitted is a function of the quantity and configuration of the material and the specific radionuclides. The level of radiation that reaches a detector is additionally a function of the radiation emitted, the amount and type of intervening material, the distance from the source, and detector size and environment. Because gamma–ray and neutron radiation can pass through significant thicknesses of surrounding materials (whereas alphas and betas cannot), detection and identification of the presence of nuclear or other radioactive materials relies on these types of radiation. Radiation may not be detected if the radioactive materials are shielded such that radiation levels fall below the detectable limits for the equipment used. Because specific pieces of equipment cannot detect all radioactive material in all possible quantities, attention must be paid to the type of equipment chosen for each application and use. The effectiveness of a detection system is a function of the equipment, how it is used, and the preparedness of the staff operating the system (see Chapter 4, Section 4.6). Various radiation detection instruments are used for different steps leading to the detection and identification of radioactive materials. These steps have the following purposes: 1. Detection: An instrument is expected to give an alarm if a certain detected radiation level is exceeded. 2. Verification: Once an alarm is triggered, it must be confirmed. Effective procedures for secondary inspection and verification of alarms are essential. 3. Assessment of safety and localization: A confirmed alarm necessitates searching for and locating the origin of the radiation source. As this is conducted, it is important to make a radiological assessment for the safety of the public and CBP officers and to determine the appropriate response. 4. Identification: Once the source is located, the radionuclide must be identified through determination of the radiation type and energy; this helps to categorize the nature of the source and determine further response.

3.2.2 instrumentation options Instrumentation options for interdiction can be divided into three general categories: radiation portal monitors, personal radiation detectors, and handheld

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instrumentation. These devices are discussed in this subsection. We begin with radiation portal monitors, to which a major portion of this book is devoted. Radiation portal monitors are robust systems designed to be used at security checkpoints for detecting the presence of radioactive material typically carried by pedestrians, either on their person or in their automobiles, transported by vehicles, typically tractor-trailer rigs, or moved on conveyers, such as at a post office or commercial shipping companies. These monitors may be fixed, portable, or mounted on a vehicle for mobile use. They provide simple information about the presence of radiation and, in some cases, the identification of isotopes in the radioactive source. Cargo and general vehicle RPMs are usually large systems and have the greatest detection sensitivity of instruments used for interdiction. Personal radiation detector devices are small, lightweight instruments used to detect the presence and strength of a radiation field caused by the presence of radioactive material. These instruments may be used for personal safety but are not intended for personal dosimetry. A personal radiation device can be used in specific situations by trained personnel. Typical use is to determine the presence and strength of a radiation field during secondary inspection of a cargo vehicle. In this case, it is used simply to warn the inspector that the radiation field is unsafe for further investigation and additional assistance is needed. Another use could be to determine the presence of radiation in an individual, for example, a person having had a recent radioisotope medical treatment, or in small packages when more sensitive instruments, such as an RPM, are not available or practical. Handheld instruments are portable devices used to detect, locate, or identify isotopes in radioactive material. These instruments are larger than the personal devices. There are specialty gamma ray-only devices that are designed for detecting and locating sources of gamma rays. There are neutron-only devices that may or may not be combined with gamma-ray devices, thereby offering more general radiation detection capability. More capability can be built into these devices, but with concomitant increases in the weight of the device, by including radioisotope identification. These devices may be used to verify an alarm triggered at a primary or secondary RPM or when a radiation field is detected with the inspector’s personal radiation detector.

3.2.3 general instrument requirements Because CBP had previously selected and deployed PRDs and handheld instruments to the field, the RPMP concentrated on the methodology and equipment requirements for optimum scanning of large volumes of cargo, vehicles, containers, packages, mail, luggage, passengers, and pedestrians. These considerations included a number of high-level requirements. The systems must provide an appropriate level of sensitivity to detect the defined threats, provide the ability to

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Radiation Detection and Interdiction at U.S. Borders

scan 100% of material and persons at U.S. ports of entry, and operate without interference to the flow of commerce. At the same time, they must merge into the existing operations without requiring a large additional staff burden, be easy to operate by officers with little additional training, and be limited in variety to reduce training, maintenance, and operating costs. They also need to be available for immediate deployment and have an appropriate cost benefit. Given these requirements, it became clear that RPMs would be the workhorse system for deployment because they provided the level of detection sensitivity required and were commercially available for immediate deployment. A modular approach was taken to allow building systems for various locations out of component subsystems. Therefore, portals for scanning cargo and automobiles could be formed from the same detector subsystems. With this approach, most primary scanning is performed with RPMs, handheld detectors are used for search and identification in secondary processing, and personal radiation devices provide a layer of safety to each CBP officer to provide a limited search capability for small packages and individuals, such as persons who have had a recent radioisotope medical treatment. The existing X-ray imaging systems deployed at ports of entry also provide complementary information to the scanning process by revealing possible shielding materials. In addition to the basic modular RPM system (as described in Section 3.3.4), new deployment requirements led to the need for new configurations. The development of a mobile RPM was spurred by the need to scan targeted containers at seaports where transport to an existing RPM is not possible or is impractical. Thus, a prototype mobile RPM was developed and tested, leading to a procurement of mobile systems that use the same RPM building blocks as the static systems so that they are compatible with other deployed systems. The visual identification system (Section 3.3) was developed in response to the potential that “port runners” exist. This system is used to read and identify the cargo containers exiting seaports on flat-bed tractor-trailer rigs or stacked on flat-bed rail road cars. Similarly, remotely operated RPM systems at seaports and airports make operation of RPMs possible in remote parts of the seaport or airport. Larger RPM and visual identification systems were needed for use on rail systems both at seaports and at remote border crossings. At seaports, it was determined that scanning cargo containers would expedite the scanning of these containers if the detection device could be integrated into the straddle carrier, large lift devices that straddle the cargo container to move the container from one place to another (Section 3.3). The need for improved identification in secondary processing of containers that caused primary alarms and the need for identification of NORM during primary scanning for rail led to the initial investigation and specification of an SPM (Chapter 4). And, finally, the need for central data storage and expert assistance in the field led to the development of a national integration plan that resulted in the deployment of the Port Radiation Inspection, Detection, and Evaluation (PRIDE) system.

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3.2.4 options considered for scanning at mail and express consignment courier facilities For mail and packages that enter the country at international airports, scanning can be done at various locations depending on the mail facility. Some facilities have entry gates where standard truck monitoring can be done, while other facilities have doorway choke points or conveyor belts where scanning systems can be used. The RPM systems are placed at locations identified by site surveys as providing the best scanning opportunity that fits into operations with minimal impact. These installations can be complicated by facility ownership and the responsibilities of various parties involved in handling the mail. Equipment options include standard RPM building blocks plus smaller but compatible RPM detector assemblies. Express consignment courier facilities located in leased or privately owned buildings with access to the airport tarmac generally have requirements and options similar to those for mail facilities. One difference is that while CBP officers usually handle clearance of cargo processed through these facilities, they do not maintain a continuous presence as they do at most international mail facilities. Instead, CBP officers typically visit on a regular schedule, or when required. This requires the addition of ancillary communication equipment, such as auto dialers (equipment that automatically dials a prescribed telephone number list) and pagers.

3.2.5 options considered for scanning at land border and rail crossings Land border crossings are typically locations where CBP has a continuous presence and include local facilities for processing cars, trucks, and people. Scanning at these sites can generally be handled by installing fixed RPM systems in the same traffic flow patterns already established for entry into the United States. Most land border crossings include multiple lanes for cars, trucks, and buses, plus substantial pedestrian traffic at some locations. Scanning this traffic involves variations of the RPM building blocks to accommodate established traffic lanes. Cargo entering the United States by rail typically crosses the northern and southern borders where CBP is located nearby. While PVT RPMs can be used for rail scanning, tests have indicated that a high rate of alarms will occur because of the large NORM masses carried on trains. This would place a heavy demand on secondary processing, which is difficult at rail sites because sidings would be required to separate the rail cars from the rest of the train, and cranes may be needed to lift containers. Rail crossings are therefore a prime candidate to benefit from spectroscopic portal monitors, which allow for simultaneous identification of alarming cargo during primary scanning. In addition, other tools such as large mobile spectroscopic systems, which allow a single CBP officer to scan a loaded train car, have been initially investigated to aid in the secondary scanning process.

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Radiation Detection and Interdiction at U.S. Borders

3.2.6 options considered for airport cargo scanning Airport cargo scanning can be handled with standard RPM configurations, but the traffic control process is complicated. Traffic flow on an airport tarmac has various pathways, and most cargo is carried on tugs—small-wheeled trains consisting of a driver-operated tractor to which multiple trailers are attached. The most viable scanning approach is deploying multiple RPM systems at key locations. These can be operated from a central site with verified operational compliance. As the cargo is scanned, the results are combined with an electronic record of cargo contents (called a “consist”) to guarantee 100% scanning. The electronic record would include imaging and character recognition technology to validate against tug consists.

3.2.7 options considered for seaport scanning Seaports represent one of the most complex environments for deploying radiation interdiction equipment because terminals are dynamically changing venues with multiple owners and stakeholders. Additionally, there is usually not a permanent CBP presence at each terminal. The expression “once you have seen one seaport, you have seen one seaport” has been used to describe the diversity of this venue. Examples of the consideration made by the RPMP of various deployment options for scanning seaport containers are described in this section. Most of the cargo-scanning operations at seaports can be performed at truck exit gates from the port with fixed primary and secondary RPM systems identical to systems deployed at land border crossings. These systems are sometimes mounted onto metal plates rather than fixed foundations so they can be moved to a new exit gate when the seaport is reconfigured. Because CBP does not usually have facilities at each port, these installations usually include CBP supervisory booths. Ancillary equipment, such as traffic controls, is often needed to direct traffic during alarm conditions. Some of these systems have the added requirement of being controlled from a CBP booth some distance from the RPMs or remotely operated RPMs. Under optimal circumstances, the ability to scan cargo immediately upon arrival would minimize the potential for diversion. One method to achieve this result is to set portal systems on the docks at locations close to the gantry cranes to monitor trucks conveying containers to the laydown yard. Toward that end, a fleet of truck-mounted mobile detectors can, in principle, be placed on the dock (Section 3.3). In stationary mode, two fully independent mobile RPMs are parked with the detectors facing one another to form a traffic lane. In mobile mode, a single mobile RPM is driven at approximately 5 km/hr (3 mph) along a row of cargo containers arranged end to end. At the end of the row, the driver turns around and repeats the process on the opposite side of the row. At a minimum, this approach should prove useful for specifically targeted, high-risk cargo; however,

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because of the very busy, dynamic operations on the docks, it is impractical at most large ports to use this approach for all cargo. A further complication is the growing use of straddle carriers. Straddle carriers are too large and too well shielded to monitor cargo directly with standard portal systems. It is possible to mount radiation detectors directly on straddle carriers (Section 3.3). Some of the larger ports have large fleets of straddle carriers; for example, Maher Terminal in New Jersey has more than 150 straddle carriers in operation. While the large numbers effectively preclude equipping the entire fleet with detectors, it should be feasible to instrument one or two dedicated straddle carriers at each of the affected terminals. The selected straddle carriers would have their lifting mechanisms removed and serve only as mobile platforms for the detectors. These straddle carriers would be used either for scanning grounded containers immediately after unloading or for scanning fully assembled trains before departure from the port. The former case requires a high degree of organization and cooperation by the terminal operator and represents a formidable challenge with respect to accountability in verifying that all containers have been scanned at least once. When containers are stacked three high, multiple scanning of the same containers and disposition of alarms produced by cross talk between neighboring containers also present serious problems. These problems may be largely eliminated by scanning only the assembled train itself with a standard rail RPM system. However, train scanning also introduces unique challenges to operations. At some seaports, cargo exits the area directly on rail lines, presenting a challenge to scanning cargo before it is placed on the train or scanning the train as it exits the port. In principle, the simplest way to deal with rail monitoring is to deploy radiation-sensing portals at the rail tracks (see Section 3.3). The portals would then be operated in a manner similar to exit gate cargo monitors as the train exits the port through a RPM. In practice, however, there are serious drawbacks to that approach. Rail portals must be set farther apart than typical truck portals to accommodate rail setback requirements. This configuration results in a larger investment in detectors than is typical for a truck exit gate. Further complicating the problem are multiple closely spaced parallel tracks, the presence of derailers (used to switch between tracks) and other switch gear, track curvature, and other features typical of rail yards. Even more problematic are operational issues connected with the disposition of alarms on moving trains leaving or already outside of the port. Terminal operators have consistently opposed any strategy that disrupts train operations, and many terminals have rail operations for “justin-time-delivery” that would be adversely affected by any disruption to train schedules. One innovative approach for scanning containers at seaports involves instrumented platforms placed under each gantry crane. Containers that would normally be grounded by the crane for collection by a straddle carrier would instead be placed on a portable table sturdy enough to hold a 40-ton container. One design

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involves a pair of detectors mounted on a drive mechanism that would scan the sides of stationary containers and then return to a protected rest position at each end of the table. Irrespective of detailed design considerations, at least one platform would be needed per crane, each one accurately positioned under the crane and moved with it. While this approach deserves study, the operational requirements appear to be prohibitively complex.

3.2.8 radiation portal monitor specifications The RPMP developed specifications for all RPM procurements. The RPM specifications (Stromswold and Kouzes 2002; Stromswold et al. 2003) describe the requirements for deployable systems consisting of hardware and software intended as the primary scanning tool for the detection of radionuclides within fully loaded intermodal cargo containers, trucks, passenger vehicles, rail cars, and mail at U.S. ports of entry. The portal monitor systems described in the specification are suitable for installation at different locations, including vehicle lanes at land border crossings, seaports, airports, rail lines, mail facility gates, express courier package–handling facility gates, and other gated entries as effective tools for the detection of the illicit movement of radioactive materials. The radioactive materials of interest include SNM such as Pu and HEU, which could be used in a nuclear device. Also of interest are a wide variety of radionuclides that might be used in an RDD, for example  Co, Sr/Y, Cs, or spent reactor fuel. The RPM procurement specification was developed with the following guiding principles: • RPMs are used in both primary and secondary scanning applications. The purpose of primary scanning with RPMs is rapid detection, as opposed to identification or quantification, of radioactive materials. The RPMs are also used in secondary scanning to verify the existence of radioactive materials and to help locate the material within the vehicle or container. In addition, other secondary inspection methods with various detection technologies are used to provide radioactive material identification. While primary scanning systems must be as sensitive as practical to containers with elevated radiation levels, nonradioactive cargo must be passed as quickly as practical so that the impact on the flow of commerce is minimized. • The definition of a single threat to be used in procurement specifications was deemed impractical for the specification because of the limitless number of combinations of source material (e.g., Pu, HEU, and RDD radionuclides), surrounding cargo and engineered shielding material, and operational constraints. Instead, the combined results of market surveys, collective experience of vendors and national laboratories, and

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an extensive set of simulations to understand the capabilities of passive radiation portals were combined to derive the portal monitor specification. • The system configuration for each border security vector (e.g., seaport, vehicle lane, rail line, airport, mail, express courier) is somewhat different. The radiation sensors are considered a “building block” to be combined in the most effective manner for each vector. The specification was written to focus on integrated performance of the radiation sensors and accompanying data-processing methods, rather than the performance of a portal monitor system operating in a stand-alone mode. • The portal monitor system is a combination of gamma-ray and neutron detectors to take advantage of the complementary nature of these two technologies for shielded plutonium detection. • The portal monitors must function correctly in both “pass-through” operation and “wait-in” (stopped vehicle up to a specified time limit) operation depending on operational requirements and technical needs at each POE. Portal monitor hardware and software support both modes of operation. The specification was written such that vendors could test their RPM systems at their own facilities with commonly available radiation sources (e.g., Am, Co,  Ba, Co, Cs, Cf) as surrogates for HEU, Pu, and other nuclides of interest. The specification contains many detailed requirements for the RPM configuration and operation. An RPM generally consists of multiple radiation sensor panels (RSPs) positioned on opposite sides of a lane through which a vehicle or container passes. The RSPs have a local control box that provides the basic control and alarm functions for each individual lane. Applications may be single lane or multilane configurations. All of the RPMs for any given location communicate their data to a single local supervisory computer over Ethernet and to a national integration network system. Monitoring computers are also generally deployed to echo the information shown on the supervisory computer. Additionally, an Ethernet network–based VIS is used for recording images of vehicles and cargo passing through the portal system. Table 3.1 gives the specification required for RPM systems to have minimum count rates in response to a variety of gamma-ray sources of different energies in a specific geometry. This requirement translated into a minimum area and thickness for the detector systems. In the table, absolute detection efficiency is given in net count rate above background per micro-Ci of source activity when measured at a distance of 2 m (6.5 ft). For determining the neutron sensitivity of the RPM system, a Cf neutron source was specified. The absolute detection efficiency for a Cf source, located 2 m (6.5 ft) from the detector, was specified to be greater than 2.5 cps/ng of Cf (equivalent to 4.6 cps/μCi of Cf).

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table 3.1 Absolute gamma-ray detection efficiency for a radiation portal monitor detector with the sources 2 m (6.5 ft) from the detector face Radionuclide

Primary Emission Energies (keV)

Minimum Absolute Detection Efficiency (net cps/μCi)



22, 25 60 122, 136 31, 356, 81, 302, 35, 383, 276 662 1173, 1333

15 25 100 200 110 220

Cd Am  Co  Ba  Cs  Co 

ANSI standards for border security equipment, discussed in Chapter 7, were developed at the same time that the RPMP specifications were utilized for procurement of deployed systems (ANSI 2006a–2006e). The RPMP procurement specification required compliance with the available draft ANSI standards.

3.3 Instruments and Capabilities Megan Lerchen and Richard Kouzes Rapid detection of vehicles and cargo with elevated radiation signatures is paramount in meeting the objective of identifying items that warrant additional examination while minimizing the overall impact to legitimate trade and travel. Primary scanning provides a quick go, no-go decision on whether cargo is emitting radiation that exceeds an alarm criterion. This primary alarm determination is performed in RPMs equipped with both gamma and neutron radiation detectors, as well as a variety of ancillary equipment as needed for efficient and effective system operation. In primary scanning, no effort is made to determine the nature of the radiological source other than to differentiate between gamma-ray emission and neutron emission. Source identification is typically performed in secondary scanning. Secondary scanning is a confirmation of the primary alarm with additional RPMs and other technologies and procedures that provide a more thorough examination of the amount and source of the radiation detected in primary scanning. This section discusses the various radiation detection technologies used in both primary and secondary scanning at ports of entry for the RPMP. Information about the ANSI standards that govern the radiation detection requirements for border security applications is presented in Chapter 7.

3.3.1 detection technologies The detectors selected by the RPMP for primary scanning use a combination of gamma-ray and neutron detection technologies for shielded plutonium detection.

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These detectors provide primary scanning as opposed to identification or quantification of radioactive materials. RPMP and other organizations are exploring other technologies that can provide augmented radiation detection and identification capabilities. Depending on site-specific requirements, secondary scanning requires detectors that can identify radioisotopes.

3.3.1.1 Gamma-Ray Detection Gamma-ray detection is predicated on detecting and measuring the interactions of gamma rays in matter. Several materials are available for this purpose. For larger-area portal monitors, commercially available detectors are almost exclusively based on polyvinyl toluene (PVT) plastic scintillators. A primary factor in selecting a gamma-ray detection technology is a high level of sensitivity to gamma rays spanning the energy range from a few keV to about 3 MeV. This is required to reduce the amount of detection time needed to complete the primary scanning step. Ideally, primary scanning may be done within the normal flow of traffic at each border crossing and does not need to impact commerce. Polyvinyl toluene is a plastic material containing two dissolved scintillators that, as a unit, functions as a gamma-ray detector (Figure 3.6) and is commonly found in commercially available, large-volume gamma detectors. The PVT-based detectors are outfitted with photomultiplier tubes to sense the scintillation light and multichannel analyzers for signal processing. PVT-based detectors tend to be relatively inexpensive and robust, yet offer high sensitivity. However, as shown in Figure 3.7, PVT offers only limited spectral information. With this limitation, PVT detectors are best suited for primary portals where large numbers of vehicles need to be scanned rapidly for elevated gamma radiation signatures.

figure 3.6 Polyvinyl toluene plastic scintillator with photomultiplier tubes attached at the right.

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figure 3.8 Measured spectra from NaI(Tl) detector.

Gamma-ray detection may also be accomplished by use of inorganic detection materials such as sodium iodide activated with thallium, or NaI(Tl) (Section 3.1). Like plastic scintillator materials, NaI(Tl) and other inorganic scintillators are generally used with photomultiplier tubes and electronics to give more detailed spectral information (Figure 3.8) than is possible with PVT. However, the NaI(Tl) detector (Figure 3.9) is less robust than PVT-based detectors, has a strong temperature dependence in its response, and is much more expensive. The additional cost to obtain the added spectral information renders the NaI(Tl) detector

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figure 3.9 Two large-volume NaI(Tl) detectors with photomultiplier tubes attached and enclosed in metal shielding.

generally more applicable to secondary scanning or to deployments where spectral information is required from the initial scanning. Deployed NaI(Tl)-based detectors are generally handheld instruments. When commercially available, portal monitor systems based on a large volume of NaI(Tl) detectors in secondary scanning are expected to provide better isotope identification than that presently available from handheld units in similar operating conditions. Deployments of portals for rail scanning are planned to be based on NaI(Tl) detectors because of the difficulty associated with secondary scanning for the railway vector. Other materials such as HPGe, thallium-activated cesium iodide (CsI[Tl]), or cadmium zinc telluride (CZT) may be used for gamma radiation detection. At this time, these materials have not been generally recommended for primary scanning because their drawbacks outweigh their potential advantages. For example, although HPGe has extraordinarily better resolution than PVT-based detectors, it requires much greater day-to-day maintenance for both environmental and calibration needs (e.g., a HPGe detector requires cooling with liquid nitrogen or cryo-coolers) than PVT-based detectors. In addition, although these detectors offer enhanced spectral capability, they may cost up to 10 times more than PVTbased detectors. Thus, detectors with these materials have not been deployed as part of the RPMP; however, their potential applications for interdiction are part of ongoing studies.

3.3.1.2 Neutron Detection Neutron detection is predicated on detecting and measuring the interaction of neutrons with detector materials (Section 3.1) and the secondary charged

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particles produced. Several detector technologies are available for neutron detection, but commercially available detectors tend to be based on He gasfilled proportional counters or, in some cases, lithium-loaded glass. The use of  He as a neutron detector material is due to its large thermal neutron capture cross section, making it very sensitive to neutrons, and to its negligible sensitivity to gamma rays when used in proportional counters. Neutron detectors deployed in the RPMP use He neutron detectors with a polyethylene moderator to reduce neutron energy to the point where the He absorption cross section is high. When a neutron interacts with the He in the gas-filled tube, the absorption of the neutron into the nucleus of He causes the prompt emission of a charged particle and ionization in the surrounding gas, which is detected as an electrical pulse. Electronic discriminator circuits are used to discriminate these neutron signals from small gamma radiation-induced signals. Due to the large increase in use of He for homeland security and basic research, the supply has dwindled and can no longer meet the demand (Kouzes 2009). The method currently used to produce the inert gas He is collecting it as a by-product from the radioactive decay of tritium [H(t/= 12.3 y) → He + ß + ν], where it is separated during the tritium purification process. Tritium was produced for use in nuclear weapons in the U.S. and Russia into the 1990s. Stores of tritium must occasionally be processed to remove the ingrown He and maintain the desired tritium concentration. At this time, tritium essentially comes from the refurbishment and dismantlement of the nuclear stockpile. The reduced production and increased demand has resulted in a significant shortage of He worldwide. The worldwide steady state production of He is about 10–20 kl/y, while the demand is anticipated to be about 65 kl/y, a very significant shortfall. This has led to the search for an alternative technology to replace the use of  He-based neutron detectors. Of the currently available, commercially produced alternatives, neutron detection technologies, BF-filled proportional detectors, boron-lined proportional detectors, Li-loaded scintillating glass fiber, or wavelength-shifting plastic fibers coated with Li and ZnS are the only possible near-term replacements for He detector technology for homeland security. Several new alternative technologies may become commercially available in the 3–5 year time range with the characteristics required for homeland security needs (Van Ginhoven 2009).

3.3.2 radiation portal and area monitors Radiation portal monitors are used in both primary and secondary scanning applications. The purpose of primary scanning with RPMs is rapid detection, as opposed to identification or quantification, of radioactive materials. They may also be used in secondary scanning to verify the initial detection of radioactive materials and to help locate the material within the vehicle or container.

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The detection technologies generally used in RPMs are PVT detectors for gamma detection and He gas detectors for neutron detection. These detectors are combined in a radiation sensor panel (RSP) as a building block unit, and several units are deployed in an RPM configuration where traffic is constrained to pass. When operated continuously without presence sensors, RPMs are referred to as area monitors. Radiation portal and area monitors are composed of one or more large-volume primary detection scanning devices called the radiation sensor panel that contains the gamma-ray detection module with internal shielding, the neutron detection module, electronics, and environmental enclosure. During the course of the RPMP, the RSP design has been standardized so that when one or more RSPs are deployed in an appropriate configuration, the result meets the applicable detection standards for both gamma and neutron detection. One RSP design is generally used in deployment, although a smaller RSP is available for limited application in scanning nonvehicle items (such as packages) with a relatively small region of interest. Unless otherwise noted, references to an RSP are to the standard large RSP. • Standard RSP – The standard RSP is a configuration with a large gamma-ray detection module and a neutron detection module, as shown schematically in Figure .. The gamma-ray detection module consists of a PVT scintillator gamma-ray detector with phototubes on one end. The neutron detection module is a polyethylene box surrounding He detector tubes. • Small RSP – In deployments where the area of interest is limited to a small area, such as mail and cargo conveyer belts, a smaller RSP has been developed for deployment. This small RSP is acceptable where the required level of sensitivity may be achieved with a smaller volume of sensor material. Small RSPs may also be used in one-sided deployments such as doorways, as long as the required sensitivity levels can be achieved.

N E U T R O N

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figure 3.10 Layout of radiation sensor panel components within the environmental enclosure.

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The following requirements are generally applicable to standard RPM configurations: • Scintillator panels and neutron detectors are mounted vertically with one or more pairs of RSPs positioned on both sides of a monitored lane. One-sided deployments and L-shaped cantilever deployments are acceptable only in limited special cases. • Horizontal spacing between panels spanning a traffic lane is recommended to be typically . m (. ft) or less. Spacing up to . m (. ft) can be accommodated in a standard deployment assuming efficiency standards can be met through threshold adjustment at the observed background. Larger panel spacing requires additional RSPs. • Horizontal spacing between panels spanning the road will be affected by the general gamma-ray background at each deployment site. Recommended and maximum portal separations are based on the gamma-ray background at PNNL. At sites with significantly higher background, the maximum portal separations must be reduced to allow the necessary sensitivity to be obtained. • Portals must be shielded with . mm (. in.) of lead and steel ( mm [. in.] on back,  mm [ in.] on the sides) to suppress background and interlane cross talk. • The portal specification allows for a wide range of weather conditions, which may affect portal performance. During deployment, basic precautions must be taken to protect the equipment from physical damage and factors such as snow accumulation on sensor units. • All deployments use equipment that meets the ANSI standard N. (ANSI d) for both neutron and gamma-ray detection. • Portals should be in line with the roadway to within  degrees. • The two sides of a portal should line up with each other to within  mm ( in.) horizontally and  mm ( in.) vertically.

Deviations from these standard configurations are considered on a case-bycase basis. As discussed above, the technology used for the routine, primary scanning of inbound traffic for radioactive materials at the ports of entry are the RSPs made from PVT-based gamma detectors paired with helium-based neutron detection tubes. Two configuration operating modes are routinely used: portal monitors and area monitors (portal monitor in area mode). A typical RPM system includes two or more RSPs installed on opposite sides of a lane through which a vehicle or container passes, control box, occupancy sensors, shielding/support stands, an annunciator assembly providing alarm indication and additional information for each portal or lane, and analysis/control software components. The operator interface is generally through the annunciator and supervisory computer, as seen in Figure 3.11 (Tinker 2010).

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figure 3.11 Alarm view scan, supervisory computer shown at a test facility.

A set of RSPs at each lane is connected to a local control box that provides the basic control and alarm functions for each lane. Applications may be single- or multilane configurations. In most deployments, at least one pair of RSPs is located at each lane or gate. The exact system configuration for each deployment depends on the infrastructure and operations at each site. Figure 3.12 depicts a conceptual drawing for a four-lane road portal configuration with RSPs located on both sides of the vehicle lanes. As shown in Figure 3.13, one or several RPM systems are connected to a supervisory computer that is used to control the system along with other functions associated with managing the system operation and logistics of vehicle scanning at a POE. In multilane RPM systems, alarms based on real-time data from an individual lane of traffic are generated at the supervisory computer, which processes data to potentially isolate the location of a source that causes alarms in multiple lanes (cross talk). One or more monitor computers may be installed elsewhere at the port or crossing to simultaneously present the information shown on the supervisory computer and in secondary scanning operations. For each RPM system, the supervisory computer and the monitor computer are capable of being connected to an area or national border security network. As increasing numbers of RPM systems are deployed, the RPMP has taken steps to improve their operational performance while simultaneously reducing the

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Radiation Detection and Interdiction at U.S. Borders

figure 3.12 Radiation portal monitor concept for cargo portals with existing U.S. Customs and Border Protection kiosks.

Additional RPMS

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figure 3.13 Basic architecture of a radiation portal monitor system with limited ancillary equipment.

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potential impact on the flow of commerce and travel. Approaches to RPM improvements are discussed in detail in Chapter 4.

3.3.2.1 Spectroscopic Portal Monitors The current generation of RPMs uses plastic-based scintillation detectors to scan cargo and people for radioactive material. Because of their poor energy resolution, these detectors can provide only limited spectroscopic data from the scanning process. This limitation hinders the filtering out of nuisance alarms caused by legitimate cargo that may contain NORM (see Chapter 2, Section 2.3) or by people being treated with medical radionuclides (Chapter 2, Section 2.4). One possible tactic that could limit the number of NORM alarms is the use of detectors that provide sufficient energy resolution to allow for isotopic identification of the alarming material. Higher-resolution detectors, such as those based on NaI(Tl) or HPGe, have the potential to perform isotopic identification (such as that currently performed during secondary scanning in identifying the source of a primary alarm). When such detectors are included in a portal monitor, the result is a spectroscopic portal monitor (SPM), an example of which is the ASP or Advanced Spectroscopic Portal. The concept of spectroscopic portals has been studied at PNNL, where a preliminary specification for an SPM system was developed in late 2003. A prototype SPM was constructed at PNNL (Figure 3.14) and consists of four 4 × 4 × 16 in. NaI(Tl) crystals mounted vertically. This detector was funded by the National Nuclear Security Administration (NNSA) (NA-22) as a system designed to detect  U. The ANSI standards for border security radiation scanning equipment include one for SPM systems, ANSI N42.38 (ANSI 2006e). With input from both the PNNL and ANSI work, the DHS Science and Technology Directorate’s Homeland Security Advanced Research Projects Agency developed a specification for a class of SPMs referred to as ASPs. This work was later transitioned to the Domestic Nuclear Detection Office (DNDO). A major drawback of SPM systems is that they cost substantially more than plastic-based systems. To justify this higher cost, three conditions should be considered before replacing a plastic-based RPM: . The SPM must increase the sensitivity of detecting threats . The SPM must reduce the operational burden on a port of entry due to nuisance alarms . The SPM must improve the isotopic identification capability over what is currently used (i.e., handheld detectors). Any or all of these conditions may be adequate to justify the use of SPM systems depending on the unique needs of the individual deployment site. Plasticbased RPM systems otherwise are generally regarded as providing adequate primary scanning capability for detecting elevated radiation levels and alerting CBP officers of the need for further action.

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Radiation Detection and Interdiction at U.S. Borders

3.3.2.2 Area Monitors Area monitors are RPMs that are set to continuously monitor the nearby surroundings for elevated radiation signatures, rather than being activated with an occupancy sensor. Area monitors check the radiation level constantly for rapid increases and accumulate background information on a periodic basis. Area monitors are used in some mail/ ECCF applications.

3.3.2.3 Mobile Radiation Portal Monitors The mobile RPM is a self-contained truck-mounted RPM system used to detect nuclear materials where mobile operation is needed. Designed at PNNL to examine cargo in trucks and shipping containers, the system can also be used as a stationary RPM with cargo passing between a pair of mobile RPM vehicles or can be driven past stationary objects. figure 3.14 Inside an NaI(Tl)-based At ports of entry such as seaports, prototype SPM radiation sensor panel cargo containers are typically unloaded developed by Pacific Northwest National from a ship to a laydown yard. With Laboratory. fixed RPM deployments, containers are not scanned until they have been loaded onto a tractor-trailer rig that then passes through fixed RPMs at the exit gates. The mobile RPM allows cargo to be scanned as it is unloaded or while stored in a laydown yard (a storage area at a seaport). Mobile RPMs also provide a temporary scanning capability at locations scheduled for future modifications that preclude installation of fixed detection systems. Figure 3.15 shows the prototype mobile RPM system, consisting of two trucks, each equipped with two mounted standard RSPs. An integrated paging system allows the CBP officer to work away from the vehicles (e.g., directing traffic in stationary mode) while still receiving indications of radiation alarms, excessive target vehicle speed, or instrument failure conditions. The handheld pager is sometimes referred to as a “pendant.” This handheld (or wearable) unit is only available for mobile RPMs.

3.3.2.4 Rail Portal Monitors Rail crossings are a type of land border crossing. However, because the geometry of rail traffic is much different than other land crossings, a significantly different,

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figure 3.15 Mobile radiation portal monitors in two-sided configuration.

more complex, rail-specific identification system is required instead of the simple break-beam presence sensors used in other applications. The rail RPM in concept is similar to the standard RPM but has two substantial towers on either side of the track. To accommodate the size of a boxcar, the conceptual rail RPMs require 12 RSPs per system with 6 on each side of the track, placed vertically end to end in two towers of three panels (Figure 3.16). Rail RPMs will use core equipment consisting of a control box, shielding/support stands, annunciator, analysis/control software components, supervisory computer, and a special rail identification system. When available, SPM systems will likely be used instead of PVT systems because of the prevalence of NORM in rail cargo and the difficulty in conducting secondary scanning. The conceptual rail identification system must be comprised of an integrated set of components, including automated equipment identification (e.g., radio frequency identification tag readers), rail car imaging, wheel/axle counting, and speed determination. Rail identification data can be cross-correlated and correlated with presence sensor data and individual rail car radiation profiles. The imaging system concept is based on area scan cameras (like a vehicle identification systems, VIS) or digital line scan cameras (similar to rail X-ray systems) so that a continuous image of an entire train may be acquired. Rail RPM deployments must also use a site surveillance system to maintain situational awareness by monitoring the pedestrian and/or vehicle traffic surrounding the area of the RPM system. Rail RPMs will be deployed at rail crossings and may be used in seaports for monitoring on-dock rail.

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Radiation Detection and Interdiction at U.S. Borders

figure 3.16 Concept for a rail radiation portal monitor system.

3.3.2.5 Remotely Operated RPM To minimize staff required to operate RPM systems, especially at seaports and airports, a technical capability based on commercial off-the-shelf technology was developed at PNNL so that RPMs can be operated from a remote location. A remotely operated, or RO-RPM, consists of one or more portal monitors, a supervisory computer, a VIS, a traffic control system with traffic lights and audio communications, a video surveillance system, and an integrated network for data communications between RPM components (Figure 3.17). Data from the remotely operated components are sent to the supervisory booth and computer by wire, fiber optic cable, or wireless transmission. Vehicles and cargo passing through an RPM trigger radiation measurements and image capture. Surveillance video is free running, and audio communications are available when needed. The VIS images are integrated with radiation alarm data from the RPM and stored on the supervisory computer in the database. Side and rear container images are acquired for rail access deployments, while rear license plate and front cab images are acquired for seaport terminal exit gate deployments. For rail access gate applications, optical character recognition (OCR) software on the supervisory computer is used to extract the container number and store it in the database. Container numbers are available for an automated comparison to train consist data provided by the terminal. An electronic comparison is made between the cargo identifications in the consists and the OCR data. Inconsistencies are shown on the computer screen for the CBP officers to ensure that all containers on the train have been scanned for radiation and to manually match the identifications.

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figure 3.17 Remotely operated radiation portal monitor.

Mismatches can occasionally occur because of misread numbers or numbers that cannot be interpreted with the OCR software (e.g., badly damaged numbers or containers that are backward on the trailer). The traffic control and audio communications systems support the interdiction process, allowing cleared vehicles to proceed or directing those that generate alarms to secondary scanning areas for further processing. Surveillance video coverage is provided for the remote portal and the secondary scanning area. Surveillance video allows inspection staff to remotely observe RPM traffic, validate the transit of vehicles to secondary inspection areas, and ensure that containers are not tampered with in the secondary inspection area. The audio communication system permits two-way communication between inspection staff and vehicle drivers. Two-way communications allow inspectors to instruct drivers to proceed to the secondary scanning area, decreasing the potential for traffic queues. Several seaport deployments have effectively used remotely operated RPM ancillary equipment.

3.3.2.6 Straddle Carrier Portal The majority of incoming intermodal cargo containers at a seaport are scanned with RPMs designed for truck-based traffic. However, not all ports that have rail exits can accommodate these configurations due to site and operational constraints for their on-dock rail operations. A limited number of seaports use straddle carriers for moving containers from dock to rail, complicating or eliminating the use of standard RPM configurations. Accordingly, two additional approaches have been examined for scanning cargo containers while they are in transit in a straddle carrier. In one approach, the RSPs are attached to the straddle carriers to scan grounded containers. In the other approach, the RSPs are attached to a pair of substantial towers to scan containers being transported by straddle carriers. Straddle carriers used by the ports are typically about 5.2 m wide (17 ft) and 13 m tall (43 ft), with 3.7 m (12 ft) of internal clearance. Straddle carriers are approximately

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Radiation Detection and Interdiction at U.S. Borders

10.4 m (34 ft) in length and can handle standard intermodal containers (e.g., 6.1, 12, and 24 m long) weighing up to 44 tons. When loaded, the straddle carriers can drive 24 km/h (15 mph). The load can be raised or lowered at approximately 0.3 m/s, and the straddle carrier can accelerate from a dead stop to 11 km/h (7 mph) in approximately 50 m (164 ft). Operational effects from both straddle carrier portal approaches were mitigated based on the results from a time-motion study and adaptation of a remotely operated RPM booth concept using logical lighting schemes for traffic control, cameras, OCR, and wireless technology. A stationary straddle carrier RPM concept is designed to scan cargo containers in transit under a straddle carrier (Figure 3.18) as opposed to grounded containers. Largely based on the design for rail RPMs, the straddle carrier portal preliminary design is fully capable of detecting radiation and notifying CBP officers of radiation detection events, performing radiation scanning on a single, elevated cargo container, one container at a time, as it is being transported through the portal via straddle carrier, monitoring the speed of cargo through the portal, and providing digital image(s) of elevated containers passing through the portal. The stationary straddle carrier RPM detectors are designed to be no more than 6.7 m (22 ft) apart. This maximum distance was established as a design limit and allows for sufficient detector sensitivity while allowing a 5.2 m (17 ft)–wide straddle carrier to pass through. The detector panels are mounted to span vertically from approximately 3 m (10 ft) to 7.6 m (25 ft) above grade. This height and span place

figure 3.18 Artist’s image of a straddle carrier portal concept.

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the effective detection area of the portal above the straddle carrier engine compartment and provide detection capability for a container up to 2.9 m (9.5 ft) tall. The cargo containers being scanned must be elevated at least 3.6 m (12 ft) above grade to eliminate shielding by the engine compartments, and the straddle carrier speed through the portal cannot exceed 8 km/h (5 mph). The system will provide immediate notification to CBP officers when a container sets off an alarm. Seaports are busy terminals, with several straddle carriers transporting containers sequentially through the stationary straddle carrier RPM. False alarms can be generated in RPMs by a container near, but not actually in, the RPM. To prevent a following container containing radioactive material from causing an alarm for a leading container, following straddle carriers are held at a stop line located about 25.9 m (85 ft) away from the portal until the leading container clears the portal. This stop line is equipped with an inductive loop presence sensor (see Section 3.3.10 for a description) to determine a lane violation (i.e., a straddle carrier not stopping). If the inductive loop detects the presence of a straddle carrier in the lane while another straddle carrier is still within the portal, the straddle carrier RPM defaults to alarming status, and both the straddle carrier in the lane and the waiting straddle carrier are required to go to secondary inspection. Such queuing violations are clearly identified to CBP officers for administrative purposes. A guide frame is included in the straddle carrier design to aid in placing cargo containers in the center of the straddle carrier portal. It is in the center of the portal and is high enough to ensure that the container is raised into the detector field of view with adequate clearance from the straddle carrier engine compartment. The guide frame is also a valuable reference for alignment, easing driver navigation through the portal. This concept was tested at a seaport and appeared acceptable to straddle carrier operators. A mobile RPM for straddle carriers has been designed to scan grounded, stacked cargo containers rather than containers in transit. In this concept, RSPs are attached directly to a straddle carrier. Based in part on consultations with RPMP staff, DOE’s Megaports Initiative (Chapter 7, Section 7.3.2) developed and fielded the mobile straddle carrier RPM (Figure 3.19). This prototype was built on a new, dedicated straddle carrier with the detectors mounted on the trailing edge and no lifting mechanism for transporting containers. The large RSPs are plastic scintillator based, similar to those in standard RPMs, but have three smaller enclosures on each side that hold a 102 × 51 × 406 mm (4 × 2 × 16 in.) volume of sodium iodide crystals.

3.3.2.7 Portable Source Identification Device To meet the need for isotopic identification, particularly in the Rail and Seaports Vectors, a concept for a portable source identification device (PSID) was developed at PNNL. Figure 3.20 illustrates the PSID design concept: a portable device consisting of a telescoping mast mounted on a small truck. The mast supports a box containing commercially available sodium iodide detectors (for near-real-time isotopic identification), plus neutron detectors.

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figure 3.19 Prototype mobile straddle carrier radiation portal monitor.

figure 3.20 Concept for a portable source identification device.

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The telescoping mast lowers the detectors to within 0.3 m (0.9 ft) of the ground and raises them to over 12 m (20 ft) in the air, allowing a wide-vertical scanning range. The mobility of the PSID allows it to move efficiently to the cargo that caused an alarm. Scenarios illustrating the need for such a device include the following: • At a seaport, a container alarms while being transported through a portal monitor. The container is grounded in an isolation area. The PSID, as guided by printed information from primary scanning, is used to identify the source of radiation in the container within a few minutes. • In space-constrained areas (seaports or other locations) or areas that have a low traffic volume, a PSID is dispatched to perform secondary scanning and isotope identification for containers or vehicles that have alarmed in a primary RPM. In some situations, using the PSID potentially avoids the cost of installing a secondary portal monitor. This need could also potentially be met by the mobile RPM with spectroscopic capability. • A series of  rail cars alarm while passing through a primary rail portal. In secondary scanning, using printed information from the primary scan, the PSID is used to rapidly identify the isotopic content of each car. In about  minutes, the train is ready to be released without decoupling or significant delay. A manual survey of the rail cars would take hours, could be dangerous to staff performing the survey, and would not allow for detector placement at the upper heights of rail cars (the top of a double-stacked rail car can be as much as  m [nearly  ft] above ground level and  m or so higher if the tracks are on a berm). Trains are often hundreds of cars in length and up to  km (. mi) long, creating an imposing inspection challenge.

3.3.2.8 Handheld Radiation Detectors Other detectors used as part of the overall radiation scanning strategy for CBP include a number of personal or handheld devices. PRDs are small, lightweight radiation monitors worn on the body. They are used to detect the presence of gamma and neutron radiation, to alert the user about the presence of radioactive and nuclear materials, and to warn of significant radiation levels. Handheld RIIDs (defined in this section) are used to detect, locate, and identify radioactive and nuclear materials while simultaneously providing sufficiently accurate gamma dose rate measurement to ensure radiation safety. A PRD (defined in Section 3.1.1 and illustrated in Figure 3.21) is a compact instrument for detecting gamma radiation exceeding the natural background level and indicating the relative strength of the radiation field. The pager alerts the inspector to the presence of gamma radiation and can be used in limited ways to locate source materials on persons or in luggage or packages. A PRD consists of a

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figure 3.21 Example personal radiation detector: Radiation Pager.®

figure 3.22 Example radioisotope identifier device (Exploranium™ GR-135).

detector, support electronics, display, and power source inside a small plastic case. A typical alarm level (with a reasonable nuisance alarm rate) is 30 μR/h in a background of 10 μR/h. The detector commonly used in a PRD is a CsI(Tl) scintillator material. The handheld RIID detects the presence of radioactive emissions in field operations. RIIDs typically use NaI gamma-ray detectors but may also include GeigerMueller detectors, neutron detectors, and cadmium zinc telluride. The RIID alarms on detecting high gamma-ray or neutron radiation. Figure 3.22 shows an



Radiation Pager is a registered trademark of Sensor Technology Engineering, Inc.

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RIID, a portable, spectroscopic isotope identifier consisting of a main housing and associated docking station for storage, calibration, recharging, and data downloading. Typical RIIDs have an energy response range of 20 keV to 3 MeV for gamma rays. New scintillators like lanthanum bromide may enhance their identification capability and are available from a few vendors.

3.3.2.9 Ancillary Equipment Additional ancillary equipment is typically deployed with RPM systems to facilitate operation or meet situational needs. Factors such as network and voice communication, traffic control, unacceptable impacts to supervisory staff presence, and other aspects in minimizing impacts to legitimate trade and travel are considered when establishing a need for additional equipment beyond the base RPM system. This section contains brief descriptions (listed in alphabetical order) of available and requested ancillary equipment and their typical deployment application, as summarized in Table 3.2.

3.3.2.10 Area Surveillance The area surveillance system is used for maintaining situational awareness in locations where supervisory presence is not maintained yet observation is warranted, as depicted in Figure 3.23. A surveillance system consists of several components, including digital area surveillance cameras (may be fixed or pan, tilt, zoom-capable), joystick controller panel, observation monitors, a digital recorder, and network components required for communication and linkage to other systems, such as the table 3.2 Ancillary equipment applications Ancillary Equipment

Area surveillance system Auto dialer Booth Gate arms Inductive loop presence sensors Intercom Network system OCR/reconciliation software Optical presence sensors Pendant PRIDE Programmable logic controllers Rail identification system Strobe/siren Traffic control Traffic lights VIS

Mail/ECCF

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Seaports

Airports

Rail

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X

X

X

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X

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X

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X X

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X X X

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X X X X X X X

X

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figure 3.23 Bank of monitors for viewing area surveillance system imagery.

figure 3.24 Auto dialer.

RPM system. The surveillance system may be configured for action on preprogrammed events, such as automatic recording on external alarm from the RPM system, or on specific activities such as movement in a laydown yard. A surveillance system is typically deployed as part of a remotely operated RPM at a seaport, for other unmanned operations, or as required for maintaining situational awareness.

3.3.2.11 Auto Dialer In response to a system signal or alert, an auto dialer (Figure 3.24) will transmit prerecorded messages to preprogrammed telephone numbers for standard telephones, cellular phones, and voice and/or numeric pager. When activated, the dialer instantly begins calling numbers in sequence, delivering each message.

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The dialer may be programmed to attempt message delivery once or a number of times for each call and, for each call, to repeat messages once or a number of times. Depending on the type of alarm, the dialer will select from a number of prerecorded messages. The auto dialer is typically used at facilities that are not continuously manned or monitored, such as ECCFs. In the event of an alarm, the auto dialer is programmed to call the appropriate supervisory office.

3.3.2.12 Booth Some RPM systems may be installed where there is no available structure for placing equipment (supervisory computer, annunciators, etc.) and staff to operate the system. In this case, a modular booth (Figure 3.25) is provided for equipment and staff. Most booths are typically installed at seaport deployment sites.

3.3.2.13 Gate Arm Gate arms (Figure 3.26) provide additional traffic control, particularly for RPM deployments that are remote from booths, such as RO-RPMs. They are intended to provide an additional indication—in conjunction with traffic lights and/or signs—that a stop is required. Typically, gate arms only activate in case of an alarm. Gate arms are typically deployed at seaports.

3.3.2.14 Inductive Loop Inductive loop presence sensors (also referred to as track circuit overlay or inground loop detectors) detect the presence of a vehicle (road vehicles, train cars, etc.)

figure 3.25 Modular booth.

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figure 3.26 Gate arm deployment.

figure 3.27 Representative inductive loop presence sensor.

from a change in the electromagnetic field in the loop area when a conductor (metal) enters. Because all vehicles that might be expected to pass through portals are constructed mostly of metal, the sensor detects the presence of a vehicle as a change in electromagnetic field that induces a current in the loop (Figure 3.27). Inductive loops evaluated in the RPMP were found to be the only acceptable alternative technology to break-beam optical sensors that could meet the reliability requirements. Inductive loop presence sensors are only used in certain land and rail crossing deployments. For the Rail Vector, track circuit overlays are used to signal the

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presence of a train, in conjunction with additional ancillary components used to recognize train segmentation. In-ground loops are also suitable when the RPM configuration does not lend itself to practical installation of break-beam presence sensors, e.g., cantilever portals at seaports or land border crossings. They are also applicable for use in land border crossings where weather may be an operational factor.

3.3.2.15 Intercom An intercom system provides communication between a supervisor in the booth and a driver in the lane, such as for RO-RPM deployments. There are two types of intercom systems: wired and wireless. Wired solutions, preferably procured through the construction subcontractors, are intended to have local maintenance support and be relatively standardized with respect to the location. The wireless intercom system is more complex and relies on the resident network system; therefore, it is standardized in accordance with the supervisory network requirements. The wireless intercom solution is based on a hardware and software platform that provides users with a set of voice communications capabilities for using the existing Ethernet-based backbone of the RPM system’s network. The wireless intercom (Figure 3.28) supports near-instant, full-duplex, multichannel, simultaneous communications on one-to-one, one–to-many, and many-to-many bases.

3.3.2.16 Network System Data communication between the RPM system and its ancillary equipment is over a local area network. RPM systems are closed networks and, with few exceptions,

figure 3.28 Lane speaker box and booth master unit for wireless intercom.

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are not linked to external networks. The RPM local area network uses standardized communication protocols. Equipment may include the following: • Network switch – a computer networking device that connects (and selects between) network segments. • Encoder/decoder – a device used to encode a signal (such as a bit stream) or data into a form that is acceptable for transmission or storage. This is usually done by means of a programmed algorithm, especially if any part is digital, while most analog encoding is done with circuitry. • Cabling – standardized protocols for data communication over cable types, including the following: – Category  cable, commonly known as Cat —a cable standard for Gigabit Ethernet and other interconnect that is backward compatible with Cat  and Cat e cable. Cat  features more stringent specifications for cross talk and system noise and is normally terminated in RJ- electrical connectors. The maximum cable length without loss of data is  m (~ ft). – Category  cable—commonly known as Cat —is an unshielded twisted-pair-type cable designed for high-signal integrity [ANSI a]. The cable is commonly rated for its Ethernet capability of  megabytes/sec. Patch leads created from Cat  are often terminated with RJ– electrical connectors. A Cat e cable is an enhanced version of Cat , developed for use with BASE-T (gigabyte) networks, or for long-distance  Base-T links ( m ( ft), compared with  m ( ft) for Cat . In locations where cable lengths exceed the Cat e limits, repeaters or media converters are required to switch to fiber optic (or other long-run protocol). • Optical fiber – a thin fiber, usually glass or plastic, for transmitting light. Although fibers can be either transparent plastic or glass, those used in long-distance telecommunications applications are always glass because of its lower optical absorption. Fiber optic cabling is preferred over copper in industrial environments where radio frequency electromagnetic interference from noise sources in the environment is a problem for other network solutions. • Power over Ethernet switch – a switch that sends electrical power to remote devices over standard twisted-pair cable (Cat e, Cat ) in an Ethernet network without affecting the data communications. • Wireless communications – used for data transmittal in ports where () the portal is distant from the supervisory computer and annunciators, and () the port configuration precludes a wired network communication solution. Wireless communications require inserting additional network components, including wireless bridges, antennas, managed switches, and possibly data converters.

The size and complexity of network systems required for all RPM systems are driven by the site needs.

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3.3.2.17 Optical Character Recognition/Reconciliation Software OCR and reconciliation software are tools for automating a procedure to ensure that intermodal containers exiting a port by rail have been scanned through an RPM system. A properly aligned VIS and a properly configured electronic list of containers from the port or terminal are required for use of the OCR/reconciliation tools. The OCR software extracts intermodal container identification numbers from images. The container identification numbers are automatically written to a database server vehicle log file on the supervisory computer. The numbers can be viewed on the supervisory computer and are printed on alarm reports. Reconciliation software is used to compare the derived container identification numbers with the electronic list provided by the terminal or railroad. The OCR software is not 100% accurate in recognizing container identification numbers for several reasons (e.g., number obscured, nonstandard placement, or failure to read all numbers accurately). Accordingly, a software reconciliation tool provides a list of close matches for the user to complete the reconciliation (see Figure 3.29). The OCR/reconciliation software is mainly applicable for use at rail terminal gates at seaports to automate cross-checking that all containers listed on the consist for a train exiting the terminal have been scanned.

3.3.2.18 Optical Presence Sensors Optical break-beam presence sensor technology (Figure 3.30) is used to detect the presence of a vehicle (road vehicles, train cars, etc.) that enters the field of view of the RPM. The presence sensors generate signals that indicate when the vehicle enters or exits the RPM and provide a means for the RPM control box to calculate vehicle speed. Presence sensors are typically mounted in pairs on opposing sides of the RSP support stands. Optical break-beam presence sensors are used for all RPM deployments unless another technology, such as inductive loop presence sensors or the rail identification system, fulfills the presence detection function.

3.3.2.19 Pendant The pendant is a pager-type device that acts as a personal, handheld annunciator. It is based on wireless pagers and logic developed at PNNL that detects an output signal from the RPM control box. This device is used as an integral component of the mobile RPM system.

3.3.2.20 Port Radiation Inspection, Detection, and Evaluation The PRIDE application is designed to collect RPM profile data, radiation spectral data, visual images, and related information from the RPM supervisory computer at a POE and transfer it over a wide-area network to a centralized data warehouse. At the POE, PRIDE matches vehicles in the primary and secondary RPMs, ensuring that alarming vehicles are validated by the secondary RPM. Chapter 6, Section 6.4, discusses PRIDE in more detail.

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figure 3.29 Optical character recognition reconciliation tool interface shown at a test facility.

figure 3.30 Optical break-beam presence sensors typically used on radiation portal monitors.

3.3.2.21 Programmable Logic Controllers Programmable logic controllers (PLCs) are hard-programmed devices used to ensure synchronized, consistent actions by the RPM system ancillary equipment. The PLCs operate based on their programming and the input from presence sensors, speed detection devices, and the RPM system itself. The RPMs use PLCs (Figure 3.31) with programming tailored to the deployment site. They are used as

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figure 3.31 Example programmable logic controller module.

figure 3.32 Rail identification system components: (1) line scan camera, (2) radar detectors, (3) RFID tag reader antenna, (4) lights for line scan camera, (5) rail portal tower.

control units where logic-driven actions are required, such as operating traffic light controllers and triggering gate arms.

3.3.2.22 Rail Identification System Rail RPM systems require a specialized identification system. The rail identification system is an integrated system of components (Figure 3.32) that provides

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automated equipment identification (radio frequency identification tag readers), rail car imaging, wheel/axle counting, and speed determination.

3.3.2.23 Strobe/Siren The strobe/siren is a low-voltage indoor/outdoor unit (Figure 3.33) that features a single-tone siren with a warble sound output and a red strobe light. These units are installed at deployments where the undivided attention of supervisory staff is not required, though they are in the area for quick response in the event of an alarm. The strobe/siren is typically used at mail/ECCF deployments that are not continuously staffed.

3.3.2.24 Traffic Control A number of technologies for traffic control have been identified in addition to the signage used routinely at deployments. Ancillary equipment that may be used in traffic control designs includes the following components, which are described throughout Section 3.3: • • • • • • • •

Area surveillance system Gate arms Inductive loop presence sensors Intercom Optical presence sensors PLCs Traffic lights VIS

Red/green traffic lights are controlled through PLCs that receive signals from the control box. Traffic lights are routinely deployed, particularly at seaport deployments, to automatically provide traffic control information to drivers.

figure 3.33 Strobe/siren unit installed at mail/ ECCF deployment.

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3.3.2.25 Visual Identification System VIS provides images of all vehicles that pass through an RPM for accurate and rapid identification of alarming vehicles. Images are captured as the vehicle moves through VIS-enabled RPMs (Figure 3.34) and are subsequently available for viewing. All images are retained for a predetermined time. The key components of VIS are the high-resolution cameras (Figure 3.35) that capture images of vehicles and trailers. In addition to the cameras, the physical integration and network communications of the VIS components are integrated with the balance of the RPM system. Additional infrastructure such as VIS equipment supports, lighting, and network equipment is required to support the VIS architecture. Three images are captured by VIS for cargo portals (front, side and rear), and two (front and rear) for POV portals. Designed as an integrated upgrade to the RPM system, VIS has been deployed at land border crossings and at RO-RPMs in the Seaports, Mail/ECCF, and International Airport Vectors.

3.4 Imaging Systems Daniel Strom (Contributors: Ofelia Bredt, Megan Lerchen, Gary McNair, and Robert Runkle) Imaging systems are effective in inspecting people, vehicles, and cargo for weapons, explosives, and contraband. A wide variety of imaging and identification

figure 3.34 Visual identification system images displayed at a test facility.

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figure 3.35 Visual identification system camera and lighting attached to a radiation portal monitor.

technologies have been deployed in recent years using ionizing and nonionizing radiation, as well as chemical and biological detection methods. This includes X-ray and gamma-ray imaging techniques (both transmission and backscatter); passive imaging; and nonionizing radiation technologies using ultrasound, radio frequency, infrared, visible, and ultraviolet radiation. A mix of these technologies is needed to detect contraband materials and, for example, to verify that the cargo manifest contains an accurate description of the container contents. The following section focuses on nonionizing and ionizing radiological techniques. It addresses traditional transmission radiography and backscatter X-ray imaging now used in a range of applications from ports to prisons (Strom and Callerame 2003, 2004). This section briefly outlines the physics of transmission and backscatter imaging technologies, reviews the kinds of radiation sources used in each type of imaging, such as radionuclide sources, X-ray generators, and linear accelerators, and discusses the benefits and drawbacks of the methods. Data are presented on radiation doses to persons being scanned, to cargo, and to workers and the public in the vicinity. Selected nonionizing radiation imaging technologies are also discussed.

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figure 3.36 The Acoustic Inspection Device is a handheld gun that transmits ultrasonic pulses and detects return echoes to determine the contents of sealed containers.

3.4.1 nonionizing radiation technologies for imaging and identification Ultrasound can be used for one- and two-dimensional imaging. Traditional ultrasound imaging has been used in medicine for many years. More advanced ultrasound applications include material identification and discrimination of contents in sealed containers. The Acoustic Inspection Device shown in Figure 3.36 was developed at PNNL. It is a handheld “gun” that transmits ultrasonic pulses and detects return echoes to determine the contents of a sealed container. As sound waves are transmitted in the container material, the time-of-flight and amplitude decay of the return echoes reflecting from the far side of the container are analyzed to identify the characteristics of the contents and compare those features with a data library in the memory of the device. This instrument is sensitive enough to distinguish between diet and regular soft drinks and can find objects or compartments in liquid-filled containers and solid materials, including shipping drums and metal ingots (PNNL 2006a). Concealed weapons made of plastic, ceramics, and metal can be detected with a radio frequency imaging system developed for the Federal Aviation Administration and known as the Personal Security Scanner (Figure 3.37) (PNNL 2006b). The wide-band millimeter-wave holographic imaging system was

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figure 3.37 An example of the radio frequency imaging system, developed by Pacific Northwest National Laboratory, installed at a security gate.

developed at PNNL to scan airline passengers for weapons and can also be used for other mass transit systems, including subways and trains. Inspection stations could be set up at the entrances to government buildings, courtrooms, embassies, offices and prisons, nuclear sites, public-gathering places, as well as at exits from business owners concerned about theft. The Personal Security Scanner can identify metal, plastic, and ceramic weapons concealed under clothing. Low-powered millimeter radio waves penetrate clothing but reflect from the body and other objects. These reflected waves are detected with a transceiver, digitized, and sent to a computer. Three–dimensional holographic image-processing techniques form high-resolution radar images of the person under surveillance along with any concealed objects on the body. Privacy concerns have led to the development of techniques to remove or obscure actual anatomical views. Privacy algorithms based on neural networks and other image-processing techniques are used to locate and segment the concealed objects and display them on a computer-generated, gender-neutral, wire frame mannequin or human silhouette. A security guard can easily spot the weapons and their locations while not viewing the person’s body in high resolution. This holographic imaging system produces images like those shown in Figure 3.38. It offers distinct advantages over surveillance systems that rely on metal detectors or X-ray imaging. Metal detectors cannot scan for plastic or ceramic weapons, and X-ray imaging systems subject the body to small doses of

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ionizing radiation. Millimeter-wave scanning poses significantly less risk. The system can be installed into existing security checkpoints without significantly altering the surroundings and, because the scanning process only requires 1 to 2 seconds, minimizes congestion in high-traffic public areas. With advanced software, a technique known as speckle detection can be displayed.

3.4.2 ionizing radiation imaging technologies A method of nonintrusive cargo inspection that uses transmitted or reflected ionizing radiation is known as radiography. This method can produce electronic digital images of vehicles, containers, and their contents. The original impetus for developing ionizing radiation inspection was to look for hidden compartments, contraband, explosives, drugs, and stowaways in vehicles and cargo containers. Now this technology is also used for the detection and identification of weapons of mass destruction. New challenges have arisen because many existing imaging systems lack the penetrating ability to detect these weapons in dense cargo such as scrap metal and thick or dense bulk cargo. Currently deployed cargo radiography systems use either gamma radiation from radioactive sources such as Co or Cs or X-radiation from conventional X-ray tubes or linear accelerators (linacs). Rarely, fast neutron radiation is used. In the future, other types of radiation, such as muons, may be employed (see Section 3.5). While gamma rays are emitted at discrete energies, X-rays are produced in a continuous spectrum ranging up to the maximum potential of the generating device, such as an X-ray tube. Thus, X-ray energies are given in terms of voltage

figure 3.38 Nonintrusive weapons detection using millimeter waves reveals hidden objects of metal and plastic.

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such as kilovolts (kV). The energies of linacs are given in terms of the energy of the electrons used to produce X-rays; for example, 10 MeV. A continuous spectrum of X-rays is also produced by a linac. Unlike conventional medical radiography, radiation beams for cargo examination are generally collimated into two-dimensional fan beams or one-dimensional line beams. The vehicle or container may move with respect to a stationary radiation source, or the radiation source may move with respect to a stationary vehicle or container. In computed tomography, a fan-shaped radiation beam generally rotates around the object being scanned, and the object is translated along the axis of rotation. Fan-beam gamma and X-radiation generators require segmented or positionsensitive detectors, usually with proprietary designs. Scanning pencil beam X-radiation generators do not require segmented or position-sensitive detectors. Neutron detectors used for imaging are generally segmented. Some systems use multiple radiation beams to establish coverage in tight quarters, examine cargo from different vantage points, or sequentially examine cargo with different energies of radiation to gain additional information. Other systems, such as combined backscatter/transmission systems, produce two completely distinct images from a single scan. Figure 3.39 shows an example transmission radiograph. All electronic imaging (as opposed to film or fluoroscopic imaging) techniques employ some image processing such as the following: displaying transmission intensities, production of backscatter images, production of three-dimensional slices with computed tomography, contrast and brightness manipulation and dynamic range stretching, pseudocoloring or false coloring, image analysis for object recognition, subtraction of dual-energy beam images, and integration

figure 3.39 Transmission of a radiographic image using a 3.8 MV linear accelerator. (This image used with permission of American Science and Engineering, Inc. © 2010. Any further use or reproduction is prohibited without proper acknowledgment and the specific permission of AS&E.)

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of image data with other measurements, such as radiation emissions from radioactivity in the cargo, characteristic X-ray emissions, and license plate photos. Three kinds of radiography images are used: transmission, backscatter, and computed tomography. Each has advantages and disadvantages. Transmission radiography systems using X- or gamma-radiation produce traditional “shadow” or “silhouette” images because photon radiation is attenuated differentially by thick, dense items of high-atomic number materials like tungsten, mercury, lead, uranium, and plutonium. Fast neutron transmission radiography also produces traditional “shadow” or “silhouette” images because neutrons are attenuated and scattered differentially by the following: thick items; dense items; low-atomic number materials like hydrogen, lithium, beryllium, and carbon that scatter fast neutrons with significant energy loss; specific isotopes with high neutron-capture cross sections, like He, Li, B, Cd, and In, that absorb low-energy neutrons; and fissile isotopes like U and Pu (see Section 3.1.2). Neutron radiographs can be used to display materials containing large fractions of hydrogen such as liquids and plastics. Transmission images typically have a great deal of distortion because part of the vehicle or cargo is very close to the radiation source while part of it is more distant. Objects close to the source are magnified relative to those farther away. Often, some of the transmitted beam returns to the image receptor by passing through the side of a container and some through the top, yielding curiously foreshortened or stretched images. Backscatter radiography systems using X- or gamma-radiation produce more photographic images because they image reflected radiation the way a camera images reflected light. However, backscatter systems can also produce somewhat distorted images because the pencil beam they use penetrates the object being imaged from an array of different angles. Figure 3.40 is a schematic representation of a backscatter X-ray imaging system. The X-ray source is collimated into a pencil beam by a rotating collimator with small radial holes. The beam scans the object in a vertical plane while either the object or the X-ray system and detectors move horizontally. A transmission image can be received on the far side of the object by a transmission radiation detector such as a scintillator. Positional information is captured by timing the object or detectors as they move. Therefore, the detector can be larger than the unscattered pencil beam, and the backscattered radiation is received by detectors on either side of the pencil beam. Backscatter radiographic imaging systems operate by sending high-energy photon radiation into an object, essentially with Compton backscattered photons (Chapter 2, Section 2.1), to X-ray the cargo after they are reflected back toward the radiation source. Because backscattered photons have energies in the range of 200 to 250 keV regardless of the initial beam energy, the density thickness (or aerial density given in terms of mass per unit area) through which they can penetrate is limited. These X-rays do not penetrate significantly through high-atomic number

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Display

Display

Number of backscattered X-rays at each position of the pencil beam. Brightness is proportional to the number of detected X-rays.

Number of penetrating X-rays (transmitted X-rays) at each position of the pencil beam. Brightness is proportional to the number of detected X-rays.

X-ray detector Beam scan direction

The vertical position of the beam on the object is determined by the position of the rotating collimator.

X-ray detectors

The horizontal position of the beam on the object is determined by the position of the conveyor belt. These two positions (recorded in units of time) define the location of the picture element (pixel) on the display.

Vertically scanning X-ray pencil beam X-ray source

Horizontally translating conveyor

in center of rotating collimators

figure 3.40 Schematic of imaging system incorporating simultaneous backscatter and transmission of X-ray images from a single source (AS&E 2005). (This image used with permission of American Science and Engineering, Inc. ©2010. Any further use or reproduction is prohibited without proper acknowledgment and the specific permission of AS&E.)

materials such as lead or uranium. Shielded SNM in dense cargo cannot be detected with backscatter X-ray systems. An example backscatter image is shown in Figure 3.41. Computed tomography systems produce a series of cross-sectional images. Such images have almost no spatial distortion and can be processed to generate high-fidelity, three-dimensional images. A two-dimensional image is shown in Figure 3.42. Four key features that determine the performance of radiographic systems are their maximum penetration, spatial resolution, contrast sensitivity, and dynamic range. Higher-energy X- and gamma radiation is generally more penetrating up to a limit in the range of 4 to 10 MeV for higher atomic number materials (Figure 3.43). The curve marked “E” in the figure has a higher mean energy than the other spectra with less filtration. But, higher-energy radiation generally produces images of lower contrast and resolution, often making interpretation more difficult. Thicker, denser, or higher–atomic number materials in an item being examined with transmission radiography require higher energy for maximum penetration. Manufacturers quote penetration in terms of thickness of steel.

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figure 3.41 Backscatter image of stowaways in a cargo container. (This image used with permission of American Science and Engineering, Inc. ©2010. Any further use or reproduction is prohibited without proper acknowledgement and the specific permission of AS&E.)

figure 3.42 Computed tomographic (cross section) image of a suitcase taken with an airport luggage scanner (Photo credit: Morpho Detection, Inc., Newark, CA).

Spatial resolution is the size of the smallest object that can be distinguished in a radiographic image and is often taken as the diameter of a wire of a specified composition, such as copper or steel. Spatial resolution is controlled by beam collimation and, when segmented detectors are used, by segment size. Because of increased forward scatter, spatial resolution will be poorer for thick, high-density

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Energy f uence per energy nterva ergs/cm2/kev

12 Characteristic K radiation

P

10

K lines of tungsten

8

no filter 1mm Al K lines of Sn

6

B D C

Q

K edge of Sn 29.25

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B E

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R

C 1mm Al +  mm Cu + 1mm Sn

D

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150 100 Photon energy (kev)

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figure 3.43 The X-ray spectrum is hardened after passing through filters of aluminum, copper, and tin (Johns and Cunningham 1974).

figure 3.44 A transmission image with a 3.5-MeV beam of a 0.46-m- (18-in.-) diameter object (far left) with alternating lead and aluminum spokes (AS&E 2005). (This image used with permission of American Science and Engineering, Inc. ©2010. Any further use or reproduction is prohibited without proper acknowledgment and the specific permission of AS&E.)

cargoes than for thin, low-density cargoes. Figure 3.44 is an example of an image showing both penetration and resolution inside a standard shipping container. The center hole in the circle is 25 mm (1 in) in diameter. The spokes indicate minimum wire-size detectability. The rectangular area has been “density expanded” to reveal a lead brick behind 0.25 m (0.8 ft) of steel.

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Contrast sensitivity is usually specified by the percentage change that can be detected in the material thickness, as specified in ASTM (2002). Contrast sensitivity also depends on radiation intensity, scan speed, and other factors. To understand the capabilities of photon (gamma-ray or X-ray) radiography, one must understand the interactions of photons as a function of energy (see Chapter 2, Section 2.1). Three properties of cargo affect photon transmission: density (r), thickness (x), and atomic number (Z). It is common to combine the first two into a quantity called density-thickness (rx also called “aerial density”). Thus, a 0.3 m–thick steel object having a density of 7.6 ⋅ 10 kg/m presents a densitythickness of 2.28 ⋅ 10 kg/m, as does a 0.119 m–thick slab of uranium having a density of 19.2 ⋅ 10 kg/m. A continuous X-ray energy spectrum is shown in Figure 3.43. By adding absorbers (or filters) through which the beam is made to pass, the less penetrating, lowerenergy photons can be absorbed, which reduces radiation dose rates without reducing the penetrating power of the radiation. This type of filtration produces more penetrating radiation beams and is referred to as “beam hardening.” A physical quantity that characterizes the penetrability of photons is the mass attenuation coefficient m/r (m/kg), which is a measure of the absorption of photons passing through a particular material. (see Section 3.1.1) A graph of these coefficients is shown in Figure 3.45 for elements from hydrogen through uranium. The gamma-radiation energies of Cs (0.662 MeV) and Co (1.17 and 1.33 MeV) are within the spectral energy ranges for X-rays produced by a conventional 450 kV X-ray tube and a 9 MeV linear accelerator. It can be seen in Figure 3.45 that, at Co energies, virtually all materials have the same mass attenuation coefficient except for hydrogen. Thus, radiography with

10,000 1000

μ/rho (cm2/g)

100 450 kV x rays

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9 MV x rays 137

0.1

H He Li Be C Al Fe Pb U

Cs 60

Co

0.1 0.01 0.001

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figure 3.45 Mass attenuation coefficients for photons (X- and gamma radiation) in various materials. Data from (Hubbell 1982; Seltzer 1993).

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

Co as a source cannot distinguish among the various elements. Radiography with Co will have difficulty distinguishing uranium from an equal density thickness of steel. The 0.3 m (0.9 ft)–thick slab of steel mentioned above would appear roughly the same as a 0.119 m (0.3 ft)–thick slab of uranium that has the same density-thickness of 2.28 10 kg/m because the transmission of Co photons through these two materials will be the same. Figure 3.45 also shows that as energy increases, another effect, known as pair production wherein a high-energy photon creates an electron–positron pair (see Chapter 2, Section 2.1), becomes a dominant mode of interaction for high-Z materials like lead, uranium, and plutonium. These phenomena lead to two arguments for higher-energy photon radiation for the radiographic detection of SNM. First, most lower-Z materials are less absorbing as photon energies rise above 1 MeV, so radiation can penetrate thicker amounts of these materials. For intermediate-Z materials like iron or steel, absorption starts to increase at about 5 MeV from the increase in pair production interactions. Second, high-Z materials become more absorbing at higher energies, so they will tend to stand out on radiographic images. Thus, lead and uranium gradually become more absorbing above 4 MeV; the higher the energy, the greater the contrast between high-Z materials and low-to-intermediate Z materials.

3.4.2.1 Considerations for Selecting Ionizing Radiation Imaging Systems In the choice of systems, logistics considerations, as well as detection and identification performance, must be considered. Logistical considerations in radiography systems include the following: • Purchase, maintenance, and operating costs • Reliability (mean time between failures) and time to repair • Operator training and need for a commercial driving license (for mobile equipment) • Throughput (vehicles per hour) • Ability to get vehicle to secondary scanning • Facility footprint (physical area required for facility) • Standoff footprint (exclusion area required for radiation safety and shielding) • Minimum distance from RPMs • Power consumption • Security concerns (Cs and Co may be targets for adversaries; X-ray tube or linacs are not) • Radiation dose to radiography system operators • Radiation dose to supervisory and ancillary personnel • Radiation dose to truck drivers or railroad engineers

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• Radiation dose to cargo and stowaways • Creation of radioactivity (“activation”) in vehicles and cargo • Production of noxious gases such as ozone.

Detection and identification performance considerations include the following: • • • • • • • • • •

Penetrating power of the radiation beam(s) Resolution and contrast of the image Dynamic range of the system Signal-to-noise performance Scatter rejection Number of views (side, top) Variable offset angle Ease of image interpretation Degree of automation of image analysis Ability to integrate findings of radiography with other detection modalities.

Some progress in penetrating power, resolution, contrast, and image processing can be expected as cargo inspection radiographic systems mature. Progress in X-ray generation, collimation, and filtration to eliminate lower-energy radiation using absorbers near the source, along with increases in intensity, will lead to improved image quality and higher vehicle throughput. Progress in image analysis, possibly making use of dual or multiple energy beams, should enhance object identification and the probability of detection. Angled views could increase the probability of finding hidden objects, as do multiple views (side and top; backscatter; and computed tomography).

3.4.2.2 Cargo Radiography Systems Existing radiography systems for cargo are deployed in secondary inspection. The majority of existing systems were developed when the objective was to find hidden compartments and contraband such as drugs and stowaways. These systems, mostly the Vehicle and Cargo Inspection System (VACIS®), are not able to penetrate very thick or dense cargo such as rolled steel, steel pipe, uncured lumber, or kitty litter. With additional focus on explosives and weapons of mass destruction, efforts are needed to maximize existing technologies and to develop and deploy new technologies. The CBP has deployed several high-energy (3.8 and 6 MeV) linac-based inspection systems and has ordered them for secondary scanning in ports. Schematic views of a VACIS are shown in Figure 3.46 and Figure 3.47. It is anticipated that new performance specifications will push technology to the limit and require significant electrical power. These anticipated detection specifications will require significantly higher-energy radiation than is now deployed, and the footprint, 18.3 × 36.6 m (60 × 120 ft), will require additional

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Moving detector tower

Source

figure 3.46 Isometric drawing of a relocatable vehicle and cargo inspection system for truck inspection at a border.

Moving source High source at 1350 mm Low source at 500 mm

+ 27 degrees + 30 degrees

Moving detector tower 6400 mm

−5 degrees

figure 3.47 Sectional view of the relocatable vehicle and cargo inspection system truck inspection station.

shielding, including shielded doors in front of and behind the vehicle, and an improved linac to produce the X-rays. Some VACIS units cannot meet this footprint (Khan et al. 2004). It is also possible that some high-energy systems may induce too much radioactivity in the cargo being imaged. Radioactivity is induced in some materials, often those with a high Z, when high-energy photons (usually above about 10 MeV) interact with a nucleus of the material and produce photoneutrons.

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The loss of a neutron from a nucleus can render that nucleus radioactive, and the photoneutrons can interact with other nuclei and cause them to become radioactive. Furthermore, cargo being examined with linac–based systems at 6 MeV and above should go through passive detection systems before being radiographed so any induced radioactivity is less likely to cause an alarm. Spectroscopic portals may be less susceptible to the signals from induced radioactivity, but there is much to learn in this area.

3.4.2.3 Gamma Radiation Security Systems for Cargo To perform noninvasive inspection of vehicles and cargo, VACIS (vehicle and cargo inspection system) with a low-level gamma-radiation source are used in the RPMP. Absorption of this low-level radiation is progressively measured with a sensitive linear array of detectors located on the opposing side of the vehicle. Absorption data are then fed to a computer where a digital image is formed for display on a high-resolution video monitor. The radiographic image is analyzed by the operator, and appropriate steps are taken if apparent contraband or threat objects are present. System operators can readily see voids, false walls or ceilings, and other secret compartments typically associated with illicit transport of drugs, explosives, and weapons. The images also are used to verify that the actual cargo is consistent with a declared manifest. The following are the basic types of VACIS: • A relocatable VACIS uses two tracks with a moving source and a moving detector that travel along parallel tracks on each side of a stationary vehicle. As the vehicle is scanned, the system generates video screen images showing any concealed areas within the vehicle. • A portal VACIS uses a permanent source and sensor, and the vehicle is driven past the sensors. This may be combined with other detection systems in an integrated container inspection system. • A mobile VACIS has the source mounted on a boom from the truck on which the detectors are mounted, allowing a vehicle to pass between the detectors and source during scanning. This is also referred to as a mobile truck gamma-ray system.

Currently deployed CBP cargo radiography systems are almost entirely Co based, though there are some mobile VACISs in which Cs is the gamma source. The stated resolution of the VACIS is 12.5 mm (0.5 in.) with a penetration of 102 mm (4 in.) of steel with a 59 GBq (1.6 Ci) Cs gamma-ray source and a penetration of 165 mm (6.5 in.) of steel with a 0.28 GBq (0.75 Ci) Co gamma-ray source. Throughput, including analysis, is limited to an effective scan rate of approximately 0.3 m/s (1 ft/sec), resulting in a typical inspection cycle time of 2 to 3 minutes for a standard tractor-trailer rig. Most existing systems can operate with a 10 degree angle offset from the perpendicular, which helps identify anomalies. Nevertheless, there are times when the VACIS reveals

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that the cargo is opaque to the radiography system, necessitating unloading or other inspection. The dose to stowaways and cargo from VACIS is very low (Khan et al. 2004), permitting a person to receive 5,000 or more scans before reaching the public dose limit of 1 mSv (100 mrem) per year. However, deliberate radiation exposure to truck drivers or others during primary scanning is not standard operating practice.

3.4.2.4 X-Radiation Security Systems for Cargo Some cargo inspection systems produce X-rays with conventional vacuum tubes and voltages up to about 450 kV, or with a linear accelerator (linac) with outputs that may range above 10 MeV. Linac-based systems have the highest cargo-penetrating power currently available. In high-energy linac-based cargo inspection systems (Figure 3.48), the driver exits and the entire vehicle is radiographed. The system has a throughput rate of 20–25 trucks per hour and a capability to penetrate 0.410 m (16.14 in.) of steel.

figure 3.48 Two examples of a large-footprint system employing a high-energy linac (Smiths Heimann http://www.heimanncargovision.com/shockedpage.html).

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3.4.2.5 Backscatter X-Ray Imaging for Cargo When a thin, pencil-like beam of X-rays illuminates volume elements, or voxels, in the object, radiation is scattered back to a detector within a narrow cone. In this way, the backscattered X-rays radiograph the cargo from the inside out. This results in very strong dependence on Z because of the lower energy of the backscattered X-rays. Among the low-Z materials highlighted with backscattered X–rays are all organic materials such as people, drugs, agricultural products, and explosives. Thus, this system provides distinct imagery that is complementary to transmission imagery. An almost photo-like image is created, as shown in Figure 3.49. The device used was the “Z Backscatter” system manufactured by AS&E (2005). As shown in Figure 3.50, a transmission image of a cargo container often appears quite cluttered, and deciphering the cargo contents from a transmission image alone can be both time consuming and limiting in its ability to detect threats. The cargo in this case is a load of durians, a spiny Southeast Asian fruit. While improved spatial resolution and contrast sensitivity are important attributes in dealing with clutter, it is the additional information obtained from backscatter images that often permits an inspector to correlate the X-ray images of a container with the manifest that accompanies it, allowing faster throughput as well as better detection. The backscatter image Figure 3.51 shows that there is contraband (small rectangular shapes under durians in bottom two rows near left side of image) concealed in the front of the load under some of the legitimate cargo.

3.4.2.6 Radiography Security Systems for People There are few deployed radiography systems for examining people. Existing systems include very low-dose (i.e., 0.05 μSv/scan) backscatter systems such as Smartcheck® by American Science and Engineering, Inc. (2005), shown in Figure 3.52, and Rapiscan 1000 by Rapiscan® Systems. These systems meet the dose

figure 3.49 Example of a high-resolution backscatter X-ray image of automobiles. (This image used with permission of American Science and Engineering, Inc. ©2010. Any further use or reproduction is prohibited without proper acknowledgment and the specific permission of AS&E.)

 

Smartcheck and BodySearch are registered trademarks of American Science and Engineering. Rapiscan Systems is a registered trademark of Rapiscan Systems.

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figure 3.50 Transmission image of a truckload of durians, a spiny Southeast Asian fruit (AS&E 2005). (This image used with permission of American Science and Engineering, Inc. ©2010. Any further use or reproduction is prohibited without proper acknowledgment and the specific permission of AS&E.)

figure 3.51 A backscatter X-ray image of the same truckload of durians (Figure 3.50) showing an anomaly (AS&E 2005). (This image used with permission of American Science and Engineering, Inc. ©2010. Any further use or reproduction is prohibited without proper acknowledgment and the specific permission of AS&E.)

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constraints of 0.1 μSv/scan specified in Radiation Safety for Personnel Security Screening Systems (ANSI 2002b). Backscatter X-ray systems produce extremely low doses of radiation but, because of privacy issues, are in limited use. Like radiofrequency imaging, backscatter X-ray images are perceived as invading people’s privacy. Privacy issues include the following: • • • •

Who sees the image? Is it a same-sex operator? Do medical devices such as prostheses appear? Is the image stored and retained? Is the image information available to others?

Transportation Security Administration has deployed this technology, and methods are being sought to improve mitigatation the privacy issues. Figure 3.53 shows the details of hidden objects that can be seen with the backscatter X-ray system. Each side image requires 12 seconds to scan, and the procedure delivers 0.05 μSv to the individual. Transmission X-ray machines for examining people include the DRS “SecureScan” and the M.M.C. International B.V. “CONPASS.” These systems have improved significantly in reducing the dose but cannot yet produce an adequate image of a person at 0.1 μSv (10 μrem). The image produced in Figure 3.54 required 0.25 μSv (25 μrem). If a person were exposed to this dose 4,000 times per year, he or she would reach the public dose limit of 1 mSv (100 mrem) per year.

figure 3.52 A backscatter X-ray imager for checking people for contraband (AS&E 2005). (This image used with permission of American Science and Engineering, Inc. ©2010. Any further use or reproduction is prohibited without proper acknowledgment and the specific permission of AS&E.)

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Plastic explosive

Handgun

Detonating device and wires for IED Ceramic knife

figure 3.53 Image from a scan with a BodySearch “Z-Backscatter™” scanner. (This image used with permission of American Science and Engineering, Inc. ©2010. Any further use or reproduction is prohibited without proper acknowledgment and the specific permission of AS&E.)

3.4.3 future of cargo scanning Several developments can be foreseen in security scanning with imaging technologies, including high-energy X-ray systems that may be deployed in secondary scanning at all ports of entry unless supplanted with successful active interrogation systems. It is expected that new accelerator and target technology (e.g., new materials and cooling methods) for X-ray production will be developed. Higherenergy, higher-intensity, and more compact linac or other types of accelerators may be developed to produce higher-energy X-radiation. Systems combining both transmission and backscatter imaging may be developed and deployed. New beam-hardening techniques (e.g., filtering) may be used to remove lower-energy photon radiation and reduce doses to operators, drivers, engineers, bystanders, cargo, and stowaways. Multiangle, high-energy radiography systems, including possible downward-looking systems, and high-energy computed-tomography systems may be developed. Another advanced system involves laser backscatter sources in which imaging photons are generated by directing a laser beam at an ion beam such as protons. The principle for this technique has been demonstrated at the Advanced Light Source at Argonne National Laboratory and may become practical. This technology

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is perhaps some years away from being deployed, but it is a technology that potentially could produce a tunable, coherent, highly collimated source of very high-energy photons. Such a source would be useful for radiography and possibly for nuclear resonance fluorescence (NRF) that could help to identify specific substances in cargo. High-energy radiography systems, active interrogation systems, and RPMs may also be located in underground facilities constructed of low radiation–background materials with curved entrance tunnels to (1) reduce the radiation leakage from the facilities, and (2) minimize background radiation for RPMs. The maze keeps out terrestrial radiation, while the underground location shields the detectors from cosmic radiation and reduces the effects of weather (Chapter 2, Section 2.2). Low radiation–background materials reduce terrestrial radiation.

3.4.4 future of scanning people figure 3.54 Image taken with

Clearly, there is a place for radio frequency holo- a transmission system during graphic scanning of individuals. Because the levels trials (plastic knife on the hip); of exposure to radio frequency energy are so far this individual received 0.25 below the thresholds for biological effects, such μSv. (Photo credit: ADANI, technologies can be justified and should not pose Minsk, the Republic of Belarus). a public health or personal risk. Radiographic scanning systems are now routinely used in airports. While the radiation dose limit for the public is 1 mSv (100 mrem) per year, common practice is to further constrain doses to 25% of that limit in situations such as security scanning radiography (NCRP 1993). Thus, a dose constraint of 250 μSv (25 mrem) might be chosen by a suitable competent authority or national regulatory body for security scanning of people. The benefit of interdicting contraband items may be considered to outweigh the small risk of additional exposure to ionizing radiation. Therefore, it may be justified to permit use of radiographic security scanning systems for air travel and for crossing international borders.

3.5 Active Interrogation Techniques Ronald Brodzinski One of the gravest scenarios for terrorism would be the illicit importation and detonation of a nuclear weapon or even an improvised nuclear device. Because

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such a device in an unshielded condition would be relatively easy to detect with existing passive technology, terrorists intent on using such a device would likely attempt to conceal its transport by using an engineered radiation shield or surrounding it with significant amounts of normal cargo, or both. Active interrogation techniques are intended to detect the presence of SNM, even when shielded sufficiently to prevent detection by typical passive radiation detection technology. To study the relative merits of various active techniques for detecting shielded SNM, the DHS convened a panel of experts from the DOE National Laboratories. A brief discussion of the panel’s findings is presented below.

3.5.1 interrogation techniques Active interrogation techniques are those that impose signals (typically electromagnetic radiation or nuclear particles) on an object or container and capture returning radiological signatures that are the “fingerprints” of the materials present. In the broadest sense, this includes interrogation with acoustic and optical signals, as well as with more penetrating particles. Imaging techniques, which typically use X-rays, gamma rays, or neutrons as the interrogation sources and appropriate detectors—such as the baggage scanners present at airports—to discern their transmission patterns through the object of interest, are also technically considered active interrogation devices. However, this section will discuss active interrogation techniques that induce unmistakable fingerprints of the materials of interest. Furthermore, while some of these techniques can elicit specific signatures from many different materials, including high explosives, which are also of major interest in interdiction efforts, the following subsections will concentrate on the use of active techniques to generate identifying signatures from special nuclear material (SNM) such as may be found in a nuclear weapon, particularly in shielded configurations. This discussion is limited to the use of nuclear particles and energetic photons as the interrogation sources.

3.5.1.1 Active Interrogation Techniques that Induce Fission The nuclear materials of most interest are those that undergo fission at modest excitation energies and are the principal components of nuclear weapons, namely  U and Pu. When these isotopes undergo fission, they promptly produce multiple gamma rays and neutrons, as well as unstable fission products that subsequently decay by emission of distinctive neutrons and monoenergetic gamma rays. The presence of SNM can be confirmed by detection of these prompt or delayed gamma rays and/or neutrons correlated with the source that induced the fission. The methods for detecting these signatures and the electronic circuitry required to correlate them with the induced fissions are the subject of a major treatise themselves but are discussed only briefly in this section. The emphasis is on the techniques that induce the fission.

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The most common method of inducing fission in fissile materials is by capture of a neutron, which imparts sufficient excitation energy to create the fission. In a noncritical situation, such as considered here, the interrogating neutrons are supplied by an external source, most often some type of accelerator. The accelerator usually produces a beam of particles, such as deuterons, that impinge on various targets to produce neutrons of various energies. For example, fairly low-energy deuterons incident on a lithium target can be used to produce neutrons in the 10–100 keV kinetic energy range. Deuterons are often used to bombard targets containing deuterons to produce neutrons in the 2 to 7 MeV range. Deuterons bombarding targets containing tritium will produce 14 MeV neutrons. Energetic neutrons can also be produced by alpha particles and high-energy gamma rays incident on a beryllium target. A typical accelerator used to produce energetic neutrons is shown in Figure 3.55. Another common method of inducing fission is high-energy gamma radiation. The fissile materials have excitation functions for photon-induced fission that start in the 6–7 MeV range and increase rapidly, passing through a maximum around 15 MeV. Accelerators are generally used to produce these high-energy gamma-ray interrogation sources. The most common interrogation source is composed of an electron accelerator impinging high-energy electrons on a high atomic mass target such as tungsten. The result is a broad bremsstrahlung X-ray spectrum of photons having a maximum energy equivalent to the electron beam energy. A typical bremsstrahlung-generating accelerator is shown in Figure 3.56. Monoenergetic high-energy gamma rays are also produced by accelerating

figure 3.55 A typical accelerator to produce a neutron beam. (Photo credit: Lawrence Livermore National Laboratory, California.)

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charged particles, usually protons, into various targets such as boron, carbon, and fluorine. Nuclear reactions in these targets generate very specific high-energy deexcitation gamma rays. Other excitation sources such as cyclotrons, plasma X-ray generators, or even more exotic particle generators are potentially capable of inducing fission in shielded SNM. These devices, however, are in the conceptual stage and far from practical deployment.

3.5.1.2 Active Interrogation Techniques that do not Induce Fission There are no proven techniques for detecting shielded SNM that do not rely on inducing fission. However, some techniques that have the potential to do so are under investigation. Perhaps the most sought after of these techniques is NRF because it has the potential to detect not only SNM but (in theory) most illicit materials, including drugs and explosives. The NRF interrogation source is a bremsstrahlung generator that excites the nuclei of interest to selected energy levels. Prompt decay from these nuclei to lower energy levels results in the emission of monoenergetic deexcitation gamma rays that serve as unique fingerprints of specific isotopes. These gamma rays are detected with a high-resolution germanium spectrometer, and identification of these unique lines in the accumulated energy spectrum clearly identifies the presence of the excited nuclei. This spectral analysis procedure is completely analogous to identifying the presence of specific radionuclides based on their signature decay lines in a gamma-ray

figure 3.56 A typical electron accelerator used to produce bremsstrahlung photons for imaging; in this case, a pallet of plywood. (Photo credit: Idaho National Laboratory, Idaho.)

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spectrum or to identifying specific elements based on their signature fluorescence X-rays following stimulation of their atomic electrons. This technique is undergoing intensive experimentation with respect to its potential for detecting SNM signatures in the presence of intense background spectrum anticipated to be generated by other cargo materials. Other nonfission techniques include relatively exotic processes such as cosmic muon imaging or spectroscopic measurement of muonic capture X-rays (Borozdin et al. 2003). These techniques and their anticipated limitations are largely theoretical and as such are not discussed further here.

3.5.2 signature detection It is somewhat axiomatic that the untested and unproven active interrogation techniques all seem to require detectors having high energy or spatial resolution, while the proven techniques that rely on inducing fission are mostly satisfied with proven detector technologies that require high efficiency and timing to identify the fission signatures. Unique detection of prompt fission gamma rays and neutrons is, more often than not, impossible. The difficulty in detecting signature gamma rays while interrogating with a bremsstrahlung spectrum, or in detecting signature neutrons while interrogating with neutrons from a generator, lies in the confusion among the bombarding and returning particles. On the surface, it may seem quite plausible to detect signature neutrons while interrogating with gamma rays or to detect signature gamma rays while interrogating with neutrons. However, in reality, the secondary gamma rays and neutrons engendered in the cargo or shielding materials are more than enough to obscure the relatively small number of signatures that might be present. Some exceptions do exist; for example, the detection of fast prompt neutrons while irradiating with 10–100 keV neutrons. These exceptions, however, are not useful over a wide variety of cargo types and configurations. Therefore, the most useful fission signature detectors are those that are sensitive to delayed gamma rays and neutrons emitted by short-lived fission products. With appropriate timing circuitry, these fission signatures can be uniquely correlated with the pulses from an interrogation source and clearly distinguished from the interrogating particles and the secondary particles they induce in the shielding and cargo. An example of a very large-area, high-efficiency neutron detector is shown in Figure 3.57. Similar large-area gamma-ray detectors are commercially available.

3.5.3 active interrogation requirements Detecting the presence of SNM in cargo or an engineered shield requires the use of an active interrogation source to induce fission, followed by detection of the gamma-ray and neutron signatures emitted by the fission fragments. This is difficult because the cargo and shielding not only absorb the interrogating particles but

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figure 3.57 Large panels of Li glass fiber thermal neutron detectors.

also the signature particles. To mitigate the influence of cargo and shielding on the interrogation source(s), a source consisting of neutrons or gamma rays, or both, needs to be as high in energy as possible. This suggests using neutrons in the 7–14 MeV range or bremsstrahlung with an end point energy of 15–20 MeV. These highenergy interrogation particles are much more penetrating than low-energy particles and have a much higher probability of getting through the cargo and shielding to create fission in the SNM. Unfortunately, the energies of the fission gamma-ray and neutron signatures are fixed, and nothing can be done to enhance their escape from the cargo and shielding. Hence, it is always possible to design an engineered shield that makes the direct detection of SNM impossible. Therefore, any active interrogation system must also be capable of detecting the presence of shielding designed to obscure the signatures of interest. The shielding in itself serves as a signature, suggesting that further investigation may be warranted. This might be done by imaging or by noting the diminished number of expected prompt source signals, which would indicate the presence of highly absorbing materials. Noting the presence of such shielding materials would not necessarily indicate the presence of SNM but simply point out that any SNM that might be present could not be observed. Finally, any active interrogation technique capable of inducing fission in SNM will also induce activation products in the accompanying cargo. These activation products may be an unacceptable byproduct in certain electronic devices, foodstuffs, or pharmaceuticals. Similarly, use of any active interrogation source requires significant infrastructure (i.e., a large building, shielding, etc.) while in operation

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to control the radiation dose to operators and other personnel and to minimize the interference with other radiation detection devices.

3.6 References ANSI. a. Commercial Building Telecommunications Cabling Standard - Part : Balanced Twisted Pair Components - Addendum  - Transmission Performance Specifications for -Pair  Ohm Category  Cabling. ANSI/TIA/EIA--B.--, ANSI, Washington, DC. Accessed October , , at http://www.tiaonline.org. ANSI. b. Radiation Safety for Personnel Security Screening Systems. ANSI Standard N., American National Standards Institute, Washington, DC. ANSI. a. American National Standard for Portable Radiation Detection Instrumentation for Homeland Security. ANSI N., American Nuclear Standards Institute, Washington, DC. ANSI. b. American National Standard Performance Criteria for Alarming Personnel Radiation Detectors. ANSI N., American Nuclear Standards Institute, Washington, DC. ANSI. c. American National Standard Performance Criteria for Hand-Held Instruments for the Detection and Identification of Radionuclides. ANSI N., American Nuclear Standards Institute, Washington, DC. ANSI. d. American National Standard for Evaluation and Performance of Radiation Detection Portal Monitors for Use in Homeland Security. ANSI N., American Nuclear Standards Institute, Washington, DC. ANSI. e. Performance Criteria for Spectroscopy-Based Portal Monitors Used for Homeland Security. ANSI N., ANSI, Washington, DC. American Science and Engineering, Inc (AS&E). . Z Backscatter. Accessed October , , at http://www.as-e.com/products_solutions/z_backscatter.asp. American Society for Testing and Materials (ASTM). . Standard Practice for Evaluating the Imaging Performance of Security X-Ray Systems. ASTM F-e, ASTM International, West Conshohocken, PA. Audi G and AH Wapstra. . The  update to the atomic mass evaluation. Nuclear Physics A :–. Berger MJ, JH Hubbell, SM Seltzer, J Chang, JS Coursey, R Sukumar, and DS Zucker. . XCOM: Photon Cross Sections Database, Version .. National Institute of Standards and Technology, Gaithersburg, MD. Borozdin KN, GE Hogan, C Morris, WC Priedhorsky, A Saunders, LJ Schultz, ME Teasdale. . “Radiographic imaging with cosmic-ray muons”, Nature , . Guss P, M Reed, D Yuan, M Cutler, C Contreras, and D Beller. . Comparison of CeBr with LaBr:Ce, LaCl:Ce, and NaI:Tl detectors. Proc. SPIE , L, San Diego, California . Hubbell JH. . Photon mass attenuation and energy-absorption coefficients from  KeV to  MeV. International Journal of Applied Radiation and Isotopes :–. Johns HE and JR Cunningham. . The Physics of Radiology, rd Ed., Charles C. Thomas, Springfield, IL. Khan SM, PE Nicholas, and MS Terpilak. . Radiation dose equivalent to stowaways in vehicles. Health Physics ():–.

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Knoll GF. . Radiation Detection and Measurement th Edition. J.Wiley&Sons, NY,  Korea Atomic Energy Research Institute (KAERI). Cross Section Plotter. Available at http:// atom.kaeri.re.kr/endfplot.shtml. Accessed April . Kouzes RT. . The He Supply Problem. Technical Rpt. PNNL-, Pacific Northwest National Laboratory, Richland, WA. McDonald JC, BM Coursey, and M Carter. . Instrumentation for the detection of illicit radioactive sources. Physics Today ():–. Mughabghab SF, M Divadeenum, and NE Holden. . Nuclear Cross Sections. Vol. : Neutron Resonance Parameters and Thermal Cross Sections, Part A, Z =  – . Academic Press, New York. National Council on Radiation Protection and Measurements (NCRP). . Limitation of Exposure to Ionizing Radiation. Report No. , NCRP Publications, Bethesda, MD. Pacific Northwest National Laboratory (PNNL). a. Acoustic Inspection Device. Accessed November ,  at http://availabletechnologies.pnl.gov/technology.asp?id=. PNNL, Richland, WA. Pacific Northwest National Laboratory (PNNL). b. Personal Security Scanner. Accessed November ,  at http://availabletechnologies.pnl.gov/technology.asp?id=. PNNL, Richland, WA. Seltzer SM. . Calculation of photon mass energy-transfer and mass energy-absorption coefficients. Radiation Research :–. Strom DJ and J Callerame. . Ionizing radiation imaging technologies for homeland security. Radiation safety aspects of homeland security and emergency response, Proceedings of th Topical Meeting of Health Physics Society, San Antonio, TX, pp. –. Health Physics Society, McLean, VA. Strom DJ and J Callerame. . Imaging and Identification Technologies for Homeland Security. Chapter  in Public Protection from Nuclear, Chemical, and Biological Terrorism, eds. A Brodsky, RH Johnson Jr., and RE Goans, –. Madison, WI: Medical Physics Publishing. Stromswold DC, JH Ely, RT Kouzes, JE Schweppe, and BS Carlisle. . Specifications for Radiation Portal Monitor Systems: Revision .. PNNL- (PIET--TM-), Pacific Northwest National Laboratory, Richland, WA. Stromswold D and RT Kouzes. . Specifications for a Radiation Detection Portal Monitor. PNNL–, Pacific Northwest National Laboratory, Richland, WA. Tinker M. . Standardisation of radiation portal monitor controls and readouts. Radiat Prot Dosim () ():–. Van Ginhoven RM, RT Kouzes, DL Stephens. . Alternative Neutron Detector Technologies for Homeland Security, Technical Rpt. PNNL-, Pacific Northwest National Laboratory, Richland, WA. Van Loef EVD, P Dorenbos, CWE van Eijk, KW Kramer, and HU Gudel. . “Highenergy-resolution scintillator: Ce+ activated LaBr.” Applied Physics Letters (): –.

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Enhancing the Effectiveness of Radiation Portal Monitor Systems

As discussed in earlier chapters, plastic scintillator-based RPM systems have many advantages, including their relative simplicity and low cost. However, they also have some limitations under normal operation. For example, the use of gross counts to trigger alarms leads to many NORM-related alarms in some operating environments. This chapter describes efforts to enhance the effectiveness of RPM systems, principally through changes to the software analysis of the count rate data. These changes allow more information to be gleaned from the collected data and, in combination with gross counts, allow better discrimination of normal commerce from materials of concern. An important element included in this chapter is the use of modeling and simulation to better understand how count rate data are collected, and how background count rates are suppressed—by phenomena such as shadow shielding—and what that means to the count rate alarm levels. In addition, information is presented about how the data might be manipulated to gain more discrimination, and about the role of spectroscopic detectors as RPMs. Lastly, the discussion focuses on the role of operators and support staff in making RPM systems more effective tools for detecting materials of concern.

4.1 Modeling and Simulation John Schweppe, Ronald McConn Jr., Richard Pagh, Sean Robinson, and Edward Siciliano The primary reason for developing computer models for simulating RPM systems and various screening scenarios is the limited amount of data from actual vehicles containing radioactive sources passing through RPMs. These data are needed to fully test detection probabilities and nuisance alarm rates. Although a limited amount of controlled, experimental drive-through data with SNM sources is available, a much larger number of data from actual vehicles at ports of entry is needed to provide a statistical set of meaningful test data. Testing of alarm algorithms with sources packed into normal cargo loads in a randomly selected tractor-trailer

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Radiation Detection and Interdiction at U.S. Borders

configuration at a port of entry is not feasible because of the impact to the flow of commerce, security and safety concerns, and the expense of preparation and execution. To fill this need, these scenarios can be modeled, and the results of the simulations can be checked against a limited set of data gathered at a test facility under controlled conditions. This process helps to validate the models used and provides confidence in their results. Once validated, the computer models can be exploited to improve the operation of the RPM systems. The following section illustrates the usefulness of these computer simulations. Further detail is provided in Siciliano et al. (2005) and Kouzes et al. (2006). These calculations cover various detector geometries and a wide range of variables, some of which would be experimentally convenient. Simulations of actual physical detector responses can be performed to augment the results from experiments. Detailed models of the detectors can be used to determine their responses to unshielded sources, and to estimate the effects that result from embedding sources in cargo-filled, steel shipping containers or the trailers of trucks. All the computer simulations shown here were obtained with the MCNP code (Briesmeister 2000). Originally developed at Los Alamos in 1977, MCNP has been, and continues to be, the principle code for simulating the interaction of radiation with various materials. The numerical results for detection efficiencies were obtained from the MCNP photon pulse-height efficiency output option (tally type 8). For the 1 keV energy bins used here, this option provides a distribution of pulses created from the energy deposited by each photon entering the detector volume. The energy deposition algorithm was constructed to simulate the response of a physical detector and is determined by subtracting the photon energy outflow from the photon energy inflow to the detector volume. The output is given in terms of counts per bin and is an efficient method because it is normalized to the number of photons sent from the source(s) used in the simulation. Equation (4.1) includes Gaussian energy-broadening parameters for both PVT and NaI(Tl) calculations. This option in MCNP is used to simulate peak-broadening effects in the detectors and is implemented through the formula for peak fullwidth at half maximum: FWHM W ( E ) a + b (E + cE 2 )

(4.1),

where the parameters (a, b, c) used in these simulations had values (0.0, 0.05086, 0.30486) for NaI(Tl) and (0.0, 0.114, 0.0) for PVT. For E = 0.662 MeV (the energy of the primary gamma ray from Cs), these values give approximate FWHM(E)/E of 7% for NaI(Tl) and 14% for PVT. No other renormalization of the MCNP

 For “peak spreading” to occur, photopeaks must be included in the calculations. For PVT, there is no significant photoelectric absorption above ~80 keV, so 14% resolution at 662 keV is somewhat misleading.

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output was implemented (e.g., to account for any light-collection inefficiency within the detectors), and the results are consistent with experimentally determined quantities. All calculations were checked for statistical convergence to satisfy the MCNP internal statistical checks used to form confidence intervals for the mean for each tally bin; additionally, most results have statistical uncertainties of <1%. The MCNP simulations discussed in this section were performed to illustrate the differences between NaI(Tl) and PVT in the basic characteristics of detection efficiencies and detector thickness, which are discussed separately for both detector types. First, results comparing the intrinsic and absolute photon detection efficiencies are shown for various detector geometries and a selected range of variables. Second, different criteria and methods of evaluation are used to estimate the optimal values for detector thickness.

4.1.1 photon detection efficiency Figure 4.1 shows the calculated intrinsic detection efficiency, eInt, for NaI(Tl) and PVT detectors over a range of incident photon energies and detector thicknesses. Values shown are thicknesses. The NaI(Tl) detectors are all 10 cm × 40.6 cm in area and encased with 0.1 cm stainless steel. The PVT detectors have large, medium, and small areas of 36 cm × 173 cm, 36 cm × 122 cm, and 15 cm × 76 cm, respectively. The photon sources for these calculations were taken as monoenergetic point sources located 2.0 m from the midpoint of the detectors’ front surface. Each value

Intr ns c detect on eff c ency (e nt)

100%

80%

NaI, 10 cm NaI, 5.7 cm NaI, 3.8 cm

60%

40%

20%

0% 101

PVT, 3.8 cm, large PVT, 5.7 cm, medium PVT, two 3.8 cm, small 102 103 Incident photon energy (keV)

104

figure 4.1 Calculated intrinsic detection efficiency for PVT and NaI(Tl). Values shown are thicknesses.

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Radiation Detection and Interdiction at U.S. Borders

shown in this figure is a total-count efficiency obtained by summing over calculated energy-deposition distributions from 5 keV to 3 MeV obtained for bin widths ΔE = 1 keV, as seen in Equation (4.2): 3000

(

)

e Int ( E x ) = ∑ N E x E j ΔE j =5

(4.2)

where the efficiency distribution function N(E, x, Ej) gives the fraction of counts per keV that deposit energy Ej = j ΔE caused by an incident photon of energy E traversing a detector of thickness x. The fraction of counts is an intrinsic efficiency because it is normalized to the total number of photons incident onto the detector. Because of the wide range in energy summation, these total-count efficiencies include Compton, photoelectric, and pair production interactions in the detectors (see Section 2.1). The physical properties of the six detectors for which modeling results are shown in Figure 4.1 are as follows. The dimensions of the three NaI(Tl) detector faces are the same, 102 mm × 410 mm (4 in. × 16 in.), but the thicknesses range from 38 mm to 102 mm (1.5 in. to 4 in.). The dimensions for the three PVT detectors correspond to some commercial RPM panels. The two large PVT panels (38 mm [1.4 in.] thick, 360 mm [14 in.] wide, 1.73 m [5.6 ft] high and 57 mm [2.2 in.] thick, 360 mm [14 in.] wide, 1.22 m [4 ft] high) are used as single units; that is, one PVT detector in each stand-alone system. The third case is for two small PVT panels (38 mm × 152 mm × 760 mm [1.4 in. × 6 in. x 30 in.] each), commercially available in a stacked configuration of two detectors within one stand-alone system. The models of the NaI(Tl) detectors have a 1 mm (0.03 in.) thick stainless-steel casing plus a 1.59 mm (0.06 in.) aluminum front entrance window that simulates the door of a portal monitor. The PVT models use the same aluminum entrance door but no stainless-steel casing. Aluminum and steel of these thicknesses attenuate the incident flux by more than a factor of 10 for gammas with incident energies below ~15 keV and below ~40 keV, respectively. As seen in Figure 4.1, the NaI(Tl) intrinsic efficiency reaches a maximum of ~90% at about 200 keV incident energy. For incident energies below ~50 keV, the aluminum door and steel casing strongly attenuate the incident flux. Between 50 keV and 200 keV, there is very little difference among the intrinsic efficiencies for the three thicknesses evaluated. However, at energies above 200 keV, the effects of detector thickness become significant due to the larger interaction distance of gamma rays in this region. The results for PVT are somewhat different, reaching a peak intrinsic efficiency of 40% to 50% (depending on thickness) at an incident gamma energy of ~100 keV. This 10% difference among the thicknesses evaluated persists throughout the energy region shown, except below ~20 keV, where the aluminum door attenuates the incident flux. From these results on NaI(Tl), it appears that a thickness greater than 38 mm (1.4 in.) would be adequate for detecting most gamma rays emitted from

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167



U (75 keV to 200 keV) and Pu (200 keV to 450 keV). When analyzing the relatively low-energy photo peaks for these sources, a thinner NaI(Tl) detector may be advantageous in reducing the Compton continuum under the peaks because higher-energy NORM gamma rays pass through with less interaction probability. The photo peak mode for NaI(Tl) does not require an energy ratio like PVT, so the statistics of higher-energy signals are irrelevant when a NaI(Tl) detector is operated in this mode. Energetic (>1400 keV) gamma rays typically associated with NORM are detected more efficiently with the 102 mm NaI(Tl) detector than with the thinner NaI(Tl) or the PVT. Because of the density and atomic make-up differences, the intrinsic efficiency for all three PVT detectors is significantly less than for NaI(Tl). The two PVT detectors that have the same thickness (38 mm [1.4 in.]) have essentially the same intrinsic efficiency, which does not include the surface area, while the thicker detector has approximately 25% greater efficiency at energies above 30 keV. Intrinsically, a NaI(Tl) detector is more efficient than a PVT detector, but large PVT detectors are more economical than large NaI(Tl) detectors, resulting in a realized absolute efficiency advantage for plastic scintillators at the same investment level. The importance of this effect is shown in Equation (4.3), in which the results for the intrinsic efficiencies, eInt, in Figure 4.1 (summed counts per total photons incident onto the detector) were converted into absolute detection efficiencies, eAbs (summed counts per total photons emitted from the source), by multiplying with the appropriate factors for the solid angles subtended:

eAbs ( E

)

eInt ( E , x ) ×

Ω 4p

(4.3)

These corresponding absolute efficiencies are shown in Figure 4.2, where the relative absolute detection efficiencies are inverted with respect to the intrinsic efficiencies shown in Figure 4.1. The PVT absolute efficiencies now dominate because of their larger size and the change in efficiency metric. In this case, the two large PVT detectors have approximately the same absolute efficiency because they have very similar volumes. The small PVT detector has much less absolute detection efficiency than the larger PVT detectors. The NaI(Tl) detectors have lowabsolute detection efficiency because of their small surface area exposed to the total (4 π) gamma-ray flux. From these results, it would take an array of five NaI(Tl) detectors of more than 38 mm (1.4 in.) thickness to attain an absolute efficiency for gross counts comparable to the large area PVT scintillator (i.e., >0.35%) in the 100 keV to 400 keV energy region. Note that a simple, but misleading, estimate from the ratio of detector areas indicates it would take an array of approximately 15 NaI(Tl) detectors!

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Radiation Detection and Interdiction at U.S. Borders

Abso ute detect on eff c ency (EAbs)

0.5%

0.4%

PVT, 3.8 cm, large PVT, 5.7 cm, medium PVT, two 3.8 cm, small

0.3%

0.2% NaI, 10 cm NaI, 5.7 cm NaI, 3.8 cm

0.1%

0.0% 101

102 103 Incident photon energy (keV)

104

figure 4.2 Calculated absolute detection efficiency for polyvinyl toluene (plastic scintillator) and NaI(Tl).

4.1.2 specific detector simulations The MCNP models discussed in this section were constructed to simulate actual PVT and NaI(Tl) detectors. The PVT model was constructed to the specifications of a commercial portal monitor, including the detector (38 mm × 360 mm × 1.73 m [1.4 in. × 14 in. × 5.6 ft]), shielding, insulation, and all enclosures. The NaI(Tl) model was constructed by replacing the PVT detector with a smaller NaI(Tl) crystal (100 mm × 100 mm × 410 mm [4 in. × 4 in. × 16 in.]) encased in a 1 mm (0.03 in.) stainless-steel shell, while retaining the same shielding, insulation, and enclosures as the PVT. In addition to the detailed detectors, the modeled sources were also constructed to simulate the actual sources in both physical extent and relative position. All of the modeled sources were single, small spheres at a distance of 2 m (6.5 ft) perpendicular from the face of the outer aluminum door and 1.63 m (5.3 ft) above a modeled pavement. This paved surface provided a reflector that accounts for ~10% of the total PVT responses for sources with gamma emissions above ~400 keV (e.g.,  Cs, Co, and Th). The 10% gain provided by the pavement was calculated relative to a spectrum with pavement replaced by a void. Gains for NaI(Tl) were somewhat less. The modeled sources included Am, Ba, Co, Co, Cs, and Th. The detailed values of the energies and percent branching fractions (%BFs) of the photons from these sources are listed in Table 4.1. Thorium was not one of the sources measured, but it is included here for reasons explained below. In the table, the entries for the dominant gamma rays are shaded.

table 4.1 Photon emission data for model sources used in MCNP simulations; dominant gamma rays are shown in the shaded boxes Source Isotope

Emitted Photon Energies ∗ Percent Branching Ratios (%BF) (∗Units for 232Th Are Gammas Emitted Per Second [gep] Per Gram)

Photons Emitted Per Second Per μCi 

keV %BF keV %BF

11.9 0.809

13.9 13.04

15.9 0.33

17.6 20.17

21 5.183

Am 26.3 2.4

32.2 0.017

33.2 0.12

42.6 0.001

55.5 0.02

59.5 35.7

69.8 0.024

97.1 0.001

98.9 0.02

101.1 0.002

102.9 0.019

122.9 0.001

125.3 0.004

43.4 0.073

28,800

36.01 4.40

53.15 2.17

102,000

14.41 9.54

122.06 85.54

59,900



keV %BF keV %BF

3.79 0.24

4.14 0.11

4.28 6.70

4.73 6.52

5.39 0.91

Ba 30.63 35.56

30.97 65.74

34.97 17.97

79.61 3.18

80.99 34.18

160.60 0.60

223.24 0.46

276.39 7.09

302.85 18.40

356.00 62.15

383.84 8.92



keV %BF keV %BF

0.62 0.07

0.63 0.05

0.70 0.65

0.73 0.4985

6.39 16.41

Co 6.4 32.5

7.06 5.842

7.17 2.2e-7

136.47 10.69

230.26 3.4e-4

339.66 4.6e-3

352.32 3.2e-3

366.74 6.1e-4

569.92 0.0146

691.98 0.1584

706.39 6.7e-3



keV %BF keV %BF

346.95 0.0076

826.33 0.0075

1173.2 99.90

1332.5 99.98

2158.9 0.0011

Co 2505.7 2.0e-6

3.95 0.01

4.33 0.01

4.46 0.3983

4.94 0.37

5.62 0.05

Cs-137 31.82 2.05

74,000

32.19 3.77

36.36 1.04

37.45 0.26

661.66 85.21

34,500

(Continued)

table 4.1 (Contd.) Source Isotope

keV geps/g keV geps/g keV geps/g keV geps/g keV geps/g keV geps/g keV geps/g

Emitted Photon Energies Percent Branching Ratios (%BF) (∗Units for 232Th Are Gammas Emitted Per Second [gep] Per Gram)

Photons Emitted Per Second Per μCi

∗

10.26 118 15.23 184 74.97 50.3 153.89 33.3 463.11 180 785.51 43.1 1496.0 41.5

10.83 256 15.38 44.2 77.11 691 209.39 163 510.61 302 794.79 181 1588.2 142

11.12 35.1 16.15 808 84.26 47.1 238.58 1700 562.65 39.9 835.60 68.5 1620.7 58.8

12.28 138 16.18 1780 87.19 244 240.76 152 581.53 59.4 840.44 37.1 1630.5 77.0

12.33 122 17.95 41.4 89.96 135 270.26 149 583.02 1200 860.30 168 2614.4 1400

 Th 12.76 750 18.80 513 90.13 72.5 277.28 95.0 677.07 33.8 904.29 35.2

12.95 596 19.11 183 93.35 220 300.03 130 726.63 34.4 911.16 1140

13.10 235 19.40 35.5 99.55 52.9 328.07 137 727.25 259 964.64 228

13.52 3950 39.85 42.8 105.36 79.5 338.42 491 755.28 52.1 968.97 688

15.15 395 74.81 411 129.04 116 409.63 86.9 772.28 43.0 1459.2 41.8

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Although unshielded and shielded sources were modeled, for brevity only the results from the unshielded source are presented here. Results from the calculations for a source mixed with cargo are also presented. Calculations for the unshielded source were performed to simulate the real detector responses both qualitatively and quantitatively. Qualitative features of the simulated spectral distributions are discussed for the unshielded source, then the total-count values that result from evaluating the area under these distributions are tabulated (in units of counts per second per μ(Ci)) and, when available, compared to the measured values. Note that all numerical results presented here are net-signal values and not gross-signal (i.e., signal plus background) values.

4.1.3 unshielded-source results Figure 4.3 shows examples of experimentally measured spectra obtained with NaI(Tl) and PVT detectors for a Ba source. Figures 4.4, 4.5, and 4.6 show examples of computer-simulated net responses of the NaI(Tl) and PVT detector models for each of three individual sources (Ba, Am, and Th) with no intervening material (except air) between the sources and the detectors. In Figure 4.4 through Figure 4.6, the PVT results are depicted as solid lines, and the NaI(Tl) results as connected circles. Their values are given in counts per emitted source photon (the left-hand ordinate) as a function of the energy deposition. Also shown (filled triangles) in these figures are the distributions of photon flux incident on the detectors after passing through the aluminum door. These flux distributions are included to provide an unambiguous reference against which

Ba-133

Counts/sec/μC

101

PVT NaI

100

10−1

0.10

0.20

0.30

Approximate energy (MeV)

figure 4.3 Measured Ba spectrum from Na(Tl) and PVT detectors.

0.40

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Radiation Detection and Interdiction at U.S. Borders

Re at ve to max

100 10−1

Am-241 Incident

10−2

Counts/em tted photon/keV

10−3

PVT response Nal response

10−4

10−5

10−6

10−7

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Energy bins (MeV)

figure 4.4 Calculated photon flux and spectra for polyvinyl toluene (plastic scintillator) and NaI(Tl) for Am.

the detector responses can be compared. Although passage through the door spreads them somewhat, the flux shapes still appear as narrow spikes on the scale of energies plotted in the figures. For easy comparison, the values of the flux are given as relative values, normalized to their maximum and read from the righthand scale. Note that both the left-hand and right-hand ordinates cover four decades and, hence, can visually distort the relative importance of minor flux values.

4.1.4 spectral distributions One immediate observation about the model spectra displayed in Figure 4.4 is that they simulate well the two main features seen in the real spectra (e.g., Ba seen in Figure 4.3)—the presence of photo peaks in the NaI(Tl) responses, and the absence of peaks along with build-up of low-energy counts in the PVT responses. Note these are net spectra, excluding the background contribution. Nevertheless, these features are indeed characteristic of all the sources, and they underscore the need for PVT-based systems to have the lowest-possible energy threshold for maximizing signal collection.

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173

Re at ve to max

100 Ba-133 Incident

10−1 10−2

Counts/em tted photon/keV

10−3

PVT response Nal response

10−4

10−5

10−6

10−7

0.10

0.20

0.30

0.40

Energy bins (MeV)

figure 4.5 Calculated photon flux and spectra for polyvinyl toluene (plastic scintillator) and NaI(Tl) for Ba.

Comparing the position and relative strengths of the simulated photo peaks to the incident flux shows the extent to which NaI(Tl) is able to resolve the dominant gamma rays. From this comparison, it is obvious that the NaI(Tl) spectra are well correlated to the energy and amplitude of the incident flux. As with the measured spectra, however, this statement requires the caveat that the simulation is allowed to run for sufficient time to obtain good statistical results. Typically, each simulation run took >1900 seconds and tracked ~4 × 10 emitted photons. This corresponds to a real world “measurement time” of ~100 seconds with a 370 kBq (10 μCi) source. Much shorter detection times (~10 seconds) would be used in a simulation of the response of a source moving at speeds typical for vehicles passing through an RPM. In contrast to the NaI(Tl) results, a cursory comparison of the incident flux distributions to the simulated responses for PVT show little correlation, either in position or relative strength. As shown in Figure 4.3, the dominant PVT response is a shift of the high-energy intensity of the incident flux toward lower energies, due to the Compton scattering energy deposition process in the PVT material. For the sources with well-isolated flux below ~100 keV, a careful examination of



Radiation Detection and Interdiction at U.S. Borders

Re at ve to max

100 Th-232 Incident

10−1 10−2

Counts/em tted photon/keV

10−3

PVT Nal

10−5

10−6

10−7

10−8 0.00

0.50

1.00

1.50

2.00

2.50

Energy bins (MeV)

figure 4.6 Calculated photon flux and spectra for polyvinyl toluene (plastic scintillator) and NaI(Tl) for Th.

the PVT spectra does show some hint of photo peak contributions, but these are mostly concealed by the Compton down-scatter contributions from higher-energy gamma rays. What can be seen, and roughly correlated in some of these PVT spectra, are the positions of the so-called Compton edge – the maximum energy deposited by back-scattered photons. These maxima single-scattering energy depositions create the broad bumps in the high-energy portions of the PVT spectra. In light of the above general comment about PVT response being poorly correlated in energy and amplitude to the incident flux, the results shown in Figure 4.4 for Am are discussed in detail. In that figure, the broad bump in the PVT response centered at 60 keV is indeed a broadened, and somewhat obscured, photo peak signal. This response is unique to this source for two reasons. First, the incident flux for this source is dominated by a single emission at ~60 keV, and thus, there are no higher-energy emissions to contribute Compton downscatter counts into this energy region. Second, although the photoelectric absorption process in PVT is not comparable to the Compton scattering process above ~20 keV, there is indeed a small (~4%) probability for photoelectric absorption in

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the region of 60 keV. However, most of the energy deposited by the 60 keV flux is more likely to be distributed to lower energies by Compton scattering. A more common example of the NaI(Tl) and PVT spectral differences is shown in Figure 4.5 for Ba. For NaI(Tl), distinct photo peaks are apparent, although the ones centered at approximately 300 keV and 350 keV are rather broad because of the incident flux at nearby energies. On the other hand, PVT shows only a continuum, with no well-defined peaks and relatively few counts above 250 keV. However, a careful inspection of this simulated PVT spectrum does indicate the presence of a Compton-buried photo peak in the 30 keV to 40 keV region, and a high-energy Compton edge at ~200 keV from the ~350 keV flux. Most of these features are readily observed in the net experimentally measured spectra for this source shown in Figure 4.3. However, note that the energy locations for the data are approximate, where the low-energy values start at ~25 keV for PVT and ~50 keV for NaI(Tl). Again, the dominance of PVT counts at energies below ~50 keV in these results illustrates the importance of having a low discriminator threshold for PVT detectors. As a final example, the simulated NaI(Tl) and PVT responses to Th are shown in Figure 4.6. The gamma emission parameters used to model this source represent 1 gram of Th in equilibrium with its numerous decay products. The Th source is of interest because it and its daughters emit appreciable flux at energies both below 1 MeV (at 13.5 keV, 16 keV, 239 keV, 583 keV, and 911 keV) and above 2.5 MeV (at 2614 keV). The presence of the 2614 keV emission not only distinguishes Th from the other five sources, but it is also present in the natural background and in most NORM. In the energy region below 1 MeV, comparison of the NaI(Tl) to the PVT simulated spectra is the same as with the other sources, (i.e., well-defined, narrow photo peaks for NaI(Tl) and a low-energy-dominated continuum for PVT). The reason for these distinctions is the dominance of Compton scattering over photoelectric absorption in PVT for incident energies above ~20 keV. However, for energies above ~2 MeV, the probability of photoelectric interactions in NaI(Tl) has decreased to <3%, with Compton scattering at ~90% and pair production rising to ~7%. Nevertheless, a weak but clearly identifiable peak centered at 2600 keV is seen in the NaI(Tl) simulated spectrum seen in Figure 4.6.

4.1.5 vehicle modeling In the previous discussion, the advantage of MCNP calculations to simulate detector response was discussed. To simulate the source of radiation from a vehicle, an accurate model of the vehicle must be constructed. This is not a trivial task,

 Comparisons of incident to transmitted flux out the back of the detector show that over 50% of 60 keV gamma rays transit a 3.8 cm (1.4 in.) thick PVT without interacting. Because these noninteracting photons deposit no energy, they are omitted from the 5 keV to 3 MeV sums. This leads to a 33% intrinsic efficiency for a 3.8 cm (1.4 in.)–thick PVT.

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Radiation Detection and Interdiction at U.S. Borders

and it is as important to model the vehicle accurately as it is to model the response of the detector. As an example of the power of MCNP calculations to estimate complex source and shielding scenarios, the trailer of a tractor-trailer rig is modeled. The picture of the trailer, the model representation of it, and a cut-away showing cargo are shown in Figure 4.7 and Figure 4.8. As can be seen from the considerable detail in these figures, the representation of the trailer is highly specific and accurate. The goal is to use MCNP computer models to generate a sufficient amount of validated drive-through simulations to assess not only how well these calculations agree with real scans obtained from similar vehicles, but also to assess the response of the RPM to various configurations of cargo and targeted material. The latter is difficult to obtain from actual runs because these experiments cannot be done at real ports of entry without seriously impacting legitimate trade and travel and are prohibitive in terms of time and cost to conduct in a laboratory setting.

figure 4.7 A photo of a  ft ( m) “dry van” trailer (left) and the three-dimensional screen model representation of the trailer vehicle.

figure 4.8 Three-dimensional rendering of the cargo as a “cutaway” of the trailer shown in Figure ..

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Several vehicles were selected to represent the “real-world” average commercial cargo vehicle, including a 20 ft (6.1 m) covered trailer and an intermodal cargo container (IMCC) on a chassis (Figure 4.7). The models include elements representing all of the significant structural components in the vehicle undercarriage, the trailer bed and walls, the additional supporting steel of the IMCC, and the standard pallets and cargo boxes contained within the trailer or IMCC (Figure 4.8). Details of their major structural components and modular cargo containment were used to create the models. To validate the results from these models, a series of static source mappings was made of the IMCC chassis model. Figure 4.9 summarizes the model results for radiation sources located as indicated in Figure 4.10. A small radioactive source was placed at one of many locations inside the IMCC with and without cargo (indicated by stars in Figure 4.10), and detection rates were measured with a set of PVT-based cargo RPMs. Simulations were also completed, matching these results for comparison. At each of the locations shown by the circles in Figure 4.10, the modeled result from a small point source was calculated. The ratio between simulated and actual data was made for each cargo and source scenario. Without cargo, the detector response for each source location was compared with the model result. This allowed a considerable refinement of the model, as

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Radiation Detection and Interdiction at U.S. Borders

figure 4.10 Source locations used with the trailer model for validation.

discrepancies in the comparison revealed the most critical components of the model and allowed for careful field measurements of those components and continued iteration of the process. In this way, a very sophisticated and accurate model for the IMCC chassis system was developed. The accuracy of this model is shown in Figure 4.9, where the ratio between measurements and model results is close to unity over the interior of the cargo container. The model result shows a small, and mostly constant, count rate above the actual detection rate, due to effects from the electronics not considered by the model.

4.1.6 model results for simulated drive-through scenarios To form a consistent model for such a complicated scenario, the cargo vehicle model is used in a “drive-through” scheme to produce output similar to a vehicle driving through an RPM at a specified speed. The system of vehicle, cargo, and source is stepped through the RPM by moving it slightly forward during each of many simulations. The incremental displacement between each step was 45 mm (1.7 in.), corresponding to the change in vehicle position at 0.1 second time intervals or a constant speed of 1.6 km/h (1 mph). Afterward, these simulations

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are combined to form a time history of detection rates and are arranged into a series. By this method, simulated results closely matching those seen in the field can be generated, and examples are shown below in Figure 4.11 and Figure 4.12. The results shown in Figure 4.11 are for a simulated nonradioactive cargo drive-through scenario with only background radiation present. The background was modeled as a 50 m (164 ft) diameter, 1.25 m (4 ft) deep disk providing a good representation of terrestrial gamma rays. Modeling experience has shown that such a large area source term must be used to accurately represent the observed background, which leads to a significant computational time. In Figure 4.11, the responses in the top and bottom detectors are shown separately and indicate that the shadow-shielding effect (discussed in Section 4.4) is greater in the top panels of the RPM because the vehicle blocks a greater portion of the field of view of background radiation in these panels. The total simulated shadow-shielding effect is similar in magnitude to that seen in physical RPMs. The results shown in Figure 4.12 are for a drive-through simulation with NORM cargo and show the effects of count rate enhancement from a source, similar to that seen in NORM field data (see Figure 4.17). The overall count rate is larger in

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figure 4.12 Simulated drive-through profile for naturally occurring radioactive material cargo.

the bottom panel because it is on average closer to the NORM cargo than the top panel. However, scenarios containing sources of interest may be significantly more complex, with asymmetric point sources as well as NORM cargo, and unknown cargo configuration.

4.1.7 observations As expected, measured and simulated spectra for NaI(Tl) show distinct peaks in response to gamma-ray sources, but the PVT spectra have limited spectral information with most of the counts being shifted to low energies and only coarse features, such as Compton edges, appearing in the energy spectra. Detailed modeling of sources in vehicles shows that, with accurate models of the vehicle and RPM structures and a good representative model of the background gamma radiation, reasonably accurate modeling of the response of a RPM to a vehicle passing through it can be performed. Comparing modeling results for the NaI(Tl) and PVT detectors considered shows that the ratio of PVT-to-NaI(Tl) total counts is about 5-to-1 for both a bare source and model-cargo configurations. This is the same value deduced by comparing the estimates of the PVT to NaI(Tl) absolute detection efficiencies shown in Figure 4.2. This type of modeling result can be used to specify the equivalent amount of material needed for various detectors to obtain a given efficiency. Scattering of gamma rays in cargo reduces the energy of many gamma rays and therefore degrades the quality of spectra that can be obtained from NaI(Tl).

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Modeling verifies that, for a source surrounded by significant amounts of cargo, the characteristic peaks used to identify specific radioisotopes can be absent or severely attenuated because of gamma-ray scattering. Cargo effects on simulated gamma-ray spectra attenuate the incident flux by different factors, depending upon the energies of photons emitted by the source. The magnitude of this flux attenuation factor can be as large as ~10 for emission energies below ~100 keV, or as small as ~10 for emission energies above ~1500 keV. Accordingly, when the source is embedded in a cargo-filled container, the spectral identification capabilities of a NaI(Tl)-based RPM system that relies upon observing the full-energy peaks may be substantially reduced and, for some sources in some cargos, may be completely eliminated. The effects of cargo on measurements with a PVT-based system can also be significant; however, because many of the scattered gammarays are degraded to low-energy photons, the effect on gross-count measurements may be somewhat mitigated by ensuring that the lowest possible energy region of the spectra is counted. Cargo effects can thus reduce the total count rates observed by detectors by factors ranging from ~3 to ~260 due to scattering and absorption, depending on the energy distribution of the emission spectrum of the source. Modeling studies can generate time profiles for vehicles containing sources passing through RPMs. With these models, a variety of scenarios can be simulated that could not otherwise be tested. Sources and cargo of various sizes and configurations can easily be simulated. These studies show that the strength of the signal in time from a source being driven through an RPM is dominated by considerations of the detector solid angle and cargo thickness variations along the line of sight with the detector panels. These temporal distributions of observed signal may be highly asymmetric with regard to location of the detector panels and in time. Validated modeling provides a method to explore a large number of scenarios that are impractical, either physically or fiscally, to measure experimentally. Where models have been validated with experimental measurements, the agreement has been very good. This use of modeling and simulation for interdiction applications is crucial to the success of these programs.

4.2 Intelligent Algorithms for Plastic Scintillator Gamma-Ray Detectors: Energy Windowing James Ely, Richard Kouzes, John Schweppe, Edward Siciliano, Denis Strachan, and Dennis Weier The simplest way to detect excess radiation is by collecting the total number of counts for time intervals during the period that a vehicle transits the RPM. The alarm decisions made on this basis use a gross-count algorithm. Under this algorithm, an alarm is activated when the number of counts exceeds a predetermined

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Radiation Detection and Interdiction at U.S. Borders

threshold. The potential problem associated with this simplistic approach is evident—a low threshold results in too many alarms, whereas a high threshold results in a detection system that is ineffective. Therefore, intelligent algorithms are methods of manipulating the same data collected at the RPM to harvest more information about the nature of the radiation source. These results, combined with the gross-count result, yield a more sensitive methodology for detecting radiation sources of concern. Because all radiation alarms at a border crossing must be investigated to determine the possible presence of threat material, the occurrence of nuisance alarms increases the cost and operational impact of radiation screening. In this section, several intelligent algorithmic methods for improving the performance of RPMs are discussed.

4.2.1 thresholds and nuisance alarms Background radiation is the primary source of radiation detected in an RPM. Background radiation levels depend on the locale but have a fairly stable average value. Atmospheric pressure changes and rain can cause background from radon daughter levels to change rapidly (see Chapter 2, Section 2.2). The dot-dashed “Background” curve in Figure 4.13 shows the distribution of a typical background counting rate. Background values associated with about 3,500 vehicles in narrow traffic lanes, and 1,900 vehicles in wide traffic lanes, are included in the distribution. A skewed Gaussian-like shape shows the statistical behavior of radioactive decay and environmental variations. Changes in the background environment surrounding the detector, including weather and nuisance sources within range of the detector, give rise to the high-count rate tail on the background distribution. The gross-count alarm threshold of the RPM is set significantly above the average level (peak of the distribution curve) of the background radiation in order to avoid false-positive alarms from statistical fluctuations. Persons or cargo within vehicles are another source of radiation detected with RPMs, but they may not necessarily be of concern. Shipments containing normal commercial items, such as tile or cement, or patients who have been treated with radiopharmaceuticals for medical reasons are examples of nonthreatening radiation sources. Depending on the alarm threshold, these sources are detected by an increased level of radiation above the average background level and may cause an alarm. Such sources are classified as “nuisance” radiation, giving rise to nuisance alarms. A general representation of the normalized distributions of the average grosscount rate when vehicles are present is shown as the dashed “Vehicles” curve in  These nuisance alarms are not the same as false alarms because they arise from the presence of actual radioactive material. False alarms originate from background fluctuations or equipment faults when no radiation is actually present.

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Background Vehicles Signal probability distributions

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figure 4.13 Example of the total gamma-ray counting rates in counts-per-second from a typical polyvinyl toluene (plastic scintillator) used in vehicle radiation portal monitor systems showing the distribution of cargo vehicles with or without naturally occurring radioactive material plus background (dashed curve), the distribution of background alone (dash-dotted curve), and a simulated test source (solid curve). The vehicle distribution is seen to be downshifted relative to the background distribution, showing the effect of shadow shielding. The data are from measurements of about , vehicles; normalization is arbitrary.

Figure 4.13 for the same sets of vehicles that generated the background data plotted in the dot-dashed “Background” curve. All portals showed this same pattern irrespective of the site, vehicle, and RPM manufacturer. The distribution of this averaged data shows a shift toward low count rate from the shadow shielding effect (Lopresti et al. 2005). Naturally occurring radioactive material and other sources in the vehicles produces the observed skewed Gaussian-like distribution with a high-count-rate tail. For comparison, as shown schematically by the solid curve in Figure 4.13, an isolated point source of sufficient strength would produce a Gaussian peak, arbitrarily placed here at about 4800 cps. The width of a Gaussian distribution is described by the fluctuation of the distribution from the mean value. This fluctuation is often given in units of standard deviation (σ). The Gaussian distribution is the large count rate limit of a Poisson distribution where σ is equivalent to the

 The RPM industry has traditionally used this simple standard deviation as a parameter for their alarm algorithms even though it is only an approximation when applied to the true skewed background distribution. This accepted practice is used here.

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Radiation Detection and Interdiction at U.S. Borders

square root of the number of counts, N, accumulated in any given counting interval as shown in Equation (4.4): s= N

(4.4)

The radiation has an average level well above the background curve centroid for the point source example assumed in Figure 4.13. A proper selection of a grosscount threshold setting easily differentiates the test source from the background distribution. For example, for 99.9% detection efficiency, the threshold should be set at about 3 σ below the mean of the detected distribution of the test source value. The test source would produce about 4800 counts for a 1-second measurement period for the count rates shown in the figure, resulting in a 99.9% detection probability at a threshold of about 4592 counts. The test source would nearly always produce an alarm (~0.1% false-negative alarms) at this threshold. However, nuisance alarms would be produced by the high-count-rate tail of the radiation from cargo vehicles shown in this example since it cannot be discriminated from the test source with a simple gross-count threshold. Simple gross-count thresholds can generate many nuisance alarms since commercial cargo contains NORM and medical radioisotopes. The gross-count thresholds might have to be adjusted to a higher (less sensitive) value to reduce the operational impact of handling too many nuisance alarms, depending on the operation of vehicle screening and the nuisance alarm rate.

4.2.2 description of naturally occurring radioactive material and special nuclear material signatures The operation of some intelligent algorithm techniques is based on the fact that most of the gamma-ray emission from NORM sources comes at a significantly higher gamma-ray energy than SNM. To illustrate this point, two NORM sources (tile and fertilizer) and two artificial sources, plutonium and HEU, were compared to background. Each bulk NORM source was contained in wooden boxes approximately 0.9 m × 1.2 m (3 ft × 4 ft) × 1.2 m (4 ft) high. The radioactive signature from tile was primarily from K (half-life 1.28⋅10 y) decay and radionuclides in the uranium and thorium decay chains. The fertilizer radiation was primarily from  K. A 99.4 g PuO WGPu source (principally Pu, half-life 2.41⋅10 y) was used. The WGPu was doubly contained in sealed schedule-80 stainless-steel pipes, providing approximately 10 mm (0.3 in.) of shielding. Radiation from the Am (present in the sample from the decay of small concentrations of Pu) was highly attenuated by the stainless-steel shielding. The 93.1% enriched HEU (U; half-life 7.04⋅10 y) source had a total mass of 123 g and consisted of a number of stacked foils in a thin container. Radiation from these four sources was measured with a single RPM panel by placing them at a distance of 2 m (6.5 ft) perpendicular to the center position of the

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front surface. No additional shielding was placed between the sources and the detector. Background measurements were taken both before and after the source measurements. The WGPu and HEU sources were measured for 5 and 20 seconds, respectively, while the NORM measurements were taken for 60 seconds. The data were normalized to 60 seconds for comparison. Note that, a typical vehicle scan with a RPM takes less than 20 seconds. Background radiation and NORM have very similar shapes over most of the PVT energy spectrum, as seen in Figure 4.14(A). The intensities of the accumulated spectra differ depending on the source (tile, road salt, fertilizer, etc), cargo (size, shape, etc), and isotope distribution (K, uranium decay series, etc). However, once a normalizing ratio is taken, these gross signal variations do not play an important role. Figure 4.14(B) also shows the gross signal distributions of the WGPu and HEU sources (plus background). Even though the data are from gross-count spectra (as opposed to net-count spectra) only the corresponding source will be used when referring to the spectra. Distinct differences between background and these sources are plainly visible in the lower channels (low energy). The count distribution in the channels with higher numbers is very similar to background because of the lack of significant high-energy gamma rays from these sources. This distinction between the energy distributions is the underlying feature that allows the intelligent algorithm discrimination of man-made sources from NORM sources.

4.2.3 algorithms for radiation detection The goal for the use of intelligent algorithms in RPM applications is to utilize the crude spectroscopic information available from PVT-based gamma-ray detectors to discriminate NORM from other radiation sources to obtain optimal detection of sources of concern while minimizing nuisance alarms. Data from RPMs are typically obtained with a multi-channel analyzer with 256 to 512 channels to accumulate counts at various “energies” from these detectors. These channels are then summed into energy regions, or windows, to obtain a rough measure of the original gamma-ray energy entering the PVT material. The number of windows can range from one up to the maximum number of channels in the analyzer, but there is a diminishing return above a certain number of windows due to the limited energy information from PVT. Alarm algorithms for PVT-based RPMs that have been investigated include the following: gross-count algorithms, energy window ratio algorithms, speed-dependent algorithms, and spatial algorithms. This section

 Additional shielding would have the effect of attenuating the source strength, down-scattering in energy the gamma rays that remain, as well has hardening the spectrum if higher-energy gamma rays are emitted. The low-energy characteristic of plutonium remains when shielded. HEU also retains its low-energy character with modest shielding, but the higher energy gamma rays from 238U will dominate with sufficient shielding.

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figure 4.14 Spectra from polyvinyl toluene (plastic scintillator) for (A) naturally occurring radioactive material radiation (fertilizer and tile) and background, and (B) highly enriched uranium, weapons grade plutonium, and background to illustrate the differences in the spectra at low energies (low-channel number). Note the different scales used in the upper and lower graphs; due to the collection time differences, the WGPu and HEU spectra have more fluctuation at high energies.

focuses on the capabilities of energy windowing, while the next section considers other algorithmic methods and their applications.

4.2.3.1 Gross-Count Algorithms A gross-count threshold algorithm that compares the gross-count rates when a vehicle is present to the background rates is the simplest first approach to an alarm algorithm. This method can be applied to each of the individual energy windows or to the total energy spectrum. As implemented in commercial RPM equipment, a typical gross-counting threshold is based on the background counts in a set time interval and the associated variations or fluctuations. A typical gross-count threshold is calculated in Equation (4.5): T

N +K N

(4.5)

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where T is the threshold counts based on the averaged background, N (counts in a time interval), and the standard deviation of that background, N , for Poisson statistics. The threshold value above background (in units of background standard deviations) is expressed as the constant K (“sigma multiplier”). As seen from the spectra shown in Figure 4.14, a windowed-gross-count threshold algorithm fails when applied to actual data. The counts from NORM sources are elevated across the entire spectrum, as seen in Figure 4.14(A), including the low energies where SNM (Figure 4.14[B]) would enhance the spectrum. Hence, discrimination of sources of concern from NORM cannot be accomplished with a simple gross-count approach whether applied to the entire spectrum or individual windows.

4.2.3.2 Energy Windowing Algorithms This section analyzes a specific class of intelligent algorithmic methods known as energy windowing. This method can be used to enhance detection sensitivity to certain sources, while minimizing the number of NORM-induced nuisance alarms from PVT-based RPMs. The energy windowing approach has been referred to as “spectral analysis” or “natural background reduction.” The various implementations of energy windowing, which differ in detail, are all based upon ratios of counts in regions of the energy spectrum. The energy windowing algorithm was first reported in a German patent application in 1997 by Trost and Iwatschenko (2002) and later by Iwatschenko-Borho (1997), Iwatschenko-Borho et al. (1998), and Rieck and Iwatschenko (2001). These papers reported the ability to discriminate NORM from artificial radiation sources in plastic scintillator gamma-ray detectors. The patent disclosed a method that involved taking the ratio of the intensity from the low-energy part of the scintillation light spectrum to the intensity in the higher-energy part of the spectrum. Since SNM is characterized by low-energy gamma rays while NORM largely gives rise to high-energy gamma rays, this ratio turns out to be different for NORM and many artificial radiation sources of interest such as SNM. This method became known as energy windowing because it required dividing the broad total-energy spectrum from a plastic scintillation material into a few smaller, nonoverlapping “windows” of energy.

 Because the value of N is determined without a vehicle present, this expression shows how any depression of the vehicle-present background, e.g., Nʹ < N, leads to a decrease in alarm sensitivity set by the value T, when it should be a smaller value. A lower threshold can compensate for this, but with a resulting higher nuisance alarm rate.  Note that the application of energy windowing in plastic scintillators is not the same as when used with spectroscopic-capable detector materials like thallium-doped sodium iodide NaI(Tl). The spectra in these applications contain full-energy photopeaks, and partitioning the full spectra into smaller

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Radiation Detection and Interdiction at U.S. Borders

During the past several years, extensive testing at PNNL has shown that, in combination with gross-count thresholds, energy windowing can reduce the rate of NORM alarms and mitigate the effects of shadow shielding (Ely et al. 2006). As will be discussed, the energy windowing algorithm allows for the use of the intrinsic efficiency of the detector while discriminating against high-energy gamma-ray sources. Noncargo applications of RPMs, such as mail screening and POV monitoring, do not typically involve the observation of NORM and, therefore, do not tend to benefit from the use of energy windowing. Several manufacturers, including Ludlum Measurements, Inc., SAIC, and Thermo Electron Corporation, have implemented an energy windowing method in commercial plastic scintillator-based RPMs. Initially, these companies are interested in the capability of energy windowing associated with reducing nuisance alarms in the screening of scrap metal. Verification of the energy windowing methodology to ensure continued sensitivity to SNM is required because the use of energy windowing for RPM screening of cargo traffic represented a new application. A more sophisticated approach than simple gross counting in each of the several possible energy windows must be used to discriminate NORM from other sources. The approach is to compare the shape of the energy distribution to that of the background spectrum, and to quantify the similarity or differences in these shapes. A simple such example is to compare the slopes of the distributions in these spectra. Although NORM sources have radiation signatures with greater gross intensity than background, Figure 4.14 shows that the energy shape distribution is very similar. This is because NORM and background materials contain the same radioactive isotopes, namely, K and isotopes in the uranium and thorium decay chains. In contrast, SNM and other artificial isotopes have spectra with more low-energy radiation. From the Figure 4.14, it can be seen that the shapes of the energy distributions for these sources are clearly not the same. The other significant nuisance source, radiopharmaceuticals, is not discussed here because, like SNM, they also produce a low-energy signature. Thus, energy windowing is not a useful technique for discriminating radiopharmaceuticals from SNM, but they can be discriminated from NORM (Kouzes and Siciliano 2005). Consider an energy windowing scheme consisting of two windows—one for the lower half of the spectrum, extending from the detection threshold up to and slightly above the Compton edge, and the other covering the remaining portion of the high-energy spectrum—and a source that emits a monoenergetic gamma ray. Placement of the window boundary just above the Compton edge in the gammaray spectrum optimizes the statistical precision of the alarm algorithm. The counts in the spectra are sorted into these two windows and summed, and then the

regions in which characteristic photopeaks are located enables rapid determination of the presence of specific radiation sources.

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numbers are compared to the values for background in the same windows. The similarity or difference of the counts in the same windows can be quantified in various ways. One simple approach is to normalize the counts in each energy window to the counts in the high-energy window and to compare that normalized ratio to the corresponding background ratio as seen in Equation (4.6): REW =

N EW NH

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where NEW is the number of counts in the specific energy window and NH is the number in the high-energy window, both when a vehicle is present. The comparison to the background values might take the form of a difference in ratios (a net ratio), or a ratio of the measurement to the background (a ratio of ratios). Another option is that the comparison might take a form similar to the gross-count algorithm (Equation 4.7): EW

RB + Ks RB

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where REW and RB = the ratios for the source being measured and background windows, respectively, K = a multiplier that accounts for the difference in sensitivity and geometry of monitors in different lanes of traffic, and RB

= the statistical standard deviation for the background ratio.

The value s RB can be derived from standard statistical variance propagation of functions of random variables as given in Equation (4.8):

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1 1 + N LB N HB

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N LB N HB

where NLB and NHB are the number of low-energy window and high-energy window background counts, respectively, and ρ is the correlation coefficient for NLB and NHB. Multiple energy windows can be handled by extending this concept. It has been demonstrated that the actual form of the energy window ratio is not critical, though an exhaustive investigation of the various possibilities for analyzing the data has not been performed. Investigations have been made of two other ratios, which are found to give equivalent results. The first is shown in Equation (4.9): REW ′ =

N EW n

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Radiation Detection and Interdiction at U.S. Borders

where Ni is the number of counts in the ith window of a multiwindow system. This is very similar to the ratio discussed above, except that the denominator is now the total count over all windows instead of just the counts in the highest window. A further method, shown in Equation (4.10), is given by Trost and Iwatschenko (2002): Rc

NL −

N LB NH N HB

(4.10)

where NL is the number in the low-energy window, and RC is the compensated ratio that is approximately equal to zero when the measured counts in the lowenergy region are equivalent to the background-compensated counts in the highenergy region (i.e., the shape is the same as the background). In this formulation, by summing groups of windows, the counts are always divided into two regions, a low and high region, even in a multiwindow implementation. Several possible variations of the energy windowing algorithms are based on ratios of regions of interest, the most common being those in the equations above. Because systematic effects tend to dominate over statistics, PNNL studies indicate that there is little or no operational advantage of one variation over another. Generally, these studies that optimize the number of energy windows and the locations of the window edges show that the window edge should be placed at the Compton edge of the source of interest. While it is possible to divide the energy spectrum into a large number of energy windows, too many windows dilute the ability to discriminate target sources. The optimal number of windows is found to depend upon the targeted sources, but three to five windows tend to be practical. From a statistical standpoint, dividing a spectrum into windows to implement energy windowing may at first appear to be a poor approach. Consider dividing a spectrum into windows. If the statistical error is simply added in quadrature (which would be the case if the errors were random and independent), the statistical error from the sum of energy windows appears to increase when compared to the total gross-count mode. However, count rates obtained by dividing the spectra into windows are maximally correlated, as they are exactly the same data obtained from different window thresholds. Thus, the statistical errors are not independent, and the correct expression for evaluating the results must include the covariance (Bevington 1969). In this case, the resulting statistical error for the energy window can be shown to be less than the statistical error for the total count rate. Consider a simple two-window system with a background count rate of 2000 cps in the low window and 1000 cps in the high window as an example of the sensitivity of the various algorithms. A total of 3000 counts is then obtained in a 1-second measurement with a standard deviation of about 55 (from 3000 ). The standard deviations for the low- and high-energy window counts are approximately 45 and 32, respectively. Both of these are less than the standard deviation for the total counts (55). Thus, if a targeted material has counts only in the low-energy

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window, dividing the counts into energy windows increases the sensitivity (signalto-background or signal-to-noise) compared to total counts, even when the same gross-count threshold algorithm is used for each window. For a three–standard deviation detection threshold, approximately 135 counts (3⋅45) above background in the low-energy window would be detectable with the gross-count algorithm. Using the approach given in Equation 4.5 to extend this simple example to a ratio algorithm, the statistical standard deviation of the ratio is approximately 0.02 (from Equation 4.8, assuming maximal correlation with ρ = 1). Then, three standard deviations for the energy windowing ratio results in a detection of approximately 60 counts in the low-energy window, which is about twice as sensitive as the gross-count algorithm. The discrimination power of any method depends both on statistical and systematic fluctuations, and the simple argument described previously only considered statistical fluctuations, ignoring any systematic variations. Systematic variations, such as shadow shielding (Section 4.4), dominate the total fluctuations and must be taken into consideration as well. The optimal window discriminator setting is generally just above the Compton edge with a window for each targeted material. Consider a selection of three energy windows encompassing channels 1–20 (low energy), 21–60 (medium energy), and 61–256 (high energy) applied to the spectra shown in Figure 4.14. The WGPu source strength appears in both the low- and medium-energy windows, while the entire HEU source strength is captured in the low-energy window. Figure 4.15 displays the gross-count rates for each of the NORM, SNM, and background sources summed in the low-, medium-, and high-energy windows. From the results, it can be seen that the WGPu counts are above background in both the low- and medium-energy bins, while the HEU counts are greater in the low-energy bin only. Assuming only a gross-count threshold applied separately to each energy window, it is evident that NORM discrimination with gross counts is difficult since NORM also has counts above background in the lower-energy windows. Thus, a more sophisticated approach is required than just gross counts to take advantage of the energy window information. Figure 4.16(A) and Figure 4.16(B) result from applying Equation 4.6 to the data shown in Figure 4.14(A) and Figure 4.14(B), illustrating ratios in each window for the background, NORM, WGPu, and HEU. Because the ratio for the high-energy window to itself has a value of 1 by definition of the normalization process (Equation 4.6), only low-energy and medium-energy ratios to the high-energy window are formed for this example of three windows. Even though the total counts for the sources are very different, the NORM ratios shown in Figure 4.16(A) and Figure 4.16(B) are very similar to the background ratios. There are, however,  The ratio of the low-energy to high-energy window counts (2000 and 1000, respectively) is 2. A 3-sigma shift would thus be a value of 2.06 for the ratio. Again with the low-energy and high-energy window values given, it takes 60 counts in the low-energy window to give this ratio.

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Radiation Detection and Interdiction at U.S. Borders

Count per second per window, 104 cps

7 6

0.3 0.2 0.1 0

5

Low

4

Medium

High

Pu HEU

3

Fertilizer

2

Tile Background

1 0 Low

Medium Energy window

High

figure 4.15 Count rates in counts per second per energy window in the three energy bins (low, medium, and high-energy) for background, highly enriched uranium, weapons-grade plutonium, fertilizer, and tile obtained by summing the spectra in Figure .. The inset is expanded along the vertical axis to better show the lower intensities of NORM and the similarity of all sources at high energy. Such a gross count approach is not effective in discriminating NORM from SNM.

large differences in the HEU and WGPu ratios compared with the background. This study illustrates the capability of these ratios to discriminate NORM from lightly shielded SNM. The energy windowing algorithm was applied to data from vehicle traffic in commercial traffic lanes with cargo RPMs. The RPM systems used for these measurements have a data acquisition subsystem that allows spectroscopic information to be obtained for subsequent data analysis. The data from these systems was analyzed with either a two-window implementation of energy windowing or a five-window implementation of energy windowing. To simplify the mathematics and explanation, the two-window energy windowing algorithm applied to traffic data is discussed first. The five-window system is discussed later in this section. For this two-window analysis, Equation (4.9) is used, and the difference ratio, Rdif, is found with the equation (4.11): Rdifi =

NL N LB − N L N H N LB N HB

(4.11)

where NL and NH are the counts in the high- and low-energy windows, and NLB and NHB are the counts in the high- and low-energy background windows, respectively. Alarming criteria are typically set on the total gross-count rate and can

Enhancing the Effectiveness of RPM Systems

A

193 WGPu

100

HEU

Low ratio value

Fertilizer Tile Background 10

1

B

WGPu

Medium ratio value

100

HEU Fertilizer Tile Background

10

1

figure 4.16 The ratio of counts in the (A) low- and (B) medium-energy windows to the counts in the high-energy window. Note the different scales between the (A) and (B) graphs. The low-energy window ratio (A) shows both HEU and WGPu being very different from NORM and background. The medium-energy window ratio (B) shows only WGPu differing from NORM and background. The NORM sources and background in both cases have very similar ratios.

additionally be set to alarm based on this ratio of window values (energy windowing alarms). Continuing with the two-window example, a typical NORM (clay material)– carrying vehicle “profile” scan is shown in Figure 4.17(A) and Figure 4.17(B). The data were accumulated from the four sensor panels in a cargo RPM every 0.1 second as the vehicle passed through the portal. The raw-count rate in the high- and lowenergy bins, that is, channels 1–100 (low energy) and 101–250 (high energy), is shown in Figure 4.17(A). The low-energy counts show a large increase as the trailer passes through the portal, with a similar increase in the high-energy bin.

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Radiation Detection and Interdiction at U.S. Borders

However, as shown in Figure 4.17(B), the ratio of the two counts indicates that this commodity resembled the shape of background; that is, the ratio showed no difference from the background ratio, indicating that the radiation source was NORM. This is one example of many vehicles analyzed in this study where NORM caused little change in the ratio profile but would have generated a gross-count alarm. One major effect of a vehicle passing through an RPM is the shadow shielding, or baseline depression effect (see Figure 4.18A), that the vehicle produces to the normal background radiation that is incident on the detectors (Chambers et al. 1974). Because the signal is suppressed below the background level as the vehicle enters the RPM, this effect complicates the detection of radiation as discussed in detail in Section 4.4. The gross-count threshold uses the unsuppressed background collected before the vehicle enters the RPM for comparison to the measured data. Because the radiation in the vehicle must be greater than the suppression plus the threshold values in order to generate an alarm, this effect lowers the sensitivity of the gross-count measurement. An appropriate energy windowing algorithm can help mitigate this shadowshielding effect (Geelhood et al. 2003). In the energy windowing approach, the shape of the energy distribution is compared to the shape of the background instead of the absolute value of the radiation response. Therefore, the energy windowing approach should be much less sensitive to background suppression. It is generally true that the shadow shielding from the vehicle does not change the relative ratio of the counts in the various energy windowing bins. Figure 4.18(A) shows a typical gross-count profile taken from the vehicle study for the highenergy and low-energy bins as a commercial vehicle containing no NORM passes through a four-panel RPM. The profile indicates no presence of radiation in the vehicle, and the background is slightly suppressed as the vehicle passes through the RPM. The ratio of the low-energy to total counts for the vehicle profile is shown in Figure 4.18(B). The ratio is statistically constant across the profile and is unaffected by the suppression of the counts. This is a typical example of the many cases analyzed in the course of this study, all of which appear very stable with regard to shadow shielding. These profiles are indicative that the energy windowing algorithm can mitigate shadow shielding to a large extent. While the above sections focused on an example of a single-vehicle profile to illustrate the advantages of the energy windowing technique, it is also useful to perform a multivehicle analysis and investigate average behaviors. Data collected by a deployed RPM with four sensor panels from over 700 vehicles are shown in Figure 4.19. The temporal signals (background-subtracted net gross-count profiles and net ratio difference profiles) from the passage of these vehicles are overlaid in the figure, with the horizontal axis being time. Because vehicles move at slightly different speeds through the RPM, the data were normalized to a 20 second occupancy to allow comparison. This small time normalization has little effect on the statistics since the count rates are large.

2000 1800 1600 1400 1200 1000 800 600 400 200 0

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Enhancing the Effectiveness of RPM Systems

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figure 4.17 Typical naturally occurring radioactive material alarm vehicle profile from a cargo radiation portal monitor. The upper graph (A) displays raw-count rate for the low-energy (top curve) and high-energy (bottom curve) windows. The lower graph (B) displays the ratio of low- to high-energy counts. While the gross-count signal is large, the ratio shows no deviation from background-like behavior during the passage of the NORM cargo.

Net counts for one top and one bottom RPM panel are seen in the two graphs on the top-left and middle-left, respectively, of Figure 4.19. The sum of the net count rates from all four panels is seen in the graph on the lower-left of the figure. Although most of these net count profiles remain “well behaved” similar to “benign” vehicles, a few of them have enough signal to present substantial elevation of counts over background. Differences between vehicle energy windowing ratios and background energy windowing ratios are shown in the graphs on the upper-right and middle-right for the same panels shown to the left. The background level determined when no vehicles are present is indicated by the dashed line shown in the left graphs; the solid curve is the statistical average of all the data from the 700 vehicles based on cubic spline smoothing. The shadow-shielding effect is clearly seen from this data average curve, which is below the background, for the two panels of this RPM.

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Radiation Detection and Interdiction at U.S. Borders Low counts per 0 1 second

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Time (seconds) 5 4.5

Rat o ow/h gh

4 3.5 3 2.5 2 1.5 1 0.5 0

B

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figure 4.18 Typical nonradioactive cargo vehicle profile showing background suppression due to shadow shielding. The upper graph (A) displays raw-count rate for the low-energy (top curve) and high-energy (bottom curve) windows. The lower graph (B) displays the ratio of low- to high-energy counts, and no shadow shielding effect is seen during the passage of the vehicle.

An approximation of a standard deviation over the vehicle profiles can be obtained from the vertical distribution of the dark band of profiles. For example, select the values at record 6 on the x-axis in the upper left graph of Figure 4.19. The range in the bulk of the data is about 400 counts, and the average (solid curve) is clearly below zero, at about –200 counts. The standard deviation of the estimated mean (standard error) can be approximated with a common statistical approximation method: by dividing this range by 4 (giving 100), then in turn dividing this result by the square root of 700. The estimated standard deviation of the mean is thus about 4 counts. Statistically, and observationally, the average counts (solid curve) are significantly different from zero for the distribution of the individual panels for the 700 vehicles as a result of shadow shielding. Equation 4.11 was applied to the data to form the energy ratio in this study. The pair of graphs in the upper-right and middle-right in Figure 4.19 shows the results.

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The average ratio difference shown (solid curve) in these graphs is also very different statistically from zero, analogous to the estimation method above for standard deviation. The fact that the lower RPM panels and upper RPM panels show depression and enhancement, respectively, of the ratio is indicative that shadow shielding does have some effect on vehicle ratio profiles. However, a simple sum over all panels eliminates the shadow-shielding effect on the average ratio since the upper and lower panels are affected in the opposite direction for the ratios. The results for

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figure 4.19 Total count profiles for each of  vehicles passing through a -panel radiation portal monitor. The two graphs on the top-left and middle-left show net count profiles for one top and one bottom RPM panel, respectively. The upper-right and middle-right graphs show profiles of energy ratio differences between vehicle ratios and background ratios for the same panels shown to the left. The lower-left graph shows the net count profiles for the sum of all four panels. The lower-right graph shows the profiles of ratio differences for the sum of all four panels.

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Radiation Detection and Interdiction at U.S. Borders

the ratios shown in the lower-right of Figure 4.19 come from applying Equation 4.11 to the sum of all panels. The figure shows that the positive and negative deviations of the average ratio from the upper and lower panels, respectively, almost completely cancel each other when summed together. Thus, the energy windowing algorithm shows no shadow-shielding effect when applied to the sum of the panels and, therefore, largely eliminates this important effect from impacting the energy windowing alarm method. This is another significant advantage of the energywindowing method over the gross-count alarm method. Consider the ratio formed by taking the channel-by-channel counts in a net-source spectrum divided by the counts from a background spectrum, both normalized to the same count time. This channel-by-channel ratio will be a constant if the source spectrum has the same shape as background. The result of such an operation on spectra taken from Co, HEU, Ba, WGPu, and depleted uranium (DU) are shown in Figure 4.20. The increasing channel number of the peak in this ratio is consistent with the increasing energy of the dominant gamma rays from these sources. It is clear that above about channel 100 (as seen in the inset of this figure) the spectra are dominated by noise and, although there are important differences, the effective area for energy windowing is found below channel 100. Five window regions were selected by inspection, as demarked by the vertical lines. These windows are labeled 1 through 5 and consist of the channel ranges 1–15, 16–25, 26–60, 61–90, and the remaining channels form Window 5. Clearly there 6 3

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figure 4.20 The channel-by-channel ratio of counts in a net source spectrum to counts from a background spectrum for spectra from Co, highly enriched uranium, Ba, weapons-grade plutonium, and depleted uranium. The circled numbers are the window numbers referenced in the text.

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are many other ways to divide these channels into windows, but these are illustrative to show the capability of energy windowing. A ratio value of 1 indicates the same number of counts in the net-source spectrum as in the background spectrum. As seen in the figure, the Co and HEU sources differ from background within Window 1, the Ba and WGPu sources differ from background within Windows 1 and 2, and the DU source differs from background within Windows 2–4. None of these sources show a significant deviation from background in Window 5. In a variation on the method used in the two-window and three-window approach discussed earlier, each of these ratios is normalized to a sum of higher windows in the injection studies discussed below. Thus, the ratios of the total counts in each of these energy windows to the counts in the remaining windows are formed (e.g., the ratio of counts in Window 1 to the number of counts in Windows 2–5 is formed). This can be expressed by a generalization of Equation 4.11 as Equation 4.12: Rj =

Nj n

Σ Ni

i j +1

−

N Bj

(4.12)

n

Σ N Bi

i j +1

where

Nj = the number of counts in a group of windows with the highest window being the jth window of a multiwindow system, NBj = the number of counts in a group of windows with the highest window being the jth window of the background spectrum, and Ni and NBi = the number of counts in higher energy windows than the jth window.

The ratio formed for the first window is referred to as “Ratio 1,” and so forth. For the five-window example system used here, the four energy window ratios are as follows: Ratio 1 is the ratio of the counts in Window 1 to the combined counts in Windows 2 through 5; Ratio 2 is the counts in Windows 1 plus 2 to those in Windows 3 through 5; Ratio 3 is the combined counts in Windows 2 and 3 to those in Windows 4 through 5; Ratio 4 is the combined counts from Windows 2 through 4 to those in Window 5. Thus far, the use of energy windowing to eliminate the impact of shadow shielding and to discriminate NORM has been discussed. However, the use of two- and three-window systems to distinguish all potential sources of interest relative to NORM and background can be limited. While laboratory experimental data exist for SNM, such data are not available from vehicles at ports of entry. Therefore, to simulate the presence of SNM in commerce, injection studies with actual vehicle data from ports of entry were performed. It is worth reemphasizing that the gross-count threshold is still utilized when the energy windowing method is used. The energy windowing threshold adds very few new NORM-related alarms. The energy-windowing approach cannot improve



Radiation Detection and Interdiction at U.S. Borders

the inherent sensitivity of the RPM, but it can provide targeted sensitivity without adding nuisance alarms by best utilizing “all” available sensitivity. The same targeted sensitivity as energy windowing could be reached with the gross-count threshold approach, but with a severe nuisance alarm rate. It is not feasible to operationally test the energy windowing concept in the field with sources packed into normal cargo loads in randomly selected tractor-trailer rigs. However, it is possible to use a large volume of collected data from deployed RPMs and randomly inject a simulated source into the data during subsequent data processing. The raw RPM data are in the form of energy spectra (as counts in each window) taken at 0.1 second time intervals during the passage of a vehicle (e.g., see Figure 4.17). Counts are injected (added) to each time slice for the study at the appropriate amplitude for the source location. Thus, these “injection study” profiles, randomly selected from archived vehicle data, now contain some amount of simulated targeted material. Varying the number of injected counts from the source, the simulation is repeated multiple times. The resulting injected spectra are then analyzed to determine the system response. As seen in Figure 4.21 through Figure 4.23, the results can be displayed as detection probability versus the number of injected counts. Threshold nuisance alarm rates less than 1.5% and 0.5% for gross-count and energy windowing, respectively, were chosen for this study. An energy windowing threshold consistent with a value of 4 to 5 standard deviations above zero was used. For one injection study, records were used from a random selection of approximately 20,000 vehicles with the source injected at random locations along the vehicle record. In these cases, the SAIC Delta statistics were used; the window deltas were calculated as the injected source was varied over several cases, including Co,  Ba, and DU. For Co, Figure 4.21 shows that Delta 1 is the most sensitive; that is, there is 95% detection at the lowest number of injected counts; Delta 2 is slightly less effective; and Deltas 3 and 4 are insensitive even relative to the assumed grosscount threshold. This is to be expected, since about 70% of the total counts for Co are in the first window. Figure 4.22 shows that Delta 2 is the most sensitive for Ba. Deltas 1 and 4 and the gross-count threshold are much less effective, while Delta 3 is intermediate in effectiveness. This is to be expected, as most of the total counts for Ba are in the first and second windows (see Figure 4.20). Figure 4.23 shows that for DU, Delta 3 is the most sensitive, followed by Delta 4, but Deltas 1 and 2 are totally ineffective. The gross-count approach is less sensitive than the optimal energy-windowing delta statistic to the presence of the targeted material. Laboratory measurements were performed with sources to increase confidence in the accuracy of the injection studies. Figure 4.24 shows the result from one such measurement with 2 kg of DU walked through an RPM. The figure shows that the delta statistic increases and declines as the source passes through the RPM. In this example, Deltas 3 and 4 are sensitive to the presence of DU in the RPM, but Deltas 1 and 2 are insensitive to DU. This is consistent with the window placement as displayed in Figure 4.20. Refinements in the algorithm can result in further performance improvements.

Enhancing the Effectiveness of RPM Systems 100

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60

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figure 4.21 Detection probability versus number of injected counts for Co. Delta  is seen to be the most sensitive; that is, % of vehicles are detected at the lowest number of injected counts with Delta .

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figure 4.22 Detection probability versus number of injected counts for Ba. Delta  is seen to be the most sensitive.

The NORM discrimination power of the energy windowing ratio is illustrated by another injection study performed with approximately 20,000 vehicles. The NORM alarm rate for the gross-count threshold was 2.65% (530 vehicles), whereas the energy windowing ratio statistic resulted in only 0.37% NORM alarms (74 vehicles), for a greater operational sensitivity to targeted sources in this study.

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figure 4.23 Detection probability versus number of injected counts for depleted uranium. Delta  is seen to be the most sensitive.

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Delta 3

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figure 4.24 Results from a single measurement with  kg (. lb) of depleted uranium walked through a radiation portal monitor. Curves are displayed as delta sigma (standard deviations) above or below zero for four delta statistics versus sample observation time.

For an extremely busy border crossing, 20,000 vehicles might represent a few days of operation. These small percentages of vehicles potentially sent to secondary inspection do not seem large but can be significant for a busy port. This work, and other experiments and injection studies not presented here, show the effectiveness of a multiwindow energy windowing implementation.

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4.2.4 summary of energy windowing studies Energy windowing can discriminate against NORM while maintaining sensitivity to materials of interest, specifically SNM in certain scenarios, as shown by experimental measurements and vehicle traffic studies. A simple three-window energywindowing scheme maintains excellent sensitivity to modest amounts of SNM while allowing NORM discrimination. The energy windowing ratio from a sum of panels reduces the loss of sensitivity due to shadow shielding. Energy windowing is always used in conjunction with a gross-count threshold to ensure alarm capability on very active signatures. The positive test results for energy windowing allow for the use of ratio and gross-count criteria together to minimize the number of nuisance alarms at ports of entry while maximizing the ability to detect the targeted sources. To date, the studies of energy windowing have focused on NORM discrimination while ensuring detection of SNM. More investigation is required for other targeted material. Since gamma emissions from patients with radiopharmaceutical treatments, the other major category of nuisance alarms, are mostly in the low-energy region of the spectrum, energy windowing does not discriminate radiopharmaceuticals from SNM. There may also be benefit from the energy-windowing technique for applications outside of commercial cargo screening, such as screening packages, or large area searches with mobile vehicles.

4.3 Other Intelligent Algorithms for Plastic Scintillator Gamma-Ray Detectors James Ely, Richard Kouzes, John Schweppe, Edward Siciliano, Denis Strachan, and Dennis Weier For the energy window algorithm, the ratio of counts in specific energy ranges (windows) is used to detect specific target materials. In this section, other algorithms are discussed that might be used to differentiate the target from the materials that cause nuisance alarms or other effects that reduce the overall ability to detect targeted materials.

4.3.1 absolute threshold algorithm Typical RPMs alarm when a radiation reading exceeds a calculated threshold that is independent of fluctuations such as shadow shielding. As discussed earlier, alarm thresholds are usually set as a multiple of the background standard deviation value. Another possible algorithm could use an absolute count rate above a background rate, rather than a multiple of sigma. This “absolute threshold” algorithm has the advantage of being unaffected by changes in the background count rate.



Radiation Detection and Interdiction at U.S. Borders

For an absolute count method, alarm thresholds are set according to the following relation in Equation (4.13): A Bt Ct

(4.13)

where A = threshold count value, C = a user-input count rate value (cps), B = average background count rate (cps), and t = time interval over which the survey is being performed. For example, if the latest detected background produced B = 100 cps, the survey time for a container is t = 1 second, and the user specifies 100 counts over the measured background as the alarm threshold (C), an alarm threshold of 200 counts would be calculated and used for that container.

4.3.2 cross-talk suppression in multilane deployments Both commercial and noncommercial vehicles are often checked through several lanes of traffic equipped with multilane detector systems. The back sides of these detector systems are shielded to reduce cross talk between lanes. Obviously, the front face of the detector cannot be easily shielded from vehicles in an adjacent lane. A vehicle with a radioactive source may be seen by the unshielded front face of a detector in an adjacent lane and, although the level of radiation in an adjacent lane detector system will be reduced by the distance factor, it may still cause an alarm. This is especially the case if the source is quite active, such as a patient who has recently had a medical procedure in which radioactive pharmaceuticals were used. Two approaches may be used to reduce these cross-lane alarms. The first is a global analysis of all vehicle lanes at a particular site to identify the lane in which the peak count rate was located. This global analysis could be used to suppress alarms in lanes adjacent to the lane with the peak response. Thus, only a single vehicle would be sent to secondary screening rather than all vehicles in lanes for which alarms were triggered. In the second approach, the two detectors in a single lane could be compared. If the radiation is from a vehicle in an adjacent lane, the response in the two detectors should be quite different because one of the detectors will be shielded. The latter analysis should be capable of determining the difference between an alarm from an adjacent lane, as well as the location of the person in the vehicle (passenger or driver side).

4.3.3 vehicle speed and detector measurement time Background radiation is continuously measured with the RPM systems whenever a vehicle is absent. When the presence sensors detect a vehicle, the system starts a measurement mode. This mode counts the radiation on a short time scale (about 0.1 second intervals) and compares the counts to the threshold. To increase

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statistical significance of the data and reduce false alarms, most detector systems average the number of counts over several of the 0.1 second time intervals. This is typically a “running” average, with each new 0.1 second measurement added while the oldest 0.1 second measurement is dropped from the average. The measurement period (the number of the 0.1 s measurements in the running average) is variable. For a moving source, the amount of radiation detected varies over time as the distance between the source and detector changes. Thus, the average count rate over the measurement period could be very different than over an individual 0.1 second measurement period. For example, the intensity of a radiation source recording 100 cps in the peak 0.1 second time interval would only be ~50 cps, averaged over a 1 second period for a source moving at 15 mph. Including the vehicle speed and detector measurement time into the alarm threshold algorithm would reduce the possibility of missed sources in rapidly moving vehicles and allow a threshold to retain a uniform meaning independent of vehicle speed. The speed can be used to adjust the threshold value to correct for the approximately square root dependence of the signal on measurement time. Similar speed-dependent algorithms have been utilized in deployed systems. It is also possible to have a speed alarm activated on RPM systems. This allows for an alarm to be generated if a vehicle exceeds a preset speed during transit of the RPM in order to guarantee adequate screening. Vehicles generating speed alarms would require additional processing.

4.3.4 tracking algorithms for background suppression from vehicles Commercial goods are primarily transported into the United States through seaports and over land border crossings. In both cases, the goods are usually contained in large transportation carriers, either intermodal containers (seaport terminals) or large trailers (land border crossings). These carriers and the tractors that pull them are constructed from dense materials (engine, axles, frame, etc.) that shield and reflect gamma rays. When one of these vehicles passes through a portal monitor, the natural background radiation is suppressed. In addition, the cargo may contribute to the background suppression. This background suppression varies with the type of material in the load and the size, make, and shape of the vehicle but can be significant, as shown schematically in Figure 4.25. At some locations, a large fraction of the commercial vehicles carry items associated with manufacturing industries that are predominantly metallic, and they can suppress the background an average of about 25%. This background suppression, therefore, requires a more active radiation source to trigger a simple constant alarm threshold and needs to be taken into consideration in an alarm algorithm. An algorithm that tracks the background count rate, or calculates the change in background based on consist data (for example, as the vehicle passes through the RPM), could have a higher sensitivity than a fixed, flat threshold value. Such approaches are



Radiation Detection and Interdiction at U.S. Borders 3200

Counts/seconds

3000 2800 2600 2400 2200 2000 0

1

2 Time (sec)

3

4

5

figure 4.25 Schematic of radiation portal monitor background count rate during a commercial vehicle passage, illustrating the dense-load background suppression effect.

complicated by the huge variation in the shape of the response profile curves to the variety of cargo routinely observed. Such background tracking methods have to date only been used in research.

4.3.5 spatial distribution of naturally occurring radioactive material versus point sources Another consideration in commercial loads containing radioactive material is the distribution of the radioactive sources in the load. If the radioactive material is uniformly distributed in the load, there is a high probability that it contains only NORM. However, if the radiation appears from a point-like source and produces a sharp peak in the truck profile, as illustrated in Figure 4.26, then there are several possibilities. The load may contain more than one type of product, some of which may be NORM material that causes a sharp peak in the profile. If the sharp peak occurs in the cab area of the tractor-trailer, a medical radioisotope signal from the driver or passenger may be the cause. A sharp peak could also be from targeted radioactive material. An alarm algorithm capable of distinguishing between uniform and nonuniform (peaked) radiation profiles could be useful in the alarm decision process to pass NORM without passing point-like sources.

4.3.6 spatial distributions for passenger (noncommercial) vehicles Passenger vehicles generate nuisance alarms primarily from medical radioisotopes, as the amount of NORM material that a noncommercial vehicle can carry is rather limited. Approximately 1 vehicle in 1300 will carry a patient who has had a recent medical treatment in which radiopharmaceuticals were used (see Section 2.4). Medical isotopes tend to produce a greater response in detectors than do NORM sources, increasing the chance for interlane cross talk. Medical radioisotopes appear

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3200 Counts/second

3000 2800 2600 2400 Point-like source Uniform source

2200 2000 0

1

2 3 Time (sec)

4

5

figure 4.26 Schematic of count rate of two commercial vehicle passages, illustrating the difference in the profiles of a uniform source and a point-like source.

as point sources in the profile, as does targeted material. Therefore, distribution information will have limited usefulness in passenger vehicle alarm decisions. However, decisions based on energy analysis could be useful in these cases, assuming medical isotopes can be distinguished from targeted materials. Because of the small amount of background suppression from most passenger vehicles, background suppression is not as large a factor in the alarming algorithm as it is for cargo vehicles. In secondary screening, detection of medical radioisotopes in a person with a recent treatment is relatively easy because the person can be isolated from the vehicle.

4.3.7 spatial optimization Typically, RPMs alarm when the count rate exceeds a predetermined threshold during vehicle screening. The threshold corresponds to a measure of gross counts over a time interval. The gross count yield is typically calculated as a rolling sum of measurements, each 0.1 second in duration. If the rolling sum exceeds the threshold, an alarm is triggered. While a rolling sum is used to reduce statistical variance, it may not be optimum. In this section, an optimal value for the rolling average duration is evaluated. The highest sensitivity of the RPM occurs when a rolling average is calculated over an interval that maximizes the signal-to-noise ratio of the profile in question. The maximum signal-to-noise ratio varies depending on the speed of the vehicle and thus is not identical for all vehicles. Hereafter, the rolling sum interval that maximizes the signal-to-noise ratio is referred to as the “optimal sum interval.” The length of the optimal sum interval depends on cargo properties (e.g., whether the source is point-like or extended), and on the material surrounding the source (e.g., shielding). Figure 4.27 schematically displays one half (one panel) of a typical RPM setup. The horizontal line marks the centerline between the two panels. The vehicle (referred to here as the “source”) moves from left to right with speed v(t).



Radiation Detection and Interdiction at U.S. Borders

X V

r

d=2m

θ

figure 4.27 Radiation portal monitor geometry.

RPM

The notation r(t) represents the distance from the RPM center to the source, while x(t) is the distance from the center axis to the source. The variables x(t) and r(t) are related by the constant distance, d, which is typically about 2 m (6.5 ft). Sources normally pass through at slow, but variable speeds, while the count rate is measured. A typical speed is 3 mph (1.3 m/s). In this discussion, the vehicle speed is assumed to be constant, and the x position of the source is correlated in time according to the following relationship in Equation (4.14): x(t ) = x o v ⋅ t

(4.14)

where xo = the x-position at t = 0 t = the elapsed time since position xo. Plotting the rolling sum as a function of time gives the temporal profile of the passing source. Because the RPM measures counts in relatively short time bins of 0.1 seconds, the temporal profile closely approximates a spatial profile. More explicitly, the RPM effectively measures the radiation yield at various positions. Because vehicles can proceed at various speeds and thus have different temporal profiles even for identical sources, it is useful to convert the temporal profile into a spatial profile. The remainder of this discussion focuses on profiles in terms of positions (spatial profiles), which can easily be converted into time profiles with Equation 4.14. For a point source, the maximum rolling-sum value corresponds to the time when the center of this maximum and the source reaches the distance of closest approach (d). During this time, the average source-to-detector distance across the rolling-sum interval reaches a minimum, and the rolling sum extends through x-positions symmetrically about the RPM panel midpoint. These values are referred to as -xmax and xmax, and thus, the total sum interval extends a distance

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209

of 2 xmax. The value xmax is also commonly referred to as the limiting position, as the rolling sum is limited to its specified range on each side of the distance of closest approach. Determining the optimal value for the sum interval will be discussed in the next section, but it is useful to introduce a few concepts here. For each speed, v, a unique optimal sum interval in time exists, but for any v the optimal sum interval in position is identical. To prevent confusion, the optimal sum interval in position is referred to as the optimal limiting position. If only a source is present without background, the optimal limiting position would be infinite because including counts from each position would increase the total number of counts without any cost. In reality, a background is present, which dilutes the source strength. Adding a small number of counts to the sum from a distant source position increases the background counts far more than it does source counts. The impact of choosing an optimal limiting position for calculating a rolling sum is shown by the curve on the left side of Figure 4.28, which displays the sigma multiplier value (m) for various limiting positions. The sigma multiplier value is a measure of the detection sensitivity and is related to the number of counts in the rolling sum shown in Equation (4.15): m=

T

B B

(4.15)

where T = total number of counts in the sum, and B = number of previously measured background counts for the same interval. An m above a predetermined threshold value results in an RPM alarm. In this example, the source strength at closest approach was 1000 cps added to a flat background of 4000 cps. If assuming a value of m equal to 40, the RPM will alarm only for limiting x-positions in a small range that maximize the detector’s sensitivity. This calculation assumed a spatial profile model shown by the upper plot in the graph on the right side of Figure 4.28 . At the optimal limiting position (the maximum in the curve of the left graph), the sum interval extends over the marked portion of the profile (upper plot in right graph). If the limiting position is too small or too large, the sigma multiplier value falls below threshold, and the source does not trigger an alarm. For example, if the threshold is set at 35, the sigma multiplier of the profile below exceeds the profile only for xmax values between approximately 1 m (3.2 ft) and 4 m (13 ft). In this case, the RPM would alarm. For xmax values below 1 m (3.2 ft) or above 4 m (13 ft), the RPM would not alarm. In this example, it is crucial to calculate the rolling sum within a specific  The sigma multiplier of 40 is used for example purposes only and does not reflect the actual settings of deployed systems.

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Radiation Detection and Interdiction at U.S. Borders

range of limiting positions. In performing a calculation such as producing the data in Figure 4.28, it is necessary to assume a model spatial profile. These models are discussed in the following sections. The simplest model for the observed count rate in an RPM as a function of x-position is to assume an inverse square of the distance dependence (r–), that is, only solid angle effects. For simplicity, one may assume the detector and source are point receiver and point source, respectively. If, at the distance of closest approach, the net count rate in the detector is No, then, as the source moves farther away, the net count rate at any x-position, N(x), decreases by the ratio of the square of the closest approach distance to the present distance, that is, by the relative solid angle: 2 ⎛ ⎞ d d N (x ) = N o ⎛ ⎞ = N o ⎜ 2 2 ⎟ ⎝r⎠ ⎝ d x ⎠

2

(4.16)

The exponent of (d/r) will hereafter be referred to as the power of the model. This is the “power = 2” model. In reality, the detector has a front surface area that directly faces the source at x = 0, the others being shielded. In this geometry, a correction to the above model accounts for the projected area in the direction of the source, which approximately has a cos(θ) dependence. This implies that: 2 2 ⎛ ⎞ d ⎛ d⎞ ⎛ d⎞ d N (x ) = N o ⋅ cos ( ) = N o ⋅ = No ⎜ 2 2 ⎟ ⎝r⎠ ⎝r⎠ r ⎝ d x ⎠

3

(4.17)

This model is the “power = 3” model. These two models provide an analytical expression for the count rate as a function of position based on simple assumptions. A more sophisticated picture of

Sigma multiplier values for various xmax positions

Example spatial profile 6000

45 40

5000

35 4000 CPS

m

30 25 20

2000

15 10

Total profile Source profile Background profile Optimal rolling sum interval

1000

m values

5 0

3000

Optimal xmax value

0

2

4 6 Position (m)

8

10

figure 4.28 Effect of limiting position on m values.

0 −10 −8 −6 −4 −2 0 2 4 Position (m)

6

8 10

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211

the spatial profile can be obtained from stochastic calculations with codes such as MCNP (Briesmeister 2000), with which the energy deposition of various gamma rays in the RSP and their penetration through the front casing and peripheral shielding can be calculated (see Section 4.1). All MCNP calculations of point sources presented here have a source geometry of a 10 mm sphere located along the center line of the RPM (Figure 4.27). Unless otherwise noted, the gamma-ray energy distribution is from Ba with the computed yield. This source was selected to match the source used to obtain the experimental results discussed below. The profiles for all bare sources, regardless of energy, are approximately equal, as effects such as attenuation in the air and effective detector thickness change little for the positions considered in this report. In all cases, the yield at closest approach was normalized to 1. Figure 4.29 compares the results from MCNP calculations with the models of power = 2 and power = 3. The power = 3 model has a spatial profile very similar to the MCNP simulation, but with significantly higher yield tails at positions greater than approximately 2 m. This result is consistent with the line of sight allowed by the shielding encasement. This comparison demonstrates that, for most applications, the power = 3 model adequately describes the bare source profile, and detailed calculations are unnecessary. Although unshielded sources are common at border crossings, and therefore important to understand, shielded sources pose a much greater threat. They are much more difficult to model and quantify because of the varying types and thicknesses of shielding. Insight into how shielding might conceptually affect the above spatial profile is discussed below. The results of MCNP calculations show that shielding plays an important role in the shape of the detected radiation profile as a function of distance from the centerline between RSP faces. Each shielding case requires a different power Spatial profile models 1.0

MCNP Power = 2 Power = 3

0.8

Yield

0.6

0.4

0.2

0.0 −5

−4

−3

−2

−1

0

1

Position (m)

figure 4.29 Comparison of spatial profile model.

2

3

4

5

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Radiation Detection and Interdiction at U.S. Borders

model, as shown in Table 4.2. The profiles narrow with increasing shield thickness, shield density, and decreasing gamma-ray energy. The degree to which a profile narrows is strongly dependent on the thickness and the density. For energies above 200 keV, the profiles do not spread rapidly. Due to the exponential nature of radiation attenuation, the “power” shown in Table 4.2 grows large for thick-shielding cases representing the narrowing of the observed spatial distribution. In order to verify the accuracy of the spatial profile simulations, experiments were performed at PNNL with an RPM and Ba source. A small truck was driven through the RPM at close to 5 mph (2.2 m/s) while the count rate was recorded at 0.1 second intervals. The data were not a running sum. Two tests—each with three trials—were performed: one unshielded, and the second with a 0.91 m thick paper shield that approximates an average cargo in a tractor-trailer rig. Figure 4.30 shows an example temporal profile (time) for the unshielded Ba source. Each temporal profile was converted into a spatial profile with the measured speed. The background was subtracted from the total counts and the resulting counts normalized to 1. An average spatial profile was then calculated, centered about its maximum. Figure 4.31 compares the experimental data with the MCNP results. The MCNP simulations agree very well for the bare source case but differ somewhat for the paper wall, which had very poor statistics. For the purposes of this study, the simulations sufficiently reproduce the experimental data. After establishing the spatial profile of any given configuration, the next step is to maximize the detection sensitivity. Although the sensitivity is frequently measured in terms of the sigma multiplier value (m), a canonical quantity is the signalto-noise ratio (S:N): S S

(4.18)

B

where S = the number of signal counts and B = the number of background counts.

table 4.2 Power values for various source shield configurations Model

Bare source simple theory Bare source (MCNP) 2.54 cm iron plate with 133Ba 7.62 cm iron plate with 133Ba 7.62 cm paper wall with 133Ba 5.08 cm iron plate with 1000 keV 5.08 cm iron plate with 400 keV 5.08 cm iron plate with 200 keV

Power

3 3.1 4.0 5.9 6.9 4.2 5.0 6.3

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213

Example temporal profile of an unshielded 133Ba source 4500 4000 3500

Counts

3000 2500 2000 1500 1000 500 0 0

1

2

3

4

5

Time (sec)

figure 4.30 Example of a measured temporal profile.

Comparision of MCNP simulations and experimental results 1.0

Unshielded - MCNP Unshielded experiment Paper wall - MCNP

0.8

Paper wall - experiment

Yield

0.6

0.4

0.2

0.0 −5

−4

−3

−2

−1

0

1

2

3

4

5

Position (m)

figure 4.31 Comparison of simulated and measured spatial profile.

This requires a background model that, for this discussion, is assumed to be constant. The source profiles are based on the power models described above. This leaves three parameters for calculating signal-to-noise ratio of any given configuration: the source strength at closest approach (Smax), the background count rate (B), and the distance over which the profile will be integrated, i.e., the limiting



Radiation Detection and Interdiction at U.S. Borders Signal to noise ratio vs. position: power = 3 60

50

SN

40

2000 CPS 1000 CPS 500 CPS 100 CPS

30

20

10

0 0

1

2

3

4

5

6

xmax (m)

figure 4.32 Effect of source strength on optimal limiting position, which is the distance over which the signal is integrated.

position (xmax). When calculating the signal-to-noise ratio for a given limiting position, the total counts from the signal and background correspond to the integral over their respective distributions from -xmax to xmax. The integral of both the source and background will increase with xmax. The contribution from the signal will initially increase relative to the background as more of the peak is contained in the integral. At some position, this effect reverses and the signal strength quickly decreases. Therefore, one would expect to see a maximum in the signal-to-noise ratio before it decreases to zero at infinite limiting positions. The signal-to-noise ratios were calculated versus limiting positions (xmax) for a few signal strengths, assuming a typical background of 4000 cps and a power = 3 model (i.e., the unshielded geometry). The results are shown in Figure 4.32, with the signal strength at closest approach noted in the legend. The optimal limiting position exists where the signal-to-noise ratio is maximized. As shown in Figure 4.32, this occurs at approximately the same limiting position for both the high and low source strengths (about xmax = 2.2 m). From these results, the optimal limiting position is approximately independent of source strength. Similarly, the optimal limiting position is only weakly dependent on the background. For example, if Smax is 1000, the optimal limiting position decreases by only 8 percent when the background changes from 2500 to 8000 cps. For most practical purposes, one can assume that the optimal limiting position is not a function of the background rate. In contrast, Figure 4.33 shows that the optimal limiting position decreases with increasing shielding and speed. The optimal limiting position is more commonly

215

Signal to noise ratio vs. sum interval: power = 3 30

Signal to noise ratio vs. sum interval: power = 10

25

25

20

20

30

SN

SN

Enhancing the Effectiveness of RPM Systems

15

1 mph 3 mph 5 mph

15 10

10 1 mph 3 mph 5 mph

5

5 0

0 0

5 10 15 Sum interval (seconds)

20

0

5 10 15 Sum interval (seconds)

20

figure 4.33 Optimal sum intervals.

discussed in terms of the speed-dependent optimal sum interval. In Figure 4.33, the signal-to-noise ratio is plotted against the sum interval for various speeds and for unshielded (left; power = 3 model) and shielded (right; power = 10 model) sources. In both graphs, Smax is 1000, and B is 4000. Two trends are apparent. First, the maximum signal-to-noise ratio is equal regardless of speed, but the maximum occurs at different sum interval values. Second, the maximum signal-to-noise ratios shift to the left for higher speeds. These facts demonstrate that, regardless of the profile, each vehicle should be corrected for its speed. Any setting maximizes the signal-to-noise ratio only for a small set of speed–power combinations. For example, 2 seconds is the optimal sum interval for a speed of approximately 5 mph (2.2 m/s) and a bare source (power = 3). Up to now, the signal-to-noise ratio of shielded and unshielded sources has been discussed. Although this is an important goal, the challenges of present portal monitoring environments are not detection sensitivity. The presence of NORM creates operational limitations and, as a result, alarm thresholds for cargo applications are determined not by the detector sensitivity, but by the amount of NORM that can be flagged. Consequently, a discussion of extended source (NORM) profiles is warranted. Most instances of NORM produce extended source profiles that are vastly different from point-like sources. The spatial profiles of extended sources compared to those of point sources and how to distinguish them are illustrated by a simple example: a 12 m (39 ft)–long trailer containing a uniform load of salt, principally  K. In this case, the source strength, and thus the spatial profile, would be approximately constant throughout a limiting position optimized for point sources. 

The optimal sum interval (tmax) is related to the optimal limiting position by the following relation:

t max =

2 ⋅ x max . v



Radiation Detection and Interdiction at U.S. Borders Point vs. constant sources: Smax = 1000 cps, B = 4000 cps 70 Point source 60

Constant source

Signal:Noise

50 40 30 20 10 0 0

1

2 3 Limiting position (m)

4

5

figure 4.34 Signal-to-noise ratio for constant (naturally occurring radioactive material) sources.

The graph in Figure 4.34 compares the signal-to-noise ratio as a function of limiting position for constant and bare point sources. In Figure 4.34, a background of 4000 cps and maximum source strength of 1000 cps for both the constant and point sources were assumed. Although the maximum signal-to-noise ratio for the point source occurs at approximately 2.1 m (6.8 ft), the constant source signal-to-noise ratio continues to rise. For limiting positions smaller than 2.1 m (6.8 ft), not only is the signal-tonoise ratio from the constant source larger, but it is also increasing at a faster rate. In an operationally limited environment, the greatest detection efficiency for point sources occurs when the point source sensitivity relative to the constant source sensitivity is maximal; in this case, the figure of merit (FOM) is shown in Equation (4.19): FOM =

S N point source S N constant source

(4.19)

The FOM for the above example is shown in Figure 4.35. At first, the FOM is relatively flat, but then it begins to fall. At the maximum signal-to-noise ratio for the point source, the FOM falls to 0.7. The results in Figure 4.35 show that, for NORM-limited sources containing a point source of a target material, the smallest limiting position that produces reasonable statistics should be used to maximize the point source detection sensitivity. In most applications, maximizing the signal-to-noise ratio also maximizes the detection sensitivity. Although signal-to-noise ratios are instructive, a more appropriate quantity in portal monitoring applications is the minimum detectable

Enhancing the Effectiveness of RPM Systems

217 Source sensitivity figure of merit

1.0

0.8

SN

0.6

0.4

0.2

figure 4.35 Figure of merit for point source sensitivity.

0.0 0

1

2

3

4

5

Limiting position (m)

amount (MDA) of a sample. The MDA is the source activity that creates enough counts in the detector to surpass a predetermined threshold, which is expressed here in terms of the number of standard deviations above background (see Equation 4.4). The effect of optimizing the sum intervals was evaluated by assuming a typical scenario of a point-sized bare Co source moving at 1.3 m/s (3 mph) through a single-panel RPM. In an idealized scenario, with a background count rate of 4000 cps and threshold of 10σ, the MDA with a 1-second sum interval is approximately 0.27 MBq (7.2 μCi). If one uses the optimal sum interval of 3.2 seconds, the MDA drops to 0.20 MBq (5.5 μCi), a 25% improvement. For the papershielded source analyzed previously, optimizing the sum interval lowers the MDA by 10%. This latter result may be generalized to most shielded sources because of their narrow spatial profiles. Decreasing the detector-to-center line distance substantially affects both the detection sensitivity and the spatial profile. The left-hand graph of Figure 4.36 compares the spatial profiles for distances of 2 m (6.76 ft), 1.8 m (6 ft), and 1.5 m (5 ft). The detection efficiencies at closest approach were normalized to one for the 2 m (6.76 ft) case. For shorter distances, the spatial profile narrows and a smaller optimal time sum interval results. If one assumes a 3 mph (1.3 m/s) source speed, the optimal sum intervals are 3.1, 2.7, and 2.2 seconds. In the right-hand graph of Figure 4.36, the MDAs calculated at the three separations are compared along with the optimal sum interval for each spacing. Significant improvement in the MDA results from optimizing the sum interval for a given configuration.



All MDAs presented in this report correspond to a 5% false-negative probability.

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Radiation Detection and Interdiction at U.S. Borders

Spatial profile for varying “d” values (Bare point source)

MDAs vs. source-to-detector spacing (Bare point source)

2.0

Yield

1.6

57CO MDA (uCi)

1.53 m 1.83 m 2.06 m

1.2 0.8

7.2

6 5 4 3

5.5 4.6 3.7

2 1 0

0.4 0.0 −5 −4 −3 −2 −1 0 1 2 x-position (m)

10 9 8 7

3

4

5

0

2

2.06 m 1.83 m 1.53 m Present setting Optimal

4 6 Sum interval (s)

8

10

figure 4.36 Comparison of (a) spatial profiles and (b) MDAs for varying d values (Figure .).

As discussed above, the methodology for calculating the rolling average of the counts from passing vehicles can be optimized. According to RPM field data, the average vehicle speed is approximately 1.3 m/s (3 mph). At this speed, the optimal sum interval is 3.2 seconds for a bare point source. In the case of the paper-shielded source (a surrogate for the average cargo), the MDA drops by approximately 10% at its optimal sum interval of 1.9 seconds. For slower speeds, the MDAs would show more improvement; for faster speeds, the improvement would be smaller. Ideally, the spatial profile for each vehicle should be calculated from the vehicle temporal profile and speed. If this were done, the signal-to-noise ratio could be maximized for each vehicle by integrating the spatial profile over the optimal limiting distance. The optimal limiting distance for a bare source is 2.2 m and, for the paper-shielded source analyzed in this report, it is 1.2 m. In NORM and point source combinations, maximizing the signal-to-noise ratio does not maximize the point source sensitivity. The smallest sum interval that produces reasonable statistics is optimal.

4.4 Baseline Suppression Charles Lo Presti, Dennis Weier, Richard Kouzes, and John Schweppe The gamma radiation–induced count rates observed for vehicles passing through RPMs are generally suppressed relative to the no-vehicle background gamma-ray count rates. This is caused by the shielding effect of the vehicle and its cargo as it moves through the portal (Ely et al. 2006; Geelhood et al. 2003; Lopresti et al. 2005). The baseline is suppressed because the vehicle and its cargo shield the detector panels from the background gamma rays emanating from the local

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219

environment, the roadway, and concrete and brick structures in proximity to the RPM. This effect is also known as shadow shielding. The configuration and density of a vehicle and its cargo influence the observed baseline suppression, as discussed earlier. The gaps between a tractor and trailer often show up as a peak(s) in the count profile because the shielding is reduced in the gap, resulting in a “double-dip” or bimodal profile. This double-dip pattern may also be related to the vehicle axles. Baseline suppression maxima may be 30% or more below the no-vehicle background count rates. In previous studies, baseline count reductions on the order of 10% were observed for the smallest vehicles and up to 27% for tractor-trailers (Fehlau et al. 1983). RPMs suffer from suppression of background by shadow shielding because it complicates and limits or reduces the operational sensitivity of some alarm algorithms, including those based on gross- or net-count-rate thresholds. This section summarizes a study conducted by Lopresti et al. (2005) that addressed baseline suppression for various port-of-entry sites, different RPM systems, vehicles in narrow and wide lanes, driver-side versus passenger-side panels, and top versus bottom panels. Results for a one-RPM model are discussed in this section. All RPM systems examined in this study used four similarly sized operational PVT panels for detecting gamma-ray gross-count events for narrow lanes. Cargo RPMs have two panels on the driver side, arranged vertically one above the other, and two panels on the passenger side, arranged in the same manner. For wide lanes, the RPMs have eight panels wired in pairs to form four sets of detectors positioned similarly. The observed shadow shielding effect was also observed to be dependent on the position of the detector panels relative to the vehicle, as defined in terms of driver or passenger side and by top or bottom panels. When the RPMs are unoccupied, the systems continuously count background. A background count rate applicable to each vehicle is established by averaging the background over the several minutes immediately before a vehicle is detected entering the RPM. Once occupied by a vehicle, the RPM system counts gamma rays detected by each panel to establish average count rate time profiles as the vehicle is driven through the portal, typically at a nominal speed of 1.3 m/s (3 mph). The time period for vehicle count profiles varies from a few seconds up to 30 seconds depending on vehicle length and transit speed. Collections of operational background and vehicle profile count data from five sites (identified here as sites A, B, C, D, and E) were obtained for analysis where RPMs from a single manufacturer were installed. Because RPMs are designed to accommodate varying numbers and configurations of traffic lanes, data from three narrow lanes come from sites A and D, three narrow lanes and one wide lane from sites B and E, and two narrow lanes from site C.

 In multitrailer rigs, another gap is expected between the trailers, but the majority of vehicles are tractor single-trailer rigs.

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Radiation Detection and Interdiction at U.S. Borders

To quantify background suppression, RPM gross-count data were analyzed as 1 second running sums of vehicle data, and the background data were taken during several seconds prior to vehicle entry. The running sums, expressed in counts per second, were compared to the background count rates by calculating percent baseline suppression (PBS) for each running 1-second sum as shown in Equation (4.20). PBS =

Vehicle Count − Background Count Background Count

×100

(4.20)

Because the background count rate is generally larger than the count rate when a vehicle is present (because of shadow shielding), the PBS value on-vehicle is generally negative unless a gamma-ray emitter is present. The arithmetically smallest PBS value for any vehicle is the maximum baseline suppression for that vehicle. For this study, two statistics—the arithmetic average PBS and the minimum PBS, representing average and maximum baseline suppression, respectively—were calculated for each gross-count running-sum profile as detected at each panel for each vehicle. Collections of these vehicle statistics were used to characterize baseline suppression for each port of entry, RPM system, lane category (wide/narrow), and panel (driver/passenger-side orientation and top/bottom panel position). Analyses focused more on maximum suppression because this value is often used when evaluating gross-count alarm algorithms. Vehicle collections that had obviously elevated 1-second running sum count profiles well above background were deliberately excluded, whether or not they actually generated an alarm. This was intended to eliminate strong profiles of NORM that would obscure the baseline suppression effect. In addition, the middle 80% of vehicles in terms of transit time were analyzed in an attempt to obtain a more homogeneous set of profiles. This would eliminate the smallest vehicles, vehicles with rapid transit times, and vehicles that actually stopped in the portals for several seconds.

4.4.1 vehicle profiles Ensemble graphs of vehicle PBS profiles (Equation 4.20) for all four panels from all narrow lanes pooled at Site A are shown in Figure 4.37. These graphs represent 979 vehicles in narrow-lane RPMs with transit times ranging from about 5 to 21 seconds, the middle 80% of profiles, without regard to vehicle type or cargo. All ensemble graphs are shown on the same PBS and profile duration scales to facilitate their comparison. The background reference is shown as a dashed line at PBS = 0%. Each profile of running sums at 0.1 second intervals is rescaled in time to 20 seconds to create an ensemble of superimposed profiles in a fashion that normalizes vehicle transit speed and length. For each panel, a spline fit to the rescaled profile data is shown as a solid white line to indicate, in a general sense, the average vehicle

Enhancing the Effectiveness of RPM Systems

221

−10 0 10 −30 −50

−30

Percent suppression

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Passenger side top panel

−50

Percent suppression

Driver side top panel

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5 10 15 20 Adjusted transit time (seconds)

0

Passenger side bottom panel −10 0 10 −50

−30

Percent suppression

−10 0 10 −30

Percent suppression

Driver side bottom panel

−50

5 10 15 20 Adjusted transit time (seconds)

0

5 10 15 20 Adjusted transit time (seconds)

0

5 10 15 20 Adjusted transit time (seconds)

figure 4.37 Ensemble plots from  vehicles showing vehicle percent baseline suppression (Equation .) profiles for all four radiation portal monitor panels at Site (A), with all narrow lanes pooled. The bimodal pattern is clearly more pronounced in the bottom panels, and the dispersion is greater in the top panels.

PBS for the ensemble. With complete vehicle baseline suppression profiles plotted, the ensemble of profiles forms an envelope in which its boundaries indicate minimum and maximum suppression over all vehicles at each adjusted time. Note that these ensemble plots and the associated spline fits are intended to serve as visual aids to display trends and dispersion of these vehicle profiles by observational group and as such are not intended for statistical inference. The bimodal pattern mentioned above is evident, and the pattern varies depending on panel position. While some information can be gleaned from the vehicle profiles, more information can be obtained from these profiles by first examining the profile for an individual vehicle. Baseline suppression profiles for a single vehicle from the four radiation sensor panels making up a cargo RPM at site A are shown in Figure 4.38. These profiles represent 135 intervals of 0.1 second grouped into 1-second runningsum counts that comprise the 13.5 seconds this vehicle transited the RPM, with the time dimension rescaled to 0 to 200 records (labeled as 20 seconds), consistent with associated ensemble plots shown in Figure 4.37. The abbreviations “Passenger Bottom (PB)” and “Passenger Top (PT)” indicate passenger-side bottom and top panels, and “Driver Bottom (DB)” and “Diver Top (DT)” indicate driver-side bottom and top panels. This can be understood from the observation that a vehicle

Radiation Detection and Interdiction at U.S. Borders

−10 −20 −30 −40

Percent suppress on

0

10

20



−50

PB PT DB DT 0

5

10 15 Adjusted transit time (seconds)

20

figure 4.38 Typical vehicle percent baseline suppression profiles. Profiles are shown for passenger bottom, passenger top, driver bottom, and driver top PBS for running sum data plotted against adjusted transit time. Background reference at PBS = % is shown as a dashed line.

shields more of the top panel from the ground compared with the bottom panel, and that the driver often tends to drive closer to the RPM on the driver’s side (CBP officer booth side). Looking at the DB profile in Figure 4.38 and corresponding values in Table 4.3, the value of –13.87% is the arithmetic average of the 135 PBS data values in the profile, and the value –25.88% is the arithmetic minimum PBS value in the profile, which occurs about at record 62 (or 6.2 seconds) into the collection. Average suppression and maximum suppression are therefore 13.87% and 25.88%, respectively. Although the suppressions found for the four panels show many similarities, each panel also appears to have its own signature of baseline suppression. First, driverside panels (DB and DT) show more baseline suppression than passenger-side panels (PB and PT). This is evident in both the envelope of profiles and in the spline fit curve in Figure 4.37. Top panels show more suppression than bottom panels. The bottom panels have more pronounced bimodality (dips in the profile) than the top panels (Figure 4.38), consistent with the 979 rescaled vehicle profiles shown in Figure 4.37. This is estimated to reflect shielding by either axles or tractor-trailer “segments,” which would shield the bottom panels more. The rise in the middle, creating the bimodality, may reflect difference in shielding between the tractor with its heavy engine, transmission, and axle structure followed by the main cargo area with its trailing heavy axle structure.

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223

table 4.3 Vehicle profile summary statistics for a single vehicle from four portal panels at site (A) as shown in Figure . DBa

Profile Statistics

DTb

PBc

PTd

Mean percent suppression over profile

–13.87%

–21.77%

–11.02%

–16.45%

Maximum percent suppression over profile

–25.88%

–32.84%

–21.32%

–27.49%

a

DB = Driver-side, bottom panel DT = Driver-side, top panel c PB = Passenger-side, bottom panel d PT = Passenger-side, top panel. b

In Figure 4.38, the small peaks in the bottom-panel suppression at approximately 4 and 5 seconds may reflect the gap between tractor and trailer. This detail is lost in the top-panel counts because of the wider angle of view. In Figure 4.37, these details are lost because of the variety of vehicle types, vehicle speeds, and cargo. However, the shape of the first depression of background is characteristically different than the second depression and consistent with the distribution of heavy shielding in the typical tractor-trailer rig. Other features, such as cargo configuration and special vehicle types, will play a role in generating profile shapes.

4.4.2 observations on baseline suppression The following is a list of results from the comparison of the five sites in Section 4.4 with RPM systems. The focus is on maximum baseline suppression because that metric is of greatest potential impact to gross-count alarm algorithms. Results are summarized from Table 2 in Lopresti et al. (2005). Stated values are group medians of maximum baseline suppression. To facilitate interpretation of these results, standard errors of group medians varied from 0.06% to 0.38%. These results suggest systematic individual differences between the systems and between lanes (wide versus narrow). • For narrow-lane configurations, the aggregate of five RPM systems on average showed intermediate baseline suppression at % to %, depending on panel position. When data from two other RPM models is evaluated, the maximum baseline suppression ranged from % up to %. • For wide-lane configurations, the aggregate of three RPM systems gave the smallest maximum baseline suppression at % to %. • Site C (narrow lanes) had the largest maximum baseline suppression values (% to %). Site D (narrow lanes) had the smallest suppression at % to %. • Site A (narrow lanes) showed the widest dispersion between panels (% to %). Site D (narrow lanes) showed the smallest dispersion between panels (% to %).

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Radiation Detection and Interdiction at U.S. Borders

• Medians of maximum baseline suppression at each site were largest for narrow lanes, ranging from % to % depending on panel and site. For wide lanes, medians of maximum baseline suppression ranged from % to %. • Wide lanes showed less dispersion than narrow lanes based on standard errors of medians.

Differences in panel effects relative to position were evident as well. The findings are based on examining ratios of medians of maximum suppression (e.g., suppression ratios) by panel position. Results showed that driver-side suppression was generally greater than passenger-side suppression. Expressed as a ratio of percent suppression, generally, the driver-side count rate is suppressed by a factor of 1.04 to 1.25 more than the passenger side, depending on the site. Suppression on the driver side is even more pronounced than on the passenger side for wide lanes when compared with narrow lanes. The suppression factor was 1.36 to 1.49 depending on site. For narrow lanes, top panels show more suppression than bottom panels at two of the five sites, with suppression ratios ranging from 1.05 to 1.27. At sites B, D, and E, the ratio was 0.92. For wide lanes, the sites showed generally greater top-panel suppression than bottom-panel suppression. Dispersions as measured with standard errors of medians were consistently greater for top panels than bottom panels, at every site.

4.4.3 baseline suppression for energy window ratios The energy window ratio baseline suppression was also investigated with the Site A data, as seen in Figure 4.39. Energy window ratios are the ratios of counts in a high-energy window to counts in a low-energy window, as discussed in Section 4.2 and by Ely et al. (2006). The energy windows discussed here are the operational windows at the five sites discussed above. This plot is similar to Figure 4.37 in format and x-axis scale but, in contrast to the preceding ensemble plots, it shows percent energy window ratio suppression (PRS) as calculated in Equation (4.21). PRS =

Vehicle Ratio − Background Ratio Background Ratio

×100

(4.21)

The PRS expresses the percent difference between a vehicle ratio to a background ratio. This formula can be applied for individual time bins or over an entire vehicle scan to generate a profile. The upper panel spline curves (the top profiles of

 Example: A suppression ratio for comparing driver to passenger side may be formed by dividing the sum of driver panel median PBS by the sum of passenger side median PBS; for example, (29.30 + 24.18)/(25.56 + 18.92) = 1.20.Suppression ratios to compare top to bottom panels may be formed in a similar fashion.

Enhancing the Effectiveness of RPM Systems

225 Passenger side top panel

−5

−5

Percent suppression 0 5

Percent suppression 0 5

Driver side top panel

0

5 10 15 Ajusted transit time (seconds)

20

0

20

Passenger side bottom panel

−5

−5

Percent suppression 0 5

Percent suppression 0 5

Driver side bottom panel

5 10 15 Ajusted transit time (seconds)

0

5 10 15 Ajusted transit time (seconds)

20

0

5 10 15 Ajusted transit time (seconds)

20

All panels summed

−5

−5

Percent suppression 0 5

Percent suppression 0 5

Average for all panels

0

5 10 15 Ajusted transit time (seconds)

20

0

5 10 15 Ajusted transit time (seconds)

20

figure 4.39 Profiles of percent ratio suppression based on vehicle energy window ratios vis à vis background energy window ratios for the narrow lanes at Site (A). The bottom-right graph (the ratio of the sum of all panels) shows almost complete mitigation of baseline suppression effects.

Figure 4.39) showed an enhancement of the energy window ratio of +1% to +2% higher in the middle of the rescaled time series. These values are statistically different from zero because the average is taken over 979 vehicles and the resulting data yield uncertainties much less than 1%. The lower panel curves showed –1% to –2% suppression (the middle profiles of Figure 4.39), also statistically different from zero. This difference suggests that shadow shielding as picked up by top and



Radiation Detection and Interdiction at U.S. Borders

bottom panels is somewhat energy dependent. One mechanism to account for this is differential scattering related to path of gamma rays through the cargo. Cargo scattering shifts gamma-ray energies differentially to lower levels as they travel through the cargo to the detectors by different paths, resulting in different observed energy window ratios as observed by top panels versus bottom panels (Stromswold et al. 2003). By generating the ratio of the sums of top and bottom panels (the lower profiles of Figure 4.39), the spline fit shows that the shadow shielding impact is effectively eliminated. This makes the energy ratio algorithm largely immune to shadow shielding, improving its sensitivity to sources. Of course, a basic assumption with the energy window approach is maintenance of detector energy calibration over both background and vehicle collection.

4.4.4 summary These results demonstrate that baseline suppression is an observable and quantifiable effect in RPMs, and that baseline suppression appears to vary in systematic ways depending on panel, system, lane width, and site. In particular, in the study cited (Lopresti et al. 2005), the RPM sites examined exhibited differences in absolute percent background suppression between top and bottom panels, and between driver-side and passenger-side panels. Each site seems to show its own unique signature. The spline fits to ensemble profiles of 979 vehicles in Figure 4.37 show an example of such differences between the four detector panels. Driver behavior may account for the additional driver-side baseline suppression at land border crossings. It is conjectured that vehicle drivers stay to the left to maintain the clearance in the narrow lane. This would result in more shadow shielding of the driver-side portal panels by the vehicle. Differential shielding may account for the additional top-panel suppression. The bottom panels would be partially shielded by the vehicle from gamma radiation from the roadway, while the top panels would be more completely shielded by the vehicle. This effect would vary depending on the truck dimensions, placement of cargo, density of cargo (e.g., high or low atomic number), and also the placement of the detector panels. Results suggest that alarm algorithms that rely on gross-count profiles may need to be tuned not only to the RPM site, but also with regard to lane and panel orientation. Alarm algorithms that are based on energy windows appear to be more robust to baseline suppression effects.

4.5 Spectroscopic Portal Monitors (SPMs) Richard Kouzes, James Ely, Kathleen McCormick, Brian Milbrath, John Schweppe, Edward Siciliano, and David Stromswold All RPM technology deployed in CBP operations has used PVT scintillators for the detection of gamma radiation. Despite the significant advantages of this

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227

technology, other gamma-ray detectors may provide better energy resolution and might be considered superior to PVT-based systems for interdiction of radioactive materials. The most notable possible detector materials are NaI(Tl) and HPGe. While a plastic scintillator has an energy broadening of about 30–50%, typical energy resolution of NaI(Tl) detectors are on the order of 5% to 10% depending on the gamma-ray energy, whereas with HPGe the resolutions are about 0.2% (typically a factor of 30 better than NaI[Tl] detectors). However, the expense of purchasing and operating HPGe detectors, coupled with the requirement for cooling the fragile detectors with liquid nitrogen, has tempered the use of HPGe detectors. Other possible detection materials are generally not currently available in sufficient physical size or quantity at a competitive price to be considered for RPM applications. When such higher-resolution detectors made of NaI(Tl) or HPGe are used in a portal monitor, the result is an SPM. Several studies have been reported in which the advantages and challenges for the use of plastic scintillator and sodium iodide to detect gamma rays in RPM applications have been assessed (Chambers et al. 1974; Fehlau et al. 1983; Siciliano et al. 2005). Three significant driving factors for considering alternative detector technologies include the potential for increased sensitivity to radioactive materials of concern, the ability to reject alarms from NORM, and the improved isotopic identification of all materials that cause alarms. As previously discussed, PVT-based systems, along with data analysis with advanced algorithms, such as energy windowing, can partially address the first two of these factors (Ely et al. 2004, 2006). Plastic scintillator-based gamma-ray detectors cannot identify the specific isotope that produces an alarm due to the limited energy resolution obtained in such detectors. This section considers additional improvements that might be possible with improved energy resolution detectors, as well as related disadvantages. Recognizing the needs for specific uses of SPM systems, the RPMP first developed an initial specification for an SPM system in late 2003 and developed a preliminary deployment plan concept for this technology. The American National Standards Institute developed a series of standards for border security radiation screening equipment (discussed in Chapter 7), including a standard for SPM systems (ANSI 2006). Based on this work and other sources, the Department of Homeland Security Science and Technology Directorate’s Homeland Security Advanced Research Projects Agency (HSARPA) created a specification in late 2004 for a program for development of spectroscopic portal monitors referred to as the Advanced Spectroscopic Portal (ASP) Program. This work was then transferred into the Domestic Nuclear Detection Office (DNDO) within DHS in Fiscal Year 2006.

 Since a plastic scintillator does not have full energy peaks above about 25 keV, and only Compton scattering features are observed, the term “energy resolution” is inappropriate. Rather, it is more precise to refer to this as an energy broadening effect whereby the signal has been spread out due to a number of physical factors.



Radiation Detection and Interdiction at U.S. Borders

The major possible drawback of SPM systems is that they have a substantially higher cost than PVT-based systems. To justify this higher cost and before replacing a PVT-based RPM with an SPM, the advantages of SPMs should be considered. The SPM increases the sensitivity of screening over what is currently deployed, potentially reduces the operational burden on a port of entry due to nuisance alarms, and improves the isotopic identification capability over the handheld detectors that are currently used. Any or all of these conditions may adequately justify the use of the SPM system depending on the needs of each individual deployment site. Plastic-based RPM systems otherwise generally provide adequate capability for the radiation screening need, especially with the use of advanced algorithms.

4.5.1 drivers and requirements for deploying spectroscopic portal monitor systems When SPM systems are deployed, they must fit within the operational constraints of a port of entry, which includes the current primary-secondary processing approach, or an acceptable modification thereof. Primary operational applications drive the possible deployment of SPM systems. Deployment of SPMs improves the secondary processing of alarms for isotopic identification where handheld instruments are currently used and may have difficulty identifying weak NORM sources. Operation in situations where isotopic identification in primary screening is especially important and deployment of a SPM may have advantages. An example of this is screening of rail cars, which carry significant NORM, where it is difficult to stop a train and remove one or more cars for separate secondary processing except when absolutely necessary. At some seaports, secondary processing space is very limited. Thus, SPMs could provide for primary screening where primary volumes are very high and drive the need to reduce secondary processing volume. Lastly, these detectors could be mounted on mobile systems for targeted screening or rapid deployments where simultaneous detection and identification is required. Deployment of SPM systems to other applications, such as POV land sites, air cargo sites, mail and express consignment courier facilities, might occur depending on results from cost and benefit analyses. Because of the limited volume of NORM at these sites, PVT-based systems can be operated at peak sensitivity, thereby maximizing the cost effectiveness of these systems versus any expected gain from SPMs. Because of spatial constraints at ports of entry, that is, no room for side-byside systems, it is necessary that SPM systems be a replacement for PVT-based systems either in secondary or in both primary and secondary. This, then, defines the need for the SPM system to have all the same capabilities as current RPM systems, including data archiving to a central location and ancillary control capability. The SPM systems must also replace the already deployed PVT-based systems without any disruption in the operational, law enforcement function of

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229

screening for radiation. The SPM systems need to be able to replace PVT-based systems in applications such as mobile portals, rail portals, and remotely operated systems. The SPM systems, like the PVT-based systems, also need to be reliable, easily serviced when needed, and resilient to a wide range of environmental conditions, with a reasonable life cycle cost, just like the PVT-based RPM systems they replace.

4.5.2 prototype spectroscopic portal system Comparisons have been made between plastic-based gamma-ray detectors and NaI(Tl)-based gamma-ray detectors (Siciliano et al. 2005). A similar comparison with HPGe-based gamma-ray detectors has also been performed (Ely, Siciliano, and Kouzes 2004). The majority of the first deployed SPM systems will be based on NaI(Tl) gamma-ray detectors, as this technology is very mature and the requirements for an SPM can be met with a natural evolution of already existing NaI(Tl)based products. Prototypical systems based on HPGe have been developed in industry, but their cost-to-benefit advantage will need to be critically evaluated. A prototype SPM was constructed and tested at PNNL (Stromswold et al. 2004) as shown in Figure 4.40. This detector was funded by NNSA (NA-22) as a system designed to look at detection of  U. The prototype consists of four NaI crystals, each 101.6 mm × 101.6 mm × 406.4 mm (4 in. × 4 in. × 16 in.), mounted with the long-axis vertical. The resolution of each detector is approximately 8% (FWHM) at 662 keV. High voltage is supplied to each individual detector with a four-channel high-voltage module. The data acquisition system consists of commercial electronics with a 512-channel multichannel analyzer readout. The whole detector assembly is located inside a steel enclosure, which is insulated with foam to minimize detector drift. The detection efficiency of this prototype SPM was tested with both unshielded and shielded radioactive sources (McCormick et al. 2005). For unshielded sources, the absolute detection efficiency was measured to allow figure 4.40 Spectroscopic portal monitor the comparison with PVT-based RPMs. prototype utilizing four NaI(Tl) logs.



Radiation Detection and Interdiction at U.S. Borders

For the shielded sources, the measured count rate versus thickness of shielding was studied, as well as the isotopic identification capability of the SPM. A series of tests was also performed with radiation sources positioned behind crates of representative cargo, some containing NORM. The absolute detection efficiency of the prototype and a PVT-based detector were measured with various sources in the same geometry. Table 4.4 shows a comparison for various sources of the counts per second per microcurie observed in the PVT-based system and the four-log NaI(Tl) system. The average ratio of the absolute detection efficiency of the NaI(Tl) prototype portal to a standard RPM is approximately 0.8. This closely agrees with the results of a simulation performed with MCNP (Briesmeister 2000), which showed that five NaI(Tl) crystals of this size would be needed to equal the gross counting efficiency of a standard PVTbased portal panel (Siciliano et al. 2005). It is an open issue whether a spectroscopic RPM needs to have the same gross counting efficiency as a PVT-based portal, because of the added spectral information it provides. The argument for equal detection efficiencies is to maintain gross detection efficiency for those situations where heavy shielding prevents the identification of full-energy peaks. Measurements were made to determine how the spectral shape observed from a source changed as a function of lead-shielding thickness between the source and the detector. Figure 4.41 shows measurements taken with a Th source behind various thicknesses of lead shielding. Each of the plots was generated with data taken during a 5-second interval, to more closely match the scanning time of real cargo in the field. As seen in the figure, layers of shielding attenuate the full-energy peaks, changing the shape of the spectrum. This redistribution of spectral strength has implications for analysis algorithms when applied to detection of shielded sources. Measurements of cargo vehicles were conducted with the prototype SPM system (Stromswold 2004). Figure 4.42 shows spectra from two NORM events (pottery and ceramic tile, both counted for 300 s in a stationary mode), along with a background spectrum taken for 1000 s. The relatively long count periods used to obtain these spectra might be similar to what could be used in secondary screening to identify the specific radionuclides that triggered a primary alarm. table 4.4 Absolute detection efficiency comparison for a PVT-Based system versus an NaI(Tl) system consisting of four  cm ×  cm ×  cm ( in. ×  in. ×  in.) crystals Source 241

Am Co 133 Ba 137 Cs 60 Co 228 Th 57

Net Rate (cps)

Source Activity (μCi)

NaI (cps/μCi)

PVT (cps/μCi)

Ratio (NaI/PVT)

116 565 899 816 1447 1217

9.84 6.49 7.15 9.29 7.78 7.60

12 87 126 88 186 160

15 98 200 119 229 199

0.80 0.89 0.63 0.74 0.81 0.80

Counts (cps)

102 Background Unshielded Th-228

0

500

1000

1500

2000

2500

3000

Net counts (cps)

Energy (keV) 102

0

500

1000

1500

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Net counts (cps)

Energy (keV) 102

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Net counts (cps)

102

0

500

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3000 Energy (keV)

Net counts (cps)

102

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3000 Energy (keV)

figure 4.41 The measured spectral shape of a Th source as a function of lead-shielding thickness. From the top down, the first graph shows an unshielded source with a background spectrum overlayed. The second graph shows the net unshielded source spectrum. The following graphs show net counts and spectral shape for various layers of lead shielding: . cm (. in.) for the third graph, . cm (. in.) for the fourth graph, and . cm ( in.) for the bottom graph in the figure.



Radiation Detection and Interdiction at U.S. Borders

100000 214Bi 40K

10000

Counts

Pottery

214Bi

1000

214Bi

208TI

100 Ceramic tile

10 Background

1 0

500

1000

1500

2000

2500

3000

Energy (keV)

figure 4.42 Background and alarming stationary vehicle spectra for an NaI(Tl) portal.

To separate the spectra for easier viewing, the data in the figure are scaled. For all three spectra, the dominant sources of gamma rays are the radionuclides K, U (with its primary gamma-ray signature from the decay product Bi), and Th (Tl), which are present in the environment and give rise to both background radiation and NORM radiation. Comparison of the relative magnitudes of the peaks marked by arrows shows the relative abundance of the primary gamma rays. For example, the pottery spectrum contains enhanced Th (2614 keV) and reduced K (1460 keV), compared with the background spectrum. The ceramic tile shows relatively enhanced counts at 609 keV, 1764 keV, 2204 keV, and 2447 keV from Bi, which is a decay product of U. Figure 4.43 shows drive-by spectra (collected in about 20 s) from two non-NORM cargos: Co and UO. The characteristic double peaks from Co are distorted by the radiation shield around the source. Uranium oxide (>4000 kg) produced a spectrum with high count-rates due primarily to 1001 keV and 767 keV gamma rays from m Pa, which is in the decay chain of U. These gamma rays are scattered in the uranium oxide, contributing to the large number oflow-energy counts.

4.5.3 specification for spectroscopic portal monitors In September 2003, the RPMP developed an initial specification for an SPM based upon the same requirements used in the PVT-based RPM specification. The concept was to allow the SPM system to be operationally similar to the deployed systems in every way but with the enhancement of spectroscopic identification from NaI(Tl) gamma-ray detectors. The specification includes the capability for filtering alarms based on user-defined criteria for rejection of NORM and radioisotopes utilized for medical procedures.

Enhancing the Effectiveness of RPM Systems

233

10000 4025;R20

UO2

Counts

1000

100 60Co

10

1 0

500

1000

1500 Energy (keV)

2000

2500

3000

figure 4.43 Drive-by spectra from Co and UO cargo.

Just as for the PVT-based RPM specification, the majority of the SPM specification dealt with operating requirements and environmental specifications. The SPM panels were required to fit into the existing support stands to allow easy implementation in deployed stands. The SPM had to be deployable to all of the same venues as the RPM systems. Compliance with the ANSI N42.35 standard for RPM systems was required, as was compliance with the ANSI N42.38 standard under development at that time. The list of isotopes to be identified with the SPM was based upon those listed in the ANSI N42.34 standard. These isotopes included: K, Co, Co, Ga, m Tc, Mo, I, I, Ba, Cs, Sm, Ir, Tl, Ra, Th, Th, U, U, U, U,  Pu (reactor grade plutonium [> 6% Pu]), and Am. The gamma-ray sensitivity for the SPM was specified to require an absolute detection efficiency for various sources, with similar sensitivity values used for the RPM specification, as shown in Table 4.5. The reason for this stringent requirement was to meet the basis outlined earlier in this chapter for detection of highly shielded sources. Based upon modeling studies by the RPMP, the configuration of detectors was specified as a specific arrangement of five 102 mm × 102 mm × 406 mm (4 in. × 4 in. × 16 in.) NaI(Tl) “logs,” which was known to meet the specified absolute detection criteria. The detectors were spaced vertically to allow for effective coverage of vehicles of all configurations. Several gamma-ray alarm algorithms were specified that could be applied to the individual detectors as well as groups of detectors. Complete data logging was required, along with state-of-health indicators to predict impending system failures. The software used to analyze the data is crucial to the success of an SPM. To date, the most promising analysis approach is known as “template matching,” in which a fit is made to the data from a library of actual spectra under a wide range of conditions. This method is used in some commercial products.



Radiation Detection and Interdiction at U.S. Borders

table 4.5 Detection efficiencies for radionuclides Radionuclide

Primary Emission Energies (keV)

Minimum Absolute Detection Efficiency (net cps/μCi)

241

60 122, 136 31, 356, 81, 302, 35, 383, 276 662 1173, 1333

20 100 200 110 220

Am Co 133 Ba 137 Cs 60 Co 57

4.5.4 comparison of thallium-doped sodium iodide and high-purity germanium detector materials Perhaps the most important consideration in a comparison of NaI(Tl) to HPGe for portal monitoring applications is the efficiency of the detectors themselves (Ely, Siciliano, and Kouzes 2004). Many comparisons of the efficiency of NaI(Tl) to HPGe have been conducted over the years, and in fact, the most common method of specifying a germanium detector is in terms of the relative efficiency compared to a 75 mm × 75 mm (3 × 3 inch) right cylindrical NaI(Tl) crystal. The relative efficiency is measured with a Co point source at a distance of 250 mm. The NaI(Tl) has greater gamma-ray absorption because of the higher effective Z compared to HPGe and, so, has a higher overall efficiency for the same size detector. For isotopic identification, full-energy peak efficiency is of considerable importance, and different analytical and semianalytical functions have been used to describe both HPGe and NaI(Tl) full-energy peak efficiencies (Sudarshan et al. 1992). For the same size detectors, the HPGe peak efficiency is less, but because of its superior energy resolution, the isotopic identification capability of HPGe is superior, and the MDA of source material is smaller. A recent investigation of the efficiencies, resolutions, and MDA of common-sized NaI(Tl) and HPGe detector systems has been made with regard to border security applications (Perez-Andujar and Pibida 2004) in which the NaI(Tl) system provided the most efficiency and lowest MDA. However, in this comparison the HPGe detector considered is a factor of 6 smaller than the NaI(Tl) detector. Other factors must also be considered in the comparison of NaI(Tl) to HPGe for vehicle RPM applications. The HPGe requires cooling, which is more expensive, but removes the problems related to ambient temperature changes that may cause instability in NaI(Tl) systems. Rapid temperature changes can crack large NaI(Tl) detectors, and the scintillation light output and resulting pulse-height spectra, are dependent on temperature. These problems can be overcome in NaI(Tl)-based systems either by providing constant temperature environments or by gain stabilization techniques with thermal insulation. Liquid nitrogen has traditionally been used for HPGe detectors, but for field applications, mechanical cooling provides a viable alternative (albeit with significant electrical power requirements). Investigation into more efficient and robust cooling for HPGe

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detectors is a current priority with HPGe system manufacturers. Durability is a consideration; NaI(Tl) systems have been designed and used in many field applications, whereas HPGe has been used primarily in laboratory settings where the thin windows of the detection system can be protected. As they will typically be used in a fixed location, portal monitors of both NaI(Tl) and HPGe can be engineered to operate in a wide variety of environmental conditions for an extended period of time. Another factor for consideration is the need for a rapid real-time decision about the presence of any source of radiation above background while the vehicle transits a primary screening portal. Because of the very short time available for data collection, spectral identification may not be possible. Thus, HPGe systems with improved energy resolution may not have a significant advantage over NaI(Tl) systems for this real-time decision mode.

4.5.5 advanced spectroscopic portal (asp) program The HSARPA organization within DHS developed a specification for a class of SPM referred to as an ASP in 2004, for deployment starting in 2007. The development of these ASP systems is taking place under the program supported by the DHS DNDO. The HSARPA Broad Agency Announcement for the ASP program resulted in 10 contracts to vendors for prototype systems. These contracts were awarded in two groups of five vendors (the first five were redirected under a previous award, and five additional vendors were selected). The 10 vendors awarded contracts, under the two HSARPA Broad Agency Announcements, and the spectroscopic technology they used were: Ancore® (NaI); Applied Research Associates® (NaI); ORTEC (HPGe); Canberra Industries, Inc. (HPGe); Constellation Technology Corporation (high-pressure xenon); Corus (NaI); Northeastern University, Boston (NaI and liquid scintillator); NucSafe (NaI); SAIC/Exploranium (NaI); and Thermo Electron Corporation (NaI). Thus, the majority of the vendors used NaI(Tl) inorganic scintillator as the gamma-ray detector material. These systems were tested in the summer of 2005 at the Nevada Test Site, and the information obtained from those tests contributed to the process of down-selecting vendors in 2006 to manufacture limited-rate initial production (LRIP) systems. Like PVT-based systems, SPM systems are complex instruments requiring the integration of many component subsystems. Because NaI(Tl) gamma-ray detector material is only made by one large producer and a few small producers in the United States, one of the key challenges is to obtain sufficient amounts of the 

Ancore is a registered trademark of Rapiscan® Systems. Applied Research Associates is a registered trademark of Applied Research Associates, Inc.  Information regarding the Nevada Test Site is available at http://www.nv.doe.gov/main.aspx 

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material for production of several hundred ASP units. Another major challenge is the development of the analysis and control software for the systems. This software needs to be effective in identifying radiation sources, and in providing this information to the operator in an easily understood interface.

4.5.6 deployment strategy The overall strategy for placement of the initial ASP systems into an operational environment should be consistent with several goals. Spectroscopic portals should be used to maintain, at a minimum, the existing level of interdiction capability while proving the operational suitability, capability, and reliability of the interdiction system. Deployment of ASPs, at least initially, should be done with the aim to gain knowledge of the enhanced capabilities when integrated into the interdiction system. This includes the ability to build conduct of operations and operational confidence centered around the ASPs integrated into the overall operation of ports of entry, which means that a plan for integration of this new technology into the overall border protection strategy would need to be developed. The systems to be initially deployed into the field will be placed at currently protected border crossings and ports in series with existing operational PVTbased detection systems. This will allow verification of ASP performance without affecting the existing screening capability of the port. The systems will be deployed at a cross section of ports-of-entry in order to sample the range of venues. As the reliability and operational readiness of the ASP is proven, and as production quantities become available, the systems will be widely deployed in a variety of venues. It is possible that some venues that have low amounts of NORM in commerce, such as mail facilities and POV lanes, will not require ASP systems. Figure 4.44 indicates a conceptual timeline of how ASP systems could be phased in and could replace many PVT-based RPM systems over a time period of

Dep oyed systems

Total systems

SPMs

PVT RPMs

Time

figure 4.44 Timeline for the introduction of advanced spectroscopic portal systems into the U.S. Customs and Border Protection environment.

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approximately 5 years. The majority of initial ASP systems deployed may replace existing PVT-based systems, as the high-priority sites are already equipped with such RPM systems. As time passes, and more ASP systems are manufactured, they will replace more of the PVT-based systems. To date (2010), the ASP systems have not been deployed.

4.6 Human Factors in Radiation Portal Monitoring Systems Thomas Sanquist, Pamela Doctor, and Christian Richard Humans ultimately operate and administer the RPM systems, and their effective interaction with the technologies is a critical variable in overall system success. Human performance is influenced by many aspects of security system design, including the equipment with which personnel interface, the training they receive, the procedures used, and the teamwork needed for personnel to work with each other to perform their various roles. These aspects of security system design are addressed by human factors engineering—the systematic application of psychology and physiology to the design of equipment and systems. The objectives of human factors engineering are to ensure that the performance requirements of systems, in terms of staff, individual operators, and user interfaces, accommodate the capabilities and limitations of operations and maintenance personnel. Complex system design efforts involving systems as diverse as nuclear power plants, aircraft, and military command and control often engage human factors experts. The Department of Defense has institutionalized human factors through a Human-System Integration process that is part of the overall engineering development cycle (Booher 2003). A number of human-technology interfaces are part of RPM systems, as illustrated in Figure 4.45. The output of RPM systems is presented to inspection officers at various locations, depending on the transportation vector. For each alarm, the officer evaluates the nature of the signal, and other information, to resolve the alarm. Several important aspects of human factors engineering as they pertain to RPM systems are described in this chapter, including the role of the inspections officer, the variables influencing operator trust in systems, the impact of false or nuisance alarms and the base rate of threat events, and the methods of information display that can enhance situational awareness of true (legitimate) threat alarms.

4.6.1 human role in radiation portal monitor security decision making The most basic signal-processing systems, such as raw radar output, involve the human operator making fine sensory discriminations on system outputs (visual or acoustic displays) to determine the presence or absence of a signal of interest.

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Radiation Detection and Interdiction at U.S. Borders Human decision & response

• Is it real? • Can I trust the sensor? • How do I respond? • What else do I need to know?

Threats exist in background of everyday human activity

Vulnerability vectors for potential threat manifestation

Sensors applied at vulnerability vectors to detect or amplify potential threats

Human operators administer and interpret sensor system data, and respond as appropriate

Security systems need to support the decision and response process

figure 4.45 General model of human element in radiation portal monitor security systems.

In these types of systems, the human decides when to issue an alert. A similar example is the use of a stethoscope by a physician—the instrument amplifies the physical signal, and the physician decides whether the resulting sounds represent normal or abnormal physiology. More sophisticated systems involve signal processing to determine the presence or absence of a condition of interest. These systems alert human operators when that condition is encountered. Alarm content may simply reflect a two-state “alarm/no-alarm” condition or may provide additional information in the alarm signal concerning the situation of interest. The human attendant’s job is to react to that information according to various procedures. This type of alerting system invariably involves elements of judgment and choice by the operator (Rankin et al. 1983). These two types of systems, visually represented in Figure 4.46, may be considered as the “amplifier model” and the “evaluator model” and are useful ways of conceptualizing the human decision process for purposes of developing decision aids. Most portal-monitoring systems deployed through the RPMP involve alerting officers by means of a two-state alarm when the radiation level at a sensor panel exceeds a threshold. The required response to alarms involves confirmation screening, more detailed isotope identification and, where applicable, examination of the manifest and inspection of the cargo contents. Once the source of the radiation alarm is resolved, the suspect vehicle or package is released. This process is best represented with the evaluator model. Because the principal job of the inspection officer is to evaluate RPM system output and render a case disposition, psychological processes involving system trust are at work. These, in turn, are influenced by the false or nuisance alarm rate of the system, and the base rate of threat events.

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Physical signal Physical signal

Amplification & processing Amplification Decision criteria

Condition present or manual

Condition absent: no alarm

Condition present: alarm Evaluate

Final disposition

A Amplifier model

B Evaluator model

figure 4.46 Two general classes of human-mediated inspection systems. (A) The amplifier model involves the human deciding whether the target condition is present or absent. (B) The evaluator model involves a person assessing the system output.

In the following sections, the pertinent human factors findings and implications in these areas are discussed.

4.6.2 system trust Currently, RPM systems are based on technology elements used by inspection officers to directly detect threats or to obtain information that forms a basis for making judgments about the potential presence of threats. Through training, experience with the technology, and other means, officers will develop a sense of trust in the technology they use, and this trust impacts the degree to which they will be willing to rely on the information obtained from the system as a basis for making decisions. Lee and See (2004) define trust as “the attitude that an agent [technology] will help achieve an individual’s goals in a situation characterized by uncertainty and vulnerability.” In an applied context, trust is the attitude that a technology will provide some functional benefit (e.g., detecting a threat) even though there is potential uncertainty that the technology will function as expected and that relying on technology potentially leaves the user in a vulnerable situation (Johns 1996). Consequently, individuals will rely on automation they trust and likely reject automation that they do not trust.

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Simply having trust in a technology is not the critical aspect of trust in automation. Rather, what is critical is that the level of trust is justified (i.e., valid) based on the actual capabilities and reliability of the technology (Lee and Moray 1994; Muir 1987). In other words, an individual’s level of trust in a system should be calibrated to match the degree to which it is safe to rely on that system to adequately perform as it should. For RPMs, a key issue in relation to the level of officer trust is the high rate of nuisance alarms; this is especially true for other systems like RPMs that are specifically designed to minimize the chance of missing a target event (Parasuraman and Riley 1997). For example, in early warning systems in aviation, excessive false alarm rates resulted in mistrust and lack of pilot usage, or pilots that found creative ways to disable alarm systems (Satchell 1993; Wiener 1988). Similar evidence is found in process control industries, where alarms from untrustworthy systems are more likely to be acknowledged (and turned off ) before their location and nature are determined (Hale and Glendon 1987; Zwaga and Veldkamp 1984). Thus, highly sensitive systems with excessive false or nuisance alarm rates eventually lead to a reduction of operator trust, and the development of various means to ignore, override, or disable alarms.

4.6.3 false and nuisance alarms The effectiveness of a warning or alerting system depends in large part on its credibility. The more credible an alarm, the more likely an operator is to take the potential threat seriously and to take action in response to the alarm. Unfortunately, the credibility of alarms can be significantly eroded by the excessive occurrence of false or nuisance alarms. This can result in an individual being less likely to respond with the same “intensity of conviction” to future alarms because of the potential that they may again represent a false alarm (Breznitz 1984). This attenuation in response or loss of credibility in a warning because of false alarms is called the “false alarm effect” (Breznitz 1984). High rates of nuisance alarms typically occur because detection sensitivity is set with a low threshold. This is a safe strategy when the costs of missing a signal or threat are very high, yet as mentioned above, the performance of operators in responding to the alarm may be less than optimal because they will likely suspect that it is just another false alarm (Breznitz 1984). It is also relatively safe when traffic volumes are very low. Even if operators are required to respond because of operating procedures, there is likely to be some incentive to “cut corners” in the investigation, especially if the search comes at a cost in terms of time and efficiency (Dzindolet et al. 2002). Thus, the strategy of trying to be sensitive to the presence of all potential threats is actually undermined somewhat by how the detection parameters influence the corresponding behaviors of individuals that respond to the alarms.

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On the surface, one strategy for reducing false alarms may be to reduce the sensitivity of the system so that fewer false alarms are triggered, which means that operators will be responding to real threats more frequently and may do a better job at remaining vigilant in their responses. However, some research suggests that under low base rate conditions, lowering the system sensitivity, and thus the false alarm rate, may not result in any real improvement (Parasuraman et al. 1997). The problem is that with very low event base rates, the probability of a true alarm (known as the posterior probability) is still so low as to be effectively the same as with higher false alarm rates. Thus, not only is the system’s ability to capture potential threats diminished by reducing sensor sensitivity, but this cost is unlikely to lead to more vigilant behavior by security officers or a reduction in the false alarm effect. A critical aspect of designing effective detection and warning systems involving human operators is the availability of a base rate of target events of interest; in fact, this element underlies the entire philosophy of a routine inspection process designed to detect target events (Parasuraman et al. 1997). The base rate significantly affects the utility of any detection system. It has been found that extremely low base rates and high-consequence signals are associated with generalized operator mistrust of detection systems, leading to creative disabling or ignoring of alarms. Developing a hypothetical base rate for illicit nuclear smuggling in the United States can be done with data from sources such as the IAEA, which suggests approximately 450 nuclear smuggling incidents in Europe during the past 10 years (Orlov 2004). If we confine the analysis to cargo trucks entering the United States from Mexico or Canada, data from the U.S. Bureau of Transportation Statistics indicate that approximately 11 million transits occur on an annual basis. Thus, the base rate of occurrence would be Number of annual smuggling incidents/number of opportunities = 45/11⋅10 = 4.1⋅10– The probability of a true threat RPM alert with this base rate of occurrence is very nearly zero (obtained by applying Bayes Theorem): P ( threat alarm) =

P ( hit )

P ( hit ) + P ( nuisance alarm) ⎡⎣( −bbbase rate ) / bas a e rate ⎤⎦

= 3.9⋅10–

(4.22)

This quantity is otherwise known as the positive predictive value of the detection system, which is virtually nonexistent for the RPM as a threat detector. Instead, the RPM simply serves to alert the operators to the presence of some radioactive material and provides no additional information concerning the likelihood of a threat. Because the rate of alarms is generally considered between 0.5% and 1% of

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vehicles, this base rate of occurrence would yield a true threat alarm on the order of every 2 to 4 years. However, because the actual base rate of occurrence of threats is probably substantially lower (because of a much larger denominator of transportation opportunities and a much smaller numerator of smuggling incidents), it is reasonable to question whether any true-threat alarms would be expected to occur over the life cycle of the system. At the present time, virtually all alarms from RPMs are a result of NORM or medical isotopes and are thus “nuisance” alarms. This situation is fraught with problems from a psychological standpoint (Sorkin 1988), because inspection officers are expected to carry out the same procedures regardless of their knowledge of the probable source and the frequency with which they see certain types of vehicles causing alarms. One method for improving the overall performance of a detection system with a low event base rate is to reduce false or nuisance alarms through a classification or response mapping procedure (Parasuraman et al. 1997) that assigns a likelihood value to specific instances of detected signals. Early experiments with this type of display (Sorkin et al. 1988) suggest that operator workload is reduced with signal classification. As described below, incorporating situational awareness information with energy spectrum and cargo data make it feasible to address alarm classification.

4.6.4 situational awareness The concept of situational awareness, which has migrated to human factors psychology from the military, is defined as “the perception of the elements in the environment within a volume of time and space, the comprehension of their meaning and the projection of their status in the near future” (Endsley 1997). Situational awareness is clearly an integrating concept that assembles basic elements of perception, interpretation, judgment, and forecasting. The definition is a reasonable description of the rapid cognitive activities performed, for example, by an inspection officer during a routine border crossing interview. They perceive the vehicle, its occupants, and their manner of interaction within the overall context of national security levels, prior history of smuggling or illegal entry at that particular location, along with any other intelligence information they may have available. A judgment is made regarding the relative safety or honesty of the vehicle occupants, and a course of action is determined. All of this happens within a span of 30 seconds or less in most instances. This type of cognition is often described as “pattern matching” since it appears that decision makers often rapidly assess critical cues and compare them to mental models. In the context of RPMs, situational awareness provides the inspection officer with the ability to interpret conflicting information and reach a conclusion.

The number of true threat alarms annually = (alarm rate × number of vehicles)(p(threat|alarm) < 0.5.

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For example, should an isotope identification system suggest the presence of RDD material, but experience and all other data suggest that the source is a medical isotope, a decision would be made to resolve the alarm as a medical isotope. Because the isotope identification technology does make misclassifications, and inspection officers are aware of this, the flexibility of their decision process allows them to interpret the situation and respond accordingly.

4.6.5 applications to radiation portal monitor systems: the likelihood display concept The implications of the concepts described above for RPM system alarms and displays are best illustrated by example. Figure 4.47 shows the basic components of the most widely implemented RPM system design with the two-state threshold processor as the basis for alarms. In this design, the alarm status is all or none, and no distinction is made for NORM. Thus, secondary scanning is required, along with a prescribed isotopic identification and inspection protocol. Successful completion of these steps resolves the alarm. and the vehicle or package can be discharged. The secondary screening and inspection process can be relatively time consuming with cargo vehicles and can be especially problematic when traffic is heavy. The activity then becomes manpower limited. Because virtually all alarms are successfully resolved, a substantial human factors concern is the added workload to process alarms that have no security significance. An alternative approach to alarm display is shown in Figure 4.48. In this design, the primary signal from the RPM is processed with statistical templates

Primary signal

Threshold processor

Alarm status all-or-none

Inspect Secondary scan

Confirm

Resolve & discharge

figure 4.47 Two-state radiation portal monitor alarm system involves four postalarm steps.

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Radiation Detection and Interdiction at U.S. Borders

Primary signal

Statistical processor: templates, spectra

Visual & meta-data

Alarm status calibrated to potential threat Resolve & discharge

Situational awareness

figure 4.48 Likelihood alarm radiation portal monitor system provides more information to officers through green, yellow, and red indications.

and/or energy spectra, and it is integrated with metadata concerning observed vehicle type and frequency and other salient information. This leads to a multistate output, color coded as green, yellow, and red to correspond to “no signal of concern,” “probable NORM—inspection optional,” and “unidentified signal—perform secondary inspection.” In this model, we have linked the alarm display to increasing levels of intervention. An unidentified signal that yields a red alarm requires additional inspection for resolution; a yellow alarm results from a signal that is highly probable NORM, so in this case inspection is left to the discretion of the inspection official. Addition of the intermediate alerting state of “probable NORM” significantly reduces the requirement for additional screening, while allowing the human operators to develop more complex and robust mental models of RPM system performance. The existence of the intermediate alarm state will lend credibility to the much lower frequency “unidentified signal” alarms. This basic approach has been termed a “likelihood display” as it presents alarm data in a probabilistic way. The need for classifying legitimate sources that generate RPM alarms is recognized as an important element by Kouzes et al. (2006), who stated: …it is necessary to anticipate and recognize the types of cargo that contain naturally occurring radioactive materials so that such “nuisance” alarms can be quickly dealt with. Clearly, any reduction in the time spent by [inspection] officers to determine that an alarm is not of concern will help in their search for actual illicit materials.

4.6.6 distinguishing between illicit material and naturally occurring radioactive material: human factors applications The physical signals underlying radiation detection in RPMs contain information that is potentially useful in developing a graded or likelihood display approach.

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The bulk of nuisance alarms are based on gamma-ray generated counts from PVTbased RPMs. Initially, alarms were based on gross-count thresholds established to determine the likely deviation of a particular signal over background radiation. However, this approach did not take into account any energy information available from the plastic scintillator (Ely et al. 2006). Application of energy windowing algorithms in PVT RPMs (see Section 4.2) to data collected in the field leads, in principle, to considerably reduced NORM alarms. This is shown in data from Weier et al. (2005), in which gross-count criteria resulted in 1731 alarms for NORM cargo, whereas energy window criteria resulted in 227 alarms—a reduction of 87%. The number of medical alarms was basically unaffected because their signatures are essentially indistinguishable from targeted materials when detected with PVT-based RPMs. While the energy windowing approach has been demonstrated in principle and has been activated in the field, practical issues related to implementation remain. The most significant of these is the need to retain a relatively low gross-count threshold in order to ensure that other threats not distinguished with energy windowing are detected. In practice, this leads to the same problem of inflated numbers of nuisance alarms. However, it is feasible to use energy windowing criteria in conjunction with metadata, such as cargo information, vehicle frequency, and other situational awareness factors to classify alarms as “highly likely NORM” or some similar designation to indicate the alarm does not imply a threat condition. Thus, with appropriate signal information mapped to alarm displays, RPM systems can be made to alert regarding the presence of a signal, inform the inspection officers regarding the likely content of the signal, and, thus, guide the intervention process. Depending on the complexity of the physical signal classification and metadata available, a considerable amount of information can be provided to the inspection officer to permit better judgment and choice regarding necessary intervention procedures. Existing data suggest that NORM can be classified with reasonable accuracy, while medical isotopes (which represent a surrogate for a radiation threat) are classified differentially based on energy spectrum ratios. Energy windowing and other statistical or logical classification rules based on various RPM signal data and metadata—such as rapidly obtained manifest information—could provide a much-needed enhancement to the current implementation of RPM systems in the field. At a minimum, the use of ratio data in the low-energy spectrum might form the basis for decision rules that can assign a probability (likelihood) that any specific RPM alarm signal is more like background (likely NORM), or more like SNM (likely threat). Coded logic for threat likelihood classification might take the following form: IF RPM gross counts > X, AND energy windowing ratio < 0.05 AND commodity = kitty litter THEN threat level = PROBABLE NORM, ELSE CHECK NEXT COMMODITY

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In order for this type of classification system to be developed for practical use in operational settings, a number of requirements must be met. The most important of these is the ability to rapidly capture and integrate metadata concerning the specific vehicle or cargo container being screened.

4.6.7 human factors impact The human factors discipline is concerned with enhancing the effectiveness of human–machine systems. With respect to the use of detection and warning systems, RPMs show a number of similarities to other technical domains. A substantial volume of literature suggests that detection systems for low-probability/high-consequence events can lead to system mistrust and lower alarm effectiveness if not properly designed. Bayesian analysis of alarm probabilities in RPM systems suggests that a true threat alarm may not occur during the life cycle of the system, because of the extremely low base rate of nuclear smuggling events and the large number of vehicles screened. To reduce the system mistrust that these circumstances are likely to engender, despite rigid response protocols, a likelihood display approach can provide additional RPM signal information to help classify alarm events. This method will enhance confidence that the system is actually working, while reducing the problems associated with high rates of nuisance alarms. Methods based on energy windowing represent one basis for this approach in PVT RPMs, which can be extended to advanced spectroscopic portals as they become available. To support the development of effective likelihood displays, collaborative research, development, and evaluation between human factors, statistics, and radiation physics specialists is required. The most likely near-term application of likelihood displays for RPMs is in cargo screening—an area where it is possible to obtain advance information concerning container contents, and where specific isotope signatures can be developed. Application of human factors principles to RPM design is an important area for continued focus, as system effectiveness is a function of both technical and human performance.

4.7 References ANSI. . American National Standard Performance Criteria for Spectroscopy-Based Portal Monitors Used for Homeland Security. ANSI Standard N ., American National Standards Institute, Washington, DC. Bevington PR. . Data reduction and error analysis for the physical sciences. McGraw Hill, New York. Booher HR. . Handbook of human systems integration. Wiley, Hoboken, NJ. Breznitz S. . Cry wolf: The psychology of false alarms. Erlbaum, Hillsdale, NJ. Briesmeister JF. . MCNP™: A General Monte Carlo N-Particle Transport Code, Version . LA--M, Los Alamos National Laboratory, Los Alamos, NM.

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Chambers WH, HF Atwater, PE Fehlau, RD Hastings, CN Henry, WE Kunz, TE Sampson, TH Whittlesey, and GM Worth. . Portal Monitor for Diversion Safeguards. LA-, Los Alamos Scientific Laboratory, Los Alamos, NM. Dzindolet MT, LG Pierce, HP Beck, and LA Dawe. . The perceived utility of human and automated aids in a visual detection task. Human Factors ():–. Ely JH, RT Kouzes, BD Geelhood, JE Schweppe, and RA Warner. . Discrimination of naturally occurring radioactive material in plastic scintillator material, IEEE Transactions on Nuclear Science ():–. Ely J, RT Kouzes, JE Schweppe, ER Siciliano, DM Strachan, and DR Weier. . The use of energy windowing to discriminate SNM from NORM in radiation portal monitors. Nuclear Instruments and Methods in Physics Research A ():–. Ely JH, ER Siciliano, and RT Kouzes. . Comparison of NaI(Tl) scintillators and high purity germanium for vehicle portal monitor applications, IEEE Nuclear Science Symposium Conference Record, Vol. , –, pp. –. Endsley MR. . The role of situational awareness in naturalistic decision making. In CE Zsambok and G Klein (eds.), Naturalistic Decision Making, –. Mahwah, NJ: Lawrence Earlbaum Associates. Fehlau PE, C Garcia, RA Payne, and ER Shunk. . Vehicle monitors for domestic perimeter safeguards. LA--MS, Los Alamos National Laboratory, Los Alamos, NM. Geelhood BD, JH Ely, R Hansen, RT Kouzes, JE Schweppe, and RA Warner. . Overview of portal monitoring at border crossings. In Nuclear Science Symposium Conference Record, Portland, OR, IEEE Transactions on Nuclear Science Volume , pp. –. Hale AR and AI Glendon. . Individual behaviour in the face of danger. Elsevier, Amsterdam, Netherlands. Iwatschenko-Borho M. . Schnellerkennung von Gammakontaminationen. International Magazine for Nuclear Energy :–. Iwatschenko-Borho M, L Dederichs, F Nürbechen, W Schiefer, and W Rieck. . Schnellerkennung von Künstlichen Gammastrahlern mit dem Nbr-Verfahren. Fachgespräch zur Überwachung der Umweltradioaktivität :–. Johns JL. . A concept analysis of trust. Journal of Advanced Nursing :–. Kouzes R, J Ely, J Evans, W Hensley, E Lepel, J McDonald, J Schweppe, E Siciliano, D Strom, and M Woodring. . Naturally occurring radioactive materials. Cargo at U.S. Borders, Packaging, Transport, Storage & Security of Radioactive Material ():–. Kouzes RT and ER Siciliano. . The response of radiation portal monitors to medical radionuclides in border applications. Radiation Measurements ():–. Lee JD and N Moray. . Trust, self-confidence, and operators’ adaptation to automation. International Journal of Human-Computer Studies ():–. Lee JD and KA See. . Trust in automation: Designing for appropriate reliance. Human Factors ():–. Lopresti CA, DR Weier, RT Kouzes, and JE Schweppe. . Baseline suppression of portal monitor vehicle gamma count profiles: A characterization study. Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment ():–. McCormick KR, DC Stromswold, J Ely, JE Schweppe, and RT Kouzes. . Spectroscopic Portal Monitor Prototype. PNNL-SA-, Pacific Northwest National Laboratory, Richland, WA.

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Radiation Portal Monitor Project Deployment Summary Megan Lerchen

5.1 Introduction The RPMP was initiated to provide CBP with technical advice on the interdiction of radiological and nuclear materials, and it grew into a deployment project to specify, procure, and install equipment to interdict illicit radioactive materials at U.S. POEs. The project therefore hinged upon the fundamental technologies and tools capable of detecting radioactive materials (Kouzes 2004; 2005). As described in Chapter 3, the PVT-based RPM is the “workhorse” for most installations. A typical RPM system includes two or more RSPs, a control box, occupancy sensors, shielding/support stands, annunciator assemblies for each portal or lane, capability for a VIS, and analysis/control software components, as shown in Figure 3.13 from Chapter 3. This detection technology was deployed in a variety of configurations depending on the vector- and site-specific needs. The term “vector” refers to the type of port of entry, and they are international mail and express consignment courier facilities, land border crossings, seaports, international airports, and rail border crossings. In most deployments, each lane, gate, or other exit is monitored with at least one pair of RSPs. The exact system configuration for each deployment varies according to the infrastructure and daily operations at each deployment site. A concept of one possible configuration with RSPs located on both sides of multiple vehicle lanes is shown in Figure 3.12. The standard RPM system is typically augmented with a variety of ancillary equipment to meet individual site needs. Factors such as network and voice communication, traffic control, unacceptable impacts to staff presence, and other aspects in minimizing impacts to legitimate trade and travel are considered when establishing a need for additional equipment beyond the base RPM system.

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Examples of ancillary equipment include visual identification systems, surveillance, intercom, wireless, optical character recognition, rail identification, and traffic control systems. A synopsis of ancillary equipment used in RPMP deployments is presented in Table 5.1; more detailed information about this equipment is provided in Chapter 3.

table 5.1 Ancillary equipment available for deployment with radiation portal monitor systems Ancillary Equipment

Description

Area surveillance system Auto dialer

A remote free-running-type camera system with logging, alarm, and viewing capabilities Based on a system signal or alert, an auto dialer will transmit a number of prerecorded messages to preprogrammed telephone numbers for standard telephones, cellular phones, and voice and/or numeric pagers. Booths are provided where RPM systems have been installed in locations where there is no available structure for placing equipment (supervisory computer, annunciators, etc.) and CBP staff who will operate the system. Gate arms are available for additional traffic control in deployments that may be at some distance from a booth. The human machine interface is a touch screen control device for operating RPM systems over the local Ethernet network. Inductive-loop presence sensors detect the presence of a vehicle by inducing a current in the loop as vehicles transit the loop. These are used when optical break-beam presence sensors cannot be used. An intercom system provides communication between a CBP officer in the booth and a driver in the lane. There are two types of intercom systems: wired and network-based (required for wireless communications). Data communication within the RPM system and its ancillary equipment is by a local area network. Ethernet computer system network components are required for network communication on the RPM subsystem and the CBP wide area network. Network components may include such components as cabling, wireless bridges, antennas, managed switches, and data converters. Optical character recognition and reconciliation software convert optical images of identification numbers on intermodal containers into digital values suitable for computerized processing and tracking of containers. The container identification numbers are electronically reconciled with the shipping consist. Optical presence sensors detect the presence of a vehicle (road vehicles, train cars, etc.) that enters the field of view of the RPM with optical break-beam technology. Presence sensors generate signals that indicate when the vehicle enters or exits the RPM and provide a means to calculate vehicle speed. PRIDE will transfer specific data from the RPM system (RPM profile data, radiation spectral data, visual images, and other related information) at a particular POE site to a centralized data warehouse managed by CBP’s Enterprise Data Warehouse organization. PRIDE is under development. At the port, PRIDE will provide a tool to match vehicles from primary scanning to those in secondary scanning. Programmable logic controllers are hard-programmed devices used to ensure synchronized, consistent actions by the RPM system. The strobe/siren alarms with a single-tone siren with a warble sound output and a red strobe light. Traffic lights are used to provide traffic control information to drivers automatically or at CBP discretion. Records images of vehicles that pass through an RPM.

Booth

Gate arms Human machine interface Inductive-loop presence sensors Intercom

Network system

OCR/reconciliation software

Optical presence sensors

PRIDE

PLCs Strobe/siren Traffic lights VIS

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5.2 Deployment Approach RPMs are deployed in conjunction with additional tools for radiation detection, such as PRDs and RIIDs. Furthermore, detection at U.S. ports of entry is part of a Department of Homeland Security multilayered approach for radioactive materials interdiction. Customs and Border Protection staff identified the ports of entry slated for RPM deployment and the priority scheme for these deployments. The deployment schedule was broken down into phases based on the vector locations (international mail and express consignment courier facilities, land border crossings, seaports, international airports, and rail border crossings). Within each phase, the deployment order was further prioritized on a port of entry basis.

5.3 Deployment Process Flow At each POE, deployments move through a standardized series of process steps from initiation through commissioning and postdeployment support. The work scope of a deployment generally consists of site surveys, design, procurement, subcontracting, construction, training, and other associated activities. Site surveys are needed to collect information from the site, identify specific local requirements, and provide an opportunity to establish initial deployment concepts. Infrastructure and RPM system designs, conceptual through final stages, need to be developed for each site. This includes holding meetings and providing needed information so all local stakeholders may review final design concepts and achieve consensus on the recommended approach. Systems and other required material and equipment need to be purchased, and subcontracts for site construction and installation need to be put in place. Then, the site and facilities to accommodate RPM system installation and operations need to be constructed and installed, including the infrastructure for communications within the site and to national centers. Since each facility is unique in many aspects, CBP officers and management need to be trained as part of the commissioning activities, during which time adjustments to operations and facilities are made and final acceptance testing and transfer the systems to CBP for sustained operation are performed. Lastly, technical and operational support to CBP is provided, as requested. The site survey is critical to the success of all subsequent steps in the process. The RPM system designs must be tailored to meet individual port infrastructure and operational requirements to effectively monitor all traffic and cargo without impeding commerce. Construction and installation activities are generally performed by local subcontractors but are managed by RPMP staff. The RPM systems are turned over (or commissioned) to the local port for operations only after CBP supervisory officers have been trained and the RPM system has been accepted, calibrated, and commissioned. In addition, a national data collection

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and integration system is under development to collect and integrate all RPM system data. The following sections present deployment strategies for each of the RPMP vectors.

5.4 Northern and Southern Land Borders The RPMP provides RPMs for radiation scanning of cargo, privately owned vehicles, and rail traffic. Each land crossing also has an RPM available for secondary scanning of vehicles that caused alarms during primary scanning. This secondary RPM may be an additional portal at most crossings, or a primary RPM through which vehicles pass a second time. This section describes the scanning for cargo and privately owned vehicles; rail crossings are sufficiently different to merit independent treatment and are described later in this section. Land crossing RPM systems for privately owned vehicles and cargo are based on a lane-by-lane RPM concept with limited exceptions. The privately owned vehicle deployments utilize standard RPM systems and typically operate with low alarm rates because there is generally no significant volume of NORM in privately owned vehicle traffic. The use of shielding walls may be necessary at some sites to prevent cross talk alarms from traffic in cargo lanes or pedestrians containing medical radionuclides (see Section 4.3). Bus lane deployments utilize standard cargo-type RPM systems. Cargo lane deployments utilize standard four-panel RPM systems as well as eight-panel wide lane systems, and a few cantilever designs where panels cannot be deployed on both sides of a road. Some cargo deployments utilize visual identification systems.

5.4.1 primary scanning The size and complexity of land border crossings vary considerably depending on the crossing configuration and traffic volume served. The RPM system complexity tends to increase with a greater number of lanes and traffic volumes. Primary radiation scanning at land crossings is with RPMs configured for the traffic and plausible threat in a particular lane (generally grouped into privately owned vehicle and commercial lanes). The privately owned vehicle lanes primarily handle cars, vans, pickup trucks, and sometimes recreational vehicles. Commercial lanes handle truck and cargo traffic but are also capable of handling other types of commercial road vehicles such as cars, light trucks, and buses. At some crossings, dual-use lanes are installed to handle both privately owned vehicle and commercial traffic. RPM designs are based on a lane-by-lane concept where RSPs are positioned on both sides of the lane. The privately owned vehicle RPMs typically have two RSPs deployed on opposite sides of the lane, while commercial traffic portals

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typically use four RSPs, with two vertically stacked panels on opposite sides of the commercial lane. Specific port conditions and operations ultimately determine the need for minor departures from these two main RPM geometries. For example, at ports with a commercial wide-cargo lane, the number of panels is doubled (four RSPs on each side of the lane) to ensure that sensitivity requirements are met despite the greater distance between RSPs. Land crossing RPM systems include infrastructure such as foundations, conduit, and cabling. In general, limited ancillary equipment is needed at land crossings for RPM system deployment as there is generally adequate infrastructure such as buildings, space, land, and devanning facilities. At some crossings, a visual identification system may also be deployed at CBP discretion. In addition to RPM systems, land crossings also have a considerable number of existing or planned technologies/programs to either improve the capabilities of CBP or to facilitate movement of traffic, such as license plate readers and the Free and Secure Trade program. Minimal, if any, interface is required with these technologies, although interference with their infrastructure and operation must be avoided. The following portal configurations represent the range of primary scanning options for land border crossings.

5.4.1.1 Privately Owned Vehicle Lanes Privately owned vehicle lanes have two panels per system, one on each side of the lane for standard spacing, as shown in Figure 5.1. Where a standard two-panel deployment is not feasible, an L-shaped cantilever configuration (with RPMs on one side and above the lane) is acceptable as long as efficiency requirements are met. A one-sided deployment is unacceptable. RPMs installed above the lane without an accompanying side panel are also unacceptable. Vehicles, such as a recreational vehicle, may pass through a privately owned vehicle deployment, but cargo trucks or car carriers must use dual-use or truck-configured lanes.

5.4.1.2 Commercial Lanes Commercial, or cargo, lanes typically have four panels per system, two on each side of the lane, placed vertically end-to-end for standard horizontal spacing, as shown in Figure 5.2.

5.4.1.3 Wide-Load Lanes Some commercial lanes must accommodate commercial wide-load traffic or a wider turning radius. The width of these lanes forces the RSP-to-RSP distance to exceed the maximum distance allowed to meet sensitivity requirements in a standard commercial lane RPM configuration. To mitigate this potential lack of operational sensitivity, the number of RSPs on each side of the lane is doubled so that for wide-lane installations, there are four panels for cars and eight panels for trucks, as shown in Figure 5.3.

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figure 5.1 Radiation portal monitors in privately owned vehicle lanes.

figure 5.2 Standard four-panel cargo radiation portal monitors.

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To further mitigate the loss of sensitivity, all vehicles that pass through wide RPMs proceed at a reduced speed, thereby increasing the detection time.

5.4.1.4 Dual-Use Radiation Portal Monitors Dual-use RPMs are used at lanes that have both commercial and privately owned vehicle traffic; the configuration is optimized for trucks. Like cargo RPMs, dualuse portals have four RSPs with two panels on each side of a monitored lane placed vertically end-to-end for standard horizontal spacing, as shown in Figure 5.4.

figure 5.3 Eight-panel wide-cargo radiation portal monitor at secondary.

figure 5.4 Dual-use radiation portal monitor.

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5.4.1.5 Dedicated Bus/Recreational Vehicle Lanes Like commercial cargo RPMs, dedicated bus/recreational vehicle RPMs have four RSPs deployed on both sides of a monitored lane, with two panels on each side of the lane placed vertically end-to-end, for standard horizontal spacing. Such lanes exist at ports where traffic volume requires it.

5.4.1.6 Cantilever Radiation Portal Monitors In a limited number of cases, port configurations and operations do not allow placement of RSPs on each side of the lane. In these instances, a cantilever RPM can be used, as shown in Figure 5.5. The RSP geometry is determined on a case-bycase basis for cantilever RPMs.

5.4.2 secondary scanning Each land crossing also has an RPM available for secondary scanning of vehicles that caused an alarm in primary scanning. This secondary RPM may be used as an additional RPM(s) at busier crossings. Vehicles may be redirected through a primary RPM a second time at low-volume sites. Crossings that accommodate privately owned vehicles only (no commercial truck traffic) use a two-panel secondary RPM; otherwise, a commercial RPM configuration is required for secondary scanning, as shown in Figure 5.6. When a vehicle passes through an RPM and sets off an alarm, the driver is directed to the secondary RPM for additional scanning. If the secondary scanning confirms the initial alarm, a further examination of the vehicle and cargo is

figure 5.5 Cantilever cargo portal.

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figure 5.6 Secondary and bus portal.

performed. Based on the secondary scanning, the alarm is either cleared or the vehicle is held for further examination and disposition. Most alarms are due to NORM and medical radioisotopes and are thus cleared relatively quickly through secondary scanning.

5.4.3 ancillary equipment Peripheral devices that may be needed for operations include auto dialers (equipment that automatically dials a telephone list, for when CBP is not present), flashing light alarm indicators, traffic control lights, portable detectors, and programmable logic controllers. The site configuration and needs of the local CBP operations determine which of these devices are deployed. Inductive-loop sensors are used only in a limited number of instances for land border crossings, generally in locations where the use of optical presence sensors is precluded for some reason. Land border crossings generally use optical break–beam presence sensors to trigger operation of the RPM system and some ancillary equipment, such as VIS cameras. Also at land border crossings, RPM systems use various programmable logic controllers to ensure consistent, logical operation. Typically, these are used to direct events for other ancillary equipment, such as VIS, based on signals from other system components, such as the presence sensor or RPM alarms. In limited cases, traffic lights are used as necessary for traffic control. Visual identification systems have been deployed to a limited number of locations to electronically provide vehicle identification. One version of VIS provides an image of vehicles passing through choke point RPMs that are out of the line-of-sight of CBP officers. More detailed information on each of these devices is provided in Chapter 3.

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In addition, the RPM systems deployed for land border crossings may be integrated into the system for national data collection and integration; more information about this system is provided in Chapter 6.

5.5 Seaports Seaports are comprised of individual, privately owned and operated terminals. Figure 5.7 shows one example of a seaport terminal. Nearly 95% of all foreign trade comes into the United States through seaports. Over 17 million cargo containers entered U.S. seaports in 2005 alone. The largest U.S. seaports each process over three million containers annually Imported goods entering through seaports are categorized as containerized cargo, bulk cargo (e.g., cement, coal), and break-bulk cargo (e.g., forest products, steel, and other loose cargo not containerized or shipped as bulk). Containerized cargo is shipped in a number of standard intermodal container configurations; bulk and break-bulk cargos are far more variable in configuration. Cargo received at seaports has considerable variability and includes significant amounts of NORM materials including tiles, granite, and other stone; ore; and consumer products like televisions. The bulk of imported goods exit seaports via truck and rail; a small fraction of the cargo is transshipped (off-loaded to another ship or barge) at a limited number of ports.

figure 5.7 Ship at dock at a seaport terminal.

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Radiation portal monitors are used at terminal exit gates and rail access gates for trucks or straddle carriers. Mobile RPMs and fixed dockside RPMs can be used for targeted cargo and transshipped cargo. Generally, there is limited room for secondary processing, and devanning space is limited. Seaport deployments utilize the widest range of ancillary equipment. Besides standard RPMs, some seaports have implemented RO-RPMs that include VIS, area surveillance, wireless communications, and intercom systems and may include optical character recognition, and manifest reconciliation systems. Mobile RPM systems are also largely deployed for use at seaports.

5.5.1 primary scanning The approach to radiation scanning at seaports is based on a “final exit gate” strategy with cargo RPMs deployed on a lane-by-lane basis, as seen in Figure 5.8. Also, mobile RPM systems are used for targeted cargo inspections at seaports. Primary and secondary RPMs are typically deployed in a manner to maintain the flow of commerce and provide a capability to detect and confirm the presence of radioactive materials. One or more primary RPMs are deployed at strategic locations on a terminal or port to provide a rapid pass/fail screen of cargo. A secondary RPM is typically deployed downstream from the primary RPMs to confirm alarms and to provide additional scanning capability. The secondary RPM is typically placed off the main traffic flow and is positioned in a protected area for conducting additional scanning of the alarm-producing truck. An extensive survey of the seaport environment is conducted prior to each RPM deployment. Based on data from terminal operators, local CBP, PNNL, and the port authority (as required), locations at seaport terminals, such as exit gates, seal check booths, natural choke points along the exit process, trailer interchange record gates, and access areas to on-dock rail yards, are considered as viable primary RPM deployment sites. Radiation portal monitors are deployed in standardand wide-cargo configurations in both permanent and relocatable configurations

figure 5.8 Radiation portal monitors at a seaport terminal.

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similar to those found at land crossings. In addition, truck-mounted mobile RPM systems are available where terminal operations may be more fluid. Seaport RPM installations typically include infrastructure, such as foundations, conduit, and cabling. Additional equipment and infrastructure, such as a booth for housing officers and equipment, network equipment, and traffic control equipment, are generally required to enable CBP operation of an RPM deployment at a seaport. Portable or handheld instruments, such as a RIIDs, are used by CBP officers to identify radioactive material that triggers alarms in primary and secondary RPMs. Because CBP staffing, space, and commercial needs are often limited, RPM systems installed at seaports require peripheral equipment and features beyond those strictly required for radiation monitoring. This additional equipment may include traffic lights and associated programmable logic controllers, seaport VIS and associated optical character recognition technology, surveillance video, and wireless communications networks. Seaport terminal exit gates are essentially commercial lanes similar to cargo lanes at land crossings (see Figure 5.9). These standard commercial cargo portals have four RSPs with two on each side of the lane placed vertically end-to-end and standard horizontal spacing. Rail access gates exist at some seaports. If the containers destined to leave the terminal by rail are transported between ship and rail yard on trucks, the rail access gate RPM is based on the standard commercial cargo RPM deployment.

figure 5.9 Seaport terminal exit gate.

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Often, these rail access gates have light usage and are only transited by vehicles operated by port or terminal employees. In these cases, additional ancillary equipment is added to result in an RO-RPM, which is operated from a remote location. In cases where there is a need for a fully functional RPM system that can be quickly deployed to different parts of the site or used in a mobile mode, seaports have mobile RPM systems available. Since some seaports have multiple terminal exit and rail access gates and these gates are rarely used, they do not warrant a continual CBP presence. In these cases, a remotely operated RPM is deployed to ensure scanning occurs while allowing optimal use of CBP officers at busier exits. Seaports are increasingly serving the needs of intermodal container transport and use specialized equipment for activity. At some ports, much of the on-dock handling and transport of containers is done with a straddle carrier, a tall vehicle that can straddle and move multiple containers in a stack. An RPM system tailored to straddle carrier operations has been designed, but none are currently commercially available. Further information on straddle carrier solutions is available in Chapter 3.

5.5.2 secondary scanning Secondary scanning of vehicles that caused an alarm in primary scanning is performed with a nearby secondary RPM. This secondary portal may be an additional portal (see Figure 5.10), or the vehicles may be redirected through a primary RPM a second time. When a vehicle that passes through a seaport RPM creates an alarm, the driver is directed by a CBP officer to the secondary RPM for additional scanning. If the secondary scanning confirms the initial alarm, further examination of the vehicle and cargo is conducted. Based on the secondary scanning, the alarm is either cleared or the vehicle is held for further examination and disposition. Most alarms are from NORM and medical radioisotopes.

5.5.3 ancillary equipment Peripheral devices that may be needed for seaport RPM operations include booths, presence sensors, network equipment, auto dialers for alarm notification, flashing light alarm indicators, and traffic control equipment. Site configurations and operational needs determine which and how these devices are deployed. Remotely operated RPMs are equipped with an area surveillance system tailored to the configuration of the port to allow CBP officers to maintain situational awareness and record events. In limited cases, the operating procedure for the use of an RO-RPM relies on designated CBP officers being informed by auto dialer or a visual and audible alert system, consisting of a strobe and siren in case of an alarm. Seaport RPM systems typically require installation of a booth, in which both operating

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figure 5.10 Primary and secondary truck exit gate portals.

equipment and the CBP officers are housed. Gate arms are used at RO–RPM installations to control traffic. In the case of multiple gates, the human machine interface unit is used for setting up, programming, operating, and reprogramming the programmable logic controller modules. This unit replaces the function of devices such as the annunciator and has audio capability for communicating programmable logic controller, network, and annunciator audio alarms to the CBP booth. In some cases, seaport configurations do not allow for installation of breakbeam presence sensors. In these cases, an inductive-loop presence sensor is used. Intercom systems are installed at RO-RPM systems to allow communication between the CBP officer in the main booth and a driver at the remote site. All RPM systems require a network for electronic communication between system components. The complexity and need for additional network components increases with the number and type of equipment. Typically, the most complex systems are installed at RO-RPM deployments with multiple gate locations. Optical character recognition and reconciliation software is used for RO-RPMs at rail exit gates to provide a tool for assuring that all containers leaving the seaport by rail have been screened. All of the RPM systems generally have optical break-beam presence sensors to trigger operation of the RPM system and various ancillary equipment, such as the VIS cameras. Various programmable logic controllers are used to ensure consistent, logical operation of ancillary equipment, such as traffic lights, strobe/ siren alarms, and VISs. Traffic lights are used where necessary for traffic control,

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typically at RO-RPM deployments. A VIS is typically deployed at RO-RPM systems where a clear record of traffic is required. A VIS is required for use of optical character recognition/reconciliation software at rail access gates. More detailed information about each of these devices is provided in Chapter 3. In addition, the RPM systems deployed at seaports may be integrated into the system for national data collection and integration; more information about this system is provided in Chapter 6.

5.6 International Mail/Express Consignment Courier Facilities International mail and ECCF are entry points into the United States for international mail and small cargo (letters, packages, air cargo containers) handled by organizations such as the U.S. Postal Service, DHL Express, Inc., and Emery Worldwide Freight Services. Handling of these materials is typically by conveyer belt, bulk mail cart, airport tug, or truck. The use of particular equipment and its configuration depends on the size and requirements of the operation in question. All RPM deployments at these facilities use systems of two sizes: a compact design for installation directly over or next to conveyer belts, and the standard larger systems for dock doors or roadways. International mail sites use systems configured for doorways, roadways, and smaller systems for belts. Some systems operate in area mode rather than being triggered by presence sensors, such as at doorways with bags moved by staff. Special needs arise for shielding from X-ray equipment. ECCF sites, generally similar to mail sites, use VIS equipment to identify tug containers.

5.6.1 primary scanning Primary scanning is typically performed with an RPM tailored to the package transport systems and geometry (e.g., conveyer belt, door, tug portals, or area monitors). Equipment deployed during the initial stages of the RPMP included a large variety of detection equipment to meet project schedules and needs. A subsequent campaign to upgrade and standardize equipment resulted in standard RSPs in a variety of configurations, including a smaller RSP appropriate for the equipment used for letter and package handling. Retrofits were driven by frequent design changes to ECCF facilities for a variety of purposes, such as streamlining and automating package handling, expansion of offices to accommodate new technology, company mergers, and subsequent decommissioning of facilities. The variety of configuration challenges for international mail/ECCFs included triple-layer conveyor belts, supervisory rooms separately located, X-ray machines in small spaces, and seasonal fluctuations in package volumes and commodities. Each facility had a unique footprint, with the primary common denominator being minimal space for placing radiation detection equipment. The unique nature

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of each deployment resulted in the use of a variety of customized stands for the RPM system equipment. Two additional RPM systems with smaller RSPs than those used for vehicles were developed to meet the space limitations of conveyor belts and inside doorways. This design was further expanded to include an area monitor for nonstop operation (also known as continuous scanning) to differentiate from vehicle presence sensor mode. Enable/disable switches, locks, and surveillance cameras were also installed to give additional administrative controls in locations where the international mail/ECCF employees had direct access to the RPMs during unstaffed periods. The following RPM applications represent the range of primary scanning options for international mail/ECCFs.

5.6.1.1 Conveyor Belts International mail/ECCFs rely on conveyer belts to move letters and packages. The size of these belts varies based on the facility needs. For purposes of radiation scanning, these may be viewed as either small or large conveyer belt types. Because materials are constantly transported on the belts, RPMs for this application are left in area monitor mode to provide continuous scanning. Conveyor belt RPM deployments can use either two RSPs mounted on opposing sides of the belt (similar to other, standard portal geometries) or a single small RSP usually mounted overhead to monitor the area of interest, as shown in Figure 5.11. In some cases, conveyer belt RPM detector panels may be mounted in a one-sided configuration provided they can meet the RPMP threshold requirements. In all cases, the detector’s view of the belt must be unobstructed.

figure 5.11 Small conveyer belt radiation portal monitor configuration in an international mail facility.

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figure 5.12 Typical radiation sensor panel configuration for a doorway in an international mail facility.

5.6.1.2 Doorways Optimal placement of RPMs at international mail/ECCF facilities is often in building entryways or facility doorways from one handling area to another. The doorways range in size from standard interior doors for foot traffic to industrialtype roll-up doors suitable for truck transit or loading/unloading. The RSPs deployed at these locations are selected to assure that the area of interest is screened. Doorway deployments require two RSPs mounted on opposing sides, similar to other standard RPM geometries. Small doorways may require only a single RSP to monitor the area of interest. For any configuration, efficiency standards are met through RPM threshold adjustment. In all cases, the detectors must have an unobstructed view of items passing through the doorway. Radiation portal monitors deployed at a door are mounted as closely as possible to the door to minimize the room between the door and monitor, preventing the possibility of mail or packages slipping through unchecked. At a door where mail bags are unloaded from a truck and then dragged away, RPMs operate in continuous mode while mail bags are in the area, and revert to measuring background when the area is clear, with appropriate buffer times. A typical doorway configuration is presented in Figure 5.12.

5.6.1.3 Tug Portal Tug portal RPM systems may be placed for scanning packages and mail while in transit from an air cargo plane to a nearby processing facility. Tug portals use either one or two pairs of RSPs located on opposite sides of the tug lane, as shown in Figure 5.13. Cargo containers no more than 1.8 m (6 ft) tall can be checked in a two-panel tug RPM, while cargo containers greater than 1.8 m (6 ft) tall must pass

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figure 5.13 Tug portal for scanning packages and mail while in transit to an international express consignment courier facility.

through a four-panel cargo RPM to meet sensitivity requirements. The speed of tugs pulling air freight containers should be no more than 8 km/h (5 mph).

5.6.1.4 Cart Mounted Because the arrival of packages and mail at international mail/ECCF facilities may be intermittent, a cart mounted, portable RPM system with a small RSP was developed, as shown in Figure 5.14. The cart-mounted system operates in area mode and is equipped with a strobe/siren that is activated in the event of an alarm (i.e., does not require active attention from a CBP officer for operation). The cartmounted RPM must be placed so that the surveillance area is 3.4 m (11 ft) or less from the monitor to meet sensitivity requirements.

5.6.1.5 Truck A small amount of international mail arrives via commercial truck. RPMs that service air containers close to the size of cars are configured as RPMs for cars, while RPMs that service air containers close to the size of trucks are configured as RPMs for trucks, as shown in Figure 5.15.

5.6.2 secondary scanning Secondary scanning at mail/ECCF facilities is typically accomplished with handheld instruments and visual inspection. Gamma- or X-ray imaging systems are also used to allow nondestructive examination of letters and packages.

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figure 5.14 Cart-mounted, portable radiation portal monitor system for international mail/express consignment courier facility.

figure 5.15 Truck portal at an international mail facility.

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5.6.3 ancillary equipment At international mail and ECCFs, peripheral devices that may be needed for operations include auto dialers, VIS, flashing light alarm indicators, traffic control lights, and portable detectors. The operating procedure at many facilities does not rely on continual CBP presence at the RPM. Instead, designated CBP officers are informed by an auto dialer or audible alert of a strobe/siren in case of an alarm. Tug and truck portals that operate on a vehicle basis use optical break–beam presence sensors to trigger the RPM systems. Various programmable logic controllers ensure consistent, logical operation. Typically, programmable logic controllers are used to direct events for other ancillary equipment, such as traffic lights, strobe/ siren alarms, and VIS. Where needed for traffic control of motorized vehicles, such as at tug and truck portals, traffic lights have been included. Depending on local needs, some international mail/ECCF deployments are outfitted with VIS equipment that generates images corresponding with the alarm records to facilitate alarm processing. Site configurations and local operational needs determine which of these devices are deployed. More detailed information on each of these devices is provided in Chapter 3. In addition, the RPM systems deployed for international mail/ECCFs may be integrated into the PRIDE system for national data collection and integration; more information about this system is provided in Chapter 6.

5.7 International Airports At the national air cargo POEs, approximately 2,500 aircraft are processed each day. Air cargo facilities are located at international airports and generally have multiple carriers with individual, separate operations located some distance from each other. In contrast, CBP offices are in a centralized location at each airport. Consequently, although airport operations are widely distributed across each airport, there is a significant driver for a substantial communications network supporting efficient air cargo RPM deployments. Airport facilities must have the capability to monitor air cargo containers from multiple carriers in an efficient fashion. This requires multiple carrier portals on the tarmac with sufficient peripherals to control, assess, and process containers that cause alarms with remote monitoring systems. Peripheral equipment is expected to be similar to that for seaports. Although only initial pilot deployments have been conducted to date, airports must incorporate systems similar to international mail and ECCFs, plus tarmac-based systems that may operate as RO-RPM systems. These can include VIS equipment, area surveillance, wireless communications, and intercom systems.

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figure 5.16 Airport tug radiation portal monitor prototype at test bed facility.

5.7.1 primary scanning Primary scanning will occur in a manner similar to tarmac-based systems employed for ECCF deployments. A prototype airport tug RPM system has been constructed at a PNNL test bed facility, as seen in Figure 5.16. The RPM systems deployed for air will be integrated into a system for national data collection and integration (see Chapter 6).

5.7.2 secondary scanning Secondary scanning of alarming vehicles, such as tugs or containers, will be performed in a manner similar to that at land border crossings or seaports. Because air cargo RPMs will be sited at various locations throughout an airport as needed to scan incoming air cargo, control of cargo that has created an alarm is critical to the success of the deployments. This is accomplished through use of ancillary equipment. After identifying a container that has alarmed, a CBP officer will direct the tug driver to the designated secondary RPM for secondary scanning. Based on the secondary scanning, the alarm is either cleared or the vehicle is held for further examination and disposition.

5.7.3 ancillary equipment Peripheral devices that may be needed for operations include presence sensors, network equipment, auto dialers for alarm notification, flashing light alarm

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indicators, and traffic control equipment, much of which has been discussed in previous sections. Site configurations and operational needs determine which of these devices are deployed. More detailed information on each of these devices is provided in Chapter 3.

5.8 Rail Crossings Rail crossings are a type of land border crossing for railroad traffic. Traffic at rail crossings includes freight and passenger rail. Approximately 70 million metric tons of cargo with a value approaching $80 billion is transported each year through these rail crossings, including 2.5 million rail containers. The bulk of rail cargo is shipped on freight trains; some light cargo and parcels may be shipped by passenger rail. Freight rail cargo consists of a wide variety of containerized and bulk cargo. Rail crossings typically consist of one or two sets of railroad tracks crossing the border. Available track, access, and other facilities for secondary scannings vary between crossings. Further, in some cases, rail traffic enters the country in remote locations, some of which are miles distant from border and port facilities. Installation of rail RPM systems at a crossing is complicated by existing track layout, trackside equipment, urban infrastructure, remote locations, and the need to be located a distance from rail X-ray systems. The rail RPM system is currently under development. Because of the difficulty of separating train cars for secondary processing, rail RPMs must provide as much radiation scanning as possible during primary scanning. This implies the need for spectroscopic identification in primary scanning. Railcar identification is also required, which creates the need for complex, redundant presence-sensing systems that include wheel counters, radio frequency tag readers, and optical character recognition systems. Rail RPMs may also require a technology for matching radiation measurements with X-ray images.

5.8.1 primary scanning Rail border crossings will be monitored with a standardized rail RPM system. The rail RPM concept is based on a RPM with two substantial towers on opposite sides of the track. Initially developed prototype rail RPMs have 12 RSPs per system, 6 on each side of the track placed vertically end-to-end, in two towers of three panels each and with standard horizontal spacing for rail applications. Because of the greater height and increased number of panels, and subsequently the added weight, the rail towers have a more robust structure than other RPM systems. A rail RPM prototype installation at a rail RPM test bed facility (shown in Figure 5.17) relies on initial use of plastic scintillator-based RSPs, although the baseline technology for routine rail deployments is expected to be spectroscopic RPMs.

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figure 5.17 Rail radiation portal monitor prototype.

Prior to initiating radiation monitoring at a rail crossing, operational details must be addressed through development of a standard operating procedure. The procedure requires understanding and addressing such issues as generation of NORM alarms, current port operations, secondary inspection capabilities for radiation sources, and site-specific factors such as inspection capabilities and available facilities. As complementary systems, RPM and X-ray imaging systems facilitate the clearing of alarms. To better integrate these systems, they will be deployed with some shared, common rail identification infrastructure. Rail automated manifest system or hard copy train manifests are also available at rail crossings at least one hour prior to trains crossing the border. This information provides cargo information that will facilitate the clearing of alarms.

5.8.2 secondary scanning Secondary examination at rail crossings of individual cars will most likely have a considerable time impact, depending on the secondary examination process. Consequently, to facilitate the clearing of alarms early in the process, the primary rail RPM scanning process is planned to be spectroscopic. If a railcar requires further examination, the railcar will generally be removed from the train and transported elsewhere. Typically, this will occur at a secondary scanning dock located at a yard or a bonded warehouse.

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In addition to the gathering of spectroscopic information during primary scanning, the CBP officers at most rail crossings have access to handheld radiation detection equipment, such as PRDs and RIIDs. Generally, railcars are too large to achieve the sensitivity needed to conduct a close examination of an entire railcar with standard handheld detection devices. Various devices to solve these problems have been proposed and are under consideration.

5.8.3 ancillary equipment Rail RPM systems require a specialized rail identification system to identify each railcar and container on a train. The rail identification system is similar to the system used in the rail radiography program. This system has been demonstrated in standard railroad operations. The rail identification system is an integrated system of components that includes automated equipment identification, railcar imaging, wheel sensors, a radio frequency identification tag reader, area cameras, a line scan camera, a radar detector, and track circuit overlay. Data from this system can be correlated to the presence sensor data and the individual railcar radiation profiles. The imaging system, based either on area scan cameras or digital line scan cameras, produces a continuous image of an entire train. The area or site surveillance system provides a means to maintain situational awareness by monitoring the pedestrian and/or vehicle traffic surrounding the area of the RPM system. This system includes surveillance cameras, a digital video recorder, and a monitor. Infrastructure used to support RPM placement and operations of the rail identification system and site surveillance systems includes power, lighting (both area and camera-specific), foundations, structural towers, equipment enclosures, communications network(s), telephone network(s), and security fencing. All RPM systems require additional site infrastructure; however, the rail RPM systems require significantly more infrastructure because they are usually placed in remote or urban locations and have more and heavier duty RPM towers and other structures than land border crossings. The RPM systems deployed for rail will also be integrated into the national data collection and integration system (see Chapter 6).

5.9 References Kouzes RT. . Radiation Detection and Interdiction for Public Protection from Terrorism. In Public Protection from Nuclear, Chemical, and Biological Terrorism, eds. A Brodsky and J Johnson, R.H. Madison, WI: Medical Physics Publishing. pp. –. Kouzes RT. . Detecting Illicit Nuclear Materials. American Scientist ():–.

.

http://www.saic.com/products/security/rr-vacis/index.html

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Operational Considerations for Radiation Interdiction James Ely, Richard Kouzes, Denis Strachan, Robert Bass, and Joseph McDonald

Radiation interdiction equipment is deployed into the complex and busy operational environment of border crossings. CBP officers are responsible for enforcing hundreds of laws, and their new major role of protecting against terrorism has added to their workload. This chapter includes some specific operational issues CBP officers encounter on a daily basis.

6.1 Overview of Operations for Radiation Interdiction Border crossings, seaport and airport terminals, mail facilities, and ECCFs operated long before radiation detection devices were needed. Therefore, the installation of radiation detectors required that these devices fit within the physical confines of the existing facilities, and, once installed, have minimal impact on facility operations. Unavoidably, there are impacts on operations from interdiction activities. However, the goal should be to minimize impacts. At border crossings, impact is minimized by incorporating the radiation scanning program into the existing standard operating procedures as much as possible. At land border crossings, this entails the use of pre-existing primary and secondary scanning approaches and the existing infrastructure. At seaports, radiation scanning is typically integrated with the exit gates, where paperwork and container numbers are verified. However, CBP generally does not have booths or other shelters for officers in place at every seaport terminal, and this makes it necessary to install facilities and infrastructure, along with the radiation detection and ancillary equipment. At high-volume crossings, it may be appealing to install RPMs where the least number would be required, such as at the entrance to a facility where a small number of traffic lanes (choke point) feed a large plaza with many booths. An alarming vehicle must then move to an inspection center. However, experience

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with this type of installation indicates that it is extremely difficult to reliably track a vehicle that causes an alarm from the RPM to the inspection gate without stopping all traffic and causing a significant impact on the traffic flow through the crossing. When each lane has an RPM, traffic flows better and the impact on commerce and CBP operations is minimized. At low-volume border crossings, it may seem practical not to install an RPM and, instead, scan cargo with a handheld radiation scanning device. This approach involves a lot of CBP personnel time because it can take several minutes to perform this task, during which time no other traffic can pass. Or, alternatively, another CBP officer must be available to monitor the other vehicles. Therefore, in both cases, it is almost always more cost effective and efficient to install RPMs at each lane of traffic at a border crossing independent of traffic volume. Primary and secondary radiation scanning stations are used to further minimize the impact on the flow of commerce because this type of action is a familiar part of preexisting operating procedures. Primary scanning takes place in the main lanes of traffic (see Chapter 3). If no radiation alarm is triggered, the vehicle is allowed to pass, subject to other conventional indicators that may lead a CBP officer to refer a vehicle for additional inspections. If a vehicle causes a radiation alarm, it is sent immediately to a secondary RPM where it is scanned again, but at a slower speed and under more control. Assuming that the radiation alarm is confirmed by the secondary RPM, a procedure is typically performed, guided by the RPM results, whereby an RIID is used to identify the specific radionuclide that produced the alarm. In the case where the radionuclide cannot be identified, or where an unusual situation produces uncertainty, further checks can be performed. Results can then be shared with a centralized site where radiation detection experts are available to review the information before the vehicle is released. At lowvolume crossings, where a secondary RPM may not be available, typically the cargo is scanned again with the primary RPM for confirmation. Installing new radiation interdiction equipment at an existing facility is challenging, especially when space is limited. At border crossings, space for operations is always at a premium. In the case of RPM installations, the challenge is to fit the extra equipment between the multiple existing traffic lanes. The least amount of space is taken with the back-to-back RPM configuration. In some situations, the RPMs must be staggered side-by-side to fit them into the existing lane spacing. This does not usually hinder the operation of the RPM, but it does increase the total length of the space where these primary scans are performed and in some cases creates operational limitations such as the need for traffic lights to moderate traffic flow. A few sites have such tight space constraints that cantilevered RPM configurations must be used because space does not allow two independent pillars. Shielding the backs of the RPMs minimizes the cross talk between adjacent lanes from radiation signals as well as the background radiation. Special lanes are required at some sites for extra-wide loads requiring larger RPM units containing up to 16 RSPs. Some RPM installations require configurations that are easily

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relocated for sites that frequently undergo reconfiguration, such as some seaport terminals. Mobile RPM systems are also used for interdiction, especially for targeted scanning. As might be expected, in an established international mail facility, an RPM must be adapted to the existing mail-handling equipment. Ideally, an RPM should be placed at a choke point through which all mail and packages pass; for example, as the tractor-trailer rig enters the facility. This is generally not always possible. Multiple systems must then be installed to guarantee that all commerce is scanned. Monitoring rail shipments is particularly difficult because the types of loads in a 2-mile long train can vary significantly. A train can easily contain a large number of cars carrying NORM, which requires time-consuming processing. The width of train cars and engines is limited by the presence of tunnels and the proximity of other tracks on which trains travel, but regulations limit the closest distance an RPM can be placed, thus requiring larger detection systems. The distance between RPM pillars and their proximity to other traffic has implications on the amount of cross talk that occurs. At some border crossings, the commercial traffic is well separated from noncommercial traffic. In these cases, there are a limited number of instances in which a vehicle that causes an alarm in its lane also causes one in a commercial lane. In cases where the commercial and noncommercial traffic lanes are in close proximity, a radioactive cargo in one lane could cause an alarm in any, or several, of the other neighboring lanes. In cases of multiple alarms, all affected vehicles report to secondary scanning—a potentially significant operational burden. A further possibility arises for the case where there is a space-constrained vehicle site and a nearby railway. Alarms can then be induced in the vehicle lanes by train cars containing NORM. When RPMs are installed (and periodically thereafter), the equipment must be calibrated to assure proper operation. This calibration operation requires trained staff and a period of time when a lane can be closed to traffic for approximately 1 hour for a typical PVT-based RPM. Calibration activities are usually performed during off-peak hours. Periodically during routine operation, data that are collected from the RPMs are downloaded to a central site where the data are further analyzed. The data sets can become quite large because of all the RPM data from four or more panels, each with their own data set, background information, metadata (information on the cargo) and, in some cases, pictures of the vehicle. At some of the high-volume border crossings, the size of the data set can be a few gigabytes per day. These data sets must be retained at a central location where terabyte disk storage capacity is required. Operationally, complications can arise before the vehicle arrives at the POE. Information on the cargo contents is needed at the port before the vehicle or ship arrives to determine which cargo will require extra processing. In the case of vehicles, this prenotification might be several hours; for ships, approximately 1 day is needed to process the hundreds of containers on the ship. Electronic manifests, or

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consists (lists of cargo contents), have to be matched to the vehicle or to individual containers on a ship. In cases where a vehicle or container causes an alarm, the consist can be examined to determine if it is reasonable to suspect that an alarm should arise. There are various reasons why a certain cargo type might cause an alarm: for example, NORM in the form of ceramic tile or TENORM such as welding rods containing thorium. If a vehicle or container causes an alarm, much of the information about the alarm and the following secondary examination must be manually entered. At some facilities, this is performed on hard copy or on computer systems. These data are potentially useful in a number of alarm analyses, such as the following: Development of load-specific alarm algorithms Examination of seasonal load variation effects on alarm trends Prediction of staff needs during certain time periods Identification of cargo (and events) that routinely and therefore predictably cause nuisance alarms • Long- or short-term forecasting of equipment functionality based on alarm descriptions and categorizations • • • •

Over the long term, the robustness of the RPM system is important. If a system has a very high mean–time-between-failure rate, there will be fewer times when lanes are removed from service; this is especially important at high-volume POEs. Additionally, at a low-volume border crossing where only a single RPM may be used, a failure might mean the entire crossing must be shut down. The RPM system must be able to perform over long time periods without failure withstanding extreme weather that ranges from hot to subzero temperatures and dry to rainy conditions. The PVT-based RPM systems currently in place are robust with a mean-time-to-failure rate of several years.

6.2 Operational Impacts of Gamma-Ray Alarms Gamma-ray detectors in RPMs are designed for detecting materials of interest with high efficiency, and consequently, they must be physically large systems. Typical plastic scintillator-based RPMs are sensitive to gamma-ray radiation sources over an energy range up to about 3 MeV, corresponding to emissions from most radionuclides of concern. Although beta particles could be detected with these devices, the short range of beta particles in materials means they are quickly absorbed. However, when beta particles are absorbed, they can raise to an energetic bremsstrahlung (X-ray) photon spectrum (see Chapter 2) that is also detectable. Energetic gamma rays (above 1 MeV) present in background radiation are extremely penetrating and require significant shielding with high-density metals. The need for high efficiency, as well as shielding, results in physically large RPMs that have a significant footprint where installed.

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At land border crossings, the main sources of RPM alarms are nuisance alarms from NORM and medical radionuclides. The alarm rate from medical radiopharmaceuticals is similar for both POV and cargo lanes, although there appear to be more alarms on the northern U.S. border than on the southern border. As described in Section 2.4, the most common medical radionuclide is mTc. Common NORM and TENORM that cause alarms include kitty litter, fertilizer, road salt, gravel, stone, ores, and ceramic tiles, as discussed in Section 2.3. Relatively few NORM alarms occur in POV lanes because only small quantities of NORM can be carried in passenger cars. Alarm rates at land border crossings vary greatly and can be in the range from 1 in 50 vehicles to 1 in 1,000 vehicles. Many ports have X-ray imaging systems that are used routinely in secondary inspections. If not positioned far enough away from the RPMs, these X-ray systems can produce primary alarms that must be resolved at secondary inspection locations. At seaport terminals, NORM commodities are encountered, but the cargo container loads tend to be more densely packed. This gives rise to a somewhat higher rate of alarms at some seaport terminals. The radioactive loads may not be equally distributed on a ship, giving rise to sporadic clusters of alarms from containers. Note that while food is transported in large quantities and some has high K content that could, in principle, produce alarms, the packing density of food is generally low. Thus, contrary to some reports, bananas do not actually produce RPM alarms. Mail handling and ECCFs have fewer alarms from NORM because the package sizes are smaller. Alarms tend to be items such as watches and instruments with radium-painted dials, pottery glazed with uranium-bearing materials, and stone countertops. The radiation background at mail handling and ECCF has less of an impact on the operation of RPMs, although nearby X-ray equipment can produce problems that need to be addressed with shielding. Operationally, alarms cause problems when the number of vehicles or containers to be handled exceeds the capacity of secondary processing. Because every alarm must be verified, and the source identified, a sufficient number of CBP officers must be available to process them to keep legitimate trade and travel flowing. Because cargos containing NORM are only weakly radioactive and only produce radiation alarms when present in large quantities, identification of NORM sources is one of the main challenges to the detection and identification of illicit radioactive materials. Strong sources of gamma radiation, such as medical radionuclides, are easily detected and can induce alarms in multiple lanes, causing several vehicles to be routed to secondary scanning areas for processing. This may mean that secondary processing must quickly clear a dozen vehicles. One approach to solving this problem is the development of intelligent data processing algorithms (as discussed in Section 4.3) that can help reduce the number of NORM alarms and thus reduce the burden in secondary scanning.

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Counts/second

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figure 6.1 A record of the gamma-ray signal observed for the passage of vehicles through a radiation portal monitor over a period of about  hours.

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Total counts (cps)

Figure 6.1 displays a record of the passages of many vehicles through an RPM over a 6-hour time period. Several observations can be made from this record. First, it is seen that the average background count rate decreases by about 7% over the 6-hour period due largely to changes in the natural radon background with increasing daytime temperature (Stranden et al. 1984). Such weather-related effects also vary depending on the season, and at times the background can change dramatically during a short period due to activity such as thunderstorms (Crozier 1969; Fujitaka et al. 1992; Gale and Peaple 1958; Miles and Algar 1988; Porstendorfer et al. 1994;). Secondly, when each vehicle transited the RPM, the background count rate was generally suppressed to a varying degree due to shadow shielding (Lopresti et al. 2005). Occasionally, the signal increases above the background, representing the passage of a radiation source. Figure 6.2 shows a 30-second time record of many vehicle passages through an RPM. The variety of temporal shapes can be seen. The majority of traces fall near the baseline, since the vehicles do not contain significant radioactivity. Some traces

0

5

10

15 20 Normalized time

25

30

figure 6.2 Record of the observed gamma-ray signal from a number of vehicles passing through a radiation portal monitor.

Operational Considerations for Radiation Interdiction

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figure 6.3 Shipments of smoke detectors can cause radiation portal monitor alarms due to the presence of Am sources in most units.

have long durations of high-count rates. These are due to evenly distributed NORM loads. Other traces show short durations at high-count rates. These represent vehicles with point sources, such as medical radionuclides. In addition to gamma-ray alarms caused by NORM and medical sources, commercial products containing radiation sources produce some alarms. Figure 6.3 shows a smoke detector that contains a Am source. Only bulk shipments of such devices can produce radiation alarms because the radioactivity of an individual smoke alarm is very small. Industrial irradiation sources are used for such applications as the sterilization of surgical tools, medical supplies, food, inspection of welds (Figure 6.4), liquid-level gauges, curing of plastics, and many other processes. These sources, when they are shipped legally, have adequate paperwork and posting to enable status confirmation. Because of its very high density, depleted uranium (containing less than 0.7%  U) is used in a variety of applications such as shipping shields for commercial radiation sources, counterweights for elevators, sailboat keel weights, airplane components (Figure 6.5), military armor, and armor-piercing bullets. Nuclear reactor fuel and its precursors (yellowcake and uranium hexafluoride) cross the borders on a regular basis. These shipments produce large radiation signatures and need to be specially handled, although there is usually adequate paperwork and vehicle posting to verify the source of the alarms.

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Radiation Detection and Interdiction at U.S. Borders

figure 6.4 Radiation source used as a density gauge for liquids flowing through the vertical cylinder. (Photo credit: Berthold Technologies, TN.)

figure 6.5 Depleted uranium is used in a number of commercial applications, including shipping shields for strong commercial radiation sources, military munitions and armor, and airplane counterweights. (Photo credit: Boeing Company, WA.)

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6.3 Operational Impact of Neutron Alarms Neutron alarms observed at border crossings are caused most often by neutronemitting radionuclides in commercial material or instruments. Neutron sources are fairly rare in commerce, occurring in about one in 10,000 vehicles, and there is a huge range of alarm rates varying from port to port. These sources include welllogging probes (used to examine oil and gas wells), concrete dryness detectors (Figure 6.6), nuclear fuel (Figure 6.7), and uranium-bearing compounds in large quantities, such as yellowcake and uranium hexafluoride. Commercial instruments containing neutron sources principally use Cf or AmBe neutron sources. Although the neutrons emitted from these sources can be relatively energetic and penetrate many materials, their enclosures will reduce the energy of the neutrons. This energy reduction, or moderation, can make the detection of these neutrons somewhat easier because the neutron detectors used in RPMs are more sensitive to low-energy neutrons. Background count rates for neutrons are usually very low, with brief spikes in the count rate associated with cosmic rays. These cosmic ray–induced events are enhanced by the presence of large masses of material such as loads of steel with

figure 6.6 An example of a gauge used to measure concrete dryness and soil density; such gauges can contain both gamma and neutron sources. (Photo credit: Troxler Electronic Laboratories, Inc., Research Triangle Park, NC.)

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Radiation Detection and Interdiction at U.S. Borders

figure 6.7 Nuclear fuel assembly (top) containing thousands of fuel pellets (bottom).

which cosmic rays can interact and produce multiple neutrons. This phenomenon is referred to as the “ship effect” (Kouzes et al. 2008). Figure 6.8 shows a brief neutron spike at channel 31 induced by cosmic rays. Such cosmic ray–induced neutron events are usually short spikes in the neutron count rate and thus can be differentiated from true neutron sources. Figure 6.9 shows the RPM response to a vehicle with both a neutron and a gamma-ray source. The trace shows a broad feature, rather than a spike, and this is characteristic of a neutron source being moved past the detector. All sustained neutron alarms are investigated thoroughly because they indicate a noncosmic source that may be of concern. Neutron sources of extreme concern include improvised nuclear weapons or actual state-produced nuclear weapons. The He proportional counters typically used for neutron detection in RPM systems are somewhat sensitive to vibration and radiofrequency interference. Alternative neutron detection technologies may have additional sensitivities to environmental factors. The neutron counters and electronics need to be designed and installed to have adequate radiofrequency shielding and to have vibration and shock isolation, as these systems can be sensitive to such effects. Requirements for these environmental factors are included in the ANSI standards used to test systems for adequate performance (ANSI 2006).

Operational Considerations for Radiation Interdiction

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Profile (0.1 second samples)

Back

Counts in 0.1second

Front

0

10

20

Gamma

30

40 50 60 70 80 90 100 110 120 130 140 150 Total vehicle duration 15.5 seconds Neutron

figure 6.8 Neutron spike event induced by a cosmic ray as seen at channel .

Profile (0.1 second samples)

Back

Counts in o.1second

Front

0

5

10 15 20 25 30 35 40 Vehicle duration 28 samples = 2.8 seconds, speed: 7.38 MPH Gamma Lo Sum Number of pre sample: 10 Gamma Hi Sum Number of post sample: 10 Neutron Sum

45

figure 6.9 Radiation portal monitor response to a vehicle containing a neutron and gamma-ray source; the center curve is the neutron response.

Because the neutron background count rate is very stable, generally varying appreciably only with altitude, a sensitive alarm threshold can be used. Functional testing of the neutron detection system of an RPM must be performed with a known neutron source to verify its correct response to sources expected to be encountered at monitoring sites.

6.4 National Integration of Radiation Portal Monitor Data The amount of data generated by deployed RPMs and associated monitoring instrumentation is enormous. Each passage through an RPM produces from 10 kB to 100 kB of data, and there are hundreds of thousands of such passages each day. Beyond the immediate use of determining if a vehicle contains radioactive material, the archive of RPM data contains useful information to improve the

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Radiation Detection and Interdiction at U.S. Borders

interdiction process and the RPM equipment. Information on some radiation alarms is communicated to a central site for further analysis. Both of these functions would be better served if the information were automatically communicated and made available electronically; however, it must be realized that the process of acquiring, storing, and analyzing large quantities of data is a problem that needs to be addressed. Early in the project, RPMP staff proposed the creation of a National Integration System (NIS) to perform this automated archive and notification process. The NIS is designed to deliver a compilation of radiation interdiction data to a central site to facilitate a variety of CBP data needs, including real-time resolution of unresolved alarms. It is the primary function of NIS to gather data generated from the RPMs, the RIID, video images, CBP officer notes, and other metadata from each of the ports and create data records that are stored in a central location and are accessible by appropriate authorities. The primary tool used in the NIS is the software program entitled PRIDE. Generally, the data flow at an RPM deployment site stems from the primary and secondary RPMs and video cameras to a site server where it is archived, along with metadata and RIID information. Information, such as the consist and commercial vehicle identification, are added during secondary inspection. Consist data and other information will eventually be merged electronically before the vehicle arrives at the POE. Railcars have electronic identification tags that make it easier to match a consist with a specific car. The PRIDE software allows the radiation profiles of all vehicles that pass through the RPM to be transmitted over a wide area network to the central data store. For those vehicles that cause an alarm at the RPM, a more complete data set is initiated. This set of data includes visual images, RIID scan, and any remaining data that are entered manually or electronically. Scans from the secondary RPM are automatically transmitted, whereas the RIID scans are entered electronically when the RIID is placed in its docking station. Because there may be more than one vehicle in secondary inspection, the secondary information must be matched to the primary data. All this information is transmitted to the centralized data store where it is added by the PRIDE software to the previously transmitted information on the vehicle. All of these data for the vehicle form a record for that vehicle. Another function of the PRIDE software is to gather information on the state-of-health of the RPMs. Information on manual events, such as RPM calibrations, and on automatic events that are part of the PRIDE system, is also transmitted. There are four CBP departments that make the greatest use of the data transmitted by PRIDE, including the following: 1. Laboratories and Scientific Services: This organization provides scientific support to the field and provides forensic capabilities. Use of the PRIDE software can enhance the quality and timeliness of the technical support that Laboratories and Scientific Services provides

Operational Considerations for Radiation Interdiction

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to the POE. With the electronic availability of the information on vehicles, Laboratories and Scientific Services staff can have a real-time display of RPM, RIID, video images, CBP officer notes, and metadata accessible in their office. Laboratories and Scientific Services staff has available sophisticated analysis software to help resolve alarms, and the expertise to provide these analyses. Similarly, the staff has available information analytics software to analyze RPM data and national trend profiles. 2. National Enforcement Equipment Maintenance and Repair: National Enforcement Equipment Maintenance and Repair staff performs the maintenance and repair of the CBP radiation interdiction instrumentation. With these data, National Enforcement Equipment Maintenance and Repair staff can evaluate the state-of-health parameters for the equipment in the system. The system will support display of system errors, remote diagnosis of system failures, and predictive analyses to identify pending component failures. 3. National Command Center: This center can use real-time data from PRIDE to status alarm data and resolution information, system status information, system response control, and trend analyses. The latter includes much better ways of understanding seasonal variations in RPM operation and background radiation. Port alarm patterns can be identified and tracked. . National Targeting Center: This center can enhance operational capabilities, day, and season, targeting analysis by commodity and carrier, and feedback to targeting models and rules. National integration of the RPM systems provides a unique centralized data resource for operations, predictive maintenance, and counterterrorism.

6.5 References ANSI. . American National Standard for Evaluation and Performance of Radiation Detection Portal Monitors for Use in Homeland Security. ANSI ., American Nuclear Standards Institute, Washington, DC. Crozier WD. . Direct measurement of radon- (thoron) exhalation from ground. Journal of Geophysical Research :–. Fujitaka K, M Matsumoto, K Kaiho, and S Abe. . Effect of rain interval on wet deposition of radon daughters. Radiation Protection Dosimetry (–):–. Gale HJ and LHJ Peaple. . A study of radon content of ground-level air at Harwell. International Journal of Air Pollution :–. Kouzes RT, JH Ely, A Seifert, ER Siciliano, DR Weier, LK Windsor. . Cosmic-rayinduced ship-effect neutron measurements and implications for cargo scanning at borders. Nuclear Instruments and Methods A , –.

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Lopresti CA, DR Weier, RT Kouzes, and JE Schweppe. . Baseline suppression of portal monitor vehicle gamma count profiles: A characterization study. Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment ():–. Miles JCH, and RA Algar. . Variations in radon- concentrations. Journal of Radiological Protection :–. Porstendorfer J, G Butterweck, and A Reineking. . Daily variation of the radon concentration indoors and outdoors and the influence of meteorological parameters. Health Physics ():–. Stranden E, AK Kolstad, and B Lind. . Radon exhalation – moisture and temperaturedependence. Health Physics ():–.

{7}

Related Work

Interdiction of nuclear and radiological materials is underway around the world. Activities include developing standards and testing equipment, two important aspects for any instrumentation planned for field deployment. For equipment that will be deployed for interdiction purposes, standards provide vendors with minimum requirements the equipment must meet. Significant efforts have been made (both domestically and internationally) in developing instrument standards for security equipment used at border crossings, and in testing this equipment against these standards. For the last decade, DOE has worked to interdict illicit radioactive and nuclear materials worldwide under the Second Line of Defense (SLD) program. The U.S. Department of Defense (DoD) is also executing the Joint Program Manager (JPM)-Guardian, an active program designed for force protection against radioactive threats. Within the U.S. Department of Homeland Security, other major efforts (beyond the RPMP) largely relate to future requirements for interdiction. In the international arena, several nations have undertaken programs similar to those in the United States, and the International Atomic Energy Agency (IAEA) has been an important leader in these international programs. This chapter summarizes selected efforts related to significant programs for interdiction of illicit trafficking in nuclear and other radioactive materials.

7.1 Testing, Evaluation, and Standards Joseph McDonald Equipment used for detection and identification of illicit radioactive sources became more widely available following September 11, 2001. Health physics instruments had been available for many years, and some of the tasks performed for radiation protection are similar to those used in radiation interdiction. However, the general purpose of radiation protection measurements is to determine the dose equivalent or dose equivalent rate produced by a source of radiation in a controlled environment, such as a research laboratory or a nuclear reactor.

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Radiation Detection and Interdiction at U.S. Borders

Radiation detection equipment initially produced for homeland security purposes was not primarily designed to measure dose equivalent but, rather, provided a quick indication of the presence of unexpected radiation. Subsequent to radiation detection, additional measurements could then be conducted to identify the radiation source. These measurements would also need to be performed in public, commercial, and other relatively uncontrolled areas where portable, batteryoperated equipment and instruments would be required. The environmental conditions to which radiation detection and identification equipment was expected to be exposed included a wide range of temperatures, humidity, and ambient interferences. For example, a handheld device could be carried by an emergency responder who was also carrying a portable transmitter or cell phone, each of which might produce radio frequency electromagnetic radiation that could interfere with the operation of the radiation detection or identification equipment. In addition, emergency conditions may require responders to wear additional protective clothing, thus making it difficult to hear an alarm produced by radiation detection equipment, or to read its panel display of data. It was also anticipated that instruments would be dropped or otherwise shocked. Therefore, it was important to determine whether commercial instruments would operate correctly when challenged by environmental conditions and interferences that might be experienced during expected use. For many years, the performance of health physics instruments has been tested and evaluated prior to purchase or use. Some of this testing experience was used to develop appropriate tests for the newly developed radiation detection and identification equipment. Existing U.S. and international standards for testing health physics instruments were useful models for developing new testing standards for radiation detection equipment and devices intended for interdiction of illicit radioactive materials. However, because of the significantly different nature of the two different types of equipment, the health physics standards could not be directly applied to the new equipment. Health physics instruments are designed to measure dose equivalent or dose equivalent rate, so their response is relatively independent of the energy of the incident radiation. Such instruments are generally less sensitive than radiation interdiction devices. In addition, health physics instruments are often used indoors and may be subject to a narrower range of temperature and humidity fluctuations. These instruments may also be somewhat larger than the radiation detection devices, which may be carried by a CBP officer who requires an inconspicuous device that produces a silent, vibrating alarm. These features are not normally found in health physics instruments. The essential differences in function and design between health physics instruments and radiation detection and identification equipment required the development of new testing and evaluation procedures and standards. The organization most familiar with the development of testing standards for radiation-measuring instruments is the American National Standards Institute

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(ANSI) Committee N42 on Nuclear Instrumentation. An N42 subcommittee addresses radiation protection instrumentation; therefore, several committee members began developing testing standards specifically for radiation detection and identification equipment for interdiction applications. The purpose, and therefore the design, of radiation detection equipment can vary. Equipment can be placed in four general categories. First, there are the small PRDs that may be worn in a pocket or on a belt, or otherwise concealed. These devices may provide an indication of a person in possession of an illicit radiation source at an airport terminal or other public area. The second category is a handheld detector that may be used when a CBP officer conducts a search, and higher sensitivity to radiation is required. Detection of radiation sources in vehicles passing a checkpoint, such as a border crossing or port of entry, is performed with the third category of equipment, large RPMs usually mounted on either side of a vehicle lane. As the vehicles pass through this detector combination, measurements are performed in a few seconds, and small quantities of radiation can be detected. When a radioactive source has been located, an identification of the type of source can be performed with the fourth category, an identification instrument typically containing a small NaI(Tl) scintillator, phototube, and multichannel analyzer to perform a spectroscopic analysis of the photons emitted by the source. The new ANSI standards for homeland security applications released in 2003, and updated in 2006, drew from previous standards such as ANSI N42.17A, Performance Specifications for Health Physics Instrumentation–Portable Instrumentation for Use in Normal Environmental Conditions (ANSI 1994b) and ANSI N42.17C, Performance Specifications for Health Physics Instrumentation– Portable Instrumentation for Use in Extreme Environmental Conditions (ANSI 1994a). In addition, international standards were consulted, including International Electrotechnical Commission (IEC) standard IEC 60395, Portable X or Gamma Radiation Exposure Rate Meters and Monitors for Use in Radiological Protection (IEC 1972). Other international standards dealing with the performance of radiation detection instrumentation also provided information used in the new ANSI standards for homeland security applications (IEC 2004; ISO 1996, 2001). The four main ANSI standards developed for homeland security applications are 1. ANSI N42.32 (2006), American National Standard Performance Criteria for Alarming Personal Radiation Detectors for Homeland Security (ANSI 2006a). 2. ANSI N42.33 (2006), American National Standard for Portable Radiation Detection Instrumentation for Homeland Security (ANSI 2006b). 3. ANSI N42.34 (2006), American National Standard Performance Criteria for Hand-Held Instruments for the Detection and Identification of Radionuclides (ANSI 2006c).

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Radiation Detection and Interdiction at U.S. Borders

4. ANSI N42.35 (2006), American National Standard for Evaluation and Performance of Radiation Detection Portal Monitors for Use in Homeland Security (ANSI 2006d). Table 7.1 provides a synopsis of the scope and purpose of these four ANSI standards and a few of the other standards developed for homeland security purposes.

table 7.1 Description of scope and purpose of American National Standards Institute standards for homeland security applications Standard

Description

ANSI N42.32 Performance Criteria for Alarming Personal Radiation Detectors for Homeland Security (ANSI 2006a)

This standard describes design and performance criteria, along with testing methods, for evaluating the performance of instruments for homeland security that are pocket sized and carried on the body for the purpose of detecting the presence and magnitude of radiation. This standard specifies the performance criteria for radiation detection and measurement instruments that may be used in a variety of environmental conditions. The performance criteria contained in this standard are meant to provide a means for verifying the capability of these instruments to reliably detect significant changes above background levels of radiation and alert the user to these changes. This standard establishes design and performance criteria, test and calibration requirements, and operating instruction requirements for portable radiation detection instruments. These instruments are used for detection and measurement of photon–emitting radioactive substances for the purposes of radiation detection and interdiction, and hazard assessment. The informative annexes of this standard provide reference information. This standard addresses instruments that can be used for homeland security applications to detect and identify radionuclides, for gamma-dose rate measurement, and for indication of neutron radiation. This standard specifies general requirements and test procedures; radiation response requirements; and electrical, mechanical, and environmental requirements. Successful completion of the tests described in this standard should not be construed as an ability to successfully identify all isotopes in all environments. This standard provides the testing and evaluation criteria for RPMs to detect radioactive materials that could be used for nuclear weapons or RDDs. RPMs may be used in permanent installations, in temporary installations for short-duration detection needs, or as a transportable system. These systems are used to provide monitoring of people, packages, and vehicles to detect illicit radioactive material transportation, or for emergency response to an event that releases radioactive material. This standard describes training requirements for homeland security personnel using radiation detection instruments.

ANSI N42.33: Portable Radiation Detection Instrumentation for Homeland Security (ANSI 2006c)

ANSI N42.34: Performance Criteria for Hand-Held Instruments for the Detection and Identification of Radionuclides (ANSI 2006b)

ANSI N42.35: Evaluation and Performance of Radiation Detection Portal Monitors for Use in Homeland Security (ANSI 2006d)

ANSI N42.37: Training Requirements for Homeland Security Personnel Using Radiation Detection Instruments (ANSI, 2006e)

(Continued)

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table 7.1 (Contd.) Standard

Description

ANSI N42.38: This standard describes the performance requirements for Performance Criteria for radionuclide identifying portal monitors, also referred to as Spectroscopy-Based Portal spectroscopic portal monitors. The requirements stated are Monitors Used for Homeland based on monitors used in support of DHS-related work. Security (ANSI, 2006f ) ANSI N42.41: This standard describes the performance criteria for Active Minimum Performance Criteria Interrogation Systems in Homeland Security applications. for Active Interrogation These systems are intended for nonintrusive inspection of Systems Used for closed containers, vehicles, and packages of a wide range of Homeland Security types and sizes. In these systems, the contents of an inspection (ANSI, 2007) zone are irradiated with penetrating ionizing radiation to determine the presence of a hidden substance of interest by the analysis of stimulated secondary radiations or nuclear resonance absorption spectra that are indicative of the chemical and/or nuclidic composition of the substance of interest. ANSI N42.42: This standard specifies the data format that shall be used for both Data Format Standard for required and optional data to be made available by radiation Radiation Detectors Used for instruments for U.S. Homeland Security applications. Homeland Security The performance for these types of instruments is described (ANSI, 2006g) in other standards.

7.2 International Atomic Energy Agency Activities Joseph McDonald and Richard Kouzes The IAEA provides recommendations for monitoring activities to member states and has developed performance specifications and test methods for radiation detection equipment used in monitoring border crossings. Beginning in 1997, a project to evaluate the performance of commercially available radiation detection and identification equipment was conducted jointly by the IAEA and the Austrian Research Centre Seibersdorf. This project was called the Illicit Trafficking Radiation Detection Assessment Program (ITRAP). Laboratory and field tests were conducted at Austrian Research Centre Seibersdorf, and as a result, significant improvements in the specifications evolved. Additional tests and specification improvements were made following a workshop on test procedures at the European Research Centre at Ispra, Italy. Additional tests and measurements were conducted at the IAEA laboratory and the Illicit Trafficking Radiation Detection Assessment Program test site at Austrian Research Centre Seibersdorf, and a final version of the specifications, Technical/Functional Specifications for Border Radiation Monitoring Equipment (IAEA 2005) was written. Because the ANSI standards were being developed at approximately the same time, members of the ANSI writing groups collaborated with the IAEA, which was helpful to both groups in synchronizing the standards.

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Radiation Detection and Interdiction at U.S. Borders

The primary difference between the IAEA specifications document (IAEA 2005) and the ANSI standards is the purpose of the IAEA document is to be a deployment specification rather than purely a test specification, which is the purpose of ANSI standards. System parameters given in the document can be used as the specification for how the equipment will actually be deployed for border security applications rather than only for use during tests and evaluations of equipment from various manufacturers. For this reason, some system parameters may differ between the IAEA document (IAEA 2005) and the ANSI standards. Table 7.2 and Table 7.3 compare a few of the IAEA and ANSI specifications for personal radiation detectors and RPMs. As an example of the difference in approach between the ANSI standards and the IAEA specification, consider the entries in Table 7.3 for the gamma-ray sources. The IAEA specification requires that a particular count rate be observed per becquerel of activity for each source centered in the RPM. In contrast, the ANSI standard simply requires that the RPM alarm 90% of the time with 95% confidence for the indicated radioactive sources passing through the monitor at a specified distance and speed. The ANSI approach allows for pass and fail type–testing for comparison of the operation of different systems, while the IAEA approach table 7.2 Personal radiation detectors Specification

IAEA

Standard background

0.1 μSv⋅h ± 50%

20, 60, 100 μR⋅h–

False alarm rate

≤1 γ/60 min ≤1 n/60 min (≤2/1 h, 95% Confidence Level)

≤1/60 min

Test speed

0.5 m⋅s–

Static tests

γ - sensitivity

0.2 + 0.3 μGy.h– within 5 ± 1 s 0.5 μSv⋅h– at detector with source passing with speed of 0.5 m⋅s– alarms alarms within <2 s with Am,  Cs, and Co in ≥45/50 passages at 0.4 m 20 MBq Activity not specified 1.0 MBq Activity not specified 0.25 MBq Activity not specified



Am Cs  Co Neutron sensitivity 

ANSI N42.32 –

Cf: 20 000 n/s,0.2 MBq, 0.01 μg moving at 10 cm distance with 0.5 m.s– alarms in ≥45/50 passages (PRD on phantom) No alarm with 300 MBq Co at 1 m (100 μSv⋅h–) during 1 h



γ - dose rate indication

0.1–10 μSv⋅h– ± 50% for Am, Cs,  Co, related to Cs (PRD on phantom)

Overrange response

Overrange alarm up to ≥10 mSv⋅h– (300 MBq Co at 0.1 m)

5 measurements from 50 μR⋅h– to 75% of maximum range with ≤30% relative error 2 times maximum range as specified

Physical dimensions

<200 cm, 120 × 65 × 25 mm <250 g

200 × 100 × 50 mm <400 g

Battery life

No alarm: >100 h nonrechargeable, >12 h rechargeable alarm: >3 h

>30 min with continuous alarm “low” indicator for lifetime <4 h

γ - sensitivity of neutron -detector



Cf: 2.5 n⋅s–⋅cm– for 2 ± 0.5 s in <3 s (PRD on phantom)

–

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table 7.3 Vehicle portal monitors IAEA

ANSI N42.35

Pedestrian:

0.1–2 m, 3 m

0.1–2 m, 1 m

Vehicle: Rail: Standard background False alarm rate Test speed

0.2–4.5 m, 5 m 0.3–6 m, 6 m

0.2–4.5 m, 5 m 0.3–6 m, 6 m

0.1 μSv⋅h– ±50% ≤1/10 (95% C.L.) 8 km⋅h– Static tests

> 5μR⋅h (~0.05 μSv⋅h–) ≤1/10 8 km⋅h– Dynamic tests 59/60 passages, trigger alarm MBq at 1 m 90% alarm with 17 MBq at 3 m 90% alarm with 3.5 MBq at 3 m 90% alarm with 0.6 MBq at 3 m 90% alarm with 0.15 MBq at 3 m –

Specification

Search region (Vertical, Horizontal)

γ - sensitivity 

Am Co  Cs  Co Neutron sensitivity static tests Neutron sensitivity dynamic tests 

γ - sensitivity of neutron detectors Innocent alarm suppression

Op. availability Overload characteristics

Occupancy sensor Environmental

Electromagnetic interference

≥200 cps/MBq centered 800 cps/MBq centered 900 cps/MBq centered 1900 cps/MBq centered Cf: 20 000 n⋅s–, 0.2 MBq, 0.01 μg ≥10 cps at 3 m (one pillar)  Cf: 15000 n⋅s–, at 3 m distance, 1 m height, 8 km⋅h– 45/50 alarms No alarm with 100 μSv⋅h– Co in 60/60 tests Rejection: 300 MBq Ra @ 0.5 m from detector (1 μSv⋅h–), 8 km⋅h– ≤ 5/50 alarms. Detection: 300 MBq Ra @ 0.5 m (1 μSv⋅h–), plus 0.3 MBq Cs (0.2 μSv⋅h–) 45/50 alarms 99%, <4 days/year – 

Cf: 2.10 n⋅s– 59/60 alarms at 2 m distance



No alarm with 100 μSv⋅h– Co (2.56 MBq 1 m) 60/60 tests –

– Alarm remains with background of 10 mR/h (11 MBq Cs at 0.1 m), <1 min recovery time, 30/30 tests 1 s interrupt in 10 s each, 100% reliable 99% reliable in 100 consecutive over 8 h also in semitransparent mode tests, based on type of sensor –15°C (–35°C) to +45°C, 90% RH –30°C to +55°C 93% RH +40°C <15% change in count rate relative to +20°C, 65% RH IEC 61000-4, 20V⋅m–∗, DC-1GHz IEC 61000-4-3–4-5… 10V/m, 20 MHz–1 GHZ, <15% change in count rate

provides a requirement for absolute efficiency. Harmonization of the two approaches means that the resulting systems are roughly comparable in performance. In addition to development of the specifications document, the IAEA has sponsored research on detection methods and techniques through its Cooperative Research Program. For example, there is a project to evaluate the detection characteristics of lanthanum bromide, LaBr:Ce+. This scintillator material has properties such as high light output, good energy resolution, and fast decay time,

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Radiation Detection and Interdiction at U.S. Borders

table 7.4 Some properties of scintillator materials Scintillator

NaI(Tl) CsI(Tl) LiI(Eu) Bi4Ge3O12 (bismuth germanate) CaF2:Eu YAP:Ce

Maximum Emission Wavelength (nm)

Decay Time (μs)

Light Yield (1000 Photons/ MeV)

415 540 470 505

0.23 1.0 1.4 0.30

38 52 11 8.2

LaBr3:Ce+

435 370 358

0.9 0.025 0.035

24 10 61 [3]

LaCl3:Ce+

330

0.025

46 [3]

Note: Adapted from Dorenbos et al. 2004; Knoll 1989.

which make it a good candidate for use in the detection and identification of illicit radioactive materials. Currently, materials such as NaI(Tl) and thallium-doped cesium iodide (CsI[Tl]) are used in conjunction with a photomultiplier that detects light pulses produced in the material from the absorption of radiation. Table 7.4 contains information on the characteristics of some scintillator materials used for the detection of radiation. It can be seen that LaBr:Ce+ has some useful properties and compares well with the most commonly used material, NaI(Tl). Through support of such research projects by the IAEA in several countries around the world, the science of interdiction is being advanced while simultaneously engaging these countries in the shared interests of interdiction of illicit nuclear and other radiological materials and other border security needs. The IAEA has a continuing effort in the development of technical documents on interdiction. One document, Combating Illicit Trafficking in Nuclear and Other Radioactive Materials (IAEA 2007), has been published. Two other documents continue to be developed on this topic by panels of international experts: 1. Nuclear Security at Major Public Events 2. National Response Plan for Unauthorized Acts Involving Nuclear and Other Radioactive Material. The IAEA provides a forum for the exchange of information between nations and has generally furthered the improvement of the capabilities of the international community to limit the potential for nuclear terrorism.

7.3 Second Line of Defense Program The SLD program plays a key role in the DOE NNSA nonproliferation mission. This is a separate but coordinated program from the DHS interdiction efforts at

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U.S. border crossings. The SLD program focus is external to the United States and operates primarily through partnerships with agencies within the U.S. government and with government agencies in other countries. The SLD program strengthens the overall capability of partner countries to deter, detect, and interdict illicit trafficking in special nuclear and other radiological materials at international border crossings, seaport terminals, and international airports. The SLD program is consistent with the broader U.S. government and DOE/NNSA defense-in-depth, or layered approach, concept. While other programs within DOE/NNSA enhance the security of weapons-usable nuclear materials in countries of concern (i.e., the “first line of defense”), the SLD program provides an additional layer of defense should vulnerable material be removed from its storage location and be moved across an international border. The SLD program furthers international nonproliferation efforts and helps reduce the probability that special nuclear and other radiological materials could be used in a weapon of mass destruction or a radiation dispersal device against the United States and/or our international partners. In cooperation with partner countries, the SLD program provides radiation detection systems that indicate the presence of special nuclear or other radioactive materials, provides training on the operation and maintenance of the equipment, and provides technical support, if needed. Once the systems have been installed and tested and training has been completed, the responsibility for operation of the equipment and evaluation and dispensation of any radiation alarms is the host government’s. The SLD program not only enhances U.S. national security but also the security of both the host nation in which the equipment is installed and the global intermodal shipping system. A successful terrorist event in the global trade network will likely have impacts on every national economy and to public confidence that are beyond our ability to adequately predict. The SLD program has two main components: the Core Program and the Megaports Initiative. The SLD mission is achieved through execution of these program components.

7.3.1 second line of defense core program The Core Program implements a comprehensive and systematic approach for upgrading detection capabilities for the interdiction of special nuclear and other radiological materials at border crossings, seaport terminals, and international airports. Working in partnership with customs agents in foreign countries, border enforcement officials, and other relevant agencies, the Core Program originally focused on Russia. The program has since expanded beyond Russian borders to include other countries of the former Soviet Union, as well as other key countries in Eastern Europe, and the Caucasus, Baltic, and Mediterranean regions. In 2002, the Core Program also assumed responsibility for providing regular repair and maintenance of legacy radiation detection systems installed by various

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Radiation Detection and Interdiction at U.S. Borders

U.S. government agencies at the end of the Cold War. Since that time, the maintenance component of the Core Program has provided sustainability support for these systems and visited 21 of the 23 countries in which this equipment was installed. One of the countries declined U.S. support for the maintenance of the equipment, and current U.S. policy restricts cooperation with another. Participating countries are located throughout Asia and Europe. Collaboration between the Russian Federation’s Federal Customs Service and DOE/NNSA began in 1998. Since that time, DOE/NNSA has provided equipment at 78 sites in Russia. In addition to the equipment provided by DOE/NNSA, the Federal Customs Service has also equipped a large number of border crossing sites throughout Russia. The initial project deployment was the installation of pedestrian RPMs at Moscow’s Sheremetyevo Airport in the summer of 1998. In addition to work in Russia, DOE/NNSA has provided equipment at sites in Greece and Lithuania. Under the Core Program, work has occurred in many countries, including the Ukraine, Kazakhstan, Slovenia, Slovakia, Georgia, Kyrgyzstan, Lithuania, Latvia, Greece, Turkmenistan, Azerbaijan, Mongolia, Estonia, and Armenia.

7.3.2 second line of defense megaports initiative The world’s largest and busiest ports (megaports) provide an opportunity for terrorists to exploit the international shipping network to deliver an improvised nuclear device or radiation dispersal device. These megaports provide law enforcement officials with the opportunity to scan the bulk of the cargo in the world trade system for SNM and radioactive material suitable for those devices before entering U.S. waters. The SLD Megaports Initiative began in 2003 in an effort to scan containerized cargo for SNM and other radiological materials as the cargo moves through the global maritime shipping network before these materials could be used in an act of terrorism against the United States or our allies. The Megaports Initiative follows a strategy of engagement that considers the volume of container cargo movement to the United States and regional terrorist threat. With this approach, the Megaports Initiative is pursuing the most effective strategy for protecting the U.S. homeland and international commerce. As with the Core Program, radiation detection equipment installed under the Megaports Initiative is designed to indicate the presence of SNM or other radiological materials, alerting foreign customs agents of the need to examine the cargo and take appropriate action. The Megaports Initiative complements DHS’s CBP Container Security Initiative in its effort to safeguard global maritime trade, and to identify and examine high-risk containers as early as possible before they reach the United States. The radiation detection equipment deployed under the Megaports Initiative provides an additional interdiction tool for U.S. CBP officers worldwide. As of 2010, the Megaports program has completed installations at 27 ports: Bahamas, Belgium, Colombia, Dominican Republic, Greece, Honduras (SFI Port),

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Israel, Jamaica, Malaysia, Mexico, the Netherlands, Oman (SFI Port), Pakistan (SFI Port), Panama, the Philippines, Portugal, Spain, Singapore, South Korea (SFI Port), Sri Lanka, Taiwan, Thailand, and the United Kingdom (SFI Port). Implementation is underway at 16 ports in the following locations: Bangladesh, China, Djibouti, Dubai-United Arab Emirate, Egypt, Japan, Jordan, Kenya, Lebanon, Malaysia, Mexico, Panama, and Spain. Agreements for cooperation under the Megaports Initiative have been completed with additional countries; installation activities are at various stages of completion in each. In addition, DOE/NNSA is actively engaged with countries in Europe, Asia, South America, the Middle East, and the Caribbean regarding cooperation under the Megaports Initiative.

7.3.3 united states interagency relationships As a key component in the layered, multiagency approach designed to prevent terrorists from acquiring, smuggling, and using dangerous materials to develop a WMD or RDD that could be used against the United States or its global partners, SLD program officials have established close partnerships with other U.S. government agencies. The DOE/NNSA works closely with the DHS, the DoD, the U.S. Department of State, and other interagencies to develop strategies, share lessons learned, and coordinate complementary activities. The DOE/NNSA coordinates with DHS on its ongoing deployment of radiation detection systems at our nation’s POEs and is an active participant in the establishment of DHS’s DNDO, which is envisioned to be the single point for coordination and resolution of actual or perceived nonmilitary nuclear threat events. The SLD program also works closely with DoD in layered defense activities conducted by the Defense Threat Reduction Agency (DTRA), and with U.S. Department of State and its Export and Border Security Program.

7.4 Department of Defense Programs Richard Kouzes, Michael Catalan, Patrick Kolbas, and David Walter Driven by terrorist incidents within the United States and abroad, the DoD has started to direct significant efforts toward preparedness initiatives designed to safeguard personnel and facilities from the threat of weapons of mass destruction. Following an initial assessment and report by the U.S. Government Accounting Office in September 2002 (GAO 2002), the Deputy Secretary of Defense called for the development of a department-wide integrated chemical, biological, radiological, nuclear, and explosives (CBRNE) approach to installation preparedness in a memorandum (DoD 2002). The memorandum called for complete integration of policies, technologies, equipment, and operational concepts. The memorandum also stated that DoD would begin providing all installation personnel—including



Radiation Detection and Interdiction at U.S. Borders

military and civilian personnel, contractors, and others living or working on military facilities—protection against the wider range of threats. In a separate but related effort, the National Defense Authorization Act for fiscal year 2003 (U.S. Congress 2002) required DoD to develop a comprehensive plan for improving the preparedness of military installations in case of a terrorist event. It required the department to address five elements of a preparedness strategy, and four elements of a performance plan. This is being addressed by the programs described below. Three organizations have responsibilities for the DoD-wide improvement initiatives. The Joint Requirements Office for Chemical, Biological, Radiological, and Nuclear Defense, under the Chairman of the U.S. Joint Chiefs of Staff, is responsible for developing CBRNE concepts of operations and standards for installation preparedness. The Joint Program Executive Office for ChemicalBiological Defense, under the Chairman of the Joint Chiefs of Staff, is in charge of implementing these standards over the next five years, including the JPMGuardian Program to safeguard 200 military installations. The cost of the JPM-Guardian Program is estimated to be $1.6 billion. Oversight of this project is assigned to the Office of the Assistant to the Secretary of Defense for Nuclear and Chemical and Biological Defense Programs. The DTRA, a combat support agency that reports to the Under Secretary of Defense for Acquisition, Technology, and Logistics, is responsible for safeguarding the United States and its allies from WMD (chemical, biological, radiological, nuclear, and high explosives) by providing capabilities to reduce, eliminate, and counter the threat, and mitigate resulting effects.

7.4.1 joint service installation pilot project and unconventional nuclear warfare defense The Joint Service Installation Pilot Project (JSIPP) was initiated by the DoD to enhance U.S. military force protection at the installation level. In September 2002, the department began the $61 million JSIPP at nine diverse DoD installations. The objectives of JSIPP were to equip the selected installations with chemical and biological defense and emergency response equipment, provide an improved emergency response capability for consequence management should a CBRNE incident occur, collect data and refine concepts of operations for chemical and biological defense of installations, and provide recommendations on development of joint chemical and biological defense requirements and budget submissions for chemical and biological defense preparedness and CBRNE emergency responder needs. Activities associated with the JSIPP were completed in fiscal year 2005. The Unconventional Nuclear Warfare Defense (UNWD) program commenced in January 2002, as directed by Congress. At the installation level, the primary objective of the UNWD program was to identify existing technologies and develop concepts of operation to counter the threat from nuclear or radiological weapons

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delivered by means other than missile or aircraft. The $75 million UNWD program—a collaborative effort of multiple contractors, government agencies, and national laboratories—showcased the integration of radiation detection and other technologies at four military bases as a proof-of-principle demonstration. The purpose of the UNWD was to demonstrate the capability for real-time detection and notification of unauthorized radioactive material movement. The project includes the following activities: • Establishing an outdoor test bed for side-by-side testing of radiation sensors with a variety of sources selecting and installing sensors in a number of outdoor locations • Developing a common sensor event reporting format for all sensors • Linking the sensors to a central reporting station with a combination of radio frequency, fiber optic, and copper wire communications links • Selecting cameras and lighting to provide still images and live video of any event that triggered any of the sensors.

Sensor data and alarm reporting are displayed in real time at the command center, resulting in immediate notification of response forces and are logged for further analysis. Stemming from the UNWD program, DTRA also manages the Technical Evaluation and Assessment Monitor Site facility at Kirtland Air Force Base in New Mexico. This flexible, multiuse facility serves as an important test bed to evaluate DTRA and interagency programs and emerging technologies to detect, combat, and defeat nuclear and radiological threats.

7.4.2 installation protection program “guardian” The DoD Installation Protection Program (IPP) constitutes one of DoD’s first efforts to field a full spectrum of chemical, biological, radiological, and nuclear installation protection capabilities designed as a family of systems for military installations and DoD-owned or -leased facilities worldwide. At an estimated cost of $1.6 billion, approximately 200 DoD-owned or -leased facilities are included on the IPP schedule for deployment by 2011. This program—commonly referred to as “Guardian”—started in October 2003. The U.S. Army serves as program manager within the Joint Program Executive Office for Chemical/Biological Defense, and activities are executed through the JPM-Guardian Program. The IPP will provide an integrated protection capability against chemical, biological, radiological, and nuclear events:

 Demonstrations of UNWD technologies took place in 2002–2003 at Kirtland Air Force Base in Albuquerque, New Mexico; Naval Submarine Base Kings Bay, Georgia; Fort Leonard Wood, Missouri; and Marine Corps Base Camp Lejeune, North Carolina.



Radiation Detection and Interdiction at U.S. Borders

• Providing an effective chemical, biological, radiological, and nuclear detection, identification, warning, and protection system for each installation • Ensuring integration of chemical, biological, radiological, and nuclear networks with existing command, control, communications, computers, and intelligence and augmenting capabilities to provide effective information management • Providing a capability that will allow for rapid restoration of critical installation operations following a chemical, biological, radiological, or nuclear event • Protecting DoD civilians, contractors, and other persons working or living on U.S. military installations and facilities from WMD events • The JPM-Guardian Program is deploying real property to provide “near-highest level” of protection at DoD installations to protect personnel, maintain critical military operations, and restore essential operations as quickly as possible following a chemical, biological, radiological, or nuclear event. The Installation Commander or Facility Director will ensure the fielded systems remain operational as they are an integral part of the incident management response system. Continuous operation of the systems allows the military installation or facility personnel to respond effectively to a WMD event.

The JPM-Guardian charter is to provide DoD-prioritized installations with an integrated protection and response systems capability if a chemical, biological, radiological, or nuclear event occurred. The system capability is designed to reduce casualties, maintain critical operations, contain contamination, and effectively restore critical operations. The project is designed to tailor requirements according to installation needs and includes the design and installation of detection systems. Responsibilities include providing an effective installation protection capability, information management, medical protection, surveillance, training, and response, and improving emergency responder capabilities. Providing an effective installation protection capability consists of systems for detection, identification, warning, protection, and decontamination of chemical, biological, and radiological agents. Information management must be provided on chemical, biological, radiological, or nuclear agents and, subsequently, medical protection, surveillance, and response. Improvement in the emergency first responder (medical, fire, hazmat, and security) response must also be included. To optimize the system, the program must leverage existing physical security, logistics, sustainment, maintenance and command, control, communications, and intelligence capabilities to maximize effectiveness while reducing the resource impact (time, funding, and personnel) on the installation. Lastly, but of almost equal importance, is the training of personnel in the use of the new systems. The Joint Chiefs of Staff have provided the JPM-Guardian Program the prioritized list of installations and facilities (by year and by branch of the military

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service) that will receive protection under the IPP. For example, the U.S. Air Force is apportioned 64 of those installations. The Joint Requirements Office worked with the U.S. armed services to prioritize 185 installations within the continental United States and 15 outside the continental United States. Presently, the DoD has committed $1.04 billion to fielding the family of systems and executing program requirements for all 200 installations, and allocated funds to each branch of military service to sustain Guardian-fielded capabilities over the fiscal year period 2004–2009 for all services. The U.S. General Accounting Office has recently reviewed the status of the DoD programs, including the JPM-Guardian Program (GAO 2004). Because of funding and resource constraints, JPM-Guardian’s focus is on mission assurance and consequence management to mitigate terrorist threats against critical missions and essential operations. JPM-Guardian will focus on point-release attacks that are limited in scope and effect. Threats include biological warfare agents, standard chemical agents, radiological material (for building dirty bombs), and toxic industrial chemicals. JPM-Guardian plans to procure government and commercial-off-the-shelf systems designed to meet the specific operational requirements.

7.5 U.S. Department of Homeland Security’s Science and Technology, and Domestic Nuclear Detection Office Efforts Sonya Bowyer The U.S. Customs Service, within the Department of the Treasury, initiated the RPMP in early 2002. In March 2003, by virtue of the Homeland Security Act of 2002, the DHS was created and the U.S. Customs Service was subsumed into DHS as part of a new organization called U.S. Customs and Border Protection. Simultaneously, the Homeland Security Act of 2002 mandated that a completely new organization, the Science and Technology Directorate, be formed within DHS to conduct research, development, testing, and evaluation (RDT&E) for DHS as a whole. The specific areas of interest within the Science and Technology Directorate were designated as portfolios. To address the RDT&E needed to reduce the potential use by terrorists of an RDD or a nuclear weapon, the Radiological and Nuclear Countermeasures (Rad/Nuc) Portfolio was established and consisted of essentially 10 program or focus areas that have been reorganized and consolidated in various ways throughout the portfolio’s existence. The 10 program areas were as follows: • • • •

Systems Architecture and Analysis Systems Integration Pre-Planned Product Improvement Passive Detection



• • • • • •

Radiation Detection and Interdiction at U.S. Borders

Active Interrogation Sensor Networks Surreptitious Entry Consequence Management Crisis Response Attribution and Forensics.

The RDT&E within the Rad/Nuc Portfolio that was directly relevant to the RPMP consisted of efforts in the Passive Detection, Systems Architecture, and Systems Integration program areas. Of particular note is the New York/New Jersey Port Authority Test Bed that originated from DOE and was transferred to the Science and Technology Directorate when DHS was established. The New York/ New Jersey Port Authority approached DOE shortly after September 11, 2001, looking for ways to engage the research and development community and, in doing so, offer their numerous and varied operational venues as operational test beds for radiation and other detection devices. This effort was later renamed the Countermeasures Test Bed, and under it, a significant number of commercially available and developmental radiation detection systems were tested in operational environments, including some POEs. These systems included RPMs, handheld isotope identifiers, and mobile isotope identifiers. Technical evaluations of detection systems, data collected for verification and calibration of models and simulations, and operational evaluations and experiences were all incorporated into the RPMP. In particular, the joint collection and evaluation efforts with the RPMP on spectroscopic portal data in the rail environment (Blaine, Washington) and at a northern land border crossing (Champlain, New York) were tremendously beneficial in providing information on the potential capabilities and requirements for these systems. Within the Passive Detection program area, the ASP program had a significant impact on the RPMP. Although this program is now under DNDO, all ASPs that were to be deployed through the RPMP are a result of the initial work performed under the ASP program. The ASP program was targeted solely at the private sector with the intent of quickly developing commercially available spectroscopic portals for homeland security mission needs. Ten vendors were originally selected to participate in the initial phase of the ASP program with the intent of energizing the somewhat stagnant RDT&E in this commercial market sector by simultaneously sponsoring a large number of individual development efforts. The results of these development efforts were evaluated, and a significantly smaller number of contracts were awarded to vendors that were evaluated favorably and met all required criteria.

 The New York/New Jersey Port was headquartered at and operated the World Trade Center. On September 11, 2001, the port lost 84 employees, including 37 officers and its Executive Director.

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Although appropriately initiated under the Active Interrogation program area, it was recognized as essential to have the capability to routinely conduct controlled operational testing and evaluation of nuclear counterterrorism technologies against high-fidelity threats. To meet this need, the Radiological and Nuclear Countermeasures Test and Evaluation Complex was established at the Nevada Test Site. The initial plans for this complex included a specific facility to support active interrogation technology testing and evaluation, and general facilities to support test and evaluation at venues representative of POEs, ports of departure, domestic checkpoints, and weigh stations. As part of the selection criteria for the commercial ASP system, testing was conducted at a temporary location next to Radiological and Nuclear Countermeasures Test and Evaluation Complex. One of the first programs established by the Rad/Nuc Portfolio was the Secondary Reachback program. This program is ongoing and is now under DNDO as well. As part of the radiation alarm resolution protocols of CBP, as required, CBP officers reach out to specialists within CBP Laboratories and Scientific Services (LSS) to resolve alarms that are difficult to determine as nonthreatening. Personnel at LSS have the capability and technology to resolve most alarms. However, in instances where further verification is required, LSS needed to have the ability to reach out to the nation’s leading spectroscopists, especially with regard to weapons and weapons materials, without raising undue concern before confirming the potential severity of the situation. The Secondary Reachback program established a means for LSS personnel to reach out to experts at the national weapons laboratories working directly for DHS while adhering to required DHS protocols. On April 15, 2005, a Presidential Directive was issued to create the Domestic Nuclear Detection Office as a jointly staffed national office reporting to the Secretary of Homeland Security. This new office would be responsible for developing the global nuclear detection architecture to detect and report attempts to import or transport a nuclear device or radiological material intended for illicit use. This office would also be responsible for acquiring and supporting the deployment of the domestic portion of the global nuclear detection architecture. In addition, the appropriations for the RPMP were transferred from CBP to DNDO in the Fiscal Year 2006 Budget. By means of the Presidential Directive, DNDO was directed to coordinate and accomplish many efforts, and specifically, it was directed to advance the science of nuclear and radiological detection through both evolutionary and transformational research and development programs. Upon its creation, DNDO subsumed the majority of the RDT&E efforts being conducted in the Science and Technology Directorate through the Rad/Nuc Portfolio. The only program areas that initially remained in Science and Technology were the Attribution and Forensics, Crisis Response, and Consequence Management program areas. The DNDO was to be substantially more focused on acquisition of detection capability, the evolutionary (spiral) development of that capability, and deployment



Radiation Detection and Interdiction at U.S. Borders

of that capability in operational environments than was the Rad/Nuc Portfolio. Hence, with the RPMP being the primary domestic radiation detection deployment project, DNDO was to remain sharply focused on ensuring its timely and successful completion. The ASP program, as mentioned above, was aimed at providing advanced capabilities to deployments at our nation’s POEs. Additionally, DNDO launched an effort early on to develop next-generation radiography capabilities with its Cargo Advanced Automated Radiography Systems (CAARS) program. Development programs in handheld isotope identifiers and other identifiers were also launched. DNDO also invested heavily in a very large transformational research and development effort with the intent of producing the required technical solutions of tomorrow; both those to be used at POEs and in the other layers of the multilayered global nuclear detection architecture.

7.6 References ANSI. a. American National Standard Performance Specifications for Health Physics Instrumentation—Portable Instrumentation for Use in Extreme Environmental Conditions. ANSI N.C- (R), American National Standards Institute, Washington, DC. ANSI. b. American National Standard Performance Specifications for Health Physics Instrumentation—Portable Instrumentation for Use in Normal Environmental Conditions. ANSI N.A- (R), American National Standards Institute, Washington, DC. ANSI. a. American National Standard Performance Criteria for Alarming Personal Radiation Detectors for Homeland Security. ANSI N.-, American National Standards Institute, Washington, DC. ANSI. b. American National Standard Performance Criteria for Hand-held Instruments for the Detection and Identification of Radionuclides. ANSI N.-, American National Standards Institute, Washington, DC. ANSI. c. American National Standard for Portable Radiation Detection Instrumentation for Homeland Security. ANSI N.-, American National Standards Institute, Washington, DC. ANSI. d. American National Standard for Evaluation and Performance of Radiation Detection Portal Monitors for Use in Homeland Security. ANSI N.-, American National Standards Institute, Washington, DC. ANSI. e. Training Requirements for Homeland Security Personnel Using Radiation Detection Instruments. ANSI N.-, American National Standards Institute, Washington, DC. ANSI. f. Performance Criteria for Spectroscopy-Based Portal Monitors Used for Homeland Security. ANSI N.-, American National Standards Institute, Washington, DC. ANSI. g. Data Format Standard for Radiation Detectors Used for Homeland Security. ANSI N.-, American National Standards Institute, Washington, DC.

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ANSI. . Minimum Performance Criteria for Active Interrogation Systems Used for Homeland Security. ANSI N.-, American National Standards Institute, Washington, DC. DoD. . Preparedness of U.S. Military Installations and Facilities Worldwide against Chemical, Biological, Radiological, Nuclear, and High-Yield Explosive (CBRNE) Attack. U.S. Deputy Secretary of Defense, Washington, DC. Dorenbos P, JTM de Haas, and CWE van Eijk. . Gamma ray spectroscopy with a circle divide × mm() LaBr : .% Ce+ scintillator. IEEE Transactions on Nuclear Science ():–. GAO. . Combating Terrorism: Preparedness of Military Installations for Incidents Involving Weapons of Mass Destruction. GAO--R, Government Accounting Office, Washington, DC. GAO. . Combating Terrorism: DoD Efforts to Improve Installation Preparedness Can Be Enhanced with Clarified Responsibilities and Comprehensive Planning. GAO--, Government Accounting Office, Washington, DC. IAEA. . Technical/Functional Specifications for Border Radiation Monitoring Equipment. IAEA–SVS-X, International Atomic Energy Agency, Vienna, Austria. IAEA. . Combating Illicit Trafficking in Nuclear and Other Radioactive Materials. Nuclear Security Series No. . International Atomic Energy Agency, Vienna, Austria. IEC. . Portable X or Gamma Radiation Exposure Rate Meters and Monitors for Use in Radiological Protection. IEC Standard , International Electrotechnical Commission, Geneva, Switzerland. IEC. . Electromagnetic Compatibility (EMC)—Part : Testing and Measurement Techniques. IEC Standard -, International Electrotechnical Commission, Geneva, Switzerland. ISO. . X and Gamma Reference Radiation for Calibrating Dosimeters and Doserate Meters and for Determining Their Response as a Function of Photon Energy—Part : Radiation Characteristics and Production Methods. ISO -:, International Standards Organization, Geneva, Switzerland. ISO. . Reference Neutron Radiations—Part : Characteristics and Methods of Production. ISO -:, International Standards Organization, Geneva, Switzerland. Knoll GF. . Radiation Detection and Measurement. nd ed., John Wiley and Sons, New York. U.S. Congress. . National Defense Authorization Act for Fiscal Year  Conference Report to Accompany H.R. , th Congress, nd Session. U.S. Congress, House of Representatives, Washington, DC.

{8}

The Future for Interdiction of Radiological and Nuclear Threats at Borders Ray Warner

Radiation detection systems and additional related technology currently deployed at ports of entry are tailored to serve their specific roles. These technologies have been designed with precise dimensions and packaging and include newly specified user interfaces, electronics, alarm algorithms, and communication capabilities. However, this equipment relies on principles that have previously been demonstrated in field applications. Previous chapters described many of the improvements made to the technology and procedures used to detect and identify illicit radioactive materials. However, in most cases, there is no presumption that this is the most effective, efficient, or economical technology available. Continuing research and development activities are essential if the government is to have affordable radiation detection technology that will ensure that CBP officers interdict attempts to smuggle nuclear or radiological materials. This chapter addresses some of the needs for research and development, and, in some cases, promising approaches for satisfying those needs. Inspection technologies at ports of entry support two key interdiction activities: 1. Primary scanning: All people, vehicles, and other objects arriving at ports must be scanned for contraband. To minimize inspection costs and impact on commerce, this scanning must not require more than a few seconds for each person, vehicle, or container. 2. Secondary inspections: Certain people or objects, whether pretargeted, flagged in primary, or randomly selected, must be examined more thoroughly. Ideally, these inspections involve only a small fraction of those scanned in primary. Secondary inspections may require anywhere from a few minutes to several hours. Technology can enhance primary scanning and secondary inspection capabilities in a variety of ways; for example, it is desirable to do the following: • Provide images of objects otherwise hidden from view • Detect (directly or indirectly) nuclear radiation spontaneously emitted by materials of concern

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• Actively induce materials to emit more easily detected radiations • Perform the above tasks autonomously or as controlled by CBP officers • Record, preserve, and communicate inspection data for remote and/or subsequent detailed analysis and interpretation to better understand the overall process, as well as individual events • Enable quick changes in the balance between speedy commerce and extreme security, on either a local or national scale, as directed by authorities.

Current technologies address all of these needs to some extent, but improvements can enhance security. And, technologies are only tools that help the most important element in the interdiction system—that is, the trained and experienced CBP officer. The technologies must be designed to make the job of the officer easier, safer, and more efficient and not burden the officer with the difficulties often associated with high-tech instrumentation and computer systems.

8.1 Detection Technologies Passive radiation detection, currently the focus of the RPMP, will be considered first, followed by a shorter discussion of imaging and other active methods.

8.1.1 signatures Both nuclear weapons and RDDs rely on the use of materials that emit energetic photons (gamma rays or bremsstrahlung). The characteristics of these emissions are specific to the radionuclides emitting them and thus serve as identifiers or signatures. As discussed in previous chapters, there are numerous natural and other legitimate sources of radiation that require finer distinctions to be made to efficiently scan commerce. Some threat objects also emit neutrons, less commonly found in ordinary commerce. Other radiation emitted by these materials include X-rays, alpha particles (fully ionized helium–4 nuclei), beta rays (electrons or positrons), and neutrinos (most frequently antineutrinos). Of these latter emissions, the most energetic X-rays, gamma rays, and the neutrinos are sufficiently penetrating to be of value for detection, but the neutrinos penetrate so well that it is unlikely a passive detector for them will ever be practical for interdiction purposes. The vast majority of the neutrinos pass through the entire Earth and do not interact with materials at all. Deferring discussion of perturbations of the natural environment, such as the possibility of mu-mesons of cosmic origin scattered by dense objects, neutrons and energetic photons are the major passively detectable radiation signatures. These are the signatures exploited by currently deployed radiation detection systems. Therefore, attention should be given to improving the detection of these radiation detection systems.

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8.1.2 detectors An individual neutron or photon carries at best only two kinds of information of value for detection. This information is its energy and its direction of flight. Both properties may be perturbed as they penetrate matter. However, in principle at least, the energy might indicate the type of source, and the direction of flight might indicate the location of the source. Few existing detectors, in the field or the laboratory, preserve either type of information for individual particles (neutrons or photons). Most neutron detectors preserve no energy information, and photon detectors usually record only a fraction of the photon energy emitted from a radionuclide. In principle, directional information can be extracted only from those particles that interact at least twice in the detector. Unlike charged particles, which can leave a discernible track in some kinds of detector media, the paths of photons and neutrons can only be recorded at those points where they scatter or are captured. These points are usually few; otherwise, the quanta would have a small chance of reaching the detector at all. However, if a detector can record the momenta and energies transferred in at least two locations where scattering or capture occurred, the initial direction and energy can, in principal, be determined. Because direction and energy information for individual particles are so difficult to acquire, scientists rely on distributions of large numbers of detected particles to learn about the source. Collimation (shielding the detector in all directions except for a narrow field of view) provides some directional information by recording only those particles that enter the detector in a specific direction. Detected particles are thus more likely to have arrived from directions that did not require passage through the shielding purposely placed near the detector. They are also more likely to have come from nearby sources than from those more distant. Thus, as long as the signal is strong enough, a moving source or a moving or rotating detector can provide information on source location. If more explicit direction information could be extracted from individual particles, far fewer would be needed to pinpoint a source (or at least identify a plane containing the source). Collimation is less useful for the most common current neutron detectors, which can detect fast, slow, and thermal neutrons from the source with a thermal-neutron detector that is surrounded with a moderator to slow the faster incident neutrons to energies that can be detected efficiently. This slowing or moderation unfortunately eliminates directional information as well as any information about the spectrum of neutrons from the source. Determination of gamma-ray energies can be immensely valuable in distinguishing background and legitimate radioactive sources from those of concern; the energy spectra from sources of concern are sufficiently different. Most current and expected progress in passive detection and identification relies on improving the capability for measuring gamma-ray energies (Knoll 1989). Different detection media have dramatically different capabilities in this regard.

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However, neutron energies are of considerably less value for passive detection. Although neutrons from natural sources, such as cosmic-ray induced neutrons, can have energies much higher than those from sources of concern, present radiation detectors are so insensitive to the high-energy neutrons that there is little to be gained from improvements in this area. New neutron detection methods will have to rely on novel methods because the interaction cross sections of most detector materials are quite small for neutron energies above about 1 MeV. Because of the shortage of He that became apparent in 2008, there is significant work on alternative technologies, which provides the opportunity for innovations to extend the capability of neutron detection (Kouzes et al. 2010).

8.1.3 passive detection For field use, there will always be a need for better radiation detectors. While most requirements are well known to radiation detection researchers, areas for improvement include • Optimizing the energy resolution of spectroscopic detectors • Increasing the efficiency of detectors. It would be desirable to have the full-energy peak efficiency as close as possible to that of detection by a single Compton scattering in plastic for medium- and high-energy photons. • Increasing ruggedness and reliability. Hand-held instruments can be dropped, and installed equipment is always subject to vibration at CBP search locations. • Decreasing price; this is necessary because thousands of instruments are needed, some will be broken in the line of duty, and others will fail as the expected lifetime is approached. • Making primary scanning systems, in particular, more intelligent to reduce dramatically the number of alarms when there is no threat present.

While large plastic scintillators, sodium-iodide crystals, and smaller detectors are most commonly used for gamma-ray detection at ports of entry, newer detection media may have advantages worthy of investigation. Alternatives to helium-3 (He) proportional counters for neutron detection must be considered. Currently there is a serious shortage of He, and the demand has far outstripped the supply (as discussed in Chapter 3).

8.2 Alarm Algorithms Improved intelligent algorithms will use all of the relevant available data to suppress alarms from legitimate sources without reducing the probability of alarming

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on a real threat. More significant than the alarms from background are the innocent alarms resulting from frequent, but legitimate, slightly radioactive materials of commerce (Kouzes et al. 2006). Certain high-energy gamma rays can carry important threat signatures. Unfortunately, the ambient terrestrial background also usually includes high-energy gamma rays from uranium and thorium decay products. Regardless of the energy resolution of the sensor (plastic or sodium iodide), there is a need to distinguish the threat signature from the ambient background, and better algorithms than those currently in use are likely possible. It is particularly important to improve the ability to detect threat signatures that could be embedded in the radiation emitted from legitimate sources. Recent research and work underway demonstrates that existing detector types may collect more useful information than has traditionally been extracted from them, as discussed in Chapter 4. New alarm algorithms for both spectroscopic detectors and those previously considered not of spectroscopic quality look promising, and it seems likely that further improvements are possible. Although this technology may be viewed as being mature and working at its practical, if not theoretical, limits, it is still possible that improvements can be made.

8.3 Signal Processing and Alarm Criteria Among the lessons learned from the voluminous quantity of radiation detection data generated at ports of entry and archived in the past few years is that the simplest detector media already deployed in primary and secondary portals deliver more information than has been routinely used to advantage. Chapter 4 describes some alarm criteria applied (and some that could be applied) to pulses from among the simplest of the gamma-ray detectors in use. Recent studies suggest significant additional capability enhancements that may require little more than further modifying algorithms, for example, deconvolution algorithms (Runkle et al. 2006; Burt and Ramsden 2009). Related schemes may also improve the utility of mediumand high-resolution detectors, although this possibility is even more speculative. A possible reason for the lack of optimization described above is because all the detector media in question have been employed in laboratories and even other field applications for decades. The reasons rest in part on the following important factors distinguishing the specific task of interdicting illicit sources at ports of entry from previous applications of such detectors: • Number of measurements: In the first -year period, during which RPMs were being installed at U.S. ports of entry, more than  million vehicles were scanned for radioactive cargo. To maintain the flow of commerce, % of these vehicles each had to be cleared in a few seconds. Primary decisions had to be made without human input, based on radiation data collected in those few seconds.

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• Presence of legitimate radioactivity: Gamma-ray detectors in all applications pick up background radiation. That the background rate seen by the detector can be altered by the presence of even an empty vehicle was known before the use of RPMs at border crossings. Although this problem had been addressed with energy window algorithms, the optimum settings for such an algorithm are only beginning to be understood. A complication rarely faced in earlier portal monitor applications is the presence of legitimate radioactivity in cargo and the significant cost of delays in clearing such shipments. • Spectrum degradation: A further complication, first faced on large scale at ports of entry, is the presence of surrounding cargo, which can alter the shape of gamma-ray energy spectra from all radioactive sources, whether legitimate or not, thus rendering the distinction more difficult.

8.4 Radioactive Isotope Identification In addition to the advantages energy spectroscopy offers for rapid clearing of traffic containing innocuous radioactive sources, more careful identification of a radioactive source that is possible when the energy spectrum has been recorded is particularly important following some alarms. Although existing radioactive isotope identification systems are appreciably advanced compared with equipment available a few years ago, there is room for improvement. Some improvements will undoubtedly come from commercial manufacturers. For example, if significantly larger sensor elements were incorporated into some of the “smartest” systems currently on the market, this improvement could measurably speed up secondary searches. Similarly, manufacturers may well further simplify the human interface of these instruments for optimal use by busy CBP officers who are constantly faced with competing responsibilities. However, significantly improving the reliability with which certain radionuclides are identified may require more extensive and risky research than is likely to be funded in the commercial sector. Solutions to these more difficult identification problems may arise from new algorithms for energy spectrum analysis, or they may hinge on the development of larger noncryogenically cooled high-resolution detectors. There have been no claims of dramatic advances in radiation detection materials. Most of the applicable research is aimed at incremental advances (Milbrath et al. 2005). While there is little reason to expect a near-term breakthrough in the development of new, efficient, high-resolution detection media, great improvements can be expected for complete instruments. Exploitation of advancements in electronics and communications (both largely driven by the larger economy), custom packaging, and other engineering advances, as well as more deliberate experience-driven choices in overall detector design, will deliver much more capable tools for search and identification, as long as government agencies are

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committed to their use. Major progress can be expected from a combination of the engineering advances mentioned and improvements in algorithms.

8.5 Vehicle Geometry Recording Vehicle presence and speed sensors must be designed and installed such that they function reliably under all-weather conditions. In addition, however, sensors and associated real-time algorithms are needed that will contribute to the unambiguous identification of the vehicle and specific portion of the vehicle from which radiation was detected. Confounding factors that may make this identification more difficult include tailgating vehicles, multisegment vehicles, and vehicles with “holes,” such as automobile carriers. Video systems with fast, automated, image analysis algorithms might provide solutions to these problems.

8.6 Identification and Tracking Subsystems Technological improvements are required to ensure that each person, vehicle, container, parcel, or other object responsible for an alarm is immediately and unambiguously identified, and the identification preserved and communicated to any CBP officer with responsibility for subsequent disposition. While the suggestion that suspect vehicles be marked with auto-fired paintball guns is unlikely to be acceptable, some other technology that accomplishes the same tagging goal is needed. Automated tagging of suspect vehicles is a technology that would be useful.

8.7 Smaller Radiation Detection Systems Regardless of how well large automated systems can detect the presence of unexpected radioactive material, there will always be a role for portable search tools— radiation detectors that can be brought close to indicated or suspicious objects in tight quarters. There will continue to be a need for hand-held or body-worn instruments that have radionuclide identification capabilities but are much more sensitive than those in current use. If an adequately capable instrument is inexpensive, small, and light enough to be routinely worn or easily carried by a CBP officer, it will always be available when needed, whether for locating or identifying radiation sources. Multiple types of portable search and identification instruments are currently used at ports and in other security applications. Many of these instruments detect both gamma rays and neutrons. The neutron detectors make use of helium-3 and solid-state materials such as LiI. Gamma-ray units use a variety of detection methods such as Geiger-Müeller tubes, scintillation crystals, and semiconductors,

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including cryogenically cooled germanium. Sizes range from belt-mounted “pager” units to bulky backpacks with capabilities being roughly proportional to their relative sizes. Few of the products currently in use existed even in prototype form 5 years ago, so improvements are possible. A better instrument is needed for rapid scanning of an intermodal cargo container more than a meter above the pavement to be performed by a CBP officer walking on the pavement around the container. Ideally, the instrument would provide feedback to the CBP officer indicating whether the scanning is being done closely, slowly, and thoroughly enough for adequate sensitivity.

8.8 Imaging and Other Active Probes Technologies that require exposing objects (or people) to radiation levels above what is encountered in the natural environment raise health concerns, and in some instances may not be permitted. Implementation of these technologies may entail substantial capital and operational costs. But, the ability to image suspect objects is invaluable. For the types of contraband that had received the bulk of attention until 2001, transmission X-ray imaging had proved valuable in searches for knives, guns, explosives, drugs, cash, illegal immigrants, illegal products, and so forth. Before the events of September 11, 2001, higher-energy transmission imaging systems were used to examine selected containers at U.S. ports of entry. Compton backscatter imaging (see Chapter 3) had also been demonstrated at ports. These systems and their successors are increasingly valuable components in our border security network. It helps that these systems are not limited to searches for a single type of threat. Systems that routinely demonstrate their value for the interdiction of minor threats and also have the potential for detecting threatening radioactive or nuclear material are extremely valuable. Active detection system approaches with the use of neutrons or photons with higher energies than those in most X-ray units are being considered for detection of fissile material or other heavy elements. Systems under development include those that cause materials of concern to emit large numbers of neutrons or gamma rays that are detected with radiation detectors similar to those currently deployed. Other active interrogation systems include some imaging capability—either by switching between the interrogating and imaging modes, or interrogating in such a way that images are generated simultaneously with the interrogating radiation. Several variations and modifications to these systems have been proposed for further development.

8.8.1 imaging There is little doubt that a picture can convey information to the human brain rapidly, and the brain has as yet unmatched flexibility for interpreting images. For

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this reason, Transportation Security officers view every X-ray image of the bags carried onto airplanes. The same process is used to image cargo containers, although the photon energies are higher, the machines are larger, and the time spent recording the image can be substantially longer. Unfortunately, current systems are still expensive to buy and operate. Therefore, there are not nearly enough of them for the job. In addition, dramatic improvements are needed in inspection speed, photon penetration depth, threat recognition, and general compatibility with operations. But, the tradeoffs among penetration power (beam energy), inspection speed (beam intensity), and radiation safety are daunting. Nevertheless, the technologies are still relatively immature, so there is reason to expect substantial improvement from a vigorous research and development program.

8.8.2 interrogation Chapter 3 includes a discussion of active techniques, some of which have been applied in nuclear safeguards situations. Some approaches may add to interdiction capabilities, and others, while intriguing, are probably impractical. The limitations of some techniques could conceivably be ameliorated by changes in operational restrictions such as constraints on energy, beam intensity, and doses for irradiations, within the limits of radiation safety regulations. As ports of entry are redesigned to efficiently accommodate increased traffic, they may incorporate safety features to handle the radiation levels needed for effective interrogation.

8.8.3 interrogation and detection with imaging Active interrogation and imaging techniques of vehicles from which occupants have been removed are most effective in detecting threats such as highly enriched uranium, massive gamma-ray shielding that might enclose a large radioactive source, and threat objects of special shapes. Objects may be scanned with beams of neutrons, X-ray or gamma-ray photons, acoustic sources, or other radiations. Detectors may be designed to detect the transmitted or scattered incident beam, or other prompt or delayed radiation produced by the interaction of the incident beam with cargo or materials of interest. Most applications require imaging detectors with high spatial resolution (or angular resolution for some schemes) if adequate sensitivity is to be achieved with minimum impact on commerce and CBP operations. Three–dimensional computed-tomography images may be valuable when generated in a timely manner compatible with operations. Systems currently in use for large vehicles and containers are effective and, although expensive relative to passive technologies, may be more useful in the detection of contraband if they were faster and more economical to use. The combination of active interrogation with imaging, whether simultaneous or sequential, appears to be the ultimate capability for discovering nuclear contraband.

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The intense radiation returns stimulated by active radiation probes could be defeated by heavy shielding, but the shield could be identified and localized by high-energy imaging. The shielded volume would then be subjected to increased scrutiny, either by more intense or more prolonged active interrogation, or by other means. Of course, these combination systems face all the development and acceptance problems with which imaging and active interrogation alone must contend.

8.9 Data Handling and System Control As discussed in Chapter 3, raw radiation detection data from instruments deployed at ports does not lose its value the instant the alarm decision is made or the automatic identification is transmitted. If an alarm does result, there will often be a spatial profile and perhaps energy information that can be used to expedite subsequent inspection activities. Optimum use of this information may require that it be transmitted to a secondary inspection location, although in some cases a regional or nationwide expert may review the data to better advise the CBP officers.

8.10 Automatic Triage with Smart Alerts to Remote Centers Alarms are currently addressed first by CBP officers on site. In the vast majority of cases, the supervisor on duty can resolve the situation without requesting outside assistance. When the situation warrants, additional advice or assistance is requested over the telephone. As detection systems become “smarter” and more reliable, under certain circumstances, they clearly should be capable of sending the alarm and all associated data to a remote regional or national center without human assistance. In the extreme case of detection of a nuclear device, the absence of such an automated notification capability could create an unacceptable delay and perhaps escalate the dangerous situation. In more routine cases, automated notification can be expected to occasionally draw expert attention to a situation that might otherwise be overlooked because of human error. In addition to improved reliability and more capable alarm algorithms, research is required to identify situations worthy of such automatic notification. In addition to individual alarms, clusters of alarms occur, for example, when an unusually strong source approaches closely spaced detectors. In the absence of data communication from each of the alarming detectors, the only avenue for resolution is to route every vehicle and object that could have caused the alarms to a secondary location. In some cases, a prompt automated collective analysis of all the relevant primary data would obviate this slowdown. There are other circumstances under which prompt collective analyses may be beneficial, and these are often specified by DHS.

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8.11 Data Fusion One type of automated data fusion already occurs in deployed portal monitors where alarm algorithms rely on input from vehicle presence sensors as well as from radiation detectors. As other kinds of data become available (e.g., cargo manifests), more complex but equally useful types of data fusion may become possible. For example, if a vehicle or container ostensibly contains legitimate radioactive material, such as a medical isotope or, perhaps, a load of NORM, an electronic manifest might trigger an automatic calculation of the expected radiation signature from such activity that is then compared with the actual spectrum from the RPM. Such a comparison could be a valuable aid to an inspector. Contributions of additional data types are easily imagined, though the chances of developing useful fusion algorithms are admittedly difficult. Data fusions might include the following: • Measured vehicle or container weight versus design of record and load manifest, and perhaps density profile from a transmission X-ray scan; • Origin and destination versus cargo type—particularly any radioactive portion; and • Radiation (and other) signatures on file from similar previous loads.

While the benefits from these data fusions appear marginal for the vast majority of cases, once the algorithms are developed, they can run in the background without interfering with operations except for an occasional alert suggesting further investigation by an officer. Data collected from RPMs over a long time period, when associated with other information about traffic entering the country, can reveal much about the state-ofhealth and effectiveness of the technology deployed, the radioactive background at various locations, and the number, strength, and type of radioactive materials or discrete sources entering the country. Even in the first few months of operation, this information proved essential for choosing more appropriate individual alarm thresholds, improving alarm algorithms, and evaluating new system concepts. These after-action analyses were originally performed with data that was periodically manually written to removable storage media and sent to an archive. This onerous but essential approach was tolerated to facilitate better-informed recommendations for change without compromising the security of the data (network communications for these sensitive data were not yet approved at that time).

8.12 Communication Standards It is worthwhile to consider whether all devices associated with RPMs should communicate as network appliances that satisfy Internet standards rather than sensor domain standards. The concept, requirements, and specification for a standardized-sensor interface should be developed, prototyped, and tested.

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A national consensus standard is being considered for this purpose. In addition, a standard (ANSI N42.42 2006) has been developed to define the manner in which data are stored and transmitted for all national security radiation detection instrumentation.

8.13 Modularization (Both Hardware and Software) Modular hardware and software with standard interfaces are essential to achieve long-term performance and affordable life cycle costs; therefore, development in this area would be beneficial.

8.14 Multithreat Interdiction Technology Integration Nuclear and radiological threats are not the only interdiction challenges being considered with new emphasis. Chemical and biological weapons and agents are also of serious concern at ports of entry, as are terrorists themselves. It is important that a few researchers and technologists remain cognizant of developments and plans for interdicting threats other than those of their primary focus, in order to be alert to possible security enhancements and economies available from integrating some of the technologies. This synergy is most obvious for developers of active interrogation and imaging technologies. In addition, the prospect of multithreat sensors at a border station, for example, will require additional capabilities for data fusion.

8.15 Remote State-of-Health Monitoring As operational experience accumulates for currently fielded systems, statistics will become available on lifetimes, rate of degradation, failure modes, optimum maintenance programs, and calibration intervals. Because there will be a very large number of complex systems in the field, remote state-of-health monitoring will be essential. While the basis for communicating state-of-health data is designed into many of the fielded systems, a program to study the state-of-health statistics will be necessary to ensure that an adequate capability is maintained at ports of entry at nearly all times, and that when the capability at one or more ports is substandard, that fact will be securely reported to a central location, along with information adequate to effect prompt and cost-effective repair or other action.

8.16 Control In addition to better data exploitation, regional or national networking of detectors offers centralized control of a large number of systems in the field.

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Two obvious options include remote updating of software and remote changes to alarm thresholds appropriate to changes in regional or nationwide threat level. Security concerns accompanying these capabilities are extremely serious, and the public is unlikely to be told the extent to which such options are implemented, much less the procedural and physical security precautions taken. Similar problems will undoubtedly arise as nonnuclear WMD inspection technologies mature.

8.17 Instruments for the Port of the Future The port of the future will be designed for efficient commerce and secure inspections. It will fully accommodate equipment to satisfy all the security and radiation detection needs identified in this report and others as well. Its equipment and infrastructure will come in permanent and mobile configurations. Manual data entry will be permitted but not required. All data gathered will be both transmitted and archived locally. All instruments will be rugged and self-diagnosing, will be commercially available from multiple sources, and will be maximally modular to simplify maintenance and upgrades.

8.18 Away from U.S. Ports of Entry Not all interdiction takes place at U.S. border crossings. Activity also takes place overseas and at illegal border crossing points.

8.18.1 advance scanning and container security As is recognized in the DOE Megaports and DHS Container Security Initiatives (discussed in Chapter 7), scanning at locations other than U.S. ports of entry can further enhance homeland security. More detection equipment is being deployed outside of the United States; some may be unattended on ships at sea. If scanning has been accomplished at a trusted port before a container (or conceivably, even an entire ship) reaches the United States, there is a question as to whether the integrity of the shipment can be continuously assured, or at least confirmed upon arrival at a port of entry. These questions raise numerous challenges, and the most appropriate technologies may not yet have been recognized.

8.18.2 small-boat scanning Border crossings at other than U.S. Customs ports of entry are generally outside the scope of this book, but it is important to note the problem of small boats of

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foreign origin entering port areas without proactively contacting CBP or the Coast Guard. Further research and development in this area is recommended.

8.18.3 automation aids New–material handling technologies are needed for rapid searches of containers, boxcars, vans, and bulk loads on trucks or ships. A related challenge is that of retrieving individual containers from a ship at sea so that the containers may be inspected before the ship enters a port.

8.19 Summary All of the challenges discussed above are important. Some key areas requiring further development can be summarized as near-term, longer-term, and advanced technology needs. In the near-term, there is a need for improved alarm algorithms, optimized isotope identification, and “smarter” RPMs. These will result in better detection of items of concern and a reduction in the current impact on commerce. Such a “holistic” approach to radiation detection at the borders can only optimize a somewhat effective process for the detection of radiation in vehicles crossing the U.S. borders at land crossings and seaports. At somewhat longer term (roughly 5 years), there needs to be improvements in handheld radiation detection devices to better and more rapidly determine the location and identification of the radiation source that caused a vehicle to be routed to the secondary inspection station. The determination of the vehicle geometry would enable better detection and location of sources by reducing the effects of shadow shielding. Advances in, and the use of, identification and tracking subsystems should also lead to better detection of isotopes and their relation to the source and destination of the cargo. The development of passive systems to detect radioactivity would allow the detection and modeling of the expected cargo. These data could be related to the manifest for the cargo, especially if there is a large mass of shielding involved. These advances would then enable the use of automatic triage and smart alerts both locally and at remote centers to more accurately identify threat cargo with minimal impact on commerce. More advanced (longer-term, greater than 5 years) developments include detection and scanning of small boats that routinely enter U.S. seaports, active interrogation and imaging for better sensitivity to items of concern independent of shielding, and automation of many of the processes that currently require the intervention of a CBP officer. The latter would remove much of the subjectivity that might currently be in the process for determining if a source is an item of concern or not. The above list is not all inclusive; some of the topics not listed may turn out to be more important than these. The authors hope that readers will give all topics

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serious consideration, and that reading this book will suggest other research topics that might otherwise have been overlooked.

five hypotheses Five hypotheses regarding the future of portal monitors have been established to stimulate thinking about what technological paths should be explored (Kouzes et al., 2005; Kouzes and Ely, 2008). These are: Hypothesis #: Currently deployed radiation portal monitors are approaching the limits of passive radiation scanning technology. To improve substantially the sensitivity to items of concern, new detection capabilities are needed. Hypothesis #: Naturally occurring radioactive material is an important factor for the operation of RPMs but is not necessarily a limiting factor. The signatures of naturally occurring radioactive material are fundamentally different from threats and, hence, should be more easily discriminated against with advanced technologies. Hypothesis #: Medical radioisotopes are an operational limiting factor for radiation scanning at borders. The signatures of medical radioisotopes are similar to threats and are more difficult to discriminate against. Hypothesis #: Spectroscopic portal monitors, such as those based on NaI(Tl), will only enhance our scanning capability by about a factor of . Hypothesis #: Imaging is crucial, but other active interrogation techniques are impractical. Some people may have more optimistic views on these topics, but it is up to the reader to make the final determination. There remains much work to be done.

8.20 References ANSI N.. . Data Format Standard for Radiation Detectors Used for Homeland Security. American Nuclear Standards Institute, Washington, DC. Burt C, D Ramsden. . “The Development of Large-area Plastic Gamma-ray Spectrometer.”  IEEE Nuclear Science Symposium And Medical Imaging Conference Record ( NSS/MIC), VOLS -, –. Knoll GF. . Radiation detection and measurement. nd ed., John Wiley and Sons, New York. Kouzes RT and JH Ely. . The Role of Spectroscopy Versus Detection for Border Security and Safeguards. In Methods and Applications of Radioanalytical Chemistry (MARC VII ). Kona, Hawaii. Journal of Radioanalytical and Nuclear Chemistry , No.  (June ). Kouzes RT, JH Ely, JC Evans, WK Hensley, E Lepel, JC McDonald, JE Schweppe, E Siciliano, D Strom, and ML Woodring. . Naturally Occurring Radioactive Materials in

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Cargo at U.S. Borders. Packaging, Transport, Storage & Security of Radioactive Material ():–. Kouzes RT, JH Ely, BD Milbrath, JE Schweppe, ER Siciliano, DC Stromswold. . Spectroscopic and Non-Spectroscopic Radiation Portal Applications to Border Security, IEEE Transactions on Nuclear Science NSS San Juan Conference Record N–, –. Kouzes RT, JH Ely, LE Erikson, WJ Kernan, AT Lintereur, ER Siciliano, DL Stephens, DC Stromswold, RM Van Ginhoven, ML Woodring. . “Neutron Detection Alternatives For Homeland Security”. Nuclear Instruments and Methods A , –. Milbrath BD, BJ Choate, JE Fast, RT Kouzes, and JE Schweppe. . “Comparison of LaBr:Ce and NaI(Tl) Scintillators for Radio-Isotope Identification Devices.” In IEEENSS  Conference Record N– –. San Juan, Puerto Rico. Runkle RC, MF Tardiff, KK Anderson, DK Carlson, LE Smith. . Analysis of Spectroscopic Radiation Portal Monitor Data Using Principal Components Analysis. IEEE Transactions on Nuclear Science ():–.

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{ contributors } All authors are from Pacific Northwest National Laboratory R.B. Bass Software and Engineering Architecture Group, National Security Directorate S.M. Bowyer Nonproliferation Systems Integration, National Security Directorate O.P. Bredt System Design and Integration Group, National Security Directorate R.L. Brodzinski Detector Systems, National Security Directorate M.A. Catalan Engineering, Mechanical and Structural Materials Group, Energy and Environment Directorate P. Doctor Applied Statistics and Computational Modeling, National Security Directorate J.H. Ely Detector Systems, National Security Directorate P.E. Keller Electromagnetics, National Security Directorate P.J. Kolbas Nonproliferation Systems Integration, National Security Directorate R.T. Kouzes Materials Sciences, Fundamental and Computational Sciences Directorate J.A. Leonowich Radiological Sciences and Engineering, Energy and Environment Directorate M.E. Lerchen System Design and Integration, National Security Directorate C.A. Lo Presti Applied Statistics and Computational Modeling, National Security Directorate R.J. McConn, Jr. Radiological Sciences and Engineering, Energy and Environment Directorate J.C. McDonald Environmental Technology, Energy and Environment Directorate K.R. McCormick Radiation Detection, National Security Directorate G.W. McNair Systems Engineering and Integration, National Security Directorate B.D. Milbrath Radiation Detection and Nuclear Science, National Security Directorate R.T. Pagh Nuclear Design, Analysis and Testing, National Security Directorate C.M. Richard Transportation Human Factors, Battelle Seattle S.M. Robinson Radiation Detection and Nuclear Science, National Security Directorate



Contributors

R.C. Runkle Detector Development, National Security Directorate T.E. Sanquist Global Security technology and Policy, National Security Directorate J.E. Schweppe Simulations and Analysis, National Security Directorate E.R. Siciliano Fluid & Computational Engineering Group, Energy and Environment Directorate D.M. Strachan Radiological Materials and Technology Development, Energy and Environment Directorate D.J. Strom Radiological Sciences and Engineering, Energy and Environment Directorate D.C. Stromswold Radiation Detection Group, National Security Directorate R.C. Thompson Chemical, Biological and Physical Sciences, National Security Directorate D.W. Walter Systems Engineering and Integration, National Security Directorate R.A. Warner Radiation Detection Group, National Security Directorate D.R. Weier Applied Statistics and Computational Modeling, National Security Directorate

{index} absolute threshold algorithm, – absorption coefficients, 93, 94–95, 94f, 95f Acoustic Inspection Device, 137, 137f active interrogation techniques fission induced, 156–58, 157f, 158f overview, 155–56 requirements, 159–61 signature detection, 159, 160f that do not induce fission, 158–59 Advanced Spectroscopic Portal (ASP) monitors, 115 deployment strategy, 236–37, 236f development of, 235–36 DNDO and, 302 rail crossings and, 101 airport cargo. See also international airports ancillary equipment, 125t interdiction options, 102 alarms. See also nuisance alarms analysis, 276 expected occurrence of, 74–77, 75f future and, 309–10 gamma-rays and, 276–79, 278f, 279f, 280f neutrons and, 281–83, 281f, 282f, 283f primary determination of, 106, 274, 276 secondary determination of, 106, 274, 276, 277 signal processing and, 310–11 algorithms. See also energy windowing algorithms absolute threshold, 203–4 background suppression and, 205–6, 206f cross-talk suppression and, 204 future and, 309–10 gross-count, 186–87, 186f noncommercial vehicles spatial distributions and, 206–7 NORM and SNM and, 184–85, 186f NORM vs. point source spatial distribution and, 206, 207f overview about, 182–83 spatial optimization and, 201f, 207–18, 208f, 211f, 212t, 213f, 214f, 215f, 216f, 217f, 218f thresholds and nuisance alarms and, 182–84, 183f

types of, 185–86 vehicle speed and, 204–5 American National Standards Institute (ANSI), 12 IAEA standards compared with, 292–93, 292t, 293t RPMs and, 19–20 standards of, 288–90, 290t, 291t amplifier model, 238, 239f ancillary equipment applications, 125t area surveillance system, 125–26, 125t, 126f, 250t auto dialer, 125t, 126–27, 126f, 250t booth, 125t, 127–30, 127f, 128f, 129f, 250t descriptions of, 250t gate arms, 125t, 127, 128f, 250t inductive loop presence sensors and, 125t, 127–29, 128f, 250t intercom system, 125t, 129, 129f, 250t at international airports, 269–70 international mail facilities and ECCFs, 125t, 268 land border, 125t, 257–58 mobile RPMs, 125t network systems, 125t, 129–30 OCR and reconciliation software, 125t, 130–31, 132f, 250t optical presence sensors, 125t, 131, 132f, 250t pendant, 125t, 131 PLCs, 125t, 132–33, 133f, 250t PRIDE, 125t, 131, 250t rail identification system, 125t, 133–34, 133f, 272 seaports, 125t, 261–63 strobe/siren, 125t, 134, 134f, 250t traffic control, 125t, 134, 250t VIS, 125t, 135, 135f, 136f, 250t ANSI. See American National Standards Institute area monitors, 116 area surveillance system, 125–26, 125t, 126f, 250t ASP. See Advanced Spectroscopic Portal; advanced spectroscopic portal monitors atomic bombs, 81, 81f

 auto dialer, 125t, 126–27, 126f, 250t automatic triage, 315 automation aids, 319

background radiation. See also cosmic background; earth overview, 35–36 sources, 35–46 background suppression, 205–6, 206f backscatter radiography advantages and disadvantages, 141–42 examples, 142f, 143f ionizing radiation technologies and, 141–42, 142f, 143f X-ray imaging for cargo, 151, 151f, 152f barometric pressure, 52 baseline depression effect, 194, 196f baseline suppression study double-dip pattern and, 219 ensemble graphs in, 220–21, 221f individual vehicle profile in, 221–23, 222f, 223t observations, 223–24 overview about, 218–20 PBS and, 220–21, 221f PRS and, 224–26, 225f summary, 226 suppression ratios, 224, 224n13 base rate, of threat events, 241–42 Becquerel, Henri, 78 Bonner, Robert C., 6 booth description of, 250t gate arms and, 125t, 127, 128f, 250t inductive loop presence sensors and, 125t, 127–29, 128f, 250t intercom system, 125t, 129, 129f, 250t network systems, 125t, 129–30 technologies, 125t, 127–30, 127f, 128f, 129f bus/recreational vehicle lanes, 256

CAARS. See Cargo Advanced Automated Radiography Systems program cabling, 130 calculation configurations, 69–70, 71f calibration of equipment, 275 cantilever RPMs, 256, 256f cargo. See also airport cargo drive-through scenario results and, 178–80, 179f, 180f gamma radiation systems for, 149–50 IMCC and, 176f, 177–78, 177f, 178f NORM and, 58–60, 60f, 61f observations concerning, 180–81 photon emission spectra from, 61, 62f, 63f, 64

Index prenotification, 275–76 radiography systems, 147–51, 148f, 150f, 151f, 152f scanning future, 154–55 VACIS and, 147, 148f, 149–50 vehicle modeling and, 175–78, 176f, 177f, 178f X-ray and, 150, 150f, 151, 151f, 152f Cargo Advanced Automated Radiography Systems (CAARS) program, 304 cart mounted RPM, 266, 267f CBP. See Customs and Border Protection, U.S. channel-by-channel ratio, 198, 198f classification mapping procedure, 242 commerce, disruption of, 273–74 commercial lanes, 253, 254f communication standards, 316–17 Compton, Arthur, 38 Compton effect, 28–29, 28f computed tomography advantages, 142 example, 143f computers, 113, 114f concrete dryness detectors, 281, 281f configurations calculation, 69–70, 71f doorway, 265, 265f four-lane, 113, 114f ionizing radiation technologies, 140 RPM, 69–70, 71f, 110–13, 111f, 113f, 114f, 115 consists, 276 construction materials, as radiation source, 48–49 contrast sensitivity, 145–46, 145f control of systems, 317–18 conveyor belts, 264, 264f Cooperative Research Program, 293–94, 294t Core Program, SLD, 295–96 cosmic background earth’s atmosphere and, 37–41, 38f, 39f, 40f major solar events and, 37f, 43, 44f, 45, 45f neutron spike, 282, 283f overview, 35–36 RPM correlations and, 45–46, 46t RPM location study of, 41, 42f, 43 Countermeasures Test Bed, 302 counting rates, gamma-ray, 18–19, 19f crane platforms, 103–4 Criteria for Alarming Personal Radiation Detectors for Homeland Security (ANSI), 289, 290t cross-talk, 275 suppression, 204 Curie, Marie, 78

Index Curie, Pierre, 78 Customs and Border Protection, U.S. (CBP) first deployment of RPMs and, 10–11 Fort Street Cargo Facility and, 10–11, 11f goals and objectives, 5–6 mission, 6–7 RPMP initial activities, 7–8 RPMs deployment beginnings and, 9–11

data fusion, 316 handling, 315 integration, 283–85 NORM border crossing, 60–61, 62f operational considerations and, 275, 283–85 two window traffic, 192–94, 195f weather study, 50–51, 50f Data Format Standard for Radiation Detectors Used for Homeland Security (ANSI), 291t DB. See driver-side panels decoder, 129 dedicated bus/recreational vehicle lanes, 256 Department of Defense (DoD) IPP and, 299–301 JSIPP and, 298–99 National Defense Authorization Act and, 298 programs overview, 297–98 UNWD program and, 298–99, 299n1 Department of Energy, U.S. (DOE), 9, 9n5 Department of Homeland Security (DHS), 301–2 depleted uranium, 279, 280f deployment advances, 11–13 approach, 251 ASP monitors and, 236–37, 236f beginnings, 9–11 cantilever RPMs and, 256, 256f cart mounted RPM, 266, 267f commercial lanes and, 253, 254f conveyor belts and, 264, 264f dedicated bus/recreational vehicle lanes at, 256 doorway configuration and, 265, 265f dual-use RPMs and, 255, 255f first, 10–11 at Fort Street Cargo Facility, 10–11, 11f international airports, 268–70 international mail facilities and ECCFs, 263–66, 264f, 265f, 266f, 267f, 268 land border, 252 Northern and Southern land borders, 252–53, 254f, 255–58, 255f, 256f, 257f overview about, 250 privately owned vehicle lanes and, 253, 254f process flow, 251–52

327 rail crossing, 270–72, 271f SAIC and, 12 seaports, 258–63, 258f, 259f, 260f, 262f SPM systems and, 228–29, 233, 236–37, 236f truck RPMs, 266, 267f tug portal and, 265–66, 266f wide load lanes and, 253, 255, 255f detection technologies ancillary equipment applications, 125t area monitors, 116 area surveillance system, 125–26, 125t, 126f, 250t auto dialer, 125t, 126–27, 126f, 250t booth, 125t, 127–30, 127f, 128f, 129f, 250t future and, 307–9 gamma-ray, 107–9, 107f, 108f, 109f gate arms, 125t, 127, 128f, 250t handheld, 123–25, 124f inductive loop presence sensors and, 125t, 127–29, 128f, 250t instruments and capabilities, 106–10 intercom system, 125t, 129, 129f, 250t mobile RPMs, 116–21, 117f, 118f, 120f, 122f, 123 network systems, 125t, 129–30 neutron, 109–10 OCR and reconciliation software, 125t, 130–31, 132f, 250t optical presence sensors, 125t, 131, 132f, 250t overview, 106–7 passive detection and, 309 pendant, 125t, 131 PLCs, 125t, 132–33, 133f, 250t PRD, 123–24, 124f PRIDE, 125t, 131, 250t PSID, 121, 122f, 123 rail identification system, 125t, 133–34, 133f, 272 rail RPMs, 117, 118f RIID, 124–25, 124f RO-RPMs, 118–19, 118f RPM configurations, 110–13, 111f, 113f, 114f, 115 signatures and, 307 SPMs, 115, 116f stationary RPM for straddle carriers, 120–21, 120f strobe/siren, 125t, 134, 134f, 250t traffic control, 125t, 134, 250t VIS, 125t, 135, 135f, 136f, 250t detector response, medical radionuclide alarm events expected occurrence, 74–77, 75f calculation configurations, 69–70, 71f modeling method, 69t, 72 modeling results, 72–73, 73f UNSCEAR and, 76–77

 DHS. See Department of Homeland Security dirty bomb. See radiological dispersal devices disruption of commerce, 273–74 diurnal cycles, 54–55, 55f DNDO. See Domestic Nuclear Detection Office DoD. See Department of Defense DOE. See Department of Energy, U.S. Domestic Nuclear Detection Office (DNDO), 6 ASP and, 302 CAARS program of, 304 RPMP funding and, 14 Secondary Reachback program and, 303 2005 Presidential Directive and, 303 doorway configuration, 265, 265f double-dip pattern, 219 driver-side panels (DB and DT), 222, 222f, 223t drive-through scenario results, 178–80, 179f, 180f DT. See driver-side panels dual-use RPMs, 255, 255f

earth atmospheric background radiation, 37–41, 38f, 39f, 40f terrestrial background radiation, 46–48, 47f ECCF. See express consignment courier facilities elastic scattering, 30, 30f electromagnetic interference effects EMP, 85–86 overview about, 81–82 RFI, 82f, 83–85, 83t sources, 82f, 83–84, 83t summary, 86 electromagnetic pulse (EMP), 85–86 encoder, 129 energy shape distribution, 186f, 188 energy windowing algorithms baseline depression effect and, 194, 196f channel-by-channel ratio and, 198, 198f energy shape distribution and, 186f, 188 first reporting of, 187 five window example of, 198–99, 198f gross-count algorithms and, 186–87, 186f injection studies, 200–202, 201f, 202f multivehicle analysis, 194–98, 197f NAI(Tl) and, 187n7 NORM and, 186–203, 186f, 192f, 193f, 195f, 196f, 197f, 198f, 201f, 202f optimal number of windows and, 190 optimal window discriminator settings, 191 overview, 187–88 shadow shielding and, 194, 196f summary about, 203 three window example of, 191, 192f two window examples using, 188–90, 190–91 two window traffic data and, 192–94, 195f ensemble graphs, 220–21, 221f

Index Ethernet switch, power over, 130 evaluation. See also Port Radiation Inspection, Detection, and Evaluation system; Radiological and Nuclear Countermeasures Test and Evaluation Complex ANSI and, 288–90, 290t, 291t radiation protection vs. detection and, 287–88 Evaluation and Performance of Radiation Detection Portal Monitors for Use in Homeland Security (ANSI), 290, 290t evaluator model, 238, 239f exit gates, truck, 102 express consignment courier facilities (ECCF) ancillary equipment, 125t, 268 cart mounted RPM and, 266, 267f conveyor belts and, 264, 264f deployment at, 263–66, 264f, 265f, 266f, 267f, 268 doorway configuration at, 265, 265f interdiction options, 101 operational considerations, 277 overview about, 263 primary scanning at, 263–66, 264f, 265f, 266f, 267f secondary scanning at, 266 truck RPMs at, 266, 267f tug portal and, 265–66, 266f

false alarms algorithms and, 182–84, 183f base rate and, 241–42 effect, 240 human factors regarding, 240–42 likelihood display concept and, 243–44, 244f response mapping procedure and, 242 two-state threshold processor and, 243, 243f “Fat Man” atomic bomb, 81, 81f figure of merit (FOM), 216, 217f fission, 80, 80n5 interrogation techniques and, 156–58, 157f, 158f five window example, 198–99, 198f FOM. See figure of merit Fort Street Cargo Facility, 10–11, 11f Frost and Sullivan Healthcare Group, 66–68, 67t future alarms and, 309–10 automatic triage, 315 automation aids, 319 away from U.S. POEs, 318–19 of cargo scanning, 154–55 communication standards, 316–17 control of systems, 317–18 data fusion, 316 data handling and system control, 315 detection technologies and, 307–9

Index detectors and, 308–9 hypotheses about, 320 identification and tracking subsystems, 312 imaging, 313–15 interrogation, 314–15 ionizing radiation technologies, 154–55 modularization, 317 multithreat technology integration, 317 overview regarding, 306–7 passive detection and, 309 POEs, 318 radioactive isotope identification and, 311–12 remote state-of-health monitoring, 317 of scanning people, 155 signal processing and alarm criteria, 310–11 signatures and, 307 small-boat scanning, 318–19 smaller detection systems, 312–13 smart alerts to remote centers, 315 summary about, 319–20 vehicle geometry recording and, 312

gamma-ray absorption coefficients, 93, 94f alarm impacts, 276–79, 278f, 279f, 280f cargo and, 149–50 counting rates, 18–19, 19f detection, 92–95, 92f, 94f, 95f, 107–9, 107f, 108f, 109f inorganic scintillation detectors and, 93 interactions with matter, 27–29, 28f minimum count rates, 105–6, 106t organic scintillation detectors and, 93–94 photomultiplier-based scintillation detector, 92, 92f radiation detection mechanisms, 92–95, 92f, 94f, 95f spectra, 32–33, 33f gate arms, 125t, 127, 128f, 250t gross-count algorithms, 186–87, 186f gross-count instrument, 18–19, 19f Guardian program, 299–301 guide frame, 121

handheld instruments overview about, 99 PRDs, 123–24, 124f RIID, 124–25, 124f 3 HE. See helium-3 neutron detector health physics instruments, 287–88 helium-3 (3HE) neutron detector, 110 Hess, Victor, 36 HEU. See highly enriched uranium high-energy linac, 150, 150f highly enriched uranium (HEU), 3

329 high-purity germanium (HPGe) detectors NaI(Tl) compared with, 234–35 overview about, 227 high-volume crossings, 273–74 HPGe. See high-purity germanium detectors human factors amplifier model and, 238, 239f evaluator model and, 238, 239f false and nuisance alarms, 240–42 impact, 246 likelihood display concept and, 243–44, 244f modeling and, 238, 238f, 239f NORM vs. illicit material and, 244–46 overview, 237 security decision making, 237–39, 239f situational awareness, 242–43 system trust and, 239–40 two-state threshold processor and, 243, 243f human machine interface, 250t humidity, 53 hypotheses, about future, 320

IAEA. See International Atomic Energy Agency identification subsystems, 312 IGY. See International Geophysical Year monitors Illicit Trafficking Radiation Detection Assessment Program (ITRAP), 291 imaging systems future, 313–15 ionizing radiation technologies, 139–51, 140f, 142f, 143f, 144f, 145f, 148f, 150f, 151f, 152f, 153, 153f, 154f, 155f overview, 135–36 radio frequency, 137–39, 138f, 139f X-ray, 151, 151f, 152f IMCC. See intermodal cargo container improvised nuclear devices (INDs), 3 individual vehicle profile, 221–23, 222f, 223t INDs. See improvised nuclear devices inductive loop presence sensors, 125t, 127–29, 128f, 250t industrial radiation sources common, 78–80, 79f operational considerations, 279, 279f, 280f overview, 77–78 inelastic scattering, 30–31 injection studies, 200–202, 201f, 202f inorganic scintillation detectors, 93 Installation Protection Program (IPP), 299–301 instruments and capabilities area surveillance system, 125–26, 125t, 126f, 250t ASPs, 115 auto dialer, 125t, 126–27, 126f, 250t booth, 125t, 127–30, 127f, 128f, 129f, 250t

 instruments and capabilities (Continued) to counter threats, 17–19, 19f detection technologies and, 106–10 gamma-ray detection and, 107–9, 107f, 108f, 109f gate arms, 125t, 127, 128f, 250t general requirements, 99–100 gross-count, 18–19, 19f handheld, 99, 123–25, 124f inductive loop presence sensors and, 125t, 127–29, 128f, 250t intercom system, 125t, 129, 129f, 250t interdiction options, 98–99 mobile RPMs, 116–21, 117f, 118f, 120f, 122f, 123 NaI(Tl) detectors, 108–9, 108f, 109f network systems, 125t, 129–30 OCR and reconciliation software, 125t, 130–31, 132f, 250t optical presence sensors, 125t, 131, 132f, 250t overview about, 106 pendant, 125t, 131 PLCs, 125t, 132–33, 133f, 250t PRD, 123–24, 124f PRIDE, 125t, 131, 250t PSID, 121, 122f, 123 PVT detectors, 107–8, 107f, 108f rail identification system, 125t, 133–34, 133f, 272 rail RPMs, 117, 118f RIID, 124–25, 124f RO-RPMs, 118–19, 118f RPM configurations and, 110–13, 111f, 113f, 114f, 115 SPMs, 115, 116f stationary RPM for straddle carriers, 120–21, 120f strobe/siren, 125t, 134, 134f, 250t traffic control, 125t, 134, 250t VIS, 125t, 135, 135f, 136f, 250t integration, data NIS and, 284 PRIDE and, 284–85 RPM, 283–85 interagency relationships, 297 intercom system, 125t, 129, 129f, 250t interdiction, radiation. See also specific subject goals and objectives, 5–6 layered approach to, 4 necessity of, 21–24 overview, 3–4 RPM requirements for, 99–100 interdiction options for airport cargo, 102 instrumentation, 98–99 for land border and rail crossings, 101

Index for mail and ECCFs, 101 overview, 97–98 purposes to consider regarding, 98 radiation detection and, 98 for seaport scanning, 102–4 international airports ancillary equipment at, 269–70 deployment, 268–70 overview about, 268 primary scanning at, 269, 269f secondary scanning at, 269 International Atomic Energy Agency (IAEA), 12 activities, 291–94, 292t, 293t, 294t ANSI standards compared with, 292–93, 292t, 293t Cooperative Research Program, 293–94, 294t example trafficking incidents, 16 ITRAP and, 291 minimum detection requirements and, 20 technical document development by, 294 Technical/Functional Specifications for Border Radiation Monitoring Equipment and, 291 intermodal cargo container (IMCC), 176f, 177–78, 177f, 178f International Geophysical Year (IGY) monitors, 41, 43 international mail facilities ancillary equipment at, 125t, 268 cart mounted RPM and, 266, 267f conveyor belts and, 264, 264f deployment at, 263–66, 264f, 265f, 266f, 267f, 268 doorway configuration at, 265, 265f operational considerations, 275, 277 overview about, 263 primary scanning at, 263–66, 264f, 265f, 266f, 267f secondary scanning at, 266 truck RPMs at, 266, 267f tug portal and, 265–66, 266f interrogation, 314–15. See also active interrogation techniques ionizing radiation technologies advantages and disadvantages, 141–42 backscatter radiography, 141–42, 142f, 143f cargo radiography systems, 147–51, 148f, 150f, 151f, 152f computed tomography, 142, 143f configurations, 140 contrast sensitivity and, 145–46, 145f for examining people, 151, 153, 153f, 154f, 155, 155f future, 154–55 overview, 139 penetration and, 142–43, 144f performance determinants, 142–46, 144f, 145f

Index photon radiography, 145–46, 145f processing techniques, 140–41 radiation types used in, 139–40 selection considerations, 146–47 spatial resolution and, 143–45, 144f transmission radiography, 140f, 141, 142f IPP. See Installation Protection Program isotopes NORM, 58 SPM systems and, 233 ITRAP. See Illicit Trafficking Radiation Detection Assessment Program

Joint Program Manager (JPM)-Guardian, 287 Joint Service Installation Pilot Project (JSIPP), 298–99

kitty litter, 63f, 64

Laboratories and Scientific Services, 284–85, 303 land border ancillary equipment, 125t, 257–58 cantilever RPMs and, 256, 256f commercial lanes at, 253, 254f dedicated bus/recreational vehicle lanes at, 256 deployment overview, 252 dual-use RPMs and, 255, 255f interdiction options, 101 Northern and Southern, 252–53, 254f, 255–58, 255f, 256f, 257f operational considerations, 273, 277 primary scanning, 252–53, 254f, 255–56, 255f, 256f privately owned vehicle lanes at, 253, 254f secondary scanning, 256–57, 257f wide load lanes at, 253, 255, 255f law enforcement agencies, 22–23 lead-shielding thickness, 230, 231f likelihood display concept, 243–44, 244f “Little Boy” atomic bomb, 81, 81f low-volume crossings, 274 Ludlum Measurements, Inc., 9

mail. See also international mail facilities ancillary equipment, 125t interdiction options, 101 major solar events, 37f, 43, 44f, 45, 45f marble tile, 62f, 64 mass attenuation coefficients, 94–95, 95f mass energy-absorption coefficients, 94–95, 95f MCNP. See Monte Carlo N-Particle MDA. See minimum detectable amount

331 mean-time-to-failure rate, 276 medical radionuclide alarm events expected occurrence, 74–77, 75f decay properties, 68–69, 69t, 70f detector response calculation configurations, 69–70, 71f detector response modeling method, 69t, 72 detector response modeling results, 72–73, 73f MCNP and, 72 operational considerations and, 277 overview about, 64–65 survey results, 67–68 UNSCEAR and, 76–77 U.S. Radiopharmaceuticals Markets and, 66–68, 67t use, 65–67, 65f Megaports Initiative, SDL, 296–97 melt salt, 63f, 64 minimum detectable amount (MDA), 216–18, 218f Minimum Performance Criteria for Active Interrogation Systems Used for Homeland Security (ANSI), 291t mobile RPMs ancillary equipment, 125t detection technologies, 116–21, 117f, 118f, 120f, 122f, 123 overview, 116 prototype, 116, 117f PSID and, 121, 122f, 123 truck-mounted, 102–3 modeling amplifier model, 238, 239f detector response, 69t, 72–73, 73f drive-through scenario results and, 178–80, 179f, 180f evaluator model, 238, 239f human factors and, 238, 238f, 239f IMCC, 176f, 177–78, 177f, 178f observations concerning, 180–81 overview, 163–65 photon detection efficiency and, 165–67, 165f, 168f specific detector simulations, 168, 169t, 170t, 171 spectral distributions and, 171f, 172–75, 172f, 174f unshielded-source results and, 171–72, 171f, 172f, 173f, 174f vehicle, 175–80, 176f, 177f, 178f, 179f, 180f modularization, 317 Monte Carlo N-Particle (MCNP), 64 IMCC modeling and, 176f, 177–78, 177f, 178f medical radionuclide and, 72 overview about, 164–65 photon detection efficiency and, 165–67, 165f, 168f

 Monte Carlo N-Particle (MCNP) (Continued) spatial optimization and, 211–12, 211f, 212t specific detector simulations, 168, 169t, 170t, 171 spectral distributions and, 171f, 172–75, 172f, 174f unshielded-source results and, 171–72, 171f, 172f, 173f, 174f vehicle modeling and, 175–78, 176f, 177f, 178f multilayer defense, 20–21 multithreat technology integration, 317

NaI(Tl). See thallium-doped sodium iodide National Command Center, 285 National Defense Authorization Act, 298 National Enforcement Equipment Maintenance and Repair, 285 National Integration System (NIS), 284 National Targeting Center, 285 naturally occurring radioactive materials (NORM). See also technologically enhanced NORM border crossing data on, 60–61, 62f cargo containing, 58–60, 60f, 61f drive-through scenario results, 179–80, 180f energy windowing algorithms and, 186–203, 186f, 192f, 193f, 195f, 196f, 197f, 198f, 201f, 202f human factors and, 244–46 isotopes present in, 58 noncommercial vehicles and, 206–7 operational considerations and, 277 overview about, 57–58 photon emission spectra from, 61, 62f, 63f, 64 point source spatial distribution vs., 206, 207f radiation sources, 57–61, 60t, 61t, 62t, 63f, 64 rail crossings and, 101 signature description, 184–85, 186f spatial optimization and, 215–16, 216f SPMs and, 228, 230, 232, 232f network cabling, 130 description of, 250t encoder/decoder, 129 optical fiber, 130 power over Ethernet switch, 130 switch, 129 systems, 125t, 129–30 wireless communications, 130 neutron alarm impact, 281–83, 281f, 282f, 283f detection, 96–97, 97f, 109–10 elastic scattering and, 30, 30f 3 He and, 110 inelastic scattering and, 30–31

Index interactions with matter, 29–31, 30f Newark Neutron Monitor of, 43, 44f, 45, 45f NM-64 monitor of, 41, 43 radiation detection mechanisms, 96–97, 97f spectra, 31, 32f spike, 282, 283f Newark Neutron Monitor, 43, 44f, 45, 45f New York/New Jersey Port Authority Test Bed, 302 NIS. See National Integration System NM-64 neutron monitor, 41, 43 nonionizing radiation technologies, 137, 137f NORM. See naturally occurring radioactive materials nuclear fuel sources, 281, 282f nuisance alarms algorithms and, 182–84, 183f base rate and, 241–42 false alarm effect and, 240 human factors regarding, 240–42 likelihood display concept and, 243–44, 244f response mapping procedure and, 242 two-state threshold processor and, 243, 243f

OCR. See optical character recognition software operational considerations alarms and, 274, 276, 277 calibration of equipment, 275 cargo prenotification, 275–76 cross talk and, 275 data collection, 275 data integration, 283–85 disruption of commerce, 273–74 ECCF, 277 gamma-ray alarm impacts, 276–79, 278f, 279f, 280f of high-volume crossings, 273–74 industrial/commercial radiation sources, 279, 279f, 280f international mail facilities, 275 land border, 273 of low-volume crossings, 274 mean-time-to-failure rate, 276 medical radionuclides and, 277 neutron alarm impact, 281–83, 281f, 282f, 283f NORM and medical radionuclides, 277 overview, 273–76 primary alarm determination, 274, 276 rail, 275 RPM size, 276 seaports, 273 secondary alarm determination, 274, 276, 277 space constraints, 274–75 optical character recognition (OCR) software, 125t, 130–31, 132f, 250t optical fiber, 130

Index optical presence sensors, 125t, 131, 132f, 250t optimal limiting position, 209–10, 210f, 214–15, 214f, 215f, 215n10 optimal sum interval, 207, 215, 215f, 215n10 organic scintillation detectors, 93–94

Pacific Northwest National Laboratory (PNNL) RPMP established at, 5 RPMP initial activities, 7–8 RPMs deployment beginnings and, 9–11 pair production, 28–29, 28f passenger-side panels (PB and PT), 222, 222f, 223t passive detection, 309 PB. See passenger-side panels PBS. See percent baseline suppression pendant, 116, 125t, 131 penetration, 142–43, 144f people future of scanning, 155 radiography security examining, 151, 153, 153f, 154f, 155, 155f percent baseline suppression (PBS) calculation of, 220 ensemble graphs, 220–21, 221f percent energy window ratio suppression (PRS), 224–26, 225f Performance Criteria for Hand-Held Instruments for the Detection and Identification of Radionuclides (ANSI), 289, 290t Performance Criteria for Spectroscopy-Based Portal Monitors Used for Homeland Security (ANSI), 291t personal radiation detectors (PRDs), 5 described, 123–24, 124f overview about, 99 Personal Security Scanner, 137–39, 138f, 139f photoelectric effect, 28, 28f photomultiplier-based scintillation detector, 92, 92f photon detection efficiency, 165–67, 165f, 168f radiography, 145–46, 145f photon emission spectra from cargo, 61, 62f, 63f, 64 of kitty litter, 63f, 64 of marble tile, 62f, 64 of melt salt, 63f, 64 photonuclear effect, 28, 29 physics gamma-ray interactions with matter, 27–29, 28f gamma-ray spectra, 32–33, 33f health instruments and, 287–88 neutron/matter interactions, 29–31, 30f neutron spectra, 31, 32f

333 of radiation sources, 26–33, 28f, 30f, 32f, 33f radionuclides and, 26–27 statistics and, 33–35, 34f PLCs. See programmable logic controllers plutonium, 3 PNNL. See Pacific Northwest National Laboratory POEs. See ports of entry point source, NORM spatial distribution vs., 206, 207f polyvinyl toluene (PVT) gamma-ray detection, 107–8, 107f, 108f MCNP and, 164–65 observations concerning, 180–81 photon detection efficiency and, 165–67, 165f, 168f specific detector simulations, 168, 169t, 170t, 171 spectral distributions and, 171f, 172–75, 172f, 174f unshielded-source results regarding, 171–72, 171f, 172f, 173f, 174f Portable Radiation Detection Instrumentation for Homeland Security (ANSI), 289, 290t portable source identification device (PSID), 13 scenarios, 123 telescoping mast, 121, 122f, 123 Port Radiation Inspection, Detection, and Evaluation (PRIDE) system, 100, 125t, 131, 250t data integration and, 284–85 ports of entry (POEs), 4 away from U.S., 318–19 defined, 4n1 deployment process flow at, 251–52 future, 318 illustration of, 10f power over Ethernet switch, 130 PRDs. See personal radiation detectors precipitation, 52–53 prenotification, cargo, 275–76 Presidential Directive, 2005, 303 PRIDE. See Port Radiation Inspection, Detection, and Evaluation system primary alarm determination, 106, 274, 276 primary scanning at international airports, 269, 269f at international mail facilities and ECCFs, 263–66, 264f, 265f, 266f, 267f at land borders, 252–53, 254f, 255–56, 255f, 256f rail, 270–71, 271f at seaports, 102–4, 259–61, 259f, 260f, 263–66, 264f, 265f, 266f, 267f privately owned vehicle lanes, 253, 254f process flow, deployment, 251–52 procurement specifications, RPM, 104–6, 106t

 programmable logic controllers (PLCs), 125t, 132–33, 133f, 250t PRS. See percent energy window ratio suppression PSID. See portable source identification device PT. See passenger-side panels PVT. See polyvinyl toluene

radiation detection difficulties with, 95, 95f gamma-ray, 92–95, 92f, 94f, 95f gamma-ray absorption coefficients and, 93, 94f inorganic scintillation detectors and, 93 mass attenuation/mass energy-absorption coefficients and, 94–95, 95f mechanisms, 91–97, 92f, 94f, 95f, 97f neutron, 96–97, 97f organic scintillation detectors and, 93–94 overview, 91–92 photomultiplier-based scintillation detector and, 92, 92f Radiation Detection Panel, DOE, 9, 9n5 Radiation Portal Monitor Project (RPMP) commercial lanes and, 253, 254f deployment advances, 11–13 deployment approach, 251 deployment beginnings, 9–11 deployment process flow, 251–52 DNDO funding of, 14 establishment of, 5 first deployment, 10–11 Fort Street Cargo Facility and, 10–11, 11f goals and objectives, 5–6 initial activities of, 7–8 international airports and, 268–70 international mail facilities and ECCFs and, 263–66, 264f, 265f, 266f, 267f, 268 mission, 6–7 moving toward completion, 13–14 Northern and Southern land borders and, 252–53, 254f, 255–58, 255f, 256f, 257f POEs, 10f privately owned vehicle lanes and, 253, 254f procurement specifications, 104–6, 106t rail crossings and, 270–72, 271f seaports and, 258–63, 258f, 259f, 260f, 262f SLD program and, 12 wide load lanes and, 253, 255, 255f radiation portal monitors (RPMs), 4. See also specific subject ANSI and, 19–20 area monitors and, 116 barometric pressure and, 52 cantilever, 256, 256f cart mounted, 266, 267f

Index common features of, 105 computers and, 113, 114f configuration requirements, 112 configurations, 110–13, 111f, 113f, 114f, 115 construction materials and, 48–49 correlations, 45–46, 46t cosmic background correlations and, 45–46, 46t cosmic background study and, 41, 42f, 43 data integration, 283–85 deployment advances, 11–13 deployment beginnings, 9–11 diurnal cycles and, 54–55, 55f dual-use, 255, 255f first deployment, 10–11 Fort Street Cargo Facility and, 10–11, 11f four-lane configuration, 113, 114f gamma-ray minimum count rates, 105–6, 106t gross-count instrument, 18–19, 19f humidity and, 53 major solar events and, 43, 44f, 45, 45f medical radionuclide alarm events, 74–77, 75f medical radionuclide calculation configurations, 69–70, 71f medical radionuclide modeling method, 69t, 72 medical radionuclide modeling results, 72–73, 73f mobile, 116–21, 117f, 118f, 120f, 122f, 123, 125t operating modes, 112 operator interface, 112, 113f overview about, 98–99 precipitation and, 52–53 procurement specifications, 104–6, 106t project history, 4–14 rail, 117, 118f requirements for interdiction, 99–100 RO, 13, 118–19, 118f seasonal variation and, 56, 56t size, 276 SNM detection history, 8 stationary, 120–21, 120f temperature and, 51 thunderstorms and, 54 truck, 266, 267f types of, 100 visibility and, 53 weather study locations, 49 wind and, 53–54 radiation protection, 287–88 radiation sensor panels (RSPs), 105, 111, 111f radiation sources background, 35–46 barometric pressure, 52 construction materials, 48–49 cosmic background, 35–41, 37f, 38f, 39f, 40f, 42f, 43, 44f, 45–46, 46t, 282, 283f

Index diurnal cycles, 54–55, 55f earth-terrestrial background, 46–48, 47f electromagnetic interference effects, 81–86, 82f, 83t EMP, 85–86 gamma-ray interactions with matter and, 27–29, 28f gamma-ray spectra and, 32–33, 33f humidity, 53 industrial, 77–80, 79f major solar events, 37f, 43, 44f, 45, 45f medical radionuclide, 64–70, 65f, 67t, 69t, 70f, 71f, 72–77, 73f, 75f neutron/matter interactions, 29–31, 30f neutron spectra, 31, 32f NORM, 57–61, 60t, 61t, 62t, 63f, 64 overview, 26 photon emission spectra from cargo, 61, 62f, 63f, 64 physics of, 26–33, 28f, 30f, 32f, 33f precipitation, 52–53 RFI, 82f, 83–85, 83t RPM correlations and, 45–46, 46t seasonal variation, 56, 56t specialized commercial, 59 special nuclear materials, 80–81, 81f statistics and, 33–35, 34f temperature, 51 thunderstorms, 54 visibility and, 53 weather-related, 49–57, 49t, 50f, 55f, 56t wind, 53–54 radio frequency imaging, 137–39, 138f, 139f radio frequency interference (RFI), 82f, 83–85, 83t radiography. See ionizing radiation technologies radioisotope identifier device (RIID), 13, 124–25, 124f Radiological and Nuclear Countermeasures (Rad/Nuc) Portfolio, 301–2, 303 Radiological and Nuclear Countermeasures Test and Evaluation Complex, 303 radiological dispersal devices (RDDs), 3, 26–27 radionuclides. See also medical radionuclide identifiers, 5 RDD and, 26–27 SPM systems and, 233, 234t Radiopharmaceuticals Markets, U.S. (Frost and Sullivan Healthcare Group), 66–68, 67t Rad/Nuc. See Radiological and Nuclear Countermeasures Portfolio rail ancillary equipment, 125t, 133–34, 133f, 272 ASP monitors and, 101

335 deployment, 270–72, 271f identification system, 125t, 133–34, 133f interdiction options, 101 NORM and, 101 operational considerations, 275 primary scanning, 270–71, 271f RPMs for, 117, 118f seaports and, 103 secondary scanning, 271–72 Rayleigh scattering, 28, 29 RDDs. See radiological dispersal devices reconciliation software, 125t, 130–31, 132f, 250t recreational vehicle lanes, 256 remotely operated RPM (RO-RPM), 13, 118–19, 118f remote state-of-health monitoring, 317 response mapping procedure, 242 RFI. See radio frequency interference RIID. See radioisotope identifier device RO-RPM. See remotely operated RPM RPMP. See Radiation Portal Monitor Project RPMs. See radiation portal monitors RSPs. See radiation sensor panels

SAIC. See Science Applications International Corporation salt, melt, 63f, 64 scanning. See also primary scanning; secondary scanning future, 154–55, 155, 318–19 people, 155 small-boat, 318–19 Science and Technology Directorate, 6, 301–2 Science Applications International Corporation (SAIC), 12 scintillation detectors. See also specific subject gamma-rays and, 92–95, 92f, 94f, 95f inorganic, 93 organic, 93–94 photomultiplier-based, 92, 92f seaports ancillary equipment, 125t, 261–63 crane platforms at, 103–4 deployment at, 258–63, 258f, 259f, 260f, 262f interdiction options, 102–4 mobile RPM for straddle carriers at, 121, 122f, 123 operational considerations, 273 overview about, 258–59, 258f primary scanning at, 102–4, 259–61, 259f, 260f, 263–66, 264f, 265f, 266f, 267f secondary scanning at, 261, 262f stationary RPM for straddle carriers at, 120–21, 120f

 seaports (Continued) straddle carriers and, 103, 119–21, 120f, 122f, 123 train and, 103 truck exit gates at, 102 truck-mounted mobile detectors at, 102–3 seasonal variation, 56, 56t secondary alarm determination, 106, 274, 276, 277 Secondary Reachback program, 303 secondary scanning at international airports, 269 at international mail facilities and ECCFs, 266 at land borders, 256–57, 257f rail, 271–72 at seaports, 261, 262f second line of defense, 21, 21n10 Second Line of Defense (SLD), 6 Core Program, 295–96 interagency relationships, 297 Megaports Initiative, 296–97 program overview, 294–95 RPMP and, 12 security decision making, 237–39, 239f September 11, 2001, 4–5 shadow shielding, 194, 196f shadow shielding study double-dip pattern and, 219 ensemble graphs in, 220–21, 221f individual vehicle profile in, 221–23, 222f, 223t observations, 223–24 overview about, 218–20 PBS and, 220–21, 221f PRS and, 224–26, 225f summary, 226 suppression ratios, 224, 224n13 signal processing, 310–11 signal-to-noise ratio, 213–16, 214f, 215f, 216f signature detection active interrogation techniques and, 159, 160f SNM and, 184–85, 186f technologies, 307 simulations drive-through scenario results and, 178–80, 179f, 180f IMCC and, 176f, 177–78, 177f, 178f observations concerning, 180–81 overview, 163–65 photon detection efficiency and, 165–67, 165f, 168f specific detector, 168, 169t, 170t, 171 spectral distributions and, 171f, 172–75, 172f, 174f unshielded-source results and, 171–72, 171f, 172f, 173f, 174f

Index vehicle, 175–78, 176f, 177f, 178f siren, 125t, 134, 134f, 250t situational awareness, 242–43 SLD. See Second Line of Defense small-boat scanning, 318–19 smart alerts, to remote centers, 315 Smartcheck®, 151, 153, 153f smoke detector, 279, 279f SNM. See special nuclear material sodium iodide detector. See thallium-doped sodium iodide software, OCR/reconciliation, 125t, 130–31, 132f, 250t solar events, major, 37f, 43, 44f, 45, 45f source shield power values, 211–12, 212t Soviet Union, breakup of, 22 space constraints, 274–75 spatial distribution for noncommercial vehicles, 206–7 of NORM vs. point sources, 206, 207f spatial optimization algorithms and, 201f, 207–18, 208f, 211f, 212t, 213f, 214f, 215f, 216f, 217f, 218f experiments, 212, 213f FOM and, 216, 217f MCNP calculations and, 211–12, 211f, 212t MDA and, 216–18, 218f NORM and, 215–16, 216f optimal limiting position and, 209–10, 210f, 214–15, 214f, 215f, 215n10 optimal sum interval and, 207, 215, 215f, 215n10 RPM geometry and, 207–8, 208f signal-to-noise ratio and, 213–16, 214f, 215f, 216f source shield power values and, 211–12, 212t spatial resolution, 143–45, 144f special nuclear material (SNM), 3 active interrogation techniques and, 156–61, 157f, 158f, 160f dangers of, 80–81, 81f energy windowing algorithms and, 186–203, 186f, 192f, 193f, 195f, 196f, 197f, 198f, 201f, 202f RPMs history regarding, 8 signature description, 184–85, 186f spectral distributions, 171f, 172–75, 172f, 174f spectroscopic portal monitor (SPM) systems ASP, 235–36 conditions justifying expense of, 115 deployment requirements, 228–29, 233 deployment strategy, 236–37, 236f detection efficiency, 229–30, 230t as detection technology, 115, 116f drawbacks of, 228

Index drive-by spectra for, 232, 233f isotopes to identify using, 233 lead-shielding thickness and, 230, 231f NaI(Tl) and HPGe comparisons, 234–35 NaI(Tl)-based, 12, 13, 229–30, 229f, 230t, 231f, 232, 232f, 233f, 234–35 NORM and, 228, 230, 232, 232f overview, 226–28 panels of, 233 prototype of, 115, 116f, 229–30, 229f, 230t, 231f, 232, 232f, 233f radionuclides detection efficiencies and, 233, 234t specifications regarding, 227, 232–33, 234t stationary vehicle spectra for, 230, 232, 232f template matching and, 233 SPM. See spectroscopic portal monitor systems standards ANSI and, 288–90, 290t, 291t comparison of ANSI and IAEA, 292–93, 292t, 293t radiation protection vs. detection and, 287–88 state-of-health monitoring, remote, 317 stationary RPM, for straddle carriers, 120–21, 120f statistics, radiation sources and, 33–35, 34f stop line, 121 straddle carriers, 103 mobile RPM for, 121, 122f, 123 overview, 119–20 stationary RPM concept, 120–21, 120f strobe, 125t, 134, 134f, 250t suppression ratios, 224, 224n13 system control, 315 system trust, 239–40

Technical/Functional Specifications for Border Radiation Monitoring Equipment (IAEA), 291 technologically enhanced NORM (TENORM), 58 telescoping mast, 121, 122f, 123 temperature, 51 template matching, 233 TENORM. See technologically enhanced NORM terrorists objectives of, 3 weapon types, 3 testing ANSI and, 288–90, 290t, 291t radiation protection vs. detection and, 287–88 thallium-doped sodium iodide [NaI(Tl)]. See also spectroscopic portal monitor systems based SPM systems, 12, 13, 229–30, 229f, 230t, 231f, 232, 232f, 233f, 234–35

337 energy windowing algorithms and, 187n7 gamma-ray detectors, 108–9, 108f, 109f HPGe comparisons, 234–35 MCNP and, 164–65 observations concerning, 180–81 photon detection efficiency and, 165–67, 165f, 168f specific detector simulations, 168, 169t, 170t, 171 spectral distributions and, 171f, 172–75, 172f, 174f unshielded-source results regarding, 171–72, 171f, 172f, 173f, 174f threats base rate and, 241–42 detecting, 14–21 example incidents, 16, 17f instrumentation to counter, 17–19, 19f multilayer defense and, 20–21 multithreat technology integration, 317 specification and standards regarding, 19–20 types of, 15–16, 15f three window example, 191, 192f thresholds algorithms and, 182–84, 183f, 203–4 two-state processor and, 243, 243f thunderstorms, 54 total gamma-ray counting rates, 18–19, 19f tracking subsystems, 312 traffic control, 125t, 134, 250t Training Requirements for Homeland Security Personnel Using Radiation Detection Instruments (ANSI), 290t transmission radiography advantages and disadvantages, 141 examples, 140f, 142f triage, automatic, 315 tritium, 110 truck exit gates, 102 mounted mobile detectors, 102–3 RPMs, 266, 267f trust, system, 239–40 tug portal, 265–66, 266f two-state threshold processor, 243, 243f two window examples, 188–90, 190–91 traffic data, 192–94, 195f

ultrasound, 137, 137f Unconventional Nuclear Warfare Defense (UNWD) program, 298–99, 299n1 United Nations Scientific Committee on the Effects of Atomic Radiation report (UNSCEAR), 76–77

 unshielded-source results, 171–72, 171f, 172f, 173f, 174f UNWD. See Unconventional Nuclear Warfare Defense program U.S. Radiopharmaceuticals Markets (Frost and Sullivan Healthcare Group), 66–68, 67t

VACIS. See Vehicle and Cargo Inspection System vehicle geometry recording, 312 speed, 204–5 Vehicle and Cargo Inspection System (VACIS), 147, 148f, 149–50 vehicle modeling drive-through scenario results from, 178–80, 179f, 180f MCNP and, 175–78, 176f, 177f, 178f VIS. See visual identification system visibility, weather and, 53 visual identification system (VIS), 125t, 135, 135f, 136f, 250t

Index weapons of mass destruction/disruption, 3 weather barometric pressure, 52 diurnal cycles and, 54–55, 55f humidity, 53 overview about, 49, 49t precipitation, 52–53 radiation sources and, 49–57, 49t, 50f, 55f, 56t seasonal variation and, 56, 56t study data, 50–51, 50f study RPM locations, 49 summary of effects of, 56–57 temperature, 51 thunderstorms, 54 visibility and, 53 wind, 53–54 weather study, 49, 50–51, 50f wide load lanes, 253, 255, 255f wind, 53–54 wireless communications, 130

X-ray, 150, 150f, 151, 151f, 152f