Tritium measurement techniques: Recommendations of the National Council on Radiation Protection and Measurements (NCRP report ; no. 47)

NCRP REPORT No. 47

TRITIUM MEASUREMENT TECHNIQUES Recommendations of the NATIONAL COUNCIL O N RADIATION PROTECTION AND MEASUREMENTS

Issued May 28, 7976 First Reprinting December 1,1982 Second Reprinting January 15,1995 National Council on Radiation Protection and Measurements 7910 WOODMONT AVENUE I BETHESDA. MD 20814

Copyright 0 National Council on Radiation Protection and Measurements 1976 All rights reserved. This publication is protected by copyright. No part of this publication may be reproduced in any form or by any means. including photocopying, or utilized by any information storage and retrieval system without written permission from the.copyright owner, except for brief quotation in critical articles or reviews. Library of Congress Catalog Card Number 76-16301 International Standard Book Number G913392-29-4

Preface As a consequence of the increasing importance of tritium resulting from nuclear fission and neutron activation, from its use in accelerators, from its use in research and industry, and from its use in the investigation of the environment and its distribution in the environment, the NCRP designated a scientific committee to prepare a report on the currently acceptable methods of measuring tritium. This report is particularly aimed in assisting a n individual to select a procedure suitable to the problem a t hand. The present report was prepared by the Council's Scientific Committee 36 on Tritium Measurement Techniques for Laboratory and Environmental Use. Serving on the Committee during the preparation of this report were: WILLIAMC. REINIC,Chuirman ROBERTV. OSBORNE J. M. ROBINHUTCHINSON H. G ~ T OSTLUND E JOHNJ . KORANDA A. AUN MOCHISSI JOHN I. PETERSON,Consultant

The Council wishes to express its appreciation to the members of the Committee and the consultant for the time and effort devoted to the preparation of this report. Lauriston S. Taylor President, NCRP Bethesda, Maryland November 12, 1975

Contents Preface ..................... . .. . . . . . . . . . . . . . . . . . . . . . . . . 1. Introduction .......................................... 2 Radioactivity of Tritium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Sample Collection for Tritium Analysis ................ 3.1 Introduction ...................................... 3.2 Collection of Tritium &om Air ..................... 3.3 Collection of Water ............................... 3.4 Collection of Urine ................................ 3.5 Collection of Biota and Soil Samples ............... 3.6 Sampling for Tritiated Particles .................... 4. Sample Preparation ................................... 4.1 Introduction ...................................... 4.2 Environmental Samples ........................... 4.3 Organically-Bound Tritium ........................ 4.4 Labeled Pharmaceuticals .......................... 4.5 Urine Samples ................... . ............... 4.6 Enrichment ...................................... 5. Measurement Techniques ............................. 5.1 Introduction ...................................... 5.2 Measurement of Discrete Samples .................. 5.3 Real-Time Measurement .......................... 5.4 Measurement of Surface Activity ................... 5.5 Miscellaneous Methods ............................ 5.6 Comparison of Measurement Methods .............. 6 . Standardization and Calibration ....................... 6.1 Introduction ...................................... 6.2 Calibration ...................................... 6.3 Use of Standards ................................. 6.4 Blanks .......................................... 6.5 Calibration of Real-Time Instruments .............. 6.6 Propagation of Errors ............................. 6.7 Expected Accuracies .............................. References ............................................... The NCRP ............................................. NCRP Reports ............................................

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2 3 3 3 8 8 8 10 11 11 11

16 20 21 22 26 26 31 40 53 56 61 62 62 62 67 68 69 72 74 75 86 92

1. Introduction In the short time since the radioactivity of tritium was discovered by AIvarez and Cornog (1939), this radionuclide has become important in many scientific and industrial fields. Tritium occurs naturally and is also produced by nuclear fission and neutron activation. The incidental production of tritium in weapons tests and by the nuclear industry and the effects of tritium on man have received considerable attention (Jacobs, 1968).New uses of tritium in research and industry and expansion of the nuclear power industry seem certain. All of these activities encourage interest in tritium measurements. This report describes and discusses methods for the measurement of tritium in a variety of media. It is intended to assist individuals and organizations in the selection of procedures best suited to particular problems and resources. Included are most of the important methods for the measurement of tritium and information on their advantages and disadvantages. Stepby-step procedures and detailed descriptions of equipment are beyond the scope of the report, and the reader is advised to refer to the extensive literature cited to obtain this information.

2. Radioactivity of Tritium Tritium (3H or T) is a radioactive isotope of hydrogen. It decays to helium by emission of a beta particle in the reaction, :H + f He + e-, with a maximum energy of 18 keV and an average energy of 5.7 keV. The half-life for radioactive decay is 12.33 years. There are 9620 Ci per gram of tritium. Most tritium analyses, especially health physics measurements, are reported in terms of curies (or fraction thereof) per unit volume or mass. In certain fields, however, it has been traditional to express the results of tritium assays in relation to the number of hydrogen atoms in the sample. The International Commission on Radiation Units and Measurements recommends the term "tritium ratio" (TR) for this purpose (ICRU, 1963),where a TR of value 1equals one tritium atom per 1018hydrogen atoms. The term "tritium unit" (TU)has been and is still used to express this same relationship, although the ICRU deprecates the use of "tritium unit" for this quantity. It should be noted that TR is a concentration ratio similar to the molar ratio used in chemistry. For example, if a sample of water a t 25°C contains 1 tritium atom per 1018hydrogen atoms, the TR is 1.00 and its specific activity is 3.2 pcilliter.' However, if a sample of benzene (also a t 25°C) contains 1tritium atom per 1018hydrogen atoms, the TR is 1.00 but the specific activity is 1.95 pcilliter. It is correct to say that a sample has a TR of 500 or its TR is 500, but not that the sample contains or has 500 TR. Because of pitfalls associated with the use of the term "tritium ratio", it is recommended that the term be restricted to fields where it has been traditionally used and not be used in other fields, such as radiation protection.

' With respect to symbols and terminology, this report follows generally the recommendations of the International Union of Pure and Applied Chemistry (1970). Prefixed symbols in both the numerator and denominator of units,e.g., pCilan3, are used in this report where they appear appropriate.

3. Sample Collection for Tritium Analysis 3.1 Introduction The collection of a representative sample for tritium analysis presents essentially the same problems encountered with any other material. For a description of sampling theory and statistics and, in particular, information on sampling site, sample size, and sampling frequency, the reader is referred to the appropriate literature (Dixon and Massey, 1969; Crow et al., 1960; Bancroft, 1957; Hald, 1952). There are, however, peculiarities in correct tritium sampling that require special attention. Three possible sources of error in the measurement of total activity or concentration are: Addition of atmospheric moisture to the collected sample through absorption, adsorption or condensation. Evaporation of water from the sample. This would cause a direct error if the total activity in a sample were being measured. Also, if the tritium concentration is of interest, then the different evaporation rates of tritiated and non-tritiated water, especially a t lower temperatures, lead to errors. Note that the vapor pressure of H,O and HTO at room temperatures differ by approximately 10 percent (Sepal1 and Mason, 1960). Exchange of organically-bound hydrogen with hydrogen in atmospheric water. This exchange is catalyzed by some metal oxides that are often available near the sample.

3.2

Collection of Tritium from Air

Tritiated water vapor and hydrogen gas are the most common forms of tritium in air. Both occur naturally and are the predominant forms of release from nuclear facilities and nuclear weapons tests. Other forms of tritium in air include naturally occurring tritiated methane and ethane (Haines and Musgrave, 1968); tritiated methane

4

1

3. SAMPLE COLLECTION FOR TRITIUM ANALYSIS

is also produced in weapons tests (Martell, 1963). Tritiated hydrocarbons can be produced in the laboratory and some of these are volatile and could become airborne. However, the greatest interest by far is in the collection of tritiated water vapor from air, especially in the vicinity of nuclear reactors and nuclear fuel reprocessing plants. 3.2.1

Collection of Tritiated Water Vapor porn Air

The collection may be preparatory to measuring the specific activity of tritium in the moisture or to measuring the concentration of tritium per unit volume of air. I t may be effected by: Bubbling the air through non-tritiated water or other appropriate solvents. Passing the air through a desiccant. Condensing or freezing. The choice of method, the precautions, and the ancillary measurements needed will be determined by the purpose of the collection. For specific activity determination, the only measurement needed is of the concentration of tritium in the moisture collected by either the second or third of the above-mentioned methods. If the HTO has been collected by the first method, then the absolute humidity of the air must be determined and the total volume of air from which the HTO was obtained must be measured. The relative humidity (or dew point) and air temperature are usually measured directly; the absolute humidity can then be determined from psychrometric tables. If the sampling period is short, or the climatic conditions are sufficiently steady, a single measurement of temperature and relative humidity may suffice. For long sampling periods, a recording hygrothermograph may be required to estimate average absolute humidity. The air volume may be mcasured directly with a wet-test meter or estimated from the average air flow rate and sampling time. Meters should preferably (always in the case of a wet-test meter) be placed downstream of the collecting device to minimize contamination (or dilution) of the sampled moisture. The air pump should also be downstream of the collector. For measurements of the concentration of tritium in air, the total tritium in the sampled air must be determined, together with the volume of air containing it. Total tritium may be estimated directly from the total collected and the collection efficiency or from the specific activity of the collected moisture and the absolute humidity. Once a sample has been collected, it should be carefully sealed in a container having low moisture absorptivity or permeability (for example,glass) to reduce dilution, evaporation or contamination.

3.2 COLLECTION O F TRITIUM FROM AIR

1

5

3.2.1.1 Collection of HTO from Air by Dilution. This widely applied technique is the simplest method for collecting HTO from air. Air is bubbled through a water-filled gas washing bottle. Provided the isotopic ratio of HTO/H,O in the water is low compared to that in the incoming air, the collection efficiency is high and predictable (0sborne, 1973). Efficiencies ranging from 90 percent to 98 percent have been reported and are easily determined by use of several bubblers in series ( McConnon, 1970; Valentine, 1968; Dannecker and Spittel, 1967; Banville, 1965). However, the collection e f f i c i e n for tritium gas by this technique is less than 0.01 percent (Eakins and Zrinzo, 1973). 3.2.1.2 Collection of HTO from Air by Desiccation. Air is drawn through a cell or column containing the desiccant. Adsorbents for the collection of tritiated water vapor should have (a) a high capacity for moisture; (b) good moisture retention until intentionally desorbed; (c) low residual retention on desorption; and (d) mechanical stability and chemical inertness. Typical desiccants for the collection of HTO from air include silica gel, molecular sieves (alumina-silicates), anhydrous calcium sulfate, and activated alumina (Trussell and Diehl, 1963). The quantity of water held by a given sorbent when equilibrium has been reached is related to the relative humidity of the air. For example, the adsorptive capacities (gram of waterlgram of adsorbent) of silica gel are 0.05 a t 10 percent relative humidity and 0.26 a t 50 percent relative humidity, in the normal range of ground-level Sampling temperatures. For molecular sieves, the adsorptive capacity is less dependent on relative humidity, varying from 0.15 a t 10 percent to 0.18 a t 50 percent relative humidity (Bolz and Tuve, 1970). Information on the adsorptive capacities of specific sorbents is usually provided by their manufacturers. The amout of desiccant needed will depend on its adsorptive capacity and the total moisture expected to be removed from the air. In sampling situations where the latter is difficult to estimate accurately, the sampling should be arranged so that less than half the cell's collection capacity would normally be required. In some cases, it may be practical to use a sorbent with a n indicator which changes color when saturation is approached. Collection efficiency is usually determined by tests with collectors in series. Ostlund (1968) determined the efficiency of a cell containing a molecular sieve (Figure 1) by an isotope dilution technique. The efficiency of this cell and other well-designed collectors is generally greater than 99 percent (Osloond et a l . , 1973; Steinberg.and Rohrbough, 1965). Sax and Gabay (1970) describe a sampling method in which silica

6

I

3. SAMPLE COLLECTlON FOR

TRITIUM ANALYSIS

MOLECULAR SIEVE OR CATALYST

STAINLESS STEEL SPONGE

Fig. 1. Adsorption cell for atmospheric moisture.

gel in a plastic-screen bag is hung in air. Because air is not drawn through the sorbent, no pump or power is required. When this method was used for environmental monitoring, the results showed good reproducibility, and were in good agreement with other sampling methods. The best water adsorbers, such as concentrated sulfuric acid and granulated P,O,, are not useful collectors of tritiated water from air because the water cannot be readily removed for subsequent analysis. Extraction of HTO from desiccants is described in Section 4.2.1.2. 3.2.1.3 Collection of HTO from Air by Condensation or Freezing. Condensation of tritiated water vapor depends upon decreasing the air temperature to or below the dew point. A trap cooled by liquid nitrogen, liquid air, or various dry ice-solvent mixtures will collect moisture from an air sample. Air should be passed through the cold trap before the flow meter and pump to avoid the loss of tritiated vapor. Fine ice particles may form in the cold trap and their subsequent flaking could cause a decrease in collection efficiency. To prevent the loss, glass wool, glass balls or steel balls are sometimes placed in the trap. When a high collection efficiency is not required, a water-cooled condenser (Gibson and Burt, 1966) or a household dehumidifier (Bal-

3.2 COLLECTION OF TRITIUM FROM AIR

1

7

lard and Ely, 1963) can be used. These collectors provide continuous extraction of moisture and the collected water can be continuously analyzed. Iyengar et al. (1965) developed a "cold strip" apparatus consisting of a n aluminum strip with its lower section immersed in liquid nitrogen and the remainder extending into the air. Water condenses and freezes on the upper section and when the liquid nitrogen is exhausted, the melted ice trickles into the Dewar flask. A similar sampler described by Koranda et al. (1971) consists of an aluminum can placed on an aluminum pan and filled with crushed dry ice. At the end of the sampling period, atmospheric water collected as frost on the side of the can is scraped into the pan, then melted and poured into a sample bottle. 3.2.2

Collection of HT or

T1from Air

For atmospheric levels of tritium gas in the general environment, sampling has been done a t air liquefaction plants. The uncondensible &action a t the top of the air distillation column, called the "raw neon" fraction, contains the atmospheric hydrogen gas of the processed air. This fraction can easily be sampled, and after purification of the hydrogen, can be analyzed for tritium (Begemann and F'riedman, 1968). In a recently developed method (Ostlund and Mason, 1974; Ostlund, 19701, that is not restricted to liquid air plants, air is passed through a thoroughly dried bed of molecular sieve to collect the atmospheric water (including HTO).The air is then mixed with tritium free hydrogen (0.3 percent by volume) and passed over a palladium catalyst supported on a molecular sieve in a container similar to Figure 1. Hydrogen gas (including HT and T& in the air burns at ambient temperature, or slightly above, forming water which is adsorbed in situ on the molecular sieve. Because of the addition ofhydrogen, a macroscopic amount (several grams) of water is collected and can subsequently be efficiently removed from the sieve for counting. Techniques have been developed for reducing the "memory" of the sieve bed, and overcoming the limited capacity of the catalyst. 3.2.3 Collection of Other Tritiated Gases in Air

Atmospheric tritiated hydrocarbons, primarily methane and ethane, have been sampled routinely from liquid air plants and

8

1

3.

SAMPLE COLLECTION FOR TRITIUM ANALYSIS

counted for tritium (Begemann and Friedman, 1968; Haines and Musgrave, 1968).

3.3 Collection of Water

In collecting water samples, especially those with low tritium concentrations compared to the tritium concentration in air moisture, care must be taken to minimize the exposure of the sample to air.' Exchange between the water and tritiated atmospheric moisture could contaminate the sample even if the sample temperature is above the dew point. Water samples containing low concentrations of tritium should be stored in well-sealed containers. Tritiated water vapor in the atmosphere can diffuse through polyethylene containers and contaminate the water (IAEA, 1967). Glass or metal should therefore be used when samples have to be stored for several weeks or longer.

3.4

Collection of Urine

Because only a few milliliters of urine are usually needed for an analysis, a single voiding is sufficient. If the urine sample is being collected to monitor occupational exposure, the worker should remove any protective clothing and wash his hands thoroughly prior to the sample collection. Several hours are needed after assimilation for HTO to become uniformly distributed in body fluids. As a result, a sample collected immediately after exposure will not represent the peak concentration (Osborne, 1970b). Ideally, a urine sample should not be collected until after a complete voiding a t least two hours after the end of the exposure. If the samples are to be analyzed within a few days after collection, no preservatives need be used. Samples that may be subjected to heat or prolonged storage should be preserved with a n aqueous solution of "Merthiolate" (Moghissi and Lieberman, 1970).

3.5

Collection of Biota and Soil Samples

If the specific activity of the water in a sample is to be measured, dilution by air moisture must be avoided. When the time between

3.5 COLLECTION OF BIOTA AND SOIL SAMPLES

1

9

collection and analysis is only a few hours, the sample should be kept a t room temperature. Biota samples that must be frozen to preserve them until analysis should be packaged before freezing to prevent condensation of air moisture on the samples. 3.5.1

Biota Samples

Plants and animals may be collected in many ways to achieve a representative sample of the population, habitat, or the experimental group under study. The foliage of trees and shrubs can be collected by taking the sample from a single plant, or by collecting a leaf or two from many plants to produce an integrated value for the collection area. Grassy vegetation may be sampled by clipping the plants from a known area, such as a square decimeter, and the entire sample analyzed to provide unit area data, or by using a "grazing" approach to collect grass from many locations in a large field. In general, it is desirable to reduce the temperature of the packaged samples as soon after collection as possible. Storage of samples a t 0°C will temporarily preserve animal and plant tissues for tritium analysis. In animals, autolysis of the organs and tissues will occur a t temperatures just above freezing, and the use of ice should not be extended for a prolonged time. Plant samples may be kept somewhat longer before putrefaction and decomposition occur, and the temperature of ice will prevent large losses of transpirational water from the leaves during transit. 3.5.2

Soil Samples

Specific methods for collecting soil samples may vary from coring with a metal pipe or tool to trenching and sampling the sidewall of the trench with a small tool such as a trowel or shovel. The mechanical characteristics of the soil usually determine the most effective methods of sampling. Coring tools will produce a useful sample from soils without much gravel or clay. Some deformation of the soil profile or stratigraphy occurs when the tool is driven into the soil. This may not be important in some quantitative studies, and it is sometimes possible to evaluate the amount of compression that takes place in the coring operation. If the entire core, for example from a depth of 0-15 cm, is analyzed in a single step, the depth profile of tritium activity is effectively integrated in the single analytical step. This may be a

10

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3.

SAMPLE COLLECTION FOR TRITIUM ANALYSIS

useful approach in environmental studies where periodic site inventories are required, and where all of the tritium activity occurs in the strata sampled by the core. Soil samples may be packaged in any container which isolates the sample from extraneous moisture.

3.6

Sampling for Tritiated Particles

In certain industrial and research facilities, tritiated particles may be present in the air, on work benches, and on other surfaces. These particles can be collected for analysis by the usual method, i.e., drawing air through a filter, except that the filter must be compatible with the liquid scintillation counting system (Moghissi et al., 1970). Filters with a pore size of 1 pm or less are commonly used. Cellulose filters and filters made from polyfluorinated hydrocarbons offer adequate compatibility with liquid scintillators. Filters made from synthetic materials that are soluble in aromatic hydrocarbons are available, and these can be dissolved in the scintillation liquid. A "smear" or "wipe" test can be made to detect the presence of particulate contamination on surfaces. For this qualitative test, the surface is wiped with dry paper, glycerol-impregnated paper (Silver and Chew, 19721, or vinyl filter paper (Porter and Slaback, 1966). The material for taking the smear, as well as its size, should be compatible with the counting system (see Section 5.4.2).

4. Sample Preparation 4.1

Introduction

Tritium in liquid or solid samples cannot be easily determined quantitatively by any kind of nondestructive analysis because its beta radiation energy is so weak that most is absorbed in the sample. Most non-aqueous samples must be treated chemically or physically to be suitable for counting. The sample material, resources of the laboratory, number of samples to be handled and the measurement method to be used will influence the choice of the preparation method. In the following, some of the more common preparation methods are described, with special emphasis on procedures suitable for handling large numbers of samples.

4.2

4.2.1

Environmental Samples

Water Extraction

The a ~ a l y s i of s an environmental sample for tritium, whether it is soil, plant, or animal, usually entails the extraction of water from the sample material. Any method that is feasible in the laboratory and which will yield reasonably pure tissue water from the plant or animal, or interstitial water from a soil sample, is potentially useful. Four methods have been found practical. The choice of method will depend on availablility of equipment and number of samples to be processed. These methods are lyophilization, distillation a t high temperature, dilution with solvent, and azeotropic distillation. 4.2.1.1 Lyophilization. This method is basically the same as conventional freeze drying, except that for tritium analysis precautions must be taken to avoid cross-contamination. Although tritiated water has a slightly lower vapor pressure than ordinary water, freeze drying is practically free of isotope effect because of the lack of mobility of molecules in the solid state (ice) as compared to the liquid state. In vaporization from the liquid phase,

12

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4.

SAMPLE PREPARATION

fractionation between the phases can occur; from the solid state. however, the flux of water molecules to the vapor phase will have the same isotopic content as the bulk solid. Koranda et al. (1971) described procedures for preparation of environmental samples by lyophilization. Because of the specific properties of the type of environmental sample and the type of subsequent counting, procedures must sometimes be modified. If electrolytic enrichment of the water sample is necessary, a larger sample of water must be obtained in the vacuum distillation step. Each environmental sample processed for tritium analysis must have its own condensing trap. Measures are taken to prevent crosscontamination of the condensate if, as is usually the case, a common source of vacuum is used. Figure 2 showsan arrangement that allows parallel distillation of many samples; glass or metal may be used for the manifold. Production line techniques can be developed for extracting sample water. Depending upon sample size .and available laboratory space and facilities, up to 50 samples per day can be processed (Stewart et al.,1972). The decontamination and washing of glassware is usually a limiting factor in such large-scale analytical programs. Water may be extracted from samples by lyophilization and counted in the field a t remote sites allowing rapid analysis (Koranda et al.,1971). The type of sample to be analyzed will determine the specific lyophilization procedure to be followed. Four sample types may be distinguished:

VACUUM MANIFOLD

COLD F W E R FREEZE- WINO FLASK

Fig. 2.

Schematic of small lyophilization system for water extraction.

4.2

ENVlRONMENTAL SAMPLES

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(a) Soil Samples: Soils may range from sand to clay and thus a wide variation in the water content may be observed. However, most soil samples contain 5 to 20 percent water. The lyophilization of drier soil samples requires precautions to avoid the transfer of dust when the vacuum is applied. In this case, the sample may be covered by a filter paper or glass wool. (b) Plant Samples: Leaves, fruits, stems and twigs have a water content from 30 to 90 percent of wet weight. In general, the samples should be cut into thin slices or shredded to promote rapid sublimation. The water from a small leaf sample weighing 10 grams can be extracted in a few hours, whereas overnight extraction may be required if a large, undivided sample is placed in the vacuum system. (c) Animal Samples: These samples may be either organs excised from large animals or whole small animals, such as rodents. They are sliced in thin sections or in small pieces, the largest dimension not exceeding 10-15 mm, and preferably frozen before vacuum is applied. Since animal tissues contain a p proximately 60-70 percent water, samples weighing 20-50 grams usually will yield an adequate sample of the tissuewater. (dl Liquid Samples: Liquid samples, such as milk, blood, and water, containing a large amount of suspended or dissolved material may be lyophilized by shell-freezing them onto the wall of a spherical flask. Suspended material may be filtered from the sample first if there is a n excessive amount in the sample. 4.2.1.2 Distillation at High Temperature. Distillation a t normal or reduced pressure is common for extraction of water from liquid samples containing large amounts of electrolytes, such as urine and seawater, or from desiccants that have collected water from air. Depending upon the desired accuracy, isotope fractionation may have to be considered. As noted in the previous section, tritiated water has a lower vapor pressure than ordinary water, and thus,if the distillation is not completed, the remaining water may be enriched in tritium and the condensate depleted. Errors of 5-10 percent could be introduced in this way (Moore and Buskirk, 1961). The error in any particular distillation will depend upon the apparatus and conditions; for example, Simpson and Greening (1960) observed no depletion in serial samples distilled from urine. Clearly, complete distillation is advisable where possible. Another possible source of error is the exchange of water between

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4.

SAMPLE PREPARATION

the sample and the moisture of the air during the distillation. To avoid this error, the air inlet of the distillation apparatus is connected to a drying tube. For vegetation, animal tissue, and similar samples, distillation a t reduced pressure is usually the preferred method. 4.2.1.3 Dilution with Solvent. I f the water content of the sample is known and the specific activity of tritium is sufficiently high, dilution with a n appropriate solvent, such as water or dioxane, may be adequate. The sample is mixed with the solvent and left for a time to assure an even distribution of water in the solvent. One or two hours is usually long enough if the mixture is agitated and heated. The dilution technique is particularly useful if a liquid scintillation solution is used as the diluent. The presence of other soluble compounds that may quench the light output from the scintillator or that may contain tritium may limit the applicability of this technique. 4.2.1.4 Azeotropic Distillation. This technique is based on co-distillation of a solvent with water containing tritium. If the solvent is properly selected, the vapor contains a constant ratio of the solvent and water, and the boiling point is constant. There are numerous solvents that form an azeotrope with water. However, the specific requirements of tritium analysis narrow the choice to aromatic hydrocarbons. Table 1contains relevant information for benzene, toluene, and p xylene. These solvents are extensively used in liquid scintillation counting and thus their presence in the sample does not interfere with the preparation of the scintillation liquid. Their solubility in water is so small (0.05-0.06 percent) that no significant error is introduced in the gravimetric or volumetric measurement of the separated water. Benzene and its homologues are produced from petroleum and thus contain no tritium. The presence of small amounts of these compounds in water will consequently be insignificant. TABLE1-Pmperties of b p w n e , toluene and p x y k n e for cwotropic di$illation

of

water A.cotmpe

Hybocarbw

Relative Amount of Waier

Boiling Point

Boiling Point 'C

Benzene Toluene Xylene

80.1 110.6 138.35

a t BP

at 20%

'C

9

8

69.4 85.0 94.5

20.2 40.0

8.9

8.0 18.0 36.6

Upper Layer Lower Layer % %

0.06 0.05 0.05

99.94 99.95 99.95

4.2 ENVIRONMENTAL SAMPLES

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Hydrogen exchange between water and these solvents represents a possible source of error. Moghissi et al. (1973) have shown that this exchange is lowest for benzene, and the effect may be ignored in most cases for all three solvents. The lower boiling point of benzene is an added advantage for preservation of biological samples which may be heat sensitive. Processing of samples using benzene will require, however, a longer time than using the other two solvents because the concentration of water in the vapor phase is 8.9 percent as compared to 20.2 percent and 40.0 percent in toluene and xylene, respectively. For most distillations, any one of these solvents will be satisfactory; Daruschy (1965) used xylene while Moghissi et al. (1973) used benzene. Benzene, and to a lesser extent xylene and toluene, can cause acute and chronic poisoning, and they also have a high fire-explosion potential. Therefore, these solvents should be used with proper precautions. The sample and solvent are placed in a flask and refluxed until all the water is distilled. The refluxing can continue, if necessary, without harming the separation or the remaining sample. A drying agent is used in the air inlet of the distillation system for the same purpose as described in Section 4.2.1.2. Table 2 shows the quantity of various environmental samples and the corresponding volume of benzene required to obtain 10-20 ml of water. Subsequent to the separation, the dehydrated sample can be advantageously stored in benzene. The separation of benzene from the remaining sample occurs by distillation of the bulk of benzene a t atmospheric pressure followed by a vacuum distillation or heating on a water bath. Azeotropic distillation is suitable for processing a small number of samples. It is also suitable for production-type facilities where a large number of samples must be processed. In these facilities benzene can be purified and reused, if necessary. The purification consists of the addition of the water followed by azeotropic distillation. TABLE2-Sample

sires and quantity of benzene mquired for various sample lypes &-obtoii 10-20 mi of &r k p k %= Sample Weight Benzene Volume

Soil Hay G m n vegetation

Urine Animal and human tissue

8

ml

200

1300

50

400 70

30 20 30

50 150

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SAMPLE PREPARATION

4.3

Organically-Bound Tritium

Tritiated non-aqueous molecules in tissues or other substances may be measured if the tritium can be converted to a form compatible with the detector. The most widely used methods are wet and dry oxidation and solubilization in basic or acid solutions. 4.3.1

Wet Oxidation

Wet oxidation is an effective way to prepare a wide variety of samples. Mahin and Loftberg (1970) have reviewed the use of strong acids and bases in this method. Acrylamide gels, frequently used for electrophoretic separations of protein and nucleic acids, may be digested with hydrogen peroxide. Blood, which is troublesome in scintillation counting because of the strong color quenching of the pigment, may be digested with perchloric acid-hydrogen peroxide (Mahin and Loftberg, 1966). Caution must be exercised when working with perchloric acid because of its high explosion potential. Blood may also be dissolved and bleached with benzoyl peroxide in a strong basic solution. The method of Sansoni and Kracke (19711, using HpOl in the presence of Fe2+ ion, is also applicable for oxidizing samples for tritium analysis. Wet oxidation is not suitable for analysis requiring great accuracy or sensitivity because losses of tritium are inevitabIe and because other hydrogenous compounds are added to the sample. Residues of strong oxidizing agents remain in the sample causing chemiluminescence in the subsequent liquid scintillation counting. 4.3.2

Dry Combustion

Oxidation of hydrogenous materials by heating in the presence of oxygen with the combustion products passing over heated copper oxide at 500'-700°C is a method dating back to Liebig in 1831. CuO acts both as a catalyst and final oxiduing agent. It is essential that the oxygen by completely dry and that the combustion train be decontaminated after each sample. This method is simple and can reliably handle samples up to several hundred grams. However, combustion of a large sample may take several days. In a recent version of this approach, the sample is.burned in a n oxygen stream, sometimes with a catalyst to aid combustion, and the resulting water is collected in a cold trap. Applications of this technique to tritium have been reported by Peets et al. (1960) and Christman et al. (1955).

4.3 ORGANICALLY-BOUND TRITIUM

1

17

The Liebig method in a more practical form is flask combustion (Schaniger, 1955). Details of its application may be found in several references (Davidson and Oliverio, 1968; Kelly et al., 1961). The method uses inexpensive, simple apparatus. A sample, suspended in a wire basket and usually contained in a cellophane envelope, is placed in a two-liter filter flask with a sealed side arm. The flask is filled with oxygen before inserting the stopper, and the sample is ignited electrically or optically. The sample burns smoothly in the oxygen and the resulting water condenses on the walls of the flask. After cooling the flask, scintillation solvent is bled into the flask in measured quantity through the side arm. The flask is swirled to ensure complete collection of the water in the solvent and some of the solution is removed for counting. There are several disadvantages to this method. Combustion may be incomplete if the sample is wet or is larger than 100 mg. The technique can become laborious when large numbers of samples must be processed because of the necessity of cleaning the flasks thoroughly after each use. F i ~ l l y ,the method can be hazardous if solvents are not completely removed. For treating a limited number of small, dry samples, the oxygen flask method remains, however, very practical. When large numbers of samples have to be processed each day, an automatic method is needed. Two types have been developed, both applicable to chemical and biological samples. One method was developed by Kaartinen (1969) and the other by Peterson et al. (1969). A discussion of the details of operation and construction of apparatus for the oxygen flask technique, the Kaartinen apparatus, and the Peterson apparatus is given by Davidson et al. (1970). In the Kaartinen apparatus (Figure 31, a wire basket serves as a sample holder and igniter. The sample, as large as a gram or more, is usually wrapped in paper tissue. The wire basket is enclosed in a borosilicate combustion chamber into which oxygen is fed at a controlled rate to regulate the combustion rate and temperature. When the combustion is finished, the oxygen is passed through a condensation chamber to collect the water. The oxygen is followed by a stream of nitrogen, to minimize oxygen quenching in the scintillation solution. A predetermined volume of scintillation counting solvent is then flushed through the condenser to collect the water and the resulting solution is delivered to a counting vial. Whereas the Kaartinen apparatus is essentially an improved flask technique, the Peterson apparatus (Figure 4) is based on the oxygen train approach. Hardened gelatin or plastic capsules, containing up to 500 mg of dry sample, are dropped through an interlock onto a

18

1

4. SAMPLE PREPARATION

Fig. 3. Kaartinen tritium sample combustion apparatus [after Davidson, J. D., Oliverio, V. T.and Peterson, J. I. (1970). "Combustionof samples for liquid scintillation counting," page 222 in The Cumnt Status of Liquid Scintillation Counting. Bransome, E . , Jr., Ed. (Grune and Stratton, New York), by permiseionl.

quartz chip bed in a vertical combustion tube, with a continuous stream of oxygen flowing past. The combustion chamber and catalytic region are kept a t about 600°C. When the sample enters the system, it ignites and bums smoothly for about a minute. A chimney surrounding the combustion region directs the oxygen flow to provide smooth burning and contains the flame. The combustion gases pass through a catalytic oxidation bed consisting first of copper oxide and finally

I)F

--Furnace tube

Fig. 4. Peterson tritium sample combustion apparatus [afterDavidson. J. D., Oliverio. V. T. and Peterson, J. I. (1970). "Combustionof samples for liquid scintillation counting," page 222 in The Current Status of Liquid ScintilLatwn Counting, Bransome, E . , Jr., Ed. (Grune and Stratton, New York), by permission]. 1s

20

1

4. SAMPLE PREPARATION

hopcalite as a finishing catalyst (the use of a catalyst is important primarily for the complete oxidation of carbon for the determination of "C, for which the apparatus is also used). The oxygen stream carrying the water of combustion passes through a spiral condenser into which scintillation solvent is pumped in a measured amount. co here the water combines with the scintillation solvent and is delivered to a counting vial. The high combustion temperature and the excess oxygen provided in both types of instrument result in more complete oxidation than is possible with the oxygen flask technique. This allows the processing of larger samples and even wet samples or other materials that are dfficult to combust. Recently, a new combustion technique for processing of samples up to 10 g has been reported (Moghissi et al., 1975). The sample is placed in a steel container, known as a Parr bomb (Figure 5), which is pressurized with oxygen to 1.5-2 MPa.' Subsequent to ignition, the pressure rises to 15 MPa, primarily as a result of the temperature increase in the container. After combustion, water is collected in a cold trap. If samples larger than one gram are to be processed, this method offers advantages over others primarily due to its speed. After each ignition, the bomb is washed and heated for a period of approximately one hour to remove residual water. 4.3.3 Solubilizatwn

A practical way to prepare samples of tissues, filtrates, or gel fractions containing tritiated molecules is by digesting them in a solubilizing medium that is compatible with scintillation counting liquid. Hansen and Bush (1967) re-evaluated this technique using strongly basic reagents. Improvements in solubilizers and compatible counting solutions have been made since then and several proprietary mixtures for this purpose are available. The details of choosing the best solubilizer for a particular application are discussed in Section 5.

4.4

Labeled Pharmaceuticals

The preparation of tritium-labeled pharmaceutical compounds for counting is very similar to that used for other materials. Pharmaceu-

' 1 Pascal = 1 Pa = 1 N/m2= 9.87

x

10-6

atm.

4.4

URINE SAMPLES

1

21

Outlet

Pig. 5. Pam-bomb tritium sample combustion apparatus (after Moghissi el d., 1975).

ticals may need dilution prior to counting; thus one must calculate the factor needed to obtain a statistically valid count rate in a reasonable period of time. An appropriate aliquot is added to a suitable liquid scintillation solution. Problems of adsorption of the radioactive sample onto the interior walls of glass counting vials can be minimized by using plastic or siliconized glass vials, adding nonradioactive carrier, and/or adjusting pH.

4.5

Urine Samples

Urine samples are conveniently analyzed by liquid scintillation counting. However, content of solid materials, color, and acidity of raw urine can affect counting efficiency (Butler, 1964). Urine is sometimes distilled, particularly in preparation for low-level counting (Moghissi et al., 1969b). As mentioned in Section 4.2.1.2, the distillation should be carried almost to dryness. If tritium is excreted in an organic compound in urine, measurement of only the distillate is inadequate. In this case, either raw urine should be used or adequate oxidation procedures should be applied (Rehnberg et d., 1972). An alternative to liquid scintillation counting is to prepare triti-

22

/

4.

SAMPLE PREPARATION

ated hydrogen from urine for measurement in a proportional or gas ionization chamber. Calcium and zinc turnings have been used for the reaction (Butler, 1964). This method is inadequate if organicallybound tritium is to be assayed.

4.6

Enrichment

The specific activity of tritiated samples may be so low that suficient activity cannot be introduced into the counting equipment to give statistically adequate counts. When this occurs with other radionuclides, a chemical method is often used to separate the radioactive compound from the other constituents of the sample. If the sample is already one single compound, nothing more can be done. With tritium, however, the situation is more fortunate because fractionation factors between tritium and protium are high. Several ways of enriching the tritium content of a sample are feasible. 4.6.1 Electrolysis For water samples, electrolysis, introduced for this purpose by Kaufman and Libby (1954), is the most popular method of enrichment. In disintegrating water molecules by electric current in the presence of an electrolyte, a fractionation factor of about 25 in favor of protium occurs. That is to say, the escaping hydrdgen has an isotope ratio, T/H, that is 25 times lower than that of the liquid water from which it emerges. With a current density below 0.1 A/cm2 and a temperature below 10°C, the volume may be reduced in one batch by a factor of 20, the remaining 5 percent of liquid containing 90 percent of the original amount of tritium. The limiting factor for the volume reduction ratio is set by the requirement that the concentration of electrolyte, usually NaOH, should be between 0.8 percent and 20 percent by weight. Before the volume may be reduced hrther, the solution must be neutralized and the sample distilled from the electrolyte. The process may then be continued with a smaller quantity of electrolyte. This batch-method for electrolysis was improved by Brown and Brummitt (1956), and has been used extensively. Great care must be taken to avoid contamination and losses in handling of the sample in multiple distillations. The enrichment procedure has been fuither modified so that a 100fold volume reduction can be achieved with less handling of the sample (Ostlund et al., 1969). The electrolysis is performed in the cell

4.6

ENRICHMENT

I

23

shown in Figure 6. The starting volume is 250 ml of carefully distilled sample-water of which 50 ml is introduced into the lower part of the cell, with 0.6 g of tritium-free sodium hydroxide, and the remaining 200 ml are stored in the dropfunnel on top of the electrolysis cell. With a current of 3 A for 24 h, 25 ml of water are disassociated, and water is then added from the storage funnel until the 50 ml mark is again reached. After nine days, the total residual volume is 25 ml in the cell. The current is then lowered to 0.3 A and electrolysis is

SUPPLY

Fig. 6. Cell for electrolytic enrichment [after Ostlund, H.G., Rinkel, M. 0. and Rooth. C. (1969).Tritium in the equatorial Atlantic current system," J . Geophys. R-. 74. 4535, copyrighted by American Geophysical Union].

24

I

4.

SAMPLE PREPARATION

continued for another nine days, a t which time the remaining quantity of water is about 2.5 ml with typically 78 percent of the total original quantity of tritium. Thus a 78fold enrichment of the tritium concentration in a sample can be achieved by this periodic-addition electrolysis. The reproducibility attainable with this method is about 2-4 percent for one standard deviation. The accuracy can be slightly improved by measuring the D/H ratio in the original and the final sample, thereby monitoring the enrichment process. The periodic-addition enrichment takes about 10 days for each factor of 10, but is not very labor-consuming. It can easily be performed with a large number of cells operating simultaneously in one chain (series coupling) from one 3-A power supply, and another chain from a 0.3-A power supply, so that samples can be a t different stages of enrichment. It is thus well suited for routine work. Bogen et al. (1973) described a n acidic electrolysis method. They report that this method is more rapid and permits enrichment of smaller volume samples than other electrolysis techniques. In any electrolysis cell, there is always a danger of explosion of the hydrogen-oxygen mixture. Thus electrical connections must be nonsparking, the cells must never be handled with the power on, and electrolysis must be stopped before dryness, or before the liquid level falls below the lower edge of the electrode. 4.6.2

Thermal Diffusion

Thermal diffision is a well-established method for separating isotopes. The functional part of the apparatus is a vertical, cylindrical tube containing a center wire. The tube is filled with the gas sample. The center wire is heated while the outer cylinder wall is cooled. Gas near the cool wall is enriched in heavier molecules; that near the hot wire in lighter molecules. For hydrogen, this method is particularly attractive because of the large ratio in molecular weight between HT and Hi.Several laboratories have tried this method for routine enrichment of tritium samples (Verhagen, 1967; Israel, 19621, and it is used routinely by a t least one. Twelve liters of hydrogen are prepared from the water sample by reduction with hot magnesium or zinc. By suitable arrangement of two diffusion columns and a container, a ten-times enrichment with better than 95 percent recovery is achieved in about twenty hours. This is attractive, but there is a disadvantage in that hydrogen gas must be made out of the entire sample, which is usually water. Thus,problems arise in the preparation system if larger amounts of water than about 20 g are handled

4.6 ENRICHMENT

1

25

because the size of the reduction furnace and the isotope separator becomes unwieldy. 4.6.3

Gas Chromatography

Hydrogen easily dissolves in metallic palladium, and several attempts have been made to use palladium as the stationary phase in a gas chromatographic column, the temperature of which is varied to separate HT and H,. Hayes and Hoy (1973)have recently developed a system that lends itself well to routine use. A water sample of up to 40 g is slowly reduced by hot magnesium to hydrogen, and the hydrogen is introduced into a system of several ovens containing palladium sponge. The ovens are alternately heated and cooled according to a pre-set program, absorbing and releasing the gas sequentially. A high degree of enrichment is obtained, about 80 percent of the tritium from the original 40 g of water being contained in about 700 cm of hydrogen gas a t the end of the gas chromatograph. This method is advantageous in that the whole procedure takes only a few hours. On the other hand, the instrumentation is somewhat complicated and reduction of large amounts of water to hydmgen is not a trivial task. 4.6.4

Distillation

Enrichment of tritium by distillation has been attempted. To obtain 10-fold enrichment, a column of about 30 theoretical plates3 is needed, with the deuterium and tritium collecting in the bottom boiler and the depleted light water in a container at the top. Columns with the necessary separating property have such a long hold-up that the yield in practice is insufficient. An idealized plate on which the outgoing vapor and liquid phases are exactly at equilibrium.

5. Measurement Techniques 5.1

Introduction

5.1.1 General

Liquid scintillation counting is the usual technique for measuring tritium in discrete samples. A wide range of tritiated solutions or suspensions may be counted directly or after the treatment described in Section 4. However, if the sample is gaseous or if greater sensitivity is required, proportional counting may be preferable. Geiger counters or ionization chambers can be used for measurements requiring less sensitivity. Several methods are available for continuous measurements of tritium concentrations in gaseous or liquid streams. The appropriate method for such real-time measurements is determined by the required sensitivity and response time and also by the presence of other radionuclides. Tritium in liquid. (usually tritiated water) is often measured with solid organic or plastic scintillators. Real-time methods using liquid scintillators are available, offering better sensitivity at the expense of simplicity. A current ionization chamber with an electrometer is the simplest and most economical method for real-time measurement of tritium in the gaseous form even though it is not specific for tritium. Methods have been established fm reducing the interferences from other radiations, from gas-borne ions, and from charged particles. Greater sensitivity, particularly when measuring tritium in hydrogen or in a hydrocarbon gas, is attainable with a flow proportional counter. Such devices can also be used, though with greater difficulty, for measurement of tritium in air. Solid scintillators may also be used for realtime measurements of tritium in gases. In the case of tritiated water vapor, these methods are not very satisfactory due to sorption of water on the scintillator surfaces. Tritiated water vapor can be measured in the liquid phase either by continuously condensing the sampled vapor or by collecting it in non-tritiated water. A monitor for tritium in water can then be used.

5.1 INTRODUCTION

1

27

Activity on surfaces is most easily detected by inclusion of the surface within a counter or by transfer of some of the activity to a counter. Direct measurement is difficult although windowed and windowless counters have been used. Several methods for detecting tritium that are of restricted application or are in the early stages of development are also noted in this Section. 5.1.2

Criteria for Comparison of Methods

5.1.2.1 Introduction. Selection of the most suitable method for a particular measurement may depend upon one or several criteria. Most likely to be important are sensitivity and precision of measurement, the time taken for a measurement, the degree of specificity of a method for tritium or for a particular chemical (or physical) form, the commercial availability of equipment, and whether discrete samples are to be handled or whether a continuous real-time monitor is required. The relative importance ascribed to the various criteria and the trade-offs necessary will depend upon the individual application. 5.1.2.2 Sensitivity and Precision. Minimum detectable activities have often been defined somewhat arbitrarily as multiples of the standard deviation of the background count. A factor of 3 has been widely used (NCRP,1961), though factors as low as two times the standard deviation have been suggested (Moghissi et al., 1969b). However, a detection limit with a more precise statistical meaning can be derived. The most suitable statistical criteria for deciding which of several methods will provide the best performance depend upon the intent of the determination. Three lower limits of activity should be distinguished. The first is a decision limit a t which one may decide whether activity is present. The second is that activity which may be detected with a given reliability. The third is that activity which may be measured with a given precision. Currie (1968) and Altshuler and Pasternak (1963) have discussed in detail the derivation of these limits. For the purpose of this report, the second limit, i-e., the detection limit, has been chosen for comparison of methods. Two probabilities are important for this limit: the probability of falsely concluding that a blank observation represents a real signal and the probability of falsely concluding that a real observation represents a blank signal. In some measurements, the consequences of overlooking activity or of falsely concluding that i t is present may be different, and the two

values of probability used in estimating the detection limit should therefore be different. For this discussion, however, these probabilities are both assigned the value of 5 percent. The "minimum detectable true activity" or "detection limit", A , may then be defined as that activity corresponding to a count above background that will be detected with 95 percent probability with only 5 percent probability of falsely concluding that a blank observation represents a "real" signal. Implicit in the criteria is that the measurement conditions are reproducible. Expressions for A (in terms of disintegration rate) derived from Cunie (1968)are as follows: When the background is "known" (i.e., a mean background has been estimated with variance small compared to the background count in the period of sample measurement, T),' and has a Poisson distribution, then the detection limit, A,, is given by

where B is the expected background counting rate during time T and E is the counting efficiency in terms of counting rate per unit disintegration rate. When the background is "unknown" (i-e., is estimated during a period equal to that in which the sample is counted), then the detection limit, A,, is given by

[Note that if estimation of the lower limit of activity that may be measured to a given precision is required, then optimal division of the counting time between background and sample plus background should be considered, as suggested by Loevinger and Berrnan (1951) and developed by ICRU (1972). Expressions for this "quantitative limit* and for minimum true detection limits for different probabilities of incorrect decisions may be derived from C u m e (1968).I The first term in the numerator of the expressions for Ak and Au may often be neglected. The detection limits, expressed as counts, are then multiples of the standard deviation of the background count, 3 . 3 ~and 4 . 7 ~when the backgrounds are known and unknown, respectively. In some instances, e.g., liquid scintillation counting, the detection limit for a concentration of activity is the limit of interest. As shown above, the minimum detectable true concentration, C, (disinterpa-

5.1

INTRODUCTION

1

29

tion rate per unit mass), when the background is unknown, is given by

C, =

2.7

+ 4.7 MET

'

(3)

where M is the mass of sample counted. If, as is often the case. 4.7 is large compared to 2.7, then

The reciprocal of the bracketed quantity has been termed the "figure of meritn for counters (Loevinger and Berman, 1951). If the counts from a detector are not presented directly on a scaler, but have a signal derived proportional to the counting rate by a ratemeter circuit, then the minimum detectable concentration will depend upon the time constant of the rate meter. If this is Tc, the effective counting time may be taken as 2T, in estimating the minimum detectable concentration from a single reading; that is, assuming the background is "well knownn from prior long-term averaging, then

Ck =

2.7

+ 3.3

a

MET

For purposes of comparing the various methods described in this section, the detection limits, Ah,A",and Ck,C,, have been estimated. where possible, from the above expressions with T = 1 min, the background being assumed unknown for discrete samples and known for real-time measurements. In some instances, sufficient data were not available and the author's quoted limits have been used. 5.1.2.3 Time for Measurement. The time taken for a measurement will include sample collection and preparation as well as the counting time. Handling of discrete samples prior to counting in a liquid scintillation counter or proportional counter has been covered in Section 4. The emphasis here is on the times that are associated with the flow of sample through a continuous monitoring instrument and those required to collect the appropriate number of counts or charge. Ifthe sample does not mix within the detector, and if a step change in the concentration of activity occurs in a sample flowing through a t constant rate, then the magnitude of the output signal (i-e., counting

30

1

5. MEASUREMENT TECHNIQUES

rate or current) will change linearly with time between initial and final values. If complete mixing occurs within the detector (and none in the input sample line), the output signal will change exponentially in time for a similar step change in the sample concentration. The time constant will be the detedor sample volume divided by the flow rate. If time (or spatial) resolution in a stream is important, as in a sample from a chromatographic column, for example, then, clearly, mixing must be avoided so that the shortest response time is attained for a given flow rate. In most applications, however, time resolution is not so critical and the geometry of the detedor allows mixing within it. A single exponential will only describe the time response over a limited dynamic range since imperfect mixing within the detector will slow the rate of signal change. Generally, little is gained by using a counting time or rate-meter time constant that is much shorter than the time constant that describes the sampling response over the requisite dynamic range. The detector size o h n has to be selected as a compromise between the requirements for high sensitivity (large detector) and fast response time (small detector). 5.1.2.4 Specificity to Tritium. Because tritium is a pure beta em% ter with a maximum energy of only 18.6 keV, its unambiguous detection and measurement is complicated by the presence of other radionuclides in the sample, by exposure of the detector to gamma radiation, and by electrical noise or instability. Techniques for reducing the first two interferences will be noted in the subsequent sections. The effect of residual interference upon the minimum detectable activities, provided that the interference has a steady mean value, may be estimated by using the appropriate value of the total background counts, since all arise from Poisson distributions. If the interference produces an output signal, Be, which has a relative standard deviation, ue say (such as might arise from electrical noise), then the variance of the total background, B, will be B, + (U,B,)~,where B, is the output from the Poissonian distributed background. The expression B,, + (veBe)' should be used for the value of B in the detection-limit equation in Section 5.1.2.2. 5.1.2.5 Specificity to Physical or Chemical Form. Because of the a. short range of the tritium beta particles, the physical form of the tritiated material has a considerable effect on the efficacy of detection. For tritium beta particles to be observed, the detector must not be more than about 0.1 mg ern+ away from the source. In water, for example, the detector must be within 1 pm. A large source volume can therefore only be attained in the liquid state by using a detector

5.1

MEASUREMENT OF DISCRETE SAMPLES

I

31

of large area or by mixing with a liquid detector; i.e., a liquid scintillator. With a solid sample, only the activity within about 0.1 mg ~ m of- the ~ surface will be measured unless the activity is high enough for the bremsstrahlung to be detected. 5.1.2.6 Commercial Availability. A few types of complete instruments for measuring tritium are available commercially, although many of the techniques described in this report can be performed without fabrication of items other than a detector, particularly if laboratory measurements only are envisaged. For example, a special ionization chamber may be needed for air monitoring, but a commercially available electrometer can probably be used with it; for water monitoring, a scintillation detector may be made that will fit into a commercially available liquid scintillation counter. The review by the Lawrence Berkeley Laboratory (1972) lists available instruments and suppliers. 5.1.2.7 Discrete Samples or Continuocrs Monitoring. Since most sample collection, handling and preparation procedures can be autumated, the decision as to whether to perform a measurement by taking discrete samples or by continuous monitoring will depend upon the degree of complexity involved in the latter and the relative costs of the two methods. Clearly, methods requiring no or very little sample processing are easily set up for continuous monitoring. For example, air often requires only filtration before being passed into a n ionization chamber for measurement of tritium content. Monitoring continuously in this manner is very practical. Conversely, methods that require more complicated processing are more likely to be carried out manually with separate samples. An example is liquid scintillation counting of water samples. Automatic systems have been designed that sample, process, and count, but these are costly and practical only in limited application.

5.2 5.2.1

Measurement of Discrete Samples

Liquid Scintillation Counting

5.2.1.1 Introduction. Detection with liquid scintillators has become established as the most convenient and practical way of measuring tritium in the liquid phase. Counters are commercially available, many with capabilities for handling several hundred samples. Numerous monographs, symposium proceedings and reviews are availa-

32

1

5. MEASUREMENT TECHNIQUES

ble on various aspects of liquid scintillation counting and the reader is referred to these extensive reviews (Lawrence Berkeley Laboratory, 1972; Horrocks and Peng, 1971; Bransome, 1970; Parmentier and Ten Haaf, 1969; Birks, 1964; Rapkin, 1964; Bell and Hayes, 1958). In this section, the counting technique is outlined in a manner suitable for the user and practical aspects are discussed. The technique consists of dissolving or dispersing the tritiated compound in a liquid scintillator, subsequently detecting the light emitted from the scintillator, and counting the number of emissions. Major efforts in developing the technique have been directed to improving the detection efficiency of the photomultipliers, distinguishing the tritium scintillation events from others, and in finding scintillator/solvent mixtures that can accommodate large volumes of sample (especially aqueous samples) without degradation of scintillation properties. 5.2.1.2 Instrumentation. Only a few photons are emitted by a scintillator as a result of absorption of a tritium beta ray, approximately 12.33, whereE is the beta energy in keV (Horrocks, 1970).Hence, the spectrum of pulses from tritium beta particles a t a photomultiplier output lies in the region of the single photoelectron "noise" spectrum originating in the phototube. Because simple pulse height discrimination is, therefore, inapplicable, counting the coincident pulses from two photomultipliers, proposed by Hiebert and Watts (19531, has become the standard method for liquid scintillation counters. The rate, Re, a t which pulses originating from single photons or uncorrelated photoelectron events, in each photomultiplier, coincide is given by where r is the time within which pulses from the two photomultipliers are detected as coincident and N,, N 2 are the uncorrelated counting rates from the two photomultipliers. A small value of r is desirable for a low random coincidence counting rate. Because a high efficiency for measurement of true coincidences arising from tritium beta rays is also desirable, the ultimate limit is set by the time during which photons are emitted in each scintillation+ few nanoseconds. However, practical values are limited to 10-30 nanoseconds because of variations in the transit times of the pulses through the photomultipliers and amplifiers and variations in the time relationship between pairs of pulses with different heights. Until a few years ago, refrigeration of the photomultipliers was needed to reduce random coincidence counting rates to less than 1 s-I. An order of magnitude

5.2 MEASUREMENT OF DISCRETE SAMPLES

1

33

reduction could be attained by operating a t 0°C rather than room temperature. With the newer photomultipliers that have bi-alkali cathodes, random coincidence counting rates may be less than 0.1 s-I a t normal room temperatures so that refrigeration is often unnecessary. Introduction of the more sensitive bi-alkali photomultipliers, the spectral response of which more closely matches the spectrum of light emitted by the scintillators, has also improved the detection eficiencies. Improvement has also resulted from the technique of adding the amplified pulses from the two photomultipliers prior to selection of pulses within the range of heights corresponding to those from tritium. A typical arrangement is shown in Figure 7. The final gate ensures that the output pulses have satisfied both the timing require ment and the pulse height requirement. Graded shields are often used. Detection efficiencies for unquenched samples may be as high as 60 percent with commercially available counters and the detection limits are currently in the range of 1-10 pCi/g for counting periods of a t least 10 minutes.

LK;HT REFLECTORS

P

m

7,

AMPLIFIER

.

-

COlNaDEKE UNIT

-

AMPLIFIER

-

qv

-

1'

SUMMNG AWLIFIER

?

'

71

SINGLE CHANNEL ANALYZER

-

OUTPUT GATE

c

Fig. 7. Main detector and electronic features of a liquid scintillation counter

34

1

5.

MEASUREMENT TECHNIQUES

5.2.1.3 Liquid Scintillators. A scintillation liquid consists of one or more fluors (or scintillators) dissolved in one or more solvents. Energy from ionizing particles is absorbed primarily by the solvents. Some energy is subsequently transferred to the fluor where a high percentage of it is converted to light that can escape from the liquid. Suitable solvents are benzene and its homologues, toluene and pxylene. Toluene is the most widely used since it is inexpensive and has a freezing point (-95°C) below the operating temperatures of refrigerated systems. As noted above (Section 5.2.1.2), in recent instruments refrigeration is unnecessary. Therefore, the use of p xylene (freezing point 1293, the most efficient solvent, is increasing. The major disadvantage of aromatic solvents is their incompatibility with water. Because water and aqueous samples represent a major portion of all tritium samples, substantial efforts have been made to develop mixtures that are capable of incorporating aqueous samples while allowing transfer of absorbed energy to the fluor. The most successful non-aromatic solvent for this purpose has been pdioxane, introduced by Kallman et al. (1955). Although dioxane itself does not t r a d e r energy to a fluor very efficiently, efficiencies comparable to those of the aromatic solvents can be attained by adding naphthalene as a secondary solvent in relatively large amounts. The best of various, empirically-determined formulations can incorporate up to 20 percent by volume of water without impractical degradation of light output (Moghissi et a1 ., 1969b). The major disadvantages of a dioxane-based system are the deterioration in light outpul when inorganic compounds are present, the occurrence of phosphorescence, and the high cost. Alkylphenol detergents have been used successfully to suspend water in aromatic solvents (Lieberman and Moghissi, 1970; Patterson and Green, 1965). Examples of these detergents, which are available from various manufacturers, are "Triton X-100" and 'Triton N-101". The majority of liquid scintillation "cocktailsn offered commercially ("Insta-gel", "Aquasoln, for example) are based on these detergents with toluene or p-xylene as solvents. Birks (1970, 1964) and Horrocks (1970) have summarized the properties of the many fluors that are being used or have been proposed for liquid scintillators. Diphenyloxazole (PPO), introduced by Hayes et al. (19551, is the one most widely used and readily obtainable. Paraterphenyl has also been widely used. Although it is not as efficient as PPO and cannot be used with naphthalene, i t is less expensive. The peak in the emission spectrum from PPO is at 370 nm; other fluors, such as p-terphenyl, are a t similar or shorter wave-

5.2 MEASUREMENT OF DISCRETE SAMPLES

1

35

lengths. Since most of the older photomultipliers are most sensitive to light in the wavelength range 400-450 nm (S-11 type photocathode), a second fluor has normally been used a s a wavelength shifter. Hayes et al. (1956) introduced p-bis-2(5phenyloxazyl)-benzene(POPOP) which has a peak emission a t 415 nm. Of the many secondary fluors proposed since then, the more soluble dimethyl derivative of POPOP (peak emission a t 424 nm)and pbis-(o-methylstyrylbbenzenehisMSB), suggested by Bush (1966), appear to be best (Moghissi et al., 1971; Moghissi and Carter, 1968). The newer photomultipliers with bi-alkali photocathodes are most sensitive in the wavelength range 380-390 nm. Hence, secondary fluors are unnecessary to match PPO to these photomultipliers but may still be useful with pterphenyl. A secondary fluor may also be useful if the scintillator container or a component of the sample absorbs some of the primary fluorescence. Self absorption by the primary fluor, which is reduced by wavelength shifting, is not usually of consequence in the usual size of scintillatorl sample mixture (-20 cm3). In summary, the following scintillation liquids are recommended for aqueous samples: (a) Solvent: Toluene or p-xylene with a n alkylphenol detergent i n the volume ratio 3 3 to 2. Primary fluor: PPO or p-terphenyl a t 5-10 mglg. Secondary fluor (if needed): bis-MSB, POPOP or dimethyl POPOP a t 0.5-2 mglg. Water content: up to 50 percent by volume possible; highest efficiency is with 40 percent water by volume in p-xylene, which contains 40 percent detergent by volume. (b) Solvent: Dioxane with naphthalene in the volume ratio 8-12 to 1. Primary fluor: PPO a t 5-10 mg/g. Secondary fluor (if needed): bis-MSB, POPOP or dimethyl POPOP at 0.5-2 mglg Water content: up to 20 percent by volume. 5.2.1.4 Zncorpomtion of Biological Tissue into Liquid Scintillator. Incorporation of samples of biological tissue into a liquid scintillator is difficult. The problems are incomplete mixing, whlch results in low, difficult-to-measure counting efficiencies, and quenching of the scintillation by the acids and bases used for tissue hydrolysis. No method is generally applicable. Table 3 (from Pollay and Stevens, 1970) summarizes the efficacy of various methods. The use of detergents to incorporate alkaline digestions into toluene-based scintillators appears to be the most promising method.

5.2.1.5 Counting Vials. The choice of counting vial or container is determined partly by performance and partly by economics. Plastic vials are less expensive than glass, exhibit lower background counting rates and allow higher counting efficiencies than even the ."low potassium" borosilicate glass vials. However, the scintillator solvents permeate the plastic vials, resulting in loss of solvent, as noted in Table 4 and, in some cases, in swelling or distortion of the vials. The standard capacity is 20-25 cm3, but smaller vials (and re-usable vial holders) are available that may be used in automatic liquid scintillation counters. These can offer appreciable saving in costs both in vials

TABLE3 -Methods

for incorpomting biological tissues inta liquid scintilluiorsa

Solubilizing Agmb

Scintilhtim Sdremta

oplmum Tissue Weight or Volume'

Rcp.ntim

-

I

Nirrie acid

PPO. POPOP, Naphthakne. Diozane. Ethylene eIy-1 Formamide Tolueneethanol PW or Toluene PPO.. POPOP 2 h' Methadie m u m PPO, WPOP. Ethykne hydrcaide glycol. Mowbutyl ether. Toluene 1.5 M Mehnolic Hy- Toluene. PPO.POPOP aminclC-X' chloride and Trim-X-IW 1.0 M Mehnolic Hy- NE21S rintilhlor baed amlnel(FX' chloride on Xylene and NaphthaRetrcatmrnt with N. lene

1

<76 mg

Wid

Pmr

cm

ma

Pmr

clOO mg

Rapid

Fair

LOO mg

Slow

Fair

<50 mg

MoQntely slow

Fair

KOH 1.0 M Mehnolic Hy- Toluene, Butoxyethano1 100-1000 mg up Slow Cmd amlnclC-X' hydroxide and PPO ' to 1 ml plasma NCS ReagenF (quater- PPO. POPOP, Toluene Sw for whole tissua but Cmd >I000 mg nay ammonium basts) moderate for homop nues 1 N NaOH and BieSoIv PPO. POPOP. Toluene Up to 150 mg Rapid BBST - Bi-Solv BBS? Up to 1 ml Cmd PPO. POPOP. Toluene Rapid '&I Polhy. M. and [email protected]. A. (1970). " S d u b i l i i m of a n i d timues for liquid mntilhtim Wtk."py 2M in T k Cvmnr S L ~ ~ofLiQvid W ScidUah Cwnr*. B r w o m e . E.. Jr.. Ed. (Grune m d Strat. m. New Ymk). by permirim. V v i - mmerhat with typ ofCiuue. 'T n d c w k . Robm and H u s . Inc. Trademark. Nudear-Chicego Corp. ' Trademark. Beckman Carp.

1-

TABLE4-LOSS of compoWIClS through liquid scintillation vinls at tempemture of 22T0 Polyethylene 'Teflon" coated Polyethylene Polyethylene thick.wall Nylon

Volume

P-rvlene

Toluene

Dioxane

H.0

ml

mgld

mgld

mgld

mgld

25

128 120 77 5.3

187 192 101 10

4.1 3.2 1.2 15

0.5 0.1 0.3 34

25 20 25

Adapted from Lieberman and Moghissi (1970).

5.2 MEASUREMENT OF DISCRETE SAMPLES

1

37

and quantity of scintillator when adequate sample activity is available (Rummerfield and Goldman, 1972). An even less expensive method, noted by Gupta (1973), is to use disposable polyethylene bags inside re-usable polyethylene vials. Scintillator volumes of as little as 50 mm3 can be counted with negligible deterioration in counting efficiency in this way by selecting the appropriate size of bag. 5.2.2

Internal Gas Proportional Counting

5.2.2.1 Introduction. The main advantages of a proportional counting system for tritium are high efficiency and low background. Although proportional counting with tritiated water vapor in the counter is practicable (Memtt, 1958), low-level laboratory measurements are usually performed on samples that have been converted into a gas which has good counting characteristics and is easily handled. Details of the design and C O I ~ S ~ N of ~ ~counters O ~ for lowlevel measurements have been extensively reviewed by Cameron (1967) and more recently by ICRU (1972). 5.2.2.2 Design and Performance. Gas proportional counters of many different sizes are presently in use. Large volumes (up to 3 liters)and high pressures (up to several atmospheres) are used to increase the amount of sample included in the counter. Practical limits to these variables are the voltage required for operation with high efficiency on the proportional counting plateau and the need to shield the device to reduce the background. Up to 5 kV have been used for the polarizing voltage on large counters (Cameron, 1967). In the better systems, the tritium in 1 to 5 cm3 of water sample may be contained in the counter in the form of a suitable counting gas (see below), of which about 80 percent may be in the active volume of the counter. Upper and lower pulse height discriminators may be positioned to define a "tritium channel" so that 95 percent of the pulses from tritium disintegrations in the active region are counted in the channel. Hence, counting rates of 2-8 min-1 are realizable from a water sample with a concentration of 1 pCi/cm3. For low-level measurements, precautions are needed to reduce the counting rate from radioactivity in the chamber construction materials, from electrical noise, and from external radiations. All these topics are covered by ICRU (1972). For metallic parts, material made before 1939 is preferable. If this is not available, oxygen-free high purity copper seems to be the best available substitute. For reduction of the counting rate from external radiation, shielding and an anticoincidence guard are used. The classical system for

38

1

5.

MEAsUREhlENT TECHNIQUES

this is outlined in Figure 8 with the gas counter at the center of a cylindrical lead shield, the lead shield being surrounded by a ring of cosmic ray counters. These usually operate in the Geiger region at atmospheric pressure with an isobutane-helium gas mixture slowly flowing through. They are electrically connected in anticoincidence with the proportional counter output. The assembly is contained inside a massive shield of iron with wall thickness of about 20 cm of iron and 10 cm of paraffin containing boron to attenuate cosmic rayproduced neutrons. An alternative anticoincidence guard ring is integral with the proportional counter (Houtermans and Oeschger, 19581. In this case, the sample gas serves as filling for both the center counter itself and its guard ring. The advantage of this system is that a very low background is obtained since there is little or no material in the walls of the center counter. However, the overall efficiency is considerably lowered because only part of the gas is supplying counts to the center wire. A useful practice is to monitor the coincident counting rate from the guard and sample counters and also the anticoincidence counting rate above the tritium-channel upper discriminator as a check on the stability of the background counting and on the counter and amplifier gain. Background counting rates in the tritium channel of less than 2 min-' can be obtained. Most other background pulses are larger than the tritium pulses and are counted above the upper limit of the tritium channel. Background pulses being counted in the tritium channel can be reduced by increasing the density of the counting gas by the addition of a hydrocarbon such a s propane. The pulse heights from tritium beta particles are not affected appreciably, but the

Fig. 8. Gas proportional counter surrounded by lead shield and ring of Geiger counters.

5.2 MEASUREMENT OF DISCRETE SAMPLES

1

39

background pulses are shifted to higher energies and out of the tritium channel. To reduce the background in gas counting of tritium further, another improvement has recently been introduced which is called "pulse shape discrimination" (Hayes, 1972). The usual anticoincidence gates, triggered by the cosmic ray counters, and the selection of only a part of the tritium spectrum, as in the other systems, exclude a large part of the unwanted background counts. Because of the short range of the tritium beta particles, the entire avalanche amves a t the center wire within a very short time interval, resulting in risetimes for the tritium pulses shorter than for pulses from other (unwanted) sources. Discrimination against the pulses with the longer rise-times is then possible. The minimum detectable activity in a one-minute count with a lowlevel proportional counter is equivalent to about 1 pCilcm3 of water sample. However, sample counting conditions can be maintained for many hours and a more realistic counting time is 1000 minutes. A typical minimum detectable activity is then in the range of 20-80 fCil cm3of water, with the best units currently in use being able to detect (by the criteria in Section 5.1.2)down to 15 fCilcm3. If the sample is enriched by a factor of 100 prior to conversion to a counting gas (see Section 4), then the theoretical minimum detectable activity is reduced to 0.2 fCilcm3. However, laboratory contamination usually limits the sensitivity to a concentration somewhat greater than this. 5.2.2.3 Preparation of Counting Gas. Hydrocarbons are excellent counting gases and methane, in particular, is used for this purpose (Anand and Lal, 1964). Preparation from aqueous samples involves the direct hydrogenation of carbon dioxide with the sample using zinc metal as reducing agent and a catalyst in a high pressure vessel. A disadvantage of methane is that its low boiling point (relative to other hydrocarbons) makes its handling within the preparation system difficult. Ethane has also been prepared successfully from a water sample. The chemical preparation includes reduction of the water with zinc or magnesium to hydrogen gas which is then combined with either acetylene or ethylene to form ethane. The reduction should not yield a metal hydroxide as a by-product because this would remove part of the sample and cause unpredictable isotope fractionation in the hydrogen gas. Magnesium is generally preferred over zinc because it may be used a t a temperature farther below its melting point (and hence be less likely to melt). Fifty grams of magnesium at about 500°C will reduce up to 15-20 grams of water. Combination with acetylene (Bainbridge et a l . , 1961)has the bene-

40

1

5.

MEASUREMENT TECHNIQUES

fit of incorporating four atoms of the sample hydrogen into each molecule of the counting gas, but acetylene free from tritium is difficult to obtain commercially because it is frequently prepared from surface water. Reaction with ethylene is usually preferred. A spherical flask large enough to store the hydrogen and the ethylene at about atmospheric pressure is used. A palladium sponge catalyst on the bottom of the flask is warmed just slightly from underneath. The reaction takes a few hours, and the conversion is essentially complete. Any slight excess of ethylene will follow the ethane into the counter. Ethane is easily moved from one point to another in a system by freezing out with liquid nitrogen. Hydrogen itself may be used in a counter without further conversion if mixed with a hydrocarbon (Ostlund et al., 1969). This method which saves one preparation step is being used more frequently. A counter containing hydrogen a t 200 kPa and propane a t 30 kPa operates reliably in the proportional mode a t about 4 kV. Hydrogen gas can be handled by condensing on activated charcoal a t liquid nitrogen temperature. Losses and cross contamination in the charcoal have not been important in practice.

5.3 Real-Time Measurement 5.3.1

Scintillation Methods for Tritium in Liquid Streams

1.3.1.1 Introduction. Scintillation methods are now the only practical approach to monitoring liquids for tritium. Most applications are associated with chromatography where the tritium is being used as a molecular label or with industrial or laboratory effluents where the tritium is most likely in the form of tritiated water. Several reviews of the methods and their associated problems are available (Osborne, 1971;Schram, 1970;Parmentier and Ten Haaf, 1969; Rapkin, 19641. Both solid and liquid scintillators have been used to detect tritium in liquids. The number of tritium beta particles reaching an interface with a thick source is only 62.2 rnin-I ern+ from a concentration (in water) of 1 kCi/cm3 (Feinendegen, 1967). Therefore, large surface areas of solid scintillators are generally needed. Liquid scintillators, intimately mixed with the tritiated liquid, possess a n obvious advantage. The comparable counting rate, with a mixing ratio of 10:1, is 2.22 x lo5 min-1 cm-3. 5.3.1.2 Solid Scintillators. A variety of tubes and cells packed with powdered anthracene have been described, mainly for chromato-

5.3 REAL-TIME MEASUREMENT

1

41

graphic applications. The reviews cited above provide a comprehensive discussion. Several manufacturers of scintillators can provide flow cells of this type. A convenient summary is in a recent survey (Lawrence Berkeley Laboratory, 1972). Spatial resolution along the stream is important in this application. Most of the dectectors have sample volumes of about one cubic centimeter and are made to fit into normal liquid scintillation counters. Detection efficiencies (i-e., counting rate per unit disintegration rate in the total sample) are typically one percent. Quoted lower limits of the concentration of tritium measurable have been above 0.1 nCilcm3. Plastic scintillators have also been used in various forms (Schram, 1970; Muramatsu et al., 1967)and flow cells are commercially available. Large surface areas of scintillator are not as easily attained as with a finely-divided fluor, such as anthracene, so that detection efficiencies and limits are generally poorer. However, in one recently developed cell, which has a stack of thin (125 pm) sheets of plastic scintillator for a total surface area of 3000 cm2, 0.8 nCi/cm3 can be detected (Osborne, 1970a). 5.3.1.3 Discrimination Against Other Beta Emitters Using Solid Scintillators. The efficiencies of both anthracene and plastic scintillator dectectors for other beta emitters are higher than that for tritium because the effective source volume increases with increasing beta energy. The pulse height spectra from most other beta emitters generally range t o higher energies so that some of the interference from these emitters in tritium measurements may be reduced by pulse height discrimination. The actual reduction will depend on the thickness of scintillator and the volume (or effective thickness) of the sample. When only one contaminant is present, the counting rate h m a second energy channel is sdcient to estimate its contribution. However, with many or unknown contaminants, such as might be expected when monitoring industrial effluents (for example, fission products, activated corrosion products), this technique may be of little avail. Under these circumstances, counting should be preceded by removal of the contaminants by ion exchange, by adsorption, or in the case of dissolved gases by purging. 5.3.1.4 Reduction of the Sensitivity of Solid Scintillator Detectors to Gamma Radiation. A few centimeters of lead shielding are normally used to reduce the counting rate from external radiation and in most laboratory applications the shielding in a commercial liquid scintillation counter is adequate. The shielding may be inadequate, however, in some monitoring applications where the gamma-ray field is high. For example, with 10 cm of lead, the counting rate in a field of

42

I

5.

MEASUREMENT TECHNIQUES

1 mR/h from '%o was equal to that from a tritium concentration of 5 nCi/cm3 in the plastic scintillator detector described above (Osborne, 1970a). Reduction in the total quantity of a scintillator, while maintaining the same surface area, is necessary for a reduction in response to gamma radiation. One approach is to use even thinner layers of scintillator; Moghissi et al. (1969a) demonstrated this by spreading anthracene powder in thin layers on lucite rods. 5.3,1.5 Response Time and Contamination of Solid Scintillators. With anthracene packed cells, flow rates are usually a few cubic centimeters per minute, normally adequate to give response times of less than a minute. In detectors with the more open plastic or anthracene layer construction, flow rates may be an order of magnitude higher. Although anthracene cells are satisfactory in many laboratory applications, in industrial applications, such as effluent monitoring, they have not been very successful because they are easily blocked by suspended material in the sample flow, as well as by movement of the anthracene powder. Plastic scintillators are better for such applications. Discoloration of the scintillator surface or accumulation of material in slimes on the scintillation surface leads to loss in sensitivity and to retention of activity. The latter has proved to be a practical problem in monitoring wastes from nuclear power stations (Osborne, 1971) and requires periodic flushing of the detector with a chelating agent or periodic replacement of the detedor. Ideally, the sample can be adequately filtered or treated but, in practice, a compromise may have to be made between this and allowable delay or response time. 5.3.1.6 Liquid Scintillators. Solid scintillators are inadequate for monitoring a t levels much below 1 nCi/cm3. A liquid scintillator is tnore sensitive by several orders of magnitude (see Section 5.2.11, has greater freedom from contamination, and allows discrimination against other beta-ray activities. Automatic mixing of the liquid scintillator and the sample stream presents practical problems, but automatic mixing of the two liquid streams has been ackomplished in several ways. Tomono et al. (1972) and Shigematsu et al. (1969) emulsified the streams by sonication. Others have used a magnetic stirrer (Ting and Little, 1973; Hunt, 1968; Scharpenseel and Menke, 1962). Osborne (1968, 1972) has used agitation by air and tumbling of the two phase segments in a mixing coil. Both dioxane- and toluenebased scintillators have been used. Provided that mixing is effective, the counting efficiencies obtained with a flow system are the same a s those obtained in conventional liquid scintillation counting, as is the discrimination against other

5.3 REALTIME MEASUREMENT

I.

43

beta-emitting nuclides and gamma background. Detection Iimits for tritium then depend upon the effective volume of the counting cell used. A compromise usually has to be made between the sensitivity desired, which determines the flow cell size, and the desired response time. The cost of liquid scintillator may be a limiting factor, particularly if continuing operation is envisaged. For chromatographic purposes, Tomono et al. (1972)used a l-mm diameter "Teflon" tube, and noted that 0.6 nCi could be measured in 0.01 cm3 of column eluate. Ting and Little (1973)could detect 0.07 nCi/cm3 with a 4.3-cm3 coiled glass cell in a n effluent monitor. With a 0.5-cm3 glass cell, the minimum detectable concentration was 0.5 nCi/cm3 (Osborne, 1972). Water contents of the mixtures have been not more than 15percent, a limit imposed more by the capabilities of the mixers and the need to maintain a viscosity compatible with the desired flow rate rather than G t h the properties of the scintillator itself. Dioxane-based scintillators have been preferred where rapid mixing is necessary. Since the accuracy of a flow monitor of this kind depends upon maintaining a constant ratio of sample and scintillator flows, metering of the liquids is important. Pumps for aqueous samples are readily available; however, a t the low flow rates that may have to be used, the volume of sample in a pump may appreciably increase the response time. At flow rates of several cubic centimeters per minute, gravity feed capillary tubes can be used to control the flow of scintillator (Ting and Little, 1973). At lower flow rates, pumping has been found necessary. With both toluene and dioxane scintillators, the choice of pump materials is somewhat limited. Peristaltic pump tubing has been found to last only a few days. 'Teflon" diaphragm, "Teflonnpiston and stainless steel peristaltic pumps have been found to be satisfactory (Osborne, 1968, 1972;Hunt, 1968).

5.3.2

Ionization Chambers for Gaseous Streams

5.3.2.1 Introduction. Ionization chambers are widely used for measuring tritium in gaseous form in laboratory, environmental, and industrial monitoring applications (Jalbert and Hiebert, 1971;Baker and Richards, 1968; Jones and Douglas, 1967; Maushart, 1967; 0 s borne and Cowper, 1966,Soudain, 1965;Reinig and Albenesius, 1962). Such devices offer the benefits of simplicity and economy, requiring only a n electrically-polarized ionization chamber, an -electrometer and a method for passing the sample through the chamber.

44

1

5.

MEASUREMENT TECHNIQUES

If the ion pairs produced by tritium beta particles in the ionization chamber are completely collected, then the resulting current per unit volume of gas is given by

where S is the concentration of tritium in the gas in pCi/cm3and W is the average energy (in eV) requird to form one ion pair in the gas. Values of W are typically in the range 25-43 eV and for air may be taken as 34 eV. A useful rule of thumb for tritium in air is that the ionization current collected at saturation is 1pA/Ci. Instruments in regular use have chambers ranging in size fiom 0.3 1to 55 1allowing, in principle, with present day electrometers capable d measuring down to 1 fA, measurement of tritium down to the order of 10 fCi/cm3. Instability of the electrometer and electrical noise are practical limits, as may be the presence of other radionuclides or gamma background to be discussed later. 5.3.2.2 Geneml Design.The basic design of ionization chambers has been covered extensively elsewhere (for example, Boag, 1966) and will not be considered in detail here. In designing devices for measurements in the sub-picoampere region, precautions are necessary to keep the electrical capacitance of the chamber as low as possible, to provide a grounded guard to isolate the collecting electrode from the chamber polarizing voltage (which should be from batteries or a well smoothed, low impedance power supply), and to maintain an electrical resistance from the collecting electrode to ground which is high compared to that of the input of the electrometer. Some insulators (e-g., Teflon") may be electrically noisy, particularly ifthey have been strained during assembly. The noise can often be reduced by exposing assembled insulators and electrodes to several thousand R of gamma radiation. Ifgas tight seals are also required a t the collectorlinsulator/guardinterfaces, soft "0"rings should be used in the interface rather than sealing with the insulators. Relative movement of the collecting electrode and grounded or other surfaces should be avoided since such changes (in electrical capacitance) result in current flow a t the electrometer input. Similarly, connecting wires should be held rigidly and, preferably, should be embedded so that accumulation of ions is prevented. Ceresine wax is a possibility when the highest impedances are needed. If particles are likely to be present in the sampled gas, a filter

5.3 REAL-TIME

MEASUREMENT

1

45

should be installed upstream of the ionization chambers. An ion trap immediately upstream of the measuring chamber may be required to prevent entry of ions into the chamber with the gas. Higher electrical fields are generally used in the ion trap, which may be a separate inline unit or may be integral with the main ionization chamber as, for example, described by Reinig and Albenesius (1962). When tritium is the major source of ion production in the sample air (i.e., ions from other sources, such as electrical discharges, are not present), an ion trap may not be needed a t low flow rates since the ions will recombine before entering the ionization chamber. Soudain (1965) suggests that this is the case for sampling air below 100 cm3/s. 5.3.2.3 Construction Materials. The nature of the tritiated compound being measured will determine the suitability of various materials for construction. In the case of tritiated water vapor, adsorption will occur on insulators and electrodes. The ionization in the chamber gas is then the sum of that caused by tritium in the vapor phase and that by tritium on the walls. The relative proportions will vary with the surfacelvolume ratio of the ionization chamber, the nature of the surface, and the specific activity of the vapor. A change in effective sensitivity (current per unit concentration) with humidity may then result. With aluminum chambers, Osborne and Coveart (1973) noted a 15 percent increase in sensitivity in changing from 60 percent to 5 percent relative humidity, corresponding to an adsorption of 0.5 pg/ cm2.Fry (1959)noted 1 pglcm2deposited on nickel and stainless steel. Soudain (19651, however, found that less than 0.033 pg/cmZ was adsorbed on the walls of a stainless steel ionization chamber. He suggested, on theoretical grounds, that only metals with a high hydrogen potential should be used. Gold or platinum coatings have been recommended (Moshlev, 1968) but may not be necessary in many cases. The adsorption of water on chamber surfaces also lengthens the response times for tritiated water vapor. Of practical importance here is that flushing with a dry gas is much less effective than flushing with a gas of high relative humidity. In the latter approach, the activity is exchanged from the surfaces. Retention of tritium on any upstream components, such as tubing, filters, or a pump, will also result in lengthening of the "memory" of the system. With tritiated water vapor, gum rubber and polyvinyl chloride tubing are very retentive; polyethylene and "Teflon" tubing are less so. Silicone-coated filter media are less retentive than noncoated media (Osborne, 1973). Attachment of the pump downstream from the chamber prevents it from contributing to the memory. m e downstream position is also desirable because the possibilities of

46

1

5. MEASUREMENT TECHNIQUES

condensing moisture in the chamber and contaminating the chamber with oil or carbon are reduced.] At very high relative humidities (>80 percent), adsorption of water on insulator surfaces, irrespective of the type of material, may lead to electrical instability in the electrometer. Heating of the assembly reduces the problem. Changes in the ambient temperature and relative humidity in the sampled air may result in transient sub-picoampere currents from the ionization chamber. The source is often difficult to determine (see, for example, Woods, 1973);the currents may be generated by chamber capacitance changes or by the insulators. The former might be reduced by electrostatically screening the collecting electrode from the high voltage electrode (Yarom et al.,19721,' but this reduces ion collection efficiencies and may not be mechanically practical. 5.3.2.4 Reduction of Response to Gamma-RayBackground. A tritium concentration of approximately 10 pCi/cm3 produces ions at the rate equivalent to 1 mR/h. Natural gamma-radiation backgrounds are, therefore, of consequence only when measurement of tritium concentrations below 1pCi/cm3 is desired. More intense gamma fields may be experienced in some locations, near reactors or accelerators, for example, and reduction in response to gamma rays may then be required. If the weight of the tritium monitor is unimportant (portability, for example, is not required), a graded lead shield can be used, as with laboratory low-level proportional counters (see Section 5.2.2.2). Where this solution is not acceptable, a widely used alternative is to expose a second ionization chamber to a related (ideally the same) gamma-ray field but not to the tritiated sample. The responses of the chambers to any gamma-ray field should be equal so that the difference in the ionization currents is then equal to that from the tritium alone. If the direction and spectrum of the gamma-ray background are fixed relative to the tritium monitor, then two separate chambers can be satisfadory (Cowper and Simpson, 1960). Equality of response to gamma radiation can be attained by adjusting the effective volume of one of the chambers, either by physically altering a dimension or by manipulation of the electric field with a separate electrode. Polarizing the chambers oppositely allows the ionization currents to be combined a t the input to a single electrometer- The electrical noise will be increased by the additional input capacity and, even with accurate cancellation of the response to a gamma field, the baseline noise will be increased by statistical fluctuations in the responses of the two chambers (Osborne and Coveart, 1973; Jalbert and Hiebert, 1971).

5.3 REALTIME MEASUREMENT

1

47

When the intensity or quality of the gamma radiation is not uniform over the volume in which the ionization chambers are positioned -the usual case with portable instruments -the spatial separation of the chambers leads to inaccurate cancellation. In some instances this could be particularly undesirable since overcancellation of a gamma background could mask the presence of tritium in the sampled gas. Positioning of one chamber inside the other improves the accuracy of the cancellation for spatially varying gamma fields, better than 95 percent cancellation can be attained in this way for a point source in any direction and as close as 0.5 m (Osborne and Coveart, 1973). With the sealed chamber inside the sampling chamber, inaccurate cancellation is more likely to result in readings additive to those from tritium. The two chambers can be formed conveniently by three coaxial hemispherical or domed cylindrical electrodes, opposite voltages being applied to the innermost and outermost electrodes, the middle one being the collector. In many instances the rate of change of gamma-ray background is small compared to the fastest rate a t which changes in tritium concentration are to be measured. If the cancelling chamber and sampling chamber are physically interchanged cyclically with a period less than the time constant of the electrometer, the gamma field will not change appreciably during a complete cycle and the ideal situation of having the cancelling chamber coincident in space and time with the sampling chamber is approached. Cancellation to better than 99.5 percent has been attainable in this way (Jalbert and Hiebert, 1971). These authors report that they can detect 25 pCi/cms in 50 s in a field of 50 mRh. All methods that use a second sealed chamber do not maintain a m t e cancellation if the pressure or temperature of the sampled gas (and hence mass)i n the tritium chamber changes. A variation by as much as 10 percent in the mass of gas in the chamber is quite likely when sampling air under normal ambient conditions. Some users have vented the cancelling chamber to allow the mass of air in it to change. Care must then be exercised to avoid contaminating it with tritium. 5.3.2.5 Measurements When Radioactive Noble Gases are Present. The radioactive noble gas nuclides most likely to be encountered if air is being sampled are "Ar (if the air has been irradiated with neutrons) and naturally occurring =Rn. Radiokrypton and radioxenon may be present if the sampled gas is contaminated with fission products. Because these radionuclides have a larger energy per disintegration, the ionization chambers are more sensitive to them per unit concentration than to tritium. The actual factor depends on the

48

1

5.

MEASUREMENT TECHNIQUES

chamber geometry and size but, for units in the range 1-40 1, factors from 2 . 2 ~to lox .have been reported (Osborne and Coveart, 1973; Osborne, 1971; Bestetti et al., 1969;Jones and Douglas, 1967; Soudain, 1965). If only one nuclide is likely to be present, a detector sensitive to that contaminant, but not to tritium, can be used to provide a cancelling signal. Soudain (1965), for example, mounted a n endwindow Geiger tube in the wall of a n ionization chamber to provide a signal proportional to the concentration of 2*Rn in sampled air. Similarly, Osborne (1971) obtained a cancelling signal for 41Arcontamination from a plastic scintillator detector. When more than one contaminant is present, the detector providing the cancelling signal has to respond to each nuclide to the same extent as does the tritium detector. This is not usually the case with dissimilar detectors (as with similar detectors i n an inhomogenous gamma field, Section 5.3.2.4). In principle, the responses of more than two detectors could be linearly combined to give an output proportional to the tritium concentration, but practical devices of this kind have not been described. When the tritium is in the form of tritiated water vapor, an. alternative method of cancellation is possible and practicable (0sborne, 1971; Jones and Douglas, 1967). The vapor may be nearly completely removed from the sampled gas stream with a desiccant, for example, silica gel or molecular sieve. The difference in ionization current from the original sample and the dried sample approximates that from the tritium. Conditions to be satisfied are that the desiccant be chosen so that the residual humidity is low compared to humidity of the sampled air and the contaminating gas is not retained on the desiccant. Such retention might occur if a cooled molecular sieve were used. The two measurements may be made sequentially with one ionization chamber or concurrently with two identical chambers. The advantage of the former is that no sample splitting is required; the disadvantage is that the time between the sequential measurements limits the rate of change of contaminant concentration for ,which the cancellation can be maintained. Since desiccants have a limited capacity, periodic renewal is necessary, either by replacement or reactivation by heat or with dry gas. The use of two containers of desiccant which are used and purged alternately provides continuous drying. Such a system, employing a commercially available '%eatless dryer", that allowed cancellation of 99 percent of the response to noble gas contamination has been described (Osborne, 1971). A disadvantage of this cancellation method, pointed out by Jones

5.3 REALTIME

MEASUREMENT

1

49

and Douglas (1967), is that it is not "fail safe". Loss of drying capability results in cancellation of a signal from tritium. 5.3.3

Gas Flow Proportional Counters for Tritium Monitoring

5.3.3.1 Geneml Design. As noted in Section 5.2.2, internal gas proportional counters are used for measuring low levels of tritium because interfering radiations may be more effectively discriminated against than with ionization chambers. If the trit.ium to be monitored is in the form of tritium gas, tritiated hydrogen, or a tritiated hydrocarbon with only a low percentage of oxygen or water vapor present, then a counter can be used for monitoring directly. The gas sample is either used as the counting gas itself or is mixed with a counting gas prior to entering the counter. Examples of this application include the measurement of the effluents from gas chromatographs (Feinendegen. 1967) and the monitoring of natural gas for tritium (Bowman et al., 1973). The most recent of these instruments has the usual form of a central active volume (670 cm3) surrounded by a n annular anticoincidence counter. The counter operates at 0.29 MPa and has a minimum detedable activity of 7 K!i/cms a t STP. Flow rates up to 0.1 m3/min may be used. 5.3.3.2 Monitoring Tritium in Air With Proportional Counters. Proportional counters may be used for direct measurement of tritium in air. The advantage of this approach over ionization chambers, namely easier reduction in response to gamma background, is gained only a t the expense of considerable complication. Driver (1956) and, more recently, Balonov et al. (1973), Waters (1972). and Ehret (1967), described monitors of this kind. The sampled counter is usually surrounded by a n anticoincidence counter (following Houtermans and Oeschger, 1958), electrically isolated from the former by a wire d but ventilated with the same gas. Because some beta particles ass through the grid, some of the pulses in the counter that are caused by tritium are coincident with pulses in the outer counter and are not registered as tritium counts. A double grid between measuring and anticoincidence counting, creating a field-free volume thick enough to absorb the tritium betas, will prevent coincident tritium counts (Ehret, 1967). Methane is usually used to dilute the air so that the counter will operate in the proportional mode. M e m t t (1958) demonstrated that the practical upper limit to the amount of water vapor in the methane counting gas was 20 jtg/cm3. Ehret (1967)found that up to 2030 percent by volume of air a t 14°C dew point can be added to the methane without the counting plateau becoming unac-

=!

50

I

5. MEASUREMENT TECHNIQUES

ceptably short or steep. For locations where the use of methane is prohibited for safety reasons, the same author suggests that a 90110 mixture of argonlmethane can be satisfactory although the counting plateaus obtained with this mixture are shorter than those with methane. Other less expensive alternatives to methane are liquid petroleum gas and commercial natural gas. However, care should be taken to ensure that the natural gas is free from tritium, krypton-85 or radon-222. The minimum detectable concentration in the largest counter described, 1000 cm3, was 50 fCi/cm3 (Ehret, 1967). 5.3.3.3 Reduction of Background and Response to Other Airborne Activity. Shielding and counting in anticoincidence with peripheral Geiger counters may both be used for background reduction (Section 5.2.2.2). Ehret (1967) has, in addition, estimated the gamma induced anticoincident counting rate from the coincident counting rate. The dependence of the accuracy of this method upon the quality of the radiation was not noted by the author. The variation should be established before applying the subtraction method so that overcompensation, and possible masking of tritium, can be avoided. Similarly, when other beta emitters accompany the tritium, coincident counts will be obtained from both sample and peripheral counkrs, and, therefore, will not be included yith the antiooincident tritium count. In contrast with the counts from tritium, the requirement here is for as few anticoincident pulses as possible. The proportion of these pulses to the total pulses in the counter from a beta emitter depends upon the counter geometry and the beta energy; the lower the energy, the higher the proportion of anticoincident pulses. and =Kr, the proportion was about 10 percent in the counters For 41Ar described by Ehret (1967). When only one contaminating nuclide is involved, its concentration can be estimated from the coincident counting rate. With more than one nuclide, this method is inaccurate. Removal of the tritiated water from a duplicate concurrent sample to enable the background to be determined separately is then required. Details of this approach are described in Section 5.3.2.5. 5.3.3.4 Stabilization. For monitoring applications where unattended performance is desired, instability is a problem, particularly with airlmethane mixtures in which the counting plateaus are shorter and steeper than with more usual laboratory mixtures. Stabilization by feedback derived from counting a known source is a and the method can be method widely used in nuclear sp~~troscopy applied here. I3owman et al. (1973), for example, used a second counter, ventilated with the sampled gas, in which is mounted a QNi

5.3 REAL-TIME MEASUREMENT

I

51

source. The counting rate due to this source, higher than any from the sample, is used to control the common high voltage. 5.3.3.5 Contamination. As with ionization chambers, construction materials should have low a n i t y for water. Merritt (1958) found that operating a counter at 90°C (by circulation of the counting gas through a heated sidearm) successfully reduced memory effects. If, however, the measuring volume is largely defined by a grid, tritium on the counter walls is less important. By arranging for the counting gas mixture to flow axially or from measuring volume to anticoincidence volume, tritium desorbed from the counter walls can be prevented from entering the measuring volume. 5.3.3.6 Windowed Gas Proportional Counters. Since the mean range of the tritium beta radiation is only 56 pg/cm2,it is difficult to make windows that are thin enough to allow some energy deposition within the counter and yet self-supporting and able to withstand small pressure differentials and the general wear and tear of use. Practical problems that might be expected with this kind of device, in addition to window fragility, are the ease of contamination and its slow response. Eficiency may be difficult to establish since tritium will be measured both through the window and, because of the poorly defined window porosity or permeability, within the counter. Recently Block et al. (1971) developed a flat assembly of four proportional counters, one side of which was covered by a "Formvai' window, thin enough (-50 pg/crn') to allow penetration by the tritium beta particles. The effective window area was 105 cm'. By maintaining the counters at slight positive pressure, the argon1 methane counting gas was forced through pores in the window, thereby preventing air from entering the counter. Tritiated water vapor did, however, diffuse through the window into the counter so that counts were from beta particles originating from within and without the counter. The minimum detectable activity in air was 0.5 pCilcm:'. A compensating signal for the response to both gamma radiation and to other radioactive noble gases could be obtained from a second counter identical to the tritium counter except that the window was aluminized "Mylarn and, therefore, excluded tritiated water vapor and tritium beta particles from the counter. Because of the separation of the counters, the gamma compensation is poorer than can be attained with concentric ionization chambers (see Section 5.3.2.4).

5.3.4 Scintillation Methods for Tritium in Gaseous Streams 5.3.4.1 Direct Measurement of Activity in Gaseous Streams. The solid scintillator methods described in Section 5.3.1.2 have also been

52

1

5. MEASUREMENT TECHNIQUES

applied to measurement of volatile tritium compounds, mainly from gas chromatography columns (Feinendegen, 1967). Anthracene and plastic scintillator have also been used with limited success for air monitoring (Osborne, 1970a; Moghissi et al., 1969a; Sannes and Banville, 1965). The minimum detectable activities for these detectors were in the range 0.1-1 pCi/cm3. . When used for monitoring tritiated water vapor, adsorption of water on the scintillator surface increases the counting rate above that from tritium gas a t the same air concentration. The count* rate from a given concentration of tritium in air will also vary with the relative humidity of the air, being highest a t very low humidities. The unit described by Sannes and Banville (1965) shows an increase of 25 percent between 60 percent and 15 percent relative humidity. This property limits its usefulness for ambient air monitoring. Yellowing of anthracene with time, resulting in loss of sensitivity, is an additional limitation. When weight presents no problem, the detectors may be shielded with lead to reduce the influence of external gamma radiation. For example, Osborne (1970a) used 10 cm of lead to reduce the counting rate caused by 1mR/h to a value equal to that from tritium in air a t 0.5 pCi/cm3. Moghissi et al. (1969a) spread anthracene powder in a thin layer reducing the response of the device to gamma radiation so that no lead shielding was. needed to detect 1 pCi/cm3 in normal gamma backgrounds. Inorganic scintillators are generally too hygroscopic for use without windows. However, C&(Eu) has been used successfully as a monitor for tritium in natural gas (Prevo and Santomassimo, 1970) and appears to offer the advantage of simplicity -only a single photomultiplier was used with an amplifier and single channel analyzerbut the sensitivity is poorer than with the other scintillation systems. The minimum detectable activity was 160 pCi/cm3. Methods involving continuous collection of volatile tritium compounds in a stream of liquid scintillator which subsequently flows through a counting cell, have also been described for chromatographic applications. For tritiated water vapor, dioxane based liquid scintillator is a very efficient collector (Cowper and Osborne, 1968). Such methods could also be applied to other monitoring requirements. This approach, however, does not appear to have been pursued for air monitoring purposes. 5.3.4.2 Method. Znuolving Conversion to the Liquid Phase. These methods are most oomrnonly applied to monitoring tritiated water vapor in air. Interference from gamma background and, more impor-

5.3 MEASUBEMENT OF SURFACE ACTIVITY

1

53

tantly, from radioactive noble gases, is reduced since a small scintillation detector is easier to shield than a large ionization chamber and the solubility of noble gases in water is low. Gibson and Burt (1966) condensed the water vapor and measured the activity with a flow cell containing anthracene. Concentrations of tritium in air down to 0.1 pCi/cm3could be measured. With 7.5 cm of lead shielding around the detector, an exposure of 1 mWh gave an increase in counting rate equal to that from a concentration of 0.15 pCilcm3. Additional correction for gamma-ray background was obtained by measuring the counting rate from three Geiger counters placed within the lead shielding. An inherent disadvantage of the condensertflow cell method is that the sensitivity in terms of concentration in air is dependent upon the humidity of the air since the flow cell measures concentration in water. This is of no consequence if the air is maintained at a fixed humidity, but for monitoring in an uncontrolled environment, either the humidity must be measured or the dew point of the sampled air increased to a predetermined (high) value before condensation. The latter technique is preferable since a d36cult measurement is avoided and consistent performance by condensers over a wide humidity range is diffcult to attain; however, i t adds considerable complication to the method. Conversion to the liquid phase is most easily arranged by passing air through a water-filled gas-washing bottle tor bubbler) as described in Section 3.2.1.1. Osborne (1971,1972)has described methods in which tritiated water vapor is collected continuously in a water stream which is subsequently counted with a plastic or liquid scintillator. The concentration of tritium in the water is proportional to that in the sampled air. A high value for the ratio of air flow rate to water flow rate is desirable for a high sensitivity. The upper limit to the ratio is determined by the requirement to be insensitive to variations in the air humidity and by mechanical considerations. With an air/ water flow ratio of 1000, the maximum effect from humidity variations is typically 2 percent, and tritium at 1 pCilcm3 in the air is collected in the water at 1 nCilcm3. The response to radioactive noble gases in the sampled air was less than 0.2 percent of that to tritiated water vapor when the liquid scintillator flow method (see Section 5.3.1.6) was used. 5.4 Measurement of Surface Activity

Introduction Four kinds of techniques for measurement of surface activity may

5.4.1

54

1

5. MEASUREMENT TECHNIQUES

be distinguished. One of the most widely used is that for measuring tritium distributions in histological sections. The art and science of autoradiography that encompasses this type of measurement has been the subject of many reviews and will not be considered in detail here. A second type of measurement is that of the tritium activity of labeled compounds on paper chromatograms, and this has prompted most instrument designs (see Prydz, 1971 and Feinendegen, 1967 for reviews). The other types of measurements are more associated with health physics; namely the measurement of tritium at the surface of a tritiated solid, as in the case of a tritiated accelerator target or a tritiated paint, and the measurement of tritium (often as tritiated water) contamination of surfaces. Instrumental methods have been developed but none has been widely accepted. Interpretation of the results from this kind of measurement is difficult since the activity on a surface a t any time will depend strongly upon the history of exposure, the nature of the surface and its treatment (Balkwell and Kubose, 1965; Hutchinson and Eakins, 1968). Normally, measurement of the variation of the surface activity with time is a minimum requirement. 5.4.2

'Smear" Sampling

A technique frequently used to estimate the activity on a surface is to wipe the surface with a filter paper of some kind (variously called "wipe", "swipe", or "smear" sampling) and assay the activity thereby removed from the surface. Liquid scintillation counting is the most convenient assay method. Precision is usually not required in this type of measurement and accurate measurement of the variation in counting efficiency and activity release rate for various filters not necessary. Glycerol and ethylene glycol can be added to filters to increase the fraction of surface activity removed and, in the case of tritiated water vapor, to reduce subsequent loss of activity from the paper (Symonds, 1959; Balkwell and Kubose, 1965). Smears taken with glycerol-loaded papers lose tritium less rapidly than those with ethylene glycol and also attain a steady counting rate in liquid scintillator more quickly (Eakins and Hutchinson, 1969). The variation in counting rate is, however, only a few percent and, in practice, probably negligible. 5.4.3

Gas Counting Methods for Surface Measurements

Windowless gas-flow proportional counters have been used in health physics for measurement of surface contamination. In some

5.6 COMPARISON OF MEASUREMENT METHODS

I

55

instruments, a solid sample can be introduced into the counter. For field monitoring, proportional counters have been made with a longitudinal slot in the cathode serving as an inlet for the tritium beta particles and an outlet for the methane counting gas which flows continuously through the counter. With a slot 4 rnrn x 36 mm, one device could detect 30 pCi/cmz on a surface. Gas-tight sealing to the monitored surface is not needed. For greater sensitivity, several counters can be mounted parallel to one another with one side screened only by a metallic mesh. Balonov et al. (1973) have recently described such a device; with a 40-cmZscreen, 2 mm from a contaminated surface, the minimum detectable surface activity was 2 pCil cm2. Windowed counters have also been used for surface monitoring but, as noted in Section 5.3.3.6, the practical problem of window fragility limits the device to situations where careful use can be assured. Seimiya et al. (1967), for example, have operated a counter in the Geiger region with a 17.5-pglcm2polycarbonate window. Also, Block et al. (1971) adapted an air monitor, a flat assembly of four proportional counters with a 100-cm2, 50-pg/cm2 "Formvar" window, for surface monitoring. From the background counting rates of 100 min-' and an estimated efficiency of 10 percent for tritium detection, the minimum detectable surface activity was 2 pCi/m2. 5.4.4

Scintillation Methods for Surface Measurements

Liquid scintillation counting may be used for measuring tritium where the substrate can be cut to a size small enough to fit into a counting vial and where the counting efficiency remains sufficiently high. The assay of paper chromatograms is an example (Feinendegen, 1967). An alternative approach for chromatograms is to soak the paper in liquid scintillator and dry it before loading with tritium. It may then be counted directly. Coating of the radioactive d a c e with plastic scintillator dissolved in toluene is yet another alternative that has been used with chromatograms and could be applied to "swipe" samples. Surface monitoring with solid scintillators is possible. The two major problems are mechanical-namely, that of implementing a coincidence counting arrangement and of ensuring light-tightness over irregular surfaces. Muramatsu et al. (1967) optically coupled a pair of photomultipliers to a 60-cm2sheet of plastic scintillator by way of a single prism so that both photomultipliers were on the same side of the scintillator. In this way, coincidence counting techniques could

56

1

5. MEASUREMENT TECHNIQUES

be used despite the problem of being able to view only one side of the detector. When spaced a few millimeters from a water surface, the counting rate was about 1 min-' for 1 nCi/cm3. 5.4.5

Photographic Methods for Surface Measurements

Photographic emulsions, widely used for detecting and locating tritium activity in tissue slices, are not easily used for quantitative measurements. The efficiency of detection is affected by many variables - humidity, temperature, chemical nature of the surface, fading of the latent image before development as well as the distribution of activity in the top few micrometers of the surface layer. Feinendegen (1967) suggests that a detection efficiency of 5 percent is typical for a labeled 3-pm tissue section. Wheeler and Shaw (1971) suggest 2 to 4 percent for a 5 - ~ msection of tissue equivalent plastic. Using the value for the flux given in Section 5.3.1, the detection efficiency for beta particles nominally entering the emulsion would be 40-70 percent. The background grain count is, similarly, very dependent upon the conditions of emulsion development and storage. As a result, a minimum detectable activity is difficult to specify. For paper or thinlayer chromatograms, 10 nCi over an area of up to 1 cm'L appears detectable in a week's exposure with dry emulsion plates. Coating the surface with wet emulsions provides better contact and, hence, higher detection efficiencies. Treatment of the surface with a scintillator (e.g., anthracene in benzene) very effectively increases the number of grains sensitized per unit time for a given surface activity. 5.5 Miscellaneous Methods

Various other methods of measuring tritium have been suggested. Although these methods have not found wide use, they have been applied to particular problems or they appear to have development potential. For these reasons, they are included here. 5.5.1

Mass Spectrometry

Direct mass spectrometry of a sample is a powerful method for hydrogen isotope analysis. However, Genty et al. (19731, recently reviewing the application of this method in analytical chemistry, concluded that gas chromatography (see Section 5.5.2) is generally simpler, more dependable and less expensive for determination of hydrogen isotope concentrations.

5.5

MISCELLANEOUS METHODS

I

57

By contrast, an indirect method, the measurement of the tritium daughter W e in a mass spectrometer, is potentially the most sensitive technique available for determining tritium. In the method described by Clarke and Kugler (1973), a water sample is degassed (to remove "He) and then stored. The 3He produced as the tritium decays distributes itself mainly in the vapor phase. The 3He and water vapor are then drawn off, dried, and the 3He is analyzed with a mass spectrometer. The spectrometer used could resolve 3He from HD, H2 and 4He and could detect an accumulation of approximately 3000 atoms of :'He apparently corresponding to 3.2 fCi in a liter water sample stored for 10 months, several orders of magnitude better than obtainable with enrichment plus proportional counting. The method has been used routinely for samples a t the 100-fCilcm3level. 5.5.2

Gar Chromatogmphy

Genty et al. (1973) suggest that chromatography is the most suitable procedure for hydrogen isotope separation, distinguishing, for example, between HT and DT. Quantitative analysis is practicable with an accuracy of 2-3 percent, the minimum detectable activity being determined by the detector used with the chromatograph. An ionization chamber was used by these authors. 5.5.3

Avalanche Semiconductor Radiation Detectors

Avalanche diodes, which are solid state analogues of the gas proportional counter, have been suggested as detectors for tritium (Johnston et al., 1970).The current state of the art can produce devices with entrance dead layers as thin as 1 p m of silicon, approximately the depth of penetration of a 10-keV beta particle. The efficiency for tritium detection, therefore, will be low, but these authors speculate that it may be as high as 1 percent. Background counting rates are typically 1 min-' so that with sensitive areas of 1-10 mrn2, such impinging on counters would detect 10-lOO beta particles min-' the detector. This corresponds approximately to a concentration in air on the order of 100 pCilcm3or the surface emission from water on the order of 1 pCilcm3. 5.5.4

Measurement of Bremsstrahlung

Detection of bremsstrahlung from tritium beta particle absorption has been suggested as a practical way of measuring tritium in sam-

58

1

5. MEASUREMENT TECHNIQUES

ples not suitable for measurement by liquidrscintillationcounting, for example, tritiated charcoal, vacuum pump oil, metals, colored aqueous wastes or biological materials (Curtis, 1972; Rosen et al., 1967; Westermark et a1 ., 1960). The most successful method described used 7.5-cm diameter, 2.0-mm thick sodium iodide crystals with 25pm aluminum windows or 1 2 - ~ r nberyllium windows (Curtis, 1972). Windows have to be thin because of the low energy of the photons and, hence, the scintillator crystals need only be about 2 mrn thick to absorb most of the tritium bremsstrahlung photons (the half value layer is 0.7 mrn water) while inefficiently absorbing higher energy photons from other sources. The minimum detectable concentration depends upon the sample size and material. For a few grams of water, it was in the range 1-10 ~ C i l g .

5.5.5 Electron Multipliers These devices have been investigated by Pry& et al. (1971) for scanning surface activity and were reported as promising for further development. A few nCi could be "easily detected in spots the order of 1 cm2. The detector and surface have to be a t low pressure (C50 mPa1 which limits the possible application. 5.5-6 Film Dosimetry

Photographic films have been suggested as being useful for personnel dosimetry (Geiger, 1968; Gibson, 1961). As with film emulsion methods applied to surface measurement (see Section 5.4.51, many variables affect the relation between film blackening and the concentration of tritiated water vapor in air to which a film might be exposed. Quantitative determinations are not very practicable. Geiger (1968) suggests a detection limit of 5 pCi/cm3 averaged over a one-month exposure. 5.5.7 Calorimetry

Calorimetry allows very accurate measurement of tritium, and was the original method by which calibration standards were produced. The technique is described in Section 6. 5.5.8 Thermally Stimulated Exoelectron Emission

Heating of ceramic beryllium oxide discs after exposure to ionizing radiation results in emission of electrons from the surface, the num-

60 1

5. MEASUREMENT TECHNIQUES

5.6 COMPARISON OF MEASUREMENT METHODS

1

61

ber of which, under some conditions, is proportional to the absorbed dose. Since this is a surface phenomenon, the effect is applicable to tritium measurement, a t least in principle. Becker (1973) has described attempts to measure tritium in air and on surfaces with a device of this nature. The electron emission from a disc exposed to a fixed concentration of elemental tritiated hydrogen was shown to be proportional to the exposure time. The minimum Tritium detectable exposure was the order of 10 rnin pCi activity on metal surfaces was reported as being detectable but the limiting activity was not estimated.

5.6 Comparison of Measurement Methods Tables 5 and 6 list most of the methods available for measuring tritium in liquid and in gaseous materials, respectively. Methods are ranked in order of the lowest minimum detectable concentration quoted for each method. Countmg times have been taken as one minute. Clearly, longer times are used in practice with some methods when sensitivity is of prime concern, particularly with discrete sample methods, namely, proportional counting and liquid scintillation counting. Typically, counting times might be 400 min in these cases, permitting a factor-of-20 reduction in detectable concentration. Some of the less sensitive methods described in Section 5.5 are not included. Particular sample preparations are noted where relevant. In practice, poorer sensitivity might be expected for most methods. Qualitative features of the methods are tabulated for intercomparison. The "times to attain the noted sensitivity" are given in minutes, hours, etc., "a few" of each being understood. Quantitative information (or its source) can be found in sections in this chapter as noted in the tables. Upper limits are not given here. In general, the counting methods can measure up to 106-10' times the minimum detectable activities listed in the tables.

6. Standardization and Calibration 6.1

Introduction

Three methods have been used for the calibration of standard samples of 3H,namely, microcalorimetry (Mann and Seliger, 1958; Gross et a l . , 1957; Hawkings and Memtt, 19541, internal gas proportional counting (Mannand Spernol, 1964; Mann et al., 1964; Spernol and Denecke, 19641, and liquid scintillation counting (Garfinkel et al., 1965). The first two methods are direct, in the sense that the disintegration rate of a solution is determined without comparison with a sample whose disintegration rate is presumed known. The latter method is comparative and requires the use of a standard which has previously been calibrated by one of the other two methods. This standard is thus termed a relative standard and the general procedure for obtaining it is also used for generating internal standards for efficiency determination of liquid scintillator sources in specific laboratory situations. Suppliers of tritium standards and descriptions of the standards are given in Table 7.

6.2 6.2.1

Calibration

Triticrted Water Standurd

The National Bureau of Standards (NBS) tritiated water standard was originally calibrated by microcalorimetric measurements in 1954. It was subsequently measured in 1961 by means of internal gas proportional counting. The value obtained by gas counting was 1.96 percent lower than that obtained by microcalorimetry. The difference was probably due to the uncertainty in the mean energy of the tritium beta ray. In 1964, the NBS standard was measured by proportional counting at the Euratom Gee1 Laboratory with agreement between the two laboratories to about 0.25 percent. NBS certificates,

TABLE7-Suppliers Producer

Czechoslovak Atomic Energy Commission Slezska 9. Prague 2 Czechoslovakia

of tritium standardsm. I Chem~adForm

AaiviLy

(a) toluene -H3 (b) tritiated water

Isocomrnen GmbH 1 tritiakd water Binnen- und Aussenhandel sunter. nehmen fur radioactive and sta. bile isotope DDR-1115 Berlin-Buch Lindenberger Weg 70 Deutsehe Dernokratische Republik Institute of Isotopes of the Hungar- (a) benzoic acid (b) toluene ian Academy of Sciences (c) tritiated water Hungary, Budapest 114. (d) 2,5 diphenyloxyazole P.O. Box 77

(PW) (a) tritiated water Institute of Nudear Rasearch Radioisotopes and Distribution Cen- (b) n-he~adacanal,%~H ter

Swierk, Wamaw, Poland Japan Radioisotope Association

tritiated water

28-45 Hon-Komagome 2-Chome

Bunkyo-Ku, Tokyo. Japan Laboratoire de MOtrologie des Ray- (a) tritiated water onnements Ionisants (b) T gas BP No. 2-91 190 Gif-sur-Yvette (c) tritiated toluene France

10-=-200 pCi/g 0.2 and 500 pCi/ml 1 pCi/g

National Bureau of Standards Room C114, Bldg. 245 Washington. D.C. 20234, U.S.A.

9 x l o a s-lg-' 9 x 105 s-'g-' 3 x 1P s-'g-I

(a) tritiated water (b) toluene (c) n-Hexadecane

Physicalishe-Technischa Bundeaan- tritiated water stalt 33 Braunschweig. Bundesallee 100 West Germany

3.7 x 10.' and 3.7 x 106 s-lg-'

Radiochemical Centre Arnersham, Buckinghamshire England

(a) n-hexadecane-1.2-'H (b)t o l ~ e n e 4 ~ H (c) tritiated water

4 x 106 min-'g-'

Soviet Union [Contact] L. G6rski IAEA Kirtner Ring 11 P.O. Box 590, A-1011 Vienna, Austtia

tritiated water in liquid scintillator

4 x 1V min-lg-l

10 pCilg and 1 pCilg

" Dr. G6rski, International Atomic Energy Agency, Vienna, assisted in preparing this compilation. In addition to the non-commercial agencies listed, there are commercial organizations that supply tritium standards.

64

1

6. STANDARDIZATION AND CALIBRATION

since 1964, have given both the microcalorimetric and the more recent gas counting results; the use of the gas counting results is to be preferred (Mann, 1973). The quoted overall accuracy, defined as one standard error plus the linear sum of systematic errors, is 0.3 percent for the gas counting measurements. 6.2.2

Microcalorimetry

The NBS calorimeter is in the form of a radiobalance developed by Callendar (1911)and is shown in Figure 9. The source can be placed in either of the twin gold cups, A and B. A copper-constantin Peltier junction is in close thermal contact with each of the cups. The two junctions are connected and mounted so as to either heat A and COO] B or heat B and cool A, depending on the direction of the current, I. The upper ends of series-connected thermopiles, TAand TB,surround and are in close thermal contact with the respective gold cups. The lower ends of TA and TBare placed below the cups in cavities KA and K,, which serve as constant temperature baths. An electrometer, G, determines the current produced by small temperature differences between TA and TB.A source is placed in cup A producing a rate of heat energy emission, W, and a dummy source is placed in cup B. The current, I, is adjusted so as to reduce the net thermopile current to zero.The heating rates are then identical and

W-m+IZRA=m+FRB,

(7)

KA KB Fig. 9. Callendar radiobalance (after Maan. 1954). (See text for explanation and operation.)

6.2 CALIBRATION

1

65

where R A and R B are the resistances of the junctions A and B, respectively, and I2 is the Peltier coefficient. The current is now reversed and the sources interchanged, whence,

m + PR, = w - ITI + PR,.

(8)

From the above equations,

W

= 2m.

(9)

The coeff~cient,n, is determined by means of accurately known, but different valued, resistances connected in series to a source of current, and placed in the gold cups. The disintegration rate is then determined from W and the known average beta-ray energy. 6.2.3

Gas PropoTtional Counting

This standardization procedure consists of reducing a tritiated water standard in a heated zinc bed and then determining the tritium evolved in the gaseous hydrogen by proportional counting using methane as the counting gas. The apparatus for preparing the hydrogen gas samples for counting is shown in Figure 10. The insert shows a sample tube used for

ATTACH TO X)INT A

m o w 0 GLASS CLOSED-END MANOMETER

I I

OPEN-END YANOYETER

I

9: Gas-COLLECTING SECTION

I

W P L E T18E

I

1 GAS-GENEAATlNG SECTION

Fig. 10. NBS apparatus for preparing tritium gas samples (after Mann et al., 1964).

66

1

6.

STANDARDIZATION AND CALIBRATION

weighing samples of tritiated water. The tube is attached to the system at joint A, which is then opened allowing the sample to be distilled through the zinc bed into the second cold trap. Warming the sample allows the water vapor to pass over the heated zinc bed. The generated gas is transferred with the Toepler pump. Gas collecting bulbs, with accurately known volumes, are then transferred and attached to the gas counting system for mixing with methane and counting. In the NBS compensated gas proportional system, three counters of different lengths are used. The counting rate measured in one of the counters and the measured gas volume and pressure are not sufficient to give the required disintegration rate per mole. In the end regions of the counter where the electric fields are distorted, the effective counting volume cannot be easily determined. Also, beta particles may be lost to the walls and hence not counted. Differences in counting rates between the various counters are therefore determined. Because all the counter ends have been carefully machined so that they give the same field distribution, the differencesin effective counting volumes between the chambers are accurately known and, hence, the counting rate per unit volume can be determined. To correct for losses to the walls, the counting rate is measured for several pressures (PI, plotted against 1IP.and extrapolated to 1IP = 0, for which pressure total absorption in the counting volume occurs. 6.2.4

Calibration of the NBS Tritiated Toluene Standard by Relative Counting

The tritiated toluene standard is calibrated by intercomparison with the tritiated water standard using liquid scintillation measurements. A scintillation solution is diluted with ethyl alcohol in the proportion 70:30, and this solution is divided into two parts, A and B. To part A is added the tritiated toluene sample to be calibrated and to part B, the NBS tritiated water standard. The chemical content of A and B are made as identical as possible by appropriate additions of non-tritiated water to A and non-tritiated toluene to B. Details of the dilution procedure, designed to completely prevent losses of activity during dilution, were described by Garfinkel et al. (1965). A significant problem observed in the measurement is the decrease of counting rate with time due either to absorption of oxygen by the source or loss of material from the counting bottles through leaks around the caps. Since evaporation removes activity in the case of tritium, as distinguished from most other radioelements which remain in solution, particular care must be taken to minimize evapora-

6.3 USE

OF STANDARDS

1

67

tive losses. Losses of several percent in a 24-hour period using commercially available bottles and bottle caps have been noted. This loss can be prevented by grinding flat the tops of the glass counting bottles and using Teflon" gaskets, with a thin covering layer of silicone grease on each surface. Significant variations (up to 3.5 percent) in efficiency, caused by inhomogeneities of the counting vials, are overcome by using the same set of bottles to count both A and B. The final p d u r e takes into account the likelihood that the gain of the system will change from one set of measurements to the next.

6.3 Use of Standards 6.3.1

Calibration in Liquid Scintillation Counters With an I n t e m l Standard

Recommendations for producing a number of standards relative to one standard have been given by Moghissi and Carter (1968). Their procedure is similar to that of Garfinkel et al. (1965) described above for the production of the tritiated toluene standards. The method avoids the drawbacks, discussed by several authors (Smithand Reed, 1965; Butler, 1964; Davidson and Feigelson, 19581, inherent in introducing a known amount of standard solution into the sample plus scintillator to be counted without compensating for the change in efficiency caused by the introduction of standard solution. A procedure for the production of relative standards is as follows: (a) Add equal amounts of source material to equal amounts of scintillation cocktail in two identical counting bottles, A and B; (b) Add a known amount of the standard spiking solution to A; (c) Add the same amount of inactive material of the same form as the standard to B; (dl The difference in the counting rates of A and B and the known counting rate of the standard spike gives the counting efficiency for the two mixtures. Using the determined efficiency, the counting rate of the solution in B can be determined. Solution A then constitutes a standard that can be used for the determination of all sources of the type B plus inactive spike.

6.3.2 Calibrcrtion in Liquid Scintillation Counters With an External Standard Tritiated toluene and tritiated water samples may be calibrated

68

1

6. STANDARDIZATION AND CALIBRATION

also by external standardization. This method of efficiency determination (Glass, 1970;Takahashi and Blanchard, 1970;Bell, 1968;de Wachter and Fiers, 1967;Horrocks, 1964) relies on the similarity of the quenching of pulses generated by tritium beta rays and by Compton recoil electrons, generated in a sample by an external highenergy gamma-ray source (' UBa, '37Cs, ZBRa, 226Ra-241Am, or others). Factors such as volume and electron density of sample and wall thickness of counting vial may affect the number of recoil electrons generated and need to be kept constant from sample to sample. The accuracy of the external standardization is better if the channels ratio (ratio of counts in two portions of the energy spectrum) is used to monitor quench correction instead of the counting rate. To obtain the relationship between efficiency and external sm.dard channels ratio, a set of standards containing varying amounts of inactive water, encompassing the range of concentrations that will be encountered, and a tritium standard are counted at the optimum settings for tritium (de Wachter and Fiers, 1967).The external standardization cycle in the counter is executed to yield the external standard channels ratio. A plot of sample efficiency against external standard channels ratio yields the correlation curve. Samples to be calibrated by external standardization must be prepared in a liquid scintillator of the same composition and counted a t the same instrument settings as those used for obtaining the correlation curve.

6.3.3 Calibration of Gas Counters In the United States, gas counters for tritium are usually standardized against a carefully prepared dilution of National Bureau of Standards Sample No. 4926 of tritiated water. The activity of that standard material is low enough so that excessive dilution is unnecessary, but special care must be taken so as not to lose tritium by evaporation when small amounts of water are handled. The diluted standard sample is brought through exactly the same preparation procedure as the unknown samples of water.

6.4 Blanks The determination of the background counting rate in any instrument requires the use of a material which is fiee of tritium. Compressed hydrocarbons, such as methane, propane and ethylene from

6.5 CALIBRATION OF REALTIME INSTRUMENTS

1

69

commercial sources, usually meet this test. Compressed hydrogen, however, contains various amounts of tritium, depending on how and where it was processed. The best tritium-free material available is aquifer water, which by other means (hydrological information, I4C measurements) is known to have a long residence time underground compared to the half life of tritium. Preferably the aquifer should be under artesian pressure so that not even small amounts of recent surface water are present. Low tritium water is, for instance, available in the so-called Floridian aquifer underlying a large part of the Florida peninsula. Another possible source is deep seawater, which has to be collected in an area where it is known that no undercurrent transports other waters to the sampling area. The quality of a blank is tested by having it first undergo enrichment. From the degree of enrichment and the observed counting rate, it is possible to calculate the original tritium concentration. In ultralow-level tritium measurements, double-checks and cross-checks should be made between "zero" water from different sources. Once water has been established as containing tritium below the desired lower level of detection, it should be kept well-sealed from the atmosphere in glass bottles with effective caps, such as "Polyseal". Cork discs under plastic screw caps are not adequate and any pmlonged storage of tritium-free samples in polyethylene containers will cause the samples to become contaminated by environmental tritium in the storage area. Accurate very low-level measurements are almost impossible to achieve in laboratory buildings where tritium in any fonn is handled (including autoradiography). In some buildings, evaporation from luminescence paints is the greatest contributor to indoor HTO levels. Such luminescent materials containing tritium are now used frequently in watches, compasses and some gauges (Kim and Vaughn, 1971).

6.5 Calibration of Real-Time Instruments 6.5.1 Introduction

The calibration of a tritium measuring instrument used in "real time" is usually determined by means of a national laboratory standard. Methods by which this may be accomplished are quite varied,

70

1

6.

STANDARDIZATION AND CALIBRATION

particularly with measurements in air. Also, quick "in field" checks of an instrument's performance can be made by determining, and correcting for, changes in the calibration constant. 6.5.2

Direct Calibration

Calibration of "real-time" instruments measuring tritium contained in liquids is relatively straightforward and can be accomplished by using a solution derived from a stock standard. The calibrated solution must be chemically identical to the liquids being measured, and any prior sample should be completely removed by thorough flushing. Calibration of instruments that measure tritium in gaseous form is somewhat more difficult. Tritiated hydrogen and tritiated methane of known specific activity are both available and may be used where these chemical forms of tritium are appropriate. A small cylinder containing compressed tritiated methane and having provision for releasing a measured mass of gas is available commercially for such calibrations. Calibration of instruments that measure tritiated water vapor in air is not so simple, especially of those instruments that condense or otherwise collect the tritiated water vapor. Tritiated water may be vaporized into an air stream by bubbling the air through the water. If the tritiated water is derived from a tritium standard, then, in principle, the concentration of tritium in the air may be easily calculated. In practice, considerableexperimental effort is required to obtain precise results. An in-line series of bubblers is required to ensure saturation of the air stream. The air must be thoroughly dried before it enters the bubblers to prevent reduction of the specific activity of the water, and must be heated (or cooled) to make its temperature a t the input to the bubbler chain equal to or slightly above that of the water in the last bubbler. Water temperature of the last bubbler should be measured directly because evaporative cooling in the upstream bubblers causes temperature differentials. The air flow rate, which must be measured with due attention to pressure and temperature corrections, must be low enough to ensure that water droplets are not carried out of the bubblers. A trap may be necessary. The isotope effect, noted earlier, will reduce the concentration of tritium in the air stream to other than the value obtained under equilibrium conditions (Osborne, 1973). Temperature and flow rates should remain steady for at least 15 minutes before starting a calibration and throughout the Pd u e . The relative humidity may be reduced by mixing with a me-

6.5

CALIBRATION OF REAGTIME INSTRUMENTS

1

71

tered flow of dry air, and the specific activity of the tritiated vapor in the air adjusted by using appropriately humidified air for dilution. Banville (1965) has described an evaporation system allowing control or measurement of most of these features, but suggests that the range in error of such a method is a 20 percent.

6.5.3 Indirect Calibration Indirect calibration is performed by comparing the response of the instrument to that of another previously calibrated instrument, rather than through the use of a standard. For calibrating air monitors with tritiated water vapor, the indirect method is preferred. An air stream can be humidified with tritiated water in the manner noted in the previous section but without the necessity for the precise measurements. The only requirement is for a steady concentration of HTO in the humidified air. This concentration is then measured by the instrument being calibrakd and also by scintillation counting after collection in water bubblers, as described in Section 3.2.1.1. Banville (1965) has described this method in detail. He suggests that the overall error in the procedure (excluding those in the liquid scintillation counting) may be easily maintained below 5 percent. 6.5.4

Field Calibration

For many instruments, any one of the methods described in Section

6.5.2 may be used away from a calibration laboratory, particularly if only a n approximate check (e-g., to 20 percent) of the calibration is required. For air or gas phase monitors, particularly portable ionization chambers, a simpler method not involving metering of gas flows from ancillary equipment may be desirable. A beta and/or gamma emitting source may be reproducibly positioned outside the ionization chamber to give a convenient instrument reading. Deviation from the designated reading (established a t the time of laboratory calibration) is an indication of malfunction. Baker and Richards (1968) have described a system whereby the ionization chamber is not irradiated dvectly but ions formed by the beta particles from a 63Ni source are swept into the chamber by the air flow. The activity is deposited on the penultimate plate of a multiplate ion-trap. By removing the voltage on the remainder of the trap downstream from the 63Ni,the ions that are normally removed are released into the ionization chamber. Air flow and calibration are therefore checked together.

72

1

6. STANDARDIZATION AND CALIBRATION

Another method uses a shuttered rnNi source positioned in the air inlet to a n ionization chamber (Osborne and Coveart, 1973). The direct irradiation of air in the chamber with no air flow checks the calibration. With air flow, ions are swept into the chamber and the subsequent increased reading verifies that the air flow rate is adequate.

6.6 Propagation of Errors There are two methods for combining the errors associated with the calibration of a measuring system into the statement of errors associated with the subsequent calibration of a sample. Method I proposes that the total quoted error in the standard be treated as a systematic error in the measurement of an unknown. Method II proposes that the random and systematic components of the total error associated with calibration of the standard be treated on an equal basis with other corresponding random and systematic errors. The essential difference between the two methods lies in the fact that statistical, or random, errors are added in quadrature whereas systematic errors are added linearly. The total error, E, is usually written: where R is the random error and Y is the systematic error. For a set of m measurements on a sample, R is calculated as where f -,is the Student "t factor" a t a given level of confidence for m measurements, and S,,,-, is the percent standard error of the value reported. Values oft,,,-, are listed by ICRU (1968)and others. Y is the linear sum of estimated systematic errors multiplied by a factor, usually 1 but sometimes 213. In Method 1, the error in the standard is included in Y and this error is intrinsic in all subsequent measurements on a system calibrated with the standard. Method II introduces the problem of weighting the standard errors in the quadratic sum. One possible method is to use the formula for R (given in percent error)

6.6 PROPAGATION OF ERRORS

1

73

where &-, and &,-,are the percent standard errors of the sample and standard, respectively, which had k and n measured values, respectively. i is the Student t factor for the sample and the standard a t the desired confidence level with f degrees of f d o m , where f is computed from

The value of f i s usually fractional, and the value of i is obtained from the table by interpolation. This expression for computing the effective degrees of freedom assumes that the activity of the standard and sample are approximately the same. The following example illustrates how these formulae should be used. Five sources taken from the same sample are measured with a liquid scintillation counter, giving x = 10.5, 9.6, 8.5, 9.8, and 10.3 nCi/l, respectively. The instrument is standardized by taking one measurement on each of eight separate relative standards and yields y = 50, 53, 51, 54, 52, 53, 52, 54 percent, respectively. In this case k = 5, n = 8, x = 9.7, and = 52.4. Then,

$-, = 100 1-I

'I2 k(k - 1)

= 3.6 percent

and

S.-,

= 100 7-I

\IZ

(Y

- ')'

n ( n - 1)

= 0.95 percent

From Table 2 in ICRU Report 12 (ICRU,1968):

t+, a t the 99 percent confidence level = 4.604 and

t,,-, a t the 99 percent confidence level = 3.499. Random errors associated with measurements on the sample and standard are t-,&-, and t,-, Sn-,, respectively, which are numerically equal to:

&-, &-, = (4.604) (3.6) = 16.6 percent t,-,

Sn-,= (3.499) (0.95) = 3.3 percent,

74

1

6.

STANDARDIZATION AND CALJBRATiON

and

? = 4.6 and i (99 percent confidence level) = 4.26. Method I thus gives for the overall error, E, at the 99 percent confidence level:

E

+ tn-ISn-, + ZY 16.6 + 3.3 + ZY = 20 percent + ZY.

= tk-,Sk-l

=

Method II gives for the overall error, E, at the 99 percent confidence level:

E

=

+

= (4.26) (3.72)

+ ZY + EY

= 16 percent

+ ZY.

Values of the various systematic errors, Y, must be estimated from the previous experience of the investigator.

6.7 Expected Accuracies

.

An intercomparative study (Ostlund et al., 1964)at natural tritium levels by three national laboratories to study dilution and calibration procedures for low-level gas counting established that reduction and measurements of a tritiated water sample can be performed within 2 4 percent. There was evidence of contamination from the atmosphere during the analysis by one of the laboratories, but otherwise all results from the three laboratories agreed well with the known values of the samples, which were dilutions with tritium-free water of standard tritium solutions. A comparison of the tritium measurement techniques of 35 laboratories is described by Florkowski et al. (1970). The study concluded that enrichment and gas counting is most accurate in the concentration range from 0 to 250 pCifl. For higher concentrations, direct gas counting is more accurate because the error in the enrichment p m ess becomes a significant factor. The accuracy of liquid scintillation and gas counting is the same (3.5 percent) for concentrations above 1 nCi/l. A standard deviation of 10 percent for single analyses of water containing 8 5 nCi/l was established by an intercomparative study including six laboratories in the United States (Knowles and Baratta, 1971).

References ALTSHULER, B. AND PASTERNACK, B. (1963). "Statistical measures of the lower limit of detection of a radioactivity counter," Health Physics 9,293. ALVAREZ, L. W. AND CORNOC, R. (1939). "Helium and hydrogen of mass 3," Physical Review [2] 56. 613. ANAND,J. S. AND LAL, D. (1964). "Synthesis of methane h m water for tritium measurement," Nature 201, 775. B A I N B ~ I IA. ~ EE., . SANDOVAL. P. AND Sums, H. E. (1961). "Natural tritium measurements by ethane counting," Science 134, 552. BAKER,R. P. AND RICHAFLDS, R. D. (1968). Reliable and Versutile Instrumentation for Detection of Tritium Hazard, Designed fir Military Application, Report SC-M-68-245 (Sandia Laboratories, Albuquerque, New Mexico). BALKWELL, W. R. AND KUBOSE,D. A. (1965). Determination of the Total Tritium Activity Adsorbed on the S u r f i of Various Metals, Report USNRDGTR-890 (U.S. Naval Radiological Defense Laboratory, San Francisco, California). BALWBD,L. F. AND ELY, R. L.,JR. (1963). "A sensitive tritium monitor," Transactions of American Nuclear Society 6 (2). 400. BALONOV, M. I., D o t c u m v , E. I., KARASIK,A. S. AND PROKOFIEV, 0. N. (1973)."Instruments and methods for day-to-day monitoring of tritium and carbon-14 in production," page 1422 in Proceedings of the Third Intermtional Congress of the International Radiological Protection Association, published as Report CONF-730907.P2 (National Technical Information Service. U.S. Department of Commerce, Springfield, Virginia). BANCROPI,H. (1957). Introduction to Bioshtistics (Hwber-Harper, New York). B. (1965). Calibration of Tritium-in-Air Monitor, Report AECL BANVILLE, 2265 (Atomic Energy of Canada Limited, Chalk River Nuclear Laboratories, Ontario). K.(1973). Annual Progress Report, Health Physics Division, Report BECKER. ORNL.4903 (Oak Ridge National Laboratory, Oak Ridge, Tennessee). F. AND FRIEDMAN, I. (1968). "Isotopic composition of atmospheric BECEMANN, hydrogen," J. Geophys. Res. 73. 1139. BELL,C. G. AND HAYES,F. N. (1958).Liquid Scintillation Counting (Pergamon Press, New York). BELL.T. K. (1968). "Liquid scintillation counting using external standard and channel ratio techniques for quench mrrectons,"Laboratory Practice 17,809. BEST=, M. E., SANCUIST, V. AND TEBRANI, S. (1969). "Calibration of an 75

76

1

REFERENCES

ionization chamber for internal gas counting," Energia Nucleare 16,41. BIRKS,J. B. (1964). The Theory and Practice of Scintillation Counting (Pergamon Press, New York). BIRKS,J. B. (1970)."Physics of the liquid scintillation process," page 3 in The Current Status of Liquid Scintillation Counting, Bransome, E., Ed. @rune and Stratton, New York). BLOCK,S., HODGEKINS, D. AND BARLOW, 0. (1971). Recent Techniques. in Tritium Monitoring by Proportional Counters, Report UCRG51131 (Lawrence Livermore Laboratory, Livennore, California). BOAG,J. W. (1966). "Ionization chambers," page 2 in Radiation Dosimetry, Vol. 11, Attix. F. H. and Roesch, W. C., Eds. (Academic Press, New York). BOGEN,D. C., HENKEL. C. A., WHITE,C. G. C. AND WELFORD, G. A. (1973). "Methods for determination of tritium distribution in environmental and biological samples," J. Radioanal. Chem. 13, 335. BOLZ,R. E. AND TUVE,G. L., Eds. (1970). Handbook of Tables for Applied Engineering Science (Chemical Rubber Co., Cleveland. Ohio). BOWMAN, C. R., CHEW,E. W. AND DOLES,A. E. (1973). "On-line monitor for natural gas from nuclear stimulation," page 548 in Tritium, Moghissi. A. A. and Carter, M. W., Eds. (Messenger Graphics, Phoenix, Arizona and Las Vegas, Nevada). BRANSOME, E.. JR., Ed. (1970). The Current Status of Liquid Scintillation Counting (Grune and Stratton, New York). BROWN, R. M. AND GRUMMITT, W. E. (1956)."The detennination of tritium in natural waters," Can. J. Chem. 34, 220. BUSH,E. T.(1966). "Relative efficiency of a new liquid scintillation fluor, pbis(o-methylstyry1)-benzene," Anal. Chem. 38, 1241. BUTLER,F. E. (1964). "Assessment of tritium in production workers," page 431 in Assessment of Radioactivity in Man, Vol. II (International Atomic Energy Agency, Vienna). CALLENDAR. H. L. (1911). "The radio-balance. A thermoelectric balance for the absolute measurement of radiation with applications to radium and its emanation," Roc. Phys. Soc. (London) 23, 1. CAMERON. J. F. (1967). "Survey of systems for concentration and low background counting of tritium in water," page 543 in Radioactive Dating and Methods of Low-Level Counting (International Atomic Energy Agency. Vienna). CHRISTMAN, D. R., DAY, N. E., HANSELL, P. R. AND ANDERSON, R. C. (1955). "Improvements in isotopic carbon assay and chemical analysis of organic compounds by dry combustion," Anal. Biochem. 27, 1935. CLARKE, W. B. AND KUGLER, G. (1973)."Dissolved helium in groundwater A possible method for uranium and thorium prospecting," Economic Geology 68, 243. COWPER, G. AND SIMPSON,S. D. (1960). A Monitor for Airborne Tritium, Report AECL 1049 (Atomic Energy of Canada Limited, Chalk River, Ontario). COWPER, G. AND OSBORNE, R. V. (1968). "Measurement of tritium in air in

REFERENCES

1

77

the presence of gamma radiation," page 285 in Proceedings, First International Congress of Radiation Protection, Rome, 1966,lst ed., Snyder, W. S., Abee, H. H., Burton, L. K..Maushart, R., Benco, A., Duhamel, F. and Wheately, B. M., Eds. (Pergamon Press. New York). M. W. (1960). Statistical Manual CROW,E. L., DAVIS,F. A. AND MAXFIELD, (Dover Publications, New York). CURRIE, L. A. (1968). "Limits for qualitative detection and quantitative determination," Anal. Chem. 40, 586. CURTIS,M. L. (1972). "Detection and measurement of tritium by bremsstrahlung counting," Int. J. Appl. Radiat. and Isotopes 23, 17. DANNECKER, A. K. AND S P ~ E LK., H. (1967). "Measurement of tritium concentration of air," page 515 in Assessment of Airborne Radioactivity (International Atomic Energy Agency, Vienna). DARUSCHY, P. (1965). "Eine schnell und einfache Methode zum Nachweis von Tritium in biologischem Material," Atompraxis 11. 273. P. (1958). "Practical aspects of internalD A V I D ~ ~J.ND. , AND FEICELSON, sample liquid-scintillation counting," Int. J. Appl. Radiat. and Isotopes 2, 1.

D A V I ~ NJ., D. AND OUVERIO,V. T. (1968). "Tritium and carbon-14 by oxygen flask combustion," page 67 in Advances in Tracer Methodology, Vol. 4 , Rothchild, S., Ed. (Plenum Press. New York). DAVIDSON, J. D., OLIVERIO, V. T. AND PETEXSON, J. I. (1970). "Combustion of samples for liquid scintillation counting," page 222 in The Current Status of Liquid Scintillation Counting, Bransome, E., Jr., Ed. (Grune and Stratton, New York). DE WACHTER, R. AND FIEBS,W. (1967). "External standardization in liquid scintillation counting of homogeneous samples labeled with one, two, or three isotopes," Anal. Biochem. 18, 351. DIXON.W. J. AND MASSEY,F. J. (1969).Introduction to Statistical Analysis (McGraw Hill. New York). DRIVER,G. E. (1956). "Tritium survey instruments," Rev. Scient. Instrum. 27,300. EAKINS,J. D., AND HUTCHINSON, W. P. (1969). The Radiological Hazard from Tritium Sorbed on Metal Surfaces, Report AERE-R-5988 (U.K. Atomic Energy Authority, Harwell, England). J. D. AND ZRINZO,M. (1973). "A reassessment of the water bubbler EAKINS, for differentiating tritiated water vapor and tritium gas." presented a t the Symposium on the Determination of Radwnucleides in Environmental and Biological Materials, London, England, April 2 3 , 1973, AED CONF-1973-085 (Zentralstelle Fitr Atamenergie Dokumentation, Karlsruhe, West Germany). EHRET.R. (1967). "Proportional flow counters for measurement of tritium in air," page 531 in Assessment of A i r b o m Radioactivity (International Atomic Energy Agency, Vienna). FEINENDECEN, L. E. (1967).Tritium-Labeled Molecules in Biology and Medicine (Academic Press,New York).

FLORKOWSKI, T., PAYNE,B. R. AND SAUZAY, G. (1970). "Interlaboratory comparison of analysis of tritium in natural waters," Int. J . Appl. Radiat. and Isotopes 21, 453. FRY,R. M. (1959). The Calibration of Flow loniratwn Chambers for Tritium Monitoring in Air, Report AERE-M-428 (U.K. Atomic Energy Authority, Harwell, England). GARPINKEL, S. B., MANN,W. B.,MEDLOCK, R. W. AND YURA,0 . (1965). "The calibration of the National Bureau of Standards tritiated-toluene standard of radioactivity," Int. J. Appl. Radiat. and Isotopes 16, 27. GEIGER,E. L. (1968). "Tritium film badge," Health Physics 14. 51. GENTY,C., SCHOTT, R., LEFEVRE,H., FROMENT, G. AND SANSON, C. (1973). "Determination of tritium in a n analytical chemistry laboratory," page 102 in Tritium, Moghissi, A. A. and Carter, M. W.,Eds. (Messenger Graphics, Phoenix, Arizona and Las Vegas, Nevada). GIBSON, J. A. B. (1961)."Detection of tritium with a film dosemeter,"Phys. in Med. and Biol. 6, 283. GIBSON,J. A. B. AND BURT,A. K. (1966). "A method for the continuous measurement of tritiated water in air," J. Nucl. Energy, Parts AIB, 20, 185. GLASS,D. S. (1970). "Automatic quench correction by channels ratio for both 14C and tritium, using a 3-channel liquid scintillation counter," Int. J. Appl. Radiat. and Isotopes 21, 531. GROSS,W., WINGATE,C. AND FAILLA,G. (1957). "Average energy loss by sulfur-35 beta rays per ion pair produced in air," Radiat. -Res. 7, 570. GUFTA, G. N. (1973). "New procedures for liquid scintillation counting of =H in the polyester plastic bags," page 186 in Tritium, Moghissi, A. A. and Carter, M. W.. Eds. (Messenger Graphics, Phoenix, Arizona and Las Vegas, Nevada). HAINES,A. AND MUSCRAVE, B. C. (1968). "Tritium content of atmospheric methane and ethane," J. Geophys. Res. 73, 1167. H m , A. (1952). Statistical Theory with Engineering Applications (John Wiley and Sons,New York). HANSEN,D. L. AND BUSH.E. T. (1967). . . "Im~rovedsolubilization Dmcedures for liquid scintillation counting of biological materials," Anal. Biochem. 18, 320. HAWKINGS, R. C. AND ME-, W. F. (1954). Some Prelimi~aryResults on the Absolute Beta Counting of Tritium, Report AECG94 (Atomic Energy of Canada Limited, Chalk River, Ontario). HAYES,D. W. (1972). A Method for Lowering Background in Tritium Gas Counters, Report DP-MS-72-17 (Savannah River Laboratory, Aiken, South Carolina). HAYES,D. W. AND HOY,J. E. (1973). "A chromatographic system for the enrichment and analysis of low level tritium samples," page 127 in Triti u y , Moghissi, A. A. and Carter, M. W., Eds. (Messenger Graphics, Phoenix, Arizona and Las Vegas, Nevada). HAYES,F. N., Om, D. G.AND KEW, V. N. (1956). "Pulse-height comparison of secondary solutes," Nucleonics 14 (11, 42.

REFERENCES

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79

HAYES,F. N., Om, D. G., K E ~V., N. AND F&IGERS, B. S. (1955). "Pulseheight comparison of primary solutes," Nucleonics 13 (121, 38. HIEBERT.R. D. AND WATTS.R. J. (1953). "Fast coincidence circuit for =Hand .14Cmeasurements," Nucleonics 11 (12). 38. HOFLROCICS, D. L. (1964). "Measurement of samples quenching of liquid scintillation solutions," Nature 202, 78. H o s a o c ~ s D. , L. (1970). "The scintillator solute," page 25 in The Current Status ofLjquid Scintillation Counting, Bransom, E., Jr., Ed. (Grune and Stratton, New York). H o m o c ~ s D. , L. AND PENG,C. T. (1971). Organic Scintillators and Liquid Scintillation Counting (Academic Press, New York). HOUTEFLMANS, F. G. AND OESCHGER, H. (1958). "hportionalz&lmhr zur Messung schwacher Aktivitiiten weicher &Strahlung," Helv. Phys. Acta 31, 117. HUNT,J. A. (1968). "Continuous flow monitor system for detection of UV absorbence, "C and aH in effluent of a column chromatogram," Anal. Biochem. 23,289. J. D. (1968). The radiological hunard from H m n m s o N , W. P. ANDEAXXNS, tritium sorbed on metal surfaces, Report AERE-RS80 (U.K. Atomic Energy Authority, Harwell, England). IAEA (1967). International Atomic Energy Agency, Tritium and Other Environmental Isotopes in the Hydrological Cycle, Tech. Rep. Series NO. 73 (International Atomic Energy Agency, Vienna). ICRU (1963). International Commission on Radiation Units and Measurements, Radioactivity, ICRU Report 10c, published as NBS Handbook 86 (U.S. Government Printing Office, Washington). ICRU (1968). International Commission on Radiation Units and Measurements, Certifkation of Standardized Radioactive Sources, ICRU Report 12 (International Commission on Radiation Units and Measurements, Washington). ICRU (1972). International Commission on Radiation Units and Measurements, Measurement of Low-Leuel Radhctivity, ICRU Report 22 (International Commission on Radiation Units and Measurements, Washington). International Union of Pure and Applied Chemistry (1970). Manual on Symbols and Terminology fir Physiochemical Quantities and Units (Butterworths, London). ISRAEL,G. (1962). "Tritium measurement in rain over the ye? 196M1 &r isotope enrichment in a separation column," 2.Naturforsch 17a. 925. T. S., SADARANGANI, S. H., SOMASUNDARAM, S. AND VAZE,P. K. IYENCAR, (1965)."A cold strip apparatus for sampling tritium in air," Health Physics 11, 313. JACOES, D. G. (1968). Sources of Tritium and Its Behavior Upon Release to the Environment, Report 'ITD-24635 (U.S. Atomic Energy Commission, Washington). JALBERT, R. A. AND HIEBEET,R. D. (1971). "Gamma insensitive air monitor for radioactive gases," Nucl. Instrum. Methods 96.61. JOHNSTON,P. A., HUTH, G. C. AND LOCKER, R. J . (1970). "Low-energy

radioactivity and avalanche semiconductor radiation detectors," Isotopes and Radiat. Tech. 7, 266. JONES,E. W. AND DOUGLAS,K. D. S. (1967). "Monitoring techniques for gaseous beta emitters," page 545 in Assessment of Airborne Radioactivity (International Atomic Energy Agency, Vienna). KAARTINEN,N. (1969). A New Oxidation Method for the Prepamtion of Liquid Scintillation Samples, Packard Technical Bulletin NO. 18 (Packard Instrument Company, Downers Grove, Illinois). KALLEAANN, H., FURST, M. AND BROWN,F. H. (1955). "Increasing fluorescence efficiency of liquid scintillation solutions," Nucleonics 13 (4), 58. KAUPMAN, S. AND LIBBY,W. F. (1954). "The natural distribution of tritium," Phys. Rev. 93, 1337. KELLY,R. G., PEETS,E. A., GORDON, S. AND BUYSKE,D. A. (1961). "Deterrnination of C-14 and H-3 in biological samples by Schiiniger combustion and liquid scintillation techniques," Anal. Biochem. 2, 267. KIM, S. M. AND VAUGHN,L. (1971). "Low level 3H measurements and tritiated luminous dial watches," Radiochem. Radioanal. Letters 8, 115. F. E. AND BABATTA, E.J. (1971). "The determination of accuracy KNOWLES, and precision limits for tritium in water," Radiolog. Health Data Reports 12, 405. KORANDA, J. J., PHELPS,P. L., ANSPAUGH, L. R. AND HOLLADAY, G. (1971). "Sampling and analytical systems for measuring environmental radioactivity," page 587 in Rapid Methods for Measuring Radioactivity in the Environment (International Atomic Energy Agency, Vienna). Lawrence Berkely Laboratory (1972). Instrcmentation for Environmental Monitoring, Report LBL1, Vol. 3 (Lawrence Berkeley Laboratory, Berkeley, California). R. AND M ~ H I S S IA. , A. (1970). "Low-level counting by liquid LIEBERMAN, scintillation-n. Application of emulsions in tritium counting," Int. J. Appl. Radiat. and Isotopes 21, 319. R. AND BERMAN, M. (1951). "Efficiency criteria in radioactivity LOEVINGER, counting," Nucleonics 9 (I), 26. MAHIN,D. T. AND LOPBERG,R. T. (1966). "A simplified method of sample preparation for determination of tritium, carbon-14 or sulfur-35 in blood or tissue by liquid scintillation counting," Anal. Biochem. 16, 500. R. T. (1970). "Determination of several isotopes MAHIN,D. T. AND LOFBERG, in tissue by oxidation," page 212 in The Current Status of Liquid Scintillation Counting, Bransome, E., Jr., Ed. (Grune and Stratton, New York). MANN,W. B. (1954). "Use of Callendars' 'Radio-Balance' for the measurement of the energy emission from radioactive sources," NBS J. of Research 52, 177. MANN,W. B. (1973). "The calibration of the National Bureau of Standards' tritium standards by microcalorimetry and gas counting," page 86 in Tritium, Moghissi, A. A. and Carter, M. W., Eds. (Messenger Graphics, Phoenix. Arizona and Las Vegas, Nevada). MANN,W. B. AND SELIGER,H. H. (1958). Preparation, Maintennnce, and Application of Standards of Radioactivity, NBS Circular 594 (U.S. GOV-

REFERENCES

1

81

ernment Printing Office, Washington). MANN,W. B. AND SPERNOL, A. (1964). 'The National Bureau of Standards tritiated water standards," Int. J. Appl. Radiat. and Isotopes 16, 628. MANN,W. B., M E D ~ C X R., W. AND YURA,0. (1964). "A recalibration of the National Bureau of Standards tritiated water standards by gas counting," Int. J. Appl. Radiat. and Isotopes 16, 351. MARTELL, E. A. (1963). "On the inventory of artificial tritium and its occurrence in atmospheric methane," J. Geophys. Res. 68. 3759. MAUSHART, R. (1967). Tritiummessungen in Strahlemchutt, Report KFK677 (Kernforschungszentrum, Karlsruhe, Germany). MCCONNON, D. (1970). The Use of Water QS a Sampling Medium for Tritium Oxide, Report BNWGCC-547 (Pacific Northwest Laboratory, Battelle Memorial Institute, Richland, Washington). MERRITT,W. F. (1958). "System for counting tritium as water vapor," Anal. Chem. 30, 1745. MOCHISSI,A.A. AND CARTER, M. W. (1968). "Internal standards with identical system properties for determination of liquid scintillation counting efficiency," Anal. Chem. 40, 812. R. (1970). "Tritium body burden of chilM o c ~ r s s A. ~ , A. AND LIEBERMAN, dren, 1967-1968," Radiolog. Health Data Reports 11, 227. MOCHISSI, A. A., BRETTHAUER, E. W. AND CARTER,M. W. (1971). "Monitoring of tritium under emergency conditions," page 263 in Rapid Methods for Measuring Radiation in the Environment (International Atomic Energy Agency, Vienna). MOGHISSI, A. A., BBETTHAUER, E. W. ,uur~COMPPON, E. H. (1973). "Separation of water from biological and environmental samples for tritium analysis." Anal. Chem. 45, 1565. E. W., PLATT,W. F. AND MCNELIS,D. N. MOCHISSI,A. A., BRETTHAUER, (1975). "Oxygen bomb combustion of environmental and biological samples for tritium analysis," Int. J. Appl. Radiat. and Isotopes 26, 339. MOCHISSI, A. A., KEUEY,H. L., PHILLIPS,C. R. AND RECNIER,J. E. (1969a). "A tritium monitor based on scintillation," Nucl. Instrum. Methods 68,159. A. A., KELLEY, H. L., REGNIER, J. E. AND CARTER, M. W. 11969b3. MOCHISSI, "Low-level counting by liquid scintillation-I. Tritium measurements in homogeneous systems," Int. J. Appl. Radiat. and Isotopes 20. 145. M. W. AND POSEY, MOCHXSSI, A. A., TOEBBER, E. D., RECNIER,J. E., CARTER, C. D. (.1970)."Health physics aspects of tritium luminous dial painting." Health Physics 18, 255. Moom, R. AND BUSHIRK,E. R. (1961). "Differential sublimation rates of light and tritium-labeled water," Nature 189, 149. MOSKALEV, Y. I., Ed. (1968).Tritium Oxide (Available as AEC-W-7298, 1972, National Technical Information Service, U.S. Department of Commerce, Springfield, Virginia). N. (1967). "A scintillation MURAMATSU, M., KOYANO, A. AND TOKUNAGA, probe for continuous monitoring of tritiated water," Nucl. Instnun. Methods 54, 325. NCRP (1961). National Council on Radiation Protection and Measurements,

82

1

REFERENCES

A Manual of Radioactivity Procedures, NCRP Report No. 28, published as NBS Handbook 80 (U.S. Government Printing Office, Washington). OSBORNE, R. V. (1968). Automatic Urinalyser for Tritium, AEP-521 6. Report AECL2702 (Atomic Energy of Canada Limited, Chalk River, Ontario). OSBORNE,R. V. (1970a). "Detector for tritium in water," Nucl. Instrum. Methods 77, 170. OSBORNE, R. V. (1970b). "Performance of an automatic analyzer for tritium in urine," Health Physics 18, 87. OSBORNE, R. V. (1971). Monitoring Reactor EfFuents for Tritium: Problems and Possibilities, Report AECL-4051 (Atomic Energy of Canada Limited, Chalk River, Ontario). OSBORNE, R. V. (1972).Development of a Monitor for Tritium in the Presence of Noble Cases, Report AECG4303 (Atomic Energy of Canada Limited, Chalk River, Ontario). OSBORNE, R. V. (1973). "Sampling for tritiated water vapor," page 1428 in Proceedings of the Third International Congress of the International Radiotion Protection Association, published as Report CONF-730907-P1 (National Technical Information Service, U.S. Department of Commerce, Springfield, Virginia). OSBORNE, R. V. AND COVEART, A. S. (1973). "A portable monitor for tritium in air," Nucl. Instrum. Methods 106, 181. OSBORNE, R. V. AND COWPER,G. (1966). Detection of Tritium in Air with Ionization Chambers, Report AECL2604 (Atomic Energy of Canada Limited, Chalk River, Ontario). OSLOOND, J . H., ECHO,J . B., POLZER, W. L. AND JOHNSON, B. D. (1973). "A tritium air sampling method for environmental and nuclear plant monitoring," page 531 in Tritium, Moghissi, A. A. and Carter, M. W., Eds. (Messenger Graphics, Phoenix, Arizona and Las Vegas, Nevada). O ~ W ,H. G. (1966). "Hurricane tritium-II. Au-sea exchange of water in Betsy 1965," Tellus 20, 577. O ~ L UH. ~G. ,(1970). A Rapid Field Sampling for Tritium in Atmospheric Hydrogen, Technical Report (University of Miami, Miami, Florida). O m W , H. G. AND MASON,A. S. (1974). "Atmospheric HT and HTO-I. Experimental procedures and tropospheric data 1968-1972," Tellus, 26.1. O r n u n , H. G., BROWN, R. M. m BAINBRIDGE, A. E. (1964). "Standardization of natural tritium measurements," Tellus, 16, 131. O ~ U N DH:, G., RINKEL,M. 0. AND R m n , C. (1969). "Tritium in the equatorial Atlantic current system," J. Geophys. Res. 74, 4535. PARMENTIER,J. H. AND TENH w , F. E. L. (1969). "Developments in liquid scintillation counting since 1963,"Int. J. Appl. Radiat. and Isotopes 20,305. PAT~ERSON, M. S. AND GBEENE.R. C. (1965). "Measurement of low energy beta-emitters in aqueous solution by liquid scintillation counting of emulsions," Anal. Chem. 37, 854. PEE% E. A., FLORINI, J. R. AND BUYSKE, D.A. (1960). "Tritium radioactivity determination of biological materials by rapid dry combustion technique," Anal. Biochem. 32, 1465.

PETERSON,J. I., WAGNER,F.,

SIEGEL, S. AND NIXON,W. (1969). "A system for convenient combustion preparation of tritiated biological samples." Anal. Biochem. 31, 189. M. AND STEVENS, F. A. (1970). "Solubilization of animal tisues for POLLAY, liquid scintillation counting," page 207 in The Current Stutus of Liquid Scintilhtion Counting, Bransome, E., Jr., Ed. (Grune and Stratton, New York). PORTER, S . W. AND SLABACK, L. A. (1966). Measurement of Tritium Surface Contamination, Report AFRRI-SP66-13 (Armed Forces Radiobiology Research Institute, Bethesda, Maryland). PREVO,C. T.AND SANTAMASSIMO, V. C. (1970).znstrumenbtion for Monitoring of =Kr and =Hin Natural Gas, Report UCRG50882 (Lawrence Radiation Laboratory, Livermore, California). P ~ Y D ZS., (1971). Some Detection Methods in Radiochmmatography a n d the Measurement of Tritium and Other Low-Energy &Emitters (University of Oslo, Oslo) (Available as IMSMF-262. International Atomic Energy Agency, Vienna). PRYDZ, S., MEW, T. B. AND KOBEN,J. F. (1971). T w o dimensional scanner for radiochromatograms employing a n electron multiplier detector in vacuo," J. Chromatography 59, 99. RAPKIN,E.(1964). "Liquid scintillation counting 1954-1963. a review," Int. J. Appl. Radiat. and Isotopes 15, 69. ~ B C B. F., , MOCHIBSI, A. A., WHEELEB,J. K. AND COLVPI,M. C. (1972). "Percutaneous absorption by rats of tritium from a luminous compound," Health Physics 22, 31. REINIG,W. C. AND AISENESIUS, E. L. (1962). Control of Tritium Health Hazards at the Savannah River Plant, Report DP-771 (Savannah River Laboratory, Aiken, South Carolina). &SEN, J. C., LAVBEB, G. R AND EISENBUD, M. (1967). "Carbon-14 and tritium measurement by means of bremsstrahlung emissions," Science 157. 77. RUMMERFXELD, P. S. AND GOLDMAN,I. H. (1972). "Some practical observations on liquid scintillation counting," Int. J. Appl. Radiat. and Isotopes 23, 353.

SANNES, R. AND BAINVILLE,B. (1965). A Portable Tritium-&-Air Monitor, Report AECL2283 (Atomic Energy of Canada Limited, Chalk River, Ontario). SANSONI,B. AND KBACKE. W. (1971). "Rapid determination of low-level alpha and beta activities in biological material using wet ashing by OH radicals." page 217 in Rapid Methbds for Measuring Radioactivity in the Environm n t (International Atomic Energy Agency. Vienna). SAX, N. I. AND GABAY,J. J. (1970). "A surveillance technique for tritiated moisture in the atmosphere," page 380 in Environmental Surveillance in the Viciniw of Nuclear Facilities, Reining, W. C., Ed. (CharlesC Thomas Publisher, Spri&ield, Illinois). SCHARPENSEEL, H. W. AND MENKE,K. H. (1962). "Radio column chromato-

84

/

REFERENCES

graphic assay of 3H-labelledsubstances," page 281 in Tritium in the Physical and Biological Sciences, Vol. 1 (International Atomic Energy Agency, Vienna). SCH~NIGER, W. (1955). "Eine mikroanalytische Schnellbestimrnung von Halogen in organische Substanzen," Mikrochim Acta, page 123. SCHUM, E. (1970). "Flow-monitoring of aqueous solutions containing weak beta emitters," page 95 in The Current Status of Liquid Scintillation Counting, Bransome, E., Jr., Ed. (Grune and Stratton, New York). SEIMIYA, T., SEKWE,K. AND SASAKI,T. (1967). "Counting of tritium with a thin-windowed Geiger-Miiller counter tube," J. Sci. Instrum. 42, 906. SEPALL,0.AND MASON,S. G. (1960). "Vaporlliquid partition of tritium in tritiated water," Can. J. Chem. 23, 2024. SHIGEMATSU, A., TOYOHARA, M. AND TOMONO, K. (1969). "A scintillation probe for efficient counting of low energy beta rays in the elution from liquid or gas chromatographs," Nucl. Instrum. Methods 76,301. SILVER,W. J. AND CHEW,M. H. (1972). "Health physics aspects of tritium control a t the Lawrence Radiation Laboratory. Livermore." page 265 in Health Physics Opemtional Monitoring, Willis, C. A. and Handloser, J. S., Eds. (Cordon and Breach, New York). SIMPSON, J. D. AND GREENING, J. R. (1960). "Preparation of tritiated water samples by distillation," Nature 186, 467. SMITH,A. H. AND REED,G. W. (1965). "The standardization of a liquid scintillation system," page 427 in Radioisotope Sample Measurement Techniques in Medicine and Bwlogy (International Atomic Energy Agency, Vienna). SOUDAIN, G. (1965).Contribution to the Dosimetryof Tritium in the Presence of Inactive and Active Gases (University of Toulouse, Toulouse, France) available as Mound Laboratory Report MLM-1825 (TR)(Monsanto Research Corporation, Mound Laboratory, Miamisburg, Ohio, 1971). SPERNOL, A. AND DENECKE, B. (1964). "High precision absolute gas counting of tritium-ID. Absolute counting of tritium in the internal gas counter," Int. J. Appl. Radiat. and Isotopes 15, 241. STEINBERG, S. AND ROHRBOUGH, S. F. (1965). "Adsorption techniques for the collection of water vapor in the upper atmosphere," page 458 in Humidity and Moisture-Measurement and Control in Science and Industry, Vol. 2 , Amdur, E. J., Ed. (Reinhold Publishing Corp., New York). STEWART, M. L., KLINE,J. R. AND JORDAN. C. F. (1972). "A tritiated water recovery system," Int. J. Appl. Radiat. and Isotopes 23. 387. SYMONDS, A. E. (1959). Removal of Tritium Contamination from Suqfaces of Metals, Report DP-367 ( E . I. du Pont de Nemours. Aiken, South Carolina). F.A. (1970). "Counting quenched liquid TAKAHASHI. I. T. AND BLANCHARD, scintillation samples using an outside-the-instrument gamma source and an external standard channel ratio method." Anal. Biochem. 35, 411. TING,P.AND L w ,R. L. (1973). "Continuous monitoring of aqueous tritium activity," page 170 in Tritium. Moghissi. A. A. and Carter, M. W., Eds. (Messenger Graphics, Phoenix, Arizona and Las Vegas, Nevada). K., TOICUNACA, N., KOYANA. A.. TOYOHARA, M. AND SHIGEMATSU, TOMONO,

REFERENCES

1

85

A. (1972). "Detailed description of a simplified radio-liquid chmrnatograph," Nucl. Instrum. Methods 102, 257. ' b u s s ~ u ,F. AND DIEHL, H. (1963). "Efficiency ofchemical desiccants," Anal. Chem. 35, 674. VALENTINE, A. M. (1968). An Investigation of a Bubbler Tritium Sampler, Report LA-3916 (Los Alamos Scientific Laboratory. Los Alamos, New Mexico). VERHAGEN,B. TH. (1967). "Rapid isotope enrichment of gases by thermal diffusion for nuclear dating," page 657 in Radioactive Dating and Methods of Low-Level Counting (International Atomic Energy Agency. Vienna). WATERS,J. R. (1972). Measurement of Low Concentrations of Tritium in Air with Discrimination Between HT and HTO, presented a t the 17th Annual Meeting, Health Physics Society, Las Vegas, Nevada, June 12-17, 1972 (Abstract Pl122, Health Physics 23, 431). WESTERMARK,T., DEVELL, L. AND GHANEM,N. A. (1960). "On the use of bremsstrahlung for the determination of tritium in aqueous and organic systems," Nucl. Instrum. Methods 9, 141. WHEELER,K.T. AND SHAW, E. I. (1971). "Determination of the efficiency of NTB nuclear track emulsion for the detection of tritium," Int. J. Appl. Radiat. and Isotopes 22, 759. WOODS, M. J. (1973). "Humidity effects in the 1383A ionization chamber," Nucl. Instrum. Methods 109, 615. YAROM,A., YAIR, R. AND SABBAH, B. (1972). "Stability improvement of ionization chambers," Nucl. Instrum. Methods 104, 257.

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tional in scope and are concerned with scientsc problems involving radiation quantities, units, measurements and effects, or radiation protection may be admitted to collaborating status by the Council. The present Collaborating Organizations with which the NCRP maintains liaison are as follows: Ameriean Academy of Dermatology American Association of Physicists in Medicine American College of Radiology American Dental Association American Industrial Hygiene Association American Insurance Association American Medical Association American Nuclear Society American Occupational Medical Association American Podiatry Association American Public Health Association American Radium Society American Roentgen Ray Society American Society of Radiologic Technologists American Veterinary Medical Association Association of University Radiologists Atomic Industrial Forum College of American Pathologists Defense Civil Preparedness Agency Genetics Society of America Health Physics Society National Bureau of Standards National Electrical Manufacturers Association Radiation Research Society Radiological Society of North America Society of Nuclear Medicine United States Air Force United States Army United States Energy Research and Development Administration United States Environmental Protection Agency United States Navy United States Nuclear Regulatory Commission United States Public Health Service

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Alpha-Emitting Particles in Lungs (1975) Tritium Measurement Techniques (1976) StructumlShielding Design and Evaluationfor Medical Use of X Rays and Gamma Rays of Energies Up to 10 MeV (1976) Environmenrul Radiation Measurements (1976) Radiation Protection Design Guidelines for 0.1-100 MeV Particle Accelerator Facilities ( 1977) Cesium-137 h m the Environment to Man: Metabolism and Dose (1977) Medical Radiation Exposum of P r e g m t and Potentidly Pregnant Women (1977) Protection of the Thyroid Gland in the Event of Releases of Radioiodine (1977) Instrumentation and Monitoring Methods for Radiation Protection (1978) A Handbook of Radioactivity Measurements Procedures, 2nd ed. (1985) Opemtional Radiation Safety Pr0gm.m (1978) Physical. Chemical, a d Biological Properties of Radiocerium Relevant to Radiution Protection Guidelines (1978) Radiation Safety Training Criteria for Industrial Radiogmphy (1978) Tritium in the Environment (1979) Tritium and Other Radionuclide Labeled Organic Compounds Incorpomted in Genetic Material (1979) Influence of Dose and Its Distribution in Time on DoseResponse Relationships for Low-LETRadiations (1980) Management of Persons Accidentally Contuminuted with Radionuclides (1980) Radiofrequency Electromagnetic Fields-Properties, Quantities and Units, Biophysical Interaction, and Measurements (1981 Radi4tion Protection in Pediatric Radiology (1981) Dosimetry of X-Ray and Gummu-Ray Beams for Radiation Therapy in the Energy Range 20 KeV to 50 MeV (1981) Nuclear Medicine-Factors Influencing the Choice and Use of Radionuclides in Diagnosis and Therapy (1982) OpemtbncJ Radiation Safety-Training (1983) Radiation Protection and Measurement for Low-Voltage Neutron Genemtors ( 1983) Protection in Nuclear Medrcine and Ultrasound Diagnostic Procedures in Children (1983)

/

NCRP PUBLICATIONS

Biological Effects of Ultnzsound: Mechanisms and Clinical Implications (1983) Iodine-129: Evaluation of Releases from Nuclear Power Genemlion (1983) Radiological Assessment: Predicting the Tmnsport, Biwccumulation, and Uptake by M a n o f Radionuclides Released to the Environment (1984) Exposures from the Uranium Series with Emphasis on R d n and Its Daughters (1984) Evaluation of Occupational and Environmental Exposures toRadon and Radon Daughters in the UnitedStates (1984) Neutron Contaminationfrom Medical Electron Accelemtors (1984) Induction of Thyroid Cancer by Ionizing Radiutwn (1985) Carbon-14 in the Environment (1985) SI Units in Raaktion Protection and Measurements (1985) The Experimental Basis for A bsorbed-Dose CcJczhtwns in Medical Uses of Radionuclides ( 1985) General Concepts for the Dosimetry of Internally Deposited Radionuclides (1985) Mammography-A User's Guide (1986) Biological Effects and Exposure Criteria for Radiofrequency Electromagnetic Fields (1986) Use of Bioassay Procedures for Assessment of Internal Radionuclide Deposition (1987) Radiation Alarms and Access Control Systems (1986) Genetic Effects from Internally Deposited Radionuclides (1987) Neptunium: Radiation Protection Guidelines (1988) Public Radiation Exposure from Nuclear Power Generation in the U&d States (1987) Ionizing Radiation Exposure of the Population of the United States (1987) Exposure of the Population in the United States and Canada from Natural Backgmund Radiation (1987) Radiation Exposure of the U S . PopuLation from C o n s u w Products and Miscellaneous Sources (1987) Comparative Carcinogenicity o f Ionizing Radiation and Chemicals (1989) Measurement of Radon and Radon Daughters in Air (1988) Guidance on Radiution Received in Space Activities (1989) Quality Assurance for Diixgrwstic Imaging (1988) Exposure of the U.S. P o p h t i o n from Diugmstic Medical Rad-n (1989)

NCRPPUBLICATIONS

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95

Exposure of the U.S. Population from Occupational Radiation (1989) Medical X-Ray, Electron Beam and Gamma-RayProtection for Energies Up to 50 MeV (Equipment Design, PeTform n c e and Use) (1989) Control of Radon in Houses ( 1989) TheRelative B i o l o g d Effectiveness ofRadiations of Different Qwlity (1990) Radiation Protection for Medical and Allied Health Personnel (1989) Limit for Exposure to "Hot Particles" on the Skin (1989) Implementation of the PTinciple of As Low As Reasonably Achievable ( M A ) for M e d i . and Dental Personnel (1990) Conceptual Basis for Calculations of Absorbed -Dose Distributions (1991) Effects of Ionizing Radicrtion on Aqturtic Organisms (1991) Some Aspects of Strontium Radiobiology (1991) Developing Radiation Ernegency Plans forAcademic, Medical or Industriul Facilities (1991 Calibmtion ofSurvqy Instruments Used in Radiation Pmtection for the Assessment of Ionizurg Radiation Fields and Radhctive Sur+&ce Conturnination (1991) Exposure Criteria for Medical Diugmstic Ultmsound: I. Criteria Based on Thennal Mechanisms (1992) Maintaining Radiotion Protection Records (1992) Risk Estimates for Radiation Protection (1993) Limita&n ofExposure to Ionizing Radiation (1993) Research Needs for Radiation Protection (1993) Radiation Protection in the Mineral Extraction Industry (1993) A P m c W Guide to the Determination of Human Exposure to Radiofrequency Fields (1993) Binders for NCRP reports are available.Two sizesmake it possible to collect into small binders the "old series"of reports (NCRPReports Nos. 8-30)and into large bindersthe more recent publications (NCRP Reports Nos. 32-119). Each binder will accommodate from five to seven reports. The binders carry the identification "NCRP Reports" and come with label holders which permit the user to attach labels showing the reports contained in each binder. The following bound sets of NCRP reports are also available: Volume I. Volume It.

NCRP Reports Nus. 8,22 NCRP Reports Nos. 23,25,27,30

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NCRP PUBLICATIONS

Volume III. Volume Dl. Volume V. Volume VI. Volume VII. Volume WI. Volume M. Volume X. Volume XI. Volume XII. Volume XUI. Volume XIV. Volume XV. Volume XVI. Volume XVII. Volume X V m . Volume XM. Volume XX. Volume XXI. Volume XXII. Volume XXIII.

NCRP Reports Nos. 32,35,36,37 NCRP Reports Nos. 38,40,41 NCRP Reports Nos. 42,44,46 NCRP Reports Nos. 47,49,50,51 NCRP Reports Nos. 52,53,54,55,57 NCRP Report No. 58 NCRP Reports Nos. 59,60, 61,62,63 NCRP Reports Nos. 64,65,66,67 NCRP Reports Nos. 68.69,70, 71,72 NCRP Reports Nos. 73,74,75,76 NCRP Reports Nos. 77,78,79,80 NCRP Reports Nos. 81,82,83,84,85 NCRF' Reports Nos. 86,87,88,89 NCRP Reports Nos. 90,91,92,93 NCRP Reports Nos. 94.95,96,97 NCRP Reports Nos. 98,99,100 NCRP Reports Nos. 101,102,103,104 NCRF' Reports Nos. 105,106,107,108 NCRP Reports Nos. 109,110,111 NCRP Reports Nos. 112,113,114 NCRP Reports Nos. 115,116,117,118

(Titles of the individual reports contained in each volume are given above. )

No. 1 2

3 4

5 6

NCRP Commentaries Title Krypton-85 in the Atmosphere-With Specific Reference.to the Public Health Signifiance of the Proposed ContrvUed Release at Three Mile Island ( 1980) Preliminary Evaluation of Criteriafor the Disposal of Tmnsuranic Contamimzted Waste (1982) Screening Techniques for Determining Compliance with Environmentul Standcutis-Releases of Radionuclides to the Atmosphere (19861,Revised (1989) Guidelines for the Release of Waste Water from Nuclear Facilities with Special Reference to the Public Health Significance of the ftoposed Release of Treated Waste Waters at Three Mile Islnnd (1987) Review of the Publication, Living Without Landfills (1989) Radon Exposure of the U.S. Population-Status of the Problem (1991)

NCRP PUBLICATIONS

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97

Mkadministrtation of Radiwctive Materinl in MedicineScientificBackground (1991) Uncertuinty in NCRP Screening Models Relating to Atmospheric Transport, Deposition and Uptake by Humans (1993)

Proceedings of the Annual Meeting No. 1

Title

Perceptions of Risk, Proceedings of the Fifteenth Annual Meeting held on March 1415, 1979 (including Taylor Lecture No. 3) (1980) Critical Zssws in Setting Radiation Dose Limits, Proceedings of the Seventeenth Annual Meeting held on April 89, 1981 (including Taylor Lecture No. 5) (1982) Radiation Protection and New Medical Diagnostic A p p m h e s , Proceedings of the Eighteenth Annual Meeting held on April 6 7 , 1982 (including Taylor Lecture No. 6) (1983) Environmental Radioactivity, Proceedings of t h e Nineteenth Annual Meeting held on April 67,1983 (including Taylor Lecture No. 7) (1983) Some Zssues Importcrnt in Developing Basic Radiation Protection Recommendutions, Proceedings of the Twentieth Annual Meeting held on April 4-5,1984 (including Taylor Lecture No. 8) (1985) Radioactive Waste, Proceedings of the Twenty-first Annual Meeting held on April 34,1985 (including Taylor Lecture No. 9) (1986) Nonionizing Electmnagndrc Radiations and Ultrasound, Promedings of the Twenty-second Annual Meeting held on April 2-3, 1986 (including Taylor Lecture No. 10) (1988) New Dosimetry at Hiroshima adNagasaki d Z t s Implications for Risk Estimates, Proceedings of the Twenty-third Annual Meeting held on April 89,1987 (including Taylor Lecture No. 11)(1988) Radon, Proceedings of the Twenty-fourth Annual Meeting held on March 30-31, 1988 (including Taylor Lecture No. 12) (1989) Radiation Protection Toduy-TheNCRPat Sixty Years,PIW ceedings of t h e Twenty-fifth Annual Meeting held on April 5 6 , 1989 (including Taylor Lecture No. 13)(1990)

98

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NCRP PUBLICATIONS

Health and Ecological Implications of Radioactively Contaminated Environments, Proceedings of t h e Twentysixth Annual Meeting held on April 4-5,1990 (including Taylor Lecture No. 14) (1991) Genes, Cancer and Radiation Protection, Proceedings of the Twenty-seventh Annual Meeting held on April 3-4,1991 (including Taylor Lecture No. 15)(1992) Radiation Protection in Medicine, Proceedings of the Twenty-eighth Annual Meeting held on April 1-2,1992 (including Taylor Lecture No.16) (1993)

No. 1

Lauriston S. Taylor Ledwes Title The Squares of theNatural Numbers in Radiation Protection by Herbert M. Parker (1977) Why be Quantitative about Radiation Risk Estimates? by Sir Edward Pochin (1978) Radiation Protection2oncepts Md Tmde by Hymer L Friedell (1979)[Available also in Perceptwns of Risk, &e above] From "Quantrty of Radiation" and "Dose" ta 'Zxpsure" and "Absorbed Dose"-An Historical Review by Harold 0. Wyckoff ( 1980) How Well Can We Assess Genetic Risk? Not Very by James F. Crow (1981)[Available also in C&iccrl Issues in Setting Radiation Dose Limits, see abovel Ethics, Trade-offs and Medical Radiation by Eugene L. Saenger (1982) [Available also in Radiation Protection and New Medical Diagnostic Apptwches, see above] The Human Environment-Past, Present and Future by Meml Eisenbud (1983)[Available also in Environmental Radioactivity, see above] Limitation and Assessment i n Radiation Protection by Harald H. h i (1984) [Available also in Some Issues Imporimt in Dewloping Basic Radiution Protection Recommendutions, see above] Truth (and Beauty) in Radiatwn Measwement by John H. Harley (1985) [Available also in Radioactive Waste, see above] Biological Effects of Non-ionizing Radiations: Cellular Properties and Interactions by Herman P. Schwan (1987) [Available also in Nonionizing Electromagnetic Radiations and Ultrasound, see above]

NCRPPUBLICATIONS

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99

How to be Quuntitdve about Radiation Risk Estimates by Seymour Jablon (1988) [Available also inNew Dosimetry at Hiroshima and Nagasaki and its I m p l ~ i o n for s Risk Estimates, see abovel How Safe is Safe Enough? by Bo Lindell (1988) [Available also in Radon, see abovel Radiobiology and Radiation Protection: The Past Century and Prospects for the Future by Arthur C. Upton (1989) [Available also in Radiation Protection T h y , see above] Radiation Protection and the Internal Emitter Saga by J . Newel1 Stannard (1990) [Available also in Health and Ecological Implications of Radioactively Contaminated Environments, see abovel When is a Dose Not a Dose? by Victor P. Bond (1992) [Available also in Genes, Cancer and Radiution Protection, see abovel Dose a d Risk in Dkgnmtic Radiology: How Big? How Little? by Edward W . Webster (1992)IAvailablealso in Radiation Protection in Medicine, see above] Science, Radiation Protection and the NCRP by Warren K. Sinclair (1993) symposium Proceedings The Control of Exposure of the Public to Ionizing Radiation in theEvent ofAccident or Attack, Rmxdings of a Sympct sium held April 27-29, 1981 (1982)

NCRP Statements No. 1

Title "Blood Counts, Statement of the National Committee on Radiation Protection," Radiology 63,428 (1954) "Statements on Maximum Permissible Dose h m Television k i v e r s and Maximum Permissible Dose to the Skin of the Whole Body," Am. J. Roentgenol., Radium Ther. and Nucl. Med. 84, 152 (1960) and Radiology 75, 122 (1960) X-Ray Protection Standatds for Home TelevisionReceivers, Interim Stutenent of the National Council on Radiation Protection and Measurements (1968) Specification of Units ofNatccral Uranium and Natuml Thorium, Stcrteinent of the NationaJ Council on Radiation Protection a d Measurements, (1973)

100 5 6

7

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NCRPPUBLICATIONS

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NCRP Statement on Dose Limit for Neutrons (1980) Control of Air Emissions of Radbnuclzdes (1984) The Probability That a Particular Malrgnuncy May Have Been Caused by a Specified I r r a d W n (1992)

Other Documents The following documents of the NCRP were published outside of the NCRP Report, Commentary and Statement series: Report Somatic Radiation Dose for the General Pop&n, of the Ad Hoc Committee of the National Council on Radiation Protection and Measurements, 6 May 1959, Science, February 19, 1960, Vol. 131, No. 3399, pages 482-486 Dose Effect Modifying Factors I n Radiation Protection, Report of Subcommittee M-4(Relative Biological Effectiveness) of the National Council on Radiation Protection and Measurements, Report BNL 50073 (T-471) (1967) Brookhaven National Laboratory (National Technical Information Semce Springfield, Virginia) The following documents are now superseded andlor out of print:

NCRP Reports No. 1 2 3 4

5

'Title X-Ray Protection (1931) [Superded by NCRP Report No. 31 R& Prdechon (1934) [Superseded by NCRP Report No. 41 X-Ray Protection (1936) [Superseded by NCRP Report No. 61 . -N 131 Raduun Protection (1938) [Supeneded by Safe Handling of Radimctwe Lwninous Compound (1941) [Out of m t 1 Medical X-Ray Protection Up to Two Million Volts (1949) [Superseded by NCRP Report No. 181 Safe Handling of Radioactive Isotopes (1949) [Superseded by NCRP Report No. 301 Recommendations for Waste Disposal ofPhosphonrs32 a n d Iodine-131 for Mediccrl Users (1951) [Out of Print] Radiological Monitoring Methods and Instnrments (1952) [Superseded by NCRP Report NO. 571 Maximum Permissible Amounts of Radioisotopes in the Human Body curd Maximum Permissible Concentmtions in Air and Water (1953) [SupefSeded by NCRP Report . No. 221 Recommendations for the Disposal of Carbon-14 Wastes (1953) [Supersededby NCRP Report No. 811

NCRP PUBLICATIONS

1

101

Protection Against Radiations from Radium, Cobalt-60 and Cesium-137 (1954) [Supersededby NCRP Report NO.241 Protection Against Betutron-Synchrotron Radiations Up to 100 Million Electron Volts (1954) [Superseded by NCRF' Report No. 511 Safe Handling of Cadavers ColrtainingRadioactive Isotopes (1953:)[Superseded by NCRP Report No. 211 Radioactive-Waste Disposal in the Ocean (1954) [Out of Flint]

Permissible Dose @om External Sources of Ionizing Radiation (1954)including Maximum Permissible Exposures to Man, Addendum to National Bureau of Standards Handbook 59 (1958) [Superseded by NCRP Report No. 391 XRay Prdection (1955) [Supemded by NCRP Report No. 261 Regulation of Radiation Exposure by Legislative Means (1955) [Out of Print] Protection Against Neutron Radiation Up to 30 Million Electron Volts (1957) [Superseded by NCRP Report No. 381 Safe Handing of Bodies Containing Radioactive Isotopes (1958) [Superseded by NCRP Report No. 371 Protection Against Radtations from Sealed Gamma Sources (1960) [Superseded by NCRP Reports No. 33,34 and 401 Medical X-Ray Protection Up to Three Million Volts (1961) [Supersededby NCRP Reports No. 33,34,35 and 361 A M a n d of Radioactivity Procedures (1961) [Superseded by NCRP Report No. 581 Exposure to Radiution in an Emergency (1962) [Superseded by NCRP Report No. 421 Shielding for High-Energy Electron Accelerator Znstullations (1964) [Superseded by NCRF' Report No. 511 Medical X-Ray and Gamma-Ray Protection for Energies up to 10 MeV-Equipment Design and Use (1968) [Superseded by NCRP Report No. 1021 Medical X-Ray and Gamma-Ray Protection for Energies Up to 10 MeV--Structural Shielding Deszgn and Eva.i&n Handbook (1970) [Superseded by NCRP Report No. 491 B a i c R&wn Protection Crderk (1971) [Superseded by NCRP Report No. 911 Review of the Current State ofRadiutwnProtection Philosophy (1975) [Superseded by NCRP Report No. 91.1 Natuml Background Radiution in the United States (1975) [Superseded by NCRP Report No. 941 Radiution Protection for Me&d and Allied Health Personnel (1976) [Superseded by NCRP Report No. 1051

102 53 56 58

66 91

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NCRP PUBLICATIONS Review ofNCRP Radiation Dose Limit forEmbryo and Fetus in Occupatiorurlly-Exposed Women (1977) [Out of Printl RadiabbnEqmsmfn,m C o n s w n e r ~ t s a nM d isadhmus S o m (1977)[Superseded by NCRP Report No. 951 A Handbook of Radioactivity Measurements Procedures, 1st ed. (1978) [Supersededby NCRP Report No. 58, 2nd 4.1 Mammography (1980) [Out of Printl R m m r n e ~ non s L i d for Erpsun?to Ionizing Radiaaon (1987) [Superseded by NCRP Report No. 1161

NCRP Proceedings No. 2

Title Quuntitative Risk in Standurds Setting, Proceedings of the Sixteenth Annual Meeting held on April 2-3, 1980 [Out of Printl

Index Accelerator targets, 54 Acetylene, 39, 40 Activated alumina, 5 Activated charcoal, 40 Air liquefaction plants, 7 Alkylphenol detergents. 34 Anthracene, 40, 41, 42, 52, 59 Anticoincident counting, 37, 38, 49 Argon-41, 47, 48, 50 Autolysis, 9 Avalanche diodes, 49, 57 Benzene, 14, 15, 34 Precautions in use. 15 Blanks, 68, 69 Bremsstrahlung measurement, 31, 57 Bubblers, 4, 5, 53, 59, 70 Calcium sulfate, 5 Callendar radiobalance. 64 Calibration. 62 External standard, 67.68 Gas counters, 68 Internal standard. 67 Real-time instruments, 69.70, 71, 72 Calorimetry, 58. 62 Catalysts Hopcalite, 20 Palladium, 7, 40 Ceresine wax, 44 Chemiluminescence, 16 Chromatograms, 54,55,56 Coincident counting, 32, 33, 38 Cold traps, 6, 7 Counting vials, 36. 37 Desiccants, 5, 6, 15, 48 Dioxane, 14, 34, 35, 42, 43, 52 Diphenyloxazole (PPO).34.35 Distillation column, 25 Electrical noise. 46 Electrolysis cells. 22. 23

Electrometer, 26, 43, 61 Electron multipliers, 58 Ethane, 7, 39 Ethylene, 68 Euratom Gee1 Laboratory, 62 Exoelectron emission, 58 Figure of merit. 29 Filters for air sampling, 10 Flow cells, 40, 41, 42, 43, 53 Gas chromatography, 25, 48, 56, 57 Gaskets. 44.67 Geiger counter, 59 Glycerol-impregnated paper, 54 Guard ring. 38 Heliurn-3, 57 Histological sections. 54, 56 Humidity, 4 Absolute, 4 Relative, 4 Hydrogen exchange, 13, 15 Internal gas proportional counting, 37.49. 59.62

Background redudion, 50 Counting gases, 39, 40 Design and performance. 37, 38, 39, 49 Windowed counters, 51 Windowless counters. 54, 55 Insulators, 44, 45, 46 Ion trap, 45 Ionization chambers, 43, 59 Construction materials, 45, 46 Design, 44 Reduction of background. 46 Kaartinen apparatus, 17 Liebig method, 16, 17 Liquid scintillation counting. 31. 32. 33. 34.35, 36. 37, 59. 62

103

104

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INDEX

Mass spectrometry, 56, 57, 59 Measurements, 26, 27, 59, 60 Continuous monitoring, 40-53 Discrete samples, 31-40 Low-level, 37 Precision, 27, 28, 29 Sensitivity. 27, 28, 29 Specificity to tritium, 30 Time, 29. 30 Methane, 7, 39, 49. 50 Microcalorimetry, 62, 64 Minimum detectable activity. 27 Molecular sieves, 5, 7, 48 National Bureau of Standards, 62, 63, 64, 65, 66 Nickel-63, 50. 71, 72 Nuclear power plants, 42 Palladium, 25 Parr bomb, 20 Peltier coefficient, 65 Perchloric acid digestion, 16 Peristaltic pump, 43 Peterson apparatus, 17 Photographic emulsions, 56, 58, 59 Photomultipliers, 32, 33 Propagation of errors, 72, 73, 74 Propane, 68 Pulse height discrimination, 37 Pulse shape discrimination, 39 Quench, 14, 16, 68 Radiokrypton. 47, 50 Radioxenon, 47 Radon-222, 47, 48, 50 Response time, 30, 42 Sample collection. 3 Air, 3, 4, 5, 6, 7, 8 Biota, 9 Collection efficiency, 5 Containers, 4, 8, 69 Condensation, 6 Coring, 9. 10 Desiccation. 5 Dilution, 5 Grass, 9 Preservation, 8, 9 Soil. 9, 10

Trees and shrubs, 9 Tritiated particles, 10 Urine, 8 Water, 8 Sample preparation, 11 Animal samples, 13 Biological samples, 17 Blood, 16 Dilution with solvent, 14 Distillation, azeotropic, 14 Distillation, high temperature, 13, 25 Dry combustion, 16, 17, 18, 19, 20 Electrolysis, 22, 23, 24 Enrichment, 22 Environmental samples, 11 Gas chromatography, 25 Gels, 16, 20 Labeled pharmaceuticals, 20, 21 Liquids, 13, 14, 22, 23, 24 Lyophilization (freeze drying), 11 Organically-bound tritium, 16 Plant samples, 13 Soil, 13 Solubilization, 20 Thermal diffusion. 24 Tissue, 20. 35. 36 Urine, 21 Water extraction, 11, 12, 13, 14, 15 Wet oxidization, 16 Scintillators Liquid, 34, 35, 42, 43, 52 Solid, 40, 41, 42. 52 Shielding for counters. 33. 37, 38, 41. 42, 46, 52, 53 Silica gel, 5, 6, 48 Smear (wipe) test, 10, 54 Standardization. 62 Microcalorimetry, 64, 65. 66 Gas proportional counting Statistical criteria, 27 Surface activity measurement, 53, 54, 55, 56

Toluene. 14, 15, 34. 35, 42, 55 Tritiated hydrocarbons, 3, 4, 7 Tritiated paint, 54 Tritiated water standard, 62 Tritium Beta energy, 2 Discovery, 1 Forms in air, 3

INDEX Half-life, 2 Radioactive decay, 2 Range of beta particles, 30 Tritium adsorption on materials, 45 Tritium ratio, 2 Tritium standards. 63, 66 Calibration, 66 Relative standards, 67 Suppliers, 63

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105

Tritium unit, 2 Tritium-free water, 69 Tubing, 45 Vapor pressure of tritiated water, 11, 13

W values. 44 Xylene, 14. 15