Opticall~y Stimulated Luminescence Dosimetry
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Optically Stimulated Luminescence Dosimetry
L. Botter-Jensen Riso National Laboratory Radiation Research Department DK-4000 Roskilde Denmark
S. W. S. McKeever Department of Physics Oklahoma State University Stillwater, OK 74078-0444 USA
A. G. Wintle Institute of Geography and Earth Sciences University of Wales Aberystwyth, SY23 3DB UK
2003
ELSEVIER Amsterdam - Boston - Heidelberg - London - New York - Oxford P a r i s - San D i e g o - San F r a n c i s c o - S i n g a p o r e - S y d n e y - T o k y o
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Dedicated to our spouses and families, who supported us and put up with us during this long process:
To Marja, Malene, Kristine and Caroline To Joan, Katie and Alison To Jack
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TABLE OF C O N T E N T S
PREFACE
.................................................................
C H A P T E R 1: I N T R O D U C T I O N .......................................... 1.1 Optically s t i m u l a t e d l u m i n e s c e n c e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 H i s t o r i c a l d e v e l o p m e n t o f O S L d o s i m e t r y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 O S L d o s i m e t r y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1 Personal dosimetry ............................................... 1.3.2 E n v i r o n m e n t a l d o s i m e t r y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.3 M e d i c a l d o s i m e t r y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.4 R e t r o s p e c t i v e d o s i m e t r y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 This b o o k . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C H A P T E R 2:
OPTICALLY STIMULATED LUMINESCENCE THEORY ..................................................
1 1
2 5 7 9 9 9 11
15
2.1
Stimulated luminescence
2.2
G e n e r a l i s e d m a t h e m a t i c a l d e s c r i p t i o n o f optically s t i m u l a t e d l um i ne sc e n c e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . T h e p h o t o i o n i s a t i o n cross-section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 O p t i c a l transitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 W a v e l e n g t h dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19 19 21
2.3.3 M e a s u r e m e n t o f the p h o t o i o n i s a t i o n cross-section . . . . . . . . . . . . . . . . . CW-OSL .............................................................. 2.4.1 M o d e l s a n d rate equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2 The one-trap/one-centre m o d e l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.3 M o d e l s containing m u l t i p l e - t r a p s a n d centres . . . . . . . . . . . . . . . . . . . . . 2.4.4 A m o r e g e n e r a l i s e d m o d e l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.5 T e m p e r a t u r e dependence effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.6 T h e r m a l quenching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . LM-OSL .............................................................. 2.5.1 First- a n d general-order-kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.2 Relationship between L M - O S L and C W - O S L .................... 2.5.3 W a v e l e n g t h dependence o f L M - O S L ............................. 2.5.4 P h o t o c o n d u c t i v i t y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23 27 27 27 30 34 37 44 47 47 52 52 54
2.3
2.4
2.5
...............................................
xv
15 17
Table o f Contents
viii 2.6
2.7
2.8
Pulsed OSL
56
2.6.1
............................................................ Principles o f pulsed O S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.2 Delayed O S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . P h o t o t r a n s f e r r e d effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
56
Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M a t h e m a t i c a l description and typical data . . . . . . . . . . . . . . . . . . . . . . . . Radiophotoluminescence ...............................................
2.7.1
60
2.7.2
61 65
2.8.1
Procedure
C H A P T E R 3: 3.1
3.2
3.3
59 60
......................................................
O S L P R O P E R T I E S OF S Y N T H E T I C M A T E R I A L S
65
......
71 71
3.1.2 Crystal growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3 O S L stimulation and emission characteristics o f A l 2 0 s C ......... 3.1.4 The O S L response o f A l 2 0 3 : C to radiation exposure . . . . . . . . . . . . . . 3.1.5 The temperature dependence o f O S L f r o m A I 2 0 s C . . . . . . . . . . . . . . . 3.1.6 Zeroing o f the O S L signal f r o m A I 2 0 3 : C . . . . . . . . . . . . . . . . . . . . . . . . . Halides ................................................................ 3.2.1 KCl ............................................................ 3.2.2 KBr ............................................................
71 73 75 77 79 81 81 82
NaCl ........................................................... RbI ............................................................ 3.2.5 CaF2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.6 B a F X ( X = Br, Cl, I) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sulphates ..............................................................
3.2.3
84
3.2.4
85
MgS04 ......................................................... GAS04 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sulphides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 A S ( A = M g , Sr, Ca, Ba) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1 BeO ............................................................ 3.5.2 Fused quartz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 3.3.2
3.4 3.5
C H A P T E R 4: 4.1
71
A1203:C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PASSIVE OPTICALLY STIMULATED LUMINESCENCE DOSIMETRY .............................................
Personal dosimetry
...................................................
86 87 90 90 90 90 90 92 92 95
101 101
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Landauer's L u x e U M personal dosimetry system . . . . . . . . . . . . . . . . . . Landauer's InLight T M personal dosimetry system . . . . . . . . . . . . . . . . .
101
4.1.2 4.1.3 4.1.4 4.1.5
Beta dosimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . P O S L imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
104
4.1.1
102 102 104
Table o f Contents 4.2
Environmental 4.2.1 4.2.2
OSL dosimetry using A1203:C
ix .........................
107
M e a s u r e m e n t o f the natural terrestrial background radiation . . . . . Measurement o f the natural space background radiation . . . . . . . . .
110
4.3
UV dosimetry
4.4
O S L a n d R L r e m o t e optical fibre d o s i m e t r y in m e d i c a l a p p l i c a t i o n s 4.4.1 4.4.2
........................................................
5.1.1 5.1.2
5.1.3
5.1.4
5.1.5
5.1.6
....
Real-time ( R T ) in vivo monitoring o f doses during radiotherapy .. Opticalfibre dosimeters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C H A P T E R 5: O S L P R O P E R T I E S O F N A T U R A L M A T E R I A L S 5.1 Quartz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
107 107
........
Crystal structure and point defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Decay curve shapes obtained under continuous s t i m u l a t i o n - CW-OSL ...................................................... 5.1.2.1 Stimulation sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2.2 Effect o f the l l O ~ trap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2.3 Dependence on power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2.4 Three components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2.5 Effect o f stimulation wavelength . . . . . . . . . . . . . . . . . . . . . . . 5.1.2.6 Effect o f stimulation temperature . . . . . . . . . . . . . . . . . . . . . . Linear modulation O S L - - L M - O S L ............................. 5.1.3.1 L M - O S L at 160~ with 470 nm stimulation . . . . . . . . . . . . 5.1.3.2 L M - O S L at different temperatures with 526 nm stimulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.3.3 L M - O S L f r o m single grains using 532 nm . . . . . . . . . . . . . Pulsed O S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.4.1 Time resolved luminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.4.2 Delayed optically stimulated luminescence or optically stimulated afterglow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Excitation spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.5.1 Bleaching response spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.5.2 Excitation spectra after bleaching by 514 + 25 nm light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.5.3 Continuous scanning o f stimulation wavelengths . . . . . . . . 5.1.5.4 Excitation using interference filters and xenon lamp . . . . 5.1.5.5 Excitation using laser lines f r o m 458 to 645 nm . . . . . . . . 5.1.5.6 Stimulation in the infra-red 7 8 0 - 9 2 0 nm . . . . . . . . . . . . . . . Emission spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.6.1 O S L emission spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.6.2 T L emission spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.6.2.1 3 6 0 - 4 2 0 nm (near U V to violet) . . . . . . . . . . . 5.1.6.2.2 4 2 0 - 4 9 0 nm (blue) . . . . . . . . . . . . . . . . . . . . . . . . 5.1.6.2.3 5 9 0 - 6 5 0 nm (orange-red) . . . . . . . . . . . . . . . . . . 5.1.6.3 Radioluminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
112 112 112
119 119
119 123 123 123 125 126 127 130 130 130
135 135 136 137 140
141 141 143 143 145 147 147 149 149 150 150 153 153
155
x
Table o f Contents 5.1.7
Dose dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.7.1 Fast component . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.7.1.1 Multiple aliquot data . . . . . . . . . . . . . . . . . . . . . . . 5.1.7.1.2 Single aliquot data . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.7.1.3 Single grain data . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.7.2 Low doses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.8 Effects o f previous thermal treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.8.1 High temperature annealing--above 500~ ............ 5.1.8.1.1 Comparison o f L M - O S L , TL, R L and E P R . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.8.1.2 C W - O S L growth curves after annealing . . . . . 5.1.8.2 Low temperature annealing--160 to 280~ ............ 5.1.8.3 Thermal stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.8.3.1 Isothermal decay . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.8.3.2 Pulse annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.8.4 Irradiation at elevated temperatures . . . . . . . . . . . . . . . . . . . 5.1.8.5 Thermal transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.9 Raised temperature OSL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.9.1 Thermal quenching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.9.2 Thermal assistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.10 The slow component . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.10.1 Thermal stability . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.10.2 Growth curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.10.3 Optical bleaching . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.10.4 TRL ...................................... 5.1.11 Photoionisation cross-section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.12 Modelling processes giving rise to OSL in quartz . . . . . . . . . . . . . . . . 5.1.13 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2
Feldspars 5.2.1 5.2.2
5.2.3 5.2.4
5.2.5
.............................................................
157 157 157 159 160 160 162 162
162 165 167 169 169
170 173 174 177 177 179 180
181 183 184 184 184 186 188 188
Crystalstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 Decay curve shape obtained under continuous stimulation--CW-OSL and C W - I R S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 5.2.2.1 Stimulation sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 5.2.2.2 Effect o f stimulation temperature . . . . . . . . . . . . . . . . . . . . . . 189 5.2.2.2.1 Initial part o f signal . . . . . . . . . . . . . . . . . . . . . . . . 189 5.2.2.2.2 Decay curve shape . . . . . . . . . . . . . . . . . . . . . . . . . 194 Linear modulation I R S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 Pulsed O S L and I R S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 5.2.4.1 Pulsed O S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 5.2.4.2 Pulsed I R S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 5.2.4.3 Optically stimulated afterglow . . . . . . . . . . . . . . . . . . . . . . . . . 197 Excitation spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 5.2.5.1 Direct measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
xi
Table of Contents
5.2.6
5.2.7
5.2.5.2 Bleaching response spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . Emission spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.6.1 I R S L emission spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.6.1.1 2 8 0 - 2 9 0 n m (near U V ) . . . . . . . . . . . . . . . . . . . . 5.2.6.1.2 3 2 0 - 3 4 0 n m (near U V ) . . . . . . . . . . . . . . . . . . . . 5.2.6.1.3 3 9 0 - 4 4 0 n m (violet/blue) . . . . . . . . . . . . . . . . . . 5.2.6.1.4 5 5 0 - 5 7 0 n m (yellow~green) . . . . . . . . . . . . . . . . 5.2.6.1.5 6 0 0 - 7 5 0 n m (red/far red) . . . . . . . . . . . . . . . . . . 5.2.6.2 T L emission spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.6.3 R L emission spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.6.3.1 Under X - r a y stimulation at low temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.6.3.2 Under X - r a y stimulation above room temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.6.3.3 Under beta stimulation f r o m a lSTCs source 5.2.6.4 Photoluminescence emission spectra . . . . . . . . . . . . . . . . . . . Effects o f previous optical treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.7.1 Bleaching at ambient temperature . . . . . . . . . . . . . . . . . . . . . 5.2.7.2 I R bleaching at elevated temperature . . . . . . . . . . . . . . . . . .
5.2.8
Effects o f previous thermal treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.8.1 Pre-heating o f laboratory and naturally irradiated samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.8.2 Pulse annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.8.3 Irradiation at elevated temperature . . . . . . . . . . . . . . . . . . . . 5.2.9 R a i s e d temperature I R S L and O S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.9.1 T h e r m a l quenching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.9.2 T h e r m a l assistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.9.2.1 A b o v e room temperature . . . . . . . . . . . . . . . . . . . 5.2.9.2.2 Below room temperature . . . . . . . . . . . . . . . . . . . 5.2.9.2.3 Wavelength dependence . . . . . . . . . . . . . . . . . . . . 5.2.9.2.4 L i n k to anomalous f a d i n g . . . . . . . . . . . . . . . . . . 5.2.10 A n o m a l o u s f a d i n g . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.10.1 T L , O S L and I R S L .................................. 5.2.10.2 A t t e m p t s to remove anomalous f a d i n g . . . . . . . . . . . . . . . . . 5.2.10.2.1 Using a p r e h e a t . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.10.2.2 Using an optical treatment . . . . . . . . . . . . . . . . . 5.2.10.3 A t t e m p t s to avoid anomalous f a d i n g . . . . . . . . . . . . . . . . . . . 5.2.10.3.1 Using time-resolved m e a s u r e m e n t s . . . . . . . . . . 5.2.10.3.2 Using different detection wavelengths . . . . . . . 5.2.10.4 5.2.10.5
C L and T L spectra o f f a d i n g f e l d s p a r s . . . . . . . . . . . . . . . . . L o w temperature phosphorescence . . . . . . . . . . . . . . . . . . . . .
5.2.10.6 5.2.10.7
Single grain I R S L f a d i n g and f a d i a plots . . . . . . . . . . . . . . . L o g a r i t h m i c signal decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
201 201 201 201 202 203 203 203 203 203 203
.
205 205 205 207 207 208 211 211 212
215 215 215 216 216 216 217 218 219 219
219 219 220 220 220 220 220 221 223 224
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Table of Contents 5.2.10.8 Correcting f o r anomalous f a d i n g . . . . . . . . . . . . . . . . . . . . . . . 5.2.11 Radioluminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.11.1 A new dating m e t h o d . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.11.2 Practical considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.11.3 M e t h o d s o f De determination . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.11.4 Thermal stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.11.5 Single grain m e a s u r e m e n t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.12 M o d e l s f o r I R S L , O S L , I R - R L in feldspars . . . . . . . . . . . . . . . . . . . . . 5.2.12.1 I R S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.12.2 O S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.12.3 I R - R L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.12.4 Comparison o f l R - R L and I R S L (or O S L ) . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CHAPTER
Part 6.1 6.2 6.3 6.4
6:
RETROSPECTIVE
OSL DOSIMETRY
..................
I: R E T R O S P E C T I V E A C C I D E N T D O S I M E T R Y . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Materials and sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sample preparation and experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . Determination of the accident dose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.1 Retrospective assessment o f environmental dose rates . . . . . . . . . . . . 6.4.2 E stimati o n o f the accident dose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Analytical protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.2 Multiple-aliquot protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.3 The single aliquot regeneration and added dose p r o t o co l . . . . . . . . . . 6.5.4 True single-aliquot protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.4.1 Introduction ......................................... 6.5.4.2 Variation o f O S L signal with pre-heat . . . . . . . . . . . . . . . . . . 6.5.4.3 Choice o f O S L signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.4.4 Sensitivity changes with regeneration cycles . . . . . . . . . . . . 6.5.4.5 The S A R protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6 Evaluation of dose-depth profiles in bricks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6.1 Continuous O S L scanning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6.2 Determination o f d o s e - d e p t h profiles f r o m Chernobyl bricks . . . . . 6.6.3 Absolute errors and e s t i m a t e d precision o f the equivalent dose in bricks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.7 Retrospective OSL dosimetry using unheated quartz . . . . . . . . . . . . . . . . . . . 6.7.1 Dose distributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.7.2 T h e r m a l transfer and sensitivity changes . . . . . . . . . . . . . . . . . . . . . . . .
224 227 227 229 229 229 229 230 230 231 231 233 234
245
245 245 246 247 247 247 249 250 250 250 250 252 252 253 253 255 255 257 258 259 259 260 261 263
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6.8 6.9
Retrospective OSL dosimetry using household and workplace chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Retrospective OSL dosimetry using porcelain . . . . . . . . . . . . . . . . . . . . . . . . . . 6.9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.9.2 The origin o f O S L in porcelain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.9.2.1 Time-decaying dose-dependent O S L signals . . . . . . . . . . . . 6.9.2.2 Time-steady P L emission spectra f r o m porcelain . . . . . . . 6.9.2.3 O S L stimulation spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . O S L dose response o f porcelain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.9.3 6.9.4 D o s e - d e p t h profiles in porcelain and the effect o f transparency 6.9.5 O S L dosimetry using porcelain dental crowns . . . . . . . . . . . . . . . . . . . . 6.10 Retrospective accident dosimetry--conclusions . . . . . . . . . . . . . . . . . . . . . . . .
...
Part II: G E O L O G I C A L A N D A R C H A E O L O G I C A L D A T I N G . . . . . . . . . . . 6.11 M e a s u r e m e n t procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.1 Multiple-aliquot m e t h o d s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.2 Single-aliquot m e t h o d s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.2.1 Feldspars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.2.1.1 Additive dose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.2.1.2 Regenerative dose . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.2.2 Q u a r t z . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.2.2.1 Additive dose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.2.2.2 Regenerative dose . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.2.3 L u m i n e s c e n c e sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.2.4 Reliability o f O S L monitoring o f sensitivity change . . . . . 6.11.3 Dose distributions f o r single aliquots . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.3.1 H i s t o g r a m s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.3.2 6.11.3.3 6.11.3.4
Probability density plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Radial plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Calculation o f De . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.12 Single grains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.12.1 M e a s u r e m e n t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.12.1.1 Feldspars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.12.1.2 Q u a r t z . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.12.2 Dose distributions f o r single grains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.12.2.1 H i s t o g r a m s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.12.2.2 Probability density plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.12.2.3 Radial plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.12.2.4
Calculation o f De . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.13 Geological and archaeological d a t i n g - c o n c l u s i o n s
.....................
265 267 267 267 267 270 271 271 272 273 275
276 276 277 280 280 281 281 281 281 285 287 291 293 293 295 296 297 298 298 298 299 299 299 300 300 301 302
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7: O S L M E A S U R E M E N T T E C H N O L O G Y ................. Stimulation modes .................................................... 7.1.1 CW-OSL ...................................................... 7.1.2 L M - O S L ...................................................... 7.1.3 POSL ......................................................... 7.2 T h e light de te c t i o n system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1 P h o t o m u l t i p l i e r tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.2 I m a g i n g p h o t o n d e t e c t o r s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.3 Solid-state detectors ............................................ 7.3 A u t o m a t e d O S L r e a d e r s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 D e v e l o p m e n t o f optical s t i m u l a t i o n sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.1 Laser stimulation .............................................. 7.4.2 I R L E D s t i m u l a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.3 I R laser diode s t i m u l a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.4 B r o a d - b a n d light s t i m u l a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.5 Optimisation of OSL detection .................................. 7.4.6 Green L E D stimulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.7 Blue LED stimulation .......................................... 7.4.8 Blue LED and cut-off filter characteristics ....................... 7.4.9 R a m p i n g the L E D s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.10 P u l s e d a n d t i m e - r e s o l v e d O S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5 W a v e l e n g t h resolved O S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5.1 Stimulation spectrometry ....................................... 7.5.2 E m i s s i o n s p e c t r o m e t r y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6 I m a g i n g systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7 Single gr a in O S L systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7.1 Introduction ................................................... 7.7.2 CCD luminescence imaging systems ............................. 7.7.3 S i n g l e g r a i n laser O S L s y s t e m s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.8 O S L scanners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.9 P o r t a b l e systems for O S L m e a s u r e m e n t s in the field . . . . . . . . . . . . . . . . . . . 7.10 T h e m e a s u r e m e n t o f R L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.11 C o m m e r c i a l l y available O S L a p p a r a t u s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.12 F u t u r e d e v e l o p m e n t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
320 321 323 325 325 326 330 330 332 334 334 334 335 335 338 340 340 343 345
SUBJECT
351
CHAPTER
7.1
INDEX
........................................................
311 311 311 311 311 312 312 313 314
315 316 316 316 317 318
PREFACE
Optically stimulated luminescence (OSL) has become the technique of choice for many areas of radiation dosimetry. The technique is finding widespread application in a variety of radiation dosimetry fields, including personal monitoring, environmental monitoring, retrospective dosimetry (including geological dating and accident dosimetry), space dosimetry, and many more. In this book we have attempted to synthesise the major advances in the field, covering both fundamental understanding and the many applications. The latter serve to demonstrate the success and popularity of OSL as a dosimetry method. The book is designed for researchers and radiation dosimetry practitioners alike. Chapter 1 sets the stage with an overview of the process and its uses. Chapter 2 then delves into the detailed theory of the process from the point of view of stimulated relaxation phenomena, describing the energy storage and release processes phenomenologically and developing detailed mathematical descriptions to enable a quantitative understanding of the observed phenomena. The various stimulation modes (continuous wave, pulsed, or linear modulation) are introduced and compared. Chapter 3 discusses the most important synthetic OSL materials beginning with the dominant carbon-doped A1203, and moving through discussions of other, less-well studied but nevertheless important, or potentially important, materials. Chapter 4 is the first of the applications chapters and deals with the use of OSL from synthetic materials in personal, environmental, medical and UV dosimetry. Chapter 5 discusses in detail the OSL properties of the two most important natural OSL dosimetry material types, namely quartz and feldspars. These discussions originate primarily from the use of these materials in geological dating and this leads naturally into Chapter 6, dealing with all aspects of retrospective dosimetry. The division of retrospective dosimetry into accident dosimetry (Part I) and dating (Part II) is a natural one, and the inclusion of both parts under one chapter heading is appropriate since the detailed methodologies are similar in many respects, with many advances in one field being transferred to the other. Finally, Chapter 7 gives the reader an overview of the developments in instrumentation that have occurred over the past decade or more. These instrumentation developments have themselves led to new experimental methodologies, particularly in the field of geological dating where the ability to analyze large numbers of small sample aliquots, and even single
xvi
Preface
grains, has led to new capabilities and possibilities undreamt of at the beginning of OSL dosimetry. We hope that the book will find use in those laboratories within academia, national institutes and the private sector where research and applications in radiation dosimetry using luminescence are being conducted. Potential readers include personnel involved in radiation protection practice and research, hospitals, nuclear power stations, radiation clean-up and remediation, food irradiation and materials processing, security monitoring, geological and archaeological dating, luminescence studies of minerals, etc. We are grateful to the various authors (as indicated in the figure captions) and the following publishers for kind permission to reproduce copyrighted or trade-marked material (in alphabetical order): the American Institute of Physics (for figures 2.5, 2.9, 2.21, 2.25, 2.26), Ancient TL (for figure 6.40), Blackwell Publishing (for figures 5.29 and 6.41), Taylor and Francis (for figure 7.15), Elsevier, Geologos (for figure 5.32a), the Institute of Physics (for figures 5.10, 5.13a, 5.34, 5.35a,b, 5.41, 5.68 and 5.88), the International Atomic Energy Agency (for figures 4.11, 4.12 and 4.13), Landauer Inc. (for figures 4.1(c) and 4.2), the National Research Council of Canada (for figures 5.46, 5.54 and 5.84), the Nature Publishing Group (for figure 5.30), Nuclear Technology Publishing (for figures 1.4, 2.29, 3.3, 3.4, 3.6-3.10, 3.12, 3.15a,b, 3.16, 3.20, 3.21, 4.6, 4.7, 4.9, 5.57, 5.60, 5.71, 5.73, 5.74, 5.83, 6.2, 6.6-6.8, 6.11-6.15, 6.17-6.21, 6.35, 6.36, 6.38, 7.3, 7.6a, 7.7-7.9, 7.13 and 7.17), and Springer-Verlag (for figures 2.16, 5.1, 5.58 and 5.78). We each thank our respective institutions for allowing us the time and facilities to work on the book (Riso National Laboratory, Denmark; The University of Wales, Aberystwyth, UK; Oklahoma State University, USA) and one of us (AGW) also thanks the Swedish Natural Science Research Council for funding a six month visiting professorship at the University of Uppsala during the book's preparation. No work of this size takes place in isolation and particular thanks need to go to several individuals. First among these is our long-suffering friend and colleague, Finn Jorgensen, who with professionalism, infinite patience and a permanent smile drew, re-drew and re-re-drew countless numbers of figures. Similar humble thanks are due to Antony Smith for skill and patience in rescuing several of our ill-copied figures and transforming them into works of graphic art. Others whose friendship was stretched beyond the bounds of decency include several of our colleagues who read sections and chapters of the text at various stages of completion, and with grace, tact and delicacy pointed out our numerous errors. All remaining deficiencies in the book are
Preface
xvii
ours and ours alone. Finally, the three authors wish to thank their students and professional colleagues from all over the globe who have enriched our research, and improved our understanding with insight, originality and common sense. We are in debt to the whole community.
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Chapter 1
Introduction 1.1. Optically stimulated luminescence Optically stimulated luminescence (OSL) is the luminescence emitted from an irradiated insulator or semiconductor during exposure to light. The OSL intensity is a function of the dose of radiation absorbed by the sample and thus can be used as the basis of a radiation dosimetry method. The process begins with irradiation causing ionisation of valence electrons and the creation of electron/hole pairs. Pre-existing defects within the material then localise the free electrons and holes through non-radiative trapping transitions. Subsequent illumination of the irradiated sample with light leads to absorption of energy by the trapped electrons and transitions from the localised trap into the delocalised conduction band. Recombination of the freed electrons with the localised holes results in radiative emission and luminescence. This is the OSL signal, the intensity of which is proportional to the dose of absorbed radiation. OSL signals are often accompanied by photoconductivity phenomena. OSL is not to be confused with the related phenomenon of photoluminescence (PL) that can be stimulated from similar materials, but which is generally not dependent upon irradiation of the sample. PL is the excitation, via the absorption of light, of an electron in a crystal defect within the material, resulting in excitation of the electron from the defect' s ground state to an excited state. Relaxation back to the ground state results in the emission of luminescence, the intensity of which is proportional to the concentration of excited defects. Ionisation of the electron from the defect (i.e., a transition from a localised to a de-localised state) does not generally occur, however, and there is no associated photoconductivity. As a consequence of the above mechanism, the wavelength of the emitted luminescence is longer than that of the excitation light (Stokes' shift). Exceptions to that latter rule (the so-called "anti-Stokes" phosphors) may be found in which energy transfer mechanisms dominate. If the defect being excited is itself created by irradiation of the sample, a PL signal that is dependent on absorbed dose may be obtained. This is termed radiophotoluminescence (RPL) and the RPL signal may be utilised in dosimetry, but the mechanism is PL, not OSL. OSL is one of a class of measurements known as stimulated phenomena. Such phenomena may be stimulated thermally (thermally stimulated phenomena or TSP) or optically (optically stimulated phenomena or OSP). TSP include thermoluminescence (TL), thermally stimulated conductivity (TSC), thermally stimulated exo-electron emission (TSEE), thermally stimulated capacitance (TSCap), deep level transient spectroscopy (DLTS), thermogravimetry (TG), differential thermal analysis (DTA)
Optically Stimulated Luminescence Dosimetry
Fig. 1.1. Schematic representation of several popular thermally and optically stimulated phenomena. Capacitance techniques (DLTS and TSCap) measure signals proportional to the concentration of charges when they reside in the traps. Conductivity techniques (TSC and PC) monitor the charges after release from the traps as they transit through the conduction band. Luminescence techniques (TL and OSL) monitor the charges as they undergo radiative recombination with charge of the opposite sign. Exo-electron processes (TSEE and OSEE) monitor the charges if they are emitted from the surface of the material. Although not the same type of stimulated phenomenon, PL is also indicated.
and others. Likewise, OSP include OSL, photoconductivity (PC) and optically stimulated exo-electron emission (OSEE). The relationship between these different phenomena is illustrated in Fig. 1.1 using a schematic energy band diagram. The reader is referred to works by Br/iunlich (1979), Chen and Kirsh (1981) and Chen and McKeever (1997) for general texts on TSPs and related phenomena. See McKeever (1998, 2001) for reviews of OSL and its use in dosimetry.
1.2. Historical development of OSL dosimetry In recent years, OSL has become a popular procedure for the determination of environmental radiation doses absorbed by archaeological and geological materials in an attempt to date those materials. In this procedure, the target samples (usually natural grains of quartz and/or feldspar) are exposed in the laboratory to a steady source of light of appropriate wavelength and intensity, and the luminescence stimulated from the mineral during this procedure is monitored as a function of the stimulation time. The integral of the luminescence emitted during the stimulation period is a measure of the dose of radiation absorbed by the mineral since it was last exposed to light. Through calibration of the signals against known doses of radiation, the absorbed dose can be obtained and through
Introduction
3
a separate determination of the environmental dose rate, the age of the sample can be determined. Huntley et al. (1985) first used the method, now known as "continuouswave-OSL" (CW-OSL), for this purpose and the latest developments in this field have been described in the triennial conferences on luminescence and ESR dating (Fain et al., 1991; Bailiff et al., 1994; McKeever, 1997, 2000). The first OSL measurements on quartz and feldspar were made using an argon ion laser (Huntley et al., 1985). However, the development of cheaper stimulation systems based, first on filtered lamps, and then on light emitting diodes (LEDs), have led to a massive expansion in dating applications. Feldspars, particularly sand-sized potassium-rich feldspars that could be isolated using heavy liquids, were the first to be investigated. Htitt et al. (1988) showed that luminescence signals could be stimulated from feldspars using near infra-red wavelengths around 880 nm, where a resonance in the stimulation spectrum had been observed. This led to the measurement of infra-red stimulated luminescence (IRSL) using clusters of inexpensive diodes (Spooner et al., 1990). Green light from filtered halogen lamps was used for quartz (BCtter-Jensen and Duller, 1992) until sufficiently powerful blue (470 nm) LEDs became available (BCtter-Jensen et al., 1999b). Since diodes can be used to give short stimulation pulses, and have far longer working lives than the lamps, it was possible to construct laboratory procedures to determine the equivalent dose (De) for single aliquots of sample. Duller (1991) developed an additive dose method for feldspars and this has been widely adopted. A similar procedure was developed for quartz using the filtered lamp system (Murray et al., 1997). More recently, following a five-year study of the OSL properties of quartz, Murray and Wintle (2000) developed the single aliquot regenerative dose (SAR) protocol that has been used in both dating and accident dosimetry. In this method, the sensitivity of all OSL measurements used to obtain De is monitored by the OSL response to a test dose. For sedimentary quartz, the method has been shown to be reliable by the accurate dating of 50 samples, for which there is independent age information (Murray and Olley, 2002). The SAR protocol has now been used for single quartz grains (Duller et al., 2000) when stimulated using a focussed solid-state laser as the stimulation source (Duller et al., 1999). This has opened up a whole new level of investigation for sedimentary deposits (Duller and Murray, 2000). The use of OSL as a personal dosimetry technique, however, is not yet so widespread, despite the fact that its use in this field has a much longer genesis. It was first suggested for this application several decades ago by Antonov-Romanovskii et al. (1956) and was later used by Br~iunlich et al. (1967) and Sanborn and Beard (1967). Since these early developments, however, the use of OSL in radiation dosimetry has not been extensively reported, perhaps due to the lack of a good luminescent material, which was both highly sensitive to radiation, and had a high optical stimulation efficiency, a low effective atomic number and good fading characteristics (i.e., a stable luminescence signal at room temperature). MgS, CaS, SrS and SrSe doped with different rare earth elements such as Ce, Sm and Eu were among the first phosphors suggested for OSL dosimetry applications (Br~iunlich et al., 1967; Sanborn and Beard, 1967; Rao et al., 1984). They possess a high sensitivity to radiation and a high efficiency under infra-red stimulation at a wavelength around 1 I~m, but they suffer from significant fading of the luminescence at room temperature. These phosphors also have a very high effective atomic number and, as a
Optically Stimulated Luminescence Dosimetry
result, exhibit strong photon energy dependence, which is unacceptable for use in personal dosimetry. Several research groups have tried to use optical stimulation as a dosimetric tool by using light to transfer trapped charge carriers from deep traps to shallow traps and then monitoring the phosphorescence at room temperature as the charge leaked away from the shallow traps. This approach was suggested for fast neutron dosimetry for which one can mix the phosphor with polyethylene to measure the absorbed dose from recoil protons and perform the luminescence measurements at room temperature. Several phosphors such as BeO (Tochilin et al., 1969; Rhyner and Miller, 1970), CaF2:Mn (Bernhardt and Herforth, 1974) and CaSO4:Dy (Pradhan and Ayyanger, 1977; Pradhan and Bhatt, 1981) were used in this mode but they each exhibited relatively low sensitivity. This OSL readout mode is often described as "Delayed" OSL (DOSL) (Yoder and Salasky, 1997). A new modification, called pulsed OSL (POSL), was introduced by McKeever, Akselrod and colleagues (Markey et al., 1995; McKeever et al., 1996; Akselrod and McKeever, 1999) using crystalline A1203:C as the luminescent material. Here, one exposes irradiated A1203:C to a pulsed light source and synchronously detects the emitted luminescence between pulses, but not during the pulse. This synchronous arrangement allows one to use less optical filtration than with CW-OSL, which is used in the latter method to discriminate between the stimulation light and the luminescence. At the same time, the POSL method allows one to bias against the slow phosphorescence processes, which make up the main signal in DOSL measurements. These features grant the POSL technique both a high sensitivity and weaker temperature dependence compared with the DOSL method. The high sensitivity and rapid readout features also allow use of the method for imaging the distribution of the dose over large area detectors (Akselrod et al., 2000). Some authors use the fact that irradiation of the detector material induces stable radiation-induced defects, and subsequent illumination of the sample with light stimulates PL from those defects. The emission is termed "radiophotoluminescence" (RPL), and the intensity is proportional to the absorbed dose. This approach is significantly different from the other OSL methods as the stimulation with light does not result in the ionisation of the defect, but only in its excitation. Thus, the dose can be read multiple times without destroying the signal. Disadvantages of this approach are that the signal cannot be reduced to zero by this procedure, and the sensitivity of the technique is relatively low because it requires a high concentration of radiation-induced defects (i.e., a high level of absorbed dose). Examples of this method are given for alkali halides (Regulla, 1972; Miller and Endres, 1990) and phosphate glasses (Piesch et al., 1990, 1993). It is clear from the above that unlike TL, OSL is blessed with several experimental approaches in which the luminescence can be stimulated. Several of these have been mentioned already, and among the more popular are: (a) the "continuous-wave OSL" (CW-OSL) method in which the stimulation light intensity is kept constant and the OSL signal monitored continuously throughout the stimulation period, (b) the so-called "linearmodulation OSL" (LM-OSL) method in which the stimulation intensity is ramped linearly while the OSL is measured, and (c) the POSL method in which the stimulation source is pulsed and the OSL is monitored only between pulses. Each of these methods is described in depth throughout the pages of this book, especially in Chapter 2. For the present,
Introduction
3
however, we illustrate in Fig. 1.2 each of these three popular methods with experimental examples corresponding to the different stimulation modes (shown in the insets).
1.3. OSL dosimetry Aside from the different readout methods available, OSL techniques have advantages over conventional TL techniques for a number of other reasons. The most obvious advantage lies in the fact that the readout method is all optical, requiring no heating of the samples (although some additional advantages may be gained by performing the optical stimulation at slightly elevated temperatures, as will be discussed in later chapters). Apart from removing the need to provide a reliable and reproducible heating scheme, this also means that problems due to thermal quenching of the luminescence efficiency are removed. Thermal quenching is a reduction in the efficiency of luminescence as the temperature of the sample increases due to the opening up of competing, non-radiative relaxation pathways (see Chapter 2). The phenomenon has been described for two important OSL materialsmnamely quartz (Wintle, 1975) and A1203:C (Akselrod et al., 1998). Adoption of A1203 as a TL material for personal or environmental dosimetry has been handicapped by a heating-rate dependence of the TL sensitivity caused by thermal quenching of the luminescence efficiency. As the heating rate increased, so the TL peak shifted to higher temperatures for which reduced luminescence efficiency was noted. The effect is not seen at lower heating rates, and thus a heating-rate dependence for the TL sensitivity is observed (Kitis et al., 1994; Kortov et al., 1994; Akselrod et al., 1998). However, by using optical stimulation, the readout of the luminescence can be performed at temperatures lower than those for which thermal quenching occurs, and thus a significant increase in sensitivity is achieved. Quartz is an important material for retrospective dosimetry. Several of its luminescence emission bands, including those active in OSL, are known to undergo luminescence quenching (see Chapter 5; Wintle, 1975). Examination of the temperature dependence of TL, PL and OSL from this material (Spooner, 1994; McKeever et al., 1997; Bailiff, 2000; Wintle and Murray, 2000) shows that thermal quenching of the luminescence takes place at elevated temperatures such that increased OSL sensitivity is obtained if measured at lower temperatures. The all-optical nature of the OSL readout process also allows the use of "plastic" dosimetersmnamely, luminescence phosphors impregnated into a plastic matrix (e.g., polytetrafluoroethylene, PTFE). In this way, robust dosimeters may be manufactured and advantages may be gained for neutron dosimetry through the interaction of neutrons with hydrogen atoms producing knock-on protons, which then yield luminescence from the phosphor through ionisation processes (Pradhan and Bhatt, 1981). The high sensitivity of OSL also leads to advantages related to multiple readings since it is sometimes not necessary to stimulate all of the trapped charge in order to read a sufficient luminescence signal. In this way, a residual remains that can be stimulated at a later time if second, or third, etc., readout of the signal is necessary for dose verification purposes. Finally, the readout process can be made very fast by adjusting the stimulating light intensity (power) leading to advantages associated with the rapid analysis of large numbers of dosimeters.
400
Optically Stimulated Luminescence Dosimetry
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Fig. 1.2. Experimental data illustrating three examples of OSL readout method: (a) CW-OSL, (b) LM-OSL, and (c) POSL. In (a) the sample was A1203:C (Luxel TM)irradiated with 0.3 Gy of beta rays (9~176 The CW-OSL was measured in a Rise TL/OSL-DA-15 reader using Hoya U-340 filters (7.5 mm) to discriminate between the green (525 nm) stimulation light and the OSL emission. The stimulation power used was --- 10 mW/cm 2. In (b) the sample was A1203:C (TLD-500) irradiated with 0.17 Gy of beta rays (9~176 The LM-OSL was measured under the same conditions as in (a), but the stimulation power was ramped from 0 to --- 10 mW/cm 2 in 1800 s. In (c) the sample was again A1203:C (Luxel TM) irradiated with heavy charged particles (100mGy of Fe, 500 MeV/u). The POSL was measured during 1s of stimulation using 300 ns pulsed stimulation light from the second harmonic (532 nm) of a Nd:YAG laser operating at a frequency of 4 kHz. The luminescence was detected between the pulses using a gated photon counting system. A Nd:YAG 532 nm laser line filter and Kopp 5-58 filters were placed between the sample and the PMT. Each of these three measurement methods are described in detail in Chapter 2.
Introduction
7
OSL has found use in several dosimetry applications, including personal, environmental, medical and retrospective (dating, accident) dosimetry. The distinction between these groups is, in some sense, arbitrary. For example, retrospective dosimetry can include evaluation of radiation exposure to natural materials for purposes of luminescence dating. This application could also be described as an example of environmental dosimetry in that the materials used are natural materials from the environment and the radiation is natural environmental radiation. In another example of retrospective dosimetry, however, one is concerned with the doses absorbed by locally available materials during radiation accidents for the purposes of estimating the doses received by people during the exposure event. This could be described as an example of personal dosimetry. For purposes of this book, however, we draw a distinction between these categories by describing personal dosimetry and environmental dosimetry to be those applications that use synthetic dosimeters (i.e., OSLDs) to measure the dose to either people, or to the environment. Retrospective dosimetry, on the other hand, is the dosimetry of natural or locally available materials and not the dosimetry of synthetically engineered OSLDs. This distinction is of value since the OSL procedures that one adopts are very much dictated by the type of material being used. We describe medical dosimetry separately from personal dosimetry, however, since although one uses synthetic OSLDs to obtain doses to people, the people concerned (patients) are intentionally irradiated, sometimes to high doses, with specific forms of radiation and at specific locations in the body. Such circumstances dictate the use of non-conventional OSL procedures and methods. The overall purposes of these various dosimetry applications are described below.
1.3.1. Personal dosimetry Personal dosimetry is concerned with the evaluation of deep dose, shallow dose and eye doses (quantifies Hp(10), Hp(0.07) and Hp(0.3)), respectively. Hp(10) is the dose equivalent absorbed by human tissue at a depth equivalent to 1000 mg/cm 2 (or about 1.0 cm deep below the skin surface). The interest here is in highly penetrating radiation such as gamma rays, high-energy beta particles, X-rays (> 15 keV) and neutrons. Hp(0.07) is the shallow dose equivalent absorbed by the skin at a depth of 5 10 mg/cm 2. Here the interest is in non-penetrating radiation (low-energy beta particles), X-rays (< 15 keV). Hp(0.3) is the dose to the lens of the eye at a tissue depth of 3 mm. The primary quantity of interest (NCRP, 1993, 1995) is the dose equivalent to a point in tissue (H, in Sv), related to the absorbed dose (D, in Gy) by the quality factor Q (ICRU, 1991) thus H = QD. Alternatively, one can consider the equivalent dose HT absorbed by tissue or organ T, related to the average absorbed dose by that tissue DT by the radiation weighting factor w, thus HT -- WDT. The whole body total effective dose E is then E = S'T WTHT, where WT is the tissue weighting factor and the sum is over all organ tissues. A major requirement of OSLDs in these applications is that they are approximately tissue equivalent. Thus, materials with effective atomic numbers (Ze~) near that of human tissue (Zeff = 7.6) are desired. The dose equivalent range of interest is from approximately 10 p~Sv to 1 Sv, with a required uncertainty better than approximately 10%. The expansion of the US, Asian and European space exploration programs is leading to increased exposure of people (astronauts) to space radiation. The sources of exposure for
Optically Stimulated Luminescence Dosimetry astronauts (and for electronic components) are from galactic cosmic rays (high-energy protons and heavy charged particles), solar particles (medium- to high-energy protons) and (for low-Earth orbit) trapped radiation belts (medium-energy protons and electrons). Thus, the radiation environment external to a spacecraft in low-Earth orbit consists of electrons, positrons, neutrons, protons and stable atomic nuclei up to charge Z - - 9 2 (Fig. 1.3). Energies range from a few eV for trapped electrons to 1014 MeV for galactic cosmic ray ions. Absorbed doses vary with activity (e.g., extravehicular activity, EVA) and location within the spacecraft. Typical dose rates in a space vehicle in low-Earth orbit are --~0.8 mSv per day, with Shuttle flights lasting 1 0 - 1 2 d a y s and sojourns in the International Space Station lasting several months (Benton and Benton, 2001). Because of the mixed radiation field and the dominance of high-energy, high-LET particles, the dose quantities of interest are the gray-equivalent (Gy-Eq) for short-term
Fig. 1.3. The integral LET flux spectra measured by a tissue equivalent proportional counter (JSC-TEPC) and plastic nuclear track detectors (CR-39, University of San Francisco) on the STS-57 mission at an inclination of 28.5~ and an altitude of 462 km, in June 1973 (from Benton and Benton, 2001).
Introduction
9
deterministic effects, where Gy-Eq = (RBE)DT. DT is the mean absorbed dose (in Gy) in an organ or tissue and RBE is the radiobiological effectiveness for a given radiation type. For long-term stochastic effects, the quantity of interest is the effective dose (E, in Sv), where E = ~T WTHT (NCRP, 2000). 1.3.2. Environmental dosimetry
Tissue equivalence is not an issue with dose estimation to the environment, for which the only quantity of interest is the absorbed dose D (in Gy). The primary interest in this field is the impact of "man-made" radiation on the general public. Sources of such manmade radiations include nuclear waste disposal, emissions from nuclear power and reprocessing plants, and the nuclear weapons industry. Political, community, health and environmental watchdog pressure has led to the continuous monitoring of such radiation "pollution", primarily using TL dosimeters (TLDs). This monitoring is deemed of importance despite the fact that the average whole body burden to the average population from "man-made" environmental sources is <0.1% of the total environmental dose (which comes primarily from natural sources). Major requirements of dosimeters for these applications include high sensitivity (to enable short- or long-term monitoring), and stability with respect to adverse weather and changing light levels and temperatures. With natural dose rates of a few mGy (or mSv) per year (Fig. 1.4), the dose range of interest 770
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11
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Fig. 1.4. Fluctuations in the natural environmental dose equivalent rates measured 1 m above the surface level
over a 14 week period using a Bitt RS02 proportional counter in meadow land near Delft (The Netherlands). The sharp peaks correspond to periods with rainfall and the horizontal lines correspond to averages over the weeks indicated (from Bos et al., 1999).
10
Optically Stimulated Luminescence Dosimetry
depends on exposure time, but a minimum measurable dose (MMD) of approximately 1 lxGy is desirable for short-term measurements. OSL techniques and materials hold significant promise for these applications due to the high sensitivity and ease of use (e.g., BCtter-Jensen and Thompson, 1995). In addition to exposures to astronauts, the expanded space exploration program and the increased use of satellites in low-Earth orbit lead to a consideration of the doses absorbed by electronic components from space radiation. The reduction in scale of modem electronic circuitry, down to active areas of a few txm2, means that device failure due to single particle absorptions (single event upsets) is of serious concern. Both TLDs and OSLDs have potential applications in the dosimetry of such effects, and the quantity of interest is the dose to silicon (expressed in Gy(Si)).
1.3.3. Medical dosimetry Although concerned with radiation exposure of people, medical dosimetry has several special features not found in conventional personal dosimetry. The most obvious is the need for very small dosimeters capable of being used on or in (in vivo) the patient during medical radiation treatment and diagnosis. For example, there is a need for small, unobtrusive radiation dosimeters capable of real-time, or near-real-time, readout when placed in vivo during radiotherapy, including external radiation treatment (teletherapy), internal brachytherapy and the use of radiopharmaceuticals. Additional medical applications of radiation for which internal dosimetry would be highly advantageous include restinosis using, for example, miniature X-ray tubes implanted in a blood vessel. All such applications require dose monitoring to assist in effective treatment. TLDs are popular dosimeters in many hospitals for external dosimetry during these treatments. In these applications, however, the TLD can only provide an integral reading of the total surface exposure to the patient after treatment. OSL has the potential for the development of near-real-time dosimetry in which the measured quantity can be either dose (Sv) or dose rate (Sv/s). Typical doses of interest can be up to 20 Sv (Huston et al., 2001).
1.3.4. Retrospective dosimetry Retrospective dosimetry is the term used when determining the dose of absorbed radiation to environmental or locally available materials in situations where conventional, synthetic dosimeters were not in place at the time of radiation exposure. Two major categories exist, namely dating and accident dosimetry. Dating of geological or archaeological artefacts using OSL relies upon the determination of the dose absorbed by natural minerals, such as quartz or feldspar, during the period that the artefact is in its last (present) location (Aitken, 1998). The dose absorbed during this period is due to absorption of radiation from the natural background and is proportional to the age of the artefact in that location. A "zeroing mechanism" is required; this is a mechanism whereby the OSL signal due to the previous exposure of the mineral to radiation before it was in its last location is removed, or zeroed, at the time of last deposition. For example, for a windblown deposit (e.g., a sand dune) all previous radiation exposure history is erased while the sediment grains are being transported in the air where they are exposed to natural sunlight. Absorption of energy from the sunlight leads to the de-trapping of all trapped charges and
Introduction
11
zeroing of the OSL signal. Burial of the sample in the sand dune shields the grains from further exposure to sunlight and thus the OSL signal is regenerated by exposure to the environmental radiation field. When the sediment grains are now excavated, the subsequent OSL signal is proportional to the time of burial. The proportionality constant is the natural environmental dose rate to which the sediment has been subjected during burial. Modern techniques (Wintle, 1997) use small, single aliquots--even single grainsm and result in large numbers of independent dose determinations. The quantity of interest is the equivalent dose, D e (in Gy). This is the beta dose (used for calibration in the laboratory) that yields the equivalent OSL signal to the natural environmental dose, which is derived from a mixed radiation field made up of alpha, beta, gamma and cosmic radiation. The other category is accident dosimetry, where one is interested in the determination of absorbed dose due to a radiation accident or other event, over and above the normal background radiation. Examples include the determination of absorbed doses during events such as nuclear weapons explosions, nuclear reactor accidents or other incidences of unintended radiation release. Since synthetic dosimeters were not in place at the time, one has to rely upon the determination of absorbed doses in locally available materials, such as bricks (in particular the quartz grains within the bricks), or porcelain (light fixtures, bathroom fixtures, etc.). Again, the quantity of interest is the equivalent dose, D e. OSL is proving to be a useful tool since most of the suitable local materials display high OSL intensity (Bailiff, 1997; Banerjee et al., 1999; BCtter-Jensen et al., 1999a).
1.4. This book Our purpose in this book is to lead the reader through a description of the theoretical background of OSL dosimetry, including the models describing the OSL properties of many common OSL materials, through a description of the applications of the methods and the detailed procedures required in the different applications. The journey starts in Chapter 2 with a detailed treatment of the theory of OSL and a description of several OSL measurement modes. Chapter 3 describes the OSL properties of several important materials that can be used as synthetic OSL dosimeters. Applications in personal and medical dosimetry follow in Chapter 4. Chapter 5 describes the OSL properties of quartz and feldspars, which are used as natural dosimeters. Chapter 6 is reserved for a description of the detailed procedures for, and applications of, retrospective dosimetry, including dating. The final chapter deals with modern instrumentation for the measurement of OSL in its various forms.
References Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford University Press, Oxford. Akselrod, M.S., McKeever, S.W.S., 1999. A radiation dosimetry method using pulsed optically stimulated luminescence. Radiat. Prot. Dosim. 81,167-176. Akselrod, M.S., Agersnap Larsen, N., Whitley, V., McKeever, S.W.S., 1998. Thermal quenching of F-center luminescence in A1203:C. J. Appl. Phys. 84, 3364-3373.
12
Optically Stimulated Luminescence Dosimetry
Akselrod, M.S., Agersnap Larsen, N., McKeever, S.W.S., 2000. A procedure for the distinction between static and dynamic radiation exposures of personal radiation badges using pulsed optically stimulated luminescence. Radiat. Meas. 32, 215-225. Antonov-Romanovskii, V.V., Keirum-Marcus, I.F., Poroshina, M.S., Trapeznikova, Z.A., 1956. Conference of the Academy of Sciences of the USSR on the Peaceful Uses of Atomic Energy, Moscow, 1955, USAEC Report AEC-tr-2435 (Pt. 1), 239. Bailiff, I.K., 1997. Retrospective dosimetry with ceramics. Radiat. Meas. 27, 932-941. Bailiff, I.K., 2000. Characteristics of time-resolved luminescence in quartz. Radiat. Meas. 32, 401-405. Bailiff, I.K., Fad'n, J., Prescott, J.R., Townsend, P.D., Vana, N., Visocekas, R. (Eds.), 1994. Proceedings of the 7th International Specialist Seminar on Thermoluminescence and ESR Dating, Krems (1993). Radiat. Meas. 23, 267-654. Banerjee, D., BCtter-Jensen, L., Murray, A.S., 1999. Retrospective dosimetry: preliminary use of the single aliquot regeneration (SAR) protocol for the measurement of quartz dose in young house bricks. Radiat. Prot. Dosim. 84, 421-426. Benton, E.R., Benton, E.V., 2001. Space radiation dosimetry in low-Earth orbit and beyond. Nucl. Instr. Meth. B. 184, 255- 294. Bernhardt, R., Herforth, L., 1974. Radiation dosimetry by optically stimulated phosphorescence of CaF2:Mn. Niewadomski, T. (Ed.), Proceedings of the Fourth International Conference on Luminescence Dosimetry, Krakow, Poland, 1091-1104. Bos, A.J.J., Winkelman, A.J.M., Moelker, C.W., Okx, W.J.C., 1999. SEAD: A TLD system for the determination of man-made photon doses in a fluctuating natural background. Radiat. Prot. Dosim. 85, 227-232. BCtter-Jensen, L., Duller, G.A.T., 1992. A new system for measuring optically stimulated luminescence from quartz samples. Nucl. Tracks Radiat. Meas. 20, 549-553. BCtter-Jensen, L., Thompson, I.M.G., 1995. An international comparison of passive dosemeters, electronic dosemeters and dose rate meters used for environmental radiation measurements. Radiat. Prot. Dosim. 60, 201-211. BCtter-Jensen, L., Banerjee, D., Jungner, H., Murray, A.S., 1999a. Retrospective assessment of environmental dose rates using optically stimulated luminescence from A1203:C and quartz. Radiat. Prot. Dosim. 84, 537-542. BCtter-Jensen, L., Duller, G.A.T., Murray, A.S., Banerjee, D., 1999b. Blue light emitting diodes for optical stimulation of quartz in retrospective dosimetry and dating. Radiat. Prot. Dosim. 84, 335-340. Br~iunlich, P. (Ed.), 1979. Thermally Stimulated Relaxation in Solids. In: Topics in Advanced Physics, 31. Springer, Berlin. Br~iunlich, P., Schafer, D., Scharmann, A., 1967. A simple model for thermoluminescence and thermally stimulated conductivity of inorganic photoconducting phosphors and experiments pertaining to infra-red stimulated luminescence. Proceedings of the First International Conference on Luminescence Dosimetry, Stanford, June 1965, USAEC, 57-73. Chen, R., Kirsh, Y., 1981. Analysis of Thermally Stimulated Processes. Pergamon Press, Oxford. Chen, R., McKeever, S.W.S., 1997. Theory of Thermoluminescence and Related Phenomena. World Scientific Publishing, Singapore. Duller, G.A.T., 1991. Equivalent dose determination using single aliquots. Nucl. Tracks Radiat. Meas. 18, 371-378. Duller, G.A.T., Murray, A.S., 2000. Luminescence dating of sediments using individual mineral grains. Geologos 5, 88-106. Duller, G.A.T., BCtter-Jensen, L., Kohsiek, P., Murray, A.S., 1999. A high-sensitivity optically stimulated luminescence scanning system for measurement of single sand-sized grains. Radiat. Prot. Dosim. 84, 325-330. Duller, G.A.T., BCtter-Jensen, L., Murray, A.S., 2000. Optical dating of single sand-sized grains of quartz: sources of variability. Radiat. Meas. 32, 453-457. Fain, J., Miallier, D., Aitken, M.J., Bailiff, I.K., Grtin, R., Mangini, A., Mejdahl, V., Rendell, H.M., Townsend, P.D., Valladas, G., Visocekas, R., Wintle, A.G. (Eds.), 1991. Proceedings of the Sixth International Specialist Seminar on Thermoluminescence and ESR Dating, Clermont-Ferrand (1990). Nucl. Tracks Radiat. Meas. 18, 5-282. Huntley, D.J., Godfrey-Smith, D.I., Thewalt, M.L.W., 1985. Optical dating of sediments. Nature 313, 105-107.
Introduction
13
Huston, A.L., Justus, B.L., Falkenstein, P.L., Miller, R.W., Ning, H., Altemus, R., 2001. Remote optical fiber dosimetry. Nucl. Instr. Meth. B 184, 55-67. Hiitt, G., Jaek, I., Tchonka, J., 1988. Optical dating: K-feldspars optical response stimulation spectra. Quat. Sci. Rev. 7, 381- 385. ICRU, 1991. 1990 Recommendations of the International Commission on Radiological Protection, International Commission on Radiological Protection, Report 60. Kitis, G., Papadopoulos, S., Charalambous, S., Tuyn, J.W.N., 1994. The influence of heating rate on the response and trapping parameters of oL-A1203:C. Radiat. Prot. Dosim. 55, 183-190. Kortov, V.S., Milman, I.I., Kirpa, V.I., Lesz, J., 1994. Some features of oL-A1203:Cdosimetric thermoluminescent crystals. Radiat. Prot. Dosim. 55, 279-283. Markey, B.G., Colyott, L.E., McKeever, S.W.S., 1995. Time-resolved optically stimulated luminescence from aA1203:C. Radiat. Meas. 24, 457-463. McKeever, S.W.S (Ed.), 1997. Proceedings of the Eighth International Conference on Luminescence and ESR Dating Conference, Canberra (1996). Radiat. Meas. 27, 75-443. McKeever, S.W.S., 1998. Optically stimulated luminescence dosimetry. SPIE 3534, 531-541. McKeever, S.W.S. (Ed.), 2000. Proceedings of the Ninth International Conference on Luminescence and ESR Dating Conference, Rome (1999). Radiat. Meas. 32, 387-880. McKeever, S.W.S., 2001. Optically stimulated luminescence dosimetry. Nucl. Instr. Meth. B 184, 29-54. McKeever, S.W.S., Akselrod, M.S., Markey, B.G., 1996. Pulsed optically stimulated luminescence dosimetry using a-A1203:C. Radiat. Prot. Dosim. 65, 267-272. McKeever, S.W.S., BCtter-Jensen, L., Agersnap Larsen, N., Duller, G.A.T., 1997. Temperature dependence of OSL decay curves: experimental and theoretical aspects. Radiat. Meas. 27, 161-170. Miller, S.D., Endres, G.W.R., 1990. Laser-induced, optically stimulated M-centre luminescence in LiF. Radiat. Prot. Dosim. 33, 59-62. Murray, A.S., Olley, J.M., 2002. Precision and accuracy in the optically stimulated luminescence dating of sedimentary quartz: a status review. Geochronometria 21, 1-16. Murray, A.S., Roberts, R.G., Wintle, A.G., 1997. Equivalent dose measurement using a single aliquot of quartz. Radiat. Meas. 27, 171-184. Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved single-aliquot regenerativedose protocol. Radiat. Meas. 32, 57-73. NCRP, 1993. Limitation of Exposure to Ionizing Radiation, National Council for Radiation Protection and Measurement, Report 116. NCRP, 1995. Use of Personal Monitors to Estimate Effective Dose Equivalent and Effective Dose to Workers for External Exposure to Low-LET Radiation, National Council for Radiation Protection and Measurement, Report 122. NCRP, 2000. Radiation Protection Guidance for Activities in Low-Earth Orbit, National Council on Radiation Protection and Measurements, Report 132. Piesch, E., Burgkhardt, B., Vilgis, M., 1990. Photoluminescence Dosimetry: Progress and present state of the art. Radiat. Prot. Dosim. 33, 215-226. Piesch, E., Burgkhardt, B., Vilgis, M., 1993. Progress in phosphate glass dosimetry: Experiences and monitoring with a modern dosimetry system. Radiat. Prot. Dosim. 47, 409-413. Pradhan, A.S., Ayyanger, K., 1977. Radiation dosimetry by photostimulated luminescence of CaSO4:Dy. Int. J. Appl. Radiat. Isotopes 28, 534-535. Pradhan, A.S., Bhatt, R.C., 1981. Photostimulated luminescence and thermoluminescence in CaSO4:Dy. Phys. Stat. Sol. (a) 68, 405-411. Rao, R.P., de Murcia, M., Gasiot, J., 1984. Optically stimulated luminescence dosimetry. Radiat. Prot. Dosim. 6, 64-66. Regulla, D.F., 1972. Lithium fluoride dosimetry based on radiophotoluminescence. Health Phys. 22, 419-421. Rhyner, C.R., Miller, W.G., 1970. Radiation dosimetry by optically stimulated luminescence in BeO. Health Phys. 18, 681-684. Sanborn, E.N., Beard, E.L., 1967. Sulfides of strontium, calcium, and magnesium in infra-red stimulated luminescence dosimetry. Proceedings of the First International Conference on Luminescence Dosimetry, Stanford, June. 1965, USAEC, 183-191. Spooner, N.A., 1994. On the optical dating signal from quartz. Radiat. Meas. 23, 593-600.
14
Optically Stimulated Luminescence Dosimetry
Spooner, N.A., Aitken, M.J., Smith, B.W., Franks, M., McElroy, C., 1990. Archaeological dating by infrared stimulated luminescence using a diode array. Radiat. Prot. Dosim. 34, 83-86. Tochilin, E., Goldstein, N., Miller, W.G., 1969. Beryllium oxide as a thermoluminescent dosimeter. Health Phys. 16, 1-7. Wintle, A.G., 1975. Thermal quenching of thermoluminescence in quartz. Geophys. J. Roy. Astr. Soc. 41, 107-113. Wintle, A.G. (Ed.), 1997. Luminescence and ESR Dating and Allied Research. Radiat. Meas. 27, 625-1025. Wintle, A.G., Murray, A.S., 2000. Quartz OSL: Effects of thermal treatment and their relevance to laboratory dating procedures. Radiat. Meas. 32, 387-400. Yoder, R.C., Salasky, M.R., 1997. A dosimetry system based on delayed optically stimulated luminescence. Health Phys. 72, S18-S19.
Chapter 2
Optically stimulated luminescence theory 2.1. Stimulated luminescence
The absorption of energy from an ionising radiation source by an insulating or semiconducting material causes the excitation of free electrons and free holes and the subsequent trapping of these electronic species at defects (trapping states) within the material. After removal of the excitation, the sample may then be stimulated in such a way that the absorbed energy causes the liberation of charge carriers of one sign, which are then able to recombine with charge carriers of the opposite sign. The radiation absorption and the excitation of charge (primarily by the Compton effect or the photoelectron effect, depending on the radiation energy and type) lead to a perturbation of the system from a state of thermodynamic equilibrium to a metastable state. The subsequent absorption of external energy by the metastable trapped charge results in the stimulated relaxation of the system back to its equilibrium condition. During the relaxation process, recombination of the electronic charge occurs and, if the recombination is radiative, luminescence is emitted. In optically stimulated luminescence (OSL) the stimulating energy source is light (UV, visible or infra-red). This general description of OSL places the phenomenon within the family of stimulated relaxation phenomena (SRP). For the specific case of OSL, the intensity of the emitted luminescence is related to the r a t e at which the system returns to equilibrium. The rate at which the equilibrium is re-established is a function of the concentration of trapped (metastable) charge and in the simplest (first-order) case the rate is linearly proportional to the trapped charge concentration. Normally, one monitors the intensity of the luminescence as a function of time, resulting in a characteristic luminescence-versus-time curve. The integral of the luminescence-versus-time curve is thus related to the trapped charge concentration, which, in turn, is proportional (in the ideal case) to the initial dose of the absorbed radiation. This is the basis for the use of OSL in radiation dosimetry. In other SRP, the form of perturbation may differ along with the property being monitored during the stimulation. For example, in the closely related technique of thermoluminescence (TL), the luminescence is stimulated thermally by warming the sample at a prescribed rate after radiation absorption. In thermally stimulated conductivity (TSC) or photoconductivity (PC), ionising radiation may still be used as the excitation source, but one detects the thermally (or optically) stimulated relaxation back to equilibrium by monitoring the freed charges during their passage through the delocalised excited state (i.e., conduction band for electrons, or valence band for holes).
Optically Stimulated Luminescence Dosimetry
16
For thermally (or optically) stimulated exoelectron emission (TSEE, or OSEE), one monitors the exoemission of electrons, usually from surface traps, during the relaxation process. Alternatively, for either deep level transient spectroscopy (DLTS) or thermally stimulated capacitance (TSCap), the excitation can either be ionising radiation or electrical energy, and one monitors the change in capacitance across a pn semiconductor junction, or a metal-semiconductor junction, during the thermally stimulated transition of the trapped charge from the traps into the delocalised bands. Each of these processes, and all related processes, can be described in terms of the free energy of the system and the perturbation of the equilibrium Fermi level in the material under study. At 0 K and in thermodynamic equilibrium one can expect from Fermi-Dirac statistics that all states above the Fermi level EF are empty while all states below EF are filled. The situation is illustrated in Fig. 2.1 in which we see a flatband energy band diagram representing the bottom of the conduction band Ec, the top of the valence band E~ and two normal distributions of energy states N(E) (one for electron and one for hole traps) in the forbidden gap between these two energy levels. Following Br~iunlich (1979) we use "filling diagrams" to represent the extent to which the energy states are filled f(E) during a thermally stimulated relaxation experiment.
e,.
I yA Before Irradiation (Equilibrium)
E~
EFe
.........................
>
i/ 9 B After Irradiation (Metastable)
EF
EV
9C During Optical Stimulation
9 D Back to Equilibrium
Fig. 2.1. A representation (following Br~iunlich, 1979) of the occupancy of forbidden gap states at absolute zero. (A) Before irradiation at equilibrium; EF is the Fermi level (assumed to be approximately mid-way between the top of the valence band Ev and the bottom of the conduction band Ec). All states below the Fermi level are full, as described by the occupancy function f(E) = 1, and all states above the Fermi level are empty (f(E) = 0). (B) After irradiation, some electron states above the Fermi level are now full, up to the quasi-Fermi level for trapped electrons, EFe, while several hole states below the Fermi level are now empty (with a quasi-Fermi level for trapped holes, E~). Normal distributions of forbidden gap states are assumed, one for electrons and one for holes. As optical stimulation of the irradiated sample proceeds (C), filled states are emptied and recombination with empty states occurs. The quasi-Fermi levels move towards the Fermi level, until finally all states return to their original equilibrium occupancies (D).
Optically Stimulated Luminescence Theory
17
Beginning with the left-hand figure (A), we see the Fermi-Dirac filling function at 0 K for which all electron traps (above the Fermi level) are empty and all hole traps (below the Fermi level) are full (represented by the shaded distribution). After perturbation by ionising radiation, we find a new filling function showing electrons trapped at localised states within the band gap above EF and an equal concentration of holes trapped below EF. One can define two quasi-Fermi levels, one each for electrons EFe and for holes E~. These are useful devices for describing the non-equilibrium state, which follows the perturbation in terms of equilibrium statistics by making the assumption that the trapped electron and hole populations are in thermal equilibrium over their available energy levels. During stimulated relaxation, i.e., during illumination of the irradiated sample with UV, visible or IR light, the filling function f ( E ) gradually returns to its preperturbation state. During this process, the quasi-Fermi levels gradually move back towards the equilibrium Fermi level as the trapped charge concentrations decay back to their equilibrium values. In Fig. 2.1 the stimulation is assumed to be optical and all diagrams are depicted at absolute zero temperature.
2.2. Generalised mathematical description of OSL The total concentration of occupied metastable states in the system at time t may be represented by/z(t), where: /x(t)=f
~ 'Yl
...~ '}/2
n(3"~,3"z,...3"n,t)d3"ld3"z...d3" m
(2.1)
]/m
where n(3"l, 3'2,... 3"m, t) is the concentration of occupied states 1 ---, m, described by state parameters 3"1,3"2,...3"m, and in general n( 3", t) = N ( 3")f ( 3", t). Here n(3") is the concentration of occupied states, N(3") the concentration of available states, and f(3") the occupancy of the state. (f = 1 when a state is full a n d f = 0 when it is empty.) Both n(3") and f(3") are time-dependent functions. (Note that in Fig. 2.1, the state parameter being represented is energy, i.e., 3' = E.) The state parameters 3"~, 3"2, ... 3"m dictate the stability of the metastable state under the prevailing conditions of temperature and illumination intensity, that is they govern the probability per unit time that the system will return to equilibrium, n(3"1,3"2, ... 3"m, t) is a weighting function, or distribution, expressing the concentration of occupied states possessing the parameters 3'1, 3'2, . . . 3'm" Eq. (2.1) is a time- and dose-dependent function since it increases during irradiation and decreases during stimulation. In stimulated luminescence measurements we monitor the emitted luminescence intensity during the return of the system to equilibrium. The luminescence intensity I is proportional to the rate at which the metastable states decay, such that: I(t) =
d/x(t) dt
(2.2)
Thus, the stimulated luminescence is time-dependent through the functional dependence of Idtx(t)/dtl upon n(3'1, 3'2,...3'm,t)- In order to evaluate I(t) we need a relationship
Optically Stimulated Luminescence Dosimetry
18
between Idl~(t)/dt] and n(yl, Y2, ... Ym, t). One possibility is an expression of the form: d/x(t) _ _/x t(t)P(t) dt
(2.3)
where P(t) is the probability per unit time of the decay of the metastable states/x(t). With l = 1 this is, of course, a first-order function. If each state n(yl, 72, ...Ym) has its own probability function P(71,72,... Ym) then, with l = 1 :
I(t) = d/x(t)dt -- ~
~ ]/2 " " f "
]/m
n(71, y2," "~/m't)p(71T2, , . ''Tm)dyl d72"" . 9
(2.4)
where we assume the superposition principle~i.e., there is no interaction between states. Eq. (2.4) relates to a fixed, time-independent probability P(71,3/2,-.-Ym)-For a timedependent probability:
l(t) = d/x(t)dt = f v, ~
]/mn(71' T2'"'Ym't)P('Yl' Y2'"'Tm't)d'Yl d72""d'Ym (2.5)
The form ofp depends on the stimulation method, i.e., thermal or optical stimulation of the metastable state back to equilibrium. For thermal stimulation we may write:
p ( f ) -- v g exp - ~-~
(2.6)
where v is the phonon lattice vibration frequency, K a transition probability constant and F the Helmholtz free energy barrier, which must be overcome for decay of the metastable state. The equation may be expanded using F = E - AST to yield:
p ( E , s ) = v K e x p { - ~ t e x p { - ~ - ~E} =
s e x p,,f - ~-~} E
(2.7)
The product v K e x p { A S / k } = s is usually termed the "pre-exponential factor" (dimensions s-l). AS is the entropy change associated with the transition and E is the internal energy barrier. Thus, the metastable states are characterised by m - - 2 parameters, namely Yl = E and Y2 = s. The Boltzmann term e x p { - (E/kT)} defines the probability that the energy delivered by the phonon interaction is sufficient to surmount the barrier of height E. At a fixed temperature T, it describes the isothermal decay of the metastable state back to the ground state. In terms of the energy states illustrated in Fig. 2.1 E represents the "trap depth" and p is thus the probability of thermal ionisation of the metastable electronic state and the return of the Fermi level to its equilibrium level. Our primary interest in this text is with optical stimulation and thus, for optical stimulation we have:
p(Eo)---- ~tr(Eo)
(2.8)
where @ is the optical stimulation intensity and tr (Eo) is the photoionisation cross-section for interaction of the metastable state with an incident photon, and Eo is the threshold optical stimulation energy required for charge release and a return of the system to equilibrium. Here m = 1 and Yl = Eo.
Optically Stimulated Luminescence Theory
19
In the above representations T, A and 4)are all fixed values independent of time. For optical stimulation, when the traps are emptied using a fixed wavelength A and a steady illumination intensity q~ the luminescence recorded is known as continuous wave OSL, or CW-OSL. However, a time dependence to p can be introduced by scanning the above terms with time--i.e., qb(t) or A(O. Thus, for a linear increase in the intensity of optical stimulation at a fixed wavelength:
ci9( t ) = ci9o + fl~t
(2.9)
with/3~ = d@/dt. OSL recorded is this manner is known as linear modulation OSL, or LM-OSL. Other schemes can be imagined in which the intensity is modulated in nonlinear manner ways. For example, one can imagine an exponentially increasing stimulation intensity (q/~(t)= 4)0 exp{t}), which is a scheme that can have advantages when emptying a range of traps with photoionisation cross-sections that differ by orders of magnitude. Alternatively, the stimulation may be pulsed, such that qls(t)-- 4)0 for to -< t < to + At, and qJ(t) = 0 for (to + At) <_ t < (to + ~') where At is the pulse width and ~"is the period. Such a scheme is known as pulsed OSL, or POSL. These various stimulation schemes are illustrated schematically in Fig. 2.2. An alternative method for introducing a time dependence to the trap emptying probability p(Eo, t) is to linearly scan the wavelength of the excitation light, at constant intensity. Thus: A(t) = Ao + fiat
(2.10)
with /3a = - d A / d t (where the negative sign indicates a scan from longer to shorter wavelengths, or smaller to larger photon energies, h v). We fully develop each of the above stimulation schemes in subsequent sections of this chapter. Before doing so, however, we first examine the concept of photoionisation crosssection, o-(Eo).
2.3. The photoionisation cross-section 2.3.1. Optical transitions The absorption of light of incident intensity lo(A) at wavelength A as a function of distance x in a solid is described by the Lambert-Beer Law, namely: I(A,x) = Io(A)exp{ - c~(A)x}
(2.11)
where a(A) is the absorption coefficient and I(A,x) is the intensity at position x. The value of a depends upon the optical absorption process occurring at that wavelength. A schematic view of the various optical absorption transitions possible and that are important for dosimetry is illustrated in Fig. 2.3. Free carrier transitions to higher states are not considered. Band-to-band optical transitions (transition (1)) are not important for wide-band-gap insulators that may be used in dosimetry (e.g., A1203, SiO2), but are important for narrow-band semiconductors (e.g., ZnSe). Exciton formation (transition (2)) can lead to charge localisation and ultimately TL and/or OSL, but such transitions
20
Optically Stimulated Luminescence Dosimetry
Fig. 2.3. Flat-band representation of the possible optical absorption transitions in an insulator: (1) ionisation (excitation across the band gap); (2) exciton formation; (3a, 3b) defect ionisation; (4a, 4b) trap ionisation; (5) internal intra-centre transition.
Optically Stimulated Luminescence Theory
21
generally occur in the vacuum ultraviolet and are not usually important in practical dosimetry. However, all other transitions in Fig. 2.3 can be important. Examples of defect ionisation (transitions of type 3) include ionisation of the F-centre in A1203:C in which absorption of a photon at approximately 6.05 eV induces an electron transition from the 1A ground state to the 2P excited state of the F-centre. Since the 2P state is accessible to the conduction band, ionisation of the electron can occur, leading to trapping and, subsequently, TL and/or OSL (Summers, 1984). Intraband transitions of type 5 are important in certain optically stimulated processes involved in the dosimetry of natural materials. For example, absorption of an IR photon (at approximately 840nm) induces a ground-state-to-excited-state transition in a radiation-induced defect in feldspar minerals. If the temperature is sufficiently high thermal ionisation of the excited state electron can then occur, leading to luminescence. This phenomenon is exploited in the dosimetry of natural radiation absorbed by feldspars in luminescence dating applications (Htitt et al., 1988; Htitt and Jaek, 1993). Transitions of type 4 yield OSL. Such transitions result from initial localisation of charge by traps during irradiation and the optically stimulated release of those trapped charges from the traps through the absorption of light. Subsequent recombination of the charge results in OSL emission. Transitions of this type also give rise to phototransfer effects (e.g., phototransferred TL or OSL) and optical bleaching of TL and/or OSL signals. In each case, the subsequent luminescence signal (OSL, TL, or phototransferred signal) is a function of the initial dose of radiation absorbed, and the intensity, wavelength and duration of the optical stimulation. Photoluminescence (PL) is the luminescence emitted following absorption of light by an internal transition, such as that depicted in transition (5). Since these do not involve transport of charge from one defect site to another such transitions do not affect the subsequent OSL signal, unless the excited state is thermally unstable, as discussed above for feldspars. However, they can yield useful information about the available luminescence sites, for example, the site symmetries of the rare-earth emitters in rare-earth doped alkaline earth fluorides. Furthermore, PL can also be used as a dosimetry method if the defect in which the internal transition occurs is itself a radiation-induced defect. Examples here include radiation-induced PL absorption in glasses (Piesch et al., 1990; 1993) and LiF (Regulla, 1972; Miller et al., 1988). The latter authors use the PL signal from F2-centres formed by the reaction between two radiation-induced F-centres for high-dose dosimetry.
2.3.2. Wavelength dependence The photoionisation cross-section o-(Eo) is perhaps the most important parameter governing transitions of type 4 in Fig. 2.3 and dictating the stability of a particular trap during optical stimulation. The absorption coefficient for a defect-band optical transition at an optical stimulation energy hu may be written as:
r
= n(Eo)a(hu , Eo)
(2.12)
where n(Eo) is the concentration of defects, each with an optical ionisation threshold energy Eo. The shape of the absorption coefficient a(hu) as a function of stimulation
Optically Stimulated Luminescence Dosimetry
22
energy h v should be edge-like since the absorption line shape contains contributions from all the relevant continuum states in the band into which the electron is being excited (Stoneham, 1975). Following the transition, the charge on the defect will change by one electronic charge and significant lattice relaxation may occur. This is also true during the inverse process (Br~iunlich, 1979). Several expressions for the spectral dependence of cr(Eo), namely o-(hv, Eo), have been derived using a variety of assumptions relating to the potential energy in the vicinity of the defect. For a shallow (hydrogenic) electron trap and a plane wave state for the free electron, one can write: ~r (hv, Eo) cc
( h v - Eo)3/2
(by) 5
(2.13)
for photon energy hv (see reviews by Blakemore and Rahimi (1984) and Landsberg (1991)). This falls off as hv -7/2 for hv > Eo, with a maximum at hv = 1.4Eo. However, for such small photon energies in which the free electron is close to the ionised defect one should take into account the coulombic attraction between the freed electron and the ionised defect (Blakemore and Rahimi, 1984). For deep traps the hydrogenic model is inappropriate prompting Lucovsky (1964) to use a delta-function potential for the defect. Such a model leads to the following expression for the spectral dependence of the cross-section:
o. (hv, Eo) CC.[ 4(hv - E~176 ] 3/2 (hv) 2
(2.14)
Here, the cross-section reaches a maximum at h v = 2Eo, with o-increasing as ( h v - E o ) 3/2 for hv < Eo, and decreasing a s (hv) -3/2 for hv > Eo. It is to be noted also that the coulombic field should be accounted for when considering defect excited states. An explicit assumption in Lucovsky's analysis is that the effective mass me of the electron in the conduction band can be used also for the electron in the localised state. Grimmeis and Ledebo (1975a,b) preferred to use the electron rest mass mo for the localised electron, which, when used with a plane-wave final state and the assumption of parabolic bands, leads to: or (hv, Eo) co.
( h v - Eo)3/2 hv [ h v - Eo(1 - mo/me)] 2
(2.15)
Banks et al. (1980) discussed the photoionisation cross-section of deep traps and Lucovksy's assumption of a single band edge and derived the following expression for the cross-section by considering the possibility of nodal properties for the deep trap wavefunction: ( h v - Eo) 1/2 o'(hv, Eo) oc hu(hv +/3)2
(2.16)
Optically Stimulated Luminescence Theory
23
1.2 Eqn. 2.14
Eqn.2.13
"O
~ 90.8 O
=..E. 0.6 c
o (1)
, 0.4
o
II/
0
0.2
l! /
0
I
0
2
I
I
4 6 Photon Energy (eV)
I
8
10
Fig. 2.4. Comparisonof the shapes of various expressions for the photoionisation cross-section as a function of stimulation photon energy, obtained with a threshold energy Eo = 3.0 eV, and mo/m e 2. Each curve is normalised to a maximum of 1. =
with a range of possible values for/3 from/3 = 0 to/3 >> Eo. Other expressions for o- exist (Blakemore and Rahimi, 1984; Ridley, 1988; Landsberg, 1991) following different assumptions for the form of the potential and defect wavefunction. Dosimetric materials are usually wide-band-gap insulators, and stable OSL signals originate from the release of electrons from deep trapping states. Thus far, the expression most frequently used to represent the photoionisation cross-section of such centres has been either Eq. (2.15), as described by Grimmeis and Ledebo (1975a,b), or the Lucovsky (1964) expression (Eq. (2.14)) (e.g., Alexander and McKeever, 1998; Whitley and McKeever, 2000, 2001; Bailey, 2001). The shapes of some of the above expressions for the photoionisation cross-section are shown in Fig. 2.4 for a threshold energy of Eo = 3.0 eV, and mo/m e = 2. For each curve, o- -- 0 for hv --< Eo. The threshold optical ionisation energy Eo is larger than the thermal ionisation energy (thermal trap depth, Et) by an amount equal to the phonon energy, namely: Eph = Shoop
(2.17)
where S is the H u a n g - R h y s factor and Wp is the phonon vibration frequency. 2.3.3. Measurement of the photoionisation cross-section The photoionisation cross-section can be determined experimentally by a number of techniques. Consider a one-trap/one-recombination centre model for a luminescent material and illumination of an irradiated sample containing n trapped electrons at N traps, each with an optical ionisation threshold energy of Eo (see Section 2.3.2). Electrons are stimulated from the traps into the conduction band (viz. transition 4a in Fig. 2.3) from
Optically Stimulated Luminescence Dosimetry
24
where free electrons (concentration nc) may either be re-trapped or recombine with trapped holes to produce OSL. If the illumination flux at wavelength A is q~(A), then under steady-state conditions (i.e., dnc/dt = 0), we have qb (A)o" (A)n = A ( N - n)n c - Amncm
(2.18)
where A is the probability (in m 3 s - l) of capture of free electrons, and Am is the probability (also in m 3 s- l) of recombination of free electrons with trapped holes of concentration m. Under conditions of weak stimulation, n and m remain approximately constant during the stimulation period (i.e., the number of charge carriers removed is much less than the initial number, or An << n). Thus, we have from Eq. (2.18)
o-(a.)--
K ) nc q,(A)
(2.19)
from which we see that the photoionisation cross-section is directly proportional to the free carrier density, nc. Using Eq. (2.19) there are three ways to determine the change in the photoionisation cross-section with incident photon energy. The first, proposed by Grimmeis and Ledebo (1975b), consists of illuminating a sample with light and monitoring the PC, which is proportional to the free carrier concentration, nc. The intensity (flux) of the stimulation light is varied as the wavelength is changed in order to maintain the same PC (i.e., maintain a constant nc at all wavelengths A). In this way, the wavelength dependence of the photoionisation cross-section is found to be inversely proportional to the required photon flux. In contrast, the second and third methods both maintain a constant photon flux (i.e., constant ~(A)) and follow the variation in nc as a function of wavelength. This can be done by monitoring either PC (proportional to nc) or OSL (proportional to nc/~" = ncAmm, where ~-is the luminescence lifetime). When using OSL, it is necessary that ~-remains constant (i.e., Amm remains constant) as the wavelength changes. Under conditions of weak stimulation (An << n) this condition holds true. A comparison of the three methods, and an indication of the shape of a typical o-(A) curve (actually a ~r(hu) curve), is shown in Fig. 2.5. Here the normalised PC and OSL stimulation spectra for irradiated (300 Gy) A1203:C single crystals are shown. The PC data were taken either with the constant-flux method (1) or the constant-PC method (2). The OSL spectrum (3) was obtained for a fixed emission wavelength of 420 nm (Whitley and McKeever, 2000). The stimulation range is wide, indicating multiple array of trapping sites giving rise to the PC and OSL signals. Thus, the data must be interpreted as a convolution of the weighted sum of multiple photoionisation cross-sections, with the weights being determined by the trapped charge populations. An additional example of this type of excitation spectrum in shown in Fig. 2.6 for OSL from natural quartz. Ditlefsen and Huntley (1994) demonstrate that for photon energies from approximately 1.9 to 2.7 eV, the excitation of OSL from quartz can be interpreted as primarily coming from a single type of trap. In contrast, Fig. 2.7 illustrates an entirely different excitation spectrum shape, from Amelia albite. Resonance absorption is observed in the latter spectrum, at approximately 1.4 and 2.5 eV. The resonance at 1.4 eV is consistent with the model of Hiitt et al. (1988) and Hfitt and Jaek (1993) for OSL in these materials in which the optical absorption at 1.4 eV corresponds to an internal transition
2~
Optically Stimulated Luminescence Theory
1
-
..J
O9
0
~2
o o
> ~
o
0,1
-
-o E
0 o o o rD.
0,01 -
/-3
2,0
i
i
i
i
i
i
i
i
i
2,2
2,4
2,6
2,8
3,0
3,2
3,4
3,6
3,8
4,0
Stimulation e n e r g y ( e V ) Fig. 2.5. Normalised PC and OSL stimulation spectra for irradiated (300 Gy) A1203:C single crystals. The PC data were taken either with the constant-flux method (1) or the constant-PC method (2). The OSL spectrum (3) was obtained for a fixed emission wavelength of 420 nm. The data were normalised to unity at a stimulation energy of 4.0 eV (from Whitley and McKeever, 2000).
from a ground state to an excited state (transition type 5 in Fig. 2.3), from where thermal stimulation to the conduction band leads eventually to the production of the OSL signal. The calculated thermal activation energies for the thermal transition are also indicated in the figure, as a function of stimulation wavelength (BCtter-Jensen et al., 1994a,b). Note that these methods do not yield absolute values for the photoionisation crosssections, but only relative values. Absolute values for o-, for a fixed stimulation STIMULATION WAVELENGTH [nm) 5
600
650
4
5,~o
500
4~0
3 -
2
J
1 0 1,8
2
2,2
2,4
2,6
2.8
STIMULATION ENERGY [eV] Fig. 2.6. OSL stimulation spectra for irradiated natural quartz obtained by the constant-flux method (from BCtter-Jensen et al., 1994).
Optically Stimulated Luminescence Dosimetry
26
Fig. 2.7. OSL stimulation spectrum for irradiated natural amelia albite obtained by the constant-flux method. The edge-like stimulation spectrum seen in Figs. 2.5 and 2.6 is not observed in this material over this wavelength range. Instead, resonance transitions are observed, at approximately 1.4 and 2.5 eV. The resonance in the infrared region is interpreted as being due to intra-band excitation (of the type denoted by transition 5 in Fig. 2.4) followed by thermal excitation into the de-localised band to cause ionisation of the trap (from BCtter-Jensen et al., 1994).
wavelength, can be obtained from LM-OSL, to be described in a later section. Alternatively, one can use the analysis of Huntley et al. (1996), who monitor the decay rate of the OSL as a function of illumination time (CW-OSL), and determine o-from the ratio of the OSL intensity to the slope of the OSL decay curve. From Eq. (2.18) with negligible re-trapping (i.e., first-order kinetics, with A ( N - n)nc << Amncm) we see that the OSL intensity, IOSL, is given by: lOSE "-- ncAm m = nCI9(h)tr (h)
(2.20)
For continuous wave excitation (CW-OSL), the incident photon flux t/~(h) is held constant with time, and thus the rate of change of the OSL intensity as the traps are depleted while the sample is being stimulated is: d/os L
dt
dn
-- SosL = @ (a)o- (a) ~ dt
(2.21)
As noted in Section 2.3.2, for first-order kinetics d n / d t = I O S L , and thus: SOSL-'- t/)(/~)O'(/~)/OSL
(2.22)
or
o-(A) =
1
SosL
(2.23)
qb(A) l o s E
and hence the photoionisation cross-section can be obtained from the ratio of the slope (SosL) of the CW-OSL decay curve to the CW-OSL intensity (IosL).
Optically Stimulated Luminescence Theory
27
2.4. C W - O S L 2.4.1. M o d e l s and rate equations The transitions of charge between energy levels during irradiation and subsequent optical stimulation of a dosimeter can be described by a series of non-linear, coupled rate equations. The equations themselves are intractable and several simplifying assumptions have to be introduced in order to arrive at analytical expressions for the evolution of the OSL intensity with time during optical stimulation and, ultimately, the dependence of the OSL signal on the absorbed dose. Several energy-level models can be imagined on which to base these analyses, and each includes transport of electrons through the conduction band in order for them to reach the trapped holes at the radiative recombination sites. The simplest is that corresponding to a system containing one type of electron trap and one type of hole trap. The trapped holes act as recombination centres at which recombination of electrons with the holes occurs, leading to the emission of luminescence. The model is called the one-trap/one-centre model. Additional complexities can be introduced in a systematic fashion to gain an appreciation of the role played by additional electron traps and/or recombination centres. These models are explored in the following sections.
2.4.2. The one-trap~one-centre model The model is shown in Fig. 2.8a. Charge neutrality for this system can be written as: nc + n = my + m
(2.24)
where nc and n are the concentrations of electrons in the conduction band and the traps, respectively, and mv and m are the concentrations of holes in the valence band and hole traps (recombination centres), respectively. If we consider thermal equilibrium at the end of the irradiation period such that nc and mv are zero, then we may write that at the start of optical stimulation no = m0, where the "0" subscripts imply time t -- 0. During optical stimulation of the electrons from the traps, transitions to the valence band do not occur and at any time t during the optical stimulation period the charge neutrality condition becomes nc + n = m from which we may write the rate of change of the various concentrations as dn c dt
-- -
dn dt
~
dm
(2.25)
dt
The terms on the right-hand side may be written explicitly as" dR
dt
-- np - n c A ( N -
n)
(2.26)
and dm
dt
= ncAmm-
nc
~-
(2.27)
28
Optically Stimulated Luminescence Dosimetry
Fig. 2.8. Simple models for OSL. (a) Simplest model; involving one trap and one radiative recombination centre. (b) Model containing an additional deep, competing trap. (c) Model containing a shallow, competing trap. (d) Model containing a competing non-radiative recombination centre.
Here, p is the rate of stimulation (in units of s -1) of electrons from the trap and is related to the incident photon flux q~ and the photoionisation cross-section o-by: p = q~o-
(2.28)
as already seen in Eq. (2.8) and where the dependence on wavelength is understood for a given optical ionisation threshold energy. The other terms in Eqs. (2.26) and (2.27) include A, the trapping probability (in units of m 3 s-l); Am, the recombination probability (also in units of m 3 s-l); N, the total available concentration of electron traps (in m-3); and z = 1/Amm , the free electron recombination lifetime (in s). With the introduction of a quasi-stationary population of free electrons in the conduction band (the so-called "quasiequilibrium approximation", or dnc/dt << dn/dt, d m / d t and nc << n, m) we have: dm dn -dt dt
(2.29)
The second major assumption to be introduced at this point is that of slow re-trapping, that is, ncA(N - n) << np, ncAmm. This leads to: dm losE = -- dt
dn dt -- np
(2.30)
nop exp{ - tp } = Io exp{ - t/Zd }
(2.31)
the solution of which is IOSL =
Here, Io is the initial OSL intensity at t = 0 and ~'d is the CW-OSL decay constant. Thus, this first-order model leads to an exponentially decaying OSL intensity as the (constant) stimulation light intensity is applied to the sample. Eventually, all the traps are depleted and the OSL becomes zero (McKeever et al., 1997a).
Optically Stimulated Luminescence Theory
29
This simple result (i.e., an exponential decay of OSL during stimulation with a fixed intensity stimulation beam, Eq. (2.31)) is obtained under the stringent conditions of slow re-trapping, or first-order kinetics (c.f. dn/dt = - n p ) . In practice, however, experimental CW-OSL decay curves show a wide variety of curve shapes, which do not conform to this simple exponential description. An obvious cause for such a deviation from this simple rule may be the addition of other optically active traps leading to two or more electron traps releasing their trapped charge at the same time, each at its own rate and described by its own photoionisation cross-section at the stimulation wavelengths used. This case of multiple traps is dealt with in the following section. Our interest here, however, is to examine the one-trap/one-centre model only to seek conditions under which nonexponential behaviour can be produced even with this simplest of models. Chen and McKeever (1997) show that if significant re-trapping into the trap during optical stimulation (see Fig. 2.8a) is included in the analysis, such that the slowre-trapping approximation (ncA(N - n) << np, ncAmm ) is no longer valid, one obtains for the OSL intensity: IOSL -- np -- n c A ( N - n) For the specific case of N >> n, and R = A / A m >> n / ( N order) function results, namely:
n2p
(2.32) n), a bimolecular (second-
dn
IOSL = NR = - d---t-
(2.34)
The solution, after integration, is: IOSL- I 0 ( 1 -
noPt) -2 NR
(2.35)
where Io -- nZp/NR. For the more general case of Io = n~p/NR we have:
1-b
n0 tNR)
236,
or b IOSL--I0( 1 -- nopt )
(2.37)
Chen and Leung (2002) examined this situation in some detail. Solving the above set of differential equations numerically with parameters A = 10 -13 m 3 s -1 , A m -10 -12 m 3 s -1, N = 1017 m -3, no -- m0 = 1016 m -3 at the start of the stimulation, and p = 1 s-1 they demonstrate that the CW-OSL curve is no longer a simple exponential but instead is best fitted by a so-called "stretched exponential" of the form: IOSL -- I0 exp{ -- (t/~'d)/3 }
(2.38)
with 0 < / 3 < 1. Normally, this type of decay behaviour is observed in metastable systems containing disorder. As the system returns to equilibrium there are distributions of decay
30
Optically Stimulated Luminescence Dosimetry
pathways and corresponding decay constants (Chen and Leung, 2002). The numerical solution to the rate equations using the above parameters was fitted using/3 = 0.5. However, Chen and Leung (2002) also showed that the fit is substantially better using the summation of two stretched-exponential functions, with the early part of the decay being best described using fl = 0.94 and the latter part using/3 - 0.35. The importance of this observation is that the fit to the early part using/3 - 0.94 (i.e., close to 1) is perhaps due to the re-trapping being weakest here such that the decay approximates an exponential. In the latter part of the decay where re-trapping may be more important, one gets the stretched-exponential behaviour. Thus, the shape of the decay curve may be dependent upon the degree of trap filling. If true, this leads to an interesting prediction, namely that the shape of the decay curve will also be dependent upon the dose, becoming more exponential-like (/3 ~ 1) as the dose increases. If this was observed experimentally, one might be tempted to describe the observation in terms of the overlap of several first-order processes, which is an interpretation placed on several experimental observations, as described in the next section. We should point out that the effects of changing probabilities of re-trapping and recombination have been studied by Sunta and colleagues for descriptions of TL kinetics (Sunta et al., 1997). Before discussing the expected effects due to the existence of more than one trap and/or recombination centre, however, it is important to point out that the decay constant and the trap emptying rate are related to the photoionisation cross-section by Eq. (2.28), namely z~-1 - - p - q~or. For a sample that is stimulated by a narrow band of monochromatic light (for example, from a laser), the decay constant will be single-valued. Some popular OSL systems used in modem experiments use a band of stimulation wavelengths. Since both q~ and o- vary with wavelength, a range of values for p - - ~.~-1 will result. However, a single exponential decay is still expected with the decay constant now given by "/'d 1 = ~ o-(h)@(h)dh and the evaluated effective decay constant is the mean of the distribution of decay constants caused by the broad band stimulation. A potentially more serious problem occurs when the intensity of the stimulation light is spatially distributed non-uniformly over the surface of the sample (e.g., the TEMoo mode from a laser). Since different parts of the sample now experience different intensities (albeit at the same wavelength), the net result is such that the measured decay constant Zd may not be single-valued and the observed OSL decay curve may be best fit by a stretched exponential, despite the fact that slow re-trapping prevails. 2.4.3. Models containing multiple traps and centres Smith and Rhodes (1994) observed non-exponential OSL decay from irradiated quartz and interpreted the observation in terms of contributions from three separate traps, with different photoionisation cross-sections at the stimulation wavelengths used. Possible experimental effects due to the use of broadband stimulation or laser stimulation were tested by Bailey et al. (1997) and found to be negligible in this instance, and the latter authors were led to the same conclusion as Smith and Rhodes (1994). They were able to interpret their data consistently by fitting the OSL decay curves to the sum of three exponential decay curves with varying values for the decay constant--termed the "fast", "medium" and "slow" components.
Optically Stimulated Luminescence Theory
31
For two optically sensitive traps, of trapped charge concentrations n l and ne and with stimulation rates Pl -- ~'dl1 and Pe = ~'d21, it is straightforward to show that (McKeever et al., 1997a,b): dm dnl . . . . dt dt
dna dt
(2.39)
and that IOSL = nl0Pl exp{ --tpl } + n20P2 exp{ --tP2 } = 11o exp{ --t/Tdl } + 120 exp{ -- t/'/'d2 }
(2.40)
using the superposition principle and no interaction between the traps. Clearly, this can be extended to three or more optically sensitive traps, each emptying at their own characteristic rate during stimulation. Extending this to a distribution of traps, Whitley and McKeever (2000) used the onedimensional Fredholm integral equation of the first kind to describe the OSL decay from a series of traps with a distribution of photoionisation cross-sections (i.e., a distribution of threshold energies, Eo). For a stimulation photon energy hi,, the Fredholm equation is written as:
IosL(hU ) -- [n(Eo)p(hu, Eo)dE o -- clg(hu) ~n(Eo)o-(hu, Eo)dE o
(2.41)
where n(Eo) is a weighting factor defining the fraction of the OSL curve that is due to a particular trap. Eq. (2.41) is in fact a re-statement of Eq. (2.4) with m = 1 and 3/= Eo. Whitley and McKeever (2000) fitted Eq. (2.41) to experimental PC-versus-wavelength data from irradiated A1203 single crystals and obtained a distribution of trapping states contributing to the PC signal in this material. Example data showing how the distribution develops as a function of absorbed radiation dose is shown in Fig. 2.9. This approach requires an assumption of slow re-trapping since the Fredholm equation is an expression of superposition, which assumes no interaction between the traps. If additional traps introduced into the energy band model are not sensitive to the optical stimulation, they can still act as traps for released charge and affect the OSL decay kinetics. An example is shown in Fig. 2.8b where we see a deep trap able to capture free charge but from which optical stimulation does not occur. (One can imagine, for example, stimulation with long wavelength light optically emptying shallow traps but with the light being of insufficient energy to empty the deep traps.) In this model, the deep traps may be viewed as competitors to the recombination centres. The OSL intensity in this case is described by: IOSL = nlOP exp{ -- t/•'} -- ncA2)(N 2 - n2)
(2.42)
where N2, n2 and A2 are the concentration of available traps, concentration of filled traps, and trapping probability, respectively, for the deep trap. Thus, the OSL intensity is reduced by an amount dictated by the relative rates of recombination and trapping into the deep, competing trap. The decay is no longer exponential since the second term in Eq. (2.42) is time-dependent and approaches zero as t ---* oo.
32
Optically Stimulated Luminescence Dosimetry (.-
0.16
O
0.12
:3 ...Q
0.08
. m
. m !,,_
a tO
. m
:3 ...Q . m
k _
(/)
a . m
tO
I
100 Gy xl0i
0.04 0.00 0.08
300Gy
0.06 0.04 0.02 0.00 0.08
k
1000 Gy
0.06 :3 .-9_ 0.04 . m
,,,i-a
!,...
s
0.02 0.00 1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Energy (eV) Fig. 2.9. Optical threshold energy distributions for an irradiated A1203 crystal obtained from a de-convolution of PC-versus-wavelength data using Eq. (2.4). Results for three different absorbed doses are shown, showing the evolution of the distribution with absorbed dose (from Whitley and McKeever, 2000).
Bailey et al. (1997) introduced some additional assumptions into this model. T h e y a s s u m e d that N~ >> n l, N2 >> n2 and m is approximated to a large constant. The latter assumption in effect means that n l << n2 -~ m for charge neutrality. Bailey et al. (1997) argue that experimental evidence indicates that these constraints are appropriate for natural quartz and, using these assumptions, introduce the following constants: Anl = A I ( N 1 - nl)
(2.43a)
A~2 -- A2(N2 - n2)
(2.43b)
Aml -- A m m
(2.43c)
and
W i t h these constants one can now write: dR 1 dt
dn2 dt
-- - n i p
-
ncAn2
+ ncAnl
(2.44)
(2.45)
and dm
dt
= -ncAml
(2.46)
Optically Stimulated Luminescence Theory
33
With the usual dnc dnl . . . . dt dt
dn2 dm ! dt dt
(2.47)
along with the quasi-equilibrium approximation, the above equations reduce to: dr/1
dt
(2.48)
-- - C n l
and dm dt
= -Dnl
= Dnlo exp{ - Ct} = Io exp{ - Ct}
(2.49)
where C and D are constants. Thus, under this set of constraining assumptions, even a model including deep traps can produce a first-order, exponential decay. If the competing centres are shallow (Fig. 2.8c), Eq. (2.42) becomes: lose -- nloP exp{ -- t/~'a} + nzs exp{ -- E / k T } - n c a z ( N 2 - nz)
(2.50)
where s and E are the pre-exponential factor and thermal trap depth for the shallow trap, and k is the Boltzmann' s constant. The last two terms in Eq. (2.50) combine to produce a long-lived, temperature-dependent component to the OSL decay curve. The form of this component is an initial increase, followed by a decrease at longer times. Depending on the size of the first term compared with the second and third terms, the overall OSL intensity can also exhibit an initial increase followed by a decrease. In such circumstances, the terms "OSL decay curve" or even "shine down" curve, as used in some publications, are entirely inappropriate. Key elements are the intensity of the stimulation light p, and the temperature T, such that at certain combinations of p and T the initial increase may be observed, whereas for other combination only a decay may be observed. A further example of energy levels in the OSL model is obtained by considering a second recombination centre such that the total trapped electron concentration n after irradiation is given by n -- ml + m2. Here, ml and m2 are the concentrations of trapped holes in the two recombination centres, respectively. We further assume at this point that only electron-hole recombination in the first centre is radiative (thus, losE -- - d m l / d t ) and that recombination at the second centre is non-radiative (or, if radiative, is outside the detector sensitivity window). Under these conditions we have: los E = n p exp{ --t/~'d} --
dm2 dt
(2.51)
where we again see that the OSL intensity is reduced by the presence of a non-radiative pathway for charge interaction. Quasi-equilibrium ( d n c / d t ~ 0) leads to: ml -- ml0 exp{ - t n c A m l }
(2.52)
m2 = m20 exp{ -tncAm2 }
(2.53)
and
where Am1 and Am2 a r e the recombination probabilities at the two centres, respectively.
Optically Stimulated Luminescence Dosimetry
34
The relative size of the recombination centres is time-dependent, thus: ml m2
"-"
m01
exp{
-- tnc[Aml - Am2 ] }
(2.54)
m02
In the circumstance that Aml ~ Am2 the ratio remains approximately constant. In this case, the CW-OSL decay curve remains approximately exponential according to: 1
losE -- ~ noP exp{ -- tp }
(2.55)
where K is a constant given by K = (ml + m 2 ) / m l . 2.4.4. A more generalised model The above analyses are for certain very special cases only. In the models we have discussed we have assumed (say) a shallow trap, or a deep trap, or a second recombination centre, etc. Real materials, however, contain multiple traps and centres, some of which may be shallow, some others "optically sensitive" (i.e., the charge may be optically stimulated from the trap at the wavelength being used), and some deep. Some of the recombination centres may be radiative, and others not. Charge transfer effects may take place between some centres, and not others. The charge transfer processes may involve the conduction band (as is assumed so far) or they may not. In other words, real materials show a complexity of behaviour, which is far greater than that indicated so far in this text. As a first step towards a more realistic model for a generic OSL material, McKeever et al. (1997a) introduced the model shown in Fig. 2.10. The model includes: a shallow trap (level 1) into which electrons may be trapped during optical stimulation (downward arrow) and from which electrons may be thermally or optically released (upward arrow); a main "dosimetry" trap (level 2) from which electrons are optically stimulated; a deep trap (level 3) into which electrons may be trapped (but once trapped remain localised); a radiative recombination centre (level 4) in which electrons may recombine with trapped holes to produce OSL; and a non-radiative recombination centre (level 5) at which recombination can also occur but without the production of an OSL photon. The transitions shown in Fig. 2.10 are those occurring during the optical stimulation phase only. Note that the single shallow trapping level indicated here can be considered as representing all the shallow, unstable traps that one might expect to find on a real material, each with its own trapped charge lifetime and thermal decay characteristics. The same holds true for both types of recombination centre. Similarly, the deep traps can be considered to represent all those deep traps that may capture charge during optical stimulation and thereby compete with the luminescence centres. Thus, although the model described is simplistic, it encompasses many of the features that might be found with a real material. The rate equations describing the flow of charge into and out of the various traps and centres depicted here form a set of six, coupled, non-linear equations. Even for this stillsimplified model, these equations are intractable. One can only gain further insight into the potential behaviour of such a system by solving the rate equations numerically under a range of parameters. At this point, the range of parameters is open ended in that an
Optically Stimulated Luminescence Theory
Fig. 2.10.
35
A model combining all the elements indicated separately in Fig. 2.8 (from McKeever et al., 1997a,b).
infinite number of values and combinations of values may be chosen. If one were attempting to model a specific material, one would have to look carefully at the available experimental data in order to define the numbers of traps and recombination centres needed and to restrict the possible values that the trapping and recombination parameters could take. At this stage, however, we have to make intelligent guesses only. Nevertheless, even this admittedly unsatisfactory approach leads us to insights into the possible experimental results that one might obtain with a real material. Such a procedure was adopted by B~tter-Jensen et al. (1994a) and McKeever et al. (1997a,b) for a range of parameters in order to determine how the OSL decay curve shape may vary as a function of dose, stimulation intensity, pre-irradiation annealing and temperature. Example results are shown in Fig. 2.11. The figure shows the main features that can be expected for OSL curves from a model such as the one described. At low temperatures (Fig. 2.11a), where the half-life of the charge in the shallow traps is much longer than the observed decay time for the CW-OSL signal, a reduced OSL signal is observed due to competitive trapping of the released charge into the shallow traps. At high temperatures, when the corresponding half-life is much smaller than the CW-OSL decay time, the OSL intensity is much higher. After the initial increase, the OSL decays approximately exponentially, as might be expected for charge release from a single trap under conditions of first-order kinetics. However, the decay departs from a simple exponential at longer times. The cause of this departure from exponential decay may be caused by the re-trapping of the charge into the shallow, dosimetric and deep traps, and by the presence of more than one recombination centre, as described in the previous sections. All can contribute to non-exponential behaviour. As noted by Chen and Leung (2002) the decay at longer times in such circumstances may be better described by a stretched-exponential function.
Optically Stimulated Luminescence Dosimetry
36
10
I
I
I
I
I
I
I
1
400 K
I
(a)
380 K 360 K _... 340 K
00 K m
,m r-
-
0
l
E"
(b) f=l
Ill
" 1.0 .e
_
v
f=0.1
m 0.5 rr._1
O
00
I
~~~.D _
%
I
1
I
= 1000
(c)
D = 100 D=10
0
20
40 60 Time (s)
80
100
Fig. 2.11. (a) Simulated CW-OSL curves using the model of Fig. 2.10 at a variety of temperatures. (b) Simulated CW-OSL curves as a function of the excitation rate p (N.B. the authors used f instead of p in this figure). (c) Simulated CW-OSL curves as a function of dose. The data are from McKeever et al. (1997a). The parameters used in the calculations are listed by McKeever et al. (1997b).
Interesting behaviour is observed at intermediate temperatures. Here, a peak is observed in the CW-OSL curve caused by trapping of charge into, and subsequent thermal release from, the shallow traps. At low temperatures the charge in the shallow traps is stable and does not contribute to the luminescence. At high temperatures, the effect of the shallow traps is negligible. At these temperature extremes, the initial decay rate of the CW-OSL is the same and is dictated by the stimulation rate p. As indicated in previous sections, the decay constant is given by ~'d -- 1/p in these circumstances. At intermediate temperatures, however, this is not the case. The decay is a convolution of a simple decay due to depletion and a term that first increases and then decreases at a rate governed by the thermal stability of the shallow traps. In these circumstances, the decay is non-exponential and is temperature-dependent (see Eq. (2.50)).
Optically Stimulated Luminescence Theory
37
In Fig. 2.1 lb we see the decay of the CW-OSL signal, at intermediate temperatures, as a function of the stimulation power. Eq. (2.50) teaches us that the relative size of the terms on the right-hand side depends not only on the temperature, but also on the stimulation power. Thus, at higher powers the first term dominates and the peak in the OSL curve disappears. A simple decay is again observed, although true exponential decay is not observed. At lower power values the decay is slow and a peak is again observed. The position of the peak in the CW-OSL shifts to longer times as the power decreases. The values of the stimulation power and the temperatures necessary for these properties to be observed experimentally depend on the values of the parameters describing the shallow trap. Finally, in Fig. 2.11 c we see the dependence of the CW-OSL curves, at intermediate temperatures and stimulation powers, on the absorbed dose. The OSL signal is a linear function of dose under the conditions reported in these calculations. A slight shift of the peak to lower times is observed as the dose increases. Example experimental data showing similar results to these are shown in Fig. 2.12 for irradiated A1203. The sample in this case exhibits significant concentrations of shallow traps, as can be seen in the TL data of Fig. 2.12a. Deep traps are known to exist in this material (Whitley and McKeever, 2000). Also to be noted is the fact that the TL signal from the shallow traps is weakly affected by the stimulation of the sample with 532 nm light, whereas the main TL peak at higher temperatures is rapidly reduced during stimulation. These observations lend justification to the use of the model of Fig. 2.10 to describe the OSL properties of this material. As a result, at room temperature and using these particular stimulation powers we see a clear peak in the CW-OSL curve for this sample. McKeever et al. (1997a) also studied CW-OSL from natural quartz and showed similar behaviour to that predicted in Fig. 2.11a for this material--as seen in Fig. 2.13. Here we see that at intermediate temperatures, a peak is observed in the CW-OSL curves, whereas at lower and higher temperatures no such peak is evident. Furthermore, the intensity of the OSL at lower temperatures is less than that at higher temperatures due to the competing effect of the shallow traps. 2.4.5.
Temperature dependence effects
The variations in OSL curve shapes described above are just some of the temperaturedependent effects that can be observed when OSL measurements are performed at different temperatures. Perhaps the most commonly observed effect is genetically called "thermal assistance" and is manifested by the observation that more OSL is stimulated, for the same excitation intensity, at higher temperatures than at lower temperatures. Shallow traps can cause this observation, as indicated in the discussion of the previous section, but an important feature of thermal assistance is not just an increase in the OSL efficiency, but also an increase in the rate of OSL decay during measurement of CW-OSL. In these cases, the CW-OSL decay rate p = 1/~'a is temperature-dependent, thus: P = Po exp - ~
(2.56)
where AE is the thermal activation energy. By monitoring either the CW-OSL intensity or the decay rate as a function of temperature and fitting the data to an equation of the form of
Optically Stimulated Luminescence Dosimetry
38
1.2 9
-
Before bleaching
~#
[] ,_,1
_
0.8
[] o
"O N
o
o.~
0.6
-
0.4
O
-
I~
0.2
~ After bleaching -
[]
[]
.
#
o _ ~1~'. ql~.~.~:~
ra_~
[] []
~t~
~t:".o
z
(a)
[]
~
% ~il~t~.~
.
.
.
.
0 -50
50
150
250
Temperature (~ 10
-09 O
6
,
0
50
,
100
,
150
200
Time (s) Fig. 2.12. (a) TL from A1203 irradiated at - 5 0 ~ and warmed at a heating rate of 1.0~ Strong low temperature TL is observed near 20~ This TL peak is not significantly reduced when an irradiated specimen is stimulated with green (532 nm) light. In contrast, the TL near 200~ (which constitutes the main dosimetric signal in this material) is sensitive to the 532 nm light as electrons are optically stimulated from these traps. Large concentrations of deep traps also exist in this material (Whitley and McKeever, 2000). (b) The CW-OSL from the same sample, stimulated with 532 nm light at room temperature. An initial peak is clearly observed in the OSL curve due to charge trapping by the shallow trap (from Polf, 2002).
Eq. (2.56) a variety of values of AE has been obtained for materials of importance of OSL dosimetry. The materials include quartz (Spooner, 1994) and a range of feldspar types (Htitt et al., 1988; Poolton et al., 1995a,b; 2002a,b). McKeever et al. (1997a) summarised the possible processes that could give rise to such observations, and these are indicated schematically in Fig. 2.14. Mechanism (a) in Fig. 2.14 gives rise to a temperature-dependent OSL through the trapping of optically stimulated charge by shallow traps, as discussed in the previous section. Markey et al. (1997) observed the effect in A1203 and modelled the process using a model similar to that described in Section 2.4.4.
39
Optically Stimulated Luminescence Theory
14000
I
E 10000 8000
'~
6000
I
50 ~
12000
:
I
100 ~
i
4000 0 2O00 0
1
0
20
1
40
1
60
80
Time (s) Fig. 2.13. CW-OSL curves from quartz stimulated at a variety of temperatures. A peak in the OSL curve is observed at intermediate temperatures, as predicted from the analysis of the model of Fig. 2.10. The sample was irradiated with 31 Gy (beta), preheated to 125~ for 20 s, and then illuminated with broad band (420-560 nm) visible light to monitor the CW-OSL curves (from McKeever et al., 1997a). In this case, the a c t i v a t i o n e n e r g y A E is identified w i t h the t h e r m a l trap d e p t h o f the s h a l l o w traps, Et. Htitt et al. ( 1 9 8 8 ) p o s t u l a t e d m e c h a n i s m (b) in w h i c h o p t i c a l e x c i t a t i o n to a d e f e c t e x c i t e d state is f o l l o w e d b y t h e r m a l e x c i t a t i o n to the d e l o c a l i s e d band. In this case, the
E.
ET
a
b
c
T d
I
E~
e
Fig. 2.14. Schematic representation (after McKeever et al., 1997a) of the processes that may give rise to a temperature dependence for OSL production. These include: (a) the effect of shallow traps (McKeever et al., 1997a; Markey et al., 1997; see also Section 2.4.4); (b) thermal assistance from an excited state (Hiitt et al., 1988); (c) donor-acceptor hopping (Poolton et al., 1994); (d) band tail states hopping (Poolton et al., 1995a,b; 2002a,b); and (e) ground state excitation (Spooner, 1994).
Optically Stimulated Luminescence Dosimetry
40
0.15 A
> -
>' L
Or
-
0.10
c LU o=
E
Ab
0.05
_
L
el-
fl
t.0
1.5
2.0
2.5
3.0
Optical Excitation Energy (eV) Fig. 2.15. Thermal activation energy dependence upon the optical stimulation energy for three different feldspars, where Or = orthoclase, Ab -- albite and An = anorthite (from Poolton et al., 1995b).
activation energy is AE = EA. This mechanism was proposed from the observation that OSL can be stimulated in irradiated feldspars by stimulating with infra-red light (i.e., infra-red stimulated OSL, or IRSL). The infra-red energies are approximately 1.4-1.5 eV and the resulting OSL is anti-Stokes shifted to shorter wavelength (visible). The OSL intensity was seen to increase with temperature, with a thermally activated probability proportional to exp{ - E A / k T }. However, a more detailed investigation of the thermal activation energies required to produce OSL by Poolton et al. (1995a,b) revealed that the thermal energies were dependent on the energy of optical stimulation, and that a small thermal activation energy was required even when high energy visible stimulation was used. The data of Poolton et al. (1995a,b) are shown in Fig. 2.15 and to explain these observations Poolton et al. (1994, 1995a, 2002a) modelled the trapped-electron defect that gives rise to the OSL signal using a simple hydrogenic model for the trap. In this model, the energy E, of the nth excited state is m*/m e
E~--Eh
(ern)2
(2.57)
where E h is the ionisation energy of the free hydrogen atom (13.6 eV), m* the electron effective mass, me the free electron mass, and er the relative permittivity. Similarly, the radius R,, of the nth excited state is: Rn =
Rhn~, r
m*//rne
(2.58)
where Rh is the radius of the free hydrogen atom. Measurements of m*/m e = 0.79 for feldspars of various types by Poolton et al. (2001) yield estimates of the first excited state of 1.48 eV for alkali feldspars (Poolton et al., 2002a). Earlier estimates using m*/me -0.76, yielded estimates of 1.441, 1.422 and 1.225 eV for orthoclase, albite and anorthite, respectively (Poolton et al., 1995a,b). These values compare very well with the measured optical energies corresponding to the thermal energy minima in Fig. 2.15--namely, 1.440 +_ 0.003, 1.422 _+ 0.003 and 1.275 +_ 0.004 eV for the same minerals, respectively.
Optically Stimulated Luminescence Theory
41
It is to be noted from Fig. 2.15 that the thermal activation energies at the minima (i.e., corresponding to optical excitation at the IR energies noted above) decrease as one goes from orthoclase to anorthite--viz. 0.105 eV (orthoclase), 0.065 eV (albite) and 0.03 eV (anorthite). This is not what one would expect with the Htitt et al. (1988) model where one would in fact expect the opposite trend to be true. Poolton et al. (1995a,b) observe, however, that the radii of the first excited states increases as one goes from orthoclase to anorthite, with the variations accounted for by differences in the dielectric constants of the materials. Poolton et al. (1995b) propose, therefore, that the thermal activation energy is in fact the hopping energy En required to hop from the first excited state of the trap (the "donor") to the recombination centre (the "acceptor") in a donoracceptor ("d-a") recombination process, as depicted in Fig. 2.14c. As the radius of the trap increases as one goes from orthoclase to anorthite, the overlap between donor states increases and the thermal activation energy is reduced accordingly, as observed. Thus, AE is identified with Eh (Fig. 2.14c) in this d - a model. In a later publication, however, Poolton et al. (2002a) interpreted the observed thermal activation energies at resonance (where the observed energies can be as low as 0.02 eV) as being caused by tunnelling from the trap to the recombination centre. Tunnelling is consistent with the observations of "athermal" fading of the stored luminescence signal from feldspars in which the OSL and TL signals from irradiated feldspars are seen to decay at rates that are not expected from the observed positions of the TL peaks associated with these signals. Tunnelling is proposed to account for the effect (Visocekas et al., 1994, 1998; Visocekas and Zink 1995). As pointed out in another publication by Poolton et al. (2002b), the thermal activation energy is not reduced to zero even at higher optical stimulation energies (in the visible). In fact, when the optical excitation is off resonance thermal activation, energies as high as 0.10-0.15 eV are generally observed. To explain this effect, the authors argue that disordered materials such as natural feldspars are characterised by band tail states, which exist below and near the conduction band edge and extend into the forbidden gap (Mott and Davies, 1971). These states arise from random fluctuations in bond lengths and angles. Optical transitions to the edge of the conduction band, therefore, stimulate the electrons into the band tail states and transport occurs via thermal hopping through these states. In this case, the activation energy AE is now identified with the band tail hopping energy EH (Fig. 2.14d). Poolton et al. (2002b) define a more exact form of the temperature dependence due to hopping through the tail states as:
IosL(hV, T)oc exp{ - hv}[exp{ - 2/3(hv)R}exp{ - h~op/kT}] 3
(2.59)
where the first exponential term accounts for the density of states of the conduction band tails, and the second term is the three-dimensional hopping probability. R is the well/ conduction-band-edge separation, Wp is the phonon vibrational frequency, and /3 = [8m* 7r2/h2(Ec - hu)] ~ where Ec is the conduction band edge and (Ec - hu) represents the electron energy in the conduction band tail states. A comparison of the shape of Eq. (2.59) with experimental data for alkali feldspar is given in Fig. 2.16. Spooner (1994) offered another interpretation of thermal assistance, this time in quartz, after noting that the measured thermal activation energy in this material varies smoothly
42
Optically Stimulated Luminescence Dosimetry
Ja,J ,
0.6 4>, ~
0.4
,,,,
r
= =... 4~
0.2
,'',
_1
O
0 1.0
;/t
.....
1.2
1.4
1.6
1.8
, 1
2.0
Optical Excitation Energy (eV) =
0.15
_
,
i
1
i
_
(bl
0.10 C
w
0.05
E 0
1.0
,
I
1.2
I
I
1.4
i
I
1.6
=
I
1.8
I
2.0
Optical Excitation Energy (eV)
Fig. 2.16. (a) Comparison of Eq. (2.59) (full line) with experimental data for alkali feldspars at 300 K. The dashed line shows the location of the IR resonance associated with a transition to the first excited state of the defect. (b) Measured thermal activation energies as a function of optical energies. The dip at resonance corresponds to tunnelling from the defect excited state, while the larger energies correspond to hopping through the band tail states (from Poolton et al., 2002b).
with the optical energy. The Hiitt et al. (1988) model leads to a prediction of a thermal energy that is either independent of optical energy, or one that varies discretely with optical energy as higher excited states (n > 1) are populated. Since this is not the case with quartz, Spooner suggests a model based on an array of ground states energies, from where optical excitation to the conduction band can occur (i.e. Fig. 2.14e). Here AE is now identified with Eg. Earlier authors examined the necessary electron coupling to the phonon vibrational modes theoretically and its effect on the shape of the photoionisation crosssection has been discussed in detail, e.g., Jaros (1977) and Noras (1980). Such effects are important to consider when e l e c t r o n - p h o n o n coupling is strong since accurate interpretation of OSL and PC spectral dependencies depends upon the correct choice of the expression for the photoionisation cross-section. Electron-phonon coupling is usually described with the aid of a configurational coordinate diagram, such as that shown in Fig. 2.17 in which we see the defect energy state corresponding to a filled trap and an empty, ionised trap. The horizontal lines represent phonon vibrational levels. Electronic transitions are represented vertically due to the supposition that absorption of a photon occurs too quickly for simultaneous re-adjustment of the lattice coordinate, Q (the F r a n k - C o n d o n principle). Since ionisation of the trap will result in a lattice re-adjustment, the minima in the ground state and empty states are not coincident in Q. Thus, a vertical optical transition is followed by phonon emission and non-radiative relaxation. A f r e e - b o u n d transition occurs from the upper minimum
~
43
Optically Stimulated Luminescence Theory
Defect empty
/
Ee (e)
t--
LIJ
Defect occupied ~ .
1
_~I_L/
ConfigurationalCoordinate,Q Fig. 2.17. Configurational coordinate diagram showing the vibronic and potential energy curves for a filled trap, and an empty trap. The potential energy curves are plotted as function of the one-dimensional configurational coordinate, Q. The horizontal lines are vibrational levels with separation htop.
vertically to the lower filled state, with the further emission of phonons. The difference in energy between the upward vertical and the downward vertical transitions is given by the Huang-Rhys factor, S and is equal to 2Shwp (where the terms have been previously defined, in Section 2.3.2). Purely electronic transitions result in ionisation of the trapped electron into the conduction band when the optical excitation energy h v is greater than the optical threshold energy Eo. Expressions for the resulting photoionisation cross-sections were given in Section 2.3.2. For purely electronic transitions (vertical transitions on a configurational coordinate diagram) no change in the lattice configuration occurs. When phonon coupling occurs, transitions to the conduction band can occur at photon energies less than Eo as long as the lattice can supply (or absorb) the corresponding energy A (i.e., h v - - Eo + A). For strong phonon coupling the photoionisation cross-section can be expressed (B6er, 1990) as: o" ( h v) oc
~oo
( x/~ FA ) b e x p { - ( A - h v - Eo 0 hv(,4r2FA + Eo(m*/me)) a
(2.60)
with the phonon broadening factor given by
F ~--
(h
)2coth
2-~
(2.61)
Optically Stimulated Luminescence Dosimetry
44
from which the exponential temperature dependence is clear. The parameter a = 2 for a short-range &function potential, and a = 4 for a long-range Coulombic potential. Parameter b = 3/2 for forbidden transitions, while b -- 1/2 for allowed transitions (Brer, 1990). Similar expressions are derived by Noras (1980) and Jaros (1977). Note that for photon energies significantly above the threshold energy (hv >> Eo + A) the purely electronic form of the cross-section suffices (Noras, 1980). Experimentally, one can observe phonon broadening effects by observing the shift in the threshold energy Eo as a function of temperature. One can expect variations in Eo with T due to thermal expansion effects, but such large shifts can be expected if thermal broadening is a significant factor. 2.4.6. Thermal quenching A final temperature-dependent effect governing the variation of OSL with temperature is thermal quenchingmi.e., the loss of luminescence efficiency with increasing temperature. In general, the effect can be observed in several ways but is seen most often in experiments that monitor the intensity of either PL or radioluminescence (RL) from a sample as a function of temperature, under conditions of constant excitation (light excitation for PL or ionising radiation excitation for RL). In both cases, thermal quenching is manifest by a decreasing emission intensity as the temperature increases. In the context of OSL, one observes a decrease in both the peak intensity (under CW excitation) and the integrated area under the CW-OSL decay curve as the sample temperature is increased. This observation assumes that for each measurement temperature the sample has been irradiated prior to OSL measurement under identical conditions such that the only variable in the experiment is the temperature of OSL measurement. The effect is manifest also in TL when a set of TL curves is obtained at a variety of heating rates. As the TL peaks shift to higher temperatures with increasing heating rate, so the luminescence efficiency decreases and a reduced TL peak (area and peak height) is obtained. Furthermore, the shape of the peak is distorted from that which is expected from kinetic considerations (e.g., see Section 2.2) since the high-temperature side of the peak is afflicted more than the lowtemperature side. Explanation of the effect is generally centred upon one of the two modelsmnamely, the Mott-Seitz model and the Sch6n-Klasens model. The Mott-Seitz model is best understood through reference to the defect configurational coordinate diagram, such as that shown in Fig. 2.18. Again, optical excitations take place vertically from the ground state to the excited state, followed by lattice relaxation and phonon emission before deexcitation and luminescence occurs. The energy difference between the emission and the excitation energy corresponds to 2hwp (the so-called Stokes shift). The lifetime of the carrier in the excited state ~-is governed by the quantum mechanical transition selection rules. Whilst in the excited state, however, the electron can absorb an amount of phonon energy W and undergo a transition over this potential barrier to decay to the ground state non-radiatively with the emission of phonons only (Dexter et al., 1955). The temperature dependence of the excited state lifetime ~"is given by:
1 "r
--
1 To
{w}
+- ~, exp k T -
(2.62)
45
Optically Stimulated Luminescence Theory
Excited state
Ee (Q)
t~
W
r
iii
Ground state _ _ _
EQ~
%! i
I
Configurational Coordinate, Q Fig. 2.18. Configurational coordinate diagram for a defect ground state Eg and excited state Ee. If thermal energy AE is absorbed by the electron whilst in the excited state, a non-radiative relaxation to the ground state occurs.
where ro is the lifetime for radiative transitions and u is a constant (frequency factor). The luminescence decay time is thus: ~- =
To 1 + ~'ov e x p ( -
W/kT)
(2.63)
The luminescence efficiency r/ is defined as the ratio of the probability for radiative transition divided by the total transition probability. Thus: --
~" To
--
1
1 + C exp{ -
W/kT}
(2.64)
where C = ~'ov. The radiative luminescence intensity I is likewise reduced according to: I =
Io 1 + C exp{ -
W/kT}
(2.65)
with Io the unquenched intensity obtained at low temperatures. The latter expression is also expected if one considers the S c h 6 n - K l a s e n s model for thermal quenching, indicated in Fig. 2.19 (McKeever, 1985). In this view, the luminescence emission results from the recombination of charge carriers from the delocalised band with trapped carriers of the opposite sign (e.g., free electrons recombining with trapped holes at recombination centres). However, in the
46
Optically Stimulated Luminescence Dosimetry
Fig. 2.19. The Sch6n-Klasens model for thermal quenching of luminescence. Free charge carriers (say, electrons in the conduction band) recombine with trapped holes to initiate the luminescence process. However, in this model the trapped holes are thermally unstable and may be released from the hole centres at a rate equal to s exp{ - Eh/kT}. This gives rise to a decreasing recombination probability. The discussion is symmetric with respect to the sign of the charge carrier and similar descriptions can be applied to recombination of holes with trapped electrons.
Sch6n-Klasens model, the trapped carriers are considered thermally unstable such that there is a significant probability of thermal release and a concomitant reduction in the concentration of recombination sites. This, in turn, leads to quenching of the luminescence process. The net result is a reduced luminescence efficiency as given by Eq. (2.64), but with the activation energy W identified with the thermal activation energy for charge carrier release (i.e., Eh in Fig. 2.19). It should be noted, however, that PL lifetime r should remain unaffected. This process, however, will affect the intensities of RL, OSL and TL. Examples of thermal quenching for quartz are given in several publications, including RL (Wintle, 1975), OSL (McKeever et al., 1997a; Murray and Wintle, 1998) and TL (Nanjundaswamy et al., 2002). A representative plot of OSL versus sample temperature for quartz is shown in Fig. 2.20. It should be noted that when one plots luminescence intensity versus temperature (either RL, OSL or TL) there is expected to be a difference in the obtained thermal quenching curve depending upon whether one is heating the sample from low temperatures, or cooling the sample from high temperatures. This is caused by the effect of shallow traps upon the recorded intensity. At low temperatures, shallow traps are filled during the initial irradiation of the sample. As the temperature increases the trapped charges are released, increasing the free charges available for recombination. The result is an initial enhancement of the luminescence intensity, due to the emptying of the shallow traps, before the expected decrease at higher temperatures is observed due to quenching. When one starts from high temperature, however, the shallow traps are empty and, although some carriers may be trapped as the temperature drops, the effect is much less noticeable. A clear example is given in Fig. 2.21 where we see luminescence from three
47
Optically Stimulated Luminescence Theory 6x105 ,-
4x105
0 0
..2 cO
0
2xl 05
0
i
i
a
I
50
100
150
200
250
Stimulation Temperature, ~ Fig. 2.20. Integrated OSL ( 1 - 1 0 0 s) under a CW-OSL decay curve for irradiated quartz, as a function of stimulation temperature. The solid line is a fit to Eq. (2.65) with W = 0.636 eV (___0.013 eV) and C = 3.4 x 107 (_+ 0.9 X 107) (from Murray and Wintle, 1998).
examples of A1203:C containing different concentrations of shallow traps. When the concentration of these traps is high, an initial increase in the luminescence is clearly observed as the temperature rises before thermal quenching sets in. The size of the effect is reduced during cooling and the peak in the emission is correlated with the position of the TL peak from the same traps. Measurements of luminescence intensity can also be affected by the degree to which the deep traps are filled. Competition effects with the deep traps lead to significant changes in the luminescence intensity curves, depending upon the degree of filling of the deep traps. This has been convincingly demonstrated for A1203:C by Milman et al. (1998). The best way to overcome such interferences, however, is to monitor the luminescence lifetime ~- as a function of temperature (Eq. (2.63)) for which competition effects are minimised. This was shown clearly for quartz by Bailiff (2000) and for A1203:C by Akselrod et al. (1998a). Analyses of the thermal quenching data for quartz indicate a quenching activation energy of approximately 0.6 eV (Bailiff, 2000) and a classical Mott-Seitz mechanism. The main (F-centre) emission from A1203 is characterised by a quenching energy of approximately 1.08 eV (Akselrod et al. 1998a) and is also adequately described by the Mott-Seitz model.
2.5. LM-OSL 2.5.1. First- and general-order kinetics The description of OSL has so far been based entirely on CW-OSL--namely OSL stimulated using a constant intensity, constant wavelength light source. Bulur (1996) introduced an alternative technique in which the intensity of the stimulation source is ramped linearly and the OSL monitored throughout the ramp. By adopting this stimulation mode, the OSL is seen as a series of peaks, with each peak corresponding to the optical release of charge from different trap types. Thus, traps for which the photoionisation
Optically Stimulated Luminescence Dosimetry
48
"~->~' v~ 1.21'4[. . . . . . . . . . . . . . . . . . '=; oa)
.=_ E
[
1
'
0
i
0.8-
-.
~
"~",',
#1 ~
-'. "
TV
0.6-
9
V
U
u
0.4"O Z
9149 " 9
00
"o
E
(a)
#2
i1 V v
0.2"
I
*
9 I
g
vv
0.0
I
1
.~ 3000
,.U
.~ 2500 r v
D
2000
9
v v I I .......... V ~ v . _ _ ~
TL for #100
(b)
9 Phosphorescence ~ during heating , 9
700x10~ 600x103 500x103 t400x103
xi
~--
v
lsoo
,,,'-'|176176176 o
500
oe--
0
o.
350
To
Phosphorescence during cooling
~_/
300x103 "g 200x103 .~ _J
100x103
400
450
I--.
=~=~--,..=,--.~J 0 500 550
Temperature (K) Fig. 2.21. (a) Thermal quenching of luminescence from three samples of A1203:C as a function of temperature. The initial peak in the luminescence is caused by thermal release of charge from traps and subsequent phosphorescence emissionmas is evidenced by the correlation of the TL peak position with the maximum in the luminescence versus temperature curve (b) (from Akselrod et al., 1998).
cross-section is large at the particular wavelength used in the experiment are emptied first and are shown as a peak in a plot of OSL versus stimulation time. Traps with smaller photoionisation cross-sections empty more slowly and give rise to OSL peaks that appear at later times. Thus, traps with fast, slow and medium rates of de-trapping may be more easily resolved using LM-OSL compared with CW-OSL. To describe the shape of an LM-OSL curve mathematically, consider a one-trap/onecentre model in which electrons of concentration n are trapped at a localised state until stimulated into the conduction band by absorption of a photon (of wavelength hvex). The freed electron is then able to recombine at a trapped hole centre, producing an emission
Optically Stimulated Luminescence Theory
49
photon (luminescence) of wavelength hvem. For first-order kinetics (negligible retrapping) the rate of de-trapping is given by Eq. (2.30), and the corresponding luminescence (CW-OSL) intensity by Eq. (2.31), where the time-constant of the decay is ~-~ = 1/o-q~, where all the terms have their usual meaning. If, however, the intensity is linearly ramped from zero to a maximum value q~m according to: qb(t) = yt
(2.66)
then Eq. (2.30) is replaced by: dn = - o-ytn dt
(2.67)
from which we obtained a Gaussian function" n = no exp ---~-
(2.68)
The luminescence intensity (i.e., the LM-OSL intensity) is then given by: trYt2 t lose = noO'yt exp { -- --~
(2.69)
Note that for first-order kinetics the principle of superposition applies (as discussed in Section 2.2) and thus, if there are K traps of type-i, following Whitley and McKeever (2001) the equation may be rewritten as: lOSE -- yt ~K
noitri exp {TOrit2] -- - - ~
(2.70)
i--1
An experimental LM-OSL curve from a sample in which several traps are emptying simultaneously, but at different rates, can thus be described as a simple sum of first-order LM-OSL curves. Simulated example LM-OSL curves for different values of the product To-, for fixed (normalised) values of no are shown in Fig. 2.22. It should be noted that each peak starts from t -- 0, no matter what values of o- and y are used. The shape of the LMOSL curve for a single trap is that of a linearly increasing function (in proportion to the linear increase in the stimulation power) followed by a Gaussian decrease in OSL intensity as the traps deplete. The time at which the maximum is achieved is given by: /
tmax
= ~ 1
(2.71)
O-T
and the LM-OSL maximum intensity is" /max
9OSL
n~
tmax
{l}
exp-
-~
(2.72)
Thus, the ionisation cross-section at the wavelength used in the experiment can be determined from the known value of y, and the observed value of tmax. We also observe that the position of the LM-OSL peak is dependent on both the wavelength (through the wavelength dependence of or) and the linear modulation ramp rate y. Specifically,
Optically Stimulated Luminescence Dosimetry
50
2,5
.~
2
.01 units
.oo un, s
1,5
0
. ,
0,5
0
0
10
20
30
40
50
60
Time (s) Fig. 2.22. Simulated LM-OSL curves for first-order kinetics, using three different values of the product 0"% For fixed ramp rates y, the LM-OSL peaks appear at shorter times as the photoionisation cross-section or increases. Similarly, for fixed o-, the peaks appear at shorter times as the ramp rate increases. All peaks start at t = 0. For a system displaying first-order de-trapping and multiple peaks, the net LM-OSL curve is a simple addition of peaks like those illustrated.
the peak will shift to shorter times at higher ramp rates or for larger values of the crosssection. If the photoionisation cross-section has a significant temperature dependence (see Section 2.4.5), the position of the LM-OSL peak will also shift with temperature. Adopting a general-order kinetics model in which the rate of re-trapping of the released charge is significant compared to the rate of recombination (Bulur, 1996) yields: dn
dt
--
oTtn b
(2.73)
nbo-1
where b is a dimensionless positive number; b > 0, b # 1. The solution is:
O")tt2 IOSL =
no~ryt (b - 1) ~ -
]b/(1-b) + 1
(2.74)
In contrast to the first-order case, the superposition principle no longer applies if there is more than one type of trap and an experimental LM-OSL curve cannot simply be described as the sum of several non-first-order processes. The maximum of a general-order LM-OSL peak is achieved at time tmax, where:
tmax
(2.75)
(ry(b + 1)
at which the maximum intensity is:
omax (2no)(')( SL= b + l ~
b+l
)bJ, b,
(2.76)
Optically Stimulated Luminescence Theory
51
Fig. 2.23. ExperimentalLM-OSL curves from a variety of materials. (a) Quartz: 10 Gy; 280~ for 10 s pre-heat; s pre-heat;75~ measurementtemperature.(c) 160~ measurementtemperature.(b) A1203:C: 100 mGy; 180~ BeO: 100 mGy; 180~ s pre-heat; 75~ measurementtemperature. (d) NaCI: 100 mGy; 225~ s pre-heat; 25~ measurementtemperature.The curveswere obtainedusingblue lightfrom a Rise TL/OSL DA- 15 system.The inset in each case shows the CW-OSL curves obtainedunder the same conditions (from Bulur et al., 2001).
The LM-OSL technique was first applied to OSL from ZnS and SrS IR-stimulable storage phosphors by Bulur and Grksu (1997). A selection of experimental LM-OSL curves (and their corresponding CW-OSL curves) is shown in Fig. 2.23. Each curve has been obtained after stimulation of the irradiated samples with blue light, under the conditions noted in the caption. The descriptions of LM-OSL and CW-OSL curves for first-order kinetics have assumed that the luminescence intensity is directly proportional to the de-trapping rate, dn/dt. These analyses lead to the realisation that the de-trapping rate is directly proportional to the stimulation intensity. Thus, from Eq. (2.31), with p = trY, we see that [d ln(Icw_osL)/dt] oc cI9 (i.e., the slope of the ln(Icw_osL)-versus-t curve is directly proportional to the stimulation power @). Bulur et al. (2001) demonstrated this to be the case for quartz, A1203:C and BeO, but not for NaC1. For the latter a non-linear, saturating exponential relationship was found. This may be due to the inadequacy of first-order kinetics or the simple one-trap/one-centre model in describing the OSL from this material. If first-order kinetics, or the simple model, do not apply, there is no longer a direct proportionality between ICW-OSL and dn/dt and, consequently, between d ln(Icw_osL)/dt and @. Bulur et al. (2001) treated the situation empirically, however, and viewed the experimentally obtained relationship between d ln(Icw_osL)/dt and @ as a true indication of the relationship between the de-trapping
52
Optically Stimulated Luminescence Dosimetry
rate and @, and modified the LM-OSL curve accordingly. The modified LM-OSL expression is found to be adequate in describing the LM-OSL curve shape for NaC1. 2.5.2. Relationship between LM-OSL and CW-OSL If the stimulation ramp in an LM-OSL experiment is arranged so that it reaches a final stimulation power q~f in time re, such that @f is equal to the fixed stimulation power used in a CW-OSL experiment (Kuhns et al., 2000), then the observed CW-OSL decay rate will be related to the observed maximum LM-OSL by: 1 ~'d --
O")ttf
t2a• --
(2.77)
tf
Bulur (2000) describes a simple mathematical transformation that allows one to convert CW-OSL curves into LM-OSL curves. First define a variable u, thus:
u-- ~/2tP
(2.78)
or /12
t=
(2.79)
2P
where P is the total measurement period in an LM-OSL experiment, and u has the dimensions of time. Substituting Eq. (2.79) in the expression for CW-OSL (Eq. (2.31)) and multiplying by u/P yields: IOSL
{
n~176 exp -p
(2.80)
which is of the same form as the expression for LM-OSL (Eq. (2.69)). Comparing Eq. (2.69) with Eq. (2.80) we see that u maps with t, while clearly cI)/P = 3t. The transformation of u to the time t domain scales with the choice of P. Note that if P is made equal to the observation time for the CW-OSL experiment, then the scaling factor is , ~ . In Fig. 2.24 we show an example CW-OSL curve, the transformed (or "pseudo") LMOSL curve, and an experimental LM-OSL curve for comparison, for IR-stimulated luminescence from potassium feldspar. The agreement between the pseudo-LM-OSL and the actual LM-OSL for this material is clear. The transformations can also be demonstrated for second-order and general-order kinetics (Bulur, 2000). Table 2.1 lists the obtained expressions of the peak position (Umax) and the peak maximum ( I ~ ) for first-, second- and general-order kinetics for the pseudo-LM-OSL curves. 2.5.3. Wavelength dependence of LM-OSL The excitation wavelength dependence is shown in Fig. 2.25. In this figure the Whitley and McKeever (2001) simulations of the LM-OSL curves to be expected for a system with three trapping states with optical threshold energies of 1.9, 2.5 and 2.9 eV, as the stimulation energy is changed are illustrated. The data show how the curves merge into
Optically StimulatedLuminescenceTheory
53
Fig. 2.24. CW-OSL,pseudo-LM-OSL and LM-OSL curves from Na-feldspar. The CW-OSL and real LM-OSL curves were obtained using IR-stimulation. A ramp time of P = 100 s was used, both in the experiments and in the transformation calculation using Eq. (2.78) (from Bulur, 2000).
each other as the positions of their peak m a x i m a change with stimulation energy. Note that for long excitation times apparent resonances are seen at stimulation energies corresponding to the three optical threshold energies. At these long times only the slowest emptying traps contribute to the L M - O S L signal at high stimulation energies, while at low energies only those for which the stimulation energy is greater than the threshold energy contribute to the L M - O S L signal. W h e n the stimulation energy is such that several first-order L M - O S L peaks merge together to form one indistinguishable peak, the shape of the net L M - O S L signal may give the appearance of one second-order (b = 2) L M - O S L curve (Whitley and McKeever, 2001). Fitting experimental L M - O S L curves, therefore, can be misleading unless there
Table 2.1 The parameters Umax and Jtos Ltmax for the pseudo-LM-OSL curves for different kinetics (from Bulur, 2000). N is the maximum number of available trapping states and no the number of filled traps, b is the kinetic order Parameter
First-order
Second-order
"m.x
J2"N 3o'q~ no
o'@
I0max SL
{ no exp Umax
1} -
~
3no 8Umax
General-order
2 (b + 1) ~rq~ no 2no
l(2b~
b/(1-b)
(b -k- 1) Umax b-+--11
Optically Stimulated Luminescence Dosimetry
54
Fig. 2.25. Simulated LM-OSL curves as a function of stimulation light energy, for a system with three traps, of optical threshold energies 1.9, 2.5 and 2.9 eV (from Whitley and McKeever, 2001).
exists a priori information about the number of trapping states contributing to the overall LM-OSL signal. If such a priori information is not available, wavelength-dependent LMOSL curves are essential. Example fittings of experimental LM-OSL curves from A1203:C to three first-order processes are shown in Fig. 2.26. The experimental data for the different samples are well fitted to three first-order peaks and suggest photoionisation cross-sections of (3.3-3.7) • 10 - 2 ~ (1.4-1.7) • 10 -19 to (5.8-7.0) • 10 -19 c m 2. Alternatively, one could selectively bleach the different trapping states at different bleaching wavelengths before monitoring the LM-OSL curve (Singarayer and Bailey, 2002). As each trap bleaches successively one can establish the wavelength dependence of the photoionisation cross-section in a similar fashion to that for CW-OSL, as described earlier in this chapter (Section 2.3.3). 2.5.4.
Photoconductivity
As an alternative to monitoring the OSL during a linear increase in stimulation light intensity, one can monitor the current flow through the sample~namely, the PC. In this case, the equivalent expressions to the first- and general-order LM-OSL curves are: /pc - - et.tF'rrlosL
(2.81)
where e is the electronic charge,/x the free carrier mobility, Ze the free cartier lifetime, F the electric field, and IOSL the LM-OSL expression for either first- (Eq. (2.70)) or generalorder (Eq. (2.74)) kinetics. Whitley and McKeever (2001) used linearly modulated PC (LM-PC) to examine traps in A1203 that did not appear in LM-OSL data. In particular, they observed two LM-PC peaks, including a large second LM-PC peak, from samples for which simultaneously
55
Optically Stimulated Luminescence Theory Time (s) 1000 1500
5OO
40
I
I
2000
I
(a)
2500
I
~o
3000 1.2
I
~
1.0
30
0.8 5 ~, b
"~ 20
,._:
0.6 ~" . J
0.4
10 0.2 0
I
~ I
30
0
(b)
25
~
500 I
0.0
I
0"-19
10-1710-18 1
10-20
G (cm 2) Time (s) 1500 2000 I I
1000 I
,
///
o?
15
2500 I
3000 1.2 1.0 0.8
10
0.4
5
0.2
0
.
.
.
.
10-17 10-18 10-19
0
30
500
1000
"
"
0
,
.
.
.
.
,
"
(cm 2)
Time (s) 1500
2000
"
'
.
.
10-20
.
.
.
2500
,
5
0.6
0.0
3000 1.2
25
1.0
20
0.8
15
0.6~
10
0.4 ~
5
0.2
0
10-1710-18
10-19
G (cm 2)
10-20
0.0
Fig. 2.26. Deconvolution of L M - O S L curves for three samples of A1203:C. The solid line corresponds to the data while the dotted line is the best fit. The bar graphs are the photoionisation cross-sections determined from the fitting algorithm. The photoionisation cross-section axis marks are displayed on a 1/0"7 scale (from Whitley and McKeever, 2001).
56
Optically Stimulated Luminescence Dosimetry
measured LM-OSL showed only one peak (Whitley and McKeever, 2001). Possible reasons for this include the suggestion that charge released from the second trap does not recombine radiatively, or that the recombination, if radiative, results in photon emission outside the wavelength detection window for the experiment. The ratio of the LM-PC to LM-OSL yields (from Eq. (2.81)):
/pc = elxF%
(2.82)
/OSL from which we see that as long as the mobility/x and % remain constant during the detrapping process, the LM-OSL signal will peak at the same time as the LM-PC signal. However, if either ~ or % are trap limited, both parameters could change as the detrapping proceeds. Under these circumstances: d/pc dt
B/z Bt
q'eJOSL + - ~
Z~Bt I~e,U
4~,IosL
(2.83)
and we see that the LM-PC signal can peak after or before the LM-OSL signal, depending upon the time dependencies of the mobility and lifetime. Thus, simultaneously measured LM-PC and LM-OSL can reveal details about the recombination and charge carrier transport dynamics that are unavailable from measurements of LM-OSL alone.
2.6. Pulsed OSL 2.6.1. Principles of pulsed OSL The third major stimulation mode, as shown in Fig. 2.2, is pulsed OSL (POSL). To describe the principle behind the measurement of POSL, we begin by considering several stimulation pulses, of different intensities ~i (i = 1,2...) and durations (pulse widths, Ti) such that ~iTi is kept constant. The stimulation rate is proportional to the stimulation power absorbed by the sample and thus, by decreasing the pulse width in proportion to an increase in the stimulation power, the absorbed energy per pulse may be maintained fixed. Furthermore, for first-order kinetics we have (from Eq. (2.30)): An --
0
no-q), dt
(2.84)
and therefore for weak stimulation (i.e., An << n) we see that An oc 45T and thus, the total charge released from the traps is approximately equal in all cases. Normally, when writing rate equations we consider that each recombination event leads instantly to a photon emission event, and thus we normally write losE oc dm/dt. However, actually there is a built-in delay between recombination and photon emission due to the fact that each recombination event leads to the excitation of the recombination centre (luminescence centre) into an excited state, where it remains for a characteristic lifetime z before relaxation occurs with the emission of a luminescence photon. Thus, if the
Optically Stimulated Luminescence Theory
57
concentration of excited states is ne then we may write: dm dt
dn e -
dt
-
ne
(2.85)
r
A photon is only emitted when the excited state relaxes to the ground state and thus ne/'r. If the relaxation time is extremely fast (i.e., ~'e is very small compared with the pulse width T--which is the usual case and is certainly true for CW-OSL) then quasiequilibrium conditions hold (dne/dt ~ 0) and we have IOSL ~ dm/dt, as usual. However, if ~-is comparable to or larger than T, the latter relationship is not true. The key to understanding POSL is to consider how many of the excited states relax during the excitation pulse, versus how many relax after the excitation pulse. If one chooses the pulse width such that T < ~-, then a larger concentration of centres exist in the excited state after a short pulse, compared with those in the excited state after a long pulse (T -> ~'), for the same energy input (q~T). At the end of the stimulation pulse, those centres in the excited state relax with a time constant ~-. The net effect is that the ratio of the photons emitted after the pulse to those emitted during the pulse, increases as the pulse width decreases, for constant stimulation energy. For T < < ~', most of the photons emerge after the pulse. This is shown schematically in Fig. 2.27. Here the simulated OSL curves stimulated by three different pulses, of intensifies 9 = 103, 102 and 20 energy/s, and corresponding pulse widths of T = 6.6, 66 and 300 ms are also shown. A luminescence lifetime of z = 100 ms was assumed. The vertical lines in each case represent the ends of the stimulation pulses. It is clear from inspection of the curves that the ratio of the area under the curves after the pulse to that during the pulse increases as the pulse width decreases. McKeever et al. (1996) demonstrated this experimentally. They stimulated irradiated
IOSL =
7 6
~5 102
0
2 1 0
~ 0
0.1
0.2
0.3
0.4
0.5
0.6
Time (s)
Fig. 2.27. Schematic illustrating the variation in the ratio of the light emitted during a pulse to that emitted after the pulse as the pulse width changes, for fixed stimulation energy per pulse. A luminescence lifetime of z = 100 ms was assumed, with pulse powers varying from 9 = 103 to 20 energy units/s. The pulse widths varied accordingly, from T = 6.6 to 300 ms. In each case it is assumed, for the purposes of this illustration, that the concentration of charge released per pulse is negligible compared with the total trapped charge concentration (i.e., An << n).
58
Optically Stimulated Luminescence Dosimetry
600
I
I
I
I
I
I
I
0.1
0.2
0.3
0.4
500
~"
400 300
._1
oo o
2OO 100 0 0.0
0.5
Time (s)
Fig. 2.28. Build up and decay of POSL from irradiated A1203 and stimulated with an Ar-ion laser (514 nm) for 0.1 s (from Markey et al., 1995).
A1203:C with a pulsed laser of varying pulse width while maintaining the delivered energy per pulse constant. The data of M c K e e v e r et al. (1996) are indicated in Fig. 2.28 in which is shown the build-up and decay of l u m i n e s c e n c e from A1203 for a 0.1 s laser stimulated pulse. The ratio of the light emitted after the pulse to that emitted during the pulse, as a function of pulse width, is shown in Fig. 2.29.
140 120 O.
_ .--
loo
..Q
~
80
0
-o
60
0
.g N n,"
40 20
0
I
I
I
2
4
6
8
Laser pulse duration (ms)
Fig. 2.29. Experimental data showing the variation in the ratio of the luminescence emitted after the pulse to that emitted during the pulse, for POSL from irradiated A1203:C (re-drawn from McKeever et al., 1996).
59
Optically Stimulated Luminescence Theory
Note that the total integral under each of the curves in Fig. 2.27 is the same, and represents the total charge released from the trap. The area under the curve after the pulse is also proportional to this charge, but the proportionality constant is different for each different pulse width. For any given pulse width the area under the curve scales with trapped charge concentration. This principle was developed into an efficient dosimetry system based on A1203:C by Akselrod and McKeever (1999) using a lOOns pulsed Nd:YAG laser. The luminescence lifetime of the OSL signal in this material is 35 ms (Markey et al., 1995; Springis et al., 1995). By using short, high-power pulses only a small fraction of the excited recombination centres (excited F-centres) decay during the pulse and the luminescence is emitted primarily after each pulse has ended. In fact it was demonstrated that > 90% of the light is emitted after the pulse if the pulse width is chosen to be small enough. By synchronizing the detection system to record only the light that is emitted after the stimulation pulse is off, the method provides for very effective separation of stimulation light and luminescence light, leading to the removal of noise due to the background signal from the intense stimulation pulse. The principles of the timing sequence required for the measurement of POSL are described in Chapter 7. 2.6.2. Delayed OSL (DOSL) The OSL signal after the laser pulse should decay according to the lifetime of the luminescence centres being stimulated. As noted, in A1203:C these are F-centres, with an excited state lifetime of 35 ms at room temperature. In some A1203 samples, however, a much longer lifetime is observed. The OSL is seen to decay much more slowly than one would expect due to re-trapping of the released charge in shallow traps. This is illustrated in the data of Markey et al. (1995) shown in Fig. 2.30; here the OSL decay after the stimulation pulse is fitted to two exponentials. The faster decay is due the intrinsic
600 "~"
500 j
C
~d
400
t~
oC 0 gO .C m
E ~
300 200 100
\
\\
~
0 0
0
0.0
~ 7
~
0.1
. . . . . .
I
0.2 Time (s)
I
0.3
0.4
Fig. 2.30. Decayof OSL, following a stimulation pulse from a laser, at 25~ in AleO3:C. The decay has been fitted to two exponentials. The lifetimeof the fast decay is 35 ms, corresponding to the decay of excited F-centres. The slower decay (545 ms at this temperature) is due to phosphorescence caused by the transfer of charge from deep traps to shallow traps (from Markey et al., 1995).
60
Optically Stimulated Luminescence Dosimetry
F-centre luminescence lifetime, and the other is a much slower (545 ms at 25~ highly temperature-dependent decay. By performing the experiment at different temperatures, it is revealed that the second slower component is due to trapping of charges in two shallow traps (with activation energies 0.65 and 0.77 eV, respectively). This "delayed" OSL emission has been given the acronym DOSL ("delayed OSL"; Yoder and Salasky, 1997) but the method was suggested as a technique in dosimetry as early as 1969 by Miller and colleagues (Tochilin et al., 1969; Rhyner and Miller, 1970) who studied the OSL emission from BeO. Later it was suggested for CaF2:Mn (Hanniger et al., 1982), CaSO4:Dy (Pradhan and Ayyanger, 1977; Pradhan and Bhatt, 1981) and A1203:C (Yoder and Salasky, 1997; Akselrod et al., 1998b), while Jaek et al. (1999) used the method to study deep traps in feldspars and quartz. In each case one relies upon the fact that the traps to which the charge has been optically transferred during the stimulation are unstable at room temperatures so that the charge leaks slowly out of these traps and recombines at luminescence centres. In this sense, DOSL may also be called optically stimulated phosphorescence.
2.7. Phototransferred effects
2.7.1. Procedure DOSL, as described above, is due to the phototransfer of charge from deep, stable traps to shallow, unstable traps. If the temperature at which the phototransfer takes place is low enough, the traps into which the charge is transferred are then stable and no phosphorescence is seen due to charge leakage. This is the essence of a family of techniques that use the phototransfer effect as a means of dosimetry. We discuss here the archetype of phototransferred effects, namely phototransferred TL, or PTTL. PTTL is observed in many luminescence dosimetry materials. It is defined as that TL signal originating from shallow traps following the optical transfer of charge from deep traps (donor traps) to the shallow traps (acceptor traps). PTTL can be observed experimentally in a number of ways. For example, one might irradiate at a temperature T~ at which all trapping centres are stable, and then heat to a temperature/'2 to empty the shallow (acceptor) traps, before cooling back to T~. At this point, the sample is illuminated with light of intensity q~ for a period P during which time charge is emptied from the deep (donor) traps. Some of this charge recombines at recombination centres (producing OSL, if radiative) and some charge is captured by the shallow, acceptor traps. Subsequent heating of the sample yields a TL signal (i.e., the PTTL signal) due to the thermal release of charge from the shallow trap. Alternative schemes, such as cooling to temperature T3, are possible instead of heating to 7"2 after the initial irradiation. At T3, traps that were too shallow to fill at temperature T~ are now stable. Subsequent illumination may then transfer charge from all-filled hightemperature (deep) traps into the empty, low-temperature (shallow) traps. Subsequent heating from low temperature (T3) releases charge from the shallow traps yielding a PTTL signal.
Optically Stimulated Luminescence Theory
61
Both these schemes have been used in dosimetry. Colyott et al. (1996, 1997) used the first of the schemes as a method of UV dosimetry. They irradiated A1203:C with gamma irradiation, and heated the sample to remove the main TL signal (near 200~ The sample was then illuminated with UVB irradiation, transferring charge from deep traps (stable above 600~ such that on heating a second time a PTTL signal was observed at 200~ The size of the PTTL peak was found to be proportional to the dose of UVB irradiation, and thus the technique can be used as a UVB dosimetry method. Miller et al. (1988) used the second scheme mentioned above as a method of ionising radiation dosimetry. The material used by Miller et al. was CaFz:Mn. The sample was irradiated at room temperature before being cooled to liquid nitrogen temperature. The dosimeter was then stimulated with light for a fixed period with a fixed intensity. The resulting PTTL signal from the shallow traps was then found to be proportional to the original dose of ionising radiation. Other similar results have been found for LiF (Buckman and Payne, 1976; Driscoll et al., 1983) and other common dosimetry materials. The early literature is reviewed by Jain (1983) and by McKeever (1985), while Sono and McKeever (2002) give some more recent examples. These practical applications of PTTL serve to illustrate that the PTTL intensity is proportional to both the original dose of ionising radiation (for fixed optical stimulation conditions), and the dose delivered during optical stimulation (e.g., optical energy fluence) for a fixed dose of ionising radiation. For this to be truly useful as a dosimetric tool, however, the PTTL response versus either ionising radiation dose, or stimulation energy fluence should be linear over several orders of magnitude. Not all dosimetric materials exhibit these properties and thus only a limited number of materials have proved useful in this regard. 2.7.2. M a t h e m a t i c a l d e s c r i p t i o n a n d t y p i c a l d a t a In general, PTTL is useful for examining the optically stimulated charge transfer processes that occur between trapping centres in OSL materials during illumination. As such, the models to describe PTTL are informative for understanding OSL processes. The simplest model necessary to produce PTTL is that of one deep trap, one shallow trap, and one recombination centre. (These are the minimum centres required for the effect to occur.) If n l and n2 are the shallow and deep electron trap populations, respectively, and m is the concentration of holes in recombination centres, then, at some point after irradiation and immediately at the start of the illumination period, one might have nl0 = 0 and n20 = m 0. The illumination is then assumed to excite electrons from the deep trap (the "donor trap") at a rate f -- o-q), after which they are captured by the shallow trap (the "acceptor traps"), such that we may write: dn2
dt
-- - n z f -k- nc(N2 - n2)A2
dn 1
dt
-
nc(N1 - hi)A1
(2.86)
(2.87)
Optically Stimulated Luminescence Dosimetry
62
and dm
dt
-- - n c m A m --
m
r
(2.88)
where ~"= (ncAm)-1 is the recombination lifetime and all other terms have their usual meaning. At quasi-equilibrium (dnc/dtn << dni/dt, dm/dt; i = 1 or 2) and assuming no re-trapping into the deep trap during illumination the solution for the growth of n~ as a function of illumination time t is: nl(t) = NI[1 - exp{ - Bt}]
(2.89)
where B = ncA1, and both n2 and m decay exponentially, thus: nz(t) = n20 exp{ - 0~}
(2.90)
m(t) -- mo exp{ - t/•'}
(2.91)
and
When considering the warming of the sample after an illumination period t* one has to take into account the competition for the released charges to either re-trap in empty deep traps (concentration N2 - nz(t*)), or recombine with trapped holes (concentration m(t*)). As described by Chen and McKeever (1997) under these conditions the PTTL signal is given by: SpTTL(t*) -- C exp{ - t*/T}NI[1 -- exp{ -- Bt*}] [Nz/n2o - exp{ - t'f}]
(2.92)
where C is a constant. Eq. (2.92) describes a monotonically increasing function~i.e., the PTTL signal grows with illumination time until either the donor traps are depleted or the acceptor traps become full. The maximum PTTL signal (at t = c~) is then SVrTL(C~)-Cn2~/N2. An example of this type of behaviour is given in Fig. 2.3 l a. However, one often finds cases in which the PTTL intensity increases to a maximum, and then decreases for prolonged illumination times. An example of this type of behaviour is shown in Fig. 2.3 l b and Milanovich-Reichhalter and Vana (1990) showed additional examples for quartz. This type of behaviour results from the simultaneous filling and emptying of the shallow trap during illumination. For the example shown in this figure (A1203:C) the wavelengths used for the PTTL (---307 nm) are known to not only empty charges from the deep, donor traps, but also empty them from the shallower, acceptor traps. As a result of the competition between trap filling and trap emptying, the PTTL signal reaches a maximum, and then decays (eventually decaying to zero). A more unusual hehaviour is illustrated in Fig. 2.32 inwhich we show the P T T L curves from a shallow trap in quartz. Here, the PTTL increases, then decreases, but, as long as the temperature is low enough such that the PTTL peak is stable, then the PTTL peak does not obviously decay to zero at long illumination times. Alexander et al. (1997) used the model of Fig. 2.10 to explain these results. Referring to the model of Fig. 2.10, level 4 represents a radiative centre and recombination of electrons with holes at this centre produces OSL during the illumination period. However, level 5 is a competing non-radiative centre and
Optically Stimulated Luminescence Theory
63
1.2
E~
//
~
2'51=+04
LI I~
~:~ ~~ 1.5E+04
|1
_~ ,0~0,
0.2 IT
~. o.oE§
~N 0.6
i04
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7000
Illumination Time (min) 1.2 [
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it II
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Illumination time (min) Fig. 2.31. Example PTTL versus stimulation time curves for (a) MgO:Cu and (b) A1203:C. The PTTL curves are shown in the insets (from Sono and McKeever, 2002).
electrons recombining here are lost from both PTTL and OSL. The PTTL signal is a result of electrons in the acceptor trap (level 1) recombining with holes in the radiative centre (level 4). It is important to realise, therefore, that the PTTL signal will follow the smaller of these two concentrations, i.e., SeTTI~ = min(nl, m4) Thus, even though the electrons in level 1 may be increasing during illumination, the number of available holes in level 4 is decreasing and, depending on the initial conditions and the relative concentrations in the various levels, one can arrive at the situation where the electron concentration in the acceptor trap ( n l ) is greater than the hole concentration in the radiative centre (m4). Thus, after an initial increase, the PTTL due to level 1 will start to follow m4 and will decrease with illumination time. At very long times, one can deplete the charge in the donor traps such that the final PTTL curve will be characterised by an increase, followed by a decrease, followed by a stable (non-zero) level.
Optically Stimulated Luminescence Dosimetry
64
6
I
~.
I
30,
,
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,
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,
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~
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,
IlluminationTime(hrs)
i 100
i 200
[ 30 0
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J 400
500
Illumination T i m e (min) Fig. 2.32. PTTL (at 363 K) from a sample of quartz versus time of illumination with 460 nm light, at 82 and 300 K. The inset shows the behaviour of the 273 K PTTL peak illuminated with 325 nm light at 100 K for 24 h (from Alexander et al., 1997).
This simple "hand-waving" argument, however, is too simplistic even for a relatively simple model such as that shown in Fig. 2.10. Alexander and McKeever (1998) analysed this and several other models in detail and solved the simultaneous differential equations numerically without introducing assumptions (such as quasi-equilibrium, or no retrapping). This analysis indicated that a variety of PTTL-versus-time curve shapes are possible, including shapes such as those illustrated in Figs. 2.31 and 2.32. The key parameters, for a particular model and illumination conditions, are the initial concentrations in the various traps at the start of the illumination period, and the photoionisation cross-sections at the wavelengths used. The important points illustrated by these analyses are that: (i) a decrease in the PTTL signal at long illumination times does not necessarily mean that there is simultaneous optical excitation out of the acceptor trap; (ii) in all cases a steady-state level will eventually be reached at long illumination times. The steady-state value may be zero, or non-zero, depending upon the relative sizes of the donor trapped electron concentration and the radiative centre concentration at the start of the illumination period. Finally, we note that the dependence of the PTTL signal on the wavelength of the stimulation light will be govemed by the photoionisation cross-section of the donor traps. By examining the PTTL intensity as a function of stimulation wavelength (for constant photon flux) one can determine the donor trap photoionisation cross-section, in a manner similar to that described above for OSL in Section 2.3.2. Thus, PTTL was used by
Optically Stimulated Luminescence Theory
65
Alexander and McKeever (1998) to determine the shape of the or(A) curve for the donor traps in quartz, using the trap responsible for the 110~ PTTL peak as the acceptor trap. Several deep traps were found to empty when stimulated with light over the range 300500 nm. Each contributed to the PTTL signal, but to different extents, according to the wavelength used. This observation is in general agreement with similar conclusions reached by Smith and Rhodes (1994) using CW-OSL, and Singarayer and Bailey (2002) using LM-OSL.
2.8. Radiophotoluminescence 2.8.1. Procedure Radiophotoluminescence (RPL) is that luminescence stimulated by light from defects which have been created by the radiation, but which do not ionise during optical excitation (Perry, 1987). The luminescence emission results from an intra-centre excitation from a ground state to an excited state of a defect, which has been produced by the radiation itself. The first published use of this principle in dosimetry is probably the work on F2-centres in LiF by Regulla (1972) and later by Miller and Endres (1990). More recently, the principle was developed by Piesch and colleagues (Piesch et al., 1986, 1990, 1993) for personal dosimetry using glass materials. A difficulty with the technique is that since the optical stimulation only excites the defects but does not ionise them, the dosimeters are not zeroed and require high temperature annealing to be able to be used again. An advantage of this, however, is that they may be archived for later re-reading of the dose if or when necessary. The use of F2-centres in LiF is particularly useful for high-dose dosimetry (Miller, 1996). F-centres are created in LiF due to the creation of self-trapped excitons. The hole is localised at two F - ions forming an F2 molecule aligned along the {110) close-packed anion direction. Relaxation of the defect results in kinetic energy transfer to the F2 molecule and a displacement sequence is initiated along the (110) direction. This leaves an F vacancy with a trapped electron (an F-centre) and a trapped interstitial F ion (known as an H-centre). The latter is most likely stabilised by an existing monovalent impurity (forming an HA-centre) or a divalent impurity (forming an Hz-centre). Increased irradiation results in the formation of pair of F-centres (F2-centres), which may be detected through optical absorption and/or PL (e.g., Waibel et al., 1993). Using the latter technique, Regulla (1972) and Miller and Endres (1990) proposed the use of the characteristic PL emission from F2_ centres for dosimetry--particularly, high-dose dosimetry. Phosphate glass dosimeters were used by Piesch and colleagues as personal dosimeters using RPL (Piesch et al., 1986, 1990, 1993). By selectively timing the luminescence reading at two points after the laser excitation pulse, the non-radiation-induced PL signal can be separated from the radiation-dependent RPL by subtraction. Repeated measurements of the absorbed dose are possible since the laser excitation only stimulates PL emission from the radiation-induced defects, and does not destroy (bleach) the defects. The system was developed by Toshiba into an automated system (Piesch et al., 1990, 1993) that was capable of measuring personal doses as low as several IxSv.
66
Optically Stimulated Luminescence Dosimetry
References Akselrod, M.S., McKeever, S.W.S., 1999. A radiation dosimetry method using pulsed optically stimulated luminescence. Radiat. Prot. Dosim. 81, 167-176. Akselrod, M.S., Agersnap Larsen, N., Whitley, V., McKeever, S.W.S., 1998a. Thermal quenching of F-center luminescence in A1203:C. J. Appl. Phys. 84, 3364-3373. Akselrod, M.S., Lucas, A.C., Polf, J.C., McKeever, S.W.S., 1998b. Optically stimulated luminescence of A1203. Radiat. Meas. 29, 391-399. Alexander, S.C., McKeever, S.W.S., 1998. Phototransferred thermoluminescence. J. Phys. D: Appl. Phys. 31, 2908-2920. Alexander, S.C., Morris, M.F., McKeever, S.W.S., 1997. The time and wavelength response of phototransferred thermoluminescence in natural and synthetic quartz. Radiat. Meas. 27, 153-159. Bailey, R.M., 2001. Towards a general kinetic model for optically and thermally stimulated luminescence of quartz. Radiat. Meas. 33, 17-45. Bailey, R.M., Smith, B.W., Rhodes, E.J., 1997. Partial bleaching and the decay form characteristics of quartz OSL. Radiat. Meas. 27, 123-136. Bailiff, I.K., 2000. Characteristics of time-resolved luminescence in quartz. Radiat. Meas. 32, 401-405. Banks, P.W., Brand, S., Jaros, M., 1980. Optical cross sections associated with deep levels in semiconductors. I. J. Phys. C: Solid St. Phys. 13, 6167-6180. Blakemore, J.S., Rahimi, S., 1984. Models for mid-gap states in GaAs. In: Willardson, R.K., Beer, A.C. (Eds.), Semi-insulating GaAs: Semiconductors and Semimetals, vol. 20. Academic Press, Orlando, pp. 233-361. BiSer, K.W., 1990. Survey of Semiconductor Physics: Electrons and Other Particles in Bulk Semiconductors. Van Nostrand Reinhold, New York. B0tter-Jensen, L., Duller, G.A.T., Poolton, N.R.J., 1994a. Excitation and emission spectrometry of stimulated luminescence from quartz and feldspars. Radiat. Meas. 23, 613-616. BCtter-Jensen, L., Agersnap Larsen, N., Mejdahl, V., Poolton, N.R.J., McKeever, S.W.S., 1994b. Luminescence sensitivity changes in quartz as a result of annealing. Radiat. Meas. 24, 535-541. Br~iunlich, P., 1979. Introduction and basic principles. In: Br~iunlich, P. (Ed.). Thermally Stimulated Relaxation in Solids. Springer Verlag, Berlin, pp. 1-33. Buckman, W.G., Payne, M.R., 1976. Photostimulated thermoluminescence of lithium fluoride as an ultraviolet radiation dosimeter. Health Phys. 31,501-504. Bulur, E., 1996. An alternative technique for optically stimulated luminescence (OSL) experiment. Radiat. Meas. 26, 701-709. Bulur, E., 2000. A simple transformation for converting CW-OSL curves to LM-OSL curves. Radiat. Meas. 32, 141-145. Bulur, E., Grksu, H.Y., 1997. IR stimulated luminescence from ZnS and SrS based storage phosphors: A reexamination using the linear modulation technique. Phys. Stat. Sol. (a) 161, R9-R10. Bulur, E., BCtter-Jensen, L., Murray, A.S., 2001. LM-OSL signals from some insulators: an analysis of the dependency of the detrapping probability on stimulation light intensity. Radiat. Meas. 33, 715-719. Chen, R., Leung, P.L., 2002. The decay of OSL signals as stretched exponential functions. Radiat. Meas. 37, 519-526. Chen, R., McKeever, S.W.S., 1997. Theory of Thermoluminescence and Related Phenomena. World Scientific Press, Singapore. Colyott, L.E., Akselrod, M.S., McKeever, S.W.S., 1996. Phototransferred thermoluminescence in ct-A1203:C. Radiat. Prot. Dosim. 65, 263-266. Colyott, L.E., Akselrod, M.S., McKeever, S.W.S., 1997. An integrating ultraviolet-B dosemeter using phototransferred thermoluminescence from ot-A1203:C. Radiat. Prot. Dosim. 72, 87-94. Dexter, D.L., Klick, C.C., Russel, G.A., 1955. Criterion for the occurrence of luminescence. Phys. Rev. 100, 603 -605. Ditlefsen, C., Huntley, D.J., 1994. Optical excitation of trapped charges in quartz, potassium feldspars and mixed silicates: the dependence on photon energy. Radiat. Meas. 23, 675-682. Driscoll, C.M.H., McKinlay, A.F., Smith, P.A., 1983. Multiple dose re-assessment of lithium fluoride by the ultraviolet phototransferred thermoluminescence technique. Radiat. Prot. Dosim. 6, 117-120.
Optically Stimulated Luminescence Theory
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Grimmeis, H.G., Ledebo, L.-,~., 1975a. Photo-ionisation of deep impurity levels in semiconductors with nonparabolic bands. J. Phys. C: Sol. Stat. Phys. 8, 2615-2626. Grimmeis, H.G., Ledebo, L.-,~., 1975b. Spectral distribution of photoionisation cross-sections by photoconductivity measurements. J. Appl. Phys. 46, 2155-2162. Hanniger, J., Horlbeck, B., Hubner, K., Prokert, K., 1982. The evaluation of CaFz:Mn-polyethylene detectors with aid of optically stimulated luminescence (OSL). Nucl. Instr. Meth. 204, 209-212. Huntley, D.J., Short, M.A., Dunphy, K., 1996. Deep traps in quartz and their use for optical dating. Can. J. Phys. 74, 81-91. Htitt, G., Jaek, I., 1993. Photostimulated luminescence of some materials and its dosimetry applications. Nucl. Tracks Radiat. Meas. 21, 95-98. Htitt, G., Jaek, I., Tchonka, J., 1988. Optical dating: K-feldspars optical response stimulation spectra. Quat. Sci. Rev. 7, 381-385. Jaek, I., Htitt, G., Streltsov, A., 1999. Study of deep traps in alkali feldspars and quartz by the optically stimulated afterglow. Radiat. Prot. Dosim. 84, 467-470. Jain, V.K., 1983. Photostimulated thermoluminescence. In: Horowitz, Y.S. (Ed.), Thermoluminescence and Thermoluminescent Dosimetry, vol. II. CRC Press, Boca Raton, pp. 173-211. Jaros, M., 1977. Wave function and optical cross sections with deep centers in semiconductors. Phys. Rev. B 16, 3694-3706. Kuhns, C.K., Agersnap Larsen, N., McKeever, S.W.S., 2000. Characteristics of LM-OSL from several different types of quartz. Radiat. Meas. 32, 413-418. Landsberg, P.T., 1991. Recombination in Semiconductors. Cambridge University Press, Cambridge. Lucovsky, G., 1964. On the photoionisation of deep impurity centers in semiconductors. Sol. State Commun. 3, 299-302. Markey, B.G., Colyott, L.E., McKeever, S.W.S., 1995. Time-resolved optically stimulated luminescence from et-AI203:C. Radiat. Meas. 24, 457-463. Markey, B.G., McKeever, S.W.S., Akselrod, M.S., BCtter-Jensen, L., Agersnap Larsen, N., Colyott, L.E., 1997. The temperature dependence of optically stimulated luminescence from ot-A1203. Radiat. Prot. Dosim. 65, 185-189. McKeever, S.W.S., 1985. Thermoluminescence of Solids. Cambridge University Press, Cambridge. McKeever, S.W.S., Akselrod, M.S., Markey, B.G., 1996. Pulsed optically stimulated luminescence dosimetry using ot-Al203:C. Radiat. Prot. Dosim. 65, 267-272. McKeever, S.W.S., BCtter-Jensen, L., Agersnap Larsen, N., Duller, G.A.T., 1997a. Temperature dependence of OSL decay curves: Experimental and theoretical aspects. Radiat. Meas. 27, 161-170. McKeever, S.W.S., Agersnap Larsen, N., BCtter-Jensen, L., Mejdahl, V., 1997b. OSL sensitivity changes during single aliquot procedures: computer simulations. Radiat. Meas. 27, 75-82. Milanovich-Reichhalter, I., Vana, N., 1990. Phototransferred thermoluminescence in quartz. Radiat. Prot. Dosim. 33, 211-213. Miller, S.D., 1996. High dose dosimetry using optically stimulated luminescence. Radiat. Prot. Dosim. 66, 201-204. Miller, S.D., Endres, G.W.R., 1990. Laser-induced, optically stimulated M-centre luminescence in LiF. Radiat. Prot. Dosim. 33, 59-62. Miller, S.D., Endres, G.W.R., McDonald, J.C., Swinth, K.L., 1988. Cooled optically stimulated luminescence in CaFz:Mn. Radiat. Prot. Dosim. 25, 201-206. Milman, I.I., Kortov, V.S., Nikiforov, S.V., 1998. An interactive process in the mechanism of the thermally stimulated luminescence in anion-defective et-Al203 crystals. Radiat. Meas. 29, 401-410. Mott, N.F., Davies, E.A., 1971. Electronic Processes in Non-crystalline Materials. Clarendon Press, Oxford. Murray, A.S., Wintle, A.G., 1998. Factors controlling the shape of the OSL decay curve in quartz. Radiat. Meas. 29, 223-229. Nanjundaswamy, R., Lepper, K., McKeever, S.W.S., 2002. Thermal quenching of thermoluminescence in natural quartz. Radiat. Prot. Dosim. 100, 305-308. Noras, J.M., 1980. Photoionisation and phonon coupling. J. Phys. C: Solid St. Phys. 13, 1779-1789. Perry, J.A., 1987. RPL Dosimetry: Radiophotoluminescence in Health Physics. Adam Hilger, Bristol.
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Optically Stimulated Luminescence Dosimetry
Piesch, E., Burgkhardt, B., Fischer, M., R6ber, H.G., Ugi, S., 1986. Properties of radiophotoluminescence glass dosimeter systems using pulsed laser UV excitation. Radiat. Prot. Dosim. 17, 293-297. Piesch, E., Burgkhardt, B., Vilgis, M., 1990. Photoluminescence Dosimetry: Progress and present state of the art. Radiat. Prot. Dosim. 33, 215-226. Piesch, E., Burgkhardt, B., Vilgis, M., 1993. Progress in phosphate glass dosimetry: experiences and monitoring with a modern dosimetry system. Radiat. Prot. Dosim. 47, 409-413. Polf, J.C., 2002. A study of optically stimulated luminescence in A1203 fibers for the development of a real-time, fiber optic dosimetry system. PhD Thesis, Oklahoma State University, Stillwater. Poolton, N.R.J., BCtter-Jensen, L., Ypma, P.J.M., Johnsen, O., 1994. Influence of crystal structure on the optically stimulated luminescence properties of feldspars. Radiat. Meas. 23, 551-554. Poolton, N.R.J., BCtter-Jensen, L., Johnsen, O., 1995a. Influence on donor electron energies of the chemical composition of K, Na and Ca aluminosilicates. J. Phys.: Condens. Matter 7, 4751-4762. Poolton, N.R.J., BCtter-Jensen, L., Johnsen, O., 1995b. Thermo-optical properties of optically stimulated luminescence in feldspars. Radiat. Meas. 24, 531-534. Poolton, N.J., Nicholls, J.E., BCtter-Jensen, L., Smith, G.M., Reidi, P.C., 2001. Observation of free electron cyclotron resonance in NaA1Si308 feldspar: direct determination of the effective electron mass. Phys. Stat. Sol. (b) 225, 467-475. Poolton, N.R.J., Wallinga, J., Murray, A.S., Bulur, E., BCtter-Jensen, L., 2002a. Electrons in feldspars I: on the wavefunction of electrons trapped at simple lattice defects. Phys. Chem. Minerals 29, 210-216. Poolton, N.R.J., Ozanyan, K.B., Wallinga, J., Murray, A.S., BCtter-Jensen, L., 2002b. Electrons in feldspars II: a consideration of the influence of conduction band-tail states on luminescence processes. Phys. Chem. Minerals 29, 217-225. Pradhan, A.S., Ayyanger, K., 1977. Radiation dosimetry by photostimulated luminescence of CaSOa:Dy. Int. J. Appl. Radiat. Isotop. 28, 534-535. Pradhan, A.S., Bhatt, R.C., 1981. Photostimulated luminescence and thermoluminescence in CaSO4:Dy. Phys. Stat. Sol. (a) 68, 405-411. Regulla, D.F., 1972. Lithium fluoride dosimetry based on radiophotoluminescence. Health Phys. 22, 419-421. Rhyner, C.R., Miller, W.G., 1970. Radiation dosimetry by optically stimulated luminescence in BeO. Health Phys. 18, 681-684. Ridley, R.K., 1988. Quantum Processes in Semiconductors. Clarendon Press, Oxford. Singarayer, J.S., Bailey, R.M., 2003. Component-resolved bleaching of quartz optically stimulated luminescence: Preliminary results and implications for dating (submitted). Radiat. Meas. Smith, B.W., Rhodes, E.J., 1994. Charge movements in quartz and their relevance to optical dating. Radiat. Meas. 23, 329-333. Sono, D.A., McKeever, S.W.S., 2002. Phototransferred thermoluminescence for use in UVB dosimetry. Radiat. Prot. Dosim. 100, 309- 312. Spooner, N.A., 1994. On the optical dating signal from quartz. Radiat. Meas. 23, 593-600. Springis, M., Kulis, P., Veispals, A., Tale, I., 1995. Photo- and thermostimulated processes in o~-A1203:C. Radiat. Meas. 24, 453-456. Stoneham, A.M., 1975. Theory of Defects in Solids. Clarendon Press, Oxford. Summers, G.P., 1984. Thermoluminescence in single crystal o~-A1203. Radiat. Prot. Dosim. 8, 69-80. Sunta, C.M., Feria Ayta, W.E., Kulkarni, R.N., Piters, T.M., Watanabe, S., 1997. General-order kinetics of thermoluminescence and its physical meaning. J. Phys. D: Appl. Phys. 30, 1234-1242. Tochilin, E., Goldstein, N., Miller, W.G., 1969. Beryllium oxide as a thermoluminescent dosimeter. Health Phys. 16, 1-7. Visocekas, R., Zink, A., 1995. Tunnelling afterglow and point defects in feldspars. Radiat. Eft. Defects Solids 134, 265-272. Visocekas, R., Spooner, N.A., Zink, A., Blanc, P., 1994. Tunnel afterglow and point defects in feldspars. Radiat. Meas. 23, 377-385. Visocekas, R., Tale, V., Zink, A., Tale, I., 1998. Trap spectroscopy and tunnelling luminescence in feldspars. Radiat. Meas. 29, 427-434. Waibel, A., Grksu, Y., Regulla, D.F., 1993. A method of individual calibration of LiF optical absorption dosemeters. Radiat. Prot. Dosim. 47, 581-583.
Optically Stimulated Luminescence Theory
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Whitley, V.H., McKeever, S.W.S., 2000. Photoionisation of deep centers in A1203. J. Appl. Phys. 87, 249-256. Whitley, V.H., McKeever, S.W.S., 2001. Linearly modulated photoconductivity and linearly modulated optically stimulated luminescence measurements on A1203:C. J. Appl. Phys. 90, 6073-6083. Wintle, A.G., 1975. Thermal quenching of thermoluminescence in quartz. Geophys. J. Roy. Astr. Soc. 41, 107-113. Yoder, R.C., Salasky, M.R., 1997. A dosimetry system based on delayed optically stimulated luminescence. Health Phys. 72, S18-S19.
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Chapter 3
OSL properties of synthetic materials 3.1. A1203:C
3.1.1. Introduction The introduction of A1203:C as a material for thermoluminescence (TL) dosimetry (Akselrod et al., 1990) opened the possibility of several promising applications for highsensitivity measurements, particularly for short-term exposure in environmental dosimetry. Of particular interest for dating applications and in retrospective dosimetry is the fact that A1203:C possesses a photon energy response nearly identical to that of quartz and feldspar (Akselrod et al., 1990, 1993). It is, therefore, ideal for monitoring the environmental photon dose rates in connection with determining the absorbed doses in natural materials and ceramics. An additional major advantage of A1203:C is that it possesses a TL sensitivity 4 0 - 6 0 times greater than that of LiF TLD-100 making it a strong candidate for low-dose, short-exposure applications (McKeever et al., 1995). However, thermal quenching processes in the material cause the TL sensitivity to depend strongly on the heating rate used, with less sensitivity being observed at higher heating rates (Kitis et al., 1994; Kortov et al., 1994). This feature can be a disadvantage in routine TL dosimetry where fast heating automatic readers are often applied. Another potential limitation in the use of this material is its sensitivity to light. In irradiated samples, light sensitivity manifests itself in two ways (Moscovitch et al., 1993; Walker et al., 1996): (a) a light-induced fading of the TL signal; and (b) the phototransfer of charge from deep states to shallower states giving rise to a phototransferred TL (PTTL) signal (e.g., Colyott et al., 1996). However, the light-induced fading of the TL signal from A1203:C and the subsequent PTTL found for shallow traps indicate the potential of this material as a sensitive optically stimulated luminescence (OSL) dosimeter. A phenomenological model (BCtter-Jensen and McKeever, 1996; McKeever et al., 1996) predicts an even higher sensitivity (i.e., emitted luminescence photons per unit radiation dose) for OSL than for TL. Given that A1203:C is already one of the most sensitive TL materials, it is not surprising to find that its OSL sensitivity is exceptionally high.
3.1.2. Crystal growth OSL dosimetry-grade A1203 is available from a number of sources. These include Harshaw Saint-Gobain (Cleveland, USA) from where it is available under the popular commercial name TLD-500, Rados (Finland), Landauer Crystal Growth Facility (Stillwater, USA) and Nextep Technologies (Stillwater, USA), in addition to several
72
Optically Stimulated Luminescence Dosimetry
research laboratory sources in Russia (Urals) and Latvia (Riga). The material from Saint-Gobain, Rados and Nextep originates from the first laboratory to offer the material for use in luminescence dosimetry, namely the Urals Polytechnical Institute in Russia (Akselrod et al., 1993). The material from Landauer, however, is newly produced. Corundum or sapphire (oL-A1203) is an important technological material in many optical and electronic applications. It is used as a host material for solid-state lasers, as optical windows, as a substrate material in semiconductor epitaxial growth and, more recently, as a radiation detector, oL-A1203 has a rigid, slightly distorted, hexagonal-closepacked 0 2- sublattice with A13+ ions occupying two out of every three octahedral interstices. Each 0 2- ion occupies a site of C2 symmetry and is surrounded by four tetrahedral nearest-neighbour A13+ ions (see Fig. 3.1). The A13+ ions occupy sites of distorted octahedral (Oh) symmetry in which they are surrounded by six 0 2- ions, arrayed in two triangles of three 0 2- ions eachmone on the plane above and an inverted triangle in the plane below. The A1-O bond lengths are 0.186 and 0.197 nm. Oxygen vacancies, both neutral and charged (F, F + and F-centre clusters), play important roles as luminescence centres. The existence of F +, in particular, is essential for efficient OSL emission since they play the role of recombination centres for electrons, resulting in excited F-centres (Summers, 1984). A1203 is usually grown from the melt ( > 2050~ using Czochralksi, Verneuil or Stepanov techniques. TLD-500 and similar materials for use in TL or OSL dosimetry are usually in the form of discs, 5 mm diameter and 1 mm thickness. The A1203 used in the Landauer Luxel T, OSL dosimeters is in the form of powder. For high-quality dosimetry material, it is necessary to grow the material in a reducing atmosphere in the presence of carbon (Akselrod et al., 1993). As a result, carbon impurities, which act as catalysts for the formation of oxygen vacancy centres, are present up to as much as 5000 ppm. Other common impurities include Ca, Cr, Ti, Ni, Si, Cu, Mg and Fe, with amounts varying depending upon the growth conditions and method used (Grimadova et al., 1990; Akselrod et al., 1990; 1993; Springis et al., 1995). Cr and Ti impurities should be kept to a minimum since they provide efficient recombination pathways for charge carriers. The emission from Ti impurities overlaps with that from the (desired) F-centre luminescence and is especially to be avoided in good quality dosimetry-grade material (Molnfir et al., 200 l b).
Fig. 3.1. Crystal structure of A1203, showing the 02- ions occupying sites of C2 symmetry arrayed in equilateral triangles, one above and one belowthe plane of the A13+ions. The A13+ions occupy sites of distorted Oh symmetryand form tetrahedral structures surrounding the 02- ions (drawing courtesy of V. Whitley).
OSL Properties of Synthetic Materials
'/3
3.1.3. OSL stimulation and emission characteristics of A1203:C Markey et al. (1995) used the different lines from an Ar-ion laser to measure the OSL stimulation spectrum from A1203:C. The power from the laser at each wavelength was adjusted to give the same number of photons per unit time per unit area incident on the sample. The stimulation spectrum showed a rising continuum at lower wavelengths with a broad stimulation feature peaking around 480 nm. BCtter-Jensen et al. (1997) used a continuous scanning monochromator (see Chapter 7) attached to a broad-band stimulation light source to measure the stimulation spectrum from A1203:C (see Fig. 3.2a) and when stimulated in this wavelength region, the OSL signal from A1203:C exhibits a bright, rapidly decaying curve (see Fig. 3.2b). The stimulation spectrum shows a rising continuum at lower wavelengths in addition to a smooth broad stimulation resonance peaking around 500 nm. A similar broad-band stimulation peak was observed in the photoconductivity spectrum observed by Walker et al. (1996), but this time appearing at approximately 430 nm. 300
(a)
.-.. 250
200 cf)
c 150 C .m
j 100 9 50 400
450 500 550 600 650 Stimulation wavelength (nm)
700
100 (b) -7-. 8O "-I v
(r / 3
60
_= 40 _.J
9 20 ,
0
20
,
,
40
60
,'
80
1 O0
Time (s) Fig. 3.2. (a) OSL stimulation spectrum (OSL versus stimulation wavelength) for A1203:C obtained using the Rise visible monochromator and a broad-band halogen lamp stimulation light source. Detection filter: U-340. (b) OSL decay curve from A1203:C after irradiation by a 9~176 beta source giving a dose of 60 mGy and stimulated with a wavelength band 420-550 nm at a density of 16 mW/cm2 (from BCtter-Jensen et al., 1997).
74
Optically Stimulated Luminescence Dosimetry
The apparent stimulation maxima observed in these different measurements is highly variable, with observations of a stimulation peak at approximately 430, 480 and 500 nm. However, the observation of an apparent resonance in the optical stimulation curve may be misleading. The stimulation curve is in fact a complex convolution of the true wavelength dependencies of the photoionisation cross-sections of the traps contributing to the OSL signal (including the main dosimetric trap) and the wavelength dependencies of the transfer of charge from deep traps into the dosimetric and other shallower traps. As a result, the obtained stimulation spectra are complex and dependent upon the radiation and readout history of sample and the extent of deep trap filling. For weak stimulation from the traps (defined in Chapter 2), Whitley and McKeever (2000) examined the OSL and photoconductivity stimulation spectra at a variety of doses and over a wide wavelength range. The net wavelength dependence of the photoionisation cross-sections of the various trapping centres was obtained, as discussed and illustrated in Chapter 2, Fig. 2.5. An isometric plot of the luminescence emission from A1203 irradiated over the shortwavelength stimulation region from 200 to 320 nm, as a function of emission wavelength, is shown in Fig. 3.3. Several emission features are observed. The main emission, peaking near 420 nm is the F-centre emission due to relaxation from the excited 3P state to the IS ground state (Evans and Stapelbroek, 1978; Summers, 1984). The most prominent stimulation peak is at 205 nm due to electron transitions from the IS ground state to the 1P level in neutral oxygen vacancy centres (F-centres). This feature is intrinsic and is not radiation induced. The radiation-induced stimulated emission (i.e., OSL) appears as a broad "ridge" from short stimulation wavelengths to long wavelengths. Although the data in Fig. 3.3 end at a stimulation wavelength of 320 nm, efficient stimulation of the 420 nm OSL emission occurs up to the red (see Chapter 2, Fig. 2.5) and even stimulation in the infra-red region has been reported (Bulur and Grksu, 1998a; Bailiff and Clark, 1999). Other emission features observed are due to F+-centres. The primary stimulation wavelengths are at 255 and 230 nm due to 1A to 1B and 2A levels in the F+-centres,
Fig. 3.3. Isometricplot of OSL intensity as a function of stimulation wavelength (from 200 to 320 nm) and emission wavelength (from 360 to 580 nm) (from Whitley and McKeever, 2000).
OSL Properties of Synthetic Materials
75
followed by 1B to 1A relaxation and emission at 326 nm. This again is an intrinsic feature and is not induced by the radiation. The F +-centre stimulation and emission bands are only just shown in Fig. 3.3 but have been well studied by others (e.g., Evans and Stapelbroek, 1978). The remaining intrinsic feature in Fig. 3.3 is an emission band at approximately 500 nm with a stimulation maximum near 300 nm. This has been ascribed to A1 interstitial ions, but may also be caused by F-centre clusters (Pogatshnik et al., 1987; Tale et al., 1996; Pelenyov et al., 2001). Markey et al. (1995) measured the time-resolved OSL emission spectrum from irradiated A1203:C following pulsed stimulation with the 514 nm line from an Ar-ion laser. They demonstrated that the emission peak near 410-420 nm remained fixed at this wavelength during the entire decay process. This suggests that the OSL process is not a simple donor-acceptor pair type of recombination, since in that case it would be expected that a wavelength shift with time would occur. In contrast, this result supports the notion that the OSL in A1203:C involves electron hole transitions via de-localised bands. The observation of photoconductivity from this material supports this assertion (Walker et al., 1996; Whitley and McKeever, 2000; 2001). The emission at 410-420 nm is also the main emission band observed in TL from this material (Akselrod et al., 1993; McKeever et al., 1999). Finally, we note that Erfurt et al. (2000) and Poolton et al. (2001) studied the radioluminescence (RL) properties of A1203:C. Each group measured the RL emission spectrum during either 137Cs(Erfurt et al.) or 9~176 beta (Poolton et al.) irradiation. In each case, the RL emission spectrum showed the distinct F-centre peak at 410-420 nm. Erfurt et al. (2000) also observed infra-red emission near 700 and 790 nm. The latter are probably due to the R-line of Cr 3+ impurities (due to relaxation from the Cr 3+ 2E state at 695 nm) and Cr-clusters, respectively (Summers, 1984). These emissions are also frequently observed in TL (McKeever et al., 1995).
3.1.4. The OSL response of A1203:C to radiation exposure The pulsed OSL (POSL) dose response for A1203:C exposed to 9~176 is shown in Fig. 3.4. The dose response is linear over several orders of magnitude of the dose, and begins to show a saturation effect around 50 Gy. The data points in the figure represent the same sample re-used multiple times. A simple calibration approach is the obvious technique for rapid determination of the integrated dose using OSL with A1203:C. In such a procedure, the same sample is irradiated to different known doses in the laboratory and the OSL signal monitored after each irradiation. Comparison of the OSL signal from the unknown dose with this calibration curve enables the unknown dose to be determined. This simplifies the OSL measurements whether made in the field for environmental dosimetry (e.g., using portable instrumentation; BCtter-Jensen et al., 1997), or in the laboratory using bench-top instrumentation. A crucial issue, however, when using calibration methods in which the sample is repeatedly irradiated and the signal read, is whether any changes in luminescence sensitivity occur as a result of repeated irradiations and readout. Some of these characteristics of A1203:C have been investigated by BCtter-Jensen and colleagues (BCtter-Jensen and McKeever, 1996; BCtter-Jensen et al., 1997; 1999). The regenerated OSL signal has been shown to vary within < 1.7% during
76
Optically Stimulated Luminescence Dosimetry
Fig. 3.4. POSL dose response. The POSL signal was measured using the procedures described in Chapter 6 for A1203:C, including using a weaker laser beam for the high-dose levels and a stronger beam for the low-dose levels (from Akselrod and McKeever, 1999).
such procedures, which is well within the uncertainties arising from other sources (such as the beta source calibration). Thus, this simple calibration approach can be used for A1203:C without any sensitivity corrections. In principle, an OSL measurement of the unknown dose and a matching laboratory calibration dose should be sufficient for dose evaluation. Repeated single-sample OSL calibration measurements of A1203:C after irradiation to a laboratory dose of 4 IxGy of 6~ gamma radiation were made by BCtter-Jensen et al. (1997) and are shown in Fig. 3.5. These authors also showed that the total light sum for the OSL is approximately two times higher than that for TL when using heating rates in the range 1-2~ and doses lower than 0.5 txGy (5 h exposure to the natural background radiation) can easily be measured with good statistics using OSL. Considering that A1203 is already one of the brightest and most sensitive TL dosimeters available (Bos, 2001), this indicates that OSL from this material is a particularly sensitive means of detecting absorbed dose. The OSL dose response has been shown to be linear up to about 50 Gy (McKeever et al., 1996). No measurable fading of the OSL signal from A1203:C at room temperature can be detected over 100 days (BCtter-Jensen et al., 1997).
77
OSL Properties of Synthetic Materials
40 .~35 :5
g
30 e-.4--.'
_.= 25 __1 if3
Background
0 20
- e - Without P r e - h e a t - o - With Pre-heat
15
|
0
2
,
|
4
,
|
,
n
,
6 8 Re-use number
|
10
,
12
Fig. 3.5. Repeatedsingle aliquot regeneration OSL measurements of A1203:C after delivery of a regeneration dose of 4 IxGy using 6~ gamma radiation. The two curves represent: (i) regeneration using a pre-heat of 100~ s and (ii) regeneration without a pre-heat. Note the background readings (undosed dosimeterreadings) before and after the regeneration cycle (from BCtter-Jensenet al., 1997).
3.1.5. The temperature dependence of OSL from AI203:C Several temperature-dependent effects can be observed in the OSL properties of A1203:C. Many of these have already been discussed in depth in Chapter 2. Some of the important features for this material are noted here. Markey et al. (1995) showed that the pulsed OSL signal from A1203:C increased with sample temperature. They analysed the delayed OSL (DOSL) decay curve after a 100 ms laser pulse and concluded that the OSL signal consists of two components: (1) a fast component that is temperature independent (for low temperatures) with a life time of about 35 ms (corresponding to the F-centre lifetime), and (2) a slower component that is temperature dependent and is due to phosphorescence from two shallow states with trap depths of 0.65 and 0.77 eV, respectively (Markey et al., 1995). The latter two traps yield TL at temperatures below that of the main TL peak in this material. Thus, if A1203:C is irradiated at 200 K ( - 73~ the TL glow curve consists of three peaksmpeak I at 265 K (---0~ peak II at 310 K (---70~ and peak III at 450 K (---200~ when heated at 0.4~ (see McKeever et al. (1999) and also Chapter 2, Fig. 2.12). Peak III is the so-called "dosimetric peak" and is the signal monitored during conventional TL dosimetry using this material (Akselrod et al., 1990; 1993; Kortov et al., 1994; McKeever et al., 1995; 1999). Measurements of the variation of the shape and position of peak III as functions of dose and pre-annealing temperature indicate the presence of several overlapping components (Walker et al., 1996; Milman et al., 1998; Agersnap Larsen et al., 1999). Deep traps also exist in this material (Molnfir et al., 200 l a). These traps, and their degree of filling, have profound effects on the sensitivity, shape and position of the dosimetry TL peak Ill due to competition effects (Akselrod et al., 1993; 1998; Milman et al., 1998; Kortov et al., 1999). As will be noted later, there exists a strong correlation between TL peak III and the OSL signal from this material and thus both shallow and deep traps also
Optically Stimulated Luminescence Dosimetry
78
affect the OSL properties of this material. In order to fully develop OSL of A1203:C in dosimetry, an understanding of the mechanisms of OSL production and the interplay between the various traps is necessary, at least at the phenomenological level. To assist in this, so-called thermo-optical luminescence (TOL) measurements on A1203:C single crystals can be used to investigate the significance of shallow traps by examining the variation of the OSL signal as a function of temperature. In a TOL experiment, the total luminescence measured is the sum of OSL and TL. The OSL temperature dependence is then calculated by subtracting the TL signal from the total signal according to the procedure used for feldspars described by Duller and BCtter-Jensen (1993). The TOL curve for A1203:C is shown in Fig. 3.6a and demonstrates that the efficiency with which OSL is produced is temperature dependent. The OSL output increases up to ---150~ beyond which a sharp decrease occurs. The decrease is
7OOO0
60000
&-
o
1400O
(a)
..-.
.
- 12000
]
-
40000 ~
/ /
-- 8000
30000 ~
- 6000
50000
10000
4-, O ..~
C
t-
4000 ~oo ~
20o001 100001
~
01" 0
- 2000 |
100
-
200
,
,
300
400
O
Temperature (~ 1 0rj TO
0 6
--•
-
105 -
(b) Regeneration
dose
1Gy
10 4
.~_
E 10 a 0
_j 10 2O3
o 101 Instrumental background
100
'
0
I
200
'
I
400 Temperature (~
'
I
600
Fig. 3.6. (a) Thermo-optical luminescence (TOL) characteristics (OSL and TL plotted against temperature) of A1203:C after 1 Gy 9~176 beta dose delivered at room temperature and heated at 2 ~ (b) OSL plotted against pre-heat temperature for AleO3:C chips given 1 Gy, l l 0 m G y and 100 lxGy using a 9~176 beta source, respectively, at room temperature. Note the logarithmic scale (from BCtter-Jensen et al., 1999).
OSL Properties of Synthetic Materials
79
interpreted as being partially due to emptying of the dosimetric traps, and partially due to strong thermal quenching of F-centre emission (Kortov et al., 1994; Akselrod et al., 1998). For comparison, the TL curve is also shown in Fig. 3.6a and the TL peaks II and III are clearly seen. Previous studies of the light-induced fading of the TL from A1203:C (Moscovitch et al., 1993; Walker et al., 1996) have indicated that illumination of an irradiated specimen with visible light will remove the main dosimetric TL peak. PTTL measurements support this observation (Akselrod et al., 1993; Colyott et al., 1996). An obvious proposition, therefore, is that the OSL signal is induced when charges optically released from the main dosimetric traps, recombine with F +-centres and produce the observed F centre emission at ---410-420 nm. However, the relationship between the TL and OSL sensitivities is more complex than this straightforward statement implies. McKeever et al. (1999) showed that there is, as expected, a general relationship between OSL (in this case, POSL) intensity and TL, but that the relationship shows considerable scatter from sample to sample. Akselrod and Akselrod (2002) demonstrated that in some materials, characterised by a narrow TL peak, there is a direct, one-to-one relationship between the two signal types. In other samples, however, characterised by a wide TL peak, the high temperature side of the TL peak is less sensitive to optical bleaching and is apparently unrelated to OSL. The photoionisation cross-section determinations by Whitley and McKeever (2000) show a complex distribution of optical trap depths. The distribution is seen to vary with dose, reminiscent of the variation in the TL peak shape, width and position as functions of dose (Walker et al., 1996; Milman et al., 1998; Agersnap Larsen et al., 1999). The deep traps directly contribute only --~2 - 3 % of the OSL signal when stimulated with wavelengths in the green region. Decreasing the wavelength increases the contribution from these deep traps, with approximately 10% of the signal coming from the deep traps when stimulated at 465 nm (Whitley and McKeever, 2000). These observations are in agreement with the data from PTTL experiments (Colyott et al., 1996). It is difficult to distinguish the temperature dependence of the OSL due to the deep traps from that of the OSL due to the dosimetric trap(s). This can be illustrated using a sample pre-irradiated with a large beta dose (1.5 Gy) and pre-heated to temperatures > 230~ This treatment fills the deep traps, but empties the dosimetric trap(s). An OSL temperature dependence similar to that shown in Fig. 3.6b is then observed. At first sight, this appears to indicate that OSL from deep traps displays a temperature dependence similar to that observed from the dosimetry traps. However, once again, the interpretation is complicated by the phototransfer of charge from the deep traps to the dosimetry traps during OSL measurements (Colyott et al., 1996), accompanied by the simultaneous optical stimulation of charge out of the dosimetry traps. Thus, the observed temperature dependence may still be that of the OSL from the dosimetry traps, rather than from the deep traps (Markey et al., 1996). 3.1.6. Zeroing of the OSL signal from AI203:C In a single-sample calibration sequence, it is necessary that the previous luminescence signals from earlier irradiations of the same sample are reduced to negligible fractions of their initial values by either thermal annealing or bleaching with stimulation light. Earlier
80
Optically Stimulated Luminescence Dosimetry
studies of the OSL characteristics of A1203:C have suggested that annealing at 900~ is necessary to avoid the effect of charge in deep traps on repeated measurements if doses higher than 1 Gy are used (Markey et al., 1996). BCtter-Jensen et al. (1997) showed that the deep trap effect is negligible in environmental dosimetry where small doses (--~ 1 mGy) are measured; this is a typical annual gamma dose from natural radiation (terrestrial + cosmic). Fig. 3.6b shows the OSL as a function of pre-heat for A1203:C chips exposed to different doses and stimulated with blue (470 nm) light. The main features seen are that no OSL comes from shallow traps, which is consistent with the weaker optical sensitivity of the shallow traps, as indicated in Fig. 2.12 of Chapter 2. Thus, even in chips exposed to doses as high as 1 Gy, the OSL falls to instrument background for pre-heats above 500~ This suggests that even for significant prior doses, an annealing temperature of 500~ is sufficient to completely zero the OSL from A1203:C (BCtter-Jensen et al., 1999). BCtter-Jensen et al. (1999) showed that using blue light (470 nm) stimulation, the OSL signal is typically depleted to less than 1% of the initial value over 35 s at a power density of 30 mW/cm 2 (see Fig. 3.7). A particularly attractive feature of erasing the OSL signal by bleaching is the potential of being able to zero effectively the OSL signal by sunlight on location in the field, e.g., when A1203:C is used for environmental dosimetry. This is particularly important when carrying out environmental dosimetry over short periods (a few weeks) where the travel dose, i.e., the dose collected by the dosimeter during the time from when it is annealed until it reaches its location in the field, becomes significant. This feature can be particularly important when dosimeters are mailed over long distances (by air, with a significant cosmic ray dose rate) to remote field sites. Bleaching with unfiltered sunlight for 8 h typically results in a residual dose in the order of 0.5 txGy, which may be considered negligible in most applications (BCtter-Jensen et al., 1997). Akselrod and McKeever (1999) used POSL to show that, for the doses expected in environmental or personal dosimetry, the degree of trap emptying caused by stimulation is independent of the absorbed dose, for the same optical stimulation energy. This allows a very effective means of estimating the degree of trap emptying for a given set of
.--. 10 5 t,.f)
CO
6 104 .c_ C 0
o J
-110 mGy
103 102
00
9 101 10 o 0
20
40
60
Diode exposure time (s)
Fig. 3.7. OSL decay curves from A1203:C exposed to 110 mGy and 100 IxGy beta radiation read at room temperature. Note the logarithmic Y-axis(from BCtter-Jensenet al., 1999).
OSL Properties of Synthetic Materials
81
stimulation parameters such that re-reading of the absorbed dose can be undertaken multiple times while maintaining sufficient accuracy for personal dosimetry (Akselrod and McKeever, 1999). At high doses, however, this may not be true. The shift of the TL peaks to lower temperatures with increasing dose at high doses, coupled with the greater optical sensitivity of the lower temperature part of these TL peaks, means that the rate of depletion of the charge from the traps may increase for very high doses, for the same absorbed stimulation energy.
3.2. Halides 3.2.1. KCI
Eu-doped KC1 has been suggested as an efficient storage phosphor for potential use in several fields, including X-ray radiography, optical logic and optical memory devices and optical neural networks (Nanto et al., 1993a,b). OSL is observed after the material has been exposed to either X-irradiation or to UV-irradiation and the material has also been suggested as a potential UV dosimeter (Nanto et al., 1993b). X-irradiated material shows OSL emission peaking at 420 nm when stimulated with light at 560 nm, which corresponds to the peak in the stimulation spectrum (Fig. 3.8a). The latter corresponds to absorption by F-centres and the OSL emission intensity is directly proportional to the F-centre concentration. The 420 nm emission is from the T2g(4f65d) electron excited state in divalent Eu 2+ to the 4f 7 ground state, and consequently it is speculated that the absorption of 560 nm light releases electrons from F-centres (and/or impurity-perturbed F-centres, i.e., Fz-centres). The freed electrons subsequently react with Eu 3+ ions to yield Eu 2+ in an excited state, followed by relaxation and light (OSL) emission. Chernov et al. (2001) studied the model further and correlated the optical bleaching of the TL peak, following X-irradiation and illumination at 560 nm, with the OSL signal at 420 nm. The 470 K TL peak (ascribed to F-centres) decays rapidly during 560 nm bleaching, while the TL peaks at 370 and 410 K (ascribed to Eu-perturbed Fz-centres) initially increase with 560 nm bleaching time, and then decrease. The photochromic effect, i.e., the F-to-Fz conversion during the initial stages of bleaching at 560 nm light, is extremely efficient. The OSL decay curve is non-exponential and thermal annealing studies of the OSL signal reveal annealing steps that correlate with the temperatures of the TL peaks. The studies confirm the participation of the Fz- and F-centres in the OSL process. However, the shape of the OSL decay curve is maintained after annealing to different temperatures and thus an explanation of the non-exponential decay as a simple superposition of exponential decays due to the release of charge from different traps is unsupported (Chernov et al., 2001). The precise mechanism of OSL production is clearly complex and requires further study. The OSL signal is thermally stable at room temperature and, thus despite the complexity of the OSL mechanism, the material has potential utility as an OSL radiation dosimeter, or X-ray storage phosphor (Nanto et al., 1993a,b). Melrndrez et al. (1996) also indicate OSL emission following oL, [3, ~ as well as X-irradiation. In addition, the same
Optically Stimulated Luminescence Dosimetry
82
(a) i
->" 1.0-
|
Excitation
.--
.
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.... " 9. -,.
Excitation " " '-. .
@
i= j
.
,_
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.
-
,'",
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i
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.
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9
9 -
Z ,
,.
,
.
0 200
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.
. ,
,
.'
,
9
i 300 400 Wavelength (nm)
0 2oo
500
,
.
,
I
-
J
300 400 Wavelength (nm}
500
(b) !
_>, ~,r_..r
•
I
I
x=0.33
-...... _.._/_:__../
fKCl:Eot
1.0 -
/
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/
x=0.5
.x-oo7
~,~%
"~" . / " " ~ ~ ' ~ _
/
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J
o') 0 E Z
Slimulation/..-~// 0.5-
~....
/ .-//l
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-
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0 40O
I
500
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,~
. . . . . .
..
%%'%~,,
I
700
800
(nm)
Fig. 3.8.
(a) O S L stimulation and e m i s s i o n spectra from KCI:Eu (left) and KBr:Eu (right). (b) Variation in the O S L stimulation spectrum from KClxBrl -x:Eu crystals as a function of composition, i.e., as a function of x (from
Douguchi et al., 1999).
420 nm OSL signal can be produced following excitation with UV at 240 nm (Nanto et al., 1993a; Mel6ndrez et al., 1996). The OSL signal, however, is non-linear with UV dose and is only produced by exposure to high-energy UVC. 3.2.2. KBr
OSL from Eu-doped KBr has also been suggested for use in X-ray imaging phosphors and radiation dosimeters. As with KCI:Eu, the OSL emission maximum is also at 420 nm corresponding to a 4f65d-4f 7 transition of Eu 2+ ions. The OSL stimulation maximum
83
OSL Properties of Synthetic Materials
at 620 nm corresponds to absorption by neutral Br-vacancy centres (i.e., F-centres; Fig. 3.8a). A study by Douguchi et al. (1999) of the variation in the wavelength of the OSL stimulation maximum in KClxBrl-x:Eu crystals reveals a steady shift in the peak OSL stimulation efficiency as the chlorine content is increased relative to bromine, from 620 (for x -- 0) to 560 nm (for x = 1) (Fig. 3.8b). However, fading of the OSL signal was observed to be approximately 20% in 60 min at room temperature for samples with an Eu-content of 0.05 mol%, but essentially zero over the same period for an Eu-content of only 0.01 mol%. The fading rate increased, however, with an increase in ambient humidity. Although, the OSL response to X-ray dose was determined to be linear over five orders of magnitude (see Fig. 3.9), an interesting observation by Douguchi et al. (1999) was that the fading rate increased as the energy of the X-ray energy decreased, from no observed fading for 40 kVp X-rays, to about 50% fading for 20 kVp X-rays. UV-irradiated samples faded the most. This was interpreted as being due to enhanced fading at the surface connected with humidity effects.
10 4
r
10 3
.,,
h
1
..---..
J
s'
.d
.a
...__.. N9
j
J
102 f
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co 0
j J
f
10
f
.r
/ ,I
1
10
10 2
10 s
10 4
Irradiation time (s)
Fig. 3.9. X-ray dose dependence of several halide crystals. Key: @, KCI:Eu; [3, BaFBr:Eu; O, KBr:Eu (0.05mo1%)" O, KBr;Eu (0.01 mol%); O, KClo.33Bro.67:Eu;4-KClo.sBro.5:Eu; V, KClo.67Bro.33:Eu (from Douguchi et al., 1999).
84
Optically Stimulated Luminescence Dosimetry
3.2.3. NaC1 Nanto et al. (1993c) examined the OSL from Cu-doped NaC1 single crystals. Following 30 kVp X-irradiation and optical stimulation, three OSL emission peaks were observed at 353, 420 and 680 nm (Fig. 3.10). The peak in the stimulation spectrum was observed at 470 nm. Cu + substitutes for Na + and the 353 nm emission is believed to be due to a 3d94s-3d ~~transition in isolated Cu + ions. The stimulation band at 470 nm is attributed to X-ray-induced F-centres. Likewise the absorption band due to Cu + ions is reduced during X-irradiation, suggesting conversion to Cu 2+ ions. Thus, the mechanism proposed by Nanto et al. (1993c) for OSL emission is the stimulated release of electrons from F-centres to Cu 2+ ions, resulting in Cu+-ion conversion and emission at 353 nm. The OSL emission was found to be linear with absorbed X-ray dose over several orders of magnitude. Bailey et al. (2000) described the use of analytical-quality NaC1 in dosimetry, for the purposes of evaluating the potential of natural halite deposits for OSL dating of paleohydrological events. They examined several properties that are important in OSL dating procedures, including thermal stability (and other thermally stimulated properties such as thermal assistance and thermal quenching), and sensitivity changes during single aliquot procedures (see Chapter 6). The OSL decay curve from NaC1 is shown in Fig. 3.11 at several different temperatures. The authors interpret the decrease in the signal as a function of OSL stimulation temperature as being due to thermal quenching. Bailey et al. (2000) also noted that OSL could be induced by stimulation with visible (420-560 nm) and IR (880 nm) light, although the latter signal was very unstable and unsuited to dosimetry. OSL stimulated in the visible range, however, was found to have considerable potential for dosimetry. This range of stimulation wavelengths covers the absorption range of F-centres in this material (see Nanto et al. (1993c) and Fig. 3.10). LM-OSL and CW-OSL measurements on irradiated NaC1 were reported by Bulur et al. (2001) (see Fig. 2.23). These authors found that the optical de-trapping rate when 1.0 ~'0.8
r \
.D C
II
.ci
~0.6--
/
t
..,....
,
4--., Or) C
~0.4-C
_.J (/.)
0 0.2-02O0
1. . . . I "" 300 400 500 600 Wavelength (rim)
700
800
Fig. 3.10. Emission(full line) and stimulationspectrum(dashedline) for OSL fromX-irradiatedNaCI:Eu(from Nanto et al., 1993c).
~
100 c ~
0.8 -
190 C
0
0.6
~,
N
4
1.0 '
160 C 175C
205 C
5
0.4 ~ 0.2 _=
E t._ 0
0
0.0 ~, 0
50
100
150
200
250
T (~
,,A
o
85
OSL Properties of Synthetic Materials
2
0
10
20
30
40
50
Illumination time (s)
Fig. 3.11. OSL decay curves measured at different temperatures. The large difference in the decay observed at 100~ (top curve) and the curve obtained at 160~ (next curve down) is interpreted as being due to thermal detrapping of phototransferred charge. No increase in decay rate is indicated (suggesting no thermal assistance mechanisms). The inset shows the variation in OSL area (under the decay curve) normalised to 1 at room temperature. The data are fitted to an equation of the form given in Eq. (2.64), with W = 0.56 eV and C = 107 (from Bailey et al., 2000).
stimulated with blue light (at 470 nm, near the m a x i m u m of the stimulation spectrum; Nanto et al., 1993c) is non-linear with stimulation power. Thomsen et al. (2002) also studied the LM-OSL and C W - O S L signals from irradiated salt as a potential to use this material in retrospective accident dosimetry (see Section 6.8). 3.2.4. R b I OSL from RbI:X (with X = T1+, In +, P b 2+ o r Eu 2+) was reported by Thoms et al. (1994). Again, the OSL stimulation spectrum is dominated by absorption from F-centres, but additional stimulation features appear, depending upon the dopant used. The emission spectrum also depends upon the dopant. The important electron trapping sites are the Fcentres, Fz-type centres (impurity-perturbed F-centres) and, in Tl-doped material, T1~ centres. Important hole trapping sites are Vwand VkA-centres. Most of the study by Thorns et al. (1994) was concerned with OSL signals below room temperature. For useful X-ray storage phosphors or dosimeters, OSL signals that are stable above room temperature are necessary. The F-centres were shown to be stable up to 350 K and thus the best stimulation wavelength at room temperature is asserted to correspond to F-centre absorption, namely 780 nm in this material. Hole stabilisation above room temperature is provided by VkA-centres.
86
Optically Stimulated Luminescence Dosimetry
It is to be noted that in all of the alkali halide materials discussed here, the stimulation spectrum is in the form of a resonance peak, rather than the edge-like absorption features noted for other materials in Chapter 2. This suggests that in each of the alkali halide materials discussed here, the primary absorption process is excitation of the F-centre into an excited state that is either mixed with the conduction band states, or is accessible to the conduction band via phonon absorption. The lack of evidence of thermal assistance in the OSL process in NaC1, as observed by Bailey et al. (2000), suggests that the latter process is not the case for NaC1 at least. However, other alkali halides (e.g., LiF) are known to have an excited state in the forbidden gap but close to the conduction band edge (McKeever, 1985). In either case, ionisation of the F-centre is apparent, as indicated by photoconductivity during stimulation (e.g., for RbI; Thoms et al., 1994).
3.2.5. CaFe CaF2 doped with Mn is a popular TLD material, usually known as TLD-400 (McKeever et al., 1995). Bernhardt and Herforth suggested its use as an OSL dosimeter as early as 1974 (Bernhardt and Herforth, 1974). Specifically, these authors monitored the optically stimulated phosphorescence, or DOSL. They stimulated gamma-irradiated CaFz:Mn impregnated Teflon TM discs with broad-band light from a tungsten lamp, and monitored the subsequent phosphorescence (DOSL) from the material over a period of 15 min at room temperature. The integrated signal under the DOSL decay curve was linear with absorbed gamma dose over five orders of magnitude, from a low dose of 10 -2 Gy. Allen and McKeever (1990) studied conventional CW-OSL from irradiated CaFz:Mn. Fig. 3.12 shows the excitation spectrum for luminescence emission from un-irradiated and irradiated (259 Gy gamma irradiation) CaFz:Mn at room temperature. In the un-irradiated spectrum, the luminescence is photoluminescence (PL) and the excitation peaks correspond to spin-forbidden absorption transitions from the 6S ground state to the various 4G, 4p and 4D excited states of Mn 2+. After irradiation, an additional broad-band stimulation feature is observed peaking near 300 nm. This is the excitation band for irradiation-induced OSL. In both cases, the emission observed is that from the 4TIg(4G ) first excited state to the 6Alg(6S ) ground state of Mn 2+ ions substituting for host Ca 2+ ions. The induced OSL excitation band is believed to be caused by FA-centres and the OSL emission to be stimulated by electron de-trapping from these centres, recombining with hole centres (possibly Mn 3+ centres) to yield Mn 2+ luminescence. The excitation band bleaches with prolonged illumination at 280-300 nm and is thermally unstable. Several thermal annealing steps are observed with the lowest being at --~50~ OSL from CaF2:Mn is probably not viable for dosimetry purposes due to the thermal instability of the OSL excitation bands, and the fact that the radiation-induced OSL excitation bands overlap with the non-radiation induced PL excitation bands. Thus, there is a strong non-radiation-induced background and low-dose measurements with OSL from this material will be difficult.
OSL Properties of Synthetic Materials
1.0
.
.
.
.
l
'
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Z
200
300 Wavelength (nm)
400
Fig. 3.12. Stimulation spectra of the 495 nm emission from Mn 2+ in CaF2:Mn. Curve 1: before irradiation. Curve 2: after irradiation with a gamma dose of 259 Gy at room temperature. The curves are normalised to give the same maximum intensity (at 400 nm) to facilitate comparison of the curve shapes. The emission spectrum is unaltered by the irradiation (from Allen and McKeever, 1990).
3.2.6. B a F X (X -- Br, C1, I) Understanding the OSL properties of the photostimulable X-ray phosphor BaFX:Eu (X-----Br, C1, I) has become important in recent years because of the use of these materials in digital radiography more so than in dosimetry. Fuji Corporation introduced BaFBr:Eu as a phosphor screen for use in photostimulable digital radiographic imaging, although the universal adoption of the technology has been limited due to constraints on resolution and price. Nevertheless, a high X-ray sensitivity and a linear response to dose are both important properties of such materials and it is, therefore, of interest to include the materials in this section as important OSL phosphors. When doped with europium these materials, of which BaFBr is the archetype, emit OSL at 385 nm (3.2 eV), originating from the same 5f-4d transition in Eu 2+ ions as noted earlier. The OSL intensity is directly proportional to the X-ray dose over several orders of magnitude (see Fig. 3.9). Typically, these phosphors show two stimulation bands, at approximately 600 and 500 nm. It has been suggested that these correspond to absorption by F(Br-)- and F(F-)-centres, respectively, although this is not a universally accepted viewpoint (Starick et al., 1993; Lakshmanan, 1996). The two stimulation bands are also said to be l s - 2 p transitions in F(Br-)-centres (Thoms et al., 1991; von Seggern, 1999). The mechanism for the production of OSL from this material is still under debate. Several mechanisms have been proposed, and have been reviewed by a number of authors (e.g., Lakshmanan, 1996; von Seggern, 1999; Lakshmanan et al., 2000). The first mechanism proposed was that of Takahashi and colleagues (Takahashi et al., 1984; Iwabuchi et al., 1994). Here, X- (or vacuum UV-) irradiation ionises Eu 2+ ions converting them to Eu 3+ ions, either directly or by the trapping of holes. The electrons are trapped by
88
Optically Stimulated Luminescence Dosimetry
Fig. 3.13. Energy level scheme proposed by Iwabuchi et al. (1994) to describe the OSL process in irradiated BaFBr:Eu 2+. The OSL emission process is stimulated by liberation of electrons from F(Br-)- and F(F-)-centres. The electrons recombine with holes trapped at Eu3+-centres to form excited Eu2+-centres, and result in emission of light at 385 nm (from Iwabuchi et al., 1994).
F+-centres converting these to F-centres. Illumination with light in the green-to-red region of the spectrum liberates the electrons from the F-centres, allowing recombination with the Eu 3+ ions and emission from excited Eu 2+ ions at 385 nm. The energy level scheme describing this model is shown in Fig. 3.13. A second proposed mechanism involves energy transfer from recombination of a selftrapped exciton (STE) to a Eu 2+ site (von Seggern et al., 1988; Thoms et al., 1991; von Seggern, 1999). These authors studied the spatial correlation between F(Br-)-centres and Eu 2+ ions and concluded that a significant portion of the OSL resulted from a tunnelling recombination mechanism in which electrons from F(Br-)-centres recombine with holes at Eu 2+ sites, leading to 385 nm emission from relaxation of the excited state of the Eu 2+ ions. The trapped holes are in the form of Vk-centres localised by the Eu 2+ ions. At low temperatures ( < 40 K), the recombination takes place exclusively via tunnelling between spatially correlated pairs of F(Br-)-centres and Eu2+/Vk-centre complexes. At room temperature, however, both tunnelling recombination (between spatially correlated pairs) and free electron recombination at those Eu2+/Vk-centres which are spatially uncorrelated with the F(Br-)-centres contribute to the OSL emission. For the latter process, the temperature has to be high enough so that thermal ionisation of an electron from the relaxed excited state (RES) of the F(Br-)-centre to the conduction band can take place. The recombination schemes are illustrated schematically in Fig. 3.14. Note that F(F-)centres are believed not to take part in the OSL process since it is assumed that too high a thermal activation energy is required for escape from the RES of these centres to the conduction band. Lakshmanan et al. (2000) discuss the effect of oxygen impurities on the OSL process. These impurities substitute for F ions forming 0 2- centres. OSL emission at 480 nm results when electrons from F(Br-) centres recombine with OF centres, while emission at
OSL Properties of Synthetic Materials
89
Fig. 3.14. Energy level scheme proposed by Thoms et al. (1991) to describe the OSL process in irradiated BaFBr:Eu 2+. The OSL emission process is stimulated by either of the two possible mechanisms. In the first, electrons are thermally liberated from the RES of F(Br-)-centres (activation energy 35 meV) and recombine with holes at spatially uncorrelated Eu2+/Vk-centres. The recombination energy is transferred to the Eu 2+ ions forming excited Eu 2+ ions, which relax yielding the emission of light at 385 nm (3.2 eV). In the second mechanism, the electrons tunnel from the RES of the F(Br-)-centres to spatially correlated Eu2+/Vk-centres, again resulting in emission at 385 nm (from Thoms et al., 1991).
400 nm results from the recombination of STEs at the O{- sites. At parts-per-million levels, oxygen appears to enhance the OSL sensitivity, whereas at higher concentrations the OSL from Eu 2+ may be quenched. As discussed by von Seggern (1999), an alternative mechanism to simple ionisation of Eu 2+ impurities (to form Eu 3+ ions) and trapping of electrons by F +-centres (as suggested by Takahashi et al. (1984) and Iwabuchi et al. (1994)) is the formation of F-centres during X-irradiation. Here the absorption of X-rays result in the formation of a STE that relaxes to form an F-centre and an interstitial halide atom (an H-centre). Both F(Br-)- and F(F-)centres may be formed in this way. Thus, the trapped-hole centre may be a Eu2+/H-centre complex rather than a Eu2+/Vk-centre complex (von Seggern, 1999). This straightforward mechanism, modelled on similar processes shown to occur in the alkali halides, removes the difficulty of requiting charged halide vacancies (F +-centres) to exist before irradiation. Although, primarily used in radiographic imaging, BaFBr:Eu has also been suggested for use in conventional ionising radiation dosimetry by Yamadera et al. (1995). These authors tested the OSL response from this material to Bremsstrahlung X-rays (with effective energies from 30 to 140 keV), as well as 6~ and 137Csgamma rays. Linearity from 1 IxSv to 10 mSv was found (with an upper limit of 50 mSv quoted), although the response was shown to be highly non-tissue-equivalent. An additional feature that makes this an unattractive material for dosimetry is the instability of the OSL signal after irradiation. Studies by Yamadera et al. (1995) indicate that the signal fades significantly. Two fading components were observed, with half-lives of 2.29 and 24.37 days, respectively.
90
Optically Stimulated Luminescence Dosimetry
3.3. Sulphates 3.3.1. MgS04 Hydrated salts of MgSO4, n H20, with n = 6 or 7, were studied by Le Masson et al. (2001) for potential applications in fast neutron dosimetry. The materials were prepared with Ce 3+ ions as activators. Although the UV-induced fluorescence signal revealed emission due to these activators, the OSL signal was found to be un-related to the Ce content. OSL could be induced from beta-irradiated samples using infra-red stimulation (830 nm) and BG-39 and U-340 detection filters. The IR-induced OSL signal was found to be linear with absorbed beta dose, but unfortunately exhibited significant fading (50% at room temperature over several hours). The fast neutron response (using a PuBe source) was separated from the gamma response using lead filtration and found to be significant. 3.3.2. CaS04 Calcium sulphate doped with dysprosium was one of the earliest materials to be suggested for use as an OSL dosimeter (Pradhan and Ayyanger, 1976). The method chosen, however, was DOSL due to the optically stimulated transfer of charge to shallow metastable traps. The irradiated sample was optically stimulated with light from a mercury lamp and the luminescence emission was measured after a delay of 15 s from the end of the stimulation. This relatively long delay was possible because of the long-lived phosphorescence signal, which decayed to 50% of its initial value after about 80 s. A minimum detectable limit of about 0.01 Gy (___3%) was measured by these authors. A much shorter delay time (50 ms) was used by Jaek et al. (2002) who used the DOSL signal from irradiated CaSO4:Dy to measure absorbed doses over a similar dose range (up to 30 Gy). The latter authors also used the DOSL signal to determine that the most efficient stimulation wavelengths were centred around 350 nm, for emission at 510 nm.
3.4. Sulphides 3.4.1. AS (A = Mg, Sr, Ca, Ba) The alkaline earth sulphide family of compounds (MgS, CaS, SrS and BaS) has been suggested for use in OSL dosimetry since the first suggested use of OSL as a potential dosimetric method. Antonov-Romanovskii et al. (1956) examined IR-stimulated OSL from Ce + Sm-doped and Eu + Sm-doped SrS. This early work was followed by studies by Br~iunlich et al. (1967) and Sanbom and Beard (1967) on related compounds. All compounds are stimulated with light in the infra-red region of the spectrum. Examples of stimulation spectra are shown in Fig. 3.15, for various MgS compounds, while OSL emission spectra are shown in Fig. 3.16 (from Rao et al., 1984; Mathur et al., 1986). The stimulation spectra are such that these materials can be effectively stimulated using the fundamental wavelength from a Nd:YAG laser (1.06 txm). The emission maximum near 590 nm is believed to be from 4f-transitions with Eu 2+ ions (Rao et al., 1984; Chakrabarti et al., 1988). Cerium doping also induces emission characteristics of transitions to the split
91
OSL Properties of Synthetic Materials (a)
40 D
30 m
oo z
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Fig. 3.15. (a) OSL stimulation spectrum for irradiated MgS:Eu,Sm (from Rao et al., 1984). (b) OSL stimulation spectra from irradiated MgS:Ce,Sm. Curve 1--30 s UV exposure; curves II and III--after 300 and 600 s stimulation with 1317 nm subsequent to UV exposure (from Mathur et al., 1986).
ground s t a t e (2F5/2 a n d 2F7/2) of C e 3+ ions (i.e., an emission doublet at ---527 and --- 580 nm; Chakrabarti et al., 1988). Thus, the OSL emission from MgS is characteristic of Eu in Eu + Sm-doped phosphors, and Ce in Ce + Sm-doped phosphors. However, Chakrabarti et al. (1988) also note that the TL emission spectra from these materials is always characteristic of Sm 3+, independent of the identity of the co-dopant. The OSL and
92
Optically Stimulated Luminescence Dosimetry
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.
.
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.
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OSL emission spectra from (A): irradiated MgS:Eu, and (B) irradiated MgS:Eu,Sm (from Rao et al.,
TL mechanisms proposed by the latter authors to account for these observations are illustrated in Fig. 3.17. Here, Sm 2+ acts as the optically sensitive OSL trap. Electrons released from Sm 2+ traps recombine with either Ce 3+ or Eu 2+, as appropriate, yielding OSL emission characteristics of these ions. These authors propose that during TL holes are released from Ce 4+ or Eu 3+ ions, leading to recombination with Sm 3+ and TL emission. A similar mechanism to the above was suggested by Nanto et al. (1999) for OSL emission from SrS:Eu,Sm. The observed emission at 600 nm was from Eu 2+ ions after stimulation with light between 800 and 1700 nm. These authors investigated the utility of this material as a UV dosimeter by monitoring the OSL emission following exposure to filtered (248 nm) sunlight.
3.5.
Oxides
3.5.1. BeO Rhyner and Miller (1970) examined the potential of BeO (discs and powder) as an OSL dosimetry material. As with many of the OSL studies from this era, however, the measurement method was actually DOSL in which the phosphorescence from the sample was monitored following a delay of approximately 2 s. More recent investigations of this material used conventional CW-OSL (Bulur and G6ksu, 1998b), or LM-OSL (Bulur et al., 2001) methods.
OSL Properties of Synthetic Materials
93
Fig. 3.17.
Models for OSL and TL emission from MgS doped with either Ce and Sm, or Eu and Sm (from Chakrabarti et al., 1988).
BeO is a material with a long history in luminescence dosimetry, being suggested initially as a TL dosimeter. Its popularity was primarily due to its near-tissue-equivalence (effective atomic number-- 7.13; McKeever et al., 1995). Several forms of the material are commercially available, although the most popular for luminescence dosimetry has been "Thermalox 995" from Brush Wellman (Brush Beryllium, USA). The crystal structure of BeO is that of wurtzite with each Be atom surrounded by four oxygen atoms in a nearly perfect tetrahedron, with each oxygen atom similarly surrounded by four TM
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Stimulation spectra for OSL from irradiated BeO, obtained at different readout temperatures, as indicated. Emission is in the range 340 ___40 nm (from Bulur and Grksu, 1998b).
94
Optically Stimulated Luminescence Dosimetry 107
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beryllium atoms. Common impurities include Mg, Si, Ca, B and A1 (see McKeever et al. (1995) for further discussion). Although the TL mechanism is not fully understood, Kortov et al. (1993) have suggested that Li impurities in Li-doped BeO play an important role as a hole trap, and that A12+ sites act as the recombination centres. Recombination induces luminescence emission near 330 nm. One of the main difficulties with the material as a TL dosimeter is the light sensitivity of the dosimetric TL signal (e.g. Gammage and Cheka, 1977). However, this leads to obvious potential of the material as an OSL dosimeter. Despite a very early study by Albrecht and Mandeville (1956) who examined optical stimulation of luminescence from X-irradiated BeO, the OSL properties of this material were not thoroughly investigated until the work
OSL Properties of Synthetic Materials
93
of Bulur and G6ksu (1998b), and apart from the studies of LM-OSL by Bulur et al. (2001) little has been done since then. The stimulation spectrum, for OSL emission within a detection window of 340 ___40 nm, is shown in Fig. 3.18 and consists of a broad band between 420 and 550 nm, with a maximum at 435 nm and a smaller peak near 400 nm. Strong thermal quenching of the signal is observed, with the signal intensity reducing significantly between 50 and 100~ The thermal quenching energy was determined to be between 0.48 and 0.52 eV. The CW-OSL signal is itself made up of several components, some of which are thermally unstable. Pre-heating the sample to 125~ after irradiation removes the unstable components, leaving a very stable OSL signal. Even the stable signal consists of multiple components, however, as can be observed using LM-OSL after removal of the unstable components (Bulur et al., 2001). Bulur and G6ksu (1998b) determined that the primary trap responsible for the stable OSL signal has a thermal activation energy of 1.74 eV (with a corresponding frequency factor of 6.4 x 1013 s-l). This appears to correspond to a TL signal near 340~ but a clear correlation has not been established. Surprisingly, even after pre-heating, the OSL signal was observed to fade by about 5% over approximately 1 h at room temperature, followed by a long period of stability. Using the stable OSL component, a linear dose response was obtained from less than a few mGy to over 10 Gy (as shown in Fig. 3.19). 3.5.2. Fused quartz A new OSL material based on fused quartz glass doped with Cu has been developed by the US Naval Research Laboratory (Justus et al., 1997; 1999a,b; Huston et al., 2002). The
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Fig. 3.20. Comparison of TL and OSL emission spectra from Cu-doped fused quartz glass. The TL data (dots) were obtained by integrating the TL signal from 130 to 185~ The OSL signal was stimulated with 790 nm light from a GaAs diode laser (from Justus et al., 1999a).
96
Optically Stimulated Luminescence Dosimetry
emission spectrum (Fig. 3.20) peaks at 505 nm, believed to be due to spin-forbidden 3d l ~ 3d94s transitions from Cu + ions in this matrix. The exact emission maximum varies with the chemical composition of the glass host. There is no evidence of multiple types of Cu + site (Justus et al., 1999a). The CW-OSL curve from this material is shown in Fig. 3.2 l a, and the response to absorbed 6~ gamma dose is shown in Fig. 3.2 lb. The OSL signal is associated with a TL peak at approximately 200~ The OSL mechanism is not fully
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OSL Properties of Synthetic Materials
97
u n d e r s t o o d but is b e l i e v e d to i n v o l v e t r a p p i n g o f h o l e s at C u +, f o r m i n g C u 2+. ( A l t e r n a t i v e l y , one m a y v i e w this as i o n i s a t i o n o f C u + to f o r m Cu2+.) T h e f r e e d e l e c t r o n s are t r a p p e d e l s e w h e r e at unidentified sites. S t i m u l a t i o n w i t h light ( O S L ) or heat ( T L ) r e l e a s e s the t r a p p e d electrons, w h i c h r e c o m b i n e at the C u 2+ sites f o r m i n g C u + in an e x c i t e d state. R e l a x a t i o n o f the e x c i t e d C u + ions leads to g r e e n e m i s s i o n n e a r 505 n m ( H u s t o n et al., 2002). T h e s t i m u l a t i o n efficiency p e a k s n e a r 390 n m and falls off b y about a factor o f 10 by 800 nm. D e s p i t e this, efficient s t i m u l a t i o n can be a c h i e v e d using an I R diode at 790 n m ( H u s t o n et al., 2002).
References Agersnap Larsen, N., BCtter-Jensen, L., McKeever, S.W.S., 1999. Thermally stimulated conductivity and thermoluminescence from A1203:C. Radiat. Prot. Dosim. 84, 87-90. Akselrod, M.S., Agersnap Larsen, N., Whitley, V., McKeever, S.W.S., 1998. Thermal quenching of F-centre luminescence in A1203:C. J. Appl. Phys. 84, 3364-3373. Akselrod, A., Akselrod, M.S., 2002. Correlations between OSL and the distribution of TL traps in A1203:C. Radiat. Prot. Dosim. 100, 217- 220. Akselrod, M.S., Kortov, V.S., Gorelova, E.A., 1993. Preparation and properties of o~-A1203:C. Radiat. Prot. Dosim. 47, 159-164. Akselrod, M.S., Kortov, V.S., Kravetsky, D.J., Gotlib, V.I., 1990. Highly sensitive thermoluminescent aniondefect oL-A1203:C single crystal detectors. Radiat. Prot. Dosim. 33, 119-122. Akselrod, M.S., McKeever, S.W.S., 1999. A radiation dosimetry method using pulsed optically stimulated luminescence. Radiat. Prot. Dosim. 81, 167-175. Albrecht, H.O., Mandeville, C.E., 1956. Storage of energy in BeO. Phys. Rev. 101, 1250-1254. Allen, P., McKeever, S.W.S., 1990. Studies of PTTL and OSL in TLD-400. Radiat. Prot. Dosim. 33, 19-22. Antonov-Romanovskii, V.V., Keirum-Marcus, I.F., Poroshina, M.S., Trapeznikova, Z.A., 1956. Conference of the Academy of Sciences of the USSR on the Peaceful Uses of Atomic Energy, Moscow, 1955, USAEC Report AEC-tr-2435 (Pt. 1), 239. Bailey, R.M., Adamiec, G., Rhodes, E.J., 2000. OSL properties of NaC1 relative to dating and dosimetry. Radiat. Meas. 32, 717-723. Bailiff, I.K., Clark, R.J., 1999. A preliminary study of the fast time-resolved luminescence in A1203:C. Radiat. Prot. Dosim. 84, 457-460. Bernhardt, R., Herforth, L., 1974. Radiation dosimetry by optically stimulated phosphorescence of CaF2:Mn. In: Newiadomski, T. (Ed.), Proceedings of the Fourth International Conference on Luminescence Dosimetry, Krakow, Poland, pp. 1091-1104. Bos, A.J.J., 2001. On the energy conversion in thermoluminescence dosimetry materials. Radiat. Meas. 33, 737 -744. BCtter-Jensen, L., Agersnap Larsen, N., Markey, B.G., McKeever, S.W.S., 1997. A1203:C as a sensitive OSL dosemeter for rapid assessment of environmental photon dose rates. Radiat. Meas. 27, 295-298. BCtter-Jensen, L., Banerjee, D., Jungner, H., Murray, A.S., 1999. Retrospective assessment of environmental dose rates using optically stimulated luminescence from A1203:C and quartz. Radiat. Prot. Dosim. 84, 537-542. BCtter-Jensen, L., McKeever, S.W.S., 1996. Optically stimulated luminescence dosimetry using natural and synthetic materials. Radiat. Prot. Dosim. 65, 273-280. Br/iunlich, P., Sch~ifer, D., Scharmann, A., 1967. A simple model for thermoluminescence and thermally stimulated conductivity of inorganic photoconducting phosphors and experiments pertaining to infra-red stimulated luminescence. Proceedings of 1s t International Conference on Luminescence Dosimetry, Stanford, June 1965, USAEC, 57-73. Bulur, E., BCtter-Jensen, L., Murray, A.S., 2001. LM-OSL signals from some insulators: an analysis of the dependency of the detrapping probability on stimulation light intensity. Radiat. Meas. 33, 715-719. Bulur, E., G6ksu, H.Y., 1998a. Infrared stimulated luminescence from A1203:C. Radiat. Meas. 29, 625-638.
98
Optically Stimulated Luminescence Dosimetry
Bulur, E., G6ksu, H.Y., 1998b. OSL from BeO ceramics: New observations from an old material. Radiat. Meas. 29, 639-650. Chakrabarti, K., Mathur, V.K., Rhodes, J.F., Abbundi, R.J., 1988. Stimulated luminescence in rare-earth-doped MgS. J. Appl. Phys. 64, 1363-1366. Chernov, V., Melrndrez Ao, R., Piters, T.M., Barboza-Flores, M., 2001. Thermally and optically stimulated luminescence correlated processes in X-ray irradiated KCI:Eu. Radiat. Meas. 33, 797-800. Colyott, L.E., Akselrod, M.S., McKeever, S.W.S., 1996. Phototransferred thermoluminescence in A1203:C. Radiat. Proc Dosim. 65, 263-266. Douguchi, Y., Nanto, H., Sato, T., Imai, A., Nasu, S., Kusano, E., Kinbara, A., 1999. Optically stimulated luminescence in Eu-doped KBr phosphor ceramics. Radiat. Prot. Dosim. 84, 143-148. Duller, G.A.T., BCtter-Jensen, L., 1993. Luminescence from potassium feldspars stimulated by infrared and green light. Radiat. Prot. Dosim. 47, 683-688. Erfurt, G., Krbetschek, M.R., Trautmann, T., Stolz, W., 2000. Radioluminescence (RL) behavior of AlzO3:Cpotential for dosimetric applications. Radiat. Meas. 32, 735-739. Evans, B.D., Stapelbroek, M., 1978. Optical properties of F+-properties in crystalline A1203. Phys. Rev. B 18, 7089-7098. Gammage, R.B., Cheka, J.S., 1977. Further characteristics important in the operation of ceramic BeO TLD. Health Phys. 32, 189-192. Grimadova, T.I., Bessonova, T.S., Tale, I.A., Avvakumova, L.A., Bodyachevsky, S.V., 1990. On the thermoluminescence mechanism of non-doped corundum monocrystals with defect structure. Radiat. Prot. Dosim. 33, 47-50. Huston, A.L., Justus, B.L., Falkenstein, P.L., Miller, R.W., Ning, H., Altemus, R., 2002. Optically stimulated luminescent glass optical fibre dosimeter. Radiat. Prot. Dosim. 101, 23-26. Iwabuchi, Y., Mori, N., Takahashi, K., Matsuda, T., Shionoya, S., 1994. Mechanism of photostimulated luminescence process in BaFBr:Eu 2+ phosphors. Jpn. J. Appl. Phys. 33, 178-185. Jaek, I., Kerikm~ie, M., Lust, A., 2002. Optically stimulated luminescence in some thermoluminescence detectors as an indicator of absorbed radiation dose. Radiat. Prot. Dosim. 100, 459-462. Justus, B.L., Merritt, C.D., Pawlovich, K.J., Huston, A.L., Rychnovsky, S., 1999a. Optically stimulated luminescence dosimetry using doped fused quartz. Radiat. Prot. Dosim. 84, 189-192. Justus, B.L., Pawlovich, K.J., Merritt, C.D., Huston, A.L., 1999b. Optically and thermally stimulated luminescence characteristics of Cu-doped fused quartz. Radiat. Prot. Dosim. 81, 5-10. Justus, B.L., Rychnovsky, S., Huston, A.L., Miller, M.A., Pawlovich, K.J., 1997. Optically stimulated luminescence radiation dosimetry using doped silica glass. Radiat. Prot. Dosim. 74, 151-154. Kitis, G., Papadopoulos, J.G., Charalambous, S., Tuyn, J.W.N., 1994. The influence of heating rate on the response and trapping parameters of c~-A1203:C. Radiat. Prot. Dosim. 55, 183-190. Kortov, V.S., Milman, I.I., Kirpa, V.I., Lesz, J., 1994. Some features of A1203:C dosimetric thermoluminescent crystals. Radiat. Prot. Dosim. 55,279-283. Kortov, V.S., Milman, I.I., Nikiforov, S.V., 1999. The effect of deep traps on the main features of thermoluminescence in dosimetric ct-Al203 crystals. Radiat. Prot. Dosim. 84, 35-38. Kortov, V.S., Milman, I.I., Slesarev, A.I., Kijko, V.S., 1993. New BeO ceramics for TL ESR dosimetry. Radiat. Prot. Dosim. 47, 267-270. Lakshmanan, A.R., 1996. Radiation-induced defects and photostimulated luminescence processes in BaFBr:Eu 2+. Phys. Stat. Sol., (a) 153, 3-27. Lakshmanan, A.R., Murase, N., Yazawa, T., Qui, J., Mitsuyu, T., Hirao, K., Tomita, A., Hoffman, A., 2000. Luminescence studies in BaFBr and BaFBr:Eu 2+. Proceedings of the ISLA-200 Conference, Baroda, February 2000, 207- 212. Le Masson, N.J.M., Bos, A.J.J., Van Eijk, C.W.E., 2001. Optically stimulated luminescence in hydrated magnesium sulfates. Radiat. Meas. 33, 693-697. Markey, B.G., Colyott, L.E., McKeever, S.W.S., 1995. Time-resolved optically stimulated luminescence from etA1203:C. Radiat. Meas. 24, 457-463. Markey, B.G., McKeever, S.W.S., Akselrod, M.S., BCtter-Jensen, L., Agersnap Larsen, N., Colyott, L.E., 1996. The temperature dependence of optically stimulated luminescence from o~-A1203:C. Radiat. Prot. Dosim. 65, 185-189.
OSL Properties of Synthetic Materials
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Mathur, V.K., Gasiot, J., Abbundi, R.J., Brown, M.D., 1986. Optically stimulated luminescence in MgS:Ce,Sm. Radiat. Prot. Dosim. 17, 333-336. McKeever, S.W.S., 1985. Thermoluminescence of Solids. Cambridge University Press, Cambridge. McKeever, S.W.S., Akselrod, M.S., Colyott, L.E., Agersnap Larsen, N., Polf, J.C., Whitley, V., 1999. Characterisation of A1203 for use in thermally and optically stimulated luminescence dosimetry. Radiat. Prot. Dosim. 84, 163-168. McKeever, S.W.S., Markey, B.G., Akselrod, M.S., 1996. Pulsed optically-stimulated luminescence dosimetry using ot-A1203:C. Radiat. Prot. Dosim. 65, 267-272. McKeever, S.W.S., Moscovitch, M., Townsend, P.D., 1995. Thermoluminescence Dosimetry Materials: Properties and Uses. Nuclear Technology Publishing, Ashford, UK. Melrndrez, R., P@ez-Slas, R., Pashenko, L.P., Aceves, R., Piters, T.M., Barboza-Flores, M., 1996. Dosimetric properties of KCI:Eu2+ under oL, [3, F, X-ray and ultraviolet irradiation. Appl. Phys. Lett. 68, 3398-3400. Milman, I.I., Kortov, V.S., Nikiforov, S.V., 1998. An interactive process in the mechanism of the thermally stimulated luminescence of anion-defective oL-A1203 crystals. Radiat. Meas. 29, 401-410. Molnfir, G., Benabdesselam, M., Borossay, J., Lapraz, D., Iacconi, P., Akselrod, M.S., 2001a. Influence of the irradiation temperature on TL sensitivity of oL-A120:C. Radiat. Meas. 33, 619-623. Molnfir, G., Benabdesselam, M., Borossay, J., Lapraz, D., Iacconi, P., Kortov, V.S., Surdo, A.I., 200lb. Photoluminescence and thermoluminescence of titanium ions in sapphire crystals. Radiat. Meas. 33,663-667. Moscovitch, M., Tawil, R.A., Svikin, M., 1993. Light-induced fading in o~-A1203. Radiat. Prot. Dosim. 47, 251-253. Nanto, H., Murayama, K., Usuda, T., Endo, F., Hirai, Y., 1993a. Laser-stimulable transparent KCI:Eu crystals for erasable and rewritable optical memory utilizing photostimulated luminescence. J.Appl. Phys. 74, 1445-1447. Nanto, H., Murayama, K., Usuda, T., Taniguchi, S., Takeuchi, N., 1993b. Optically stimulated luminescence in KCI:Eu single crystals. Radiat. Prot. Dosim. 47, 281-284. Nanto, H., Sato, T., Kashiwagi, N., Miyazaki, M., Nasu, S., Kusano, E., Kinbara, A., Douguchi, Y., 1999. A UV dosimeter utilising photostimulated luminescence in SrS:Eu,Sm phosphor ceramics. Radiat. Prot. Dosim. 85, 305-307. Nanto, H., Usuda, T., Murayama, K., Nakamura, S., Inabe, K., Takeuchi, N., 1993c. Emission mechanism of optically stimulated luminescence in copper-doped sodium chloride single crystals. Radiat. Prot. Dosim. 47, 293-296. Pelenyov, V.E., Kortov, V.S., Milman, I.I., 2001. The interaction of deep traps in anion-defective c~-A1203. Radiat. Meas. 33, 629-631. Pogatshnik, G.J., Chen, Y., Evans, B.D., 1987. A model of lattice defects in sapphire. IEEE Trans. Nucl. Sci. NC34, 1709-1712. Poolton, N.R.J., Bulur, E., Wallinga, J., BCtter-Jensen, L., Murray, A.S., Willumsen, F., 2001. An automated system for the analysis of variable temperature radioluminescence. Nucl. Instr. Meth. B 179, 575-584. Pradhan, A.S., Ayyanger, K., 1976. Radiation dosimetry by photostimulated luminescence in CaSO4:Dy. Int. J. Appl. Radiat. Isot. 28, 534-535. Rao, R.P., de Murcia, M., Gasiot, J., 1984. Optically stimulated luminescence dosimetry. Radiat. Prot. Dosim. 6, 64-66. Rhyner, C.R., Miller, W.G., 1970. Radiation dosimetry by optically-stimulated luminescence of BeO. Health Phys. 18, 681-684. Sanborn, E.N., Beard, E.L., 1967. Sulfides of strontium, calcium, and magnesium in infra-red stimulated luminescence dosimetry. Proceedings of 1s t International Conference on Luminescence Dosimetry, Stanford, June 1965, USAEC, 183-191. von Seggern, H., 1999. Photostimulable x-ray storage phosphors: a review of present understanding. Braz. J. Phys. 29, 254-268. von Seggern, H., Voight, T., Kniipfer, W., Lange, G., 1988. Physical model of photostimulated luminescence of x-ray irradiated BaFBr:Eu 2+. J. Appl. Phys. 64, 1405-1412. Springis, M., Kulis, P., Veispals, A., Tale, I., 1995. Photo- and thermostimulated processes in o~-A1203. Radiat. Meas. 24, 453-456.
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Starick, D., Gurvich, A.M., Myagkova, M.G., Riidiger, J., Herzog, G., 1993. The influence of preparation conditions on the performance of BaFBr-Eu storage phosphors. Nucl. Tracks Radiat. Meas. 21, 39-41. Summers, G.P., 1984. Thermoluminescence in single crystal oL-A1203:C. Radiat. Prot. Dosim. 8, 69-80. Takahashi, K., Kohda, K., Miyahara, J., 1984. Mechanism of photostimulated luminescence in BaFX:Eu 2+ (X = C1, Br) phosphors. J. Lumin. 31 & 32, 266-268. Tale, I., Piters, T.M., Barbosa-Flores, M., Perez-Salas, R., Aceves, R., Springis, M., 1996. Optical properties of complex anion vacancy centres and photo-excited electronic processes in anion defective o~-A1203. Radiat. Prot. Dosim. 65, 235-238. Thoms, M., von Seggern, H., Winnacker, A., 1991. Spatial correlation and photostimulability of defect centres in the x-ray-storage phosphor BaFBr:Eu2+. Phys. Rev. B 44, 9240-9247. Thoms, M., von Seggern, H., Winnacker, A., 1994. Optical and thermal properties of electron- and hole-trapping sites in the x-ray storage phosphor RbI:X (X = T1+, In +, Pb 2+, Eu2+). J. Appl. Phys. 76, 1800-1808. Thomsen, K.J., BCtter-Jensen, L., Murray, A.S., 2002. Household and workplace chemicals as retrospective luminescence dosemeters. Radiat. Prot. Dosim. 101, 515-518. Walker, F.D., Colyott, L.E., Agersnap Larsen, N., McKeever, S.W.S., 1996. The wavelength dependence of lightinduced fading of thermoluminescence from o~-A1203:C. Radiat. Meas. 26, 711-718. Whitley, V.H., McKeever, S.W.S., 2000. Photoionisation of deep centers in A1203. J. Appl. Phys. 87, 249-256. Whitley, V.H., McKeever, S.W.S., 2001. Linearly modulated photoconductivity and linearly modulated optically stimulated luminescence measurements on A1203:C. J. Appl. Phys. 90, 6073-6083. Yamadera, A., Kim, E., Miyata, T., Nakamura, T., 1995. Development of high sensitivity X-ray and ~/-ray personal dosimeter using photostimulated luminescent detector. Appl. Radiat. Isot. 46, 467-468.
Chapter 4
Passive optically stimulated luminescence dosimetry 4.1. Personal dosimetry 4.1.1. Introduction
There are currently approximately five million personal dosimetry badges being used by radiation workers around the world (2002 data), including thermoluminescence dosimeters (TLDs), film badges, optically stimulated luminescence (OSL) dosimeters and others. Although, it may be argued that the use of OSL in personal dosimetry is not yet widespread in terms of the number of different laboratories running OSL instruments, it has to be recognised nevertheless that OSL is indeed a major player in the personal dosimetry arena since approximately 25% of the five million badges are in fact OSL dosimeters, primarily based on pulsed OSL (POSL) from A1203:C. As already described in Chapter 1, OSL has been suggested as a personal dosimetry method for several decades, with various forms of the OSL method being suggested at one point or another in the published literature. None of these early pioneering studies, however, led to a commercial-scale personal dosimetry system until the advent of POSL of A1203:C (Akselrod and McKeever, 1999), and the subsequent development of Luxel TM personal dosimeters by Landauer Inc. in the late 1990s. A possible exception to this statement is the use of radiophotoluminescence (RPL) of phosphate glasses in personal dosimetry throughout the 1980s (Perry, 1987; Piesch et al., 1986; 1990; 1993). However, as discussed elsewhere (Chapter 2), RPL and OSL are different physical phenomena and we limit the discussion here to a description of OSL personal dosimetry. Following the success of POSL in this application, newly developed commercial OSL personal dosimetry systems are now available using continuous wave-OSL (CW-OSL) as a readout mode. Descriptions of some commercial OSL personal dosimetry systems are given in the following sections. In general, the key features of OSL that make it so attractive for personal dosimetry include the ability to tune the performance across a wide dynamic dose range by being able to vary the stimulation power. This allows optimum sensitivity at both the low-dose and the high-dose ends of the dose response. Furthermore, by adjusting the stimulation power appropriate to the dose range, one has the ability to re-read the OSL signal since the high sensitivity allows dose assessment even though only a fraction of the stored charge is depleted during the measurement. Thus, a complete re-analysis is achieved and true, independent dose readings are possible. This is to be distinguished from film
102
Optically Stimulated Luminescence Dosimetry
badge dosimetry in which the developed image can be read a second time, but the film can only be developed once; thus the dose readings are not independent in this sense. With thermoluminescence (TL), of course, the entire signal is destroyed (100% depletion of the trapped charge) during one reading and re-reading is not possible. Both fast readout and fast analysis, important for processing large numbers of dosimeters, are made possible with the speed of POSL, although at the cost of expensive equipment. Multiple analyses are possible with cheaper LEDs as the stimulation source, but at a cost of slower analysis time. In addition, the all-optical nature of the OSL process removes the need for heating and allows for imaging capabilities normally only expected with film badges. 4.1.2. Landauer's Luxel TM personal dosimetry system The Luxel ~Mbadge is illustrated in Fig. 4.1 a-c. The heart of the badge, i.e., the detector itself, is shown in Fig. 4.1a and is a thin layer of A1203:C powder deposited onto a clear polyester film base. The size of the active area of the detector is 16.5 mm by 18.5 mm and the grain size of the A1203:C powder is in the range 20-90 lxm. The powder layer is protected by a thin, clear polyester tape. The detector is located inside a filter pack (Fig. 4.1b) consisting of an open window, a copper filter (250 txm thick), a tin filter (500 Ixm thick) and a 500 Ixm-thick copper grid consisting of a 5 x 5 array of 1.0 mm diameter holes. When the filter pack is folded over the detector, each of the filters sits over its own quadrant, coveting part of the A1203:C powder film. In normal analysis mode, a POSL reading is taken from those three parts of the A1203:C film that sit under the three circular filters (open, Cu and Sn). From these three independent POSL readings deep dose (Hp(10)), shallow dose (Hp(0.07)) and eye dose (Hp(0.3))--see Chapter 1 for definitions--may be evaluated through use of a suitable algorithm of the form:
D = alow -t- BIcu + Clsn
(4.1)
where Ii (i = OW, Cu, Sn) is the POSL intensity measured under the open window, copper or tin filters, respectively, and A, B and C are the coefficients unique to each of the different filters. The POSL readout procedure (Akselrod and McKeever, 1999) is described in detail in Chapter 7, while the theory behind the POSL technique is described in Chapter 2. An example POSL versus dose curve for A1203:C is illustrated in Chapter 3, Fig. 3.4. A typical POSL measurement takes 350-1000 ms per reading, and each reading depletes the POSL signal by only a fraction of the stored information available. Thus, second, third, or more readings may be performed if required, in order to achieve true second dose readings from one dosimeter (Akselrod and McKeever, 1999). 4.1.3. Landauer's InLight TM personal dosimetry system A new development in OSL personal dosimetry is the InLight TM system, also by Landauer. This is a bench-top, CW-OSL system specifically designed for personal dosimetry and uses bright LEDs (green) to stimulate the OSL. It is again based on A1203:C. The use of LEDs results in longer readout times compared with POSL, but
103
Passive Optically Stimulated Luminescence Dosimetry
Fig. 4.1. The Landauer Luxel personal dosimetry badge. (a) The A1203:C detector. (b) The filter pack, showing the open window, the Cu filter, the Sn filter and the Cu-grid filter. (c) The complete Luxel badge (courtesy of Landauer Inc., USA). TM
TM
enables dose readings for very little depletion of the signal. This in turn leads to a different paradigm for personal dosimetry badge wearers--namely that the InLight TM badges are normally distributed to individuals for a full year, with periodic readings taken twice every month during normal operation. In this way, not only are the doses corresponding to the individual wear periods evaluated, but the last reading gives the individual's total accumulated dose for the whole year. The stimulation power is such that the twicemonthly readings deplete the signal by only approximately 10% over one year. The InLight TM badge is illustrated in Fig. 4.2. Four A1203:C powder films are used as dosimeters. The operational dose range is up to 10 Gy, for use with photons > 5 keV, and
104
Optically Stimulated Luminescence Dosimetry
Fig. 4.2. The Landauer InLight personal dosimetry badge. Four A1203:C detectors, constructed in the same way as a Luxel detector, fit within the badge holder (courtesy of Landauer Inc., USA). TM
TM
with electrons > 150 keV. As with all A1203:C dosimeters, the system is insensitive to neutrons. InLight TMis designed for organisations that wish to perform their own dosimetry, whereas LuxelT~' is designed for use by radiation dosimetry service providers.
4.1.4. Beta dosimetry Akselrod et al. (1999) demonstrated the ability to measure Hp(0.07) for low-energy beta radiation using OSL from A1203:C thin dosimeters. A1203:C powder grains (20-25 txm) were crushed into the surface of an aluminium substrate resulting in a surface density of A1203:C powder between 7.5 and 9.3 mg/cm 2. Both CW-OSL and POSL measurements sources. were performed following beta irradiation from 147pm, 2~ and 9~176 Through use of appropriate Mylar thin-film filters, having an effective filter thickness of 3.8 mg/cm 2, a flat normalised energy response for Hp(0.07) was obtained from 0.224 (for 147pm) to 2.280 M e V (9~176 to within both EU and US DOELAP limits.
4.1.5. POSL imaging By using a series of radiation-absorbing filters, of different atomic number and thickness as described above, differential absorbed dose measurements can be made and information pertaining to the quality of the absorbed radiation is produced. For some dose evaluations, however, further analysis is warranted. By examining the POSL signal from that part of the detector under the copper grid (Fig. 4.1b) one can take advantage of the imaging capabilities of the OSL method to map a "radiation image" in order to detect abnormal exposure conditions. A possible abnormal exposure condition of interest might be the intentional, or accidental, exposure of the badge to a radiation source when a person is not wearing the badge. Someone placing the radiation film badge close to a radiation source in order to expose the badge (but not the person) to the radiation, would provide an example. This could be described as a "static" exposure and is to be distinguished from a "dynamic" exposure in which a person wears the film badge on his/her clothing over an
Passive Optically Stimulated Luminescence Dosimetry
105
extended period. Other potential abnormal exposure conditions include the unintended shielding of the detector by external objects (coins, paper-clips, etc.) due to the badge being worn incorrectly (say) in a person's pocket rather than being worn correctly on the outside of a person's clothing. Still other abnormal exposure conditions include contamination of the badge by radioactive contaminants, or a radiation filter being physically damaged. It is desirable, therefore, to be able to provide additional information about the conditions of the radiation exposure such that abnormal exposures may be identified. Akselrod et al. (2000) describe an analysis using Luxel TMbadges that had been exposed to a variety of radiation fields under a wide range of dynamic, static and other abnormal conditions. The approach taken by Akselrod et al. (2000) was to de-focus the stimulation laser beam during POSL readout and to image the pattern of luminescence emission from that part of the detector that was under the copper grid. Under conditions of static exposure, the image obtained (using an intensified, cooled CCD camera) of the POSL emission from the detector revealed a series of "sharp" peaks, corresponding to the penetration of the radiation field through the holes in the grid, and the absorption of the field by the copper parts of the grid (see Fig. 4.3a). On the other hand, detectors that had been moved during exposure were shown to give a broad, "blurred" image with no obvious image of the filter holes. The distinction between sharp and blurred image was assisted by performing a Fourier transform to obtain the spatial frequency of the obtained POSL image (see Fig. 4.3b). The two-dimensional discrete Fourier transform (DFT) used in the analysis by Akselrod et al. (2000) was: ~1~1
{
(nn 1
1 f(n, m) exp - j 2rr - ~ F ( n l , m l ) - NM n=O m=O
mml) } + ---M-- '
(4.2)
where N x M is the resolution of the spatial image and the transform represents the power spectrum in the frequency domain (wn, O)m), where the wn and o)m axes correspond, respectively, to the n and m axes. The low-frequency components of the DFT represent gradual variations in the POSL image, while the high-frequency components are a result of the pixel noise, sharp contrast features, etc. Thus, by selectively filtering unwanted frequencies, one can reduce the influence of unwanted noise or background signals.
Fig. 4.3. A POSL image from the Luxel A1203:Cdetector, obtained with a static 1 mGy exposure of M30 X-rays. (b) The DFT of the image pattern (from Akselrodet al., 2000). TM
Optically Stimulated Luminescence Dosimetry
106
Akselrod et al. (2000) applied a weighting function W(nl, ml), which may be viewed as a simple filter, such that its value was set to zero for all spatial frequencies above 20% of the spatial frequency range, and below 2% of the frequency range. This had the effect of eliminating the high-frequency components associated with pixel noise, etc., and the lowfrequency components associated with the dc signal in the image. Thus, only that part of the frequency spectrum corresponding to the filter pattem was retained. Inverse transformation then yields an almost-noise-free image. The image shown in Fig. 4.3a was obtained in this manner. After application of a suitable filter, numerical values may be extracted from the DFT before inverse transformation in order to calculate a parameter that represents the probability of static or dynamic irradiation. Two numerical parameters were defined to achieve this goal. The first was defined as a "shape parameter" ~:, which is a number representing the shape of the filtered DFT. The shape parameter is thus defined as: N
M
Z r __
2
nl = 0
IF(nl' m l)W(nl, m l)l
m 1= 0
Max g/1 ~ml
{[F(nl,ml)W(nl,ml)]}
(4.3)
where Max{ 9} represents the maximum of the DFT spectrum over the indicated range, and I~ represents the complex magnitude of the argument. Defined in this way, s is a general measure of the spatial frequency distribution of the image. An alternate parameter was defined as a "relative modulation" /x. From a priori knowledge of the filter pattern used in the definition of the POSL image, the ideal static image would consist of a regular array of bright POSL locations, the positions of which are dictated by the pattern of the filter used (Fig. 4. lb). This unique pattern is characterised by well-defined frequencies in the n- and m-directions. For the pattern shown in Fig. 4.3a the DFT (shown in Fig. 4.3b) is defined by eight "satellites" surrounding a central zerofrequency component. The satellites are located in the DFT at those frequencies (i.e., at those pixels) corresponding to the spatial separation of the holes in the filter in the n- and m-directions. The relative modulation, /x, is defined by extracting the intensities of the dominating four satellites along the o)n and o)m axes, thus: 4
k=l
IX-- Mean{F(nl,ml)}"
(4.4)
Here, IF(n~,m~)[ is the magnitude of the DFT at those pixels k(k-1...4) corresponding to the four satellites characterizing the filter used, and Mean{. } is the average values of those pixels in the DFT corresponding to the high-frequency range. In the determination of/~, a filter is not applied, i.e., W(n~,m l ) = 1 for all (n l, m 1). An advantage of the relative modulation method is that even if the image is partially obscured by an unwanted object (coin, etc.) the signal at the four characteristic pixels still represents only that signal from the irradiation filter, and not from the obscuring object. Thus, the pattern can still be recognised and the value of/~ can still be determined, even in this case.
Passive Optically Stimulated Luminescence Dosimetry
107
Akselrod et al. (2000) extracted values for ~: and ~ from a series of POSL images obtained under a variety of exposure conditions. They then defined a discrimination value for each parameter (~:aiscrand ~discr) to distinguish between static and dynamic exposures. Since ~discr and /[J,discr are dose-dependent, the discrimination levels were determined by plotting the sc and ~ values as a function of dose for all irradiations--i.e., for all energies and for both static and dynamic irradiations, and the dose dependencies of the discrimination levels were determined as ~discr : 623-6D-~ and ]~discr--50-12D~ The data are shown in Fig. 4.4. These functions are the loci of the sc or/x values at the lower (for ~ or upper (for ~) edges of the data for dynamic irradiations and form a clearly defined discrimination level to distinguish between static and dynamic irradiation. The procedure was found to be successful for > 90% of the cases tested when using/Xaiscr, and > 80% successful when using ~:aiscr, even when the images were partially obscured by unwanted objects (coins, paper clips, etc.).
4.2. Environmental OSL dosimetry using A1203:C 4.2.1. Measurement of the natural terrestrial background radiation The high sensitivity of A1203:C has been exploited for environmental dosimetry using TL. For example, Budzanowski et al. (1996) reported the ability to measure integrated environmental photon doses of the order of 4 IxGy using the TL signal from this material. The capability of using the OSL response of A1203:C to measure the environmental photon radiation over short periods was tested by BCtter-Jensen et al. (1997). The response of A1203:C compares well with that of a high-pressure ionisation chamber (Reuter Stokes RS-111). As seen from Table 4.1, even at low doses, the OSL uncertainties are small and the OSL measurements agree very well with those obtained with the ionisation chamber (BCtter-Jensen et al., 1997). To illustrate the OSL yield, the decay curves from A1203:C exposed to integrated natural environmental radiation doses of 0.98 and 5.10 txGy obtained over 15 and 72 h, respectively, compared with that from a 44 I~Gy 6~ gamma calibration dose are shown in Fig. 4.5 (from BCtter-Jensen et al., 1997). It has been shown that A1203:C can be completely emptied of trapped charges by exposure to daylight for some hours. This observation effectively improves the minimum detection limit of the dosimeter; by zeroing the A1203:C immediately before field measurement, any dose absorbed during travel to the site is removed. Sensitivity changes during normal measurement cycles have been shown to be negligible; this allows a simple regeneration dose measurement, without the need for any special procedures to deal with sensitivity changes (BCtter-Jensen et al., 1997). 4.2.2. Measurement of the natural space background radiation The radiation environment in space consists of a broad spectrum of low to high energy, light and heavily charged particles (HCP) (Benton and Benton, 2001). OSL is potentially a useful technique for monitoring the doses to astronauts in this environment, especially for
Optically Stimulated Luminescence Dosimetry
108 100
(a)
90 discr
80 :a. ,.0
-
50.12D
~
70
N 6o o
50
>
40
9 n~
30 20 10 I
I
I
I
1
2
3
4
Dose (mGy) 2500
(b) 2000 O O
L.
"~ 1500 E
x
s_
g_
|
9
~ discr= 623.6D -~
looo
t-
co
5OO
0
1
2
3
4
5
6
Dose (mGy)
Fig. 4.4. Discrimination levels/-/,discr (a) and ~discr (b). The levels have been selected to be the upper (in the case of/x) or the lower (in the case of ~ limits of the dynamic irradiation values, respectively, and are given by the functions /~discr = 50.12D~ and Caiscr = 623.6D-~ Thus, static irradiations yield/~ or ~ values which are above (in the case of/x) or below (in the case of ~) the discriminator levels/Xdi~cr and SCdisc,.,respectively. The datum points in the figures represent different filtrations for X-irradiation, thus: o , M30; O, M60; A, M100; 9 M150; • H150. The H150 irradiations are not possible to classify as either static or dynamic using these discrimination levels since copper has a weak attenuation coefficient at these energies (re-drawn from Akselrod et al., 2000).
long-duration portable OSL dosimeters on A1203:C, and
flights on the International Space Station, since it may be possible to use readers on board the spacecraft to enable the astronauts to read the OSL a regular basis. The only material suggested as an OSL dosimeter in space is to be useful as a dosimeter in these environments it is first necessary to
Passive Optically Stimulated Luminescence Dosimetry
109
Table 4.1 Comparison between the response of high-pressure ionisation chamber (HPIC) and A1203:C dosimeters after short-term exposure to natural environmental photon radiation. Nine A1203:C chips were used for each dose evaluation Exposure time (h)
HPIC (txGy)
A1203:C (IxGy)
15 72
1.04 ___0.05 5.13 --- 0.01
0.98 +__0.03 (n = 9) 5.10 _+ 0.02 (n = 9)
calibrate the OSL response of the material to HCP over a wide range of energies and linear energy transfer (LET). A suggestion as to the high LET response of OSL from this material may be gained from an examination of the gamma (or high-energy beta) dose response. Both the TL and OSL dose responses of A1203:C show saturation effects setting in around 50 Gy, depending on the sample. From track interaction theory (e.g., Horowitz, 2001; Horowitz et al., 2001), one can expect a significantly reduced efficiency to HCP of high LET. Early measurements of the TL efficiency to alpha particles supported this conclusion (e.g., Mukherjee and Lucas, 1993). Yasuda and Kobayashi (2001) recently investigated the possibility of using A1203:C in passive OSL dosimeters for monitoring the absorbed dose in high LET radiation fields. They exposed Landauer Luxel TM A1203 detectors to He (150 MeV/n), C (290 MeV/n), Ar (500 MeV/n) and Fe (500 MeV/n) ion beams in a heavy ion medical accelerator. As expected, a notable reduction of the OSL efficiency as a function of LET was found, except for the He ions for which an efficiency > 1 was reported (compared to 137Cs radiation). For an effective dosimetry package, the reduced efficiency of the OSL signal at high LET may be compensated by using the dosimeters in combination with plastic nuclear track detectors (e.g., Doke et al., 1995). The reasonably good linear dose response and angle dependence at high LET energies, however, make OSL from A1203:C a promising candidate for use in space dosimetry.
2oo 150
60 t~ v
O
.100 (/9 ..d (.9
"•
43.5ILtGy Co-60 calibration dose
~ ~
72 hrs Environmental radiation (5.1 pGy)
50-
|
0
20
,
,
40
,
Time (s)
,
60
,
,,
|
80
,
100
Fig. 4.5. OSL decay curves from A1203:C dosimeters exposed over 15 and 72 h to the natural environmental background radiation representing evaluated integrated doses of 0.98 and 5.10 lxGy, respectively, compared to that from a 44 I~Gy 6~ gamma calibration dose (from BCtter-Jensen et al., 1997).
110
Optically Stimulated Luminescence Dosimetry
4.3. UV dosimetry Until recently, atmospheric ozone has absorbed all but approximately 1% of wavelengths below 320 nm. Recent evidence of ozone depletion in the stratosphere has generated interest in the biological impact on plants and animals resulting from increased exposure to these wavelengths (e.g., Lubin and Jensen, 1995; Driscoll, 1996). In addition, increases in erythema (sunburn), skin cancers, eye disorders and DNA damage have shown to increase as the UVB level increases. This has created a need for ultraviolet dosimetry of wavelengths shorter than 320 nm, particularly at southern latitudes where ozone depletion appears to be most significant (e.g., Quintern et al., 1994). UV dosimetry using TL has been suggested in the past (e.g., Chakrabarti et al., 1990; Yeh and Su, 1996; Trinkler et al., 2000) and offers the advantage of being able to place the dosimeters in situ, without requiring any special monitoring or logistical considerations (e.g., portable field power source for any electronics which other UV measuring systems may require). Colyott et al. (1997) designed a UVB field dosimeter that measures absorbed ultraviolet dose, for the wavelength band centred at 307 nm, based upon the phototransferred TL properties of thin layer A1203:C detectors. More recently, Colyott et al. (1999) used the phototransferred OSL (PTOSL) signal from A1203:C to measure the integrated UVB dose. OSL from A1203:C involves the optical stimulation of charge carriers trapped at the so-called main dosimetric trap, which is thermally stable up to 465 K (see Chapter 3). However, deeper traps, thermally stable up to 900 and 1200 K, also exist and the stimulation efficiency has been shown to be wavelength- dependent for both deep traps and the main dosimetric traps (BCtter-Jensen and McKeever, 1996). Since the wavelength dependence is different for each type of trap, careful selection of the optical stimulation wavelength allows each type of trap to be probed selectively. In general, PTOSL involves the production of OSL by phototransfer of charge to empty traps from deeper, filled traps. Thus, by filling the deeper traps (as well as the dosimetry traps) of A1203:C with a pre-dose of irradiation and subsequently preheating the sample to remove any trapped charge from the dosimetry traps, charge can be transferred from the deep traps to the dosimetry traps by exposing the sample to UV wavelengths. When subsequently stimulated with blue-green wavelengths (470-530 nm), a PTOSL signal is observed from the sample as the transferred charge is released from the dosimetry traps. The PTOSL signal is proportional to the light exposure, as well as to the initial pre-dose of irradiation. The PTOSL UVB dosimeter designed by Colyott et al. (1999) is shown schematically in Fig. 4.6. The dosimeter container that includes an optical interference detection filter centred at 307 nm, is made of Teflon, is watertight and can thus be used both in air and water. The UVB dosimeter normalised PTOSL response as a function of natural sunlight exposure is shown in Fig. 4.7. The data show a nearly linear relationship over at least three orders of magnitude in UVB exposure. As a result, the dosimeter has a nearly linear response from some minutes to several weeks total sunlight exposure. Trinkler et al. (2000) showed that aluminium nitride, A1N:Y203, is an ultra-sensitive TL and OSL dosimeter material with a sensitivity one order of magnitude higher than that of A1203:C. Trinkler et al. (2000) investigated the blue-light stimulated OSL properties of this material after UVB exposure and they concluded that even
Passive Optically Stimulated Luminescence Dosimetry
111
Fig. 4.6. Schematic diagram of the UVB dosimeter. Dimensions are: cap/base diameter, 25.4 mm; window diameter, 12.7 mm; height 19.1 mm (from Colyott et al., 1999).
though significant fading of the OSL signal was observed, the very high OSL sensitivity of this material makes it attractive as a UVB dosimeter when very short exposure times can be applied. This would be practical when using portable readout instrumentation in the field.
Fig. 4.7. UVB dosimeter normalised PTOSL response as a function of natural sunlight exposure (from Colyott et al., 1999).
112
Optically Stimulated Luminescence Dosimetry
4.4. OSL and RL remote optical fibre dosimetry in medical applications 4.4.1. Real-time (RT) in vivo monitoring of doses during radiotherapy Therapeutic radiation oncology treatments, including teletherapy and brachytherapy as well as related procedures such as the treatment of restinosis, require the delivery of highly localised doses of radiation to patient target organs. The efficacy of the radiation treatment, however, requires knowledge of the absorbed dose at the organ of interest to within ___5%, with a higher risk of local recurrence or a higher risk of complications resulting from incorrect exposure. Furthermore, since it is inevitable that healthy organs and tissue will also be exposed during treatment, over-exposure carries with it a concomitant risk of secondary cancers. It is necessary that all possible measures be taken to reduce the toxicity effects of undesired exposure to as low as possible. This requires both the accurate calibration of radiotherapy sources and the accurate assessment of dose at critical locations on or within the body. Thus, determination of the spatial distribution of dose to tissue from the source is an essential aspect of effective health care and treatment. As a result of international regulations (International Commission on Radiological Protection, 2000) there is a growing demand to improve methods for in vivo measurements of the absorbed doses to patients. So far, patient monitoring has been performed using one of the several available detector systems; radiochromic dye films, plastic scintillators, TL dosimeters, diode detectors, or MOSFET detectors. However, each of these systems has significant disadvantages for general-purpose external dosimetry. Apart from the scintillators, the readout system is not coupled directly to the detectors, and this in turn requires a separate post-irradiation evaluation of the dosimeters. The result is that no RT dose or dose rate information is provided. Ideally, a RT in vivo dosimeter is needed to measure absorbed doses during exposure, mainly to provide feedback of important information to the physician during treatment. OSL is an obvious method for RT in vivo measurements of absorbed doses because the stimulation of a dosimeter can be made simultaneously with the detection of luminescence, e.g., via light fibres using remotely placed light sources. Since the stimulation wavelength is different from that of the emitted luminescence, CW-OSL measurements can be carried out using only a single optical fibre in connection with a suitable detection filter placed in front of a photomultiplier tube cathode. Furthermore, in addition to absorbed dose information as provided by OSL, the prompt radioluminescence (RL) signal generated by the therapy radiation source directly reflects the dose rate at any time during the treatment. Thus, in addition to a small-size sensor, the main advantages of an optical fibre dosimeter over the currently available radiation detectors used in clinical applications are the capabilities of measuring both the RT dose rate and the absorbed dose. When the sensor size is similar to the field size, the sensor will provide information on the average dose distribution, rather than the dose at the centre of the radiation field. 4.4.2. Optical fibre dosimeters Roy et al. (1997) designed a remote OSL dosimeter system based on rare earth doped alkaline earth sulphides (e.g., MgS) as the dosimeter material coupled to the end of an optical fibre and stimulated with an infrared laser diode. This dosimeter system yielded
Passive Optically Stimulated Luminescence Dosimetry
113
a dose response from 40 lxGy to 10 Gy. Justus et al. (1999a,b) developed Cu+-doped quartz rods, which they used as remote fibre OSL dosimeters under stimulation with 790 nm light from a GaAs laser. The OSL characteristics of Cu+-doped quartz are significantly different from those of natural quartz, which makes it possible to use lowenergy light for stimulation. Huston et al. (2001) constructed fibre dosimeters by drawing a 20 mm diameter by 1 m long Cu+-doped glass rod into a 1 km length of 400 txm diameter. This fibre was used to construct a four-channel fibre optic dosimeter system for monitoring the dose delivered to patients undergoing cancer radiotherapy. The radiation-sensitive portion of the optical fibre dosimeter consists of a 2 mm long, 0.4 mm diameter piece of Cu-doped glass that is fusion spliced to one end of a 1 mm long, 0.4 mm diameter optical fibre. A black Teflon jacket surrounds most of the fibre assembly to prevent external light from entering the fibre. The end of the fibre is coated with aluminium to reflect both the stimulation and the signal light to improve the efficiency of the dosimeter. The system, which is shown in Fig. 4.8, has been tested successfully over a range from 10 mGy to 10 Gy. Ranchoux et al. (2002) reported a fibre-based remote OSL system based on A1203:C single crystals, Cu+-doped silicate fibres and alkaline earth sulphide (MgS). A titaniumsapphire tuneable laser was used to stimulate the Cu+-doped silicate fibre (860 nm) and MgS (980 nm) whereas the A1203:C was stimulated using 514.5 nm light from an argon laser. Polf et al. (2002) examined A1203:C optical fibres for their potential use as RT luminescence dosimeters for use in radiotherapy. The OSL and RL responses of the A1203:C fibre probes were measured and it was found that both RL and OSL signals increased linearly with dose rate and the absorbed dose, respectively, within the actual To/From PC
PMT's 7-
Laser
Filter
To/From DosimeterProbe Fig. 4.8. Schematicdiagram of a remote multiple optical fibre dosimeter system (from Huston et al., 2001).
114
Optically Stimulated Luminescence Dosimetry
Fig. 4.10. OSL as function of dose measured using the dual-fibre dosimeter. See text for explanation (from Polf, 2002).
Passive Optically Stimulated Luminescence Dosimetry
Optically Stimulated Luminescence Dosimetry
116 2000
I
I
I
,...-., (fJ Q.
.o.
tO Q.
/
60Co ' 137 Cs
/
1000
.f"
0
._I
.,-.. cO
./
, , o/
.J
/
(a) L
/=
o
/
0
./~
n,"
90Sr
10-
0o~
002
O
/
./
/,
/"
/"
/
O"
oJ e
0z
J o
Dose rate [Gy/min]
(b)
q
~
Dose [Gy]
Fig. 4.12. (a) RL versus gamma dose, and (b) OSL versus beta dose rate for the single optical fibre system (from Andersen et al., 2002).
OSL response to 0.5 Gy Deviation from mean value for 6 MV photons Fig. 4.13. Reproducibility measurements using the single optical fibre system in clinical beams. The OSL responses are normalised to that obtained using 6 MV photons. As seen, the precision is < 0.5 % (from Andersen et al., 2002).
Passive Optically Stimulated Luminescence Dosimetry
117
range in radiotherapy ( 0 . 5 - 8 5 Gy). Polf et al. (2002) designed a dual-fibre system using the green light from a 40 m W Nd:YAG laser to stimulate the OSL signal. RL was experimentally generated using a 100 mCi 9~176 source. The OSL response as a function of dose was linear in the dose range from 50 mGy to 10 Gy and the OSL signal increased for doses up to 100 Gy. Fig. 4.9 shows a schematic diagram of the dual-fibre system. Fig. 4.10 shows the OSL response as a function of absorbed dose. Fig. 4.10a compares the RT OSL signal with the standard OSL signal from the same fibre. The latter was obtained by irradiating to the doses indicated and reading the corresponding OSL. Fig. 10b shows the same data but with the RT signal corrected for depletion during the RT measurement. The under-correction at higher doses is due to a dose-dependent increase in the OSL depletion rate over this range. Andersen et al. (2002) and Aznar et al. (2002) used a single-fibre system combined with a 2 0 m W Nd:YAG (532 nm) laser as a remote optical fibre dosimetry system for radiotherapy (see Fig. 4.11). To produce OSL, the laser beam is focused through a wavelength-discriminating beam-splitter positioned at a 45 ~ angle relative to the incident beam, and via the light fibre into the dosimeter. The OSL is transmitted from the dosimeter in the same fibre and reflected through 90 ~ onto the photocathode of a miniature PM tube. A narrow UV band transmission filter (e.g., Hoya U-340) mounted in front of the photocathode rejects the scattered green light from the laser. Fig. 4.12a and b shows the RL response versus 137Cs gamma dose rate and the OSL response versus 9~176 beta dose, respectively, obtained with the one-fibre system. Fig. 4.13 shows that a reproducibility of less than 0.5% could be obtained when exposing the A1203:C fibre to clinical beams of 6 and 18 MV photons, and 20 MV electrons.
References Akselrod, A., Akselrod, M.S., Agersnap Larsen, N., Banerjee, D., BCtter-Jensen, L., Christensen, P., Lucas, A.C., McKeever, S.W.S., Yoder, C., 1999. Optically stimulated luminescence response of A1203 to beta irradiation. Radiat. Prot. Dosim. 85, 125-128. Akselrod, M.S., McKeever, S.W.S., 1999. A radiation dosimetry method using pulsed optically stimulated luminescence. Radiat. Prot. Dosim. 81, 167-176. Akselrod, M.S., Agersnap Larsen, N., McKeever, S.W.S., 2000. A procedure for the distinction between static and dynamic radiation exposures of personal dosimetry badges using pulsed optically stimulated luminescence. Radiat. Meas. 32, 215-225. Andersen, C.E., Aznar, M.C., BCtter-Jensen, L., Baeck, S.A.J., Mattson, S., Medin, J., 2002. Development of optical fibre luminescence techniques for RT in-vivo dosimetry in radiotherapy. Presented at the IAEA symposium on standards and codes of practice in medical radiation dosimetry, Vienna, November 25-28, 2002 (IAEA-CN-96-118). Aznar, M.C., Polf, J.C., Akselrod, M.S., Andersen, C.E., Baeck, S.A.J., BCtter-Jensen,L., Mattson, S., McKeever, S.W.S., Medin, J., 2002. Real-time optical fibre dosimetry in radiotherapy. Presented at the American Association of Physics in Medicine (AAPM) 44th annual meeting in Montreal, July 14-18, 2002. Benton, E.R., Benton, E.V., 2001. Space radiation dosimetry in low-earth orbit and beyond. Nucl. Instr. Meth. B. 184, 255- 294. BCtter-Jensen, L., McKeever, S.W.S., 1996. Optically stimulated luminescence dosimetry using natural and synthetic materials. Radiat. Prot. Dosim. 65, 273-280. BCtter-Jensen, L., Agersnap Larsen, N., Markey, B.G., McKeever, S.W.S., 1997. A1203:C as a sensitive OSL dosemeter for rapid assessment of environmental photon dose rates. Radiat. Meas. 27, 295-298.
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Optically Stimulated Luminescence Dosimetry
Budzanowski, M., Bilksi, P., BCtter-Jensen, L., Delgado, A., Olko, P., Sfiez-Vergara, J.C., Waligorski, M.P.R., 1996. Comparison of LiF:Mg,Cu,P (MCP-N, GR-200A) and (et-A1203:C TL detectors in short-term measurements of natural radiation. Radiat. Prot. Dosim. 66, 157-160. Chakrabarti, K., Mathur, V.K., Abbundi, R.J., Hill, M.D., 1990. UV induced trapping in powder and sintered CaSO4:Tm and CaSO4:Dy. Radiat. Prot. Dosim. 33, 35-38. Colyott, L.E., Akselrod, M.S., McKeever, S.W.S., 1997. An integrating ultraviolet-B dosemeter using phototransferred thermoluminescence from ct-A1203:C. Radiat. Prot. Dosim. 72, 87-94. Colyott, L.E., McKeever, S.W.S., Akselrod, M.S., 1999. An integrating UVB dosemeter system. Radiat. Prot. Dosim. 85, 309- 312. Doke, T., Hayashi, T., Nagaoka, S., Ogura, K., Takeuchi, R., 1995. Estimation of dose equivalent in STS-47 by combination of TLDs and CR-39. Radiat. Meas. 24, 75-82. Driscoll, C.M.H., 1996. Solar UVB measurements. Radiat. Prot. Dosim. 64, 179-188. Horowitz, Y.S., 2001. Theory of thermoluminescence gamma dose response: the unified interaction model. Nucl. Instr. Meth. B 184, 68-84. Horowitz, Y.S., Satinger, D., Gamboa-deBuen, I., Buenfil, A.E., Ruiz-Trejo, C., 2001. The extended track interaction model: supralinearity and saturation He-ion TL fluence response in sensitized TLD-100. Radiat. Prot. Dosim. 84, 29-34. Huston, A.L., Justus, B.L., Falkenstein, P.L., Miller, R.W., Ning, H., Altemus, R., 2001. Remote optical fibre dosimetry. Nucl. Instr. Meth. B 184, 55-77. International Commission on Radiological Protection, 2000. Prevention of accidental exposures to patients undergoing radiation therapy. ICRP Publication 86, Annals of the ICRP 30, 1-70. Justus, B.L., Pawlovich, K.J., Merritt, C.D., Huston, A.L., 1999a. Optically and thermally stimulated luminescence characteristics of Cu+-doped fused quartz. Radiat. Prot. Dosim. 81, 5-10. Justus, B.L., Merritt, C.D., Pawlovich, K.J., Huston, A.L., Rychnovsky, S., 1999b. Optically stimulated luminescence dosimetry using doped fused quartz. Radiat. Prot. Dosim. 84, 189-192. Lubin, D., Jensen, E.H., 1995. Effects of clouds and stratospheric ozone depletion on ultraviolet radiation trends. Nature 377, 710-713. Mukherjee, B., Lucas, A.C., 1993. Light conversion efficiency of aluminium oxide dosimeters irradiated with 241A1TIalpha particles. Radiat. Prot. Dosim. 47, 177-179. Perry, J.A., 1987. RPL Dosimetry: Radiophotoluminescence in Health Physics. Adam Hilger, Bristol. Piesch, E., Burgkhardt, B., Fischer, M., Rrber, H.G., Ugi, S., 1986. Properties of radiophotoluminescence glass dosimeter systems using pulsed laser UV excitation. Radiat. Prot. Dosim. 17, 293-297. Piesch, E., Burgkhardt, B., Vilgis, M., 1990. Photoluminescence dosimetry: progress and present state of the art. Radiat. Prot. Dosim. 33, 215-226. Piesch, E., Burgkhardt, B., Vilgis, M., 1993. Progress in phosphate glass dosimetry: experiences and monitoring with a modern dosimetry system. Radiat. Prot. Dosim. 47, 409-413. Polf, J.C., 2002. A study of optically stimulated luminescence in A1203 fibres for the development of a real-time fibre optic dosimetry system. PhD. Thesis, Oklahoma State University, Stillwater. Polf, J.C., McKeever, S.W.S., Akselrod, M.S., Holmstrom, S., 2002. A real-time, fibre optic dosimetry system using A1203 fibres. Radiat. Prot. Dosim. 100, 301-304. Quintern, L.E., Puskeppeleit, M., Rainer, P., Weber, S., E1 Naggar, S., Eschweiler, U., Horneck, G., 1994. Continuous dosimetry of the biologically harmful UV-radiation in Antarctica with biofilm technique. Photochem. Photobiol. B. 22, 59-66. Ranchoux, G., Magne, S., Bouvet, J.P., Ferdinand, P., 2002. Fibre remote optoelectronic gamma dosimetry based on optically stimulated luminescence of A1203:C. Radiat. Prot. Dosim. 100, 255-260. Roy, O., Magne, S., Gaucher, J.C., Albert, L., Dusseau, L., Bessiere, J.C., Ferdinand, P., 1997. All optical fibre sensor based on optically stimulated luminescence for radiation detection. Presented at the 12th International Conference on Optical Fiber Sensors OFS'97. Oct. 28-31, 1997, Williamsburg, Virginia, USA. Trinkler, L., BCtter-Jensen, L., Christensen, P., Berzina, B., 2000. Studies of aluminium nitride ceramics for application in UV dosimetry. Radiat. Prot. Dosim. 92, 299-306. Yasuda, H., Kobayashi, I., 2001. Optically stimulated luminescence from A1203:C irradiated with relativistic heavy ions. Radiat. Prot. Dosim. 95, 339-343. Yeh, S.-M., Su, C.-S., 1996. UV induced thermoluminescence in rare earth oxide doped phosphors: possible use for UV dosimetry. Radiat. Prot. Dosim. 65, 359-362.
Chapter 5
OSL properties of natural materials 5.1. Quartz 5.1.1. Crystal structure and point defects Quartz is the most common mineral in our environment. It is found in granite, hydrothermal veins and volcanic rocks, as well as in sedimentary deposits derived from such solid materials. If undisturbed, these sediments contain a record of past environmental change, both natural and anthropogenic, in terms of organic remains or artefacts. These sediments are also made into building materials, such as bricks, and pottery. Thus, the potential use of a dose reconstruction technique based on quartz grains is enormous, whether as a dating tool in archaeology and Quaternary geology, or in nuclear accident dosimetry. Studies of the luminescence behaviour of synthetic and amorphous quartz are also important, since they play a role in the manufacture of electronic devices. Quartz has a simple molecular structure, SiO2, in which the Si and O atoms are linked as shown in Fig. 5.1. When formed as a result of processes beneath the Earth's surface, quartz contains particular impurities that act to create electron traps and provide recombination centres. Most quartz is in the structural form known as alpha-quartz. On heating, the quartz can be restructured as it passes through two phase transitions, at 575 and 870~ to form beta-quartz and tridymite, respectively. The alpha-beta structural change at 575~ is displacive only and is fully reversible when cooling takes place slowly. Since the alpha-beta phase transition involves an increase in the size of the crystal lattice, it might be expected to have a significant effect on the optically stimulated luminescence (OSL) behaviour. The defect structures of natural minerals are complex. Many defects within the silicate minerals are based on the SiO4 tetrahedral structure common among them. The archetype of these minerals, quartz, is one of the most well studied natural minerals in terms of its luminescence properties, primarily because of its utility in luminescence dosimetry, particularly dating. One can define two basic types of defect within quartz--extrinsic defects related to impurities, and intrinsic defects related to structural imperfections such as missing oxygen or silicon atoms. The 40% ionic and 60% covalent nature of the S i - O bond in SiO2 leads to a rather rigid structure and misplaced lattice ions tend to associate with the impurities and, thus, combinations of impurities and structural defects exist forming complex structures. Such defects may well play important roles in the thermoluminescence (TL) and OSL properties of this material, and related materials. Among the most ubiquitous of impurities in quartz is aluminium, in the form of A13+ ions. These substitute with ease for the host tetravalent Si 4+ ions, with the charge
120
Optically Stimulated Luminescence Dosimetry
silanol groups
0
0
/ Si \
/ Si \
I
O\si I 0I
0
i0
OH
I
O\ i0
Si (+) oxygen vacancy - ~ (+)
/Si\o\
/ 0 / Si \
O~ / 0 hole on a .Si Ge nonbridging~l~ (o)0 / I oxygen -., 0(-) Li+ 0 ,71 I nonbridging /Si~ /Si oxygen
0
O \ Si f 0 I
0
0
I
O\
HO~Si ~ 0 Si ~ O H I 0I
/ Si \
I
/ Si \
.,, i 0 Si I 0I
O\ /0 Si I 0
/ AI(4
/1(_) Li+
I
Si n T ~
Na +
O\ /0 Si I 0
/ Si \
I
~ O \ Si t O i l \ O \ s i / O
0
O\ i 0
I
0 substitutional
/
Si
peroxy linkage
O\
Si / I 0 I
\O\si/O
/Si
0
peroxy
"~O~ 0 radical
I
0
and interstitial cations
Fig. 5.1. Schematicof quartz lattice showing common defects (from G6tze et al., 2001).
imbalance provided by monovalent impurities (M +) such as H +, or alkali ions Na + and Li +, forming neutral centres of the type (A104/M+) ~ (Nassau and Prescott, 1975; Weil, 1975; Cohen and Makar, 1982; McKeever, 1984; Martini et al., 1995). Charge compensation by H leads to the formation of OH-bonds that can be observed in infrared spectroscopy (Brown and Kahan, 1975; Subramaniam et al., 1984). Irradiation of quartz at low temperatures (e.g., liquid nitrogen temperature (LNT)) results in the capture of a hole by these defects, leading to the formation of (A104/M+) + defects. However, subsequent warming to room temperature or, alternatively, irradiation at room temperature, leads to the formation of (A104) ~ in which the charge compensation is provided by a trapped hole. This defect gives rise to the well-known smoky colouration of smoky quartz and of heavily irradiated quartz (Nassau and Prescott, 1975; McKeever, 1984; McKeever et al., 1985). Radiation-induced conductivity measurements (Hughes, 1975; Martini et al., 1986) indicate that the alkali ions, or the H + ions, diffuse away from the A13+ impurity along the c-axis channels during irradiation (at room temperature) or during the post-irradiation heating (if irradiated at low temperature). Other common impurities in quartz include Ge and Ti, both of which form electron-trapping centres. During the process of warming irradiated quartz, or during irradiation at room temperature, electron traps of the (Ge4+/M+) ~ are formed when alkali ions diffuse away from the A1 site to become trapped at (Ge4+) - centres. Similarly, (Ti3+/H+) - electron traps may be created (McKeever et al., 1985). Additional important defects within SiO2 include the so-called E1-centres, of which there are several. The E~ centre consists of an unpaired electron trapped at one of the nonequivalent Si sites next to an oxygen vacancy (Isoya et al., 1981). The E~-centre also consists of an oxygen vacancy, but with an associated proton, and with the electron located
OSL Properties of Natural Materials
121
on the other non-equivalent Si site. A third related defect, the E~-centre, is identified as an oxygen vacancy with a hydride ion bonded to one of the Si atoms, and with an electron shared between the two Si atoms (Isoya et al., 1981). Correlating specific defects with specific TL peaks in quartz has proved to be extremely difficult. In a study of synthetic quartz McKeever et al. (1985) studied the annealing curves of electron spin resonance (ESR) signals and correlated the 110~ TL peak with the electron trap (GeO4)-. Few other correlations of this type exist in the literature. From the point of view of the origin of the luminescence bands from quartz, identification of the exact defect causing the observed emissions is difficult. This is primarily due to the broad-band nature of the emissions, and the slight variations that appear in the wavelengths for maximum emission from one quartz sample to another, perhaps due to different degrees of overlap and strengths of the various emissions. As a result, one can never be sure that the observed emission reported for one quartz sample matches the reported emission observed from another. McKeever (1984) gives a detailed review of the early (pre-1984) luminescence literature on quartz, and Krbetschek et al. (1997) give a comprehensive discussion of the literature up to 1996. The luminescence bands of importance for radiation dosimetry lie in the visible-UVA part of the spectrum, specifically from approximately 360 to about 420 nm (UVA-violet), 460 to 480 nm (blue) and between 610 and 630 nm (orange) (Krbetschek et al., 1997). In any one sample, the observed emissions in these regions may be composite bands. Of major importance for TL and OSL dosimetry are the UVA-violet emissions. In OSL measurements at room temperature, these have been reported at approximately 365 nm (Huntley et al., 1991). In TL at 110~ the emission appears nearer 380 nm, while higher temperature TL peaks show emissions at 410 nm (for TL at 200-220~ and 430 nm (for TL at 305-325~ Franklin et al. (1995) suggest that the traps giving rise to each of these signals form a "family" of centres, with each using the same recombination centre. The emission wavelength depends upon the temperature at which the signal appears and is thus highly temperature-dependent, implying significant phonon coupling with the host lattice. Not surprisingly, therefore, this emission is seen to undergo strong thermal quenching, with an apparent activation energy near 0.6-0.65 eV (Wintle, 1975, 1997; Nanjundaswamy et al., 2002). The source of this emission is uncertain, even after many experimental studies have been devoted to identifying the defects causing the various emissions in quartz. Yang and McKeever (1990) attempted to identify the point defects associated with the so-called "pre-dose effect" (Zimmerman, 1971) in this material using electron spin resonance and concluded that the 380 nm emission was due to the recombination of electrons with holes trapped at H304 centres. These centres consist of three protons occupying a Si 4+ site, with a trapped hole for charge neutrality. The precursors to the H304 centres are believed to be H404 centres (Nuttall and Weil, 1980) in which a hydrogen ion is replaced by a hole during irradiation. Others relate the 380 nm emission to electron recombination with holes trapped at (A104) ~ centres (Halperin and Sucov, 1993; Martini et al., 1995), or at (A104/M +) + defects (Luff and Townsend, 1990; Kalceff and Phillips, 1995). However, as discussed earlier, the latter defect is only stable below room temperature and cannot be the cause of the above-room-temperature TL signals of interest in dosimetry. (This highlights the difficulty of identifying specific
122
Optically Stimulated Luminescence Dosimetry
emissions in quartz with particular defects, since many defects give rise to emissions in the same wavelength region.) The blue emission (from 460 to 480 nm) has most frequently been identified with the (A104) ~ centre (Nassau and Prescott, 1975; McKeever, 1984; McKeever et al., 1985; Yang and McKeever, 1990; Rink et al., 1993; Woda et al., 2002) although Hashimoto et al. (1987) reported an anti-correlation of the blue TL emission with A1 content, and others have connected the blue emission to intrinsic features, such as self-trapped-exciton relaxation (Luff and Townsend, 1990; Kalceff and Phillips, 1995). The orange emission (from 610 to 630 nm) has been correlated with non-bridging oxygen hole centres (Kalceff and Phillips, 1995) and U-centres (Luff and Townsend, 1990). The lack of certainty regarding the identification of the defects involved in the luminescence processes in quartz has led to a corresponding uncertainty in describing the TL and OSL emission mechanisms. While some identifications are reasonably well placed, e.g., the identification of the 110~ TL with the electron centre (GeO4) ~ none are certain. Of particular interest in luminescence dating and dosimetry studies has been the pre-dose effect first described by Zimmerman (1971). The effect is an observed increase in sensitivity of the 110~ TL peak following irradiation (the predose) and heating ("activation"). Zimmerman (1971) described a phenomenological model for the process, but others since then have attempted to identify specific defects and defect interactions that may cause the effect. McKeever et al. (1985) and Yang and McKeever (1990) noted that the l l0~ TL peak was emitted at both 380 and 470 nm, but that only the 380 nm emission was involved in the pre-dose process; the 4 7 0 n m emission did not sensitise. This was confirmed in later emission spectra studies by Akber et al. (1988). From measurements of IR absorption spectra and ESR, Yang and McKeever (1990) suggested that the activation of the pre-dose effect took place via the thermally stimulated movement of hydrogen ions in the lattice from hydrogen ion traps, created during the pre-dose irradiation, to the H304 centre precursors. A similar mechanism has been suggested recently by Itoh et al. (2001; 2002) who prefer to identify the 380 nm emission with (A104) ~ centres and relate the activation process to the movement of alkali ions and their trapping at unidentified defects (X) to form (XM) ~ electron traps. A suggested impurity for the unknown defect is Ge, and thus the l l0~ peak would then be a (Ge4+M+) ~ centre. As shown by McKeever et al. (1985) however, such defects are stable up to 300~ and cannot be the origin of the 110~ TL peak; nevertheless, there are several attractive features of the models of Itoh et al. (2001, 2002) that deserve further study. In addition to those centres that are directly related to the production of luminescence emission, many other defects exist that affect the TL and OSL emission only indirectly. For example, if recombination occurs without the emission of a photon (non-radiative recombination), then such centres act as competitors to the luminescence centres and their concentration can dictate the strength of the luminescence emission observed. Such defects can be observed by non-luminescence techniques such as ESR and optical absorption, but their existence can only be inferred from luminescence measurements. It is with such a background that OSL studies of quartz need to be introduced.
OSL Properties of Natural Materials
123
5.1.2. Decay curve shapes obtained under continuous stimulation--CW-OSL 5.1.2.1. Stimulation sources Under steady optical stimulation, the OSL from sedimentary quartz is observed to decrease continuously, though not as a single exponential (Fig. 5.2). This type of OSL has been termed continuous wave-OSL (CW-OSL) (see Chapter 2). The non-exponential decay was first noted by Huntley et al. (1985) in their study using 514.5 nm light from an argon-ion laser with the sample held at room temperature. Blue-pass filters were placed in front of the photomultiplier tube in order to reject the stimulation wavelength; the Coming 7-59 and 5-58 filter combination passed emission that peaked around 410 nm. More recently, stimulation has been performed increasingly with cheaper light sources, such as filtered halogen lamps (BCtter-Jensen and Duller, 1992) and blue light emitting diodes (LEDs) (BCtter-Jensen et al., 1999a,b). The use of stimulation wavelengths from 420 to 550 nm, and at 470 nm, respectively, has resulted in the need for a shorter wavelength detection filter (e.g., 7.5 mm of Hoya U-340, which transmits from 250 to 390 nm, peaking at 340 nm in the near ultra-violet or a 1 mm Corning BG-39 and two 2.5 mm Hoya U-340 filters). Bailey et al. (1997) reported the OSL decay rates to be similar for both an argonion laser and a filtered halogen lamp when similar power is delivered to the sample. Typical power levels at the samples have been reported as --- 50 mW/cm 2 for an argon-ion laser (Huntley et al., 1985; Huntley et al., 1996), ---16mW/cm 2 for the filtered halogen lamp on a Rise reader (BCtter-Jensen, 1997) and --~35 mW/cm 2 for 49 blue LEDs on a Rise reader (BCtter-Jensen et al., 2000). More detailed considerations of the experimental constraints on the measurement of OSL from quartz can be found in Chapter 7. 5.1.2.2. Effect of the llO~ TL trap Analysis of the OSL decay curve from quartz provides information on the optical stability of the various OSL signal components. This information is fundamental to their suitability for dating. Once a signal has been selected, its other properties, such as thermal
Fig. 5.2. Natural OSL decay curves for sedimentary quartz obtained for stimulation with a filtered halogen lamp when the sample is held at elevated temperatures. Prior to stimulation, each aliquot was held for 100 s at that temperature to minimise any thermally stimulated signal (from Murray and Wintle, 1998).
124
Optically Stimulated Luminescence Dosimetry
stability can be investigated. For OSL from quartz it was recognised soon after the early studies that the signal measured at temperatures below 100~ was affected by the presence of the 110~ TL trap. Fig. 5.2 shows OSL decay curves obtained for naturally irradiated (58 Gy) quartz, stimulated at temperatures ranging from 25 to 175~ The curves have been normalised using a 0.1 s OSL measurement at 25~ prior to the measurement at elevated temperature. The non-exponential behaviour is clearly seen in the log-linear plot. McKeever et al. (1997a) obtained a set of decay curves for thermally annealed (600~ for 20 s) and laboratory-irradiated (31 Gy) quartz aliquots that had been pre-heated (125~ for 20 s) before being measured at the given stimulation temperatures (Fig. 5.3). These data are normalised to the initial signal value and plotted on a linear scale. Spooner (1994a) presented his OSL decay curves, measured at temperatures from 20 to 253~ on a linear-log plot (Fig. 5.4). For all three data sets (Figs. 5.2, 5.3 and 5.4), an obvious change in decay rate can be seen between stimulation at 75 and 100~ as would be expected if charge was entering the 110~ trap and was then being optically stimulated from there at a slower rate. Studies of the phototransferred thermoluminescence (PTTL) observed at 110~ at the end of room temperature stimulation confirm such charge trapping (Smith and Rhodes, 1994; Wintle and Murray, 1997). In addition, Bailey (1997) and Wintle and Murray (1997) also reported the optical stimulation of charge from the 110~ TL peak. Bailey (2000a) modelled the effect of charge phototransfer during OSL measurement, with particular regard to methods of equivalent dose (De) determination. He compared simulations at 1.0 I
i
I
I
40 o.8
z
-
0.8
so~
o
200
,oo
-
Temperature (~
I//~ N 0.4
: ooo
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0
5
10
15
20
Time (s)
Fig. 5.3. OSL decay curves for aliquots of annealed (600~ for 20 s) sedimentaryquartz given 31 Gy and a 20 s pre-heat at 125 ~ Aliquots were stimulated using green light from a filtered halogen lamp (---16 mW/cm2) for 20 s whilst held at different temperatures from room temperature to 250 ~ Inset is integrated OSL intensity as a function of illumination temperature (from McKeever et al., 1997a).
125
OSL Properties of Natural Materials
120000 [ 10(i
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80 6(:] 40
8O0O0 A
(a
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20
60000
0
40o00 20000 o 1
10
100
1000
Illumination Time (seconds) Fig. 5.4. Isothermal decay curves obtained using 514.5 nm stimulation (20 mW/cm 2) on samples held at the temperatures shown (~ Data are normalised by 0.4 mJ short exposures at 20 ~ (from Spooner, 1994a).
20 and 160~ for his model, in which parameters were chosen as being similar to those obtained for real samples. The effect of re-trapping was observed for the simulated decay curve at 20~ There was also a small effect ( < 6%) on the De estimate obtained for increasing stimulation time, as the phototransferred component became more important. However, at these stimulation times, the slow component of the OSL signal will have a far greater effect (see Sections 5.1.2.4 and 5.1.11). Alexander et al. (1997) also modelled the phototransfer process and concluded that the data obtained by Wintle and Murray (1997) were consistent with a significantly lower optical stimulation rate from the l l0~ (acceptor) trap compared to that from the deeper OSL (source) traps. 5.1.2.3. Dependence on power
Simple models (Chapter 2) predict a proportional change in the decay rate with stimulation power. Several studies have been carried out to investigate the effect of changing the stimulation power delivered to a quartz sample on the shape of the decay curve. The results are most easily compared when the OSL is plotted as a function of energy delivered to the sample (mJ) and the OSL is given in counts per unit energy (cts/mJ). For example, Duller and B~tter-Jensen (1996) showed that the OSL decay curves
126
Optically Stimulated Luminescence Dosimetry
obtained for a sedimentary quartz using the Ris0 filtered halogen lamp were almost identical over a 100-fold change in illumination intensity (0.11-12 mW/cm2). The decay curves in Fig. 5.3 were obtained using a stimulation power of---16 mW/cm 2, whereas similar data, already shown in Fig. 2.13, were obtained with the power decreased by a factor of 0.0125 to --~ 2 mW/cm 2. For the latter, the signal intensity (as shown by the noise level of the measurements; compare Fig. 2.13 and Fig. 5.3) and the decay rate was much less (much shallower decay curves). An additional feature can be observed when the lower stimulation power is applied at 50 and 100~ a peak in the OSL is seen at around 3 s stimulation time. This is a further evidence of re-trapping in the 110~ TL trap and was predicted from the accompanying computer model (McKeever et al., 1997a). See Chapter 2 for a full discussion of these effects. The effects of changing illumination intensity have also been carried out using short exposures to laser light for both the fast component (Spooner, 1994a) and the slow component observed after the fast and medium components had been removed by light exposure (Bailey, 2000b). For definitions of "fast" and "slow" components, see Section 5.1.2.4. For the fast component, linearity was observed over the power range 0.28238 mW/cm 2 and for the slow component, linearity was observed over the range 0 - 1 2 0 mW/cm 2. In both studies, the linearity of the response was taken as an evidence for electron un-trapping which was the result of a single-photon absorption process. Using blue (470 nm) LEDs, Bulur et al. (2001) measured the OSL decay curves using 11 stimulation powers. They calculated the de-trapping probabilities (bleaching rates) from the initial slopes of the ln(OSL) versus t plots, and Fig. 5.5 shows the linear relationship between the de-trapping probability and the stimulation power. This is fundamental for the linear modulation OSL (LM-OSL) experiments that will be discussed in Section 5.1.3.1. 5.1.2.4.
Three components
From the discussion in Section 5.1.2.2, it can be inferred that in order to observe the behaviour of the deep OSL trap(s), it is necessary to stimulate at temperatures that keep the
Fig. 5.5. De-trappingprobabilitiesplotted as a function of stimulationpowerfor quartz and two otherphosphors with data obtained from CW-OSL measurements (from Bulur et al., 2001a).
OSL Properties of Natural Materials
127
l l0~ TL trap empty. Several temperatures have been used, e.g., 220~ (Smith and Rhodes, 1994), 160~ (Bailey et al., 1997) and 125~ (Wintle and Murray, 1997). Using laser stimulation at 220~ for a naturally irradiated quartz, Smith and Rhodes (1994) identified three exponential components, designated fast, medium and slow, according to their rate of decay under optical excitation. More recently, Bailey et al. (1997) used a filtered halogen lamp and stimulation at 160~ to investigate these three signals. Bailey et al. (1997) carried out an empirical analysis by identifying the slow component, and then subtracting it from the remaining data; this procedure was repeated for the medium component. The results for one sample, stimulated at 160~ using --~ 12 mW/cm 2 of 420-560 nm light from a filtered halogen lamp, is shown in Fig. 5.6a,b. The slow component shows negligible decay over the first 70 s of stimulation (Fig. 5.6a), but can be seen as the sole component from 70 s onwards (inset in Fig. 5.6b). Fig. 5.6b shows that the experimental data are well fitted by the sum of three exponential decays. For the three samples studied, the major contribution to the total area under the decay curve was from the fast component, with the slow component being the next important. For the sample in Fig. 5.6, the fractions are 0.75, 0.09 and 0.16, respectively. However, as seen in Fig. 5.6a, the contribution of the slow component to the initial part of the signal (e.g., in the first 1 s) is < 1%. Also, for the sample shown, the medium component only contributes 3% towards the initial 1 s OSL signal. The fast and medium components are removed by heating to 400~ leaving the slow component, which will be discussed in Section 5.1.10.
5.1.2.5. Effect of stimulation wavelength Ditlefsen and Huntley (1994) measured both the initial slope and the initial intensity as a function of wavelength for a sample of Australian sedimentary quartz. They used a detection filter combination made up of two Coming 7-37, a Coming 7-60 and a Schott BG-39 filters to provide a 30 nm wide transmission band at 360 nm. This enabled them to stimulate using seven laser lines with wavelengths up to 458 nm, whilst observing the major emission at 365 nm. They demonstrated that the initial slope of a decay curve, So, is proportional to the square of the initial OSL intensity (I2) when the intensity is expressed as photon counts per mJ/cm 2 and plotted as a function of the energy received by the sample (in mJ/cm2). They plotted the square root of So versus the initial intensity for six wavelengths, Fig. 5.7. The linearity of response was taken as evidence of the initial OSL signal being derived from a single electron trap. Taking this relationship further, Huntley et al. (1996) showed that the ratio So/Io is related directly to the excitation cross-section. A sensible value of around 10 - 1 7 cm 2 was calculated for the initial OSL, but it was also noted that the decay curve deviated from being a simple exponential as the signal depleted, and other traps with different cross-sections became significant contributors to the OSL signal. In a related experiment, Alexander et al. (1997) measured the initial slope of the l l0~ PTTL decay curve as a function of wavelength. They showed that for green stimulation there was one source trap, but, as the wavelength was reduced, additional traps contributed to the PTTL. In a study of some New Zealand quartz, Duller and BCtter-Jensen (1996) varied the stimulation wavelength (with 25 nm bandwidth) from 425 to 575 nm. Having first demonstrated that it was possible to correct for variations in power by plotting the OSL as a function of illumination energy (rather than time), Duller and BCtter-Jensen (1996) made
128
Optically Stimulated Luminescence Dosimetry
Fig. 5.6. (a) OSL decay curves obtained for naturally irradiated sedimentary quartz held at 160~ during stimulation for 70 s from a filtered halogen lamp. The signal has been described by the sum of three exponential components, termed fast, medium and slow. (b) The observed data are compared with the sum of three exponential components, with the inset showing the decay out to a stimulation time of 870 s (from Bailey et al., 1997).
such plots at seven stimulation wavelengths (Fig. 5.8). It can be seen that more energy is required to reduce the signal as the stimulation wavelength increases. The efficiency of stimulation was monitored by calculating the amount of energy required to reduce the OSL by 50%. When this value is used as a normalisation factor and the resulting curves are plotted as a function of the product of the energy and the efficiency, the form of the OSL
129
OSL Properties of Natural Materials I
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Fig. 5.7. Square root of the initial rate of decrease in luminescence ~/(-S0) plotted against initial luminescence intensity (I0) for stimulation using several different monoenergetic stimulation sources (redrawn from Ditlefsen and Huntley, 1994). 6000 Stimulation w a v e l e n g t h 5000
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130
Optically Stimulated Luminescence Dosimetry
decay curves is identical in the range 425-575 nm. This suggests that OSL studies should be equivalent, whether stimulation is provided by a 514.5 nm laser line or broad-band wavelengths, such as 420-560 nm from the filtered halogen lamp on the Rise reader (BCtter-Jensen and Duller, 1992). 5.1.2.6. Effect of stimulation temperature OSL decay curves obtained using high-stimulation intensities show a strong thermal dependence. Two main effects can be seen in Figs. 5.2 and 5.4. Although, there is a small increase in the initial signal intensity as the temperature is raised from 25 to 50~ the dominant effect is the significant decrease in intensity of the initial signal as the temperature is raised further. Also, when the signals are normalised using the initial signal intensity (obtained at elevated temperatures) as in Fig. 5.3, it can be seen that the decay curves become steeper with increasing temperature. Similar behaviour has been reported by others (e.g., Smith and Rhodes, 1994; Spooner, 1994a). The decrease in intensity of the fast component as the temperature is raised is the result of the reduced efficiency of the luminescence centres due to thermal quenching (see Sections 5.1.6.2 and 5.1.9.1; see also Chapter 2). Smith and Rhodes (1994) reported that both the fast and medium components showed thermal quenching. Bailey (1997) used the 110~ TL peak response to a test dose to measure the sensitivity during measurement of the decay curve and found that no optical de-sensitisation occurred as the slow component was measured, whereas it was observed during the decay of the fast and medium components. This observation led Bailey (2000b) to suggest that the slow component used different recombination centres from those of the other components and the 110~ TL peak. This apparently conflicts with the data of Murray and Wintle (1998), which demonstrated that both the initial signal (0.4 s, 5.2 mJ/cm 2) and that at the end of their 100 s decay curve were affected by thermal quenching, unless the latter was from the medium component. The increase in initial intensity and the initial decay rate with increasing temperature have been explained in terms of thermal assistance (Spooner, 1994a; Murray and Wintle, 1998). This results in an increased rate of trap emptying. The activation energy for thermal assistance has been obtained as a function of wavelength (Spooner, 1994a; Huntley et al., 1996) and will be discussed in Section 5.1.9.2. 5.1.3. Linear modulation OSL--LM-OSL 5.1.3.1. LM-OSL at 160~ with 470 nm stimulation Another way of optically de-trapping electrons is by linearly increasing the stimulation light intensity from zero to a maximum value (Bulur, 1996), instead of stimulating with a constant illumination intensity. The resulting luminescence is termed LM-OSL (see Chapter 2). The LM-OSL from a single trap initially increases and then decays after reaching a maximum. The time at which the OSL reaches the maximum value depends on the ramp rate and the photoionisation cross-section. The latter is dependent on both the wavelength and temperature (see Section 5.1.11). If more than one trap contributes to the CW-OSL decay curve, as found by Bailey et al. (1997), then more than one peak will be seen in the LM-OSL plots. Singarayer and Bailey
OSL Properties of Natural Materials
131
(2002) measured the fast and medium components of the CW-OSL for a sedimentary quartz sample, and converted them to LM-OSL curves using a mathematical transformation (Bulur, 2000). These pseudo-LM-OSL data sets were derived after the grains had been exposed to light of different wavelengths. This enabled Singarayer and Bailey to determine the photoionisation cross-section for each component as a function of wavelength (see Section 5.1.11) Some LM-OSL curves for quartz are shown in Fig. 5.9. Fig. 5.9a shows the LM-OSL measured at 160~ using blue LEDs (470 nm); the power was raised from zero to a maximum of 50 mW/cm 2 in 50 s (Bulur et al., 2001). This sample of quartz had been irradiated and heated many times (in order to stabilise the sensitivity) before being given a 10 Gy dose. A pre-heat for 10 s at 280~ was applied before measurement of the LM-OSL.
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Fig. 5.9. (a) LM-OSL as a function of diode power ramp time on a lin-lin plot. Pre-annealed quartz was given 10 Gy, pre-heated at 280~ for 10 s, and the LM-OSL measured at 160~ as the illumination intensity from blue LEDs (470 nm) was increased from zero to 25 mW/cm e. Three components from computer de-convolution are shown. The inset shows on a log-linear plot the OSL decay under continuous stimulation (from Bulur et al., 2001). (b) LM-OSL as a function of diode power ramp time on a lin-lin plot; illumination intensity from blue LEDs (470 nm) is increased from zero to 25 mW/cm a in 3600 s. Measurements were made immediately after delivery of a 25 Gy dose, with samples held at 25 and 125~ (from Bulur et al., 2000). (c) LM-OSL data from (a) displayed on a log-log plot (from Bulur, pers. comm.). (d) LM-OSL data from (a) displayed on a lin-log plot (from Bulur, pers. comm.). (e) LM-OSL data from (a) displayed on a log-lin plot (from Bulur, pers. comm.). (f) LM-OSL data on log-log plot obtained using a 25 Gy dose, a 10 s pre-heat at 280~ a stimulation temperature of 160~ and a ramp time of 3600 s. The experiment was repeated a further six times (from Bulur et al., 2000).
132
Optically Stimulated Luminescence Dosimetry
Fig. 5.9 (continued)
Three components were determined by curve fitting (Bulur et al., 2001). The inset in Fig. 5.9a shows the OSL decay curve for an equivalent aliquot obtained for full power in CW mode at room temperature. The advantage of using LM-OSL is that it enables separation of signals from traps with different photoionisation cross-sections, i.e., different decay rates under CW stimulation. The ramp rate can be chosen to obtain the best separation of the peaks. Fig. 5.9b shows the LM-OSL measurements made immediately after irradiation (Bulur et al., 2000). The stimulation power was increased from zero to 25 mW/cm 2 over a period of 3600 s. In one case, the sample was held at 25~ and in the other at 125~ In each experiment, there is a sharp peak appearing shortly after the power is switched on. BCtter-Jensen et al. (1999a) deduced that this peak corresponded to the fast component found under constant stimulation (Bailey et al., 1997). The second peak is missing from the sample measured at 125~ Bulur et al. (2000) take this as evidence for it being related to the trap responsible for the 110~ TL peak, as previously suggested by BCtter-Jensen et al. (1999a). Besides linear plots, the data from Fig. 5.9a can be displayed on log-log plots, Fig. 5.9c, as linear-log plots, Fig. 5.9d and as log-linear plots, Fig. 5.9e. In Fig. 5.9a,c, it is just
133
OSL Properties of Natural Materials
possible to see the slow c o m p o n e n t starting to appear at the end of the 50 s stimulation time. However, ramping the OSL over a longer time (3600 s) and using a 25 Gy dose and a stimulation temperature of 160~ Bulur et al. (2000) obtained the data set in Fig. 5.9f. Under these conditions, the m a x i m u m for the slow component was observed. However, it was not totally emptied, as can be seen by the results of repeated ramped stimulation of the same sample. Poolton et al. (2000) presented results for a n u m b e r of annealed quartz samples in their comparison with E P R measurements. For a sample that had been given a 3 kGy g a m m a irradiation after being annealed at 400~ L M - O S L curves obtained using blue LEDs ( 4 7 0 n m ) are shown as a l o g - l o g plot in Fig. 5.10a and a l i n e a r - l o g plot in Fig. 5.10b. These were obtained using a slower ramp rate, going up to 35 m W / c m 2 in 7200 s with stimulation at 160~ For this sample, fast and slow components can be identified as the OSL that is released between 10 and 100 s and between 400 and 7200 s, respectively (Poolton et al., 2000). It is interesting to note that although the intensity of the fast component appears in these plots, the integrated photon count is
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Time (s) Fig. 5.10. (a) LM-OSL as a function of diode power ramp time on a log-log plot. Data taken for a northern European sedimentary quartz that had been held at a temperature of 400~ for 1 h, prior to being given a 3 kGy gamma dose. Stimulation is provided by blue LEDs (470 nm) ramped from zero to 35 mW/cm2 in 7200 s with the sample held at 160~ (from Poolton et al., 2000). (b) LM-OSL data from (a) displayed on a lin-log plot with four components fitted (redrawn from Poolton et al., 2000).
134
Optically Stimulated Luminescence Dosimetry 25000
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OSL Properties of Natural Materials
135
in fact small compared with that of the slow component. This conclusion was also high reached by Bailey et al. (1997) in their study of CW-OSL measurements of quartz. 5.1.3.2. LM-OSL at different temperatures with 526 nm stimulation Kuhns et al. (2000) used green LEDs (526 nm) ramped from 0 to 29 mW/cm 2 in 2000 s to obtain the LM-OSL curves from a variety of quartz samples--from sediments, natural rock crystals and synthetic single crystals. The OSL was recorded through a 1 mm Schott BG-39 and two Hoya U-340 filters. Fig. 5.11a-c shows the results for samples given a 93.6 Gy dose and pre-heated to 162~ at l~ prior to ramped stimulation with the sample held at 127~ Fig. 5.12a-c shows the CW-OSL for identically treated samples obtained using the green LEDs to give 20 mW/cm 2 at the samples, which were also held at 127~ In addition, Fig. 5.11d shows the LM-OSL obtained at room temperature without pre-heating for the rock crystal samples. Considerable differences in both absolute intensity and form can be seen when comparing the behaviour of either LM-OSL or CW-OSL for different types of quartz. Interesting comparisons can be made between the LM-OSL at room temperature (Fig. 5.11d) and that obtained at 127~ (Fig. 5.11b) for the rock crystals. Firstly, a nonzero initial luminescence signal can be seen for the room temperature measurement that was made immediately after irradiation. This shows the presence of phosphorescence. Secondly, three peaks are observed in the LM-OSL curves at room temperature (maxima occur at t - - 1 0 0 , 400 and 1500s), whereas only two peaks (at 40 and 1400 s) are observed at 127~ The middle component seen in Fig. 5.11d corresponds to the charge being optically released from the trap equivalent to the 100~ TL peak, previously reported by BCtter-Jensen et al. (1999a). The fast component appears at different ramp times for the two temperatures, 100 s at RT and 4 0 s at 127~ Kuhns et al. (2000) related this behaviour to the temperature dependence of the photoionisation cross-section, i.e., the thermal assistance of charge release seen for the initial part of the CW-OSL (Spooner, 1994a; Murray and Wintle, 1998). 5.1.3.3. LM-OSL from single grains using 532 nm Bulur et al. (2002) observed the LM-OSL from single grains when stimulated at 160~ using a green (532 nm) diode-pumped solid-state laser, with the power being raised from 0 to 10 mW in 10 s. Three different types of behaviour were recorded for the 81 grains measured. One type contained only the easy-to-bleach component, as identified in the bulk sample measurements; another grain showed only a hard-to-bleach component; while the third type showed both the components.
Fig. 5.11. LM-OSLas a function of diode power ramp time on a lin-lin plot. Quartz was given 93.6 Gy and stimulation was with green LEDs (526 nm) ramped from zero to 20 mW/cm2 in 2000 s. (a)-(c) were measuredat 127~ after pre-heating to 162~ at l~ and (d) was measured at RT with no pre-heat. (a) Sedimentaryquartz, (b) and (d) natural rock crystal quartz, (c) synthetic single crystals (from Kuhns et al., 2000).
Optically Stimulated Luminescence Dosimetry
136
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5.1.4. Pulsed OSL
Optical stimulation can also be provided in discrete pulses. The signal can be observed during the pulse to provide pulsed OSL (POSL) (see Section 2.6.1; McKeever, 2001) This is also known as time resolved luminescence (TRL) (Bailiff, 2000; Chithambo
OSL Properties of Natural Materials
137
and Galloway, 2000a). The pulse width is chosen to be less than the intrinsic lifetime of the luminescence centre. It is also possible to measure a delayed OSL signal (DOSL; see Section 2.6.2). This is also known as optically stimulated afterglow (OSA) (Jaek et al., 1999).
5.1.4.1. Time resolved luminescence Time resolved luminescence (TRL) is observed when the stimulation is pulsed, using a pulsed, tunable laser (Bailiff, 2000) or using pulsed LEDs (Chithambo and Galloway, 2000a-c; Galloway, 2002). Chithambo and Galloway (2000a) designed a control circuit that was capable of generating pulses from 3 to 30 Ixs using 16 LEDs (emitting at 525 nm) to provide a light intensity of 1 mW/cm 2 at the sample. Using this system they obtained the time-resolved spectrum shown in Fig. 5.13a (Chithambo and Galloway, 2000a); this quartz had been annealed (500~ for 2 min), irradiated (150 Gy) and then pre-heated (220~ for 5 min) before being measured at room temperature. This time-resolved spectrum has a dynamic range of 64 txs. The luminescence was observed using a conventional photomultiplier tube (EMI9635QA) with a Schott BG-39 and a Schott UG11 filter combination. Fig. 5.13a shows the data obtained for two runs, one with a sample present and the other without. The first part of the background signal, obtained whilst the diodes are switched on, has a component derived from the scattered light from the diodes. After the diodes are switched off, the background signal is derived from the photomultiplier noise. The OSL is well above these background signals. When the diode is switched on, the luminescence can be seen to increase. After the diodes are switched off, the luminescence (L(t)) decays exponentially. The lifetime, ~', was obtained by fitting a single exponential function. L(t) = L(tl)exp{ - ( t - tl)/'r } where tl is the stimulation pulse width, and t is the time since the start of the stimulation pulse (in Fig. 5.13a this was 11.2 txs). Using this equation, Chithambo and Galloway (2000a) obtained luminescence lifetimes in the range of 3 0 - 4 0 txs. Bailiff (2000) used a laser pulsed at 10 Hz for 5 ns using 470 nm wavelength photons. He used detection filters that passed in the near ultraviolet, with FWHM transmission from 280 to 380nm in front of a fast linear-focussed photomultiplier tube (EMI9831QB). Bailiff (2000) obtained similar lifetimes (33 +_ 0.3 txs) for seven samples of natural quartz extracts from sediments and ceramics. In each case, the decay was fitted by a single exponential (Fig. 5.13b). Similar measurements on synthetic quartz gave a slightly longer lifetime (40 _+ 0.6 txs). However, varying the stimulation wavelength from 450 to 650 nm did not cause any change in the lifetime, but only in the yield. Both Bailiff (2000) and Chithambo and Galloway (2000c) made pulsed measurements whilst holding the quartz samples at a range of temperatures from 20 to 200~ and observed that the lifetime of the time-resolved spectra decreased with increasing stimulation temperature (Fig. 5.14a,b). Bailiff (2000) noted that the integrated intensity decreased concomitantly. On the basis of the comparison of the thermal response for both the radioluminescence (RL) (RL observed at 360 nm) (see Section 5.1.6.3) and the lifetime, Bailiff (2000) concludes that the behaviour is due to the changing competition between radiative and non-radiative transitions within the luminescence centre as the temperature is increased, i.e., thermal quenching (see Section 5.1.9.1). This view is
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supported by Chithambo (2002) and Nanjundaswamy et al. (2002) who conclude that these experiments confirm the relationship of Eq. (2.64), thus supporting the Mott-Seitz mechanism for thermal quenching (see Section 2.4.6). Chithambo and Galloway (2000b) reported small changes in values of ~-(from 29 to 39 Ixs) as a function of cumulative stimulation time for quartz, given a 150 Gy dose after heating to 500~ for 2 min or 220~ for 5 min; however, Galloway (2002) subsequently suggested that this may have been an instrumental artefact. In a further study on the same material, Galloway (2002) studied the effect of high-temperature annealing on the lifetime measured at 20~ For quartz annealed for 7 min, the lifetime was constant (41.5 txs) for temperatures up to 500~ but showed a decrease when the 7 min annealing runs were
OSL Properties of Natural Materials 50
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raised in temperature up to 100~ (Fig. 5.15). The extent of reduction in lifetime could be modified by applying short pre-heats (60 and 300 s) after delivery of the beta dose. The reduction in lifetime was interpreted in terms of a model with two luminescence centres for the OSL process, rather than one, each with a different lifetime (41.5 and 31.5 lXS,if there is no pre-heat prior to measurement). In Galloway' s model, the change in observed lifetime relates to holes being moved from the high-lifetime centre to the lowlifetime centre. This was seen as analogous to the transfer of holes from non-radiative centres to luminescence centres in models of TL (Zimmerman, 1971) and OSL sensitisation with thermal treatment (BCtter-Jensen et al., 1995). The temperature
Optically Stimulated Luminescence Dosimetry
140 42-
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annealing temperature (C) Fig. 5.15. Luminescence lifetime for a 25 Gy beta dose measured as a function of previous annealing temperature (7 min anneals) with no pre-heat after dose ( 9 ) and also for pre-heats at 220~ of 60 s (D) and 300 s (O) after beta dose of 25 Gy (from Galloway, 2002).
dependence of this process was characterised by an energy W = 1.4 + 0.4 eV, taken to be the energy above the valence band of the high-lifetime centre. Galloway (2002) repeated previous measurements (Chithambo and Galloway, 2000c) of the influence of stimulation temperature on lifetime, whilst simultaneously recording the OSL intensity. For un-annealed quartz, the decrease in lifetime and the decrease in OSL intensity occurred with the same activation energy (0.74 _+ 0.06 eV and 0.73 +_ 0.14 eV, respectively) when the data were fitted with an equation of the form used for thermal quenching. These values relate to the behaviour of the highlifetime centre, and an energy of 0.79 _+ 0.15 eV was deduced for the short-lifetime centre. By opening and closing the beta source, it is possible to obtain pulsed RL curves whilst heating the sample (Poolton et al., 2001). Data for such a measurement on quartz are shown in Fig. 7.18a (see Section 7.10), where thermal quenching is clearly seen.
5.1.4.2. Delayed optically stimulated luminescence or optically stimulated afterglow Delayed optically stimulated luminescence (DOSL) was observed for quartz by Jack et al. (1999) who termed it optically stimulated afterglow (OSA). OSA is considered to be due to the recombination of electrons released from shallow traps as the sample is held at room temperature (Jack et al., 1999). These traps were filled by electrons put into the conduction band by the optical stimulation. OSA measurements were made for stimulation wavelengths between 400 and 250 nm (Hiitt et al., 2001) and demonstrated a continuous increase in luminescence signal in response to shorter wavelength stimulation (see Fig. 5.21 of Section 5.1.5.4). For OSA measurements, it is necessary either to store the sample after irradiation--Hiitt et al. (2001) used 3 weeks--or to pre-heat to empty the shallow traps filled by the laboratory irradiation.
OSL Properties of Natural Materials
141
5.1.5. Excitation spectra Several different experimental approaches have been taken to observe the response of the OSL signal to exposure to light of various wavelengths. They can be divided into those in which the OSL is measured using the same stimulation wavelength, and those in which stimulation is made at different wavelengths.
5.1.5.1. Bleaching response spectrum The first spectral information concerning the initial fast component of the OSL came from detailed studies of the bleaching of the OSL signal (Spooner, 1994a). The OSL signal was measured using short exposures to the 514.5 nm argon-ion laser line and observed at room temperature with 2 mm of Schott BG-39 and 5.1 mm of Coming 7-51 filter glass in front of the photomultiplier tube. Bleaching was achieved with narrow-band (10 nm FWHM) interference filters in front of an 800 W halogen lamp. The resulting decay curves for wavelengths from 400 to 900 nm are given in Fig. 5.16. It can be seen that there is negligible ( < 5%) bleaching for wavelengths greater than 690 nm. Thus, no signal would be expected when quartz is stimulated at room temperature using infra-red LEDs (880 nm). Based on these findings, the lack of an infra-red stimulated 100
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Optically Stimulated Luminescence Dosimetry
142
luminescence (IRSL) signal at room temperature is regularly used as a test for purity of quartz extracted from sediments. From Fig. 5.16 it can also be seen that although bleaching can be achieved by exposure to wavelengths around 600 nm, the bleaching is more efficient at shorter wavelengths. The decay in the OSL signal for exposure to 514.6 nm can be seen close to the centre of the data set. Spooner (1994a) reported that this decay was equivalent to the OSL decay curve obtained using the 514.5 nm argon-ion laser line as a stimulation source in dating applications. For wavelengths around 400 nm, the stimulation is 10 times faster than 514.5 nm, with 50% depletion being reached after --- 10 mJ/cm 2 for the former, compared with 200 mJ/cm 2 for the latter. Thus, it can be inferred that it would be advantageous, both for more rapid measurement and for an increased signal to noise ratio, to use shorter stimulation wavelengths. As a result, BCtter-Jensen et al. (1999a) selected blue LEDs with peak emission intensity at 470 nm. The filtered halogen light source developed for quartz stimulation by BCtter-Jensen and Duller (1992) is a broad-band source, with wavelengths from 420 to 550 nm. Matching their measurements of the thermal assistance energy obtained using this source to the wavelength-dependent data of Spooner (1994a), Murray and Wintle (1998) concluded that the effective stimulation wavelength for the filtered lamp was 468 nm. The similarity of the OSL decay curves for quartz using these two light sources was shown by BCtter-Jensen
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OSL Properties of Natural Materials
143
(1997) for powers of 6 and 28 mW/cm 2, respectively. Using Fig. 5.16, the 50% depletion level is reached with 40 mJ/cm 2. More recently, Singarayer and Bailey (2002) measured the decrease in the blue (470 nm) stimulated OSL after room temperature exposure to 430, 470, 500, 525 and 590 nm light from LED sources. Fig. 5.17 shows the exponential nature of the decays for the fast and medium components that had been separated using pseudo-LM-OSL plots (see Section 5.1.3.1). A significant advantage of obtaining the bleaching spectrum for the OSL is that it can be compared directly with the bleaching of the TL peaks in quartz. TL bleaching characteristics had been reported by Spooner et al. (1988). They noted that by the time the 325~ TL peak had been removed, the other TL peaks (at 370 and 480~ had only been depleted by 20%, as shown in Fig. 5.18a, in which the OSL signal reduction has been added (Spooner, 1994a). From these results, Spooner (1994a)concluded that bleaching of the 325~ TL peak and the prompt emission of OSL are both related to single-photonabsorption photo-ionisation direct to the conduction band. Huntley et al. (1996) disagree with the claimed relationship, on the basis of the relative reduction of the two signals caused by a single laser (488 nm) exposure of 80 mJ/cm 2. However, they expected complete removal of all the TL in the vicinity of 325~ Wintle and Murray (1997) observed the decay of a small peak centred at 325~ found on the shoulder of a higher peak that did not bleach (as also seen in the data of Huntley et al. (1996)). Wintle and Murray (1997) observed this signal and the OSL during exposure to blue/green light from a filtered halogen lamp. Both signals decayed proportionally (Fig. 5.18b) and were effectively erased by an exposure of 10 s, corresponding to 130 mJ/cm 2. Thus, the correlation between the two signals is confirmed. 5.1.5.2. Excitation spectra after bleaching by 514 + 25 nm light Duller and BCtter-Jensen (1996) used a scanning monochromator to investigate the OSL as a function of stimulation wavelength from 400 to 560 nm, the range of the instrument (BCtter-Jensen et al., 1994a). The sample of sedimentary quartz had received a dose of 10.8 Gy and a pre-heat (220~ for 5 min)after being heated to 450~ Using a scan speed of 2.5 nm/s, they observed the luminescence using two Hoya 2 mm Hoya U-340 filters. The excitation spectra were obtained after a set of samples had been bleached for various lengths of time by 514 + 25 nm light from the same monochromator (0.4 mW/ cm2). The data thus obtained (Fig. 5.19) were then re-plotted as a set of almost identical bleaching curves. Duller and BCtter-Jensen (1996) concluded that the similarity of the decay curves in the range 450-610 nm and over two orders of magnitude of signal reduction indicated that the same trap (or traps) were being probed by all these wavelengths. 5.1.5.3. Continuous scanning of stimulation wavelengths BCtter-Jensen et al. (1994b) used the same light source and scanning monochromator to obtain continuous OSL output, as the stimulation wavelengths were changed from 400 to 750 nm. The signal was measured using two Hoya U-340 filters. The OSL output from an irradiated (8 Gy) sedimentary quartz that had previously been heated to 850~ is shown in Fig. 2.6 as a function of stimulation energy. BCtter-Jensen et al. (1994b) concluded that the
144
Optically Stimulated Luminescence Dosimetry 108
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OSL Properties of Natural Materials
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Fig. 5.19. Stimulationspectra obtained after exposure to 514 _+ 25 nm light (obtained using a narrow band-pass monochromator in front of a halogen lamp) for periods of time ranging from 0 to 3200 s, as indicated in legend. The spectra were obtained using a resolution of 25 nm and have not been corrected for variations in stimulation power as a function of wavelength (from Duller and BCtter-Jensen, 1996).
deviation from true exponential behaviour for photon energies beyond 2.5 eV may be related to the presence of an absorption band in this region. A similar deviation can be seen in Spooner's data (Fig. 5.18a).
5.1.5.4. Excitation using interference filters and xenon lamp Kuhn et al. (2000) used a set of interference filters, in combination with a heat absorbing and an edge cut filter, to stimulate OSL from quartz. They observed the OSL between 300 and 390 nm, obtained using MUG-2 and Hoya U-340 filters. Data were obtained for sedimentary quartz samples from 13 locations around the world and a typical OSL stimulation spectrum in given in Fig. 5.20. It shows a break in response at ---2.5 eV similar to that reported in the previous section. However, Kuhn et al. (2000) put forward a different model to that of BCtter-Jensen et al. (1994b), with two types of traps providing the source of the electrons. They also recommend that stimulation in this region ( 5 0 0 - 5 2 0 nm) should be avoided. It should be noted that this includes the 514.5 nm line from an argon-ion laser and the wavelengths from the blue plus green ( 4 2 0 - 5 5 0 n m ) light OSL attachment for the Rise TL/OSL system (Markey et al., 1997). Kuhn et al. (2000) suggest that the energy at which this behavioural break occurs may be dose (or dose rate) dependent. If so, this would lead to problems with age determination procedures. Htitt et al. (2001) obtained a stimulation spectrum from 950 to 420 nm for OSL from both natural and synthetic quartz (doped with Cu) using a xenon lamp and a monochromator (Hfitt et al., 1988). They measured the OSL with a filter (UFS) that has
Optically Stimulated Luminescence Dosimetry
146
photon energy in eV 2.8 2.6 2.4
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a transmission window centred on 330 nm, but with its half width from 300 to 380 nm (Fig. 5.21). Using a novel approach, they extended their investigation of the stimulation of electrons from the OSL trap by observing the delayed luminescence (OSA) (see Section 5.1.4.2) occurring at least 0.05 s after optical stimulation is ended.
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OSL Properties of Natural Materials
147
5.1.5.5. Excitation using laser lines from 458 to 645 nm In an early study, Godfrey-Smith et al. (1988) reported OSL measurements of the equivalent dose for quartz made using krypton laser lines from 413 to 799 nm. However, quantification of photon output was only later reported by Ditlefsen and Huntley (1994) who used selected wavelengths from argon-ion, H e - N e , krypton and diode lasers to stimulate OSL from quartz; the OSL was observed with a filter combination (two Corning 7-37, one Coming 7-60 and one 2 mm Schott BG-39) that transmitted at 360 +_ 30 nm. Fig. 5.22 shows the log of the initial intensity (corrected for the different incident energies) as a function of stimulation photon energy. In a later paper, Huntley et al. (1996) show the initial intensity in terms of the number of 360 nm photons observed per incident photon as a function of stimulation energy. The data formed a smooth curve, and indicated a production rate approaching 1 in 10 l~ for the highest stimulation energy (2.71 eV, 458 nm) (see Fig. 5.54 in Section 5.1.11). 5.1.5.6. Stimulation in the infra-red 7 8 0 - 9 2 0 nm As discussed in Section 5.1.5.1, wavelengths greater than 690 nm appear to cause negligible reduction in the 514.5 nm OSL signal stimulated at room temperature (Spooner, 1994a). However, Godfrey-Smith and Cada (1996) reported a stimulation peak at 840 nm for two sedimentary quartz samples included in their study of IR stimulated spectra from feldspars. In contrast, for sedimentary quartz, doped with Cu to give a higher sensitivity, Hiitt et al. (1999) found a monotonic decline in the OSL intensity from 600 to 1000 nm.
1000000
~,:
700 '
600 '
500 '
(nm)
E
100000-
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S
.-.3
E t/) .t.-, t"O
10000-
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1000-
IO0 1.60
c-
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1.80
2.b0
2.~0
2.:~0
2.~0
2.80
Photon energy (eV) Fig. 5.22. Initial OSL intensity plotted as a function of stimulation photon energy on a log-lin plot. Data were obtained using discrete laser line stimulation. The sample was naturally irradiated Australian sedimentary quartz. The spectrum is corrected by plotting the OSL as counts per mJ/cm 2 (redrawn from Ditlefsen and Huntley, 1994).
148
Optically Stimulated Luminescence Dosimetry
1000000
a
489.9 A 6.8 nm
9 514.6 & 7.2 nm 9 581.1 & 8.3 nm
0 639.2 9 695.3 [] 750.1 9 861.1
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9
9.6 nm 12.4 nm 13.7 nrn 12.5 nm
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Fig. 5.23. The thermal dependence, over temperaturesranging from 100 to 500 K, of the OSL stimulatedby a variety of narrow wavebands obtained using a filtered light source. Background count rates were subtracted and the data were corrected for depletion by the cumulative short exposures used to obtain each data set (redrawn from Spooner, 1994a).
Spooner (1994a) showed that a weak OSL signal could be stimulated by wavelengths greater than 700 nm (Fig. 5.23). Seven narrow wavebands (10 nm FWHM) in the range from 480 to 860 nm were used to stimulate an Australian sedimentary quartz at temperatures from - 170 to 200~ OSL, stimulated by the 860 nm light, can be seen when stimulation is carried out at temperatures above 50~ As shown in Fig. 5.23, the IR stimulated signal obtained using the same detection filters is about four orders of magnitude less than for stimulation at 515 nm. It must be noted that this factor is specific to the measurement system, since the data were not normalised for incident beam energy. Use of a 10 times more powerful stimulation source, e.g., a 1 W solid-state infra-red laser diode giving about 400 mW/cm 2 of 830 nm IR at the sample, has been found to remove the fast component without affecting the medium component when the exposure is performed at 160~ (Singarayer and Bailey, 2002). Bailey (1998) reported that IR stimulation at 875 nm only gave rise to IRSL when stimulation was carried out at temperatures greater than 200~ When this was observed, the OSL signal (stimulated with blue/green light) decayed at the same rate following IR exposure. This suggests that both the IRSL and the fast component of the OSL are derived from the same trap. As will be shown in Section 5.1.9.2 (Fig. 5.49), the thermal activation energy determined for the IRSL output fits the thermal activation energy versus wavelength curve determined for the visible wavelengths (Spooner, 1994a). Thus, it must be concluded that pure quartz has an IRSL signal and that merely determining the presence or absence of IRSL is not an adequate test for feldspar contamination. A more appropriate test is whether the OSL signal can be reduced by IR exposure (Duller, pers. comm.).
149
OSL Properties of Natural Materials 5.1.6. Emission spectra
Spectral characteristics of luminescence emitted from quartz are of vital importance for the correct choice of optical filters to optimise detection of weak signals, as are often found in retrospective dosimetry. Additionally, in the case of OSL, it is important to minimise wavelengths from the stimulation source. In TL, it is the blackbody radiation that needs to be rejected, as temperatures are raised. Table 7.1 (in Chapter 7) and Table B 1 in the appendix of Aitken (1998) provide useful summaries of the most commonly used band-pass filters for OSL measurements. The latter gives characteristics such as peak wavelength and F W H M for particular filter thicknesses. For other studies, e.g., of kinetic behaviour, a narrower bandwidth can be selected using a long-pass filter together with an interference filter. Some combinations are discussed below. 5.1.6.1. OSL emission spectra Using a spectrometer based on a micro-channel plate detector, OSL emission spectra of quartz from Australian dune sands have been obtained using excitation at 647 nm from a krypton laser (Huntley et al., 1991). Fig. 5.24 shows the single emission band that is centred on 365 nm when the measurements were made at room temperature, confirming earlier studies using colour glass filters (Huntley et al., 1989). The results in Fig. 5.24 have provided the base for optical filter selection in routine measurements, e.g., Hoya U-340.
200
i
i
I
i
1
|
1
!
!
i
i
i
i
!
1
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i
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i
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SESA Quartz 647 nm stimulation
"
150
i
.
Photon counts 100
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0 | 300
A
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O ka
350
"
9 ~ -l~-Wl..,,,,F~
!
~
~ ~-!
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400 450 500 Wavelength (nm)
w ~ |
i
550
Fig. 5.24. Emissionspectra of several samples of sedimentary quartz from south-east South Australia aged from 0 to 800 ka, obtained for stimulation using the 647 nm line from a krypton laser (from Huntley et al., 1991).
150
Optically Stimulated Luminescence Dosimetry
No experiments have been performed at elevated temperatures to confirm or deny the claim by Franklin et al. (1995) that the luminescence centres used are the same as those used by the lower TL peaks, as discussed in Section 5.1.6.2. 5.1.6.2. TL emission spectra There have been many studies of TL spectra, on quartz crystals (McKeever, 1984), quartz of different geological origins (Rink et al., 1993) and quartz extracted from sediments (Scholefield et al., 1994; Scholefield and Prescott, 1999). An extensive review of emission spectra has been provided by Krbetschek et al. (1997). Fig. 5.25 shows the main TL emission bands as a function of temperature in the range 50-400~ The colour definitions used in the following sections (violet, blue, orange) are those from the German Industry Norm (DIN 5031) as reproduced by Aitken (1998). It should be noted that spectra for natural samples are obtained using sub-samples made up of a large number of grains. Several studies suggest that individual grains are dominated by either blue or "red" emission (e.g., Hashimoto et al., 1986; Huntley et al., 1988a,b). As the result of a recent study involving thermal annealing of natural quartz, Hashimoto et al. (1996) have suggested that the blue luminescence may originate from grains of a-quartz, whereas the red emission may relate to grains that have been heated beyond the phase transition at 870~ Thus, it is not clear to what extent any spectra will have been influenced by mixing of such grains as would result from sediment-forming processes. 5.1.6.2.1. 3 6 0 - 4 2 0 n m (near UV to violet). Emission in the near UV to violet (360-420 nm) is characteristic of the low-temperature TL peaks (100-210~ that are present subsequent to laboratory irradiation. In Fig. 5.25, an apparent shift in peak emission to lower energies with increasing temperatures is seen. The UV-violet data in Fig. 5.25 are based on spectral studies made by Franklin et al. (1995) on sedimentary quartz. This sample gave a strong emission at 470 nm for the higher temperature TL peak. In Fig. 5.26, four TL peaks (at 100, 180, 220 and 305~ are marked by crosses. Franklin et al. (1995) concluded that they form a family of traps that use the same recombination
Fig. 5.25. Main TL emission bands of quartz from 50 to 400~ obtained from a literature survey (from Krbetschek et al., 1997).
OSL Properties of Natural Materials
151
Wovetength ( n m )
500 400 ~---~)/~~~
400
300
,
....
_
.-- 300
"~ 200
1(30
X
9 2.5
1
!
3.0 3.5 P h o t o n e n e r g y {eV)
(b)
1
4.0
Fig. 5.26. Contour map of three-dimensional TL spectrum for unbleached natural quartz from Australia that has been given an additional 60 Gy dose. The crosses mark approximate peak locations, and the OSL emission at 22~ from Fig. 5.24 (from Franklin et al., 1995).
centre and suggested that the emission peak shifts to lower energies as the temperature is increased. The 365 nm OSL emission at 22~ (Huntley et al., 1991) is also shown in Fig. 5.26. The near UV to violet luminescence has a particular feature that can be seen during TL and OSL measurements made from just above room temperature to 300~ There is a decrease in luminescence efficiency with increasing temperature due to the increased probability of non-radiative transitions. This is known as thermal quenching (Chapter 2, and Chen and McKeever, 1997). It was first observed in luminescence studies on quartz by Wintle (1975) and more recently has been documented for the three peaks at 180, 220 and 305~ by Franklin et al. (1995) and for both the 325~ TL peak and OSL by Spooner (1994a). Spooner' s data are shown in Fig. 5.27a. The TL data were obtained by varying the heating rate from 10~ to as low as 0.002~ For each TL data point, the 325~ signal was obtained by the subtraction of glow curves from identical aliquots bleached with light from which the ultraviolet and blue components had been removed. The resulting data are shown in Fig. 5.27b. From Fig. 5.27a it can be seen that thermal quenching affects the luminescence output from about 30 to 250~ This figure demonstrates the equivalence of the luminescence from the "325~ '' TL peak and the integrated fast component of the OSL. Nanjundaswamy et al. (2002) investigated thermal quenching of the UV emission (340 + 30 nm) related to the 110~ TL peak. Heating rates from 0.01 to 9~ were used and the TL peak areas were plotted as a function of peak temperature. The resulting graph agreed with the predicted thermal quenching behaviour, based on previously determined physical constants (McKeever et al., 1997a). It is interesting to note that Nanjundaswamy et al. (2002) corrected their data for the shift in emission wavelength with temperature,
152
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Optically Stimulated Luminescence Dosimetry
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OSL Properties of Natural Materials
153
suggested by Franklin et al. (1995). Further information on thermal quenching is found in Sections 5.1.6.3 and 5.1.9.1. 4 2 0 - 4 9 0 nm (blue). Scholefield et al. (1994) used a modified T w y m a n - G r e e n interferometer to separate the emissions from the 325 and 375~ TL peaks (Fig. 5.28) by making two measurements, and looking at the difference in the output. Fig. 5.28a shows the spectrum for a natural sedimentary quartz sample that has been given a small dose to reveal the 110~ TL peak. The high-temperature TL, from 300 to 400~ is dominated by an emission peak at around 480 nm: however, a broader emission is also present at the lower end of that temperature range. This is removed by bleaching with a light source that has been filtered to give wavelengths greater than 475 nm. Photons of these wavelengths have been shown to have little effect on the TL peak at 335~ (nominally the 375~ peak) (see Fig. 5.18a). Fig. 5.28b shows the spectrum of the TL remaining after this light exposure. The emission peak just below 500 nm is clearly seen. The difference between the two spectral runs is shown in Fig. 5.28c. A broad peak centred at 430 nm was obtained for the TL that peaked at 305~ (nominally the rapidly bleaching peak at 325~ (Franklin, 1998)). A blue emission band is commonly reported for the '375~ ' TL peak, i.e., the slowly bleaching peak (Hornyak et al., 1992; Franklin, 1997) (Fig. 5.25). This peak can be seen in Fig. 5.27b as the component that is left after the optical bleach. From their study of several sedimentary quartz samples, Scholefield et al. (1994) found the emission to be at 482 ___ 4 nm. For their study of the blue TL of quartz, Spooner and Questiaux (2000) chose 3 m m Schott GG-455 and an interference filter (500A80) as the preferred detection filter combination. In a study of quartz from volcanic rocks, Woda et al. (2002) showed the TL to be dominated by the 375~ TL peak. This TL peak was dominated by emission at 470 nm, and when measured using a 462 nm interference filter, the TL signal continued to grow with dose beyond 1 kGy. From Fig. 5.24 it appears as though there is very little OSL emission in the 4 2 0 - 4 9 0 nm wavelength region. Nanjundaswamy et al. (2002) repeated their study of the 110~ TL peak using a blue filter (450 ___ 40 nm). They reported thermal quenching, but with an activation energy of 0.9 eV. This was found in a different temperature region to the thermal quenching for 470 nm observed in RL and TL above 270~ for the latter an even higher thermal activation energy was reported (see Section 5.1.6.3). 5.1.6.2.2.
5 9 0 - 6 5 0 nm (orange-red). Emission at these wavelengths has been reported from sedimentary quartz (Fig. 5.25) (Scholefield et al., 1994; Prescott et al., 1995;
5.1.6.2.3.
Fig. 5.27. (a) The light sum of the isothermal OSL decay curves and the light sums of the separated 325~ TL glow peaks. The detection filters were 2 mm BG-39 and 5.1 mm Coming 7-51. All data are normalised using the light sum of the OSL decay curve measured at 20~ (293 K) (from Spooner, 1994a). (b) Glow curves of both natural and bleached quartz measured at heating rates of 10, 0.2 and 0.0 l~ The detection filters were 2 mm BG-39 and 5.1 mm Coming 7-51. Note that the natural peak shifts from about 350~ for 10~ to about 250~ for 0.0 l~ Also shown are the 325~ TL peaks obtained by subtracting the two experimentally determined curves. For the fastest heating rate, the 325~ TL peak is relatively small but it becomes dominant at 0.01~ (from Spooner, 1994a).
154
Optically Stimulated Luminescence Dosimetry W a v e l e n g t h (nm) 500
400
300 . . . . . . . .
400
(a)
300
200
~ o o [..,
100
2.5
3.0
3.5
4.0
JO 5.0
4.5
P h o t o n e n e r g y (eV) W a v e l e n g t h (nm) 500
400
300
(b)
400
3O0 r~
200 o [..,
100
2.5
3.0
3.5
4.0
0 ;.0
4.5
Photon e n e r g y (eV) W a v e l e n g t h (nm) 500
400
300 .......
400
(c) 300 o 200 0
o
100
2.5
3.0
3.5
4.0
Photon e n e r g y (eV)
4.5
5.0
OSL Properties of Natural Materials
155
Singhvi and Krbetschek, 1996; Scholefield and Prescott, 1999). However, it has also been observed for quartz of volcanic origin (e.g., Hashimoto et al., 1987; Rink et al., 1993). In an extensive study of Australian dune sands, Scholefield and Prescott (1999) reported that most TL peaks, apart from the l l0~ peak, had both blue (475 nm) and red (650 nm) emissions. Based on this study, Spooner and Questiaux (2000) chose a 3 mm Schott OG-590 together with an interference filter (600A40 nm) as the filter combination for measurement of the red TL signal in quartz. Franklin et al. (2000) used a similar set of filters in their comparison of the kinetic behaviour of red and blue TL signals. Using the same filter system, Spooner and Franklin (2002) measured activation energies for thermal quenching of 0.14 + 0.02 and 0.21 ___ 0.03 eV for the rapidly and slowly bleaching peaks. It is not yet known whether there is any OSL emission in this wavelength region. 5.1.6.3. R a d i o l u m i n e s c e n c e
Under constant irradiation, electrons are maintained in the conduction band and can recombine with recombination centres, with the latter being continually replenished by holes from the valence band. The resulting luminescence is known as radioluminescence (RL). Using a beta source to provide continuous ionising radiation, Wintle (1975) measured the RL emitted from quartz from room temperature to 400~ Using a filter centred on 465 nm, the RL was found to decrease steeply between 100 and 250~ The RL was the same whether the measurements were made as the temperature was increased or decreased, as expected for a signal dominated by RL. Fig. 5.29a shows the RL signal obtained as a function of temperature, and re-plotted with the high-temperature steady component subtracted. Other studies indicated that the stable component was the result of emission from another luminescence centre (Wintle, 1975). The thermally decreasing signal was re-plotted in Fig. 5.29b and fitted to an equation of the form r/(T) = (1 + K exp-W/~T) -1 The energy for the thermal quenching process (W) was determined as 0.64 eV and the factor K as 2.8 x 107. More recently, Schilles et al. (2001) reported both TL and RL measurements made using a spectrometer (Rieser et al., 1999). For the emission at 360 nm, the energy of quenching was found to be 0.65 eV. The blue emission, monitored at 470 nm, also showed quenching but with a higher associated energy, W -- 1.32 eV and K = 1.28 • 1012. Loss of luminescence efficiency for this signal was not observed until a temperature of 260~ was reached. Negligible thermal quenching was detected for the red emission (630 nm) up to 325~ (the limit of the experimental measurements). Woda et al. (2002) also reported the RL spectra for quartz from Quaternary volcanic rocks. They found two emission peaks,
Fig. 5.28. (a) Contourmap of three-dimensional TL spectrum of unbleached natural quartz from Australia, preheated at 240~ for 1 min and then given 1 Gy to activate the 110~ peak. Heating rate 5~ (b) TL spectrumof similar aliquot but bleached in yellow light for 30 min before the pre-heat. (c) Difference spectrum formed by subtracting the data of (b) from (a) (from Scholefield et al., 1994).
Optically Stimulated Luminescence Dosimetry
156
(a)
m
o~ 17.
.=_ 3 ..J fir.
B
I
I00
#*/" ~
1.00,8-
0.6-
I
I
400
3O0
200 T/"C
.
u
u
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(b)
0.40.3
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0.080.06
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2.'0
2'.2
2'.4
2.'6 2'.8 1/Txl03(K -1)
3'.0
3'.2
OSL Properties of Natural Materials
lb'/
at 360 and 490 nm. The growth of the blue (490 nm) RL emission as a function of dose up to 5 kGy mirrored the growth of the ESR signal from the [A104] centre, indicating that the latter is the recombination site for the blue emission.
5.1.7. Dose dependence The dose dependence of a particular luminescence signal is the fundamental basis of its use for either dating or retrospective dosimetry. Within the dose region of interest, the response should be, at the very least, reproducible as a function of dose, and preferably linear. Application to dating and dosimetry may stretch the limits at the high and low ends of the dose curve, respectively. The OSL signal is interpreted as being derived from one, or more, electron traps. It is thus the build up of the trapped electrons in proportion to radiation exposure that permits palaeodosimetry. It is assumed that the effective dose received by the grains, from whatever radiation field they may have been subjected to (e.g., alpha, beta or gamma), can be determined by comparison of the natural luminescence signal with that resulting from exposure to a laboratory source. An accurate and precise determination of this effective dose, the equivalent dose De, is the desired outcome of such measurements. As summarised in a recent paper by Murray and Wintle (2000), an accurate estimate of the equivalent dose is possible only if: (i) any competition for charge during trap filling is in the same proportion during laboratory irradiation as during natural irradiation, (ii) the OSL sensitivity (luminescence per unit trapped charge) is the same during measurement of both the natural and test-dose-induced signals, and (iii) the traps are stable over the relevant archaeological or geological time periods (in dating applications). These factors are taken as true in the following discussion, and will be discussed separately in later sections.
5.1.7.1. Fast component 5.1.7.1.1. Multiple aliquot data. The OSL signal that has received the greatest attention has been the fast component, i.e., one which is measured immediately after the stimulating light has been applied. The dose-dependent response has been plotted using both multiplealiquot additive-dose methods and single-aliquot regenerative-dose. Huntley et al. (1996) presented a plot of the initial OSL intensity as a function of added dose for sand from a dune in South Australia. The data, shown in Fig. 5.30a, could be well fitted by a saturating exponential function I = I0(1 - e x p ( - D / D o ) ) where I is the OSL intensity resulting from a dose D, and Io is the infinite value of OSL intensity reached at saturation. Do is the characteristic dose at which the OSL signal I = I0(1 - e-l).
Fig. 5.29. Radioluminescence observed at 465 nm when a heated archaeological quartz was continually irradiated with a beta source whilst being heated from room temperature to 475~ (a) Raw data and that after subtraction of the steady RL emission observed above 300~ (b) lnr/versus T -1 (redrawnfrom Wintle, 1975).
Optically Stimulated Luminescence Dosimetry
158
40000
(a) I
9
t
|
II
I:: o o
J 09 20000 O
/DeI = 61 + 5 G,y 0
200
i
!
400
600
800
Added dose, Gy 400 (b)
300 "7 g~
= O
200
.J
o~ O
100
A
0
20
40
60
a
80
100
Dose (Gy) Fig. 5.30. (a) Growth curve of initial OSL obtained using 514.5 nm laser stimulation for quartz from a 122 ka dune in Australia. Data were obtained using multiple aliquots and additive doses (redrawn from Huntley et al., 1996). (b) Growth curve of integrated OSL obtained using filtered halogen lamp stimulation for quartz from a 9 ka sandy sediment in Australia. Data were obtained using multiple aliquots and additive doses (from Roberts et al., 1998).
For the sample shown in Fig. 5.30a, the equivalent dose, De, is obtained by extrapolation of the OSL data to the light intensity (near zero) obtained from a wellbleached and pre-heated sub-sample. Combining D e = 61 + 5 Gy and the dose rate of 0.58 +- 0.02 Gy/ka, an age of 105 _ 10 ka was obtained. The natural OSL signal is about 50% of the saturation value. Hence, this approach would be inappropriate for older samples, and thus the additive dose technique using the initial OSL signal is limited to samples <--~ 100 ka. In addition, this dose rate, as measured for the dune sand, is low compared with those measured for other types of deposits. Hence, a much smaller maximum age limit may be expected when different types of samples are examined. The limitation is related to the value of Do, in this case close to 90 Gy.
OSL Properties of Natural Materials
159
Another additive dose growth curve obtained using small aliquots of a y o u n g e r (--~ 10 ka) Australian sand is shown in Fig. 5.30b. In this case, the O S L signal was the integrated O S L signal from 250 s of stimulation from a 4 2 0 - 5 5 0 n m filtered h a l o g e n lamp with the samples kept at r o o m temperature and detection being through two 3 m m H o y a U-340 filters (Roberts et al., 1998). D e is 6.0 • 0.7 G y and the curvature can be seen throughout the growth curve obtained for doses up to 96 Gy.
5.1.7.1.2. Single aliquot data. The advent of the single aliquot regenerative (SAR) dose procedure (see Chapter 6) for D e determination (Murray and Wintle, 2000) has led to more growth curves appearing in the literature. The SAR protocol (see Section 6.5.4.5) permits the generation of a growth curve by making repeated m e a s u r e m e n t s after the signal has been erased in the course of the previous measurement. After each O S L m e a s u r e m e n t , the O S L response to a test dose is measured, and this is used to correct for any sensitivity changes that occur during repeated cycles of irradiation, pre-heating and light exposure. Fig. 5.31 a shows the O S L m e a s u r e m e n t s as a function of dose and the O S L response to the test dose that is related to each of the previous measurements. Fig. 5.3 l b shows the ratio of
sedimentary: _e- - e - e 3x105- 98251~_e~ -e--- e~ ~m~i 014~"
2x105-
i..I....~Im"
/i
105i]i 0~0 10
lr jl..
Natu-r~l-
(a) I
5~)0
1000
(b) 6
_
I
2 -~-0-~r-
0
I--
100 t
200 J
Natural I
500 1000 Regenerative dose, Gy
Fig. 5.31. (a) Initial OSL measurements plotted as a function of regenerative dose for a water-lain sedimentary quartz. The squares are the OSL signals from those doses, and the circles are the OSL signals in response to a test dose of 11 Gy given after the main OSL measurement. A 10 s pre-heat at 260~ was applied after each main dose (from 20 to 1000 Gy) and a cut heat to 160~ was applied after each test dose. The sensitivity data were multiplied by 25 for clarity. A, the response for a step for which no dose was given and C], the values for the lowest dose (20 Gy) that was repeated twice, in the middle and at the end of the sequence (from Murray and Wintle, 2000). (b) The ratio of the main OSL signal to the corresponding test dose response is plotted as a function of regenerative dose, i.e., the ratio of the two curves from (a). A the ratio when no dose was given, and the superimposed ~ are those of the repeated 20 Gy doses (from Murray and Wintle, 2000).
160
Optically Stimulated Luminescence Dosimetry
the two data sets in Fig. 5.31 a, i.e., the sensitivity-corrected growth curve. It grows much less steeply than the uncorrected data set (filled squares in Fig. 5.31a) for doses above 200Gy. The best fit for the corrected data is an equation of the form Y-A(1 - e x p ( - B / X ) ) + CX, i.e., a combination of a saturating exponential and a linear response. If such a linear component can be found, and proved reliable, then dating could be possible for doses above 100 Gy. However, it should be noted that the reduced slope would decrease the precision attainable, and would be prone to inaccuracy if there were any systematic errors in the correction procedure, as discussed by Murray and Wintle (2000). Similar growth curves extending to large doses were obtained for sedimentary quartz from East Greenland (Hansen et al., 1999). 5.1.7.1.3. Single grain data. It is also possible to apply the SAR protocol to single grains of quartz (Duller and Murray, 2000). Fig. 5.32a shows the OSL from a single grain of quartz extracted from dune sand from Tasmania. The signal is obtained using a 10 mW Nd:YVO4 diode pumped laser emitting at 532 nm. The beam is focussed down to a spot about 50 l~m in diameter and results in an optical power density of---50 W/cm 2 at the sample. This is about 1000 times higher than the illumination intensity from the commonly used blue LEDs, and results in the very rapid decay of the OSL. For this grain, the sensitivity-corrected growth curve is shown in the inset to Fig. 5.32a. An exponential fit has been applied to the regenerative dose data set. It suggests that this grain would not be useful for dating if the sample had received a natural dose above 60 Gy. By making large numbers (972) of measurements on single grains from this sample, Duller et al. (2000) showed that about 50% of the grains were bright enough to be measured. They found that it was possible to fit the growth curves of the bright grains with a single saturating exponential function, as shown in the inset. However, the constant Do in the equation I -- I0(1 - exp(-D/Do)) was not the same for each grain. In a similar study of grains from a periglacial depositional environment in Denmark, Duller et al. (2000) reported even greater variability in form and an even smaller proportion of bright grains. For the examples given in Fig. 5.32b, the combination of an exponential and a linear function were required. For the 37 grains with measurable signals, the values of Do ranged from 40 to 600 Gy. Selection of grains with high Do may allow dating of sediments older than 100 ka, provided they are representative of the sediment unit. The selection of such "supergrains" has been investigated by Yoshida et al. (2000).
5.1.7.2. Low doses For heated materials, such as pottery or brick used in retrospective dosimetry, the lowdose regions of the growth curve are of particular interest. Banerjee (2001) constructed a growth curve in 0.5 Gy steps up to 29 Gy using the SAR protocol (see Section 6.5.4.5), and extended it up to 56 Gy using 2.4 Gy steps. A test dose of 24 mGy was used to monitor the OSL sensitivity. The uncorrected OSL data (Fig. 5.33a) show a supra-linear response in the dose range 0 - 5 Gy (see inset in Fig. 5.33a). However, on applying the OSL response to the test dose as a sensitivity correction, the supra-linear behaviour disappears (Fig. 5.33a). The shape of the uncorrected growth curve can be explained by the dependence of the OSL sensitivity on either regeneration cycle or prior dose. To obtain the data in Fig. 5.33a, Banerjee (2001) employed a total of 71 regeneration cycles. The sample was a 30-year-old brick, i.e., all the traps had been emptied by the initial firing. During the experiment, the light
161
OSL Properties of Natural Materials
4000 - (a) ._3
"~ 30000 0
O
gl 2000-
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0 v0 d
0
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Laboratory
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I
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0 Z
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o
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Dose (Gy) Fig. 5.32. (a) OSL decay curve for single grains of dune sand with a dose of 26 Gy obtained by stimulating with solid-state laser diode at 532 nm and with 50 W / c m 2 delivered to the grain. The inset shows a sensitivitycorrected growth curve obtained using SAR protocol and fitted with a single exponential equation (from Duller and Murray, 2000). (b) Sensitivity-corrected growth curves for two grains from a periglacial sediment obtained using the same procedure. Note curve fitting is with combined exponential and linear functions and response shows growth, even at 1000 Gy (from Duller et al., 2000).
exposures during each of the 140 O S L measurements (including the test dose response) wo u ld have left an increasing trapped charge population in optically insensitive traps. Fig. 5.33b shows the O S L test dose response that was measured as a function of both cycle and previous irradiation history. Comparing these data with those of S ton eh am and Stokes (1991), w h o used a single cycle but a wide range of regeneration doses ( 0 - 1 4 4 Gy), Banerjee (2001) concluded that his data for the brick were best explained in terms of a d e p e n d e n c e on the total dose received prior to the measurement.
Optically StimulatedLuminescenceDosimetry
162 6x106
r
(a)
1000
9 Uncorrected o Corrected
000
4xl 06
._1
o o J co 2x10 6 0
0
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Or) - 500 O
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60
Regeneration cycle number 20
40
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prior regen close vs Test close OSL regen cycle vs Test close OSk !
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Previous regeneration dose (Gy)
Fig. 5.33. (a) OSL growth curves for heated quartz showing supra-linearity of uncorrected OSL ( 9 ) and no supra-linearity for sensitivity corrected data (O). The inset shows an expansion of the uncorrected data for the regeneration dose range 0-10 Gy. (b) OSL test dose responses as a function of regeneration cycle ( 9 ) and prior regeneration dose (O). Data for (b) were used in obtaining the data for (a) (from Banerjee, 2001).
5.1.8. Effects of previous thermal treatment
5.1.8.1. High temperature annealing--above 500~ 5.1.8.1.1. Comparison of LM-OSL, TL, RL and EPR. Heating a - q u a r t z b e y o n d the first and s e c o n d phase transitions, at 575 and 870~ respectively, has a significant effect on the l u m i n e s c e n c e sensitivity, as first shown in colour p h o t o g r a p h s by H a s h i m o t o et al. (1996). F o r several samples of natural quartz, BCtter-Jensen et al. (1995) reported that large O S L sensitivity increases occurred w h e n the grains were p r e v i o u s l y heated to t e m p e r a t u r e s b e t w e e n 575 and 870~ M o r e recent studies by P o o l t o n et al. (2000) and Schilles et al.
163
OSL Properties of Natural Materials
(2001) have attempted to determine the origins of this sensitivity increase by the use of TL, RL, LM-OSL and EPR measurements, in addition to CW-OSL measurements. For both TL and RL, only the UV emission (360 nm) is enhanced by heating to 700~ for 1 h (Fig. 5.34), whereas the red (630 nm) emission is enhanced by heating to 1060~ i.e., above the second phase transition. Schilles et al. (2001) used a range of pre-heat temperatures up to the second phase transition (870~ and observed the magnitude of the LM-OSL and 225~ TL signals, measured in response to a test dose and using a UV detection system. Using LM-OSL enabled them to compare both the fast and slow components of the OSL signal with the TL. Fig. 5.35a shows that both OSL signals are linearly related to the TL signal, with both signals showing changes of over two orders of magnitude. However, the RL at 20~ also measured in the UV, showed a different relationship to the 225~ TL signal. This difference is important since RL relates only to the recombination part of luminescence production, since the electrons are derived from the conduction band. When TL or OSL are produced, the electrons are derived from traps. Hence, the data of Fig. 5.35a imply that the larger enhancement seen for TL (and also OSL) is related to a change in the number of traps. Schilles et al. (2001) also observed the changes in the EPR signals of the identical quartz sub-samples. They used high-frequency (93 GHz) EPR measurements at 15 K so that they could work with 2 mg samples. The EPR signal, derived from [TiO4/Li+] ~ and [TiO4/H+] ~ electron trapping centres, also increased with annealing temperature (Fig. 5.35b); the order of magnitude increase was similar to that for the increased TL and OSL sensitivities when compared to the RL (Fig. 5.35a). The [TiO4/Li+] ~ and [TiO4/H+] ~
(a)
~
9
+::i 400
(b)
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Wavelength (nm)
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Wavelength (nm)
+ooI
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~.--~ 800
Wavelength (nm)
Fig. 5.34. Comparisonof TL emission spectra (shown as temperature versus wavelength) and RL spectra at 21~ for samples given a 3 kGy dose. (a) Unheated sedimentarysample, (b) Sampleheated to 700~ (thus having passed throughthe alpha-beta phase transition) and (c) sampleheated to 1060~ thus having passed throughthe second phase transition (from Schilles et al., 2001).
Optically Stimulated Luminescence Dosimetry
164
(a)' "' . . . . . . . . . . . . . . . . .
10000
~>~ 1000
v
.~
0
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10
60
~.
40
v
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v
o
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1
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ul 200 ~ ~ 0
200
400
600
800
1000
1200
Anneal Temperature (*C) Fig. 5.35. (a) OSL ( 9 slow component and o , fast component) and RL plotted as a function of the 225~ TL, all emissions in the UV. (b) EPR measurements of E / centre ( 9 and RL response in the UV ( 9 ) given as a function of annealing temperature, with the phase transition temperatures marked (from Schilles et al., 2001).
centres were also found to be optically bleachable when exposed to visible light. Schilles et al. (2001) tentatively conclude that these centres are involved in the luminescence processes. The other EPR signal monitored by Schilles et al. (2001) was from the [A104] ~ recombination centre. Interestingly, the signal was not found to be changed by any thermal treatment up to 1100~ Thus, the change in RL (and the concomitant change in TL and OSL), must relate to changes in the concentrations of competing recombination centres. Such a centre might be the oxygen vacancy E I centre studied by Poolton et al. (2000). These defects are removed as the annealing temperature increases from 500 to 870~
165
OSL Properties of Natural Materials
(Fig. 5.35b). From their spectral work, Schilles et al. (2001)conclude that although the E I centre acts as a recombination centre, it is one which must be either non-radiative or emit outside the spectral range of their spectrometer ( 2 8 0 - 9 0 0 nm).
5.1.8.1.2. CW-OSL growth curves after annealing. Chen et al. (2001) investigated how the sensitivities of the OSL and the l l0~ TL signals changed as a result of annealing for 10 min at temperatures between 160 and 1000~ In their experiments, the same aliquot was used repeatedly; the annealing temperature was increased after the sensitivity measurements had been made. The data obtained for the natural quartz sample showed increase in sensitivity of about two orders of magnitude as the temperature was increased from 500 to 900~ and a decrease once the temperature of the second phase transition (870~ was passed (Fig. 5.36a). The OSL sensitivity increase was greater than that of the 110~ TL by almost a factor of 3 (Fig. 5.36b). Using the same sample of sedimentary quartz, Chen et al. (2000) measured growth curves following different thermal treatments. The growth curves were constructed for a single aliquot using the SAR procedure (see Section 6.5.4.5), first for the un-annealed sample, and then following successive 10 min pre-heats at 100~ intervals from 500 to 1000~ Fig. 5.37a shows the uncorrected OSL data. Fig. 5.37b shows the OSL
120
(a) Natural
80 0 "O
--e-- OSL o 110 ~
40
!
!
300
I
600
900
1200
Annealing temperature (~ 2.5 -o I._1 J
09 O
2.0-
(b)
1.51.0" 0.5" .0
0
I
I
I
300
600
900
1200
Annealing temperature (~
Fig. 5.36. Responseto test dose as a function of annealing temperature for sample having been bleached, but not previously heated. (a) OSL ( 9 ) and 110~ TL (O). (b) Ratio of the two signals OSL/TL for data from (a) (from Chen et al., 2000).
166
Optically Stimulated Luminescence Dosimetry
.J
40x10
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8 ](c) 09 0 -o o -9 o 0
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V
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400
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Regenerative Dose (Gy)
Fig. 5.37. (a) OSL growth curves obtained for unhyphen;annealed samples ( 9 ) and after pre-heating samples to 500 (O), 600 (Y), 700 (V), 800 (11), 900 (El) and 1000 (@) ~ (b) OSL sensitivity as measured using a test dose after each measurement in (a); (c) sensitivity-corrected growth curve, obtained by dividing each data point in (a) by the corresponding data point in (b) (from Chen et al., 2001).
sensitivity as measured by the test dose responses obtained during the SAR runs. The degree of sensitivity change is again seen to be dependent upon the annealing temperature, with the greatest increase ( - • 1500) being observed for temperatures of 900 and 1000~ Applying the sensitivity corrections from Fig. 5.37b to the growth curve data sets in Fig. 5.37a, gives a set of remarkably similar growth curves (Fig. 5.37c), given that the correction factors spanned three orders of magnitude. Thus, the growth curve shape is relatively independent of annealing temperature and appears to be unaffected by the phase changes at 573 and 870~
OSL Properties of Natural Materials
167
5.1.8.2. Low temperature annealingml60 to 280~ Effects of annealing sedimentary quartz at temperatures between 160 and 280~ have been studied as part of the development of a reliable dating procedure for quartz. In dating, it is necessary to heat both naturally and laboratory-irradiated samples prior to OSL measurements (see Sections 5.1.8.3 and 5.1.8.4 on thermal stability and thermal transfer). The pre-heat time used in the dating sequence is typically 10 s, i.e., one convenient to use in a Rise TL/OSL reader. The upper limit of this pre-heat will be 280~ above which the OSL trap is thermally depleted. The lower limit is usually 160~ in order to guarantee that the l l0~ peak is empty prior to the optical stimulation at 125~ The effects of these pre-heats, and also the effects of holding the sample for longer times at the same temperature, have been investigated by Wintle and Murray (1999). For a sample that has had its OSL zeroed prior to the measurement sequence, the OSL response to a test dose can be measured (Fig. 5.38a). It can be seen that for this sample, a 30 ka sand from Australia, the maximum signal increase is the same, independent of the pre-heat temperature. In this case, the signal enhancement is a factor of 2, much less than the increase observed for sedimentary quartz heated through the first phase change at 573~ It can also be seen that the sensitivity increase occurs in progressively shorter times as the temperature is raised. Wintle and Murray (1999) fitted the laboratory data sets with saturating exponential functions. The resulting lifetimes were used in an Arrhenius plot to determine the thermal energy (E) and pre-exponential factor (s) for the main process of enhancement (E -- 1.24 _ 0.06 eV and lOgl0(S) - 9.9 _ 0.6). These predict a lifetime of --~40 ka at 20~ It should, thus, be expected that samples over ---10 ka taken from a sediment in a warm climate should have exhibited a considerable degree of sensitisation in nature. Younger samples, and those from colder climates, will show less natural sensitisation. For irradiated samples, containing either a natural or a laboratory dose, it is not possible to use the OSL response to a test dose. However, it is possible to measure the luminescence sensitivity using the 110~ TL peak, even though there is not a complete correspondence between it and the subsequent OSL measurement (Wintle and Murray, 1999). For the naturally irradiated sample, the increase in sensitivity from such TL measurements is indeed smaller (Fig. 5.38b). Fig. 5.38b shows a similar trend to that in Fig. 5.38a, but the 2 0 - 3 0 % increase as a result of laboratory pre-heating suggests that a considerable amount of sensitisation has already occurred in its burial environment. Others who have observed sensitivity changes with pre-heating of sedimentary quartz include Li et al. (1999). Prior to the development of the SAR protocol (see Section 6.5.4.5 and Section 6.11.2.2.2), two procedures to deal with these sensitivity changes were adopted. In the first, it was suggested that a sufficiently strong pre-heat is applied, so that full sensitisation occurs for both natural and laboratory-irradiated samples (Murray et al., 1997). For the 30 ka sample which was the main subject of their experiments, Murray et al. (1997) found that 10 s pre-heats at temperature of 280 and 300~ were needed to equalise the degree of sensitisation. This was used for dating with the single-aliquot additive-dose protocol. For younger samples, they found that lower pre-heat temperatures would suffice, as there had been less time for sensitisation in the natural environment. The second procedure involved the use of the 110~ TL
168
Optically Stimulated Luminescence Dosimetry 22000
(a)
9
20000 go
18000
c5
16000
gO
14000
0 0
J o9
12000
0
10000 8000 0.001
0.01
0.1
1
10
Cumulative time at temperature (hr) 1.4
1.3 -i-, > go
= O0
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0.9
0.001
.
0.01
.
.
0.1
.
1
10
100
Cumulative time at temperature (hr) Fig. 5.38. (a) Sensitisation of the OSL from a sample previously heated to 500~ and given a dose of 51 Gy before storage times of up to 22 h at 160 ( 9 ), 180 (u 200 (11), 220 ( . ) , 240 (A), 260 ( 9 ) and 280 (0) ~ (b) Sensitisation of the 110~ TL peak as a function of time held at various temperatures: 160 ( 9 ), 180 ( 9 200 (I?), 220 (V), 240 (11), 260 (D) and 280 ( , ) ~ (redrawn from Wintle and Murray, 1999).
peak as a sensitivity monitor; this was used by Murray and Roberts (1998) in a singlealiquot regenerative protocol. Vartanian et al. (2000) investigated the changes in OSL sensitivity for quartz from archaeological material and synthetic hydrothermal quartz. For the latter, different concentrations of aluminium and lithium were added as the crystals were grown, and the concentrations of these elements and alkali ions (e.g., Na and K) were measured. A correlation was found between these elemental concentrations and luminescence sensitivity. Vartanian et al. (2000) suggested that the sensitivity change caused by
OSL Properties o f Natural Materials
169
pre-heating in the range of 200-250~ is irreversible and is due to defect migration. They suggested the selection of low-temperature pre-heats in order to avoid sensitisation, unless the change can be monitored (as in the case of the SAR procedure).
5.1.8.3. Thermal stability 5.1.8.3.1. Isothermal decay. Isothermal decay of the OSL can be used to monitor the thermal emptying of the trap that gives rise to the OSL signal. However, these measurements are only correct either when no sensitivity change occurs during the thermal treatment (e.g., the sample has been fully sensitised by thermal annealing; Smith et al., 1990) or when any sensitivity change is monitored (e.g., using the 110~ response to a test dose; Murray and Wintle, 1999a) or using data taken after the initial sensitisation has ceased (Spooner and Questiaux, 2000). The latter two studies were undertaken on naturally irradiated quartz in order to avoid measuring OSL from traps that are only filled by laboratory irradiation and to avoid sensitivity changes resulting from the laboratory irradiation. Li et al. (1999) also showed exponential decay for storage at 260~ provided a storage time of 30 s was exceeded, allowing the major change in sensitivity to have taken place. Murray and Wintle (1999a) used single aliquots to determine the decay curves of the initial OSL signal for storage at temperatures between 160 and 280~ Fig. 5.39 shows the sensitisation-corrected decay curves of the natural OSL of a sedimentary quartz (---30 ka with D e "~ 51 Gy). Storage times of up to 25 h were used. Ninety-nine percent of the signal can be represented as a single exponential decay, implying that a single trap is being emptied by these thermal treatments. Using the corrected data set (c.f. earlier measurements by Wintle and Murray (1998)), a trap depth of 1.59 ___0.05 eV and a preexponential factor given as lOgl0 s -- 12.9 + 0.5 were obtained. Spooner and Questiaux (2000) reported a trap depth of 1.59 eV and lOgl0 s -- 12.5, giving a calculated lifetime of 21 x 106 years at 20~ This makes it suitable for dating samples up to 1 million years old. The exponential decays shown by Smith et al. (1990), Murray and Wintle (1999a) and Spooner and Questiaux (2000) support their conclusions that the major part (> 95%) of the initial OSL signal from a naturally irradiated sample is derived from a single trap. This is in disagreement with the conclusions of Huntley et al. (1996) who did not observe exponential decays and thus considered several traps with different photoionisation crosssections to be involved. Their isothermal decay data were also obtained on natural quartz, but no allowance for sensitivity changes was made. Murray and Wintle (1999a) also made measurements on the same material that had been optically bleached and given 51 Gy and also heated to 500~ and given 51 Gy. The sensitivity-corrected data were again plotted, but in these cases, two components were seen in the thermal decay. The components contributed 61 and 38% for the bleached and 69 and 29% for the heated samples. The values of E for the dominant component (also that which decayed more slowly) were higher than that for the natural, namely 1.73 ___0.09 and 1.69 ___0.04 eV, respectively. The cause of this behaviour for laboratory-irradiated samples was not explained. Instead a best estimate of 1.66 ___0.03 eV was used for the component that depleted most slowly in the laboratory experiments and assumed to be that which remained in nature. The lifetime at 20~ proposed for this component was ---1.1 • 108 years. For the other component, seen only in laboratory-irradiated
Optically Stimulated Luminescence Dosimetry
170
10000
c
1000
\
100 ~
&240~
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0
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Cumulative time at temperature, hr Fig. 5.39. Isothermal decay curves for OSL signal from natural quartz. The OSL signal was corrected for sensitisation using the 110~ TL peak response to a test dose given after each measurement. Results are shown on three time scales (from Murray and Wintle, 1999a).
samples, the equivalent lifetime was --- 380 years. These results suggest that a pre-heat is necessary to remove the unstable charge. Based on the trap depth of 1.14 + 0.14 eV and pre-exponential factor given as logl0s = 9.5 + 1.5, pre-heats of 16 h at 160~ or 5 min at 220~ or 10 s at 280~ would be sufficient to remove it. Using these trap depth data, and the sensitisation data, Murray and Wintle (1999b) conclude that for their 30 ka Australian quartz sample, it would not be possible to select a pre-heat time and temperature that would provide complete sensitisation of natural and laboratory-irradiated aliquots, without causing some loss of charge from the OSL trap.
5.1.8.3.2. Pulse annealing. Another approach in obtaining and displaying the data on thermal stability is to plot the OSL signal remaining after a fixed time at various temperatures. Rhodes (1988) used 5 min heat treatments at a range of temperatures up to
171
OSL Properties of Natural Materials
240~ on a naturally irradiated sedimentary quartz and on the same material that had been given an additional laboratory irradiation. However, interpretation of such plots in terms of thermal erosion of OSL traps is complicated by the sensitivity changes brought about by the thermal treatment. Wintle and Murray (1998) carried out a similar experiment but used the response of the 110~ TL peak to a test dose (Fig. 5.40b) to correct for such sensitivity changes. In their experiment, a single aliquot was heated for 10 s at progressively higher temperatures. The uncorrected data in Fig. 5.40a suggest that the laboratory-irradiated sample has a large thermally transferred OSL signal. However, after correction, this effect is much reduced (Fig. 5.40c).
J O9 ,e--
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100 200 300 Temperature, ~
0 100
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Preheat Temperature, ~
Fig. 5.40. (a) Initial OSL obtained at 125~ after pre-heating for 10 s at temperatures from 160 to 500~ for natural sample (O) and for sample that had been bleached at 125~ for 200 s and irradiated with 56 Gy ( 9 ); (b) 110~ TL peak from 0.1 Gy dose given after each OSL measurement in (a); (c) OSL data from (a) corrected for thermal activation using data from (b) (from Wintle and Murray, 1998). (The inset shows the ratio of the two data sets.)
Optically Stimulated Luminescence Dosimetry
172
1.2
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r_..!
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0.0 100
200
300 Temperature (~
400
500
Fig. 5.41. Pulse annealing curves using different heating rates; (a) natural quartz, and (b) quartz annealed at 500~ and given 50 Gy (from Li and Chen, 2001).
Li and Chen (2001) made OSL measurements on both natural and laboratory-irradiated quartz after heating at rates from 0.5 to 3~ to progressively higher temperatures from 50 to 450~ in 10~ steps (Fig. 5.41). They did not maintain the temperature, but cooled the sample immediately, a process termed "pulse annealing". No sensitivity measurement was made, but they used the data in Fig. 5.41 to obtain the percentage reduction in OSL signal per ~ (Fig. 5.42). Positive data points indicate the decay of the OSL signal and relate to thermal untrapping, whereas negative data points indicate enhancement of the signal and relate to sensitivity increase. When these data are obtained at more than one heating rate, it is possible to obtain the thermal activation energy and frequency factor for the process (Li et al., 1997). Li and Chen (2001) obtained a trap depth of 1.75 + 0.03 eV for thermal depletion and 1.38 eV for the sensitisation process that peaked at 250~ for the laboratoryirradiated sample, and confirmed the latter value with isothermal decay experiments. The lifetime at 20~ associated with this process, was about 30 ka and is thus likely to be the
173
OSL Properties of Natural Materials
0.3 (a) Natural
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tO o = "o .
(b) Annealed 9
o 9
1.0 ~
2.0 ~ o
3.0 C/s
I~
1 I
_.d
I
I
_
.._!
0
0.1 0.0 -0.1
200
300
400
500
Temperature (~ Fig. 5.42. Percentagereduction in OSL signal plotted as a function of pulse anneal temperature; data for (a) natural and (b) annealed quartz from Fig. 5.41 (from Li and Chen, 2001).
cause of the natural sensitisation that occurred for sedimentary samples deposited several tens of thousands of years ago. 5.1.8.4. Irradiation at elevated temperatures Since shallow traps remain empty during environmental irradiation, but continue to fill during laboratory irradiation, it is possible that the electron-trapping probability for the OSL traps may be different under the two conditions. In particular, some effect would be seen if the shallow traps associated with the 110~ TL peak saturated during the laboratory irradiation. This possibility was investigated by Wallinga et al. (2002) by performing laboratory irradiation at temperatures from 35 to 260~ on a sample of sedimentary quartz that had been repeatedly heated to 500~ to stabilise the sensitivity. A 20% monotonic decrease in the electron-trapping probability was found for temperatures from 35 to 185~ Although OSL traps fill more slowly at higher irradiation temperatures, there is no step change; this suggests that shallow trap saturation does not affect the OSL signal from the deeper trap. The observed decrease in OSL with irradiation temperature may be related to
174
Optically Stimulated Luminescence Dosimetry
a reduction in the capture cross-section for the OSL traps as the temperature is increased; this will have no effect on equivalent dose determination when irradiation is made at ambient temperature. Bailey (2002) suggested that irradiation at elevated temperature may also provide an additional SAR procedure. He proposed the use of the RL obtained at the end of the regeneration irradiation as a monitor of the luminescence efficiency. For the natural data point, it would be necessary to give a small dose to enable the appropriate RL to be measured. This approach was tested using his computer model for quartz (Bailey, 2002), but has not been applied to real samples.
5.1.8.5. Thermal transfer A number of dating studies involving quartz samples from glaciated areas have reported an increase in OSL signal following application of a pre-heat (Rhodes and Pownall, 1994; Rhodes and Bailey, 1997). The pre-heat causes charge to be transferred from thermally stable light-insensitive traps in the region of 300~ to the OSL trap via the conduction band. For material taken from streams in glaciated areas of the Himalayas, Rhodes and Pownall (1994) found D e values in the range 2 4 - 30 Gy, and these values were only halved by an 8 h exposure to daylight before D e measurement. However, in these studies, multiple aliquot dating procedures were used, allowing no account to be taken of any sensitivity change and not permitting D e t o be determined as a function of temperature. A more comprehensive study on a variety of glacial sediments and an aeolian sand was reported by Rhodes (2000). The SAR protocol was applied in order to account for sensitivity change, though very little was found for these samples for pre-heats up to 280~ as monitored by both the OSL response to the test dose in the SAR procedure and the 110~ TL peak. Pre-heats of 10s were used, with the temperature range of 160-360~ being covered in 40~ steps. All the samples were optically bleached with 200 s of 4 2 0 560 nm light at 125~ using a stimulation power of 12 mW/cm 2, in order to make comparison between the responses of samples from the Himalayas and from West Greenland. The results are shown in Fig. 5.43, together with the result for a sample of 24 ka dune sand from Alabama. The latter gave a negligible value of D e ( < 0.1 Gy) until a pre-heat of 280~ was exceeded. (Note that this pre-heat would not be used in a dating run since all the OSL charges would have been thermally erased (see Section 5.1.8.3)). The glacigenic quartz gave non-zero values of D e , with the Himalayan samples (L6 and L8) giving increasing De values (up to 20 Gy) as the pre-heat temperature is increased. In further studies on one of these samples (L6), the TL was measured as the sample was heated to the pre-heat temperature, ranging from 200 to 280~ in 20~ steps. The OSL was then measured at 160~ The pre-heating was measured many times, until the TL and OSL signal reached negligible levels. Plots of the initial OSL versus the TL from the preceding pre-heat were made (Fig. 5.44). The relationship was taken to imply the charge transfer via the conduction band (Rhodes, 2000). Charge transfer was also reported for quartz from aeolian deposits (e.g., Rhodes, 1988); however, in these early studies, no sensitivity monitoring was carried out. More recently, studies of young dune sands by Bailey et al. (2001) have shown negligible thermal transfer for temperatures below 260~ (Fig. 5.45), with a modem sample giving D e values of 0.03 _+ 0.02 Gy (corresponding to an age of 20 _ 10 years). For pre-heats above 260~ D e values up to 2 Gy were obtained. Likewise, Murray and Clemmensen (2001) report no
OSL Properties of Natural Materials
a) L6 & L6 *
4O 3O
5
20 o
g~
3
~
2
10 ,
0
100
200
,
,
300
400
b) HM46
o
|
0
100
(.9
|
.
300
400
d) A122~
0-2t
c) G R 6
015
.,
10
o%'t 0.1
(.9 4
v
cl
n
200
10s Pre-heat Temperature (~
10s Pre-heat Temperature (~ 15
175
o
v
5
~; o { "
.
.
.
200
100
.
300
400
a
. 0
.
100
.
.
200
0 300
0
10S Pre-heat Temperature (~ 4O
e)L6*
3O
~'~~
~ []
,
0
100
-
p.
200
[]
[] .
300
10s Pre-heat Temperature (~
100
9
9 ~ 200
-=,
.
300
400
10s Pre-heat Temperature (~
20 10
,
203010
400
f) L8*
[] [] t
D .
.
400
[]
.
0
100
200
[]
=
i
300
400
10s Pre-heat Temperature (~
Fig. 5.43. De as a function of 10 s pre-heat temperature for glaciofluvial samples from the Himalayas (L6, L8 and HM46) and from West Greenland (GR6) and for a dune sand from Alabama (A122). All samples were exposed to laboratory light before measurement (from Rhodes, 2000).
increase in D e values with pre-heat temperature for their aeolian dune sands, including a very young sample for which they obtained a D e of 0.082 _ 0.009 Gy over the range of 160-280~ (corresponding to an age of 100 _+ 30 years). The data for this and another sample from the same area of Denmark are shown in Fig. 5.46. Vartanian et al. (2000) also concluded that thermal transfer was not the explanation for the increase in OSL that resulted from pre-heating samples of archaeological ceramic. Besides charge being transferred from deep traps, around 300~ it is also necessary to consider whether charge may be transferred from shallower traps to the main OSL trap. In this case, the charge from shallower, but optically stable traps may be transferred during burial. Indeed, one of the reasons suggested for pre-heating prior to OSL measurement was to ensure that the amount of charge transferred was equal for both natural and laboratory samples. These traps were thought to relate to the TL peaks, sometimes observed at ---230~ Once again, using SAR it is possible both to carry out D e measurements over a range of temperatures and to be sure that different sensitivity changes for natural and laboratory-irradiated material does not cause error. In the literature there are a number of
Optically Stimulated Luminescence Dosimetry
176
.o
t
(a)
~" 0.1 "~ 0.08
L6 -Initial n o r m a l i z e d O S L vs PH TL 240~ 220~
o.o6t
~ J 200 ~ o-- ~o 0.04 .~_ o "a "6 0.02
--
260~
0
0
2000
4000
6000
8000
10000
Total TL during PH ramping (counts) (b)
L6 - I n i t i a l n o r m a l i z e d O S L vs PH TL
.s A 0.12 " 1 ~
, , o~
o.1 4 , "
240~
260~
~ 0.08 0 =~ 0.06
.~ 8 0.04 = "6 0.02
--
0
|
0
10000
20000
30000
40000
Total TL during PH ramping (counts) Fig. 5.44. Initial OSL signal as a function of TL measured during pre-heat. Values obtained from repeated heating to the given temperature (a) for lower temperature pre-heats and (b) data for higher temperature pre-heats (from Rhodes, 2000).
2.5
Sample AB2-1
2.0
.-,9
1.5
1.0 $
0.5
0
140
*
§
1~0
1~0
260
2;20
2a,0
t
§§
260
280
300
320
Pre-heat temperature (~ Fig. 5.45. Plot of De as a function ofpre-heat temperature fora young dune sand withDe = 0.41 --- 0.13 Gy (for 160-260~ and giving an age of 310 ___90 years (from Bailey et al., 2001).
177
OSL Properties of Natural Materials
0.20 (.9
(a)
De (160-300~ 0.15 - =0.082+0.009 Gy
O
s
t" I1) t~
0.10 -
xlI
,
m
0.05
O" UJ
0.00
100+30 years !
!
!
!
150
200
250
300
(b)
(.9 O n
--
t
w
w
i
t@ t~ >
m
{7" UJ I
I
I
i
150
200
250
300
Pre-heat Temperature, ~ >,
60 -
(c)
@ if) 0
121
40 -
1-i1) >
.m
ID"
20 -
D e (200 to 3 0 0 ~
LLI
= 4 4 . 8 + 1.4 Gy i
150
I
200
i
250
|
3OO
Pre-heat Temperature, ~ Fig. 5.46. Plots of D e as a function of pre-heat temperature for (a) 0.082 and (b) 4.42 Gy (from Murray and Clemmensen, 2001), and (c) for 44.8 Gy (from Murray and Olley, 1999).
plots of D e as a function of pre-heat temperature, see for example Figs. 5.45 and 5.46. None of these show deviation from a plateau in a way that would imply the presence of interference from a shallower trap. 5.1.9. Raised temperature OSL
5.1.9.1. Thermal quenching As mentioned in Section 5.1.6.2.1, the luminescence emission at 3 6 0 - 4 2 0 nm is quenched as the room temperature is raised. It was first noted by Wintle (1975) who observed a discrepancy in measurements of the trap depth of the 325~ peak in quartz.
178
Optically Stimulated Luminescence Dosimetry
The isothermal decay method and Hoogenstraaten's method of trap depth determination gave values of 1.7 _ 0.1 and 1.69 - 0.03 eV, respectively, whereas the initial rise method gave 1.05 ___ 0.03 eV. The latter predicted a very low value of the mean life at 20~ as only ---200 years. This was clearly inappropriate for such a deep trap with independent evidence of its stability from dates obtained for archaeological samples. This difference in trap depth energy, 0.64 eV, can be explained by the luminescence centre having a luminescence efficiency r/that is temperature-dependent, such that r / = K expW/~, where W -- 0.64 eV. Thermal quenching should not affect the isothermal TL decay measurements, as long as no sensitivity changes occurred. However, thermal quenching would have a substantial impact on data derived with the luminescence measured at different temperatures. This was demonstrated in studies of the 325~ TL peak by Spooner (1994a) (see Section 5.1.6.2.1). Duller et al. (1995) measured the OSL from sedimentary quartz obtained on stimulation at temperatures ranging from 20 to 450~ They reported thermal quenching with W -0.63 eV and K -- 2.8 x 107. In a more detailed study, Huntley et al. (1996) measured the luminescence output as a function of temperature from 20 to 220~ They observed the OSL using filters centred at 356 nm, after subtraction of the TL signal that is observed above 150~ as the sample is heated. Several stimulation sources were used, as shown in Fig. 5.47 using lines from several laser sources, namely 454, 488, 514.5, 633 and 674 nm.
10-10tO
2.54 ev
.t.-,
0
'=ca- 10 .!,.,.,
2.73 ev
-11-
2.41 ev
tor
i,.,,. to
10-12-
r
0 0 cO
~r.- 10-13-
10-14-
1.96 ev
1.84 ev
6
5'0
16o
Temperature (~C)
260
2so
Fig. 5.47. Temperature dependence of the luminescence during excitation with selected lines from argon, He-Ne, and diode lasers (redrawn from Huntley et al., 1996).
179
OSL Properties of Natural Materials
Thermal quenching gives rise to the decreases in OSL observed for stimulation temperatures above 140~ Huntley et al. (1996) reported determining values for W and K similar to those presented by Wintle (1975). Murray and Wintle (1998) used their OSL decay curves obtained at elevated temperatures (50-175~ to demonstrate thermal quenching. They plotted both the initial OSL (first 0.4 s of their decay curve) (fast component) and the total integrated signal (100 s) as a function of temperature (Fig. 5.48). Both data sets show thermal quenching, though the initial OSL signal also shows the effect of thermal assistance (see Section 5.1.9.2). The similarity in thermal quenching behaviour implies that the slow component uses the same luminescence centres as the fast component. Using the integrated OSL signal, they fitted the equation for thermal quenching, with the parameters being W -0.61 __+0.02 eV and K -- 2.0(___1.2)• 107. Omitting the data points at 25 and 50~ the data were fitted again; the data used in this analysis are shown in Fig. 2.20. Wintle and Murray (2000) gave the values as W = 0 . 6 3 6 _ 0.013 eV and K - 3.40(___0.9)x 107. McKeever et al. (1997a) also carried out OSL measurements as a function of temperature and determined the parameters to be W - 0.60 eV and K -- 7.9 • 106. As part of a timeresolved luminescence study, Chithambo (2002) reported thermal quenching for quartz that had been heated above the phase-change temperatures. He calculated the thermal quenching energy to be W - 0.65 +-0.10eV, when using green (525 nm) LEDs for stimulation. This was in agreement with the value of 0.63 _ 0.07 obtained by Chithambo and Galloway (2001) when using blue (470 nm) LEDS. 5.1.9.2.
Thermal assistance
OSL can be stimulated at temperatures below or above room temperature, the range being limited by the apparatus used. Most studies have been in the temperature range from room temperature up to about 200~ (e.g., Murray and Wintle, 1998). At higher temperatures, particularly above 280~ the OSL trap is significantly thermally emptied (see Section 5.1.8.3). More importantly, the OSL signal is drastically reduced by thermal
0.8
8
o.6
d0
0.4
9
0.2 0.0 0
100
200
300
400
500
Stimulation Temperature, ~
Fig. 5.48. InitialOSL (first 0.4 s of decay curve, 9 and integrated signal (0-100 s, 9), normalisedto unity for values at 25~ plotted against stimulation temperature (from Murray and Wintle, 1998).
180
Optically Stimulated Luminescence Dosimetry
quenching, as discussed in Section 5.1.9.1. This can be seen for the OSL decay curves in Fig. 5.4 that have been normalised by a short OSL exposure at 20~ prior to the elevated temperature measurements. Besides the effect of the 110~ TL trap (discussed in Sections 5.1.2.2 and 5.1.2.6), the initial decay of the OSL is also more rapid at higher temperatures. This can also be seen in the normalised data set of McKeever et al. (1997a) reproduced in Fig. 5.3. This change in initial decay rate is the result of thermal dependence of the process of photoeviction (Spooner, 1994a). Spooner (1994a) used a cryostat to examine the temperature dependence from about 100 to 473 K ( - 173 to 200~ Over this temperature region, he measured the OSL signal (in the UV) using several different stimulation wavelengths (Fig. 5.23). Using these data he was able to determine the thermal activation energy (Eth) for each wavelength used, and the dependence of thermal assistance energy on stimulation photon energy is given in Fig. 5.49. A similar analysis was reported by Huntley et al. (1996). These results are important for understanding the optical stimulation mechanism (see Section 5.1.11). Besides using monochromatic light in such experiments, it is also possible to obtain the thermal activation energy for a mixed blue and green light source, as used in dating. Using the initial part of the OSL signal, and considering the thermal quenching, Murray and Wintle (1998) obtained a thermal activation energy (Eth) of about 0.045 eV. This value is consistent with an effective optical stimulation energy of 2.65 eV (468 nm), obtained by projecting the data given in Fig. 5.49, or using the calculation of Huntley et al. (1996). Chithambo (2002) reported activation energies for thermal assistance to be 0.06 +-- 0.01 eV for several samples of thermally annealed quartz. 5.1.10. The slow component
As mentioned in Section 5.1.2.4, there is a component of the OSL signal that remains after the initial part of the OSL decay curve has been removed by light exposure. This slow 0.40
.
.
.
.
.
.
.
'"
"
I
"
I
9
~" 0.35 >"
0.30
~) 0 . 2 5 -
0.20 "N
~"~
0.15
"~ 0.10
0.05 1~ i-0.00
1.6
' 1.7
.........
' 1.8
"
Optical
= 1.9
....... ' 2.0
Stimulation
"
' 2.1
photon
"
' 2.2 energy
"
' 2.3
"
' 2.4
"
~ 2.5
"
2.6
(eV)
Fig. 5.49. Thermalassistanceenergyas a function of stimulationenergy obtainedfrom data in Fig. 5.23 (from Spooner, 1994a).
181
OSL Properties of Natural Materials
component underlies the main signal and usually contributes only a few percent to the initial part of the main decay curve. It is best observed using a stimulation temperature that keeps the 110~ trap empty, e.g., 160~ (Bailey, 2000a,b). After 100 s of exposure to the blue and green ( 4 2 0 - 5 6 0 nm, 16 m W / c m a) light from the filtered halogen lamp in a Rise TL/OSL reader, the remaining OSL signal is the slow component. It is usually measured with stimulation taking place at 160~ or even 250~ (Bailey, 2000b; Singarayer et al.,
2000). As shown in Sections 5.1.3.1 and 5.1.3.2, the slow component is also seen when the power to the light source, usually blue (Bulur et al., 2000; Poolton et al., 2000) or green (Kuhns et al., 2000) LEDs, is increased linearly with time during stimulation. Chithambo and Galloway (2001) investigated the time-resolved luminescence of the slow component in their quartz sample. They found the lifetime to be strongly dependent upon stimulation temperature, with values remaining constant at 36 ___ 2 txs from 20 to 125~ but decreasing to only 8 txs by 200~ 5.1.10.1. Thermal stability A significant difference between the slow component and the rapidly bleachable components (fast and medium together) relates to the thermal stability. The rapidly bleachable component relates to a trap that empties at around 325~ thus it is removed by heating to 400~ However, irradiated quartz that has been heated to 500~ shows a slowly-decaying OSL component (Fig. 5.50). Further information on the thermal stability can be found by taking irradiated samples to progressively higher temperatures in an attempt to thermally erode the slow component. Bailey (2000b) used one aliquot of a sample to measure the effect of incremental temperature increases. These showed an increase in the slow component of the OSL as the temperature was raised from 400 to 500~ and then a decrease as the temperature is raised further to 700~ (Fig. 5.51).
100000
~,
0
10000
1000
. . . . . . . . t (s) 100
............................
I 0('~)
1000()
1 hr
,
,
|
10
100
1000
10hr
1 ........ |
10000
,1 ......
100000
Illumination time (s)
Fig. 5.50. Slowcomponent OSL for sample of modem sand given a 50 Gy beta dose and then heated to 500~ The main measurement was made with a 514.4 nm laser line; the inset measurement was made using blue and green light from an incandescent lamp. Both stimulations were with the sample at 160~ (from Bailey, 2000b).
Optically Stimulated Luminescence Dosimetry
182
25 OSLR measurement procedure
~ / ~
I100sos.,250~
/
~
o
20 .~
\,,'
15 ~,
leat to T, hold 10sI 1 0 0 s1 6O0S~L ,
10 " |
9O.....'
200
I
~
I
300
400
O.. ~ ~
I
500
'
I
J
600
I
700
Activation temperature, T (~ Fig. 5.51. Effect of pre-heat temperature on the initial level of slow component OSL, measured using sequence shown in inset. Thermal activation characteristics of the 110~ TL peak as measured using the response to a 0.5 Gy test dose (from Bailey, 2000b).
The 110~ TL peak response was monitored during this experiment, but its thermal response is clearly not the same as that of the slow component. Singarayer et al. (2000) used a multiple aliquot measurement procedure for other samples. They found similar behaviour (Fig. 5.52), but greater signal at 300~ a behaviour that they attribute to "recuperation"--transfer of charge during pre-heating (Bailey, 2000b). The mechanism giving rise to the peak in the slow component OSL between 550 and 600~ is not known.
1.4 1.2
#
TQG
---
SAQ1
-"
OJ2 SL205
0.8 0.6
"
0.4
"
0.2
-
,
!
!
|
"~
300
400
500
600
700
Pre-heat Temperature (~ Fig. 5.52. Effect of pre-heat temperature on the initial level of slow component OSL measured using separate aliquots on different samples obtained by pulse annealing (from Singarayer et al., 2000).
OSL Properties of Natural Materials
183
Interpretation is complicated by the likely opposing mechanisms of sensitisation and thermal erosion. However, Singarayer et al. (2000) used these data to choose a pre-heat temperature of 450-500~ for thermal removal of the rapidly bleachable component. 5.1.10.2.
Growth curve
Another significant difference between the slow component and the rapidly bleachable component is the far higher saturation level found for the slow component. The higher saturation level is demonstrated by the data of Singarayer et al. (2000) shown in Fig. 5.53, where the response for the slow component obtained with a single-aliquot additive-dose
25000
(a)
0
20000 0
G~ 15000
0
9
10000
.=
5000 ,
D e = 377_+23Gy D o = 2790Gy
r" -500
u
!
|
5OO
1500
Added dose (Gy) 3.5
~(b)
v
2.5 2 9 1.5 9
1 0.5
De = = 278_+37Gy D o = 88_+3Gy w
0
I
200
|
400
600
Dose (Gy) Growth curves for a sample of quartz. (a) Single-aliquot additive dose growth curve for the slow component OSL, and (b) single-aliquot regenerative dose (SAR) growth curve for initial part of the fast component of the OSL. Both are fitted with saturating exponential curves (from Singarayer et al., 2000).
Fig. 5.53.
184
Optically Stimulated Luminescence Dosimetry
procedure is compared with that using the SAR procedure for the rapidly bleaching component. The natural signal for the latter is effectively indistinguishable from the measured OSL saturation level. Both data sets were fitted to a saturating exponential curve, I = I 0 ( 1 - exp(-D/Do)). The values of Do obtained for this sample were 88 ___ 3 Gy for the fast component and 2790 Gy for the slow component. Although these results look promising for the use of slow component, Singarayer et al. (2000) also reported more complex behaviour with added dose for a modem sample. For these samples, the OSL signal was found to decrease with repeated pre-heat/stimulation cycles. This behaviour violates one of the basic assumptions of the additive dose measurement protocol. However, Singarayer et al. (2000) devised a measurement procedure that appears to permit correction for this decay, which has the added complication that the extent of decrease is dose-dependent.
5.1.10.3. Optical bleaching An additional problem with the use of the slow component for dating sedimentary quartz is the implied extra time that would be required to zero the signal at deposition. For experiments using white light from a solar simulator, Singarayer et al. (2000) showed the bleaching of the slow component to be sample dependent, with times between 17 h and 1 week needed to reduce signals to a negligible level. This would limit the applicability of this signal for dating sediments.
5.1.10.4. TRL Chithambo and Galloway (2001) used pulsed blue (470 nm) LEDs to observe the TRL from the slow component of their quartz sample, which had been given a laboratory irradiation and then bleached for 150 s with the diodes in order to remove the fast and medium components. Both thermal quenching and thermal assistance were observed (see Section 5.1.9 for equivalent data for the fast component). First, the thermal quenching was observed directly from the luminescence intensity and the parameters were similar to those for the fast component, W = 0.68 ___0.11 eV and K -- 2 x 107. Then the luminescence lifetimes were measured as a function of temperature and were found to decrease from 36 to 7.8 Ixs as the temperature was increased from 100 to 200~ and the values of W and K derived from these data were consistent with those for the direct measurements.
5.1.11. Photoionisation cross-section One of the most fundamental parameters that relates to OSL is the photoionisation cross-section, o-. It is a function of stimulation wavelength and thus can be obtained experimentally only for near-monochromatic stimulation (see Chapter 2 for discussions of the various procedures to measure o-; Bailey, 2002). Huntley et al. (1996) derived an equation that equated the excitation cross-section, o-, to the ratio So/Io where So is the initial slope of the CW-OSL decay curve and I0 is the initial luminescence intensity. These values were obtained for an Australian sedimentary quartz using seven laser stimulation lines, with energies from 1.92 to 2.71 eV (646-458 nm, respectively). The ratio So/Io is plotted as a function of stimulation energy in Fig. 5.54 (~). The values for the vertical axis range from 10 . 2 0 to 10 -17 cm 2 for energies of 1.92 and 2.71 eV, respectively. Huntley et al. (1996) point out that their value of 2.71 eV is
185
OSL Properties of Natural Materials 10-10 r
o o r
10-17
Q.
E 10 -11
tO .,i,..,
(9 "O
o
.J~
O c-
10-18
i_
(9 Q.
N
10 -12
E
r"
0
0
0 tO
10-19 % ~0
0
x= 10-la __o
1.8
210
212 214 216
10 -2o 2.8
Photon energy (eV) Fig. 5.54. Initial luminescence intensity ( 9 ) Io and relative slope, So/Io (G) as a function of incident photon energy (redrawn from Huntley et al., 1996).
close to the value calculated as an approximation for the cross-section in studies of the photoelectric effect. By combining simple equations to describe both LM-OSL and CW-OSL, Kuhns et al. (2000) calculated the photoionisation cross-section for the three components they found in the stimulation of an aeolian quartz by green LEDs (526 nm). The fitting of three first order components to both LM-OSL and CW-OSL data sets is shown in Fig. 5.55. For the fast component (shown as component 1 in each figure), o - w a s calculated to be 1.48 X 10 -18 cm 2. This is the value for 526 nm (2.36 eV) and it can be found to be similar to the value of 3 x 10-18 cm 2 from Fig. 5.54. Larsen et al. (2000) observed the LM-OSL from a glaciofluvial quartz that had been irradiated, illuminated and heated to 550~ in the laboratory. This treatment was repeated until the OSL sensitivity and the CW-OSL curve shape were constant. The sample was then given a dose of 25 Gy and a pre-heat of 10 s at 220~ The LM-OSL was observed at 160~ for stimulation with blue LEDs (470 nm). The most rapidly bleached component was determined to have a photoionisation cross-section of 9.0 x 10-17 cm 2. This value is similar to the value of 2 • 10 -17 cm 2 from Fig. 5.54 for stimulation with a photon of energy 2.64 eV. Using pseudo-LM-OSL plots (Section 5.1.3.1) to separate the fast and medium components, Singarayer and Bailey (2002) observed the depletion of the blue (470 nm) stimulated luminescence when samples were exposed to 430, 470, 500, 525 and 590 nm (Fig. 5.17). From these plots, they calculated the photoionisation cross-section for each component as a function of bleaching wavelength (Fig. 5.56a) . The ratio of the values
186
Optically Stimulated Luminescence Dosimetry 1800 (a)
~-. 1600 "~
1400 1200 1000 800
~"
600
O
400
200 0 200
400
600
800
1000
Time (s)
100000
(b)
Sum C
10000
0 0
1000 0
10
20
30
40
50
Time (s) Fig. 5.55. (a) LM-OSL, and (b) CW-OSL data for a dune sample, showing how each signal has been split into three first-order components (from Kuhns et al., 2000).
changes radically with photon energy (Fig. 5.56b). This finding led Singarayer and Bailey (2002) to propose selective removal of the fast component by IR stimulation. For their sample, IR exposure times of ---8000 s (830 nm, 1 W) with the sample held at 160~ resulted in the complete removal of the fast component, whilst the medium component remained untouched. 5.1.12. Modelling processes giving rise to OSL in quartz McKeever et al. (1997a) developed a model for OSL production (discussed in Section 2.4.4), and included a discussion of its applicability to quartz. This model was used by McKeever et al. (1997b) to model OSL sensitivity changes during single-aliquot procedures. The large volume of experimental data published on quartz OSL in the following years, led Bailey (2001) to formulate a model specifically for this material. To encompass the width of information on OSL and TL, Bailey (2001) proposed that there
187
OSL Properties of Natural Materials 1E-16
(a)
1E-17 ~lff
A IN
1E-18
l
1E-19
/ i~!
1E-20
-'
~1~ "'" ~ l q . . . . . . . ' ~ ,
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,
%
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. . . .
3
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.I
.................................
il 2.2
2.4
2.6
2.8
3
3.2
3.4
P h o t o n E n e r g y (eV) Fig. 5.56. (a) Photoionisation cross-section as a function of stimulation energy for the fast and medium components of quartz when exposed to light at room temperature; (b) ratio of data from (a) also as a function of stimulation energy (from Singarayer and Bailey, 2002) 9
should be five electron trapping centres and four recombination centres. The electron traps were those for the 110~ TL peak, the 230~ TL peak, the fast and medium components of the OSL and an additional deep trap. Electrons in the latter and the 230~ TL trap were not able to be optically stimulated. Two of the recombination centres are thermally unstable and non-radiative. In contrast, the third recombination centre is thermally stable and radiative. The fourth centre is thermally stable, but non-radiative. Experimental data were used to constrain the model parameters and include behaviour such as thermal quenching. Subsequently, Bailey (2002) incorporated three optically active electron trapping centres that give rise to the slow components observed experimentally (Bailey, 2000b; Singarayer and Bailey, 2002). The new model also includes experimental values for the optical photoionisation cross-section (o-) (Section 5.1.11) and the thermal assistance energy (Eth) (Section 5.1.9.2). Banerjee et al. (2002) have taken Bailey's (2001) model and used it to test the SAR protocol. They used a modelled pre-heat of 220~ for 10 s, OSL measurement at 125~ a test dose of 0.5 Gy with a cut heat to 170~ and four regeneration doses to construct the sensitivity-corrected OSL growth curve. They recovered a dose of 2.04 Gy, 2% higher than the stimulated environmental dose of 2.00 Gy. This level of accuracy was achieved despite a 58% increase in the observed OSL sensitivity during the four regeneration dose cycles. The model was also used to investigate the dependence of the OSL growth curve on dose rate. Below the saturation level (attained by ---500 Gy), the
188
Optically Stimulated Luminescence Dosimetry
sensitivity-corrected OSL signal was independent of dose rates that were varied by two orders of magnitude.
5.1.13. Summary Quartz is the most extensively studied mineral, owing to it being found in materials that require dating (such as pottery and sediments) and those that could act as dosimeters in the case of a nuclear accident. Its OSL characteristics make it suitable for dating sedimentary material that is up to 150,000 years old, with equivalent doses of around 100 Gy. At the same time, it has been used to obtain doses of about 10 mGy for recently fired materials. Both applications have been achieved using the SAR measurement procedure developed on fundamental experimental studies, as summarised by Wintle and Murray (2000) and given in more detail in Chapter 6.
5.2. Feldspars 5.2.1. Crystal structure Feldspars are aluminosilicates made up of A104 and SiO4 tetrahedral units, with the oxygen atoms being shared between adjacent tetrahedra. This structure allows chargecompensating cations (e.g., K +, Na + and Ca 2+) to be accommodated within the tetrahedral framework and gives rise to a range of feldspars with different chemical compositions. In addition, there are many elements than can substitute for Si or A1, as well as for K, Na and Ca, and these affect the luminescence emission. For the purposes of luminescence dating, naturally formed feldspars are described in terms of being either plagioclase or alkali depending upon their chemical composition. These two types have different density ranges and a degree of separation can be achieved when a mixture of grains is introduced into a heavy liquid made up to an appropriate density (2.58 g/cm3). The lighter potassium-rich (alkali) feldspars, such as orthoclase (KA1Si308), will float and thus be separated from the plagioclase feldspars that form a series with varying amounts of sodium and calcium. The end members of the plagioclase series are sodium-rich albite (NaA1Si308) and calcium-rich anorthite (CaA12Si208). This classification of feldspars is based on chemical composition, but feldspars can also be classified according to their structure. The degree of ordering of the A1 and Si atoms in the crystal lattice depends upon their mode of formation. Feldspars formed at high temperatures tend to have a disordered structure, with sanidine being an example. Sanidine is a highpotassium content feldspar found in volcanic rocks that cool rapidly from temperatures in excess of 1000~ Slower cooling, e.g., of a granite body, will result in a more ordered structure, resulting in other high-potassium minerals such as orthoclase or microcline. Similarly, sodium-rich feldspar will form a range of albite structures depending upon the cooling rates. In addition, cooling slowly will cause mixed crystals to be formed, with alternating zones of potassium-rich and sodium-rich feldspars; these are known as perthites. The zoning usually occurs at a scale that is too small to affect dosimetry. However, perthitic
OSL Properties of Natural Materials
189
feldspars may be expected to have mixed luminescence properties. Also, plagioclase feldspars may be found with inter-grown anorthite and albite layers. Further information on classification of feldspars can be found in the literature (e.g., Deer et al., 1992). Duller (1997) and Krbetschek et al. (1997) discuss luminescence properties related to structure. 5.2.2. Decay curve shape obtained under continuous stimulationmCW-OSL and CW-IRSL
5.2.2.1. Stimulation sources Photostimulation of feldspars can be achieved using both visible and infra-red wavelengths (see Section 5.2.5). The first OSL signals from feldspars were obtained with 514 nm light from a laser (Huntley et al., 1985), and the first IRSL signals were obtained using selected wavelengths from a xenon lamp (Htitt et al., 1988). Routinely, IRSL stimulation is now achieved using IR LEDs. Numerous continuous wave (CW) IRSL decay curves have been published and none are found to be exponential. Bailiff and Poolton (1991) fitted their IRSL decay curves with power functions of the form (1 + Bt)-1 or (1 + Bt) -2, where B is a constant that depends upon the initial charge population. These result in decay curves that do not approach zero in the same way as OSL from quartz (Section 5.1.2). Bailiff and Barnett (1994) used a more general function of the form (1 + Bt) -P, where 1 < P < 2. These could be used down to 10% of the initial signal intensity. For the development of single-aliquot dating techniques for feldspars (see Section 6.11.2.1), it is necessary to characterise the first few percent of the decay. Galloway (2000) measured the decay in IRSL brought about by 10 successive IR stimulations; after 10 measurements the signal was reduced to no less than 85% of the initial value. When expressed as a function of total IR stimulation time, the data were fitted by a straight line; the rate of this initial decay was shown to depend upon the temperature and duration of the previously applied pre-heat. The decay curve shape obtained with visible stimulation is also not exponential; the shape of the CW-OSL can be used to distinguish between quartz and feldspar grains when they are stimulated at 532 nm in single-grain OSL systems (Section 7.7.3).
5.2.2.2. Effect of stimulation temperature 5.2.2.2.1. Initial part of signal. Using the initial part of the signal, it is possible to follow the IRSL signal as a function of temperature. Fig. 5.57a,b shows the luminescence from repeated 0.1 s IR stimulation of potassium-rich feldspar samples when they are heated from room temperature up to 500~ The TL signal is recorded between the IRSL measurements, and the net IRSL can thus be obtained as the difference between the two data sets (Duller and BCtter-Jensen, 1993). This type of measurement, called thermooptical luminescence (TOL), by Duller and Winfle (1991) was first proposed by Htitt et al. (1988) who used the data to support a mechanism for IRSL production. The data in Fig. 5.57 were obtained for violet emission (340-440 nm), but later studies by Rieser et al. (1997) using other K feldspars showed different behaviours for different emission wavelengths (e.g., 410 nm versus 560 nm).
190
Optically Stimulated Luminescence Dosimetry
x 103 400
(a)
300tO
ot"- 2000 (/)
.c_ E := 100_.1
0~,-
50
150
250
350
450
Temperature (~
X 10 3
700
Boot
(b)
-~g500t o~ 4001 ,--=~ 300] g
~: 200 ._J
100 ~
50
i
150
|
2~i0
|
350
450
Temperature(~ Fig. 5.57. TOL measurements of TL and IRSL (for 0.1 s every 10~ whenheating at 10~ for sedimentaryKfeldspar (a) naturally irradiated, (b) given an additional dose of 18 Gy (from Duller and BCtter-Jensen, 1993).
These properties have practical implications for dating. The rise in IRSL with temperature up to about 220~ could be used to enhance the magnitude of the signal during dating procedures. However, it should be noted that the emission spectra show small peak shifts when IRSL measurements are performed at elevated temperatures (Duller and BCtter-Jensen, 1997; see Section 5.2.6.1). More importantly, Poolton et al. (2002b) showed
191
OSL Properties of Natural Materials
both theoretically and experimentally that using an elevated measurement temperature is inappropriate. They determined D e for a sample of K-feldspar expected to have received a dose of 0.5 Gy since its documented deposition 300 years ago. Values of D e obtained with the SAR protocol (see Section 6.11.2.1.2) are shown in Fig. 5.58. The increasing values of D e with stimulation temperature were speculated to be caused by thermally induced accessing of recombination centres that had not been bleached at deposition. Poolton et al. (2002b) concluded that the most accurate evaluation of dose would result from IRSL measurements made at the temperature at which bleaching had occurred in nature (e.g., 10~ for the sample from the Netherlands in Fig. 5.58). This is in contrast to the widespread use of 50~ as the stimulation temperature in single-aliquot procedures to ensure fixed temperature stimulation in sequences that use pre-heating after irradiation. The varying dependence of IRSL on temperature for different feldspars has been proposed as a method of distinguishing between different feldspar types, e.g., microcline and orthoclase (Krbetschek et al., 1997). Fig. 5.59 shows measurements of blue and yellow emission as a function of temperature for a sample of microcline (Duller, 1997). The rapid drop of the 560 nm (yellow/green) IRSL near 100~ was not due to the emptying of the electron traps, as can be shown by pulse annealing experiments (Duller, pers. comm.). Duller and BCtter-Jensen (1993) carried out TOL measurements with IR (875A80 nm) and blue/green light (420-550 nm) stimulation of a potassium-rich feldspar extract (Figs. 5.57 and 5.60). For the natural samples, both the IRSL and OSL signals increase with temperature up to about 250~ (Fig. 5.57a and Fig. 5.60a) and the authors suggested that both processes involve a thermally assisted transition. BCtter-Jensen (2000) plotted the two signals against each other and found that they increased in proportion between 50 and 200~ (Fig. 5.61). The OSL signal for the laboratory-irradiated grains increases very little with temperature (Fig. 5.60b), suggesting that the OSL signal is affected by the presence or absence of charge in the low-temperature traps. This is in direct contrast to the IRSL signal
"
l l l l l l , l t l l ' l l l ' l l ' l l ' l l
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ii.
0
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, I , I I , l l l l , , , , I , J , , l l , , ,
0
50
100
150
200
250
Temperature (~
Fig. 5.58. Equivalentdose for a sedimentaryK-feldspar measured as a function of the stimulation temperature for IRSL used in the SAR protocol. The expected dose level is based on the historical age of the specimen (300 ___20 yr) and the measured dose rate (from Poolton et al., 2002b).
Optically Stimulated Luminescence Dosimetry
192 (a)
25000-
Blue Emission
CO
o
9
20000-
~
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Temperature (~ (b) 4000 -
Yellow Emission I~'q..
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.
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b
(0 0 v
:9
-
2000
0
1000
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100
200
300
400
Temperature (~
Fig. 5.59. TOL measurements of TL and IRSL (for 0.1 s every 10~ when heating at 4~ for a microcline sample given a 2.2 kGy dose 2 years previously (a) obtained using blue luminescence and (b) using yellow emission (from Duller, 1997).
for laboratory-irradiated grains (Fig. 5.57b), which shows the same increase with temperature as seen following natural irradiation (Fig. 5.57a). Since the temperature dependence of the OSL signal is affected by the degree of shallow trap filling, whereas the IRSL signal is not, this indicates that the OSL process involves transport via the conduction band, as opposed to a localised transition model for IRSL. The temperature dependencies in each case are, therefore, dominated by different phenomena, viz., hopping transportation for IRSL (Poolton et al., 2002a) and trapping/de-trapping in shallow traps
193
OSL Properties of Natural Materials
xlO 3 140
(a)
1207u~ t,--
lOO-
::3 0
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t.--
m
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150
250
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Temperature (~
x 103 400
(b)
300t.0 o o r
200-
escence
0
E _J
100-
0
50
150
250
350
450
Temperature (~
Fig. 5.60. TOL measurements of TL and OSL (for 0.1 s every 10~ when heating at 10~ for sedimentary K-feldspar with optical stimulation at 420-550 nm: (a) naturally irradiated, (b) given an additional dose of 18 Gy (from Duller and BCtter-Jensen, 1993).
for OSL. Temperature dependencies of this type were discussed in Section 2.4.5 and illustrated in Fig. 2.14. This behaviour also contrasts with that obtained for quartz w h e n stimulated with blue/green light ( 4 2 0 - 5 6 0 nm) (Duller et al., 1995). For quartz, the OSL decreases
Optically Stimulated Luminescence Dosimetry
194 70
60
.~
50 o x
40
o _J
30
v
O
~ e 250~ 200~
S 150~ f O l 0~ 50oc
20 10
|
50
|
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|
|
150
200
250
300
350
IRSL (cps x 103) Fig. 5.61. OSL versus IRSL for the natural signals from 50 to 250~ using data from Figs. 5.57 and 5.60 (from BCtter-Jensen, 2000).
monotonically as it is heated from room temperature to 250~ the consequence of thermal quenching (see Section 5.1.9.1). Although, the data of Duller et al. (1995) were obtained for quartz and feldspar grains that had similar OSL signal intensities at 50~ the changes in OSL output as the samples are heated could be used to provide information on the relative proportions of quartz and feldspar present in the mixture. The lack of an OSL signal from quartz when measured at stimulation temperatures above 250~ implies that OSL measured above 250~ is likely to be dominated by the signal from feldspars. 5.2.2.2.2. Decay curve shape. McKeever et al. (1997a) measured both IRSL and OSL decay curves for feldspars for stimulation carried out over a range of temperatures. When plotting normalised IRSL decay curves, it can be seen that the initial part of the curve became steeper with increasing stimulation temperature (Fig. 5.62a). Similar data were presented by Poolton et al. (2002b). For the equivalent OSL measurements McKeever et al. (1997a) found that there was no change in the decay curve shape (Fig. 5.62b) and concluded that unlike IRSL, OSL production in feldspars does not involve a process of thermal activation. However, the absolute signal increases with temperature, as can be seen for the integrated OSL signals (inset to Fig. 5.62b). This increase corresponds to the increase seen in the TOL measurements using blue/green light stimulation (Fig. 5.60).
5.2.3. Linear modulation IRSL Linear modulation IRSL (LM-IRSL) of irradiated feldspar grains from a heated sediment was first observed by Bulur (1996) and this study was followed by the presentation of LM-IRSL data for both potassium- and sodium-rich feldspars (Bulur and
195
OSL Properties of Natural Materials 1.0
I
I
I
I
(a) 0.8
..J
co O 0.6
50~
"O N
100~
E ,.- 0.4 o z 0.2
o.o
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I
I
I
20
40
60
80
100
Time (s) 1.0
0.8 u~
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z
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I
i
I
I
5
10
15
20
Time (s) Fig. 5.62. (a) IRSL decay curves obtained for IR (880 nm) stimulation at temperatures from 50 to 200~ for a feldspar sample given 83.3 Gy and then pre-heated at 220~ for 10 s. (b) OSL decay curves obtained for green light stimulation at temperatures from 50 to 225~ for a feldspar sample given 62 Gy and then pre-heated at 220~ for 10 s. Both sets of curves are normalised to the initial intensity (from McKeever et al., 1997a). Inset in (b) shows the integrated (0-20 s) OSL intensity as a function of temperature.
196
Optically Stimulated Luminescence Dosimetry
G6ksu, 1999). The LM-IRSL curves were obtained for grains with sensitivities that had been stabilised by repeated heating and irradiation, and given a laboratory irradiation and then heated to 200~ for 5 min to remove thermally unstable components. The LM-IRSL curves for sodium and potassium feldspars appear similar and both result in three firstorder peaks when standard curve fitting procedures were applied (Fig. 5.63). However, the peak maximum for the LM-IRSL from the sodium feldspar occurs at 76 s, compared with
=.,
6
_
..
(a)
K-Feldspar o Measurement Y
~
~4
-
S
-
S 1
......
S 2
. . . .
S 3
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'
+S2+S
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8 3 82 g. 1
,
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9
\,
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0 I
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....i -
,
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' .... -I--
100
'
i-
,
,
I
200
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'
'
I
300
~
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'
400
Time (seconds) 12 (b)
.._.,10-
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Na-Feldspar
r
o
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--s,,s~+s~
5
. . . . .
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==6
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I
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'
'
'
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1O0
'
'
~
'
I ' ""'
200
'" -'
'
I
300
'
'
'
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400
Time (seconds) Fig. 5.63. LM-IRSLcurves obtained with linearly increasing excitation energy over 400 s. (a) K-feldspar and (b) Na-feldspar. Also shown are the three components deduced by curve fitting (from Bulur and G6ksu, 1999).
OSL Properties of Natural Materials
197
91 s for the potassium feldspar. This implies that the IRSL of the sodium feldspar decays more rapidly, possibly due to a larger photoionisation cross-section at this wavelength. Bulur and G6ksu (1999) determined the dose response curves for the three components of each sample. The position of the peaks did not move as increasing doses up to 180 Gy were given, indicating that each component follows first-order kinetic behaviour. Over this dose range, each component of the sodium feldspar was more linear than that for the potassium feldspar. Bulur and G6ksu (1999) also measured the LM-IRSL at a variety of elevated temperatures, from 30 to 150~ and obtained thermal activation energies for each of the three peaks (see Section 5.2.9.2). 5.2.4. Pulsed OSL and IRSL
5.2.4.1. Pulsed OSL Sanderson and Clark (1994) obtained time-resolved spectra (with stimulation at 470 nm) for a potassium feldspar standard from the International Atomic Energy Agency and a feldspar from a sample of volcanic lava. They observed peaks in the spectrum, occurring at a few hundred nanoseconds, about a microsecond and several tens of microseconds. 5.2.4.2. Pulsed IRSL Clark et al. (1997) selected 850 nm stimulation light from a tuneable IR laser and observed the luminescence emitted after the end of the pulse. These time-resolved measurements were made at temperatures ranging from 25 to 100~ and the luminescence was observed at several different emission wavelengths selected using colour glass filters. No peaks were observed, but multi-exponential non-linear regression analysis suggested the presence of up to five exponentially decaying components with different lifetimes, ranging from 30 ns to more than 15 txs. This study was extended by Clark and Bailiff (1998) who used a set of bandpass interference filters to obtain greater resolution for the CW-IRSL emission spectrum, before measuring the time-resolved luminescence emitted at 550A40 nm. The 560 nm emission has previously been linked to electronic transitions in Mn 2+. However, intrinsic Mn 2+ transitions are known to have lifetimes --~ms these are long compared with the lifetimes of 22 ns up to 164 l~s found when a dwell time of 5 ns per channel was used in the time-resolved IRSL measurements (Fig. 5.64). 5.2.4.3. Optically stimulated afterglow OSA is the luminescence observed following a pulse of photons and is caused by retrapping at, and subsequent release from, shallow traps. Htitt et al. (1999, 2001) and Jaek et al. (1999) used a xenon lamp and monochromator to obtain selected wavelengths from 250 to 900 nm for stimulation. A shutter system was employed to obtain a pulse of 2 s. The OSA is observed at least 0.05 s after the stimulation pulse ends using a near-UV (--- 330 nm) filter. For measurements made on a Cu-doped single crystal of microcline, an emission peak was found in the near IR around 880 nm. An increasing OSA signal strength was observed as the wavelengths were varied from 750 to 360 nm (Fig. 5.65) and a stimulation peak was seen near 360 nm. The OSA measurements enable the stimulation response to be observed in the region of the spectrum where the luminescence is being detected. These data can be compared with the bleaching response spectrum for IRSL
Optically Stimulated Luminescence Dosimetry
198
l0 s ra~
0
r~
10 2
o
4,-.'
0
9 .
.: ..-.-
:..........
l01
10 ~
.
0
10
.
.... . :-. : : .-: . . . . . . . . . .
20
30
._-,.., . . .
._.,
40
50
Time / Fig. 5.64. Time-resolved luminescence from a sample of Amelia albite observed at 550A40 nm and obtained for pulsed stimulation at 850 nm and using a 5 ns dwell time (from Clark and Bailiff, 1998).
15
>,
OSA
~
--
1
cq.
8 g
ii/
o
},,\ i ,\
i\ i i
~
tr
',~, ;\
~
:
_
:
~
,,
\ \
/,, /,
,
,~I
~, \~
,
" 0
0
I 300
'
I 400
'
I 500
'
I 600
'
Wavelength,
I 700
'
I 800
;
I 900
'
I 1000
nm
Fig. 5.65. OSL and OSA stimulation spectra obtained for a microcline crystal showing an IR stimulation peak at 880 nm and an OSA stimulation peak at 360 nm (redrawn from Hiitt et al., 2001).
OSL Properties of Natural Materials
199
(and OSL) (see Section 5.2.5.2) and the direct measurements of the IRSL excitation spectra (see Section 5.2.5.1). 5.2.5. Excitation spectra 5.2.5.1. Direct m e a s u r e m e n t s
In the visible part of the spectrum, the stimulation characteristics are similar to those of quartz, with the OSL signal intensity increasing rapidly as the energy of stimulation is increased. Htitt et al. (1988) and Godfrey-Smith et al. (1988) showed that luminescence could be stimulated from feldspars using wavelengths in the visible and in the near-infrared. Using a variety of laser lines, Ditlefsen and Huntley (1994) observed that the luminescence response of a K-feldspar extract from a sediment was not a linear function of stimulation energy, when the signal intensity was plotted on a log scale. They concluded that two traps contributed to the OSL signal. Htitt et al. (1999) obtained the OSL stimulation spectra for potassium-rich feldspars extracted from sediments and single crystals of microcline. In order to improve detection efficiency, the luminescence sensitivity was increased by doping with activators such as T1 (Jaek et al., 1997a) or Cu (Jaek et al., 1997b). Electrodiffusion, particularly of Cu, can be carried out at relatively low temperatures (--- 550~ and does not appear to alter the crystal structure. In Fig. 5.65, the OSL observed using a near-UV (---330 nm) filter is shown for stimulation from 1040 to 450 nm (Htitt et al., 2001). Bctter-Jensen et al. (1994b) obtained high-resolution stimulation spectra from museum specimens (Fig. 5.66a) and sedimentary feldspars (Fig. 5.66b) using a continuous scanning monochromator together with a tungsten halogen lamp (see Section 7.5.1). The luminescence was detected in the near UV (--~340 nm) as the stimulation wavelengths were varied from 400 to 1000 nm. In the visible region, the dominant feature is a steeply rising continuum, but between 450 and 650 nm there is evidence of stimulation peaks at 500 and 600 nm for the albite samples (Fig. 5.66a). Peaks at these wavelengths are also visible for the feldspar separates (Fig. 5.66b). Clark and Sanderson (1994) also used a scanning monochromator, together with a xenon lamp to study a selection of feldspars. They detected the luminescence in the wavelength region 300-400 nm. Excitation bands occurred at 500-540, 550-650 and 800-1000 nm; the relative strengths of these contributions varied amongst the 20 feldspars that were measured. Clark and Sanderson (1994) also measured the excitation spectra after the samples had been heated for 30 s at temperatures from 150 to 440~ in 50~ steps. The whole excitation spectrum was reduced progressively, but by 400~ only the excitation band at 500-540 nm remained. It is this component which was identified as having a measurable thermal activation energy (see Section 5.2.9.2). Both data sets show that besides the monotonic decrease in OSL with increasing wavelength, there is a peak for stimulation in the near infra-red (--~880 nm). Other studies, using a tuneable laser (Bailiff, 1993; Bailiff and Barnett, 1994; Barnett and Bailiff, 1997a), show a peak at about 855 nm (1.46 eV). Barnett and Bailiff (1997a) were able to show that this peak is well defined in a number of different types of feldspar, e.g., albite, oligoclase, orthoclase and sanidine. For some potassium-rich feldspars they also found a weaker
200
Optically Stimulated Luminescence Dosimetry
0.8 ~D 0.6 __J oo 0.4 O 0.2
~l
0 400
AMELIAALBITE ANORTHITE / / ~ ~ )
500
600 700 800 STIMULATION WAVELENGTH[nm}
900
(b)
/----PI.F
0,8
1000
d 0.6 -0.4 0 0.2 I..I..I oO
0 40O
500
600 700 800 STIMULATION WAVELENGTH(nm)
900
1000
Fig. 5.66. OSL stimulation spectra (corrected for power density at sample) observed with a Hoya U-340 filter; (a) museum specimens of the feldspars anorthite and albite, (iii) showing the difference between the measured spectrum for albite (i) and the continuum (ii) on an expanded scale; (b) sedimentary feldspar separates, with K.F* showing the difference between the measured spectrum for K.F and the continuum on an expanded scale (from BCtter-Jensen et al., 1994b).
stimulation peak at higher energies (---745-800 nm), whereas BCtter-Jensen et al. (1994) observed a stimulation peak in a sample of anorthite at 970 nm (Fig. 5.66a). These stimulation spectra have led to the use of IR LEDs emitting at 880A80 nm and a laser diode emitting at 830 nm for IRSL measurements in dating studies (see Sections 7.4.2 and 7.4.3). Stimulation in the near infra-red region enables detection of IRSL with a wide window in the visible region of the spectrum. However, using a filter with a broad transmission band, such as the Schott BG39 (see Fig. 7.2b), to observe the IRSL may lead to complicated responses since several emission bands will be observed (Rieser et al., 1997; see Section 5.2.6). A summary of published stimulation peaks and mineral types was authored by Krbetschek et al. (1997), together with the detection wavelength region used for the measurements. In a study of museum specimens of feldspars and feldspar-dominated sediment extracts, Godfrey-Smith and Cada (1996) found single stimulation peaks. The peaks occurred at 845 nm for all microclines, most plagioclases and all sedimentary feldspars and at slightly lower wavelengths (840 nm) for two nearly pure Na plagioclases. This led Godfrey-Smith and Cada (1996) to suggest the use of a semi-conductor diode laser tuned to 845 nm, instead of 880 nm LEDs for feldspar dating. This would provide increased photoeviction efficiency for microclines, feldspars that are less likely to be affected by anomalous fading (see Section 5.2.10).
OSL Properties of Natural Materials
201
Links between the IR stimulation peak and chemical composition were explored by Poolton et al. (1995a). They found that whilst sodium and potassium feldspars had similar stimulation spectra, calcium-containing feldspars exhibited a range of values for the stimulation peak wavelength. For low temperature, more ordered, plagioclase feldspars there was a direct correspondence between the OSL intensity and anorthite content.
5.2.5.2. Bleaching response spectrum Measurements of optical bleaching of the IRSL of four feldspar samples were carried out by Bailiff and Poolton (1991). They observed a resonance peak between 850 and 900 nm. Spooner (1994b) studied the effect of optical bleaching on the IRSL signal (stimulated at 880z~80 nm) using 28 interference filters to provide narrow wavelength bands ranging from 400.6A10.1 to 1065.3A15.0 nm. Fig. 5.67 shows, for a specimen of microcline, the energy required at each wavelength to reduce the IRSL signal by a given percentage. The IRSL was measured using a broad-band Schott BG39 filter. It can be seen that the amount of energy required to reduce the IRSL signal decreases rapidly with decreasing wavelength. In addition, a resonance can be seen at about 860 nm. 5.2.6. Emission spectra OSL emission spectra obtained from feldspars were published by Huntley et al. (1989) when stimulation was carried out using the 514 nm line from an argon laser and the 633 nm line from a H e - N e laser. Subsequently, IRSL spectra were obtained by Huntley et al. (1991) for IR diode stimulation (880A80 nm) and stimulation with the 647 nm line from a krypton laser. These authors reported that most feldspars showed emission bands at 330, 400 and 570 nm.
5.2.6.1. IRSL emission spectra BCtter-Jensen et al. (1994) used a small scanning monochromator to measure the IRSL emission spectrum from 370 to 640 nm for a museum specimen of albite and for feldspar extracts from sediments. They found two main emission bands centred at 400 and 550 nm, with the blue signal being dominant for the sedimentary feldspars. Krbetschek et al. (1996) reported emission bands at 280, 330, 410, 560 and 700 nm for feldspars from a number of museum specimens and sedimentary feldspars. Using IR as the optical stimulation source means that it is possible to observe the emission spectrum throughout the visible and near UV regions of the spectrum. Krbetschek et al. (1997) provided a comprehensive review of luminescence emission from feldspars, including TL and CL spectra as well as IRSL spectra. 5.2.6.1.1. 280-290 nm (near UV). The IRSL spectrum of alkali feldspars was studied by Clarke and Rendell (1997a), following a previous study of the TL and IRSL emission spectra for sedimentary feldspars (Rendell et al., 1995). Of particular interest are the results for the 280-290 nm emission, seen in laboratory-irradiated feldspars, but not in naturally irradiated feldspars, except for some from the low-temperature environment of Antarctica (Krause et al., 1997). The short thermal stability of this signal has been investigated by Clarke and Rendell (1997b). They found that the signal was removed by pre-heating at 220~ for 5 min after irradiation. Thus, it is removed by the typical pre-heat used in feldspar dating, and its effect can also be avoided by using a filter that does not
Optically Stimulated Luminescence Dosimetry
202
106 ~
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permit transmission of wavelengths from 280 to 290 nm. However, Clarke and Rendell (1997b) also showed evidence for changes in the magnitude of other luminescence signals as a result of application of this pre-heat.
5.2.6.1.2. 320-340nm (near UV). This emission is seen primarily in sodium-rich plagioclase and alkali feldspars, but not in calcium-rich feldspars (Krbetschek and Rieser, 1995; Krbetschek et al., 1997). However, using colour glass filters in dating studies, it is difficult to measure in this wavelength region, in isolation from the 280 and 390-440 nm emissions.
OSL Properties of Natural Materials
203
5.2.6.1.3. 3 9 0 - 4 4 0 nm (violet~blue). Emission in this range is found for all feldspars, with emission in the region 400-410 nm being the most common (Krbetschek and Rieser, 1995; Krbetschek et al., 1996). For potassium-rich feldspars, this spectral component is usually dominant for specimens from mineral collections (Huntley et al., 1991) and from sediments (Jungner and Huntley, 1991; Wiggenhom and Rieser, 1996). However, the latter was not found to be the case for the sediments examined by BCtter-Jensen et al. (1994). Krbetschek et al. (1997) suggested that additional peaks at 390 and 430 nm may be present, and that some combination of these three emission bands may relate to the signal observed when dating is carried out with violet/blue colour glass filters. Duller and BCtter-Jensen (1997) measured the emission spectrum as a function of stimulation temperature for IRSL stimulated at 880 nm. For the dominant emission peak at 400 nm (3.0 eV) they found a small, but consistent, shift of the peak emission energy to higher energies (from 2.992 to 3.015 eV) as the temperature was increased from 50 to 400~ They concluded that this small shift would have no effect on measurements of the thermal activation energy of the IRSL. 5.2.6.1.4. 5 5 0 - 5 7 0 nm (yellow/green). IRSL emission at 560 nm has been found in nearly all feldspars (Krbetschek et al., 1997). It will be observed, together with the violet/blue emission, when a filter such as the Schott BG-39 is used (Fig. 7.2b). IRSL emitted at 560 nm is bleached more quickly by daylight than the IRSL emitted at 410 nm (Krause et al., 1997). However, Krbetschek et al. (1996) provided evidence that the 560 nm IRSL signal was less thermally stable than that measured at 410 nm and may thus be unsuitable for dating samples over 10,000 years old. 5.2.6.1.5. 6 0 0 - 7 5 0 nm (red~far red). Krbetschek et al. (1996) reported IRSL in the red (690-750 nm), but could not obtain conclusive data because of interference from the IR diode stimulation source (880A80 nm). Poolton et al. (1995b) measured the OSL in the red (600-700nm) and found that it exhibited strong thermal quenching above room temperature (see Section 5.2.9.1). Studies by Fattahi (2001) suggest that it is possible to select detection filters and a photomultiplier tube to enhance detection of the far-red signal from natural feldspars. The advantage of this IRSL signal for dosimetry is that it appears not to exhibit anomalous fading, unlike the IRSL signals observed at other emission wavelengths. 5.2.6.2. TL emission spectra There have been many studies of TL spectra for feldspars from mineral collections, as well as for feldspars separated from sediments and volcanic rocks (e.g., Zink et al., 1995). A review is beyond the remit of this book and the reader is recommended to read the comprehensive review by Krbetschek et al. (1997). 5.2.6.3. RL emission spectra 5.2.6.3.1. Under X-ray stimulation at low temperature. As part of the study to understand IRSL production in feldspars at temperatures below room temperature (Bailiff and Barnett, 1994), Barnett and Bailiff (1997b) investigated the prompt luminescence during X-irradiation (RL) at temperatures from 80 to 300 K. Three detection windows were usedmblue/UV (350-500nm), green/orange (500-580nm) and red (580-610nm).
204
Optically Stimulated Luminescence Dosimetry
Of the six feldspars studied, all except one (a sanidine) showed a decrease in blue/UV RL as the temperature was raised from 90 to 200 K; thereafter the RL remained constant until 300 K. This can be explained in terms of thermal quenching (see Section 5.2.9.1). The quenching was observed to be higher for the potassium-rich feldspars (orthoclase and microcline microperthite), than for the plagioclases (albite, labradorite and oligoclase-the latter two having roughly equal Na and Ca contents). These data sets (Fig. 5.68) were used to obtain the thermal activation energy for quenching, W, by fitting to the equation r / - 1/(1 + C exp (-w/~)) given in Section 2.4.6, Eq. (2.64), where r/ is the luminescence efficiency, C a dimensionless constant, k the Boltzmann' s constant and T the absolute temperature. For the orthoclase and microcline microperthite, the values of W were 0.064 and 0.061 eV, respectively. For both the labradorite and the oligoclase, the value of W was 0.043 eV. Barnett and Bailiff (1997b) compared these values for the thermal quenching activation energy with the energies of the IR absorption bands for feldspars, which are in turn related to lattice vibrations related to the S i - O bonds. They concluded that in labradorite and
10
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Fig. 5.68. Temperaturedependence of blue/UV RL resulting from X-irradiation for two samples of orthoclase (ff], II), oligoclase (~) and labradorite (~). Data fitted with equations for thermal quenching (from Barnett and Bailiff, 1997b).
OSL Properties of Natural Materials
205
oligoclase, thermal quenching is correlated with deformation of S i - O - S i or S i - O - A 1 bonds, while, in albite, it is correlated with stretching of the N a - O bonds. As in the case with quartz (see Section 5.1.9.1), thermal quenching will affect measurements of thermal activation energies (see Section 5.2.9.2) in the temperature region that is affected by thermal quenching. 5.2.6.3.2. Under X-ray stimulation above room temperature. Rendell and Clarke (1997) obtained RL spectra for a suite of well-characterised alkali feldspars. They reported that the RL, TL and CL, emission spectra were similar for each of the samples. The relative strengths of the emission bands at 290, 340, 380-450, 550 and 700 nm varied. In a related study, Clarke et al. (1997) showed the RL spectra for perthitic feldspars (potassium feldspar with separated phases of sodium feldspars) to be dominated by the emission from 380 to 450 nm, although there was also a contribution at 290 nm. 5.2.6.3.3. Under beta stimulation from a t~7Cs source. Spectra have also been obtained for the prompt luminescence observed when feldspars are stimulated using a 137Cs internal conversion (beta) source, which forms part of the RL dating instrument (see Section 7.10 and Fig. 7.16). Spectra were obtained using a spectrometer (200-800 nm) linked to a liquid nitrogen cooled CCD detector. Trautmann et al. (1998) investigated about 15 feldspar samples from a museum collection and a sedimentary sample of labradorite. The RL spectra were collected over 600 s for irradiated and non-irradiated (thermally emptied) samples, shown in Fig. 5.69 as light and dark lines, respectively. The potassium feldspar, microcline and orthoclase, show very strong IR emissions. Other emission bands can be seen at about 3.0 eV (410 nm) for all samples and at 2.2 eV (560 nm) and 2.7 eV (330 nm) for the albite and plagioclase feldspars (Fig. 5.69). A red emission at 1.7 eV (730 nm) can also be seen, though this is unstable at room temperature, as demonstrated when an external beta source was used to give a laboratory dose. More detailed spectral measurements from 600 to 1000 nm have also shown that this emission peak at 700 nm increases its relative intensity and interferes with the 865 nm emission when a 500 Gy dose is added (Krbetschek et al., 2000). This effect is particularly pronounced since the IR peak intensity decreases when the dose is added and the red peak increases. Although, this peak has been shown not to be stable at room temperature (Trautmann et al., 1998), decaying in a matter of minutes after RL stimulation is stopped, it needs to be avoided in routine RL dating. This is particularly true if using an IR-sensitive photomultiplier or avalanche photodiode detector as proposed by Krbetschek et al. (2000). In this case, a narrow band interference filter would be needed in front of the detector to reduce the effect of red emission. 5.2.6.4. Photoluminescence emission spectra Photoluminescence (PL) is also observed for feldspars. These PL signals do not require exposure to ionising radiation and do not decay with time during optical stimulation (see Section 1.1). Under UV (340 nm) stimulation, two main emission bands have been observed, corresponding to internal transitions in the transition metal ions Mn 2+ and Fe 3+. The Mn 2+ emission usually occurs in the yellow/green (--- 560 nm) with the exact energy having little dependence upon mineralogy. Mn 2+ is often found to be the dominant luminescence defect relating to OSL. This information is derived from cathodoluminescence (CL) (G6tze et al., 1999) and optical absorption measurements (White et al., 1986).
0 0
0 0
8 8
0 0
8 X
0 0
sgun .qae I ma
8 P
Optically Stimulated Luminescence Dosimetry
8
m o
0 0
sgun eq.181
8 Z
0 0
g a g w a s
OSL Properties of Natural Materials
207
The Fe 3+ emission occurs in the far-red (around 750 nm), with the exact wavelength depending upon the chemical composition of the feldspar (White et al., 1986; Brooks et al., 2002). The PL emission peaked at 745 nm for the alkali feldspars investigated by Poolton et al. (1996). This Fe 3+ emission was found to have two stimulation resonances, at 2.95 (420 nm) and 2.75 eV (450 nm). These resonances can be related to two transitions from ground to excited states in Fe 3+. An additional resonance at 1.9 eV (650 nm)is observed for alkali feldspars, but was absent from the calcium-rich plagioclase feldspars, thus providing information on mineral identification. Based on a comparison of OSL data and PL data (energy separations), Poolton et al. (1996) concluded that the Fe 3+ and OSL defects were to be found in the same part of the crystal. Under visible light stimulation, Poolton et al. (1995b) showed that most feldspars showed strong luminescence in the far-red. Under UV stimulation, this luminescence was obscured by broad luminescence emission resulting from other transitions, thought to arise from direct recombination between the conduction band and the ground state of the defect. For one sample of alkali feldspar this was not the case, and thermal quenching of the 780 nm PL was observed when stimulation at 340 nm was carried out at temperatures from room temperature to 300~ (Fig. 5.70a). The activation energy was determined to be W -- 0.34 eV. Poolton et al. (1995b) considered that this energy was too large for lattice vibrations. In a plagioclase sample, PL emission from both Fe 3+ and Mn 2+ was observed. The Fe 3+ emission at 780 nm showed thermal quenching, but the Mn 2+ emission at 560 nm was relatively unaffected by raising the temperature from 17 to 227~ (Fig. 5.70a). As previously mentioned, UV (340 nm) stimulation tends to produce broad PL spectra that span the visible region (Fig. 5.70b). These broad-band emissions are also thermally quenched in the temperature region from room temperature to 400~ When observed at 500 nm, the two samples shown in Fig. 5.70b have energies for thermal quenching of W = 0.14 and 0.05 eV. These are close to the energies of the lattice vibration modes. 5.2.7. Effects of previous optical treatment 5.2.7.1. Bleaching at ambient temperature The effects of IR (875A80 nm) and green light (515-560 nm) on both the IRSL and OSL (stimulated at 515-560 nm) signals are shown in Fig. 5.71a,b, respectively (Duller and BCtter-Jensen, 1993). Also shown are the effects of the same exposures on the two main TL peaks at 270 and 330~ Under IR exposure, the OSL signal falls less rapidly than the IRSL and at the end of the experiment (after 1000 s IR exposure), a larger fraction of the initial OSL signal remains, 13% compared with 0.3% of the IRSL. Both TL peaks are reduced, but by no more than 10% which occurs in the first 10 s of IR exposure. Under green light illumination, a greater reduction in the TL signals occurred and both IRSL and OSL decayed at more similar rates than under IR illumination. Duller and BCtter-Jensen (1993) concluded that green light was able to evict charge from traps that remained un-emptied by IR. Galloway (1994) also examined the effect of IR stimulation at room temperature on the OSL and found a similar level of depletion (--~10%) for extended periods of IR exposure. The same room temperature exposures did not affect OSL from
Optically Stimulated Luminescence Dosimetry
208
::j
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i
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l
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i i i i I 400
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I I I I I I I I I I I
500
Wavelength
600
700
(nm)
100
80 :3
60
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40
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i 500
Wavelength
600
700
(nm)
Fig. 5.70. (a) PL emission spectra for a plagioclase feldspar stimulated at 340 nm and recorded at the temperatures stated. The Mn2+ and Fe3+ emission bands are identified (from Poolton et al., 1995b). (b) Room temperature PL emission spectra stimulated at 340 nm for samples of albite (R34) and adularia (R28) (from Poolton et al., 1995b). quartz and this feature was used by Liritzis et al. (1997) to reduce the effect of any feldspar contamination in quartz OSL dating. 5.2.7.2. IR bleaching at elevated temperature Jain and Singhvi (2001) investigated the relationship between OSL (stimulated using blue/green 4 2 0 - 5 6 0 nm light) and IRSL (880A80 nm). When IR is applied during a preheat, the OSL and IRSL signals are depleted at different rates, depending upon the temperature of the pre-heat, leading Jain and Singhvi (2001) to conclude that there were two distinct trap populations. Fig. 5.72a shows the percentage OSL remaining as a function of the percentage IRSL remaining following IR stimulation carried out at 100, 150 and 220~ for a laboratory-irradiated sample of orthoclase. Fig. 5.72b shows the reduction in OSL as a function of IR stimulation time at the temperature indicated. Rapid depletion occurs in the first 100 s, but continued loss is seen for longer exposure times, particularly at higher temperatures (e.g., 220~ Jain and Singhvi (2001) concluded that
209
OSL Properties of Natural Materials
(a) ~
rL
.=_ .=_ E
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-
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~ rm i' ' i
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(b)
A v
TL
c ..,.. r .D
E
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0
10
10 2 10 3 Green light exposure (s)
10 4
Fig. 5.71. Reductionin luminescence signals, TL (at 270~ /~ and 330~ x ), OSL (4') and IRSL (D), as a function of exposure time to (a) IR (875Anm, 42 mW/cm2) and (b) green light (515-560 nm, 6.5 mW/cm2) (from Duller and B~tter-Jensen, 1993).
elevated temperature infra-red (ETIR) exposure at 220~ could reduce the OSL signal in feldspars, thus leading to improved selectivity in measuring the OSL from a sample of quartz contaminated with feldspar. However, Bailey (1998) reported a 12% depletion in quartz OSL following IR stimulation for 300 s at 200~ Hence, a lower temperature ETIR may be more appropriate.
Optically Stimulated Luminescence Dosimetry
210
K-Feldspar OMS
lOO
~
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.
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09
(.9 133 tr
- , - 30oc 100~ -o- 150~ 220~
10
.--.,
E (D
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1
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200
400
600
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Time (sec) Fig. 5.72. (a) Reduction in OSL (stimulated at 420-560 nm) as a function of the reduction in IRSL (stimulated at 880A80 nm) observed after exposing to IR at the three temperatures shown for 100, 300, 500 and 700 s. (b) Reduction in OSL as a function of IR stimulation time at the temperatures given. The orthoclase sample was given a 30 Gy dose and then pre-heated before IR exposure (from Jain and Singhvi, 2001).
Similar measurements of OSL (470 nm stimulation) as a function of IR stimulation time at 50, 125 and 225~ were made by Poolton et al. (2002b). They compared their experimental data with the effects of such bleaching predicted by their model of luminescence production in feldspar. They predicted that the rate of bleaching with IR is increased at higher stimulation temperatures because more recombination sites are accessed at elevated temperature. This has implications for the choice of IRSL measurement temperature for dating (see Section 5.2.2.2.1).
OSL Properties of NaturalMaterials
211
5.2.8. Effects of previous thermal treatment The OSL and IRSL signals of feldspars are not derived from the same set of traps, as shown by their different responses to IR exposure (see Section 5.2.7). In addition, the optical decay curves for each are not exponential, suggesting that there is more than one component in each signal. For these reasons, isothermal decay curves have not been successful in the investigation of thermal stability (Trautmann et al., 1997). There is also evidence from the TOL curves that shallow traps contribute to the OSL signal (see Section 5.2.2.2), but not to the IRSL. When using feldspars for dating, it is important that a thermally stable signal can be isolated.
5.2.8.1. Pre-heating of laboratory and naturally irradiated samples One way of investigating the thermal stability is to compare the effect of progressively higher pre-heats on laboratory and naturally irradiated samples. Li (1991) measured the responses for laser (514 nm) stimulated luminescence from feldspars, and suggested that once the ratio between the two samples was constant, a thermally stable component had been isolated. Duller and BCtter-Jensen (1993) carried out similar experiments for TL (using the signal integrated from 0 to 450~ OSL (stimulated at 515-560 nm) and IRSL (stimulated at 875A80 nm) with 10 min pre-heats at a range of temperatures up to 250~ Fig. 5.73 shows that the depletion is greatest for the TL signal for which there was a pronounced peak at 150~ The OSL signal reaches a constant value after pre-heats of 200~ or higher, thus confirming the contribution to the OSL signal from shallow traps. No such effect is seen for IRSL, suggesting that it is derived from a trap that is thermally stable on the timescale of a few tens of thousands of years, the depositional age of the sediment
_
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i
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200
-
!
'
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Pre-heat temperature (~ Fig. 5.73. The ratio of (N + 36 Gy)/N signals (A, TL integrated from 0 to 450~ El, IRSL and , , OSL) as a function of pre-heat temperature (from Duller and BCtter-Jensen, 1993).
212
Optically Stimulated Luminescence Dosimetry
from which the feldspars were separated. This result suggests that pre-heating after laboratory irradiation should not be necessary in order to isolate a thermally stable signal.
5.2.8.2. Pulse annealing Instead of isothermal decay measurements, the effect of heating samples of feldspar to progressively higher temperatures has been employed. In this procedure, known as pulse annealing, the OSL or IRSL signal is measured at a fixed temperature, e.g., 50~ BCtter-Jensen et al. (1993) showed such curves for the OSL and IRSL of the same sedimentary feldspar that had been given a laboratory dose to induce a TL peak at 150~ (Fig. 5.74). Both signals appeared to relate to the emptying of a TL peak that was found at 270~ when the grains were heated at 8~ in contrast to the data shown in Fig. 5.73. However, the interpretation is complicated by having normalised the data to the initial measurement, for which the OSL will have an additional contribution relating to the 150~ TL trap. Similar plots of percentage signal remaining can be used to compare natural and laboratory-induced signals and the effect of pre-heats on these signals. Duller (1994) obtained data for the IRSL of two aliquots of sedimentary feldspar (D e --- 40 Gy), one of which had been given an additional dose of 45 Gy (Fig. 5.75a). Both IRSL signals remained constant up to 250~ indicating no contribution from low-temperature traps. However, there is an offset between the two data sets above this temperature, with the natural IRSL signal staying higher, suggesting that the laboratory-irradiated sample has a small contribution from a thermally unstable trap. The effect of this trap can be removed
120 100
[] +
[]
8O >-, o'} C
60
C _.1
or)
9
40
20
0
100
200
300
400
500
Temperature(~
Fig. 5.74. Percentage of the OSL and IRSL remaining as a function of pre-heat temperature for a natural geological sample given 10 Gy (from BCtter-Jensen et al., 1993).
OSL Properties of Natural Materials
213
(a)
.=-- 1 o o c
E
.~ 80t--
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--
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Temperature (~
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Natural N+45Gy N+ph N+45Gy+ph
"~arr-~ 9 c 4c~
2 _
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o-
s
0
100
200 300 Temperature (~
400
500
Fig. 5.75. (a) Pulse annealing curves for IRSL from sedimentary K-feldspars with pre-treatment as stated; preheat was 220~ for 10 min. (b) Data from (a) plotted as the percentage of the IRSL signal that is lost for each annealing run (from Duller, 1994).
by pre-heating aliquots at 220~ for 10 min. Following this treatment, both signals behave identically. This suggests that this pre-heat should be used when dating this sample. Additional information can be obtained by plotting the percentage of IRSL lost for consecutive measurements during the construction of each pulse anneal curve (Fig. 5.75b). Two components can be seen in the natural signal, with an additional component in the laboratory-irradiated sample. Li et al. (1997) obtained similar pulse anneal curves for the natural IRSL of a sedimentary K-feldspar, but using different heating rates (Fig. 5.76a). This results in the maximum loss in signal (expressed as percentage IRSL lost per ~ moving to higher temperatures with increasing heating rate (Fig. 5.76b). The relationship
Optically Stimulated Luminescence Dosimetry
214
12
] (a)
'~'~
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300
-i
i
~
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Temperature (~ 1.2
--o- 0.5 ~
(b)
4-- 1 ~
10.80.6 @
0.4 0.2
0
_0.2190
200
i
I
300
400
. . . . . . . . .
500
Temperature (~ Fig. 5.76. (a) Pulse annealing curves, obtained using different heating rates, for natural IRSL from sedimentary K-feldspars; pre-heat was 220~ for 10 min. (b) Data from (a) plotted as the percentage of the IRSL signal lost per ~ (from Li et al., 1997).
between this peak temperature and the heating rate can be used to determine the activation energy and frequency factor of the signal. Tso et al. (1996) applied this approach to the natural IRSL from K- and Na-rich feldspars extracted from the same sediment. Much lower trap depths were obtained for the Na-rich feldspars than for the K-rich samples, with predicted lifetimes at 10~ of 0.9 x 104 year for the Na-rich feldspar and 2.4 x 109 year for the K-rich feldspar. Li and Tso (1997) used a similar approach on IRSL and OSL signals from the same K-feldspars extracted from a sediment. They found the OSL signal to be two orders of magnitude less stable than the IRSL signal, with the predicted lifetimes at 10~ of 1.3 x 106 year and 6.6 x 108 year, respectively. It should be pointed out that this method of analysis is based on the signal behaving as a single trap with first-order kinetics.
OSL Properties of Natural Materials
215
5.2.8.3. Irradiation at elevated temperature In their study of the TL of feldspars, Mejdahl et al. (1992) suggested that the trap corresponding to the TL peak at 130~ competed for charge in the natural environment, but not during laboratory irradiation once it had been filled. They predicted an underresponse of the growth of the high temperature TL. Wallinga et al. (2002) investigated whether this TL trap competed with those traps giving the IRSL. For samples irradiated at a range of temperatures from 35 to 260~ no step was seen in the IRSL signal and the small steady change observed was not dose-dependent. The former piece of evidence suggests that the shallow trap does not compete and the steady change is probably due to small temperature-dependent changes in the trapping cross-section, as suggested for quartz (Wallinga et al., 2002).
5.2.9. Raised temperature IRSL and OSL 5.2.9.1. Thermal quenching Thermal quenching refers to the changing efficiency of luminescence production as a sample is heated (see Section 2.4.6). For measurements made at temperatures between 90 and 200 K, Barnett and Bailiff (1997b) observed thermal quenching of the blue/UV signal when they observed the prompt luminescence during X-irradiation (see Section 5.2.6.3). No change in luminescence efficiency was found from 200 to 300 K (room temperature). Above room temperature, effects due to thermal quenching are swamped by the effects of thermal assistance (see Section 5.2.9.2.1). However, Poolton et al. (1995b) used UV and visible light to stimulate Stokes-shifted PL from a range of well-characterised alkali and plagioclase feldspars from a museum collection. These experiments showed the thermal quenching behaviour of feldspars to be extremely complex in the temperature region from 20 to 400~ As discussed in Section 5.2.6.4, the activation energy for thermal quenching of the Fe 3+ emission is W = 0.34 eV. For the broad emission band observed under UV (340 nm) stimulation of most other feldspars, thermal quenching was observed, but the energies were smaller, ---0.1 eV. More complex behaviour was found for two samples characterised by single PL emission bands at 610 (red) and 435 nm (blue), emission bands commonly observed in the IRSL spectra. Both these showed thermal quenching to occur in two steps, with the higher temperature quenching processes being seen to be irreversible when the measurement cycle was repeated. The irreversible change is suggested by Poolton et al. (1995b) to be caused by the thermal destruction of the defect itself. Poolton et al. (1995b) demonstrated that the PL (340 nm stimulated) and IRSL (850 nm stimulated) spectra were similar, irrespective of whether narrow- or broad-band emission occurs. The similarity of the PL and IRSL behaviour is shown by the thermal quenching measurements made for both types of signal (Fig. 5.77). The PL data in Fig. 5.77 can be linked to PL data on orthoclase feldspar obtained by White et al. (1986). They observed a large ( x 80) increase in red luminescence efficiency as feldspars were cooled from room temperature to 78 K. The implication is that IRSL sensitivity will be increased if measurements are made below room temperature. Visocekas et al. (1994) observed a reduction factor of 10 in red-IR phosphorescence as samples previously irradiated at room temperature were cooled to 80 K (see Section 5.2.10.5).
216
Optically Stimulated Luminescence Dosimetry
120 ~__l
I
m
I
I
I
I
I
I
I
I
I
o
F
30
0 0
I
I
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m
60
I
440 nm
90 A
I
PL m
600-700 nm
m
100 Temperature
m
IRSL m
PL
200
300 (~
Fig. 5.77. IRSLobservedat wavelengthsof 440 nm (O) and 600-700 nm (m) from an alkali feldspar measured as a function of temperature and PL (340 nm stimulation) observedunderthe same conditions (fromPoolton et al., 1995b).
5.2.9.2.
Thermal assistance
5.2.9.2.1. Above room temperature. As shown in Fig. 5.57, thermal activation is the
dominant process, when IRSL is observed, on raising the temperature from room temperature to about 200~ Indeed, Poolton et al. (1995b) concluded that the thermal activation energies obtained were hardly affected by the weak thermal quenching found in this temperature region. Bailiff and Poolton (1991) determined a thermal activation energy of 0.10 + 0.02 eV when stimulating using monochromatic IR at 930 nm. Duller and Wintle (1991) calculated a value of 0.15 _+ 0.02 eV for IRSL measured above room temperature for their particular feldspar samples. Htitt and Jaek (1993) measured the thermal activation energy as 0.2 +_ 0.1 eV in the temperature range from room temperature up to 200~ Clark and Sanderson (1994) measured the thermal activation energies for the three excitation bands that they obtained with their scanning monochromator (see Section 5.2.5.1). They obtained 0.2, 0.17 and 0.15 eV, respectively, for stimulation at 910, 624 and one component at 500 nm. 5.2.9.2.2. Below room temperature. As part of their study of IRSL stimulated from
orthoclase at temperatures between 80 and 300 K, Bailiff and Barnett (1994) found a component that had a thermal activation energy of 0.10 eV for temperatures above 220 K. In the temperature region from 80 to 140 K, Bailiff and Barnett (1994) could not calculate a thermal assistance energy because of the effects of thermal quenching. Subsequently, Barnett and Bailiff (1997b) obtained an activation energy for thermal quenching (W) of 0.064 _+ 0.003 eV for the IRSL for this feldspar in this temperature region. This allowed them to calculate an activation energy for thermal assistance of 0.061 _+ 0.004 eV (Barnett and Bailiff, 1997b).
217
OSL Properties of Natural Materials
Poolton et al. (2002a) re-plotted the orthoclase data of Bailiff and Barnett (1994) (Fig. 5.78a). They found that above 220 K they were consistent with a thermal activation energy of 0.1 eV, as found by Bailiff and Barnett (1994). Below this temperature Poolton et al. (2002a) calculated a thermal activation energy of 0.02 eV. Rieser et al. (1997) also made measurements below room temperature and obtained two activation energies for microcline, 0.15 and 0.05 eV. Poolton et al. (2002a) believe that these two components (--- 0.14 and --- 0.01 eV) indicate that non-tunnelling and tunnelling processes co-exist and can be identified (see Sections 5.2.9.2.4 and 2.4.5).
5.2.9.2.3. Wavelength dependence. BCtter-Jensen et al. (1994) used a scanning monochromator to measure thermal activation characteristics over the entire stimulation range of 400-1000 nm (3.0-1.25 eV). They made TOL measurements at 25 nm intervals on a sample of Amelia albite over the temperature range 50-180~ As previously shown in Fig. 2.7 (Chapter 2), at each energy where there is a peak in the excitation spectrum, there is a dip in the thermal activation energy curve (e.g., at 1.5, 2.1 and 2.5 eV). Poolton et al. (1994) suggest that these features reflect the complex nature of the conduction bands in feldspars. Poolton et al. (1995c) made further measurements on this and other feldspar
E -3.0
t,-
~
I
'
I
'
I
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(a)
= 0.02 eV _
-4.5
'
I
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=o.1 eV
0.11113
0.005
0.007
0.009
0.011
0.013
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10
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._I
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lIT (K -1) Fig. 5.78. Arrhenius plots of the thermal dependence of IRSL. (a) Data taken by Bailiff and Barnett (1994) from 80 to 300 K for orthoclase. (b) Data taken from 300 to 480 K for a range of single-crystal mineral (M) and sedimentary (S) feldspar samples (from Poolton et al., 2002a).
Optically Stimulated Luminescence Dosimetry
218
samples (orthoclase and oligoclase) in the temperature region from 20 to 200~ Fig. 5.79 shows the thermal activation energies obtained for the three samples as a function of the optical stimulation energy. These thermal activation energies can be compared with the energies at which the main lattice vibrational modes occur in feldspars (Salje, 1993); these phonon modes are shown in Fig. 5.79 and are based on data from Raman and infra-red absorption spectroscopy experiments. Also shown are the optical transition energies for albite, calculated using the Bohr hydrogen model (see Sections 2.4.5 and 5.2.12.1). 5.2.9.2.4. Link to anomalous fading. It thus seems that the value of thermal activation energy obtained varies rapidly near the peak of the IR resonance. This situation can be used to advantage. Since tunnelling is most likely to occur at the resonance, the determination of low thermal activation energies for IRSL measurements (preferably made below room temperature) could be linked to anomalous fading (see Section 5.2.10). Values of thermal activation energies have been suggested by Poolton et al. (2002a) as a means of predicting whether anomalous fading is likely to occur. If the thermal activation energy is low (e.g., ---0.02 eV), then this is assumed to be related to tunnelling between excited states. Conversely, if the thermal activation energy is around 0.12 eV, then this is related to a non-tunnelling process. Poolton et al. (2002a) observed the IRSL (stimulated at 830 nm) as a function of temperature from room temperature to 200~ for several sets of samples. The Arrhenius plots are given in Fig. 5.78b for single-crystal minerals (identified as M) and sediment extracts (identified as S). The samples with the lowest values of activation energy (---0.02 eV), exemplified by Ca26NaT1K (M), faded by ---65% on storage for 7 days at 100~ following a 100 Gy laboratory irradiation. The group with the highest values of thermal activation energy ( > 0 . 1 2 e V ) , exemplified by K(M) and K(S) showed no
0.15 A
> v
>, 0") L
0.10
e" LU ...... t~
E
0.05
(1) r
I-
0 1.0
1.5
2.0
2.5
3.0
Optical Excitation Energy (eV)
Fig. 5.79. Thermal activation energy as a function of optical stimulation energy for three different types of feldspar, namely: Ab---albite, Or---orthoclase and An26Ab74--oligoclase. Horizontal lines represent calculated energies of different phonon modes. Vertical lines on the optical energy axis represent the allowed optical transitions of a donor defect calculated for albite. All energies abovethe optical ionisation energy of 1.896 eV are allowable (from Poolton et al., 1995c).
OSL Properties of Natural Materials
219
observable fading under the same conditions. The intermediate group with energies of 0.04-0.08 eV, exemplified by Na(M) and Na(S), showed some fading (around 10-15%). 5.2.10. Anomalous fading 5.2.10.1. TL, OSL and IRSL In some dating studies using either TL or IRSL signals from feldspars, ages are underestimated compared with the ages given by radiocarbon dating, or with ages based on OSL of quartz (Wallinga et al., 2001). The age underestimation is normally attributed to anomalous fading, a phenomenon that was first reported for TL of feldspars from volcanic rocks (Wintle, 1973). The term anomalous fading was used to indicate behaviour at odds with the predicted thermal stability deduced from laboratory-based kinetic studies. Feldspars of volcanic origin (e.g., sanidine) were observed to have a reduced TL signal at all glow curve temperatures, with losses as great as 50% occurring in a few days storage at room temperature (Wintle, 1973). However, measured losses are frequently much less, particularly in the case of K-feldspars extracted from sediments, although precise measurements of TL signal loss were hampered by the limited reproducibility that could be achieved using multiple aliquots. Templer (1986) suggested that the thermal dependence of the fading rate for zircon was consistent with localised transitions. However, quantum mechanical tunnelling has been proposed as the more likely mechanism for anomalous fading in feldspar (Wintle, 1977; Visocekas, 1985; Aitken, 1985 Appendix F), since it would result in the type of athermal behaviour that is reported. 5.2.10.2. Attempts to remove anomalous fading 5.2.10.2.1. Using a pre-heat. The application of elevated temperature storage as a means of removing anomalous fading in zircon TL signals was proposed by Templer (1985) by assuming that the anomalous fading observed could be explained in terms of a localised transition model (Templer, 1986). In this model, spatially localised electron traps and recombination centres share energy levels, allowing electrons to move from the trap to the recombination centre without going into the conduction band. This model would predict that a thermal treatment that would cause these electrons to recombine could be found, leaving stable electrons at other traps. Tyler and McKeever (1988) considered this mechanism to apply to the TL of an oligoclase. However, Spooner (1992) reported that the loss of signal for a labradorite for the OSL and IRSL was similar, irrespective of whether the sample was stored at 10 or 100~ behaviour that is consistent with the mechanism being quantum mechanical tunnelling. For one sanidine and an oligoclase, slightly faster loss of IRSL with storage time was found for 100~ however, no stable level was reached within 2 months, and only 50% of the initial signal remained. In a further study, Spooner (1994b) systematically examined OSL signals from a set of 24 well-characterised feldspars. Twenty showed similar fading rates for OSL (stimulated at 514 nm) and IRSL (stimulated at 880A80 nm) for storage at 10~ with signal loss at 100~ being slightly faster than at 10~ over the storage period of 15 months. This slight thermal dependence can be explained within the terms of a tunnelling model (Huntley, 1985).
220
Optically Stimulated Luminescence Dosimetry
Spooner (1994b) concluded that the anomalous fading of IRSL and OSL that he observed was consistent with quantum mechanical tunnelling, not a localised transition model. According to Visocekas (1985) quantum mechanical tunnelling is likely to occur on a wide range of time scales, since the range of trap lifetimes is likely to be infinite, with tunnelling distances ranging from 5 to 15 nm (Visocekas, 2002). Thus, the use of a preheat will not remove anomalous fading; it will lessen its effect in a dating study, but will also reduce one's ability to observe fading in laboratory experiments.
5.2.10.2.2. Using an optical treatment. In the study of zircon, Templer (1985) observed that the unstable TL signal could be removed by bleaching with red light (--- 600-660 nm). With a view to developing a similar approach, Spooner (1994b) measured the reduction of IRSL as a function of wavelength of the bleaching light. He found that the bleaching response was similar for those feldspars that faded and those that did not. In both cases, there was no resonance in the visible region (up to 750 nm) that could be linked to the removal of a specific component of the IRSL signal. Besides not providing a way to remove an unstable signal, the lack of resonance was taken as further evidence for there not being a localised transition. 5.2.10.3. Attempts to avoid anomalous fading 5.2.10.3.1. Using time-resolved measurements. Sanderson and Clark (1994) investigated the pulsed OSL signals (stimulated at 470 nm) from feldspars with the aim of identifying a luminescence component that was associated with long-range charge transport; such a component would not be affected by anomalous fading, according to the localised transition model. They carried out their experiments on a sample of feldspar from volcanic lava that was shown to lose more than 50% of its TL signal in 4 days storage following laboratory irradiation. They concluded that signals associated with components on the 40 n s - 8 ~s time scale showed the greatest fading. Both faster and slower components did not appear to show fading, at least not over the 4 day storage period. However, further studies using IR (850 nm) stimulation did not support the claim that it would be possible to select a signal component that did not fade (Clark et al., 1997; Clark and Bailiff, 1998). 5.2.10.3.2. Using different detection wavelengths. Spooner (1992, 1994b) used a detection filter (Schott BG39) that passed wavelengths from 340 to 610 nm for his IRSL studies that showed anomalous fading. However, there have been claims that better agreement of IRSL ages with independent ages can be obtained with particular detection filters (Aitken, 1998; Appendix D). More recently, it has been suggested that a non-fading IRSL signal can be obtained by observing red IRSL from 665 to 700 nm (Fattahi, 2001). This may provide a way to avoid anomalous fading in dating studies, whilst making use of the simple IR stimulation system and single aliquot measurement procedures. 5.2.10.4. CL and TL spectra of fading feldspars Visocekas et al. (1994) compared the room temperature CL spectra of six samples of sanidine that showed fading, three museum specimens (microcline, albite and adularia) that did not fade and three sedimentary K-feldspars that had been dated by IRSL and TL to over 100,000 years (and thus presumably did not fade). All samples showed a range of spectral peaks in the visible region, with emission at about 420 nm being dominant.
OSL Properties of Natural Materials
221
However, a major difference was seen in the infra-red, with all the sanidine samples having an emission peak at 720 nm (Fig. 5.80). It thus appears that there is a connection between the presence of infra-red CL and anomalous fading. Zink et al. (1995) measured the TL spectra of several volcanic sanidines that showed strong anomalous fading. For TL glow curves above RT, they observed well-separated peak emissions at 710 and at about 450 nm. They attributed the IR emission to Fe 3+ substituted for A13+ in the feldspar lattice.
5.2.10.5. Low-temperature phosphorescence Strong support for quantum mechanical tunnelling as an explanation for anomalous fading was provided in measurements of phosphorescence when recently irradiated samples were taken down to LNT. This phosphorescence was termed "tunnelling afterglow" by Visocekas (1985) when he cooled a labradorite from room temperature down to 80 K and similar behaviour was later reported for sanidine (Visocekas, 1993). This luminescence is observed at wavelengths from 590 to 890 nm, but is not seen when optical filters are used to restrict the observation to wavelengths from 305 to 590 nm. When the "red-IR" is obtained as a function of time during a typical experiment, the behaviour is typical of that shown in Fig. 5.81. The sample was irradiated at RT, following which a decaying signal is seen as traps just above room temperature empty. As the sample is cooled, the signal at first decreases, as these traps can no longer be thermally emptied. However, the signal does not reach zero, indicating the presence of an additional source of luminescence, the tunnelling afterglow. A minimum level is reached at about 250 K and on further cooling the signal increases. This increase is due to reduced thermal quenching of the red luminescence centres; as the sample is cooled, more luminescence centres produce photons when electrons recombine at those sites. On storage for 2 h at LNT, the tunnelling afterglow decays with time. At the end of the experiment, the sample is heated up to 400 K; from LNT to 250 K the luminescence decreases, due to thermal quenching, and then the TL glow curve is obtained. During the experiment "blue" luminescence is also observed, but is only significant when the final TL glow curve is measured. The glow curves for the two emissions peak at different temperatures, a fact previously noted by Visocekas (1985) for labradorite. Visocekas et al. (1994) carried out these experiments on 23 additional feldspars and compared the presence, or absence, of afterglow with the measured fading behaviour. Tunnelling rates, in terms of the percentage loss per decade of time, were calculated and a correction for thermal quenching was employed in order to permit comparison with the IRSL fading rate observed for RT storage (Spooner, 1994b). Visocekas et al. (1994) found a correlation between those samples that showed tunnelling afterglow and those for which fading had been detected. They also found that samples that did not fade did not give significant afterglow (corrected rates of < 2%). It thus appears possible to detect the likely presence of anomalous fading by monitoring the luminescence at LNT. This process is not only quicker than carrying out storage experiments, but is also more sensitive since one is not looking at small decreases in large absolute values and since the luminescence efficiency at LNT is enhanced by the lack of thermal quenching of the red-IR emission.
222
Optically S t i m u l a t e d L u m i n e s c e n c e D o s i m e t r y 180160140-
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OSL Properties of Natural Materials
223
Fig. 5.80. Cathodoluminescencespectraof some feldspars: (a) Sanidines known to display tunnel afterglowand fading; (b) feldspars known to display neither fading nor tunnel afterglow; and (c) sedimentaryK-feldspars with low tunnel afterglow, dated to about 100 ka (redrawn from Visocekas et al., 1994).
5.2.10.6. Single grain IRSL fading and fadia plots Lamothe et al. (1994) found two populations of equivalent dose for individual Kfeldspar grains from a marine sand using the single-aliquot additive-dose method. Most of the grains gave values of De that were in excess of the values expected on the basis of the age and radioactivity of the sample, and Lamothe et al. (1994) concluded that these were not fully bleached at deposition. The remaining grains gave De values that were about half the expected value and it is suggested that the cause of this latter phenomenon was anomalous fading. Lamothe and Auclair (1997) measured the initial IRSL signal (LN) and the IRSL signal measured immediately after an additional g a m m a irradiation (LN+~), using a pre-heat of 250~ for 1 min before each measurement. They determined the ratio of these values as RI and found that it varied from grain to grain, suggesting differing amounts of fading for each grain. Lamothe and Auclair (1999) applied a gamma dose of 1.25 kGy to a Pliocene sample for which the IRSL would be expected to be in saturation, thus giving R] -- 1. However, three groups of grains that had values of RI < 2, 2 < RI < 4
Temperature
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Fig. 5.81. Typicalafterglow for a sanidine sample. The lower scale shows the time since the end of a 30-min beta irradiation. The upper scale is temperature (redrawn from Visocekas et al., 1994).
224
Optically Stimulated Luminescence Dosimetry
were observed, and RI > 4, thus indicating fading (Fig. 5.82a). Measurements over storage times of up to 50 days showed the IRSL signal loss to be characterised by a power law. The ratios measured at 4 h, RI (tl), and 10 days, RI (t2), for 48 individual feldspar grains from a much younger sediment were plotted against each other (Fig. 5.82b). Lamothe and Auclair (1999) called this a "fadia plot". All except three data points lie on a line that falls below the 1:1 line, but which intersects it for Ri(tl) - Ri(t2) = 5.18 --- 0.16, representing a point where no fading occurs and which represents the ratio of the IRSL signal (LN+~) divided by (LN) for the unfaded sample. The three open circle data points are from grains that were not fully bleached at deposition as shown by the ratio of their natural and regenerated IRSL signals (shown by arrows in the inset). The value at which R~(tl) = R~(t2) can be used to correct growth curves obtained for multiple-grain aliquots, and for this sample yielded an age of 68 + 7 ka, compatible with the U - T h date on a fossil coral from the same unit. This is twice the IRSL age estimate of 35 ka obtained using routine multiple-aliquot additive-dose procedure.
5.2.10. 7. Logarithmic signal decay Luminescence data for feldspars stored at room temperature are best plotted using semi-logarithmic axes, such that the percentage signal remaining is plotted as a function of the log of the storage time (Spooner, 1994b). TL data for two feldspars are shown in Fig. 5.83 (Wintle, 1973; Visocekas, 2000), together with lines for the theoretical fading rate, expressed in terms of percentage per decade of time. This approach was proposed by Aitken (1985), who defined these rates as g. From Fig. 5.83 it can be seen that age underestimation of--~ 20% would be expected for a sample of 1 million years in age if g = 1.5% per decade. If samples had g = 5% per decade, then age underestimation between 40 and 60% would be expected for samples in the range from 1 ka to 1 Ma. For the feldspar TL signals that have g--~ 20%, dating would be impossible. Thus, it is important to monitor the fading rate of any luminescence signal to be used for dating and determine whether the loss is greater than 1.5% per decade.
5.2.10.8. Correcting for anomalous fading Huntley and Lamothe (2001) developed further the model proposed by Aitken (1985) in order to predict the extent of fading of the natural signal compared with the signals produced by laboratory irradiation and used to obtain D e . They derived an equation
Tf/T = 1 - K[ln(T/tc)- 1],
(5.1)
where T is the true age and can be obtained by iteration. Tf is the age obtained when the laboratory-irradiated aliquots are measured at a time tc after the laboratory irradiation. K is related to g as K - g/100 In(10). K is obtained in a separate experiment to measure fading on the appropriate laboratory time scale and is given by the equation I = Ic[1 - K ln(t/tc)],
(5.2)
where I is the luminescence intensity at a time t after a short irradiation, lc is the intensity when t = to. It should be noted that the value of K depends upon the choice of to. For their study, Huntley and Lamothe (2001) chose tc to be 2 days, and Fig. 5.84 shows the
223
OSL Properties o f Natural Materials
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/
0 2 4 6 8 Natural/Regenerated IRSL /
/
30
je o
20
lO[/f 0 0
I 10
/
/
/
f
f
/
0
R I = 5.18 + o.16
1 20
I 30
.1. 40
I 50
I
60 Ri(tl)
Fig. 5.82. (a) RI (ratio of delayed to immediate IRSL) as a function of room temperature storage time for groups of typical single grains and for aliquots made up of multiple grains. (b) RI for a delay of 10 days plotted versus RI for a delay of 4 h for individual feldspar grains following a gamma dose of 1005 Gy. The inset is a histogram for the same grains showing the ratio of natural IRSL to IRSL regenerated by a beta dose of 100 Gy; grains indicated by arrows were not fully bleached (from Lamothe and Auclair, 1999).
226
Optically Stimulated Luminescence Dosimetry
IO0 9O
_
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'-. \ \
'-.
"'~'~....~.
\\
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-...... .~.
8O \\
\\
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_
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..... 9
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~,
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k
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-
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V,
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0
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1015
1017
t(s) Fig. 5.83. Percentage TL left after anomalous fading as a function of storage time in semi-logarithmic coordinates, showing experimental data for labradorite (V from Wintle, 1977) and sanidine (A from Visocekas, 2000) samples and theoretical fading rates (redrawn from Visocekas, 2002).
measured loss in IRSL (corrected for optical depletion) for samples given a dose, preheated for 16 h at 120~ and then measured at different times. The samples shown cover the range of behaviour exhibited by K-feldspar separates from a number of sites in North America. Values of g ranged from 2 to 10% per decade. Huntley and Lamothe (2001) used Eq. (5.1) to correct for fading of a number of samples they had attempted to date. A test study was carried out on four samples with independent age control and for which the natural IRSL was in the linear part of the dose response curve. Corrected ages were also calculated for a further 24 samples from one site with four radiocarbon ages for peat marker horizons. The upper sample (SN4s) in Fig. 5.83 represents their behaviour, with g = 3.4 + 0.4% per decade. The uncorrected ages were 10-20% lower than expected; on application of the correction, the radiocarbon ages were found to lie very close to the trend line for the IRSL ages. For the sample SW6-01, with
OSL Properties of Natural Materials
1.0
t
i
i | i iiii1
i
i i iiiii I
'
227 '
' ' '"'1
'
SN4s ~
SW6-01
09
_
EIDS
-~
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-
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0.8
I
I
I IIilll
I
10
I
I llllll
I
100
I
I IIIIll
I
1000
Time since laboratory irradiation (days) Fig. 5.84. Fractional IRSL left after anomalous fading as a function of laboratory storage time, showing data for five samples of K-feldspar separated from sediments collected in North America (from Huntley and Lamothe, 2001).
g = 4.7 ___0.3% per decade, the correction increased the age from 4.19 +__0.20 to 5.3 +__0.3 ka, in far better agreement with the quartz OSL age of 5.4 +__0.6 ka.
5.2.11. Radioluminescence 5.2.11.1. A new dating method A strongly increasing RL signal was observed as the limit of the spectrometer (800 nm) was reached in a study of museum specimens of potassium-rich feldspars (Trautmann et al., 1998) when stimulated using a 137Cs source (see Section 5.2.6.3.3). Using a spectrometer with an extended spectral range (from 300 to 1000 nm), Trautmann et al. (1999) were able to observe the peak of the RL emission in the near infra-red at 865 nm (1.43 eV) for K-rich feldspars extracted from sediments. Fig. 5.85a shows RL spectra obtained for one such sample. The most important feature is the decrease in the IR-RL signal when it is exposed to laboratory irradiation and the increase in the IR-RL when it is exposed to sunlight. This is completely opposite to the behaviour of the RL emitted at other wavelengths, as also seen in Fig. 5.85a. Any sediment dating method (relying on exposure to sunlight and environmental radiation) based on the IR-RL signal would thus work in the completely opposite sense to those based on TL (Aitken, 1985) or OSL and IRSL (Aitken, 1998).
Optically Stimulated Luminescence Dosimetry
228
wavelength / nm
1000800
600
500
5~I .........
"~ 40 ~ :1~" [ i/l:" L :1 l"
1.5
400
300
/ Ca) 1
9. . . . sun-bleached (18 hours) natural - - - irradiated (50 hours)
2.0
2.5
3.0
| I 1
3.5
4.0
Ephoton] e V 9I
.
.
.
.
I
.
.
.
.
I
.
.
.
.
I
.
.
.
.
I
.
.
.
.
I
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.
.
.
.
.
.
0.6
0.5
.
.
.
.
Ookl (> 1 Ma) 9 H61 (> 200 ka) ---------- Gr68 (- 100 ka) . . . . . . Bur4 (~ 40 ka) Co3 (~ 35 ka) 9 Esl (~ 30 ka)
:t 0
200
400
600
800
1000
Dp/Gy
Fig. 5.85. (a) RL emission spectra for a sand sample (--~35 ka) treated as indicated in legend, and (b) normalised dose response curves for IR-RL from six samples (ages given in legend) for K-rich feldspars (apart from Gr68) (from Trautmann et al., 1999).
This is demonstrated in Fig. 5.85b, where laboratory irradiation is given to several naturally irradiated feldspar samples. Using the 137Cs source, not only to produce the RL but also as the laboratory-irradiation source, results in continuous plots of IR-RL as a function of dose. For the younger samples, the reduction in IR-RL is the greatest. Conversely, exposure to 2 - 3 h of sunlight results in an increase in the RL to a level that cannot be increased by further sunlight exposure; in this case, the greatest increase is for the oldest sample. Trautmann et al. (1999) used these concepts to obtain values of De for
OSL Properties of Natural Materials
229
these samples and the values were in agreement with those obtained using standard IRSL (observed at 410 nm and using a pre-heat) methods.
5.2.11.2. Practical considerations Krbetschek et al. (2000) constructed dose response curves for 30 samples using the 137Cs source (0.05 Gy/min) and found them to be characterised by a single exponential function that allowed precise De determinations to be made up to about 500 Gy. More recent studies have indicated that a stretched exponential function gives an even better fit (Krbetschek, pers. comm.). It should be noted that to reach 500 Gy using the 3.7 MBq 137Cs source would take 7 days. The maximum IR-RL signal level, attained by light exposure, could be obtained either by placing the grains in sunlight for 5 h or by using a 300 W sunlamp. Movement of the 4 mg aliquots out of the equipment for light exposure led to scatter in the maximum value of measured IR-RL. This was a major source of error in D e determination, and appears to be induced by changes in grain position. This scattering was particularly important when samples from recently deposited sedimentary environments were investigated (Krbetschek et al., 2000). For routine dating an instrument that would not require moving the sample aliquot for each exposure to light is needed. An advantage of the relatively high sensitivity for the IR-RL signal at low doses, provided that the bleached level can be obtained with precision, is that it should be easier to obtain depositional ages for young samples. The rate at which the IR-RL signal is altered by light exposure depends upon the power and wavelengths present. Krbetschek et al. (2000) found that a 200 W mercury vapour lamp (combined with a heat absorbing filter) at 20 cm from the sample would cause the IRRL to reach its maximum value within 10 min. Trautmann et al. (2000a) found that wavelengths below 500 nm were necessary to "zero" the IR-RL, with the procedure increasing in efficiency the higher the UV content. 5.2.11.3. Methods of De determination In addition to the additive-dose/total-bleach procedure for determining D e described above and exemplified in Fig. 5.86a, Krbetschek et al. (2000) also used the total-bleachregeneration procedure (Fig. 5.86b). Similar values of De w e r e obtained by the two approaches for two sedimentary samples. Because of the large radiation times involved (about 5 days for each dose response curve), IR-RL measurements in these examples were not made continuously, thus freeing the spectrometer to be used for other measurements. 5.2.11.4. Thermal stability Trautmann et al. (2000a) investigated the thermal stability of the IR-RL signal (and other RL emissions) by pulse annealing. They found that between 100 and 450~ there was no change in the IR-RL, thus indicating high-thermal stability. In addition, no fading of the IR-RL was observed when irradiated samples were stored at room temperature for several months. 5.2.11.5. Single grain measurements By increasing the spectral accumulation time from 300 to 900 s, Trautmann et al. (1999) were able to obtain RL spectra for naturally irradiated single grains (200-300 txm
230
Optically Stimulated Luminescence Dosimetry
additive
1700-
Os~
1600;:
1500-~
dose
(a)
.~ 1400r
:n 1300-
"~ 1200-
,ooo i1
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e
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.
.
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.
.
.
.
13-dose in Gy
18001600-
.
regeneration
%~
dose
1
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1400-
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._1
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looo-
v
800-
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600 '
1()0
200
'
3()0
400
500
'
6b0
13-dose in Gy Fig. 5.86. (a) Additive-dose and (b) regeneration-dose IR-RL dose response curves for K-feldspars from a fluvial sediment, giving values of De of 120 and 100 Gy, respectively (from Krbetschek et al., 2000).
in diameter). All the K-feldspar grains showed an emission peak at 865 nm, though there was a factor of two variation in intensity and the peak structure for visible wavelengths showed even more variation. In a later study, Trautmann et al. (2000b) used an accumulation time of 300 s to measure single grain spectra after exposing the grains to sunlight for 10 h. They also constructed dose response curves for the different RL signals. 5.2.12. Models for IRSL, OSL, IR-RL in feldspars
5.2.12.1. IRSL Having observed the IRSL as a function of temperature from room temperature up to 200~ Htitt et al. (1988) proposed a model in which electrons were optically
OSL Properties of Natural Materials
231
stimulated by IR photons to an excited state of the electron trap, and from where they were elevated to the conduction band by thermal energy. Electrons from the same trap could also be excited directly into the conduction band. However, energies for thermal activation for IRSL of around 0.1-0.2 eV (see Section 5.2.9.2) seem far too small, when compared with the energy gap of about 0.8 eV suggested by Htitt et al. (1988). Clark and Sanderson (1994) found at least two stimulation peaks for the feldspars examined by them, and proposed a model consisting of at least two traps. The traps have one or more excited states from which electrons can be ejected into the conduction band by the lower energy photons. In their more detailed study of thermal activation energies for three feldspars, Poolton et al. (1995c) show that for the peaks of infra-red transitions, around 1.44 eV for albite and oligoclase and at 1.23 eV for orthoclase, thermal activation occurs (Fig. 5.79). The thermal activation energies are 0.105 eV for orthoclase, 0.065 eV for albite and 0.03 eV for oligoclase. For optical transitions at higher energies, above the optical ionisation energies of 1.896, 1.921 and 1.633 eV for albite, orthoclase and anorthite, respectively, a correspondence of the experimentally determined thermal activation energies and the lattice vibration frequencies can be seen (Fig. 5.79). As already discussed in Chapter 2, Poolton et al. (1995c) used the Bohr hydrogen model, and estimated that for albite the minimum thermal activation energy for the infra-red transition at 1.422 eV (the lowest excited state shown in Fig. 5.79) is 0.175 eV. This is three times larger than the measured value of 0.065 eV. From this Poolton et al. (2002a,b) concluded that the simple model put forward by Htitt et al. (1988) needed modification and instead suggested a donor-acceptor hopping model for the electron transport. See Section 2.4.5 for further discussion. 5.2.12.2. OSL As discussed in Section 5.2.2, the OSL decay curves do not alter their shape with increasing stimulation temperature. This behaviour led McKeever et al. (1997a) to propose that visible light is able to stimulate electrons directly into the conduction band. 5.2.12.3. IR-RL IR-RL is the infra-red RL emitted when potassium feldspars are exposed to ionising radiation, e.g., beta radiation from a 137Cs source (Trautmann et al., 1999). The IR-RL is observed when an electron in the conduction band is trapped at an electron trap. These trapped electrons can then be excited with IR to produce IRSL. A major difference exists between these two signals as they are used in the dating of sediments. IRSL is assumed to result from electrons being released from traps that are thermally stable over the period of interest and then recombining at recombination centres that are similarly thermally stable. IR-RL is observed as photons that are released when electrons from the conduction band are trapped at a thermally stable trap and provides a direct measure of the number of electrons in that trap. This fundamental difference results in IRSL dose response curves that grow with dose and are reduced to a near-zero level by exposure to light, whereas IRRL dose response curves decrease to a fixed level as the dose increases and increase to a fixed level as the sample is exposed to light (see Section 5.2.11). Trautmann et al. (1999) explained the production of IR-RL and IRSL in terms of a band model, with one trap and two recombination centres (Fig. 5.87). In Fig. 5.87a, ionising
232
Optically Stimulated Luminescence Dosimetry
Fig. 5.87. Band model for (a) RL production under beta stimulation and (b) IRSL production under IR stimulation. The model includes one trap (N2) which has an excited state (N2*) and two recombination centres (M4 radiative and M5 non-radiative) between the valence band (VB) and the conduction band (CB). Transition pathways are described in the text (from Trautmann et al., 1999).
radiation produces electrons and holes. The electrons in the conduction band may recombine at a non-radiative centre (Ms), at a radiative centre (M4) with the resulting luminescence being in the UV/visible part of the spectrum, or may be trapped at an electron trap (N2). When the latter occurs, an infra-red photon is emitted and IR-RL is observed. For IRSL to be observed, electrons in the trap (N2) need to be stimulated by IR photons (---1.45 eV, 855 nm) (Fig. 5.87b). The measurement of a thermal activation energy for stimulation in the infra-red (see Section 5.2.9.2) suggests that not all electrons are excited directly into the conduction band, but some recombine with a neighbouring recombination centre via an excited state (N2*), i.e., via donor-acceptor-pair recombination (viz., Poolton et al., 2002a,b). Electrons that reach the conduction band are able to recombine either non-radiatively at M5 centres or radiatively at M4 centres. Both these type of centres are made active by the trapping of holes formed in the valence band during irradiation (Fig. 5.87a). The prompt recombination (path [b] in Fig. 5.87a) and trap filling (path [c]) are competing processes. During irradiation (either in the natural environment or in the laboratory) the number of empty traps (N2) will decrease and the number of transitions via path [c] will also decrease. This explains the decrease in the dose response curves of Figs. 5.85b and 5.86. Since fewer traps become available during irradiation, more conduction
OSL Properties of Natural Materials . . . . . . . .
i
. . . . . . . .
i
. . . . . . . .
i
233 . . . . . . . .
i
1.6
N -~
E
365 nm 435nm 575 nm 880 nm
--,--
(D
--.--
. = _
1.4
--D--
...-..
t,_
or
(a)/ ~ .~ ,'" 7 "" /
9
1.2 J
n,
d: 1.0 . . . . . . . .
0.1
i
. . . . . . . .
1
i
. . . . . . . .
10
i
. . . . . . . .
i
100
1000
tstimulation / S . . . . . . . .
i
. . . . . . . .
i
. . . . . . . .
i
. . . . . . . .
i
(b)
-O 1.2 N . . . ~ . ' 2 ~- . . . . . . . . . .
E o
r"
1.0
_.1
n,|
--,---,--
m
@ 0.8
. . . . . . . .
0.1
365nm 435nm i
1
--o-...A..
. . . . . . . .
i
575 nm 880nm . . . . . . . .
10
tstimulation/
i
100
, \ \0 ,
,
,
....
11
1000
S
Fig. 5.88. RL signals, (a) IR (855 nm) RL and (b) blue (410 nm) RL, as a function of bleaching time when samples are exposed to light at the wavelengths given in the legend (from Trautmann, 2000).
band electrons are free to recombine at the recombination centres (Ms or M4) and thus the RL in the UV/visible emission bands (from type M4 recombination centres) increases with added dose (Fig. 5.85a). When exposed to light, electrons are removed from the trap (N2) and thus more traps are available. This is the cause of the high IR-RL levels following exposure to sunlight. Path [f] (Fig. 5.87a) is required to permit the observation of a high, but finite, level of IR-RL following optical stimulation.
5.2.12.4. Comparison of lR-RL and IRSL (or OSL) The ability to observe the RL spectrum, not just for IR but also for visible wavelengths, has provided much information about the recombination processes. This can be compared with the IRSL measured at different emission wavelengths. Trautmann et al. (1999) made
234
Optically Stimulated Luminescence Dosimetry
comparisons of thermal stability. For one sample they demonstrated that whilst the IR-RL was independent of temperature up to 450~ both the yellow and blue IRSL signals decreased rapidly between 260 and 360~ Trautmann (2000) investigated the effects of previous light exposure on IR-RL and blue-RL for times of 10-1000 s (Fig. 5.88). IR exposures had no effect on the IR-RL (Fig. 5.88a), in spite of the fact that this exposure should have caused a large reduction in the IRSL. Only wavelengths of 435 nm, or less, caused the IR-RL to increase. In contrast, the blue (410 nm)-RL decreased for all visible wavelengths (Fig. 5.88b). Trautmann (2000) further developed the approach of Trautmann et al. (1999) by introducing a localised transition into the model, consistent with the concepts proposed by Poolton et al. (1995a, 2000a,b). Based on measurements of a very old (> 1 Ma) sample, Trautmann (2000) concluded that only a very small number (< 1.5% for this sample) of electron traps were involved in the production of IRSL. In the localised transition model, IRSL can only be produced when there is a neighbouring recombination centre. Electrons that are in traps with no neighbouring recombination centre will still be excited by IR into the excited state of the trap, but cannot escape from it. However, they can be ejected into the conduction band if higher energy photons are used. Measurements of the dose dependence of the blue (410 nm)-RL and the IR-RL for samples of different age suggest that the recombination centre (M4) responsible for the blueRL is thermally unstable. As this is the recombination centre used in measurements of IRSL, this is a cause of concern for IRSL dating of K-feldspars. Trautmann (2000) suggested incorporating the thermal instability of the blue (M4) centres into a modified model. Trautmann (2000) stimulated the IR-RL and IRSL using the modified model and two ionisation rates, one related to the low-dose rate in the environment and the other to the higher dose rate used in laboratory irradiations. Trautmann used IR-RL to provide information on the trapped electron population and the blue (420 nm)-RL to provide information on the recombination centres. The IRSL signal was predicted for such conditions as environmental irradiation over different time periods (25-600 ka) and for pulse annealing experiments. The thermally unstable M4 radiative recombination centres provided the dominant control on the resulting IRSL signal. The trapped electron population (monitored by the IR-RL) continued to increase during environmental irradiation, long after the number of M4 centres reached an equilibrium level. Thus, Trautmann (2000) concluded that only IR-RL will be a suitable method for dating samples over 20 ka.
5.3. Conclusions
In the period since the publication of Aitken's book (1998) on optical dating, much has been learnt about the fundamental OSL properties of both quartz and feldspars. Almost all natural quartz samples have been shown to have a strong fast component in the OSL signal, best seen in LM-OSL measurements (Section 5.1.3). Provided this signal is dominant, sedimentary or heated quartz can be dated back to about 100 ka, with the limit related to the saturation of the electron trap (Section 5.1.7). The luminescence sensitivity can be altered by thermal treatment (Section 5.1.8), and any sensitivity changes that occur during a dating procedure can be monitored and corrections made. In particular, reliable
OSL Properties of Natural Materials
235
ages are obtained using the SAR protocol (see Sections 6.5.4.5 and 6.11.2.2.2). To go back further in time, one of the slow components (Section 5.1.10) may be useful as it shows greater range in OSL response to dose (Section 5.1.10.2). However, being a slow component it is less easily optically bleached in nature (Section 5.1.10.3) and requires longer laboratory measurement sequences. Many advances have also been made in the understanding of feldspar IRSL behaviour (Section 5.2.12). The likely presence of anomalous fading can be detected using the red-IR phosphorescence (tunnelling afterglow) measured at LNTs following room temperature irradiation (Section 5.2.10.5). It has also been suggested that anomalous fading will be observed for samples that have IRSL thermal activation energies of around 0.02 eV, rather than 0.12 eV, for IRSL stimulated at the peak of the IR resonance in the stimulation spectrum (Section 5.2.9.2.1). Quantification of IRSL fading rates over laboratory storage times has led to two new ways of dealing with anomalous fading, namely a graphical approach (fadia) for single grains with different fading rates (Section 5.2.10.6) and a quantitative approach based on a tunnelling model (Section 5.2.10.8). The latter is limited to regions of linear growth with dose, with extrapolation of anomalous fading curves being restricted to four decades of time beyond the laboratory fading measurements used to obtain the fading factor. A more fundamental limitation of IRSL or OSL dating of feldspars is strongly suggested by the RL emission spectra (Section 5.2.12.4). The radiative recombination centres used in IRSL dating emit in the visible, and the RL measurements made following pre-heats suggest that they are only stable enough to date samples back to 20 ka. This would imply that apart from samples for which anomalous fading is a problem, ages back to 20 ka should be accurate, but older samples would give age underestimation. Thus, the development of correction factors for anomalous fading in samples over 20 ka would still not yield correct ages. For these samples, dating using the IR-RL emission appears to be more appropriate and more reliable (Section 5.2.11).
References Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, London. Aitken, M.J., 1998. Introduction to Optical Dating. Oxford University Press, Oxford. Akber, R.A., Robertson, G.B., Prescott, J.R., 1988. The 100~ thermoluminescence emission from high-fired ceramics: a three dimensional view. Nucl. Tracks Radiat. Meas. 14, 21-25. Alexander, C.S., Morris, M.F., McKeever, S.W.S., 1997. The time and wavelength response of phototransferred thermoluminescence in natural and synthetic quartz. Radiat. Meas. 27, 153-159. Bailey, R., 1997. Optical detrapping of charge from the 110~ quartz TL region. Ancient TL 15, 7-10. Bailey, R.M., 1998. Depletion of the quartz OSL signal using low photon energy stimulation. Ancient TL 16, 33-36. Bailey, R.M., 2000a. The interpretation of quartz optically stimulated luminescence equivalent dose versus time plots. Radiat. Meas. 32, 129-140. Bailey, R.M., 2000b. The slow component of quartz optically stimulated luminescence. Radiat. Meas. 32, 233-246. Bailey, R.M., 2001. Towards a general kinetic model for optically and thermally stimulated luminescence of quartz. Radiat. Meas. 33, 17-45. Bailey, R., 2002. Stimulations of variability in the luminescence characteristics of natural quartz and its implications for estimates of absorbed dose. Radiat. Prot. Dosim. 100, 33-38.
236
Optically Stimulated Luminescence Dosimetry
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Poolton, N.R.J., B0tter-Jensen, L., Johnsen, O., 1996. On the relationship between luminescence excitation spectra and feldspar mineralogy. Radiat. Meas. 26, 93-101. Poolton, N.R.J., Smith, G.M., Riedi, P.C., Bulur, E., BCtter-Jensen, L., Murray, A.S., Adrian, M., 2000. Luminescence sensitivity changes in natural quartz induced by high temperature annealing: a high frequency EPR and OSL study. J. Phys. D: Appl. Phys. 33, 1007-1017. Poolton, N.R.J., Bulur, E., Wallinga, J., Bctter-Jensen, L., Murray, A.S., Willumsen, F., 2001. An automated system for the analysis of variable temperature radioluminescence. Nucl. Instr. Meth. B 179, 575-584. Poolton, N.R.J., Wallinga, J., Murray, A.S., Bulur, E., BCtter-Jensen, L., 2002a. Electrons in feldspar I: on the wavefunction of electrons trapped at simple lattice defects. Phys. Chem. Min. 29, 210-216. Poolton, N.R.J., Ozanyan, K.B., Wallinga, J., Murray, A.S., BCtter-Jensen, L., 2002b. Electrons in feldspar II: a consideration of the influence of conduction band-tail states on luminescence processes. Phys. Chem. Min. 29, 217-225. Prescott, J.R., Scholefield, R.B., Franklin, A.D., 1995. Three-dimensional thermoluminescence spectra and their application in the study of some sedimentary quartz. Scanning Microsc. Suppl. 9, 245-254. Rendell, H.M., Clarke, M.L., 1997. Thermoluminescence, radioluminescence and cathodoluminescence spectra of alkali feldspars. Radiat. Meas. 27, 263-272. Rendell, H.M., Townsend, P.D., Wood, R.A., 1995. TL and IRSL emission spectra of detrital feldspars. New experimental data. Phys. Stat. Sol. 190, 321-330. Rhodes, E.J., 1988. Methodological considerations in the optical dating of quartz. Quat. Sci. Rev. 7, 395-400. Rhodes, E.J., 2000. Observations of thermal transfer OSL signals in glacigenic quartz. Radiat. Meas. 32, 595-602. Rhodes, E.J., Bailey, R.M., 1997. The effect of thermal transfer on the zeroing of the luminescence of quartz from recent glaciofluvial sediments. Quat. Geochron. (Quat. Sci. Rev.) 16, 291-298. Rhodes, E.J., Pownall, L., 1994. Zeroing of the OSL signal in quartz from young glaciofluvial sediments. Radiat. Meas. 23, 581-585. Rieser, U., Htitt, G., Krbetschek, M.R., Stolz, W., 1997. Feldspar IRSL emission spectra at high and low temperatures. Radiat. Meas. 27, 273-278. Rieser, U., Habermann, J., Wagner, G.A., 1999. Luminescence dating: a new high-sensitivity TL/OSL emission spectrometer. Quat. Geochron. (Quat. Sci. Rev.) 18, 311-315. Rink, W.J., Rendell, H.M., Marseglia, E.A., Luff, B.J., Townsend, P.D., 1993. Thermoluminescence spectra of igneous quartz and hydrothermal vein quartz. Phys. Chem. Min. 20, 353-361. Roberts, R.G., Bird, M., Olley, J., Galbraith, R., Lawson, E., Laslett, G., Yoshida, H., Jones, R., Fullagar, R., Jacobsen, G., Hua, Q., 1998. Optical and radiocarbon dating at Jinmium rock shelter in northern Australia. Nature 393, 358-362. Salje, E.K.H., 1993. Phase transitions and vibrational spectroscopy in feldspars. In: Feldspars and Their Reactions. Parsons, I. (Ed.). Kluwer, pp. 103-106. Sanderson, D.C.W., Clark, R.J., 1994. Pulsed photostimulated luminescence of alkali feldspars. Radiat. Meas. 23, 633-639. Schilles, T., Poolton, N.R.J., Bulur, E., BCtter-Jensen, L., Murray, A.S., Smith, G.M., Riedi, P.C., Wagner, G.A., 2001. A multi-spectroscopic study of luminescence sensitivity changes in natural quartz induced by hightemperature annealing. J. Phys. D: Appl. Phys. 34, 722-731. Scholefield, R.B., Prescott, J.R., 1999. The red thermoluminescence of quartz: 3-D spectral measurements. Radiat. Meas. 30, 83-95. Scholefield, R.B., Prescott, J.R., Franklin, A.D., Fox, P.J., 1994. Observations on some thermoluminescence emission centres in geological quartz. Radiat. Meas. 23, 409-412. Singarayer, J.S., Bailey, R.M., 2002. Component-resolved bleaching spectra of quartz optically stimulated luminescence: preliminary results and implications for dating. Personal communication. Singarayer, J.S., Bailey, R.M., Rhodes, E.J., 2000. Potential of the slow component of quartz OSL for age determination of sedimentary samples. Radiat. Meas. 32, 873-880. Singhvi, A.K., Krbetschek, M.R., 1996. Luminescence dating: a review and a perspective for arid zone sediments. Annals Arid Zone 35, 249-279. Smith, B.W., Rhodes, E.J., 1994. Charge movements in quartz and their relevance to optical dating. Radiat. Meas. 23, 329-333.
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Optically Stimulated Luminescence Dosimetry
Smith, B.W., Rhodes, E.J., Stokes, S., Spooner, N.A., 1990. The optical dating of sediments using quartz. Radiat. Prot. Dosim. 34, 75-78. Spooner, N.A., 1992. Optical dating: preliminary results on the anomalous fading of luminescence from feldspars. Quat. Sci. Rev. 11, 139-145. Spooner, N.A., 1994a. On the optical dating signal from quartz. Radiat. Meas. 23, 593-600. Spooner, N.A., 1994b. The anomalous fading of infrared-stimulated luminescence from feldspars. Radiat. Meas. 23, 625-632. Spooner, N.A., Franklin, A.D., 2002. Effect of the heating rate on the red TL of quartz. Radiat. Meas. 35, 59-66. Spooner, N.A., Questiaux, D.G., 2000. Kinetics of red, blue and UV thermoluminescence and opticallystimulated luminescence from quartz. Radiat. Meas. 32, 659-666. Spooner, N.A., Prescott, J.R., Hutton, J.T., 1988. The effect of illumination wavelength on the bleaching of the thermoluminescence (TL) of quartz. Quat. Sci. Rev. 7, 325-3329. Stoneham, D., Stokes, S., 1991. An investigation of the relationship between the 110~ TL peak and optically stimulated luminescence in sedimentary quartz. Nucl. Tracks Radiat. Meas. 18, 119-123. Subramaniam, B., Halliburton, L.E., Martin, J.J., 1984. Radiation effects in crystalline SiO2: infrared absorption from OH-related defects. J. Phys. Chem. Sol. 45, 575-579. Templer, R.H., 1985. The removal of anomalous fading in zircon. Nucl. Tracks Radiat. Meas. 10, 531-537. Templer, R.H., 1986. The localised transition model of anomalous fading. Radiat. Prot. Dosim. 17, 493-497. Trautmann, T., 2000. A study of radioluminescence kinetics of natural feldspar dosimeters: experiments and simulations. J. Phys. D: Appl. Phys. 33, 2304-2310. Trautmann, T., Rieser, U., Stolz, W., 1997. Activation energies of IRSL traps in feldspars. Radiat. Meas. 27, 193-197. Trautmann, T., Krbetschek, M.R., Dietrich, A., Stolz, W., 1998. Investigations of feldspar radioluminescence: potential for a new dating technique. Radiat. Meas. 29, 421-425. Trautmann, T., Krbetschek, M.R., Dietrich, A., Stolz, W., 1999. Feldspar radioluminescence: a new dating method and its physical background. J. Lumin. 85, 45-58. Trautmann, T., Krbetschek, M.R., Dietrich, A., Stolz, W., 2000a. The basic principle of radioluminescence dating and a localised transition model. Radiat. Meas. 32, 487-492. Trautmann, T., Krbetschek, M.R., Stolz, W., 2000b. A systematic study of the radioluminescence properties of single feldspar grains. Radiat. Meas. 32, 685-690. Tso, M.-Y.W., Wong, N.W.L., Li, S.-H., 1996. Determination of lifetime of infrared stimulated signals from potassium and sodium feldspars. Radiat. Prot. Dosim. 66, 387-389. Tyler, S., McKeever, S.W.S., 1988. Anomalous fading of thermoluminescence in oligoclase. Nucl. Tracks Radiat. Meas. 14, 149-154. Vartanian, E., Guibert, P., Roque, C., Bechtel, F., Schvoerer, M., 2000. Changes in OSL properties of quartz by pre-heating: an interpretation. Radiat. Meas. 32, 647-652. Visocekas, R., 1985. Tunnelling radiative recombination in labradorite; its association with anomalous fading of thermoluminescence. Nucl. Tracks Radiat. Meas. 10, 521-529. Visocekas, R., 1993. Tunnelling radiative recombination in K-feldspar sanidine. Nucl. Tracks Radiat. Meas. 21, 175-178. Visocekas, R., 2000. Monitoring anomalous fading of TL of feldspars by using far-red emission as a gauge. Radiat. Meas. 32, 499-504. Visocekas, R., 2002. Tunnelling in afterglow: its coexistence and interweaving with thermally stimulated luminescence. Radiat. Prot. Dosim. 100, 45-54. Visocekas, R., Spooner, N.A., Zink, A., Blanc, P., 1994. Tunnel afterglow, fading and infrared emission in thermoluminescence of feldspars. Radiat. Meas. 23, 377-385. Wallinga, J., Murray, A.S., Duller, G.A.T., Tornqvist, T.E., 2001. Testing optically stimulated luminescence dating of sand-sized quartz and feldspar from fluvial deposits. Earth Planet Sci. Lett. 193, 617-630. Wallinga, J., Murray, A.S., Wintle, A.G., BCtter-Jensen, L., 2002. Electron-trapping probability in natural dosimeter as a function of irradiation temperature. Radiat. Prot. Dosim. 101,339-344. Weil, J.A., 1975. The aluminum center in alpha-quartz. Radiat. Effects 26, 261-265. White, W.B., Matsumura, M., Linnehan, D.G., Furukawa, T., Chandrasekhar, B.K., 1986. Absorption and luminescence of Fe3+ in single-crystal orthoclase. Am. Mineral. 71, 1415-1419.
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Wiggenhorn, H., Rieser, U., 1996. Analysis of natural IRSL and TL emission spectra of potassium-rich feldspars with regard to dating applications. Radiat. Prot. Dosim. 66, 403-406. Wintle, A.G., 1973. Anomalous fading of thermoluminescence in mineral samples. Nature 245, 143-144. Wintle, A.G., 1975. Thermal quenching of thermoluminescence in quartz. Geophys. J. Roy. Astr. Soc. 41, 107-113. Wintle, A.G., 1977. Detailed study of a thermoluminescent mineral exhibiting anomalous fading. J. Lumin. 15, 385-393. Wintle, A.G., 1997. Luminescence dating: laboratory procedures and protocols. Radiat. Meas. 27, 769-817. Wintle, A.G., Murray, A.S., 1997. The relationship between quartz thermoluminescence, photo-transferred thermoluminescence and optically stimulated luminescence. Radiat. Meas. 27, 611-624. Wintle, A.G., Murray, A.S., 1998. Towards the development of a preheat procedure for OSL dating of quartz. Radiat. Meas. 29, 81-94. Wintle, A.G., Murray, A.S., 1999. Luminescence sensitivity changes in quartz. Radiat. Meas. 30, 107-118. Wintle, A.G., Murray, A.S., 2000. Quartz OSL: Effects of thermal treatment and their relevance to laboratory dating procedures. Radiat. Meas. 32, 387-400. Woda, C., Schilles, T., Rieser, U., Mangini, A., Wagner, G.A., 2002. Point defects and the blue emission in fired quartz at high doses: a comparative luminescence and EPR study. Radiat. Prot. Dosim. 100, 261-264. Yang, X.H., McKeever, S.W.S., 1990. The predose effect in crystalline quartz. J. Phys. D: Appl. Phys. 23, 237-244. Yoshida, H., Roberts, R.G., Olley, J.M., Laslett, G.M., Galbraith, R.F., 2000. Extending the age range of optical dating using single 'supergrains' of quartz. Radiat. Meas. 32, 439-446. Zimmerman, J., 1971. Radiation-induced increase of the 110~ thermoluminescence sensitivity of fired quartz. J. Phys. C: Sol. State Phys. 4, 3265-3276. Zink, A., Visocekas, R., Bos, A.J.J., 1995. Comparison of 'Blue' and 'Infrared' emission bands in thermoluminescence of alkali feldspars. Radiat. Meas. 24, 513-518.
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Chapter 6
Retrospective OSL dosimetry Part I: R E T R O S P E C T I V E A C C I D E N T D O S I M E T R Y 6.1. Introduction
In the event of a large-scale nuclear radiation accident, a quantitative assessment of the radiation dose to the general population requires the availability of suitable techniques and procedures for reconstruction of doses. The main purposes of dose reconstruction, or retrospective dosimetry, in relation to the local population after a nuclear accident, can be summarised as follows: 9 to guide the provision of proper medical treatment and protection for people exposed to radiation, 9 to provide input data for epidemiological studies, 9 to provide information to the population, and 9 to help carry out research to improve dosimetry and preparedness. The methods used for dose reconstruction have been based on: (1) Dose modelling, e.g., Monte Carlo simulations based on direct measurement results obtained from local active dose rate meters (GM counters etc.) (e.g., Meckbach and Chumak, 1996). (2) Application of luminescence methods with ceramics (thermoluminescence (TL) and optically stimulated luminescence (OSL)) including modelling and photon transport calculations (Bailiff and Stepanenko, 1996; BCtter-Jensen, 2000a). (3) Direct measurement of accumulated doses in human tissues using: (a) Electron paramagnetic resonance (EPR) on tooth enamel (Wieser et al., 2000). (b) Chromosome analysis of lymphocytes in blood; Fluorescence In Situ Hybridisation (FISH) painting methods (Lloyd et al., 1996). Solid-state dosimetry methods based on luminescence, including TL and OSL, are particularly useful because they enable the integrated absorbed dose to be measured. In the case of external sources of radiation, materials found within the accident area can be used, e.g., bricks, tiles and pottery collected from local buildings. The absorbed dose may be evaluated many years after the accident. Consequently, luminescence methods have the potential to provide data essential for dose reconstruction in areas and locations where radiation-monitoring measurements were not carried out. Haskell
246
Optically Stimulated Luminescence Dosimetry
(1993a,b) and Bailiff (1995, 1997) have recently reported the basis and operation of TL techniques and critical factors concerning their use. In the mid 1990s the application of OSL techniques to ceramics for the retrospective assessment of accident radiation doses was suggested (BCtter-Jensen, 1996, 2000b). The following sections describe OSL techniques and analytical procedures, particularly for the measurement of doses from materials collected in a nuclear accident area.
6.2. Materials and sampling The types of ceramic generally found to be suitable for retrospective dosimetry include fired materials such as bricks, glazed and unglazed tiles, roof tiles, interior floor tiles, porcelain fittings (e.g., sanitary ware) and exterior fittings such as lamp holders and electrical power line insulators (see Fig. 6.1). These materials can usually be found in various locations, allowing an investigation of both the nature of the external field and the degree of shielding within the interiors of buildings. Brick buildings offer the highest degree of flexibility in choice of samples because ceramic material is available at a range of depths within the wall. The composition and form of suitable material varies according to the geographical location and the nature of the building environment. Ceramic materials have so far proved to be the only candidates for measurement of doses at the 10 mGy level. In most cases, bricks contain a high proportion of quartz and variable amounts of feldspar of 9 0 - 1 5 0 Ixm grain diameter, the size range most suited to the use of coarse-
Fig. 6.1. Schematic of a house showing ceramic materials potentially usable as dosimeters for retrospective dose evaluations.
Retrospective OSL Dosimetry
247
grain techniques (see Sections 6.5 and 6.11). However, the quality of the bricks may vary significantly depending upon the manufacturing procedures used. Some manufacturers add roughly crushed quartzite to the clay and this can result in poor characteristics from the point of view of dosimetry. A homogeneous distribution of quartz grains is essential when, for example, a dose-depth profile is to be measured.
6.3. Sample preparation and experimental details To estimate the dose in mineral grains, the sample must be extracted without exposure to light to avoid any bleaching of the luminescence signal. In the laboratory, the samples are handled in dim red or orange light (similar to that in a photographic darkroom). Any outer bulk material that may have been exposed to daylight must be removed; this may be kept in reserve for dose rate determinations and particle size analysis. Bricks can be sliced into 10 mm sections that are crushed, and the sand-sized grains (90-180 Ixm) extracted by sieving and treated with HC1. Quartz grains are then concentrated by heavy-liquid separation using sodium polytungstate (2.62-2.65 g/cm3). These are then etched in 40% HF to remove any residual feldspars. Acid-soluble fluorides are subsequently removed in 15% HC1. If only quartz is to be measured, satisfactory results can be obtained without heavy-liquid separation; the selected particle size range is treated with HC1 and H202, and then placed directly in concentrated HF.
6.4. Determination of the accident dose All bricks contain radionuclides that contribute to the dose of all mineral grains that make up the brick. This dose component will increase with time. Following a nuclear accident in the vicinity of the bricks, an additional dose will be recorded, namely, the accident dose. It is the aim of retrospective dosimetry studies to obtain the accident dose as accurately as possible. This requires measurements to determine the natural dose rate and the age of the building, either from documentary information or other measurements. 6.4.1. Retrospective assessment of environmental dose rates The dose absorbed by a mineral grain is a function of the radioactivity both in the grain and in the surrounding material. For quartz the internal dose rate, derived from within the grain, is usually very small (Aitken, 1985). The external dose rate depends mainly on the concentrations of 4~ 238U and 232Th series radionuclides in the surrounding material, plus a component due to cosmic rays. Consequently, to determine the dose rate, it is necessary to measure the radionuclide concentrations in the surrounding material. These calculations are summarised by Aitken (1985, 1998). The water content of the material may affect the dose derived from the natural radioactivity surrounding the grain, by absorbing some energy, and thereby decreasing the dose rate to the mineral grain. Thus, the water content must be estimated and a modified dose rate calculated. The dose received by quartz grains from natural radionuclides within the brick material can be as much as 5 mGy/year. The background dose can thus make up a considerable part
248
Optically Stimulated Luminescence Dosimetry
of the total dose, particularly if bricks are collected from older buildings. As the background component is a major source of error in estimating the accrued accident dose, it is important that this background component be minimised and accurately determined. The annual dose rate resulting from internal emitters can be determined in different ways, e.g., by gamma spectrometry (Murray et al., 1987) and beta counting (BCtter-Jensen and Mejdahl, 1985, 1988). The cosmic ray contribution is calculated from relationships given by Prescott and Hutton (1988). The thickness of the overlying material, and the latitude and longitude of the location are required for these calculations. Having measured the radionuclide concentrations in the surrounding material and calculated the cosmic ray contribution, the total dose rate is calculated using the conversion factors of Olley et al. (1996). Sensitive artificial TL/OSL phosphors such as A1203:C (Akselrod et al., 1990) have also been used to perform in situ measurements of the integrated gamma doses over short periods without the need to correct for differences in mass absorption characteristics at low photon energies (BCtter-Jensen and McKeever, 1996; BCtter-Jensen et al., 1997, 1999). Such measurements help to confirm the modelled natural dose rates, and thus support the calculation of the natural background contribution to the total dose recorded after an accident (except in the case of very short lived contamination, where this background cannot be measured directly after the accident). Experiments have been designed where A1203:C dosimeters were placed in house walls. The observed doses were compared with: (1) doses derived from OSL measurements of quartz samples extracted from the same bricks, and (2) dose rates determined from laboratory measurements of the natural radionuclide concentrations. BCtter-Jensen et al. (1999) placed A1203:C dosimeters in bricks in two different house walls with known ages of 37 and 72 years, respectively, for a period of time to integrate the environmental dose rates. Aluminium tubes, 12 cm long, with an inner diameter of 6 mm and a wall thickness of 1 mm (to absorb beta particles) were packed with 30 freshly annealed A1203:C pellets (arranged in groups of three) distributed over the length of the aluminium tube using 10 mm plastic spacers; this provided 10 measurement points over the entire cross-section of the brick. The aluminium tubes were then placed in holes drilled in the middle of bricks in the two walls and left there for 18 days. At the end of the exposure period, the dosimeters were immediately removed from the aluminium tubes and their OSL signals measured. The bricks that held the aluminium tubes were then removed from the walls and quartz grains extracted for OSL measurements. Remaining bulk material was crushed and the natural radionuclide concentrations were determined using a high-resolution gamma spectrometer (Murray et al., 1987). The two dose-depth profiles measured with A1203:C are shown in Fig. 6.2. A1203:C measurements include a cosmic ray component (Prescott and Hutton, 1988), which in this case was estimated as 0.27 mGy/year, and which has to be subtracted to give the observed gamma dose rate. The quartz-derived dose rate includes the same cosmic component, an infinite matrix beta dose rate (derived from the measured concentrations) and an internal alpha component, estimated using 10% of the bulk concentrations, and an 'a' value of 0.15 (this alpha contribution amounts to approximately 5% of the total). When these contributions are subtracted from the total quartz dose rate, the so-called quartzderived gamma dose rates are obtained. Experiments have shown that it is not possible to
249
Retrospective OSL Dosimetry
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tell the difference between the gamma dose rates measured directly using A1203:C, and the quartz-derived gamma dose rates. 6.4.2. Estimation of the accident dose As discussed above, the total absorbed equivalent dose (De) is the dose absorbed by the mineral inclusions in the ceramics (e.g., quartz and feldspar) and is built up of two components: (1) the background dose accrued since the manufacture of the ceramics (brick) due to naturally occurring radioactive isotopes in the surrounding material and (2) the accident dose (also occasionally termed the fall out or transient dose in the literature) due to sources introduced into the local environment by the radiation accident. The accident dose is the difference between the total equivalent dose (De) delivered to the minerals (evaluated by luminescence measurements) and the accrued natural background dose. Thus, the total dose D e is expressed as:
De = Da + t(D~ + D~ + D.y + Dc)
(6.1)
where Da is the cumulative gamma dose observed by the ceramics due to the accident, t is the time since manufacture of the sample in years; D~, D~, D r and Dc are the effective annual alpha, beta, gamma and cosmic ray doses, respectively, due to natural sources of radioactivity. Evaluations of Da for quartz inclusions in ceramics can be related to dose in air at an external reference location by the use of conversion factors that are derived from computational modelling (Bailiff and Stepanenko, 1996). A large part of the OSL work performed to date has been concerned with dose evaluation in bricks. This is largely due to the predominance of that material at the sites studied so far (BCtter-Jensen, 1996; Bailiff, 1997; BCtter-Jensen and Jungner, 1999; Banerjee et al., 1999; BCtter-Jensen and Murray, 1999, 2001, 2002; BCtter-Jensen, 2000a,b).
250
Optically Stimulated Luminescence Dosimetry
6.5. Analytical protocols 6.5.1. Introduction All the measurement protocols used in accident dosimetry are based on those developed for dating archaeological materials (pottery, heated stones, etc.) and geological sediments. Until recently, evaluation of the equivalent dose (De) using OSL was undertaken using multiple-aliquot methods, either the additive-dose or the regenerative-dose procedures developed earlier for TL (Wintle, 1997). These methods require tens of sub-samples for a single estimation of D e. A single-aliquot additive-dose (SAAD) protocol was developed for feldspar (Duller, 1991, 1995), and more recently regenerative-dose single-aliquot methods have been developed for quartz (Mejdahl and BCtter-Jensen, 1994, 1997; Murray and Roberts, 1998; Murray and Mejdahl, 1999). In the latter, the OSL is first measured, and in the process, the light-sensitive traps are emptied. A regeneration or calibration dose is then given, approximately equal to the natural dose, and the OSL is measured again. Unfortunately, there is often a significant sensitivity change in such a cycle of measurements, especially if the sample is heated between irradiation and measurement, and this initially prevented the application of this very simple approach. 6.5.2. Multiple-aliquot protocols One of the earliest measurement protocols to be developed was that of multiple-aliquot additive-dose (MAA). The MAA protocol gets its name from the fact that many aliquots are needed, and also because laboratory doses are added on to the natural dose, to generate that portion of the dose-response curve, or growth curve, which lies above the natural dose. In its simplest form, this approach requires a minimum of two (in practice more, perhaps as many as 100) sub-samples (or aliquots) of identical characteristics. One sample is given a laboratory dose in addition to the natural dose, and the luminescence signal (TL or OSL) of both is measured. The two signals are plotted against laboratory dose, and the equivalent dose determined by extrapolation. Figure 6.3 illustrates this procedure with a practical example, in which were used 21 aliquots of quartz grains extracted from a Chernobyl brick, and six different laboratory doses. Because of the extrapolation, the actual value of De will clearly depend on the algebraic relationship used to fit the data. For the low dose levels encountered in accident dosimetry, the responses were usually found to be linear. More detailed descriptions of various multiple-aliquot techniques are given in Part II of this chapter (Section 6.11.1). 6.5.3. The single-aliquot regeneration and added dose protocol The Single-Aliquot Regeneration and Added Dose (SARA) method introduced by Mejdahl and BCtter-Jensen (1994, 1997) is based on repeated measurements on each of a small number of aliquots and has the advantages of: (1) being able to be applied to a small sample, (2) giving high precision, and (3) needing no normalisation. Duller (1991) applied single-aliquot measurements with both regeneration and added dose procedures, but abandoned regeneration because of the sensitivity changes found as a
Retrospective OSL Dosimetry
251
16
=12
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200
,
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l
,
400
600 Dose ( mGy )
l
,
800
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1200
Fig. 6.3. Typical MAA growth curve obtained from extracted brick quartz. Aliquots were each about 4 mg and normalised using the OSL signal from a brief stimulation of all aliquots, prior to addition of any laboratory doses. The solid line is the best linear fit. The intercept gives De as 98 mGy (from BCtter-Jensen, 2000b).
result of the re-use of aliquots. Sensitivity changes associated with regeneration have been further studied by Jungner and BCtter-Jensen (1994), McKeever et al. (1996, 1997), Murray and Mejdahl (1999), who identified them to be due to the transfer of electronic charge (electrons and holes) between the various traps and recombination centres taking part in the TL and OSL processes. The SARA method requires a minimum of two aliquots and thus is not a true singlealiquot method. The procedure can be summarised as follows (Fig. 6.4): (1) add beta doses (0, B1, B2, B3) to aliquots containing their natural dose; (2) carry out single-aliquot regeneration measurements on these aliquots to obtain doses (Do, D~, D2, D3); (3) plot these doses as a function of the known added doses (0, B1, B2, B3); and (4) extrapolate the regression line through the points to intersect the added dose axis and Q~ r
.Q
= ~
D3
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r--
L.
~
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252
Optically Stimulated Luminescence Dosimetry
obtain the intercept I. The intercept will then represent the true equivalent dose De (referred to as ED in Fig. 6.4), irrespective of any sensitivity change introduced during the regeneration procedure. There is one important restriction: any sensitivity change must be the same for the doses Do-D3 independent of the beta doses added initially. A simple procedure for testing this is as follows (see Fig. 6.4): From the two triangles O-Do-I and I-B3-D3 in Fig. 6.4, one can deduce that a condition for the sensitivity change being independent of the added dose is given by the following expression: Do~De = D 3 / ( D e -t- B3).
(6.2)
Three regeneration doses are usually used to determine each value of De; these are adjusted so that the "natural" signal falls within the signal interval determined by the regeneration doses. This is necessary because the regeneration growth curves are not always linear. By repeating the measurements, the dose interval can be narrowed so that the interpolation errors are negligible. The SARA method has been used with fired (archaeological) materials, ceramics, bricks and burnt stones, which are relevant to retrospective dosimetry. Mejdahl and BCtter-Jensen (1994, 1997) applied their OSL SARA protocol to both heated and non-heated quartz and feldspar samples and Murray (1996) used it on sedimentary quartz. Murray (1996) showed that the growth curve will be linear, even in the presence of some non-linearity in the OSL dose-response curve. Because SARA is a regeneration-based method, no inter-aliquot normalisation is necessary, and the precision of De estimates obtained using only a few aliquots (typically 9) was a significant improvement over the earlier multiple-aliquot methods. As described earlier, it is implicitly assumed that the operator has some prior notion of the equivalent dose (De) level of the material being studied. To overcome this problem, an automated version of the SARA procedure was developed by Duller et al. (1999) to automatically adjust the radiation dose levels that are administered to several aliquots in an automated reader. An initial value for the first radiation dose was entered, and then the algorithm adjusted this value for the subsequent aliquots until the induced OSL fell close (within + 2%) to the initial light level. Only one single regeneration measurement was used on each aliquot in order to eliminate the risk of non-uniform sensitivity changes during different regeneration cycles. The iterative procedure uses each measurement of the natural-to-regenerative luminescence signal ratio for each aliquot to make an improved estimate of De for the next aliquot. This improved estimate is used in tum to adjust the regeneration dose for the last aliquot to improve the matching of the natural and regenerative light levels. The iteration was completed in less than three measurements. 6.5.4. True single-aliquot protocols
6.5.4.1. Introduction The advantages of single-aliquot procedures over multiple-aliquot techniques are: (1) improved precision, (2) the ability to study the dose distribution within a sample, (3) rapid measurement, (4) no need for normalisation, (5) no correction for supra-linearity (in the case of regeneration protocols), and (6) smaller samples needed.
Retrospective OSL Dosimetry
253
Single-aliquot protocols allow all measurements required for the estimation of D e to be made on one sub-sample (or aliquot). There has been a rapid development in this area, with additive-dose protocols receiving attention first (Duller, 1991; Galloway, 1996; Murray et al., 1997) (see Sections 6.11.2.1.1 and 6.11.2.2.1). More recently, regenerativedose single-aliquot procedures have been developed for quartz (Murray and Roberts, 1998; Murray and Mejdahl, 1999; Murray and Wintle, 2000) (see Section 6.11.2.2.2). Regenerative methods are conceptually the simplest--the OSL is first measured, and in the process the light sensitive traps are emptied. A regeneration or calibration dose is then given, approximately equal to the natural dose, and the OSL is measured again. If there has been no change in sensitivity, then D e is given by the ratio of the two OSL signals, multiplied by the known laboratory dose. As was discussed above, in practice there is usually a significant sensitivity change, especially if the sample is heated between irradiation and measurement as may happen when pre-heating is used to empty thermally unstable traps. However, recent work has shown that a precise correction for sensitivity can be made, based on the OSL signal from a test dose given immediately after measurement of the natural or regenerated OSL signal. In this case, there is no heat treatment between irradiation and measurement (other than a fixed heat to 160~ to empty the 110~ TL trap). It has been shown (Murray and Wintle, 1998; Murray and Mejdahl, 1999) that the OSL from the test dose provides a signal that is proportional to the luminescence sensitivity relevant to the preceding natural or laboratory-induced OSL signal. Thus, dividing the latter by the test dose OSL removes the effects of any changes in sensitivity. This procedure has been extensively applied to estimate the equivalent dose of quartz extracts from brick in retrospective dosimetry (Banerjee et al., 1999; 2000; BCtter-Jensen et al., 1999).
6.5.4.2. Variation of OSL signal with pre-heat In a previous study aimed at determining the effect of pre-heating on the OSL signal of porcelain and brick quartz, Godfrey-Smith and Haskell (1993) observed very small sensitivity changes with pre-heats of 180 and 200~ Jungner and BCtter-Jensen (1994) found that in regeneration experiments using quartz, an increase in the sensitivity was observed when pre-heating for 10 s in the temperature interval of 200-250~ Above this temperature the sensitivity remained relatively constant. Keeping the quartz sample at an elevated temperature of 120~ during OSL measurements, to keep the trap associated with the 110~ TL peak empty, had no effect on the sensitivity change. However, Wintle and Murray (1999) found significant sensitivity changes in both the natural and regenerated OSL signals from a sedimentary quartz, after heating to various temperatures above 160~ for 10s.
6.5.4.3. Choice of OSL signal Murray and Wintle (1998) and Banerjee et al. (2000) have discussed which part of the OSL decay curve should be used for D e evaluation. The measurement procedure is based on the subtraction of an underlying background taken as the signal observed at the end of the stimulation period. Banerjee et al. (2000) found that for both dim and bright signals, the smallest statistical uncertainty in the net OSL signal is achieved using the first few seconds of the decay curve. Although the initial and the total OSL signal behave
254
Optically Stimulated Luminescence Dosimetry
similarly, it has been demonstrated that there may be a significant (10%) hard-to-bleach component in the total integrated signal. This contributes much less to the initial signal. It is important in a regenerative protocol using OSL that the signal used in calculations is dominated by the most rapidly decaying component of the quartz OSL signal (see Chapter 5), and thus the initial signal is used after subtracting the underlying slow component measured over an equivalent number of channels at the end of the signal ( Fig. 6.5a). Fig. 6.5b (after Banerjee et al., 2000) shows the random uncertainty arising from counting statistics as a function of total integration time, where the random uncertainty is
OSL Intensity (Counts/s) (a)
I~ I/
Channels (25) usedfor estimationof average backgroundper channel
Diode Exposure time (s)
o~
(b)
(fi r
._~
10
c" 4..., C
o r,.3
1
'Dim' sample (5 counts/s/mGy)
-k.__
Typical sample (150 counts/s/mGy)
o
>,,
E 0.1 0 c"
I
I
I
I
I
10
20
30
40
50
60
Stimulation time, s
(0.24 s channel width)
Fig. 6.5. (a) OSL decay curve indicating the initial signal and the underlying slow component (from Bctter-Jensen, 2000b). (b) Random uncertainty in the net OSL signal plotted as a function of integration time. See text for definition of the uncertainty (from Banerjee et al., 2000).
Retrospective OSL Dosimetry
255
estimated using the expression"
)1/2/Y.iS i -
or--
~'.iS i -Jr-2B n
(6.3)
Bn ,
where S i is the signal from the ith channel (i = 1,2, ...n) and Bn is the average background. 6.5.4.4. Sensitivity changes with regeneration cycles For any single-aliquot regenerative-dose method to be applicable, it must be demonstrated that luminescence sensitivity changes are negligible or that they can be corrected for by measuring the OSL sensitivity, or a proxy for it. Fig. 6.6 shows the dependence of both the 110~ TL peak area and the OSL signal (first few seconds) from the second test dose, after the measurement of the regeneration OSL for a heated brick quartz sample. The same regeneration dose of 2.5 Gy was given 10 times and a pre-heat of 160~ for 10 s employed (Banerjee et al., 1999). The OSL from the test dose correlates very well with the regenerated OSL, whereas an off-set is observed when using the 110~ TL peak. It has thus been shown that a single measurement of the OSL test dose signal can be used to correct for sensitivity changes and a sensitivity-corrected growth curve can be obtained by dividing natural and regenerated OSL signals by the subsequent test dose OSL signals (Banerjee et al., 1999; Murray and Mejdahl, 1999; Wintle and Murray, 1999). The use of either the OSL response or the 110~ TL response to a test dose, to account for sensitivity changes in sedimentary materials is discussed in detail in Section 6.11.2.3. 6.5.4.5. The SAR protocol The Single-Aliquot Regenerative-dose (SAR) protocol for bricks could employ as few as four OSL measurements. The sample extracted from the brick is first pre-heated to an arbitrary temperature between 160 and 300~ for 10 s. The material has already absorbed a dose before sampling, i.e., the sum of the accident and the natural background dose. The OSL signal (first few seconds) due to this dose is measured to give signal Ln. A test dose is then applied (10-20% of the natural dose) and the sample heated to 160~ to empty the 4x10 s
9
o'J
E 0
x_
110~
Test Dose.signal
v
8000 c
3xl 0 5
- 6000
2x105
4000
0
10 5
2OO0
oE
o o
r
o0 O.c-
-g
I-- o
0
-
,r- 0 ~'-'0
0
0
5000
10000
15000
0
20000
Regeneration dose OSL, counts Fig. 6.6. Dependence of test dose 110~ TL peak area and test dose OSL on the regenerated OSL for a brick quartz sample (from Banerjee et al., 1999).
256
Optically Stimulated Luminescence Dosimetry
charge from the 110~ TL trap; this thermal treatment is often referred to as the 'cut heat'. The OSL signal is measured again, to give Tn. A regeneration dose (Dr) is then applied, which is followed by pre-heating and measurement of the regenerated OSL (L0. The test dose is given again, heated to 160~ and the OSL signal measured to give Tr. Using the observation in Fig. 6.6 that the correlation between L and T is linear and passes through the origin, the natural dose D e is then given by: De -- ( t n / t r ) ( Z r / Z n ) O r .
(6.4)
This calculation assumes that the OSL d o s e - r e s p o n s e curve is linear, or that Dr "-" De. To avoid the need for this assumption, least three regeneration doses chosen to encompass De, are normally applied in sequence to the same disc and D e is then estimated by interpolation. To verify that the OSL has been adequately corrected for any sensitivity changes during measurement, a dose equal to the first regeneration dose is then given to the sample and its OSL measured. A ratio of the sensitivity-normalised signals of the first and the fourth regeneration measurements close to unity (within + 10%) confirms that sensitivity changes, if any, have been properly accounted for (i.e., within 10%) in the evaluation of the equivalent dose. Finally, the OSL signal is also measured without giving an additional regeneration dose before pre-heating and measurement (the "zero-dose" measurement). The sensitivity-normalised zero-signal gives an indication of the degree of thermal transfer from the hard-to-bleach traps to the OSL trap. The SAR protocol (Murray and Wintle, 2000) is outlined in Table 6.1. Banerjee et al. (2000) demonstrated the robustness of the SAR measurement protocol by giving several increasing regeneration doses in the range 0 . 5 - 5 6 Gy to a heated brick quartz sample. A pre-heat of 160~ for 10 s and a test dose of 24 mGy were applied. Fig. 6.7 presents the uncorrected and sensitivity corrected OSL growth curves. The main distinction between the uncorrected and the sensitivity-corrected growth curves is a clear removal of a supra-linear growth in OSL after sensitivity correction. Fig. 6.8a presents a routine application of the SAR protocol to the measurement of De for a heated quartz sample (Banerjee et al., 1999). Three sensitivity-corrected OSL signals
Table 6.1 Outline of a typical SAR measurementsequence Natural + accident dose (De) Regeneration dose 1 (< De) Regeneration dose 2 ( ~ De) Regeneration dose 3 (> De) 5. 6.
Regeneration dose 4 ( = Regenerationdose 1) Regeneration dose 5 ( = 0 Gy)
Pre-heat (180...280~ for 10 s), OSL at 125~ Test Dose, TL to 160~ OSL at 125~ Pre-heat (180...280~ for 10 s), OSL at 125~ Test Dose, TL to 160~ OSL at 125~ Pre-heat (180...280~ for 10 s), OSL at 125~ Test dose, TL to 160~ OSL at 125~ Pre-heat (180...280~ for 10 s), OSL at 125~ Test dose, TL to 160~ OSL at 125~ Pre-heat (180...280~ for 10 s), OSL at 125~ Test dose, TL to 160~ OSL at 125~ Pre-heat (180...280~ for 10 s), OSL at 125~ Test dose, TL to 160~ OSL at 125~
Retrospective OSL Dosimetry 6x10 6
9 Uncorrected o Corrected
o
4xl 06
O0
1000
0000 ~
E O
o~176
0000000
O
-
257
o
O0 0 0 - 500
0
8 2x106 g
E
~ g~
0~ 0
0 20
40
m
60
Regeneration dose, Gy
Fig. 6.7. Sensitivity corrected and uncorrected OSL growth curves for a Chernobyl brick quartz sample (from Banerjee et al., 1999).
(S1, S2, S3) are plotted against their corresponding regeneration doses (Drl, Dr2, Dr3). The latter were chosen so that Drl < De, Dr2 "~ De and Dr3 > Dr2. The equivalent dose (De) is then interpolated from this limited section of the regenerated dose-response curve. A fourth regeneration dose (Dr4) equal to the first (Dr4 = Drl) is administered to the same aliquot. The corrected luminescence signal ($4) corresponding to dose Dr4 is shown as an open triangle in Fig. 6.8a. The ratio of the fourth corrected luminescence signal to the first ($4/S~) gives a measure of how well the sensitivity correction has performed over the first four regeneration cycles (in this case $4/S~ --0.998). A fifth regeneration dose (D r = 0) is then given. Ideally $5 should be zero, but some recuperation may be observed. The zero-dose-corrected regeneration signal is shown as a filled circle at the origin in Fig. 6.8a. After sensitivity correction, the fourth and first regeneration dose signals ($4 and S~) are indistinguishable, signifying that sensitivity changes have been satisfactorily corrected for. The sensitivity-normalised zero dose signal is negligibly small. The sensitivitycorrected growth curve is linear in this dose range (R2 = 0.998) and passes through the origin. Figure 6.8b presents a typical plot of the variation of De with pre-heat temperatures (each for 10 s) between 180 and 280~ (Banerjee et al., 1999). The equivalent dose is independent of pre-heat temperature. This result is consistent with earlier observations (Murray and Wintle, 1998; Murray and Mejdahl, 1999) that after sensitivity correction, there is no evidence of significant thermal transfer of charge from light-insensitive traps to lightsensitive traps when pre-heating before measurement of the OSL signal due to the natural plus accident dose. Had transfer occurred in nature, a systematic change would have been observed in De with increasing pre-heat temperatures until a constant value was reached. 6.6. Evaluation of dose-depth profiles in bricks In the retrospective assessment of accident doses using luminescence methods with bricks, measurements of the dose-depth profiles into the brick material give information about the energy of the incident photon radiation (BCtter-Jensen et al., 1995a). For this reason it is desirable to compare such dose-depth profiles with those obtained from bricks
258
Optically Stimulated Luminescence Dosimetry 4.0
i I i"
"2".
~d r.,e 9 3.5
I--I
I o r
3.0
. ,,.,~ .,,-~
r
/
.,.,~ r.~
/, /
T~.TJ De = 464 mGy I
0.0
//, 0
I 500
400
600
Regeneration Dose, mGy
500 -
B d 9
400300 -
,.o
200> .,..~
~r 100 Mean D e = 376 _ 4 mGy (n = 20) 0
160
I
I
I
200
240
280
Temperature, ~
Fig. 6.8. (a) Sensitivity-corrected OSL growth curve for a Chernobyl quartz sample following the sequence given in Table 6.1, ( 9 ) regenerated OSL, (A) 4th generated OSL, (T) natural OSL. R 2 = 0.998, D e = 464 mGy. (b) Variation of the equivalent dose with pre-heat temperature for another Chernobyl quartz sample (from Banerjee et al., 1999).
irradiated using known gamma sources in the laboratory. Laboratory-irradiated bricks also provide a basis for comparison with modelling using Monte Carlo simulations. Such Monte Carlo simulations have been performed for Chernobyl bricks (BCtter-Jensen et al., 1995a, 1999; Bailiff and Stepanenko, 1996); these simulations ultimately predict the absorbed dose in air at an external reference location for a given source energy and configuration. If assumptions on the source configuration can be made, the dose-depth profiles are expected to reflect the time-integrated energy spectrum of the external radiation field. Thus, measurements of dose-depth profiles are important because they provide support for the assumption used to convert from dose in brick to dose in air. 6.6.1. Continuous OSL scanning Although beta and alpha radiation are rapidly absorbed in the outer layers of brick, gamma radiation penetrates tens of centimetres. By monitoring the attenuation of the
Retrospective OSL Dosimetry
259
radiation-induced luminescence, information on both the dose and the energy spectrum of the gamma rays can be obtained. A method was developed for measurement of dosedepth profiles in brick, tile and porcelain cores, without the need for sample separation techniques (BCtter-Jensen et al., 1995a). Using brick cores, profiles were generated by laboratory radiation using different photon energies from 137Csand 6~ gamma sources; the measured depth dependency was then compared with theoretical calculations derived from Monte Carlo simulations, and with experimental measurements made using conventional optically stimulated luminescence methods of analysis. An automatic OSL scanner is described in Chapter 7 for these applications. Examples of dose-depth profiles obtained fromtwo brick cores that had received i37Cs and 6~ gamma laboratory doses, respectively, are shown in Fig. 6.9. 6.6.2. Determination of dose-depth profiles from Chernobyl bricks Optimum sensitivity is usually attained by using samples of pure minerals (quartz and feldspar) extracted from the bulk material. Such extraction techniques have been used extensively to measure the dose-depth profiles in a variety of brick samples, collected from inhabited sites in the Chernobyl accident area. Sub-samples for measuring the dosedepth profiles are prepared by slicing a cross-section of the brick into 10 mm thick subsections, coarse-grain (90-150 ~m) quartz samples are then extracted from each section. The thickness of each slice represents the limit on spatial resolution. Thinner slices can be used, but the cross-sectional area would have to be increased in proportion. Figure 6.10 shows three dose-depth profiles obtained from Chernobyl bricks measured using the OSL SAR protocol (BCtter-Jensen, 2000b). The two upper curves represent the results from bricks that have been exposed from one side to external accidental photon doses and the exponential decay rates compare well with that obtained from a brick irradiated with 660 keV 137Csphotons in the laboratory. The lower curve represents a brick that has not received any significant dose other than that from the internal radionuclides in the matrix and the ambient photon radiation including the cosmic radiation. 6.6.3. Absolute errors and estimated precision of the equivalent dose in bricks The absolute precision of the total dose estimates obtained using the SAR protocol has been demonstrated for known-age house bricks (of an age < 100 years) and a precision of < 3% is readily achievable. The absolute uncertainties in the accident dose (i.e., D e minus the natural background dose) are dominated by systematic uncertainties, such as those arising from the calibration using laboratory doses (typically --~ 3%), and the estimation of the background dose. The overall uncertainty associated with the latter component is around 4% for a known-age sample, depending on the analytical method used to determine the dose rate (Banerjee et al., 1999). The accident dose is given by Da - - D e - B , where B is the background dose. Thus, for a sample with D e - - 1 0 0 -+- 3 mGy, and background B = 50 +__2 mGy (equivalent to the natural dose in a 15-20 year old brick), the accident dose is 50 +__4 mGy. A typical minimum detection limit for a fallout dose in these circumstances would be about 12 mGy (three standard errors). This detection limit can be optimised most easily by selecting
Optically Stimulated Luminescence Dosimetry
260
(a) A
:i ei
O cI
0,75
0,5
_i
ILl IZ:
0,25
0
50
100
150
200
250
DISTANCE ALONG THE CORE (mm)
(b) A
~J
O E3
0,75
0,5
..i
LU n,"
0,25
0
50
100
150
200
250
DISTANCE ALONG THE CORE (mm) Fig. 6.9. Relative dose-depth profile into a brick from (a) 137Csand (b) 6~ gamma radiation from one side, as calculated by the Monte Carlo code MCNP (bold lines). For comparison, the relative dose-depth profiles measured with the automatic OSL core scanner system are also shown (from Bc~tter-Jensen et al., 1995a).
buildings that were built immediately before the accident. Obviously, measurement should be made as soon after the accident as possible. Close to detection limits, the largest single source of uncertainty will probably arise from the estimation of the background dose-rate, and it is in this area that effort should be concentrated to improve accuracy.
6.7. Retrospective OSL dosimetry using unheated quartz Most attempts to apply retrospective dosimetry to building materials have made use of heated (sensitised) items such as brick or tile ceramic. Unfired materials, such as concrete and mortar, are much more widespread in the office and industrial environments, but unfortunately these cannot be assumed to contain a negligible dose at the time of construction.
Retrospective OSL Dosimetry
261
500
400
E
300
0 200
r9
~"
-
--
100
I
0
~
'
I
30
'
'
'
'
I
60
~
~
'
'
I
'
90
'
'
'
120
Mean depth of section (mm) Fig. 6.10. Typical dose-depth profiles measured from Chernobyl bricks using the SAR protocol on extracted quartz. Note the error bars are within symbols. See text for further details (from BCtter-Jensen, 2000b).
Sand for building materials is quarried from geological deposits, which contains a natural dose, in some cases up to > 100 Gy depending on the age of the deposit. However, the sand is exposed to light during quarrying and use. As a result, grains of quartz extracted from a modern mortar or concrete will often show a wide distribution of doses, with only some completely bleached grains giving effectively zero dose. The challenge in using such materials as retrospective dosimeters is in identifying well-bleached grains at the time of the accident dose, which is then superimposed on the original dose distribution. 6.7.1. Dose distributions Analyses have been described of dose distributions derived from OSL measurements of a variety of unheated samples using techniques based either on small aliquots (i.e., < 100 grains per aliquot) or single sand-sized quartz grains (Olley et al., 1998; 1999; Duller and Murray, 2000). BCtter-Jensen et al. (2000) reported the use of small aliquots (---60 grains) to measure the dose distribution of quartz extracted from the bulk mortar in a wall of a low-level radioactive waste storage facility containing distributed sources of 6~ and 137Cs. The average value obtained compared very well with that derived from a dosedepth profile measured using OSL on extracted quartz from an adjacent brick, and from a separate TL dosimeter record. However, the availability of single-grain OSL apparatus (see Section 7.7.3) has made it possible to measure large numbers of quartz and feldspar grains extracted from building materials. Recently, Jain et al. (2002) and BCtter-Jensen
Optically Stimulated Luminescence Dosimetry
262
and Murray (2002) measured the dose distributions in quartz extracted from a crosssection of a mortar sample collected from an outer wall of a radioactive waste storage facility using both small aliquots (< 100 grains per aliquot) and individual grains. Examples of dose distributions obtained using the SAR method on small aliquots and single grains of quartz from a sample of mortar are shown in Fig. 6.11. Only those aliquots with uncertainties of < 15% on the dose estimate are included. Although, the expected dose is about 9.3 Gy (Jain et al., 2002) the doses measured using small aliquots form a broad, approximately Gaussian, distribution with an average value about 14 Gy and a standard deviation of---17%, considerably more than expected on the grounds of individual uncertainties. However, the doses derived from the single grain measurements of the same sample (also only using doses determined with uncertainties of < 15%) seem to show two distributions: one having approximately the fight average value of about 9 Gy and the other having an average value of about 14 Gy. Although, only 137 out of 11,000 25
9
(a)
>., 20" O C (D 15' 13" (1) Lt_ 10"
95 80 60
.... 5
10
~ , ~ .
15
20
25
,~ . . . . . . .
30
35
40
45
o-
40
~
20
U..
5
="
] 0
o r
n o.1 50
~
E
= O
Dose (Gy)
(b) >~ o c :3 O"
6
99
o~
95
o
80
I
60
4'
40
I
L,_
ii
20
t
5
WlFnlll~
0
t-
10
20
4'o
,1
~ii a) > :m
E 0
50
Dose (Gy) Fig. 6.11. Dose distributions from (a) small aliquots and (b) single grains of quartz extracted from a poorly bleached mortar sample. See text for explanation (from BCtter-Jensen and Murray, 2002).
Retrospective OSL Dosimetry
263
grains (---1.2%) provided results that meet the acceptance criteria, these results suggest that single grain analysis is capable of identifying two different dose populations that seem to be merged when using small aliquots. Jain et al. (2002) made a comprehensive comparison of small-aliquot and single-grain OSL measurements using quartz extracted from mortar and bricks taken from a cross-section of the same wall. The measured dosedepth profiles are shown in Fig. 6.12a. Quartz grains extracted from a commercial dry pre-mix concrete have also been studied using the SAR protocol on small aliquots (---60 grains) and single grains. Thomsen et al. (2002a) prepared a simulated concrete brick consisting of a number of 10-mm thick layers of pre-mixed concrete, inter-spaced with thermally annealed quartz to provide a dosedepth profile through the brick. The brick was irradiated in the laboratory with 662 keV 137Cs photons. In this experiment, the dose distribution in the concrete after the addition of an "accident" dose can be compared with that obtained before the accident dose. Olley et al. (1998) suggested the use of the lowest 5% of a dose distribution to identify the wellbleached grains. OSL data from both small aliquots and single grains can be plotted either as a histogram or as a radial plot (Galbraith, 1990). In a histogram, all data points are weighted equally, irrespective of the precision with which they are known. A radial plot, where each result is plotted together with its relative statistical error, may be more informative. Thomsen et al. (2002a) found that about 80% of the natural OSL comes from only 2% of the single grains of an aliquot and only 2.5% of the grains gave a statistical uncertainty on the natural test dose response of < 30%. A small aliquot normally contains about 65 grains, which means that on average each aliquot only contained 1 - 2 detectable grains. Fig. 6.12b shows a comparison of small-aliquot results derived from the first 5% of histograms, radial plots and single grain results and the Monte Carlo calculated dose-depth profile into the dry-concrete-mixture-simulated brick (Thomsen et al., 2002a). 6.7.2. Thermal transfer and sensitivity changes The SAR procedure for quartz (Murray and Wintle, 2000) has successfully demonstrated that the OSL signal can be corrected for sensitivity changes occurring during repeated measurement cycles by using the OSL response to a small test dose. In the case of poorly bleached materials, the pre-heat stages in these cycles can cause charge transfer from light insensitive but thermally stable traps to the main OSL trap (associated with the 325~ TL peak) (Spooner, 1994). This thermally induced charge transfer can, in some cases, create significant problems in dating young materials, i.e., aliquots with a small natural dose (Rhodes, 2000). In retrospective dosimetry, where it is desirable to measure doses as low as few tenths of a mGy with high precision, thermal transfer could give a significant dose offset (Jain et al., 2002). The optimal pre-heat temperature, constrained by minimum thermal transfer from stable traps, can be investigated by measuring the absorbed dose as a function of the pre-heat temperature. Jain et al. (2002) analysed the thermal transfer in unheated quartz taken from a mortar sample using two different grain size ranges. Also, the thermal transfer from the test dose itself was measured. The results are shown in Fig 6.13a. Thermal transfer is insignificant for temperatures up to 240~ subsequently, it increases to about 0.5 Gy at higher
264
Optically Stimulated Luminescence Dosimetry
Fig. 6.12. (a) Dose-depth profiles measured using small aliquots and single grains of quartz, and polyminerallic fine-grained aliquots extracted from cross-sections of mortar and bricks taken from a nuclear waste storage building. The obtained data are fitted with an exponential curve (bold line) and all results compare well with an independent environmental TLD record (from Jain et al., 2002). (b) Dose-depth profiles measured using extracted quartz from pre-mix concrete. Comparison of small aliquot (SA) results derived from the first 5% of the histograms and radial plots, and single grain (SG) results versus depth with the Monte Carlo calculated dosedepth curve (from Thomsen et al., 2002a).
t e m p e r a t u r e s . T h e dose contribution due to t h e r m a l transfer f r o m the test dose is seen to be insignificant at any t e m p e r a t u r e . T h e plot of p a l e o d o s e as a function of pre-heat for the s a m e s a m p l e is s h o w n in Fig. 6.13b and forms a plateau in the range 1 6 0 - 2 6 0 ~
As the
t h e r m a l transfer is insignificant at low t e m p e r a t u r e s and there exists a stable plateau up to 260~
a standard pre-heat at 200~
is a d e q u a t e for this sample.
Retrospective OSL Dosimetry
A 150-212 microns 0 250-300 microns A 150-212 microns (TD)
1.1 A
it_
I-
265
0.8
(a)
9 250-300 microns (TD)
0.5
"~
0.2
I--
-0.1
200
160
240
280
Preheat Temperature (*C) 12 A
10
>h
(,.5 q)
i
8
o "o o
6
m
4
Ix.
(b)
2 140
i
i
i
i
i
i
i
160
180
200
220
240
260
280
Preheat Temperature (*C) Fig. 6.13. (a) Thermal transfer for different quartz grain sizes. The standard error (SE) is calculated from five aliquots for each grain size. TD represents the thermal transfer signal from the test dose alone. (b) Pre-heat plateau for the 150-212 t~m grain size. The SE is calculated from at least 10 aliquots (from Jain et al., 2002).
6.8. Retrospective OSL dosimetry using household and workplace chemicals
There are other crystalline materials found in the domestic and industrial environment, which may also act as retrospective dosimeters. Bailey et al. (2000) and Bulur et al. (2001) investigated some OSL properties of common salt (NaC1) which seems to be the most obvious of these materials. The OSL characteristics of several other household and workplace chemicals have also been investigated, including washing powder, dishwashing powder, and water softener (Thomsen et al., 2002b). Such chemicals are often held in a light-tight packaging (important for stability of the OSL signal), and are likely to have been manufactured recently, which limits the size of the likely background dose. Figure 6.14 presents typical linearly modulated OSL (LM-OSL) data for common salt, Glauber salt, washing powder and water softener, all after a radiation dose of 500 mGy and a pre-heat of 150~ for 10 s. The CW-OSL curves obtained using constant stimulation power are also shown as insets in Fig. 6.14. All materials show a strong and easily
266
Optically Stimulated Luminescence Dosimetry
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Time (s) Fig. 6.14. LM-OSLcurves for four commonhousehold chemicals(a) commonsalt; (b) Glauber salt; (c) washing powder; (d) water softener, with stimulationpower increasedfrom 0 to 100% in 500 s. The insets show CW-OSL decay curves, measured at a constant 100% stimulation power (from Thomsen et al., 2002b).
stimulated signal, which decayed to negligible proportions after < 4 s of continuous stimulation with blue light (30 mW/cm2). The LM-OSL curves demonstrate that a single trap dominates the decay curve, although a weak slow component is also visible. Thomsen et al. (2002b) investigated the stability of the signals from such materials over periods of 24 h and 2 weeks and they controlled the sensitivity changes before and after storage by monitoring the OSL response to a smaller test dose, to ensure that any signal loss during storage was not an artefact of sensitivity change. The results showed that for most of the materials tested, negligible fading over the two-week period was found.
Retrospective OSL Dosimetry
267
BCtter-Jensen and Murray (2002), and Thomsen et al. (2002b) also applied the SAR protocol to determine the growth curves and natural doses from a variety of chemicals. Common salt showed no significant sensitivity changes during the generation of the growth curve, whereas other samples showed overall sensitivity changes of about 20%. However, one can completely compensate for this effect by using the SAR method. As an example, a SAR growth curve from a common dish washing powder (Blue Care) is shown in Fig. 6.15. This material was given a dose of 500 mGy before measurement, and the evaluation of the dose using the lower region of the growth curve resulted in a value of 495 ___ 14 mGy, in good agreement with the given dose. It is clear that an accident dose of a few hundred mGy can be accurately measured using most of these materials. Furthermore, these measurements can be carried out several days after the accident, and in some cases much longer. A practical average lower detection limit found for several of the household and workplace chemicals was of the order of 10 mGy and the fading characteristics varied from 0 to 40% over two weeks (Thomsen et al., 2002b).
6.9. Retrospective OSL dosimetry using porcelain 6.9.1. Introduction Porcelain is potentially a very important material in retrospective dosimetry because it is widespread in the domestic and industrial environment (Bailiff, 1997). The potential of OSL for dose measurements on various porcelain ceramic materials has been investigated (Poolton et al., 1995; BCtter-Jensen et al., 1996), and Htibner and Grksu (1997) have reported their use of the OSL-pre-dose effect in porcelain from electric-power insulators to retrospectively assess accident doses. Although the principal raw materials used in the manufacture of porcelain are quartz, feldspar and china clay (kaolinite), A1203 is often added as a minor component. As described in section Chapter 3, A1203 can be a very sensitive OSL radiation dosimeter. However, the sensitivity of any of the potentially usable dosimeters contained within porcelain ceramic is likely to depend strongly on the production conditions (firing temperature, atmosphere, etc.), as well as the exact composition of the starting materials. 6.9.2. The origin of OSL in porcelain In general, optical stimulation of both the main porcelain matrix and the glaze gives rise to two types of luminescence signals. These are the time-decaying dose-dependent OSL signals, in which the stimulation energy is less than the emission energy, and the time-steady dose-independent photoluminescence (PL), in which the stimulation energy is greater than that of the emission.
6.9.2.1. Time-decaying dose-dependent OSL signals A link has been shown between the OSL signal and the TL peak at 110~ in quartz (Stoneham and Stokes, 1991; BCtter-Jensen and Duller, 1992; BCtter-Jensen et al., 1995b). In porcelain, an indication that at least part of the dosimetric information arises from the quartz phase of the material is obtained by monitoring the TL at 110~ both before and after OSL (Poolton et al., 1995). As shown in Fig. 6.16a, illumination of a porcelain
268
Optically Stimulated Luminescence Dosimetry
Fig. 6.15. SAR growth curves from a sample of dish-washing powder (Blue Care) that had been exposed to a gamma dose of 500 mGy. A pre-heat of 100~ was applied before each regenerated OSL measurement. (a) The sensitivity-corrected growth curve between 0 and 16 Gy. (b) The lower region of the same growth curve as in (a) from which a dose of 490 +__ 14 mGy was derived for the 'unknown' initial dose by interpolation. The error bars are hidden by the symbols (from BCtter-Jensen and Murray, 2002).
sample causes phototransfer to this low temperature TL trap (PTTL), a process that is typically associated with OSL in quartz (BCtter-Jensen et al., 1993, 1995b). However, the quartz component is certainly not the only OSL-active material present. Thermal annealing of fired porcelain samples to successively higher temperatures following irradiation indicates that the time decaying OSL signals are composed of at least three components. Fig. 6.16a also shows the result from an experiment where a porcelain sample (from a Chernobyl toilet tank) was given a 20 Gy dose using 6~ gamma radiation and OSL was measured at 20~ for 0.1 s, a period not long enough to significantly deplete
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the OSL signal. Subsequently, the sample was pulse-annealed in steps of 50~ in the range 50-350~ for 10 s, with the OSL monitored each time for 0.1 s at room temperature. For the thermal annealing between 50 and 150~ the OSL is greatly reduced and it can be deduced that a significant portion of the initial OSL probably arises from low temperature TL traps. However, the TL curve in Fig. 6.16a does not show these, since the sample here was heated to 120~ prior to measurement. For thermal annealing between 150 and 250~ no significant change in the OSL trap population is normally observed, but for heating beyond 250~ the OSL decreases rapidly. This indicates that an unstable OSL signal is present in a freshly irradiated porcelain sample and an appropriate pre-heat treatment is required for obtaining a stable OSL signal suitable for dosimetry (Poolton et al., 1995). Fig. 6.16b shows the time decay characteristics of OSL in porcelain after pre-heating at different temperatures.
Optically Stimulated Luminescence Dosimetry
270
6.9.2.2. Time-steady PL emission spectra from porcelain BCtter-Jensen et al. (1996) examined the emission characteristics of different porcelain samples by recording the time-steady PL emission spectra using a continuous scanning monochromator. UV stimulation was provided using a halogen lamp, filtered with a U-340 filter (peak transmission at 340 nm). Analyses of the spectral emission features of the crockery porcelain and glazes allow the possibility of identifying both the principal luminescent matrix, and luminescent defects contained within it. The PL emission spectra (excited by 340 nm light) from the bulk porcelain and the glaze are shown in Fig. 6.17a,b (BCtter-Jensen et al., 1996). The structures observed in the PL spectra of porcelain were identified by comparing these with TL emission spectra obtained from known artificial phosphors such as calcium sulphate doped with dysprosium (CaSO4:Dy) and aluminium oxide doped with carbon (A1203:C). Such TL spectra are shown in Fig. 6.17c. The bright emission peak near 700 nm from A1203:C is consistent with the observations by Akselrod and Kortov (1990) and Kortov et al. (1994) who identified this emission as an internal transition of Cr 3+, a very common impurity of this material (see also, Chapter 3, Section 1 0.8
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Wavelength (nm) Fig. 6.17. (a) PL plotted against wavelength for two domestic bulk porcelain samples. The emission from A1203 is demonstrated by the typical peaks at 410 and 700 nm (from BCtter-Jensen et al., 1996). (b) PL spectra for two glaze samples. Sample 2G is a white glaze and shows emissions from A1203 and Dy 3+. Sample 6G is a clear glaze also showing emissions from A1203 (from BCtter-Jensen et al., 1996). (c) TL spectra from two artificial phosphors, namely (i) CaSOa:Dy and (ii) A1203:C. The results suggest a similarity of the porcelain with that of A1203:C, with some Dy 3+ impurities present (from Poolton et al., 1995).
Retrospective OSL Dosimetry
271
3.1). Typically, this comprises a main emission at 693 nm, with satellite lines at 670, 714 and 740 nm, at relative intensities depending on the doping concentrations (Lapraz et al., 1991). The broad emission band from A1203:C peaking at --~410 nm (also seen in Fig. 6.16c) corresponds directly with the well-known F-centre emission arising from the 3P ---, IS transition (Lee and Crawford, 1979; Akselrod and Kortov, 1990, and Chapter 3). The spectra obtained from CaSO4:Dy clearly show the sharp emissions at 490 and 580 nm, which represent the distinct blue-green and yellow emission signals caused by the Dy 3+ dopant (e.g., McKeever et al., 1995). The PL spectra from the bulk porcelain samples (see Fig. 6.17a) show identical emissions at 410 and 700 nm and thus indicate that the principal luminescent matrix of bulk porcelain is A1203. The PL spectra from the glaze (see Fig. 6.17b) show peaks at 410, 490 and 580 nm that identify emissions from both A1203 and Dy 3+. It is well known that A1203 is a frequently used component of bulk porcelain matrixes and both A1203 and Dy 3+ are components often included in glazes used as decorations on crockery porcelain.
6.9.2.3. OSL stimulation spectra The OSL stimulation spectra, i.e., OSL versus stimulation wavelength, for a Chernobyl toilet porcelain and the associated glaze have been obtained using a scanning monochromator in the visible range using a U-340 detection filter. These spectra are shown in Fig. 6.18a (Poolton et al., 1995). A prominent broad transition is observed peaking around 540 nm (particularly in the glaze), together with a rising continuum at lower wavelengths. It is noted that the occurrence of the 540 nm feature is unlikely to arise from quartz, where only structureless excitation characteristics have been reported previously (BCtter-Jensen et al., 1994). An OSL excitation spectrum from another typical porcelain sample is shown in Fig. 6.18b (BCtter-Jensen et al., 1996) and a similar smooth stimulation resonance is seen, but around 500 nm in this case, well matched to stimulation sources producing light around 470 nm. 6.9.3. OSL dose response of porcelain Typical OSL decay curves for a porcelain sample, given 6~ gamma doses from 30 mGy to 2 Gy, are shown in Fig. 6.19a (BCtter-Jensen et al., 1996). As quartz and A1203 are considered major OSL sensitive components in the porcelain, pre-heating at 150~ for 30 s is recommended to remove unstable components before any OSL readout in attempting to stabilize and reproduce the signal. The dose-response curves, i.e., OSL versus 6~ gamma dose, are shown for three porcelain samples in Fig. 6.19b. In general, the OSL sensitivity of porcelain glaze is more than one order of magnitude higher than that of bulk porcelain; this effect is ascribed to the high content of A1203 and Dy 3+. Unfortunately, glazes are not suitable for OSL dosimetry since the OSL signal from this surface material will, in most cases, be bleached by the ambient light. For most porcelain samples, the OSL signal increases linearly from 10 mGy up to about 20 Gy and shows a further sub-linear increase up to at least 200 Gy. Using blue-green light simulation with sensitive fired porcelain samples has allowed doses lower than 50 mGy to be measured with a statistical uncertainty of 10% and the lower detection level was determined to be
272
Optically Stimulated Luminescence Dosimetry 2.5 I~~
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about 10 mGy (BCtter-Jensen et al., 1996). The fading of the OSL signal from a porcelain sample has been shown to be negligible over one month (Poolton et al., 1995). 6.9.4. Dose-depth profiles in porcelain and the effect of transparency BCtter-Jensen et al. (1997) measured the OSL dose-depth profile from a ceramic fuse collected at the nuclear accident site in Chernobyl. An 8 mm diameter (12 mm long) core was drilled across the fuse and sliced into lmm-thick discs. The normalised doses evaluated from each disc as a function of the depth into the material are shown in Fig. 6.20a. The dose-depth curve shows a bleaching effect on the OSL signal in the outer layers of the material. Thus, the transparency of porcelain and the consequent bleaching effect caused by ambient daylight has to be considered. BCtter-Jensen et al. (1997) consequently carried out an experiment using a 12 mm long porcelain core that was given a uniform 137Cs gamma dose of 2 Gy at fight angles to the long axis and subsequently placed in sunlight for 8 h so that only one end of the core was illuminated. Discs (1 mm thick) sliced from the core had their OSL signals measured. For comparison TL measurements were made on the same discs. The doses evaluated by OSL and TL are plotted against depth into the ceramic in Fig. 6.20b. It is clear that samples for both TL and OSL measurements must be taken at a depth of at least 2 mm in order to be unaffected by
Retrospective OSL Dosimetry
273
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25
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Fig. 6.19. (a) Typical OSL decay curves from domestic porcelain representing different doses from 30 mGy to 2 Gy 6~ gammaradiation. Stimulation: broad-band (420-550 nm), 16 mW/cm2. (b) OSL versus 6~ gamma dose for three different domestic porcelain samples (from BCtter-Jensen et al., 1996). the ambient daylight. This suggests that thin porcelain items (crockery) are unsuitable in dosimetry applications, because the entire body of the material will be significantly affected by daylight exposure. 6.9.5. OSL dosimetry using porcelain dental crowns Bailiff et al. (2002) investigated the OSL properties of porcelain dental crowns with the aim of using these as retrospective dosimeters after nuclear accidents. Dental ceramics, because of their intimate contact with the human body, are of interest as a luminescence dosimeter material because of their potential to provide a means of determining cumulative exposure to external gamma radiation arising from accidents or large-scale incidents involving population groups. The term dental ceramics is used to describe materials including porcelain and glass-ceramic materials that are employed in the construction of tooth crowns, restorative components of teeth and prosthetic teeth. Dental ceramics may have some luminescence characteristics in common with domestic porcelain, although the composition of the former generally differs from that of domestic porcelain, having a high proportion of feldspar (80% versus 15%) relative to kaolin (15% versus 70%) to achieve translucent quality (Bailiff et al., 2002). Previous work by Davies (1979) demonstrated the feasibility of the use of both thermally stimulated exo-electron
274
Optically Stimulated Luminescence Dosimetry
(a)
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2,5
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Depth (mm)
Fig. 6.20. (a) Relative accident OSL dose versus depth into a ceramic fuse. Procedure: slicing of cross-section core into discs and subsequent measurementof individual OSL signals. (b) Relative OSL and TL doses versus depth into a ceramicfuse after exposing a core to a uniform dose of 2 Gy 137Csgammaradiation and subsequently placing the core in daylight for 8 h such that only one end was illuminated. See text for details (from BCtter-Jensen et al., 1996). emission (TSEE) and TL techniques for determination of absorbed dose using dental porcelain. Later work by Mauricio et al. (1985) further underlined the potential of the material for accident dosimetry; a TL peak located at 270~ was found to be linear with the absorbed dose over a wide range (400 m G y - 4 0 0 Gy). Bailiff et al. (2002) measured OSL and IRSL decay curves from prosthetic tooth and crown enamel using different optical stimulation sources: (i) filtered spectrum ( 4 2 0 5 5 0 n m ) from a halogen lamp, (ii) IR LEDs (875 ___ 8 0 n m ) and (iii) blue LEDs (470 ___ 40 nm). The OSL intensity was generally found to be too weak for samples of crown ceramic using either halogen lamp or IR LED stimulation; however, significant improvements were obtained by using blue LED stimulation instead of the halogen lamp and by measuring the infra-red stimulated luminescence (IRSL) at an elevated temperature of 140~ (70% increase). The forms of the OSL and IRSL decay curves measured with either type of dental ceramic were not described by first-order kinetics, i.e., not of single exponentials. Examples of IRSL decay curves measured with prosthetic tooth are shown in Fig. 6.21a. However, the initial part of the decay approximates to an exponential form,
Retrospective OSL Dosimetry ,
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Absorbed Dose [Gy] Fig. 6.21. (a) IRSL decay curves measured with aliquots of prosthetic tooth after administration of different beta doses. The aliquots were pre-heated at 160~ for 100 s before measurement and held at 140~ during IR stimulation. (b) IRSL growth characteristics obtained with aliquots of prosthetic tooth at room temperature (open squares) and at 140~ (filled squares) (from Bailiff et al., 2002).
with a halving of the initial intensity after 4 and 8 s for OSL and IRSL decay curves, respectively. The general form of the dose-response characteristics for the OSL and IRSL signals were found to be linear (experimental error of ___5%) within the dose range 100 m G y - 1 0 Gy for samples that had been [3-irradiated and then subjected to a pre-heat treatment. An example of an IRSL growth characteristic for prosthetic tooth ceramic is shown in Fig. 6.2 lb where the measurements were performed at both ambient and elevated (140~ sample temperatures.
6.10. Retrospective accident dosimetry--conclusions OSL techniques with ceramic materials, as discussed so far in this chapter, appear to have widespread suitability for retrospective accident dosimetry. The application of the SAR method for the estimation of equivalent dose in quartz extracts from modem house bricks exposed during a nuclear accident has been illustrated. High precision (1% S.E. for n = 15) in the measurement of equivalent dose is readily achievable, and with present methods a detection limit of about 10 mGy for the accident component on a natural
276
Optically Stimulated Luminescence Dosimetry
background of 50 mGy can be estimated. To improve this detection limit significantly, uncertainties in the estimates of the natural dose-rates must be reduced. The use of bricks from walls to obtain dose-depth profiles provides important information concerning the nature of the time-averaged incident external photon field. Because of their speed and high precision, the new OSL measurement techniques have shown major advantages in routine dose evaluations and in the evaluation of dose-depth profiles using cut brick sections. Porcelain provides a widely available dosimeter material for measurements in shielded locations and its glazed surface also provides the advantage of low fallout retention when used in exterior locations. In the case of populated areas that have received radioactive fallout, the combined use of luminescence and computational modelling provides a means of validating calculated values of absorbed dose in air for contaminated areas and to provide dose values for subsequent modelling of dose to population groups within the area studied. The validation and use of models supported by direct measurements is crucial to epidemiological investigations and subsequent arrival at a more accurate assessment of risk to members of the population exposed to ionising radiation. The selection of appropriate samples is one of the most important aspects of retrospective accident dosimetry since the interpretation and use of the results relies heavily on assumptions made concerning the relationship between the sample and the radiation sources contributing to the transient dose. So far a comprehensive sampling methodology is yet to emerge. Wider use of the method is likely to accelerate the demand for standard procedures to be established so that the selection of appropriate samples for both accrued dose and dose rate evaluation, according to the type of building and dosimetry problem, can be optimised.
P a r t II: G E O L O G I C A L
AND
ARCHAEOLOGICAL
DATING
Another major application of retrospective dosimetry is the dating of unfired sedimentary materials and heated archaeological ceramics. Luminescence dating offers the only direct method for dating geological and archaeological sedimentary events that have occurred in the last 250,000 years (250 ka) and it is becoming increasingly the method of choice (Murray and Olley, 2002). To calculate an age requires a knowledge of both the dose and the dose rate. The latter is derived from either direct measurement or radionuclide concentrations (Aitken, 1985). This section discusses methods of determining the dose received since the event of interest.
6.11. Measurement procedures Considerable detail regarding various OSL measurement procedures has already been given in Section 6.5 with reference to the measurement of nuclear accident doses, particularly those received by fired materials such as bricks. Here we consider both multiple-aliquot procedures, and various single-aliquot procedures that have been more fully developed for the dating of sediments.
Retrospective OSL Dosimetry
277
6.11.1. Multiple-aliquot methods Multiple-aliquot procedures for determining the radiation dose received by mineral grains involve the use of a number of nominally identical sample aliquots, some of which would have received only the dose that needs to be determined, whilst the others would have received a laboratory dose. This approach was adopted for OSL dating from preexisting methods used in TL dating. In TL dating, the luminescence signal is destroyed by the act of measurement, thus only one measurement can be made per aliquot, apart from the measurement of a signal derived from a subsequent test dose. This type of measurement was used to provide a method of normalisation based on the TL sensitivity of the grains making up each aliquot; it is termed "second glow normalisation". In OSL dating, the signal may also be totally removed by the measurement, with the decay curve being taken until the OSL is less than 1% of the initial value. These decay curves for the natural- or laboratory-irradiated aliquots can then be used to construct the growth curves. Either the integrated OSL signal can be used or the signal can be broken down into components that correspond to different parts of the stimulation curve. Two multiple-aliquot approaches have been derived, the so-called additive-dose and regenerative-dose methods. In regenerative-dose procedures the OSL signal is zeroed in a way that is analogous to the zeroing that took place in nature (i.e., exposure to light) and doses are subsequently given to construct an OSL growth curve up to and just above the natural OSL level. The equivalent dose is then obtained by projecting the natural OSL level onto the growth curve (Fig. 6.22a). In additive-dose procedures, the aliquots are given additional radiation doses that will increase the signal above the level due to the natural irradiation. In this method, the equivalent dose is obtained by extrapolation (Fig. 6.22b). Used in this simple way, both approaches have their limitations, which can be discussed in terms of the OSL properties of quartz that were discussed in Chapter 5. First, in order to be able to plot such curves, it is assumed that each aliquot is identical to every other, or that some method to normalise between the aliquots is available. Weighing every aliquot (--~5 mg) to the required level of precision (1%) to use as a correction factor is tedious. For many sandy samples, weighing would be inappropriate since only a small percentage of the grains give an OSL signal. Fig. 6.23 gives the total light sum for the natural OSL of three samples of dune sand. Only about 30% of the grains in each case had OSL signals that were distinguishable from the background signal of the photomultiplier tube. For all three samples, 10% of the grains would give rise to 9 0 - 9 5 % of the signal. These results imply that 9 0 - 9 5 % of the natural signal from a 5 mg multiple-grain aliquot would come from 50 grains out of the 500 being stimulated. This leads to the use of a normalisation procedure based on the initial natural OSL signal (e.g., resulting from a 0.1 s exposure to the light source) from each aliquot. This requires the grains to be firmly mounted on the discs, so that the grains do not move during subsequent irradiation and heating prior to each OSL measurement, or during light exposure for multiple-aliquot regenerative-dose measurements. This "natural normalisation" requires the grains making up each aliquot to have received the same dose during burial. Secondly, it is assumed that the response as measured for the laboratory-irradiated aliquot, is identical to that relating to a naturally irradiated aliquot. There are several reasons why this might not be the case. For example, the dose rates employed in the
Optically Stimulated Luminescence Dosimetry
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70
Fig. 6.22. Multiple and single-aliquot growth curves for quartz OSL from a Holocene dune sand from Germany. (a) Data set for MAR protocol, using 13 natural measurements (open circles) and 35 laboratory-irradiated aliquots (filled circles) giving a De of 9.7 +- 1.3 Gy, (b) data set for MAA protocol, giving a D e of 8.0 -- 0.7 Gy obtained using similar number of aliquots, (c) raw data for one aliquot obtained using SAR protocol, with the natural signal shown as a horizontal line, and (d) the data from (c) corrected using the OSL signal from the test dose given after each measurement. The D e obtained was 9.4 _ 0.3 Gy. The insets in (c) and (d) are the data for doses below 3 Gy, and demonstrate the minimal amount of recuperation (open squares) and good reproducibility after correction shown in (d) (open diamonds and triangles) (from Hilgers et al., 2001).
laboratory are m a n y orders of m a g n i t u d e higher than those experienced by grains in their natural environment. This m a y lead to thermally unstable traps being filled during a laboratory irradiation, and it has been suggested that laboratory irradiation should be carried out at elevated temperatures in order to maintain trapping conditions similar to those in nature (Bailey, 2001). Also, as concluded in Section 5.1.8.2, the efficiency of l u m i n e s c e n c e production for a naturally irradiated aliquot of quartz m a y be higher than that for an equivalent laboratory irradiated aliquot. M u r r a y and Wintle (2000) suggested that this is the result of a sensitisation process that occurs during storage at ambient temperature. It is of particular importance for samples that are more than a few thousand years old and are from hot climatic regions. The effect can be o v e r c o m e by using the O S L response to a test dose, as in the SAR protocol (see Section 6.5.4.5). Thirdly, any multiple-aliquot procedure requires a large n u m b e r of aliquots in order to obtain a single value for De. This m e a n s that it is only practicable to m a k e a single
Retrospective
OSL
100
Dosimetry
.~.. .~.. . . . . . . . . . . . . . . . . . . . . . . . . . . .
//--
279
80
E
60
,.i-, t--
~
40
i ZB15 (N=1892) ........... ZB 13 (N=4332) ZB20 (N=2737)
20
,
.
20
40
/
/ // _ _ . 80100
% proportion of grains
Fig. 6.23. Distribution of the natural OSL intensity from over 1000 single grains from three samples. The percentage of the total light sum is plotted as a function of the specified percentage of all the grains measured (from Jacobs, pers. comm.).
determination of De and thus it is not possible to build in any checks that might permit assessment of whether an appropriate pre-heat has been applied. The data in Fig. 6.22 are for a sample of dune sand from north-eastern Germany (Hilgers et al., 2001). The laboratory-irradiated quartz grains used in the multiple-aliquot regenerative-dose (MAR) growth curve (Fig. 6.22a) were exposed to sunlight for several hours prior to gamma irradiation. Natural normalisation was used for the data shown in Fig. 6.22a,b, which is the equivalent data set for the MAA method. For this sample, the values of D e w e r e quoted as 9.7 __+ 1.3 Gy for MAR and 8.0 _ 0.7 Gy for MAA. It is clear from the figures that there is a large degree of scatter, despite the use of a normalisation procedure. For the 11 samples measured in the same way, similar scatter was found leading to uncertainty in D e o f "~ 10-50% (Hilgers et al., 2001). Given the degree of reproducibility found for SAR measurements on the same set of samples (see Sections 6.5.4.5 and 6.11.2.2.2), Hilgers et al. (2001) concluded that the scatter for the multiple-aliquot data was not caused by the presence of grains with a wide range of dose. This was to be expected intuitively for a clearly aeolian deposit. Another possibility for scatter could be related to the fact that the natural normalisation measurements were made on grains that had not been heated. Thus, grains with different degrees of natural sensitisation may have been present, and their properties were altered to different extents by thermal treatment applied prior to the main OSL measurement, namely 220~ for 300 s. This could have been taken into account if the OSL response to a test dose was obtained after each OSL measurement.
280
Optically Stimulated Luminescence Dosimetry
6.11.2. Single-aliquot methods 6.11.2.1. Feldspars 6.11.2.1.1. Additive dose. Single-aliquot procedures were first explored for the dating of sand-sized feldspar grains using the IRSL signal by Duller (1991; 1994; 1995) (see Section 5.2). Duller tried two approaches. One was based on an additive-dose method, which is feasible when only the initial part of the IRSL decay curve is used as the signal. The second procedure involved complete removal of the IRSL signal by the IR exposure used for measurement, followed by repeated cycles of irradiation and IRSL measurement in order to construct the regenerated OSL versus dose curve. In each case, only a single aliquot was needed to produce a growth curve. This possibility was first mentioned by Huntley et al. (1985). Both procedures require the application of a pre-heat to remove any thermally unstable signal. Studies by Li (1991) indicated that a pre-heat for 10 min at 220~ would be appropriate and this was widely adopted. The short stimulation required to make the measurement is typically an IR exposure of--~ 20 mW/cm 2 (e.g., 0.5 s with the power at the sample being --~40 mW/cm2). This stimulation will cause a 4% drop in the IRSL signal (Duller, 1994). In addition, this pre-heat causes the signal to undergo thermal decay. Repeated heating and IRSL measurement results in a decay curve similar to that shown in Fig. 6.24b, for which the initial drop is close to 20%. In the additive-dose method it is necessary to correct for the loss that occurs for every measurement used to construct the growth curve. The uncorrected OSL measurements are shown in Fig. 6.24c, which also shows the effect of the "luminescence correction" method described by Duller (1994). In this correction, the signal from each additional irradiation is treated separately. The correction factors applied to each component are derived from the decay curve shown in Fig. 6.24b, which is obtained using an additional aliquot of natural sample. This method is appropriate for those samples with a dose response that is close to linear. For non-linear growth of OSL with dose, another correction procedure needs to be appliedmnamely, the "dose correction" method (Duller, 1994), which is illustrated in Fig. 6.24d. The appropriateness of each of the correction procedures can be ascertained by continuing the cycles of pre-heat and IRSL measurement, but giving no additional irradiation. For the "luminescence correction" method, the values obtained after correction should be identical. For the dose correction method, the corrected data should fall on the growth curve, but not on top of each other (Duller, 1994). It should be noted that two sub-samples are actually needed for these additive-dose methods, one being required to quantify the decay, as in Fig. 6.24b. Galloway (1996) developed an empirical approach that used only one aliquot. It was based on there being a fixed signal loss as a function of each measurement cycle. This loss is determined using repeated cycles with no dose at the end of the additive-dose measurements. The loss per cycle is then applied to each data point used to construct the additive-dose growth curve. A similar approach was taken by Zhang et al. (2001). 6.11.2.1.2. Regenerative dose. The single-aliquot regenerative-dose procedure outlined by Duller (1994, 1995) is simpler to apply, since it does not require a correction for signal loss with repeated measurement. A growth curve is constructed as in the multiple-aliquot procedure; however, measurements are made consecutively. To determine
Retrospective OSL Dosimetry (a)
(c)
~- 150000
~r 150000
c 0 o
c
v ,_1 00 nm
o N+B 3
c
o
[] N+B 1
20
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Added Beta Dose (Gy)
._1
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40
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_.. []
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20
30
40
Added Beta Dose (Gy)
so
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~- 150000c 0
0.9
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Preheat number
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8
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o N+B 4
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8 [] N+B 5
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o
281
[] Uncorrected 9 Dose corrected
!1"
-10
0
I'0
20
3'0
4'0
5'0
Added Beta Dose (Gy)
Fig. 6.24. IRSL data set obtained for the SAAD protocol as applied to feldspars from a dune sand from New Zealand. (a) Raw data with no correction for the effect of pre-heating and the IRSL measurement, (b) the loss of IRSL due to repeated heating and IRSL measurement, with data obtained for seven other aliquots, (c) data from (a) corrected using the data in (b) with the luminescence correction method of Duller (1994), and (d) as for (c) but using the dose correction method of Duller (1994) (from Duller, 1995).
the correct value of De, no sensitivity changes must occur as a result of repeated cycling. This was not found to be the case, with the measured sensitivity increasing with cycle. There is also a dependence upon the spectrum of the light used to remove the IRSL signal (Richardson, 1994). This procedure was thus abandoned, until sensitivity monitoring within a SAR protocol was used (Wallinga et al., 2000a). With this procedure, reproducible values of De were given by potassium-rich feldspars that had been separated from sediments. However, ages derived using these values were consistently too young compared to both the quartz OSL ages and independent ages (Wallinga et al., 2000b). The age underestimation has been suggested to be related to changes in optical absorption as a result of pre-heating and light exposure (Wallinga and Duller, 2000), changes in electron trapping probability as a consequence of heating (Wallinga et al., 2000b) or anomalous fading (Huntley and Lamothe, 2001) or a combination of all three processes. 6.11.2.2. Quartz 6.11.2.2.1. Additive dose. The possibility of using single-aliquot procedures was put forward by Smith et al. (1986). They suggested that it would be possible to construct an additive-dose growth curve using very short stimulation times. Stokes (1994) presented
282
Optically Stimulated Luminescence Dosimetry
experimental results that showed an apparent lack of dose-dependent sensitivity changes. He thus deduced that it would be possible to derive a SAAD procedure. The experiments used a pre-heat of 16 h at 160~ which would not be practicable in routine dating. Galloway (1994) applied Duller's method to samples of heated quartz, but gave no initial dose. He used a short pre-heat (200~ for 1 min) and used 10 s exposure to the green LED stimulation source (approximate power at sample 0.2 mW/cm 2 and peak emission at 565 nm). The growth curves that he constructed for single aliquots were similar to those obtained using multiple aliquots. The approach developed by Galloway (1996) for feldspars (see Section 6.11.2.1.1) was adopted for quartz (Liritzis et al., 1997). Indeed the application to quartz was simpler than for feldspars. The decay of the OSL signal on repeated cycles of heating (220~ for 1 min) and OSL measurement was found to be exponential with cycle (Liritzis et al., 1997); thus, the percentage correction is the same for each component of signal resulting from each added dose. This exponential decay can be determined from additional measurements at the end of the additive-dose sequence, making it a true single-aliquot procedure. These data are used in an iterative least squares fitting correction (Galloway, 1996). This procedure has been applied to samples of sedimentary quartz and to quartz from small fragments of pottery taken from sub-surface drill cores. The OSL results for the ceramics fitted well with the radiocarbon chronology for the cores from which the fragments were taken (Liritzis et al., 1997, 2001). Other studies on pottery gave results in agreement with the archaeological evidence (Hong et al., 2001). However, for the sediments, no independent age control was available to confirm the results obtained for quartz (Hong and Galloway, 2000). However, Hong and Galloway (2000) demonstrated that values of De obtained using blue LEDs (420 nm and with 5 mW/cm 2) were in agreement with those obtained on the same sample using green LEDs (475 nm) giving the same power to the sample. Using the blue diodes gave better precision, however, owing to the higher OSL output achieved using this wavelength. Although these procedures gave acceptable results for the ceramic samples, it is necessary for the absolute luminescence sensitivities to be identical for both natural- and laboratory-irradiated aliquots. Wintle and Murray (1999) have shown the sensitivity for a sample of sedimentary quartz to be critically dependent upon time and temperature, whether in nature or relating to laboratory pre-heats. For the dating studies reported by Liritzis et al. (1997, 2001) and Hong et al. (2001), it may have been fortuitous that the 1min pre-heat at 220~ resulted in similar luminescence efficiencies. To support the method, Hong et al. (2000) carded out experiments to characterise the behaviour of the OSL signal when subjected to repeated cycles of pre-heat and OSL readout. Each measured decrease could be expressed as a function fin), where f(n) = exp[-c(n - 1)], where n is the number of measurements made on the aliquot and c is a parameter that depends upon pre-heat temperature and duration. This can be rewritten as f ( n ) = r (n-l~, where r = exp[-c] is the ratio of two successive measurements. Measurements made without the pre-heat showed the loss in OSL to be 5% from the optical stimulation alone. In 1997, Murray et al. also reported the exponential decay of OSL with stimulation cycle for several sedimentary quartz samples (Fig. 6.25). This led them to propose a single-aliquot additive dose (SAAD) method, again evaluating the exponential decay with
Retrospective OSL Dosimetry E
10 4
283
N+215 Gy
O O
J co o e0
C5
200~ 103 N+215 Gy
"13
00
m t~
zo
10 2
280~
(a) DS2 i
i
2
4
i
I
i
i
6
8
10
12
14
Stimulation Cycle E
10 4
O tO
co O ,,_
10 3 N+215 Gy
"13 Q) ._
E
zo
10 2
(b) WIDG8 I
I
I
I
I
i
i
2
4
6
8
10
12
14
Stimulation Cycle
Fig. 6.25. OSL signals obtained by 0.1 s stimulation after repeated 10 s pre-heats. (a) Data shown for two preheat temperatures, 200 and 280~ and (b) for another sample for 280~ only. The semi-log plot indicates the loss is exponential. Data are shown for the natural OSL and for aliquots given an additional dose ranging from 1.7 to 215 Gy. The data suggest only weak dependence upon the dose (from Murray et al., 1997).
measurements at the end of the sequence (Fig. 6.26). This approach is essentially identical to that of Liritzis et al. (1997). One of the assumptions in the SAAD method is that the depletion rate is dose independent. In fact, careful inspection of the data in Fig. 6.25 suggests that there is a weak dependence of the decay constant on dose, as was pointed out by Murray et al. (1997). For the temperatures shown in Fig. 6.25a, the direction of change is different for different temperatures. For the 200~ pre-heat (10 s) the slope decreases from 0.090 to 0.073 with increasing dose, whereas for 280~ the slope increases from 0.24 to 0.29 over the same dose range. For both the samples in Fig. 6.25, the laboratory dose of 215 Gy far exceeds the natural values of De, which are --- 3 Gy for DS2 (Figs. 6.25a and 6.27c) and --- 50 Gy for WIDG8 (Figs. 6.25b and 6.27e). If the effect were dose dependent at this level, then it would have a negligible effect on SAAD dating of DS2. Given the different response for the two pre-heats, the effect is most likely to be related to changes in the luminescence efficiency brought about by continuous application of the 10 s pre-heats. Additional studies by Murray et al. (1997) showed the value of De to be independent of the stimulation temperature employed (using 25, 110, 160 or 200~ However, different values of De were obtained when different pre-heat conditions were employed (Fig. 6.27).
284
Optically Stimulated Luminescence Dosimetry 2x104
t"
2x10 4 -
~
w
~
~
O O _J (O O
c5
104
-
5X103 -
/
/ I
0
!
5 10 Cumulative Added Dose, Gy
Decay Cycle
Fig. 6.26. Additive-dose growth curve obtained using a single aliquot of quartz. Squares show the data after correction using the data points from the decay cycle. De ("~ 3 Gy) is obtained by extrapolation (from Murray et al., 1997).
For young sediment samples, giving De values of 0.4-3 Gy (Figs. 6.27b,c) there is little dependence on the pre-heat temperature. For the very young sample (Fig. 6.27a) with De ~" 0.03 Gy, the De plateau is destroyed by thermal transfer effects (see Section 5.1.8.5). For the older samples, shown in Fig. 6.27d,e, the values of De for low temperature preheats (up to 280~ are severely overestimated. This overestimation was shown to be due to the effects of luminescence sensitivity change (Wintle and Murray, 1999), rather than thermal transfer from a peak at --~280~ as originally suggested by Murray et al. (1997). From such data, it can be seen that higher pre-heats are required for older samples in order for the laboratory sensitivity to be made equivalent to that pertaining to the naturally irradiated sample. Although the SAAD approach was reported to work on 13 (out of 15) Australian sedimentary quartzes (Murray et al., 1997), some problems have been reported. Stokes et al. (2000) report a 64% failure rate, particularly for fluvial quartz from Egypt and the River Loire in France. These samples could be identified by a dip in their uncorrected growth curve (Fig. 6.28). The problem could also be seen by analysis of the decay in OSL signal with cycle (Fig. 6.29). This graph shows the decay observed for 15 repeated cycles, using a pre-heat of 10 s at 250~ and blue LED stimulation with 24 mW/cm 2 at 125~ The exponent obtained for the initial five points and the last five points is different. After 6 cycles at the beginning of the decay curve (using exp- o.1122,,, where n is the cycle number) 51% of the signal would be left, whereas using the exp- 0.0839n, 60% of the signal would be left. Clearly this deviation from a single exponential function prevents the application of the SAAD protocol. A check for exponential decay over the 15 or so cycles used to construct the growth curve and the final decay curve should be carried out as a preliminary check. By measuring the 110~ TL peak generated by a small test dose, Stokes et al. (2000) demonstrated that the sensitivity did not remain constant for the duration of the measurement cycles. Having discovered the magnitude of the sensitivity changes caused by even a short preheat (10 s), and more importantly the different magnitudes of the sensitivity changes for
Retrospective OSL Dosimetry 0,2
285
(a) DS1
De ( 1 6 0 - 2 0 0 ~ = 0.026+0.006 Gy
0,1 0,0
D e (220-300~ = 0.41+0.03 Gy
0,8 0,4 (.9 0r)
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0,0 321-
uJ
0
w
o+.~- =
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m
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z
I
40
[]
D e (160-270~ = 2.69+0.04 Gy
(c) DS2
80-
0
I
(b) NEST6/3
{
De (260-300~ = 22.4+0.7 Gy i w
(d) AC 150
A w
A w
i w
200 100
(e) WIDG8 D e (280-290~ = 52.1+1.0 Gy
9 9 o ~
l
l
u
l
150
200
250
300
Preheat Temperature, ~ Fig. 6.27. De obtained using SAAD as a function of pre-heat temperature for five samples of Australian quartz (from Murray et al., 1997).
natural- and laboratory-irradiated aliquots of Australian quartz, Wintle and Murray (1999) attempted to correct for the sensitivity change. They used the 110~ TL response to a small test dose given at the end of each OSL measurement in the additive-dose measurement sequence. The OSL data were corrected by dividing by the subsequent TL response, and the additive-dose growth curve was then constructed as before. The result is shown in Fig. 6.30. It can be seen that D e is now independent of the pre-heat temperature and the mean value of De is close to that obtained using other methods, i.e., 52 Gy. 6.11.2.2.2. Regenerative dose. The SAR protocol for OSL dating of quartz has been described in Section 6.5.4.5, where it was applied to heated quartz. The main feature of the protocol is the use of an OSL response (Tx) to a test dose given immediately following the natural OSL measurement (Ln) and after each regeneration-dose measurement (Lx). It is implicitly assumed that both the main and test-dose OSL signals are derived from the same electron trap, i.e., that giving rise to the fast component of the OSL (Section 5.1.2.4).
286
Optically Stimulated Luminescence Dosimetry 50
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"6
~
4O 30
2o
0
0
10 I
I
I
I
I
100
200
300
400
500
600
Dose (Gy)
Fig. 6.28. SAADdata set for quartz from a fluvial sediment from the Loire. Open symbols show the raw data and the closed symbols show the data after correction using the decay function determined from the last five data points (from Stokes et al., 2000).
If other components are present, e.g., an ultra-fast component is present following laboratory irradiation or the fast component is not dominant, the SAR protocol may be inappropriate. The experimental procedure, outlined in Table 6.1 for heated quartz (Section 6.5.4.5), is that originally proposed for sediments. The pre-heat made prior to measurement of the main OSL signals is designed to detect, and allow the removal of, any thermally-unstable component of the OSL signal that may be induced by laboratory irradiation. The pre-heat, selected from within the range 160-300~ is applied for 10 s, a time convenient for automated OSL readers. Such thermal treatments have been shown to result in sensitivity change in some samples (e.g., Wintle and Murray, 1999; Section 5.1.8.2). Repeated application of the pre-heat during the course of the SAR cycle has been shown to result in
~,.
y = 1.0842e-0.1122x ----
0.8 E 'O
0.6
co 0
0.4
t-
.
y = 1.0246e-0.0839x
0.2
R 2 = 0.9881
0
5
10
15
Measurement cycle
Fig. 6.29. Depletion of OSL signal caused by repeated pre-heating and optical stimulation (but no added dose) showing that the data points are not fitted by a single exponential (from Stokes et al., 2000).
Retrospective OSL Dosimetry
287
(9
d
ro 600 D
t - Uncorrected additive 9 dose
t"
_.e 400 ._> C7"
w 200 e--
Sensitivity corrected additive dose 9
Q.
<~
0
i
150
9
"9
A -
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i
!
i
200
250
300
Preheat Temperature, ~ Fig. 6.30. D e obtained using SAAD as a function of pre-heat temperature. Squares were obtained without correction for sensitivity change (as also in Fig. 6.27e), whilst circles were obtained with correction using the 110~ TL peak (from Wintle and Murray, 1999).
progressive sensitisation. This is best demonstrated when the same regeneration dose is given in order to reduce additional changes due to dose dependence. Sensitivity changes are observed in Lx, with signals either increasing for decreasing depending on the pre-heat temperature; this is best investigated using the response to the test dose, Tx, (Section 6.11.2.4). Prior to the measurement of the OSL response to each test dose, a brief heating is carded out to prevent a contribution to the luminescence from the isothermal decay of the 110~ TL peak. This is usually performed as a so-called cut heat, i.e., heating up at 5~ and then cooling immediately once 160~ is reached. This also permits simultaneous measurement of the 110~ TL peak. An alternative thermal treatment is to heat to 125~ hold for about 10 s to make sure the traps corresponding to the 110~ peak are empty, and then measure the OSL at 125~ Both Tx and Lx (and Ln) are measured at 125~ with the aim of increasing the initial signal as a result of thermal assistance (see Section 5.1.9.2) and keeping the 110~ TL traps empty during optical stimulation. The equivalent dose is obtained by projecting the sensitivity-corrected natural luminescence (Ln/T,) onto the sensitivity-corrected growth curve (given by multiple measurements of Lx/Tx).
6.11.2.3. Luminescence sensitivity The discussions in Sections 6.11.2.1 and 6.11.2.2, together with Section 5.1.8.2, have demonstrated the importance of luminescence sensitivity changes in quartz when using various experimental procedures to determine radiation doses delivered to quartz in the natural environment. Empirical methods of correcting for the different luminescence responses of naturally-irradiated (and heated) aliquots and laboratory-irradiated (and preheated) aliquots have been developed. The use of the 110~ TL peak, and/or the OSL signal from quartz, in response to a test dose, to monitor the OSL sensitivity was mentioned already in Section 6.5.4.4. The specific application in the discussion was for heated materials (from bricks) for purposes of accident dosimetry. Here we explore the appropriateness of these two monitor signals in more detail.
288
Optically Stimulated Luminescence Dosimetry
The similarity of the sensitivity changes of the 110~ TL and the OSL, when step heated to 300~ was first reported by Aitken and Smith (1988). As a result of his study of the ll0~ TL peak in quartz, Stokes (1994) suggested that it may be possible to correct for sensitivity changes in a single-aliquot additive-dose protocol. An experimental sequence was devised by Murray and Roberts (1998) to measure both the 110~ TL and OSL responses to a test dose at several points during a SAR cycle. The sequence is listed in Table 6.2, with 0.2 Gy test doses being interposed before and after each pre-heat (280~ for 10 s) or cut heat (to 160~ This results in four 110~ TL and one OSL measurement that could be used as a potential monitor of the sensitivity of the main OSL signal. Steps 1 - 8 of the sequence represent the first measurement cycle of the SAR protocol, with measurement of Ln in step 5 and Tn in step 8. Steps 1 - 1 0 are repeated 15 times to build the data sets for each signal and the responses for three samples are shown in the left-hand column of Fig. 6.31, with the symbol coding given in Table 6.2. The samples (DS2, K166 and WIDG8) were of Australian sedimentary quartz and had equivalent doses of about 3, 20 and 50 Gy, respectively. For each cycle the regeneration dose was the same, and chosen to be similar to the equivalent dose. The high pre-heat temperature, 280~ was chosen to ensure that relatively large sensitivity changes occurred over the 15 measurement cycles. All signals show increasing absolute values with repeated cycles. By plotting the luminescence values of each of the five luminescence signals as a function of the main OSL signal, it is possible to judge which is the best monitor of the luminescence efficiency. The graphs in the right-hand column of Fig. 6.31 show these plots, with data taken from the left-hand column. To a first approximation, all the signals show linear correlation, but many do not pass through the origin. The signals passing closest to the origin, and thus likely to be the best monitors, are the OSL signals from the test dose (solid diamonds). The 110~ TL signal measured following regeneration and during the subsequent cut heat to 160~ (solid triangles) is reasonable for DS2 and for WIDG8, but gives a large intercept for K166. Another point to note from these graphs concerns the 110~ TL response to the initial test dose (open circle). This is measured as the first pre-heat is made and only for the
Table 6.2 Procedure used to examine dependence of sensitivity change on repeat regeneration cycle
1 2 3 4 5 6 7 8 9 10 11
Procedure
Observe
0.2 Gy test dose (Td) 280~ pre-heat for 10 s 0.2 Gy Td 160~ cut heat 100 s OSL at 125~ 0.2 Gy Td 160~ cut heat 100 s OSL at 125~ Regeneration dose 160~ cut heat Repeat from step 1
O 110~ TL 9 110~ TL 9 Net OSL 9 110~ TL 9 Net OSL 9 110~ TL -
Retrospective OSL Dosimetry 3x104 9
2xl 04 j T x O . O 9 x O . O 9
104
104
0 I. . . . .
^--a) ' %USzl -
1 05
10 5
-9 5x104
5x104
0 r "E=
. . . .
(b,) K1661
4
x~_-*~'~-v"~-~
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.........
o
0 I
_x0-041
.- i- c) 7,7
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0
1 3 5 7 9 11 13 15
"J
~ ,
0
8 o
Z ~ I I I I I ~
0
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~
(d)
3x104
2xl 04
289
2x106 4x106 6x106
2x104
104
"'1 , 1
1 3 5 7 9 11 1315 Treatment Cycle 9 regen.TL before preheat 9 TL from Td during preheat 9 TL from Td before OSL
o 0 5x105 106 OSL from nat. or regen, dose 9 TL from Td after OSL 9 OSL from nat. or regen. ~I~ OSL from Td
Fig. 6.31. Left-hand column gives luminescence signals measured by repeating 15 times the measurement cycle in Table 6.2. Right-hand column gives each signal plotted as a function of the main OSL signal. Data are for three samples of Australian sedimentary quartz with De of --~ 3, --~ 20 and --- 50 Gy (from Murray and Roberts, 1998).
youngest sample (DS2) does it fall on the line defined by subsequent TL sensitivity measurements prior to pre-heating (solid circles). The implication of this is that the luminescence sensitivity of the natural OSL has been sensitised to some degree in nature. Indeed for K166 and WIDG8 the open circles lie on the line defined by the solid squares, namely the 110~ TL response to the test dose after application of the pre-heat. For DS2, a possibility is that there had been insufficient time for this sensitisation to occur. Several other plots using similar repeated cycles have been published, e.g., for heated materials (Murray and Mejdahl, 1999; Murray and Wintle, 2000) and coastal sediments (Armitage et al., 2000). Another example was given in Fig. 6.6 for quartz from a house brick in a lowdose study (Section 6.5.4.4). The 110~ TL and OSL responses can be compared when test doses are given to optically bleached samples. Wintle and Murray (1999) took an optically bleached sample and held aliquots at different temperatures for different lengths of time in experiments to observe thermal sensitisation. After the delivery of a 0.5 Gy test dose, the sensitivity was monitored using the 110~ TL peak area and the initial 0.4 s of the OSL at 125~ They used six different storage temperatures (180-280~ in 20~ steps), all of which resulted in an increase in luminescence response. The 110~ TL is plotted against the 0.4 s OSL in
290
Optically Stimulated Luminescence Dosimetry
Fig. 6.32. The data sets show that the signals are well correlated, but the best fit straight line does not pass through the origin. Plots of 110~ TL versus OSL signal (first 0.4 s) derived from a test dose can also be obtained from thermal annealing plots for optically bleached samples. Wintle and Murray (1998) used a 0.2 Gy test dose and a 10 s pre-heat, the temperature of which was increased by 10~ every cycle over the temperature range from 160 to 500~ Fig. 6.33 shows the linear relationship between the two signals for temperatures up to 310~ again, the best fit straight line does not pass through the origin. Similar plots were obtained for quartz from heated stones (Murray and Mejdahl, 1999). The 110~ TL and OSL responses to a test dose for two sedimentary samples were also investigated by Chen et al. (2000) during the construction of growth curves up to high dose (---670 Gy). There was good agreement between the two signals for all regeneration doses, and similar agreement was achieved when the growth curve was regenerated after heating the sample to 500~ for 10 min. However, following 10 min at 900~ (i.e., above both quartz phase changes), the ratio of the two signals became dose-dependent, with the direction of change being different for each of the two samples. Other studies have shown the OSL and 110~ TL sensitivities to be different when samples of sedimentary quartz were subjected to experiments involving thermal annealing (Chen and Li, 2000; Chen et al., 2001; Li, 2002) and laboratory irradiation (Li and Chen, 2001). Based on these studies, Li (2002) proposed a model involving three electron traps, one type of luminescence centre and three non-radiative recombination centres. The model was based on that originally proposed by Zimmerman (1971) to account for the pre-dose behaviour of the 110~ TL peak. Li (2002) proposed that the same luminescence centre was used to produce both the 110~ TL and OSL, and that one of the non-radiative recombination centres was common to the charge released from both the 110~ TL trap and the fast component OSL trap. Li (2002) proposes that these non-radiative centres are less thermally stable than the radiative centres, or the luminescence centre. Thus heating between 160 and 300~ causes holes to be released from the non-radiative centre, resulting in a rise in both 110~ TL and OSL sensitivities. The relative increases found for -// C 0 0
"~ C
J
0 d
10 5 8X104
6x104 r
-/,.,, 0
i
i
i
10000
15000
20000
OSL, counts/O.1 s
Fig. 6.32. The 110~ TL response to a test dose plotted as a function of the initial OSL due to the sametest dose. Increases in signalresponse are causedby increasingthe time that each opticallybleached aliquot is held at 160~ (circles), 180~ (hexagons), 200~ (diamonds), 220~ (squares), 240~ (upwardtriangles) and 280~ (downward triangles) (from Wintle and Murray, 1999).
291
Retrospective OSL Dosimetry
t-
--,
0
,60j#d,-
10 5
0
I
460
<
220
_.1 ~ 5xl 0 4 o
-
o
o
0
0
I
10 4
I
2x 10 4
I
3x10 4
4x10
First 0.4 s OSL, counts
Fig. 6.33. The 110~ TL response to a test dose plotted as a function of the initial OSL due to the sametest dose. The change in signal is caused by heating for 10 s at progressively higher temperatures from 160 to 460~ between test doses (from Wintle and Murray, 1998). the two signals depend upon the concentration of the other non-radiative centres. This means that the l lO~ TL peak may not always be an appropriate monitor of OSL sensitivity change.
6.11.2.4. Reliability of OSL monitoring of sensitivity change The SAR protocol will be valid if the OSL response to a test dose (Tx) at the end of the main OSL measurement (Lx) is an appropriate monitor of the sensitivity. The simplest test is to make repeated measurements using the same conditions, as in Section 6.11.2.3. The OSL signals can be plotted against each other (as in Fig. 6.31) or the ratio (T~/Tn) can be plotted as a function of cycle. Both increases and decreases in OSL sensitivity are observed when aliquots are given repeated regeneration doses and pre-heats for 10 s. Data for two sediments are given in Fig. 6.34. For low temperature pre-heats ( < 240~ the sensitivity decreases by up to 40%, whereas for temperature above 240~ there is either little change in sensitivity (Fig. 6.34a) or an increase with repeated cycling (Fig. 6.34b). For the sample shown in Fig. 6.34b, the equivalent dose obtained using SAR was found to be independent of pre-heat temperature (Armitage et al., 2000). Thus, the test dose OSL signal (T~) can be used to correct for a range of sensitivity changes (up to a factor of 2) that can be induced by repeated pre-heats during the SAR protocol. There have been a few reports of the SAR equivalent dose not being independent of the pre-heat, or cut heat temperatures (e.g., Bailey, 2000). This appeared to be related to the non-proportional nature of the main OSL (Lx) and test dose (T~) as determined using a repeated regeneration dose (c.f., the 1:1 relationship seen for the test dose OSL signal in Fig. 6.31 (right-hand column)). Bailey (2000) showed that the relationship changed depending upon the preheats given prior to measurement of L~ and Tx. The smooth nature of the sensitivity change shown in Fig. 6.34 and the stabilization that is shown after six repeated pre-heat cycles has been found by others. Bailey (2000) found
292
Optically Stimulated Luminescence Dosimetry (a) A
X
1,1 High Temperature Preheats
1,0 0,9
._ 0,8 "" 0,7
9 0,6 "~ 0,5
Low Temperature Preheats
0,4 0
(b)
1,6
~
1,4
~
1,2
2
4 6 12 Cycle number
10
--0- 160~
- I - 180~
~
200~
- O - 220~
- O - 240~
~
-A-- 280~
-<>- 300~
260~
12
{9 1,0
0,8 ~,, 0,6 0,4 0
. 2
.
Low Temperature Preheats . . . 4 6 8 10 12 Cycle number
Fig. 6.34. Relative sensitivity changes (Tx/Tn) obtained by using the same regeneration dose in a SAR procedure. Data are shown for eight different pre-heat temperatures and are normalised to the first test dose response (Tn) obtained after measurement of the natural OSL for two samples from southern Africa (from Armitage et al., 2000).
that both the rising and falling data sets could be fitted by the empirical equation f (i) - a + b e x p ( - i/d),
where i is the number of the SAR cycle and a, b and d are fitting constants. This led Bailey (2000) to propose a new regenerative dose procedure. Following measurement of the natural OSL, a laboratory dose is given and the OSL measured. This step is repeated until negligible change in sensitivity occurs, and the data thus obtained are fitted by the above equation. A correction factor for the natural OSL signal is obtained by extrapolation of these data to that relating to the first measurement cycle. The regeneration growth curve is then measured and the corrected natural OSL signal is projected onto the growth curve to
Retrospective OSL Dosimetry
293
obtain De. This approach does not require the application of test doses after each OSL measurement. However, it does require that the sensitivity related to the natural OSL changes in a way that is consistent with the changes in the OSL response to laboratory dose brought about by repeated irradiation, pre-heating and measurement. No experimental testing of this approach has yet been carried out.
6.11.3. Dose distributions for single aliquots With the development of single-aliquot dating procedures, particularly SAR, it has been possible to obtain multiple values of D e for each sample. These values can be combined to obtain a single value of De for calculating an age, obtaining a weighted mean and standard deviation (o-), and, if each value forms part of a normal distribution, the standard error on the mean can be calculated (~r/n), where n is the number of D e values. A large data set may also be displayed graphically, permitting the observer to see whether the distribution is normal or skewed.
6.11.3.1. Histograms The simplest method of displaying the data is as a histogram. To construct a histogram, the individual D e values must be entered into bins of finite D e width. Construction of a histogram does not follow any particular rule. When more observations are made, the bin width is made smaller so that the shape of the distribution has better resolution (Lyons, 1991). As the number of measurements is increased even further and the bin width decreased, the histogram will approach a continuous distribution. For a normal distribution this will approximate to a Gaussian distribution. As discussed by Lepper et al. (2000), an objective bin width can be determined for a particular data set by defining it as the median value of the standard deviation (o') obtained for that data set. The mean value of o- was not used, as this can be skewed if the data set includes a small number of poorly determined D e values. As Lepper and McKeever (2002) point out, a range of o- values is obtained for any SAR data set. If the bin width is much larger than o-, there is loss in the resolution of the data set, and, if the bin width is much smaller, no meaningful histogram results. Olley et al. (1998) compared D e histograms for small aliquots from very young aeolian and fluvial sands. For the aeolian sample, little asymmetry was observed, whereas for the fluvial sample there were a number of De values that formed a high D e "tail". When they examined the De distributions for samples taken progressively deeper down a core, they found that the leading edge of each distribution moved systematically to larger dose values. Olley et al. (1999) found similar asymmetric distributions for other fluvial sediments and this led them to calculate an age for each sediment based on the lowest 5% of De values. These ages were in agreement with independent age estimates. Olley et al. (1999) concluded that the higher De values were the result of poorly bleached grains being found in fluvial deposits. Lepper and McKeever (2002) also compared D e histograms (constructed using the median value of o-as the bin width) for 100-125 aliquots of young aeolian and fluvial sands; they showed that a Gaussian curve could be fitted to the leading edge of both
Optically Stimulated Luminescence Dosimetry
294
distributions (Fig. 6.35). From the second derivative of each curve, they obtained the dose at the inflection point; this value was taken as an objectively determined radiation dose that represents the dose acquired by the grains since deposition. In each case, this value was larger than that obtained using the lowest 5% of De values, but smaller than that obtained using the mean of all De values. For palaeochannel sediments in south-eastern Australia, Banerjee et al. (2002) used the leading edge approach and the lowest 5% approach to obtain ages of 250 ___50 year and 230 + 50 year, respectively, for a sample expected to be less that 200 years old. Assuming that the observed distribution of values found for the fluvial sediments results from a combination of the experimental error in D e determination and the residual signals resulting from incomplete bleaching at deposition, Lepper et al. (2000) used a nonparametric technique to de-convolve these distributions. The experimental error was determined using the OSL data for the final regenerated point (termed the check dose), which was obtained using a dose at the midpoint of the regenerated dose range of the SAR measurements. This is equivalent to determining the recycling ratio. Histograms have also been employed to display De values obtained for small aliquots of unfired quartz grains from mortar used in the construction of a low-level radioactive-waste storage facility (Jain et al., 2002). Fig. 6.36 shows the histograms obtained for at least 80 aliquots of 75-100 grains taken from slices cut out at 0.7 and 11.2 cm from the inner face of the wall. In addition, the cumulative frequency plot for each data set is given. Such plots have several advantages, the main one being that a normally distributed data set would plot as a straight line, with a mixed population showing up as a change in the slope of the line (Jain et al., 2002). A wider distribution is found for the slice at the face (0.7 cm); this is probably the result of the non-flat surface of the mortar presented to the radioactive sources in the storage facility. For the slice at 11.2 cm depth, the slope of the cumulative frequency plot is straight, indicating a single population of De values. The doses of 9.45 +__0.21 Gy at 0.7 cm and 2 . 4 7 _ 0.03 Gy at 11.2 cm depth correspond to the
4O
40
" (a) Aeolian #2
(b) Fluvial #1 I
c v >.., 0 c
3O
3o
I >,, O c
=
g 2o
O"
20
L H-
U_
lO
o
|
I |
0
0.52
! |
1.04
o -
|
1.56
|
2.08
T!
o
,
,
1 ~
!
~
2.55 5.10 7.65 Equivalent dose (Gy)
"7
Equivalent dose (Gy)
Fig. 6.35. Histogram of D e values for single aliquots of (a) an aeolian dune sand and (b) a sand sample taken from a floodplain deposit. The bin width is the median value of o-for each sample. The bold line represents a Gaussian curve fitted to the leading edge of the distribution (from Lepper and McKeever, 2002).
Retrospective OSL Dosimetry
295
accumulated doses over 11 years of radioactive-waste storage and are in agreement with the dose rates measured over shorter timescales using artificial dosimeters. Histograms are unable to display the precision with which each De value is obtained. Indeed, the data used in Fig. 6.36 were deliberately selected to have uncertainties of less than 15% to ensure the data were not skewed by the inclusion of imprecisely known De values. Instead, values of De and o- can be presented as ranked in order of increasing De (e.g., Fig. 6.37). If values of or were similar, the most likely value of De would be that for which the data set is most vertical.
6.11.3.2. Probability density plots Another way in which the results can be displayed is also shown in Fig. 6.37, namely as a probability density plot. These were originally introduced for fission-track dating of zircons (Hurford et al., 1984) and the statistical basis for using these plots was set out by Brandon (1996). Each De value is represented by a Gaussian density function with a standard deviation equal to the standard error of the estimate. These functions are then s u m m e d to produce a continuous "frequency" curve. This approach has been heavily criticised by Galbraith (1998) on the basis that when values obtained with high precision
0.7cm depth
40
(a)
30
.__~~
99.5 95
h"
g 20 g (I)
1
70
g g if-
10. 0 !
5
0
10 15 20 25 30 35 40 Dose (Gy) 11.2 cm depth
40.
99.5
Q
30"
95
or
g g 20"
70 40
o~
u_ 10.
10
0
0
(b)
5
1 01 10 15 20 25 30 35 40
> -.~
~ -5 E o
Dose (Gy) Fig. 6.36. Histogramsof De values for single aliquots of unheated quartz grains from mortar slices taken at two different depths from a building exposed to radioactive materials for 11 years. For the slice at 11.2 cm depth, 86 aliquots were used to obtain an average dose of 2.47 _ 0.03 Gy; at 0.7 cm the average dose was calculated to be 9.45 ___0.21 Gy. Also shown is the cumulative frequency plot for each data set (from Jain et al., 2002).
296
Optically Stimulated Luminescence Dosimetry
Fig. 6.37. (a) Probability density plot, together with De values ranked in increasing value, and (b) radial plot for the same values obtained by applying SAR to 30 aliquots of multiple grains of an aeolian sand from South Africa (Jacobs, pers. comm.).
are combined with those obtained with low precision, the information from the precise result is obscured. He also argues that the apparent peaks obtained in such plots do not necessarily correspond to discrete component values.
6.11.3.3. Radial plots A more appropriate method of displaying large numbers of De values is a radial plot, again developed for displaying fission track ages (e.g., Galbraith, 1990). Radial plots have been used for De values obtained for single aliquots of quartz grains from fluvial deposits in Australia (e.g., Olley et al., 1999), from fluvial or colluvial deposits in central Africa (Feathers and Migliorini, 2001) and from sediments deposited against limestone rock faces in New Zealand (Holdaway et al., 2002). An example for multiple grain aliquots of an aeolian sand in South Africa is shown in Fig. 6.37b, and is used as example for the following discussion. In a radial plot, each De value is plotted as an individual point on the graph. The position on the x-axis is a measure of the precision with which De is known. The more precisely the De value is known, the further it plots to the fight of the diagram. This axis
Retrospective OSL Dosimetry
297
can also be expressed in terms of the relative error (expressed as a percentage). The value plotted on the y-axis, labelled as 'standardised estimate', is the difference between the De for that aliquot and some reference value, with the calculations carried out in logarithmic values. The difference is then divided by the uncertainty in De; thus the yaxis is the number of standard deviations that D e value lies away from the reference value. The choice of reference value is arbitrary, but the mean value of D e is the value usually chosen, in this case 61.4 Gy. Altering the reference value causes the data points to rotate about the origin. A mathematical consequence of plotting these two parameters is that data for those aliquots with the same De will fall on a line radiating from the origin. Thus, it is convenient to draw a third 'radial' axis on the right-hand side of the graph that gives D e. Such graphs are particularly useful for plotting data that have widely varying uncertainties. By drawing a band from the ' 4- 2' and ' - 2' points on the y-axis, one can de-limit the area within which the data will lie if they are consistent with a given value of De within 2o-. Thus, for a single population, 95% of the points should fall within this band. For the sample in Fig. 6.37, 30 D e values were obtained and all but four (shown as open circles) fell within the __+2o- band, i.e., 87%. Radial plots have also been used to display the range of doses found when small aliquots of quartz are extracted from concrete to examine their suitability as retrospective dosimeters for buildings. Doses over 100 Gy were obtained for a very small number of the 183 aliquots measured (Thomsen et al., 2002a); these derived from the incorporation of poorly bleached grains when the concrete was made. However, when a dose of 2.69 + 0.03 Gy was delivered to the concrete, a leading edge could be seen in the histogram of 157 aliquots (Fig. 6.38). When the ___20-band was placed on the lower dose edge of the radial plot, it included results for 24 aliquots, for which a weighted mean dose of 2.85 ___ 0.07 Gy was calculated. In another investigation of building materials, both single grain and single-aliquot radial plots have been published for quartz grains extracted from mortar between bricks (Jain et al., 2002).
6.11.3.4. Calculation of D e Having displayed the data in one of the formats discussed above, it is necessary to calculate the value of De that is most representative of the dose that has accrued since the event of interest. For samples that were well bleached at deposition, it is usually assumed that all grains would have received the same dose. Thus, a mean value would be appropriate, and if the measurements are part of a normal distribution, then the appropriate error would be the standard error of the mean, as discussed at the beginning of Section 6.11.3. When the De values are measured with different precisions, i.e., are spread along the horizontal axis of the radial plot, the weighted mean would be appropriate. These calculations assume that the distribution of the measured values of D e is normal. However, some radial plots indicate that the measured De values are not consistent with all aliquots having a single common value of De. This led Galbraith et al. (1999) to develop a 'central age model' to obtain the best estimate of D e. This model assumes that there is a distribution of true D e values that is described by a normal distribution of the logarithmic
298
Optically Stimulated Luminescence Dosimetry
Fig. 6.38. (a) Histogram of 157 aliquots of unheated quartz from an unfired concrete brick exposed to artificial sources and given a dose of 2.69 ___0.03 Gy. (b) Radial plot of the same data set, with the reference value chosen to be the given dose; 24 aliquots were located within the ___2o-band and these resulted in a weighted mean dose of 2.85 + 0.07 Gy (from Thomsen et al., 2002a).
values which are characterised by a mean and standard deviation. These parameters are determined from the measured data set according to the equations developed by Galbraith et al. (1999). This approach has been applied to non-skewed data sets of small aliquots (composed of only a few grains) (e.g., Roberts et al., 2001; Turney et al., 2001; Holdaway et al., 2002). For asymmetrical data sets, Galbraith et al. (1999) suggested using complex statistical packages to calculate a minimum age, and this approach was applied by Roberts et al. (2001) for some of their megafauna sites. Other approaches based on histogram displays (e.g., using the lowest 5% of De values) have been discussed in Section 6.11.3.1. However, in some situations, this approach may not be valid, e.g., if there is downward migration of grains from an overlying, younger deposit as a result of bioturbation.
De
6.12. Single grains 6.12.1. Measurements 6.12.1.1. Feldspars Single grain IRSL measurements of feldspars have been used to investigate the presence of poorly bleached grains and anomalous fading. Lamothe et al. (1994) observed
Retrospective OSL Dosimetry
299
a wide range of natural IRSL signals from 120 feldspar grains in the size range 5001000 Ixm extracted from a marine sand with radiocarbon age control. Fifteen selected grains were dated and half of these showed age overestimation (of 200-600%), whereas the remaining showed a ---50% age underestimation, attributed to anomalous fading. Lamothe and Auclair (1997) extended their study to single feldspar grains from other sediments of known age. Instead of measuring the natural IRSL decay curve in full, they measured the initial part of the decay curve for each grain. They then gave the grains a gamma dose and re-measured the initial part of the curve. The ratio of these two measurements was used to check for inadequate bleaching. The grains were then measured again after six weeks of storage at room temperature, in order to look for anomalous fading. Up to 40% loss of IRSL was observed for the two samples investigated. The ability to measure this decay, and the range of fading observed within a sample, led Lamothe and Auclair to develop a procedure to correct for anomalous fading, the so-called "fadia" protocol (Lamothe and Auclair, 1999, 2000). No deliberate studies have yet been made of the OSL from individual feldspar grains stimulated with visible wavelengths, though signals have been observed from contaminant grains in dating studies of quartz (e.g., Henshilwood et al., 2002).
6.12.1.2. Quartz Single quartz grain OSL dating measurements have been made either by placing single grains on a 1 cm diameter disc on a hot plate in a conventional OSL/TL reader (e.g., Roberts et al., 1998a,b, 1999) or using the single grain laser OSL system described in Section 7.7.3 (e.g., Henshilwood et al., 2002). In the first approach, stimulation was made using blue/green (420-550 nm) light with a power density of --- 16 mW/cm -2 at the disc surface. Using the focussed laser (532 nm) each grain is exposed to ---50 W/cm -2, thus producing an improved signal-to-noise ratio for the OSL signal. Further advantages of the single grain laser system are related to the number of grains per disc (e.g., 100) that can receive simultaneous irradiation or heating. This reduces measurement time, and has led to the SAR protocol being applied to several thousand quartz grains (e.g., Henshilwood et al., 2002). The size of these data sets requires consideration of their display, in order to provide maximum information regarding the reliability of individual measurements. Also, graphical representations provide an assessment of whether or not a single population of grains is present. This is important when considering whether the OSL signals of all grains have been optically zeroed prior to deposition and whether there has been any disturbance of the grains, e.g., by biological activity. Plots of the De values for well bleached and undisturbed sediments should also provide information on natural variation in dose rate received by individual grains. This is addressed by the central age model of Galbraith et al. (1999) (see Sections 6.11.3.4 and 6.12.2.4). 6.12.2. Dose distributions for single grains
6.12.2.1. Histograms For well-bleached samples, such as aeolian dunes, histograms are constructed with D e displayed on a linear scale. However, for samples that include a number of poorly
300
Optically Stimulated Luminescence Dosimetry
bleached grains, it may be necessary to display the histogram with De given on a logarithmic scale. Roberts et al. (1999) found a wide distribution of De values in their study of single grains from Jinmium; the use of histograms was made even more difficult by the larger errors obtained for single grains from that site.
6.12.2.2. Probability density plots An example of the use of this type of plot for single grains of an aeolian sand was given by Duller et al. (2000). The D e values were obtained for each of the 408 grains with a mean dose of about 23 Gy (Fig. 6.39). Duller et al. (2000) also looked at the radial plot, and found that only 81% of the values fall in the ___20- band, and took this to imply that there is an additional source of scatter.
6.12.2.3. Radial plots Radial plots for De values were introduced by Galbraith et al. (1999), as a means of displaying single grain De values for quartz grains from the complex sedimentary sequence at Jinmium, northern Australia (Roberts et al., 1998a; 1999) and a similar age site at Malakunanja (Roberts et al., 1998b). Data for the latter are shown in Fig. 6.40. Galbraith et al. (1999) presented a radial plot for 95 single quartz grains that had been bleached in the course of SAR measurements and then given a dose of 2.1 Gy. When the OSL data for the subsequent measurement were used as if they were the natural data, the values of De are shown in Fig. 6.41. As displayed, the reference value was chosen to be the known given dose of 2.1 Gy and 93 of the 95 points fell within the ___20-band. Even for aeolian sediments, plots of the natural dose, obtained for either single grains or single aliquots composed of at least 100 grains, are not as good as this, with a higher percentage of points falling outside the ___20- band.
Fig. 6.39. Probabilitydensity plot for 408 grains of Tasmanian dune sand (from Duller et al., 2000).
Retrospective OSL Dosimetry
301
Palaeodose
S~indaatrediSed
(Gy)
- - - - ' / / / / ~ 5~0
-
9
0
Relative error (%) , 0
50 i
,
20 I ' 5
'
10 I 10
7 "'
ll5
Precision
Fig. 6.40. Radial plot of De values for 18 grains from an aeolian sand from Malakunanja showing a range of De values and a range in precision, linked to the individual grain sensitivities; the reference value is 42.7 Gy (from Roberts et al., 1998b).
Radial plots have also been used to investigate the results obtained when synthetic mixtures were created from two or three sub-samples of grains that had been bleached by sunlight and then given different laboratory doses (Roberts et al., 2000). The different contributions could be recognised, and their relative proportions calculated, though a few of the grains with the lowest dose had calculated doses that were too high.
6.12.2.4. Calculation of De
Within the context of single grain OSL measurements, Galbraith et al. (1999) assumed that a more plausible distribution is obtained for the common logarithm of the De values. Assuming that all grains received the same dose, they developed their "common age model", in which the weighted average of the logarithm of De is used to obtain the best estimate of De. However, some radial plots indicate that the De values are not consistent with a common value, e.g., the values shown in the radial plot for grains of aeolian sand from Malakunanja (Fig. 6.40) (Roberts et al., 1998b). The dispersion in the single grain De values is --~ 17% and the central age model was used to calculate a value of De that is the geometric mean of the true single-grain De values (Roberts et al., 1998b; Galbraith et al., 1999); see discussion in Section 6.11.3.4. Even larger dispersion (21-27%) was found for De values for grains blown into another Australian cave site (Murray and Roberts, 1997) when the central age model was used to calculate the best estimate of De. Use of the central age model enables the width of the dose distribution to be determined.
Optically Stimulated Luminescence Dosimetry
302
calculated dose (Gy) 5 4 standardized estimate
~ -
3
2.5
9 9 9 9. . . . . . . .L. ] 2 -2 -J ....... :....... ~.-...............................................
/__
;
relative standard error 0.5 0.2 0.1 I I I
5
precision
lo
0.07 '
1.6
~ .
0.8
Fig. 6.41. Radial plot of dose values for 95 quartz grains given a known laboratory dose that is then determined using the SAR plot previously obtained for each grain. The reference value is the known applied dose of 2.1 Gy, and 93 of the points lie within the ___2o- band. Note that in this plot the relative standard error (not the percentage error) is given on the x-axis (from Galbraith et al., 1999).
6.13. Geologicaland archaeologicaldating-conclusions Since the first use of OSL as a dating tool (Huntley et al., 1985), a large number of papers reporting OSL ages for quartz and IRSL ages for feldspar have appeared in the geological literature and are beginning to appear in the archaeological literature. The single-aliquot measurement procedures such as SAR and SAAD (see Sections 6.11.2.1 and 6.11.2.2) have in-built checks on the ability to make appropriate corrections for sensitivity change and signal depletion on repeated measurements, respectively. These checks relate to construction of the OSL growth curve with doses given in the laboratory. However, to obtain a reliable value of the equivalent dose, it is necessary to be certain that the natural OSL (or IRSL) signals can be related to that laboratory dose response. One test that can be carried out is to optically bleach the grains and give them a known dose in the laboratory, prior to application of the method of De determination that is to be used in the dating project. This test is referred to as a "dose recovery" test, and has been applied both to OSL measurements of quartz grains (Roberts et al., 1999) and to IRSL measurements of feldspar grains (Wallinga et al., 2000b). The ability to recover the laboratory dose should be a pre-requisite for any dating procedure.
303
Retrospective OSL Dosimetry
The best check that the laboratory procedures are correct is to apply them to a range of samples whose ages are known independently. Unfortunately, there are relatively few sedimentary deposits for which the ages are well constrained by independent dates. Often the ages are on material from an under- or overlying deposit, thus providing only an upper or lower age limit. Also, the relevance of the material on which the age is obtained needs to be confirmed, making sure that it is in situ. Some recent comparisons for sand-sized quartz ages obtained using SAR and calibrated radiocarbon ages have been reported by Murray and Clemmensen (2001) for dune sands in Denmark (ages ranging from about 0.1 to 4.4 ka) and by Murray and Olley (2002) for older fluvial and lacustrine sediments in Denmark (ages ranging from 13 to 30 ka). These paired ages are shown in Fig. 6.42, together with paired ages for fine grained quartz extracted from a marine core (Stokes, pers. comm.) and a loess section (Watanuki, pers. comm.). There is no evidence for any systematic difference between the OSL ages and the independent age estimates over the entire age range (back to about 300 ka). In Fig. 6.43, an additional data set is presented for quartz and K-rich feldspars extracted from fluvial deposits of the Rhine-Meuse system as it crosses the Netherlands. The youngest sediment was constrained by historical information and the three sediments of Holocene age were constrained by a number of radiocarbon ages on terrestrial plant macrofossils (Wallinga et al., 2001). The SAR OSL ages on quartz were in agreement with the radiocarbon ages, but for the youngest sample with an historical age of --~300 years, the OSL age is too high, which was interpreted as being due to poor bleaching. The SAR IRSL ages show a systematic age underestimation. Wallinga et al. (2001) concluded that this may have been related to a sensitivity change that is caused by the pre-heat prior to measurement of the natural IRSL, but one that is not properly accounted for by the IRSL response to the first test dose in the SAR procedure. Systematic age underestimation was found for multiple-aliquot IRSL dating of K-rich feldspars from a sequence of Holocene dunes taken from a coastal spit in Massachusetts (Huntley and Lamothe, 2001). Application of a correction for anomalous fading (see 9 aeolian freshwater 9 marine <> glacial ~ v
100
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Fig. 6.42. Age comparison using SAR applied to quartz grains from sediments with independent age control. Inset shows the same data using linear axes (from Murray and Olley, 2002).
304
Optically Stimulated Luminescence Dosimetry
14
9 quartz
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0
2
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Independent age estimate, ka Fig. 6.43. OSL ages obtained on quartz using the SAR protocol with a 10 s 200~ pre-heat (filled symbols) and IRSL ages obtained on K-rich feldspar separates using SAAD procedure (open symbols) plotted against the independent age of the samples. All errors indicate 2o-confidence intervals (re-drawnfrom Wallinga et al., 2001). Section 5.2.10.8) gave much closer agreement with the ages expected on the basis of radiocarbon evidence. In contrast, multiple-aliquot OSL dating of quartz from aeolian sands in other parts of the USA was in good agreement with the radiocarbon ages (Stokes and Gaylord, 1993). At the time of writing, it seems that SAR applied to the fast component of the quartz OSL signal provides the most reliable age estimates. Application of SAR permits multipledose determinations to be made, and statistical procedures have been developed along the lines of those used in fission track dating. Technological developments have permitted the SAR procedure to be applied to single grains. Besides many geological applications in the late Quaternary period, OSL has played a major role in providing chronological control for human behaviour in southern Africa about 70 ka (Henshilwood et al., 2002), human occupation of Australia about 50 ka (Roberts et al., 1998a, 1999; Turney et al., 2001) and possible human linkage to megafauna extinctions in Australia about 46 ka (Roberts et al., 2001).
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Richardson, C.A., 1994. Effects of bleaching on the sensitivity to dose of the infra-red stimulated luminescence of potassium-rich feldspars from Ynyslas, Wales. Radiat. Meas. 23, 587-591. Roberts, R.G., Bird, M., Olley, J., Galbraith, R., Lawson, E., Laslett, G., Yoshida, H., Jones, R., Fullagar, R., Jacobsen, G., Hua, Q., 1998a. Optical and radiocarbon dating at Jinmium rock shelter in northern Australia. Nature 393, 358-362. Roberts, R., Yoshida, H., Galbraith, R., Laslett, G., Jones, R., Smith, M., 1998b. Single-aliquot and single-grain optical dating confirm thermoluminescence age estimates at Malakunanja II rock shelter in northern Australia. Ancient TL 16, 19- 24. Roberts, R.G., Galbraith, R.F., Olley, J.M., Yoshida, H., Laslett, G.M., 1999. Optical dating of single and multiple grains of quartz from Jinmium rock shelter, northern Australia: Part II, Results and implications. Archaeometry 41,365-395. Roberts, R.G., Galbraith, R.F., Yoshida, H., Laslett, G.M., Olley, J.M., 2000. Distinguishing dose populations in sediment mixtures: a test of single-grain optical dating procedures using mixtures of laboratory-dosed quartz. Radiat. Meas. 32, 459-465. Roberts, R.G., Flannery, T.F., Ayliffe, L.K., Yoshida, H., Olley, J.M., Prideaux, G.J., Laslett, G.M., Baynes, A., Smith, M.A., Jones, R., Smith, B.L., 2001. New ages for the last Australian megafauna: continent-wide extinction about 46,000 years ago. Science 292, 1888-1892. Smith, B.W., Aitken, M.J., Rhodes, E.J., Robinson, P.D., Geldard, D.M., 1986. Optical dating: Methodological aspects. Radiat. Prot. Dosim. 17, 229-233. Spooner, N.A., 1994. On the optical dating signal from quartz. Radiat. Meas. 23, 593-600. Stokes, S., 1994. The timing of OSL sensitivity changes in a natural quartz. Radiat. Meas. 23, 601-605. Stokes, S., Gaylord, D.R., 1993. Optical dating of Holocene dune sands in the Ferris Dune Field, Wyoming. Quat. Res. 39, 274-281. Stokes, S., Coils, A.E.L., Fattahi, M., Rich, J., 2000. Investigations of the performance of quartz single aliquot DE determination procedures. Radiat. Meas. 32, 585-594. Stoneham, D., Stokes, S., 1991. An investigation of the relationship between the 110~ TL peak and optically stimulated luminescence in sedimentary quartz. Nucl. Tracks Radiat. Meas. 18, 119-123. Thomsen, K.J., BCtter-Jensen, L., Murray, A.S., Solongo, S., 2002a. Retrospective dosimetry using unheated quartz: a feasibility study. Radiat. Prot. Dosim. 101,345-348. Thomsen, K.J., BCtter-Jensen, L., Murray, A.S., 2002b. Household and workplace chemicals as retrospective luminescence dosemeters. Radiat. Prot. Dosim 101, 515-518. Turney, C.S.M., Bird, M.I., Fifield, L.K., Roberts, R.G., Smith, M., Dortch, C.E., Grtin, R., Lawson, E., Ayliffe, L.K., Miller, G.H., Dortch, J., Cresswell, R.G., 2001. Early human occupation at Devil's Lair, southwestern Australia 50,000 years ago. Quat. Res. 55, 3-13. Wallinga, J., Duller, G.A.T., 2000. The effect of optical absorption on the infrared stimulated luminescence age obtained on coarse-grain feldspar. Quat. Sci. Rev. 19, 1035-1042. Wallinga, J., Murray, A., Wintle, A., 2000a. The single-aliquot regenerative-dose (SAR) protocol applied to coarse-grain feldspar. Radiat. Meas. 32, 529-533. Wallinga, J., Murray, A.S., Duller, G.A.T., 2000b. Underestimation of equivalent dose in single-aliquot optical dating of feldspars caused by pre-heating. Radiat. Meas. 32, 691-695. Wallinga, J., Murray, A.S., Duller, G.A.T., Tornqvist, T.E., 2001. Testing optically stimulated luminescence dating of sand-sized quartz and feldspar from fluvial deposits. Earth Planet. Sci. Lett. 193, 617-630. Wieser, A., Onori, S., Aregno, D., Fattibene, P., Romanyukha, A., Ignetiev, E., Kostha, A., Skvortzov, V., Ivannikov, A., Stepanenko, V., Chumak, V., Sholom, S., Haskell, E., Hayes, R., Kenner, G., 2000. Comparison of sample preparation and signal evaluation methods in EPR analysis of tooth enamel. Appl. Radiat. Isotop. 52, 1059-1064. Wintle, A.G., 1997. Luminescence dating: laboratory procedures and protocols. Radiat. Meas. 27, 769-817. Wintle, A.G., Murray, A.S., 1998. Towards the development of a pre-heat procedure for OSL dating of quartz. Radiat. Meas. 29, 81-94. Wintle, A.G., Murray, A.S., 1999. Luminescence sensitivity changes in quartz. Radiat. Meas. 30, 107-118. Zhang, J.F., Li, S.H., Tso, M.Y.W., 2001. Improvement of the equivalent dose determination using aliquots of potassium feldspar. Radiat. Meas. 33, 65-71.
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Chapter 7
OSL measurement technology 7.1. Stimulation modes
The basis for OSL measurements is the stimulation of an irradiated sample with a light source providing a selected wavelength or wavelengths and the monitoring of the emission from the sample at a different wavelength using a sensitive detector (e.g., a photomultiplier (PM) tube). Different modes of stimulation can be used, e.g., continuous wave-OSL (CWOSL), linearly modulated OSL (LM-OSL), and pulsed OSL (POSL). 7.1.1. CW-OSL So far, the most common way to obtain an OSL signal is to illuminate the sample with a light source of constant intensity and to simultaneously monitor the luminescence emission (CW-OSL). The luminescence emission is monitored continuously while the stimulation light beam is on and narrow band filters are used to discriminate between the stimulation light and the emission light, and to prevent scattered stimulation light from entering the detector. The OSL is usually monitored in the form of an exponential-like decay until all the traps are emptied and the luminescence ceases. The integrated emission is recorded and is used to determine the absorbed radiation dose. A simplified diagram of a set up for measuring CW-OSL is shown in Fig. 7.1a and a typical OSL decay curve obtained from a sedimentary (natural) quartz sample stimulated with broad-band (420550 nm) blue-green light is shown in Fig. 7.lb. Most decay curves show more than one component. 7.1.2. LM-OSL
If the stimulation light intensity is increased linearly with time during OSL readout, LM-OSL is produced. The OSL output is observed to increase, initially linearly, as the stimulation power increases until the traps start to become depleted, after which the OSL intensity decreases non-linearly to zero. This method is useful for distinguishing between OSL originating from different traps. 7.1.3. POSL
Pulsed OSL results when the stimulation source is pulsed at a particular modulation frequency, with a particular pulse width appropriate to the lifetime of the luminescence being observed. In this mode of stimulation only the OSL emission between the pulses is measured, rather than during the pulses.
Optically Stimulated Luminescence Dosimetry
312
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7.2. The light detection system 7.2.1. Photomultiplier tubes As in TL measurements, OSL is normally detected using a photomultiplier (PM) tube. PM tubes are still the most sensitive light detectors and are used in most luminescence measurement systems. The PM is a vacuum tube that includes a photosensitive cathode, a number of electron-multiplying dynodes and an anode normally held at about 1000 V. Light photons interact with the photoelectric cathode material (e.g., potassiumcaesium) causing the emission of electrons that are then attracted to the positive voltage of the first dynode. Depending on the dynode material (e.g., antimony-caesium), two or three electrons are emitted for each electron striking it. These electrons are again attracted by the next dynode and so on, resulting in several million electrons reaching the anode for each electron emitted from the cathode. Thus, a light photon reaching the photocathode is converted to an electrical pulse at the anode. However, the PM is not equally sensitive to photons emitted at different wavelengths, i.e., not all photons are converted to pulses. This results in a varying quantum efficiency of up to 25% depending on the wavelength.
OSL Measurement Technology
313
Typically, a bialkali PM tube, such as EMI 9235, has a selective response curve with a maximum detection efficiency peaking around 400 nm, which is suitable for measurement of luminescence emission spectra. Other types of PM tubes, such as EMI 9558Q, EMI 9202Q and RCA 31034, are available with an extended sensitivity in the red region (S-20 cathode), which is particularly suitable for the investigation of the red emission from different materials (e.g., Visocekas, 1993; Scholefield and Prescott, 1999; Fattahi and Stokes, 2000). S-20 cathode PM tubes normally need cooling below room temperature to reduce the dark noise. Commonly used PM windows are fused silica and quartz. 'Q' on the PM tube designation refers to a pure quartz window, which allows for detection of the near UV emission from materials, e.g., the 365 nm OSL emission from quartz (Huntley et al., 1991). Gallium-arsenide PM tubes for detection of IR emission are also available (e.g., Schilles and Habermann, 2000). In principle, the PM tube can be operated in two modes. One method is based on smoothing the pulses arriving at the PM anode and thereby generating a DC current signal that is proportional to the number of photons reaching the photocathode. This DC signal may be digitised using a current-to-pulse rate converter, which allows a wide response range of the order of seven decades, and the possibility of off-setting the dark current to zero. However, a more sensitive approach is to directly count the single pulses generated from the photons interacting with the photocathode. A fast pulse amplifier and a pulse height discriminator are used to feed a rate-meter or scaler (e.g., Aitken, 1985). Modem bialkali PM tubes, such as the EMI 9235QA, are now available with a dark count-rate of less than 20 counts per second at room temperature. The dark count-rate is proportional to the PM cathode area and therefore PM tubes with small diameters (e.g., 10 mm) should be used where an improved signal-to-noise ratio is desirable. PM tubes are available with cathode diameters from 75 mm down to a few mm. However, small-diameter PM tubes require that the light to be detected is focussed to optimise the light collection. 7.2.2. Imaging photon detectors Ultra-sensitive imaging systems such as imaging photon detectors (IPDs), have been used to measure the spatially resolved luminescence from natural samples (Smith et al., 1991; Burggraaf and Haskell, 1994; McFee and Tite, 1994). An IPD utilises a positionsensitive micro-channel plate detector for image capture allowing time encoding for each detected photon. It consists of a photocathode similar to that used in a PM tube, which produces a photoelectron when struck by an incident photon. The photoelectron is accelerated towards a stack of micro-channel plates, which are placed close (0.7 mm) behind the photocathode. These are discs of metal-impregnated glass, which have many narrow holes (micro-channels) passing through them, each 10 txm in diameter. The photoelectron enters a micro-channel in close proximity to the position on the photocathode from which it originated, thus preserving the spatial information. Reflections off the walls of the micro-channel produce a shower of secondary electrons and a typical IPD consisting of five plates gives a total gain of 3 • 107 (Smith et al., 1991). At the back ofthe tube is a sheet of uniform resistive material, which is terminated by linear resistances in the form of circular arcs at each comer. When an electron burst hits this resistive anode, the centroid is calculated from the charge (and hence, the voltage) passed at each resistor. The signal is
314
Optically Stimulated Luminescence Dosimetry
amplified, discriminated and passed to the software as a photon count with a given x - y coordinate. IPDs retain the high sensitivity of a PM tube. However, they are rather expensive and difficult to operate and have also been shown to degenerate as a function of use. 7.2.3. Solid-state detectors The development of solid-state imaging systems based on charge coupled device (CCD) technology offers an alternative to IPDs for imaging work. A typical CCD chip (Kodak KAF-0400) consists of an array of 768 x 512 pixels, with 16 bit analogue to digital converter allowing up to 65,535 grey levels. Each pixel is 9 Ixm square, giving a total active area of 6.9 x 4.6 mm 2. The CCD chip is cooled (to approximately 250 K) using a thermoelectric cooler. An electromechanical shutter is placed in front of the CCD so that it is possible to control the period of time for which the CCD is exposed. Digitisation and storage of a whole image (393,216 pixels, or 768 kb of data) takes approximately 3 s, during which time the sensor is not exposed to light in order to prevent streaking across the image as the data are read from the CCD. A special facility known as "on-chip binning" allows the signal, in groups of adjacent pixels, to be summed within the CCD readout register, prior to conversion from an analogue to a digital signal and subsequent transmission to a host computer. This feature allows the spatial resolution of the system to be altered, via software control, to any multiple of a pixel. Additionally, the use of binning reduces the download time and file size, and hence fewer reads are required. Duller et al. (1997) and Spooner (2000) have constructed CCD cameras for measuring the spatially resolved luminescence signals. A CCD camera has a similar sensitivity to that of a PM tube, though the spectral responses are very different. The quantum efficiency of a typical CCD is highest between 600 and 800 nm. The active surface of the CCD is shielded by a silicon substrate upon which the device is fabricated. Since silicon absorbs wavelengths shorter than approximately 500 nm, the sensitivity of the device drops rapidly below this point. This can be compensated to some extent by coating the CCD with a phosphor that absorbs photons with a wavelength shorter than 460 nm and then emits at 520 and 560 nm (Cowens et al., 1980). Erfurt et al. (2000) used a CCD camera to measure radioluminescence (RL) emission spectra from aluminium oxide; Rieser et al. (1994) and Krbetschek et al. (2000) used a CCD camera for the measurement of emission spectra from sediments. Akselrod et al. (2000) used a CCD for spatial imaging of OSL from large area A1203 detectors. There is a large demand for developing solid-state photodiodes with sufficient sensitivity for detection of luminescence signals. Such devices are especially required in connection with the design of portable luminescence measurement equipment to avoid the constraint of using high-voltage supplies needed for PM tubes. Silicon avalanche photo diodes (APDs) have been used as fast photon-counting detectors in astronomical photometry (Dravins et al., 2000). Absolute quantum efficiencies up to 76% at 700 nm have been measured for APDs in single photon counting mode (Kwait et al., 1994). Silicon PIN diodes are widely used in optical fibre communication systems. A PIN diode is a semiconductor diode in which a high-resistive intrinsic I is sandwiched between a P-type and an N-type region. When the PIN diode is forward biased, holes and electrons are injected into the 1-region. However, both APDs and PIN diodes have nearly the same spectral
OSL Measurement Technology
315
response as CCDs and are so far not sensitive enough in the visible range to compete with PM tubes in OSL measurements.
7.3. Automated OSL readers
The demand to carry out a large number of OSL measurements routinely has accentuated the need for equipment with automatic changing of samples. The development of automated OSL readers has been described by BCtter-Jensen (1997, 2000), BCtter-Jensen et al. (2000, 2002) and Bortolot (1997, 2000). These readers are controlled by slave computers and programmed using a host computer with a dedicated measurement sequence editor. In order to be able to perform regenerative-dose measurement procedures, such as the single-aliquot regenerative-dose measurement protocol (see Chapter 6) it is important to have an attachable beta irradiator for in-situ irradiations of samples. This beta source is typically based on the radionuclide 9~176 with an activity of 1.5 GBq (40 mCi) providing a dose rate of approximately 0.1 Gy/s at the sample position. It is important to keep the cross-talk as low as possible to avoid unwanted irradiation of neighbouring samples waiting in the sample changer. The cross-talk is typically of the order of 0.2% when using a 48sample carousel (BCtter-Jensen et al., 2000). More recently the cross-talk caused by the attachable beta irradiator of the automated Rise TL/OSL reader has been reported based on highly sensitive A1203:C dosimeters used in environmental dosimetry studies (Kalchgruber et al., 2002), and quartz from beach sands used for sediment dating (Bray et al., 2002). In OSL dosimetry using natural materials, it is important to be able to perform accurate pre-heating of samples. Also, OSL readout requires a constant temperature during stimulation, e.g., in CW-OSL at 125~ for quartz (see Sections 5.1.2.6 and 5.2.2.2), in LM-OSL measurements (see Section 5.1.3) and in studying isothermal decay (see Section 5.1.8.3). Heating may also be necessary for associated TL measurements. Linear heating is normally performed using a low-mass heater strip made of high resistance alloys (e.g., nickel and Kanthal) by feeding a controlled current through the heating element. A feedback control of the temperature is made using a thermocouple (e.g., Cr/A1) welded to the heater strip. Normally, heating is controlled by an electronic ramp that can generate various pre-heat functions and linear heating rates, e.g., in the range 0.1-30~ (BCtter-Jensen, 2000). The maximum temperature normally used for quartz and feldspar investigations is 500~ but for special investigations on deep trap effects, heating of samples up to 700~ is necessary. Although the heating strip is usually located directly beneath the PM tube, an additional heater may be useful when placed beneath the irradiation source (BCtter-Jensen et al., 2000; Bortolot, 2000). This permits investigations of possible effects due to keeping the shallower traps empty during irradiation, as was the case in the natural environment (Wallinga et al., 2002) (see Sections 5.1.8.4 and 5.2.8.3). Other heating systems have been used for a fast readout of conventional solid TL dosimeters, e.g., in personal and environmental dosimetry. The dosimeter material may be lifted into a stream of hot nitrogen (300-400~ and the luminescence signal is released as a non-linear function depending on the heat conduction of the material (e.g., BCtter-Jensen, 1978). A CO2 laser beam has also been used for the non-linear heating of solid TL dosimeters (Bratinlich et al., 1981) and more
316
Optically Stimulated Luminescence Dosimetry
recently powerful halogen lamps have been used with parabolic mirrors to indirectly transfer heat into samples (Lucas, 1993).
7.4. Development of optical stimulation sources 7.4.1. Laser stimulation Huntley et al. (1985) used the 514 nm line from an argon ion laser to stimulate OSL from quartz. Later studies characterised the OSL properties of quartz using argon lasers as the stimulation light source (e.g., Aitken and Smith, 1988; Godfrey-Smith et al., 1988; Rhodes, 1988; Aitken, 1990). Godfrey-Smith (1991) built an automatic 50-sample OSL reader based on a variety of argon, krypton and red dye lasers as the stimulation light sources. Bailiff (1993), Bailiff and Barnett (1994) and Godfrey-Smith and Cada (1996) used Ti:sapphire tuneable lasers to determine the IR stimulation spectra from feldspars. McKeever et al. (1996), and more recently Akselrod and McKeever (1999), used pulsed Nd:YAG lasers (2nd harmonic: 532 nm) to stimulate A1203:C for personal radiation dosimetry. 7.4.2. IR LED stimulation Htitt et al. (1988) first showed that infra-red (IR) light could stimulate luminescence from feldspars, and because it was technically easier to provide IR stimulation than green laser stimulation, several laboratories rapidly published descriptions of practical stimulation units. Poolton and Bailiff (1989), Spooner et al. (1990) and BCtter-Jensen et al. (1991) described the use of IR light emitting diodes (LEDs) for stimulation of feldspars. BCtter-Jensen et al. (1991) described an IR add-on unit to be mounted directly between the PM tube assembly and the glow oven of the automated Rise TL apparatus. In this unit, 32 IR LEDs are arranged in two concentric tings one above the other, with each LED pointing at the sample. IR stimulated luminescence (IRSL), emitted vertically through the ring of diodes, is then measured with the same PM as used for the TL measurements. A Schott BG39 detection filter rejects the scattered IR light. The IR stimulation unit is shown in Fig. 7.2a and the published BG-39 transmission and IR LED emission characteristics are shown in Fig. 7.2b. In the above systems, IR LEDs peaking at 870-880 nm are used for stimulation of feldspars. This matches well with the resonance peaks of most feldspars at 860 and 875 nm with the former being found by Barnett and Bailiff (1997) to be independent of feldspar composition. However, Godfrey-Smith and Cada (1996) measured the IR resonance of several feldspars to be around 845-850 nm. IR LEDs peaking at 840 nm are available. An array consisting of 30-40 such IR diodes close to the sample (20 mm), typically delivers about 4 0 - 5 0 mW/cm 2 at the sample position; this provides a useful luminescence signal in most applications. Recently, powerful IR LEDs emitting at 875 nm, providing more than six times higher stimulation power than the older types, have become available. An array of 21 such diodes at 20 mm distance from the sample has been shown to deliver > 135 mW/cm 2 at the sample (BCtter-Jensen et al., 2002). This high power, in addition to the LED emission wavelength being near the peak resonance of most feldspars, has significantly enhanced the depletion rate resulting in higher depletion efficiency.
317
OSL Measurement Technology
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7.4.3. IR laser diode stimulation
Generally, the decay rate of the IRSL signal is slow using conventional IR LEDs (about 50 s to reduce the initial luminescence by about 50%). In measurement protocols based on regenerating the IRSL signal (e.g., Wallinga et al., 2000), high power is needed to be able to completely deplete the IRSL signal in a reasonable time. The potential of solid-state IR laser diodes has been investigated based on a 1.0 W laser diode emitting at 830 nm (BCtter-Jensen et al., 2000). A cylindrical focal lens is needed to convert the slit-shaped emission from the diode laser to a nearly uniform illumination area of approximately 1 cm 2 at the sample position. Maximum power at the sample position was measured to be
318
Optically Stimulated Luminescence Dosimetry
approximately 400 mW/cm 2 (BCtter-Jensen et al., 2000) but only a maximum power of 200 mW/cm 2 was recommended in routine use because of heat-induced instability. It has further been shown that the emission wavelength typically increased by approximately 4 nm, e.g., from 829 to 833 nm, when running the laser diode at 400 and 1200 mA, respectively (BCtter-Jensen, 2000). Typical IRSL decay curves from a feldspar sample obtained with: (1) an IR laser diode running at 400 mW/cm 2 at the sample; and (2) conventional IR LED array (40 mW/cm 2) are shown in Fig. 7.3. The greater stimulation power produced by the laser diode is seen to give an enhanced initial light output and a steeper initial OSL decay curve in accordance with theoretical expectations (see Chapter 2); the relative depletion after 75 s is much greater (BCtter-Jensen et al., 2000). However, experience with presently available IR laser diodes has shown that it is particularly difficult to reproduce the focussing of the slit-shaped emission from a laser diode uniformly onto a particular sample area, and more seriously, they have a relatively short lifetime (on average one year in normal use). Also, laser diodes emitting at wavelengths higher than 810 nm are rarely available and in any case very expensive. Therefore, the continuous development of new powerful LEDs emitting at 875 nm (close to the average IR resonance range in most feldspars), and new long-lived laser diodes emitting at higher wavelengths (e.g., 870 nm), is important to meet the requirements for IR stimulation light sources that are both powerful and have long-term stability. 7.4.4. Broad-band light stimulation The demand for OSL dating of quartz, and an inexpensive alternative to the 514 nm laser stimulation, led to the development of stimulation systems based on green light wavelength bands filtered from incandescent broad-band lamps. BCtter-Jensen and Duller (1992) developed a commercial compact green light OSL system for dating applications based on the light from a simple low-power halogen lamp. A low-power (75 W) tungsten halogen lamp, filtered to produce a stimulation wavelength band from 420 to 550 nm, delivered a power of 16 mW/cm 2 to the sample. The filtered halogen lamp was stable
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319
OSL Measurement Technology
within 2% after an initial stabilisation period of about 2 min. This means that the system is not suitable for repeated stimulations unless an electromagnetic shutter is incorporated in front of the light source. The main feature of this construction is that the halogen lamp does not require complex cooling and the small 2 x 2 mm 2 filament can be easily imaged onto the sample, thus providing good energy transfer to the sample. A schematic of the broad-band stimulation system is shown in Fig. 7.4a.
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320
Optically Stimulated Luminescence Dosimetry
Selection of appropriate filters for use with specific stimulation wavelengths and emission bands is essential for optimum OSL signal-to-noise ratios. For example, Table 7.1 lists a comparison of six filter configurations for the detection of the 365 nm OSL emission from quartz. The characteristics of the optimal excitation wavelength band and detection filter combinations are shown in Fig. 7.4b. An enhanced power delivery can be achieved by using optical fibres to carry the stimulation light to be focussed close to the sample position (BCtter-Jensen, 1997; Bortolot, 1997). The signal-to-noise ratio may be further improved by using a multi-layer metal oxide coated (ZrOz/SiO2) Hoya U-340 detection filter (specially made by DELTA Light and Optics, Denmark) or a Schott DUG 11 filter (Galloway et al., 1997), which attenuates any light passed through a transmission window found in the red regions of normal U-340 and UG 11 filter characteristics. The observed characteristics of the metal oxide coated U-340 filter are compared with those of a normal U-340 filter in Fig. 7.5 (BCtter-Jensen and Murray, 1999). Certain experimental advantages may be gained by combining IR and broad-band stimulation sourcesmfor example, to screen the purity of quartz samples by detecting feldspar contamination. Examples of commercial systems are provided by Rise (BCtter-Jensen, 1997) and Daybreak (Bortolot, 1997) using a variety of visible and IR light stimulation sources. Schematics are given in Fig. 7.6a and b. 7.4.5. Optimisation o f OSL detection In principle, a single luminescing grain emits light in all directions, i.e., in a nearly 4-rr geometry. If the sample is heated or illuminated on a metal support, the maximum light signal is then reduced to about 50% (to 2"rr geometry), unless the support for the sample is reflective and the sample transparent. Sample-to-PM tube distance is thus very important, since only a small increase will lead to loss of light collected. If a greater sample-to-PM tube distance is needed, suitable optics are required to retain the sensitivity of the design. Ditlefsen and Huntley (1994) constructed an OSL apparatus that included a spherical mirror to improve the light collection angle in the measurement of luminescence from
Table 7.1 Potential excitation and detection filter combinations for optimumdetection of 365 nm OSL from natural quartz. Signal-to noise ratios (S/N) obtainedas the mean values from three heated quartz samplesare given relative to that for the first combination. The background noise signals (N) taken as the OSL signal from heated samples, the mean PM dark signals (D) and the ratios (N/D) are also listed (from BCtter-Jensen and Duller, 1992) Excitation filter
Detectionfilter
S/N (rel.)
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D (cps)
N/D
6 mm GG-420 6 mm GG-420 6 mm GG-455 6 mm GG-455 5 mm GG-455 6 mm OG-515
5 mm U-340 4 mm UG-11 5 mm U-340 4 mm UG-11 5 mm U-360 12 mm 7-59 + 5 mm BG-39
1.00 0.39 0.88 0.18 0.48 0.01
68 241 74 306 94 195
52 58 54 53 55 76
1.31 4.16 1.37 5.76 1.71 2.57
cps: counts per second.
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1 E+02 1 E+01
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400
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|
600
800
1000
1200
1400
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Fig. 7.5. The observed transmission characteristics of a normal Hoya U-340 detection filter overlain with that of a metal oxide coated U-340 filter. The effect of suppressing the transmission window at 700 nm is clearly seen (from BCtter-Jensen and Murray, 1999).
quartz and feldspars. Markey et al. (1996) designed and tested OSL attachments to the automated Rise system based on reflecting the luminescence from ellipsoidal mirrors; these provide the greatest flexibility for the incorporation of different excitation sources. By lifting the samples into the focal point of the ellipsoidal mirror, a gain in sensitivity of 4 was achieved compared to the standard Rise TL/OSL system whether thermally or optically stimulated. Readout systems based on metallic mirrors are dependent on a stable reflectivity and thus the choice of a pure metal surface such as nickel electroplated with rhodium is of great importance. In the full-reflector system reported by Markey et al. (1997) excitation illumination is introduced by up to four optional light guides. Rieser et al. (1999) designed a high-sensitivity OSL emission spectrometer based on light collection using a rhodium-coated elliptical mirror, which in combination with optical lenses provides a nearly full 4o measurement geometry. Schilles et al. (1999) used a doubleellipsoidal reflector system to improve both the sensitivity and the signal-to-noise ratio in single aliquot applications. A sensitive mirror and lens system was also used for single grain studies by McCoy et al. (2000). 7.4.6. Green LED stimulation Visible LEDs offer several advantages over existing laser or broad-band stimulation sources. They are inexpensive and compact, and their heat dissipation is negligible. Switchon and -off times are much shorter than can be obtained with the electromechanical shutters used with halogen lamps, and with optical feedback no warm up period is necessary. From an experimental point of view, the illumination power density can be controlled electronically. This offers the possibility of software control during experimental sequences, e.g., to provide reduced power during the brief illuminations used in normalisation, or to provide a time-varying stimulation power (BCtter-Jensen et al., 1999b). Galloway (1993, 1994) described initial investigations into the use of green-light LEDs for stimulation of quartz and feldspars. The relatively low power that could be delivered to
322
Optically Stimulated Luminescence Dosimetry
Fig. 7.6. (a) Schematic diagram of the compact Ris~ liquid light-guide-based combined IR/blue-green stimulation unit attachable to the automatic Rise TL reader (from BCtter-Jensen and Murray, 1999). (b) Schematic of the Daybreak combined fibre-optic/IR LED OSL illuminator (from Bortolot, 1997).
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the sample at that time and the heavy filtering of the PM photocathode (necessary to avoid stray light from the LED emission band reaching the PM) resulted in slowly decaying OSL curves that required readout times in the order of 2000 s to give useful signals for dose assessment. However, these initial investigations of green LEDs for OSL dosimetry provided a good basis for later investigations of more powerful green and blue LEDs. Galloway et al. (1997) reported the testing of a newer type of green LED with enhanced brightness. They further investigated the use of detection filters consisting only of Schott UG- 11 filters that were coated with metal oxide on each side (Schott DUG- 11). These have the same advantage as described for the coated U-340 filters in Section 7.4.4, namely the attenuation of light from the transmission windows found in the red region of a normal UG-11 filter (see Fig. 7.5). The enhanced illumination power achieved in combination with the DUG-11 detection filters improved the overall sensitivity by a factor of 1000 compared with their previous green LED system. Bortolot (2000) reported a new OSL stimulation unit that uses arrays of bright green LEDs (Nichia NSPG310) in connection with Schott GG475 and BG39 filters to remove the out-of-band emission tails that could pass through the UG11 detector filter. The resulting emission peak of 515 nm provides 30 mW/cm 2 at the sample. However, the excitation power achieved using green LEDs is still below that obtained with filtered lamps and lasers. 7.4.7. Blue LED stimulation Shorter wavelength light will stimulate luminescence with greater efficiency because of the almost exponential increase of stimulation with decreasing wavelength due to the dependence of the photoionisation cross-section on the wavelength (Spooner, 1994; Chapter 2). Because of this, considerable benefits are gained from using the blue light from LEDs (BCtter-Jensen, 1997; BCtter-Jensen et al., 1999a,b). It was found that for similar power densities, the higher energy light provided by the blue LEDs (470 nm) gives a rate of stimulation in quartz that is orders of magnitude greater than that from blue-green light filtered from a halogen lamp. Blue LEDs are available from Nichia (e.g., type NSPB-500S). The manufacturer describes these devices as having a peak emission of 470 nm (half width 20 nm), an emission angle of 15 ~ and a power output of about 2 - 4 cd at 20 mA current; the luminance of individual diodes from a batch (for a given current) may vary by up to a factor 2. In a batch of 100 diodes, it was found that 25% delivered more than 2.5 mW/cm 2 at a distance of 2 cm, compared with the average of 1.9 mW/cm 2. It is clear that individual selection of diodes can easily provide a 50% increase in power over that from a random choice. The testing of different blue LED configurations (BCtter-Jensen et al., 1999a,b) resulted in the design of a compact OSL attachment for the automated Rise TL reader. This unit is built up of clusters of blue LEDs contained in interchangeable tubes arranged in a ring between the sample heater plate and the PM tube. Each cluster consists of seven blue LEDs placed in a holder machined so that all individual diodes focus onto the sample. The ring-shaped holder can contain up to seven clusters making a total of 49 diodes illuminating the sample at a distance of about 30 mm. However, one position is normally
Optically Stimulated Luminescence Dosimetry
324
occupied by the focussed IR laser diode. A schematic diagram of the combined blue LED cluster and IR laser diode OSL unit is shown in Fig. 7.7a (BCtter-Jensen and Murray, 1999). A green long-pass Schott GG-420 filter is fitted in front of each blue LED cluster to minimise the directly scattered blue light from reaching the PM photocathode (see Section 7.4.8). The total power (seven clusters) delivered to the sample position was measured as > 6 0 mW/cm 2. Detection is through two 3 mm Hoya U-340 filters, one of which is coated with metal oxide. To ensure stability of the output power, the blue diode array should be equipped with an optical feedback servo-system (BCtter-Jensen, 1997). An extra diode connected in the current chain of the LED array is arranged to face an optical fibre
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Fig. 7.7. (a) Schematic diagram of a combined blue LED and IR laser diode OSL unit. Thirty-six blue LEDs (in six clusters) emitting at 470 nm deliver max 20 mW/cm 2 at the sample and the IR laser diode emitting at 830 nm delivers max 550 mW/cm 2 at the sample (from BCtter-Jensen and Murray, 1999). (b) The measured blue LED emission spectrum overlain with the transmission curves for the Schott GG-420 green long-pass filter and the detection filter Hoya U-340 (from BCtter-Jensen et al., 1999b).
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light-guide, which in turn is connected to a phototransistor. The phototransistor output regulates the feedback comparator/amplifier that controls the LED current. 7.4.8. Blue LED and cut-off filter characteristics The incorporation of a green long-pass filter has a very significant effect on background luminescence count rate (BCtter-Jensen et al., 1999b). Fig. 7.7b shows the observed blue LED emission spectrum overlain with the published transmission curve of the blue longpass filter GG-420 and the U-340 detection filter. There is a significant tail in the diode emission, extending to shorter wavelengths, but this is attenuated effectively by the GG420 filter, at the cost of about 5% attenuation in the peak emission wavelength. There is also a long wavelength tail in the blue diode emission spectrum, which extends above 600 nm (Fig. 7.7b); because of the red transmission window in the U-340 response, this could contribute to the scattered light reaching the PM tube. This problem has been solved by using metal oxide coated U-340 filters. 7.4.9. Ramping the LEDs LED arrays need not be run at constant power, as has been assumed in Section 7.4.3. Instead, it is possible to increase the power from zero to a pre-determined maximum value over a given period of time. This results in LM-OSL. Such an approach has been applied to feldspars using IR diodes (Bulur, 1996; Bulur and Grksu, 1999) and to quartz using blue LEDs (BCtter-Jensen et al., 1999b; 2000; Bulur et al., 2000; Singarayer and Bailey, submitted) and to A1203:C (Whitley and McKeever, 2001). As an example, Fig. 7.8 shows a LM-OSL curve obtained from a heated quartz sample using linearly increasing stimulation light intensity from blue LEDs (0-15 mW/cm2).
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Power mW / cm 2 Fig. 7.8. An exampleof OSL versus linearlyincreasing stimulationlight intensity (0-15 mW/cm2) obtainedfor a heated quartz sample using the ramp facility of a blue LED OSL unit (from BCtter-Jensen et al., 1999b).
326
Optically Stimulated Luminescence Dosimetry
In LM-OSL, the stimulation light intensity is commonly controlled using an analogue servo-mechanism employing a feedback photodetector. Due to the analogous nature of the circuits, the average light power on the sample can be affected by the non-linearity and the DC offset problems in the circuit elements. Bulur et al. (2001) described a frequency modulated pulsed stimulation system capable of controlling the average stimulation power by varying the repetition rate of pulses of fixed width. The analogue control voltage from the OSL system computer is converted to pulses 10 Ixs wide with a repetition rate of up to 10 kHz, which gives a duty-cycle ratio of 10. A special feature of such a system is that if the pulses are sufficiently short, and the switch-off time sufficiently long, the LEDs can withstand much higher currents than the recommended continuous rating. It should, however, be emphasised that a shift in wavelength of the diodes may occur when the LED current is ramped. Thus the integrated power is not linear with time, whereas the intensity using CW stimulation at a fixed wavelength is stable. Thus, the stimulation rate is non-linear even if the power is linear (p = o-q~,see Eq. (2.8)), requiting correction when analysing LM-OSL curves. A more detailed examination of the OSL response of different materials to ramped stimulation power is described in Chapters 2 and 3. 7.4.10. Pulsed and time-resolved OSL In the applications discussed so far, the light from the excitation sourcesmeither lasers, diodes or filtered lampsmis emitted continuously and the luminescence is monitored during the period when the sample is exposed to the stimulation source. As discussed, this requires the use of filters to discriminate between the stimulation light and the emitted light. This prevents the use of stimulation wavelengths, which are the same as, or close to, those observed in the emission. McKeever et al. (1996) reported a pulsed OSL (POSL) technique, in which the stimulation source is pulsed and the OSL is only monitored after the end of each pulse, i.e., only the afterglow is measured. Since the emission is not detected while the pulse is on, this arrangement extends the potential range of stimulation wavelengths that can be used. As discussed in Chapter 2, POSL involves the synchronised detection of OSL following a pulse of stimulation. The key to the effective use of POSL is to stimulate the sample with a high-intensity pulse of light using a pulse width T that is much shorter than the intrinsic luminescence lifetime z of the recombination centre excited state. Under these conditions, a large population of charge is placed in the centre' s excited state during the pulse. This population relaxes, with the emission of photons (POSL), after the stimulation pulse. Experimentally, one can excite an irradiated sample with a stream of pulses, recording the POSL after each one of them. In this way, multiple readings can be taken and, if enough pulses are used, the trap population can be entirely depleted. If one were to excite with a stream of pulses in this way one could allow each excited state population to completely decay after each pulse, before exciting the sample with a second pulse. To ensure this, one would need to wait a minimum of (say) 3z between pulses. For long lifetimes (e.g., the F-centre lifetime in A1203:C is 35 ms) this restricts the pulse frequency that can be used and prolongs the readout time. However, rather than waiting for the POSL to decay after each pulse, one can multiplex the signal by exciting with the second pulse
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before the luminescence from the first pulse has decayed. The second pulse, therefore, adds an additional excited state population to that not yet relaxed from the first pulse and, consequently, the POSL signal from the second pulse adds to the POSL signal from the first. This is continued with additional pulses and eventually an equilibrium excited state population is reached. As the process continues, the trapped charge population depletes and the excited state population starts to reduce. This will continue as longas the pulsed excitation continues, until the trap is finally emptied. A typical timing sequence for the process is illustrated in Fig. 7.9a. The times indicated in the figure are those appropriate to detection of F-centre luminescence from A1203:C. Pulse widths of 300 ns are used. The PM detector gate is closed for a 15 Ixs period just ahead of the pulse, and opened after a short delay just after the pulse. The gate remains open, during which time the POSL emission is measured, for a period of 235 Ixs, before closing the gate to allow the next laser pulse. A pulse frequency of 4 kHz is used in this illustration. The timings illustrated in Fig. 7.9a were used by Akselrod and McKeever (1999) as the basis of a dosimetry system using A1203:C and a pulsed Nd:YAG laser (2nd harmonic: 532 nm). Since the lifetime of the luminescence from A1203:C is long (35 ms) the luminescence emission is essentially constant during each data acquisition period (235 ~s). Thus, with each laser stimulation pulse, the POSL intensity increases. This process continues until equilibrium is achieved between the generation of an excited state population and the decay of this population via luminescence emission. Continued excitation of the sample leads to depletion of the trapped charge concentration and a decay in the POSL. Example data showing these effects in A1203:C are displayed in Fig. 7.9b. The timing sequences illustrated in Fig. 7.9a are appropriate for A1203:C, but not necessarily for other materials. The POSL timing sequence needs to be tuned to the lifetime of the particular system being studied. For example, the intrinsic lifetime of OSL emission in quartz is measured to be of the order of several microseconds (Bailiff, 2000). Thus, for quartz, each pulse width should be of the order of a few nanoseconds (maximum) while the pulse frequency needs to be of the order of MHz for effective POSL readout. A typical POSL experimental arrangement is shown in block diagram in Fig. 7.10. POSL has been adopted as a commercial dosimetry system by Landauer Inc: (USA) for their LuxelTM personal dosimeters. A pulsed stimulation method has been used by Piesch and colleagues for radiophotoluminescence (RPL) readout from phosphate glass dosimeters (Piesch et al., 1990, 1993). Short, 4 ns pulses (using a pulsed UV excitation source) at low frequency (1-20 Hz) were used and by synchronising the detection system, the resulting luminescence signal is integrated in two time regimes. The first integration period (2-7 lxs) records the radiationinduced RPL signal plus the background signal, while the second integration period (40-45 Ixs) records the non-radiation-induced background signal only. If l(t) is the measured time-dependent luminescence signal, then the net, radiation-induced signal M is given by the difference in the two integrated signals, according to:
M = ~11(t)dt-fps
~2 l(t)dt
(7.1)
where fps is the so-called 'pre-dose suppression factor' and accounts for the change in the background signal between that measured in the 40-45 Ixs interval and that occurring
328
Optically Stimulated Luminescence Dosimetry
Fig. 7.9. (a) Schematic timing sequence for the measurement of POSL from A1203:Cshowing a data acquisition time of 235 txs and a pulse frequency of 4 kHz, with a pulse width of 300 ns (from Akselrod and McKeever, 1999). (b) POSL data from irradiated A1203:C obtained using the timing parameters noted in Fig. 7.9a. The stimulation period was 1 s (i.e., 4000 pulses were incident on the sample). At the end of the stimulation period, the OSL decays with a lifetime of 35 ms. The POSL signal during the 1 s period is seen to build-up after each pulse until equilibrium is reached (see text) followed by a decrease due to depletion of the traps (from Akselrod and McKeever, 1999).
during the 2 - 7 Ixs interval. T o s h i b a Glass Ltd c o m m e r c i a l i s e d this R P L d o s i m e t r y s y s t e m for p e r s o n a l d o s i m e t r y . R h y n e r and M i l l e r (1970), B e r n h a r d t and Herforth (1974), P r a d h a n and A y y a n g e r (1977) and Y o d e r and Salasky (1997) each u s e d p u l s e d excitation of irradiated d o s i m e t e r s
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Fig. 7.10.
329
Schematic layout of a typical POSL apparatus.
(BeO, CaF2:Mn, CaSO4:Dy and A1203:C , respectively). However, in each case what was actually measured is not POSL, as defined before and in Chapter 2, but a delayed optically stimulated luminescence (i.e., DOSL--see Chapter 2). The delay between the end of the excitation pulse and the start of the luminescence reading in each of these cases meant that the fast OSL component had already decayed and what was monitored was the OSL due to trapping in shallow traps. Sanderson et al. (1994, 1995) developed and used what they called photostimulated luminescence methods based on IR stimulation (the same as IRSL) to identify irradiated food. An instrument for rapid screening of irradiated food was developed at Scottish Universities Reactor Research Centre (SURRC) based on pulsed IR stimulation, which is designed to allow direct measurements of OSL signals from mineral contaminants in herbs and spices for screening purposes, without the need for sample preparation or reirradiation. Samples are introduced directly in Petri dishes and the instrument produces a qualitative screening measurement over 15 s. The principle of the technique is to pulse stimulate a sample using IR diodes. The pulsing allows higher current and thus larger illumination power at the sample than is possible using continuous wave stimulation. In addition, no filters are required, thus giving higher detection efficiency. The background is measured without illumination between the pulses, while the diodes cool, and subtracted automatically (Sanderson et al., 1996). Other pulsed readout methods have been used for OSL, primarily as a means of examining the time-resolved OSL emission from various dosimetric materials. For
330
Optically Stimulated Luminescence Dosimetry
exampte; Sanderson and Clark (1994) used pulsed light from a xenon lamp to measure the time-resolved photo-stimulated luminescence in alkali feldspars. Clark et al. (1997) and Clark and Bailiff (1998) used a pulsed Nd:YAG laser to examine the time-resolved emission of OSL from various feldspar minerals. Similar approaches were adopted by Bailiff (2000) who used 5 ns pulses from a tuneable Q-switched laser to examine the timeresolved OSL emission from natural and synthetic quartz. Galloway and colleagues (Chithambo and Galloway, 2000a,b; Galloway, 2002) used much longer (14 Ixs) pulses of stimulation from light-emitting diodes to monitor the luminescence lifetimes of the OSL from natural quartz. In each of these applications, however, the purpose of the measurement was the examination of the luminescence lifetimes from these natural samples as a means of understanding more about the OSL characteristics from these important dosimetric materials. Using POSL in dosimetry per se was not the goal.
7.5. Wavelength resolved OSL 7.5.1. Stimulation spectrometry Stimulation and emission spectroscopy are traditional tools for examination of the mineralogy of natural materials (e.g., Marfunin, 1979). With respect to OSL from materials of use in retrospective dosimetry, Htitt et al. (1988) demonstrated the importance of analysing the optical stimulation spectra (i.e., OSL versus stimulation wavelength) of feldspars and Poolton et al. (1996b) showed that stimulation spectra of natural samples provided information about the mineralogy. As the OSL signal decays under constant illumination due to trap depletion, consideration of procedures for correcting the stimulation spectra produced must be made. Measurement of stimulation spectra under conditions of weak excitation was discussed mathematically in Chapter 2. Bailiff (1993) and Bailiff and Barnett (1994) used a titanium-sapphire laser, tuneable between 700 and 1000 nm, to analyse the time-decaying OSL stimulation spectra from feldspars, both at room temperature and at low temperatures. Ditlefsen and Hunfley (1994) used argon, krypton, He-Ne, and argon-pumped dye lasers operated in CW mode to study optical excitation characteristics of quartz and feldspars. A general problem when obtaining stimulation spectra is that the OSL obtained at each wavelength has to be normalised and corrected for variations in beam power and instrument response. Clark and Sanderson (1994) performed OSL excitation spectroscopy using filtered light from a 300 W xenon lamp coupled to a computer-controlled, stepper-motor-driven f 3.4 monochromator. Stimulation spectra can be obtained using specific wavelength lines from lasers, especially tuneable lasers, or by using energy dispersive spectrometers with diffraction gratings or prisms. Commercially available systems, such as the Jobin-Yvon Fluoleg spectrofluorometer, may also be used to measure stimulation spectra. Alternatively, variable wavelength absorption filters may be used. An example of the latter is described by BCtter-Jensen et al. (1994a) who used a compact scanning monochromator (BCtter-Jensen et al., 1994b) based on variable interference filters coveting the wavelength band 380-1020 nm (see Fig. 7.11a). An example of an optical stimulation spectrum
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Fig. 7.11. (a) Schematic diagram showing monochromators mounted in both excitation and detection positions of the Rise broad-band OSL unit for obtaining stimulation spectra and emission spectra, respectively (from BCtter-Jensen et al., 1994a). (b) Optical stimulation spectrum (In(/) versus stimulation energy) for a sedimentary quartz annealed at 850~ Detection filter: U-340. Beta dose: 8 Gy. The spectrum is plotted, corrected for unit incident energy at the sample (from BCtter-Jensen et al., 1994b).
obtained in the wavelength band 420-650 nm (1.9-2.9 eV) from a sedimentary quartz sample using this system is shown in Fig. 7.1 lb. Godfrey-Smith and Cada (1996) used monochromatic stimulation from a tuneable Ti:sapphire laser and a Newport Corp. linear speed controller to measure stimulation spectra from plagioclase and potassium feldspars and quartz. Barnett and Bailiff (1997) also used a tuneable Ti:sapphire laser to investigate the temperature dependence of IR stimulation spectra from some feldspars.
332
Optically Stimulated Luminescence Dosimetry
7.5.2. Emission spectrometry A simple TL glow curve (TL versus temperature) or an OSL decay curve does not always yield unambiguous information, for instance, when the emission spectrum changes with temperature during a TL measurement or when there are multiple components during CW optical stimulation. This may be due to the radiative recombination of the released charges occurring at more than one defect site within the crystal. For this reason, it is important to obtain for example 'three-dimensional glow curves' when measuring TL, i.e., emission spectra in which the intensity is displayed as a function of both temperature and wavelength. Isometric displays of TL versus wavelength versus temperature thus give information both about the trap distribution and the charge recombination centres. Several instruments based on different optical principles have been developed and described in the literature. There are two factors common to all such instruments, which govern the sensitivity: the optical throughput, also known as ~tendue (beam area in IIIITI 2 X solid angle) and the resolution, i.e., the ability to distinguish between wavelengths. To some extent, these two factors work against each other, because good wavelength resolution is achieved by limiting the input beam throughput by means of a slit. A third factor is the change of output intensity with time, inherent in the method. This affects the sensitivity because it determines the amount of time that can be devoted to data collection during each part of the measurement. The great majority of instruments are based on spectral dispersion by a diffraction grating. Rapid-scanning dispersive systems based on gratings were described in the early 1970s by Harris and Jackson (1970) and Mattern et al. (1971). Insofar as they successively measure one wavelength region at a time, they are of limited sensitivity. By measuring all wavelengths simultaneously in many channels, a much higher effective sensitivity is achieved. This is known as the 'multiplex' or 'Felgett' advantage. Huntley et al. (1988) built a spectrometer based on a concave holographic grating in connection with a microchannel plate PM tube and image converter to obtain wavelength-resolved spectra of a variety of mineral samples. Luff and Townsend (1993) used two gratings, each with a multi-channel position-sensitive PM tube to cover the spectral range of 200-800 nm (6.20-1.55 eV). Piters et al. (1993) used a dispersive grating and an intensified diode array. Martini et al. (1996) developed an instrument based on wide angle mirror optics, a flat field holographic grating and a two-stage micro-channel plate detector followed by a 512 element photodiode array. Recent improvement in the sensitivity of charge-coupled devices (CCDs) means that CCDs have been increasingly used for the multiplexing detector. An early instrument of this type was described by Bakas (1984). Rieser et al. (1994, 1999) reported a versatile, high sensitivity, TL/OSL spectrometer based on a liquid nitrogen cooled CCD camera with simultaneous detection over the range 200-800 nm. In this instrument, thermal stimulation can be performed to 700~ and optical stimulation from UV to IR with monochromatic light from a 200 W mercury lamp. An example of its use with feldspar is given by Krause et al. (1997). More recently Bos et al. (2002) described a compact TL/OSL emission spectrometer where light emitted from the sample is transmitted to an optical fibre onto a fixed enhance slit of a spectrograph and measured by a CCD-linear array detector.
OSL Measurement Technology
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As mentioned earlier, a fundamental limitation to the sensitivity of wavelength dispersive instruments is the need to limit the input beam with a slit in order to achieve adequate resolution. Generally in OSL dosimetry, the emission from different materials at or above room temperature have line widths > 20 nm. Nevertheless, it remains an inherent limitation. Jacquinot (1954) long ago pointed out that spectral information can be extracted from an extended source, as distinct from a slit or point source. Bailiff et al. (1977) exploited this with a spectrometer using narrow band optical filters on a tumtable, which presented the filters sequentially to uncollimated TL emission. Although lacking the multiplex advantage, it has a sensitivity comparable with that of dispersive instruments of the same resolution. BCtter-Jensen et al. (1994a) used the Rise monochromator based on variable interference filters to measure OSL emission spectra from different feldspar samples. Bortolot (2000) described a four PM tube, multi-band, slit-less detector, which uses a solid quartz light guide and four miniature PM tubes, each with a different narrow-band filter, to measure the wavelength-resolved luminescence (Fig. 7.12). This could be described as a hybrid, combining the Jacquinot, broad source, advantage with limited wavelength selection. A different principle was exploited by Prescott et al. (1988) who developed a Fourier transform spectrometer. It makes use of the throughput advantage and is based on a Twyman-Green, Michelson type, interferometer. The data for all wavelengths of light are collected simultaneously from an extended source and the output is recorded as an interferogram by a single PM. This interferogram is the Fourier transform of the input
Fig. 7.12. The outputend of the Daybreakspectrometershowingthe light guides and PMs for spectral sampling (from Bortolot, 2000).
334
Optically Stimulated Luminescence Dosimetry
power spectrum, which can be found by inverting the transform. The original instrument had glass optics but is now fitted with quartz optics and covers the range 250-750 nm (5.00-1.65 eV). Because of the need for slits, the dispersive spectrometers discussed above have an 6tendue of about 1 or less. For example the Adelaide Fourier transform spectrometer (Prescott et al., 1988) has an effective &endue of 20 for the same resolution, which gives it an advantage of sensitivity across the spectral range. A similar instrument, with the addition of an auxiliary interferometer for automatic calibration, has been described by Haschberger (1991). In those spectrometers with optical excitation, the variables in the three-dimensional display are either photon emission energy/wavelength versus energy/wavelength and time interval after the start of depletion or excitation energy/wavelength versus energy/ wavelength. An example of the latter is given for A1203:C in Fig. 3.3.
7.6. Imaging systems The majority of luminescence measurements are made using PM tubes with bialkali photocathodes. These devices offer high sensitivity in the blue and near ultra-violet region. However, the PM tube used for such measurements integrates the luminescence signal from the entire sample and gives no indication of any spatial variation in luminescence intensity within a sample. Hashimoto et al. (1995) obtained OSL images of some X- and gamma-irradiated granite slices using photon detection through a 570 nm bandpass filter with diode-laser excitation at 910 nm. Several other laboratories have attempted to develop systems capable of imaging the luminescence signal from a sample. Three groups have used IPDs, two at the University of Oxford (Smith et al., 1991; McFee and Tite, 1994) and another at the University of Utah, Salt Lake City (Burggraaf and Haskell, 1994). These instruments retain the high sensitivity of a PM tube, but are rather expensive and difficult to operate. The development of the solid-state imaging systems based on CCD technology offers an alternative. Duller et al. (1997) and Spooner (2000) constructed CCD camera-based imaging systems that could be attached directly to the automated Rise TL/ OSL reader. The CCD has a similar sensitivity to that of a PM tube, though the spectral responses are very different. Akselrod et al. (2000) used a CCD for spatial imaging of OSL from large-area A1203 detectors for monitoring radiation deposition patterns. Greilich et al. (2002) used a CCD system and an IR laser diode to measure the spatially resolved IRSL from ground slices of artificial (archaeological) granite for dating and determination of the mineralogy.
7.7. Single grain OSL systems 7.7.1. Introduction When performing dosimetry on natural materials, consideration of variability in the OSL properties is important. Recent work has shown that the natural bleaching of quartz
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and feldspar grains is not homogeneous, as had been assumed for studies of aeolian sediments, but is markedly heterogeneous (Li, 1994; Rhodes and Pownall, 1994; Murray et al., 1995; Olley et al., 1998). The effects of this heterogeneity become more pronounced as the number of grains in an aliquot is reduced. Because of the time required to hand pick hundreds of grains for measurement using a conventional reader, there have been relatively few studies that have looked at individual grains of feldspar (e.g., Lamothe et al., 1994) and quartz (e.g., Murray and Roberts, 1997; Roberts et al., 1998). It is clear from these studies that there are considerable benefits in examining the dose distributions on a grain-by-grain basis, and particularly in identifying those grains that have been most completely bleached prior to deposition. 7.7.2. CCD luminescence imaging systems The ability to image the luminescence signal from a sample and isolate the signal from a specific part of the sample have initially been achieved using CCD cameras (Duller et al., 1997; Spooner, 2000). Such cameras consist of a CCD chip and the associated electronics, which allow the interfacing to a personal computer. The digital signal measured for any given pixel by the host computer is affected by various sources of noise and bias within the CCD camera. These can be divided into two sources of uncertainty in the final value that is read. Under normal conditions, charge accumulates in the various pixels of the CCD during light exposure. However, electrons may also be accumulated within these pixels as a result of thermal noise, and these electrons cannot be distinguished from those generated by exposure to light. This thermal noise makes a source of uncertainty. 7.7.3. Single grain laser OSL systems In the last few years, a system has been developed that enables routine measurement of the OSL signal from single grains of quartz or feldspar in the size range from 100 to 300 Ixm (Duller et al., 1999a,b). BCtter-Jensen et al. (2000) described a single grain OSL system that employs a 10 mW Nd:YVO4 solid-state diode-pumped laser emitting at 532 nm (measured stability < 1.5% over 8 h), and producing a spot of approximately 50 Ixm diameter at the surface of the sample disc. To give power control, the beam from the laser can be electronically attenuated between zero and full power (software controlled). Individual grains are located in 300 txm wide and 300 txm deep holes drilled as a 10 • 10 hole grid pattern on the surface of a 9.7 mm diameter, 1.0 mm thick aluminium disc, giving a total of 100 grain positions on each sample disc. The location of the laser spot on the sample disc is controlled by two mirrors, which are mounted orthogonally, making it possible to illuminate only one grain and measure the OSL signal it produces. A schematic of the single grain OSL attachment and the grain sample disc are shown in Fig. 7.13. Precise positioning is achieved by using the laser beam at low power to locate the positions of the three holes at the periphery of the sample disc (Fig. 7.13). A phototransistor is built into the measurement chamber, mounted in such a way that it is struck by the laser beam if reflected from the sample disc surface. This search routine is the starting point for each measurement of a sample disc since it defines the position and angle
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Fig. 7.13. Schematic of the single-grain laser OSL attachment to the automated Rise TL/OSL apparatus. Above to the right is shown the sample disc that can accommodate 100 grains (from BCtter-Jensen et al., 2002).
of rotation of the disc. The laser beam is then moved to the calculated position of the first grain and an OSL measurement is made using full laser power. Measurement of each grain is very rapid since the power density of the stimulation light is approximately 50 W/cm 2, once scattered in a grain hole. This power level is three orders of magnitude higher than for standard stimulation sources. Typically, the OSL signal from quartz decays by a factor of 2 in under 0.1 s (Duller et al., 1999a). As well as providing rapid measurement of the OSL signal from 100 grains per single disc, machine time is saved in irradiation and pre-heating since these are undertaken simultaneously on all grains. The throughput of the single grain
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OSL system is nearly two orders of magnitude higher than that obtained using a conventional reader. McCoy et al. (2000) developed an automatic apparatus for sorting single grains for routine age determinations by green light stimulation. The apparatus automatically introduces single grains via a dispenser system in sequence on top of a movable steel needle into a measurement chamber where the OSL signals from the individual grains are stimulated using a focussed, pulsed, green laser beam (the 511 nm line filtered from a CuBr laser). The UV emission from the quartz grains is detected via a U-360 transmission filter and a cooled UV-sensitive PM tube (EMI 9734QB). A schematic of the system is shown in Fig. 7.14. McCoy et al. (2000) used their single grain OSL instrument to determine the dose distributions of dune sand samples and concluded that the single grain sorting system efficiently discards the dim grains and selects the bright ones. Bortolot (2000) described a single grain scanning laser exciter using new sub-miniature galvanometer servos in an X - Y scanner mount. The beam from a 1 mW green diode
Fig. 7.14. Schematic of the single-grain laser OSL system developed at the University of Adelaide (from McCoy et al., 2000).
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Optically Stimulated Luminescence Dosimetry
pumped solid-state laser is focussed down to a 10-200 Ixm diameter spot using focussing lenses; this beam is further reflected down onto the sample disc by a rotating mirror. A segmented octagonal cone reflector, made of a thin glass sheet with magnesium-fluorideprotected aluminium coating, collects the light from the sample and couples it to the PM tube. One of the characteristics of the OSL of single grains is the wide spread of brightness intensities. Brightness distributions are log-normal or quasi log-normal over four orders of magnitude (McCoy et al., 2000; Duller et al., 2000).
7.8. O S L scanners
The ability to focus stimulation light sources to illuminate only a fraction of the sample surface lends itself to the possibility of analysing position-resolved luminescence at other scales besides single grains. An integral optical sensor system was developed for rapid analysis of split sediment cores in geological applications (Poolton et al., 1996a). The basis for the design is a core-logger system with a conveyer belt allowing optical sensors to be moved along the length of the split sediment cores up to a length of 1.7 m. The optical sensor consists of a photo-excitation and detection module, together with lamps for bleaching and regenerating the OSL (the exact regeneration mechanism is unknown, but is likely to be related to phototransfer of charge from deep traps to shallow traps). Both IR and blue-green stimulation is provided and these probe the feldspar, and combined feldspar and quartz components of the sediment, respectively. The ratio of the two signals may thus provide rapid information on the silicate mineralogy. The sensor head maintains a constant height above the core surface by means of a motor and cam mechanism (with height measured using a linear displacement voltage transducer). A schematic diagram of the apparatus is shown in Fig. 7.15, along with details of the sensor head design. Luminescence is detected using a 30 mm bialkali PM tube (type EM19924) in photon counting mode, with the emissions passing through 6 mm of standard Hoya U-340 filter (transmitting between 280 and 370 nm, with a peak at 340 nm). These filters were used in detection when exciting with either green or IR light. Their advantage is that, since of necessity the detector is open and exposed, measurements can proceed under the normal ambient lighting levels typically used in luminescence dating laboratories (i.e., similar to that in a photographic dark room) as the detector is rendered effectively blind to red/orange light. The luminescence regeneration lamp is a cold-running 20 W low pressure Hg gas discharge tube (Osram HNS-20UOZ) that yields an output of 5.5 W at 254 nm (4.9 eV) and 0.8 W at 185 nm (6.7 eV). Poolton et al. (1996a) describe the performance of the core scanner in detail and present results from measurements of a Danish sediment core. Also, a small OSL core scanner was designed for the measurement of dose-depth profiles directly on the surface of 10 mm diameter cores drilled out of bricks for retrospective dose determination after nuclear accidents (BCtter-Jensen et al., 1995). This system used a blue green (420-500 nm) stimulation band filtered from a halogen lamp focussed onto the brick core using cylindrical lenses to illuminate the surface in the form
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Fig. 7.15. Schematic diagram (above) of the Rise-developed OSL core scanner system and (below) detail of the luminescence excitation/detection head. The use of a light guide allows the luminescence excitation source to be physically removed from the main light collection optics. Sediment cores up to 1.7 m in length can be analysed in the system (from Poolton et al., 1996a).
of a slit of 10 x 1 mm 2 giving a resolution of 1 mm. The brick core scanner was used for rapid determination of dose-depth profiles in bricks that had been exposed to gamma radiation with different photon energies in the laboratory. Habermann et al. (2000) used a split-optical fibre connected between an IR laser diode, a PM tube and a movable measuring head to directly measure the IRSL signals from surfaces of irregularly shaped granite rocks.
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7.9. Portable systems for OSL measurements in the field
When dating thick sediment deposits, it is often difficult to decide in the field how many samples should be taken for preparation. Sampling is normally based on the visible features of the material. However, subtle changes in the sedimentological features relating to breaks in the deposition history are often not obvious to the naked eye, but will show up as discontinuities in the luminescence determination of the equivalent absorbed dose De as a result of either different depositional ages, or different bleaching histories or both. As a consequence, it is desirable to be able to rapidly assess the luminescence properties of the sediment at regular intervals down a section, preferably on site. The advent of optical stimulation methods allows the measurement of mineral grains at ambient temperatures, thus making portable equipment practical. Although for obtaining precise ages this material must be prepared under laboratory conditions, field measurement of untreated sediment may provide sufficient information to provide a sampling guide. A compact and lightweight computer-controlled system was developed by Poolton et al. (1994) that allows for the measurement of IRSL of samples in the field, whether in the form of loose grains or compressed pellets. The unit uses an IR diode for excitation with bleaching and the OSL regeneration is provided by cold-running gas discharge lamps. Colyott et al. (1999) designed a portable OSL system for the measurement of phototransferred OSL (PTOSL) in A1203:C in an attempt to measure the UV-B exposure from sunlight directly in the field. This portable system uses Nichia green LEDs as the excitation source, arranged in a ring to focus onto a 9.5 mm diameter spot size. The resulting OSL signal is detected with a Thorn EMI Electron Tubes model P30CWAD5 Photodetector Package (which incorporates a miniature bialkali PM tube), positioned normal to the sample. The whole system is powered from a simple laptop PC.
7.10. The measurement of RL
Radioluminescence (RL) is the prompt luminescence emitted from insulators during exposure to ionising radiation and is used for investigating radiation-induced defects and luminescence emission mechanisms in wide band-gap materials (e.g., Alanso et al., 1983; Hohenau, 1985). The phenomenon arises due to ionisation of the lattice under excitation from X- or gamma sources or high-energy particles, and subsequent prompt recombination of electrons and holes, either directly across the band gap, or via charge trapped at defect sites. TL and OSL dosimetry, using wide band-gap insulators, is based on the trapping of charges at deep lying defect centres during irradiation; these charges accumulate with increasing irradiation dose and are ideally time-stable. Thus, the TL/OSL defects will act competitively with free electrons and holes generated during irradiation, which would otherwise recombine to produce RL. As a result, the intensity of RL may not depend on the accumulated dose, and can change as competition with the TL/OSL traps changes. Marazuev et al. (1995) proposed that analysis of the dose evolution of the RL in quartz could itself be used as a dosimetric tool, and demonstrated the possibility by making retrospective dose analysis on bricks collected in the Chernobyl accident area.
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More recently, Trautmann et al. (1998, 1999, 2000a), Krbetschek et al. (2000) and Schilles and Habermann (2000) have suggested that RL may also be used for the dating of feldspar extracted from sediments, and Erfurt et al. (2000) have demonstrated the use of RL for dosimetry in A1203:C. Trautmann et al. (1999) detected the IR emission of the RL signal from various feldspar samples using a high-sensitivity luminescence spectrometer (Rieser et al., 1994) in a range up to 1000 nm (1.2 eV). RL stimulation was achieved using a 137Cs source with an activity of 3.7 MBq in close proximity to the sample and the luminescence was collected by a condensing system and connected to the spectrometer by fibre optics. Trautmann et al. (2000b) used the same equipment with a modified sample holder to study the RL properties of single feldspar grains. Schilles and Habermann (2000) constructed an RL reader using elliptical mirrors to focus the emitted light onto the cathode of a cooled IR-sensitive GaAs PM tube. The RL is stimulated using a 3.7 MBq 137Cs s o u r c e which is covered by a 5 Ixm titanium-foil that rejects any possible luminescence from the source itself. The sample holder consists of an aluminium ring that can be fixed on top of the radiation source. The equipment and sample holder are shown schematically in Fig. 7.16. The monitoring of RL in addition to TL and OSL measurements from artificial and natural dosimeters thus provides a powerful approach for analysing luminescence emission processes and can be used for real-time determination of integrated doses and dose rates in both natural and synthetic materials during irradiation. Poolton et al. (2001) developed two RL units as attachments to the existing Rise TL/OSL system; the most sensitive system delivers a dose rate of 4.5 Gy/min to materials such as quartz, using a 2.96 GBq (80 mCi) 9~176 beta source. The units further enable the measurement of RL in the temperature range 25-500~ using both continuous and pulsed radiation exposures (Poolton et al., 2001). Measurements are fully automated, allowing up to 48 samples to be measured in any one experimental run. The first of the two RL systems can be installed directly above the existing heater unit at the TL/OSL position. The luminescence is monitored by an access port drilled in the RL flange containing the necessary f 2 quartz optics to focus the beta induced RL into a 4 mm diameter light guide. The light guide transmitting the RL signal is fed into the PM tube assembly which, in this configuration, is separated from the reader. The second system enables a more comprehensive set of features to be analysed using a different exchangeable flange unit, also with a luminescence access port equipped with optics and light guide. This RL system is mounted below the existing beta irradiator in its standard position, where also the extra heater (see Section 7.3) is built into the reader under the RL unit (BCtter-Jensen et al., 2002). In this configuration, the RL signal is fed via the light guide into the detection optics using an empty port in the OSL unit where it is reflected by a mirror into the cathode of the PM tube. The latter configuration allows for combined RL, TL and OSL measurements and is shown schematically in Fig. 7.17 (BCtter-Jensen et al., 2002). As examples, the pulsed RL curves for quartz and A1203:C, obtained by opening and closing the beta source while heating the samples at a rate of 1 ~ s, are shown in Fig. 7.18a and b, respectively (from BCtter-Jensen et al., 2002). The temperature dependence of the derived pure RL signals (insets to Fig. 7.18) reveals the thermal quenching profiles of the two samples.
Sample
5 prn Titanium foil
/ Active area 6 mm d~ameter
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\ Self adhesive plast~cfo~l
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Fig. 7.16. Left: Cross-section of the RL reader developed in Heidelberg and a schematic description of the components. Right: Cross-section of the stimulation source and the sample holder (from Schilles and Habemann, 2000).
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Fig. 7.17. Schematic of the RL configuration that enables combined measurements of RL, TL and OSL signals. A light guide transfers the prompt RL signal into the general TL/OSL detection unit (from BOtter-Jensen et al., 2002).
7.11. Commercially available OSL apparatus Three main distributors of TL/OSL dating equipment are: ELSEC-Littlemore Scientific Engineering Company, UK, Daybreak Nuclear and Medical Systems, USA and Rise National Laboratory, Denmark. Landauer, USA has developed OSL measurement systems for A1203:C. The Littlemore Company has two standard automated luminescence dating instruments available. One is a 24-sample automated TL reader (without OSL attachments) and the other is a 64-sample optical dating system (without TL facilities) that is available with either IR LED stimulation or visible light stimulation using a filtered lamp module. An attachable beta irradiator is provided for the automated TL reader. The address is: Littlemore Scientific Engineering, ELSEC, Railway Lane, Littlemore, Oxford OX4 4PZ, UK. The Daybreak instrument programme includes a standard 20-sample automatic TL reader (model 1100) using an on-board computer and serial interface to a host computer. The samples are moved by a sweep arm from the sample turntable to the heating/reading position and back. An upgraded model 1150 TL reader is available with a capacity of 57 samples achieved by vertically stacking three 20-sample platters. Various OSL attachments are available based on xenon and halogen lamps. A compact fibre-optic illuminator attachment has been reported by Bortolot (1997) and a new OSL reader design (without TL facilities) based on 60-sample capacity is under development (Bortolot, 2000). The address is: Daybreak Nuclear and Medical Systems, Inc., 50 Denison Drive, Guildford, CT 06437, USA.
344
Optically Stimulated Luminescence Dosimetry 6xl 044 ******
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Rise National Laboratory provides an automatic combined TL/IR~lue light OSL dating system, with on-plate attached alpha and beta irradiators, that can accommodate turntables containing 48 samples. The most recent model incorporates a unit containing a powerful IR laser diode providing 400 mW/cm 2 at the sample and a blue LED array (470 nm) providing > 50 mW/cm 2 (BCtter-Jensen et al., 2000). New features are (1) a dual green/IR (532/830 nm) solid-state laser based single grain OSL attachment that can accommodate 48 sample discs, each containing 100 grain hole positions in a 10 x 10 grid (Duller et al., 1999b; BCtter-Jensen et al., 2000) and (2) a RL attachment with an extra heater beneath the irradiator for providing temperature-dependent RL measurements (Poolton et al., 2001). A software-controlled beta irradiator attachment for in situ irradiation of samples is also provided. A recently developed sequence editor software has significantly extended the flexibility and measurement capabilities. The address is: Rise
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National Laboratory, Radiation Research Department, Building 201, DK-4000 Roskilde, Denmark. Landauer provides an InLight TM system that utilises OSL technology known as 'Sapphire Technology'. The system is based on a LED light source for stimulating the A1203:C detector material. The InLight TM system has a linear dose response up to 10 Gy, and uses two beams, one weak and one strong, for extending the dynamic range.
7.12. Future developments In this chapter, techniques and methods applied in luminescence dating and dosimetry at many laboratories around the world have been reviewed and an attempt has been made to describe the state-of-the-art in instrument and method development. One outstanding problem, which remains to be addressed in the development of combined TL/OSL instrumentation using different stimulation light spectra, is the design of a flexible optical detection filter changing system to allow for rapid (automatic) selection of the optimal detection window whether using IR or visible light stimulation. Changing of excitation or detection filters may, if not properly protected either by hardware or software, cause serious damage to the PM tube because of insufficient suppression of stray light from the stimulation light source. The growing industrial interest in ultra-bright LEDs as light indicators (e.g., from automobile manufacturers) may soon make visible LEDs commercially available with a greater variety of emission wavelengths and substantially higher emission power than is available today. These LEDs should provide sufficient power to be considered a real alternative to lasers and powerful incandescent lamps as light sources in OSL. Major effort will no doubt still be put into the development of sensitive systems capable of measuring luminescence from small aliquots, even down to single grains so that variations in dose from grain to grain can be studied in detail. The latter feature will be especially valuable in studies of incompletely bleached materials in dating and retrospective accident dosimetry. In particular, methods for single grain spectroscopy at doses found in field samples still have to be developed. Further developments in, and investigations of, luminescence imaging systems for obtaining spatially resolved TL and OSL signals from multi-mineral samples are also foreseen. These systems give rapid and valuable information about the mineralogy of the sample and enable individual analysis of luminescence signals from single grains of a sample. This has the potential to avoid the cumbersome mechanical and chemical separation processes presently required. Thus, it will be possible to map solid surfaces containing grains with different OSL sensitivities and different doses.
References Aitken, M.J., 1985. ThermoluminescenceDating. Academic Press, London. Aitken, M.J., 1990. Optical dating of sediments: Initial results from Oxford. Archaeometry32, 19-31. Aitken, M.J., Smith, B.W., 1988. Optical dating: recuperation after heating. Quat. Sci Rev. 7, 378-393.
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Akselrod, M.S., McKeever, S.W.S., 1999. A radiation dosimetry method using pulsed optically stimulated luminescence. Radiat. Prot. Dosim. 81, 167-176. Akselrod, M.S., Agersnap Larsen, N., McKeever, S.W.S., 2000. A procedure for the distinction between static and dynamic radiation exposures of personal dosimetry badges using pulsed optically stimulated luminescence. Radiat. Meas. 32, 215-225. Alanso, P.J., Halliburton, L.E., Kohnke, E.E., Bossoli, R.B., 1983. X-ray induced luminescence in crystalline SiO2. J. Appl. Phys. 54, 5369-5375. Bailiff, I.K., 1993. Measurement of the stimulation spectrum (1.2-1.7 eV) for a specimen of potassium feldspar using a solid state laser. Radiat. Prot. Dosim. 47, 649-653. Bailiff, I.K., 2000. Characteristics of time-resolved luminescence in quartz. Radiat. Meas. 32, 401-405. Bailiff, I.K., Barnett, S.M., 1994. Characteristics of infrared-stimulated luminescence from a feldspar at low temperatures. Radiat. Meas. 23, 541-545. Bailiff, I.K., Morris, D.A., Aitken, M.J., 1977. A rapid interference spectrometer: application to low level thermoluminescence emission. J. Phys. E: Sci. Instr. 10, 1156-1160. Bakas, G.V., 1984. A new optical multichannel analyser using a charge coupled device for thermoluminescence emission measurements. Radiat. Prot. Dosim. 9, 301-305. Barnett, S.M., Bailiff, I.K., 1997. The temperature dependence of luminescence in some feldspars (80-300 K). J. Appl. Phys. 30, 683-689. Bernhardt, R., Herforth, L., 1974. Radiation dosimetry by optically stimulated phosphorescence of CaF2:Mn. In: Newiadomski, T. (Ed.), Proceedings of the Fourth International Conference on Luminescence Dosimetry, Krakow, Poland, pp. 1091-1104. Bortolot, V.J., 1997. Improved OSL excitation with fiberoptics and focused lamps. Radiat. Meas. 27, 101-106. Bortolot, V.J., 2000. A new modular high capacity OSL reader system. Radiat. Meas. 32, 751-759. Bos, A.J.J., Winkelmann, A.J.M., Le Masson, N.J.M., Sidorenko, A.V., van Eik, C.W.E., 2002. A TL/OSL emission spectrometer extension of the Ris~ reader. Radiat. Prot. Dosim. 101, 111-114. BCtter-Jensen, L., 1978. A simple, hot N2-gas TL reader incorporating a post-irradiation annealing facility. Nucl. Instr. Meth. 153, 413-418. BCtter-Jensen, L., 1997. Luminescence techniques: instrumentation and methods. Radiat. Meas. 17, 749-768. BCtter-Jensen, L., 2000. Development of optically stimulated luminescence techniques using natural materials and ceramics, and their application to retrospective dosimetry. Rise-R-1211 (EN), DSc. Thesis, p.185. BCtter-Jensen, L., Duller, G.A.T., 1992. A new system for measuring OSL from quartz samples. Nucl. Tracks Radiat. Meas. 20, 549-553. B Ctter-Jensen, L., Murray, A.S., 1999. Developments in optically stimulated luminescence techniques for dating and retrospective dosimetry. Radiat. Prot. Dosim. 84, 307-316. BCtter-Jensen, L., Ditlevsen, C., Mejdahl, V., 1991. Combined OSL (infrared) and TL studies of feldspars. Nucl. Tracks Radiat. Meas. 18, 257-263. BCtter-Jensen, L., Duller, G.A.T., Poolton, N.R.J., 1994a. Excitation and emission spectrometry of stimulated luminescence from quartz and feldspars. Radiat. Meas. 23, 613-616. BCtter-Jensen, L., Poolton, N.R.J., Willumsen, F., Christiansen, H., 1994b. A compact design for monochromatic OSL measurements in the wavelength range 380-1020 nm. Radiat. Meas. 23, 519-522. BCtter-Jensen, L., Jungner, H., Poolton, N.R.J., 1995. A continuous OSL scanning method for analysis of radiation depth-dose profiles in bricks. Radiat. Meas. 24, 525-529. B~tter-Jensen, L., Mejdahl, V., Murray, A.S., 1999a. New light on OSL. Quat. Sci. Rev. 18, 303-309. BCtter-Jensen, L., Duller, G.A.T., Murray, A.S., Banerjee, D., 1999b. Blue light emitting diodes for optical stimulation of quartz in retrospective dosimetry and dating. Radiat. Prot. Dosim. 84, 335-340. BCtter-Jensen, L., Bulur, E., Duller, G.A.T., Murray, A.S., 2000. Advances in luminescence instrument systems. Radiat. Meas. 32, 523-528. BCtter-Jensen, L., Bulur, E., Murray, A.S., Poolton, N.R.J., 2002. Enhancements in luminescence measurement techniques. Radiat. Prot. Dosim. 101, 119-124. Braiinlich, P., Gasiot, J., Fillard, J.P., Castagn~, M., 1981. Laser heating of thermoluminescent dielectric layers. Appl. Phys. Phys. Lett. 39, 769-771. Bray, H.E., Bailey, R.M., Stokes, S., 2002. Quantification of cross-irradiation and cross-illumination using a Rise TL/OSL-DA-15 reader. Radiat. Meas. 35, 275-280.
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Bulur, E., 1996. An alternative technique for optically stimulated luminescence (OSL) experiments. Radiat. Meas. 26, 701-709. Bulur, E., G6ksu, H.Y., 1999. IR stimulated luminescence from feldspars with linearly increasing excitation light intensity. Radiat. Meas. 30, 505-512. Bulur, E., BCtter-Jensen, L., Murray, A.S., 2000. Optically stimulated luminescence from quartz measured using the linear modulation technique. Radiat. Meas. 32, 407-411. Bulur, E., B0tter-Jensen, L., Murray, A.S., 2001. Frequency modulated pulsed stimulation in optically stimulated luminescence. Nucl. Instr. Meth. B 179, 151-159. Burggraaf, D., Haskell, E.H., 1994. A software package for TL/OSL spectrometry and extraction of glow curves from individual grains. Radiat. Meas. 23, 537 (abstract only). Chithambo, M.L., Galloway, R.B., 2000a. A pulsed light-emitting diode system for stimulation of luminescence. Meas. Sci. Technol. 11, 418-424. Chithambo, M.L., Galloway, R.B., 2000b. Temperature dependence of luminescence time-resolved spectra from quartz. Radiat. Meas. 32, 627-632. Clark, R.J., Bailiff, I.K., 1998. Fast time-resolved luminescence emission spectroscopy in some feldspars. Radiat. Meas. 29, 553- 560. Clark, R.J., Sanderson, D.C.W., 1994. Photostimulated luminescence excitation spectroscopy of feldspars and micas. Radiat. Meas. 23, 641-646. Clark, R.J., Bailiff, I.K., Tooley, M.J., 1997. A preliminary study of time-resolved luminescence in some feldspars. Radiat. Meas. 27, 211-220. Colyott, L.E., McKeever, S.W.S., Akselrod, M.S., 1999. An integrating UVB dosemeter system. Radiat. Prot. Dosim. 85, 309- 312. Cowens, M.W., Blouke, M.M., Fairchild, T., Westphal, J.A., 1980. Coronene and lumogen as VUV sensitive coatings for Si CCD imagers: a comparison. Appl. Optics 19, 3727-3728. Ditlefsen, C., Huntley, D.J., 1994. Optical excitation of trapped charges in quartz, potassium feldspars and mixed silicates: the dependence on photon energy. Radiat. Meas. 23, 675-682. Dravins, D., Fario, D., Nilsso, B., 2000. Avalanche diodes as photon-counting detectors in astronomical photometry. In: Iye, M., Moorwood, A.F. (Eds.), Optical and IR Telescope Instrumentation and Detectors, vol. 4008. SPIE, pp. 298-307. Duller, G.A.T., B0tter-Jensen, L., Markey, B.G., 1997. A luminescence imaging system based on a charge coupled device (CCD) camera. Radiat. Meas. 27, 91-99. Duller, G.A.T., BCtter-Jensen, L., Kohsiek, P., Murray, A.S., 1999a. A high-sensitivity optically stimulated luminescence scanning system for measurement of single sand-sized grains. Radiat. Prot. Dosim. 84, 325-330. Duller, G.A.T., BCtter-Jensen, L., Murray, A.S., Truscott, A.J., 1999b. Single grain laser luminescence (SGLL) measurements using a novel automated reader. Nucl. Instr. Meth. B 155, 506-510. Duller, G.A.T., BCtter-Jensen, L., Murray, A.S., 2000. Optical dating of sand-sized grains of quartz: sources of variability. Radiat. Meas. 32, 453-457. Erfurt, G., Krbetschek, M.R., Trautmann, T., Stolz, W., 2000. Radioluminescence (RL) behaviour of A1203:C-potential for dosimetric applications. Radiat. Meas. 32, 735-739. Fattahi, M., Stokes, S., 2000. Extending the time range of luminescence dating using red TL (RTL) from volcanic quartz. Radiat. Meas. 32, 479-485. Galloway, R.B., 1993. Stimulation of luminescence using green light emitting diodes. Radiat. Prot. Dosim. 47, 679-682. Galloway, R.B., 1994. On the stimulation of luminescence with green light emitting diodes. Radiat. Meas. 23, 547-550. Galloway, R.B., 2002. Luminescence lifetimes in quartz: dependence on annealing temperature prior to beta irradiation. Radiat. Meas. 35, 67-77. Galloway, R.B., Hong, D.G., Napier, H.J., 1997. A substantially improved green light emitting diode system for luminescence stimulation. Meas. Sci. Technol. 8, 267-271. Godfrey-Smith, D.I., 1991. Optical dating studies of sediment extracts, Ph.D. Thesis, Simon Fraser University, Canada. Godfrey-Smith, D.I., Cada, M., 1996. IR stimulation spectroscopy of plagioclase and potassium feldspars and quartz. Radiat. Prot. Dosim. 66, 379-385.
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SUBJECT
INDEX
Absorbed dose 1, 2, 4, 7, 8, 11, 27, 42, 37, 65, 71, 76, 80, 81, 84, 86, 90, 96, 104, 107, 109, 112, 117 Accident dose 249 Accident dosimetry 3, 7, 10, 11, 85, 245-249 Activation energy 25, 37, 39-42, 46, 47, 60, 88, 89, 95 Additive-dose 157, 250, 253, 277, 280, 281 Adularia 220 Afterglow 221,223 A1203:C 4-6, 19, 21, 24, 25, 31, 32, 37, 38, 47-49, 51, 54, 55, 58, 59-63, 71-81,101,102-105, 107-111,113, 114, 117, 248,270, 327, 340, 341 Albite 188, 199, 201,204, 206, 217-218, 220, 231 Alkali (feldspars) 188, 201,202, 205, 215-216 AIN 111 Alpha quartz 119, 150, 162 Annealing 162-169 Anomalous fading 218, 219-227, 303 Anorthite 188, 200 Anti-Stokes 1, 40 Argon-ion laser 123, 141, 316 Automated OSL readers 315- 316 BaFX (X= Br,C1,F) 87-89 BaS 90 BeO 51, 60, 92-95 Beta dosimetry 104 Bleaching of the OSL signal (quartz) 141-143 Bleaching at ambient temperature (feldspar) 201-202, 207-209 Bleaching by IR at elevated temperature (feldspar) 208-210 Blue LED stimulation 323-325 Brachytherapy 10, 112 Bremsstrahlung x-rays 89 Brick(s) 246-249, 253-260, 338-339, 340 Broad-band light stimulation 318-320 CaF2 60, 61, 86, 87 CaS 3, 90 CaSO4 60, 90, 270 Central age model 297, 299, 301 Ceramic(s) 246
Charge Coupled Device (CCD) 314, 334, 335 Chernobyl 250, 258, 259, 268, 272 Common age model 301 Concrete 260, 263, 297 Conductivity 120 Configurational coordinate 42-45 Continuous-Wave OSL (CW-OSL) 3, 4, 6, 19, 20, 26-29, 34-39, 44, 47-57, 65, 84-86, 92, 95, 96, 101, 102, 104, 112, 123-130,311 Cores 338-339 Cross-section 127, 184-186 Dating 3, 7, 10, 11, 21, 71, 84, 276-304 Decay curve(s) 123-130, 194-195, 271,273 274-275, 318 Deep dose (Hp(10)) 7, 102 Deep level transient spectroscopy (DLTS) 1, 2, 16 Delayed OSL (DOSL) 4, 59, 60, 140, 329 Delocalised bands 15, 16, 39, 45, 75 Dental crowns 273-275 Deterministic effects 8 Differential thermal analysis 1 Disordered structure 188 Donor-acceptor recombination 39-41, 75, 231 Donor traps 60-65 Dose correction method 280 Dose-depth profiles 248-249, 257-261, 263-264, 338-339 Dose dependence 157-161,234 Dose distributions 261,293 asymmetric (distributions) 293, 298 Dose equivalent 7, 8, 10 Dose recovery test 302 Dose response 75, 76, 94-96, 101, 109, 110, 113 E ~centre(s) 120, 164 Effective atomic number 3, 8, 93 Effective dose 7, 8 Electron spin resonance (ESR, EPR) 3, 162-165 Elevated temperature IRSL 191 Elevated temperature OSL 177-180 Elevated temperature infrared (ETIR) 209
352
Subject Index
Emission from quartz UVA-violet 121,150-151 blue 122, 153 orange 122, 153-155 Emission spectra quartz 149-157 feldspars 201-207 Emission spectrometry 332-334 Energy band diagram 16, 88, 93 model 31, 88, 93 Energy response 104 Environmental dose 3, 8, 10, 11, 71 dose rates 247-249, 250-257, 277-287, 293-302 dosimetry 5, 7, 8, 71, 75, 80, 107 radiation 2, 8, 11,109 Equivalent dose (De) 3, 7, 11, 157 Excitation spectra 141-148, 199-200 Eye dose (Hp(0.3)) 7, 102 Fadia 223-224, 299 Fading athermal 41,219-227 light-induced 71, 79 thermal 79, 83, 89, 90, 112 Fast component 127, 133-135, 151, 157-161 Fe 3+ 205-207, 221 Feldspar(s) 21, 38, 40-43, 50, 59, 71, 78, 188-235 Fermi level 16-18 Fermi-Dirac statistics 16, 17 Filling diagrams 16 Film badges 101, 102, 104 First-order kinetics 26, 29, 35, 49-51, 56, 214 Frank-Condon principle 42 Fredholm equation 31 Galactic cosmic rays 8 Gaussian 262, 293-294 General-order kinetics 50-54 Glass(es) 21, 95, 96, 101, 113 Glauber salt 265-266 Glaze 270-272 Gray-equivalent 8
Green LED stimulation 321-323 Growth curve 157-162, 166, 183-184, 250-252, 256-258, 267-268, 277-281,284-285 Half-life 35 Heavy charged particles (heavy ions) 109 Helmholtz free energy 18 High dose dosimetry 21, 65 Histograms 293-295, 299-300 Household chemicals 265-268 Huang-Rhys factor 23, 43 Hydrogenic trap model 22, 40, 218, 231 Imaging photon detectors (IPD) 313-314, 334 Infrared stimulated luminescence (IRSL) feldspars 3, 26, 189-203, 207-221, 223-227, 230-234 quartz 142, 148 Initial slope 127, 184 InLight TM102-- 104 International Space Station 8, 108 IR diodes 189, 316-318 Irradiated food 329 Irradiation, at elevated temperature(s) 173-174, 215 Irradiation, at low temperature(s) 120 Isothermal decay 169-170, 211,287 KBr 82, 83 KCI 81-83 Labradorite 204, 205-206, 219, 221 Lambert-Beer law 19 Laser 135, 178, 184, 211,316-330, 335-338 Laser diode (IR) 148, 200, 317- 318 Lattice vibrational modes 218 LiF 21, 61, 65, 71, 86 Lifetime excited state 44, 59, 56, 59, 77 free carrier 54, 56 luminescence 24, 46, 47, 57, 59, 60 radiative 45 recombination 28, 62, 169, 214 trapped charge 34 Linear energy transfer 109
Subject Index Linear modulation IRSL 194-197 Linear modulation OSL (LM-OSL) 4, 6, 19, 20, 26, 27, 48-56, 65, 84, 85, 92, 95, 130-136, 185, 265-266, 311,325-326 Linear modulation photoconductivity (LM-PC) 54, 56 Localised transition model 192, 219 Low-Earth orbit 8 Luminescence correction method 280 Luminescence efficiency 5, 109 Luminescence sensitivity change(s) 287-293 Luxel 72, 101,102-105, 109, 327 TM
Medical dosimetry 7, 9-12, 112-117 MgS 3, 90-93, 113, 117 MgSO4 90 Mn 2+ 205-207 Median value of the standard deviation 293 Medium component 127-128 Microcline(s) 191,192, 197-199,200, 201-202, 205-206, 217, 220 Microcline microperthite 204 Modelling 186-188, 230-234 Monte Carlo simulations 258-260 Mortar 260-264, 297 MOSFET detectors 112 Mott-Seitz 44, 47 Multiple-aliquot 157-159, 250, 277-279 NaC1 51, 52, 84-86, 265-266 Natural normalisation 277 Non-radiative 5, 28, 33, 34, 42, 44, 45, 62 Oligoclase 199, 204-206, 218, 219, 231 Optical bleaching 184, 201-202 Optical fibre dosimetry 112-117 Optically stimulated afterglow (OSA) 140, 146, 197-198 Optically stimulated exo-electron emission (OSEE) 2, 16 Optically stimulated phenomena (OSP) 1, 2 OSL emission spectra 149-150, 201-203 OSL scanner 338-339 Orthoclase 191,199, 205-206, 208-210, 215-218, 231
353
Personal dosimetry 3-5, 7, 9, 11, 80, 81, 101-107 Perthite(s) 188, 205 Phase transition 119, 150 Phosphorescence 48, 59, 60, 77, 86, 90, 92 Photochromic effect 81 Photoconductivity 1, 2, 54, 56, 73-75, 86 Photoelectron effect 15 Photoionisation cross-section 18, 19-26, 130-131,135, 184-186 Photoluminescence (PL) 1, 21, 86, 205-207, 215, 270-271 Photomultiplier tube 312-313 Phototransfer 21, 60-65, 71, 79, 85, 110, 124-125 Phototransferred thermoluminescence (PTTL) 124, 127, 268-269 Plagioclase(s) (feldspars) 188, 200, 202, 205, 215 Porcelain 246, 253, 267-275 Portable system 340 Pre-dose effect 121, 122 Probability density plots 295-296, 300 Pseudo-LM-OSL 131, 143, 185 Pulse annealing 170-173, 212-214, 229 Pulsed IRSL 197 Pulsed OSL (POSL) 4, 6, 19, 56-60, 75, 76, 79, 80, 101,102, 104-107, 136-140, 197, 311,326-330 Pulsed RL 140, 341- 344 Quality factor 7 Quartz 24, 25, 30, 32, 37, 39, 41, 42, 46, 47, 51, 59, 62, 64, 65, 71, 95, 96, 113, 119-188 Quasi-equilibrium 33, 62, 64 Quasi-Fermi level 16, 17 Radial plots 263-264, 296-298, 300-302 Radiochromic dye films 112 Radioluminescence (RL) 137, 155-157, 163-164, 174, 227-234, 340-343 Radioluminescence emission spectra 203-205 Radioluminescence dating 227-230 Radiophotoluminescence 1, 4, 65, 101 Radiotherapy 10, 112-117 RbI 85, 86
354
Subject Index
Red IRSL 220 Regenerative-dose 253, 277, 280-281, 285-287, 292 Resonance, in feldspar stimulation spectrum 201-202, 218, 316 Retrospective dosimetry 5, 7, 10, 11, 71, 85, 245-276 SAAD (single-aliquot additive-dose) 276, 282-285 SAR (single aliquot regeneration) 3, 159-160, 253, 255-257, 285-287, 291-293, 303-304 SARA (single aliquot regeneration and added dose) 250-252 Sanidine 199, 219, 220, 221,223 Scanning monochromator 199, 201,330-331 Sch6n-Klasens 44 Scintillators 50, 112 Self-trapped exciton 65, 88 Sensitivity change(s) with preheating 167-168, 255 monitoring of by OSL 291-293 ll0~ TL 167-168, 171,182 Sensitization in nature 167 Shallow dose (Hp(0.07)) 7, 102, 104 Shuttle 8 Single aliquots(s) 159, 252-257, 280-293 Single event upsets 9 Single grain(s) 160, 223-225, 229-230, 298-302, 334-338 Single grain laser OSL 160, 335-338 Single photon absorption 126, 143 SiO2 19, 119-120 Slow component 127, 130-135, 180-184 Solar particles 8 Space radiation 8, 107-110 SrS 3, 51, 90, 92 SrSe 3 Standard deviation 293 Standard error on the mean 293, 297 Stimulated relaxation phenomena (SRP) 15-17 Stimulation spectrum (spectra) 26, 73, 74, 81-87, 90-95, 141-148, 199-200, 330-331 Stimulation temperature 35- 39, 130, 189-194
Stochastic effects 8 Stokes' shift 1, 44 Stretched exponential 29, 30, 35 Superposition principle 18, 31, 49, 50 Supra-linear response 160-162 Teletherapy 10, 112 Thermal assistance general 37, 39, 41, 84-86 feldspars 216-217 quartz 130, 179-180, 184 Thermal quenching general 5, 44-48, 71-79, 84, 95 feldspars 204, 207, 215, 221 quartz 121,130, 137, 140, 151-153, 155-156, 177-179, 184, 204-205 Thermal stability 169-170, 181, 211,229 Thermal transfer 171, 174-177, 263-265 Thermally stimulated capacitance (TSCap) 16 Thermally stimulated conductivity (TSC) 1,2,15 Thermally stimulated exo-electron emission (TSEE) 1, 2, 16 Thermally stimulated phenomena (TSP) 1,2 Thermogravimetry (TG) 1 Thermoluminescence (TL) general 1, 2, 4-8, 71-81, 91-97, 102, 107, 109, 110-112, 189-193 emission spectra 150, 203 ll0~ 121-122, 123-126, 130, 267, 285, 287-291 325~ 143-144, 151-153, 177-178 375~ 153 Thermo-optical luminescence (TOL) 78, 189193 Time-resolved luminescence (TRL) 137-140 179, 181, 184, 220, 326 Tissue-equivalence 89 Trap depth 18, 23, 33, 39, 169-170, 172, 177-178 Traps deep 22, 31, 32, 34, 37, 38, 47, 59, 60-62, 65, 71, 74, 77, 79, 80 shallow 31, 35-39, 46, 47, 59-61, 71, 74, 77, 78, 80, 90
Subject Index Trapped radiation belts 8 Tuneable laser 199, 316, 331 Tunnelling 217, 218, 219, 220
Wavelength dependence 21-23, 24, 49, 52-54, 74 Weighted mean 293, 297
UV 15, 17, 61, 81-83, 87, 90-92, 110-112, 117, 340
X-ray storage phosphor 81
Washing powder 265-268 Water softener 265-266
ZnS 51 ZnSe 19
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