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On the dose-rate dependence of the thermoluminescence response of quartz
Nuclear Instruments and Methods 175 (1980) 216-218 © North-Holland Publishing Company
ON THE DOSE-RATE DEPENDENCE OF THE THERMOLUMINESCENCE RESPONSE OF QUARTZ
G. VALLADAS and J. FERREIRA Centre des Foibles Radioactivitds, Laboratoire m i x t e C N R S - C E A , 91190 - Gif-sur-Yvette, France
The dose-rate dependence of the thermoluminescence of different types of quartz in the range from 0.8 to 900 rad/min has been measured. This dependence, which is strong for massive quartz, decreases with the irradiation temperature of the samples.
1. Introduction
3. Results
Quartz is among the most useful minerals for thermoluminescence (TL) dating. In using this method it is assumed that an artificial dose delivered to the mineral in a necessarily short time induces the same TL as the dose received in its natural environment. However, as shown by Groom and co-workers [1,2], this assumption may not always be valid. The purpose of this paper is to compare the responses of quartz irradiated with the same total dose (1) at a dose-rate commonly used in dating ("high dose rate"), and (2) at a dose-rate about 1000 times smaller ("low dose rate"). Quartzes of various types have been used, namely massive quartz (rock crystal, pink quartz and milky quartz pebbles), and quartz grains (extracted from granite or detrital quartz). The principal TL peaks have been separated by means of optical filters.
Glow curves of sample of the rock crystal annealed at 500°C are shown in fig. 1. The full lines correspond to the low dose-rate irradiation. The three main glow peaks around 3 2 0 , 3 6 0 and 370°C are best observed through the use of the blue filter (fig. 1B), the UV filter (fig. 1A) and the green one (fig. 1C) respectively. The dashed lines correspond to the high
__
....
Low dose rate Hi9h dose rate
A
/-', /
\
/J/
a ~o~o o
0 I
ot
2. Experimental
Blue F i l t e r " - . _ . "
The experimental procedure consisted of: 1. grounding and sieving of the sample ( 1 0 0 - 1 6 0 ~m grain size), 2. annealing at 500°C and 600°C for one hour, 3. artificial irradiation with 6°Co or 137Cs sources at room temperature (high dose-rate 900 rad/ min, low dose-rate 0.8 rad/min, total dose for each sample 3200 rad). 4. TL measurements with 2 or 5 mg samples at a heating rate 5 ° / s , with optical filters for "green" ( 5 3 0 < X < 560 nm), "blue" (380 < X < 480 nm) and "UV" (320 < X < 400 nm).
I
Green
2bo
'
4bo
3oo
400
Filter
26o Fig 1
216
r~
T~'C
217
G. Valladas, J. Ferreira / The thermoluminescence response o f quartz
Fontainebleau
Sand
",,,,
UV F i l t e r
\, Blue
,,
2~o
Green
abo
/¢
,,
,6d
j"
/
2bo
300
46O4T
Fig. 2
dose-rate irradiation. As shown in fig. 1B, the 320 ~ peak decreases by roughly 40% when the dose-rate is increased while the 360 ° peak increases by 60% (fig. 1A). Using the green filter, the 370 ° peak presents only a small increase (fig. 1C). Samples of the rock crystal annealed at 600°C and of the quartz pebble annealed at 500 and 600 ° have quite similar behaviour and the determination of the decrease (increase) o f the 320 ° (360 ° ) glow peaks o f those samples are easy. A small decrease o f the 320 ° peak is observed also with the granite and detrital
quartz. However, the behaviour of the corresponding 360 ° peak is not easily defined because the 320 ° peak dominates the glow curve obtained with the UV filter (fig. 2). The 360 ° peak o f the pink quartz strongly increases with the dose-rate, but as it dominates the glow-curve relative to the blue filter, the 320 ° peak intensity is difficult to evaluate. Table 1 (columns 3, 4, 5) sunmaarizes the respective percentage variations (0) o f the three peaks due to the increase of dose-rate from 0.8 to 900 rad/min. Additional measurements have been made on a quartz pebble (500 ° annealed) to obtain the response difference corresponding to a change from the low dose-rate to 240 rad/min. They were found to be very similar to the difference between low dose-rate and 900 rad/min. The influence of fractionation has also been investigated. A sample was irradiated in five separate exposures (always at 240 rad/min) with intervals o f a few hours to get the same total dose o f 3200 tad within 48 h, a time comparable to the duration o f the low dose rate irradiation (67 h). As a result, 0 changed from - 3 8 to - 2 0 % for the 320 ° peak and from 74 to t 1% for the 360 ° peak. Obviously time is an important factor and some relaxation process during the intervals between irradiations must be assumed. It can be expected that this process would be accelerated by temperature increases [2]. A preliminary experiment demonstrated indeed that when a 116 rad/min exposure took place at room temperature and another at 100°C, the 320°C peak rose by about a factor o f two and the 360°C peak decreased by 50%.
Table 1 Percentage increase (high/low dose-rate) for 3200 rad total dose Specimen
Annealing
320 ° emission
temp. (°C)
0 (%)
error
Detrital quartz
500 600
- 14 -13
5 4
Rock crystal
500 600
-50 -60
10 I0
+40 +90
Quartz pebble
500 600 500 600
-39 -38 not determined
3 3
Pink quartz Granite
600
8
360 ° emission
4
0 (%)
370 ° emission error
not determined
0 (%)
error
5 ~1
4 5
10 15
+7 +10
2 2
+35 +74
10 10
0 -9
6 4
+250 +350
50 50
-6 0
4 5
2
3
not determined
XII. ARCHAEOLOGY
218
G. Valladas, J. Ferreira / The thermoluminescence response o f quartz
4. Conclusion Some experiments concerning the influence o f dose-rate on TL response of some quartz types have been carried out. The behaviour o f the three main TL peaks around 3 2 0 , 3 6 0 and 370°C (heating rate 5°/s) was studied by using blue, UV and green filters. The 320 ° peak decreases at increased dose-rates, while the 360 ° peak increases. This effect is more pronounced in the massive quartz, and tends to disappear when the sample temperature during irradiation is increased. The 370 ° peak shows but a slight dependence on the dose rate and appears therefore more suitable for dating.
G. Valladas acknowledges stimulating discussions with M.J. Aitken.
References [1] P.J. Groom, S.A. Durrani, K.A.R. Khazal and S.W.S. McKeever, A specialist Seminar on TL dating, Oxford 1978; J. of the European Study Group on Phys. Chem. and Meth. Techniques Applied to Archaeology 2 (1978) 201. [2] S.A. Durrani, P.J. Groom, K.A.R. Khazal and S.W.S. McKeever, J. Phys. D: Appl. Phys. 10 (1977) 1351.
ON THE DOSE-RATE DEPENDENCE OF THE THERMOLUMINESCENCE RESPONSE OF QUARTZ
G. VALLADAS and J. FERREIRA Centre des Foibles Radioactivitds, Laboratoire m i x t e C N R S - C E A , 91190 - Gif-sur-Yvette, France
The dose-rate dependence of the thermoluminescence of different types of quartz in the range from 0.8 to 900 rad/min has been measured. This dependence, which is strong for massive quartz, decreases with the irradiation temperature of the samples.
1. Introduction
3. Results
Quartz is among the most useful minerals for thermoluminescence (TL) dating. In using this method it is assumed that an artificial dose delivered to the mineral in a necessarily short time induces the same TL as the dose received in its natural environment. However, as shown by Groom and co-workers [1,2], this assumption may not always be valid. The purpose of this paper is to compare the responses of quartz irradiated with the same total dose (1) at a dose-rate commonly used in dating ("high dose rate"), and (2) at a dose-rate about 1000 times smaller ("low dose rate"). Quartzes of various types have been used, namely massive quartz (rock crystal, pink quartz and milky quartz pebbles), and quartz grains (extracted from granite or detrital quartz). The principal TL peaks have been separated by means of optical filters.
Glow curves of sample of the rock crystal annealed at 500°C are shown in fig. 1. The full lines correspond to the low dose-rate irradiation. The three main glow peaks around 3 2 0 , 3 6 0 and 370°C are best observed through the use of the blue filter (fig. 1B), the UV filter (fig. 1A) and the green one (fig. 1C) respectively. The dashed lines correspond to the high
__
....
Low dose rate Hi9h dose rate
A
/-', /
\
/J/
a ~o~o o
0 I
ot
2. Experimental
Blue F i l t e r " - . _ . "
The experimental procedure consisted of: 1. grounding and sieving of the sample ( 1 0 0 - 1 6 0 ~m grain size), 2. annealing at 500°C and 600°C for one hour, 3. artificial irradiation with 6°Co or 137Cs sources at room temperature (high dose-rate 900 rad/ min, low dose-rate 0.8 rad/min, total dose for each sample 3200 rad). 4. TL measurements with 2 or 5 mg samples at a heating rate 5 ° / s , with optical filters for "green" ( 5 3 0 < X < 560 nm), "blue" (380 < X < 480 nm) and "UV" (320 < X < 400 nm).
I
Green
2bo
'
4bo
3oo
400
Filter
26o Fig 1
216
r~
T~'C
217
G. Valladas, J. Ferreira / The thermoluminescence response o f quartz
Fontainebleau
Sand
",,,,
UV F i l t e r
\, Blue
,,
2~o
Green
abo
/¢
,,
,6d
j"
/
2bo
300
46O4T
Fig. 2
dose-rate irradiation. As shown in fig. 1B, the 320 ~ peak decreases by roughly 40% when the dose-rate is increased while the 360 ° peak increases by 60% (fig. 1A). Using the green filter, the 370 ° peak presents only a small increase (fig. 1C). Samples of the rock crystal annealed at 600°C and of the quartz pebble annealed at 500 and 600 ° have quite similar behaviour and the determination of the decrease (increase) o f the 320 ° (360 ° ) glow peaks o f those samples are easy. A small decrease o f the 320 ° peak is observed also with the granite and detrital
quartz. However, the behaviour of the corresponding 360 ° peak is not easily defined because the 320 ° peak dominates the glow curve obtained with the UV filter (fig. 2). The 360 ° peak o f the pink quartz strongly increases with the dose-rate, but as it dominates the glow-curve relative to the blue filter, the 320 ° peak intensity is difficult to evaluate. Table 1 (columns 3, 4, 5) sunmaarizes the respective percentage variations (0) o f the three peaks due to the increase of dose-rate from 0.8 to 900 rad/min. Additional measurements have been made on a quartz pebble (500 ° annealed) to obtain the response difference corresponding to a change from the low dose-rate to 240 rad/min. They were found to be very similar to the difference between low dose-rate and 900 rad/min. The influence of fractionation has also been investigated. A sample was irradiated in five separate exposures (always at 240 rad/min) with intervals o f a few hours to get the same total dose o f 3200 tad within 48 h, a time comparable to the duration o f the low dose rate irradiation (67 h). As a result, 0 changed from - 3 8 to - 2 0 % for the 320 ° peak and from 74 to t 1% for the 360 ° peak. Obviously time is an important factor and some relaxation process during the intervals between irradiations must be assumed. It can be expected that this process would be accelerated by temperature increases [2]. A preliminary experiment demonstrated indeed that when a 116 rad/min exposure took place at room temperature and another at 100°C, the 320°C peak rose by about a factor o f two and the 360°C peak decreased by 50%.
Table 1 Percentage increase (high/low dose-rate) for 3200 rad total dose Specimen
Annealing
320 ° emission
temp. (°C)
0 (%)
error
Detrital quartz
500 600
- 14 -13
5 4
Rock crystal
500 600
-50 -60
10 I0
+40 +90
Quartz pebble
500 600 500 600
-39 -38 not determined
3 3
Pink quartz Granite
600
8
360 ° emission
4
0 (%)
370 ° emission error
not determined
0 (%)
error
5 ~1
4 5
10 15
+7 +10
2 2
+35 +74
10 10
0 -9
6 4
+250 +350
50 50
-6 0
4 5
2
3
not determined
XII. ARCHAEOLOGY
218
G. Valladas, J. Ferreira / The thermoluminescence response o f quartz
4. Conclusion Some experiments concerning the influence o f dose-rate on TL response of some quartz types have been carried out. The behaviour o f the three main TL peaks around 3 2 0 , 3 6 0 and 370°C (heating rate 5°/s) was studied by using blue, UV and green filters. The 320 ° peak decreases at increased dose-rates, while the 360 ° peak increases. This effect is more pronounced in the massive quartz, and tends to disappear when the sample temperature during irradiation is increased. The 370 ° peak shows but a slight dependence on the dose rate and appears therefore more suitable for dating.
G. Valladas acknowledges stimulating discussions with M.J. Aitken.
References [1] P.J. Groom, S.A. Durrani, K.A.R. Khazal and S.W.S. McKeever, A specialist Seminar on TL dating, Oxford 1978; J. of the European Study Group on Phys. Chem. and Meth. Techniques Applied to Archaeology 2 (1978) 201. [2] S.A. Durrani, P.J. Groom, K.A.R. Khazal and S.W.S. McKeever, J. Phys. D: Appl. Phys. 10 (1977) 1351.