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The thermoluminescent properties of Brazilian topaz
Appl. Radiar. hr. Vol. 37, No. 2, pp. 1355137, 1986 ht. J. Rod&. Appl. Instrum. Parr A
0883-2889/86
$3.00 + 0.00
Copyright 0 1986 Pergamon Press Ltd
Printed in Great Britain. All rights reserved
The Thermoluminescent Properties of Brazilian Topaz C. A. FERREIRA Instituto
LIMA,
de Radioprote9lo CEP. 22602,
L. A. R. ROSA e Dosimetria, Rio de Janeiro,
(Received
11 July
and
CNEN Brazil
P. G.
CUNHA
C.P. 37025,
1985)
The TL properties of Brazilian topaz were investigated. The glow curve up to 573 K presents peaks close to 373, 403, 473 and 523 K. A linear response from 7 to at least lo4 mGy is observed. The TL sensitivity is about 150 times lower than that of LiF-TL 100 Harshaw. The fading behaviour was investigated during a period of 15 days, at room temperature. The third peak intensity, after 24 h, becomes 80% of its initial value and then seems to stabilize.
Introduction
A + B, with nitrogen equal to 10 K/s.
It is well known that some geological materials exhibit thermoluminescent properties.“’ In the present work, the thermoluminescent properties of transparent colorless topaz (Al(F, OH)),SiO, from the region of Governador Valadares, State of Minas Gerais, Brazil, were investigated. Similar studies have been performed by Moss and McKlveen”) on topaz crystals collected in the vicinity of Topaz Mountain, Utah, U.S.A., and by Juan Azorin et al.,(*) on topaz crystals obtained from various localities of Mexico. However, the TL characteristics described by these authors, e.g. glow curves and fading behaviour, are not exactly the same. This is expected since the crystals used in these researches have different origins and probably different types and different amounts of impurities. These results indicate that additional work is necessary to determine the different TL responses for different types of topaz and the influence of impurities. Experimental
purging,
Experimental
using a heating
rate
Results
Glow czme It was found that the time span between preparation and irradiation of the sample has an influence in the structure of the glow curve. For the samples irradiated immediately after their preparation from the bulk material, the glow curve in the interval range 30&573 K presents two well defined peaks close to 403 K (peak 2) and 473 K (peak 3), and a third peak close to 523 K (peak 4) which is not always clearly visible since it is incorporated in the peak at 473 K. However, the glow curve obtained from a sample irradiated with the same dose some months after preparation, showed a slightly different glow curve, the peak at 403 K (peak 2) being so much less intense, that another peak at 373 K (peak 1) became visible (Fig. 1). For instance, for a sample re-irradiated 9 months after preparation, the height of peak 2 was
Procedures
Large topaz crystals were cleaned with a chemical solution with 50% water and 50% (3HCl+ HNO,). The impurity content of these crystals was determined by neutron activation analysis. Table 1 shows the results obtained with 2 samples. To investigate the topaz TL properties, the crystals were cleaved and samples were chosen with masses ranging from 18 to 24 mg and sizes close to (3 x 3 x 1 mm). Before irradiation they were annealed at 673 K for 30 min. This was sufficient to eliminate residual natural TL radiation. The irradiation was performed in a 6oCo and (90Sr-90Y) irradiator and the TLD reader was a Harshaw 200 135
I. Impurity content of topaz crystals from Minas Gerais, Brazil, determined by neutron activation analysis
Table
Concentration Element Ta Th Sb cs SC Rb Fe Ba u Na
Sample
I
9.7 14.1 12.8 315 336 974 0.0147% 3.86 ppm 82.0 0.37%
(ppb) Sample 2 21.0 67. I 56.8 426 308 II40 0.00965% 3.41 ppm 63.0 0.36%
C.
A.
FERREIRA LIMA et
al.
Peak 3
,x
‘iii 605 E 40-
Peak2
c
20-
I
I
5
0
Storage
Fig. 3. Fading 300
I
I
350
400
I
450
Temperature
I
500 (K )
I
10
behaviour
15
time ,(days)
of peak 3 at room
temperature.
I 600
I
550
Fig. 1. Topaz glow curve.
reduced to 20% of the expected value, but peaks 3 and 4 were unaltered. This could perhaps be explained by a decrease with time, at room temperature, of the concentration of the traps responsible for peak 2, due to a defect annealing process. These traps are probably introduced during the samples preparation. A post-irradiation annealing at 393 K for 15 min completely eliminates the peaks at 373 and 403 K, leaving peaks 3 and 4 unchanged. The TL response was not studied at temperatures above 573 K. The sensitivity of the topaz samples was found to be approximately 150 times lower than that of LiFTLD 100 Harshaw.
Fading behaviour The fading of peak 3 was studied during a period of 15 days. A pre-annealing at 673 K/l 5 min and a post irradiation annealing at 393 K/ 15 min were used. The reference values were obtained immediately after the post annealing. It can be seen (Fig. 3) that most of the fading occurs in the first 24 h and then it becomes negligible. After 15 days the fading was about 20%. Although not studied in detail, the rate of fading of peak 2 was found to be higher as compared to peak 3 (70% decrease in peak height after 90 h).
Discussion Linear behaviour The linearity of the TL response was studied in the dose range 7-10,OOOmGy. A (90Sr-90Y) source was used from 7 to 420 mGy, and a Vo irradiator from 500 to 10,000 mGy. Peaks 3 and 4 were found to have a linear behaviour, as shown in Fig. 2.
!I 10'
and Conclusion
The typical glow curves obtained for the topaz samples investigated in the present work present some differences from those determined by Moss and McKlveen”) and Juan Azorin et al.‘2) For instance, these papers show glow curves with 2 peaks only, at the temperatures of 413 and 578 K, and 453 and
Peaks 3 and 4
Peak 3 /
i x
x'"
/
Dose
Fig. 2. TL response
I"
10~
10'
(mGy)
of topaz
as a function
of dose.
137
TL properties of Brazilian topaz
573 K, respectively. detected
In the present work, 4 peaks were at temperatures of 373 K (lOOC>, 403 K
(13O”C), 473 K (200°C) and 523 K (250°C). Our results concerning the rate of fading of topaz are more in accordance with those of Juan Azorin et al.(‘) Surely several other TL properties must still be investigated and correlated to the impurities contained in our samples. This is essential for the explanation of the different TL behaviour of topaz crystals obtained from minerals of different origins. However, it can be concluded that topaz crystals from the region of Governador Valadares, Brazil, possess favorable TL characteristics.
Acknowledgements-We wish to thank Dr Carlos Pires Ferreira, Director of the Department of Mineral Resources of CNEN, Brazil, for kindly providing the topaz mineral used in this work, and Dr Marina B. A. Vasconcellos, from the Institute for Energetic and Nuclear Research, of CNEN, Brazil, for performing the neutron activation analysis.
References I. Moss A. L. and McKlveen J. W. Health Phys. 34, 137 (1978). 2. Juan Azorin N., Roberto P. C. Salvi and Alicia Gutierrez C. Health Phys. 43, 599 (1982).
0883-2889/86
$3.00 + 0.00
Copyright 0 1986 Pergamon Press Ltd
Printed in Great Britain. All rights reserved
The Thermoluminescent Properties of Brazilian Topaz C. A. FERREIRA Instituto
LIMA,
de Radioprote9lo CEP. 22602,
L. A. R. ROSA e Dosimetria, Rio de Janeiro,
(Received
11 July
and
CNEN Brazil
P. G.
CUNHA
C.P. 37025,
1985)
The TL properties of Brazilian topaz were investigated. The glow curve up to 573 K presents peaks close to 373, 403, 473 and 523 K. A linear response from 7 to at least lo4 mGy is observed. The TL sensitivity is about 150 times lower than that of LiF-TL 100 Harshaw. The fading behaviour was investigated during a period of 15 days, at room temperature. The third peak intensity, after 24 h, becomes 80% of its initial value and then seems to stabilize.
Introduction
A + B, with nitrogen equal to 10 K/s.
It is well known that some geological materials exhibit thermoluminescent properties.“’ In the present work, the thermoluminescent properties of transparent colorless topaz (Al(F, OH)),SiO, from the region of Governador Valadares, State of Minas Gerais, Brazil, were investigated. Similar studies have been performed by Moss and McKlveen”) on topaz crystals collected in the vicinity of Topaz Mountain, Utah, U.S.A., and by Juan Azorin et al.,(*) on topaz crystals obtained from various localities of Mexico. However, the TL characteristics described by these authors, e.g. glow curves and fading behaviour, are not exactly the same. This is expected since the crystals used in these researches have different origins and probably different types and different amounts of impurities. These results indicate that additional work is necessary to determine the different TL responses for different types of topaz and the influence of impurities. Experimental
purging,
Experimental
using a heating
rate
Results
Glow czme It was found that the time span between preparation and irradiation of the sample has an influence in the structure of the glow curve. For the samples irradiated immediately after their preparation from the bulk material, the glow curve in the interval range 30&573 K presents two well defined peaks close to 403 K (peak 2) and 473 K (peak 3), and a third peak close to 523 K (peak 4) which is not always clearly visible since it is incorporated in the peak at 473 K. However, the glow curve obtained from a sample irradiated with the same dose some months after preparation, showed a slightly different glow curve, the peak at 403 K (peak 2) being so much less intense, that another peak at 373 K (peak 1) became visible (Fig. 1). For instance, for a sample re-irradiated 9 months after preparation, the height of peak 2 was
Procedures
Large topaz crystals were cleaned with a chemical solution with 50% water and 50% (3HCl+ HNO,). The impurity content of these crystals was determined by neutron activation analysis. Table 1 shows the results obtained with 2 samples. To investigate the topaz TL properties, the crystals were cleaved and samples were chosen with masses ranging from 18 to 24 mg and sizes close to (3 x 3 x 1 mm). Before irradiation they were annealed at 673 K for 30 min. This was sufficient to eliminate residual natural TL radiation. The irradiation was performed in a 6oCo and (90Sr-90Y) irradiator and the TLD reader was a Harshaw 200 135
I. Impurity content of topaz crystals from Minas Gerais, Brazil, determined by neutron activation analysis
Table
Concentration Element Ta Th Sb cs SC Rb Fe Ba u Na
Sample
I
9.7 14.1 12.8 315 336 974 0.0147% 3.86 ppm 82.0 0.37%
(ppb) Sample 2 21.0 67. I 56.8 426 308 II40 0.00965% 3.41 ppm 63.0 0.36%
C.
A.
FERREIRA LIMA et
al.
Peak 3
,x
‘iii 605 E 40-
Peak2
c
20-
I
I
5
0
Storage
Fig. 3. Fading 300
I
I
350
400
I
450
Temperature
I
500 (K )
I
10
behaviour
15
time ,(days)
of peak 3 at room
temperature.
I 600
I
550
Fig. 1. Topaz glow curve.
reduced to 20% of the expected value, but peaks 3 and 4 were unaltered. This could perhaps be explained by a decrease with time, at room temperature, of the concentration of the traps responsible for peak 2, due to a defect annealing process. These traps are probably introduced during the samples preparation. A post-irradiation annealing at 393 K for 15 min completely eliminates the peaks at 373 and 403 K, leaving peaks 3 and 4 unchanged. The TL response was not studied at temperatures above 573 K. The sensitivity of the topaz samples was found to be approximately 150 times lower than that of LiFTLD 100 Harshaw.
Fading behaviour The fading of peak 3 was studied during a period of 15 days. A pre-annealing at 673 K/l 5 min and a post irradiation annealing at 393 K/ 15 min were used. The reference values were obtained immediately after the post annealing. It can be seen (Fig. 3) that most of the fading occurs in the first 24 h and then it becomes negligible. After 15 days the fading was about 20%. Although not studied in detail, the rate of fading of peak 2 was found to be higher as compared to peak 3 (70% decrease in peak height after 90 h).
Discussion Linear behaviour The linearity of the TL response was studied in the dose range 7-10,OOOmGy. A (90Sr-90Y) source was used from 7 to 420 mGy, and a Vo irradiator from 500 to 10,000 mGy. Peaks 3 and 4 were found to have a linear behaviour, as shown in Fig. 2.
!I 10'
and Conclusion
The typical glow curves obtained for the topaz samples investigated in the present work present some differences from those determined by Moss and McKlveen”) and Juan Azorin et al.‘2) For instance, these papers show glow curves with 2 peaks only, at the temperatures of 413 and 578 K, and 453 and
Peaks 3 and 4
Peak 3 /
i x
x'"
/
Dose
Fig. 2. TL response
I"
10~
10'
(mGy)
of topaz
as a function
of dose.
137
TL properties of Brazilian topaz
573 K, respectively. detected
In the present work, 4 peaks were at temperatures of 373 K (lOOC>, 403 K
(13O”C), 473 K (200°C) and 523 K (250°C). Our results concerning the rate of fading of topaz are more in accordance with those of Juan Azorin et al.(‘) Surely several other TL properties must still be investigated and correlated to the impurities contained in our samples. This is essential for the explanation of the different TL behaviour of topaz crystals obtained from minerals of different origins. However, it can be concluded that topaz crystals from the region of Governador Valadares, Brazil, possess favorable TL characteristics.
Acknowledgements-We wish to thank Dr Carlos Pires Ferreira, Director of the Department of Mineral Resources of CNEN, Brazil, for kindly providing the topaz mineral used in this work, and Dr Marina B. A. Vasconcellos, from the Institute for Energetic and Nuclear Research, of CNEN, Brazil, for performing the neutron activation analysis.
References I. Moss A. L. and McKlveen J. W. Health Phys. 34, 137 (1978). 2. Juan Azorin N., Roberto P. C. Salvi and Alicia Gutierrez C. Health Phys. 43, 599 (1982).