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On the phototransferred thermoluminescence in MgB4O7:Dy
Appl. Radial. hot. Vol. 38, No. 4, pp. 313-314, 1987 ht. J. Radtat. Appl. Instrum. Part A 8 Pergamon Journals Ltd 1987. Printed in Great Britain
0883-2889/87$3.00+ 0.00
On the Phototransferred Thermoluminescence in MgB,O, : Dy ROBERT G. RICHMOND, OLABODE T. OGUNLEYE,* BERNARD L. CASH’ and KAY L. JONES’ Radiation Dosimetry Laboratory, NASA-Johnson Space Centre, Houston, TX 77058 and ‘Lockheed-EMSCO, Houston, TX 77058, U.S.A. (Received 26 March 1986; in revised form 7 August 1986)
Since the individual U.V. response of the discs varied widely, an initial selection was made by giving a large quantity a nominal U.V. exposure and grouping those with TL signals within f 10%. They were evaluated using the Harshaw TLD readers models 2000A and B employing a linear heating rate of about 5”C/s from room temperature up to 300°C. Glow curves were acquired by interfacing the readout system with a desk-top computer. For the PTTL measurements, the samples were given different y-ray exposures ranging from 0.01 to 7.0 Gy, using a calibrated 13’Cssource. All measurements were made with a nitrogen tlow of 0.04 (dm)’ s-l. It had been shown in previous studies that the readout process alone is sufficient to empty the main dosimetric traps in the phosphor.c’-6) However, this process could be very cumbersome in investigations involving a large number of dosimeters. Different annealing processes were therefore studied to find an optimum temperature that best restores their initial responses. The best results were obtained by annealing the dosimeters at 300°C for 30 min. Higher temperatures tend to lead to a decrease in the sensitivity and discoloration of the dosimeters.@)
Introduction There have been several reports on the characteristics of magnesium borate thermoluminescent dosimeters (TLD) as a material for radiation dosimetry. Some of the attractive features such as high sensitivity, low fading rate, tissue equivalence, no need for annealing and the abscence of light sensitivity, which were initially reported,“) have not been found in subsequent studies. (26)In particular we have noted some batch-related differences between those produced by the Boris KidriE Institute of Nuclear Sciences, VinEa, Belgrade, Yugoslavia, and those commercially available from Harshaw-Filtrol Partnership, Solon, Ohio, U.S.A.@’ One such finding was the considerably higher light sensitivity of the batch produced by Harshaw-Filtrol. The potential was demonstrated for their use as U.V.dosimeters. On exposure to ionizing radiations, the normal traps in MgB,O,:Dy can be emptied by the readout process only or by annealing at suitable temperatures, leaving the deep traps still intact. Subsequently, it is possible to repopulate the dosimetric peak by illuminating with U.V. light. Charge carriers released from the traps during the second readout give rise to a thermoluminescent output proportional to the ionizing radiation exposure and the U.V.radiant energy. This is called phototransferred thermoluminescence (PTTL). The PTTL method can be utilized for the reassessment of the initial y-ray exposure. This paper presents the measurements of the intrinsic response of the commercially available MgB,O,: Dy to U.V. radiation, with a demonstration of its use for y-ray dose re-estimation using the PTTL technique.
Results and Discussion Figure I shows typical glow curves from virgin magnesium borate dosimeters exposed to gamma radiation and U.V. light. The glow curve from the gamma radiation consists of a main dosimetric peak at about 19O”C,while that resulting from the U.V. exposure has two peaks at 80” and 200°C. The results in Fig. 2 show that the inherent response
100
0
200 Temperature
PC
300
)
Fig. 1. Thermoluminescence glow curves of MgB,O,: Dy after exposure to y-rays (-----) and U.V. radiation (-).
Experiment MgB,O,:Dy dosimeters used in this study were obtained as sintered solid discs, 4.5 mm in diameter and 0.9 mm thick, from Harshaw-Filtrol. T’he U.V. light exposures were made from a UVGL-58 20 W lamp (manufactured by LJVP Inc., San Gabriel, California, U.S.A.) with more than 98% of the radiant output at a wavelength of 254nm. The measured irradiance at the point of exposure 8.2cm from the lamp was 10.3 W me2.
IO0 * Present address and for correspondence: Department of Radiation Biology & Radiotherapy, College of Medicine, University of Lagos, P.M.B. 12003, Lagos, Nigeria. A.R.I.
38/4-F
IO'
102 Time
IO3
(min)
Fig. 2. Ultraviolet-induced TL response vs exposure time at 10.3 W/m2.
Technical Note
314
x, -xl 3 8 cc
x-----X
40
X t
2ot 0
1
I 100
I 300
I 200 Stomge
time
I 400
(h)
Fig. 3. Fading of u.v.-induced TL signal after storage in the dark.
Conclusion
8
c’ t
0.01
I
I
J
0.1
1
10
Gamma
MgB,O,:Dy showed a higher intrinsic sensitivity to U.V. radiation that is very reproducible following several uses as a U.V. dosimeter, provided they are annealed at 300°C between exposures. The response is very linear and covers the U.V.radiant exposures that might be of interest in bioligical research. Its use will however require an appropriate fading correction and matching of U.V. response characteristic of individual dosimeter samples. In this work, the entire glow curve area has been integrated to obtain the signal. Better stability might be obtained by first annealing out the low-temperature peaks before evaluation or by taking the heights of the 200°C peak only. It has been shown that there is good correlation between PTTL and prior y-ray absorbed dose. This will allow y-ray dose re-estimation, but only for doses much higher than those routinely encountered in personnel dosimetry. References 1. ProkiC M. Nucl. Instrum. Methods. 175, 83 (1980). 2. Driscoll C. M. H., Mundy S. J. and Elliot J. M. R&at. Profection Dosim. 1, 135 (1981). 3. Prokic M. S. Health Phys. 42, 849 (1982). 4. Jones A. R. Radiut. Protection Dosim. 6, 71 (1984). 5. Oduko J. M., Harris S. J. and Stewart J. C. Radiat. Protection Dosim. 8, 257 (1984).
6. Ogunleye 0. T., Richmond R. G. and Cash B. L. Health Phys. 49, 527 (1985).
0883-2889/87$3.00+ 0.00
On the Phototransferred Thermoluminescence in MgB,O, : Dy ROBERT G. RICHMOND, OLABODE T. OGUNLEYE,* BERNARD L. CASH’ and KAY L. JONES’ Radiation Dosimetry Laboratory, NASA-Johnson Space Centre, Houston, TX 77058 and ‘Lockheed-EMSCO, Houston, TX 77058, U.S.A. (Received 26 March 1986; in revised form 7 August 1986)
Since the individual U.V. response of the discs varied widely, an initial selection was made by giving a large quantity a nominal U.V. exposure and grouping those with TL signals within f 10%. They were evaluated using the Harshaw TLD readers models 2000A and B employing a linear heating rate of about 5”C/s from room temperature up to 300°C. Glow curves were acquired by interfacing the readout system with a desk-top computer. For the PTTL measurements, the samples were given different y-ray exposures ranging from 0.01 to 7.0 Gy, using a calibrated 13’Cssource. All measurements were made with a nitrogen tlow of 0.04 (dm)’ s-l. It had been shown in previous studies that the readout process alone is sufficient to empty the main dosimetric traps in the phosphor.c’-6) However, this process could be very cumbersome in investigations involving a large number of dosimeters. Different annealing processes were therefore studied to find an optimum temperature that best restores their initial responses. The best results were obtained by annealing the dosimeters at 300°C for 30 min. Higher temperatures tend to lead to a decrease in the sensitivity and discoloration of the dosimeters.@)
Introduction There have been several reports on the characteristics of magnesium borate thermoluminescent dosimeters (TLD) as a material for radiation dosimetry. Some of the attractive features such as high sensitivity, low fading rate, tissue equivalence, no need for annealing and the abscence of light sensitivity, which were initially reported,“) have not been found in subsequent studies. (26)In particular we have noted some batch-related differences between those produced by the Boris KidriE Institute of Nuclear Sciences, VinEa, Belgrade, Yugoslavia, and those commercially available from Harshaw-Filtrol Partnership, Solon, Ohio, U.S.A.@’ One such finding was the considerably higher light sensitivity of the batch produced by Harshaw-Filtrol. The potential was demonstrated for their use as U.V.dosimeters. On exposure to ionizing radiations, the normal traps in MgB,O,:Dy can be emptied by the readout process only or by annealing at suitable temperatures, leaving the deep traps still intact. Subsequently, it is possible to repopulate the dosimetric peak by illuminating with U.V. light. Charge carriers released from the traps during the second readout give rise to a thermoluminescent output proportional to the ionizing radiation exposure and the U.V.radiant energy. This is called phototransferred thermoluminescence (PTTL). The PTTL method can be utilized for the reassessment of the initial y-ray exposure. This paper presents the measurements of the intrinsic response of the commercially available MgB,O,: Dy to U.V. radiation, with a demonstration of its use for y-ray dose re-estimation using the PTTL technique.
Results and Discussion Figure I shows typical glow curves from virgin magnesium borate dosimeters exposed to gamma radiation and U.V. light. The glow curve from the gamma radiation consists of a main dosimetric peak at about 19O”C,while that resulting from the U.V. exposure has two peaks at 80” and 200°C. The results in Fig. 2 show that the inherent response
100
0
200 Temperature
PC
300
)
Fig. 1. Thermoluminescence glow curves of MgB,O,: Dy after exposure to y-rays (-----) and U.V. radiation (-).
Experiment MgB,O,:Dy dosimeters used in this study were obtained as sintered solid discs, 4.5 mm in diameter and 0.9 mm thick, from Harshaw-Filtrol. T’he U.V. light exposures were made from a UVGL-58 20 W lamp (manufactured by LJVP Inc., San Gabriel, California, U.S.A.) with more than 98% of the radiant output at a wavelength of 254nm. The measured irradiance at the point of exposure 8.2cm from the lamp was 10.3 W me2.
IO0 * Present address and for correspondence: Department of Radiation Biology & Radiotherapy, College of Medicine, University of Lagos, P.M.B. 12003, Lagos, Nigeria. A.R.I.
38/4-F
IO'
102 Time
IO3
(min)
Fig. 2. Ultraviolet-induced TL response vs exposure time at 10.3 W/m2.
Technical Note
314
x, -xl 3 8 cc
x-----X
40
X t
2ot 0
1
I 100
I 300
I 200 Stomge
time
I 400
(h)
Fig. 3. Fading of u.v.-induced TL signal after storage in the dark.
Conclusion
8
c’ t
0.01
I
I
J
0.1
1
10
Gamma
MgB,O,:Dy showed a higher intrinsic sensitivity to U.V. radiation that is very reproducible following several uses as a U.V. dosimeter, provided they are annealed at 300°C between exposures. The response is very linear and covers the U.V.radiant exposures that might be of interest in bioligical research. Its use will however require an appropriate fading correction and matching of U.V. response characteristic of individual dosimeter samples. In this work, the entire glow curve area has been integrated to obtain the signal. Better stability might be obtained by first annealing out the low-temperature peaks before evaluation or by taking the heights of the 200°C peak only. It has been shown that there is good correlation between PTTL and prior y-ray absorbed dose. This will allow y-ray dose re-estimation, but only for doses much higher than those routinely encountered in personnel dosimetry. References 1. ProkiC M. Nucl. Instrum. Methods. 175, 83 (1980). 2. Driscoll C. M. H., Mundy S. J. and Elliot J. M. R&at. Profection Dosim. 1, 135 (1981). 3. Prokic M. S. Health Phys. 42, 849 (1982). 4. Jones A. R. Radiut. Protection Dosim. 6, 71 (1984). 5. Oduko J. M., Harris S. J. and Stewart J. C. Radiat. Protection Dosim. 8, 257 (1984).
6. Ogunleye 0. T., Richmond R. G. and Cash B. L. Health Phys. 49, 527 (1985).