Comparison on Characteristics of Radiophotoluminescent Glass Dosemeters and Thermoluminescent Dosemeters
short running title：Comparison on Characteristics of RPLGD and TLD
Shih-Ming Hsu1, Shann-Horng Yeh2, Meei-Shiow Lin3,
Wei-Li Chen1,*
1Department of Medical Radiation Technology, National Yang-Ming University, 155, Li-Nong St. Sec. 2, Pei-tou, Taipei 112, Taiwan, ROC.
2Department of Radiological Technology, Tzu-Chi College of Technology, 880,Sec.2,Chien-kuo Rd. Hualien ,970,Taiwan, ROC.
3Institute of Nuclear Energy Research, Long-tan 325, Taiwan, ROC.
*Corresponding author: Dr. Wei-Li Chen
National Yang-Ming University
Institute of Radiological Science
Department of Medical Radiation Technology
155, Li-Nong St., Sec 2
Pei-tou, Taipei 112
Taiwan, Rep. of China
Email:
Tel: 886-2-28267281
Fax: 886-2-28201095
ABSTRACT
The radiophotoluminescent glass dosemeter (RPLGD) system is applicable for measurement of radiation dose of X-rays and gamma rays using radiophotoluminescent glass（silver activated phosphate glass）. When the radiophotoluminescent glass is exposed to ionizing radiation, stable luminescent centers are created. During pulsed ultra-violet laser excitation (337.1 nm) in the reader, the centers emit a radiation induced orange fluorescent light (600~700 nm). The phenomenon is called radiophotoluminescence. This study compared the RPLGD system with LiF thermoluminescence dosimetry (TLD) system and results of the study revealed that the RPLGD had not only good basic characteristics for reproducibility of readout value, dose linearity, energy dependence and fading but also infinite repeatable measurements and could be one of the most important radiation dose measurement instruments.
Keywords: radiation dose, radiophotoluminescent glass dosemeter, thermoluminescent dosemeter
INTRODUCTION
The radiophotoluminescent glass dosemeter was developed in the late 1960s（1,2）. In the 1990s, Asahi Techno Glass Corporation and Karlsruhe Nuclear Research Center created a new readout system. The new system gave readout fully automatically and used pulsed UV laser excitation and predose suppression, which replaced the conventional RPLGD system（3）. The RPLGD has been used in environmental and personal monitoring as an alternative technique besides automatic thermoluminescent dosemeter (TLD) systems or film dosemeters（4）.
During the last ten years, TLD was mainly used for personal monitoring in Taiwan. The RPLGD system was introduced into Taiwan by the Institute of Nuclear Energy Research (INER, Taiwan) in Dec. 2002. The luminescent centers of TLD disappeared because of heating but the centers of RPLGD would exist after receiving pulsed UV laser irradiation. This was because pulsed UV laser beam excitations did not eliminate the luminescent centers in the RPLGD. That way, RPLGD could be read infinitely. This study focused on characteristics comparison of radiation detection systems of RPLGD and TLD covering reproducibility of readout value, fading effect, build-up effect, dose linearity, energy dependence and angular dependence.
MATERIALS AND METHODS
Radiophotoluminescent glass dosemeter
The RPLGD system was composed of silver activated phosphate glass, a stainless holder (glass card), capsule （energy compensation filters）, and a full automatic readout system. A glass dosemeter SC-1(Asahi Techno Glass Corporation, Shizuoka, Japan) and a full automatic readout system FGD-202 (Asahi Techno Glass Corporation, Shizuoka, Japan) were used in this work. The RPLGD SC-1 consisted of a glass element FD-7 (16 × 16 × 1.5 mm3) fixed in a stainless steel glass card and a plastic capsule (40 × 30 × 9 mm3) with energy compensation filter on both sides by plastic and tin. The glass FD-7 material contained Na (11.0 %), P (31.55 %), O (51.16 %), Al (6.12 %) and Ag (0.17 %) by weight. The effective atomic number was 12.039（5）. SC-1 was a flat type of RPLGD and it could measure radiation dose of X-rays and gamma rays. The FGD-202 was the readout system for SC-1 glass dosemeter using pulsed nitrogen gas UV laser excitation.
Thermoluminescent dosemeter
Crystalline chips of 7LiF:Mg, Ti (BICRON/Harshaw, OH, USA) with dimensions of 3 × 3 × 0.38 mm3 were used in this study. A Model 6600 automatic TLD reader system (BICRON/Harshaw, OH, USA) was used to evaluate the 7LiF:Mg, Ti chips. The effective atomic number was 8.2. The programmed heating cycle used during the TLD readout was set as heating rate 25 °Cs-1; preheating constant temperature 150 °C for 5 s; constant temperature for reading 300 °C for 15 s. Before each experiment, TLDs were pre-annealed at 400 °C for 1 h and then 100 °C for 2 h and kept at room temperature at last.
The 60Co source, 137Cs gamma source and X-ray generator (Pantak HF 420) were established at the National Radiation Standard Laboratory (NRSL) and used to irradiate the RPLGD and TLD.
RESULTS AND DISCUSSION
Dosemeters were irradiated at the NRSL using a 137Cs source. The coefficients of variation (CVs) of RPLGD and TLD were 0.63, 0.48, 0.51 and 1.90, 1.88, 1.50, respectively after irradiated by doses of 0.6 mGy, 6 mGy and 20 mGy. The same dosemeter was read repeatedly in decdruplicate (Table 1). In measurement precision, difference of readout values could be as small as about 1 % for RPLGD and 2 % for TLD. The CVs of reproducibility of RPLGD were better than TLD. The reproducibility of readout values would not be affected whether or not RPLGD and TLD expose dose ranges were from 0.6 mGy to 20 mGy.
Degree of fading factors would affect evaluation accuracy of radiation dose. The fading effect in percentage (%) in a month for RPLGD SC-1 was within 1, for TLD-700 with preheat was about 12, and for TLD-700 without preheat was about 18 when the RPLGD and TLD were placed in the dark and at room temperature of 22 °C (Figure1). The long-term experiment confirmed that the RPLGD would not cause fading effect. However, RPLGD was suitable for long-term monitoring of dose accumulation.
If RPLGD was operated without preheat process, the intensities of radiophotoluminescence signal would stabilize for a long time after exposure. This is because electrons did not get into correct holed traps (color centers). The RPLGD must get heat treatment at 70 °C before giving out readout signals(6). The glass dosemeters irradiated by dose ranging from 0.6 mGy to 20 mGy had the same build-up when handled without heat treatment and stored at room temperature of 22 °C (Figure2). After exposure, the radiophotoluminescence signal reading increased with time and 90 % of the value at an air-conditioned laboratory temperature (22 °C) was expected on the 10th days.
Linearity to air kerma in the range from 0.01 mGy to 500 mGy of 137Cs gamma-ray source was shown in figure 3. After running the Microsoft excel software, R-squared values between exposure dose and readout value were close to 1 for RPLGD and TLD. On the other hand, the irradiated dose and readout of RPL or TLD signal intensity were a direct proportion function.
This study set 7 points within the energy range of 32 keV to 1250 keV in X-ray generator, 137Cs and 60Co, including 32 keV, 67 keV, 119 keV, 142 keV, 210 keV, 662 keV (137Cs) and 1250 keV (60Co). The SC-1 glass dosemeter energy dependence response was between –2 ﹪to +15.8 ﹪from 32 keV to 1250 keV and between –8 ﹪to +23 ﹪for TLD, as shown in figure 4. The relative response was normalized to the RPLGD and TLD reading of the 137Cs irradiation.
By moving a plastic protractor angle from the horizontal down direction to 0, ±40, ±60, ±70 and ±80 degrees, the angular dependence were measured and shown in figure 5. The relative response was normalized to the RPLGD and TLD reading of the horizontal axis at 0 degree. Table 2 compared the basic characteristics of radiation detectors between SC-1 glass dosemeter and 7LiF:Mg, Ti detector.
CONCLUSIONS
Either film badge or TLD can measure external dose of radiation workers and RPLGD is another choice. In comparison to TLD, the advantages of the RPLGD system are the good reproducibility of readout value, long-term stability (little fading effect), low energy dependence, better dose linearity and capability of repeating readouts. The RPLGD system is quite viable for dosimetry application in radiation diagnosis, radiation therapy, high energy accelerators, and nuclear power plants. In summary one can say that, according to the present work, RPLGD appears to be one of the most stable dosemeter for an application in all fields of personnel and environmental monitoring.
ACKNOWLEDGEMENTS
This study was supported by a grant from the INER of the Republic of China.
REFERENCES
1. Piesch, E., Burgkhardt, B., Fischer, M., Rober, H. G. and. Ugi, S Properties of radiophotoluminescencent glass dosemeter systems using pulsed laser UV excitation. Radiat. Prot. Dosim. 17, 293-297 (1986).
2. Burgkhardt, B., Festag, J. G., Piesh, E. and Ugi, S New aspects of environmental monitoring using flat phosphate glass and thermoluminescence dosemeters. Radiat. Prot. Dosim. 66(1), 187-192 (1996).
3. Asahi Techno Glass Corporation, Technical information for RPL glass dosemeter type SC-1. Tokyo, Japan (2000).
4. Piesch, E., Burgkhardt, B., and Vilgis, M. Photoluminescence dosimetry: progress and present state of art. Radiat. Prot. Dosim. 33(1-4), 215-226 (1990)
5. Araki, F., Ikegami, T., Ishidoya T. and Kubo, H. D. Measurements of Gamma-Knife helmet output factors using a radiophotoluminescent glass rod dosimeter and a diode detector Med. Phys. 30(8), 1976-1981. (2004).
6. Asahi Techno Glass Corporation, New RPL glass dosemeter system for large scale personal monitoring. Tokyo, Japan (2001).
Figure and Table Captions
Figure 1. Fading after 137Cs irradiation of RPLGD and TLD at room temperature.
Figure 2. The build-up characteristics of RPLGD readout at room temperature.
Figure 3. Linearity in response of the RPLGD and TLD systems to 137Cs γ-radiation.
Figure 4. Relative energy response of the SC-1 glass dosemeter and 7LiF:Mg, Ti detector.
Figure 5. The angular dependence of RPLGD and TLD.
Table 1. Reproducibility of readout value of RPLGD and TLD.
Table 2. Dosimetric properties of the SC-1 glass dosemeter and 7LiF:Mg, Ti detector.
Figure 1.
Figure 2.
Figure 3
Figure 4.
Figure 5.
Table 1.
Irradiated dose（mGy）0.6 / 6 / 20
RPLGD / TLD / RPLGD / TLD / RPLGD / TLD
C.V. / 0.63 / 1.90 / 0.48 / 1.88 / 0.51 / 1.50
C.V. =（）（）-1 = S /
Table 2.
Properties / RPLGD / TLDReproducibility of readout value（C.V.） / 0.6 mGy：0.63
6 mGy：0.48
20 mGy：0.51 / 0.6 mGy：1.90
6 mGy：1.88
20 mGy：1.50
Fading effect (30 d) / ＜1 % / with preheat：12 %
without preheat：20 %
Build-up effect / 90 %/7 d, 99 %/100 d / -
Dose linearity / 0.01 mGy ~ 500 mGy
y=0.977x+0.1142 / 0.01 mGy ~ 500 mGy
y=1.0528x-0.2236
Energy dependence / 32 keV ~ 1250 keV
-2 %to +15.8 % / 32 keV ~ 1250 keV
-8 % to +23 %
Angular dependence / -800 ~ +800
within 8 % / -800 ~ +800
within 3 %
4