The light sensitivity of thermoluminescent materials: LiF:Mg,Cu,P, LiF:Mg,Ti and Al2O3:C
Introduction
In the past two decades, two new thermoluminescence dosimeters (TLDs), namely LiF:Mg,Cu,P and Al2O3:C, have emerged in the fields of environmental and diagnostic radiology dosimetry (Sáez-Vergara and Romero, 1996, Budzanowski et al., 1996, Duggan et al., 1999). These materials have the advantage of 20–30 and 20–60 times (heating rate dependent) the sensitivity of LiF:Mg,Ti, respectively (Prokic and Bøtter-Jensen, 1993). In environmental dose monitoring a large over- or under-estimation of the environmental radiation could occur due to significant, light-induced signal gains or losses. The aim of the present study was to investigate the effects of light exposure on these more recently prepared TLD materials and gain insight into the potential problems that could arise during their use in practical dosimetry. Light-induced fading of the TL signal can occur due to the optical excitation of the charge from the traps (McKeever et al., 1995). In contrast to the optical fading effect, light-induced thermoluminescence can occur.
The light-induced fading and light-induced thermoluminescence (TL) of aluminium oxide has been quoted to be a major disadvantage of this material for use in dosimetry, although this property has been exploited by using it as a sensitive optically stimulated luminescence (OSL) dosimeter (e.g. McKeever et al., 1996). Moscovitch et al. (1993) stated that for Al2O3:C more than 95% of the dose information (0.01 Gy 90Sr–90Y beta rays) was erased by 400 lx fluorescent light as compared to LiF:Mg,Ti where light induced fading was minimal. The remaining 5% TL signal observed following several hours exposure of Al2O3:C to light was assumed to be due to light-induced signal rather than ionising radiation induced TL.
The light sensitivity of LiF:Mg,Cu,P has not been studied extensively however Wang et al. (1986) reported 11% fading after 3 h of exposure to direct sunlight (∼9000 lx). Osvay and Lembo (1993) compared the light sensitivity of GR-200 (LiF:Mg,Cu,P, Beijing China), MCP-N, TLD-100 (LiF:Mg,Ti, Harshaw-Bicron, USA) and MTS-N (LiF:Mg,Ti, TLD Niewiadomski & Co., Poland). The main conclusion was that GR-200 had a significantly higher UV dependence than MCP-N, up to 240°C at a dose level of 1–20 mGy. The increasing order of UV sensitivity included MCP-N, TLD-100, MTS-N and GR-200A. The increased UV sensitivity of GR-200 relative to TLD-100 is in conflict with Driscoll (1987). The photo-transfer efficiency of LiF:Mg,Cu,P (GR-200A) was studied by Driscoll et al. (1987) at 253.7 nm (dose level of 200–1500 mGy), and found both productions of LiF:Mg,Cu,P — MCP-N and GR-200 — had low UV sensitivity.
Section snippets
Field exposure to sunlight
Firstly, measurements were performed with MCP-N dosimeters prepared at the Institute of Nuclear Physics and exposed at Nicholas Copernicus University, Torun Poland. MCP-N detectors were irradiated to 10 mGy with controls for background, zero light exposure, temperature and humidity. A group of five detectors were left outside (Poland, mid-summer) for approximately 2 weeks to see the effects of light exposure, in the context of environmental monitoring conditions. Another set of detectors was
Field exposure to sunlight
Experiments performed in Torun, Poland showed a reduction in MCP-N thermoluminescence intensity of (45±5)% after direct sunlight exposure in approximately 2 weeks. The effects of sunlight were far more dramatic than that of the simulated Polish light source. Peak 3 followed a simple exponential decay and peak 4 (main dosimetric peak) followed a dual exponential drop-off. The high temperature component remained unchanged (read out to 310°C).
General
Fig. 2, Fig. 3 show, qualitatively, the drop in total
Conclusion
Sunlight has a substantial effect on stored radiation-induced TL intensity, which could have a significant effect on the accuracy of delayed dose assessment. LiF:Mg,Cu,P (MCP-N) showed lesser UV sensitivity than LiF:Mg,Ti (GR-100) which resulted in reduced light-induced fading. Since GR-100 and GR-200 exhibit greater UV sensitivity than TLD-100 and MCP-N, this indicates that there could exist a dependence of light induced effects on manufacturer’s specifications. Al2O3:C showed an extreme
Acknowledgements
The principle author would like to thank the NSW Cancer Council and the Radiation Oncologists at the Newcastle Mater Hospital for the opportunity to visit the Institute of Nuclear Physics and Queensland University of Technology (QUT). This work was also partly supported by Research Grant No. 4P05D 030 13 from the Polish State Committee for Scientific Research (KBN). The authors wish to thank Dr Jose Maria Gomez Ros for the possibility of using the GCA software.
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