Performance of Thermoluminescent Dosimeters Under As-Deployed Conditions.

Abstract

This dissertation examines the properties of long-term, passive radiation dosimetry systems incorporating thermoluminescent dosimeters (TLDs) for use in diverse interior and exterior settings. In so doing, a multitude of factors affecting thermoluminescence calibrations and measurements were examined. TLD calibration precision was studied, including the time variability of TLD reader measurements, irradiation inhomogeneity, and the optimal number of calibrations. It was also determined that the visible physical quality of TLDs, such as the presence of small fractures, scratches and discolorations, does not significantly affect sensitivity. A process for identifying useful TLDs from otherwise poor batches was formulated. Post-annealing TLD sensitivity changes, commonly referred to as pre-irradiation fading, and post-irradiation signal fading were examined for six common TL materials. To assist in data analysis, a generally useful glow curve analysis computer program was written using a common mathematics parser. Ratios of glow curve peaks, commonly used as self-normalizing quantities in TL dosimetry, were found for fading durations up to 30 d. Pre- and post-irradiation fading were individually fit to empirical functions of exponential form. For groups of LiF:Mg,Ti TLDs, it was found that a single fading function adequately described group behavior. For some deployments, the effects of high ambient temperature on fading must be known. LiF:Mg,Ti TLDs were subjected to controlled temperatures of 30°C to 50°C for up to 30 to 60 d. The sum of the areas of the two most stable glow curve peaks was found to be constant up to 50°C where it fades slightly over long periods. The effects of high ambient radon on TLDs had not previously been well characterized. Radon was found to have a significant effect on bare TLDs and TLDs encapsulated in dosimetry cards, but no measureable effect on cards in environmental badges for ~1 MBq m^(-3) h. The minimum detectable dose for a TLD system was analyzed based upon a dose-response linearity experiment and other results elsewhere in the dissertation. To conclude, a prototype system based upon integrating dosimetric materials is proposed as future work.