Thermoluminescence of CaF2:Mn exposed at elevated temperatures to a constant rate of low-level radiation

An experiment was performed to examine the adequacy of the Randall-Wilkins theory for describing long-term thermal fading in CaF2:Mn during continuous low-level irradiation and to determine any temperature related restrictions on the use of this dosimeter for environmental monitoring. The effect was studied for glass-encapsulated E.G. and G. Model TL-015 TLDs at 52, 66, 93, 107 and 1210C. Dosimeters were annealed to remove all residual signal, immediately placed in a temperature controlled chamber in which the irradiation rate was less than 10 μR/h gamma-equivalent, removed after exposure periods ranging from two days to one month, cooled naturally to room temperature, and immediately read out. Twenty individually-calibrated dosimeters were rotated to produce at least fifty separate measurements for each temperature. The exposure rate inside the chamber was determined independently with an estimated accuracy of +-2%. The results differ significantly from the predications of the Randall-Wilkins model. An enhanced response was obtained between room temperature and 660C where the model indicates essentially no fading. An effect similar to this has been observed for relatively short-term experiments (approximately 1 hour) in which LiF was irradiated with high doses at the temperature in question. Beyond 930C the results indicate fading in excess of that predicted by the Randall-Wilkins model. These results, along with those of other investigators in which fading observed for conditions of simultaneous irradiation and storage has been found to exceed that observed for room temperature irradiation followed by storage at the temperature in question, further indicate that the Randall-Wilkins model cannot adequately represent these phenomena. The results demonstrate that for typical environmental conditions of practical interest, i.e., low exposure rates and measurement periods of several days to several weeks, the effects produced by elevated temperatures are not a significant deterrent to the use of CaF2:Mn for environmental radiation monitoring. (author)