Thermal history and reusability of CaSO4: Dy Teflon TLD discs.

The TL sensitivity of CaCO4 : Dy Teflon TLD discs was found not to change after isothermal annealing for 1 h in the temperature range 220--400 degrees C. The reusability of these TLD discs after two separate annealing treatments (240 degrees C and 400 degrees C, each 1 h) were studied for 20 cycles of use. The former annealing treatment resulted in a 12% loss in TL sensitivity after 20 cycles, mainly due to the increase in optical density of these discs. A decrease in the maximum temperature from 300 to 250 degrees C while clamped for read-out in the TLD reader has prevented any change in the optical density of these discs, and their TL sensitivity did not show any change after 20 cycles. The darkening of PTFE-based TL dosemeters was attributed to the reorientation of molecules in PTFE when the temperature during readout exceeds 250 degrees C; isothermal annealing at 400 degrees C restored their original colour. Their TL sensitivity decreased gradually with the number of cycles (over 15% in 20 cycles) after the 400 degrees C anneal, mainly because of the loss in TL sensitivity of the CaSO4 : Dy phosphor itself. This study has demonstrated that under controlled conditions, the Teflon TL dosemeters can be reused a number of times without any change in TL sensitivity.