ANNEALING EFFECTS ON THE THERMOLUMINESCENCE OF SYNTHETIC CALCITE POWDER

We have studied the effect of annealing temperature on the nature and the kinetics of the thermoluminescence (TL) trapping centers of synthe tic calcite. Samples of high purity calcite powder were annealed in ai r and in the temperature range 300-700 degrees C. The samples were sub sequently irradiated and the effect of the annealing on the TL of the samples was studied using several techniques. The energy values for th e traps associated with the observed TL peaks were measured using the initial rise method and were used as input for a least-squares fit pro cedure, which expresses the TL glow curves as the sum of five individu al peaks of general order kinetics. It was found that all the experime ntal TL glow curves for different annealing temperatures and various d oses could be fitted to the same set of kinetic parameters E,s and b i ndicating that the annealing process probably does not change the natu re of the trapping centers. The first- and second-glow TL growth curve s were measured for four TL peaks and were fitted to equations of the form y = y(infinity)(1 - exp(- beta t)), where t is the irradiation ti me. The value of the growth constant, beta, was found to be independen t of the annealing temperature for at least two of the TL peaks studie d and for annealing temperatures up to 600 degrees C. The activation e nergy for the change in sensitivity with annealing temperature was fou nd using Arrhenius-type plots for each TL peak. Our results can be exp lained with an energy scheme similar to the one used in the predose mo del of quartz; according to this model the change of sensitivity to io nizing radiation observed after annealing is due to changes occurring within the luminescence centers while the trapping centers remain unaf fected. Annealing in air for temperatures at or above 700 degrees C ca used a collapse in the TL sensitivity, in agreement with the energy sc heme of the predose model.