The effect of annealing atmosphere on the thermoluminescence of synthetic calcite

Abstract

Samples of high purity calcite powder were annealed in air, nitrogen and carbon dioxide atmospheres in the temperature range 300–700°C and in atmospheric pressure. The samples were subsequently irradiated and the effect of the annealing atmosphere and temperature on the thermoluminescence (TL) of the samples was studied. Our results show that both carbonate and oxygen ions play an important part in the TL of calcite annealed in this temperature range. The intensities of the TL signal in the nitrogen and carbon dioxide anneals rise continuously with the annealing temperature. For all annealing temperatures it was found that the carbon dioxide atmosphere caused an increase in the observed TL signal as compared with anneals in an inert nitrogen atmosphere, while the shape of the TL glow curves remained the same. This increase in the observed TL signal is explained via the surface adsorption of carbonate ions. The shape and location of the TL peaks suggest that samples annealed in air exhibit a different type of TL center than samples annealed in nitrogen and carbon dioxide atmospheres. A possible mechanism for the role of oxygen ions involves a surface adsorption process and a subsequent diffusion of oxygen ions in the bulk of the crystal. Annealing of the samples in air at temperatures T>600°C causes a collapse of the TL signal, in agreement with previous studies of calcite powders. No such collapse of the TL signal is observed for the nitrogen and carbon dioxide anneals, suggesting that a different type of TL center and/or recombination center is involved in air anneals. Arrhenius plots for the air anneals yield an activation energy E=0.45±0.05 eV, while the carbon dioxide and nitrogen anneals yield a lower activation energy E=0.28±0.04 eV.