THE EFFECT OF ANNEALING ATMOSPHERE ON THE THERMOLUMINESCENCE OF SYNTHETIC CALCITE

Samples of high purity calcite powder were annealed in air, nitrogen a nd carbon dioxide atmospheres in the temperature range 300-700 degrees C and in atmospheric pressure. The samples were subsequently irradiat ed and the effect of the annealing atmosphere and temperature on the t hermoluminescence (TL) of the samples was studied. Our results show th at 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 wi th the annealing temperature. For all annealing temperatures it was fo und that the carbon dioxide atmosphere caused an increase in the obser ved TL signal as compared with anneals in an inert nitrogen atmosphere , while the shape of the TL glow curves remained the same. This increa se in the observed TL signal is explained via the surface adsorption o f carbonate ions. The shape and location of the TL peaks suggest that samples annealed in air exhibit a different type of TL center than sam ples annealed in nitrogen and carbon dioxide atmospheres. A possible m echanism for the role of oxygen ions involves a surface adsorption pro cess and a subsequent diffusion of oxygen ions in the bulk of the crys tal. Annealing of the samples in air at temperatures T > 600 degrees C causes a collapse of the TL signal, in agreement with previous studie s of calcite powders. No such collapse of the TL signal is observed fo r the nitrogen and carbon dioxide anneals, suggesting that a different type of TL center and/or recombination center is involved in air anne als. Arrhenius plots for the air anneals yield an activation energy E = 0.45 +/- 0.05 eV, while the carbon dioxide and nitrogen anneals yiel d a lower activation energy E = 0.28 +/- 0.04 eV. (C) 1998 Elsevier Sc ience Ltd. All rights reserved.