Thermoluminescence from a distribution of trapping levels in UV irradiated calcite

Abstract

Both geological and synthetic calcite samples exhibit a low temperature thermoluminescence (TL) peak around 80°C; this TL peak has been the subject of several investigations since it is believed to involve localized transitions in the crystal rather than the more usual electronic transitions through the energy bands. We performed a detailed study of the kinetics of this TL peak in a high purity synthetic calcite powder. Annealing of the samples at temperatures above 500°C causes the TL peak at 71°C to become more narrow and the TL intensity to change dramatically between 400 and 700°C. Our experimental results are consistent with the presence of a distribution of activation energies and a first order TL process associated with this TL peak. Several commonly used methods of analysis such as the initial rise, heating rate, whole glow, isothermal decay and peak shape methods were used to evaluate the activation energy E, the frequency factor s and the order of the kinetics b. Good agreement was obtained between all methods except the isothermal decay method. The kinetic parameters associated with this low temperature TL peak were found to be E = 0.78 ± 0.03 eV, s = (1–7) × 10 10 s −1 and b = 1. A theoretical model based on a Gaussian distribution of energies provides a self-consistent description of the TL glow curves and the dose dependence of the observed TL height. When the calcite sample is annealed at temperatures above 500°C, the width of the Gaussian distribution of energies becomes more narrow. Two possible mechanisms are suggested for the observed changes in the TL glow curves.