THERMOLUMINESCENCE FROM A DISTRIBUTION OF TRAPPING LEVELS IN UV IRRADIATED CALCITE

Both geological and synthetic calcite samples exhibit a low temperatur e thermoluminescence (TL) peak around 80 degrees C; this TL peak has b een the subject of several investigations since it is believed to invo lve localized transitions in the crystal rather than the more usual el ectronic transitions through the energy bands. We performed a detailed study of the kinetics of this TL peak in a high purity synthetic calc ite powder. Annealing of the samples at temperatures above 500 degrees C causes the TL peak at 71 degrees C to become more narrow and the TL intensity to change dramatically between 400 and 700 degrees C. Our e xperimental results are consistent with the presence of a distribution of activation energies and a first order TL process associated with t his TL peak. Several commonly used methods of analysis such as the ini tial rise, heating rate, whole glow, isothermal decay and peak shape m ethods were used to evaluate the activation energy E, the frequency fa ctor s and the order of the kinetics b. Good agreement was obtained be tween all methods except the isothermal decay method. The kinetic para meters associated with this low temperature TL peak were found to be E = 0.78 +/- 0.03 eV, s = (1-7) x 10(10) s(-1) and b = 1. A theoretical model based on a Gaussian distribution of energies provides a self-co nsistent description of the TL glow curves and the dose dependence of the observed TL height. When the calcite sample is annealed at tempera tures above 500 degrees C, the width of the Gaussian distribution of e nergies becomes more narrow. Two possible mechanisms are suggested for the observed changes in the TL glow curves.