General order kinetics of thermoluminescence-a comparison with physical models

Abstract

The general order kinetics expression of thermoluminescence (TL) contains two empirical parameters, namely the kinetics order (KO) and the pre-exponential factor (PF). In this paper thermoluminescence glow curves are calculated by assuming well defined physically meaningful models and the KO and the PF values applicable to these glow curves are calculated. The approaches used to find these values are either analytical or based on the shape of the glow curves or their isothermal decay behaviour depending on the type of the model used. The results show that the KO and the PF parameters are in general not constant for a given glow peak. They vary with the change in occupancy of the traps except under the two limiting conditions, namely KO equal to 1 or 2. This means that, when the KO of a given glow peak is not equal to either 1 or 2, its numerical value as well as that of the PF would depend on the sample dose. It also means that these parameters change continuously when the glow curve is being recorded. At very low trap occupancies these parameters approach limiting values. For the simple one-trap model this limiting value of KO is 2 whereas that for a multi-trap model is 1. The corresponding changes in the quantitative values of the PF are by large orders of magnitude and the variation is in the direction opposite to that of the KO. Furthermore, the dimensions for the PF also change. These results bring the general order kinetics approach into conflict with the physical models used to describe the TL glow curves. Implications of these theoretical results for experimental observations are discussed.