A critical look at the kinetic models of thermoluminescence: I. First-order kinetics

Using a generalized scheme of multiple traps, thermoluminescence (TL) glow curves are calculated for different sets of systems parameters. In particul ar, the conditions under which glow peaks of first-order kinetics are produ ced are highlighted. The major findings and conclusions are as follows. (1) In the generalized scheme the glow peaks always reduce to first order at l ow trap occupancies. It is therefore suggested that the peak analysis to de termine the parameters should be carried out only at low doses. (2) Glow pe aks which follow first-order kinetics can be obtained irrespective of wheth er the recombination rate (R-rec) is faster, equal to or slower than the re trapping rate (R-ret). (3) Quasi-equilibrium (QE) of free carriers in the d elocalized band, which is the essential condition for the derivation of the conventional analytical expressions of TL and thermally stimulated conduct ivity, can be realized irrespective of whether R-rec greater than or less t han R-ret. (4) The realization of the QE condition depends on the concentra tions of the traps and the recombination centres (RCs) and their cross sect ions for free carrier capture. It is discussed and shown that, in doped ins ulating and semiconducting materials, the values of these parameters are su fficiently high for the QE condition to be comfortably held. It is thus con cluded that the doubts raised by earlier workers regarding the validity of the QE assumption in the derivation of the analytical expressions are unnec essary as far as these materials are concerned. (5) It is shown that a syst em in which some of the untrapped charge carriers recombine within the germ inate centres and some become delocalized may satisfactorily explain the me chanism of TL emission in most of the phosphors. The properties of first-or der, supralinearity and pre-dose sensitization may be easily explained unde r the framework of this system. (6) Conclusions (2) and (3) above disprove those of earlier workers who had concluded that QE and fast retrapping toge ther do not form a consistent set of conditions and that the apparent domin ance of first-order kinetics in nature is due to slow retrapping.