Study of the stability of the TL and OSL signals

In the study of thermoluminescence (TL) and optically stimulated luminescence (OSL), and in particular in the applications of archaeological and geological dating as well as dosimetry, the issue of stability of the signal at ambient temperature following excitation is of paramount importance. In many cases, one determines the activation energy (E) and frequency factor (s) of a TL peak, and tries to evaluate the lifetime of the excited signal. This is meaningful if the process is of pure first order, and may not be so in non-first-order situations. In the present work, we study this matter for both first-order and the more general one-trap-one-recombination-center (OTOR) cases using numerical simulations. The conventional numerical solution of the relevant set of coupled differential equations may not work when the traps are deep and the length of time is, say, thousands of years or more, and we therefore resort to a Monte-Carlo approach. It is obvious that in instances of dominating recombination, the long-time decay is exponential, and the decay constant is as expected from the first-order behavior and the E and s values. However, in cases of substantial retrapping, the fading is slower, sometimes very significantly, and is not exponential. Thus, one may deduce from the evaluated E and s shorter decay times than occur in fact. This may lead to an apparent effect of unexpected stability, namely, that a signal is stable much longer than expected from the evaluated trapping parameters. Possible implications concerning applications in archaeological and geological dating are obvious.