The role of simulations in the study of thermoluminescence (TL)

The traffic of charge carriers in a luminescent material during its excitation by irradiation and during readout either in the measurement of thermoluminescence (TL) or optically stimulated luminescence (OSL) is governed by sets of coupled nonlinear differential equations. The analytical solution of these sets is usually not possible, and one can resort to one of two options. Some researchers preferred to make simplifying assumptions and thus got approximate solutions whereas others performed simulations by solving the simultaneous equations numerically. Each of these routes has its pitfalls. The simplifying assumptions, mainly the quasi-equilibrium assertion or the assumption that certain relations between the relevant parameters and functions hold, may be valid in certain ranges of the TL or OSL curve, and may cease to be valid, say at the high-temperature range in TL. Performing simulations using the numerical solution of the relevant set of equations may yield results which are accurate, but cannot be considered as being general because they depend on the specific choice of the parameters. Repeating the simulations with several sets of the physically plausible parameters would add credibility to the conclusions drawn. The combination of the two approaches is highly recommended, i.e, if similar results are found by approximations and simulations, the validity of the conclusions is strengthened. Evidently, the comparison of these theoretical results to experimental effects is essential. In the present work we consider the occurrence of unusually high and unusually low values of the activation energy and the effective frequency factor. In particular, we can simulate a recently discovered behavior of TL in LiF:Mg, Cu, P at the ultra-high dose range and get qualitatively the main elements of the experimentally found results.