Time and dose-rate dependence of TL and OSL due to competition between excitation and fading

During the excitation period of thermoluminescence (TL) and optically stimulated luminescence (OSL), competing effects of fading may take place, in particular in situations of long irradiation with a relatively low dose rate. In this work, we study, by the use of numerical simulations, this possible occurrence, using a model with two trapping states and one recombination center. The dependence on the time of excitation of the filling of the active trap, to which the TL or OSL signal is proportional, has been followed. Using plausible sets of trapping parameters, the solution of the relevant set of coupled differential equations revealed a time dependence of an increase up to a maximum value and then a decrease toward an equilibrium value where the rates of production and decay are equal. The equilibrium value reached by the simulations has been found to be consistent with a direct comparison of the excitation and the thermal decay rates. The results are somewhat similar to previous reports on non-monotonic dose dependence with two main differences. The previous model included two trapping states and two centers whereas the present one has only one center. Also, previously, the dependence was on the dose with no distinction whether its variation is by changing the dose rate of excitation or the length of irradiation. With the present work, the non-monotonic and final equilibrium behavior have been specific to the time dependence whereas when the dose was varied by changing the dose rate, the curve increased monotonically until it reached the saturation value. Similar results have been reached by a quasi-analytical method, using some plausible simplifying assumptions.