The decay of OSL signals as stretched-exponential functions

Abstract

Optically stimulated luminescence (OSL) and pulsed OSL have been utilized broadly for luminescence dosimetry as well as archaelogical and geological dating. It has been pointed out that in many cases, the decay of the OSL during continuous stimulating light (cw-OSL) and that of pulsed OSL following a pulse of stimulating light, do not generally behave according to a simple exponential function. In the present work, it is shown by the use of numerical simulation, that with the simplest model of a single trapping state and a single kind of recombination center, a decay curve significantly slower than a “normal” exponential can emerge. These results could be fitted to a stretched-exponential law, exp[−( t/ τ) β ] with 0< β<1 with surprisingly good agreement for the decay of OSL following a stimulating light pulse. As for the decay of OSL during the exposure to stimulating light, a typical behavior found in the simulation is an initial nearly exponential decay, followed by stretched-exponential decay at longer times. In particular, the cases where β is significantly smaller than unity (e.g., β∼0.5) are of interest. It is to be pointed out that several relaxation phenomena in complex condensed-matter systems have been found to follow the mentioned stretched-exponential decay law. This includes some reports in the literature of stretched-exponential decay of luminescence, usually in the very short time range. It has been suggested, however, that this behavior is always associated with some kind of disorder in the sample, e.g. the disorder occurring in porous silicon. The main new points in the present work are that this kind of relaxation can be expected to occur in the two kinds of OSL mentioned above and that they result from a single crystal with only one trapping state and one kind of recombination center. The concept of half-life of the decay in these cases is considered in view of the present results.