Towards a non-fading signal in feldspar: Insight into charge transport and tunnelling from time-resolved optically stimulated luminescence

Feldspars are an attractive alternative to quartz for extending the dose range, and for dating volcanic terrains such as on Mars and Iceland. Unfortunately, charge stored in the feldspar lattice undergoes anomalous fading leading to an underestimation in the dose estimates. In this paper we use the time-resolved optically stimulated luminescence (TR-OSL) technique to investigate the processes that give rise to the signal following infrared (IR), green and blue stimulation, with an objective to understand tunnelling and charge transport during thermo-optical excitations. We show that the TR-OSL shape is governed by the energy of excitation and the subsequent charge recombination route through the excited state of the trap, the band tail states or the conduction band. The role of band tail states in charge recombination is specifically examined using the signal shown to decay over several ms: we identify two dominant recombination routes, viz., phonon (0.05-0.06 eV) assisted diffusion, and quantum mechanical tunnelling, depending on the energy state of the detrapped electron. As would be expected, diffusion in the band tails is identical for both resonant and non-resonant excitations, where in the latter case the band tail state occupancy likely arises from thermalisation of conduction band electrons. The important outcome of this study is a comprehensive physical model based on a single dosimetric trap that successfully explains wide-ranging luminescence phenomena in feldspars, in particular, the luminescence efficiency and thermal partitioning of charge in different energy states and the subsequent recombination routes. The model predicts three different systematic approaches to preferentially sampling the most stable signal. We finally present evidence for a non-fading signal using one of these methods based on pulsed IR stimulation. (C) 2011 Elsevier Ltd. All rights reserved.