Optically or thermally stimulated luminescence (OSL or TL) from feldspar, the most ubiquitous mineral in the Earth's crust is widely used in dosimetry and geochronology. Despite decades of research, the mechanism of OSL and TL dose underestimation arising from anomalous fading in feldspars remains poorly understood. It is commonly assumed that anomalous fading arises from the tunnelling of electrons from the principal trap, however, this causal mechanism has never been confirmed because of the lack of methods available to directly probe the trapped electrons. The recent discovery of infrared photoluminescence (IRPL), a radiophotoluminescence (RPL) signal produced by intra-defect resonant excitation in the electron trap, allows direct testing of this assumption. The unique coupled RPL-OSL physical system available in feldspar can provide insights into the dynamics of both electron and hole traps individually and, thus, improve our understanding of electron-hole recombination in this complex material. Here we use the differential IRPL signal (ΔIRPL), a quantity proportional to the number of electrons leaving the principal trap upon optical excitation, to directly measure the OSL recombination efficiency (luminescence emission per detrapped electron) for the first time. Our investigations on nine potassium(K)-rich natural feldspar samples, extracted from known-age sediments, suggest that there exist at least two competitive recombination pathways for a detrapped electron: one that produces Infrared Stimulated Luminescence (IRSL or IR-OSL) by electron-hole recombination at the blue-emission centre, and the other that leads to either luminescence quenching or emission in a different wavelength region. Contrary to the widely accepted anomalous-fading model, our results demonstrate that anomalous fading originates primarily from a change in the recombination efficiency, likely due to the athermal loss of holes at the IRSL (blue) recombination centre. A direct loss of electrons in the principal trap is relatively minor compared to the change in the recombination efficiency. Importantly, our new approach allows us to decouple the individual losses in the electron and hole traps and provide the very first direct insights into the fate of the electrons when they leave the trap.
|Journal||Journal of Luminescence|
|Number of pages||8|
|Publication status||Published - 2022|
- Anomalous fading
- Infrared photoluminescence (IRPL)
- Infrared stimulated luminescence (IRSL)
- Luminescence dating