Thermal quenching of luminescence processes in feldspars

The technique of optically stimulated luminescence has important uses in the dose evaluation of irradiated feldspars. The luminescence process involves the eviction of electrons from donor traps, charge transfer through the conduction band, and recombination at acceptor sites; each of these processes is, in general, thermally dependent, and leads either to enhancement or quenching of the luminescence with increasing temperature. Previous studies have measured the combined thermal activation characteristics of all three processes, and show a strong dependence on stimulation energy. In this article, an initial attempt is made to isolate only the recombination part of the luminescence cycle, and determine its thermal characteristics separately. A variety of luminescence transitions are examined in a range of both alkali and plagioclase feldspars; three distinct emission types are identified, which display very different behaviour. The first involves the internal transitions of common transitions metal ions. The second is typical of centres not displaying excited states within the band gap that are likely to arise from direct recombination between the conduction band and the ground state of the defect. The third category is the most difficult to analyse since it involves irreversible changes in the luminescence characteristics with increasing temperature; we consider these to be due to thermally-induced destruction or creation of luminescence centres. Most of the alkali and plagioscale feldspars studied showed such an effect, to a greater or lesser extent, and the process complicates analysis of the reversible sensitivity changes.