Phototransferred thermoluminescence of calcite: Principles, analytical methods and mechanisms.

Calcite, as do all carbonates, derives from the (CO3)2- anion. Calcite produces intense thermoluminescence (TL) which is mostly attributed to presence of Mn2+ anions. The emission spectra of calcite [1] and carbonatite [2] consist of broadband features at lower temperatures and line structures at high temperatures. These wavelength multiplexed differences are attributed to the solubility of Mn within calcite [1]. Although there has been considerable progress in understanding mechanisms of TL in calcite, corresponding advance related to optical stimulation has been halting. In this study we combine the utility of optical stimulation with the facility of thermal stimulation to study the phototransferred thermoluminescence (PTTL) of calcite induced by 470 nm blue-, 525 nm green- and 405 nm illumination. The conventional glow curve measured during heating to 600oC has an indeterminate number of glow peaks only three of which stand out and are regenerated under phototransfer. These are the ones studied. PTTL time-response profiles, that is, the dependence of PTTL intensity on the duration of illumination, are analysed using three methods, namely, a matrix- based analytical model, by use of vector fields and by theoretical modelling. We also address attendant effects such as backscattering to donor traps and competition effects, where supposed donor electron traps suppress electron trapping at acceptor electron traps. The time-response profiles corresponding to blue and UV light illumination displays the archetype increase through a maximum whereas that induced by green light is drawn out and, in some cases, tends to saturation. Surprisingly, the PTTL induced from deep electron traps by 405 nm illumination counterintuitively increases monotonically. We consider the extent to which this behaviour reflects the influence of slow bleaching components at the higher end of the corresponding PTTL peak or the effect of emission over a range of excited states as predicted by the modified Orgel (1955) diagram rather than the principle 4P(4T1g) relevant here. The intensity of PTTL induced from deep electron traps increases with temperature of illumination with an activation energy of thermal assistance of 0.052 ± 0.002 eV and decreases at elevated temperatures with an activation energy of thermal quenching of 0.75 ± 0.06 eV. The long term behaviour of the PTTL as studied by stability theory shows unstable critical points. The dosimetric characteristics of the latter including pre-dose effects are described. [ABSTRACT FROM AUTHOR]