Optical detrapping of charge from the 110°C quartz TL region

A series of experiments has shown that the mechanism responsible for the enhanced rate of decay under illumination of PTTL (photo-transferred thermoluminescence) in the ~110°C region is most likely to be optical detrapping of charge. It has been demonstrated that an alternative explanation for the decay, related to changes in acceptor site concentrations, is unlikely to be the cause. An optical half-life of ~600s is deduced for the ~110°C PTTL emission for the conditions described (420-560nm, 12 mW.cm stimulation). Introduction The aim of this work was to investigate the optical stability of electrons held in the quartz TL peak found at ~110°C. More specifically, the main concern was the issue of whether or not the decrease in PTTL as a function of OSL exposure time is due to optical bleaching (of the 110°C PTTL) or to a change in the concentration of available acceptor sites (such as the loss of luminescence centres or an increase in competition from non-radiative centres or electron traps) during optical exposure. Basis of experiments Following an exposure to the OSL stimulation light of 75s (at 20°C, stimulating with 2.2-2.9eV photons (420-560 nm) at ~12mW.cm, within a standard automated TL-DA-12 Ris∅ reader) the measurable OSL signal is generally depleted to a level of ≤1% of the initial intensity (I0). During such an OSL shinedown a proportion of the mobile charge is transferred to the TL trap responsible for the ~110°C PTTL emission (Aitken and Smith ,1988). As the OSL signal has been reduced to such a low level, any further light exposure after this point (75s) will add only a minute fraction to the PTTL already present. If following 75s OSL the excitation light is turned off, the thermal decay of the PTTL (at 20°C) can be measured by making TL measurements of individual aliquots following time t. A plot of PTTL versus t both with and without illumination will show the effect of the light exposure on the PTTL. Sensitivity change as a function of optical exposure time It is necessary to quantify the degree to which the apparent loss of PTTL upon optical exposure (after the initial 75s shinedown) is due to changes in the concentration of available acceptor sites. An attempt to indirectly measure such effects, as a function of OSL measurement time, was made using the ~110°C TL response. Individual aliquots of natural Chaperon Rouge (724g2) quartz (a sample chosen for its relatively strong TL / PTTL peaks at ~110°C), having a natural dose of ~12 Gy, were given ~0.1 Gy of beta dose and then immediately heated to 160°C (2°C.s) to measure the TL at ~110°C (this procedure will be termed the standard sensitivity test). The aliquots were then exposed to the OSL excitation light for a range of times at 160°C. Following the OSL shine, the standard sensitivity test was performed again, the rationale being that during OSL shinedown a proportion of the de-trapped electrons recombine at the available L-centres (luminescence centres), hence the OSL, and the OSL traps are substantially depleted. Therefore, following the OSL measurement, the probability of any free electron moving around the lattice recombining at an L-centre is lessened, (both because of the reduced concentration of available L-centres and the increased competition for charge from the empty OSL traps), which should be expressed as a loss of sensitivity. This assumes that electrons released from the OSL and 110°C TL/PTTL traps recombine at the same L-centres a view supported by the spectral emission work of Franklin et al. (1995). The results (shown in Figure 1) are interpreted to mean that the loss of luminescence centres following OSL shinedown (to < 1% of I0) is only a small fraction of the total, in the order of 9%. This is consistent with the findings of Bailey et al. (1997) who report an effectively constant charge transfer 8 Ancient TL vol. 15 No. 1 1997 rate throughout the OSL shine-down. There is a slight dose dependency in the results, showing less of a drop in sensitivity for aliquots dosed (see Figure 1 for details of administered doses) and preheated (220°C for 300s) prior to measurement. This can be tentatively interpreted in a number of ways (including thermal sensitisation via the mechanism described first by Zimmerman (Zimmerman, 1971) or there being an increased likelihood of trapping holes in the appropriate L-centres during dosing rather than trapping electrons at the OSL source traps). More work however is needed here and this is a relatively subsidiary point. 0.88 0.90 0.92 0.94 0.96 0.98 1.00