Changes in natural OSL sensitivity during single aliquot regeneration procedure and their implications for equivalent dose determination

Abstract

Measurement of low temperature (90ºC-120ºC) Thermoluminescence (TL) sensitivity of natural quartz samples subjected to pre-heating and optical stimulation indicate that significant sensitivity changes can occur during measurement of the natural Optically Stimulated Luminescence (OSL). During the measurement of natural signal, the luminescence sensitivity of samples can change by 40%. The sensitivity changes both during the initial preheat and the measurement of natural OSL. The currently used version of Single Aliquot Regeneration (SAR) protocol measures and corrects for the sensitivity changes after preheat and readout of natural OSL. However, it does not take into account the changes in sensitivity during the readout of the natural signal. We therefore developed a correction procedure so that both the natural and the regenerated OSL intensities can be measured and plotted with the same sensitivity and suggest that in the absence of such a correction, a considerable fraction of the SAR based ages could have systematic errors. The correction for the sensitivity is based on the use of sensitivity of 110ºC TL quartz peak, which is correlated to OSL signal (Murray and Roberts, 1998). The use of 110ºC peak provides a reasonable measure of the changes in OSL sensitivity of quartz. A modified Natural Sensitivity Corrected-SAR (NSC-SAR) procedure, that comprises the measurement of, 1) the TL intensity of 110ºC peak for a test dose on sample as received (i.e. natural sample) and, 2) the sensitivity of the 110ºC peak of the same sample after the preheat and read out of the natural OSL, is proposed. This ratio, termed as Natural Correction Factor (NCF), then provides a way to correct for sensitivity changes. Results on samples from diverse depositional environments indicated that the NSC-SAR consistently (without exception) provided improved distribution in paleodoses i.e. a lower scatter compared to the standard SAR protocol. In addition, the use of this protocol also resolved anomalous cases where the intensity of natural OSL was significantly above the saturation intensity of the regenerated OSL. Implicitly, this study implies a caution on the use of palaeodoses obtained from single grains as such a correction is not possible in the currently used automated single grain OSL measurement systems. The only way now on will be to analyze aliquots with only a grain on them.

Residual doses and sensitivity change of post IR IRSL signals from potassium feldspar under different bleaching conditions

“…Negligible difference for sensitivity decrease is observed between PB and WB after the first cycle, suggesting that the sensitivity decrease mainly occurs in the initial measurement cycles, especially between the first and the second cycles. Sensitivity change from the measurement of natural signal to immediately following T x measurement has also been reported for quartz OSL signals (Singhvi et al, 2011).…”

“…Sensitivity changes have been investigated during OSL measurement of natural signal in the quartz SAR procedure (e.g. Murray and Wintle, 2000), and it has been reported that sensitivity correction does not take into account the changes in sensitivity that could happen during the readout of the natural signal (Singhvi et al, 2011). It is noted that the luminescence mechanism for these two minerals is significantly different, but for reliable sensitivity correction to be met in the SAR protocol, possible sensitivity changes between L n and T n measurement must be carefully considered for both minerals.…”

Distribution in SAR palaeodoses due to spatial heterogeniety of natural beta dose

“…3) the maximum spread in mean of palaeodose distribution of aliquots due to fluctuation in the spatial distribution of beta emitters. 4) ratio of maximum to minimum palaeodoses, which can be used to identify the spread in palaeodoses that arises exclusively due to spatial fluctuation of beta emitters, such that any spread beyond this can be attributed to other factors such as incomplete bleaching and sensitivity changes (Galbraith, 1990;Singhvi et al, 2011). In the following, we quantify the spread in palaeodoses using statistical parameters such as mean, standard deviation and the ratio of minimum to maximum value.…”

“…In luminescence dating of sediments, the Single Aliquot Regeneration (SAR) protocol permits palaeodose measurement using single grains and single aliquots (having several grains), (Murray and Wintle, 2000;Singhvi et al, 2011). Several aliquots/grains of a given sample are analyzed to obtain a statistically rigorous palaeodose.…”

Modelling dose rate to single grains of quartz in well-sorted sand samples: The dispersion arising from the presence of potassium feldspars and implications for single grain OSL dating

“…Our study has not considered issues related to changes in OSL sensitivity of the grains between 593 nature and laboratory irradiations; any such changes can be a source of additional intrinsic over-594 dispersion in D e populations (undetected by gamma dose recovery experiments; see, e.g., Stokes,595 1994a, b; Singhvi et al, 2011). But given that there is a satisfactory agreement between the observed 596 OD and the predicted OD (from intrinsic OD and beta dose rate variation), we think this sensitivity 597 change effect must be negligible.…”