Towards quantifying beta microdosimetric effects in single-grain quartz dose distribution

Abstract

In luminescence dating, the single grain approach offers a promise for identification of the most bleached grains for age calculations. A proper interpretation of single grain dose distribution however is still awaited. Ideally the palaeodose distribution should be a sharply peaked distribution with its width determined by the experimental errors. However, a large range of values of the relative standard deviation (RSD), even for well-bleached samples, indicates the presence of additional effects, not considered so far in the literature. We suggest that microscopic fluctuations in the spatial distribution of feldspar containing ⁴⁰K β emitters (termed as hotspots) can cause heterogeneous distribution of dose rate. This paper models these fluctuations and quantifies their effect on the distribution of doses in quartz. In this approach the dose to a quartz grain from a given spatial configuration of the hotspots within a sphere of maximum beta range is computed and this is then averaged over all possible configurations, to arrive at the dose rate/palaeodose distribution. The dose distribution is positively skewed. The net effect of such a distribution is estimated as a function of potassium concentration (lower the potassium, higher the hotspot heterogeneity and larger is the palaeodose distribution). For well bleached samples with low potassium content, the computations suggest that: (a) the probability of grains receiving zero doses is negligible and (b) distribution of doses is large. We suggest a minimum dose concept through a percentile analysis of the distribution function, viz., as the lowest doses received by a small fraction of the grains. This study therefore may imply a paradigm shift in the manner, in which the ages are computed from single grain paleodose distributions.