Beta dose heterogeneity in sediment samples measured using a Timepix pixelated detector and its implications for optical dating of individual mineral grains

Abstract

Single-grain optical dating has been widely used to estimate depositional ages for Quaternary sediments. An understanding of the sources influencing the dispersion of equivalent dose (D e ) estimates from individual grain measurements is essential for accurate age determination. Beta microdosimetry (i.e., beta dose deposition at the sub-millimetre scale) is a known cause of spread in single-grain D e values, so the detection and assessment of beta dose rate ( D ˙ β ) variation is important to properly interpret these data. Here we demonstrate the application of Timepix, a pixelated detector that directly measures in situ beta emissions based on cluster analysis, to determine the beta microdosimetry of natural sediment samples from a cave deposit in Russia and a sand dune in Australia. We describe a Timepix measurement and data processing procedure for natural sediments, and establish sample-specific calibration curves using associated sample radioelement concentrations to convert Timepix count rates into estimates of D ˙ β . On the basis of the Timepix analysis, a 2D ‘heat map’ of D ˙ β at sub-millimetre resolution was obtained for each sample. Our results show that the D ˙ β estimates are heterogeneous and their non-uniformity gives rise to 9 ± 4 to 26 ± 5% overdispersion in the single-grain D e distributions for the samples examined here. We discuss the likely sources of D ˙ β heterogeneity, based on micromorphological investigations of these sediment samples, which include a variety of materials present in natural deposits associated with D ˙ β ‘hot’ and ‘cold’ spots. A comparison of the D ˙ β dispersion with that of the corresponding single-grain D e values shows that the scatter among the latter can be fully or partly explained by beta microdosimetry and other known sources of overdispersion. • Timepix pixelated detector used to measure beta dose rates of natural sediments. • Measurement and data analysis procedure proposed and tested on four samples. • Spatially-resolved ‘heat map’ of beta dose rates obtained at sub-millimetre resolution. • Beta heterogeneity gives rise to ∼9–26% overdispersion of single-grain D e values. • Beta heterogeneity can fully or partly explain overdispersion of single-grain D e values.