Beta dose variability and its spatial contextualisation in samples used for optical dating: An empirical approach to examining beta microdosimetry
Beta microdosimetry (BM) has long been considered a leading cause in the amount of spread that is observed within samples dated using optically stimulated luminescence (OSL). We examined two samples from the site of MacCauley's Beach, NSW, Australia that were considered to be influenced by BM but not quantifiably demonstrated. The beta dose rate (β-Dr ) environment of both samples was investigated using a series of methods that employ common laboratory equipment. Beta counting and inductively coupled plasma optical emission and mass spectrometry (ICP-OES/MS) measurements of constituent components (e.g., coarse sand quartz, fine sand heavy minerals, etc.) of the bulk sediment samples was used to assess beta dose rate (β-Dr ) variability within each sample. The result of this test showed significant variation in beta flux of sedimentary components, with the heavy mineral fraction of both samples having the highest β-Dr values (11.13 ± 0.40 Gy/ka, SP2; 22.03 ± 0.07 Gy/ka, SP5) of any of the measured components. Second, the spatial distribution of β-Dr was investigated using a geographic information system (GIS) analysis of sedimentary thin sections and portable x-ray fluorescence (pXRF) analysis of resin-impregnated sediments. The GIS results revealed a statistically significant non-uniform distribution of heavy mineral grains and associated hot- and coldspot regions. The pXRF results enabled the graphical display of a heterogeneous distribution of U, Th and K concentrations and also the resulting β-Dr map. However, the detection limit of the device meant that the lower β-Dr end of the distribution could not be resolved. It was concluded that BM plays a significant role in the OD of the single-grain OSL De distributions of those samples investigated, but could only be partially explained. It is recommended that characterisation of the beta dosimetric environment be carried out before assignation of BM is applied to De distributions.