Luminescence dating techniques determine the length of time a mineral has been buried by measuring the total radiation dose that mineral has acquired from surrounding sediments and cosmic rays from outer space. So when we collect samples in the field, it’s important to consider where that radiation is coming from, and whether nearby features in the sampling environment may deliver radiation at different rates.
Buried minerals absorb radiation from cosmic rays, gamma rays, beta particles and alpha particles, each penetrating the lithosphere to different extents. Cosmic rays can penetrate sediments to several meters and attenuate by ~14% per meter of ~2 g/cm3 sediment. Gamma rays can travel up to 30 cm, beta particles ~3 mm, and alpha particles, the most destructive particles of all, travel only ~25 µm (Aitken, 1998).
Because the sphere of influence of gamma rays is ~30 cm, we prefer to extract luminescence samples from areas where the composition and granulometry of the surrounding sediments is consistent (or homogeneous) within 30 cm of the sample. If it is not, the dose rate that we measure from the sample may not accurately reflect the dose rate of the surrounding sediments. Examples of ideal and non-ideal sampling sites are shown below.
Often times, especially in archaeology, the feature we’d like to date does not provide a homogeneous dose field for sampling. In these cases, it’s necessary to: i) subsample individual lithostratigraphic layers/features within 30 cm of the sample site individually to estimate their dose rates so that the total dose rate of the sample can be modelled, and/or ii) measure the gamma dose rate to the sample directly in the field using a portable gamma spectrometer.
Heterogeneous dose fields are not only possible at the scale of a sedimentary exposure, but also at the scale of a thin section. This microscale heterogeneity is caused by spatial variations in beta dose rates, or “beta microdosimetry”. Spatial variations in K-40 concentrations (e.g., from K-feldspar grains) or U and Th in heavy minerals may lead to beta microdosimetry (e.g., Martin et al., 2015; Jankowski & Jacobs, 2018). This likely contributes to the spread (or overdispersion) of De and age distrubutions from single grains.