Rapid dose rate estimation for trapped charge dating using pXRF measurements of potassium concentration

Abstract. Quantifying environmental radiation dose rates is an essential step in age calculation using trapped charge dating methods. A means of rapid dose rate estimation would therefore be useful for a variety of reasons, especially in contexts where rapid equivalent dose estimates are available. For instance, for informing sampling strategy, providing initial age estimates, or supporting portable luminescence studies. However, high-precision methods often used to calculate dose rates are typically time consuming and expensive and are impractical for such “range-finder” applications. Portable X-ray fluorescence (pXRF) offers a rapid means of measuring the potassium (K) concentration of sediment, although the other radionuclides typically used to calculate dose rates, uranium (U) and thorium (Th), fall beneath its detection limits at the quantities at which they are usually present in sediments. In this study, we investigate whether pXRF measurements of K concentration alone can be used to estimate total environmental dose rates. A large, global dataset of 1473 radionuclide samples is used to generate a set of regression relationships between (1) K concentration and external beta dose rate; (2) external beta and gamma dose rates; and (3) external gamma and alpha dose rates. We test the utility of these relationships by measuring the K contents of 67 sediment samples with independent, high-precision radionuclide data from a variety of contexts using pXRF. The resulting K concentrations are then converted to external dose rate estimates using these equations. A simplified set of attenuation parameters are used to correct infinite matrix dose rate estimates, and these are combined with cosmic ray and internal contributions to rapidly calculate total environmental dose rates for a range of theoretical, common luminescence-dating scenarios (such as 180-250 µm quartz that has undergone etching). Results show that pXRF can accurately measure K concentrations in a laboratory setting. The regression equations can predict external beta dose rates to a good degree of accuracy based on K content alone, whilst external gamma dose rates are predicted less accurately, and external alpha dose rates are predicted the least accurately. In combination, total estimated dose rates show good agreement with their counterparts calculated from high-precision methods, with 95 % of our results lying within uncertainties of ±10 % of unity for scenarios where the alpha dose rate contribution is assumed to be negligible. Whilst alpha dose rate contributions are predicted the least accurately, scenarios including an alpha component result in at least 80 % of predictions lying within uncertainties of ±10 % of unity. The use of simplified attenuation factors to correct estimated infinite matrix dose rates does not contribute significantly to resulting scatter. This study serves as a proof of concept that pXRF measurements, along with a set of regression equations and a simplified correction procedure, can be used to rapidly calculate range-finder environmental dose rates.