Detecting and quantifying uranium-series disequilibrium in natural samples for dosimetric dating applications

Isotopic fractionation is a process that may occur in natural aquatic environments. Samples originating from these open-system environments may contain radioisotopes with activity ratios between daughter and mother nuclide being not at unity. For the purpose of dosimetric dating, small deviations from unity may have negligible effect, but the mobility of 226Ra and 234U over longer time scales affects the accuracy of the dating result. Here we use the Canberra LabSOCS model in association with a standard 60 mm diameter BEGe detector in a 50 mm thick graded lead shield to carry out gamma spectroscopic measurements on the 238U decay series in a series of environmental samples. These samples were characterised in terms of density, filling height and chemical composition and peak interferences were corrected using the gf3 software. The model calculations have been shown to be in agreement with experimental transmission measurements taken with point sources and with results from standard IAEA materials and through a comparison with ICP-MS measurements. The results presented show excellent agreement between outputs from model and independent control measurements. This suggests that the method presented here is robust and allows quantifying and correcting isotopic fractionation using readily available gamma spectrometric setup and software.