Radio-sulphur (35S) detection by LSC – How to deal with interfering natural radionuclides

Naturally occurring radio-sulphur (³⁵S) is suitable as an aqueous environmental tracer for the dating of groundwater and surface waters with residence times of less than one year. As a β-decaying radionuclide, ³⁵S is detected using liquid scintillation counting (LSC). When extracting ³⁵S as sulphate from large-volume water samples, there is the possibility of unintentional co-extraction of other naturally occurring radionuclides, which interfere with the measurement of ³⁵S by LSC. The most important of these radionuclides are (i) ²²⁶Ra, (ii) the short-lived progeny of ²²²Rn, (iii) ²¹⁰Pb and its progeny and (iv) ³H. In addition, ¹⁴C, which might be present in scintillation cocktails or LSC plastic vials, and ⁴⁰K, which is likely to be present in LSC glass vials, can have a significant impact on the LSC detection result. There are a few publications that address sample preparation for ³⁵S detection with LSC. However, the published datasets do not contain sufficiently detailed information to pursue the issue of potential interferences of the said naturally occurring radionuclides with the ³⁵S signal. In our study, we measured standardized samples containing the said radionuclides by LSC, evaluated location, shape and overlap of the associated energy peaks, and assessed the possible influences of the individual nuclides on the ³⁵S detection results. The findings of our study show that when measuring ³⁵S obtained from a natural water sample, counts detected in the ³⁵S energy window cannot be unconditionally interpreted as actual ³⁵S counts. Interfering nuclides (especially ³H, ¹⁴C, ⁴⁰K, and ²¹⁰Pb) can lead to counts in the ³⁵S energy window and thus to an overestimation of the ³⁵S activity concentration of the water sample (and consequently to an underestimation of the water age). We therefore recommend generally a complete evaluation of the LSC spectrum in order (i) to be able to infer the potential presence of other radionuclides in the measured sample, and (ii) to evaluate the DPM counted in the ³⁵S energy window accordingly, as these may contain counts of the aforementioned interfering radionuclides.