Measurements of the natural background of radioxenon isotopes in soil gas

Radioisotopes of xenon are used as indicators for an underground nuclear explosion (UNE), and it is essential to be able to distinguish a potential signal from an UNE from the natural background, i.e. from spontaneous fission of uranium. The natural occurring background of radioxenon isotopes have been studied in Sweden in three separate campaigns in the period 2019-2022. In total more than 100 samples were collected and analysed for radioxenon and especially ¹³³Xe was detected in most of the samples. This gives a good data set for understanding of the natural background of xenon in the soil. The extended sampling was performed with the goal to study any potential trend with different weather conditions, e.g. ambient pressure and rainfall, and during different seasons. The sampling method and logistics were optimized over the years to improve the sensitivity of the most short-lived isotope in this study, ¹³⁵Xe. One method was to shorten the time between collection and measurement which lead to the first detections of ¹³⁵Xe in the natural background of soil gas.