Radioxenon observations

Radioxenon detections and non-detections made by the International Monitoring System were used for the inverse modelling. In order to confine the possible source regions of the radioxenon detections as much as possible, many observations should be used, since each detection or non-detection carries some information. However, detections can be contaminated by the radioxenon background from civilian emitters. It is challenging to estimate the correct radioxenon contribution from civilian emitters for every individual sample^27,28 (especially if not all civilian sources of radioxenon in the region are known). This highlights the importance of understanding the radioxenon background and reducing radioxenon emissions from civilian sources²⁹. With the above considerations in mind, we selected all ¹³³Xe observations taken between 5 April 2013 and 15 April 2013 for the IMS noble gas stations RN20, RN38, RN45 and RN58 (see Fig. 2 for their location). In total, 57 observations were used for the inverse modelling (Supplementary Information).

The IMS noble gas stations RN20 and RN45 are equipped with the Spalax system^30,31 and measure radioxenon activity concentrations every 24 h, whereas RN38 is equipped with the Sauna system³² and measures radioxenon activity concentrations every 12 h. Finally, RN58 is equipped with the Arix system and measures radioxenon activity concentrations every 12 h³³.