STATISTICAL HYPOTHESES AND ASSOCIATED THRESHOLDS FOR CTBTRELEVANT NUCLEAR EVENT SCREENING BASED ON RADIOXENON ACTIVITY RATIOS MEASURED IN THE INTERNATIONAL MONITORING SYSTEM RADIONUCLIDE NETWORK

Noble gas samples are routinely collected at radionuclide stations in the international monitoring system (IMS) noble gas network and the radioxenon activity concentrations in the plume of air passing over IMS stations are estimated accordingly. Anomalous radioxenon activities collected in the samples are mostly caused by emissions from nuclear facilities, but they could also indicate releases from an underground nuclear explosion (UNE). Nuclear events may be screened using the time evolution of isotopic activity ratios of CTBT-relevant radioxenon isotopes. The evolution goes from a detonation to release of an assumed UNE, through atmospheric transport, to sample collections and measurements at IMS stations. Event screening can be performed based on three kinds of plots: the four-radioxenon plot, twodimensional plots of activity concentrations of paired radioxenon isotopes, and evolution plots of isotopic activity ratios. Based on activity evolution of assumed release scenarios of UNEs as well as distributions of isotopic activity ratios measured at the IMS stations, the thresholds for event screening are updated with respect to the early release scenarios.