Isotopic activity ratios of radioxenon measured in IMS noble gas samples are routinely obtained that might indicate a gas release from an underground nuclear test, although actually caused by atmospheric emissions from nuclear facilities. A robust method is required that tests the isotopic activity ratios of samples of special interest against a set of all relevant release scenarios that could possibly explain the source. This presentation treats nuclear explosions as the source. The method presented here combines the two ends of the lifetime of radioxenon isotopes and their activity ratios. One end is the radioisotope generation by a nuclear explosion, the other end is their measurement in IMS samples. Mathematical modelling is used to create the relationship between both ends. Recent research results on the source mechanisms including in-growth and decay, cavity-melt fractionation and seepage of cavity gases are used to develop best-estimate input source terms as well as minimum and maximum activity ratio boundaries as a function of time. The ratios in the IMS sample are reconstructed as a function of the time of release into the atmosphere and the sampling time. The output is a score for the consistency of a set of scenarios with the observation.
Investigating IMS noble gas samples for possible nuclear test signatures requires all possible underground nuclear test scenarios to be considered. This presentation shows how the aggregated signature of all scenarios can be connected with IMS observations of a specific event.