Investigation on Sample Association by using Anomalous Concentration Episodes and Decay-Consistent Isotopic Ratios at IMS Radionuclide Stations

Robin Schoemaker - CTBTO
Jolanta Kumierczyk-Michulec - CTBTO
Martin Ertl - CTBTO
Anne Tipka - CTBTO
Martin Kalinowski - CTBTO

For enhancement of the International Data Centre (IDC) products such as the Standard Screened Radionuclide Event Bulletin (SSREB), there is a need to associate the detections of CTBT-relevant nuclides in samples at the International Monitoring System (IMS) radionuclide stations with the same release to characterize its source for the purpose of nuclear explosion monitoring. Episodes of anomalous activity concentrations at the stations are the best first guess for being related to the same release. For multiple isotope observations, the consistency of their isotopic ratios in subsequent samples with radioactive decay is another plausible hint at coming from the same source. The basic approach assumes a puff release with short duration. This presentation explains the concept and provides statistics about the frequency of radioxenon samples fulfilling the sample association criteria. A case study is used to demonstrate the effectiveness of these two basic methods for sample association. We use the multiple observations at two IMS noble gas systems following late releases from the nuclear test announced by the DPRK in 2013. Atmospheric transport modelling (ATM) is applied to get further evidence and gain confidence in this case study for sample association by identifying the air masses that link the two distinct releases to two sets of multiple samples associated to the same release. Both sets together are related to the same nuclear fission event. We focused on the basic approach as well as the criteria for automatic sample association for the SSREB. This approach can be further refined to apply to more complex situations including contributions from multiple sources and time-extended releases. This is of relevance for expert technical analysis applied to observations of special interest.