Consecutive radioxenon detections as a trigger for further analysis

The prevalence of isotopes of radioxenon in the atmosphere poses a problem for the International Monitoring System (IMS) of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The atmospheric radioxenon background has accumulated due to emissions from civil nuclear facilities and as a result, the IMS frequently detects isotopes that might be considered a signal of a nuclear explosion. The UK National Data Centre (NDC) at the Atomic Weapons Establishment (AWE) analyses all data from the IMS radionuclide network and through a new ‘event analysis’ pipeline, works to determine the source of each detection of interest. The pipeline consists of sample screening, sample association and source reconstruction methods. There are various methods to determine which detections are worthy of further analysis, such as activity concentration magnitude, number of isotopes detected, isotopic activity ratios or consecutive detections. Once the detections have been identified, atmospheric transport and dispersion modelling (ATDM) simulations can be used to identify and characterise the source. Not all sources are known to the Treaty-verification community so work to identify new emitters and their impact on the IMS is critical to the international effort to monitor for nuclear explosions. This work presents a study of the phenomenon of consecutive ${}^{\mathrm{133}}$Xe detections (here referred to as ‘plumes’), which are frequently identified on the IMS. We consider the likelihood of a plume from various radionuclide release scenarios and conduct an analysis of a database of IMS measurement data, using the outputs of the automatic Radionuclide (RN) and Event Analysis Pipelines.