CTBT radioxenon monitoring for verification: today’s challenges

The Preparatory Commission of Comprehensive Nuclear Test-Ban-Treaty Organization is setting up a global network capable to monitor treaty compliance. Specific monitoring systems and methodologies that match the needs of the International Monitoring System (IMS), namely to clarify the nuclear character of suspect explosions, had to be developed for monitoring purposes during the last decade. Four xenon isotopes, namely 133Xe, 135Xe, 133mXe and 131mXe play a key role here. A complex background from medical isotope production facilities and nuclear power plants, varying over four orders of magnitude, challenges the system’s capability to distinguish these from treaty-relevant events. Available measurement data are compared with model calculations. The importance of atmospheric transport modelling is demonstrated both for completely understanding the civilian background and for explaining peak concentrations and abnormal events. New methodologies for backtracking nuclide detections improved the capability to locate sources and corroborate the role of radioxenon monitoring.