Sensitivity gains and quality control in radioxenon monitoring via laboratory Re-measurements

As a part of the Comprehensive Nuclear Test Ban Treaty (CTBT) the International Monitoring System (IMS), to monitor the compliance of member states to CTBT, is currently being established. The IMS consists of different monitoring techniques, of which the sampling and detection of radioactive noble gases, mainly radioxenon, is one. As a complement to the monitoring stations, a number of radionuclide laboratories are included in the IMS. Samples collected at a station can be sent to a laboratory for further analysis. However, due to the short half-lives of radioxenon isotopes, the benefits of conducting re-measurements in the laboratory are unclear, as the isotope may decay significantly during transport. The FOI facilities in Sweden host a SAUNA III IMS system and a radioxenon laboratory. The laboratory is equipped with a SAUNA Laboratory system for re-analysis of xenon samples collected using the IMS systems. To investigate both the advantages and limitations of laboratory re-measurements, several radioxenon samples from the IMS station are re-measured at the laboratory system, and compared to the SAUNA III results. This work shows that, in addition to ensuring quality control of the sampling and xenon purification process, laboratory re-measurements also provide a better detectability for most of the radioxenon isotopes. For ¹³³Xe and ^131mXe the detectability at the laboratory is as good as at the IMS station up to at least a week after the collection period, and can be improved further by increasing the measurement time at the laboratory. The higher sensitivity at the laboratory measurements also results in a better estimation of the isotopic ratios, which is important for classification of the detection.