Overview of temporary radioxenon background measurement campaigns conducted for the CTBTO between 2008 and 2018.

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) specifies that an overall network of at least 40 International Monitoring System (IMS) stations should monitor the presence of radioxenon in the atmosphere upon its entry into force. The measurement of radioxenon concentrations in the air is one of the major techniques to detect underground nuclear explosions. It is, together with radionuclide particulate monitoring, the only component of the network able to confirm whether an event originates from a nuclear test, leaving the final proof to on-site inspection. Correct and accurate interpretation of radioxenon detections by State Signatories is a key parameter of the verification regime of the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). In this context, the discrimination between the highly variable radioxenon background generated by normal operations of nuclear facilities and CTBT-relevant events is a challenging, but critical, task. To this end, the radioxenon background that can be expected at IMS noble gas systems must be sufficiently characterized and understood. All activities conducted to study the global radioxenon background are focused on the calibration and performance of the verification system as described in the Treaty. The unique CTBTO noble gas system network is designed to optimally covering the globe. By the end of 2019, 31 systems were put in operation, 25 of which being already certified. It took two decades from the first experimental setup of noble gas system in the field to reach this stage of maturity. In the meantime, it was an urgent need to gain empirical evidence of atmospheric radioxenon concentrations with the full spectrum of characteristics that IMS noble gas systems may be observing. This experience was significantly advanced through temporary measurement campaigns. Their objective was to gain the additional necessary knowledge for a correct understanding and categorization of radioxenon detections. The site selection for these campaigns put emphasis on regions with low coverage by the initially few experimental noble gas systems at IMS locations or where potential interferences with normal background might be observed. Short-term measurements were first initiated in 2008. Sites of potential interest were identified, and campaigns up to few weeks were performed. Based on the findings of these short campaigns, transportable systems were procured by the CTBTO. Longer temporary measurement campaigns were started afterwards and operated by local hosts in different regions of the globe. Site selections were based on purely scientific criteria. Objectives of the measurement campaigns were continually reassessed, and projects were designed to meet the scientific needs for radioxenon background understanding as required for nuclear explosion monitoring. As of today, several thousands of samples have been collected and measured. Spectra of temporary measurement campaigns were (and are still) analysed in the International Data Centre (IDC). As they are not part of the CTBT monitoring system, no IDC product is generated. Analysis results are stored in a non-operational database of the CTBTO and made available, together with raw data, to authorized users of States Signatories through a Secure Web Portal (SWP) and to scientific institutions for approved research projects through a virtual Data Exploitation Centre (vDEC) after signing a cost-free confidentiality agreement (https://www.ctbto.org/specials/vdec). This paper aims at providing an overview of the temporary measurement campaigns conducted by the CTBTO since the very first field measurements. It lays out scientific results in a systematic approach. This overview demonstrates the asset of radioxenon background measurement data that have been collected with a wide variety of characteristics that may be observed at IMS stations. It bears a tremendous opportunity for development, enhancement and validation of methodologies for CTBT monitoring. In 2018, a campaign started in Japan with transportable noble gas systems in the vicinity of the IMS station RN38 in Takasaki. It will be described separately once the measurements are completed.

Classification of radioxenon spectra with deep learning algorithm.

In this study, we propose for the first time a model of classification for Beta-Gamma coincidence radioxenon spectra using a deep learning approach through the convolution neural network (CNN) technique. We utilize the entire spectrum of actual data from a noble gas system in Charlottesville (USX75 station) between 2012 and 2019. This study shows that the deep learning categorization can be done as an important pre-screening method without directly involving critical limits and abnormal thresholds. Our results demonstrate that the proposed approach of combining nuclear engineering and deep learning is a promising tool for assisting experts in accelerating and optimizing the review process of clean background and CTBT-relevant samples with high classification average accuracies of 92% and 98%, respectively.

New evaluated radioxenon decay data and its implications in nuclear explosion monitoring.

This work presents the last updated evaluations of the nuclear and decay data of the four radioxenon isotopes of interest for the Comprehensive Nuclear-Test-Ban Treaty (CTBT): Xe-131 m, Xe-133, Xe-133 m and Xe-135. This includes the most recent measured values on the half-lives, gamma emission probabilities (Pγ) and internal conversion coefficients (ICC). The evaluation procedure has been made within the Decay Data Evaluation Project (DDEP) framework and using the latest available versions of nuclear and atomic data evaluation software tools and compilations. The consistency of the evaluations was confirmed by the very close result between the total available energy calculated with the present evaluated data and the tabulated Q-value. The article also analyzes the implications on the variation of the activity ratio calculations from radioxenon monitoring facilities depending on the nuclear database of reference.

Setting the baseline for estimated background observations at IMS systems of four radioxenon isotopes in 2014.

Worldwide monitoring of radionuclides is an essential part of the verification system of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) as it can provide a direct evidence of the nuclear nature of an explosion. In the case of underground nuclear testing, the radioactive noble gases, specifically radioxenon, have the highest probability to escape to the atmosphere. The detection capability of the CTBT noble gas network, which is being built, is weakened due to the presence of a worldwide civilian radioxenon background. Improving the understanding and knowledge of civilian radioxenon sources and their impact on the noble gas systems background is crucial to strengthen their verification capabilities. Two major civilian radioxenon sources have been identified in past research, namely: Medical Isotope Production Facilities (MIPFs) and Nuclear Power Plants (NPPs). In this study, a 2014 baseline radioxenon emission inventory is proposed for all four CTBT relevant radioxenon isotopes (Xe-131m, Xe-133m, Xe-133 and Xe-135) on the basis of a literature review for both the Medical Isotopes Productions Facilities and Nuclear Power Plants. This 2014 baseline radioxenon emission inventory relies on peer-reviewed information on the facility location and corresponding radioxenon emission. The baseline radioxenon emission inventory is used along with Atmospheric Transport Modelling (ATM) to estimate the radioxenon activity concentrations at the noble gas systems. The results reveal the complexity and the geographical dependence of the civilian radioxenon background. The estimations are compared to the observations for CTBT noble gas systems that were operational in 2014. It is demonstrated that the estimated Xe-133 activity concentrations are, for most systems, in the same order of magnitude than observed detections. Non-detections of Xe-131m, Xe-133m, Xe-133 and Xe-135 are, for most samples, well reproduced by the estimation. To our best knowledge, this study is the first attempt to propose, a baseline emission inventory for all four CTBT relevant radioxenon isotopes and compare the estimated Xe-131m, Xe-133m, Xe-133 and Xe-135 activity concentrations with all observations at CTBT noble gas systems during the full 2014 calendar year.