The Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) relies on its International Monitoring System (IMS) to detect radionuclide releases, which can indicate potential nuclear tests. By using atmospheric transport modelling (ATM), the CTBTO aims to establish links between detecting stations and corresponding source locations. When detection is limited to a single event within a narrow time window or between neighbouring stations, operational analysis typically generates large possible source regions that require further refinement. Multiple detections offer a unique opportunity for a more detailed analysis, allowing advanced methods to be applied for more accurate identification of the source location.Recently, elevated levels of radioxenon were detected at multiple IMS locations in and around the Japanese region, including Takasaki, Wake Island, and the non-IMS system at Horonobe. These detections exceeded historical levels, emphasizing the need for a more detailed analysis. The dense network of measurement stations in this area presents an opportunity to explore advanced methods for source localization, reducing the uncertainty, and to discuss the implications of these findings on the monitoring of radioxenon isotopes.

The determination of activity concentrations of the CTBT-relevant radioxenon relies on a robust calibration method. A procedure is outlined using four radioxenon spikes for beta-gamma detector-systems with 4π geometry. Detection efficiencies of beta-gamma coincidences in the net count calculation method, including the interference matrix between radioxenon and radon, are determined by three measurement channels: beta singles, gamma singles and beta-gamma coincidences, without reference activity values.

The International Monitoring System (IMS), installed and maintained by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) with the support of States Signatories, is a global system of monitoring stations based on four complementary technologies: seismic, hydroacoustic, infrasound and radionuclide. One of the IMS radionuclide stations is located in Spitzbergen, the largest island of the Norwegian Svalbard Archipelago, which borders the Barents Sea and the Northern Atlantic Ocean. It has been demonstrated that signs of climate change are particularly noticeable in that region. As many other radionuclides observed in environmental measurements, 212Pb is always observed at IMS stations, in varying quantities. This is also the case for the IMS station RN49, Spitzbergen, where it can be demonstrated that the average concentration of the measured lead 212Pb increases. This is observable specifically October through December. This paper demonstrates the asset of IMS data to study climate change effects. Our conclusions are supported by global temperature anomaly data from NOAA’s Global Surface Temperature Analysis, covering the period 1850 to 2023.

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.

Radionuclide monitoring is one of the verification technologies of the global verification system of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). This global network of sampling stations senses the air 24/7 for suspect noble gases and/or particulates. For noble gases this task is non-trivial due to the ever-present and highly variable background levels of the four radioxenon isotopes that are relevant for CTBT monitoring. An extensive, global effort was initiated to better estimate the civil radioxenon background based on known sources and end up with a more reliable event screening. This challenge, called “1st Nuclear Explosion Signal Screening Open Inter-Comparison Exercise 2021,” provided an assessment of a chain of multilevel, multidisciplinary scientific analyses and built on three previous atmospheric transport modelling (ATM) Challenges. It’s a first since it explored integrating both ATM and radionuclide statistical expertise to characterize the detection, time, location, and source strength of an anomalous event. The exercise ran through 2022 and was a collaboration between participants from around the world who utilized a comprehensive pre-developed test data set based on explosion release scenarios, xenon measurements and emission inventories, and atmospheric transport data provided by the ATM software FLEXPART. The data set was composed of synthetic activity concentrations of the simulated nuclear explosion signals added to the radioxenon measurements at the International Monitoring Station (IMS). Three levels of participation were offered, requiring different areas of expertise: 1) ATM expertise only, where participants simulated radioxenon background time series at the 23 IMS stations to be used as input for screening synthetic radioxenon measurements based on a set of predefined statistical methods; 2) radionuclide expertise, where participants provided their own methods and results for detection, screening, and timing powers; and 3) higher-level ATM and statistical expertise, where, in addition to Level 2, results were provided for location and magnitude estimates for a few selected test cases. This paper gives a general overview of the exercise and provides highlights and discusses the key results.

The radioactivity measurements registered in the aftermath of the Fukushima accident by the International Monitoring System (IMS) network provide evidence of North-to-South air mass transport. On a smaller scale, similar evidence is also provided by the episodic releases of both radioactive particles and noble gas from civil nuclear facilities. These anthropogenic releases are collected by the stations of the IMS radionuclide (RN) network during normal routine operations. Atmospheric transport modelling (ATM) calculations enable identification of possible source areas of these radionuclide detections. It is demonstrated that the IMS stations located in the band between the Equator and the Tropic of Capricorn can occasionally be influenced by anthropogenic radioactive emissions from facilities located in the Northern Hemisphere. North-to-South air mass transport is however a rather rare - but observed - phenomenon. It is more frequently observed between January and April. ATM backward simulations show that the actual pathways of air masses depend on the prevailing wind direction and vary from station to station. In most cases air masses need less than 14 days (which is the maximum number of days for ATM operational backward simulations at the CTBTO) to be transported from the emission source and go across the Equator. However, for more isolated IMS stations, strongly influenced by trade winds like RN51 (Kavieng, Papua New Guinea) or RN26 (Nadi, Fiji), it can be demonstrated that this operational criterion is often not adapted as transport of air masses from release to collection points can be much longer. It is therefore recommended that the ATM backward simulations performed at the CTBTO are extended to at least 21 days in routine operations.

Radioxenon can be produced with a high fission yield during a nuclear explosion, making it an important tracer to demonstrate the nuclear origin of an explosion. For this reason, it is continuously monitored by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) as part of the verification regime. Radioxenon is emitted by civil nuclear facilities, like nuclear power plants (NPPs) or isotope production facilities (IPFs), providing significant but variable contribution to the noble gas background. The discrimination between CTBT-relevant radioxenon detections and the background is then a challenging task. This work aims at estimating the radioxenon background at 8 East Asian noble gas stations of the International Monitoring Systems (IMS) (out of 26 certified and 14 others foreseen) based on known sources and atmospheric transport modelling (ATM). For the purpose of this study, the transportable system in Mutsu, Japan, was also included. The results demonstrate a predominant contribution of NPPs to the radioxenon background at most of the East Asian IMS stations, especially during summertime. In autumn, as a result of large-scale atmospheric circulation, the contribution of remote IPFs starts to dominate. In the summertime, up to 80% of the Xe-133 detections at a station may be explained by contributions from NPPs. The detections even rise to 100% in some specific cases. At some stations under investigation in this study, a transition from NPP to IPF domination is observed in September and continues during the autumn season. It has also been shown that, for some stations, simulated concentrations above the detection limit may include observable contributions from up to 19 different sources per daily sample; at the same time the sample being sensitive to 80 or more possible sources of radioxenon. This indicates that the accumulation of many weak sources can lead to a measurable result in a single air sample. This might also explain observations at very remote stations. Another important conclusion is that, despite limited knowledge about release patterns of NPPs, the agreement between simulated and measured values was good in many cases. Availability of IMS measurements allowed for validation of simulations. This comparison revealed that approximately 76% of simulated values were underestimated. Based on the paired t-test, a 95% confidence interval for the true mean difference between measurements and simulations was constructed. It was estimated that for data dominated by NPPs contribution (i.e. NPPs contribution exceeds 70%), the overall uncertainty of simulated results lies between 0.07 and 0.10 mBq/m3. For data dominated by IPFs contribution (i.e. IPFs contribution exceeds 70%), the uncertainty for the simulations is in the range between 0.03 and 0.12 mBq/m3.

noble gas background measurement that develop means to distinguish the source of detected radioactive isotopes of the noble gas xenon (radioxenon) as either peaceful activities or test explosions. Although these new TXLs cannot be used for core CTBT verification reports, they nevertheless will enable more effective use of the IMS’s monitoring capacity. Four isotopes of radioxenon are particularly relevant to the detection of a nuclear explosion; the “smoking gun” evidence of whether a nuclear test explosion has occurred. TXLs observe the background level of radioxenon at different locations and in combination with Atmospheric Transport Modeling (ATM) help CTBTO understand its behavior.

After performing a first multi-model exercise in 2015 a comprehensive and technically more demanding atmospheric transport modelling challenge was organized in 2016. Release data were provided by the Australian Nuclear Science and Technology Organization radiopharmaceutical facility in Sydney (Australia) for a one month period. Measured samples for the same time frame were gathered from six International Monitoring System stations in the Southern Hemisphere with distances to the source ranging between 680 (Melbourne) and about 17,000 km (Tristan da Cunha). Participants were prompted to work with unit emissions in pre-defined emission intervals (daily, half-daily, 3-hourly and hourly emission segment lengths) and in order to perform a blind test actual emission values were not provided to them. Despite the quite different settings of the two atmospheric transport modelling challenges there is common evidence that for long-range atmospheric transport using temporally highly resolved emissions and highly space-resolved meteorological input fields has no significant advantage compared to using lower resolved ones. As well an uncertainty of up to 20% in the daily stack emission data turns out to be acceptable for the purpose of a study like this. Model performance at individual stations is quite diverse depending largely on successfully capturing boundary layer processes. No single model meteorology combination performs best for all stations. Moreover, the stations statistics do not depend on the distance between the source and the individual stations. Finally, it became more evident how future exercises need to be designed. Set-up parameters like the meteorological driver or the output grid resolution should be pre-scribed in order to enhance diversity as well as comparability among model runs.

After performing a first multi-model exercise in 2015 a more comprehensive and technically more demanding atmospheric transport modelling challenge was organized in 2016. Release data were provided by the Australian Nuclear Science and Technology Organisation radiopharmaceutical facility in Sydney (Australia) for a one month period. Measured samples for the same time frame were gathered from six International Monitoring System stations in the Southern Hemisphere with distances to the source ranging between 670 (Melbourne) and 13,500 km (Rio de Janeiro). Participants were encouraged to work with unit emissions in pre-defined emission intervals (daily, half-daily, 3-hourly and hourly emission segment lengths) and in order to perform a blind test actual emission values were not provided to the participants before submitting their results to the challenge organization team. Similar features were detected when comparing the results of the previous and the current multi-model exercise, despite the quite different settings, like the characteristics of the measured signals, the station distances to the source and the different climatological conditions at the individual monitoring stations. Especially, there is some evidence that using daily resolved emissions has no significant disadvantage compared to using higher resolved ones, even for stations located at a few hundreds of kilometres away from the source. Furthermore, an uncertainty of up to 20% in the daily stack emission data turns out to be acceptable for the purpose of a study like this. Finally, it became more evident how future exercises need to be designed in order to end up with generally accepted conclusions reaching beyond those for single test cases.

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.

In view of assessing natural radioactivity with on-site quantitative gamma spectrometry, efficiency calibration of NaI(Tl) detectors is investigated. A calibration based on Monte Carlo simulation of detector response is proposed, to render reliable quantitative analysis practicable in field campaigns. The method is developed with reference to contact geometry, in which measurements are taken placing the NaI(Tl) probe directly against the solid source to be analyzed. The Monte Carlo code used for the simulations was MCNP. Experimental verification of the calibration goodness is obtained by comparison with appropriate standards, as reported. On-site measurements yield a quick quantitative assessment of natural radioactivity levels present (40K, 238U and 232Th). On-site gamma spectrometry can prove particularly useful insofar as it provides information on materials from which samples cannot be taken.

The Compton spectrometer is a device in which radiation coming from an X-ray generator is scattered at a known angle in order to reduce the flux and to allow spectrometric measurements without saturating the detector. A small-size prototype Compton spectrometer usable in the range 20–150 keV was developed, using a pen-type NaI detector. The source spectrum is reconstructed by unfolding the measured spectrum, using the response matrix calculated by the Monte-Carlo method. The Monte-Carlo geometrical model was validated with gamma-ray point sources. Unfolding is performed using a modified version of the GRAVEL algorithm. Results are presented for the unfolded spectrum obtained with a test measurement on a medical X-ray device.

An inverse technique has been designed to unfold the x-ray tube spectrum from the measurement of the photons scattered by a target interposed in the path of the beam. A special strategy is necessary to circumvent the ill-conditioning of the forward transport algebraic problem. The proposed method is based on the calculation of both, the forward and adjoint analytical solutions of the Boltzmann transport equation. After testing the method with numerical simulations, a simple prototype built at the Operational Unit of Health Physics of the University of Bologna was used to test the method experimentally. The reconstructed spectrum was validated by comparison with a straightforward measurement of the X-ray beam. The influence of the detector was corrected in both cases using standard unfolding techniques. The method is capable to accurately characterize the intensity distribution of an X-ray tube spectrum, even at low energies where other methods fail.