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The majority of the energy in a nuclear explosion is released in the immediate blast and the initial radiation accounts. The remaining fraction is released through radioactive decay of the explosion's fission products and neutron activation products over a longer time span. This allows for the detection of a nuclear explosion by detecting the presence of residual decay. Radionuclide monitoring stations for detection of radioactive emissions to the atmosphere is thereby an important tool in the verification of compliance with nuclear disarmament treaties. In particular, the globally spanning radionuclide station network of the International Monitoring System (IMS) has been implemented for verification of the Comprehensive Nuclear-Test-Ban Treaty. High Purity Germanium (HPGe) detectors are workhorses in radionuclide monitoring. The detection of characteristic gamma rays can be used to disclose the presence of signature nuclides produced innuclear weapon tests. A particular development that has potential to improve the sensitivity of radionuclide monitoring is the coincidence technique where decaying nuclides that emit several coincident gamma rays can be detected at much smaller activity concentrations than with conventional gamma spectroscopy. In this project, dedicated gamma-gamma coincidence detectors are being developed, utilizing electronically segmented HPGe detectors. These detectors are expected to be highly sensitive to low-activity samples of nuclides that present coincident emissions of gamma rays. In this paper we present the concept, define performance parameters, and explore the performance of such detectors to a subset of radionuclides of particular CTBT relevance. In addition, we discuss the path forward in developing a next generation gamma-gamma coincidence spectroscopy system of segmented HPGe.

Under the AMC, a range of activities covering education, research and outreach are foreseen. One of them concerns education and the build-up of competence related to disarmament, and for that reason collaborative efforts have been ongoing during 2021 and 2022 to develop a PhD-level course in verification of nuclear test explosions, and to offer it during September-October 2022. The course has developed by Uppsala University and the Swedish Defence Reserach Agency (FOI) and corresponds to 7.5 credits. It is a cross-disciplinary course that spans over several disciplines. It introduces the participants to treaties and verification regimes governing nuclear weapons and it explains identification, calculation and analysis of signatures from nuclear weapon explosions. Furthermore, effort has been made to let the participants actively work with data collection, aggregation, analysis and with the interpretation and evaluation of data. The course includes also both a laboratory exercise on detection of radionuclides, and a project work in which the participants analyze a test explosion scenario and summarize their findings and conclusions in a manner very similar to how this is done in reality. This poster will describe the details of the course and its content. Since the course is planned to be offered just before this conference, we also hope to provide some information on its execution, as well as feedback from the participants.

We discuss a novel method to address the issue of nuclear safety of the Shelter Object in the New Safe Containment at Chernobyl Nuclear Power Plant, using ultra-sensitive radioxenon measurements to monitor the fission activity. The automatic radioxenon measurement system SAUNA QB, originally developed for nuclear explosion monitoring, could be well suited for the proposed method, and theoretical modelling indicates that the radioxenon activity concentration resulting from fission in the fuel containing material might be at detectable levels inside the New Safe Containment. A simplified sampling and measurement method was developed in order to get a first indication of the, still unknown, radioxenon activity concentration levels inside the shelter. Using this system, air was collected close to fuel containing material in the Shelter Object, and the activity of the collected air was measured. No radioxenon could be detected, but the result indicates an upper activity concentration limit of about one Bq/m3, which is inside the dynamic range for the SAUNA QB. Additional measurements are desirable to confirm the result. To further, and more thoroughly, investigate the feasibility of the proposed method, it is necessary to measure for a longer time period using more sensitive equipment, such as the SAUNA QB., Vi diskuterar en ny metod som har potential att öka säkerheten med avseende på kritikalitetsincidenter i sarkofagen som täcker den havererade reaktorn in Tjernobyl. Metoden använder sig av känsliga mätningar av radioaktiv xenongas för att övervaka fissionsaktiviteten. Det automatiska systemet för mätning av radioaktivt xenon, SAUNA QB, som utvecklats för att detektera kärnvapenprov, skulle kunna vara väl lämpad för detta ändamål. En teoretisk modell indikerar att de aktivitetskoncentrationer som skulle orsakas av fission i det nukleära materialet skulle kunna vara detekterbara.Ett förenklat system för insamling och mätning utvecklades för att kunna erhålla en första indikation på aktivitetsnivån för radioaktivt xenon i sarkofagen, något som fortfarande inte är känt. Ett prov från luft i anslutning till det fissila materialet samlades in och mättes med hjälp av detta system. Inget xenon kunde detekteras, men resultatet indikerar en maximal aktivitetsnivå på ca. 1 Bq/m3, vilket är inom det dynamiska mätområdet för SAUNA QB. Ytterligare mätningar behövs för att bekräfta resultatet.En mer noggrann förstudie kräver mätningar under en längre tidsperiod med hjälp avbetydligt känsligare mätutrustning som tex SAUNA QB.

A method is described to determine the depth distribution of buried activity offering an alternative to conventional soil sampling and laboratory analysis by in-situ measurements with small detectors inserted into the ground. As ad demonstration of the method small-scale variations in the distribution of Cs-137 in a riparian marsh in central-eastern Sweden are determined using lanthanum bromide detectors (LaBr3). The results show variations of the activity ranging between 400 and 2200 kBq/m^2 over an area not exceeding 350 m^2. The decrease of the average activity since 2003 coincides with the physical decay indicating that no net redistribution of activity into this part of the marsh has occurred during the inter weening years.

As part of a program investigating means to improve gamma-ray spectra obtained with CdTe detectors, we report results obtained with two 10×10×5 mm^3 CdTe detectors, one planar the other of quasi-hemispheric geometry. Applying a combination of cooling and pulse rise-time discrimination with the quasi-hemispheric detector, we obtain a resolution of 4 percent at 662 keV and a peak-to-valley ratio of 92 at a loss of approximately 25 percent of the counts in the full-energy region.

In case of a release of radioactive nuclides into the environment it is necessary to have reliable methods to estimate the potential effect on population and ecosystem. In this context the total activity deposited, the elemental composition and its depth distribution are of importance. An efficient in situ method to estimate the average contamination over larger areas (typically ≥1000 m^2) using high purity germanium (HPGe) detectors has been developed. The method combines simulation of photon transport with measured detector properties. Comparisons regarding the total activity of Cs-137 determined from gamma spectra recorded in situ to results from soil sampling are presented. Ground properties, topography and the process of deposition of anthropogenic radionuclides can lead to large fluctuations even over small distances. Large volume cadmium telluride (CdTe) detectors are interesting candidates for local (<1 m^2) in situ measurements but are hampered by poor charge collection. The possibility to improve the energy resolution for planar and hemispheric CdTe detectors has been investigated along with laboratory experiments investigating the possibility to determine the activity of a buried radioactive source. Results derived using a standard sodium iodide detector are presented as a comparison.

We are currently investigating the possibility to use CdTe detectors for in situ determinations of radionuclide concentration in soil. Buried activity can be reliably determined by a comparison of the count rate in the photo peak and the region between the photo peak and Compton edge. However, the pulse-height spectrum from CdTe detectors is severely deteriorated, due to poor charge collection, in particular for high gamma-ray energies. Our efforts have, therefore, been concentrated on improving the peak to valley ratio for such detectors. A simple, non-discriminating, algorithm for the analysis of output from two amplifiers with different shaping times is described. By means of this algorithm the peak-tovalley ratio for a small planar CdTe detector is improved by more than a factor of six compared to the uncorrected ratio without loss of efficiency.