A plastic scintillator and HPGe ß-γ coincidence detection system.

A network of specialist laboratories support the International Monitoring System (IMS) of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) with re-measurements of radionuclide samples, including xenon gas. The measurement of four xenon fission product radionuclides (133Xe, 135Xe, 131mXe and 133mXe) can be used to detect an underground nuclear explosion. Laboratories use a range of techniques to measure the radionuclides, including beta-gamma (ß-γ) coincidence spectrometry. These highly-sensitive measurements are capable of detecting concentrations of down to 500 atoms of 133Xe in a few cm3 of xenon. In some detector systems, detection of the metastable isomers (131mXe and 133mXe) can be more challenging due to interferences between the signatures of different radionuclides. Recent work has shown that using high-purity Germanium (HPGe) high-resolution gamma detectors, these interferences can be reduced, lowering the dependence of the detection limits on radionuclide sample isotopic composition. One downside of these detectors is the reduction in detection efficiency, which impacts the overall detection sensitivity; so assessing different detector systems is a priority for radionuclide laboratories. This work presents a coincidence detector system comprising of a plastic scintillator gas cell and a large-crystal high-purity germanium detector. The energy resolution, coincidence detection efficiency, MDA and interference factors are determined from measurements of synthetic radioxenon gas samples.

Next-generation particulate monitoring.

Operated by the Comprehensive Nuclear-Test-Ban Treaty Organisation, the International Monitoring System is used by almost 200 nations to monitor for nuclear weapons tests. The IMS is still under development, and the Comprehensive Nuclear-Test-Ban Treaty has not yet entered into force, however the radionuclide component has proved instrumental in radically changing both nuclear verification science and researchers' understanding of the dynamic global radiation background. After more than 20 years, the network is mostly complete, however the technology utilised for the particulate monitoring component remains practically the same, despite a number of laboratories developing coincidence systems that can offer orders of magnitude improvements in detection sensitivity and reliability. This paper describes the status of the technology, and the advantages of implementing this within the International Monitoring System. Furthermore, the performance of a prototype system developed by the Comprehensive Nuclear-Test-Ban Treaty Organisation is presented, and the implications of introducing this technology considered.

Improving the sensitivity and reliability of radionuclide measurements at remote international monitoring stations.

The Comprehensive Nuclear-Test-Ban Treaty is supported by a global network of monitoring stations that perform high-resolution gamma-spectrometry on air filter samples. The UK CTBT Radionuclide Laboratory has utilised cosmic veto systems to improve the sensitivity of measurements since 2010. During this study, a second detector system (with a cosmic veto) was deployed at the CTBT IMS station RN67, alongside the standard detector. This is an incredibly remote IMS station on the island of St Helena in the South Atlantic. A duplicate system was also tested at AWE to benchmark the remote systems performance. The cosmic veto system improved detection sensitivities by up to 10% across a range of radionuclides. As a system to re-measure samples 7 days after the primary measurement, detection sensitivities were improved by an order of magnitude, allowing a potentially crucial confirmation of signatures when timely transport to a laboratory is not feasible. Utilising the second detector in coincidence with the primary detector system (which would require reengineering of the shield), sensitivity improvements of up to two orders of magnitude can be achieved. These improvements are maintained even when the measurement takes place without any decay, potentially allowing a highly sensitive treaty measurement within 2 h of the end of collection.

Incorporating X-ray summing into gamma-gamma signature quantification.

A method for quantifying coincidence signatures has been extended to incorporate the effects of X-ray summing, and tested using a high-efficiency γ-γ system. An X-ray library has been created, allowing all possible γ, X-ray and conversion electron cascades to be generated. The equations for calculating efficiency and cascade summing corrected coincidence signature probabilities have also been extended from a two γ, two detector 'special case' to an arbitrarily large system. The coincidence library generated is fully searchable by energy, nuclide, coincidence pair, γ multiplicity, cascade probability and the half-life of the cascade, allowing the user to quickly identify coincidence signatures of interest. The method and software described is inherently flexible, as it only requires evaluated nuclear data, an X-ray library, and accurate efficiency characterisations to quickly and easily calculate coincidence signature probabilities for a variety of systems. Additional uses for the software include the fast identification of γ coincidence signals with required multiplicities and branching ratios, identification of the optimal coincidence signatures to measure for a particular system, and the calculation of cascade summing corrections for single detector systems.

A high-efficiency HPGe coincidence system for environmental analysis.

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) is supported by a network of certified laboratories which must meet certain sensitivity requirements for CTBT relevant radionuclides. At the UK CTBT Radionuclide Laboratory (GBL15), a high-efficiency, dual-detector gamma spectroscopy system has been developed to improve the sensitivity of measurements for treaty compliance, greatly reducing the time required for each sample. Utilising list-mode acquisition, each sample can be counted once, and processed multiple times to further improve sensitivity. For the 8 key radionuclides considered, Minimum Detectable Activities (MDA's) were improved by up to 37% in standard mode (when compared to a typical CTBT detector system), with the acquisition time required to achieve the CTBT sensitivity requirements reduced from 6 days to only 3. When utilising the system in coincidence mode, the MDA for (60) Co in a high-activity source was improved by a factor of 34 when compared to a standard CTBT detector, and a factor of 17 when compared to the dual-detector system operating in standard mode. These MDA improvements will allow the accurate and timely quantification of radionuclides that decay via both singular and cascade γ emission, greatly enhancing the effectiveness of CTBT laboratories.

Maximising the sensitivity of a γ spectrometer for low-energy, low-activity radionuclides using Monte Carlo simulations.

Monte-Carlo simulations have been utilised to determine the optimum material and thickness for a γ spectrometer to be used for the assay of radionuclides that emit radiation in the 50-300 keV energy range. Both HPGe and LaBr3(Ce) materials were initially considered for use, however the additional background radiation and lack of resolution in the latter drove the selection of HPGe for further optimisation. Multiple thicknesses were considered for the HPGe detector, with the aim of improving the sensitivity of the system by maximising the efficiency for low energy emissions, and reducing the probability of interaction with (and therefore the continuum from) higher energy photons. The minimum amount of material needed to achieve this was found to be 15 mm for a source that is dominated by high energy (>2.614 MeV) photons, and 20-30 mm for a typical reference source (with photons of energy 59.54 keV-2.614 MeV).