Abstract
Noble gas fission products are produced during a nuclear explosion and are the radionuclides most likely to escape an underground nuclear weapon test. Radioactive isotopes of xenon, known as radioxenon, are an especially good signature and as such, are monitored globally via the International Monitoring System (IMS) – the primary tool for the verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The most sensitive method for detection and quantification of radioxenon fission products is β−γ (including electron–photon) coincidence spectroscopy, as is used by the IMS. The United Kingdom (UK) CTBT-certified radionuclide laboratory (known as GBL15) is operated at AWE Aldermaston, on behalf of the UK Ministry of Defence. This laboratory uses β−γ coincidence spectroscopy to measure isotopes of radioxenon in gas samples from the IMS, as well as high-resolution γ-ray spectroscopy for high-volume air filters. Through the development of new detector systems, it is possible to improve the accuracy and sensitivity of radionuclide measurements. This thesis describes how new, high-resolution detection systems can be used to improve the sensitivity of measurements of noble gas fission products such as radioactive isotopes of xenon. Improved energy resolution means reduced interference between emissions of radionuclides and lower backgrounds which result in improved detection limits. Measuring radionuclides from the atmosphere is now more difficult due to a significant and dynamic atmospheric radionuclide background from the emissions of civil nuclear facilities; measurements of key radionuclides in the presence of such background signals is the way to improve the detectability of underground nuclear tests. Through this work, noble gas fission products have been produced from neutron induced fission of U-235 and fission product noble gases (Xe-133, Xe-135, Xe-133m, Xe-131m, Xe-135m, Kr-88, Kr-85, Kr-85m) have been measured on a novel high-resolution β−γ coincidence detector system. Measurements performed as part of this work have demonstrated the detection limit can be improved for key radionuclides such as Xe-131m and Xe-133m (<0.2 mBq). This work demonstrates how a detector system setup and configured at GBL15 can be used to better discriminate between background sources and a nuclear explosion, and proposes new detector systems for consideration in the future GBL15 process.