High resolution β — γ coincidence spectrometry at the UK CTBT Radionuclide Laboratory

Abstract

Detection of radioxenon is often considered the most probable indicator of an underground nuclear explosion. GBL15 is the UK’s Comprehensive Nuclear-Test-Ban Treaty Certified Radionuclide Laboratory, operated at AWE Aldermaston and has a history of developing high fidelity coincidence detection systems for particulate radionuclides. The Laboratory also operates a SAUNA II system, using NaI(Tl) and plastic scintillator detectors to measure β – γ coincidences from the decay of the four radioxenon isotopes, namely 133Xe, 135Xe, 131mXe & 133mXe. Here the efforts to date in exploring new technologies for next generation laboratory-based β – γ coincidence spectrometry for radioxenon measurements are discussed. Results are presented from preliminary measurements using a PIPSBox detector with a high purity germanium γ detector and the output compared to that of lower resolution systems. This investigation will be used to scope future programmes on the technology used for the measurement of radioactive noble gas nuclides at GBL15.

Production and measurement of fission product noble gases

“…To quantify the metastable isomer 133m Xe, the coin- correction for radioactive decay (see [13]). given in Table 3.…”

“…A network of highlysensitive detection systems, known as the International Monitoring System (IMS) use a process of sampling, purification and spectroscopy to monitor for atmospheric radionuclides that may be indicative of a nuclear explosion [2]. This includes four specific radioisotopes: 133 Xe (t 1/2 =5.2474(5) d [3]) and 135 Xe (t 1/2 =9.14 (2) h [4]), and 16 the long-lived isomers: 131m Xe (t 1/2 =11.962(20) d [5]) 17 and 133m Xe (t 1/2 =2.198 (13) d [3]).…”

“…Some newer systems have focussed on improving the gas-processing capability to collect and measure more xenon gas from the atmosphere in a shorter period [9]; other systems have included highresolution spectrometry setups which have the benefit of measuring electron-photon coincidences with much less signal interference than scintillator-based detector systems (see [10,11]). Previous work has shown how such systems can be configured and calibrated in a laboratory setting for measurement of radioxenon samples (see [13]).…”

“…electron-X-ray coincidence signatures, which are specific to the radioisotopes under study. A more detailed account of how the laboratorybased high-resolution coincidence system has been used for previous measurements is given in [13]. This project seeks to explore the use of electron-photon coincidence spectrometry to measure fission product gases that may As part of these current measurements, the decay half- for the 133 Xe and 135 Xe ground-state feeding transitions respectively [3,4].…”

“…133 Xe, 131m Xe, 133m Xe and 135 Xe. For full detail, refer to[13]. † Beta end-point energies associated with the decay to the lowest excited energy level in the daughter nucleus.…”