135Xe measurements with a two-element CZT-based radioxenon detector for nuclear explosion monitoring

Measurement of elevated concentrations of xenon radioisotopes (131mXe, 133mXe, 133Xe and 135Xe) in the atmosphere has been shown to be a very powerful method for verifying whether or not a detected explosion is nuclear in nature. These isotopes are among the few with enough mobility and with half-lives long enough to make their detection at long distances realistic. Existing radioxenon detection systems used by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) suffer from problems such as complexity, need for high maintenance and memory effect. To study the response of CdZnTe (CZT) detectors to xenon radioisotopes and investigate whether it is capable of mitigating the aforementioned issues with the current radioxenon detection systems, a prototype detector utilizing two coplanar CZT detectors was built and tested at Oregon State University. The detection system measures xenon radioisotopes through beta-gamma coincidence technique by detecting coincidence events between the two detectors. In this paper, we introduce the detector design and report our measurement results with radioactive lab sources and 135Xe produced in the OSU TRIGA reactor. Minimum Detectable Concentration (MDC) for 135Xe was calculated to be 1.47 ± 0.05 mBq/m3.