Design and Modeling of a Compton-suppressed Phoswich Detector for Radioxenon Monitoring

By measuring the concentration of four xenon radioisotopes in the atmosphere, the International Monitoring System (IMS) can verify nuclear weapons tests around the world. To measure ultra-low concentrations of radioxenons, many detection systems, such as the Automated Radioxenon Sampler and Analyzer (ARSA) and the Swedish Automatic Unit for Noble gas Acquisition (SAUNA), have been developed and are currently under field tests. These systems employ the beta/gamma coincidence technique to facilitate background rejection. Since all of these systems use two separate detection systems for detecting beta-particles and gamma-rays, the calibration process is usually a tedious task. A phoswich detector, however, can simplify radioxenon detection by measuring both radiations with a single detector. A phoswich detector with Compton suppression capability for measuring xenon radioisotopes has been designed and simulated. The expected performance of the phoswich detector in suppressing Compton interactions and shielding against background radiation was modeled using MCNPX Version 2.5.0. The Compton suppression mechanism is integrated into the phoswich design to effectively reduce the Compton continuum in 2D gamma/beta coincidence spectra and significantly improve the Minimum Detectable Concentration (MDC) of the xenon radioisotopes. The phoswich detector has been designed with three scintillation layers: a thin plastic scintillator layer to detect beta and conversion electrons, a CsI(Tl) crystal layer for detecting X-rays and gamma-rays and a BGO crystal, which surrounds the CsI(Tl) layer, to identify scattered photons and to shield the CsI(Tl) crystal against external gamma-ray background. 2010 Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies