Preliminary Experiments with a Triple-layer Phoswich Detector for Radioxenon Detection

The International Monitoring System (IMS) employs some special radiation detectors to monitor atmospheric or underground nuclear weapons tests around the world. These detectors should be able to detect ultra low concentrations of xenon radioisotopes in the atmosphere where the background level is relatively high. Exploiting the fact that the most interesting xenon radioisotopes (Xe, Xe, Xe, and Xe) emit a beta or conversion electron (CE) in coincidence with an X-ray or gamma-ray, these detectors have been designed and optimized to record coincidence events from the radioxenon isotopes. The IMS currently uses detection systems in which beta/CE and X-ray/gamma-ray are measured in separate detectors (as in the Automated Radioxenon Sampler/Analyzer [ARSA] system developed at Pacific Northwest National Laboratory [PNNL]). Although experimental results show that the ARSA system is able to detect very low concentrations of radioxenon, its complexity makes the beta and gamma-ray energy calibration very difficult. Phoswich technology, accompanied by digital signal processing of photomultiplier tube (PMT) pulses, can simplify radioxenon detection. If well designed, like other current sensitive radioxenon detectors, a phoswich detector is also capable of detecting beta/gamma coincidence events using digital-pulse-shape analysis. We have designed a two-channel triple-layer phoswich detector for radioxenon detection. Each phoswich detector consists of three scintillation layers: a thin plastic (1.5mm) scintillator for detection of beta and CE, a CaF2 layer (2mm) for X-ray detection and a NaI layer (25.4mm) for gamma-ray measurement. A two-channel FPGA-based Digital Pulse Processor, DPP2.0, (250 MHz, 12 bits) has been designed and constructed for capturing and transferring valid phoswich pulses to the PC. A graphical user interface (GUI) also has been developed to control the DPP2.0, digitally analyze phoswich pulses, and reconstruct the 2-D beta/gamma coincidence spectra. In this paper, our digital pulse shape discrimination technique, the DPP2.0, and the GUI are introduced. At the end, our preliminary measurements with the prototypic phoswich detector are discussed. 2008 Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies