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Preliminary Experiments with a Triple-Layer Phoswich Detector for Radioxenon Detection
OREGON STATE UNIV CORVALLIS
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The International Monitoring System IMS employs radiation detectors to monitor atmospheric or underground nuclear weapons tests. These detectors should be able to detect ultra low concentrations of xenon radioisotopes in the atmosphere. Exploiting the fact that the most interesting xenon radioisotopes 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 betaCE and X-raygamma-ray are measured in separate detectors. Although 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 betagamma 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 betagamma 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.
APPROVED FOR PUBLIC RELEASE