Low Level Radioactive Xenon Monitoring by Phoswich Detector System

76252S The detection of dilute radioactive xenon isotopes in the atmosphere is an important component of both national and international efforts to detect clandestine underground nuclear weapons tests. Pacific Northwest National Laboratory has developed an Automated Radioxenon Sampler/Analyzer (ARSA) that concentrates Xe from the atmosphere and then counts it using a nuclear beta-gamma coincidence counter. Although extremely sensitive, this system employs 12 photomultiplier tubes to count 4 sample cells and requires far more complex calibration and gain stabilizations efforts than would be compatible with remote, unattended operations. Therefore, this project will construct a composite ¿Phoswich¿ scintillator by joining a fast plastic scintillator for beta detection to a slow CsI(Tl) scintillator for gamma-ray detection, and coupling this composite to a single photomultiplier tube. Phase I demonstrated that a phoswich detector and digital signal processing could be used to detect beta-gamma coincidences characteristic of Xe radioactive decay, achieved energy resolutions equivalent to typical CsI detectors, and found an upper limit for wrongly detecting coincidences of 0.1%. Monte Carlo simulations of the radiation transport and light collection in the detector were used to produce an optimized design. Phase II will build a replacement module for the current ARSA detector system, consisting of four phoswich detectors along with their readout and high voltage electronics, a pulse-shape analyzer integrated with the readout electronics, and a control computer with software for interfacing with the current ARSA system. Commercial Applications and Other Benefits as described by the awardee: An upgraded ARSA system would be a key component of the U.S. monitoring network for detecting nuclear explosions. The phoswich technology also should find use in the detection of radioactive Xenon emitted by nuclear power plants, providing U.S. and international agencies with a sensitive tool to confirm compliance with environmental regulations. The technology also should have application to the environmental monitoring of key isotopes of nuclear processing activities, and to medical imaging.