Low-Level Radioactive Xenon Monitoring by Phoswich Detector System

76252-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 alkali-halide scintillator for gamma-ray detection, and coupling this composite to a single photomultiplier tube. By making background rejection measurements, the performance of the present ARSA could be matched with only a single photomultiplier per sample cell. Phase I will demonstrate feasibility by constructing a model phoswich detector, capturing its output signals when exposed to beta-gamma decay events, and applying off-line digital signal processing to show differentiation between beta-gamma events and pure gamma background events. Different phoswich structures and digital processing algorithms will be investigated to indicate the best approach to Phase II optimization. Commercial Applications and Other Benefits as described by the awardee: The new system should reduce the effort required to install and maintain ARSA operability as part of the US Atomic Energy Detection System (USAEDS). It should greatly enhance national security through the advanced detection of nuclear proliferation activities well before they reach the deliverable weapons stage. In addition, the beta-gamma phoswich coincidence concept could provide a new approach for identifying specific radionuclides in monitoring environmental radiation and or for providing first response capability in nuclear spill or dirty bomb scenarios.