Characterization of Phoswich Well Detectors for Radioxenon Monitoring

Devices to measure the amount of radioactive xenon in the atmosphere have been installed in several locations around the world as part of the International Monitoring System to detect nuclear weapons testing. These devices extract small samples of xenon from large volumes of air and look for characteristic radioxenon isotopes emitting beta and gamma radiation in coincidence. To detect these coincidences, they currently employ a complex system of separate beta and gamma detectors which is very sensitive, but which requires careful calibration and gain matching of several detectors and photomultiplier tubes to achieve desired detection limits. An alternative to separate beta and gamma detectors is the use of a single phoswich detector in which beta-gamma coincidences are detected by pulse shape analysis. The phoswich detector consists of a plastic scintillator (absorbing betas) optically coupled to a CsI(Tl) scintillator (absorbing gammas) and thus requires only a single photomultiplier tube and electronics readout channel, greatly simplifying setup and calibration. In this paper, we present the results from an experimental evaluation of two phoswich well detector prototypes, including energy resolution, 2 D beta/gamma energy histograms from a variety of test sources, and background count rates. From these measurements, we derive detector properties such as coincidence detection efficiency, background rejection and the ability to separate beta only, gamma only, and coincidence events. We will further discuss setup and calibration procedures and compare them to those for existing detector systems.