Development of a COTS Radioxenon Detector System Using Phoswich Detectors and Pulse Shape Analysis

Several of the radioxenon detection systems developed for the International Monitoring System use betagamma coincidence detection to achieve high sensitivity. These systems use an arrangement of separate beta and gamma detectors to detect betagamma coincidence events characteristic of the different radioxenon isotopes. While very sensitive to small amounts of radioxenon, they also require careful calibration and gain matching of several detectors and photomultiplier tubes. An alternative approach is the use of a single phoswich detector in which beta-gamma coincidences are detected by pulse shape analysis PSA. We previously reported on prototype phoswich well detectors, consisting of a fast plastic scintillator absorbing betas optically coupled to a slower CsITl scintillator absorbing gammas. These detectors require only a single photomultiplier tube and an electronics readout channel. Betagamma coincidences create characteristic fastslow signals that can easily be distinguished from slow only or fast only non-coincident interactions. We describe the development of a commercial off-the-shelf COTS radioxenon detector system based on such phoswich detectors. The PSA functions were implemented using the digital signal processor in a set of commercial readout electronics that is also compatible with ARSA and SAUNA detectors. These functions detect coincidences in real time and accumulate 2D histograms in on-board memory. The acquisition and the PSA functions can also be integrated into larger-scale acquisition and control software. Several phoswich detectors have been characterized for energy resolution, separation of coincidence events in 2D betagamma energy histograms, and detection efficiency. A test pulser module has been developed to support monitoring and testing of the electronics for state of health during operation. The phoswich detector, readout electronics, and software are now available as a COTS product package.