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 beta/gamma coincidence detection to achieve high sensitivity. These systems, e.g., the Automated Radioxenon Sampler and Analyzer (ARSA) or the Swedish Automatic Unit for Noble gas Acquisition (SAUNA), currently use an arrangement of separate beta and gamma detectors to detect beta/gamma coincidence events characteristic of the different radioxenon isotopes. While they are 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 the development of prototype phoswich well detectors, consisting of a fast plastic scintillator (absorbing betas) optically coupled to a slower CsI(Tl) scintillator (absorbing gammas). These detectors thus require only a single photomultiplier tube and an electronics readout channel. Beta/gamma coincidences create characteristic “fast/slow” signals that can easily be distinguished from “slow only” or “fast only” non-coincident interactions. In this paper, we will 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 are operated through a C driver library that is called from a graphical user interface but can also be integrated into larger-scale acquisition and control software. Several phoswich detectors have been manufactured in a small-scale production run, and have been characterized for energy resolution, separation of coincidence events in 2D beta/gamma energy histograms, and detection efficiency using a variety of test sources, and for background count rates. In addition, 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 (“PhosWatch”) that is currently being beta-tested at several radioxenon monitoring research laboratories.