Development of a Phoswich Detector for Radioxenon Field Measurements

The Comprehensive Nuclear-Test-Ban Treaty Organization deploys a variety of radioxenon detect ion systems as part of its International Monitoring System to d etect nuclear explosions. To achieve the high sensitivity require d, the systems extract xenon from several cubic meters of air and look for characteristic radioactive emissions, using either high resolution high purity germanium gamma detectors or multiple scintillators for high efficiency beta/gamma coincidence detectio n. The high sensitivity comes at the expense of heavy lead shie lding, and for the latter, calibration and gain matching of multip le photomultiplier tubes as well as a memory effect of the plastic scintillator used for beta detection which absorbs Xe. Existing systems are also stationary by design, though in so me applications, for example on-site inspections, a po rtable detector is required. In this work, we therefore redesigned a previously developed phoswich detector to reduce size, weight, cost, complexity and memory effect with only minor impact on the sensitivity. The phoswich design requires only a si ngle photomultiplier tube with beta/gamma coincidences being detected by digital pulse shape analysis. Additiona l gain stabilization addresses varying environmental field conditions, such as temperature changes. Three phoswich geometries were modeled through Monte Carlo simulations, the most promising was built and tested. In this paper we describe eac h of the initial designs, their simulated performances and the facto rs that lead us to the chosen design. Preliminary results from testing of the prototype detector are presented and compared with simulation.