Development of a Phoswich Detector for Radioxenon Field Measurements

The Comprehensive Nuclear-Test-Ban Treaty Organization deploys a variety of radioxenon detection systems as part of its International Monitoring System to detect nuclear explosions. To achieve the high sensitivity required, 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 detection. The high sensitivity comes at the expense of heavy lead shielding, and for the latter, calibration and gain matching of multiple 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 some applications, for example on-site inspections, a portable 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 single photomultiplier tube with beta/gamma coincidences being detected by digital pulse shape analysis. Additional 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 each of the initial designs, their simulated performances and the factors that lead us to the chosen design. Preliminary results from testing of the prototype detector are presented and compared with simulation.