Redesigned β–γ radioxenon detector

Abstract

The Automated Radio-xenon Sampler/Analyzer (ARSA), designed by Pacific Northwest National Laboratory (PNNL) collects and detects several radioxenon isotopes, and is used to monitor underground nuclear explosions. The ARSA is very sensitive to 133Xe, 131mXe, 133mXe, and 135Xe (<1 mBq/SCM) [M. Auera et al., Wernspergera, Appl. Radiat. 6 (2004) 60] through use of its compact high efficiency β–γ coincidence detector. For this reason, it is an excellent treaty monitoring system and it can be used as an environmental sampling device as well. Field testing of the ARSA has shown it to be both robust and reliable, but the nuclear detector requires a detailed photomultiplier tube (PMT) gain matching regime difficult to implement in a field environment. Complexity is a problem from a maintenance and quality assurance/quality control (QA/QC) standpoint, and efforts to reduce these issues have led to development of a simplified β–γ coincident detector. The new design reduces the number of PMT's and the complexity of the calibration needed in comparison to the old design. New scintillation materials (NaI(Tl), CsI(Na), and CsI(Tl)) were investigated and a comparison of three different gamma sensitive well detectors has been completed. A new plastic-scintillator gas cell was constructed and a new method of forming the scintillator gas cell was developed. The simplified detector system compares favorably with the original ARSA design in spectral resolution and efficiency and is significantly easier to set up and calibrate. The new materials and configuration allow the resulting β–γ coincidence detector to maintain the overall performance of the ARSA type β–γ detector while simplifying the design.