PNNL

Abstract

Atmospheric measurement of noble gases has been extensively used for monitoring clandestine nuclear weapon explosions for many years. The ratios of four xenon isotopes of interest (131mXe, 133mXe, 133Xe, and 135Xe) help in discriminating regular reactor operations from nuclear tests. A new coincidence-based detection system using stilbene and strontium iodide [SrI2(Eu)] for electron and photon detection respectively was developed at Oregon State University to address some of the challenges of the radioxenon systems deployed in the field such as memory effect, and poor energy resolution. Silicon photomultipliers (SiPMs) were used for sensing optical photons from all scintillation media. Real-time coincidence identification was achieved using the eight-channel digital pulse processor. The detection system was evaluated using lab check sources and Oregon State TRIGA reactor irradiated radioxenon samples. A 48-hour background coincidence spectrum was collected yielding a coincidence count rate and background rejection rate of 0.0174 ± 0.0003 counts per second (cps) and 98.9% respectively. The minimum detectable concentration (MDC) of the system was evaluated to be 0.11 ± 0.01, 0.13 ± 0.02, 0.20 ± 0.02, and 0.73 ± 0.08 for 131mXe, 133mXe, 133Xe, and 135Xe respectively. The memory effect of the detection system was found to be 0.069 ± 0.015%, which is almost a 70-fold reduction compared to traditional plastic scintillators. The detection elements, custom-designed electronics, and the detector response to radioxenon are detailed in this work.