The Comprehensive Nuclear-Test-Ban Treaty (CTBT) prohibits the testing of nuclear weapons on the face of the earth. The detection of atmospheric radioxenon (131mXe, 133m/133Xe, and 135Xe) plays an important role in the identification of sub-surface clandestine nuclear weapon explosions. Since the radioxenon identified above decay via two radiation in coincidence, it allows the discrimination of background single events which leads to achieving a very low minimum detectable concentration (MDC) of below 1 mBq/m3. Some of the significant drawbacks of the systems on the International Monitoring System (IMS) network include memory effect in the electron detection cell, environmentally susceptible electronics, bulky detection systems, poor energy resolution, and high cost.
The Stilbene-SrI2(Eu) detection system aims to improve some of these shortcomings by incorporating the use of digital pulse processing, small form factor SiPMs compared to PMTs, and better detection elements in terms of energy resolution and memory effect. The Stilbene and SrI2(Eu) detectors operate in coincidence and these events are identified in real-time using a Field Programmable Gate Array (FPGA). Geant4 and DETECT2000 optical photon transport simulations were performed to study the distribution of the photons on the SiPMs. The four radioxenons of interest were irradiated in the Oregon State University TRIGA reactor and injected in the gas cell for evaluating the detector performance. The unique signatures from all the four radioxenon were uniquely identified. This detection system also yielded a memory effect of about 0.069 ± 0.015% which is almost a 70 times improvement compared to conventional plastic scintillators. This was followed by performing the background measurements and MDC calculations. A background coincidence count rate of 0.0174 ± 0.0003 counts per second was recorded with a background rejection rate of about 98.89 ± 0.02%. The MDC of 131mXe, 133mXe, 133Xe, and 135Xe was determined to be 0.15 ± 0.02, 0.12 ± 0.02, 0.30 ± 0.03, and 0.74 ± 0.08 mBq/m3 respectively. These values meet the requirements of the CTBTO of achieving an MDC below 1 mBq/m3 for all radioxenon of interest. The performance of the Stilbene-SrI2(Eu) detection system also compares well with the state-of-the-art detectors on the IMS network.