Graduate Thesis Or Dissertation

A Compact Radioxenon Detection System Using CZT, an Array of SiPMs, and a Plastic Scintillator

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  • Several radioxenon isotopes (¹³¹ᵐXe, ¹³³Xe, ¹³³ᵐXe, ¹³⁵Xe) are characteristic byproducts of nuclear explosions, and due to their chemically nonreactive nature can easily escape from tests occurring underground and enter the atmosphere. It has been shown that by utilizing beta-gamma coincidence techniques, the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO) can reliably detect the presence of these isotopes in the atmosphere to verify the nuclear nature of clandestine explosions. In an effort to mitigate conversion electron backscatter effects observed in a Two-Element CZT (TECZT) coincidence radioxenon detection system previously designed at Oregon State University, a new prototype beta-gamma coincidence detection system has been developed. The detection system consists of a coplanar CZT detector, an array of silicon photomultipliers (SiPMs), and a well-type plastic scintillator. Radioactive gas samples are injected via a tube into the plastic scintillator gas cell, the base of which is coupled to the SiPM array for light readout. This scintillator is used to detect the full energy deposition of beta particles and conversion electrons without significantly attenuating gamma and X-rays. A coplanar CZT detector, chosen for its excellent energy resolution, simple readout electronics, and room temperature operation, is positioned beside the scintillator to detect gamma radiation and X-rays that are emitted in coincidence with the beta particles and conversion electrons. The system is directly mounted onto a custom printed circuit board (PCB) for low noise readout using a custom field-programmable gate array (FPGA)-based dual-channel digital pulse processor. This allows for the capture of coincident events in real time. This thesis details the design, construction, and characterization of this prototype radioxenon detection system using lab check sources and radioxenon samples produced in the OSU TRIGA reactor, as well as a background measurement and a Minimum Detectable Concentration (MDC) calculation for ¹³⁵Xe ¹³³Xe, and ¹³³ᵐXe. The system was shown to perform effective beta-gamma coincidence detection. Preliminary experiments yielded photopeak resolutions of 33.4% full width at half max (FWHM) for 30 keV, 12.4% FWHM for 81 keV, 5.7% FWHM for 250 keV, and 3.1% FWHM for 662 keV, and an MDC of 6.931±0.104 for ¹³⁵Xe, 0.648±0.059 for ¹³³Xe, and 0.169±0.054 for ¹³³ᵐXe. These resolutions and the two latter MDCs are competitive with other radioxenon detection systems utilized in the International Monitoring System (IMS), and stand to improve with further efforts towards system optimization.
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