Graduate Thesis Or Dissertation
 

Development of a CZT-Silicon Detection System in Support of the Comprehensive Nuclear-Test-Ban Treaty

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/wp988q418

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  • The Comprehensive Nuclear-Test-Ban Treaty (CTBT) bans all nuclear explosion tests for military or civilian purposes. The International Monitoring System (IMS) was established to verify compliance with the treaty. It consists of several monitoring stations that detect: seismic activities, hydrocoustic activities, infrasound waves, and radionuclide particles and noble gases. Radioxenon detection provides the most robust evidence of a nuclear weapon test. There are four radioxenon isotopes of interest: 131mXe (t1 2 = 11.93 days), 133mXe (t1 2 = 2.19 days), 133Xe (t1 2 = 5.25 days) and 135Xe (t1 2 = 0.38 days). All of these radioxenons emit beta and gamma radiation in coincidence or conversion electrons and X-rays in coincidence during their decay process. In this research, a new radioxenon detection system was developed based on Si and CZT detectors. The system is made of the “PIPSBox” silicon gas cell recently developed by Canberra to detect beta and conversion electrons, and two coplanar CZT detectors to detect X-rays and gamma rays. The PIPSBox silicon gas cell offers many advantages such as: (1) increasing the frequency of air sampling at IMS stations because memory effect does not affect the PIPSBox gas cell like it does with plastic gas cells currently used at IMS stations, (2) reducing the Minimum Detectable Concentration (MDC) for radioxenons due to better energy resolution of silicon, and minimal background interference from previous measurements. The detection system was simulated using MCNP6 and was characterized by 131mXe to determine optimum operating voltages, proper gain, and the length of the coincidence window.Pulse waveforms of the silicon and CZT detectors were analyzed using two digital pulse processors: DPP2 and DPP8. DPP2 is a two-channel digital pulse processor with a 200 MHz sampling frequency and a 12-bit ADC resolution. DPP8 is an 8-channel, 125 MHz digital signal processor with a 14-bit ADC resolution. A coincidence firmware was implemented in the on-board FPGA to identify specific coincidences events between silicon and CZT detectors to generate 2D spectra for the four radioxenons of interest. The resolution of the 129 keV conversion electron was measured to be 16.66% in silicon1 and 16.87% in silicon2. These resolutions are the best-known values reported from other radioxenon detection systems that were included in the literature review of this research. The minimum detectable concentration (MDC) of PIPSBox CZT detection system four all radioxenons of interest was measured to be less than the 1 mBq m3 IMS requirement. Specifically, 0.25 mBq m3 for 131mXe, 0.26 mBq m3 for 133mXe, 0.39 mBq m3 for 133Xe, and 0.72 mBq m3 for 135Xe.
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