List of Recent Graduates

Cadmium zinc telluride (CZT) is the most promising material to detect X-ray and gamma-ray due to its high detection efficiency, good energy resolution, and operability at room temperature without any cooling device. However, detector responses of the CZT depend on the interaction position because of the distributions of weighting potential in a planar electrode structure and material defects, and it causes the degradation of the performance. We developed a detection system based on a 3D position-sensitive detector to overcome this problem. A 5 × 5 × 12 mm3 CZT was used, and the virtual Frisch-grid (VFG) method was applied. 3D position information was calculated by signals from the anode, cathode, and side electrodes. The detector responses were corrected by the position information, and as a result, the energy resolution of 0.83% at 662 keV was achieved. We developed a prototype Compton camera by expanding the single VFG CZT detector to a 2 × 2 array. This array structure allows us to use small but high-yielding crystals, so a large-area detection system with a low cost can be achievable. Four 6 × 6 × 19 mm3 CZTs and a commercialized signal processing system were used. Positional information acquisition and response correction were performed the same way as the previous system. Various sources were used for spectroscopy, and the energy resolutions at 356, 511, 662, 1,275, and 1,332 keV were 3.17, 2.37, 2.03, 1.49, and 1.14%, respectively. In Compton imaging experiments, single and multiple sources at various positions were measured, and the weighted list-mode MLEM method was applied for image reconstruction. The results showed that our Compton camera could correctly reconstruct the source positions of either single or multiple sources. The intrinsic efficiency and spatial resolution evaluated by single 137Cs results were (1.43 ± 0.28) × 10-3 and 16.42 ± 5.35°, respectively. It was also identified that the different sources could be distinguished by applying energy windows. Name of graduate: Lee, Chanki Affiliation: Department of Nuclear Engineering/Ulsan National Institute of Science and Technology/Korea Graduation date: Feb. 2022 Degree: Doctor of Philosophy Name of academic advisor: Kim, Hee Reyoung Title of thesis: In Situ YAlO3(Ce) Gamma Spectrometry System for Underwater Survey by Remotely Operated Vehicle Abstract: Rapid surveys and assessments of environmental radioactivity and radiation doses are required for efficient and effective response to radiological emergencies. Emergency responses under underwater conditions to illegal ocean dumping, accidents, or sabotage of marine nuclear reactors are probably going to increase in the future. Therefore, in this study, we designed, developed, and tested a mechanical, chemical, and radiologically robust in situ YAlO3(Ce) gamma spectrometer survey system, that is remotely operated by an unmanned vehicle to be used for short-range and high-dose contaminations. In particular, the system is optimally designed by prediction based on the figure of merit, which is developed by correlating the scan minimum detectable concentrations (MDCs). After calibration by Monte Carlo simulation and a water tank experiment, the response function and relevant G-factor of the spectrometer were estimated to calculate the static and scan MDCs. The calculation results show that it can satisfy concentration and dose target values for four cases (i.e. , static sealed source characterization, sealed source scanning, static effluent measurement, and localized hotspot mapping) within 10 min with 95 % confidence, and 0.2 m/s scan speed with 95 % true positive and 60% false positive. In addition, the complete system operation when mounted on an underwater vehicle with a 200 m length wired communication was tested in a laboratory mockup facility by controlling the salt concentration, flow, wave, and temperature. It was found that the standard deviation of the mechanical noise (in the CPS) was proportional to the drag force, which followed a quadratically increasing trend. Temperature coefficient was found to be -0.193 ± 0.020 %/°C, and overall noise effects to spectra were minor, except for electrical noise formed in the low-energy region below tens of keV. In summary, the developed system based on YAlO3(Ce) gamma spectrometry could be used for various radiological emergencies. List of Recent Graduates Rapid surveys and assessments of environmental radioactivity and radiation doses are required for efficient and effective response to radiological emergencies. Emergency responses under underwater conditions to illegal ocean dumping, accidents, or sabotage of marine nuclear reactors are probably going to increase in the future. Therefore, in this study, we designed, developed, and tested a mechanical, chemical, and radiologically robust in situ YAlO3(Ce) gamma spectrometer survey system, that is remotely operated by an unmanned vehicle to be used for short-range and high-dose contaminations. In particular, the system is optimally designed by prediction based on the figure of merit, which is developed by correlating the scan minimum detectable concentrations (MDCs). After calibration by Monte Carlo simulation and a water tank experiment, the response function and relevant G-factor of the spectrometer were estimated to calculate the static and scan MDCs. The calculation results show that it can satisfy concentration and dose target values for four cases (i.e. , static sealed source characterization, sealed source scanning, static effluent measurement, and localized hotspot mapping) within 10 min with 95 % confidence, and 0.2 m/s scan speed with 95 % true positive and 60% false positive. In addition, the complete system operation when mounted on an underwater vehicle with a 200 m length wired communication was tested in a laboratory mockup facility by controlling the salt concentration, flow, wave, and temperature. It was found that the standard deviation of the mechanical noise (in the CPS) was proportional to the drag force, which followed a quadratically increasing trend. Temperature coefficient was found to be -0.193 ± 0.020 %/°C, and overall noise effects to spectra were minor, except for electrical noise formed in the low-energy region below tens of keV. In summary, the developed system based on YAlO3(Ce) gamma spectrometry could be used for various radiological emergencies. List of Recent Graduates