Development of a high energy γ-rays distribution monitoring system for nuclear power plant decommissioning using pinhole gamma camera

초록

In June 2025, Kori Unit 1, the first commercial nuclear power plant in the Republic of Korea, received final approval for decommissioning. To address this need, we developed a portable high-energy gamma camera for nuclear decommissioning and high-radiation environments, and we evaluated its detector characteristics and imaging performance. Using flood maps, we confirmed uniform spatial response of the GAGG-SiPM detector array, with clearly separated pixel peaks and negligible crosstalk. The measured full width at half maximum (FWHM) of 0.58 mm, corresponding to 41% of the pixel pitch, confirmed stable spatial resolution across the array. Energy resolution measurements using Co-60 yielded values of 7.42% and 6.68% for the 1.17 MeV and 1.33 MeV photopeak, respectively, outperforming previously reported high-energy gamma imaging systems. Angular resolution tests verified that two adjacent high-energy sources were resolvable at an angular separation of approximately 10 degrees, demonstrating precise directional discrimination. Field-of-view (FOV) evaluation showed an effective FOV of 50 degrees, corresponding to 83% of the theoretical 60 degrees pinhole aperture, indicating that the compact design still enabled wide-area coverage. Image sensitivity experiments confirmed that a 62 mu Ci Co-60 source could be successfully imaged at a distance of 3 m. Additional isotope imaging using Co-60, Cs-137, Na22, Ba-133, and Co-57 demonstrated stable reconstruction performance across low-, medium-, and highenergy gamma emissions, and FWHM analysis further confirmed consistent spatial resolution regardless of isotope energy. Overall, we demonstrated that the proposed gamma camera provides high imaging performance, portability, and operational versatility, making it suitable for practical deployment in nuclear decommissioning and other high-radiation monitoring environments.

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