Optimization of Shape-Variable Gamma Camera to High-Dose-Rate Regions

Abstract

To proceed with the decommissioning of the Fukushima Daiichi Nuclear Power Station, it is necessary to visualize radioactive substances using a gamma camera in high-dose-rate regions inside a nuclear reactor building. We developed an umbrella-shaped Compton camera, whose sensitivity distribution can be adjusted by changing its shape. When used on a robot, the moving distance of the robot can be reduced compared to a pinhole camera that is sensitive only to the front surface. In addition, unlike an omnidirectional Compton camera, the sensitivity in a specific direction can be increased, which is expected to expand the degree of freedom of measurement. In this study, the characteristics of the developed system were evaluated through simulations and experiments for application in a high-level radioactive environment inside a reactor building. It is estimated that this system can operate at a dose rate of approximately 2.68 mSv/h. Furthermore, we devised a method to compensate for the effect of the dead time of the SiPM (MPPC) in the high-dose-rate region by monitoring the current of the SiPM drive power supply and evaluating its applicability to high dose rates. This correction method was validated in the case where the photoelectric peak of the energy spectra fluctuated by approximately 60 keV due to dead time.