Multi-pixel photon counter-based gamma camera with pinhole collimator to locate Cs-137 sources at high dose rates for the Fukushima nuclear power plant

Abstract

The Fukushima Daiichi Nuclear Power Plant was severely damaged during the 2011 Great East Japan Earthquake. However, the ongoing decommissioning work has been limited by the complexity of the reactor's internal structure and very high radiation levels; locating radioactive sources is essential for efficient decommissioning. Conventional gamma cameras are mainly designed for low radiation dose (∼mSv/h) and using them under high radiation conditions is difficult (>Sv/h). Therefore, we developed a pinhole gamma camera with a gamma radiation detector consisting of a high-speed YGAG scintillator array and multi-pixel photon counters to locate radioactive sources at high dose rates. The gamma-ray photon signals captured by the developed two-dimensional detector array can be processed at a speed as high as >1 MHz/pixel using the developed large scale integrated circuit. Herein, we report the measurement results of an extremely high radioactivity of 137Cs (∼34 TBq) using the developed gamma camera. The gamma-ray source position was determined using an angular size of ∼4.6°, with images obtained at 2 m from the radioactive source and at a dose rate of 0.3 Sv/h. The direct gamma rays with a photoelectric peak at 662 keV and scattered component of gamma rays can be distinguished from the measured spectrum. We also characterize the imaging capability of the 137Cs depending on the detected gamma-ray energies and discuss related details.