Abstract
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.