A cubic CeBr₃ gamma-ray spectrometer suitable for the decommissioning of the Fukushima Daiichi Nuclear Power Station

In the decommissioning of the Fukushima Daiichi Nuclear Power Station (FDNPS), the retrieval of the nuclear fuel debris is a critical step, and the localization of these debris speeds up the decommissioning operation and prevents criticality. Our work focused on the passive gamma-ray analysis (PGA) of the nuclear fuel debris based on measuring gamma rays with an energy greater than 1 MeV. The PGA requires gamma-ray spectrometers to be used under the high dose rates in the FDNPS, then we fabricated a small cubic CeBr₃ spectrometer with dimensions of 5 mm × 5 mm × 5 mm, which was coupled to a Hamamatsu R7600U-200 photomultiplier tube (PMT). We investigated the performance at dose rates of 4.4 to 750 mSv/h in a⁶⁰Co field. The energy resolution of the full width at half maximum at 1333 keV ranged from 3.79% to 4.01%, with a standard deviation of 6.9%, which met the narrow gamma decay spectral lines between¹⁵⁴Eu (1274 keV) and⁶⁰Co (1333 keV). However, the spectra shifted to a higher energy level as the exposure dose rate increased, there was a 51% increase at the dose rates of 4.4 to 750 mSv/h. The spectral shifts were caused by the increase in the PMT gain due to the large direct current flows.