Nuclear Inst. and Methods in Physics Research, A 971 (2020) 164118 Contents lists available at ScienceDirect Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima A cubic CeBr 3 gamma-ray spectrometer suitable for the decommissioning of the Fukushima Daiichi Nuclear Power Station Masaaki Kaburagi a,∗ , Kenji Shimazoe b , Yutaka Otaka b , Mizuki Uenomachi b , Kei Kamada c,d , Kyoung Jin Kim d,e , Masao Yoshino e , Yasuhiro Shoji d , Akira Yoshikawa c,d,e , Hiroyuki Takahashi b , Tatsuo Torii a a Collaborative Laboratories for Advanced Decommissioning Science, Japan Atomic Energy Agency, Japan b School of Engineering, University of Tokyo, Japan c New Industry Creation Hatchery Center (NICHe), Tohoku University, Japan d C & A Corporation, Japan e Institute for Materials Research (IMR), Tohoku University, Japan ARTICLE INFO Keywords: Cerium bromide High energy gamma-ray spectrometry High dose-rate Passive gamma-ray analysis Fukushima Daiichi Nuclear Power Station Accident ABSTRACT 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 CeBr 3 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 a 60 Co 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 between 154 Eu (1274 keV) and 60 Co (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. 1. Introduction The large earthquake that occurred in the east of Japan on March 11, 2011 triggered a powerful tsunami that struck the Fukushima Daiichi Nuclear Power Station (FDNPS). The earthquake broke the electrical power supply lines to the site, and the tsunami destroyed the operational and safety infrastructure at the site [1]. On-site and off-site electrical power supplies were lost and the cooling of the reactors thus became difficult, which caused a temperature rise in the containment vessels. The reactor cores in Unit 1–3 melted, and these melted nuclear fuels were mixed with structural materials, which solidified as nuclear fuel debris, inside the reactors of Unit 1–3. Almost all nuclear fuel debris spread within the primary containment vessel (PCV) in Unit 1, but a large amount of nuclear fuel debris is at the bottom of the reactor pressure vessel (RPV) in Unit 2. In Unit 3, a certain amount of nuclear fuel debris is at the bottom of the PCV and RPV. The retrieval of the Abbreviations: DPP, Digital pulse-processing; FDNPS, Fukushima Daiichi Nuclear Power Station; FPGA, Field programming gate array; FWHM, Full width at half maximum; IMR, Institute for Materials Research; JAEA, Japan Atomic Energy Agency; PCV, Primary containment vessels; PGA, Passive gamma-ray assessment; QE, Quantum efficiency ∗ Correspondence to: 2-4, Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan. E-mail address: kaburagi.masaaki@jaea.go.jp (M. Kaburagi). nuclear fuel debris is a critical step in the decommissioning operation. Then, the localization of the nuclear fuel debris with spatial resolution by smaller than 1 cm will be useful to clean up the reactors. So, the operation can be carried out quickly, and to prevent criticality. The highest dose rate in the PCV was greater than 5000 Sv/h; however, Tokyo Electrical Power Company Holdings reported that the dose rates at the bottom of the pedestal in Unit 2, where the nuclear fuel debris could have been spreading, were lower than 10 Sv/h [2]. These radiation fields mainly came from 137 Cs and the associated Compton background, and these photon spectra were lower than 662 keV. In contrast, 154 Eu is a fission product with a relatively prominent gamma-ray line at 1274 keV. A study on passive gamma-ray analysis based on measuring 154 Eu [3] showed that high-energy gamma rays can penetrate greater quantities of shielding. Also, spontaneous fissions occur in heavy nuclides such as 244 Cm, 238 U, 240 Pu, and 242 Pu, which leads high-energy (∼10 MeV) gamma-ray and neutron emissions. Small https://doi.org/10.1016/j.nima.2020.164118 Received 29 September 2019; Received in revised form 4 May 2020; Accepted 6 May 2020 Available online 8 May 2020 0168-9002/© 2020 Published by Elsevier B.V.