Speaker
Description
For safe use of nuclear reactor core, slowing down of neutrons and reaction rates inside the core should be predicted accurately. They have made tremendous efforts to improve the calculation methods and the nuclear data for the prediction. However, the calculated neutron spectrum and the reaction rates have been validated only by post irradiation measurement of gamma rays. As an in-situ measurement technique, the authors have focused on spectrum measurement of prompt gamma rays. Nauchi et al. have succeeded in estimation of 238U(n,γ) reaction rates by detection of 4060 keV gamma rays [1]. Then they tried to determine energy of neutron which induces capture reactions of 238U. For that, neutron energy dependent gamma ray spectrum was measured for 238U at KURNS LINAC neutron source facility. Uranium metallic samples of the natural enrichment were irradiated by the pulsed neurons of white spectrum. The gamma rays were measured with a HP-Ge detector of 35% relative efficiency. The energy of neutrons which induces capture was measured by the time of flight technique. The gamma ray spectrum measured for the neutrons of thermal energy and resonances (En=6.67eV and 20.9eV) are shown in Fig. 1. Compared to the published literature [2], the spectrum was obtained with better resolution, and differences in peak gamma ray spectra for the thermal and resonances energy neutrons were observed. By measuring the gamma ray spectrum from a reactor core, the reaction rates ratio of the thermal and the resonance neutron energy might be unfolded taking advantage of the difference of the gamma ray spectrum.
[1] Y. Nauchi, T. Sano, H. Unesaki, et al., Proceedings of 11th International conference on nuclear criticality safety, ICNC 2019, September 15-20, 2019 – Paris, France.
[2] H. Harada S. Goko, A. Kimura et al., Journal of Korean Physical Society 59(2) 1547-1552, 2011.