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Report No.
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Simulation of $$^{235}$$U(n,fission) gamma-ray energy spectrum by FIFRELIN fission event generator

Ogawa, Tatsuhiko ; Litaize, O.*; Mancusi, D.*; Chebboubi, A.*; Serot, O.*

The gamma-ray spectrum of $$^{235}$$U(n,fission) reactions measured by Makii et al., at PF1B cold-neutron beam facility at the Institut Laue Langevin (ILL) in Grenoble (France) was compared with the data calculated by the fission event generator FIFRELIN to get the insight into the mechanisms which determine the gamma-ray energy spectrum. The features structures of the spectrum, a bump at 4 MeV, a shoulder at 6 MeV agreed between experiment and calculation while the shape of the tail beyond 10 MeV did not agree well. The bump around 4 MeV is mostly attributed to the deexcitation of closed shell nuclei such as Sn-132 from their excited state around 4 MeV to their ground state. The shoulder near 6 MeV is explained by the fact that the neutron separation energy of most of fission fragments are around 6 MeV. The excitation energy of fragments is randomly distributed, therefore the fragments with excitation energy higher than the neutron separation energy favor neutron emission, whereas those with low excitation energy emit gamma-rays. Consequently, gamma-ray spectrum drops around 6 MeV. According to FIFRELIN, gamma-rays above 10 MeV are mainly from fragments with mass below 90. Meanwhile, the multiplicity of gamma-rays from the fragments with mass lower than 90 was overestimated according to the FIFRELIN previous results. Therefore reducing the multiplicity of gamma-rays from fragments with mass below 90 can explain both discrepancies. We show in this work that by increasing the spin cut-off parameter of fragments with mass lower than 90 by a factor of 5, FIFRELIN reproduced the measured gamma multiplicity mass dependence as well as the measured gamma-ray energy spectrum.

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