Effects of the nuclear structure of fission fragments on the high-energy prompt fission γ-ray spectrum in 235U(nth, f)

The prompt fission gamma-ray energy spectrum for cold-neutron-induced fission of U-235 was measured in the energy range E-gamma = 0.8-20 MeV, by gaining a factor of about 10(5) in statistics compared to the measurements performed so far. The spectrum exhibits local bump structures at E-gamma approximate to 4 MeV and approximate to 6 MeV, and also a broad one at approximate to 15 MeV. In order to understand the origins of these bumps, the gamma-ray spectra were calculated using a statistical Hauser-Feshbach model, taking into account the deexcitation of all the possible primary fission fragments. It is shown that the bump at approximate to 4 MeV is created by the transitions between the discrete levels in the fragments around Sn-132, and the bump at approximate to 6 MeV mostly comes from the complementary light fragments. It is also indicated that a limited number of nuclides, which have high-spin states at low excitation energies, can contribute to the bump structure around E-gamma approximate to 15 MeV, induced by the transition feeding into the low-lying high-spin states.