Measurement of prompt γ-ray spectrum for ²³⁵U(n_th,f)

Prompt fission γ-ray spectra (PFGS) are important as they allow us to study the structure and de-excitation process of neutron-rich fission fragments (FFs). They are also required as nuclear data to design new types of reactors, such as the Generation-IV reators. For spontaneous fission of ²⁵²Cf, the PFGS show broad structures at around γ-ray energy of 15 MeV, which was explained by incorporating the giant dipole resonance γ-ray emission. For neutron-induced fissions (n_th,f), however, no data are available in the energy range higher than 7 MeV. With a goal to measure the PFGS for ²³⁵U(n_th,f) up to energies of 20 MeV, we have developed a new measurement system. The system consists of two position-sensitive multi-wired proportional counters (MWPCs) to detect both FFs in coincidence, and two large volume LaBr₃(Ce) scintillators to measure the γ-rays in fission. The measurement has been carried out at the PF1B cold-neutron facility of the Institut Laue Langevin (ILL), Grenoble, France. Throughout the 437-hour run the total number of recorded γ rays in coincidence with FFs was reached to 1.7x10⁹. The obtained PFGS reached about 20 MeV, high enough to reveal the structure associated with the GDR of the FFs, for the first time in (n_th,f). The resulting PFGS do not follow the linearly decreasing trend as a function of γ-ray energy in logarithmic scale, but reveal the bump structures at about 4 MeV, 6 MeV, and 15 MeV. In this contribution, we will discuss the origins of the bump structures in comparison with the statistical Hauser-Freshbach model calculation.