Investigation of the structure in ²³⁵U(n_th,fis) prompt gamma energy spectrum by FIFRELIN

The energy spectrum of prompt gamma-rays from 235U thermal fission is reproduced by a fission reaction model FIFRELIN to identify the origin of spectral features. This spectrum is characterized by a bump at 4-4.5 MeV, a shoulder at around 8 MeV, and a tail above 10 MeV, which are attributed to 132Sn and 133Sb, fission fragments with mass from 130 to 134, and those with mass from 99 to 104, charge from 40 to 42, and spin from 12 ℏto 16 ℏ, respectively. Among prompt gamma-rays, high-energy part is important to get insight into scission, because the quanta are emitted by highly excited fission fragments, which still keep the memory of the scission configuration. Our analysis suggests that coincidence detection of high energy gamma-rays and their source fission fragments identifying their isotopic species, spin and excitation energy with particular focus on those with mass number less than 104 will be useful to make conclusions about the origin of high energy gamma-rays.