Measurement of level density parameters of fission fragments following thermal neutron induced fission of uranium-235

Level density parameters of fission fragments following the thermal neutron induced fission of U-235 were determined as a function of the fragment mass by measuring the energy and multiplicity of prompt neutrons from a specified mass. This measurement gave the absolute values of the parameters for the neutron-rich nuclei covering the mass range from 80 to 155 u except the region from 115 to 125 u. The obtained parameters showed a saw-tooth trend against the mass with the minimum value around the double magic number, 132 u, and this trend was close to that for Cf-252(s.f) by Budtz-Jorgensen et al. The analysis by a phenomenological description of the level density parameters including the shell and collective effects suggested the existence of a collective motion of the fission fragment at the moment of prompt neutron emission.