Measurement of Level Density Parameters of Fission Fragments Following Thermal Neutron Induced Fission of Uranium-235

摘要

Level density parameters of fission fragments following the thermal neutron induced fission of 235U 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 155u except the region from 115 to 125u. The obtained parameters showed a saw-tooth trend against the mass with the minimum value around the double magic number, 132u, and this trend was close to that for 252Cf(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.