MEASUREMENT OF LEVEL DENSITY PARAMETERS OF FISSION FRAGMENTS FOLLOWING THERMAL-NEUTRON INDUCED FISSION OF U-235

Level density parameters of fission fragments following the thermal ne utron induced fission of U-235 were determined as a function of the fr agment mass by measuring the energy and multiplicity of prompt neutron s from a specified mass. This measurement gave the absolute values of the parameters for the neutron-rich nuclei covering the mass range fro m 80 to 155 u except the region from 115 to 125 u. The obtained parame ters 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 tha t for Cf-252(s.f) by Budtz-Jorgensen et al. The analysis by a phenomen ological description of the level density parameters including the she ll and collective effects suggested the existence of a collective moti on of the fission fragment at the moment of prompt neutron emission.