Fission studies with 252Cf: half-lives of excited states, isomeric structures in 113,115,117Pd, and neutron multiplicities

Abstract

Several experiments were performed with a spontaneous fission source of 252Cf with Gammasphere. Triple coincidence gamma-gamma-gamma data were recorded in each of these experiments and analyzed to obtain detailed nuclear structure information from the neutron-rich nuclei produced in the fission. In addition to binary fission gamma-ray spectroscopy, ternary fission data were also collected in a light charged particle-gamma-gamma coincidence experiment. The half-lives of several excited states were measured with a new technique. It was necessary to correct the measurements for low-energy transitions and short half-lives. The corrected results are in agreement with previous measurements. The half-lives of excited states in 104Zr and 152Ce are reported for the first time. The results indicate super deformed ground-states in these nuclei. Theoretical calculations support these super deformations. This is interpreted as evidence for the existence of shell gaps at large deformation in this region. The level structures of 113,115,117Pd were re-examined to be consistent with the higher statistics data set. New transitions are found at low energy (39.0, 49.0, 63.7, and 85.1 keV), in addition to higher energy transitions. These are placed in the negative parity bands that feed the isomeric states. The spin and parity of the ground states and some of the low-spin states are re-assigned using these new data. In addition, new levels are assigned to a high spin side band in each nucleus. The existence of a “hot” second fission mode associated with large neutron evaporation has been reported in the Ba-Mo binary split for 252Cf but its intensity has been questioned in subsequent studies. Our high statistics data confirm the existence of this mode, with an intensity of 5% of the regular mode. The gamma-gamma-gamma coincidence data were analyzed to give the first yields of correlated pairs in alpha-ternary fission. In alpha-ternary fission, the “hot” mode is present for the Ba-alpha-Zr split, but not the Xe-alpha-Mo or Ru-alpha-Te splits. The second “hot” mode is stronger (15%) than in binary fission (5%), but with larger uncertainty.