Shell-Driven Fission across the Nuclear Chart

Results are presented from a broad, systematic study of heavy-ion induced fusion-fission mass distributions carried out at the Australian National University, covering a significant part of the chart of the nuclides. Fission characteristics of isotopes of every even-Z compound nucleus (ZCN) from 16464Gd to 21290Th were measured. Systematic evidence of shell-driven structure is present in every fission mass distribution. The changing shape of the heavy-ion fission mass distributions with ZCN is visualised through the residuals from single Gaussian fits. These results are consistent with quantitative fitting of the measured 2-D mass and total kinetic energy spectra using multiple components. Both approaches demonstrate that fragment proton shell gaps around ZFF = 34, 36 and around ZFF = 44, 46 are major drivers of fission mass distributions for nuclei below the actinide region. Significantly, the mass distributions show enhanced yields at mass-symmetry for values of ZCN equal to two times these favoured ZFF values. The same shell gaps that favour mass-asymmetric fission thus also affect mass distributions at and near mass-symmetry. For all systems, a second more mass-asymmetric fission mode is required to fit the fission mass distributions. If driven by a single shell gap, it appears to be in the light fragment, around ZFF ~28,30 or possibly NFF ~44.