Shell-driven Fission across the Nuclear Chart

¹ Department of Nuclear Physics and Accelerator Applications, Research School of Physics, Australian National University, Canberra, ACT 2601, Australia
² Department of Fundamental and Theoretical Physics, Research School of Physics, Australian National University, Canberra, ACT 2601, Australia

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Published online: 13 May 2026

Abstract

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 (Z_CN) 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 Z_CN 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 Z_FF = 34, 36 and around Z_FF = 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 Z_CN equal to two times these favoured Z_FF 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 Z_FF ~28,30 or possibly N_FF ~44.