Universality of Shell Effects in Fusion-Fission Mass Distributions

Abstract

We present the results of a broad, systematic study of heavy-ion induced fission mass distributions for every even-Z compound nucleus ($Z_{\mathrm{CN}}$) from ${}^{144}$Gd to ${}^{212}$Th. We find systematic evidence of shell-driven structure in every fission mass distribution. The change in shape of the mass distributions with $Z_{\mathrm{CN}}$ is consistent with the results of quantitative simultaneous fitting in mass and total kinetic energy, demonstrating that fragment proton shell gaps at $Z_{\mathrm{FF}}=34,36$ and $Z_{\mathrm{FF}}=44,46$ are \textit{both} major drivers of fission mass distributions below the actinide region. The mass distributions show enhanced yields at mass symmetry for values of $Z_{\mathrm{CN}}$ equal to two times these favoured $Z_{\mathrm{FF}}$ values. Thus, the same shell gaps that are drivers of mass-asymmetric fission 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 $Z_{\mathrm{FF}}=28,30$.