Universality of shell effects in fusion-fission mass distributions

We present the results of a broad, systematic study of heavy-ion induced fission mass distributions for every even-Z compound nucleus ( Z 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 CN is consistent with the results of quantitative simultaneous fitting in mass and total kinetic energy, demonstrating that fragment proton shell gaps at Z FF = 34 , 36 and Z FF = 44 , 46 are both major drivers of fission mass distributions below the actinide region. The mass distributions show enhanced yields at mass symmetry for values of Z CN equal to two times these favoured Z 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 FF = 28 , 30 .