Evolution of shell-driven fission below the actinides

The results of a systematic measurement of heavy-ion induced fission of every even-Z nucleus from ¹⁴⁴Gd to ²¹²Th ($Z_{\mathrm{CN}}=64\text{ – }90$) along a line of constant $N/Z$ is presented. We identified shell-driven fission in all systems studied. The evolving structure of the fission mass distributions with increasing $Z_{\mathrm{CN}}$ and two-dimensional mass-kinetic fitting indicates that deformed proton shell effects in the nascent fission fragments of $Z_{\mathrm{FF}}=34,36$ and $Z_{\mathrm{FF}}=44,46$ as the primary drivers of mass-asymmetric fission in this region. For light systems ($Z_{\mathrm{CN}}<80$) $Z_{\mathrm{FF}}=34,36$ can be seen to drive fission as either the heavy or light fission fragment and also at mass-symmetry with changing $Z_{\mathrm{CN}}$.