Shell-Driven Mode Evolution in the Fission of Sub-Lead Nuclei

Systematic measurements and analysis of the heavy-ion fusion-fission of 148,152,154,158Dy, 160,164,168 Yb, and 176,178,180,184,186Pt isotopes are presented, spanning a wide range of neutron-to-proton ratios in the sub-lead region. We reconstruct two-dimensional MR-RTKE distributions and perform a multi-modal Gaussian decomposition to determine the shell effects driving fission fragment formation. All systems exhibit clear evidence of shell-driven fission beyond the liquid-drop mode. A persistent inner asymmetric mode is observed at a fission fragment proton number of Z = 36 across all isotopic chains. In addition, Dy and Yb isotopes exhibit an outer asymmetric mode correlated with a fission fragment neutron number of N = 36, providing the first experimental evidence for a neutron-driven shell gap influencing fission below lead. These results demonstrate that multiple microscopic shell structures, both proton and neutron, coexist and influence fission fragment formation across isotopic chains in the sub-lead region.