Zakopane 2026 Conference on Nuclear Physics

Light charged particle energy spectra were measured in coincidence with fission fragments in the $^{11}\mathrm{B} + {}^{232}\mathrm{Th}$ reaction at $E_{\mathrm{lab}} = 64$~MeV. The pre-scission proton multiplicity ($\pi_{\mathrm{pre}}$) was determined using the Moving Source Disentangling Analysis (MSDA). A comparison of the obtained $\pi_{\mathrm{pre}}$ and previously reported value for the $^{16}\mathrm{O} + {}^{232}\mathrm{Th}$ reaction with all the existing heavy-ion induced fission data reveal that $\pi_{\mathrm{pre}}$ is nearly an order of magnitude lower in $^{11}\mathrm{B}$ and $^{16}\mathrm{O}$ induced fission of $^{232}\mathrm{Th}$. Statistical model calculations using the code \textsc{JOANNE2} successfully reproduce the experimental $\pi_{\mathrm{pre}}$ values using standard fission delays. The primary distinction between the $^{232}\mathrm{Th}$ systems and other heavy-ion induced fission lies in the high neutron-richness of the $^{232}\mathrm{Th}$ target. By normalizing existing $\pi_{\mathrm{pre}}$ data with a factor that accounts for isospin-related effects, a robust, linearly increasing trend is established. This work reports the first observation of such distinct isospin-driven quenching effects on pre-scission proton emission. These results demonstrate that the high isospin of neutron-rich systems in Superheavy elementsts (SHE) synthesis using neutron-rich RIBs effectively shuts down the proton de-excitation channel. This enforces a neutron-only cooling regime, which maintains high fissility throughout the decay chain and fundamentally constrains the survival probability of SHEs.