Evolution of fission-fragment mass distributions in the neutron-deficient lead region

Abstract

Low-energy $\ensuremath{\beta}$-delayed fission of $^{194,196}\mathrm{At}$ and $^{200,202}\mathrm{Fr}$ was studied in detail at the mass separator ISOLDE at CERN. The fission-fragment mass distributions of daughter nuclei $^{194,196}\mathrm{Po}$ and $^{202}\mathrm{Rn}$ indicate a triple-humped structure, marking the transition between asymmetric fission of $^{178,180}\mathrm{Hg}$ and symmetric fission in the light Ra-Rn nuclei. Comparison with the macroscopic-microscopic finite-range liquid-drop model and the self-consistent approach employing the Gogny D1S energy density functional yields discrepancies. This demonstrates once more the need for dynamical fission calculations, because for both models the potential-energy surfaces lack pronounced structures, in contrast to those for the actinide region.