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

Low-energy beta-delayed fission of At-194,At-196 and Fr-200,Fr-202 was studied in detail at the mass separator ISOLDE at CERN. The fission-fragment mass distributions of daughter nuclei Po-194,Po-196 and Rn-202 indicate a triple-humped structure, marking the transition between asymmetric fission of Hg-178,Hg-180 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.