  Background: Recent experiments on beta-delayed fission reported an asymmetric
mass yield in the neutron-deficient nucleus 180Hg. Earlier experiments in the
mass region A=190-200 close to the beta-stability line, using the (p,f) and
(\alpha,f) reactions, observed a more symmetric distribution of fission
fragments. While the beta-delayed fission of 180Hg can be associated with
relatively low excitation energy, this is not the case for light-ion reactions,
which result in warm compound nuclei. Purpose: To elucidate the roles of proton
and neutron numbers and excitation energy in determining symmetric and
asymmetric fission yields, we compute and analyze the isentropic potential
energy surfaces of 174,180,198Hg and 196,210Po. Methods: We use the
finite-temperature superfluid nuclear density functional theory, for excitation
energies up to E*=30MeV and zero angular momentum. For our theoretical
framework, we consider the Skyrme energy density functional SkM* and a
density-dependent pairing interaction. Results: For 174,180Hg, we predict
fission pathways consistent with asymmetric fission at low excitation energies,
with the symmetric fission pathway opening very gradually as excitation energy
is increased. For 198Hg and 196Po, we expect the nearly-symmetric fission
channel to dominate. 210Po shows a preference for a slightly asymmetric pathway
at low energies, and a preference for a symmetric pathway at high energies.
Conclusions: Our self-consistent theory suggests that excitation energy weakly
affects the fission pattern of the nuclei considered. The transition from the
asymmetric fission in the proton-rich nuclei to a more symmetric fission in the
heavier isotopes is governed by the shell structure of pre-scission
configurations.
