  We model fission at barrier-top energies in a simplified model space that
permits comparison of different components of the residual nucleon-nucleon
interaction. The model space is built on particle-hole excitations of reference
configurations. These are Slater determinants of uniformly spaced orbitals
characterized only by their quantum numbers and orbital energies. The residual
interaction in the Hamiltonian includes the diabatic interaction connecting
similar orbitals at different deformations, the pairing interaction between
like nucleons, and a schematic off-diagonal neutron-proton interaction. We find
that the fission reaction probability is sensitive to the off-diagonal
neutron-proton interaction much more than to the pairing and the diabatic
interactions. In particular, the transmission coefficients become insensitive
to th e strength of the pairing interaction when the neutron-proton interaction
is large. We also find that the branching ratio is insensitive to the
final-state scission dynamics, as is assumed in the well-known Bohr-Wheeler
theory.
