Symmetric uranium neutron-induced fission

The symmetric SL-mode fission cross sections of 235U(n, f) and 238U(n, f) are analyzed up to the emissive fission threshold within a Hauser-Feshbach statistical model. A separate outer fission barrier is assumed for the SL-mode, while the inner one is assumed to be the same for the symmetric SL- and the asymmetric S1- and S2-modes. Axial asymmetry and mass-symmetry is assumed for the outer saddle of the SL-mode, as distinct from the S1- and S2-modes, which are axially symmetric and mass-asymmetric. The observed fission cross section behaviour is modelled using the interplay of few-quasiparticle excitations in the level density of the fissioning and residual nuclei. The SL-mode fission cross section seems to be controlled by a rather high outer fission barrier with significant transparency