Symmetric/asymmetric [formula omitted] neutron-induced fission up to 200 MeV

Abstract

The symmetric SL-mode and lumped asymmetric (S1+S2)-mode fission cross sections of 238 U (n, f) reaction are calculated up to incident neutron energy of 200 MeV within a Hauser–Feshbach statistical model. For each U nuclide, contributing to the observed fission cross section via 238 U (n, xnf) reaction separate outer fission barrier is assumed for the SL-mode fission, while the inner one is assumed to be the same either for the symmetric SL- and lumped asymmetric (S1+S2)-mode fission. Inner saddle is axial asymmetric and mass-symmetric for fissioning U nuclei with number of neutrons N>144, for lower N values axial symmetry is assumed to persist. Axial asymmetry and mass-symmetry are assumed for the outer saddle of the SL-mode, as distinct from asymmetric fission modes, for which axial symmetry and mass-asymmetry are assumed. The SL-mode fission cross section up to emissive threshold is controlled by a rather high outer fission barrier with significant transparency. Damping of collective modes contribution to the level density at intrinsic excitation energies higher than 20–30 MeV for axial symmetric saddle and equilibrium deformations seems to be essential for the 238 U (n, F) observed fission cross section description. Ratio of symmetric to all 238 U neutron-induced fission events is described assuming damping of triaxial collective modes contribution to the level densities at outer saddles relevant for the SL-mode. High sensitivity of the ratio of symmetric and asymmetric fission cross sections to the relative contributions of emissive fission chances with high and low number of pre-fission (n, xnf) neutrons is observed.