Symmetric/asymmetric 238U neutron-induced fission up to 200 MeV

The symmetric SL-mode and lumped asymmetric (S1 + S2)-mode fission cross sections of U-238(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 U-238(n,xnf) reaction separate outer fission barrier is assurned 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 U-238(n, F) observed fission cross section description. Ratio of symmetric to all U-238 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. (C) 2003 Elsevier Science B.V. All rights reserved.