^225-232Th symmetric/asymmetric neutron-induced fission up to 200 MeV

Abstract
The transition from asymmetric to symmetric fission as a function of the excitation energy and the nucleon composition of fissioning Th nuclei is investigated for the ²³²Th(n,f) reaction. Observed neutron-induced fission cross section is described in a fission/evaporation approximation. A separate relatively high outer fission barrier E_fB^SL with significant curvature was assumed for the symmetric (SL) mode, while the inner one (E_fA^SL(AS)) was assumed to be the same for symmetric and asymmetric (AS) modes. Axial asymmetry and mass symmetry is assumed for the outer saddle of the SL-mode, as distinct from the AS-mode. A drop of (E_fA^SL-E_fA^AS) = 3.5 MeV to 1--1.5 MeV for Th nuclei with A ≤ 226 is responsible for the increase of the symmetric fission contribution to ^{225,229,230,231,232}Th(n,f) fission cross sections with neutron energy up to E_n = 200 MeV. It is shown above 80 MeV the ^{229,230,231,232}Th(n,f) fission cross sections are no longer dependent on the Th target nuclide fissility.