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

Abstract

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 232 Th(n,f) reaction. Observed neutron-induced fission cross section is described in a fission/evaporation approximation. A separate relatively high outer fission barrier E S L f B with significant curvature was assumed for the symmetric (SL) mode, while the inner one (E S L(AS ) f A ) 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 S L f A − E AS f A ) = 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.