225-232Th symmetric/asymmetric neutron-induced fission up to 200 MeVV.M. Maslov
Joint Institute for Nuclear and Energy Research, 220109 Minsk-Sosny, Belarus
Published online: 21 May 2008
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 EfBSL with significant curvature was assumed for the symmetric (SL) mode, while the inner one (EfASL(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 (EfASL-EfAAS) = 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 225,229,230,231,232Th(n,f) fission cross sections with neutron energy up to En = 200 MeV. It is shown above 80 MeV the 229,230,231,232Th(n,f) fission cross sections are no longer dependent on the Th target nuclide fissility.
© CEA 2008