Symmetric vs asymmetric nucleon-induced fission of thorium up to 200 MeV

The transition from asymmetric to symmetric fission as dependent on the excitation energy and nucleon composition of fissioning Th and Pa nuclei is investigated for the Th-232(n, F) and Th-232(p, F) reactions, respectively. Observed neutron-induced fission cross sections are described in a fission/evaporation approximation. A separate relatively high outer fission barrier E-fB(SL) with significant curvature was assumed for the symmetric mode, while the inner one (E-fA(SL(AS))) was assumed to be the same for symmetric (SL) 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 similar to 1-1.5 MeV for Th and Pa nuclei with A <= 226 is responsible for the increase of the symmetric fission contribution to Th-225,Th-229,Th-230,Th-231,Th-232(n, F) or Th-232(p, F) fission cross sections with nucleon energy up to E-n(p) = 200 MeV. It is shown that with increase of the E-n(p) above similar to 80 MeV the 229,230,231,232Th(n, F) fission cross sections are no longer dependent on the Th target nuclide fissility. The dependence of Th-232 fission cross section on the incident particle (n or p) is predicted to be different than in case of U-238 target nuclide. (c) 2007 Elsevier B.V. All rights reserved.