Speaker: Vladimir Maslov (Joint Institute for Nuclear and Energy Research, Minsk-Sosny, Belarus)

Actinide nucleon-induced fission up to 200 MeV

Neutron-induced fission cross sections of Th, U, Np, Pu, Am and Cm target nuclides are analyzed in fission/evaporation approximation up to 200 MeV excitation energy. It is shown that observed irregularities in neutron-induced fission cross section data above fission threshold could be attributed to the interplay of few-quasiparticle excitations in the level density of fissioning and residual nuclei. Saddle asymmetries relevant to shell correction model calculations influence fission barriers, extracted by cross section data analysis. The extracted inner barrier asymmetry is correlated with shell correction model calculations of potential energy surfaces. Fission and total level densities modelling along with double-humped fission barrier parameters allow to describe available actinide neutron-induced fission cross section data in the emissive fission domain. Estimates of first-chance fission cross section and secondary neutron spectrum model were validated by consistent fission, (n,2n) and (n,3n) data description up to En = 20 MeV.

Pre-fission (n,xnf) reaction neutron spectra are calculated within Hauser-Feshbach statistical model up to En = 20 MeV. The prompt fission neutron spectra (PFNS) component due to pre-fission neutrons is evidenced in shape of measured PFNS data. We show it to be strongly correlated with emissive fission contribution to the observed fission cross section. The dependence of this contribution on target nuclide fissility and incident neutron energy is shown to be pronounced in PFNS shapes for 232Th(n,f), 238U(n,f) and 235U(n,f). High energy tails of 1st neutrons of (n,nf) and (n,2nf) reactions are shown to be pronounced in PFNS measured data trends.

For higher excitations damping of collective modes contribution to the level density at intrinsic excitation energies higher than ca. 20 MeV for saddle and equilibrium deformations is shown to be essential. It influences contributions of emissive fission chances with high and low number of prefission (n,xnf) neutrons to the observed fission cross section. Even higher 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.

The symmetric SL-mode and lumped asymmetric (S1+S2)-mode fission cross sections of 238U(n,f) reaction are analyzed up to 200 MeV. For each U nuclide, contributing to the observed fission cross section via 238U(n,xnf) reaction separate outer fission barrier is assumed 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. Ratio of symmetric to all 238U 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. Ratio of symmetric to all nucleon-induced fission events for other actinide targets is predicted.

Differences of measured proton- and neutron-induced fission cross sections of 238U target nuclide are interpreted.