Symmetric∕asymmetric p- and n-induced fission of Th, Pa, U and Np

Abstract

The excitation energy and nucleon composition dependence of the transition from asymmetric to symmetric scission of fission observables of Th(Pa) and U(Np) nuclei is interpreted for nucleon‐induced fission cross sections of 232Th(p,F)(232Th(n,F)) and 238U(p,F) (238U(n,F)) reactions at En(p) = 1–200 MeV. Predominantly symmetric fission in 232Th(p,F) and roughly equal contributions of symmetric and asymmetric modes in 238U(n,F) at En(p) = 200 MeV as revealed by experimental branching ratios is reproduced. Steep transition from asymmetric to symmetric fission with increase of nucleon incident energy is due to fission of neutron‐deficient Th(Pa) (A<229) nuclei, since about 20 neutron‐deficient nuclides might contribute to the fission reaction at En(p)∼200 MeV. A structure of the potential energy surface (a drop of symmetric and asymmetric fission barriers difference (ESYMf−EASYMf) from ∼3.5 MeV to ∼1 MeV) of N‐deficient Pa nuclides (A<226) and available phase space at outer fission saddles, are shown to be re...