New approximation of the energy dependences of total cross sections for proton-induced fission of actinide nuclei

The energy dependences of the cross sections for the fission of 232Th to 239Pu nuclei that is induced by protons of energy in excess of 50 MeV are proposed to be approximated by the sum of two functions. Of these, the first decreases exponentially with increasing proton energy. It represents the contribution of two-step fission, which competes with particle evaporation. The second function decreases exponentially as the energy grows to about 200 MeV, whereupon it increases up to an energy of 1 or 2.5 GeV. After that, it again decreases for 232Th to 233U nuclei or remains nearly unchanged for 237Np and 239Pu nuclei. This function is likely to represent the sum of the contributions from three modes of single-stage fission.