Proton-induced fission of actinide nuclei at intermediate energies

Dynamics of large scale collective motion is determined by the interplay between global liquid drop properties and nuclear shell effects giving specific charge, mass and kinetic energy distributions of fission fragments. The investigation of light-particle-induced fission of heavy nuclei at intermediate energies is mainly connected with the problem of understanding the fission process with increasing excitation energy of the compound nucleus. The other important aspects of the fission dynamics are the contribution of various fission modes and diminishing of shell structure effects with increasing projectile energy. In particular, the search for super asymmetric fission mode, connected with the influence of the nuclear shells N=50 and Z=28 (78Ni fission mode) is of great interest. The multi-parameter correlation study of the proton-induced fission of 232Th, 238U, 242Pu at the projectile energy range from 13 up to 55 MeV has been carried out. The fission fragment mass, total kinetic energy distributions, double differential neutron spectra and γ-ray spectra have been measured. Pre-equilibrium, pre-scission and post-scission neutron and γ-ray multiplicities, measured in coincidence with primary fission fragments, gave possibility to investigate the fission dynamics in more detail. The post-neutron and γ-ray multiplicities as functions of fission fragment mass show clear sawtooth structure that is gradually washed out with increasing proton energy. Nevertheless, shell structure effects were observed in fission fragment mass distributions even at high excitation energy. Some indications on the existence of the nuclear shell Z = 28 near fragment mass Afr = 78 has been observed.(author)