Influence of multichance fission on fragment angular anisotropy in the 232Th(n, f) and 238U(n, f) reactions at intermediate energies

Fission fragment angular distributions have been measured for the Th-232(n, f) and U-238(n, f) reactions in the neutron energy range 20-100 MeV. The fragment angular anisotropy for Th-232 was found to be systematically larger than that for U-238. The obtained results have been analyzed in the framework of the statistical saddle-point model combined with pre-equilibrium and Hauser-Feshbach calculations of partial fission cross sections. The calculations have revealed that fission following multiple neutron emission takes place in both reactions, resulting in a considerable contribution of high chances to the total fission fragment angular anisotropy. This gives grounds to expect that the observed difference is mainly due to nuclei fissioning at the end of the neutron evaporation chain. (c) 2005 Elsevier B.V. All rights reserved.