Angular anisotropy of intermediate energy nucleon-induced fission of actinide and subactinide nuclei

Abstract

New results of measurements of the angular distributions of proton-induced fission fragments of 182,183,184,186 W in the proton energy region 50-180MeV are presented. The results are discussed together with earlier data, obtained for subactinide nuclei 204,206,207,208 Pb and 209 Bi and actinide nuclei 232 Th and 233,235,238 U, for both proton- and neutron-induced fission. Similarity is noted of values and energy dependences of angular anisotropy of fission induced by protons and neutrons (taking into account a trivial effect of Coulomb factor) as well as influence on the anisotropy value of the emission character of fission of actinide nuclei and its connection with parameter of fissionability Z 2 /A. Studies of fission fragment angular anisotropies is a way to determine the state of a fissioning nucleus at the saddle point - shape, angular momentum and temperature. For the under- standing of dynamics offission process the knowledge of these key characteristics is necessary in wide range of fissioning nuclei, their excitation energies and angular momenta. With this goal, in framework of ISTC projects measurements have carried out of the angular anisotropies in neutron-induced fission of 232 Th and 238 U in the 20-160MeV energy region and 209 Bi at 75MeV (1), and in proton-induced fission of 204,206,207,208 Pb and 209 Bi in the 50-180MeV energy region (2). The results have been analyzed together with compiled data on angular anisotropies in proton-induced fission of 232 Th and 233,235,238 U (3) and subactinides (4). Analysis of the experimental data for actinides have shown that the value of the anisotropy depends on ratio of neutron and fission widths, Γn/Γf, of fissioning nuclei which determine the multi-chance structure of fission of these nuclei (its dependence on the fissionability of nucleus). In the present work new results of measurements of the angular distributions of proton-induced fission fragments of 182,183,184,186 W in the energy range 50-180MeV are presented and compared with compiled data for adjacent nuclei, 185 Re and 181 Ta, and for subactinides in lead-bismuth region. The discussion is carried out in frame of the standard statistical (transition state) model taking into account the characteristics of the intermediate compound nuclei formed in process of interaction of nucleons with target nuclei.