Anisotropy of fission fragments for the reaction [sup 16]O+[sup 208]Pb

Abstract

The cross section and angular distributions of fission fragments have been measured for the compound nucleus 224Th produced in the reaction 16O+208Pb at four energy values in the range from 5 to 15 MeV below the fusion barrier calculated in terms of the Bass model. The performed measurements allowed one for the first time to obtain data for the region of interaction of heavy ions with nuclei characterized by the cross section being by 8 orders of magnitude lower than the geometric cross section. It can be seen that at the energy of oxygen ions of up to Elab⩽82 MeV, account should be taken of the contribution made by the lowest vibration states of interacting partners. The measured angular distributions of fission fragments are in agreement with the calculations performed within the framework of the standard statistic transition state model for the saddle point. The data do not support the hypothesis that deep-subbarrier reactions are characterized by fission fragments showing anomalously high anisotropy. However in the investigated energy range there is a plateau characterized by the relation W(180°)/W(90°)=1.2 in the angular anisotropy of fission fragments. This implies that the available angular packet of harmonics, which govern the production cross section of a compound nucleus, is approximately the same.