Investigation of the fusion-fission reaction 208Pb + 16O at subbarrier energies

A fission cross section of a 224Th compound nucleus has been measured in the reaction 16O + 208Pb in four energy points in the interval of 5-15 MeV below the fusion barrier, which made it possible to measure cross sections by 8 orders of magnitude lower than the geometrical cross section of heavy ion interaction with nuclei. It is evident that for ion energies of 16O ≤ 72 MeV the behaviour of the fission cross section deviates from the exponential drop, which testifies to the fact that structural effects have influence on the fusion cross section in the deep subbarrier case of the interaction of two spherical nuclei. The analysis performed made it possible to describe the fusion cross sections down to the lowest energies and to make certain statements on the necessity to consider in the calculation of the fusion cross sections the lowest vibration states of coupled nuclei. The sensitivity of the method enables one to go down in the cross section by four orders of magnitude to 10-36 cm2 which is by 12 orders of magnitude less than the geometric cross section of the nuclei interaction. The interest to further investigations in this energy region is explained by a possibility of obtaining additional information on the compound states responsible for the cluster decay of nuclei. 9 refs., 2 figs., 2 tabs