Fission of ra Isotopes Produced in 12C + 204,206,208Pb Reactions

The paper deals with a study of the formation and decay of Ra compound-nuclei and continues the work started in 1998. The present investigation was aimed at obtaining experimental data on the excitation functions for the ¹²C + ^204,206,208Pb reactions and studying of fragments mass and energy distributions (MED) in order to understand a nature of the fission process in At-Th region. The evaporation residues cross-sections (σ_ER) as well as the fission cross-sections (σ_tis) were measured simultaneously. It was found that the σ_ER/σ_tis ratios for all the measured reactions are about two orders of magnitude higher than one obtained for ¹⁶O + ²⁰⁸Pb reaction.

The analysis of the MED of the fission fragments has been performed in order to separate symmetric and asymmetric components of the distributions. The analysis shown that there are three components present in fission of Ra compound-nuclei. The symmetric component dominates but two asymmetric components connected with the influence of the nuclear shells Z = 28, N = 50, Z = 50 and N = 82 were also observed. Thus the statement that the multimodal nature of the fission process is an inherent characteristic of the whole At-Th region has been reinforced. The ratios of the symmetric component yield to the asymmetric one (Y_s/Y_a) have been obtained. The dependence of the Y_s/Y_a ratio on the excitation energy of the compound-nucleus showed that Y_s/Y_a does not depend on the neutron number in compound-nucleus when the excitation energy is higher than 50 MeV. On the contrary, the behavior of the Y_s/Y_a dependences becomes strongly dependent on the compound-nucleus neutron number at lower excitations. The values of the excitation energy where the asymmetric component has its maximum yield are different for different isotopes and increases as the neutron number in compound-nucleus decreases. Thus, the statement that the more neutron-deficient compound-system has the higher asymmetric fission barrier has been reinforced.