HINDRANCE IN DEEP SUB-BARRIER FUSION REACTIONS

摘要

The steep falloff in the fusion cross section at energies far below the Coulomb barrier is discussed in the frame of the Coupled Channels metod for various symmetric and asymmetric projectile-target combinations where this phenomenon was very recently discovered. We incorporate a repulsive core in the nuclear potential which accounts for the saturation of nuclear matter and therefore provides the correct value of the incompressibility. The result of the incorporation of the nuclear Equation of State is that the internal part of the sudden heavy-ion potential becomes more shallow and consequently the fusion cross sections decreases at bombarding energies far below the barrier. The importance of the inclusion of the low-lying 2~(+) and 3~(-) states in both target and projectile as well as mutual and two-phonon excitations of these states is highlighted. In overall we obtain a very good fit to the data for the cases ~(58)Ni+~(58)Ni, ~(64)Ni+~(64)Ni and ~(64)Ni+~(74)Ge and a satisfactory fit for the more asymmetric case ~(64)Ni+~(100)Mo. We also present the analysis of the data using diagnostic tools well suited for deep sub-barrier energies, such as the astrophysical factor and the logarithmic derivative ans spin distribution. We predict, in particular, a distinct double peaking in the S-factor for the far subbarrier fusion of ~(58)Ni+~(58)Ni which should be tested experimentally. The relation between anti-resonances and the maximum in the average angular momentum is discussed.