Microscopic study of Ca 40 + Ni 58,64 fusion reactions

Background: Heavy-ion fusion reactions at energies near the Coulomb barrier are influenced by couplings between the relative motion and nuclear intrinsic degrees of freedom of the colliding nuclei. The time-dependent Hartree-Fock (TDHF) theory, incorporating the couplings at the mean-field level, as well as the coupled-channels (CC) method are standard approaches to describe low energy nuclear reactions. Purpose: To investigate the effect of couplings to inelastic and transfer channels on the fusion cross sections for the reactions Ca40+Ni58 and Ca40+Ni64. Methods: Fusion cross sections around and below the Coulomb barrier have been obtained from CC calculations, using the bare nucleus-nucleus potential calculated with the frozen Hartree-Fock method and coupling parameters taken from known nuclear structure data. The fusion thresholds and neutron transfer probabilities have been calculated with the TDHF method. Results: For Ca40+Ni58, the TDHF fusion threshold is in agreement with the most probable barrier obtained in the CC calculations including the couplings to the low-lying octupole 31- state for Ca40 and to the low-lying quadrupole 21+ state for Ni58. This indicates that the octupole and quadrupole states are the dominant excitations while neutron transfer is shown to be weak. For Ca40+Ni64, the TDHF barrier is lower than predicted by the CC calculations including the same inelastic couplings as those for Ca40+Ni58. TDHF calculations show large neutron transfer probabilities in Ca40+Ni64 which could result in a lowering of the fusion threshold. Conclusions: Inelastic channels play an important role in Ca40+Ni58 and Ca40+Ni64 reactions. The role of neutron transfer channels has been highlighted in Ca40+Ni64.