Barrier distribution functions for the system 6Li+64Ni and the effect of channel coupling

Background: The barrier distribution function is an important observable in low-energy nucleus-nucleus collisions because it carries the distinct signature of the channel-coupling effect that is dominant at low energies. It can be derived from the fusion excitation function as well as from the back-angle quasi-elastic excitation function. The barrier distribution functions derived from the two complimentary measurements, in general, appear to peak at an energy close to the Coulomb barrier for strongly bound systems. But for weakly bound projectiles, like Li-6, a relative shift is observed between the distributions. Purpose: The present work investigates the barrier distribution functions from fusion as well as from the back-angle quasi-elastic excitation function for the Li-6 + Ni-64 system. The purpose is to look for the existence of a shift, if any, between the two measured distribution functions, as reported for Li-6 collision with heavy targets. A detailed coupled-channel calculation to probe the behavior of the distribution functions and their relative shift has been attempted. Measurement: A simultaneous measurement of fusion and back-angle quasi-elastic excitation functions for the system Li-6 + Ni-64 was performed. The fusion excitation function was measured for the energy range of 11 to 28 MeV while the quasi-elastic excitation function measurement extended from 11 to 20 MeV. The barrier distribution functions were subsequently extracted from both the excitation functions and compared. Results: A small shift of around 450 keV peak to peak is observed between the barrier distribution functions derived from the complementary measurements. Detailed coupled channel and coupled reaction channel calculations reproduced both the excitation functions and barrier distributions. The shift of about 550 keV resulted from the model predictions corroborate the experimentally observed value for Li-6 + Ni-64 system. Conclusions: The coupling to inelastic channels are found to be sufficient to describe the fusion-barrier distribution. The positive Q-value one-proton and one-neutron stripping channels, leading to three-body final states, on the other hand, play dominant roles in reproducing the barrier distribution from the back-angle quasi-elastic excitation function.