Reaction mechanisms of the 18 O + 63 Cu system at near-barrier energies

A precise quasielastic excitation function for the $^{18}\\mathrm{O}+^{63}\\mathrm{Cu}$ system has been measured at energies around the Coulomb barrier at ${\\ensuremath{\\theta}}_{\\mathrm{LAB}}={161}^{\\ensuremath{\\circ}}$. The corresponding quasielastic barrier distribution has been derived. Two-neutron-, one-proton-, and \\ensuremath{\\alpha}-transfer-excitation functions have also been measured at the same energies and angle. Coupled reaction channels calculations were performed to describe the experimental data. Large-scale shell-model calculations were performed to derive most of the spectroscopic amplitudes. No surface imaginary potential was necessary for the interaction potential because almost all relevant reaction channels were explicitly included in the calculation. The theoretical results were compared to the experimental quasielastic barrier distribution and a very good agreement was achieved. The comparison of the coupled reaction channel calculations and data has put in evidence several important details of the reaction mechanism of the $^{18}\\mathrm{O}+^{63}\\mathrm{Cu}$ system. The collectivity of the $^{63}\\mathrm{Cu}$ nucleus has important contribution to the reaction mechanism of this system, mainly due to its first $5/{2}^{+}$ and $7/{2}^{+}$ states. It was also observed a striking influence on the reaction dynamics of the $^{18}\\mathrm{O}$(${2}^{+}$) state, the two-neutron transfer and the reorientation of the target ground-state spin. The best agreement to data was achieved when the nuclear matter diffuseness for the $^{18}\\mathrm{O}$ was assumed equal to 0.60 fm, value that we have derived in a previous paper and that is 10% greater than the $^{16}\\mathrm{O}$ diffuseness. Another significant result was that the two-neutron transfer process is much more relevant than the one-neutron-transfer process, which suggests that the pairing correlation could play an important role in the transfer process of this system.