Sub-barrier fusion of Ca-40+Zr-94: Interplay of phonon and transfer couplings

A precise fusion excitation function has been measured for Ca-40+Zr-94 at near- and sub-barrier energies, and the fusion barrier distribution has been extracted. Comparing with the existing data for Ca-40,Ca-48+Zr-90,Zr-96 shows that couplings to inelastic excitations determine the fusion cross sections and the shape of the barrier distributions near the barrier. At lower energies, the two systems possessing neutron-transfer channels with large positive Q-values (Ca-40+Zr-94,Zr-96) have remarkably similar cross sections, and both show a large enhancement with respect to the other systems and to coupled-channels calculations including up to three quadrupole and octupole phonons in the Zr targets.