Nuclear structure dependence of the sub-barrier fusion enhancement for nuclei in the mass 90 region

Cross sections for evaporation residue production have been measured for the systems 58,64Ni + 78,86Kr and 58,64Ni + 92,100Mo at energies below and above the Coulomb barrier. Coupled-channels calculations including one-and two-phonon excitations of low-lying collective states in the projectile and target give excellent agreement for systems involving the closed shell nuclei 86Kr and 92Mo. Fusion cross sections for systems involving the transitional nuclei 86Kr and 92Mo, however, show an additional fusion enhancement indicating the need for the inclusion of higher order processes in the calculations