Sub-barrier fusion of 27Al+70,72,73,74,76Ge: Evidence for shape transition and structure effects

Fusion excitation functions were obtained for 27Al+70,72,73,74,76Ge at energies from about 6 MeV below to 7 MeV above the Coulomb barrier. One-dimensional barrier penetration model calculations with one free parameter yield parameters in good agreement with the systematics for fusion above the barrier. Large low-energy enhancements are observed whose trend suggests the presence of a structural change between 70,72Ge and 73,74,76Ge. Within the context of simple model calculations, this trend is explained as arising from the odd-A structure of 73Ge on one hand, and from a spherical (or oblate) to prolate shape transition between 70,72,73Ge and 74,76Ge, on the other hand