Dominance of Collective over Proton Transfer Couplings in the Fusion of 32 S and 34 S with 89 Y

Fusion excitation functions have been measured to high precision for the reactions 32S+89Y and 34S+89Y. The sub-barrier fusion cross sections for 32S+ 89Y are found to be more enhanced over single-barrier penetration calculations than those for 34S + 89Y, and the shapes of the two experimental barrier distributions are significantly different. The effects of proton transfer and double phonon excitations have been examined using exact coupled channels calculations. These suggest that the difference between the two systems lies mainly in the collectivity of the projectiles, whilst proton transfer has only a minor effect