Dominance of collective over proton transfer couplings in the fusion of ³²S and ³⁴S with ⁸⁹Y

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.