Observation of mean-spin barrier bump in sub-barrier fusion of 28Si with 154Sm.

We have measured the fusion excitation function and gamma-ray multiplicities, M{sub gamma}, for the {sup 28}Si + {sup 154}Sm system. We have also measured M{sub {gamma}} for the {sup 16}O + {sup 166} Er system that leads to the same compound nucleus, {sup 182}Os. This is used to calibrate the connection between M{sub {gamma}} and the first moment of the spin distribution of the compound nucleus, {l angle}{ell}{r angle}. We find that the deduced {l angle}{ell}{r angle} in {sup 28}Si + {sup 154}Sm agrees reasonably well with theoretical expectations, and in particular exhibits the barrier bump expected when another degree of freedom is strongly coupled to the relative motion. 17 refs., 2 figs.