Observation of mean-spin barrier bump in sub-barrier fusion ofSi28withSm154

We have measured the fusion excitation function and γ-ray multiplicities ${\mathit{M}}_{\gamma }$ for the ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ ${+}^{154}$Sm system. We have also measured ${\mathit{M}}_{\gamma }$ for the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ ${+}^{166}$Er system that leads to the same compound nucleus, ${}_{}{}^{182}{}_{}{}^{}\mathrm{Os}$, as a calibration of the connection between ${\mathit{M}}_{\gamma }$ and the first moment of the spin distribution, 〈l〉. We find that the deduced 〈l〉 for ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ ${+}^{154}$Sm agrees reasonably well with theoretical calculations, and in particular its energy dependence exhibits the barrier bump expected when a shape degree of freedom is strongly coupled to the relative motion.