Anomalously Broad Spin Distributions in Sub-barrier Fusion Reactions

The mean square spin value of the compound nucleus ${}_{}{}^{248}{}_{}{}^{}\mathrm{Cf}$ has been determined from fissionfragment angular distributions for the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{236}{}_{}{}^{}\mathrm{U}$ and ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ + ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ reactions. The anisotropy and, hence, the mean square spin values are much larger than expected. Models which reproduce the enhancement observed in the sub-barrier fusion excitation functions fail to account for the spin-distribution observations.