Fission probes of sub-barrier fusion cross section enhancements and spin distribution broadening

The mean-square spin value of the compound nucleus ${}_{}{}^{248}{}_{}{}^{}\mathrm{Cf}$ has been determined from fission fragment angular distributions for the ${}_{}{}^{12}{}_{}{}^{}$C${+\mathrm{}}^{236}$U and ${}_{}{}^{16}{}_{}{}^{}$O${+\mathrm{}}^{232}$Th reactions at sub-barrier bombarding energies. The anisotropy and hence the mean-square spin values are much larger than predicted by models which reproduce the cross section enhancement observed in the sub-barrier fusion excitation functions. A similar experiment with the spherical target ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ also leads to larger anisotropies and mean-square spin values than predicted by various fusion models, including an especially complete coupled channels calculation. Supplementary fragment-fragment coincidence experiments have been performed with the heavier targets to confirm that the observed fission is associated with full momentum transfer. The mass distribution for the ${}_{}{}^{12}{}_{}{}^{}$C${+\mathrm{}}^{236}$U reaction has been measured at two angles and found to be independent of angle. This last result suggests that quasifission processes are not playing a significant role.