Anomalous anisotropies of fission fragments for theO16+232Th sub-barrier fusion-fission reaction

Fission cross sections and angular distributions have been measured for the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ ${+}^{232}$Th reaction at the bombarding energies from 76 to 86 MeV. The fission excitation function is rather well reproduced on the basis of the Wong model. However, the model that reproduces the enhancement observed in the sub-barrier fusion excitation function fails to account for the experimental anisotropies. It is found that the observed anisotropy is much larger than expected, and as a function of center-of-mass energy it exhibits a peak around 76 MeV with a width (FWHM) of 3.7±0.5 MeV. A comparison with our previous data of ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ ${+}^{232}$Th reaction reveals that the anomaly is strongly related to the nuclear structure property of the projectile nucleus.