Anomalous anisotropy of fission fragments in near- and sub-barrier complete fusion-fission reactions ofO16+232Th,F19+232Th, andO16+238U

Using the fragment folding angle technique, the complete fusion-fission is separated from the transfer-induced fission. The cross sections of the complete fusion-fission and fragment angular distributions for the systems of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ ${+}^{232}$Th, ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ ${+}^{232}$Th, and ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ ${+}^{238}$U at near- and sub-barrier energies have been measured. The observed fission excitation functions can be fitted very well by coupled-channels theory calculations. It is found that fragments from complete fusion fission show a smaller angular anisotropy as compared with our previous measurements in which the transfer-induced fission component was not excluded. However, the measured angular anisotropies of complete fusion-fission are obviously greater than expected on the basis of theoretical models in which the effect of prefission neutron emission was taken into account. Also the peak in the variation of the anisotropy with incident energy still persists in the ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ ${+}^{232}$Th case.