CONCLUSIVE EVIDENCE FOR THE INFLUENCE OF NUCLEAR ORIENTATION ON QUASI-FISSION

Fission fragment anisotropies and mass distributions have been measure d to high accuracy, over a wide range of angles, for the O-16 + U-238 reaction. The bombarding energies spanned the fusion barrier distribut ion, in steps of 1 MeV. Fission following transfer reactions was rejec ted by making use of the deduced velocity vectors of the fissioning nu clei. The resulting mass distributions for full momentum transfer fiss ion show a small but significant skewness, which increases as the beam energy falls through the fusion barrier region, displaying a similar energy dependence as the fission fragment angular anisotropies. This i s conclusive evidence for the interpretation that collisions with the tips of the deformed U-238 target nuclei lead to quasifission, while c ollisions with the sides result in fusion-fission.