Conclusive evidence for the influence of nuclear orientation on quasifission

Fission fragment anisotropies and mass distributions have been measured to high accuracy, over a wide range of angles, for the ^16O + ^238U reaction. The bombarding energies spanned the fusion barrier distribution, in steps of 1 MeV. Fission following transfer reactions was rejected by making use of the deduced velocity vectors of the fissioning nuclei. The resulting mass distributions for full momentum transfer fission 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 is conclusive evidence for the interpretation that collisions with the tips of the deformed ^238U target nuclei lead to quasifission, while collisions with the sides result in fusion-fission. 1996 The American Physical Society.