Fission before K equilibration

It is demonstrated experimentally that the reaction systems on the different side of Businaro-Gallone critical mass asymmetry (αBG) have different characteristics in fusion-fission reactions. Fragment anisotropies resulting from the reactions with the entrance-channel mass asymmetry α approx-gt αBG are well described by the saddle-point transition-state model. However, the measured fragment anisotropies for the systems with α<αBG are obviously greater than the predictions of this model at subbarrier and near-barrier energies and gradually tend to coincide with the theoretical expectations as the bombarding energy increasing over fusion barrier. These observations have led us to a suggestion of preequilibrium fission for low angular momentum. The predictions of such a preequilibrium fission model are compared with the available experimental data and it is shown that they can satisfactorily reproduce the observed trends as a function of the bombarding energy for the reaction systems studied. copyright 1996 The American Physical Society