Fission fragment angular distributions for 11B and 19F+238U systems

The fission fragment angular distributions were measured at energies above the fusion barrier, for the systems 11B and 19F + 238U. An analysis of the present data along with those already available for the systems 6,7Li, 12C, and 16O + 238U was made in terms of the saddle-point statistical model. While the anisotropies were ''normal'' for 6,7Li, 11B, 12C+238U systems, the ones for 16O and 19F+238U systems were found to be ''anomalous.'' The entrance channel mass asymmetry dependence of the anisotropies as observed here is consistent with the expectations of preequilibrium fission dynamics. This result emphasizes the importance of preequilibrium fission in heavy-ion induced fusion-fission reactions