Prescission neutron multiplicities in fusion-fission reactions of B+ Np, B+ Th, C+ Th, and 0+ Th, lying on either side of the Businaro-Gallone mass asymmetry (uso), have been measured. The present data along with those available in literature for compound systems spanning the fissility range from 0.70 to 0.84 were analyzed in a consistent manner to deduce fusion-fission time scales for all systems. From the systematic behavior of all the data, the three components of total dynamical fusion-fission delay, namely, transient delay, saddle-to-scission delay, and formation delay, have been deduced. It is found that the formation delay depends on the entrance channel mass asymmetry relative to Businaro-Gallone point. The variations of the fusionfission time scales with fissility, ratio of fission barrier to temperature, and entrance channel mass asymmetry have been studied.PACS number(s): 25.70.Jj

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In the present work we have measured fission fragment angular distributions and fusion excitation functions for the systems 12 C, 16 Oϩ 232 Th at energies around the Coulomb barrier. These results along with the results of our earlier measurements on 19 F ϩ 232 Th system were analyzed following Hinde et al. ͓Phys. Rev. Lett. 74, 1295 ͑1995͔͒ where it was pointed out that the anomalous fission fragment angular distributions seen in heavy ion reactions below the Coulomb barrier can be due to possible occurrence of quasifission events caused by the interaction of the projectile with the tip of the deformed target nucleus. The present data do not show the expected saturation behavior of the fission fragment anisotropy at sub-barrier energies for the 12 Cϩ 232 Th system which exhibits different behavior of the fission fragment anisotropy in the sub-barrier region as compared to the other two systems. The present measurements using different projectiles on the same target nucleus do not seem to conform to the expected behavior of the quasifission mechanism to be caused by the interaction of the projectile with the tip of the deformed target nucleus.

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