Systematics of fusion-fission time scales.

Abstract

New pre-scission neutron multiplicity (..nu../sub pre/) data for /sup 7/Li-, /sup 16/O-, and /sup 20/Ne-induced fission are presented, spanning a fissility range from 0.60 to 0.85. Fission time scales have been deduced for two extreme assumptions regarding the mean excitation energy during fission. It is found for fusion-fission reactions that the fission time scale is independent of fissility, within a factor 1.5. A comparison of time scales deduced from ..nu../sub pre/ measurements for fast-fission with quasifission time- scales deduced from the rotation angle of the composite system allows a minimum fusion-fission time scale of 30/times/10/sup /minus/21/ s to be determined. For the most fissile system, fast-fission (fission without barrier) is shown to be up to three times faster than fusion-fission. Using a model to interpret the fusion-fission time scales, it is concluded that motion in the fission direction is strongly overdamped. This means that the dynamics of fission are dominated by a slow diffusion towards scission, and not by the potential energy surface. It is shown that such a picture can explain the observed lack of dependence of the fission time scale on fissility.