Neutron multiplicities in heavy-ion-induced fission: Timescale of fusion-fission

Neutron fission-fragment angular correlations have been measured for the compound nuclei 168Yb, sup(192,198,200)Pb, 210Po, 213Fr and 251Es formed at excitation energies between 45 and 90 MeV by fusion reactions induced by beams of sup(16,18)O, 19F and sup(28,30)Si. Analysis allows the determination of the mean multiplicities of neutrons originating from the fused system (νsub(pre)) and the fission fragments (2νsub(post)). The total multiplicities (νsub(pre)+2νsub(post)) are in good agreement with calculated values. However, calculations of νsub(pre) using the statistical model code ALERTI underestimate the multiplicities, the discrepancy increasing with excitation energy and fissility. This is interpreted in terms of the dynamics of the fission process, and can be resolved by including in the calculations the effects of a delayed onset of fission (delay time 70 x 10-21s) or a slow saddle-to-scission transition (transit time 30 x 10-21s). Simple theoretical estimates suggest that both effects are significant, and their characteristic times may be similar. With these constraints, the data are fitted when both effects are applied, each for 20 x 10-21s. Such long times are not inconsistent with the results of pure one-body dissipation calculations, and are supported by recent results on the timescale of quasi-fission. The quoted results are for a level-density parameter αsub(n)=1/10 A. Use of 1/12 A (1/8 A) results in shorter (longer) times; however, it is concluded that for all reasonable values, motion in the fission direction is overdamped. (orig.)