Attosecond time delays in heavy-ion induced fission measured by crystal blocking

The time delays in fission induced by bombardment of W with 180 MeV S32, 240-255 MeV Ti48, 330-375 MeV Ni58, and 390 MeV Ge74 have been measured by observation of crystal blocking. Nearly all results are consistent with exponential decay with lifetimes of order 10-18 s which depend weakly on the atomic number of the composite nucleus. This is inconsistent with the Bohr-Wheeler model of fission from a compound nucleus in statistical equilibrium at each stage in a neutron evaporation cascade and supports a picture of strongly damped quasifission. A simple diffusion model with one-body dissipation reproduces roughly the observed time scale and the exponential decay. It suggests that the outer fission barrier could play a significant role in the observed, very slow decays.