Mass asymmetry dependence of scission times in the reactions of 18.5A MeV 136Xe+48Ti.

The multiplicities of p and a particles detected in coincidence with fragments emitted in fully relaxed collisions in the reactions of 18.5A MeV "Xe+ 'Ti have been measured for different exit channel mass asyrnrnetries. A kinematic source analysis of the spectra and angular distributions of the light particles has been used to separate the total multiplicities into prescission and postscission contributions. From these results, the excitation energies at scission are determined using an empirical technique based upon previous measurements of light charged particle multiplicities observed in coincidence with evaporation residues. These excitation energies are found to decrease from -400 MeV to 110 MeV as the fragment mass asymmetry, AII /Al, varies from 4.8 to 1.0. A corresponding increase of the mean lifetime of the scissioning nucleus from -5 X 10 s to —1 X 10 s is derived using calculated statistical model decay widths. The extent to which this variation of lifetime with mass asymmetry may be attributed to completely damped deep inelastic collisions or to dynamic delays in the decay of a compound nucleus is discussed as is the need for inclusion of dynamics in the deexcitation calculations for hot nuclei. Observed three fragment events are also discussed.