TIME SCALES FOR FUSION-FISSION AND QUASI-FISSION FROM GIANT-DIPOLE RESONANCE DECAY

Giant dipole resonance (GDR) gamma-rays in coincidence with fission fragments were measured in the reactions O-16 + Pb-208 at 120 and 140 MeV, Mg-24 + Pt-196 at 150 MeV, and S-32 + (nat)W at 185 MeV. gamma-ray-fission angular correlation measurements for O-16 + Pb-208 (120 and 140 MeV) and S-32 + (nat)W confirm the strong contribution of prescission gamma-rays to the total gamma-ray spectrum. The data were analyzed with a modified statistical model code which includes the effects of nuclear dissipation in the fission process, gamma decay at equilibrium and between saddle and scission, and the decay of the mononucleus formed in quasifission reactions. The extracted value for the nuclear friction coefficient of gamma = 10 +/- 3 at temperatures between 1 and 2 MeV is much larger than the upper limit for the ground state and indicates strongly overdamped large-scale mass motion. The extracted total fission time scale tau-fiss(gamma = 10) congruent-to 2.9 x 10(-19) s at 64 MeV excitation energy agrees with that extracted from recent neutron multiplicity experiments. A strong dependence of the GDR gamma-ray yield on the target-projectile mass asymmetry is interpreted in terms of the quasifission process. gamma-ray spectra and correlations could be fitted with the assumption of a GDR contribution from the mononucleus but required an arbitrary reduction of the total fission fragment excitation energy by 30 MeV. The quasifission lifetime was estimated to tau-qf = (2-9) x 10(-20) s.