Time scales for fusion-fission and quasifission from giant dipole resonance decay

Giant dipole resonance (GDR) γ rays in coincidence with fission fragments were measured in the reactions ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ ${+}^{208}$Pb at 120 and 140 MeV, ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ ${+}^{196}$Pt at 150 MeV, and ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$ ${+}^{\mathrm{nat}}$W at 185 MeV. γ-ray–fission angular correlation measurements for ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ ${+}^{208}$Pb (120 and 140 MeV) and ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$ ${+}^{\mathrm{nat}}$W confirm the strong contribution of prescission γ rays to the total γ-ray spectrum. The data were analyzed with a modified statistical model code which includes the effects of nuclear dissipation in the fission process, γ 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 γ=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 ${\mathrm{\tau }}_{\mathrm{fiss}}$(γ=10)≃2.9×${10}^{\mathrm{-}19}$ s at 64 MeV excitation energy agrees with that extracted from recent neutron multiplicity experiments. A strong dependence of the GDR γ-ray yield on the target-projectile mass asymmetry is interpreted in terms of the quasifission process. γ-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 ${\mathrm{\tau }}_{\mathrm{qf}}$=(2–9)×${10}^{\mathrm{-}20}$ s.