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

Abstract

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