Time scales for binary fragmentations of highly excited, fissile nuclei fromS32+197Au,Th232

Fission of ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ ${,}^{232}$Th+838-MeV ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$ projectiles was studied by measuring fragment coincident neutrons. Neutron energy spectra were decomposed into preequilibrium, prescission, and postscission contributions with a constrained moving source analysis. Excitation energies deduced from the transferred linear momentum are consistent with calculations applying the Boltzmann master equation, and with an energy balance based upon the experimental neutron multiplicities and charged particles from evaporation calculations. The time scale of fission derived from the prescission neutron multiplicities extends from 5×${10}^{\mathrm{-}21}$ to 3×${10}^{\mathrm{-}20}$ s; it does not depend on the initial excitation energy, is about a factor of 2 longer for symmetric than for asymmetric fragmentations, and increases with the fissility of the primary reaction system. For all mass splits, the excitation energy left at scission is 50–60 MeV.