Three-Particle Exclusive Measurements of the ReactionsU238+U238andU238+Cm248

Kinematically complete experiments have been performed on the three-body exist channels in 7.5 MeV/amu ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ + ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ and ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ + ${}_{}{}^{248}{}_{}{}^{}\mathrm{Cm}$. No evidence was found for an instantaneous three-body breakup. Instead, a two-step nature with minimum scission-to-scission times of ${10}^{-20\mathrm{}}$ s has been derived even for intermediate nuclei $Z>\mathrm{}110\mathrm{}$. However, the observed fission-fragment angular distributions in this region are incompatible with the usual saddle-point concept.