Fission of ' Au, ' Th+838-MeV ' 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 X 10 " to 3 X 10 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.

The time scale of fission at initial nuclear temperatures of about 5 MeV is deduced from the number of neutrons evaporated prior to and after scission in the reactions 144 Sm and 154 Sm+ (838 MeV) 32 S. The prescission lifetime for fission with symmetric mass splits is longer than for asymmetric mass splits. For symmetric mass splits the excitation energy at scission is only about 60 MeV and independent of the initial excitation energy which is consistent with a prescission lifetime in the order of 10 ~2 0 s. PACS numbers: 25.70.Jj, 25.70.Gh, 25.85.Ge, 27.70.+q Fifty years after the discovery of nuclear fission the dynamics of the division of the nucleus into two parts of roughly equal size is still one of the most interesting processes of collective flow of nuclear matter and an ideal example of the yet unsolved nuclear many-body problem. The time scale of nuclear fission has recently been studied 1 " 4 very intensively up to nuclear temperatures of about 2.7 MeV by several groups. One method employed 1 " 3 the number of neutrons evaporated prior to scission to deduce the prescission lifetime by calculating the mean evaporation time and thus relating the prescission neutron multiplicity with an absolute time scale. A second method 4 exploited the giant-dipole-resonance y rays emitted from the composite systems produced in heavy-ion-fusion reactions. Both methods led to the conclusion that nuclear fission at temperatures up to 2.7 MeV is very slow, about a few 10 ~2 0 s. In other words, nuclear fission, or rather scission, occurs at the very end of the nuclear deexcitation chain. An exciting question, then, is whether this finding prevails also at higher excitation energies or nuclear temperatures. In the present experiment we have investigated for the first time the fission time scale of nuclei at initial temperatures of about 5 MeV or excitation energies of about 500-600 MeV in nuclei of mass close to 180. Furthermore, we have measured for the first time the time scale of fission with symmetric and asymmetric mass splits. At such high temperatures one might expect that the temperature dependence of the nuclear surface tension 5 and/or nuclear dissipation 6 could drastically influence the time scale of nuclear fission. In the following we will briefly discuss the experiment and then present the results for the number of neutrons emitted prior to and after scission, which we use to establish a relative and, with certain assumptions, absolute time scale.Targets of isotopically enriched 144 Sm and 154 Sm with thicknesses of 185 and 216 ^g/cm 2 , respectively, on carbon backings of 50 //g/cm 2 were bombarded with 838-MeV 32 S ions at the VICKSI accelerator in Berlin. Two heavy fragments were detected in coincidence with two x and y position-sensitive low-pressure multiwire chambers providing also good time resolution of about 0.2 ns. The absolute time-of-flight resolution in reference to the cyclotron frequency was 0.8 ns. The centers of these detectors were positioned to the left and right of the beam at 0i = -4...