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Velocity correlations of intermediate mass fragments (IMF), produced in collisions of Au+Au at 100, 150, 250, and 400{ital A} MeV beam energy, are extracted from measurements with the 4{pi} detector system (FOPI) in construction stage I at Schwerionen-Synchrotron (SIS) at the Gesellschaft fuer Schwerionenforschung (GSI) in Darmstadt. The IMF correlation functions of peripheral and semicentral events are found to be strongly affected by the collective sideward motion of nuclear matter. The sideflow causes an enhancement of correlations at small relative velocities. This enhancement results from the mixing of differently azimuthally oriented events; it vanishes if the events are rotated into a unique reaction plane. Selecting violent central collisions, the comparison of the data with a Coulomb dominated final-state interaction model points to a radius of the expanding and multifragmenting source of {ital R}{sub {ital s}}{congruent}13 fm for 100{ital A} MeV which appears shrinking by 20% when increasing the projectile energy to 400 MeV per nucleon. The deduced source radii are found to depend on the radial explosion energy used in the model. The inclusion of such a collective expansion is necessary for a reasonable description of the experimental single-particle spectra of the IMF. The unique Coulomb suppression of small relativemore » IMF velocities, found for the given beam energy range, is attributed to rather constant averaged next-neighbor distances {l_angle}{ital d}{sub IMF}{r_angle}=8.6{plus_minus}0.2 fm of the IMF charge centers within the source at breakup time.« less

Triple-differential cross sections have been measured as a function of product mass, total kinetic energy, and center-of-mass scattering angle in reactions induced by $^{238}\mathrm{U}$ on $^{16}\mathrm{O}$, $^{26}\mathrm{Mg}$, $^{27}\mathrm{Al}$, $^{32}\mathrm{S}$, $^{35}\mathrm{Cl}$, $^{40}\mathrm{Ca}$, $^{48}\mathrm{Ca}$, and $^{\mathrm{nat}}\mathrm{Zn}$ targets at several bombarding energies between 4.6 and 7.5 MeV/nucleon. The analysis focuses on binary processes in which the product masses are substantially different from the target-projectile masses. These include the complete fusion followed by fission as well as quasifission processes in which large mass transfers occur on a short time scale. The relative contributions of these two components are estimated from the mass-angle correlations and analyzed within the extra and extra-extra push concepts. The time scale for mass transfer in quasifission reactions is derived from turning angles of the intermediate complex, and it is found that the mass drift toward symmetry shows the characteristics of an overdamped motion with a universal time constant independent of scattering system and bombarding energy. This is consistent with the one-body nuclear dissipation mechanism being responsible for the damping in the mass asymmetry degree of freedom. Also the average total kinetic energy of reaction products in quasifission is independent of temperature, supporting the one-body dissipation hypothesis.