EXCITATION-ENERGY DIVISION IN V-51-197 COLLISIONS AT AND NEAR THE BARRIER(AU)

Mass- and charge-yield distributions for 19 < Z < 84 were determined r adiochemically for the binary collision products of V-51 + (197)AU col lisions at a bombarding energy corresponding exactly to the Bass-model barrier, E(cm) = B, and at E(cm) = B + 25 MeV. The average excitation energies as a function of Z are determined by comparing the centroids of the experimental, secondary mass distributions for given values of Z with the calculated primary centroids from minimization of the pote ntial energy of the di-nuclear system, i.e. from the missing masses. A t the barrier, in striking contrast to a thermal equilibrium, we find an extreme donor-acceptor asymmetry in the excitation-energy division reminiscent of the ''sawtooth'' phenomenon in low-energy nuclear fissi on. Here, the excitation energy sharing is apparently dominated by sha pe fluctuations at scission. At the slightly higher bombarding energy, E(cm) = B + 25 MeV, we observe a rapid change toward equipartition of the excitation energy indicating that, here, the excitation energy di vision due to shape fluctuations is already covered up by the dissipat ive exchange of nucleons. Also, the balance of integral cross sections for fusion fission, deep-inelastic scattering, and quasi fission is i nvestigated and is shown to contain important information about the dy namical evolution of the V-51 + Au-197 system after having passed the entrance channel barrier.