Search

We have investigated the fragmentation of $^{197}\mathrm{Au}$ projectiles at 0.85 to 1 GeV/nucleon in collisions with targets (${\mathrm{CH}}_{2}$${)}_{\mathit{n}}$, C, and Pb. Using plastic nuclear track detectors all projectile fragments with charge 6\ensuremath{\le}${\mathit{Z}}_{\mathit{F}}$\ensuremath{\le}77 produced in a collision were observed in coincidence. The combination of C and (${\mathrm{CH}}_{2}$${)}_{\mathit{n}}$ data allows us to extract the hydrogen component in measured distributions by the use of the statistical subtraction method. Our data show a target dependence of the multifragmentation process which indicates a nonequilibrated source of the fragments and/or the influence of the target nucleus on the relation between size and excitation energy of the spectator. In comparison to the prediction of the statistical model, the transverse momentum dispersions are enhanced with a dominant contribution by Coulomb repulsion between the fragments. Based on the sum of transverse momentum vectors of all fragments we conclude that multifragmentation is observed in collisions with a large momentum and energy transfer to the prefragment. Correlations in the azimuthal emission angles of the fragments are caused by mutual repulsion and momentum conservation. The idea that a liquid-gas phase transition of nuclear matter is the origin of multifragmentation is supported by our observations.

We analyze the largest set of fragment coincidence data at present available, that of the preceding paper. This set contains all projectile fragments 6\ensuremath{\le}Z\ensuremath{\le}65 produced in 6610 events of Au+Ag and Au+CR-39. We introduce a classification scheme based on the number of heavy fragments in order to define multifragmentation, spallation, and fission events. The multiplicity distribution P(${\mathit{m}}_{\mathit{Z}}$) is Poissonian, and the mean multiplicity as a function of Z can be described either by a power law or by an exponential. The slopes are independent of the projectile energy but depend on the target. While all investigated observables indicate that the multifragmentation in the case of a Ag target is a statistical process, we observe angular correlations in the case of the CR-39 target which indicate that probably the geometry and not the excitation energy is essential. Charge correlations are strong and only partially understood as yet. We find no positive sign for a liquid gas phase transition.