No abstract

The mean multiplicity of intermediate mass fragments (IMF) (M«") produced by fragmentation of Au projectiles interacting with targets of C, Al, Cu, and Pb at an incident energy of E/A =600 MeV is compared to predictions of statistical multifragmentation and sequential evaporation models. The initial conditions for the calculations were provided by Boltzmann-Uehling-Uhlenbeck simulations. In the high excitation energy regime where the IMF multiplicity reaches its maximum the observed universal correlation between (M«") and the total charge Zb, ""~o f projectile fragments with charges Z~2 cannot be reproduced by a sequential evaporation code. In this regime the data are better described by statistical decay calculations which assume the formation of an expanded nuclear system and a rather fast breakup.

Abstract. Events with 2, 3 and 4 heavy fragments (A > 20) detected in the reactions ~~176 + l~176 at 18.7, 23.7 A" MeV and ~2~ + ~Z~ at 18.4 A' MeV were analyzed by means of an improved version of the kinematic coincidence method. The phase-space distributions prove that 3-(and possibly 4-) body events predominantly originate from a two-step mechanism and are compatible with the hypothesis of a binary deep-inelastic interaction followed by the further fissionlike decay of one (or both) of the primary fragments. The characteristics of the fission step -mass asymmetry, relative velocity, in-plane and out-of-plane angles -have been reconstructed for the 3-body events and indications are found that nonequilibrium effects at the end of the deep-inelastic phase may influence the fissionlike decay.

We have studied the fragmentation of Au projectiles interacting with targets of C, A1 and Cu at an incident energy of E/A = 600 MeV. The employed inverse kinematics allowed a nearly complete detection of projectile fragments with charge Z > 2. The recorded fragmentation events were sorted according to three observables, the multiplicity Mtp of light charged particles, the largest atomic number Zma x within an event, and a new observable, Zbound, representing the sum of the atomic numbers Z of all fragments with Z>__ 2. Using these observables, the impact parameter dependence of the fragmentation process was investigated. For all three targets, a maximum mean multiplicity of 3 to 4 intermediate mass fragments (IMFs) is observed. The corresponding impact parameters range from central collisions for the C target to increasingly peripheral collisions for the heavier targets. It is found that the correlation between the IMF multiplicity and Zbouna, extending from evaporation type processes (large Zbouna) to the total disassembly of the projectile (small Zbound) , is independent of the target nucleus. This universal behaviour may suggest an -at least partial -equilibration of the projectile fragment prior to its decay.

Events with 2, 3 and 4 heavy-fragments (A>20) have been detected in the reactions l~176 l~176 at E/A = 18.7, 23.7 MeV and 12~ t2~ at E/A= 18.4 MeV.The experiments were performed with an array of 12 detectors which together covered a large fraction of the forward hemisphere and allowed a high detection efficiency for these events. Masses and energies of all fragments have been reconstructed by means of an improved version of the kinematic coincidence method. The probabilities P~ and P4 of producing 3-and 4-body events were found to depend mainly on the dissipated energy rather than on the bombarding energy, thus indicating that their origin lies more in the decay properties of the excited fragments than in the dynamics of the interaction. Emission of light particles from the composite system is shown to become more relevant with increasing bombarding energy and may explain the drop of the P3 and P4 curves at high energy losses. Small deviations of the P3 and P4 curves at 23.7 A. MeV from those at lower bombarding energies were used to estimate the amount of a possible pre-equilibrium light particle emission as a function of impact parameter.