Fragment production and collective motion in central Au+Au collisions

The light charged particles (Z=1,2) and intermediate mass fragments (3 ≤ Z ≤ 15) emitted within the forward cone θ ≤ 30deg in Au+Au collisions at 150 A.MeV beam energy have been measured at SIS-Darmstadt, using the phase I set-up of the 4π detector. The central collisions have been selected by a cut on the total charged-particle multiplicity and a cut on the 'transverse momentum directivity' D, a vector sum of the transverse momenta of the measured charge particles. For the events of the central collisions, large transverse momenta are observed especially for clusters (Z≥3), and the multiplicity of intermediate mass fragments in the acceptance of our setup is about 4. The calculations of a statistical model, a modified version of FREESCO, are compared to the experimental results. A reasonably good agreement has been found between the calculations and the experimental data for the fragment productions and corresponding velocity distributions. The separation of collective and thermal motion for the emitted fragments has been attained. (orig.)