Stopping and radial flow in central 58Ni+58Ni collisions between 1A and 2A GeV

The production of charged pions, protons. and deuterons has been studied in central collisions of Ni-58 on Ni-58 at incident beam energies of 1.06A, 1.45A, and 1.93A GeV. The dependence of transverse-momentum and rapidity spectra on the beam energy and on the centrality of the collison is presented. It is shown that the scaling of the mean rapidity shift of protons established for between 10A and 200A GeV at the Brookhaven AGS and the CERN SPS accelerators energies is valid down to 1A GeV. The degree of nuclear stopping is discussed ; quantum molecular dynamics calculations reproduce the measured proton rapidity spectra for the most central events reasonably well, but do not show any sensitivity between the soft and the hard equation of state. A radial Bow analysis, using the midrapidity transverse-momentum spectra, delivers freeze-out temperatures T and radial flow velocities beta(r) which increase with beam energy up to 2A GeV ; in comparison to the existing data of Au on Au over a large range of energies, only beta(r) shows a system size dependence.