Reaction dynamics and multifragmentation in Fermi energy heavy ion reactions

The reaction systems, Zn-64+Ni-58, Zn-64+Mo-92, Zn-64+Au-197, at 26, 35, and 47 A MeV, have been studied both in experiments with a 4pi detector array, NIMROD, and with antisymmetrized molecular dynamics model calculations employing effective interactions corresponding to soft and stiff equation of state (EOS). Direct experimental observables, such as multiplicity distributions, charge distributions, energy spectra and velocity spectra, have been compared in detail with those of the calculations and a reasonable agreement is obtained for both EOS's. No conclusive preference for either EOS has been observed. Neither of the above direct observables nor the strength of the elliptic flow are also sensitive to changes in the in-medium nucleon-nucleon cross sections. A detailed analysis of the central collision events revealed that multifragmentation with cold fragment emission is a common feature predicted for all reactions studied here. A possible multifragmentation scenario is presented; after the preequilibrium emission ceases in the composite system, cold light fragments are formed in a hotter gas of nucleons and stay cold until the composite system underdoes multifragmentation. For reaction with Au-197 at 47A MeV a significant radial expansion takes place. For reactions with Ni-58 and Mo-92 at 47A MeV semitransparency becomes prominent. The differing reaction dynamics drastically change the kinematic characteristics of emitted fragments. This scenario gives consistent explanations for many existing experimental results in the Fermi energy domain.