Reaction dynamics and multifragmentation in Fermi energy heavy ion reactions

The reaction systems, 64Zn + 58Ni, 64Zn + 92Mo, 64Zn + 197Au, at 26A, 35A and\n47A MeV, have been studied both in experiments with a 4$\\pi$ detector array,\nNIMROD, and with Antisymmetrized Molecular Dynamics model calculations\nemploying effective interactions corresponding to soft and stiff equations of\nstate (EOS). Direct experimental observables, such as multiplicity\ndistributions, charge distributions, energy spectra and velocity spectra, have\nbeen compared in detail with those of the calculations and a reasonable\nagreement is obtained. The velocity distributions of $\\alpha$ particles and\nfragments with Z >= 3 show distinct differences in calculations with the soft\nEOS and the stiff EOS. The velocity distributions of $\\alpha$ particle and\nIntermediate Mass Fragments (IMF's) are best described by the stiff EOS.\nNeither of the above direct observables nor the strength of the elliptic flow\nare sensitive to changes in the in-medium nucleon-nucleon (NN) cross sections.\nA detailed analysis of the central collision events calculated with the stiff\nEOS revealed that multifragmentation with cold fragment emission is a common\nfeature predicted for all reactions studied here. A possible multifragmentation\nscenario is presented; after the preequilibrium emission ceases in the\ncomposite system, cold light fragments are formed in a hotter gas of nucleons\nand stay cold until the composite system underdoes multifragmentation. For\nreaction with 197Au at 47A MeV a significant radial expansion takes place. For\nreactions with 58Ni and 92Mo at 47A MeV semi-transparency becomes prominent.\nThe differing reaction dynamics drastically change the kinematic\ncharacteristics of emitted fragments. This scenario gives consistent\nexplanations for many existing experimental results in the Fermi energy domain.\n Comment: 34 pages, 43 figures