Statistical multifragmentation of non-spherical expanding sources in central heavy-ion collisions

We study the anisotropy effects measured with INDRA at GSI in central collisions of sup 1 sup 2 sup 9 Xe+ sup n sup a sup t Sn at 50 A MeV and sup 1 sup 9 sup 7 Au+ sup 1 sup 9 sup 7 Au at 60, 80, 100 A MeV incident energy. The microcanonical multifragmentation model with non-spherical sources is used to simulate an incomplete shape relaxation of the multifragmenting system. This model is employed to interpret observed anisotropic distributions in the fragment size and mean kinetic energy. The data can be well reproduced if an expanding prolate source aligned along the beam direction is assumed. In the model, the anisotropy is the result of correlations between the charge of a fragment and its location in the freeze-out configuration, created by the mutual Coulomb interactions inside the non-spherical source. An either non-Hubblean or non-isotropic radial expansion is required to describe the fragment kinetic energies and their anisotropy. The qualitative similarity of the results for the studied reactions suggests that the concept of a longitudinally elongated freeze-out configuration is generally applicable for central collisions of heavy systems. The deformation decreases slightly with increasing beam energy.