Energetic proton emission in heavy ion collisions at intermediate energy: Pre-equilibrium and cooperative effects

Energetic proton emission has been investigated as a function of the centrality in the reaction Ni-58 + Ni-58 at 30 AMeV. Protons with energy extending up to a relevant fraction of the total available energy in the reaction were measured and studied. The dependence on the reaction centrality has been extensively investigated and data have been compared with the results of microscopic transport calculations. The more striking observation concerns the extremely energetic proton (E-P(NN) greater than or equal to 130 MeV) multiplicity which is found to increase almost quadratically with the number of participant nucleons thus indicating the onset of a mechanism beyond one and two-body dynamics.