Shape parameters of the participant source in Ru + Ru collisions at image Original Research Article

New results on nuclear collective flow are presented for central and semi-central Ru + Ru collisions at 400 A MeV measured with the FOPI detector at GSI-Darmstadt. The source shape parameters, flow angle and aspect ratios, are extracted from Gaussian fits to in-plane and out-of-plane momentum distributions. The orientation and the shape of the source exhibit different trends according to the investigated phase space region. The shape parameters of the participant source are studied as a function of the particle mass and collision centrality. The flow angle is found to be independent of the particle mass. Both the flow angle and the aspect ratios depend sensitively on the impact parameter. Detailed comparisons with the predictions of the isospin quantum molecular dynamics model are performed. It is shown in particular that the source shape parameters permit to extract information on the in-medium nucleon–nucleon cross section.