Flow angle from intermediate mass fragment measurements

Directed sideward flow of light charged particles and intermediate mass fragments was measured in different symmetric reactions at bombarding energies from 90 to 800 A MeV. The flow parameter is found to increase with the charge of the detected fragment up to Z = 3–4 and then turns into saturation for heavier fragments. Guided by simple simulations of an anisotropic expanding thermal source, we show that the value at saturation can provide a good estimate of the flow angle, Θflow, in the participant region. It is found that Θflow depends strongly on the impact parameter. The excitation function of Θfolw reveals striking deviations from the ideal hydrodynamical scaling. The data exhibit a steep rise of Θflow to a maximum at around 250 – 400 A MeV, followed by a moderate decrease as the bombarding energy increases further.