Flow angle from intermediate mass fragment measurements

Directed sideward flow of light charged particles and intermediate mass fra gments was measured in different symmetric reactions at bombarding energies from 90 to 800 A MeV. The flow parameter is found to increase with the cha rge of the detected fragment up to Z = 3-4 and then turns into saturation f or heavier fragments. Guided by simple simulations of an anisotropic expand ing thermal source, we show that the value at saturation can provide a good estimate of the flow angle, Theta(flow), in the participant region. It is found that Theta(flow) depends strongly on the impact parameter. The excita tion function of Theta(flow) reveals striking deviations from the ideal hyd rodynamical scaling. The data exhibit a steep rise of Theta(flow) to a maxi mum at around 250 - 400 A MeV, followed by a moderate decrease as the bomba rding energy increases further. (C) 1999 Elsevier Science B.V.