Fast electron production at intermediate energies: Evidence for Fermi shuttle acceleration and for deviations from simple relativistic kinematics

We measured fast electron velocity spectra with the multidetector ARGOS in a large angular range, for atomic collisions induced by a 45A MeV Ni-58 bea m on different solid targets. The velocity centroids of binary encounter el ectrons are shifted towards: lower velocities than predicted by simple two- body relativistic kinematics, independently of target material and thicknes s. With increasing target atomic number, the high energy tails of the spect ra exhibit an unexpectedly large number of very fast electrons. A multiple scattering (Fermi-shuttle) mechanism is invoked to explain this effect.