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 beam on different solid targets. The velocity centroids of binary encounter electrons are shifted towards: lower velocities than predicted by simple two-body relativistic kinematics, independently of target material and thickness. With increasing target atomic number, the high energy tails of the spectra exhibit an unexpectedly large number of very fast electrons. A multiple scattering (Fermi-shuttle) mechanism is invoked to explain this effect.