An accurate investigation of the validity of the first Born approximation to describe high-energy (e, 2e) cross sections at small and intermediate momentum transfer

The authors present new high-accuracy measurements of the triple differential cross sections for ionisation of helium by several keV electron impact, obtained in fully asymmetric conditions at small ejected electron energies and small and intermediate momentum transfer values. Comparison is made with two accurate calculations within the first Born approximation. It is shown that this approximation correctly describes the experimental findings and that higher-order effects (if any) are smaller than 10%.