Appearance and disappearance of the second Born effects in the $\mathrm{}(e,3e)\mathrm{}$ reaction on He

We demonstrate, both experimentally and theoretically, clear manifestation of the second Born effects in the angular distributions of two ejected electrons produced by a 500 eV electron impact on the He atom in the so-called $\mathrm{}(e,3e)\mathrm{}$ reaction. The second Born contribution, due to subsequent interaction of the projectile with the target, is most prominent for glancing collisions with small momentum transfer. However, these effects are absent for hard knock-out collisions with large momentum transfer.