On the role of double collisions in $(е, 2е)$ processes

The region of efficiency of the second Born approximation (SBA) in the theory of ionization of atoms by fast electrons is considered. A model (ASBA) is constructed for approximate calculations of the SBA contribution to the differential cross section $d^3\sigma$. The calculations are compared with a large array of experimental values of $d^3\sigma$ in the reaction $Н(е,2е)Н^+$.