Abstract
A detailed comparison has been made between the binary-encounter and Bethe theories for ionization of the (2p, 0) hydrogen atom by a fast incident charged particle. For this purpose we have mathematically reformulated the binary-encounter theory to treat the non-isotropic velocity distribution in the initial state of the atom considered. The pronounced dip found before in the Bethe theory is reproduced in the binary-encounter theory and is shown to be caused solely by the angular part of the initial-state wave function. Good agreement between the theories is found for energy transfers 
> 1 ryd. Applications to other initial states and other atoms are discussed.