[en] The ionization of the 3p orbital of argon by 100 eV incident electrons and positrons is studied with the DWBA-WM approximation. The variation of our double differential ionization cross sections with the angles of the ejected electrons is found to be in reasonable agreement with the experiment. We found that the inclusion of the post-collision interaction in the theoretical model is necessary to obtain the size of the difference between the experimental double differential cross sections for electron and positron impact ionization. Graphical abstract: .