Measurement of argon (e, 2e) differential cross sections in the perpendicular plane from 5 to 200 eV above the ionization threshold

New (e, 2e) differential cross-section measurements from argon are presented in the perpendicular plane, where the incident electron beam is orthogonal to both detected electrons that map out a detection plane as their angles are varied. New data were obtained at energies from 40 to 200 eV above the ionization potential (IP), with the scattered and ejected electrons having equal energies. These data are compared to previous measurements from 5 to 50 eV above the IP as well as to theoretical calculations from different models in this energy range. A significant discrepancy between the prediction of theory and experiment 50 eV above the IP was retested, and the new experiments confirmed the results from previous measurements. Additional data spanning this energy are presented which show a deep minimum in the cross section under these conditions. Results for the evolution of the cross section into the intermediate-energy regime are also discussed.