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Absolute cross sections for the ejection of electrons by ;proton impact that are differential in the angle and energy of the electrons have been measured for :helium, neon, and krypton. The primary energy ranged from 7.5 to 150 keV, the angles from 10° to 160°, and the measured energies from 1 to 550 eV. These data along with earlier data on argon are used to study the systematics of proton impact ionization of the rare gases. An analytical model is employed to correlate and systematize the cross-section differential in ejected-electron energy. For low-energy collisions the angular distribution of electrons ejected into the forward hemisphere is found to be independent of the electron energy and largely independent of the target. The Massey adiabatic criterion yields results for the energy of the maxima of the electron-ejection functions, in disagreement with experiment. Doubly differential cross sections for neon calculated from the distorted-wave Born approximation show only fair agreement with experimental data. A method of determining electron multiplier efficiency based on Poisson statistics is described.