Total cross sections for positron scattering in argon, nitrogen and hydrogen below 20 eV

Total cross sections for positron scattering on argon, nitrogen and hydrogen have been measured at 0.4–20 eV by an absolute, transmission method. The apparatus uses a weak magnetic field but is characterised by small entrance and exit apertures of the scattering cell, assuring a good angular resolution down to the sub-eV energy range. Present measurements in all three gases show a fall of the cross section from the zero energy limit, then constant cross section values, within the error bar, up to the positronium formation threshold. Agreement with other experiments and theories is good but present data in the low energy limit are generally higher.