Measurement of differential atomic collision cross sections— the influence of apparatus geometry

Abstract

We examine certain problems that are inherent in the measurement of cross sections for small angle scattering of atomic projectiles in a gas target. It is shown that the use of rectangular apertures to select the scattered particles will inevitably distort the measurements so that they are not representative of the true cross section. Finite aperture height must be considered in the derivation of the geometrical collection efficiency exhibited by any pair of rectangular apertures. This height also causes distortion so that the measured cross section does, in general, lie below the true value. It is suggested that experiment and theory may best be compared by convoluting the theoretical predictions into the experimental geometry. A previous publication of the cross section for scattering of 10 keV H+ in helium indicated that a theoretical prediction, using a ``static potential,'' was not in agreement with experiment at low scattering angles. A reassessment of these data has been made and the theoretical prediction is convoluted into the experimental geometry. Theory and experiment are now found to be in complete agreement.