Impact-parameter dependence of helium atom double ionisation by high-energy protons and antiprotons

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Abstract
The authors have previously reported (Reading et al 1987, Ford et al 1988) coupled-channels calculations of helium double ionisation in which the cross sections for proton and antiproton projectiles show a difference of the same sign and of approximately the same magnitude and energy dependence as is measured experimentally. These calculations are done in the impact-parameter method with a classical projectile trajectory. in the present paper the authors present the impact-parameter dependent probabilities P(b) for double and for single ionisation, in order to better elucidate the collisional mechanism responsible for the proton-antiproton differences. They find that the double to single ionisation ratio and the proton-antiproton difference is much larger in P(b) at small impact parameter b than observed in the cross sections.