Positron-atom doubly differential ionization cross sections

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Abstract
Doubly differential ionization cross sections for positron impact on argon atoms were determined by energy- and angle-resolved measurements of ejected electrons in time correlation with the produced ions. Corresponding measurements with incident electrons were made for comparison. With primary particles (positrons or electrons) of 100 eV and ejected electrons of 15 eV, measurements were extended over electron-emission angles from 0^\circ\else\textdegree\fi{} to 90^\circ\else\textdegree\fi{}. Lacking theoretical predictions for the ionization of argon we compared our data with the doubly differential ionization cross sections of Klar and Berakdar computed for positron and electron impact on hydrogen. The measured angular dependence of positron and electron cross sections for Ar agrees well with the theory for H. In particular, we find that at small angles of electron ejection the cross section for positron impact exceeds that for electron impact by an order of magnitude in accordance with the calculations of Klar and Berakdar (unpublished).