POSITRON-ATOM DOUBLY DIFFERENTIAL IONIZATION CROSS-SECTIONS

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. Corr esponding measurements with incident electrons were made for compariso n. With primary particles (positrons or electrons) of 100 eV and eject ed electrons of 15 eV, measurements were extended over electron-emissi on angles from 0 degrees to 90 degrees. Lacking theoretical prediction s for the ionization of argon we compared our data with the doubly dif ferential ionization cross sections of Klar and Berakdar computed for positron and electron impact on hydrogen. The measured angular depende nce of positron and electron cross sections for Ar agrees well with th e theory for H. In particular, we find that at small angles of electro n ejection the cross section for positron impact exceeds that for elec tron impact by an order of magnitude in accordance with the calculatio ns of Klar and Berakdar (unpublished).