Doubly and triply differential ionization studies using positrons and electrons R.D. DuBois a, * , A.C.S. Santos b , M.A. Thomason a , J. Gavin a a Department of Physics, University of Missouri – Rolla, Rolla, MO 65401, USA b Instituto de Fisica, Universidade Federal do Rio de Janeiro, Cx Postal 68528, RJ, Brazil Available online 12 September 2005 Abstract Methods and results from experimental studies of single and multiple ionization of atoms induced by positron and electron impact are discussed. Coincidences between recoil ions, forward scattered projectiles and electrons ejected into a cone of angles are measured. From these data, doubly and triply differential cross-sections are extracted. Comparisons of the differential cross-sections and/or differential charge state fractions are being used to extract information about projectile charge effects on the collision dynamics as well as about the importance of various ionization mechanisms. Examples for ionization of argon and krypton atoms and nitrogen molecules are presented. Ó 2005 Elsevier B.V. All rights reserved. PACS: 34.80.Dp; 34.85.+x Keywords: Positron; Electron; Ionization; Cross-section 1. Introduction At high velocities, first order theories such as the Born approximation predict that the ionization cross-section depends on the square of the charge of the incoming particle, q. Therefore, for single ionization by electrons and positrons the cross- sections are expected to be the same. However, experiments [1] have shown that double ionization cross-sections are roughly a factor of two larger for electron impact than for positron impact; similar differences have been found for proton and antiproton impact with again the negative projectile yielding the higher cross-section [2]. At intermediate to lower velocities, these differences have been attributed to interference effects between the so-called shake-off and two-step mechanisms. Other conceptual arguments such as change in binding energies of the target electrons, change of the projectile trajectory, momentum transfer to the target, inner shell contributions or differ- ences coming from post-collision interactions have 0168-583X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.07.232 * Corresponding author. Tel.: +1 573 341 4708; fax: +1 573 341 4715. E-mail address: dubois@umr.edu (R.D. DuBois). Nuclear Instruments and Methods in Physics Research B 241 (2005) 19–22 www.elsevier.com/locate/nimb