Triply differential measurements of single ionization of argon by 1-keV positron and electron impact

Abstract

By establishing coincidences between target ions and scattered projectiles, and coincidences between target ions, scattered projectiles and ejected electrons, triply differential (TDCS) information was generated in terms of projectile energy loss and scattering angles for interactions between 1 keV positrons and electrons and Ar atoms. The conversion of the raw experimental information to TDCS is discussed. The single ionization TDCS exhibit two distinguishable regions (lobes) where binary and recoil interactions can be described by two peaks. A comparison of the positron and electron impact data shows that the relative intensity of both binary and recoil interactions decreases exponentially as a function of the momentum transfer and is larger when ionization is induced by positron impact, when compared with electron impact.

Few Body Effects in the Electron and Positron Impact Ionization of Atoms

“…In less differential measurements, these effects will be swamped by the gross features of the interactions. Thus far, complementary studies of electron and positron impact ionization have been restricted to asymmetric geometries [1][2][3] with E 1 >> E 2 , and θ 1 small where the triple differential cross section tends to have the same character as the first Born approximation, being symmetric about the direction of momentum transfer with the only significant structures coming from target wave function effects [4]. In these kinematics, it is particularly difficult to disentangle the different few body contributions [5,6]; this problem is enhanced because what few differences there are tend to be in the absolute size of the cross section which is extremely demanding to measure accurately [7].…”

Electron- and positron-impact ionization of inert gases

“…We have presented triple-differential cross sections for the electron-impact ionization of argon(3p), krypton(4p), and xenon(5p), and comparison with experiment is generally encouraging. Agreement with the recent experiments of [3] is less encouraging, but we have argued that since the apparent discrepancy between theory and experiment is seen in both their electron and positron experiments and there is a wealth of evidence that the DWBA performs very well for electron-impact ionization in these kinematics that the problem most likely lies with the measurements. For the xenon(5p) target in coplanar asymmetric geometry we have confidence in the theoretical description for electron scattering.…”

“…In Fig. 3 we show a comparison between our calculations (DWBA, DWBA with N e ± e − ) and the recent experiments of [3]. The experiments are relative, and we have averaged our calculations over the experimental uncertainties of ±10 eV in the energy and ±0.5 • in θ 1 .…”

“…More recent pioneering work using antimatter projectiles, e.g., [1,2], has opened up the possibility of performing complementary experiments which could effectively isolate competing interactions which cannot be separately detected in an experiment with a single projectile. Such experiments [3,4] have recently been performed for electron and positron impact of Ar(3p) [5] at an impact energy of 1 keV, which is sufficiently high that annihilation and capture effects for the positron can be safely neglected. A coincidence experiment places a much more severe test on theory than less differential measurements, and while there have been some interesting experimental and theoretical results presented for doubledifferential cross sections [6][7][8][9][10], the focus here will be exclusively on the triple-differential cross section.…”

“…Also shown, solid line, DWBA, and dashed line, DWBA+N ee calculations. Experiment[3] was relative and was normalized to give the best visual fit to the DWBA.FIG. 4.Triple-differential cross sections for the electron-impact ionization of Ar(3p), E 0 = 1015.8 eV,E 2 = 120 eV, θ 1 = 8 • .…”