Anatomy of the Barkas effect

The stopping force on a swift charged particle penetrating through matter differs from that on its anti-particle. This difference, commonly called Barkas effect, has been studied theoretically as a function of velocity and projectile-target combination. In Bohr's harmonic-oscillator model the Barkas ratio L+/L- is governed by one single variable. An explicit estimate of this dependency has been found from binary stopping theory. With the addition of shell and quantum effects, more complex scaling properties emerge, but for a given material, essentially the same Barkas ratio has been found for hydrogen, lithium and argon ions when plotted as a function of the Bohr parameter mv(3)/\Z(1)\e(2)omega. Projectile screening reduces the Barkas effect and may even invert its sign. We conclude that the effect is larger for protons than for heavier ions. This finding is shown not to contradict recently reported Barkas corrections under channeling conditions. (C) 2003 Elsevier B.V. All rights reserved.