Direct measurements of the stopping power for antiprotons of light and heavy targets

Abstract

Measurements of antiproton stopping powers around the stopping-power maximum are presented for targets of Al, Si, Ti, Cu, Ag, Ta, Pt, and Au. The Low Energy Antiproton Ring antiproton beam of 5.9 MeV is degraded to 50-700 keV, and the energy loss is found by measuring the antiproton velocity before and after the target. Target thicknesses have been determined accurately by weighing and Rutherford backscattering techniques. The antiproton stopping powers are found to be reduced by around 35% for both light and heavy elements near the electronic stopping-power maximum as compared to the equivalent proton stopping power. The antiproton stopping powers and the Barkas effect; that is, the difference in stopping power between protons and antiprotons is compared to theoretical estimates, based on a harmonic-oscillator model and an electron-gas model, and good agreement is obtained.

Vanishing gap in LiF for electronic excitations by slow antiprotons

“…At high projectile energies, where perturbation theory applies, differences due to corrections beyond the first-order term, the so-called Barkas corrections, have been studied in detail. [6][7][8][9] At low energies where perturbation theory fails and a fully selfconsistent treatment of the electronic structure of the projectile-target complex is required, the analysis of stopping-power differences is far from well understood. Antiprotons play a special role as they represent the simplest realization of a Coulomb point charge.…”

Nonlinear, Band-Structure, and Surface Effects in the Interaction of Charged Particles with Solids

“…1 we show, as a function of the projectile velocity, our full RPA calculations for the separate Z contributions to the stopping power exhibit a linear dependence on the projectile velocity up to velocities approaching the stopping maximum. This linear dependence is also exhibited by full nonlinear DFT calculations of the stopping power of a FEG [25] and by recent measurements of the electronic energy loss of protons and antiprotons [13].…”

“…A quantitative comparison of our theory with existing measurements of the energy loss of antiprotons [13] (which unlike protons carry no bound states) in a variety of target materials can be achieved by combining our first-principles calculations of the Z 2 1 (linear-response) stopping power with Z 3 1 corrections in a FEG. Nevertheless, a comparison with experiment still requires the inclusion of losses from the inner shells, xc effects, and higherorder nonlinear terms.…”

Electronic energy loss of channeled ions: The giant Barkas effect

“…Later on, with the advent of antiproton beams, a direct measurement of the Barkas effect was possible using both proton and antiprotons beams [7,8]. Although some attempts were made for direct measurements using ion beams, it was found that the Barkas contribution to the stopping power in such cases was extremely small and difficult to quantify [9].…”