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.
Experimental data for single, double and triple ionization of Ne, Ar, Kr and Xe by 30-1000 keV antiproton impact are presented and compared with existing proton data. It is found that the current phenomenological understanding of ionization developed from studies of light targets generally apply to heavy atomic targets as well. Inner-shell ionization followed by Auger decay is found to be an important channel for multiple ionization of the heaviest targets, and the projectile charge dependence of their multiple ionization cross sections must be understood as stemming from the underlying inner-shell ionization cross sections.
Hydrogen formation has been observed following proton impact on positronium. This is the first observation of charge exchange involving a positronium target. The cross section for hydrogen formation has been experimentally determined at proton energies of 11.3, 13.3, and 15.8 keV. Values of s H 26͑69͒, 7.8͑62.3͒, and 7.6͑64.4͒ 3 10 216 cm 2 were obtained, in reasonable agreement with recent calculations. [S0031-9007(97)02936-0]
The cross sections for direct and total dissociative and non-dissociative ionization of the nitrogen molecule by positron impact have been measured for projectile energies from threshold to 2000 eV. The results are compared with corresponding data for impact of electrons, protons and antiprotons. The comprehensive model which during recent years has been developed to explain the behaviour of the single-and multiple-ionization cross sections of atoms by charged particle impact was found to also apply for this case of a molecular target. In particular, the interference and factorization models which have been found to explain important features of the cross sections for double ionization of atoms apply also to the dissociative ionization of nitrogen molecules.
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