DIFFERENTIAL CROSS-SECTIONS FOR THE MULTIPLE IONIZATION OF NE AND AR BY PROTONS

Differential cross sections dsigma(q+)(E,phi)/dOMEGA for multiple ioni zation of Ne and Ar by protons have been measured for final charge sta tes q=1-6, impact energies E=0.4-3 MeV, and scattering angles phi=0.28 -6.5 mrad. Within these ranges of energies and scattering angles, the angular dependence of the differential cross section for single ioniza tion at constant energy follows a power law, dsigma+/dOMEGA is-proport ional-to phi(-k). The power k is found to be near 3.8 for Ne and 3.65 for Ar. A monotonic increase of all the multiple-ionization cross sect ions dsigma(q+)/dOMEGA, q greater-than-or-equal-to 2 relative to the s ingle-ionization cross section dsigma+/dOMEGA, is observed for increas ing scattering angle. This behavior is most likely due to the combined effect of two multiple-ionization mechanisms. At the most distant col lisions, the multiple ionization is dominated by the direct removal of valence electrons by the projectile. At the closest collisions, inner -shell ionization, followed by vacancy cascades, adds to the productio n of high-charge states. For Ne, this interpretation is supported by q ualitative agreement with a model calculation based on the assumption that the multiple ionization of each shell follows binomial statistics . Quantitative agreement between experiment and theory requires a more thorough analysis of the many-body problem.