| Popis: |
Differential cross sections d${\mathrm{\ensuremath{\sigma}}}^{\mathit{q}+}$(E,cphi)/d\ensuremath{\Omega} for multiple ionization of Ne and Ar by protons have been measured for final charge states q=1--6, impact energies E=0.4--3 MeV, and scattering angles cphi=0.28--6.5 mrad. Within these ranges of energies and scattering angles, the angular dependence of the differential cross section for single ionization at constant energy follows a power law, d${\mathrm{\ensuremath{\sigma}}}^{+}$/d\ensuremath{\Omega}\ensuremath{\propto}${\mathit{cphi}}^{\mathrm{\ensuremath{-}}\mathit{k}}$. 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 sections d${\mathrm{\ensuremath{\sigma}}}^{\mathit{q}+}$/d\ensuremath{\Omega}, q\ensuremath{\ge}2 relative to the single-ionization cross section d${\mathrm{\ensuremath{\sigma}}}^{+}$/d\ensuremath{\Omega}, is observed for increasing scattering angle. This behavior is most likely due to the combined effect of two multiple-ionization mechanisms. At the most distant collisions, 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 production of high-charge states. For Ne, this interpretation is supported by qualitative 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. |
