Absolute cross sections for multiple ionization of noble gases by swift proton impact

Absolute cross sections, σ^q+, for multiple ionization of He (q = 1), Ne (q = 1, 2, and 3), Ar (q = 1, 2, 3, and 4), Kr (q = 1, 2, 3, and 4), and Xe (q = 1, 2, 3, 4, and 5) atoms by protons in the 0.75–3.5 MeV impact energy range have been measured. The present results are in good agreement with the previously published data in the case of the single-and double-ionization cross sections, but some non-systematic discrepancies appear for higher charge states. A detailed comparison with electron impact cross sections in these gases is performed, and it shows that the cross sections for single ionization by high-velocity protons are substantially smaller than those corresponding to equi-velocity electrons for heavier targets.