Ionization of helium by highly charged ions at 1.4 MeV/amu

Experimental cross sections for ionization of helium by projectile ions with charge Z up to Z=44 are presented at a fixed velocity v=7.48 a.u. corresponding to 1.4 MeV/amu. Total cross sections, summed over projectile charge states, for single ionization, ${\ensuremath{\sigma}}^{+}$, are compared to Born and Glauber calculations for point projectiles of charge Z. The Glauber calculations give a better fit to the Z dependence of the single-ionization cross sections than do the Born calculations. For double ionization, the ratio of ${\ensuremath{\sigma}}^{2+}$/${\ensuremath{\sigma}}^{+}$ increases as ${Z}^{2}$ for the smaller Z and less rapidly than ${Z}^{2}$ for larger Z projectiles, suggesting that the so called ``direct'' mechanism for double ionization is dominant, but that the first Born approximation is breaking down for the larger Z.

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