Double ionization of helium by high-velocity<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">U</mml:mi></mml:mrow><mml:mrow><mml:mn>9</mml:mn><mml:mn>0</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>ions

Double ionization of helium is investigated for 60-, 120-, and 420-MeV/u (v/c=0.34--0.72)${\mathrm{U}}^{90+}$-ion impact. The measured double-to-single ionization ratios indicate that, even for these very high velocities, double ionization of the He target results predominantly from independent interactions of the projectile with both target electrons. It is concluded that the asymptotic high-velocity regime for one-step double ionization (i.e., ``shakeoff'') has not yet been reached even for ${\mathrm{U}}^{90+}$ projectiles at 420 MeV/u, and, in fact, cannot be reached for projectiles with q\ensuremath{\gtrsim}7, thereby verifying that the ionic charge is fully as important as the velocity in determining the importance of a given ionization mechanism.

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