High-velocity limit for the ratio of helium double-to-single ionization for highly charged, bare-ion impact

The ratio R of helium double-to-single ionization cross sections was measured for ${\mathrm{Ne}}^{10+}$ and ${\mathrm{Ni}}^{28+}$ projectiles at velocities of 0.39≤${\mathit{v}}_{\mathit{P}}$/c≤0.93 intending to explore the high-velocity limit of R for highly charged ion impact. For ${\mathrm{Ne}}^{10+}$ projectiles R was observed to become independent of ${\mathit{v}}_{\mathit{P}}$ for ${\mathit{v}}_{\mathit{P}}$/c≳0.73 and an asymptotic experimental value R=(2.57±0.10)×${10}^{\mathrm{-}3}$ has been established. This is in good agreement with experimental results for proton, antiproton, electron, and positron impact and in excellent accordance with the theoretical prediction of (2.59±0.03)×${10}^{\mathrm{-}3}$.