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

The ratio [ital R] of helium double-to-single ionization cross sections was measured for Ne[sup 10+] and Ni[sup 28+] projectiles at velocities of 0.39[le][ital v][sub [ital P]]/[ital c][le]0.93 intending to explore the high-velocity limit of [ital R] for highly charged ion impact. For Ne[sup 10+] projectiles [ital R] was observed to become independent of [ital v][sub [ital P]] for [ital v][sub [ital P]]/[ital c][approx gt]0.73 and an asymptotic experimental value [ital R]=(2.57[plus minus]0.10)[times]10[sup [minus]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[plus minus]0.03)[times]10[sup [minus]3].