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 m easured for Ne10+ and Ni28+ projectiles at velocities of 0.39 less-tha n-or-equal-to v(P)/c less-than-or-equal-to 0.93 intending to explore t he high-velocity limit of R for highly charged ion impact. For Ne10+ p rojectiles R was observed to become independent of v(P) for v(P)/c gre ater than or similar to 0.73 and an asymptotic experimental value R = (2.57 +/- 0.10) x 10(-3) has been established. This is in good agreeme nt with experimental results for proton, antiproton, electron, and pos itron impact and in excellent accordance with the theoretical predicti on of (2.59 +/- 0.03) x 10(-3).