An experimental investigation of double ionisation of helium atoms in collisions with fast, fully stripped ions

Experimental cross sections for single and double ionisation of helium by 0.13-15 MeV AMU-1 ions of charge 1-8 are presented. For high ion velocities, double ionisation takes place either through a shake-off or through a two-step process. The perturbative shake-off mechanism dominates only for the lowest ion charges and projectile energies above 10 MeV AMU-1. A reliable semi-empirical expression which determines the ratio between the double and single ionisation cross sections over a wide region of ion charge and velocity is given. The difference between double ionisation by equi-velocity electrons and positive ions is discussed. (author)