Double ionization of helium by fast fully stripped ions

A complete momentum-space map of the double ionisation events in the collision of 100 MeV/u C6+ ions with helium has been obtained. From this map the angular distribution of two ejected electrons is generated as a function of the momentum transferred by the projectile to the atom. Analysis of the angular distribution of the fragments in the plane transverse to the projectile axis (the azimuthal plane) shows a separation of events into two domains, depending on the momentum transferred by the projectile to the target. For momentum transfers smaller than 1.2 a.u., both electrons are distributed independent of the azimuthal angle. For momentum transfers larger than 1.2 a.u., the electron with larger energy is distinctly emitted along the direction of momentum transfer, and the one with smaller energy is distributed isotropically. The value of 1.2 a.u. is approximately equal to the mean value of the momenta of the bound electrons in the helium atom and demarcates indirect, soft collisions and direct, hard collisions respectively. For soft collisions, the electron angular distribution shows certain similarities with the angular distributions in photo-double ionization. (orig.)