Double Ionization of Helium by Impact of Particles with Positive and Negative Charge

Double ionization of helium in collisions with 6 MeV protons (vP = 15.5 a.u.) has been explored in a kinematically complete experiment using a so‐called “reaction microscope”. For the first time fully differential cross sections for positively charged particle impact have been obtained and compared to previous (e,3e) results with 2 keV electrons (vP = 12 a.u.). Differences have been found in the angular distribution of the two emitted electrons, which originate from projectile charge effects that are not considered in the first Born approximation.