DOUBLE-IONIZATION OF HELIUM BY 40 MEV PROTONS

Differential ionization cross-sections for 40 MeV H+ + He collisions w ere measured as a function of electron energy, electron ejection angle and final recoil ion charge state. Coincidences between recoil ions a nd emitted electrons were taken for electron ejection angles of 55-deg rees and 125-degrees and energies between 3 and 500 eV. Good agreement is found between measured energy distributions of ejected electrons a nd results of standard first-order perturbation theory in the case of single ionization. The experimental double-ionization cross-sections, however, are found to exceed independent-electron model predictions by more than an order of magnitude. Furthermore, evidence is provided th at this deviation is mainly due to initial-state correlation.