Projectile angular dependence of the ratio of double to single ionization of helium by fast proton impact
We investigate differential cross sections for single and double ionization and ratios of double to single ionization of He by proton projectiles at a few MeV/amu. A factor-of-two difference is found in the T-matrix at large deflection angles, with the target nucleus-electron interaction in the final state turned on and off. The differential cross sections for single ionisation, calculated in the first Born approximation with the effect of nuclear deflection of the projectile included, are in good agreement with the experimental data. The shakeoff mechanism is calculated on an s-wave basis. The two-hard-hit mechanism, in which two energetic recoiling electrons are produced, is also calculated and a sharp peak in the ratio is found. The magnitude of the calculated peak is discussed. The ratio calculated by combining different mechanisms is compared to the experimental data and discussed.