Effect of exit surface on neutral fraction in mega-electron-volt hydrogen beams transmitted through aluminum epitaxial foils under a planar channeling condition

Abstract

Neutral fractions are measured for (0.4-0.8) MeV H{sup +} ion beams transmitted through aluminum epitaxial foils under the (100) planar channeling condition. Two foils are prepared: the first ({theta}=90 deg.) is almost perpendicular to the direction of the beam and the second ({theta}=30 deg.) is inclined at 60 deg. compared with the first. The measured neutral fractions, which are in the 10{sup -3} range, exhibit a minimum at the (100) channeling direction, however, the channeling effect is enhanced for the foil with {theta}=30 deg. The fraction is calculated for the two foils from the equilibrium fraction and the charge exchange between the ions and the surface atoms in a vacuum. Both the equilibrium fraction and the charge exchange depend on the exit positions of ions from the midplane of the channel. The measured energy dependence of the ratio (channeling/random) of the fractions is reproduced by calculation. Thus the results indicate that the transmitted charge-state distributions are modified by the charge exchange after the emergence of ions in a vacuum.