Charge-state distributions in collisions of H and He with magnesium.

Abstract

Charge equilibrium fractions have been measured for hydrogen on magnesium vapors in the energy range 4-40 keV. A large difference is seen to exist between our results and those on charge distributions o f H on Mg foils. This is attributed to the fact that charge distributions in solids are determined by surface interactions rather than by multiple charge changing collisions. For He+ and He projectiles, equilibrium could not be attained even at target thicknesses x of the order of 10 16 atom s/cm 3. This can be explained assuming a raise and subsequent fall of the population of metastable He with x, these atom s being the main producers of He* and doubly excited He~. Our results indicate that Mg, and probably other alkaline-earths, are most convenient for use in the source stage of tandem accelerators as they are good producers of negative ions at the more efficient injection energies.