Equilibrium charge state distributions of MeV He ions backscattered from continuously evaporated targets

The equilibrium charge state distributions of He ions emerging from surfaces of Pb, Sn and Al targets were measured in the energy range 0.8-2.3 MeV. Fresh surfaces of these targets were prepared by continuous evaporation during the measurement. As the evaporation rate was increased, charge state distributions changed gradually and reached the equilibrium ones which were characteristic of the respective targets. The results were compared with the calculation based on the gas model using Bohr's cross sections. The present data support oscillatory behaviour of mean charge with increasing target atomic number.