Charged Fraction of 5 keV to 150 keV Hydrogen Atoms after Emergence from Different Metal Surfaces

Abstract

The charged fraction of hydrogen atoms backseattered from Be, V, Cu, Nb, Mo and Ta surfaces has been measured for energies between 5 keV and 150 keV and a wide range of angles of emergency. Hydrogen particles with energies above 20 keV are counted and energy analysed by a surface barrier detector. Charged particles are separated from the neutrals by means of electrical deflection plates between target and detector. Neutrals with energies below 20 keV are partly ionized in a calibrated gas stripping cell. They are energy analysed in a subsequent electrostatic spectrometer and counted by a channeltron multiplier. The backscattered ions were recorded with no gas in the stripping cell. Only small differences are found for the charged fraction for different materials as long as the surface is covered by a layer of adsorbed impurities. There is, however, for most materials a change in the charged fraction due to annealing the target. For emergence energies above ~ 40 keV it is lower than for unannealed targets. An observed dependence of the charged fraction on the angle of emergence was generally just slightly above the experimental error.

The measured results are compared with theoretical curves of Zaidins, Trubnikov et al., and Brandt and Sizmann. The best agreement is found with values given by Zaidins. The peculiarities observed at annealed surfaces are not predicted by theory.