Technique for accurately measuring the stopping cross sections of reactive metals by ion back-scattering: He ions in erbium

Measurements of the absolute stopping cross sections of reactive metals have been made by ion back-scattering using in situ film deposition. The success of the method rests on the dual features of (1) an absence of impurities in the film (particularly oxygen) and (2) the measurement of the target areal density to a high degree of accuracy in the vacuum. Contamination was minimized by vacuum depositing the reactive metal onto the target substrate in an ultrahigh vacuum chamber attached to the beam line of a 2.5 MeV Van de Graaff accelerator. In addition, two independent quartz microbalances built into the substrate holder provided accurate measurements of the film areal density. Using this technique, the stopping cross sections for helium ions in erbium have been determined from 250 keV to 2.5 MeV; these cross sections were used to determine the parameters for the analytic expression for the electronic stoppinf cross section formula developed by D.K. Brice. The values of the parameters are: Z = 2.997, a = 0.3197 and n = 3.168.