Measurement of the stopping cross sections for protons and 4He ions in erbium and erbium oxide: A test of Bragg's rule

Abstract

Measurements of absolute stopping cross sections of erbium and erbium oxide for 250 keV to 2.5 MeV protons and 4He ions have been made by ion backscattering. Contamination of the erbium films was minimized by in situ deposition of the reactive metal onto the target substrate in an ultra-high vacuum chamber attached to the beam line of a 2.5 MeV Van de Graaff accelerator. Oxidation was accomplished by in situ annealing in an oxygen atmosphere. The oxidation state of the film was determined by both X-ray diffraction and elastic proton scattering. A stringent test of Bragg's Rule was made for erbium oxide by measuring the energy loss of the ion beam in an erbium film before and after oxidation. The measurements confirmed Bragg's Rule at the highest velocities of this experiment (21 × 10 8 cm/s), the rule was in error by 13%. This is believed to be the first stringent test of Bragg's Rule in solids using absolute cross sections in which significant disagreement was observed between experiment and theory. The measured cross sections were also used to determine the parameters of the electronic stopping cross section formula developed by Brice.