Thermionic cathodes studied by low-energy ion scattering spectroscopy

Low energy ion scattering (LEIS) is used to study the surface-composition and structure of B-type dispenser cathodes. Measurements between room temperature and 1150°C gave identical results, which shows that the surface structure of Ba and O on the cathodes is not dependent on temperature. The results rule out any structure with O in the outermost layer (at the same height or above Ba). On cathodes poisoned by oxygen we found that the results are in agreement with a model where O is positioned on top of the Ba. In addition we demonstrate how LEIS may be used to determine the sputter yield of Ba from a cathode by Ar+ ions by monitoring the Ba coverage as a function of ion dose.