Auger characterization of new and aged dispenser cathode surfaces

The major changes in dispenser cathode surface signatures caused by aging were studied on B-type dispenser cathodes by operating them at high temperatures. By means of a computerized scanning Auger microprobe (SAM), the surface characteristics of three aged cathodes were measured and compared with those of a new cathode. Several hundred points were monitored on each sample to develop a good statistical description of each cathode surface, including both the impregnated pore ends and the tungsten regions between the pores. A method of discrimination between impregnant regions and tungsten regions has been developed, based on differences in the oxygen Auger peak shape and position. The results show that a surprisingly large fraction (greater than 70%) of the new cathode surface could be similar to the impregnant. This fraction decreases to near 20% with age. The elemental composition of these different regions was examined by correlating the presence of each element to others. The elemental correlation showed that the dominant fraction of the cathode surface in early life is similar to a barium-oxide on tungsten, and the cathode changes to an increasing fraction of surface tungsten oxide with aging.