The distribution of autelectronic emission from single crystal metal points - II. The adsorption, migration and evaporation of thorium, barium and sodium on tungsten and molybdenum

The adsorption, migration and evaporation of thorium, barium and sodium on tungsten and molybdenum have been studied by means of the autelectronic patterns from single crystal points of these metals. It is found that migration occurs at relatively low temperatures, and that for an optimum thermionic covering there is a unique distribution of the migrating atoms on the metal crystal. Certain crystal zones are avoided by the migrating atoms. The atoms evaporate more readily from certain crystal zones, and the differences in the temperatures for complete migration and evaporation of the atoms lead to an explanation of why migration is not always observed with barium (where the difference between migration and evaporation energy is small) and is always observed with thorium (where the differences are much greater) on multicrystalline surfaces. In the studies of thorium on tungsten and thoriated tungsten it is shown that diffusion of thorium occurs through the crystal lattice along certain preferred directions and that the final distribution of thorium is identical with that obtained by depositing thorium from an external source.