Electron stimulated thorium adatom enrichment on the surface of thoriated tungsten below 2300Â K

The evolution of the surface composition of thoriated tungsten has been followed up by means of Auger electron spectroscopy (AES) upon various heat treatments performed below 2300 K. It turned out that the surface composition dramatically depends on the current density of the electron beam exciting the Auger electrons, and no Th adatom enrichment has been observed without electron beam irradiation in this temperature range. It has been shown that the sources of Th adatoms are the thoria particles attached to the free surface of the wire. It is proposed that electron irradiation induced oxygen depletion of the W-ThO2 system is the main reason for thorium release onto the free surface from the thoria particles. The relation of the topic to the electrode performance in metal halide lamps is also discussed.