The Removal of Thorium from the Surface of a Thoriated Tungsten Filament by Positive Ion Bombardment

Sputtering of thorium from the surface of a thoriated tungsten filament.—The method of measurement depends upon the fact that by suitable thermal treatment a layer of thorium one atom thick can be formed on the surface of the filament and that the electron emission of such a layer at 1500° K is ${10}^5$ times that of the tungsten and decreases according to known laws with the amount of thorium sputtered off as a result of bombardment by positive ions. The tube used contained, besides the central thoriated filament which was cold during the sputtering, two other filaments used to emit the electrons needed to produce the positive ions by collision. The gas was introduced at a low pressure, 6 bars, and the effect of argon, caesium, helium, hydrogen, neon, and mercury ions as a function of voltage and time was studied. H ions produced no sputtering even up to 600 volts energy. Ar, Cs, Hg and Ne ions all started to sputter the thorium ions at about 50 volts energy. The number of impacting ions of 150 volts energy per sputtered thorium atom varied from 12 for Ar and Cs to 45 for Ne and 7000 for He. The amount of thorium sputtered at first did not increase linearly with the time, and the rate of sputtering was greater when 0.95 of the surface was covered with thorium than when it was completely covered. In explanation, it is suggested that the removal of thorium takes place around the edges of holes or depression in the thorium coating; that for the removal of the first few atoms, two successive impacts on the same thorium atom are necessary; and that the first impact depresses the atom from the surface while at the second impact the ion is reflected from this depressed atom and knocks off one of the surrounding thorium atoms. A computation of the energies involved seems to bear out this theory. In the case of He the sputtering is so slow that a different mechanism must be involved, perhaps radiation, as suggested by Thomson.