The composition of a thorium metal surface has been monitored using Auger electron spectroscopy following Ar bombardment at different temperatures. After extended Ar bombardment, enough contaminated overlayers were removed to expose a surface region containing only thorium, bulk impurities, and imbedded argon. The main impurities, carbon and oxygen, differed in their behavior when the sample was annealed following bombardment. The amount of surface carbon either increased or remained constant

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... ring annealing depending upon the temperature of the Bample during bombardment. The amount of surface oxygen decreased rapidly when the sample was heated above 500°C regardIBSB of the sample temperature during bombardment. These experiments Indicate that preparation of clean, annealed thorium surfaces requires ion bombardment at temperatures >400°C.