The Poisoning of Thoriated Tungsten Cathodes†

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Abstract
An experimental study has been made of the poisoning of the thermionic omission from thoriated tungsten cathodes by oxidizing gases. It is shown that rapid poisoning takes place when a critical partial pressure is exceeded. For a fully carburized cathode this critical pressure measured, using an omega-tron mass spectrometer, is as follows: O2 10−5torr, CO2 2·0 × 10−5 torr, and water vapour 6 × 10−6 torr. In the experiments the total pressures measured by ion gauge were considerably higher due to the formation of CO at the cathode surface, Ther esistance to poisoning depends almost entirely on the presence of carbon on the surface, and any decarburization lowers the critical pressure. For a truly uncarburized cathode the critical pressures are loss than 10−9 torr, only the arrival of fresh thorium at the surface counteracting the adsorption of the poisoning gas. It appears probable that in the past so-called uncarburized cathodes must have had a small amount of curbon in the surface.