Efficiency and mechanism of barium getters at low pressures

The gettering rates of barium getters for oxygen, nitrogen, hydrogen, carbon-monoxide, carbon-dioxide and water vapour are measured in the pressure range between 5 × 10 -8 and 5 × 10 -5 mm of mercury. Values for the decay in gettering rate during use of the getter are given. It is shown that the gettering effect at these low pressures is almost entirely due to the take-up of atoms or metastable molecules produced by the impact of electrons on the stable gas molecules. Ions play a negligibly small part in the gettering process. The position of the getter with respect to the electronic discharge influences the getter efficiency due to two causes: re-formation of stable molecules and flow resistance between discharge and getter. Examples for the two effects which affect different gases in different ways are given.