The production of very high vacua by the use of getters

Abstract

An attempt has been made to integrate all information which might lead to a fuller understanding of the physico-chemical processes occurring on and in getter materials. Both original research results and published work from other sources are included. The characteristics of getters are defined and correlations between them are derived. Methods for measuring the characteristics are described and values of these for different getters and gases are given. Two main mechanisms of the gettering process are distinguished, namely contact gettering and discharge gettering, of which the latter, on account of the higher gettering rates attainable, is found to be of chief importance for the production of very high vacua. The processes contributing to discharge gettering are investigated in detail, and an attempt is made to determine the parts played by adsorption, diffusion and chemical reaction in different systems of getters and gases. The efficiency of flash getters is compared with that of coating getters, particularly thorium. It is found that gases like oxygen and carbon dioxide are taken up irreversibly during discharge gettering with thorium. The pressures attainable in valves using flash and coating getters are measured, and the influence of some parameters such as baking time on the pump or pressure during sealing-off is investigated.