THE INFLUENCE OF THE RESIDUAL-GAS ATMOSPHERE ON THE LIFETIME OF TUNGSTEN HAIR PIN CATHODES

Concerning the influence of the residual gas atmosphere on the lifetime of tungsten hair pin cathodes in electron optical instruments, e.g. electron microscopes, no reliable quantitative measurements have been made up to now. With a new type of hair pin cathodes, proposed by the author, the degradation by thermal evaporation takes place in a different way, and a much longer life time is achieved. Under these conditions the reaction with oxygen containing residual gases could now become more important. In order to clarify this question measurements of the recession rates in the relevant temperature range with different oxygen and water vapour pressures have been made. It is shown, that also with the new cathode the influence of the residual gas atmosphere on the lifetime at pressures below 10(-3) Pa is negligible.