MEASUREMENT OF RESIDUAL CURRENTS IN IONIZATION GAUGES AND RESIDUAL-GAS ANALYZERS

The residual current in a hot cathode ionization gauge, resulting from processes other than gas-phase ionization, limits the lowest measurable pressure. The following processes contribute to the residual current: a photoelectron current due to soft x rays; electron stimulated desorption (ESD) from adsorbed gas layers; and evaporation from the cathode. Four methods of measuring residual currents are examined and compared; (i) reduction of pressure to near zero, (ii) collector current versus electron energy plots, (iii) modulation methods, and (iv) collector current versus pressure measurements using a reference gauge with low residual current. The measurement of ESD currents in mass spectrometers (residual gas analyzers) is also discussed.