New deflected-beam gauge for pressures below 10−12 Torr

Abstract

Design and performance of a new hot-filament ionization gauge, specially developed for total pressure measurements down to 10−14 Torr, are described. The gauge features an electrostatically deflected ion beam which is directed at the funnel cathode of a small channel electron multiplier (CEM). The spurious signal due to electronic desorption of ions from the anode is extremely small, because these ions are typically of higher initial total energy than those from the residual gas, and may, therefore, be evicted from the high-energy side of the ion beam that is energetically dispersed by the deflecting electric field. The x-ray background corresponds to a pressure in the 10−14 Torr range, and since it can be measured just by inverting the deflector potential, it may easily be zeroed out electronically. Almost none of the secondary particles, released by x rays or ions from the deflecting electrodes, is counted, because the CEM cathode is operated at high negative potential and because most positive secondary ions are collected at the negative deflecting electrode, due to their small kinetic energy. The gauge’s sensitivity for nitrogen is about 1017 counts/s⋅Torr⋅A, corresponding to 105/Torr for a CEM gain of 107. It is mounted on a 113.5-mm metal flange with ceramic feedthroughs and may be baked up to about 300 °C.