Hot-Cathode Magnetron Ionization Gauge with an Electron Multiplier Ion Detector for the Measurement of Extreme Ultra-High Vacua

Abstract

A hot-cathode magnetron ionization gauge capable of measuring pressures as low as 10−14 Torr has been described in previous publications. The low-pressure limit of this gauge has been extended by the addition of an electron multiplier ion detector. The ions generated in the magnetron gauge are focused onto the first dynode of the multiplier by an electrostatic lens system. The multiplier gain is high enough to permit the use of an ion counting technique for the measurement of very low pressures. The lens system focuses the ions from the gauge into a beam that passes through two limiting apertures before striking the first dynode. This arrangement prevents the soft x rays generated at the magnetron gauge anode from falling directly on the first dynode. The x-ray photoemission from the first dynode has been reduced by a factor of 50 000 below that observed for the magnetron gauge alone. Measurements of sensitivity and x-ray photocurrent indicate that this device should have a linear response down to pressures of 2×10−17 Torr. Ability to measure low pressures with the gauge appears to be limited by the inherent dark current of the multiplier. The gauge may be used to advantage for the measurement of higher pressures by operating at reduced emission current levels to minimize pumping and degassing effects. Photoemission from the first dynode caused by incident light from the magnetron gauge filament has been reduced to values below the x-ray photoemission by the use of a lanthanum hexaboride cathode which operates at only 675°C. This low operating temperature reduces the problem of gas reactions with the filament. Radiation from gas molecules excited by electron impact in the magnetron gauge has been observed in the 10−9 Torr pressure range.