A New Method for the Measurement of Very Low Partial Pressures

Partial pressures below 10-15 Torr were measured with recurrent multi-channel scaling of ion current synchronized with the scanning of the analyzer magnet current. Mass spectra of residual gases at total pressures below 1×10-9 Torr were recorded for wide mass ranges with resolutions of better than 110. The method minimized the effect of the time variation of ion counting efficiency for long counting intervals of more than 100 hours and made it possible to measure an ion current below 10-20 A with high reliability. The false peaks formed by accidental bursts of noise pulses were easily discriminated in the computer-programmed calculation of partial pressures. An Allen-type secondary electron multiplier of 12 stage Cu-Be dynodes connected to a low-noise charge-sensitive preamplifier was used for detecting single ions of which energies were less than 4 keV. Pulse height distributions for various residual gas ions were studied to determine a suitable pulse height discriminating level to give optimum signal to noise ratio.