Abstract
A new method has been investigated for ultrahigh-vacuum calibration (molecular beam calibration) of vacuum gauges and mass spectrometers over the range 10−6−10−15 Torr with less than 5% probable error. The method is applicable, in principle, to all gases but limited in practise to gases with critical temperatures greater than or equal to that of nitrogen. It combines the techniques of pressure attenuation and molecular beaming from a nearly ideal aperture to generate a precisely known molecular density in a volume, the walls of which have a capture coefficient of near unity. The upstream (reference) pressure is measured by an instrument for which NBS has a test schedule. Direct beam to background density ratios in excess of 800:1 were measured for a 304 K argon beam. The conductance of the porous Vycor pressure-attenuating element is constant to pressures exceeding 5000 Torr. Determination of the magnitude of deviations from free molecular flow as the gas mean-free path λ in the furnace approaches the furnace orifice diameter d showed no greater than 12% deviation for λ/d≥25. The measured density distribution in the cryopumped beam tunnel agrees with the expectations of kinetic theory.
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