ANOMALIES IN THE MEASUREMENT OF THE RESIDUAL GASES IN A LARGE UHV SYSTEM USING A QUADRUPOLE MASS ANALYZER

A small quadrupole mass analyser was used to measure the residual gases in a stainless steel ultra-high vacuum system (vacuum baked at 150-degrees-C for 10 days) of 7500/volume, pumped by a NEG pump (nominal 100/s-1). A series of experiments is described which shows that the indicated partial pressures of afl of the residual gases, except hydrogen, are increased due to their production in the analyser. It has not been possible to accurately quantify the effect, but upper limits for the specific outgassing rates (mbar/s-1 cm-2) are: 2 x 10(-13) for hydrogen, less-than-or-equal-to 1 x 10(-18) for methane, less-than-or-equal-to 3 x 10(-17) for water, less-than-or-equal-to 4 x 10(-16) for carbon monoxide and less-than-or-equal-to 2 x 10(-17) for carbon dioxide.