Stable cancellation of x-ray errors in Bayard-Alpert gauges

X-ray induced emission of electrons from the ion collectors of Bayard-Alpert gauges has long been recognized as a cause of falsely high readings at low pressures. Less well known is the existence of a reverse x-ray effect leading to a superimposed, but usually smaller, error signal in the opposite direction. First explained by Redhead and others in the 1960s as being caused by photoemission from the gauge envelope, this phenomenon was immediately recognized as potentially useful in cancelling the forward x-ray effect. Although promising experimental results were obtained with special gauge tubes, these early researchers concluded that the cancellation process was too unstable to be of any practical use. We report the results of an attempt to stabilize the cancellation process by the use of identical materials at the two photoemission sites. Experimental Bayard-Alpert gauge tubes were built with gold coatings on the insides of the gauge envelopes and on the surfaces of the ion collectors. The envelope coatings were operated at approximately 15-25 V below the potential of the ion collector. Stable reduction of the x-ray errors by a factor of more than 20 has been demonstrated. Short-term improvements by a factor of 100 have been achieved. Useful improvements have also been observed in sensitivity and in stability at high pressures. Results of long-term stability tests at pressures in the 10(-10) Torr range are described. (C) 1998 American Vacuum Society.