Measurement of x-ray currents in Bayard–Alpert type gauges

Abstract

The collector current in a hot cathode gauge at low pressure is the sum of the pressure dependent ion current and a nonpressure dependent residual current. A portion of this residual term is photocurrent caused by soft x-ray production within the gauge. At ultrahigh vacuum, the photocurrent can be comparable to the ion current. For accurate measurements, it is necessary to determine the pressure equivalent of the photocurrent and subtract it from the indicated pressure. Several methods have been used to determine the x-ray limit of a gauge. Using the variation of electron energy method, also known as the Alpert method, and the comparison method, the x-ray limits were found for a commercial Bayard–Alpert (BA) gauge and several experimental nude BA gauges. X-ray limits from both methods were in good agreement for gauges with tungsten filaments. In gauges with oxide coated cathodes, agreement varied and the residual current was noted to depend upon operating history. The x-ray limit for the commercial BA gauge was found to be 3.7×10−10 Torr at 1.0 mA grid current and 2.8×10−10 Torr at 10.0 mA. For the experimental gauges, x-ray limits were about 1.5×10−10 Torr.