Ion Gauge for Vacuum Pressure Measurements below 1×10−10 Torr

Low pressure measurements by Bayard-Alpert gauges are limited by x-ray photoelectric currents at the ion collector, equivalent to a pressure reading of about 1×10−11 Torr. Through efforts to reduce this x-ray current, the Bayard-Alpert structure has been modified, whereby a beam of ions is extracted from the grid and bent around a corner to a remote collector. By this geometry the collector is shielded from the x-ray source at the grid, and the photoelectric current is reduced by a factor of 100, without incurring a loss in gauge sensitivity. Bending of the ion beam is accomplished by a cylindrical, electrostatic field, in which the ion trajectories are independent of e/m. 100% modulation of the ion beam intensity is achieved by turning off the bending field, thereby permitting pressure measurements as low as 1×10−14 Torr. A total pressure of 5×10−13 Torr, nitrogen equivalent, has been measured in a stainless steel vacuum system employing ion sputtering and titanium sublimation as the means of evacuation.