Point collector ionization gauge with spherical grid for measuring pressures below 10−11 Pa

Abstract

A new hot-filament ionization gauge using a point collector and spherical grid has been developed to reduce the low-pressure limit. Since the development of the Bayard–Alpert gauge, a number of attempts have been made to overcome the x-ray limit. In all these attempts, however, the gauge collectors are either wires or plates, even if they are removed from the ionization region and hidden from x rays. If, however, a very small point collector could be used in an ionization gauge, without reducing sensitivity, the small surface area of the collector could yield a very low x-ray limit. Therefore, in the present design a point collector is approximated using a needle tip≂0.03 mm in diameter and ≂ 0.05 mm long projected from a thin tapered sleeve. A 26-mm-diam spherical grid is formed by joining two hemispheres made of finely woven molybdenum mesh. The collector is placed about 1.5 mm inside the pole of the spherical grid. A ring filament is stretched around the equator of the spherical grid. The x-ray limit of the new gauge, calculated theoretically from geometric considerations, is about 2.3×10−12 Pa; and its experimental result is about 2.5×10−11 Pa, with a sensitivity factor of 0.4 Pa−1 (53 Torr−1). If a short wire modulator is added near the collector, the pressure limit of the new gauge is reduced below 10−11 Pa, with a modulation factor of 0.95.