GB1398167A - High pressure ion sources - Google Patents

Abstract

1398167 Ion sources H KNOF 29 Aug 1972 [31 Aug 1971] 40028/72 Heading H1D In an ion source comprising an ionization chamber 1Q wherein gas or vapour at a pressure of 1 Torr is ionized by an electron beam issuing from a cylindrical anode tube 13, the path of the ions leaving the chamber is screened by an electrode 1 maintained at substantially the same potential as the ionization chamber. Gas or vapour is supplied to the chamber from a source 12 via a pressure reducing valve 11 and a duct 2 which is electrically connected to the chamber and insulated from the outer enclosure; the valve 11 is preferably metallic but may be non-metallic. The outer chamber QR is exhausted to 1À5 x 10<SP>-3</SP> Torr by two diffusion pumps 16 and 17, and a third pump 18 may exhaust a mass spectrometer to which the ions are supplied. The anode tube 13 may be cooled by liquid nitrogen in a flask 21 and may be exhausted through a metallic screen near the cathode by means of a fourth pump 22. Electrons from the cathode are accelerated to about 800 volts by the anode 13 and focused by a magnetic field and by electrode 3 which extends into the ionization chamber, and a further focusing magnet (19, Fig. 2, not shown) may be provided behind collector A. The chamber and electrode 1 may be at - 2-9 kV, the envelope being earthed, and electrodes S and H may be at - 2-9 kV, 2À895 kV and - 2À830kV to focus the ions on an output slit 14; further electrodes F focus the ion beam on the output multiplier SEM of the mass spectrometer.