GB907511A - Method of detecting ions - Google Patents

Abstract

907,511. Mass spectrometers. UNITED KINGDOM ATOMIC ENERGY AUTHORITY. July 4, 1960 [July 31, 1959], No. 26450/59. Class 39 (1). In a method of detecting ions, e.g. in mass spectrometry, the ions strike a thin foil of such thickness that only desired ions can penetrate to the back surface of the foil to liberate secondary electrons which pass to an electron detector such as a scintillometer. In this way it is possible to distinguish between ions of the same mass and charge but having different numbers of constituent nuclei, e.g. H 2 + and D+ ions, H 3 + and HD+ ions or He+ and " air " ions, or, according to the Provisional Specification, atomic and molecular potassium ions. In Fig. 1, a thin aluminium foil is supported on a nylon film 14 which is transparent to secondary electrons. The foil is mounted in a polished stainless steel holder 12 and receives ions passing from a mass spectrometer through the slit 4 in a plate 3. Holes 5 are pumping holes. The secondary electrons liberated at the back surface of the disc 14 pass through a slit 13 in the holder 12 to a scintillometer comprising an organic scintillator 8, e.g. polystyrene, and a photomultiplier tube 11. A layer of aluminium 0.05 Á thick is provided on the surface of the scintillator and a silicone oil provides good optical contact between the scintillator and the photo-multiplier. The holder 12 is held at a high negative potential to accelerate both the ions on to the disc 14 and the secondary electrons away from the disc. An anti-corona ball 21 is provided on top of the glass metal seal 16. In Fig. 2, the ions 27 are accelerated on to the disc 14, secondary electrons 28 liberated at the back surface are detected by scintillator 8 and photo-multiplier 11 while secondary electrons 29 liberated at the front surface are detected by scintillator 8<SP>1</SP> and photo-multiplier 11<SP>1</SP> to provide a measure of the total number of ions in the beam 27. An anti-breakdown polystyrene guard ring 25 is fixed on the insulator 26 containing the lead 22. An aluminium foil may be prepared by vacuum coating on glass and then floating off in water and lifting on a small ring which is push fitted in the holder. A carbon or gold foil may be used.