Patent GB-1172304-A (GB)
Title: Electron Multiplier
IPC: H01J43/18
Filed: 1967-02-24
Abstract: 1,172,304. Electron multipliers. INTERNATIONAL STANDARD ELECTRIC CORP. 24 Feb., 1967 [2 March, 1966], No. 8971/67. Heading H1D. An electron multiplier is formed by two opposed insulating discs coated on their facing surfaces with continuous films of resistive material, permitting a radially directed field to be maintained between the discs, the resistive material being either secondary emissive or coated with secondary emissive material, and one of the discs being centrally apertured to provide a passage for charged particles or for radiation opposite a secondary emissive or photo-emissive target area on the other disc, for initiating radial secondary electron trajectories having multiple radially spaced impact points on the disc surfaces, the electrons being collected by an electrode adjacent the peripheral portions of the surfaces. Glass, ceramic, or lava discs 1 and 2 have their inner faces coated with layers 10 and 11 of a material which is both resistive and secondary emissive, e.g. aluminium oxide doped with molybdenum or chromium, the layers being connected to peripheral terminal rings 14 and 15, the discs being hermetically sealed together by a peripheral collector electrode 18 and insulating members 16 and 17. The central aperture 3 in the disc 1, which is provided with a conductive sleeve 4 and is sealed by a window, admits an electron beam from a gun disposed in an outer evacuated container to impact on the centre of layer 11, potentials being applied to the two layers by peripheral contacts 14 and 15, the sleeve 4, and a central pin electrode 20. To permit controlled radial variation of potential, annular electrodes 28 to 37 may be mounted in grooves in the two discs. In an alternative arrangement (Figs. 3 and 4, not shown), the facing surfaces of the discs are provided with coaxial annular grooves, mutually staggered to provide electrostatic focusing, secondary emissive silver/magnesium material being deposited, only in the groove valleys, upon a continuous resistive layer of metal or carbon, to provide a plurality of spaced, annular secondary-emissive areas of radially increasing potential. The groove valleys of one disc are disposed opposite the groove crests of the other disc, and the outermost grooves in each disc are coated with silver and constitute the collector electrode. In a modified arrangement constituting a photomultiplier, the central region of disc 2 opposite aperture 3 is coated with photo-emissive material, or is the underside of an optically transparent window sealing the sleeve 4. The arrangement may be modified to provide on indication of the position of an electron beam with respect to the axis of the aperture 3 by either forming the opposed secondary-emissive surface in a cone shape (Fig. 5, not shown) or by providing an opposed, centrally positioned and negatively charged electrode (Fig. 6, not shown), and using a collector in the form of a plurality of circumferentially spaced segments, whereby the asymmetric angular distribution of secondary electron trajectories produced by an offcentre beam may be detected. The layers 10 and 11 may be of non-uniform thickness or resistivity, to facilitate the production of a required radial potential variation, and in the arrangement of Fig. 7 a layer of secondary emissive material superposed upon a resistive layer may be employed for each of the layers 10 and 11. All the components of the device may be constructed of optically transparent materials.