| 摘要 |
504,927. Electron multipliers. McGEE, J. D. Oct. 28, 1937, Nos. 29426 and 33884. [Class 39 (i)] [Also in Group XXII] An electrode for use in electron multipliers comprises a thin film supported on a mesh structure ; the impingement of electrons on one side of the film causes emission of secondary electrons from the other side. The interstices of mesh may be 0À1 mm. in diameter with a shadow ratio of 10 per cent and are sufficiently small to support the film against air currents encountered in a room during assembly. The mesh, which may be made by electrodeposition and mounted and stretched as described in Specification 479,024, is immersed in a solution of cellulose acetate or collodion in acetone and withdrawn at an angle so as to leave solution in the meshes. When the solvent has evaporated, the metallic film is applied, as by sputtering, and the cellulose or collodion film is dissolved away, leaving the metal film on the mesh. To avoid local variations of surface tension likely to disrupt the film, the solution is performed slowly by starting with a nearly saturated solution and gradually diluting it. The film may comprise more than one metal layer; for example, caesium may be applied to oxidized silver. The thickness of the film is preferably such as just to prevent the incident electrons emerging from the other side. The electrodes may be used in an image intensifier with the final screen fluorescent or mosaic, focusing coils being provided between each electrode. In a modification to avoid focusing, the electrodes 4-7, Fig. 2, are placed so close together that lateral spreading cannot occur. The mesh supports are aligned so that they reduce the impenetrable area and assist focusing. If the meshes are coated with insulation such as lithium borate or alkali fluoride, the electrodes may be placed in contact. In the form shown the electron stream from photo-cathode 2 is amplified by the electrodes 4 ...7 and falls on a double-sided mosaic 9. Focusing coil 8a and grid 8 may be dispensed with and the mosaic (or a fluorescent screen) placed close to electrode 7. The large capacity between the mosaic and the other electrodes may reduce its sensitivity and this is avoided by high resistances placed in the leads to electrodes 4 ... 7. Specification 457,493 is referred to. |
