Patent GB-1175579-A (GB)
Title: An Electron Discharge Device including a Secondary-Emission Conductivity Target with Improved Scanning Surface
Inventor: BLAMOUTIER MICHEL
IPC: H01J29/44
Filed: 1967-05-05
Abstract: 1,175,579. Cathode-ray tubes. COMPAGNIE FRANCAISE THOMSON HOUSTON-HOTCHKISS BRANDT. 5 May, 1967 [5 May, 1966], No. 21097/67. Heading H1D. In a discharge tube having a secondary emission conductivity target member with, in operation, high energy electrons incident upon a first side, to develop a change pattern, and a low energy charge neutralizing electron beam incident upon the other side-e.g. a television camera tube or storage tube-the target member includes a main storage layer of porous, secondary emissive material which is bonded on the first side to a plate of conductive material which is permeable to the incident high energy electrons, and on its other side to a porous surface layer which is substantially thinner than the main layer, and which has a voltage threshold of secondary emission which exceeds that of the main layer, whereby to reduce the production of secondary emission by the low energy beam at regions of the target corresponding to, and driven to a high positive potential by, the incidence upon its first side of a high density of the high energy electrons. The main storage layer may be of potassium chloride, barium fluoride, or magnesia, of 2% apparent density and 25 Á thickness, coated on the first side with electron permeable aluminium or magnesium foil, and on the second side with a 5 Á thick layer of calcium fluoride, silicon dioxide, or silicon monoxide, of 20% apparent density. The target may be fabricated by the vapour deposition of the main storage layer on the conductive plate followed by the vapour deposition of silicon in an argon atmosphere, the deposited silicon being oxidized in situ in an oxygen atmosphere. The coating on the second side may alternatively be of silver, gold, or platinum, deposited sufficiently thinly to provide an adequate resistivity in its plane; in this case, a further layer which may be of the same material as the main storage layer but of a higher apparent density, produced by lowering the pressure towards the end of the deposition process for the main layer, may be interposed between the main layer and the metal film, to provide a relatively smooth base for the deposition of the metal, and prevent its penetration into the pores of the main layer. The target may further include a supporting alumina layer above the conductive plate. In the camera tube shown, having a photo-cathode 11 coated on the envelope end wall, the target 17 is supported, in a ceramic ring 19, by lead-in rods 22, 22a, and the anode 30 of the scanning gun extends to adjacent the target, where it is terminated by a secondary electron collecting mesh grid 32, the output signal being generated across the load resistor 25 connected to the conductive plate 1. A further, stabilizing, mesh grid 40 may be included, and the output signal may alternatively be obtained from an electron multiplier positioned to receive the reflected scanning electrons. The target may also be employed in a storage tube in which the photo-cathode is replaced by a writing electron gun, or in an image converter or intensifier tube.