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Improvements in or relating to screens for producing an electron image having a space-distribution which is a replica of the space-distribution of an image pattern of radiations
Improvements in or relating to screens for producing an electron image having a space-distribution which is a replica of the space-distribution of an image pattern of radiations
719,762. Luminescent screens. WESTING- HOUSE ELECTRIC INTERNATIONAL CO. Nov. 27, 1952 [Dec. 5, 1951], No. 30073/52. Class 39(1) A screen comprises a sheet of glass 0.004-0.01 inch, preferably 0.005-0.008 inch thick having on one side a layer of an inorganic phosphor powder 5, Fig. 2, which can be excited by X or rays and consisting of zinc cadmium sulphide, zinc sulphide, cadmium tungstate, calcium tungstate or barium lead sulphate with a binder of sodium silicate, potassium silicate, lithium silicate or ethyl silicate, and having on the other side a transparent layer 9 of material of resistance not greater than one megohm per square which is coated with a photoemissive layer 6. The screen is used in an X-ray image intensifier, Fig. 1 (not shown), the photoemissive layer being cesiated antimony, and is shaped as a watch glass, as shown in Fig. 2. The photoelectrons form an intensified image of reduced size on a second phosphor screen at the other end of the tube. Transparent layer 9 is of conducting tin oxide (stannic) and the glass support 3 is soda lime glass. The phosphor particle size is 10 - 20 microns or more. The binder is preferably a 3 - 15 per cent solution of potassium silicate in water, the ratio of silica to potassium oxide being 5:2. In one method a watch glass is heated to 600‹C and sprayed with a solution of stannic chloride having equal proportions of stannic chloride methyl alcohol and glacial acetic acid to form the tin oxide coating. After cooling it is placed convex side up in the potassium silicate solution and phosphor powder sprinkled in dry or added as a suspension in additional solution. A masking cylinder may be placed on the watch glass to keep the periphery free from phosphor. A wetting agent may be added to prevent phosphor powder from floating on the surface. The solution is finally removed by siphoning or draining, and surplus potassium silicate removed from the concave side of the screen with a jet of distilled water. The screen is then baked to 400‹C in vacuum to remove moisture. The phosphor layer may be coated with additional potassium silicate to protect it more adequately against chemical attack, e.g. by caesium vapour when the photoemissive layer 6 is being applied. A thin aluminium layer may be applied to the phosphor layer as foil or by evaporation. The tin oxide layer may contain small quantities of other materials or may be formed from tin compounds other than stannic chloride, and the compounds may be applied by dipping and washing or by vaporization instead of by spraying. The phosphor layer may be applied by painting or flowing instead of by settling. The binder is 1 - 10 per cent, preferably about 3 per cent by weight of the weight of the phosphor powder. It is stated that maximum efficiency of ZnCdS results when the proportion of CdS is such as to yield green or green-yellow light. It is stated that zinc cadmium sulphide does not give rise to background glow while zinc sulphide does owing to its property of storage of some X-ray energy and emission of light which excites the photoemissive layer for some hours after the exciting X-rays are cut off.
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