GB621729A - Improvements in or relating to devices having dark trace screens - Google Patents

Abstract

621,729. Electric discharge devices. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Nov. 14, 1944, No. 22500. Convention date, July 22, 1942. [Class 39 (i)] A device comprises a screen consisting of crystalline alkali metal halide together with a smaller quantity of a compound of a polyvalent element in order to reduce the persistence of the dark trace produced upon the screen by bombardment with radiation. The proportion of the polyvalent element is not more than 30 per cent and may be approximately 5 per cent, based on the molweight of the alkali metal. The polyvalent element may be thorium, titanium, zirconium, hafnium, barium, cadmium, tin, lead, uranium, manganese, oxygen, selenium or sulphur. In the device shown in the drawing, the screen 5 on the window 6 of the cathode-ray tube 1 is viewed from the position 7 or 7a through window 6 or window 2, and consists of potassium chloride with thorium chloride. In preparing the screen, 0.462 grams of pure potassium chloride and 0.045 grams of pure octo - hydrated thorium chloride are weighed into a platinum container, thoroughly mixed and heated, preferably by radiation from a hot body, to a red heat (500-1000‹ C.), to avoid decrepitation until the two salts have melted or are well sintered together. The crystals of the mixed salts are supported at 10 in the small platinum container 12 with its open end facing the window 6 on which the halide screen is to be produced. The platinum container is preferably surrounded by an insulated resistance heater coil 14 which vaporizes the mixed salts 10 which arc condensed upon the window 6 to produce the screen 5. A screen prepared as described above or in an evacuated vessel other than the tube 1, and then transferred to the tube 1 through an atmosphere devoid of all moisture, if desired, may be subjected to an atmosphere containing either water vapour, ammonia or both, followed by sealing and evacuating the bulb. This treatment decreases the transparency of the target to white light and renders it more milky in appearance and increases the contrast between the appearance of the bombarded and unbombarded areas of the screen. The crystals of the mixed salts may also be comminuted and deposited upon the screen foundation by settling from a suitable gas or liquid suspension such as air or amyl-acetate. Several polyvalent elements may be present simultaneously such as both thorium and oxygen, in thorium oxide. The screen may be actuated by X-rays or ultra-violet light as well as a cathode-ray beam. Specification 513,776 is referred to. Reference has been directed by the Comptroller to Specifications 511,796 and 513,776.