Patent GB-811157-A (GB)
Title: Improvements in or relating to radiographic devices for use with x-rays and like penetrating rays
IPC: G03C5/17
Filed: 1956-05-14
Abstract: 811,157. X-ray apparatus, SIEMENSREINIGER-WERKE A.G. May 14, 1956 [May 14, 1955], No. 14982/56. Drawings to Specification. Class 98(1) In a radiographic device for use with X- rays or the like consisting of a number of light-sensitive layers one behind the other and intensifier foils containing fluorescent substances interleaved therewith, at least the foils of a group disposed in the part of the device next to the radiation source and have amounts of florescent substance per unit area reduced below the amounts required for maximum intensifying effect (preferably below 40 mg./sq. cm.), to an extent sufficient to reduce substantially the absorption of radiation by each foil and means are provided for so varying the light emitting properties of the foils or the sensitivity of the layers without substantially affecting the penetration of the radiation, as to produce uniform exposure of the light-sensitive layers when they are simultaneously exposed to radiation. The lightsensitive layers may consist of photographic film, or of selenium or a like semiconductor. The intensifier foils are coated with equal amounts of fluorescent substance per unit area, preferably less than 30 mg. of fluorescent substance per sq. cm. The effect of decreasing intensity of the radiation as it penetrates one foil after another is compensated by making the intensifying effect of the foils decrease with decreasing distance from the source of radiation in one of the following ways: (1) by adding "killer" substances, e.g. nickel additives, in amounts varying from foil to foil; (2) by varying the mean grain size of the fluorescent material from foil to foil; (3) by using in different foils similar substances of different activation; and (4) by using a mixture of two fluorescent substances (of different grain size or of different chemical composition) of different sensitivity and varying the proportion of the more sensitive substance to the less sensitive substance from foil to foil. Alternatively, the foils may be similar and the sensitivity of the light-sensitive layers may be varied from layer to layer. In photographic layers, the sensitivity variation is obtained by varying the degree of ripeness, the sensitization to the light emitted by the intensifier foils, or the silver bromide content. Each layer may have intensifier foils on both sides, -but preferably there is one foil only for each layer, placed at the side, of the layer remote from the radiation source. The remote side of each foil may be coated with a substance which partially absorbs and partially reflects the fluorescent light, and the degree of absorption and reflection may decrease and increase respctively with increasing distance from the source of radiation. It is preferable to use fluorescent substances containing elments of low atomic number, such as zinc-activated or or silver-activited zinc sulphide, for softer radiation; since the rays become harder as they penetrate successive foils the foils may contain elements of increasing atomic weight with increasing distance from the radiation source. When X-rays of energies greater than 100 Kv are used, the nearer foils may consist of zinc sulphide, while the further foils consist of calcium tungstate or lead-barium tungstate. When the X-rays have energies less than 100 Kv, it is preferable to use calcium tungstate for the nearer foils and zinc sulphide for the further foils.