Patent GB-779592-A (GB)
Title: Improvements in or relating to laminar screens for substantially reducing the proportion of scattered rays in an x-ray beam
IPC: G21K1/02
Filed: 1953-09-06
Abstract: 779,592. X-ray apparatus. VIERKOTTER, P. Sept. 6, 1955 [Sept. 7, 1954], No. 25573/55. Class 98(1) A laminar screen for reducing the proportion of scattered radiation in an X-ray beam is made by coating a screen with at least one layer of small particles which are arranged in lattice formation, the particles being such that X-rays which pass through the coat approximately at right angles are weakened substantially less than X-rays which impinge on the coat at an oblique angle Preferably, the particles consist of a material having a high atomic number. One form of the screen is formed by developing and fixing a uniformly exposed double-coated photographic film, so as to obtain an even distribution of particles of silver on both sides of the film. Whilst still wet, the film is placed in a weak solution of mercuric chloride, whereby mercury is deposited on the silver particles, probably with the formation of a silver-mercury amalgam, which may be lacquered when dry. Alternatively, aluminium foil is cleaned and treated with dilute mercuric chloride solution, so that a coating of mercury particles having lattice-type structure is formed on its surface The foil is then exposed to the air, so that the portions thereof not covered by mercury are oxidized, and varnished to prevent vaporization of mercury. In another example, mercury particles are deposited on activated carbon powder by absorption of mercury vapour. The treated carbon powder is applied in a thin coating to a carrier such as aluminium foil and covered under slight pressure with a corresponding top foil. The mercury particles may be converted into mercuric iodide by treatment with iodine vapour. Further, a screen may be formed by colloidal dispersion of gold particles in a glass coating or foillike carrier. A second type of screen is of use where the scattered rays are at least partly polarized. Such rays are weakened by passing them through a screen formed of a paraffin coating which has been converted from the liquid to the solid state by slow cooling. A third type of screen, useful for X-rays in certain ranges of wavelengths, consists of a multi-layer structure of crystalline particles, such as a thin marble plate, a layer of fine grained marble powder, or a layer of chemically precipitated crystalline calcium carbonate powder. The three kinds of screen referred to above may be combined in various ways. A fourth kind of screen consists of a thin coat 'of material, the molecules of which have a regular three dimensional arrangement and consist of heavy metal atoms combined with atoms of a low X-ray absorption. Suitable materials are pulverized thorium oxide and lead oxide. A fifth kind of screen consists of a coating of a mixture of two powders, one of which absorbs X-rays strongly, and the other only slightly, for example lead dust and carbon dust. A sixth type of screen is formed by forming a layer of homologeous pulverized material which absorbs X-rays, the thickness of the layer varying in accordance with a lattice pattern. Such layers are made by precipitating a powdered substance from a liquid or allowing a sediment to form whilst vertical ultrasonic waves are produced in the liquid from at least two intersecting directions. A higher concentration of particles is deposited at the nodal points which are thus formed in the ultrasonic field. One or more screens such as are described above may be incorporated permanently or exchangably in film holders or in a fluorescent screen, and may be deposited on a photographic film in such a way that they are dissolved during the processing of the film. Intensifying foils for use with the screens described above may be made by depositing a single layer coating of very fine crystals of luminescent material. The crystal layer may be deposited, for example, on an exposed, developed and fixed coat of a photographic film, and orientated so that only the crystal edges or points project. This orientation is achieved by embedding the crystals under action of an electric field running at right angles to the carrier. This causes the individual crystals to stand upright. If the luminous crystals are para-magnetic or di-magnetic, a magnetic field can be used instead of the electric field to orientate the particles. Alternatively, the luminous particles can be produced as a coating by sedimentation from a liquid, the particles being erected by their own weight. The intensifying screen so produced may be incorporated, together with one of the anti-diffussing screens, permanently in a film holder.