Patent GB-763190-A (GB)
Title: Intensifying and storing images formed by x-rays or atomic particles
IPC: H01J31/49
Filed: 1952-05-28
Abstract: 763,190. Television pick-up tubes. SHELDON, E. E. May 28, 1952 [June 1, 1951], No. 13502/52. Class 39(1). [Also in Group XL(b)] A discharge tube comprises means for converting an X-ray or atomic particle image into changes of electrical conductivity and a low velocity scanning beam for producing video signals corresponding to the changes in conductivity. A composite photo-cathode screen 6 comprises a thin X- ray transparent light transmitting layer 7 e.g. aluminium, a fluorescent layer 8 of zinc silicate, zinc selenide or sulphide, calcium tungstate or barium lead sulphate mixed with glass, a barrier layer 9 of mica, glass, silica, ZnF2, a metal or a plastic and a photo-emissive layer 10 of caesium oxide/silver or of caesium potassium or lithium with either antimony or bismuth. Alternatively an X-ray sensitive electron-emissive screen of lead, gold or uranium may be used as the photo cathode with or without an adjacent secondary emissive layer of silver or magnesium. The next stage in the tube may be an electron multiplier 18, 19 of one or more stages. The amplifying section 12 comprises successive composite screens each consisting of layers as for screen 6 e.g. an electron pervious light reflecting layer 13, a fluorescent layer 14, a barrier layer 15 and a photo emissive layer 16. Inorganic and organic phosphors e.g. anthracene and naphthalene are suitable materials for layer 14. Magnetic and electrostatic means focussing and demagnifying the electron image are provided. The storage target 21 comprises an electron pervious light reflecting layer 21a, e.g. aluminium or silver, a fluorescent layer 21b, a conductive layer 21c and a photo-conductive layer 21d; the materials for the latter including selenium, Cu2O, germanium, thallium sulphide and lead sulphide or selenide. The layer 21c is transparent and functions as a signal plate. The target is scanned by a low velocity beam 23 from electron gun 25 through a mesh 65 and the signals fed via an amplifier to a receiver tube 90a. This image may be further amplified by a pick-up tube 92 or fed back to the X-ray pick-up tube 1 whilst the X-ray beam is suppressed. In a modification, Fig. 2 (not shown), the electrons of the low velocity scanning beam reflected by the target are returned to an electron multiplier, and an additional helical motion may be imparted to the returning electrons so that only the scattered electrons enter the multiplier, the reflected electrons being stopped by the edge of the multiplier aperture. Alternatively an additional electrode is provided to separate the scattered and reflected electrons. The signals may be fed to a tube where they may be stored for a predetermined time before transmission to the receiver. The storage target is a metal or wire mesh upon which barium or calcium fluoride and a thin overlayer of metal are deposited by evaporation. and it is located between two conducting meshes. The reading beam passes through the storage mesh to the signal plate in proportion to the charge laid down by the writing beam, the signal current being amplified and fed to kinescopes, facsimile receivers or electrographic cameras. Erasure of the positive stored image is obtained by control of the target potential to give secondary emission less than unity. In a further modification, the X-ray image impinges directly on the composite photo-conductive target: In all systems, the resistance and insulation of the composite target are made high by using specially prepared selenium produced by condensing on a cold surface or by introducing an additional dielectric layer of mica, silica or glass between the photo-conducting and conducting layers. The tube may be made sensitive to protons and neutrons by including neutron reactive materials e.g. boron, lithium gadolinum, uranium and the paraffins in the composite photo-cathode. Cadmium or copper are also reactive to neutrons to give gamma emission which actuates the fluorescent layer. Alternatively the protons and electrons from the neutron reactive layer act on electron-emissive layers such as beryllium, magnesium or silver. The fluorescent layers may have cone- or pyramid-like depressions on their surface to give increased emissive areas. Specifications 763,191 and 763,192 are referred to.