Patent GB-842318-A (GB)
Title: Luminescence intensification system
IPC: H01J29/18
Filed: 1956-09-12
Abstract: 842,318. Electroluminescence. GENERAL ELECTRIC CO. Sept. 12, 1956 [Sept. 12, 1955], No. 27918/56. Addition to 790,409. Class 39(1). A field-enhanced luminescence intensification system comprises a source of radiant energy-cathode rays, X-rays, U.V. light or visible light - and means for directing the radiant energy upon a luminescent screen comprising an electrically continuous semi-conductor phosphor layer, there being a first region of low resistivity adjacent to one side of a middle region and comprising a sulphide of zinc or cadmium impregnated with 0À001 to 3, preferably 0À01% by weight each of silver and chlorine as activators, a second region of low resistivity adjacent to the opposite side of the middle region and comprising a sulphide of zinc impregnated with 0À001 to 0À3, preferably 0À01 to 0À1% by weight each of copper and chlorine as activators, and a third or the middle region of unactivated high resistivity sulphide of zinc or a mixture of zinc and cadmium sulphides, intermediate and contiguous with the first and second regions, the variations in electrical resistivity from the high resistivity region through the low resistivity regions being gradual and continuous, and means being provided for impressing an electric field between opposite surfaces of the phosphor layer. In the construction shown in Fig. 2, the following layers are deposited on the glass plate 6-a transparent conducting layer 7 of titanium dioxide, a vapour-deposited film 11, about 0À01 micron thick-of copper chloride, a thin transparent layer 8 of zinc sulphide formed by the interaction of zinc sulphide vapour and hydrogen sulphide gas in the vapour phase, a sprayed or vapour-deposited thin layer 10 of silver chloride, and a thin metallic conducting film 9. The assembly is placed in an oven and heated for about one-half hour at a temperature of about 1100‹C to cause copper and chlorine activator atoms to diffuse into the region 14, and silver and chlorine activator atoms to diffuse into the region 12. In the case of X-ray radiation the phosphor layer 8 can comprise a single crystal or a plurality of properly orientated single crystals of zinc sulphide of the order of 100 to 1,000 microns thick. The conducting layers 7 and 9 are connected to a current source. The bulk resistivity of the phosphor layer 8 rises from about 10<SP>12</SP> ohm/ cms at the outer surface 10 of the region 12 to about 10<SP>20</SP> ohm/cms throughout the region 13 and falling again to the surface 11 of the region 14. Impressed electric fields of the order of 10<SP>4</SP> to 10<SP>7</SP> volts per cm. are referred to. The invention is described with reference to the multiplication of free electrons through the high resistivity phosphor region and the subsequent excitation of the centres of activation. It is stated that when electron-hole pairs are created within the first region of the phosphor, the polarity of the impressed electric field may be arranged to draw positive holes into the intermediate high resistivity phosphor region causing the creation of an avalanche of positive holes which subsequently excite the centres of activation.