Oct. 16, 1951 R. L. LONGIN 2,571,905 ZINC SULFIDE X-RAY PHOSFHORS Filed Aug. 28, 1947

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UNITED STATES PATENT office 2,571,905 ZINC SULFIDEX-RAY PHosPHORs Richard L. Longini, Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, East Pitts burgh, Pa., a corporation of Pennsylvania, Application August 28, 1947, Serial No. 71,113 4 Claims. (C. 252-301.6) 1. 2 My invention relates to materials which be In making measurements of the luminous effi come fluorescent to produce visible light under ciency of Such materials, I have found that while the impact of X-rays and, in particular, relates Zinc sulphide produces a much larger yield of to a method of combining different substances visible radiation than does calcium tungstate for to produce a maximum yield of visible radia a given absorption of X-ray energy, the luminous tion for a given X-ray energization. intensity of the zinc sulphide screens is made In the medical and other X-ray arts, it is fre undesirably low because zinc sulphide has a very quently desirable to make visible the X-ray pat low absorption coefficient. I have, however, found terns produced by irradiating various objects that this difficulty can be corrected by admixing Which are opaque to visible radiation by streams 10 With the zinc sulphide an ancillary material or of X-rays, and screens covered by fluorescent ma “flux' in the form of an alkali halide. Among terial on which the transmitted X-ray pattern the alkali halides useful for this purpose are is incident have been used for this purpose. caesium iodide (CsI), caesium bromide (CSBir), However, difficulties have been found in that the rubidium iodide (Rb1), caesium chloride (CsCl), brightness of the image produced on the fluores. 15 potassium iodide (KI), sodium iodide (NaI), rubi cent Screen is undesirably low, so that it has been dium bromide (RbBr), and rubidium chloride necessary to view the screens in darkened rooms, (RbCl). Such materials have a strong tendency and to delay observation until the eyes of the ob to absorb X-rays and may be referred to as heavy Server have had time to accommndate themselves alkali halides in view of their atomic weights. to the low luminous intensity of the view being 20 These materials increase the X-ray absorption observed. coefficient of the mixture by amounts having the One object of my invention is, accordingly, to same order in which I have just enumerated provide a fluorescent material in which the lumi them, the caesium iodide producing the greatest nous intensity of the fluorescent image for a relative absorption. A given one of these addi given intensity of X-ray radiation is increased 25 tion agents appears to increase the relative ab Over that possible in screens of the prior art. Sorption by an amount linearly proportional to Another object of my invention is to produce the mole fraction of the addition agent. a material which produces visible light of a highly It has also been found that such addition desirable character and intensity when irradiated agents likewise increase the luminous efficiency with X-rays. 30 of the fluorescent material, provided they are Still another object of my invention is to pro not added in too great amounts; that is to say, duce a Screen for X-ray purposes in which the they increase the ratio of the energy emitted intensity of the visible image resulting from a as visible radiation to the energy of the X-radia given amount of incident X-radiation is of a tion absorbed. There is found to be a maximum higher order than that found in screens of the 35 luminous efficiency for a certain mole fraction prior art. of the addition agent, the efficiency first increas Still another object of my invention is to pro ing as Small amounts of the addition agent are vide a method of admixing fluorescent materials incorporated with the pure zinc sulphide, then of the prior art with such a proportion of ancil rising to a maximum, and then decreasing ulti lary substances as will produce a maximum yield 40 mately to a lower value than that of the pure of visible radiation for a given intensity of X Zinc Sulphide at large molecular fractions of radiation. the addition agent. However, I have found that Other objects of my invention will become ap the amount of addition agent which produces parent upon reading the following description, the maximum luminous efficiency is lower than taken in connection with the drawing in which 45 that which produces a maximum ratio of the the single figure is a graph showing the effect on energy of visible light to the energy of the in X-ray absorption and luminous intensity of war cident X-rays. It appears that the criterion for ious mole fractions of addition agents incorpo a maximum yield of visible radiation when X rated in fluorescent materials of the prior art. rays are used as the energizing agent is propor Among the materials which have been used in 50 tional to the product of the luminous efficiency by the production of fluorescence under the action the X-ray absorption coefficient. It is one of of X-rays are zinc Sulphide, and zinc-cadmium the important features of my invention, there Sulphide. Other materials of somewhat analo fore, that I proportion the amount of the addi gous chemical classification are known, the name tion agent to the pure phosphor in such a way as 'X-ray phosphors' being frequently applied to 55 to make the above-mentioned product a maxi materials for Such purposes. 2,571,905 4. This is illustrated by the curves in the draw 2. An X-ray Screen consisting essentially of ing which represent a typical phosphor. In these a Support coated with a fired reaction product the abscissae represent mole fraction of the ad of a Substance drawn from the group consisting dition agent and the Ordinates in curve I repre of zinc sulphide and zinc cadmium Sulphide, sent luminous efficiency of the resulting phosphor. 5 caesium chloride, and an activator Selected from Curve II shows the variation of X-ray absorp the group consisting of the metals silver and tion and curve III shows the product of the Or copper, said caesium chloride being present in dinates of curves I and II. The maximum on the amount of 0.2 of a mole fraction of the curve III is seen to occur at a larger mole frac mixture. tion than the maximum for curve I. 10 3. The fired reaction product of a Substance While I have described zinc sulphide as the drawn from the group consisting of zinc Sul particular fundamental material with which the phide and zinc cadmium Sulphide mixed with addition agents have been incorporated, the in a substance drawn from the group consisting of proved results due to use of such materials may caesium iodide, caesium bromide, rubidium iodide, be attained with other fiuorescent materials. Such 5 caesium chloride, potassium iodide, Sodium as zinc-cadmium-sulphide. In particular, the iodide, rubidium bromide and rubidium chloride, principle of proportioning the addition agent So and an activator selected from the group which that the product of the luminous efficiency for consists of the metals silver and copper, Said electron energization by the X-ray absorption co caesium chloride being present in the amount efficient is a maximum is applicable to other naiX 20 of 0.2 of a mole fraction of the mixture. tures. than that which I have Specifically de 4. The fired reaction product of zinc sulphide Scribed above. mixed with caesium chloride and an activator To give a very specific embodiment of the selected from the group which consists of the principles of my invention, I have found that metals silver and copper, said caesium chloride a mixture of zinc sulphide with caesium chloride 25 being present in the amount of 0.2 of a mole frac present in an amount constituting of 0.2 mole tion of the mixture. fraction of the mixture produces a highly de RICHARD L.LONGINI, sirable screen for X-ray work. It is conventional to provide Such X-ray phos REFERENCES CITED phors with what are known as “activators' which 30 The following references are of record in the are usually present to the order of 0.01%, and file of this patent: such activators may be used with the mixtures which are described as embodiments of my in UNITED STATES PATENTS vention. For example, silver is a suitable acti Number Name Date vator; the element copper is another which may 35 2,310,740 Leavy ------Feb. 9, 1943 be used. 2,435,435 Fonda ------Feb. 3, 1948 I claim as my invention: 2,435,436 Fonda ------Feb. 3, 1948 1. The fired reaction product of a mixture of FOREIGN PATENTS zinc sulphide phosphor with caesium chloride, the latter being present in an amount of 0.2 40 Number Country Date of a mole fraction of the mixture. 449,392 Great Britain ------June 19, 1936