Abstract:
A novel X-ray intensifying screen is detailed comprising Li 3  Ta 1-x  Nb x  O 4  wherein x is 0.0 to 1.0. Further detailed is a radiographic recording element employing the same screen. This novel X-ray intensifying screen has improved speed when compared to X-ray intensifying screens comprising CaWO 4 .

Description:
FIELD OF INVENTION 
     This invention relates to radiographic intensifying screens for converting image-wise modulated X-radiation into ultraviolet and/or blue radiation. More specifically this invention is related to lithium tantalate phosphors for use in a radiographic intensifying screen. 
     BACKGROUND OF THE INVENTION 
     X-ray excited phosphors are well known for their use in medical radiography. Medical X-ray films are typically sensitive to UV, blue or green light and therefore the X-radiation containing the diagnostic information must be converted to lower energy to optimally expose the film. This conversion is done by an intensifying screen comprising an appropriate phosphor as the principal active ingredient. 
     Phosphors which are suitable for converting X-ray energy to the appropriate lower energy are well known. The specific desirable properties include: high conversion efficiency, low granularity and a low quantity of afterglow emission. Particularly useful are phosphors for use in x-ray intensifying screens which have an emission in the near UV and blue. The most widely known phosphor for this purpose is CaWO 4  which has become the industry standard. There is an ongoing desire in the art to provide UV or blue emitting medical x-ray intensifying screens which have a higher speed than intensifying screens comprising CaWO 4 . 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an x-ray intensifying screen for use in radiographic elements. Other advantages, as will become apparent from the teachings herein are provided in an x-ray intensifying screen comprising a support at least one active layer coated on said support; wherein said active layer comprises a binder and a phosphor corresponding to the formula 
     
         Li.sub.3 Ta.sub.1-x Nb.sub.x O.sub.4 
    
     wherein x is 0 to 1.0. 
     A particularly preferred embodiment is obtained when the x-ray intensifying screen of the present invention is used in combination with a suitable film as provided in a radiographic recording element comprising at least one x-ray intensifying screen in operative association with a photosensitive film element wherein said x-ray intensifying screen comprises; a support; at least one active layer coated on said support and said active layer comprises a phosphor corresponding to the formula: 
     
         Li.sub.3 Ta.sub.1-x Nb.sub.x O.sub.4 
    
     wherein x is 0.0 to 1.0. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a graph illustrating the relative speed of Li 3  Ta 1-x  Nb x  O 4  as x increases from 0.0 to 0.10. 
     FIG. 2 is a graph illustrating the relative speed of Li 3  Ta 1-x  Nb x  O 4  as x increases from 0.0 to 0.60. 
     FIG. 3 is a graph illustrating the maximum emission of an X-ray excited Li 3  Ta 1-x  Nb x  O 4  wherein x is 0.50 to 1.0. The discontinuity represents the structural transition from monoclinic morphology to cubic morphology. 
     FIG. 4 is a graph illustrating the maximum emission of an X-ray excited monoclinic Li 3  Ta 1-x  Nb x  O 4  wherein x is 0.0 to 0.6. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The screen of the present invention comprises a phosphor which corresponds to the formula: 
     
         Li.sub.3 Ta.sub.1-x Nb.sub.x O4 
    
     wherein x is 0 to 1.0. Preferred is a phosphor where x is 0 to 0.7, more preferred is a phosphor where x is 0 to 0. 1 and even more preferred is a phosphor where x is 0 to 0.07. The most preferred phosphor is obtained when x is between 0.002 and 0.010. When x is outside of the range of 0.002 to 0.010 a drop in intensity is observed as indicated in FIG. 1 wherein &#34;Sp&#34; is speed relative to High Plus arbitrarily defined as 1.0. 
     Preparation of the phosphor of the present invention is well documented in the art and yet the use of this phosphor in an X-ray intensifying screen has not been previously contemplated. The preparation of the phosphor is known to proceed according to following reaction: 
     
         (1-x)Ta.sub.2 O.sub.5 +xNb.sub.2 O.sub.5 +3Li.sub.2 CO.sub.3 →2Li.sub.3 Ta.sub.1-x Nb.sub.x O.sub.4 +3CO.sub.2 
    
     The preparative procedure typically involves intimately mixing the proper ratios of the phosphor precursors, such as Li 2  CO 3 , Ta 2  O 5  and Nb 2  O 5  in a suitable flux such as Li 2  SO 4  or LiCl. This mixture is heated in an alumina crucible at 1000° C. The flux is removed by dissolving in water, the phosphor is recovered and a preselected particle size distribution is used for the preparation of an X-ray intensifying screen. 
     After firing in the conventional manner, flux and soluble reaction products are removed by water leaching or some other suitable method. The resulting Li 3  Ta 1-x  Nb x  O4 (x=0 to approximately 0.7) crystallizes in a monoclinic C2/c space group or in a P2/n space group as detailed in Journal of Solid State Chemistry, 48, 420-430 (1983). The crystallographic forms are also detailed in J. of Solid State Chemistry, 53, 277-278 (1984). At stoichiometric levels of niobium above approximately 0.7 the structure is cubic with a space group symmetry of 123 and the properties of the phosphor become less desirable for use in an x-ray intensifying screen. 
     The stoichiometric level of niobium directly impacts the emission maximum of the phosphor. From a stoichiometric level of 0.0 to 0.6 the emission maximum, under x-ray excitation, increases from approximately 410 nm to approximately 435 nm as illustrated in FIG. 4. At niobium levels above approximately 0.7 the cubic structure is observed and the emission, under x-ray excitation, increases with niobium level. The emission is lower for the cubic structure than for the monoclinic structure. 
     There are a host of fluxes which can be used to prepare the phosphor described above. These include the alkali metal salts, e.g., lithium, sodium, potassium, etc., of sulfates, phosphates, carbonates, metaborates, etc. The halides are also known to be beneficial when added to the flux in combination with sodium, lithium, potassium, calcium, strontium, barium and the like. Particularly suitable fluxes are described, for example, in Hedden and Zegarski, U.S. Pat. No. 4,250,365. The amount of flux is not limiting. Typically a flux is present in the reaction mixture in about 30% to about 60% by weight, preferably about 45% to about 55% by weight, based on the total weight of oxide (phosphor starting material). The optional flux and phosphor precursors are intimately mixed by any of a number of means including ball milling, shaking, or by the use of a flowing gaseous or liquid medium such as air, nitrogen, water, fluorochlorinated hydrocarbons or other inert fluids as detailed in U.S. Pat. No. 5,154,360. The mixture of phosphor starting materials, and optional flux can be fired, e.g., for at least about three hours, at elevated temperatures, e.g., from 750° C. to 1500° C., before washing to remove a majority of the flux and recovering the phosphor. 
     After firing, pulverizing and washing, the phosphor is mixed with a suitable binder in the presence of a suitable solvent and coated on a support. All of these steps are described in the U.S. Pat. No. 4,225,653 and all are well-known in the prior art. A protective topcoat may also be applied over this phosphor coating, in fact it is preferred. 
     The phosphor may be dispersed in any of the commonly known binders, e.g., polyvinyl butyral or the acrylates or methacrylates, using a solvent, e.g., an alcohol, chlorinated hydrocarbon, ketone, butyl acetate, etc. Small amounts of fumed silica may be present in the phosphor to enhance handling and make the phosphor easier to pour. After dispersing in the binder, the phosphor is then cast on a conventional support, e.g., cardboard, polyester film, thin metal sheets, etc. A brightener may also be present within the phosphor and various reflective materials may be present as an underlayer or within the support itself to enhance the generation of light when the phosphor is struck by X-radiation. TiO 2  dispersed in a binder and cast on the support is conventional as well as the use of small particles of rutile TiO 2  directly in a film support. The phosphor is typically applied to the support in an amount ranging from 15 to 110 mg/cm 2 . All of these procedures are well-known in the art. Over the phosphor layer which has been cast on the support, a conventional protective topcoat may be applied and, in fact, is preferred. These topcoats are also well-known in the prior art and serve to protect the rather expensive phosphor layer from stains and defects during the handling thereof. A particularly suitable topcoat is achieved when a styrene/acrylonitrile copolymer is coated supra to the phosphor layer and dried. Conventional supports, binders, mixing and coating processes for the manufacture of typical X-ray intensifying screens are, for example, described in Patten U.S. Pat. No. 4,387,141, the pertinent disclosure of which is incorporated herein by reference thereto. 
     The screens of the present invention may be used in pairs in cooperation with double-side coated medical X-ray silver halide photographic film elements, although it is contemplated that single-side coated silver halide photographic film elements may be used for some applications. If a pair of screens is used the coating weights of each screen may be different, if required. Thus, an asymmetric pair of screens can be used to get the best results. Medical X-ray evaluations represent the predominant commercial use for X-ray intensifying screens of this invention. A dimensionally stable, polyethylene terephthalate film support into which small amounts of rutile or anatase titanium dioxide have been incorporated is the preferred support for the x-ray intensifying screen of this invention. 
     In operation, the intensifying screen absorbs X-rays that impinge thereon and emits energy having a wavelength that is readily captured by the photographic silver halide X-ray film associated therewith. These screens are generally prepared according to the methods of Brixner, U.S. Pat. No. 4,225,653. 
     In the radiological process, it is conventional to employ a photosensitive silver halide film element with the above described X-ray intensifying screens. In the practice of this invention, the silver halide element will be comprised of silver halide grains. These elements are also well-known in the prior art and the preparation of grains are also known and taught therein. The grains are generally made into an emulsion using a binder such as gelatin, and are sensitized with gold and sulfur, for example. Other adjuvants such as antifoggants, wetting and coating aides, sensitizing dyes, hardeners etc. may also be present if necessary. The emulsion may be double-side coated on the support and a thin, hardened gelatin overcoat is usually applied over each of the emulsion layers to provide protection thereto. If required, a small amount of a sensitizing dye might advantageously be added. Additionally, it is also conventional to add a sensitizing dye to tabular emulsions in order to increase their ability to respond to light. 
     The silver halide emulsion may employ any of the conventional halides but preferred are pure silver bromide or silver bromide with small amounts of iodide incorporated therein (e.g. 98% Br and 2% I by weight for example). Any grain morphology is suitable for demonstration of these teachings including, but not limited to, grains which are formed by splash techniques and those formed by techniques involving spray techniques (i.e. single and double jet procedures). Tabular grains are most preferred. 
     Tabular grain silver halide products are well-known in the prior art with exemplary methods of manufacture described by Maskasky in U.S. Pat. No. 4,400,463; Wey, U.S. Pat. No. 4,399,205; Dickerson, U.S. Pat. No. 4,414,304; Wilgus et al., U.S. Pat. No. 4,434,226; Kofron et al., U.S. Pat. No. 4,439,520; Nottorf, U.S. Pat. No. 4,722,886; and Ellis, U.S. Pat. No. 4,801,522. 
     After the grains are made, it is usually preferable to disperse the grains with a binder (e.g. gelatin or other well-known binders such as polyvinyl alcohol, phthalated gelatins, etc.). In place of gelatin other natural or synthetic water-permeable organic colloid binding agents can be used as a total or partial replacement thereof. Such agents include water permeable or water-soluble polyvinyl alcohol and its derivatives, e.g., partially hydrolyzed polyvinyl acetates, polyvinyl ethers, and acetals containing a large number of extralinear -CH 2  HOH-- groups; hydrolyzed interpolymers of vinyl acetate and unsaturated addition polymerizable compounds such as maleic anhydride, acrylic and methacrylic acid ethyl esters, and styrene. Suitable colloids of the last mentioned type are disclosed in U.S. Pat. Nos. 2,276,322, 2,276,323 and 2,347,811. The useful polyvinyl acetals include polyvinyl acetalaldehyde acetal, polyvinyl butyraldehyde acetal and polyvinyl sodium o-sulfobenzaldehyde acetal. Other useful colloid binding agents include the poly-N-vinyllactams of Bolton U.S. Pat. No. 2,495,918, the hydrophilic copolymers of N-acrylamido alkyl betaines described in Shacklett U.S. Pat. No. 2,833,650 and hydrophilic cellulose ethers and esters. Phthalated gelatins may also be used as well as binder adjuvants useful for increasing covering power such as dextran or the modified, hydrolyzed gelatins of Rakoczy, U.S. Pat. No. 3,778,278. 
     It is most preferable to chemically sensitize the grain with salts that are well known in the art. The most common sensitizers are salts of gold or sulfur. Sulfur sensitizers include those which contain labile sulfur, e.g. allyl isothiocyanate, allyl diethyl thiourea, phenyl isothiocyanate and sodium thiosulfate for example. Other non-optical sensitizers such as amines as taught by Staud et al., U.S. Pat. No. 1,925,508 and Chambers et al., U.S. Pat. No. 3,026,203, and metal salts as taught by Baldsiefen, U.S. Pat. No. 2,540,086 may also be used. 
     The emulsions can contain antifoggants, e.g. 6-nitrobenzimidazole, benzotriazole, triazaindenes, etc., as well as the usual hardeners, i.e., chrome alum, formaldehyde, dimethylol urea, mucochloric acid, and others as recited in Research Disclosure, No. 308,  December 1989, Item 30819. Other emulsion adjuvants that may be added comprise matting agents, plasticizers, toners, optical brightening agents, surfactants, image color modifiers, non-halation dyes, and covering power adjuvants among others. 
     Thermographic imaging emulsions comprising silver salts of carboxylic acids may also be used with the screens of the present invention. Exemplary examples are provided in U.S. Pat. No. 5,028,518. 
     The film support for the emulsion layers used in the process may be any suitable transparent plastic. For example, the cellulosic supports, e.g. cellulose acetate, cellulose triacetate, cellulose mixed esters, etc. may be used. Polymerized vinyl compounds, e.g., copolymerized vinyl acetate and vinyl chloride, polystyrene, and polymerized acrylates may also be mentioned. Preferred films include those formed from the polyesterification product of a dicarboxylic acid and a dihydric alcohol made according to the teachings of Alles, U.S. Pat. No. 2,779,684 and the patents referred to in the specification thereof. Other suitable supports are the polyethylene terephthalate/isophthalates of British Patent 766,290 and Canadian Patent 562,672 and those obtainable by condensing terephthalic acid and dimethyl terephthalate with propylene glycol, diethylene glycol, tetramethylene glycol or cyclohexane 1,4-dimethanol (hexahydro-p-xylene alcohol). The films of Bauer et al., U.S. Pat. No. 3,052,543 may also be used. The above polyester films are particularly suitable because of their dimensional stability. 
     When polyethylene terephthalate is manufactured for use as a photographic support, the polymer is cast as a film, the mixed polymer subbing composition of Rawlins, U.S. Pat. No. 3,567,452 is applied and the structure is then biaxially stretched, followed by application of a gelatin subbing layer. Alternatively, antistatic layers can be incorporated as illustrated, for example, by Miller, U.S. Pat. Nos. 4,91 6,011 and 4,701,403, Cho, U.S. Pat. Nos. 4,891,308 and 4,585,730 and Schadt, U.S. Pat. No. 4,225,665. Upon completion of stretching and application of subbing composition, it is necessary to remove strain and tension in the base by a heat treatment comparable to the annealing of glass. 
     The emulsions may be coated on the supports mentioned above as a single layer or multi-layer element. For medical X-ray applications, for example, layers may be coated on both sides of the support which conventionally contains a dye to impart a blue tint thereto. Contiguous to the emulsion layers it is conventional, and preferable, to apply a thin stratum of hardened gelatin supra to said emulsion to provide protection thereto. 
     Medical X-ray film processing is well documented in the art as exemplified in Wuelfing, U.S. Pat. No. 4,741,991. A medical X-ray film to be processed is typically developed to convert latent image centers within the silver halide grain into elemental silver. Unreacted silver halide is then removed by dissolving in a suitable fixer and the film is washed and dried to provide a suitable image. 
     The speed in a photographic system is broadly defined as the exposure required to obtain a predetermined density under standard processing conditions. For a medical X-ray system the specific procedure is detailed in ANSI Standard, PH2.9, 1964. It is widely accepted in the art to report a relative speed wherein the speed is determined relative to PAR which is arbitrarily assigned a speed value of 100. PAR speed is determined with a standard CaWO 4  screen with a 84 μm phosphor thickness in combination with a Du Pont Cronex® 4 film. High Plus screens and Par screens utilize a CaWO 4  phosphor and are available from E. I. DuPont de Nemours, and Co. Relative speed for a medical X-ray system is determined at a density of 1.0 above base plus fog density by exposure modulation techniques as exemplified in SPSE HANDBOOK OF PHOTOGRAPHIC SCIENCE AND ENGINEERING, Woodlief, Ed.; John Wiley and Sons, New York, 1973, pp. 798-800. 
     EXAMPLES 
     Phosphor/Screen Preparation 
     7.9384 g. of Ta 2  O 5 , 0.0240 g. of Nb 2  O 5 , 4.0000 g. of Li 2  CO 3 , and 5.98 g. of Li 2  SO 4  were ground together in a mortar and transferred to a covered alumina crucible. The sample was heated in air to 1000° C. over a period of 1.75 hours and held at 1000° C. for 10 hours followed by furnace cooling. The mixture was then washed with water to remove the flux and the resulting Li 3  Ta 0 .995 Nb 0 .005 O 4  was dried and collected. The X-ray powder diffraction pattern of this compound shows only the lines for the monoclinic (psuedo-tetragonal) structure. The phosphor was incorporated into a screen using a polyvinyl butyral binder in accordance with the teachings of Brixner U.S. Pat. No. 4,225,653. The speed was measured using Cronex®10T medical X-ray film (available from E.I. DuPont de Nemours, and Co. of Wilmington, Del.) exposed at 70 kVp with a tungsten tube. The results are contained in Table 1. 
     
                       TABLE 1______________________________________Relative Speed of Inventive X-ray Intensifying ScreenPhosphor    Coating Weight                   Speed*______________________________________Par         --          100      ComparativeHigh Plus   --          216      ComparativeLi.sub.3 Ta.sub.0.995 Nb.sub.0.005 O.sub.4       48.4 mg/cm.sup.2                   271      InventiveLi.sub.3 Ta.sub.0.995 Nb.sub.0.005 O.sub.4       36.9 mg/cm.sup.2                   234      InventiveLi.sub.3 Ta.sub.0.995 Nb.sub.0.005 O.sub.4       28.2 mg/cm.sup.2                   192      Inventive______________________________________ *Speed is relative to Par which is arbitrarily defined as a speed of 100. 
    
     Emission changes as a function of niobium level 
     A series of samples were prepared as detailed above with the level of niobium corresponding to x values of 0.0, 0.002, 0.005, 0.01, 0.02, 0.10, 0.50, 0.60, 0.80, 1.00. The emission maximum was obtained with a Hamamatsu R928 photomultiplier tube. FIG. 2 contains a plot wherein x=0.0 to 0.6. In FIG. 2 &#34;Sp&#34; is speed relative to High Plus which is arbitrarily defined as 1.0. FIG. 3 contains a graph of speed relative to High Plus arbitrarily defined as 1.0 (Sp) wherein x=0.5 to 1.0. In FIG. 3 the transition period between monoclinic and cubic structure is indicated by the void between 0.6 and 0.8. This region has not been analyzed to determine the exact transition composition. The corresponding emission maximum (EM) is shown in FIG. 4 for x=0.0 to 0.6.