X-ray intensifying screen

Abstract: An intensifying screen for producing a latent image in a silver halide radiographic element when imagewise exposed to X-radiation is disclosed. The screen contains a phosphor having a hafnia host containing zirconia in concentrations higher than those found in optical grade hafnia. The phosphor can include as an activator one or a combination of titanium, rare earth, and alkali metal ions. Phosphor preparation processes are also disclosed.

Description: The invention relates to novel X-ray intensifying screens. More specifically, the invention relates to fluorescent screens of the type used to absorb an image pattern of X-radiation and to emit a corresponding pattern of longer wavelength electromagnetic radiation. A developable latent image is formed in a silver halide emulsion layer of a radiographic element when it is imagewise exposed to X-radiation. Silver halide emulsions, however, more efficiently absorb and consequently are more responsive to longer (300 to 1500 nm) wavelength electromagnetic radiation than to X-radiation. Silver halide possesses native sensitivity to both the near ultraviolet and blue regions of the spectrum and can be sensitized readily to the green, red, and infrared portions of the electromagnetic spectrum. Consequently it is an accepted practice to employ intensifying screens in combination with silver halide emulsion layers. An intensifying screen contains on a support a fluorescent phosphor layer that absorbs the X-radiation more efficiently than silver halide and emits to the adjacent silver halide emulsion layer longer wavelength electromagnetic radiation in an image pattern corresponding to that of the X-radiation received. While the phosphor layer and emulsion layer can be integrated into one element, in most instances the adjacent silver halide emulsion layer is coated on a separate support to form a separate radiographic element. In this way, the intensifying screen, which is not permanently altered by exposure, can be reused. The most common arrangement for X-radiation exposure is to employ a dual coated radiographic element (an element with silver halide emulsion layers on opposite sides of a support), each emulsion layer being mounted adjacent a separate intensifying screen. Phosphors employed in intensifying screens consist of a host compound, often combined with a small amount of another element that changes the hue and/or improves the efficiency of fluorescence. It is generally conceded that the reliable and reproducible creation of phosphors is among the most difficult of chemical investigations. H.W. Leverenz, An Introduction to Luminescence of Solids , John Wiley & Sons, Inc., New York, 1950, p. 61 states: Synthesis and Symbolism of Phosphors Efficient general research on phosphors requires (1) an exceptionally clean chemical laboratory equipped to synthesize very pure inorganic substances, (2) one or more furnaces capable of attaining at least 1600°C with either oxidizing atmospheres, and (3) a physical laboratory having suitable sources of photons and charged material particles and means for controlling and determining the energies and numbers of these particles. The chemical facilities are necessary to prepare luminescence-pure (LP) substances, who approximate degree of purity is indicated in the following series: approx. 1% ...ore, approx. 90% technically pure, 99.9% chemically pure (CP), 99.99% reagent-grade pure (RP), 99.999% spectroscopically pure (SP), 99.9999% luminescence pure (LP), 100% ...completely pure. Leverenz at page 62 and subsequently in his text goes on to suggest that research should be conducted with luminescence pure (LP) materials to reduce discrepancies in results. Leverenz is offered as an example of the general practice in the phosphor art of employing materials of much lower permissible levels of impurities than in many other fields of chemistry, ideally impurity levels at or below that of LP materials. It has been recognized that the phosphors of highest absorption efficiencies are those in which the host compound contains at least one element from Period 6 of the Periodic Table of Elements. For example, barium sulfate, lanthanide oxyhalides and oxysulfides, yttrium tantalate, and calcium tungstate, are widely employed phosphor host compounds. One family of phosphor host compounds that have shown promise in terms of performance, but have been little used are rare earth hafnates. L.H. Brixner, "Structural and Luminescent Properties of the Ln₂Hf₂O₇-type Rare Earth Hafnates", Mat. Res. Bull., Vol. 19, pp. 143-149, 1984, describes investigations of such phosphor host compounds. Ln is defined to include not only lanthanides, but also scandium and yttrium. A significant practical disadvantage in formulating these host phosphor compounds is that firing to temperatures in the 1800 to 1900°C range is required to obtain a single phase composition. These firing temperatures render rare earth hafnates burdensome to prepare as phosphor host compounds. One hafnium containing phosphor host compound that has been recognized to possess high efficiency in its absorption of X-radiation, but has enjoyed no practical use is optical grade hafnia. Kelsey U.S. Patent 4,006,097, issued May 5, 1975, discloses to be useful in the absorption of X-radiation a phosphor satisfying the formula: HfO₂:Yb with Yb being present in a concentration of 5 X 10⁻³ to 1 X 10⁻¹. Brixner, cited above, after reporting the properties of Ti⁺⁴ as an activator for rare earth hafnates, stated: We also looked at this same activator in pure HfO₂. Under 30kVp Mo radiation x-ray excitation, this composition also emits in a broad band centered around 477 nm as seen in Fig. 5. This emission has an intensity of about 1.6 times that of PAR CaWO₄ and could therefore be of interest as an x-ray intensifying screen phosphor, especially in light of the superior absorption of HfO relative to CaWO as seen in Fig. 6. Unfortunately, the price of optical grade HfO is so prohibitive that it cannot be used on screen applications. (Emphasis added.) Optical grade hafnia contains less than 3 X 10⁻⁴ mole of zirconia per mole of hafnia. It is the difficulty in separating zirconium and hafnium that primarily accounts for the cost of optical grade hafnia. Zirconium and hafnium are known to be atoms of essentially similar radii, 1.454Å and 1.442Å, respectively. Practically all known compounds of zirconium and hafnium correspond to the +4 oxidation state. The chemical properties of the two elements are essentially identical. The elements are found together in nature and can not be entirely separated. Zirconia and hafnia both exist predominantly in a stable monoclinic crystalline phase at room temperature, with the size of the crystal cell being very similar. As reported by by E. Iwase and S. Nishiyama, "Luminescence Spectra of Trivalent Rare Earth Ions", Proc. Intern. Sym. Mol. Struct. Spectry. , Tokyo, 1962, A-407-1 to 7, the crystal lattice constants of monoclinic hafnia and zirconia are as follows: Oxide a-axis b-axis c-axis β HfO₂ 5.11 5.14 5.28 99°44' ZrO₂ 5.21 5.26 5.375 99°55' Iwase and Nishiyama investigated "high purity Hf and Zr compounds" for cathodoluminescence-i.e., fluorescence response to electron bombardment. D.K. Smith and R.W. Newkirk, "The Crystal Structure of Baddeleyite (Monoclinic ZrO₂) and its Relation to the Polymorphism of ZrO₂", Acta Cryst. , 18 , 1965, pp. 983-991, demonstrates that large single crystals of monoclinic zirconia can be produced by heating ZrO₂ with a flux of Li₂Mo₂O₇ in an oven at 1400°C and withdrawing the sample when it reaches 900°C. Chenot et al U.S. Patents 4,068,128 and 4,112,194 disclose a variety of phosphors formed of varied ratios of phosphorus, hafnium, oxygen, and, optionally, zirconium. The various phosphor hosts produced by phosphorus in combination with hafnium are, of course, crystallographically dissimilar from hafnia host phosphors and offer no reliable indication of the effect of zirconium on the luminescence of monoclinic hafnia crystals. It is one object of the invention to provide screens varied in their ingredients to provide higher luminescence intensities upon stimulation. The object of the invention is satisfied by a screen comprised of a support and a fluorescent layer containing a phosphor capable of absorbing X-radiation and emitting longer wavelength electromagnetic radiation comprised of monoclinic crystals of a hafnia phosphor host. The screen is characterized in that zirconium ions are present in the hafnia phosphor host in concentrations higher than 3 X 10⁻⁴ mole of zirconia per mole of hafnia. An essential and novel feature of the screens of this invention is a hafnia host phosphor which contains higher levels of zirconium than is present in optical grade hafnia. As employed herein the term "optical grade hafnia" refers to hafnia sold as optical grade or prepared from any optical grade hafnium source. Optical grade hafnia is the purest form of hafnia commercially available and is therefore sometimes referred to in the literature as "pure hafnia" and in the phosphor art, where high levels of purity are the normal practice, simply as hafnia. Optical grade hafnia contains less than 3 X 10⁻⁴ mole of zirconia per mole of hafnia. The preferred phosphor host in the intensifying screens of this invention consists essentially of hafnia and zirconia with the hafnium and zirconium ions in the phosphor host satisfying the relationship (I) Hf 1-z Zr z where z is in the range of from 4 X 10⁻⁴ to 0.3, most preferably from 1 X 10⁻³ to 0.2, and optimally from 2 X 10⁻³ to 0.1. Stated another way, the present invention contemplates intensifying screens in which the phosphor host is prepared from reagent grade hafnia, which, referring to relationship I, is commercially available with z being slightly less than 2 X 10⁻². Surprisingly, this material forms a phosphor having a higher level of luminescence than optical grade hafnia. From the Examples below it is apparent that by blending reagent grade hafnium sources with optical grade hafnium sources so that z is about 1 X 10⁻² peak phosphor luminescence intensities are realized. The Examples below also demonstrate that by blending optical or reagent grade zirconium compounds with reagent grade hafnium compounds to form zirconium rich hafnia host phosphors, higher luminescence intensities than demonstrated by optical grade hafnia are also realized, until z reaches a level greater than 0.3. The fact that zirconia exhibits a lower luminescence intensity than hafnia when exposed to X-radiation is, of course, to be expected, since zirconium lies in Period 5 of the Periodic Table of Elements. That a limited range of zirconium concentrations in a hafnia host can increase phosphor luminescence has not been realized prior to this invention. The small amounts of other elements found in commercially available reagent grade hafnium and zirconium source compounds are not detrimental to intensifying screen performance. Therefore, other possible impurities of the phosphor host need be given no further consideration. In the simplest form of the invention monoclinic reagent grade hafnia can be purchased and formed into an intensifying screen. It is also possible to purchase reagent grade hafnium compounds corresponding to the optical grade hafnium compounds employed by Kelsey and Brixner, cited above, and to prepare phosphors according to the invention by those conventional procedures. To form monoclinic phosphor particles containing a selected ratio of hafnium and zirconium, commercially available sources of zirconium and hafnium are intimately intermixed, preferably by being dissolved in a common solvent, followed by coprecipitation. The hafnium and zirconium containing mixture is chosen so that upon firing only hafnium, zirconium, and oxygen atoms remain as residue, any other moieties of the compounds being thermally decomposed or otherwise driven off in firing. Common sources of hafnium and zirconium include the dioxides, the basic carbonates, the oxychlorides, the oxynitrates, the sulfates, and the tetrachlorides. While the dioxides, the basic carbonates, and the sulfates can ,be used as purchased to produce phosphors, it is advantageous for both handling and phosphor performance to convert the other sources to less soluble solids that can be fired to give the monoclinic Hf 1-z Zr z O₂ phosphor desired. For example, treatment of aqueous hafnium and zirconium ion containing solutions with base (e.g., alkali or ammonium hydroxide) gives a precipitate which is a mixture of hydrous hafnia and hydrous zirconia, the relative proportions of which depend upon those present in the starting materials. Other useful solids satisfying Hf 1-z Zr z requirements can be produced by treating hafnium and zirconium ion containing solutions with organic precipitating agents, since organic materials consisting of carbon, hydrogen, and optionally nitrogen and/or oxygen leave no objectionable residue upon thermal decomposition. Hafnium and zirconium can be conveniently coprecipitated as carboxylates, such as those containing from about 2 to 20 carbon atoms. The carboxylate moieties are in one preferred form aliphatic carboxylates containing from about 2 to 10 carbon atoms, including both monocarboxylates and polycarboxylates―particularly dicarboxylates, such as oxalates, succinates, fumarates, etc. Aromatic carboxylates, such as benzoates, phthalates, and their ring substituted homologues, are also convenient to use. A particularly preferred class of carboxylates are α-hydroxycarboxylates containing from 2 to 10 carbon atoms, such as glycolates, lactates, and mandelates. Oxalic acid can be viewed as either a dicarboxylic acid or an α-hydroxycarboxylic acid. Oxalates are particularly preferred moieties for forming not only hafnium and zirconium compounds, but also compounds of other metals to be incorporated in forming preferred forms of the phosphor more particularly described below. The carboxylate moieties can form simple carboxylates with the hafnium or zironium or can form hafnium or zirconium carboxylate complexes including additional cations, such as alkali metal or ammonium ions. The hafnium and zirconium carboxylates can be conveniently formed by reacting in a common solvent the acid, salt, or ester of the carboxylate with hafnium and zirconium containing compounds in the ratios desired in the phosphor. The hafnium and zirconium containing compounds to be reacted can be selected from among compounds such as hafnium tetrachloride, zirconium tetrachloride, hafnium oxychloride, zirconium oxychloride, hafnium basic carbonate, zirconium basic carbonate, hafnium nitrate, zirconium nitrate, zirconium carbonate, hafnium sulfate, zirconium sulfate, and mixtures thereof. It is also contemplated to employ hafnium and zirconium alkoxides as starting materials. Preferred hafnium and zirconium alkoxides are those which satisfy formula II: (II) D(OR)₄ where D represents zirconium or hafnium and R represents a hydrocarbon moiety containing from about 1 to 20 (preferably about 1 to 10) carbon atoms. The hydrocarbon moieties can be chosen from any convenient straight or branched chain or cyclic saturated or unsaturated aliphatic hydrocarbon moiety―e.g., alkyl, cycloalkyl, alkenyl, or alkynyl. Alternatively the hydrocarbon moiety can be an aromatic moiety―e.g., benzyl, phenyl, tolyl, xylyl, naphthyl, etc. In a specifically preferred from R is in each instance lower alkyl of from 1 to 4 carbon atoms. Hafnium and zirconium alkoxides are disclosed in U.S. Patents 3,297,414; 3,754,011; 4,525,468; and 4,670,472. In addition to alkoxide and carboxylate moiety containing hafnium and zirconium compounds various chelates, such as hafnium and zirconium β-diketones and diaminecarboxylates can be employed. Exemplary useful hafnium starting materials are set forth under heading III below. All the compounds have otherwise identical zirconium analogues. Further, although water of hydration has been omitted, it is to be understood that under normal ambient conditions most of the compounds exist as hydrates. (III) Exemplary Hafnium Starting Materials H-1 Hafnyl oxalate HfO(C₂O₄) H-2 Hafnyl oxalic acid H₂[HfO(C₂O₄)₂] H-3 Dioxalatohafnium Hf(C₂O₄)₂ H-4 Trioxalatohafnic acid H₂[Hf(C₂O₄)₃] H-5 Ammonium trioxalatohafnate (NH₄)₂[Hf(C₂O₄)₃] H-6 Potassium tetraoxalatohafnate K₄[Hf(C₂O₄)₄] H-7 Sodium tetraoxalatohafnate Na₄[Hf(C₂O₄)₄] H-8 Ammonium hafnyl oxalate (NH₄)₂[HfO(C₂O₄)₂] H-9 Polyoxalatopolyhafnic acids H-10 Potassium hafnyl tartrate K₂[HfO(C₄H₄O₆)₂] H-11 Tetramandelatohafnic acid H₄[Hf(O₂CCHOC₆H₅)₄] H-12 Triglycolatohafnic acid H₃HfOH(OCH₂COO)₃ H-13 Trilactohafnic acid H₃HfOH(OCHCH₃COO)₃ H-14 Trioxodihafnium stearate Hf₂O₃(O₂C(CH₂)₁₆CH₃)₂ H-15 Trioxodihafnium 2-ethylcaproate Hf₂O₃(O₂CCHC₂H₅(CH₂)₃CH₃)₂ H-16 safnium acetylacetonate Hf(C₅H₇O₂)₄ H-17 Potassium bisnitrilotriacetohafnate K₂{Hf[N(CH₂CO₂)₃]} H-18 Hafnium ethylenediaminetetraacetic acid Hf[(O₂CCH₂)₂NCH₂]₂ H-19 Hafnyl malonate HfO(O₂CCH₂CO₂) H-20 Hafnyl phthalate HfO(O₂C₆H₄CO₂) H-21 Hafnium tetraisopropoxide Hf(OC₃H₇)₄ H-22 Hafnium tetra- t -amyloxide Hf(OC₅H₁₁)₄ H-23 Hafnium tetra(phenoxide) Hf(OC₆H₅)₄ H-24 Hafnium di(isopropoxide) bis(2-ethoxyethoxide) Hf(OC₃H₇)₂(OC₂H₄OC₂H₅)₂ H-25 Hafnium tetra(cyclohexoxide) Hf(OC₆H₁₁)₄ H-26 Hafnium di(isopropoxide) bis[2-(2-n-dodecanoxyethoxy)ethoxide] Hf(OC₃H₇)₂(OC₂H₄OC₂H₄OC₁₂H₂₅)₂ Formation of the zirconium rich monoclinic hafnia phosphor host is achieved by heating the zirconium and hafnium compounds to temperatures up to and including 1400°C. Higher firing temperatures can, of course, be undertaken, since the phosphor possesses high thermal stability. However, it is a distinct advantage of this invention that firing temperatures above 1400°C are not required. Preferred firing temperatures are in the range of from about 900 to 1300°C. Firing is continued until conversion to the monoclinic phase is achieved. For maximum firing temperatures the duration of firing can be less than 1 hour. While extended firing times are possible, once the phosphor has been converted to the monoclinic crystalline form, extending the duration of firing serves no useful purpose. Generally firing times in the range of from 1 to 10 hours, more typically 2 to 5 hours, provide full conversions of the starting materials to the phosphor composition sought. Since the starting materials are in most instances decomposed at temperatures well below the 900°C minimum temperature level contemplated for monoclinic crystal growth, it is generally convenient to heat the starting materials to a temperature above their decomposition temperature, but below 900°C, for an initial period to purge volatilizable materials before progressing to the higher crystallization temperatures. Typically, a preliminary heating step in the range of from about 300 to 900°C, preferably in the range of from 400 to 700°C, is undertaken. It is also often convenient to divide firing into two or more consecutive steps with intermediate cooling to permit grinding and/or washing the material. Intermediate grinding can facilitate uniformity while intermediate washing, typically with distilled water, reduces the risk of unwanted contaminants, such as starting material decomposition by-products. It has been discovered that firing the hafnia phosphor in the presence of a flux of one or a combination of akali metal ions incorporates alkali metal ion in the phosphor and dramatically increases its luminescence intensity. A preferred class of phosphors according to the present invention are those that satisfy the relationship: (IV) Hf 1-z Zr z M y where M represents at least one alkali metal; y is in the range of from 1 X 10⁻⁴ to 1 (preferably 0.2); and z is as defined above. Investigations have revealed that the benefits of alkali metal ion inclusion are fully realized at relatively low concentrations and incorporation of alkali metal ions in concentrations above those required for maximum luminescence enhancement are not detrimental to luminescence. There is no phosphor performance basis for limiting y to values of 1 or less. Rather it is primarily a phosphor preparation convenience. Alkali metal ion inclusion in the phosphor can be conveniently accomplished by forming a mixture of the hafnium and zirconium starting materials discussed above and a compound capable of releasing alkali metal ions on heating. The amount of the alkali metal compound employed is chosen to supply alkali metal ion in a concentration in excess of that sought to be incorporated in the phosphor. Thus, the following is contemplated as a starting material relationship: (V) Hf 1-z Zr z M m wherein M represents at least one alkali metal; m is greater than 3 X 10⁻² (preferably from 1 X 10⁻¹ to 6); and z satisfies any of the values noted in connection with relationships I and II. The alkali metal compounds can be alkali metal analogues of the hafnium and zirconium starting materials discussed above. Preferred alkali metal compound starting materials include alkali metal carbonates, sulfates, oxalates, halides, hydroxides, borates, tungstates, and molybdates. Mixtures of alkali metal starting materials are contemplated, particularly when different alkali metals are being concurrently incorporated in the phosphor. Since in one form the hafnium and zirconium complexes of formula II can contain alkali metal ion, the alkali metal can wholly or in part be provided by these complexes. A convenient preparation approach is to employ alkali metal containing hafnium and zirconium complexes satisfying formula II and to increase the alkali metal content of the starting materials by adding other alkali metal compounds, as indicated above. In relationship V, m can range of up to 10 or more. Most of the excess of alkali metal is removed during phosphor preparation. When an excess of alkali metal is incorporated in the phosphor, it is preferred to divide firing into two or more sequential steps with intermediate grinding and washing to remove soluble alkali metal compounds. This reduces the level of alkali metal compounds available for release during heating in a corrosive volatilized form and also reduces the possibility of forming less desirable secondary phases. Investigation of alkali metal containing zirconium rich hafnia phosphors indicates that they exhibit increased levels of luminescence even after extended washing has reduced the alkali metal content to very low levels, approaching detection limits. While it is believed that the alkali metal is incorporated into the monoclinic crystals of the phosphor, this has not been conclusively established. It is possible that the alkali metal content of the phosphor is at least partially a surface remnant of the alkali metal flux on the surf,ace of the monoclinic crystals during their formation during firing. The highest levels of phosphor luminescence have been obtained by employing lithium as an alkali metal. In a preferred form lithium containing phosphors according to this invention satisfy the relationship: (VI) Hf 1-z Zr z Li y wherein y is in the range of from 8 X 10⁻⁴ to 0.15 and z is selected from any one of the ranges indicated above. Lithium containing phosphors according to this invention are preferably prepared by selecting starting materials so that the hafnium, zirconium, and lithium ions present prior to heating satisfy the following relationship: (VII) Hf 1-z Zr z Li m wherein m is in the range of from 4 X 10⁻² to 2.0 (optimally from 7 X 10⁻² to 1.5) and z is selected as described above. When lithium is selected as the alkali metal, it has been observed that, in addition to forming the zirconium rich hafnia phosphor with lithium included, a second phase of lithium hafnate can be formed, depending upon the proportion and selection of lithium compound starting materials. Since lithium hafnate lacks the luminescence intensities of titanium and lithium containing hafnia, a preferred embodiment of the invention, lithium starting materials and their concentrations are selected so that any overall luminescence of the two phases remains higher than that attained in the absence of lithium. As demonstrated in the Examples, increasing levels of lithium carbonate employed as a starting material results first in an increase in overall luminescence eventually followed by a decrease in overall luminescence attributed to the formation of increasingly larger proportions of lithium hafnate. On the other hand, employing lithium sulfate as a starting material, increasing proportions result in peak luminescence with still higher proportions of lithium sulfate resulting in a relatively constant high level of luminescence, indicating that the proportion of lithium hafnate which is formed as a second phase is limited at higher lithium sulfate concentrations in the starting materials. Sodium and potassium compounds employed as starting materials in place of lithium compounds also result in markedly increased levels of phosphor luminescence. These alkali metal starting materials, of course, avoid any possibility of forming a lithium hafnate second phase and can therefore be employed well above the preferred maximum concentration levels of lithium starting materials without any performance penalty. On the other hand, it has been observed that sodium and potassium ions are quite effective at lower concentrations. Therefore, when M in relationship IV represents at least one of sodium and potassium, y is preferably in the range of from 6 X 10⁻⁴ to 7 X 10⁻² (optimally from 8 X 10⁻⁴ to 7 X 10⁻²). The alkali metals cesium and rubidium are also effective to increase phosphor luminescence, but to a lesser extent that than lithium, sodium, and potassium. Combinations of any and all of the alkali metals can be employed in preparing the phosphors of this invention. Particularly useful are combinations of at least two of lithium, sodium, and potassium ions. Lithium and potassium ion combinations have produced particularly high levels of luminescence. It is generally preferred to increase the fluorescence efficiency of the phosphor by blending with the phosphor host before firing a small amount of an activator. Any known activator for optically pure hafnia phosphors can be employed. Titanium (e.g., Ti⁺⁴), taught for use in optical grade hafnia by Brixner, cited above, is specifically contemplated for use as an activator. In one preferred form of the invention titanium is incorporated in the phosphor as an activator. Thus, in one preferred form of the phosphor hafnium, zirconium, and titanium are present and satisfy the relationship (VIII) Hf 1-z Zr z Ti x where x is the range of from 3 X 10⁻⁴ to 1.0 (preferably 0.5 and optimally 0.25) and z satisfies any of the ranges previously indicated. It is possible to introduce the titanium activator by physically mixing titania with any of the host phosphor forming materials described above. It has been discovered, however, that higher luminescence levels at lower titanium concentrations are possible when the titanium activator in the form of a thermally decomposable compound is physically blended with thermally decomposable hafnium and zirconium compounds. The thermally decomposable moieties of the titanium activator compounds can be selected from among the same compound classes described in connection with hafnium and zirconium. Titanium carboxylates, where the carboxylates are chosen as described above, are particularly preferred starting materials for the incorporation of titanium. The inclusion of titanium in the zirconium rich hafnia host phosphor not only greatly increases the total luminescence of the phosphor, but also shifts the maximum emission wavelength of the phosphor from the ultraviolet to the blue portion of the spectrum. Emissions in the blue portion of the spectrum are more useful for intensifying screen use, since the silver halide emulsions of radiographic elements which are employed in combination with intensifying screens possess native blue sensitivity and/or can be readily spectrally sensitized to these wavelengths while the organic vehicle of the emulsion is transparent in the blue portion of the spectrum. In a specifically preferred form of the invention the zirconium rich hafnia phosphors include both alkali metal ion and titanium, each introduced as described above. In this form the phosphor satisfies the relationship: (IX) Hf 1-z Zr z M y Ti x where x, y, and z are as previously defined. It has been surprisingly discovered that disproportionately large enhancements of luminescence are realized when both alkali metal ion and titanium are incorporated in the phosphor. That is, the luminescence increases imparted by each of the alkali metal ion and titanium alone when added together do not equal or even approach the magnitude of the luminescence increase imparted by a combination of alkali metal ion and titanium employed together in the phosphor. Rare earth activators for the phosphors of this invention are also contemplated. As applied to this invention the term "rare earth" is intended to include scandium, yttrium, and the lanthanides. The following rare earths, taught by Iwase and Nishiyama, cited above, for use in high purity hafnia are also contemplated: praseodymium (e.g., Pr⁺³), samarium (e.g., Sm⁺³), europium (e.g., Eu⁺³), terbium (e.g., Tb⁺³), and dysprosium (e.g., Dy⁺³). Ytterbium, suggested by Kelsey, cited above, can also be employed as an activator. Gadolinium (e.g., Gd⁺³) is also shown to be an activator in the Examples below. Rare earth activators can enhance the intensity of phosphor luminescence, as demonstrated in the Examples below, and can be employed to shift the emission spectra of the phosphors of the invention. Rare earth activated zirconium rich hafnia phosphors according to this invention preferably satisfy the relationship: (X) Hf 1-z Zr z L w or (XI) Hf 1-z Zr z M y L w wherein L represents at least one rare earth element; w is in the range of from 3 X 10⁻⁴ to <5 X 10⁻², preferably from 1 X 10⁻³ to 2 X 10⁻²; and y and z are as previously described. As illustrated in the examples below, the zirconium can, as a function of its concentration, also permit fine tuning of the peak emission wavelength of the phosphor. Fine tuning to match the peak emission wavelength of the phosphor to the peak absorption wavelength of the silver halide emulsion layer to be exposed can have a significant impact on the efficiency of the overall imaging system. Thus, the zirconium a plays an important role not only in increasing the luminescence of the phosphor while reducing its cost, but also in optimizing its performance. By selection, specific combinations of zirconium concentrations and activator can produce phosphors with peak emission wavelengths that match dye absorption peaks in silver halide emulsion layers of radiographic elements. The zirconium rich hafnia phosphors, once formed to satisfy the composition requirements of this invention, can be employed to form an intensifying screen of any otherwise conventional type. In its preferred construction the intensifying screen is comprised of a support onto which is coated a fluorescent layer containing the zirconium rich hafnia phosphor in particulate form and a binder for the phosphor particles. Zirconium rich hafnia phosphors can be used in the fluorescent layer in any conventional particle size range and distribution. It is generally appreciated that sharper images are realized with smaller mean particle sizes. Preferred mean particle sizes for the zirconium rich hafnia phosphors of this invention are in the range of from from 0.5 µm to 40 µm, optimally from 1 µm to 20 µm. It is, of course, recognized that the zirconium rich hafnia phosphor particles can be blended with other, conventional phosphor particles, if desired, to form an intensifying screen having optimum properties for a specific application. Intensifying screen constructions containing more than one phosphor containing layer are also possible, with the zirconium rich hafnia phosphor particles being present in one or more of the phosphor containing layers. The fluorescent layer contains sufficient binder to give structural coherence to the zirconium rich hafnia layer. The binders employed in the fluorescent layers can be identical to those conventionally employed in fluorescent screens. Such binders are generally chosen from organic polymers which are transparent to X-radiation and emitted radiation, such as sodium o -sulfobenzaldehyde acetal of poly(vinyl alcohol); chlorosulfonated poly(ethylene); a mixture of macromolecular bisphenol poly(carbonates) and copolymers comprising bisphenol carbonates and poly(alkylene oxides); aqueous ethanol soluble nylons; poly(alkyl acrylates and methacrylates) and copolymers of alkyl acrylates and methacrylates with acrylic and methacrylic acid; poly(vinyl butyral); and poly(urethane) elastomers. These and other useful binders are disclosed in U.S. Patents 2,502,529; 2,887,379; 3,617,285; 3,300,310; 3,300,311; and 3,743,833; and in Research Disclosure , Vol. 154, February 1977, Item 15444, and Vol. 182, June 1979. Particularly preferred intensifying screen binders are poly(urethanes), such as those commercially available under the trademark Estane from Goodrich Chemical Co., the trademark Permuthane from the Permuthane Division of ICI, Ltd., and the trademark Cargill from Cargill, Inc. The support onto which the fluorescent layer is coated can be of any conventional type. Most commonly, the support is a film support. For highest levels of image sharpness the support is typically chosen to be black or transparent and mounted in a cassette for exposure with a black backing. For the highest attainable speeds a white support, such as a titania or barium sulfate loaded or coated support is employed. Any one or combination of conventional intensifying screen features, such as overcoats, subbing layers, and the like, compatible with the features described above can, of course, be employed. Both conventional radiographic element and intensifying screen constructions are disclosed in Research Disclosure , Vol. 184, Aug. 1979, Item 18431. Research Disclosure is published by Kenneth Mason Publications, Ltd., Emsworth, Hampshire PO10 7DD, England. In one specifically preferred form of the invention, illustrating intensifying screens satisfying the requirements of the invention intended to be employed with a separate silver halide emulsion layer containing radiographic element, the zirconium rich hafnia phosphor can be substituted for any of the conventional phosphors employed in either the front or back intensifying screens of Luckey, Roth et al U.S. Patent 4,710,637. Similar modification of any of the conventional intensifying screens disclosed in the following patents is also contemplated: DeBoer et al U.S. Patent 4,637,898; Luckey, Cleare et al U.S. Patent 4,259,588; and Luckey U.S. Patent 4,032,471. While the zirconium rich hafnia phosphors can be employed for their prompt emission following exposure to X-radiation, they can also be employed as storage phosphors―that is, for their ability to emit electromagnetic radiation in a chosen wavelength range after being exposed to X-radiation and then stimulated by exposure to radiation in a third spectral region. For example, the phosphors of this invention can be employed in imaging systems of the type disclosed by Luckey U.S. Patent 3,859,527. When employed in such a system the refractive indices of the phosphor and binder are preferably approximately matched, as disclosed by DeBoer et al U.S. Patent 4,637,898. Examples The invention can be better appreciated by reference to the following specific examples. Examples 1-9 Phosphors Containing Varied Ratios of Hafnium and Zirconium (Hf 1-z Zr z ) The purpose of presenting these investigations is to demonstrate that, by varying the zirconium content in a hafnia host phosphor, enhanced phosphor luminescence intensity is achieved over a limited zirconium concentration range in which the zirconium content is higher than that found in optical grade hafnium sources, but still only a minor constituent. Hafnia phosphor samples containing varied amounts of zirconium substituted for hafnium were prepared by the decomposition of the appropriate trilactohafnic and trilactozirconic acid complexes. The complexes were prepared by the general method described in W. B. Blumenthal, "The Chemical Behavior of Zirconium," VanNostrand, Princeton, N. J., 1958, p 333. The varying Hf:Zr ratios are obtained by using the appropriate mixtures of zirconium and hafnium oxychlorides in the precipitation reactions. The oxychlorides were obtained from Teledyne Wah Chang Albany (located at Albany, Oregon) and used as received. The Hf:Zr ratios in the samples were determined from the analytical batch analyses provided by the supplier. The preparation of trilactohafnic acid for Example 1 was carried out in the following manner: Optical grade (Hf 1-z Zr z , z = 0.000276) hafnium oxychloride (40 g) and ACS reagent lactic acid (44 g) from Eastman Kodak Company were each dissolved in about 120 ml of distilled water. The hafnium oxychloride solution was added to the lactic acid solution with rapid stirring to form a precipitate, and the resulting mixture was heated to 80°C with continued stirring for about 0.5 hours. The cooled mixture was filtered, and the collected solid was washed with distilled water. After drying for 15 hours at 80°C, the solid weighed 42 g. (for C₉H₁₆O₁₀Hf: theory, C=23.4%, H=3.5%; found, C=22.7%, H=3.5%). Approximately 13 g of the trilactohafnic acid was placed in a 50 mL alumina crucible, covered with an alumina lid, heated in air to 700°C for one hour in an ashing furnace, then cooled to room temperature. The solid was transferred to a 20 mL alumina crucible, which was covered with an alumina lid. The covered 20 mL alumina crucible was placed into a 50 mL alumina crucible, which was thereafter covered with an alumina lid. The crucible assembly was heated to 1000°C and maintained at that temperature for 2.5 hours before cooling to room temperature. The resulting solid was ground with an agate mortar and pestle to give a powder that was returned to the 20 mL alumina crucible. The 20 mL crucible was covered with its alumina lid and then heated to 1400°C and maintained at that temperature for 1.5 hours before cooling to room temperature. The resulting solid was ground with an agate mortar and pestle to give a uniform phosphor powder. The Example 1 phosphor powder sample was made from optical grade hafnium oxychloride and contained the lowest amount of zirconium. The Example 5 sample was made from reagent grade (designated by the supplier as Reactor Grade Special and subsequently also referred to as R.G.S.) hafnium (Hf 1-z Zr z , z = 0.019) oxychloride. The Example 2, 3, 4A, and 4B samples were made by mixing appropriate amounts of the optical grade and reagent grade hafnium oxychlorides. The Example 6 to 9 samples were made by mixing appropriate amounts of reagent grade hafnium and zirconium oxychloride to obtain a zirconium content indicated in Table II. The luminescence response of the phosphor powder was in this and all subsequent Examples measured by placing the phosphor powder sample in aluminum planchets (2mm high x 24 mm diam) at a coverage of about 1.1 g/cm² and exposing to X-radiation. The X-ray response was obtained using a tungsten target X-ray source in an XRD 6™ generator. The X-ray tube was operated at 70 kVp and 10 mA, and the X-radiation from the tube was filtered through 0.5 mm Cu and 1 mm Al filters before reaching the sample. The luminescent response was measured using an IP-28™ photomultiplier tube at 500 V bias. The voltage from the photomultiplier was measured with a Keithley™ high impedance electrometer and is proportional to the total light output of the sample. The major luminescence peak of the phosphor samples was centered at about 280 nm. This value was obtained by taking the prompt emission spectrum of the powder using the unfiltered X-ray source described above. The tube was operated at 70 kVp and 30 mA. The spectrum was acquired with an Instruments S.A. Model HR 320™ grating spectrograph equipped with a Princeton Applied Research Model 1422/01™ intensified linear diode array detector. The data acquisition and processing was controlled by a Princeton Applied Research Model 1460 OMA III™ optical multichannel analyzer. The spectrum was corrected for the spectral response of the detector-spectrograph combination. The relative luminescence intensity of the phosphor powder samples as a function of their zirconium content is set out in Table II. Hf 1-z Zr z EXAMPLE NO. Zr CONTENT (z) RELATIVE INTENSITY 1 (Control) 0.000276 100 2 0.00040 231 3 0.0010 238 4A 0.01 710 4B 0.01 743 5 0.019 365 6 0.10 350 7 0.20 155 8 0.30 224 9 (Control) 0.50 80 The data of Table II demonstrate that there is an enhancement in hafnia phosphor performance when the zirconium level increased over that found in optical grade hafnium sources (represented by the Control 1). Ranges of z of from 4 X 10⁻⁴ (0.0004) to 0.3 are demonstrated to exhibit higher luminescence intensities than optical grade hafnia. Best results are demonstrated when z is in the range of from 1 X 10⁻³ (0.001) to 0.2, optimally in the range of from 5 X 10⁻³ (0.005) to 0.1. Examples 10-14 Preparation of Phosphors in the Presence of an Alkali Metal Ion (Hf 1-z Zr z M m ) The purpose of presenting these investigations is to demonstrate that the performance of hafnia host phosphors with an elevated zirconium level shown to be effective in Examples 1-9 can be further dramatically improved by preparing the hafnia phosphor in the presence of an alkali metal ion. In each example a sample consisting of 14.72 grams of trilactohafnic acid (prepared as described in Examples 1-9 from RGS hafnium oxychloride, z = 0.019) was thoroughly ground with an agate mortar and pestle with K₂CO₃ or Li₂CO₃ (Alfa Products; Ultra Pure grade). The mole percent of the alkali carbonate flux, based on hafnium, was chosen as indicated below in Table III. The mixtures prepared were heated as described above in Examples 1-9, except for the addition of a washing step after firing to 1000°C. This step involved washing the charge with 150 mL of distilled water for 1 hour. The solid was collected and dried for 5 minute intervals at 20, 35 and 50% power in a 500W CEM model MDS-81™ microwave oven. The procedure described above in Examples 1-9 was then completed. X-ray diffraction analysis of the samples confirmed the presence of monoclinic hafnia. The presence of alkali metal ion in the phosphor powder samples prepared in the presence of alkali carbonate flux was confirmed by atomic absorption analysis. Hf 1-z Zr z M m Example M m Intensity (Ex. 1 = 100) 5 - - 365 10 K 0.2 520 11 K 0.5 510 12 K 2.0 545 13 K 4.0 1005 14 Li 0.14 1005 A 140 to 275 percent increase in luminescence intensity relative to Example 5 is seen in the above examples containing alkali metal ion. Referring back to Example 1, it is apparent that the hafnia phosphor samples containing both zirconium in higher levels than found in optical grade hafnium sources and alkali metal ion exhibit luminescence intensities ranging from >5 to >10 times those demonstrated by the hafnia phosphor prepared from an optical grade hafnium source. Examples 15-18 Titanium Activated Phosphors (Hf 1-z Zr z Ti x ) The purpose of presenting these investigations is to demonstrate the utility of titanium as an activator for the hafnia phosphors of this invention containing higher than optical grade concentrations of zirconia. The titanium also shifts the maximum spectral emission band of the phosphor to visible wavelengths in the blue portion of the spectrum. In each example a sample consisting of 14.72 grams of trilactohafnic acid (prepared as described above in Examples 1-9, z = 0.019) was thoroughly ground with varying portions of ammonium bis(oxalato)-oxotitanium (IV), (NH₄)₂TiO(C₂O₄)₂2H₂O, from Johnson Matthey (99.998%). The mole percent titanium, based on hafnium, is indicated below in Table IV. The mixtures were heated and further examined as in Examples 1-9. X-ray diffraction analyses of Examples 17 and 18 each showed traces of unreacted TiO₂. A small amount of hafnium titanate was detected as an impurity phase in Example 18. The relative luminescence outputs of Examples 5 and 15-18 are set out in Table IV. Not only were the luminescence outputs greatly increased in Examples 15-18, but the luminescence band maximum shifted to 475 nm, thereby providing increased emissions of visible spectrum wavelengths more advantageous for intensifying screen applications. Hf 1-z Zr z Ti x Example x Intensity (Ex. 1 = 100) 5 - 365 15 0.02 5330 16 0.05 4000 17 0.10 2730 18 0.25 1680 From Table IV it is apparent that the inclusion of titanium in the hafnia phosphor samples containing higher than optical grade zirconium concentrations resulted in large increases in luminescence intensities. Thus, the titanium acted as an activator for the phosphor samples. Examples 19-33 Preparation of Titanium Activated Phosphors in the Presence of Lithium Carbonate (Hf 1-z Zr z Ti x Li m ) The purpose of presenting these investigations is to demonstrate that the performance of hafnia host phosphors with an elevated zirconium level (z = 0.19) and containing titanium as an activator can be further improved by preparing the hafnia phosphor in the presence of an alkali metal ion. A sample consisting of 12.26 g of trilactohafnic acid (prepared as in Examples 1-9) was thoroughly ground with 0.1 g (5 mole percent, x = 0.05) of TiO₂ (EM Chemicals; Optipur grade) and a selected amount of Li₂CO₃ (Alfa Products; Ultrapure grade). The mixtures were processed and tested similarly as in Examples 10-14. In Examples 21-23 the size of the trilactohafnic acid sample was 13.00 grams with the titania increased to 0.106 g to maintain the titanium at 5 mole percent (x = 0.05). The relative intensity of the titanium activated phosphor samples as a function of the alkali metal flux employed is given in Table V. Hf 1-z Zr z Ti x M m Example m Intensity (Ex. 1=100) 19 0 2520 20 0.01 2210 21 0.02 1000 22 0.06 3380 23 0.10 6370 24 0.10 5960 25 0.20 13500 26 0.20 14000 27 0.40 13700 28 0.50 13300 29 0.50 13500 30 1.0 8695 31 1.5 5610 32 2.0 3155 33 4.0 735 Samples in which more than 10 mole percent (m = 0.20) Li₂CO₃ was added revealed the presence of lithium hafnate in the X-ray powder patterns. The amount of lithium hafnate formed in the samples increased with the Li₂CO₃ amount. At 200 mole percent (m = 4.0) Li₂CO₃ added, lithium hafnate is the primary phase. From Table V it can be appreciated that values of m of from about 4 X 10⁻² (0.04) to 2.0 gave significantly improved results, with values of m of from about 1 X 10⁻¹ (0.10) to 1.5 providing the highest luminescence intensities observed in these comparisons. In these comparisons it should be noted that Example 19 did not provide luminescence intensity as high as that reported in Table IV for Example 16, even though both contained 5 mole percent titanium (x = 0.05) and neither was prepared in the presence of an alkali metal flux. This difference is attributed to the less efficient incorporation of the titanium activator in Example 19 resulting from employing titania rather than a titanium carboxylate salt as a starting material. Examples 34-43 Preparation of Titanium Activated Phosphors in the Presence of Lithium Sulfate (Hf 1-z Zr z Ti x Li m ) The purpose of presenting these investigations is to demonstrate that the proportions of lithium hafnate formed as a second phase can be controlled and reduced by substituting another lithium salt for lithium carbonate. The same procedures were employed as in Examples 19-33, except that for Li₂CO₃ there was substituted Li₂SO₄ (Aldrich anhydrous: 99.99%). The relative intensity of the titanium activatived phosphor samples as a function of the lithium sulfate flux employed is given in Table VI. In Table VI the performance data from Table V is also represented for samples prepared using lithium carbonate at the concentration levels as the lithium sulfate. Hf 1-z Zr z Ti x M m Li₂CO₃ Li₂SO₄ Example m Intensity Example m Intensity 20 0.01 2210 34 0.01 1545 21 0.02 1000 35 0.02 1545 36 0.04 2105 22 0.06 3380 37 0.06 3605 23 0.10 6370 38 0.10 7645 24 0.10 5960 25 0.20 13500 39 0.20 9115 26 0.20 14000 28 0.50 13300 40 0.50 12400 30 1.0 8695 41 1.0 9820 32 2.0 3155 42 2.0 9330 33 4.0 735 43 4.0 9185 The most important advantage of employing lithium sulfate as a flux as compared to lithium carbonate is that a reduced amount of the lithium hafnate phase is produced. This results in significant improvements in phosphor luminescence when higher proportions of the lithium flux are employed during phosphor formation. At lower, preferred flux concentrations the lithium carbonate flux yields higher luminescence. Examples 44-47 Preparation of Phosphors in the Presence of Varied Alkali Metal Ions The purpose of presenting these investigations is to demonstrate that all of the alkali metals significantly enhance phosphor luminescence. Example 25 was repeated, except that 10 mole percent (m = 0.2) of another alkali metal carbonate was substituted for lithium carbonate: Na₂CO₃ (0.265 g; EM Chemicals Suprapur Reagent), K₂CO₃ (0.346 g; Alfa Products Ultrapure grade), Rb₂CO₃ (0.5774 g; AESAR 99.9%), or Cs₂CO₃ (0.8146 g; AESAR 99 9%). The luminescence intensities measured for the resulting samples are set out in Table VII. Example Carbonate source Intensity (Ex. 1 = 100) 19 None 2520 25 Li₂CO₃ 13500 44 Na₂CO₃ 10400 45 K₂CO₃ 5400 46 Rb₂CO₃ 3645 47 Cs₂CO₃ 4840 From Table VII it is apparent that all of the alkali metals are effective to increase the luminescence of the hafnia phosphors prepared from sources having higher zirconium contents than found in optical grade sources of hafnium. From Table VII it is observed that the lower the atomic number alkali metals lithium, sodium, and potassium offer a significant performance advantage over the heavier alkali metals rubidium and cesium when equal starting concentrations are employed. Examples 48-51 Preparation of Phosphors Using Varied Alkali Metal Compounds The purpose of presenting these investigations is to demonstrate the utility of alkali metal compounds completed by moieties other than sulfate and carbonate. Example 25 was repeated, except that one of the following lithium sources was substituted for lithium carbonate: 0.2548 g Li₂C₂O₄ (10 mole percent, m = 0.2, Alfa Products reagent grade), 0.212 g LiCl (20 mole percent, m = 0.2, Alfa Products anhydrous Ultrapure grade), 0.4343g LiBr (20 mole percent, m = 0.2, MCB anhydrous) or 0.21 g LiOH-H₂O (20 mole percent, m = 0.2, MCB reagent). The luminescence intensities are given in Table VIII. Example Lithium Cmpd. Intensity (Ex. 1 = 100) 19 None 2520 48 Li₂C₂O₄ 12695 49 LiCl 6730 50 LiBr 9400 51 LiOH:H₂O 13185 From Table VIII it is apparent that all of the lithium compounds improve the luminescence of the phosphor. While both lithium hydroxide and lithium oxalate produced significantly higher levels of luminescence than the lithium halides, alkali carboxylates are clearly more convenient to handle than alkali hydroxides. Examples 52-54 Enhancement of Phosphor Luminescence by a Combination of Titanium and Alkali Metal Ion The purpose of presenting these investigations is to demonstrate the synergistic improvement of luminescence produced by the combination of an alkali metal ion and the titanium activator. Example 52 A sample consisting of 13.475 g of trilactohafnic acid (prepared as described in Examples 1-9) was thoroughly ground in an agate mortar and pestle with 0.2032 g Li₂CO₃ (10 mole percent, m = 0.2, Alfa Products Ultrapure grade) and processed as in Examples 10-14. Example 53 Example 15 was repeated, except that 13.475 g of trilactohafnic acid was used with 0.44 g of TiO₂ (2 mole percent, x = 0.02, EM chemicals Optipur grade). Example 54 Example 53 was repeated, except for the addition of 0.2032 g Li₂CO₃ (10 mole percent, m = 0.2, Alfa Products Ultrapure grade) in the starting mixture. The luminescence performances of Examples 5 and 52-54 are compared in Table IX. Example Additions Intensity (Ex. 1 = 100) 5 none 365 52 10 mole % Li₂CO₃ 1120 53 2 mole % TiO₂ 5690 54 10 mole % Li₂CO₃+ 2 mole % TiO₂ 14600 From Table IX it is apparent that a disproportionately large increase in luminescence was realized by employing both the titanium activator and the alkali metal ion. While each of the titanium and alkali metal alone enhanced luminescence, a larger increase in luminescence was attained when titanium and alkali metal ion were employed together than could have been predicted assuming the separate enhancements of luminescence to be fully additive. Examples 55-62 Phosphors Containing 5 Mole Percent or Less Titanium The purpose of presenting these investigations is to demonstrate the enhancements in luminescence produced by the use as starting materials of titanium at concentrations of 5 mole percent ( x = 0.05) and less, thereby presenting a better performance definition of the lower ranges of titanium concentrations. Potassium tetraoxalatohafnate (IV) 5-hydrate was prepared as described in Inorg. Syn., VIII, 42 (1966) using R.G.S. hafnium oxychloride 8-hydrate (z = 0.019). Upon drying at 70-90°C for 1-16 hours in a convection oven, the product analyzed at closer to a 3-hydrate composition and all subsequent use of this material was calculated as the 3-hydrate. Fifteen grams of the material was thoroughly ground in an agate mortar and pestle with 0.03-5 mole percent of potassium bis(oxalato)oxotitanate (IV) 2-hydrate (Alfa Products, recrystallized from ethanol). The mixtures were placed in 20 mL alumina crucibles, covered with alumina lids, and then placed in 100 mL alumina crucibles, which were covered with alumina lids. The samples were heated in air to 1000°C for 2.5 hours, then cooled to room temperature. The resulting solids were removed from the crucibles, broken into small pieces with an alumina mortar and pestle and washed by stirring in 50 mL of distilled water. The solids were then collected and dried in a convection oven at 80°C. The charges were placed in 10 mL alumina crucibles with alumina lids and heated in air to 1300°C for 2 hours, followed by cooling to room temperature. The luminescence intensities of the samples are set out in Table X. Example Mole percent Ti Intensity (Ex. 1 = 100) 5 None 365 55 0.03 5750 56 0.3 6128 57 1 9470 58 2 10500 59 3 8420 60 3 9820 61 4 8060 62 5 9120 From Table X it is apparent that even at the lowest concentrations of titanium (Hf 1-z Z r Ti x where x = 3 X 10⁻⁴, Example 55) much higher levels of luminescence are observed than in Example 5, which lacked titanium. While some of the enhancement in luminescence as compared to Example 5 can be attributed to the presence of potassium, comparing luminescence values from Table III, in which potassium was introduced without titanium being present, it is apparent that a part of the luminescence enhancement must be attributed to additional presence of the titanium. Examples 63-68 Varied Levels of Zirconium in Phosphors Prepared in the Presence of Alkali Metal Ion The purpose of presenting these investigations is to demonstrate the effect of varied levels of zirconium in the hafnia host phosphor when the hafnia phosphor was prepared in the presence of alkali metal ion. Two grades of potassium tetraoxalatohafnate (IV) 3-hydrate were prepared as in Example 55 from optical grade hafnium oxychloride 8-hydrate and R.G.S. hafnium oxychloride 8-hydrate. Potassium tetraoxalatozirconate 3-hydrate was prepared as in Example 55 from R.G.S. zirconium oxychloride 8-hydrate. A series of Hf 1-z Zr z O₂ samples in which z was varied from 2.76 x 10⁻⁴ to 6.84 x 10⁻² were prepared from mixtures of the above precursors. The powders were combined and ground in an agate mortar and pestle. The procedures of Examples 55-62 were employed, with the addition of 10 mole percent K₂CO₃ (Alfa Products Ultrapure grade) to each sample. Luminescence intensities as a function of zirconium levels (z) are given in Table XI. Example z Intensity (Ex. 1 = 100) 63 (Control) 2.8 x 10⁻⁴ 380 64 4.3 x 10⁻⁴ 165 65 9.6 x 10⁻³ 770 66 1.9 x 10⁻² 520 67 4.0 x 10⁻² 595 68 6.0 x 10⁻² 610 Note that Example 66 was identical to Example 10, except for employing a different final firing temperature, and the luminescence measured was identical. Table XI demonstrates that hafnia prepared from optical grade sources as in Control Example 63 yields inferior luminescence as compared to samples in which the zirconium content z is equal to at least 1 X 10⁻². Comparing Tables II and XI, it is apparent that the presence of potassium ion is responsible for a significant increase in luminescence at zirconium levels equal to that in R.G.S. hafnia (z = 0.019) and above. Examples 69-72 Determinations of Alkali Metal Ion Incorporation in Phosphors Differing in Zirconium Levels The purpose of presenting these investigations is to provide quantitative determinations of alkali ion incorporation levels (y) in several phosphors satisfying the general relationship Hf 1-z Zr z Ti x M y and having differing zirconium levels (z) satisfying the requirements of the invention. Samples were prepared as in Examples 63-68, except for the further addition of 0.2151 g of recrystallized potassium bis(oxalato)oxotitanate (IV) 2-hydrate (Alfa Products) to satisfy the ratio x = 0.03. Proportions of zirconium, titanium, and potassium ion in the completed phosphor samples were determined by atomic absorption analysis and inductively coupled plasma spectrometry. The luminescence of the phosphors together with their alkali ion content observed on analysis, y(obs), are reported in Table XII. The amounts of zirconium and titanium present in the starting materials, z(calc) and x(calc), are compared in Table XII to the amounts of zirconium and titanium found on analysis, z(obs) and x(obs). Hf 1-z Zr z Ti x M y Ex. Intensity (Ex. 1=100) z(calc) z(obs) x(calc) x(obs) y(obs) 69 9820 4.3 x 10⁻⁴ 4.31 x 10⁻⁴ 0.03 0.022 0.022 70 9820 9.6 x 10⁻⁴ 8.79 x 10⁻⁴ 0.03 0.026 0.019 71 9820 1.9 x 10⁻² 1.78 x 10⁻² 0.03 0.031 0.025 72 9820 4.0 x 10⁻² 3.87 x 10⁻² 0.03 0.027 0.023 Although all samples exhibited similar luminescence, when a corresponding phosphor was formed from optical grade hafnium starting materials [z(obs)= 2.91 X 10⁻⁴], a significantly lower luminescence was observed. Example 73-77 Rare Earth Activated Phosphors The purpose of presenting these investigations is to demonstrate enhanced performance of rare earth activatored zirconium rich hafnia phosphors―Hf 1-z Zr z L w , where L represents a rare earth, z = 0.019. Example 73 A sample consisting of 15.00 g of potassium tetraoxalatohafnate (IV) 3-hydrate prepared as in Example 55 was combined and ground thoroughly with 0.0861 g (x = 0.03) of Eu(NO₃)₃.6H₂O (Johnson Matthey REacton grade, 99.99%). The mixture was fired in a double alumina crucible setup (described in Example 1) to 1000°C in air for 2 hours. The recovered ingot was broken into small chunks and washed in 70-150 mL of distilled water. The relative luminescence intensity was 770. When Example 73 was repeated with increased levels of zirconium, small shifts in emission maxima were observed, indicating that zirconium content can be usefully varied in preparing these phosphors for use in intensifying screens. In intensifying screens intended to be employed with spectrally sensitized silver halide radiographic elements optimum imaging efficiency occurs when the emission peak of the phosphor corresponds with the absorption peak of the spectral sensitizing dye or dye combination present in the radiographic element. Example 74 A sample consisting of 14.82 g of potassium tetraoxalatohafnate (IV) 3-hydrate prepared from R.G.S. hafnium oxychloride 8-hydrate was ground with 0.0042 g (x = 0.001) SmF₃ (Johnson Matthey REacton grade) and processed as in Example 73. The relative luminescence output was 735. Example 75 Example 74 was repeated, however, 0.083 g (x = 0.02) of SmF₃ was used. The relative luminescence output was 400. Example 76 A sample consisting of 15.00 g of potassium tetraoxalotohafnate (IV) 3-hydrate prepared from R.G.S. hafnium oxychloride 8-hydrate was ground with 0.11 g (x = 0.015) of Gd₂O₃ (Rhone Poulenc 99.99%). The mixtures were fired as in Example 73, and then reheated in air at 1300°C for 2.5 hours. The relative luminescence output was 430. Example 77 Example 76 was repeated, except that 0.22 g (x = 0.03) of Gd₂O₃ was used. The relative luminescence output was 490. Example 78 Intensifying Screen The purpose of presenting these investigations is to demonstrate enhanced performance of intensifying screens containing zirconium rich hafnia phosphors. Hafnyl oxalic acid 3-hydrate was prepared for use as a starting material in the manner described in Zhurnal Neoorganicheskoi Khimi , Vol. II, p. 980 (1957) using R.G.S. hafnium oxychloride, z = 0.019. Two samples each consisting of 55.29 g of hafnyl oxalic acid 3-hydrate were thoroughly ground with 1.11 g of Li₂CO₃ (Aldrich; reagent grade), 4.36 g of K₂SO₄ (J.T. Baker; reagent grade) and 0.50 g of TiO₂ (EM Chemicals; Optipur grade). The samples were placed in 250 mL alumina crucibles, covered with alumina lids, and heated in air to 1000°C for 2.5 hours, then cooled to room temperature. The resulting solids were removed from the crucibles, broken into small pieces with a mortar and pestle, and washed by stirring in 500 mL of distilled water. Each sample was then collected and dried in a convection oven at 80°C. The charges were placed in 50 mL alumina crucibles with alumina lids and heated in air to 1400°C for 1.5 hours, followed by cooling to room temperature. The two samples (combined weight, 53.1 g) were ground together and shaken through a nylon sieve with 60 µm openings to give 28.6 g of product phosphor. The phosphor was mixed with 13% Permuthane™ polyurethane solution in a methylene chloride and methanol mixture to produce a dispersion with 25 parts of phosphor and 1 part of binder by weight. The dispersion was coated on a blue tinted transparent poly(ethylene terephthalate) film support to produce a coating with about 6.25 g/dm² of the phosphor. This coating when excited with unfiltered X-rays from a tungsten target tube operated at 70 kVp and 10 mA gives a speed that is about 3.5 times larger than that obtained from a commercial CaWO₄ (PAR screen) when the emission is compared using an IP-28™ photomultiplier tube.

Claims: A screen comprised of a support and a fluorescent layer containing a phosphor capable of absorbing X-radiation and emitting longer wavelength electromagnetic radiation comprised of monoclinic crystals of a hafnia phosphor host, characterized in that zirconium ions are present in the hafnia phosphor host in concentrations higher than 3 x 10⁻⁴ mole of zirkonia per mole of hafnia. A screen according to claim 1 further characterized in that hafnium and zirconium ions in the phosphor host satisfy the relationship Hf 1-z Zr z wherein z is in the range of from 4 x 10⁻⁴ to 0.3. A screen according to claim 2 further characterized in that z is in the range of from 1 X 10⁻³ to 0.2. A screen according to claim 3 further characterized in that z is in the range of from 2 X 10⁻³ to 0.1. A screen according to any one of claims 1 to 4 inclusive further characterized in that said phosphor additionally includes an activating amount of alkali metal ions. A screen according to any one of claims 1 to 5 inclusive further characterized in that the hafnium, zirconium, and alkali metal ions satisfy the relationship Hf 1-z Zr z M y wherein M represents ions of at least one alkali metal and y is in the range of from 1 X 10⁻⁴ to 1. A screen according to claim 6 further characterized in that y is in the range of from 1 X 10⁻⁴ to 0.2 A screen according to any one of claims 5 to 7 inclusive further characterized in that said alkali metal ions include lithium ions. A screen according to claim 8 further characterized in that y is in the range of from 8 X 10⁻⁴ to 0.15. A screen according to claim 8 further characterized in that said fluorescent layer additionally includes lithium hafnate as a second phosphor. A screen according to any one of claims 5 to 7 inclusive further characterized in that said alkali metal ions include at least one of sodium and potassium ions. A screen according to claim 11 further characterized in that y is in the range of from 8 X 10⁻⁴ to 7 X 10⁻². A screen according to any one of claims 5 to 11 inclusive further characterized in that said alkali metal activators include lithium and potassium ions. A screen according to any one of claims 5 to 7 inclusive further characterized in that said alkali metals include at least one of cesium and rubidium ions. A screen according to any one of claims 1 to 14 inclusive further characterized in that said phosphor includes an activating amount of titanium ions. A screen according to claim 15 further characterized in that the hafnium, zirconium, and titanium ions satisfy the relationship Hf 1-z Zr z Ti x wherein x is in the range of from 3 X 10⁻⁴ to 1.0. A screen according to claim 16 further characterized in that x is in the range of from 3 X 10⁻⁴ to 0.5. A screen according to claim 17 further characterized in that x is in the range of from 3 X 10⁻⁴ to 0.25. A screen according to any one of claims 1 to 14 inclusive further characterized in that said phosphor includes an activating amount of rare earth ions. A screen according to claim 19 further characterized in that the hafnium, zirconium, and rare earth ions satisfy the relationship Hf 1-z Zr z L w wherein L represents at least one rare earth element and w is in the range of from 3 X 10⁻⁴ to <5 X 10⁻².

Inventors: BRYAN PHILIP STEVEN C O EASTMA, JARROLD GREGORY SCOTT EASTMAN, LAMBERT PATRICK MADDOCK EASTMA, TOWERS CHRISTINE MAY EASTMAN K, BRYAN, PHILIP STEVEN,C/O EASTMAN KODAK COMPANY,, JARROLD, GREGORY SCOTT, EASTMAN KODAK COMPANY, LAMBERT, PATRICK MADDOCK, EASTMAN KODAK COMPANY, TOWERS, CHRISTINE MAY, EASTMAN KODAK COMPANY
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1990
Main classification: C09K11
IPC codes: C09K11, G01T1, G21K4, G03C5