X-ray intensifying screen with enhanced emission

Abstract: An intensifying screen for producing a latent image in a silver halide radiographic element upon exposure to an image pattern of X-radiation is disclosed. The intensifying screen contains a fluorescent layer containing a phosphor capable of absorbing X-radiation and emitting longer wavelength electromagnetic radiation comprised of monoclinic crystals of a titanium activated zirconium hafnium oxide phosphor host. The phosphor contains in an amount sufficient to enhance emission of the longer wavelength electromagnetic radiation at least one alkali metal in combination with at least one Group 14 dopant chosen from Periods 3, 4 and 5.

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. 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 in 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 Angle 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. Kroger et al U.S. Patent 2,542,336 discloses phosphors containing titanium as an activator and having a matrix comprised of one or more of the oxides of zirconium, hafnium, thorium, germanium or tin. Titanium activated zirconium oxide, magnesium stannate, calcium zirconate and zirconium phosphate are each specifically disclosed. In addition, Kroger et al suggests the formulation of phosphors which contain germanium, zirconium or hafnium and oxygen complexed with other nonmetals, such as sulfur, boron, phosphorus, silicon and the like; however, inclusions of these nonmetals as components of the host phosphor produce distinctly different crystal structures than GeO₂, ZrO₂ and HfO₂ and are not considered relevant to this invention. European Patent Application 381231 which is a conflicting European application under Art 54(3)EPC, describes a phosphor containing Hafnium, Zirconium, Lithium and optionally rare earth, titanium and tin. It is an object of the invention to provide a titanium activated zirconium hafnium oxide phosphor that exhibits increased emission intensity. In one aspect this invention is directed to an intensifying 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 titanium activated zirconium hafnium oxide phosphor host. The phosphor contains in an amount sufficient to enhance emission of the longer wavelength electromagnetic radiation at least one alkali metal in combination with at least one Group 14 dopant chosen from Periods 3, 4 and 5 in accordance with Claim 1. In the discussion which follows all references to elements in terms of Group and Period designations are based on the designations assigned by the Periodic Table of Elements as adopted by the American Chemical Society. In the rest of the description, the different relationships represent the ratio of two or more components constituting the monoclinic crystals of the present invention. The intensifying screens of this invention employ a titanium activated zirconium hafnium oxide host phosphor capable of absorbing X-radiation and emitting longer wavelength electromagnetic radiation, with peak emission typically being in the 450 to 500 nm region of the spectrum. It has been discovered that the inclusion of at least one alkali metal in combination with at least one Group 14 dopant chosen from Periods 3, 4 and 5 enhances the emission of the longer wavelength electromagnetic radiation. The zirconium content of the host phosphor can range from the lowest obtainable impurity levels of zirconium in hafnia, but preferably does not exceed 30 mole percent, based on zirconium and hafnium combined. The preferred hafnia host phosphors contain higher levels of zirconium than are 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 (HfO₂) and zirconia (ZrO₂) 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 by others. 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. It is also possible to combine hafnium and zirconium from commercial sources in any ratio satisfying relationship I above to produce the monoclinic host phosphor crystal structure desired. The hafnium and zirconium containing mixture is chosen so that upon firing only hafnium, zirconium, selected activators and dopants, 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 host 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 zirconium 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 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-ethylcarproate Hf₂O₃(O₂CCHC₂H₅(CH₂)₃CH₃)₂ H-16 Hafnium 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 crystallization of the monoclinic phase, 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. In order to prepare the phosphor of claim 1, the incorporation of alcali-metal ions in the phosphor can be realized by firing the hafnia phosphor in the presence of a flux of one or a combination of alkali metal ions incorporates alkali metal ion in the phosphor. The phosphors employed in the practice of 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 0.3; optimally 0.05); 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 IV. 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 III 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 III 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 the alkali metal containing 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 surface of the monoclinic crystals during their formation during firing. High 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 0.3 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 P wherein P 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. It has been discovered quite unexpectedly that the highest levels of luminescence are achieved when at least one Group 14 element is employed as a dopant in combination with more than one alkali metal. Further, very high levels of luminescence are achieved in a single firing. This has the highly practical advantage of allowing the second firing step discussed above to be eliminated while still achieving fully satisfactory phosphor emission levels. In one preferred form of the invention the alkali metals present in the phosphor satisfy the relationship: (VIII) (M¹) y 1(M²) y 2 where M¹ is an alkali metal chosen from the group consisting of lithium, sodium and potassium; M² is at least one alkali metal other than that represented by M¹; y¹+y² = y ; and y¹ and y² are each at least 1 X 10⁻⁶. Surprisingly, the heavier alkali metals cesium and rubidium when employed in combination with one or more of the lighter alkali metals in the Group 14 element doped phosphor provide performance levels equal to those obtained with combinations of the lighter alkali metals. In formula VIII above M¹ is preferably lithium and is specifically contemplated to satisfy relationship VI above. The remaining alkali metal forming M² can be any one or combination of sodium, potassium, cesium and rubidium. Surprisingly, in the Group 14 element doped phosphors combinations of at least three alkali metals yield the highest levels of observed performance. Although ternary combinations which exclude lithium are possible, it is preferred that M¹ in relationship VIII be lithium with (M²) y 2 satisfying the relationship: (IX) M′ y′ M″ y″ where M′ is at least one of sodium and potassium; M′ is at least one other alkali metal; and y′ and y″ are each is at least 1 X 10⁻⁶. In a specifically contemplated ternary alkali metal form of the invention M¹ is lithium and M′ is potassium with the remaining alkali metal being chosen from among sodium, cesium and rubidium. When the third akali metal is cesium or rubidium, y″ is preferably in the range of from 1 X 10⁻⁵ to 0.l and optimally in the range of from 5 X 10⁻⁵ to 5 X 10⁻³. The phosphors of this invention include a titanium activator. Any conventional level of titanium known to be useful as an activator for zirconia, hafnia or germania can be employed. Titanium(e.g., Ti⁺⁴), taught for use in optical grade hafnia by Brixner and Kroger, each cited above, is specifically contemplated for use as an activator. Thus, in one preferred form of the zirconium hafnium oxide phosphors of the invention hafnium, zirconium, and titanium are present and satisfy the relationship (X) 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 specifically contemplated as starting materials for the incorporation of titanium. The inclusion of titanium in the 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. Since the zirconium hafnium oxide phosphors include both alkali metal ion and titanium, each introduced as described above, the relationship of the metal ingredients discussed above satisfies the relationship: (XI) Hf 1-z Zr z M y Ti x where M, 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. In addition to the ingredients described above the phosphors can optionally contain one or a combination of indium and scandium both to increase speed and to decrease afterglow. In addition to the ingredients described above the phosphors can optionally contain one or a combination of rare earths to reduce phosphor afterglow. The rare earths yttrium, neodymium, samarium, europium, gadolinium, holmium, erbium, thulium and ytterbium have been found to be effective. In the practice of this invention one or more of the rare earths are optionally incorporated in the Group 14 element doped titanium activated zirconium hafnium oxide phosphor in an amount sufficient to reduce afterglow. The rare earth elements satisfy the following relationship to the phosphor elements described above: (XII) Hf 1-z Zr z M y Ti x L w wherein L is one or more rare earth selected from among those having an atomic number of 39, 60, 62 to 64 inclusive, and 67 to 71 inclusive; w is from 0 to 10⁻²; and M, x, y, and z are as previously defined. Effective levels of selected rare earth vary, depending upon the specific rare earth chosen. When L is yttrium (Y), ytterbium (Yb), or lutetium (Lu), w is in the range of from 1 X 10⁻⁶ to 1 X 10⁻³, preferably 2 X 10⁻⁶ to 5 X 10⁻⁴; and optimally from 3 X 10⁻⁶ to 3 X 10⁻⁴. When L is erbium (Er) or holmium (Ho), w is in the range of from 1 X 10⁻⁶ to 5 X 10⁻⁴, preferably 1 X 10⁻⁵ to 2 X 10⁻⁴. When L is samarium (Sm), w is in the range of from 1 X 10⁻⁴ to 5 X 10⁻³, preferably 2 X 10⁻⁴ to 3 X 10⁻⁴. When L is europium (Eu), w is in the range of from 1 X 10⁻⁶ to 1 X 10⁻², preferably 1 X 10⁻⁵ to 5 X 10⁻³, and optimally from 2 X 10⁻⁴ to 2 X 10⁻³. When L is gadolinium (Gd), w is in the range of from 2 X 10⁻⁶ to 1 X 10⁻², preferably 1 X 10⁻⁵ to 2 X 10⁻³, and optimally from 4 X 10⁻⁴ to 5 X 10⁻⁴. When L is thulium (Tm), w is in the range of from 1 X 10⁻⁶ to 1 X 10⁻³, preferably 2 X 10⁻⁶ to 3 X 10⁻⁴, and optimally from 5 X 10⁻⁵ to 2 X 10⁻⁴. In a specific preferred form of the invention the rare earth is neodymium. Neodymium has the capability of reducing and, in preferred concentrations, eliminating for all practical purposes phosphor afterglow. In a preferred form the neodymiun containing phosphor satisfies the following relationship to the phosphor elements described above: (XIII) Hf 1-z Zr z M y Ti x Nd w′ wherein M, x, y, and z are as previously defined and w′ is up to 5 X 10⁻⁴; preferably from 5 X 10⁻⁸ to 2 X 10⁻⁴ and optimally from 2 X 10⁻⁶ to 1 X 10⁻⁵. It has been further observed that prompt emission intensity of the Group 14 doped titanium activated zirconium hafnium oxide phosphors of this invention can be increased and afterglow reduced by incorporating one or a combination of indium and scandium. In the practice of this invention one or both of indium and scandium are optionally incorporated in the Group 14 element doped titanium activated zirconium hafnium oxide phosphor in an amount sufficient to increase prompt emission and reduce afterglow. Indium and scandium satisfy the following relationship to the required phosphor elements described above: (XIV) Hf 1-z Zr z M y Ti x G v wherein M, x, y, and z are as previously defined; G is at least one of indium and scandium; and v is in the range of from 0 to 1 X 10⁻². When G is indium, v is preferably 1 X 10⁻⁶ to 5 X 10⁻³; optimally 2 X 10⁻⁶ to 1 X 10⁻³. When G is scandium, v is preferably 1 X 10⁻⁶ to 1 X 10⁻²; optimally 2 X 10⁻⁶ to 5 X 10⁻³. It is specifically contemplated to employ indium and scandium in combination with one or more rare earth elements. In this instance the following relationship applies: (XV) Hf 1-z Zr z M y Ti x L w G v wherein G, L, M, v, w, x, y, and z are as previously defined. In a specifically preferred form of the invention the rare earth selected is neodymium and the following relationship applies: (XVI) Hf 1-z Zr z M y Ti x Nd w' G v wherein G, M, v, w', x, y, and z are as previously defined. According to one embodiment Group 14 elements of Periods 3, 4 and 5 when incorporated in titanium activated zirconium hafnium oxide also containing at least one alkali metal further enhance phosphor emission. These Group 14 elements are silicon, germanium and tin, and it is specifically contemplated to incorporate these elements in the phosphor as a dopant in any concentration capable of enhancing phosphor emission. It has been observed that incorporating levels of the Group 14 element during phosphor preparation in excess of that capable of being incorporated as a dopant in the phosphor crystal structure does not detract from the effectiveness of the dopant. It is thus contemplated to incorporate the Group 14 element in the phosphor host in any effective concentration up to its solubility limit in the host phosphor. Silicon and germanium are the preferred Group 14 dopants. These dopants when used in the phosphor of claim 1 increase prompt emission to higher levels than can be achieved in their absence. It is broadly contemplated to incorporate one or a combination of Group 14 elements to satisfy the relationship: (XVII) Hf 1-z Zr z E e where E is at least one Group 14 dopant chosen from Periods 3, 4 and 5; e is in the range of from greater than 1 X 10⁻³ up to the solubility limit of the Group 14 dopant in the phosphor host; and z is as previously defined. In a specifically preferred form of the invention E is germanium or silicon and e is in the range of from greater than 1 X 10⁻³ (most preferably at least 2 X 10⁻³) to 0.1. Optimally e is in the range of from 5 X 10⁻³ to 8 X 10⁻². According to the present invention, the following relationships based on the phosphor of claim 1 exist: (XVIII) Hf 1-z Zr z M y E e (XIX) Hf 1-z Zr z M m E e (XX) Hf 1-z Zr z Li y E e (XXI) Hf 1-z Zr z Ti x E e (XXII) Hf 1-z Zr z M y Ti x E e (XXIII) Hf 1-z Zr z M y Ti x L w E e (XXIV) Hf 1-z Zr z M y Ti x Nd w′ E e (XXIV) Hf 1-z Zr z M y Ti x G v E e (XXV) Hf 1-z Zr z M y Ti x L w G v E e (XXVI) Hf 1-z Zr z M y Ti x Nd w′ G v E e wherein G, L, M, v, w, w′, x, y, and z are as previously defined. In a preferred form of the invention the monoclinic phosphor crystals incorporated in the intensifying screens of the this invention consist essentially of oxygen and elements satisfying the relationship: (XXVII) Hf 1-z Zr z M y Ti x L w G v E e wherein E represents at least one of germanium and silicon; e ranges from greater than 1 X 10⁻³ to 0.1; G represents at least one indium and scandium; v ranges from 0 to 1 X 10⁻²; L represents a rare earth selected from among those have an atomic number of 39, 60, 62 to 64 inclusive, and 67 to 71 inclusive; w ranges from 0 to 1 X 10⁻²; M represents at least one alkali metal; y is in the range of from 6 X 10⁻⁴ to 0.3; x is in the range of from 3 X 10⁻⁴ to 1.0; and z is in the range of from 4 X 10⁻⁴ to 0.3. In a more specifically preferred form of the invention the monoclinic phosphor crystals incorporated in the intensifying screens of the this invention consist essentially of oxygen and elements satisfying the relationship: (XXVIII) Hf 1-z Zr z M y Ti x Nd w In v E e wherein E represents at least one of germanium and silicon; e ranges from greater than 1 X 10⁻³ to 0.1; v ranges from 0 to 5 X 10⁻³; w′ ranges from 0 to 5 X 10⁻⁴; M represents at least one alkali metal; y is in the range of from 6 X 10⁻⁴ to 0.3; x is in the range of from 3 X 10⁻⁴ to 1.0; and z is in the range of from 4 X 10⁻⁴ to 0.3. In still more specifically preferred forms of the invention the preferred and optimum concentration ranges of the elements included in relationships XXVII and XXVIII are present. The phosphors described above, 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 phosphor in particulate form and a binder for the phosphor particles. The 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 phosphors of this invention are in the range of from from 0.5 mm to 40 mm, optimally from 1 mm to 20 mm. It has been observed that presence of a Group 14 dopant reduces the size dispersity of particles obtained. In other words, the coefficient of variation of the particles is reduced. This is helpful both in terms of improving the physical uniformity of fluorescent layers and the uniformity of emission from these layers. It is, of course, recognized that the particles of the phosphor described above 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 phosphor described above being present in one or more of the phosphor containing layers. The fluorescent layer contains sufficient binder to give structural coherence to the fluorescent 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. Specifically preferred reflective supports offering the highest attainable balance of speed and sharpness are those containing reflective microlenslets, disclosed by Roberts et al U.S. Patent 4,912,333. 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 P010 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 phosphors described above 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. Examples The invention can be better appreciated by reference to the following specific examples. Percentages in the examples are in each instances mole percentages based on hafnia, unless otherwise stated. Example 1 The purpose of presenting this investigation is to demonstrate the preparation and performance of a phosphor prepared from a hafnium source without the inclusion of either a Group 14 element or cesium metal in the synthesis. The starting material complex hafnyl oxalic acid 3-hydrate was prepared in the manner described in Zhurnal Neoorganicheskoi Khimi, Vol II, 980 (1957) using Reactor Grade Special hafnium oxychloride 8-hydrate. Zirconium oxychloride 8-hydrate was added as needed to adjust the molar zirconium level to Hf=0.99 and Zr=0.01. The oxychlorides were obtained from Teledyne Wah Chang Albany (located at Albany, Oregon) and used as received. A sample consisting of 0.0265 mole of hafnyl oxalic acid was ground together with 0.00318 mole of Li₂CO₃ (Aldrich Chemical Co., 99.997%), 0.00398 mole of K₂SO₄ (Mallinckrodt Analytical Reagent), 0.00106 mole TiO₂ (Aldrich Chemical Co., 99.99%), and 0.0000033 mole of In₂O₃ (Alfa, Ultrapure) in a glass mortar and pestle. The mixture was placed in a 50 mL alumina crucible which was covered with an alumina lid. The crucible was placed in an alumina boat which was loaded into a mullite tube contained in a tube furnace. The sample was heated in air to 1000°C over 2.5 hours and then held at that temperature for an additional 2.5 hours. After cooling to room temperature, the ingot was removed, broken into small chunks, and ground to a powder in an agate mortar and pestle. The powder was stirred in 150 mL of distilled water for 1 hour. The solid was collected by suction filtration and dried in a 500W CEM Model MDS-81™ microwave oven. The luminescence response of the phosphor powder was in this and all subsequent Examples measured by placing the phosphor powder sample in aluminum planchets (2 mm 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. To facilitate the comparison with the phosphors prepared in subsequent examples, the phosphor of this Example was assigned a relative luminescence of 100. The luminescence response of all subsequent examples (but excluding the examples of the Appendix) are reported relative to this response. To simplify the writing of the preparations the source of the starting reagents will not be given again after they are first listed unless the reagents are obtained from a new source. Example 2 This sample illustrates the improvement that can be obtained in relative response by the addition of cesium to the preparation of Example 1. The sample was prepared as described in Example 1 except that 0.00053 mole of Cs₂SO₄ (Alfa, Ultrapure) was added and ground with the initial mixture. The relative intensity is given in Table I. Example 3a This sample illustrates the improvement that can be obtained in relative response by the addition of germanium to the preparation of Example 1. The sample was prepared as described in Example 1 except that 0.00040 mole of GeO₂ (Alfa, Ultrapure) was added and ground with the initial mixture. The relative intensity is given in Table I. Example 4 This sample illustrates the improvement that can be obtained in relative response by the addition of both cesium and germanium to the preparation of Example 1. The sample was prepared as described in Example 1 except that 0.00040 mole of GeO₂ and 0.00053 mole of Cs₂SO₄ was added and ground with the initial mixture. The relative intensity is given in Table I. Second Firing of Examples 1-4 The samples described above were treated further. They were placed in 20 mL alumina crucibles in ambient air, covered, and heated to 1200°C in a box furnace and maintained at that temperature for 1.5 hours before being allowed to cool to room temperature. The relative intensities are given in Table I. Example 5 This sample illustrates the improvement that can be obtained in relative response by the addition of cesium to the preparation of Example 3. The sample was prepared as described in Example 3 except that 0.000265 mole of Cs₂SO₄ (Alfa Inorganics, Ultrapure) was added and ground with the initial mixture. The relative intensity and the relative afterglow are given in Table II. The afterglow characteristics of each phosphor sample were determined by placing the sample in a chamber and exposing it to X-radiation from a tungsten target, beryllium window tube operated at 70 kVp and 10 mA, filtered with 0.5 mm Cu and 1 mm Al. The phosphor samples were prepared by placing the phosphor powder in aluminum planchets (2 mm deep x 24 mm diameter) at a coverage of about 1.1 g/cm². The emitted light was detected by a photomultiplier tube, the output current of which was measured by a voltmeter across a load resistor. The voltmeter reading served as an input to an x-y recorder which plotted the variation of voltage as a function of time. Constant irradiation by X-rays of each sample produced a steady state reading on the voltmeter, which was adjusted to 100% on the x-y recorder. The X-radiation was then shut off, and the decay of the light emitted by each sample was monitored. The elapsed time required for the signal to decay to 1% of its steady state value was then read off the x-y plot. The minimum elapsed time measurable by this technique was 0.35 second. The afterglow value obtained for Example 6 was defined as 100. Example 6 This sample does not contain indium, and is provided to demonstrate, when compared to Example 5, the observed increase in relative response upon indium addition. This sample was prepared as described in Example 5, except that indium oxide was not added to the preparation. The relative intensity is given in Table II. The afterglow value of this sample was defined as 100. Example 7 This sample illustrates the improvement that can be obtained in relative afterglow by the addition of neodymium to the preparation of Example 5. This sample was prepared as described in Example 5, except that 0.0000050 mole of neodymium was added to the preparation. The neodymium ions were provided in the form of aqueous solutions of Nd(NO₃)₃.5H₂O (Alfa, 99.9%). The relative intensity and the relative afterglow are given in Table II. Examples 8-15 These samples illustrate the improvement that can be obtained in relative response by the addition of cesium to the preparation of Example 3. The addition of increasing amounts of cesium beyond those listed in Table III is expected to give substantially similar phosphor performance. To provide the best possible control for purposes of comparison, several Example 3 control samples were prepared. During each firing set one of the control samples was present so that the control would experience exactly the same firing conditions as samples under investigation. For the sample sets described in Tables III and IV, Example 3 was an average of the controls fired with this group of phosphor samples. The samples were prepared as described in Example 3 except that varying amounts of Cs₂SO₄ (Alfa Inorganics, Ultrapure) were added and ground with the initial mixtures. The relative intensity and the mole percent of cesium sulfate (based on Hf) added to each sample are given in Table III. Examples 16-25 These examples illustrate the improvement in relative intensity that can be obtained compared to Example 3 by the addition of alkali metal salts other than cesium, the effect of which is demonstrated in Examples 8-15. Examples 25a and b show that these lithium salts are not effective, while Examples 25c and d show that these alkaline earth salts are also not effective The samples were prepared as described in Example 3 with the exception that a certain mole percent (based on Hf) of various alkali metal salts as listed in Table IV were added and ground with the initial mixtures. The sources for the various reagents are as follows: cesium chloride, Alfa, 99.9%; cesium oxalate, Alfa; cesium fluoride, Varlacoid Chemical Co., 99%; cesium carbonate, Aesar, 99.9%; rubidium sulfate, Alfa, Ultrapure; rubidium bromide, Cabot, 99.9%; rubidium chloride, Alfa, 99.7%; sodium sulfate, Baker, Analyzed Reagent; potassium chloride, Alfa, Ultrapure; lithium molydate, Aesar, 99%; lithium chloride, Alfa, Ultrapure; calcium chloride, Great Western Inorganics, 99.99%; strontium chloride, Alfa, 99%. The relative intensities of the examples are given in Table IV. Example 26 This example is provided both to show the effect of the addition of cesium to Example 1 and to serve as a control for Example Set 26-31. Example 26 does not contain GeO₂ or SiO₂. The sample was prepared as described in Example 1, except that 0.000265 mole of Cs₂SO₄(Alfa, Ultrapure) was added and ground with the initial mixture. The relative intensity is given in Table V. Examples 27-30 These examples illustrate the improvement in relative intensity as a function of mole percent of added GeO₂. The samples were prepared as described in Example 26 with the exception that a certain mole percent of GeO₂ was added and ground with the initial mixture. The relative intensities along with the mole percent (based on Hf content) of germanium oxide are given in Table V. Examples 31a-31c This example illustrates the effect on relative intensity as a function of mole percent of added SiO₂. The sample was prepared as described in Example 26 with the exception that a certain mole percent of SiO₂ was added and ground with the initial mixture. The relative intensity along with the mole percent (based on Hf content) of silicon oxide are given in Table V. Second Firing The samples described above were treated further. They were placed in 20 mL alumina crucibles in ambient air, covered, and heated to 1400°C in a box furnace and maintained at that temperature for 1.5 hours before being allowed to cool to room temperature. The relative intensities are given in Table V. Example 32 This example is presented as a control for Examples 33-52 and for Examples 53-61. It contains neither germanium nor silicon. To provide the best possible control for purposes of comparison, several Example 32 control samples were prepared. During each firing set, one of the control samples was present so that the control would experience exactly the same firing conditions as samples under investigation. For the sample sets described in Tables VI and VII, Example 32 was an average of the controls fired with this group of phosphor samples. A hydrous hafnia precursor was prepared by a conventional preparation method. Suitable methods are those disclosed for preparing hydrous zirconia by M. Shibagaki, K. Takahasi, and M. Matsushita, Bull. Chem. Soc. Japan, 61, 3283 (1988) and A. Benedetti, G. Fagherazzi, and F. Pinna, J. Am. Ceram. Soc., 72, 467 (1989). One mole of R.G.S. hafnium oxychloride from Teledyne Wah Chang Albany was dissolved in 1.0 liter of distilled water. The mole ratio was adjusted to 0.99 Hf/0.01 Zr with zirconium oxychloride if the Zr level of the starting hafnium oxychloride was lower than 0.01. Sodium hydroxide pellets (2.1 mole) were dissolved in 1.0 liter of distilled water. The solutions were added simultaneously to a precipitation vessel with rapid stirring. The resulting gelatinous solid was collected by vacuum filtration and then dried using a rotary evaporator. The solid was washed three times with 4 liters of distilled water. The collected material was then dried for 16 hours at 50°C in a convection oven. A sample consisting of 0.0265 mole of precursor hydrous hafnia was ground together with 0.00318 mole of Li₂CO₃ (Aldrich Chemical Co., 99.997%), 0.00398 mole of K2SO4 (Mallinckrodt Analytical Reagent), and 0.00132 mole TiO₂ (Aldrich Chemical Co., 99.99%) in an agate mortar and pestle. The mixture was placed in a 20 mL alumina crucible which was covered with an alumina lid. The crucible was placed in an alumina boat which was loaded into a mullite tube contained in a tube furnace. The sample was heated in air to 1000°C over 2.5 hours and then held at that temperature for an additional 2.5 hours. After cooling to room temperature, the ingot was removed, broken into small chunks, and ground to a powder in an agate mortar and pestle. The powder was stirred in 150 mL of distilled water for 1 hour. The solid was collected by suction filtration and dried in a 500W CEM Model MDS-81™ microwave oven. The sample was then returned to its 20 mL crucible in ambient air, covered, and heated to 1000°C in a box furnace and maintained at that temperature for 2.5 hours before being allowed to cool to room temperature. The resulting powder was ground to give a uniform phosphor powder. The relative intensity of luminescence was measured as described in Example 1. Examples 33-52 The purpose of this investigation is to demonstrate the performance of a phosphor prepared with varying amounts of germanium. The samples were prepared as described in Example 32, except that varying amounts of GeO2 were added and ground with the initial mixtures. The relative intensity and the mole percent of germanium oxide (based on Hf content) added to each sample are given in Table VI. Examples 53-61 The purpose of this investigation is to demonstrate the performance of a phosphor prepared with varying amounts of silicon. The samples were prepared as described in Example 32 except that varying amounts of SiO₂ (Cabot, CAB-O-SIL™, M-5, Scintillation Grade) were added and ground with the initial mixtures. The relative intensity and the mole percent of silicon dioxide (based on Hf content) added to each sample are given in Table VII. Example 62 The purpose of this investigation is to provide a control for Example 63. The sample was prepared as described in Example 32 except that the second furnace firing was heated to 1300 instead of 1000°C and the charge was held at that temperature for 1.5 hour instead of 2.5 hours. The relative afterglow is defined to be 100 for this sample. The values are displayed in Table VIII. Example 63 The purpose of this example is to demonstrate the effect of the addition of germanium on the afterglow and the particle size distribution of the phosphor. The sample was prepared as described in Example 62 except that 0.0008 mole of germanium oxide (3.0 mole percent based on Hf content) was added to the initial mixture before grinding. The particle size effect was determined from photomicrographs taken of the particles at a magnification of 600X. Three hundred particles were measured for each determination. Particles less than one micron in diameter were excluded. The results are given in Table VIII. Example 64 When compared to Example 3, this sample illustrates the effect on relative luminescence of the addition of potassium to the phosphor formulation. Example 64 contains no potassium. The sample was prepared as described in Example 3 except that no potassium sulfate was added to the mixture. The relative intensity is given in Table IX. Example 65-69 These examples show the effect of adding cesium to the preparation of Example 64. Improvements are realized at the higher loadings of cesium. Example 68 shows an improvement over Example 3f which contains potassium sulfate at the same molar level of cesium sulfate. These samples were prepared as described for Example 64, except that various amounts of cesium sulfate as given in Table IX were added to the initial mixture before grinding. The relative intensities are given in Table IX. Example 70 This example is provided as a comparison for Example 68. This example contains no germanium. This sample was prepared as described for Example 68, except that no germanium oxide was added to the initial mixture. The relative intensity is given in Table IX. Example 71 This example is provided both to show the effect of temperature (compare Example 3) and to be a control for Examples 72-74. This sample was prepared as described for Example 3 except that the sample was heated and held at 1040 instead of 1000°C. The relative intensity is given in Table X (Example 1 = 100). Examples 72-74 These examples are presented to show the effect of using cesium or rubidium instead of potassium at 1040°C. These examples were prepared as described for Example 71 except that the molar amounts of either cesium sulfate or rubidium sulfate given in Table X were added and ground in the initial mixture in place of the potassium sulfate. The relative intensities are given in Table X.

Claims: An intensifying 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 titanium activated zirconium hafnium oxide phosphor host characterized in that the phosphor contains in an amount sufficient to enhance emission of the longer wavelength electromagnetic radiation at least one alkali metal in combination with at least one Group 14 dopant chosen from Periods 3, 4 and 5, which satisfies the relationship: Hf 1-z Zr z Ti x M y E e where z is in the range of from 4 x 10⁻⁴ to 0.3 ; x is in the range of from 3 x 10⁻⁴ to 1.0 ; z is in the range of from 4 X 10⁻⁴ to 0.3 ; M is an alkali metal ; y is in the range of from 6 x 10⁻⁴ to 1.0 ; E is at least one Group 14 dopant chosen from Periods 3, 4 and 5 ; e is in the range of from greater than 1 X 10⁻³ up to the solubility limit of the Group 14 dopant in the phosphor host. An intensifying screen according to claim 1 further containing a metal selected from the class consisting of scandium and indium in the range of from 1 X 10⁻⁶ to 1 X 10⁻². An intensifying screen according to claim 1 or 2 further containing a rare earth selected from among those having an atomic number of 39, 60, 62 to 64 inclusive, and 67 to 71 inclusive in an amount in the range of from 1 x 10⁻⁶ to 1 x 10⁻². An intensifying screen according to claim 1 further characterized in that E is germanium. An intensifying screen according to claim 1 further characterized in that E is silicon. An intensifying screen according to any one of claims 1 to 5 inclusive further characterized in that the monoclinic crystals consist essentially of oxygen and elements that satisfy the relationship Hf 1-z Zr z M y Ti x Li w G v E e wherein E represents at least one of germanium and silicon; e ranges from greater than 1 X 10⁻³ to 0.1; G represents at least one indium and scandium; v ranges from 0 to 1 X 10⁻²; L represents a rare earth selected from among those have an atomic number of 39, 60, 62 to 64 inclusive, and 67 to 71 inclusive; w ranges from 0 to 1 X 10⁻²; M represents at least one alkali metal; y is in the range of from 6 x 10⁻⁴ to 0.3; x is in the range of from 3 x 10⁻⁴ to 1.0; and z is in the range of from 4 x 10⁻⁴ to 0.3. An intensifying screen according to claim 6 further characterized in that e is at least 2 x 10⁻³. An intensifying screen according to claim 6 or 7 further characterized in that v represents a range of from 1 x 10⁻⁶ to 5 x 10⁻³. An intensifying screen according to any one of claims 6 to 8 inclusive further characterized in that the monoclinic crystals consist essentially of oxygen and elements that satisfy the relationship Hf 1-z Zr z M y Ti x Nd w In v E e wherein L is Nd and w ranges from 0 to 5 X 10⁻⁴. An intensifying screen according to any one of claims 1 to 9 inclusive further characterized in that M y is a mixture of alkali metals satisfying the relationship (M¹) y 1(M²) y 2 where M¹ is an alkali metal chosen from the group consisting of lithium, sodium and potassium; M² is at least one alkali metal other than that represented by M¹; y¹+y² = y ; and y¹ and y² are each at least 1 x 10⁻⁶. An intensifying screen according to claim 10 further characterized in that y¹ and y² are each at least 1 x 10⁻⁵. An intensifying screen according to claim 10 further characterized in that M¹ is lithium. An intensifying screen according to claim 12 further characterized in that (M²)y² satisfies the relationship: M' y' M'' y'' where M' is at least one of sodium and potassium; y' is at least 1 x 10⁻⁶; M'' is at least one of cesium and rubidium; and y'' is in the range of from 1 x 10⁻⁵ to 0.1.

Inventors: BRYAN PHILIP S C O EASTMAN KOD, JARROLD GREGORY SCOTT C O EAST, LAMBERT PATRICK M C O EASTMAN, TOWERS CHRISTINE MAY C O EASTM, BRYAN, PHILIP S., C/O EASTMAN KODAK COMPANY, JARROLD, GREGORY SCOTT, C/O EASTMAN KODAK COMPANY, LAMBERT, PATRICK M., C/O EASTMAN KODAK COMPANY, TOWERS, CHRISTINE MAY, C/O EASTMAN KODAK COMPANY
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1991
Main classification: C09K11
IPC codes: G21K4, C09K11, G03C5