A radiographic phosphor panel contain a layer of phosphor, a crystalline material which responds to X-radiation on an image-wise basis. Like many other crystalline materials, radiographic phosphors have a crystal matrix which allows for the replacement of some atoms by other similar atoms, but does not readily accept other atoms or moieties. Radiographic phosphor panels can be classified, based upon their phosphors, as prompt emission panels and image storage panels.
Intensifying screens are the most common prompt emission panels. Intensifying panels are used to generate visible light upon exposure of the intensifying panel to X-radiation. A sheet of photographic film is positioned to intercept the visible light generated and commonly is pressed against the intensifying panel within a light-tight cassette. Other prompt emission panels operate similarly, but in place of the photographic film have some other means for visualizing the X-radiation.
Storage panels have storage phosphors, that have the capability of storing latent X-ray images for later release, apparently by locally trapping electron-hole pairs created by incident X-rays. Storage phosphors are distinguishable from the phosphors used in X-ray intensifying or conversion screens. In the latter, a latent image is not stored and X-radiation causes the immediate release of visible light from irradiated phosphor crystals.
Radiation image storage panels are used in computed radiography. The panel is first exposed to X-radiation to create a latent image. The panel is then stimulated with longer wavelength radiation, resulting in the emission of radiation at a third wavelength. Typically a laser having a red or infrared beam is scanned over the panel, resulting in the emission of green or blue radiation. The emitted light is collected and the resulting signal is processed electronically to produce a final image.
Degradation of final images due to panel discoloration has long been recognized for intensifying screens. There has not, however, been agreement as to the source of that discoloration. What has been noticed is that screens subject to prolonged exposure to photographic film have tended to become discolored. U.S. Pat. Nos. 4,374,905 and 4,360,571 state that the discoloration is due to "volatile organic constituents escaping from the associated photographic film" (U.S. Pat. No. 4,374,905, column 1, lines 40-59 and U.S. Pat. No. 4,360,571, column 1, lines 46-64). Great Britain Patent Application No. GB 2 017 140 A states:
"[I]t has been discovered that screens containing lanthanum-oxy-halide phosphors tend to discolor rapidly when in use and in particular when held in contact with an X-ray film, . . . PA1 "Gadolinium-oxy-halides are similar . . . PA1 "In spite of intensive research into this discolouration defect the cause of it is not yet clearly known but it appears to be a complex reaction caused, in part at least, by the hydroscopic nature of the lanthanum-oxy-halide phosphors or gadolinium-oxy-halide phosphors, the nature of the binder and the presence of the X-ray film held in contact with the screen for a period of time. PA1 "Furthermore, under somewhat different conditions of use X-ray screens and in particular X-ray screens which contain lanthanum-oxyhalide or gadolinium-oxyhalide phosphors can lose speed due to a different defect which appears to involve only the phosphor. This is hydrolysis of the phosphor which is caused by water present in the phosphor layer due either to atmospheric moisture or aqueous cleaning fluid penetrating the protective layer of the screen. It is thought that quantities of halide or more surprisingly, the free halogen, released by hydrolysis may actually catalyse the discolouration of the binder or of compounds having migrated from the film." (page 1, lines 14-33) PA1 "[A]ctivated iodide phosphors are extremely hydroscopic. Absorption of small amounts of water rapidly reduces the conversion efficiency to a vanishingly small value. In order to employ activated iodide phosphors it is therefore necessary to provide the activated iodide phosphor in the screen in a form in which it remains stable for long periods of time. PA1 "Various ways have been taught for using thallium activated potassium iodide and protecting the iodide from moisture." (Bates et al, column 1, lines 20-30) PA1 at least one alkaline earth metal fluoride selected from: BaF.sub.2, CaF.sub.2, and SrF.sub.2 ; PA1 at least one alkaline earth metal iodide selected from BaI.sub.2, CaI.sub.2, and SrI.sub.2 ; PA1 at least one alkaline earth metal halide selected from BaCl.sub.2, BaBr.sub.2, CaCl.sub.2, CaBr.sub.2, SrCl.sub.2, and SrBr.sub.2 ; PA1 at least one trivalent europium-(halide, oxide, nitrate, or sulfate); PA1 at least one transition metal compound selected from compounds of manganese, vanadium, chromium, iron, cobalt, and nickel with halide, oxide, nitrate or sulfate; PA1 at least one oxosulfur reducing agent selected from thiosulfates of barium, cesium, strontium, and ammonium. The claimed invention is not limited by the scope of this example. PA1 "The mixing procedure is generally carried out in the form of a suspension using distilled water. The distilled water contained in the suspension is then removed to obtain a dry cake. The removal of the distilled water is preferably carried out at room temperature or not so high temperature (for example, not higher than 200.degree. C.) under reduced pressure, or under vacuum, otherwise a combination of under reduced pressure and subsequent under vacuum. The obtained dry mixture is pulverized finely by means of a mortar or the like.
U.S. Pat. No. 4,374,905, to Rabatin, teaches a solution to both discoloration by "volatile organic constituents" and attack by water. The phosphor for an intensifying screen was milled with anhydrous MgSO.sub.4 or ZnSO.sub.4 during preparation of the screen. It was proposed that the protective action was based upon the reaction: EQU MgSO.sub.4 +2HOH.fwdarw.Mg(OH).sub.2 +2H.sup.+ +SO.sub.4.sup.2-2.
U.S. Pat. No. 3,836,784, to Bates et al, teaches that small amounts of "stabilizers", such as sodium thiosulfate or potassium thiosulfate can be included in the fluorocarbon binder of an intensifying screen. Bates et al, which used an iodide containing phosphor, noted:
U.S. Pat. No. 3,023,313 to De La Mayer et al teaches the addition of small amounts of sodium thiosulfate or potassium thiosulfate to the polymer binder of an intensifying screen. Examples list 2 grams and 6 grams of sodium thiosulfate per 200 grams of potassium iodide phosphor.
In U.S. Pat. No. 4,360,571, to Rabatin, phosphors were treated with fatty acids or metal salts of fatty acids to prevent discoloration by "volatile organic constituents" and attack by water. In GB 2 017 140 A, intensifying screens were stabilized against discolouration and hydrolysis by incorporation of a compound containing a free epoxy group and, optionally, a dialkyl tin compound such as dibutyl tin diocytl as an additional stabilizer.
Radiation image storage panels, unlike intensifying screens, are subject to degradative losses of both emitted light and stimulating radiation. Since these effects are cumulative, discoloration can be an even more serious issue in storage panels than in intensifying screens.
Yellowing of a phosphor layer of a radiation image storage phosphor panel, in which the phosphor contains iodine, is described in European Patent Specification No. EP 0 234 385 B1. The yellowing is ascribed to liberation of free iodine. The phosphor is divalent europium activated alkaline earth metal fluorohalide phosphor containing iodine or bismuth activated alkali metal halide phosphor containing iodine. The solution described for the yellowing problem, is incorporation in the phosphor layer of a compound containing a free epoxy group and/or a compound selected from: phosphites, organotin compounds, and specific metal salts of organic acids.
Alkaline earth metal fluoro-iodide storage phosphors are described in a number of patent publications. For example, U.S. Pat. No. 4,239,968 teaches a radiation image storage panel including storage phosphors designated by the general formula: (Ba.sub.1-x M.sup.II x)FX:yA; in which M.sup.II is Mg, Ca, Sr, Zn, or Cd; X is Cl, Br, I, or a combination thereof; A is selected from Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb, and Er; x is from 0 to 0.6; and y is from 0 to 0.2. U.S. Pat. No. 4,926,047 to Takahashi et al, teaches that metal oxides may be included in the starting materials for MFX type storage phosphors. Although it is not completely understood how the oxide improves the performance of the phosphor, advantages such as increased luminescence, improved afterglow, and sinter prevention have been mentioned. It is also not known how metal oxide is incorporated into phosphor. There are a number of possibilities, for example, incorporation within the host lattice, and adherence to the surface of crystals.
European Patent Application No. 0 107 192 A1 teaches a radiation image storage panel including storage phosphors designated by the general formula: EQU M.sup.II FX.aM.sup.I X'.bM'.sup.II X".sub.2.cM.sup.III X'".sub.3.xA:yEu.sup.2 +;
in which M.sup.II is Ba, Sr, and/or Ca; M.sup.I is Li, Na, K, Rb, and/or Cs; M'.sup.II is Be and/or Mg; M.sup.III is Al, Ga, In and/or Tl; A is metal oxide; X is Cl, Br, and/or I; X', X", and X'" are F, Cl, Br, and/or I; a is from 0 to 2; b is from 0 to 10.sup.-2 ; c is from 0 to 10.sup.-2 ; a+b+c is greater than or equal to 10.sup.-6 ; x is from 0 to 0.5; and y is from 0 to 0.2.
U.S. Pat. No. 4,505,989 to Umemoto et al teaches a radiation image storage panel including storage phosphors designated by the general formula: EQU M"FX.xNaX':yEu.sup.2 +:zA;
in which M" is at least one alkaline earth metal selected from the group consisting of Ba, Sr, and Cs; each of X and X' is at least one halogen selected from the group consisting of Cl, Br, and I; A is at least one transition metal selected from the group consisting of V, Cr, Mn, Fe, Co and Ni; and x, y, and z are numbers satisfying the conditions of 0&lt;x&lt;=2, 0&lt;y&lt;=0.2 and 0&lt;z&lt;=10.sup.-2.
U.S. Pat. No. 4,138,529 to Mori et al teaches a complex halide phosphor represented by the composition formula EQU MeF.sub.2.aM'X.sub.2.bKX'.cMe"SO.sub.4 :dEu.sup.2 +;
wherein Me, Me' and Me" each represent at least one of the alkaline earth metals of the group consisting of barium, strontium and calcium, X and X' each represent at least one of chlorine and bromine; and a, b, c and d are defined as follows: 0.3&lt;=A&lt;=1.50; 0.10&lt;=B&lt;= 2.00; 0.01&lt;=C&lt;=1.00; 0.001&lt;=D&lt;=0.20.
It would be desirable to provide improved preparation methods, phosphor intermediates, storage phosphor panels, and storage phosphor in which yellowing related degradation is reduced.