Patent Number: 061880736
Section: summary

FIELD OF THE INVENTION The present invention relates to a radiographic intensifying screen employable for radiography. BACKGROUND OF THE INVENTION In a variety of radiography such as medical radiography for diagnosis, a radiographic intensifying screen is generally used in combination with a radiographic film. The radiographic intensifying screen generally comprises a support, a phosphor layer and a surface protective layer overlaid in order. Since the surface protective layer is provided to keep the phosphor layer from chemical and physical deterioration, the protective layer must have a thickness enough to protect the phosphor layer. However, if the surface protective layer is too thick, the sensitivity lowers and further the resultant image is liable to shows poor sharpness. In order to solve this problem, many studies have been done. In a generally employable radiographic intensifying screen, a typical material for the surface protective layer is a polyethylene terephthalate film having a haze of 5 to 10. German Patent Publication No. 3,111,831 discloses a surface protective layer containing .gamma.-alumina particles in an amount of less than 0.1 wt. %. Japanese Patent Publication No. 60-34720 discloses a surface protective layer wherein an organic matting agent is introduced to improve slip property of its surface. Japanese Patent Provisional Publication No. 62-137599 discloses a surface protective layer in which polymer fine particles are introduced so as to improve slip property of its surface. Japanese Patent Provisional Publication No. H3-28798 discloses a radiographic intensifying screen which comprises a protective layer having a great number of very small convexes or concaves on its surface. Japanese Patent Provisional Publication No. 51-127688 discloses a radiographic intensifying screen which comprises a protective layer having a great number of very small convexes of matting agent. Japanese Patent Provisional Publication No. 53-66392 discloses that a light-scattering layer is provided between the phosphor layer and a silver halide emulsion layer so that production of black spots by radioactive isotope can be prevented. Japanese Patent Provisional Publication No. 58-58500 discloses a radiographic intensifying screen which has a white light-scattering layer provided on the phosphor layer, and a transparent protective layer provided on the light-scattering layer. Japanese Patent Provisional Publication No. H3-255400 discloses a radiographic intensifying screen in which metal oxides are provided between the phosphor layer and the surface protective layer so that the screen can have electroconductivity. The known surface protective layers such as described above have been developed in consideration of protection against chemical and physical deterioration (e.g., scratch resistance, stain resistance and abrasion resistance), as well as sharpness of the resultant radiation image. However, although these known surface protective layers are improved to a certain extent, their properties are still unsatisfactory. SUMMARY OF THE INVENTION It is an object of the present invention to provide a radiographic intensifying screen which has good surface durability such as high stain resistance and high abrasion resistance and which gives a radiation image of high sharpness with high sensitivity. The present invention resides in a radiographic intensifying screen comprising a support, a phosphor layer containing phosphor and a surface protective layer over-laid in order, wherein the surface protective layer shows a scattering length in the range of 5 to 80 .mu.m, said scattering length being measured at main wavelength of light emitted from the phosphor. The scattering length is used to mean an average distance in which light travels straight until it is scattered, and hence a small value means that the light is highly scattered. In accordance with Kubeluka-Munk theory, the scattering length can be calculated from the date obtained in the following measurement. First, three film samples are prepared. All film samples have a thickness differing from each other, but each consists of the same components as the target surface protective layer. The thickness (.mu.m) and the diffuse transmittance (%) of each sample are then measured. The diffuse transmittance (%) can be measured by means of a spectrophotometer equipped with an integrating sphere. In the below-described examples of the present specification, an automatic recording spectrophotometer (U-3210, manufactured by HITACHI, Ltd.) equipped with an integrating sphere of 150 .phi. (150-0910) was used. The diffuse transmittance must be measured at a wavelength corresponding to the main peak of the luminescence (light) emitted from phosphor contained in the phosphor layer on which the target surface protective layer is provided. From the thickness (.mu.m) and the diffuse transmittance (%) obtained in the above measurement, the scattering length is calculated in accordance with the following formula (A) derived from Kubeluka-Munk theory. The following formula (A) can be easily derived, under the boundary condition giving the diffuse transmittance (%), from the formulas 5.1.12 to 5.1.15 described in "Keikotai Handbook [Japanese, Handbook of Phosphor]", published by Ohm-sha, 1987, pp.403. Formula (A): EQU T/100=4.beta./[(1+.beta.).sup.2.multidot.exp(.alpha.d)-(1-.beta.).sup. 2.multidot.exp(-.alpha.d)] in which T represents the diffuse transmittance (%), d represents the thickness (.mu.m), and .alpha. and .beta. are defined by the formulas: .alpha.=[K.multidot.(K+2S)].sup.1/2 and .beta.=[K/(K+2S)].sup.1/2, respectively. The formula (A) is applied to the measured T (diffuse transmittance) and d (thickness) of each film sample, and thereby the values of K and S are determined. The scattering length (.mu.m) and the absorption length (.mu.m) described below are values defined by 1/S and 1/K, respectively. Preferred embodiments of the present invention are as follows. (1) The scattering length is in the range of 10 to 70 .mu.m, particularly 10 to 60 .mu.m. (2) The surface protective layer contains light-scattering fine particles having a grain size of 0.1 to 1 .mu.m and s refractive index of more than 1.6. (3) The surface protective layer contains light-scattering fine particles having s grain size of 0.1 to 1 .mu.m and a refractive index of not less than 1.9. (4) The surface protective layer contains light-scattering fine particles comprising at least one material selected from the group consisting of zinc oxide, zinc sulfide, titanium dioxide (particularly, anatase type titanium dioxide), and lead carbonate; and the particles have a mean grain size of 0.1 to 1 .mu.m. (5) The surface protective layer comprises a binder containing fluorocarbon resin or polyester resin and light-scattering fine particles dispersed therein. (6) The surface protective layer has the thickness of 2 to 12 .mu.m, particularly 3 to 9 .mu.m. (7) The phosphor contained in the phosphor layer is represented by the following formula: EQU M.sub.2 O.sub.2 X:Tb in which M is at least one element selected from the group consisting of Y, Gd and Lu; and X is at least one element selected from the group consisting of S, Se and Te. (8) The phosphor layer exhibits a scattering length of 5 to 50 .mu.m, particularly 7 to 30 .mu.m. (9) A light-reflecting layer is provided between the support and the phosphor layer. (10) The phosphor layer comprises a binder and the phosphor dispersed therein, and the weight ratio of the binder to the phosphor is in the range of 1/12 to 1/200 , particularly 1/16 to 1/100. DETAILED DESCRIPTION OF THE INVENTION The radiographic intensifying screen of the invention is now described in detail. The radiographic intensifying screen of the invention has the same structure as the known intensifying screen comprising a support, a phosphor layer and a surface protective layer overlaid in this order. The support employed in the invention can be optionally selected from those employed in the conventional radiographic intensifying screens. Examples of the support include polymer films containing white pigment (e.g., titanium oxide) or black pigment (e.g., carbon black). The phosphor layer may be directly provided on the top face of the support. Otherwise, the phosphor layer may be provided via a subbing layer containing light-reflecting material (i.e., light-reflecting layer). The light-reflecting layer generally comprises a polymer binder and a white pigment (e.g., titanium dioxide) dispersed therein. A variety of phosphors employable for a radiographic intensifying screen are known, and any of them can be used in the invention. Examples of the phosphor employable for the invention include CaWO.sub.4, YTaO.sub.4, YTaO.sub.4 :Nb, LaOBr:Tm, BaSO.sub.4 :Pb, ZnS:Ag, BaSO.sub.4 :Eu, YTaO.sub.4 :Tm, BaFCl:Eu, BaF(Br,I):Eu, Gd.sub.2 O.sub.2 S:Tb, Y.sub.2 O.sub.2 S:Tb, La.sub.2 O.sub.2 S:Tb, (Y, Gd).sub.2 O.sub.2 S:Tb and (Y,Gd).sub.2 O.sub.2 S:Tb,Tm. Those phosphors may be used singly or in combination. Preferred are terbium activated rare earth oxychalcogenide phosphors represented by the formula: M.sub.2 O.sub.2 X:Tb (in which M is at least one element selected from the group consisting of Y, Gd and Lu, and X is at least one element selected from the group consisting of S, Se and Te). Terbium activated rare earth oxysulfide phosphors are more preferred. Examples of the preferred phosphors include Gd.sub.2 O.sub.2 S:Tb, Y.sub.2 O.sub.2 S:Tb, La.sub.2 O.sub.2 S:Tb, (Y,Gd).sub.2 O.sub.2 S:Tb, and (Y,Gd).sub.2 O.sub.2 S:Tb,Tm. U.S. Pat. No. 3,725,704 describes in detail the terbium activated rare earth oxysulfide phosphors. The phosphor represented by Gd.sub.2 O.sub.2 S:Tb is particularly preferred for the present invention. The phosphor layer can be formed by the steps of dispersing the phosphor particles in an organic solution of binder resin to prepare a coating liquid, applying the liquid onto the support directly or via the subbing layer such as light-reflecting layer, and then drying the applied liquid to form the phosphor layer. The phosphor layer may be formed by other steps, namely, applying the above coating liquid onto a temporary support, drying the applied liquid to form a phosphor sheet, peeling off the phosphor sheet from the temporary support, and then providing the phosphor sheet with adhesive onto the support directly or via a subbing layer. The binder resins, organic solvents, and other optional additives employable for the above procedures are described in a variety of known publications. The weight ratio of the binder (total amount of organic compounds contained in the phosphor layer) to the phosphor is not restricted, but the present invention is very effective in the phosphor layer containing a small amount of binder. Accordingly, the preferred ratio of binder/phosphor is in the range of 1/12 to 1/200, more preferably 1/16 to 1/100, and particularly preferably 1/22 to 1/100. The thickness of the phosphor layer can be desirably set according to the target sensitivity. In the case that the intensifying screen is placed in front of the radiographic film, the thickness preferably is in the range of 70 to 150 .mu.m. On the other hand, the screen placed behind the film preferably has a thickness of 80 to 400 .mu.m. The volume filling content of the phosphor in the phosphor layer is preferably in the range of 60 to 85%, more preferably 65 to 80%, and particularly preferably 68 to 75%. The X-ray absorption of the phosphor layer depends on the content of the phosphor particles. On the phosphor layer, the surface protective layer characterizing the present invention is formed. The surface protective layer exhibits a scattering length of 5 to 80 .mu.m which is measured at the main wavelength of the luminescence emitted from the phosphor contained in the phosphor layer. The scattering length preferably is in the range of 10 to 70 .mu.m, more preferably 10 to 60 .mu.m. The surface protective layer preferably contains dispersed light-scattering fine particles having a mean grain size of 0.1 to 1 .mu.m and the refractive index of not less than 1.6. The refractive index preferably is not less than 1.9. Examples of the light-scattering fine particles include fine particles of magnesium oxide, zinc oxide, zinc sulfide, titanium dioxide, niobium oxide, barium sulfate, lead carbonate, silicon oxide, poly(methyl methacrylate), polystyrene, and melamine resin. Zinc oxide, zinc sulfide, titanium dioxide and lead carbonate are preferred. Titanium dioxide is particularly preferred. The binder employable for the surface protective layer is not restricted, but it is required for the binder to keep the surface durability such as stain resistance and abrasion resistance even if the light-scattering fine particles are introduced. In consideration of this, following materials are preferably employable as the binder. Examples of the binders include polyethylene terephthalate, polyethylene naphthalate, polyethylene, polyvinylidene chloride, polyamide, aramide, a solvent-soluble fluorocarbon resin, cellulose derivatives (e.g., cellulose acetate, nitrocellulose, and cellulose acetate butyrate), polyvinyl chloride, polyvinyl acetate, vinyl chloride-vinyl acetate copolymer, polycarbonte, polyvinyl butyral, poly(methyl methacrylate), polyvinyl formal, and polyurethane. More preferred are fluorocarbon resin, cellulose derivatives, and biaxial oriented polymers such as polyethylene terephthalate, polyethylene naphthalate, polyamide and aramide. Particularly preferred are fluorocarbon resin, biaxial oriented polyethylene terephthalate and biaxial oriented polyethylene naphthalate. The binder employable for the invention is by no means restricted by those examples. The surface protective layer can be formed by the steps of dispersing the above light-scattering particles in an organic solution of the binder resin to prepare a coating liquid, applying the liquid onto the phosphor layer directly or via a desired auxiliary layer, and then drying the applied liquid to form the protective layer. The surface protective layer may be formed by other steps, namely, applying the above coating liquid onto a temporary support, drying the applied liquid to form a protective sheet, peeling off the protective sheet from the temporary support, and then providing the protective sheet with adhesive onto the phosphor layer directly or via a desired auxiliary layer. The surface protective layer may contain known additives such as an antistatic agent. The thickness of the surface protective layer is not restricted, but the layer having a thickness of less than 2 .mu.m can not keep satisfactory surface durability. On the other hand, if the thickness is more than 12 .mu.m, the resultant image exhibits unsatisfactory sharpness although it is improved as compared with the image given by a conventional screen having a protective layer of the same thickness. Accordingly, the thickness of the surface protective layer preferably is in the range of 2 to 12 .mu.m, more preferably 3 to 9 .mu.m, and particularly preferably 4 to 9 .mu.m. The absorption length (which indicates a mean distance in which light travels straight until it is absorbed) of the surface protective layer is not restricted. From the viewpoint of sensitivity of the screen, it is preferred for the protective layer not to absorb light. However, in order to make up for shortage of the scattering, the surface protective layer may be made to slightly absorb the light. The absorption length preferably is more than 800 .mu.m, more preferably more than 1,200 .mu.m. General descriptions of the process for preparation of a radiographic intensifying screen and the materials employable for performing the process are given in detail in Japanese Patent Provisional Publications No. H9-21899 and No. H8-184946. A radiographic intensifying screen is generally used in combination with a radiographic film employing silver halide photosensitive material. The radiographic film used together with the radiographic intensifying screen of the invention is now described below. Any kinds of radiographic films can be used together with the screen of the invention, but a "both-sided emulsion film" is preferred. The "both-sided" radiographic film comprises a silver halide emulsion layers provided on both faces of the support. The light cross-over of the film preferably is less than 15 %, more preferably less than 10 %, particularly preferably in the range of 3 to 7 %. The radiographic film showing a low light cross-over can be produced by providing a cross-over shielding layer between the emulsion layer and the support, and is commercially available from Fuji Photo film Co., Ltd. (e.g., UR-1, UR-2, UR-3, Super HRS 30, Super L 30, Super G 30, Super C.sub.30, and Super A 30 [trade names]). The cross-over shielding layer contains a dye selected in consideration of its sensitive wavelength. Any kinds of dye can be used unless it causes disturbing absorption after development. The dye is preferably used in the form of dispersed fine solid particles in accordance with, for example, Japanese Patent Provisional Publications No. H2-264936, No. H3-210553, No. H3-210554, No. H3-238447, No. H4-14038, No. H4-14039, No. H4-125635, No. H4-338747 and No. H6-27589. Examples of the dye include dyes represented by the formulas (I) to (VII) and the compounds (I-1) to (I-37), (II-1) to (II-6), (III-1) to (III-36), (IV-1) to (IV-16), (V-1) to (V-6), (VI-1) to (VI-13) and (VII-1) to (VII-5) in Japanese Patent Provisional Publication No. H4-211542; dyes represented by the formula (1) in Japanese Patent Provisional Publication No. H8-73767; and dyes represented by the formulas (VIII) to (XII) and the compounds (VIII-1) to (VIII-5), (IX-1) to (IX-10), (X-1) to (X-21), (XI-1) to (XI-6) and (XII-1) to (XII-7) in Japanese Patent Provisional Publication No. H8-87091. The dye may be added by known methods such as the method in which the dye is adsorbed onto mordant, the method in which the dye is dissolved in oil to give an emulsion, the method in which the dye is adsorbed onto surface of an inorganic compound (method described in Japanese Patent Provisional Publication No. H3-5748), and the method in which the dye is adsorbed onto polymer material (method described in Japanese Patent Provisional Publication No. H2-298939). The crossover-shielding layer can be formed on the radiographic film in the known manner described in the above publications. Examples of the radiographic film and its materials preferably employable in combination with the intensifying screen of the invention are as follows. 1) The radiographic film described in Example 1 of Japanese Patent Provisional Publication No. H6-332088, and the radiographic films described in Examples 1 and 2 of Japanese Patent Provisional Publication No. H7-219162. 2) The emulsion of tabular silver chloride having {100} principal plane described in Examples 3 and 4 of Japanese Patent Provisional Publication No. H5-204073, that described in Example 2a of Japanese Patent Provisional Publication No. H6-194768, and that described in Example 1 of Japanese Patent Provisional Publication No. H6-227431. 3) The photosensitive silver iodobromide, silver bromide and silver bromide chloride particles having {111} principal plane described in Example 1 of Japanese Patent Provisional Publication No. H8-76305, and the emulsions described in Examples A to K of Japanese Patent Provisional Publication No. H8-69069. 4) The mono-dispersed cubic particles (whose dispersion degree is preferably in the range of 3 to 40% in terms of variation coefficient of projected area diameters) described in Example 1 of Japanese Patent Provisional Publication No. H8-76305. In addition, preferred radiographic films and their materials are described in detail in Japanese Patent Provisional Publication No. H6-67305. A radiographic film can be used in combination with a single intensifying screen, but usually a "both-sided emulsion type" radiographic film described above is used in combination with two intensifying screens placed on both faces of the film. The intensifying screen placed in front of the radiographic film is generally called "front screen", and that placed behind the film is called "back screen".