Patent Number: 
Section: description

A stimulable phosphor sheet of the invention comprises at least two partitioned stimulable phosphor films laminated one on another. The partitioned stimulable phosphor film comprises plural partitions that divide the stimulable phosphor film on its plane to give plural stripe sections, and a stimulable phosphor layer placed in each stripe section. The partitioned stimulable phosphor films are laminated in such manner that the partitions of one stimulable phosphor film are arranged to cross the partitions of another stimulable phosphor film. The partitioned stimulable phosphor film is described in detail by referring to FIG. 1 of the attached drawings. In FIG. 1, the partitioned stimulable phosphor film 1 is composed of partitions 2 and stimulable phosphor layers placed in the areas between the partitions. For accomplishing appropriate resolution and image quality, the mean width of the partition preferably is in the range of 0.5 to 50 xcexcm. The stripe of the phosphor layer preferably has a mean width in the range of 5 to 300 xcexcm. A ratio of a total surface of the stimulable phosphor layer 3 to a total surface of the phosphor film 1, that is a ratio of effective phosphor layer, preferably is in the range of 40% to 98%. In the stimulable phosphor sheet of the invention, the partition of the stimulable phosphor film preferably has a light-scattering length for the stimulating rays which is shorter than that of the stimulable phosphor layer. In particular, the partition preferably has a light-scattering length of 0.05 to 20 xcexcm for the stimulating rays and a light-absorption length of 1,000 xcexcm or longer for the stimulating rays, while the stimulable phosphor layer has a light-scattering length of 20 to 1,000 xcexcm for the stimulating rays and a light-absorption length of 1,000 xcexcm or longer for the stimulating rays. The term of light-scattering length indicates a mean distance in which a light travels straight until it is scattered, and therefore a shorter light-scattering length means that the phosphor layer or partition highly scatters a light. The term of light-absorption length indicates a mean free distance in which the stimulated emission is absorbed, and therefore a longer light-absorption length means that the phosphor layer or partition shows a lower light absorbance. The light-scattering length and light-absorption length can be determined by calculation according to Kubeluka-Munk theory. In FIG. 1, the top and bottom of the partition 2 are exposed over each surface of the stimulable phosphor film 1. However, the top and/or bottom of the partition 2 may be buried in the stimulable phosphor sheet. The partition preferably has a height corresponding to 1/3 to 1/1 of the thickness of the stimulable phosphor film 1. In FIG. 2, a set of four partitioned stimulable phosphor films 1 which are to be laminated in the illustrated mode to give a stimulable phosphor sheet in which the partitions of one stimulable phosphor film are arranged to perpendicularly (namely at an angle of approximately 90xc2x0) cross the partitions of another stimulable phosphor film. Each partitioned stimulable phosphor film preferably is identical to each other. In FIG. 3, a set of four partitioned stimulable phosphor films which are to be laminated in the illustrated mode to give a stimulable phosphor sheet. In the illustrated mode, the partitions of the top stimulable phosphor film are arranged to cross the partitions of the second stimulable phosphor film at an angle of 45xc2x0; the partitions of the second stimulable phosphor film are arranged to cross the partitions of the third stimulable phosphor film at an angle of 90xc2x0; and the partitions of the third stimulable phosphor film are arranged to cross the partitions of the fourth (i.e., bottom) stimulable phosphor film at an angle of 45xc2x0. Each partitioned stimulable phosphor film preferably is identical to each other. The stimulable phosphor sheet of the invention comprises plural partitioned thin stimulable phosphor films which are so arranged as to form square, rhombic, or triangle plural cells which are observed in the direction perpendicular to the plane of the phosphor sheet. Thus, the specific arrangements of partitions of the adjoining partitioned stimulable phosphor films results in producing imaginary cell structures in the phosphor sheet, to define scattering of the stimulating rays in the cells. In the radiation image reproducing procedure, the stimulable phosphor sheet of the invention is preferably moved in the direction parallel to the partitions of the top partitioned phosphor film, while the stimulating rays are scanned in a direction perpendicular to the direction of movement. The stimulable phosphor sheet of the invention can be preferably produced in the process illustrated in FIG. 4 through FIG. 7. The preferred process is further described below, by referring to the case that the stimulable phosphor layer comprises stimulable phosphor particles and binder, and the partition comprises low light-absorbing fine particles and polymer material. In the first step, a stimulable phosphor film is prepared. As the stimulable phosphor, a phosphor giving a stimulated emission of a wavelength in the region of 300 to 500 nm when it is irradiated with stimulating rays of a wavelength in the region of 400 to 900 nm is preferably employed. In Japanese Patent Provisional Publications No. 2-193100 and No. 4-310900, some examples of the stimulable phosphors are described in detail. Examples of the preferred stimulable phosphors include divalent europium or cerium activated alkaline earth metal halide phosphors (e.g., BaFBr:Eu, BaF(BrI):Eu), and cerium activated oxyhalide phosphors. Most preferred stimulable phosphors are rare earth metal activated alkaline earth metal fluorohalide phosphors having the following essential formula (I): MIIFX:zLnxe2x80x83xe2x80x83(I) in which MII is an alkaline earth metal such as Ba, Sr, or Ca; Ln is a rare earth metal such as Ce, Pr, Sm, Eu, Tb, Dy, Ho, Nd, Er, Tm, or Yb; X is a halogen atom such as Cl, Br, or I; and z is a value satisfying the condition of 0 less than zxe2x89xa60.2. MII of the formula (I) preferably comprises Ba in an amount of 50 atomic % or more. Ln preferably is Eu or Ce. It should be noted that the formula (I) does not mean F:X=1:1, but means to have a crystalline structure of BaFX. Thus, the formula (I) does not accurately indicate stoichiometric amounts of the constitutional elements. It is generally preferred that F is slightly rich in comparison with X, because F+ center (Xxe2x88x92 center) produced in such composition efficiently gives a stimulated emission when the phosphor is stimulated with a light in the region of 600 to 700 nm. The stimulable phosphor of the formula (I) can further contain one or more of the following additive components: bA, wNI, xNII, yNIII  In the above formulas, A is a metal oxide such as Al2O3, SiO2 or ZrO2, in which source of the metal oxide preferably is extremely fine particles having a mean diameter (of primary particles) of 0.1 xcexcm or less and has little reactivity to MIIFX particles to keep the MIIFX particles from coagulation; NI is a compound of an alkali metal such as Li, Na, K, Rb, or Cs; NII is a compound of an alkaline earth metal such as Mg and/or Be; and NIII is a compound of a monovalent or trivalent metal such as Al, Ga, In, Tl, Sc, Y, La, Gd, or Lu. The metal compounds preferably are halide compounds such as those described in Japanese Patent Provisional Publication No.59-75200. In the formulas, each of b, w, x, and y is a value which means an amount of each source material, based on one molar amount of MIIFX, under the conditions of 0xe2x89xa6bxe2x89xa60.5, 0xe2x89xa6wxe2x89xa62, 0xe2x89xa6xxe2x89xa60.3, and 0xe2x89xa6bxe2x89xa63. Accordingly, the value of b, w, x, or y does not necessarily mean the amount of each element or compound existing in the finally produced phosphor. Further, each additive compound may exist as such in the finally produced phosphor or may react with MIIFX in the course of the preparation of the stimulable phosphor. Furthermore, the stimulable phosphor of the formula (I) may contain one or more of the following compounds or reaction products thereof: Compounds of Zn and Cd described in Japanese Patent Provisional Publication No. 55-12145; Metal oxides such as TiO2, BeO, MgO, CaO, SrO, BaO, ZnO, Y2O3, LA2O3, In2O3, GeO2, SnO2, Nb2O5, Ta2O5, and ThO2 described in Japanese Patent Provisional Publication No. 55-160078; Compounds of Zr and Sc described in Japanese Patent Provisional Publication No. 56-116777; Compounds of B described in Japanese Patent Provisional Publication No. 57-23673; Compounds of As and Si described in Japanese Patent Provisional Publication No. 57-23675; Tetrafluoroborate compounds described in Japanese Patent Provisional Publication No. 59-27980; Hexafluoro compounds such as monovalent or divalent salts of hexafluorosilicic acid, hexafluorotitanic acid, or hexafluorozirconic acid described in Japanese Patent Provisional Publication No. 59-47289; and Compounds of transitional metals such as V, Cr, Mn, Fe, Co, and Ni described in Japanese Patent Provisional Publication No. 59-56480. Moreover, other additives may be incorporated, provided that the incorporated additives do not disturb the preparation of the essential phosphor composition of the formula (I). The rare earth activated alkaline earth metal fluorohalide phosphors of the formula (I) generally have an aspect ratio of 1.0 to 5.0. The stimulable phosphor particles favorably employed for the production of the stimulable phosphor sheet of the invention have an aspect ratio of 1.0 to 2.0, more preferably 1.0 to 1.5. The particle size preferably is in the range of 1 xcexcm to 10 xcexcm, more preferably 2 xcexcm to 7 xcexcm, in terms of Median diameter (Dm), and "sgr"/Dm ("sgr" is a standard deviation of the particle size distribution) preferably is not more than 50%, more preferably not more than 40%. The particles may be in the form of parallelepiped, regular hexahedron, regular octahedron, tetradecahedron, intermediate polyhedron, or amorphous. The phosphor particles of tetradecahedron are preferred. Examples of the binders include natural polymers such as proteins (e.g., gelatin), polysaccharides (e.g., dextran) and gum arabic; and synthetic polymers such as polyvinyl butyral, polyvinyl acetate, nitrocellulose, ethyl cellulose, vinylidene chloride-vinyl chloride copolymer, polyalkyl (meth) acrylate, vinyl chloride-vinyl acetate copolymer, polyurethane, cellulose acetate butyrate, polyvinyl alcohol, linear polyester, and thermoplastic elastomers. The polymer material may be crosslinked. The stimulable phosphor particles and binder are placed in an appropriate solvent to prepare a dispersion. The ratio of binder and stimulable phosphor particles in the phosphor dispersion generally is in the range of 1:1 to 1:100 (binder:phosphor, by weight), preferably 1:8 to 1:40. The phosphor dispersion is coated on a temporary support such as glass plate, metal plate, or plastic sheet, and dried to give a stimulable phosphor film A illustrated in FIG. 4. The produced stimulable phosphor film may be compressed under heating, so as to increase the density of the phosphor film. Alternatively, the stimulable phosphor film can be prepared by depositing or sintering stimulable phosphor material on a heat-resistant support such as metal plate. The partition film B is described below. Examples of the low light-absorbing fine particles are fine particles of white inorganic materials such as aluminum oxide (i.e., alumina), titanium dioxide, yttrium oxide, zirconium oxide, gadolinium oxide, tellurium oxide, ruthenium oxide, and lead oxide. Certain stimulable phosphor particles may be employed as the low light-absorbing fine particles. Preferred are alumina particles and yttrium oxide. The low light-absorbing fine particles preferably have a mean particle diameter of 0.01 to 5.0 xcexcm. There are no specific limitation with respect to the polymer binder for production of the partition film B, and the binders described hereinbefore for the production of the stimulable phosphor film A can be employed. In order to shorten the light-scattering length of the partition, however, a ratio of Kf (i.e., refractive index of the low light-absorbing fine particles) to the refractive index of the polymer binder preferably is in the range of 1.1 to 3.0. Therefore, the polymer binder preferably is polyurethane, polyacrylate, polyethylene, polystyrene, or a fluororesin. For the production of the partition film, a dispersion is prepared by mixing the low light-absorbing fine particles and the polymer binder in a solvent. The polymer binder and the low light-absorbing fine particles are mixed generally at a ratio of 1:80 to 1:3 (by weight), preferably 1:20 to 1:10 (by weight). The dispersion is coated on a temporary support, and dried to give a partition film B illustrated in FIG. 4. The preparation of the stimulable phosphor film A is repeated to produced a plurality of stimulable phosphor films, and the preparation of the partition film B is repeated to produced a plurality of partitions films. The stimulable phosphor films and partition films are then placed alternately to give a laminate illustrated in FIG. 5. The laminate of FIG. 5 is then heated under pressure in the manner illustrated in FIG. 6, to give a laminate block in which the stimulable phosphor films and partition films are bonded to each other. The laminate block is then sliced along the surface perpendicular to the planes of the phosphor films and partition films, so that a partitioned stimulable phosphor film such as that illustrated in FIG. 1 is produced. The slicing procedure is repeated to give a plurality of partitioned stimulable phosphor films. The partitioned stimulable phosphor films are then laminated in the manner illustrated in FIG. 2 or FIG. 3, and heated under pressure in the manner such as that illustrated in 6, to produce a stimulable phosphor sheet of the invention. The stimulable phosphor sheet of the invention may have a support and a transparent cover film as illustrated in FIG. 8 in which the stimulable phosphor sheet 4 has a support 5 and a transparent cover film 6, so as to keep the phosphor sheet from deterioration and to facilitate handling of the phosphor sheet. The stimulable phosphor sheet also can have a light-reflective layer on one surface side (or between the phosphor sheet and the support, if the support is provided), so as to increase the sensitivity of the phosphor sheet. As the support, a sheet or a film of flexible resin material having a thickness of 50 xcexcm to 1 mm is generally employed. The support may be transparent or may contain light-reflecting material (e.g., alumina particles, titanium dioxide particles, and barium sulfate particles) or voids, for reflecting the stimulating rays or the stimulated emission. Further, it may contain light-absorbing material (e.g., carbon black) for absorbing the stimulating rays or the stimulated emission. Examples of the resin materials include polyethylene terephthalate, polyethylene naphthalate, aramid resin and polyimide resin. The support may be a sheet of other material such as metal, ceramics and glass, if needed. On the phosphor sheet-side surface of the support, auxiliary layers (e.g., light-reflecting layer, light-absorbing layer, adhesive layer, electroconductive layer) or many hollows may be provided. On the other side surface, a friction-reducing layer or an anti-scratch layer may be formed. On the surface not facing the support, the stimulable phosphor sheet may have a protective cover film. In order not to affect the simulating rays or the stimulated emission, the cover film preferably is transparent. Further, for efficiently protecting the stimulable phosphor sheet from chemical deterioration and physical damage, the protective film should be both chemically stable and physically strong. The cover film can be provided by fixing a before-hand prepared transparent plastic film (e.g., polyethylene terephthalate) on the stimulable phosphor sheet with adhesive, or by coating the phosphor sheet with a solution of cover film material and drying the coated solution. Into the cover film, fillers of fine particles may be incorporated so as to reduce blotches caused by interference and to improve the quality of the resultant radiation image. The thickness of the cover film generally is in the range of approx. 0.1 to 20 xcexcm. For enhancing the resistance to staining, a fluororesin layer is preferably provided on the cover film. The fluororesin layer can be formed by coating the surface of the cover film with a solution of a fluororesin in an organic solvents, and drying the coated solution. The fluororesin may be used singly, but generally a mixture of the fluororesin and a film-forming resin is employed. In the mixture, an oligomer having polysiloxane structure or perfluoroalkyl group can be further added. Into the fluororesin layer, a filler of fine particles may be incorporated so as to reduce blotches caused by interference and to improve quality of the resultant radiation image. The thickness of fluororesin layer generally is in the range of 0.5 to 20 xcexcm. In the formation of the fluororesin layer, additives such as a crosslinking agent, a film-hardening agent and an anti-yellowing agent can be used. In particular, the crosslinking agent advantageously improves durability of the fluororesin layer. The light-reflective layer can comprise a white pigment such as alumina pigment, titanium dioxide pigment, or a barium sulfate pigment, or phosphor particles giving no stimulated emission. In the light-reflective layer, the pigment or particles are dispersed and supported in a binder. The present invention is further described by the following examples. 1) Stimulable phosphor (BaF(Br,I):Eu) particles (median of the particle sizes; 5 xcexcm) and a thermoplastic high molecular weight-polyester resin were dispersed in an organic solvent in a weight ratio of 5:1. The prepared phosphor dispersion was coated on a temporary support having a releasing surface, and dried to give a dry phosphor film. The phosphor film thus formed was then peeled from the temporary support to give a stimulable phosphor film A (thickness: approx. 100 xcexcm). The stimulable phosphor film A was subjected to measurement of transmittance at a stimulating wavelength (600 nm) and a stimulated emission wavelength (400 nm), to determine the light-scattering length and light-absorbing length. It was confirmed that the light-scattering length at the stimulating wavelength was such long as to give 66 xcexcm and the light-absorbing length was such long as a length of longer than 1,000 xcexcm. 2) Particles of yttrium oxide (mean particle size: 0.6 xcexcm) and a thermoplastic high molecular weight-polyester resin were dispersed in an organic solvent in a weight ratio of 15:1. Thus prepared yttrium oxide particle-containing dispersion was coated onto a temporary support having a releasing surface, and dried to give a yttrium oxide particle-containing dry film. The dry film was then peeled from the temporary support to give a partition film B (thickness: approx. 30 xcexcm). The partition film B was subjected to measurement of transmittance at a stimulating wavelength (600 nm) and a stimulated emission wavelength (400 nm), to determine the light-scattering length and light-absorbing length. It was confirmed that the light-scattering length at the stimulating wavelength was such short as to give 4 xcexcm and the light-absorbing length was such long as a length of longer than 1,000 xcexcm. 3) The stimulable phosphor films A and the partition films B were alternately piled up to form a laminate consisting of 360 films. The la te was then heated under pressure (pressure: approx. 1 kg/cm2, temperature: 100xc2x0 C.) for 1 hour to produce a laminate block. 4) The laminate block was repeatedly sliced in the manner illustrated in FIG. 7 using a wide microtome, to produce plural partitioned stimulable phosphor film (thickness: 100 xcexcm, width of partition: approx. 30 xcexcm, width of stimulable phosphor layer: approx. 100 xcexcm). 5) Three, six or nine partitioned stimulable phosphor films are laminated in such manner that the partitions of the adjoining films perpendicularly cross each other, as illustrated in FIG. 2. Thus, three stimulable phosphor sheets (thickness: approx. 300 xcexcm, approx. 600 xcexcm, or approx. 900 xcexcm) were prepared. The procedures of Example 1 were repeated except for placing a light-reflecting layer on one surface of the stimulable phosphor sheet of approx. 600 xcexcm thick by coating the yttrium oxide particle-containing dispersion employed in 2) above, to produce a stimulable phosphor sheet having a light-reflecting layer on one side. The procedures of Example 1 were repeated except for fixing a transparent polyethylene terephthalate film (thickness: 300 xcexcm) on one surface of the stimulable phosphor sheet of approx. 600 xcexcm thick, using an adhesive, to produce a stimulable phosphor sheet having a transparent cover film. The laminate block prepared in Example 1-3) was sliced in the manner illustrated in FIG. 7 to give three stimulable phosphor sheets (thickness: 320 xcexcm, 590 xcexcm, and 910 xcexcm) having partitions extending one dimensionally. Stimulable phosphor (BaF(Br,I):Eu) particles (median of the particle sizes: 5 xcexcm) and a thermoplastic high molecular weight-polyester resin were dispersed in an organic solvent in a weight ratio of 20:1. The prepared phosphor dispersion was coated on a temporary support having a releasing surface, and dried to give a dry phosphor film. The dry phosphor film was peeled off from the temporary support to obtain a stimulable phosphor sheet having a thickness of approx. 290 xcexcm, approx. 600 xcexcm, or approx. 880 xcexcm which had no partitions on its surface. The stimulable phosphor sheets obtained in Examples 1 to 3 and Comparison Examples 1 and 2 were exposed to irradiation of X-rays (tube voltage: 80 kVp, 80 mA, radiation dose: 10 mR). Subsequently, a He-Ne laser beam was scanned on the irradiated stimulable phosphor sheet, and the stimulated emission was collected on the side on which the laser beam was scanned. The amount of the stimulated emission was detected to determine the sensitivity. Independently, the stimulable phosphor sheet was exposed to X-rays in the same manner except for placing a CTF chart on the phosphor sheet. The scanning with the laser beam and the collection of the stimulated emission were carried out in the same manner to obtain radiation image data. The sharpness was evaluated using the obtained radiation image data. In the exposure to X-rays, the CTF chart was so placed on the stimulable phosphor sheet that the partitions of the phosphor sheet on the top partitioned phosphor film and the stripes of the CIF were arranged perpendicularly to each other (first run) or aligned in parallel (second run). The stimulated emission produced from the stimulable phosphor sheet of Example 3 was collected from both surface sides The results are set forth in Table 1. From the results of Table 1, it is confirmed that the stimulable phosphor sheet of the invention (Example 1) gives a reproduced radiation image of high sharpness not only in the first run but also in the second run, as compared with the stimulable phosphor sheet of Comparison Example 1. This is favorable for reproducing a radiation image for diagnosis. The stimulable phosphor sheet of Comparison Example 2, which is a conventional stimulable phosphor sheet) also gives a reproduced radiation image of relatively high sharpness not only in the first run but also in the second run, as compared with the stimulable phosphor sheet of Comparison Example 1. However, in the case that the thickness of the stimulable phosphor sheet, the phosphor sheet of Comparison Example 2 decreases the sensitivity rapidly, while its sensitivity increases only slightly. The stimulable phosphor sheets of Examples 2 and 3 show sensitivity apparently higher than that of the stimulable phosphor sheet of Example 1. The procedures of Example 1 were repeated except for laminating, in order, three (six or nine) partitioned stimulable phosphor films in such manner that the partitions of a upper film crossed the partitions of a lower film at an angle of 60xc2x0. Thus, three stimulable phosphor sheets (thickness: approx. 300 xcexcm, approx. 600 xcexcm, or approx. 900 xcexcm) were prepared. The procedures of Example 1 were repeated except for so slicing the laminate block as to give plural partitioned stimulable phosphor films having different thicknesses in the range of 50 to 600 xcexcm, in the step 4). The plural partitioned stimulable phosphor films having different thickness are laminated in such manner that the partitions of one of the adjoining films crossed perpendicularly the partitions of another film. In this manner, four stimulable phosphor sheets set forth in Table 2 were prepared. The three stimulable phosphor sheets of Example 4 and the four stimulable phosphor sheets of Example 5 were evaluated in their sensitivity and sharpness in the same manner as described above. It was confirmed that these stimulable phosphor sheets showed satisfactory sensitivity and sharpness.