Radiation image storage panel

Abstract: A radiation image storage panel comprising a support, a phosphor layer which comprises a binder and a stimulable phosphor dispersed therein and a protective film, superposed in this order, characterized in that said protective film contains an antistatic agent of an inorganic salt of a metal.

Description: The present invention relates to a radiation image storage panel employed in a radiation image recording and reproducing method utilizing a stimulable phosphor. More particularly, the invention relates to a radiation image storage panel comprising a support, a phosphor layer which comprises a binder and a stimulable phosphor dispersed therein, and a protective film, superposed in this order. For obtaining a radiation image, there has been conventionally employed a radiography utilizing a combination of a radiographic film having an emulsion layer containing a photosensitive silver salt material and a radiographic intensifying screen. As a method replacing the above-described radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U S -A- 4,239,968, has been recently paid much attention. In the radiation image recording and reproducing method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is used, and the method involves steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at considerably smaller dose, as compared with the conventional radiography. Accordingly, the radiation image recording and reproducing method is of great value especially when the method is used for medical diagnosis. The radiation image storage panel employed in the radiation image recording and reproducing method has a basic structure comprising a support and a phosphor layer provided on one surface of the support. Further, a transparent film is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock. The phosphor layer comprises a binder and stimulable phosphor particles dispersed therein. The stimulable phosphor emits light (gives stimulated emission) when excited with stimulating rays such as visible light or infrared rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or having radiated from an object is absorbed by the phosphor layer of the radiation image storage panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating (scanning) the panel with stimulating rays. The stimulated emission is then photoelectrically detected to obtain electric signals, so as to reproduce a visible image from the electric signals. Such phosphor layer of the radiation image storage panel is conventionally prepared by procedures of adding stimulable phosphor particles and a binder to an appropriate solvent to prepare a homogeneous coating dispersion comprising phosphor particles dispersed in a binder solution, applying the coating dispersion onto a support using a doctor blade, a roll coater, etc., and heating a layer of the coating dispersion to dryness. Otherwise, the phosphor layer is initially formed on a sheet by applying the coating dispersion thereonto and then the phosphor layer is combined with the support. The phosphor particles are uniformly dispersed in the prepared phosphor layer, and therefore, the surface of the phosphor layer is provided with finely protruded and depressed portions because of the appearance of phosphor particles. To protect the stimulable phosphor on the surface of the phosphor layer from chemical deterioration or physical shock and to make the panel have a smooth surface, a protective film is usually provided on the surface of the phosphor layer by applying a solution of transparent polymer material thereonto or combining a previously prepared transparent film with the phosphor layer using an adhesive agent. It is desired for the radiation image storage panel employed in the radiation image recording and reproducing method to have a high sensitivity and to provide an image of high quality (high sharpness, high graininess, etc.), as well as a radiographic intensifying screen employed in the conventional radiography. However, when the radiation image storage panel has a protective film, the light (stimulating rays) tends to be scattered on the interface of the phosphor layer and the protective film. Particularly in the case that the protective film is provided on the phosphor layer via an adhesive layer, the scattering of stimulating rays occurs on each interface among the phosphor layer, adhesive layer and protective film. Thus, the quality of the resulting image is apt to be deteriorated. Further, the efficiency in the detection of light emitted by the phosphor on the side of the panel surface (surface of the protective film) is reduced because the emitted light is also scattered on the interface(s), so that the sensitivity of the panel decreases. The radiation image storage panel, generally, is used repeatedly. When the protective film is provided on the phosphor layer via an adhesive layer, the adhesive layer tends to mechanically or thermochemically deteriorate as the use of the panel is repeated. As a result, the protective film has a tendency of peeling off the panel. In other words, the bonding strength is not so sufficient between the phosphor layer and the protective film and the panel is not sufficiently resistant to the repeated use. More in detail, the radiation image storage panel is repeatedly used in a cyclic procedure comprising steps of: exposing the panel to a radiation (recording a radiation image thereon), irradiating the panel with stimulating rays (reading out the recorded radiation image therefrom) and irradiating the panel with light for erasure (erasing the remaining radiation image therefrom). The panel is transferred from a step to the subsequent step through a transfer system and piled on other panels to be stored after one cycle is finished. The radiation image storage panel is furthermore apt to be charged on its surface because the contact and friction between panels repeatedly take place in the continuous cycle comprising transfering and piling of the panel. The static electrification on the panel brings about the adhesion with another panel to transfer two panels together in layers from the piling into the transfer system, whereby the subsequent procedure cannot be normally conducted. The charged surface of the panel is likely to be attached with dust in the air, so that the stimulating rays are also scattered on the dust attached thereon and the quality of the resulting image lowers. The object of the present invention is to provide a radiation image storage panel improved in the antistatic effect and transfer properties. This object is accomplished by a radiation image storage panel of the present invention comprising a support, a phosphor layer which comprises a binder and a stimulable phosphor dispersed therein and a protective film, superposed in this order, characterized in that said protective film contains an antistatic agent of an inorganic salt of a metal. Fig. 1 schematically shows a sectional view of a two-hopper type-coating apparatus, which is an example of the coating apparatus employable for the preparation of a radiation image storage panel according to the present invention. Fig. 2 is a photograph showing a partial section of the phosphor layer observed by using a scanning electron microscope, with respect to the radiation image storage panel according to the present invention. A phosphor layer is conventionally prepared by applying a binder solution containing phosphor particles homogeneously dispersed therein (namely, a coating dispersion) onto a support using a known method. The phosphor particles are uniformly dispersed in the whole phosphor layer with the binder. Then, a protective film is provided by combining the phosphor layer with a transparent thin film made of polymer such as polyethylene terephthalate by an adhesive agent. In the present invention, a binder solution-I containing phosphor particles and a binder solution-II not containing phosphor particles but containing an antistatic agent are simultaneously applied onto a support (or onto another plane sheet) in such a superposing manner that the binder solution-II is arranged on the binder solution-I. The binders in the binder solution -I and -II are incompatible with each other. Since the binders in the binder solutions -I and -II are incompatible with each other, the simultaneous superposition-coating brings about the simultaneous formation of two layers consisting of a phosphor layer and a protective film. According to the invention, it is not necessary to further provide a protective film on the phosphor layer, which is different from the conventional process for the preparation of a radiation image storage panel, and the process can be simplified. After applying onto a support both layers of the binder solutions-I and -II are simultaneously heated to dryness to form a phosphor layer and a protective film, so that the bonding strength between the phosphor layer and the protective film can be enhanced and the peeling of the protective film due to the deteriorated adhesive layer can be prevented. In a conventional radiation image storage panel, dust is apt to be introduced into the area between the phosphor layer and the protective film in the procedure of providing (laminating) the protective film, and therefore careful attention is required. Such problem does not occur in the manufacturing of the radiation image storage panel of the present invention, whereby an image of high quality is easily obtained. Furthermore, the static electrification phenomenon of the surface of the radiation image storage panel which occurs during the transfer system for recording and reading out the panel is prevented by introducing the antistatic agent of an inorganic salt of metal into the binder solution-II in the process for the preparation thereof. That is, the antistatic agent is dispersed on the surface of the protective film in the panel of the present invention. The presence of the antistatic agent on the surface of the panel effectively prevents the panel from being charged and brings about the smooth transfering of the panel in the system, so that the satisfactory recording and reproducing of the radiation image are achieved. The radiation image storage panel of the present invention having the above-described advantages can be prepared, for instance, in the following manner. The phosphor layer basically comprises a binder and stimulable phosphor particles dispersed therein. The stimulable phosphor, as described hereinbefore, gives stimulated emission when excited with stimulating rays after exposure to a radiation. From the viewpoint of practical use, the stimulable phosphor is desired to give stimulated emission in the wavelength region of 300 - 500 nm when excited with stimulating rays in the wavelength region of 400 - 900 nm. Examples of the stimulable phosphor employable in the radiation image storage panel of the present invention include: SrS:Ce,Sm, SrS:Eu,Sm, ThO₂:Er, and La₂O₂S:Eu,Sm, as described in U S -A- 3,859,527; ZnS:Cu,Pb, BaO·xAℓ₂O₃:Eu, in which x is a number satisfying the condition of 0.8 ≦ x ≦ 10, and M²⁺O·xSiO₂ :A, in which M²⁺ is at least one divalent metal selected from the group consisting of Mg, Ca, Sr, Zn, Cd and Ba, A is at least one element selected from the group consisting of Ce, Tb, Eu, Tm, Pb, Tℓ, Bi and Mn, and x is a number satisfying the condition of 0.5 ≦ x ≦ 2.5, as described in U S -A- 4,326,078; (Ba 1-x-y ,Mg x ,Ca y )FX:aEu²⁺, in which X is at least one element selected from the group consisting of Cℓ and Br, x and y are numbers satisfying the conditions of 0 < x+y ≦ 0.6, and xy 0, and a is a number satisfying the condition of 10⁻⁶ ≦ a ≦ 5x10⁻², as described in JP-A- 55(1980)-12143; LnOX:xA, in which Ln is at least one element selected from the group consisting of La, Y, Gd and Lu, X is at least one element selected from the group consisting of Cℓ and Br, A is at least one element selected from the group consisting of Ce and Tb, and x is a number satisfying the condition of 0 < x < 0.1, as described in the above-mentioned U S - A- 4,236,078; (Ba 1-x ,M II x )FX:yA, in which M II is at least one divalent metal selected from the group consisting of Mg, Ca, Sr, Zn and Cd, X is at least one element selected from the group consisting of Cℓ, Br and I, A is at least one element selected from the group consisting of Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb and Er, and x and y are numbers satisfying the conditions of 0 ≦ x ≦ 0.6 and 0 ≦ y ≦ 0.2, respectively, as described in U S -A- 4,239,968; M II FX·xA:yLn, in which M II is at least one element selected from the group consisting of Ba, Ca, Sr, Mg, Zn and Cd; A is at least one compound selected from the group consisting of BeO, MgO, CaO, SrO, BaO, ZnO, Aℓ₂O₃, Y₂O₃, La₂O₃, In₂O₃, SiO₂, TiO₂, ZrO₂, GeO₂, SnO₂, Nb₂O₅, Ta₂O₅ and ThO₂; Ln is at least one element selected from the group consisting of Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb, Er, Sm and Gd; X is at least one element selected from the group consisting of Cℓ, Br and I; and x and y are numbers satisfying the conditions of 5x10⁻⁵ ≦ x ≦ 0.5 and 0 < y ≦ 0.2, respectively, as described in JP-A- 55(1980)-160078; (Ba 1-x ,M II x )F₂·aBaX₂:yEu,zA, in which M II is at least one element selected from the group consisting of Be, Mg, Ca, Sr, Zn and Cd; X is at least one element selected from the group consisting of Cℓ, Br and I; A is at least one element selected from the group consisting of Zr and Sc; and a , x , y and z are numbers satisfying the conditions of 0.5 ≦ a ≦ 1.25, 0 ≦ x ≦ 1, 10⁻⁶ ≦ y ≦ 2x10⁻¹, and 0 < z ≦ 10⁻², respectively, as described in JP-A- 56(1981)-116777; (Ba 1-x ,M II x )F₂·aBaX₂:yEu,zB, in which M II is at least one element selected from the group consisting of Be, Mg, Ca, Sr, Zn and Cd; X is at least one element selected from the group consisting of Cℓ, Br and I; and a , x , y and z are numbers satisfying the conditions of 0.5 ≦ a ≦ 1.25, 0 ≦ x ≦ 1, 10⁻⁶ ≦ y ≦ 2x10⁻¹, and 0 < z ≦ 2x10⁻¹, respectively, as described in JP-A- 57(1982)-23673; (Ba 1-x ,M II x )F₂·aBaX₂:yEu,zA, in which M II is at least one element selected from the group consisting of Be, Mg, Ca, Sr, Zn and Cd; X is at least one element selected from the group consisting of Cℓ, Br and I; A is at least one element selected from the group consisting of As and Si; and a , x , y and z are numbers satisfying the conditions of 0.5 ≦ a ≦ 1.25, 0 ≦ x ≦ 1, 10⁻⁶ ≦ y ≦ 2x10⁻¹, and 0 < z ≦ 5x10⁻¹, respectively, as described in JP-A- 57(1982)-23675; M III OX:xCe, in which M III is at least one trivalent metal selected from the group consisting of Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, and Bi; X is at least one element selected from the group consisting of Cℓ and Br; and x is a number satisfying the condition of 0 < x < 0.1, as described in JP-A- 58(1983)-69281; Ba 1-x M x/2 L x/2 FX:yEu²⁺, in which M is at least one alkali metal selected from the group consisting of Li, Na, K, Rb and Cs; L is at least one trivalent metal selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Aℓ, Ga, In and Tℓ; X is at least one halogen selected from the group consisting of Cℓ, Br and I; and x and y are numbers satisfying the conditions of 10⁻² ≦ x ≦ 0.5 and 0 < y ≦ 0.1, respectively, as described in U S Patent Application No. 497,805; BaFX·xA:yEu²⁺, in which X is at least one halogen selected from the group consisting of Cℓ, Br and I; A is at least one fired product of a tetrafluoroboric acid compound; and x and y are numbers satisfying the conditions of 10⁻⁶ ≦ x ≦ 0.1 and 0 < y ≦ 0.1, respectively, as described in U S Patent Application No. 520,215; BaFX·xA:yEu²⁺, in which X is at least one halogen selected from the group consisting of Cℓ, Br and I; A is at least one fired product of a hexafluoro compound selected from the group consisting of monovalent and divalent metal salts of hexafluoro silicic acid, hexafluoro titanic acid and hexafluoro zirconic acid; and x and y are numbers satisfying the conditions of 10⁻⁶ ≦ x ≦ 0.1 and 0 < y ≦ 0.1, respectively, as described in U S Patent Application No. 502,648; BaFX·xNaX′:aEu²⁺, in which each of X and X′ is at least one halogen selected from the group consisting of Cℓ, Br and I; and x and a are numbers satisfying the conditions of 0 < x ≦ 2 and 0 < a ≦ 0.2, respectively, as described in JP-A- 59(1984)-56479; M II FX·xNaX′:yEu²⁺:zA, in which M II is at least one alkaline earth metal selected from the group consisting of Ba, Sr and Ca; 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 < x ≦ 2, 0 < y ≦ 0.2 and 0 < z ≦ 10⁻², respectively, as described in U S Patent Application No. 535,928; and M II FX·aM I X′·bM′ II X″₂·cM III X‴₃·xA:yEu²⁺, in which M II is at least one alkaline earth metal selected from the group consisting of Ba, Sr and Ca; M I is at least one alkali metal selected from the group consisting of Li, Na, K, Rb and Cs; M′ II is at least one divalent metal selected from the group consisting of Be and Mg; M III is at least one trivalent metal selected from the group consisting of Aℓ, Ga, In and Tℓ; A is at least one metal oxide; X is at least one halogen selected from the group consisting of Cℓ, Br and I; each of X′, X″ and X‴ is at least one halogen selected from the group consisting of F, Cℓ, Br and I; a, b and c are numbers satisfying the conditions of 0 ≦ a ≦ 2, 0 ≦ b ≦ 10⁻², 0 ≦ c ≦ 10⁻² and a+b+c ≧ 10⁻⁶; and x and y are numbers satisfying the conditions of 0 < x ≦ 0.5 and 0 < y ≦ 0.2, respectively, as described in U S Patent Application No. 543,326; M II X₂·aM II X′₂:xEu²⁺, in which M II is at least one alkaline earth metal selected from the group consisting of Ba, Sr and Ca; each of X and X′ is at least one halogen selected from the group consisting of Cℓ, Br and I, and X X′; and a and x are numbers satisfying the conditions of 0.1 ≦ a ≦ 10.0 and 0 < x ≦ 0.2, respectively, as described in U S Patent Application No. 660,987; and M II FX·aM I X′:xEu²⁺, in which M II is at least one alkaline earth metal selected from the group consisting of Ba, Sr and Ca; M I is at least one alkali metal selected from the group consisting of Rb and Cs; X is at least one halogen selected from the group consisting of Cℓ, Br and I; X′ is at least one halogen selected from the group consisting of F, Cℓ, Br and I; and a and x are numbers satisfying the conditions of 0 < a ≦ 4.0 and 0 < x ≦ 0.2, respectively, as described in U S Patent Application No. 668,464. The above-mentioned M II X₂·aM II X′₂:xEu²⁺ phosphor described in U S Patent Application No. 660,987 may further contain the following additives in the following amount to 1 mol. of M II X₂·aM II X′₂: bM I X″, in which M I is at least one alkali metal selected from the group consisting of Rb and Cs; X″ is at least one halogen selected from the group consisting of F, Cℓ, Br and I; and b is a number satisfying the condition of 0 < b ≦ 10.0, as described in U S Patent Application No. 699,325; bKX″·cMgX‴₂·dM III X″″₃, in which M III is at least one trivalent metal selected from the group consisting of Sc, Y, La, Gd and Lu; each of X″, X‴ and X″″ is at least one halogen selected from the group consisting of F, Cℓ, Br and I; and b , c and d are numbers satisfying the conditions of 0 ≦ b ≦ 2.0, 0 ≦ c ≦ 2.0 and 0 ≦ d ≦ 2.0, and 2x10⁻⁵ ≦ b+c+d, as described in U S Patent Application No. 723,819; bA, in which A is at least one oxide selected from the group consisting of SiO₂ and P₂O₅; and b is a number satisfying the condition of 10⁻⁴ ≦ b ≦ 2x10⁻¹, as described in U S Patent Application No. 727,972; and yB, in which y is a number satisfying the condition of 2x10⁻⁴ ≦ b ≦ 2x10⁻¹, as described in U S Patent Application No. 727,974. Among the above-described stimulable phosphors, the divalent europium activated alkaline earth metal halide phosphor and rare earth element acitivated rare earth oxyhalide phosphor are particularly preferred, because these show stimulated emission of high luminance. The above-described stimulable phosphors are given by no means to restrict the stimulable phosphor employable in the present invention. Any other phosphors can be also employed, provided that the phosphor gives stimulated emission when excited with stimulating rays after exposure to a radiation. Examples of the binder to be contained in the phosphor layer 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, ethylcellulose, vinylidene chloride-vinyl chloride copolymer, polyalkyl (meth)acrylate, vinyl chloride-vinyl acetate copolymer, polyurethane, cellulose acetate butyrate, polyvinyl alcohol and linear polyester. Particularly preferred are nitrocellulose, linear polyester, polyalkyl (meth)acrylate, a mixture of nitrocellulose and linear polyester, and a mixture of nitrocellulose and polyalkyl (meth)acrylate. These binders may be crosslinked with a crosslinking agent. The phosphor layer and the protective film can be formed on the support, for instance, by the following procedure. In the first place, stimulable phosphor particles and a binder are added to an appropriate solvent, and then they are mixed to prepare a coating dispersion-I comprising the phosphor particles homogeneously dispersed in the binder solution. Examples of the solvent employable in the preparation of the coating dispersion include lower alcohols such as methanol, ethanol, n-propanol and n-butanol; chlorinated hydrocarbons such as methylene chloride and ethylene chloride; ketones such as acetone, methyl ethyl ketone and methyl isobutyl ketone; esters of lower alcohols with lower aliphatic acids such as methyl acetate, ethyl acetate and butyl acetate; ethers such as dioxane, ethylene glycol monoethylether and ethylene glycol monomethyl ether; and mixtures of the above-mentioned compounds. The ratio between the binder and the stimulable phosphor in the coating dispersion may be determined according to the characteristics of the aimed radiation image storage panel and the nature of the phosphor employed. Generally, the ratio therebetween is within the range of from 1 : 1 to 1 : 100 (binder : phosphor, by weight), preferably from 1 : 8 to 1 : 40. The coating dispersion may contain a dispersing agent to improve the dispersibility of the phosphor particles therein, and may contain a variety of additives such as a plasticizer for increasing the bonding between the binder and the phosphor particles in the phosphor layer. Examples of the dispersing agent include phthalic acid, stearic acid, caproic acid and a hydrophobic surface active agent. Examples of the plasticizer include phosphates such as triphenyl phosphate, tricresyl phosphate and diphenyl phosphate; phthalates such as diethyl phthalate and dimethoxyethyl phthalate; glycolates such as ethylphthalyl ethyl glycolate and butylphthalyl butyl glycolate; and polyesters of polyethylene glycols with aliphatic dicarboxylic acids such as polyester of triethylene glycol with adipic acid and polyester of diethylene glycol with succinic acid. The coating dispersion may further contain such a colorant that the mean reflectance thereof in the wavelength region of stimulating rays for the stimulable phosphor is lower than the mean reflectance thereof in the wavelength region of light emitted by the stimulable phosphor upon stimulation thereof, to enhance the sharpness of an image provided by the resulting panel. Examples of the colorant include those disclosed in U S -A- 4,394,581 and U S Patent Application No. 326,642. The coating dispersion may contain such a white powder as described in U S -A- 4,350,893 for the same purpose. In the second place, a binder and an antistatic agent are added to an appropriate solvent and they are mixed to prepare a coating solution-II. Examples of the binder employable for the preparation of the coating solution-II include cellulose derivatives, polyvinyl chloride, polyvinylidene chloride, polyvinyl formal, melamine, a phenol resin and an epoxy resin, as well as the above-mentioned binders. The binder employable in the coating solution-II is required to be substantially incompatible with that used in the coating dispersion-I. For example, when some kinds of binders are employed together, it is not necessary that each of the binders is incompatible with the binder of the coating dispersion-I, but that as a whole the binders are non-compatible therewith. From the viewpoint of the physical strength of the panel surface, the binder of the coating solution-II is preferred to be relatively rigid. As the solvent employable for the coating solution-II, there can be employed the above-mentioned solvents, and the solvent of the coating solution-II may be the same or different from the solvent employed in the coating dispersion-I. Both the solvents employed in the coating solution-II and the coating dispersion-I are desired to be miscible with each other in order to dry superposed layers of the coating solution-II and the coating dipersion-I at the same speed. The coating solution-II may contain a variety of additives such a dispersing agent, a plasticizer and a colorant as employed for the coating dispersion-I. The antistatic agent prevents the static electrification of the surface of the panel. The antistatic agent employable in the invention are inorganic salts of metals. The introduction of the inorganic salt of metal enables electric charge on the panel to be easily transferred outside (statically discharged), whereby the static electrification of the panel is prevented. Namely, the electrical resistance of surface of the panel can be decreased. Representative examples of the inorganic salt of metal employable for the antistatic agent include LiCℓ, NaCℓ, NaBr, NaI, NaNO₃, Na₃PO₄, CsI, MgBr₂, BaBr₂, BaI₂ and AℓBr₃. These metal inorganic salts may be in the state of an anhydride or of a compound containing water of crystallization, provided that the metal salt is stable at room temperature. Among the metal salts, preferred are LiCℓ, NaBr, NaI, NaNO₃, MgBr₂ and AℓBr₃. From the viewpoint of colorlessness and high solubility in solvents such as water and alcohol, etc., particularly preferred are LiCℓ, NaBr, and MgBr₂. These metal salts are so colorless as not to absorb stimulating rays and so soluble in the binder solution as to be uniformly dispersed on the surface of the resulting phosphor layer (or protective film). The amount of the antistatic agent to be contained in the coating solution-II varies depending on the kind thereof, the kind of the binder and the characteristics of the radiation image storage panel. Generally, the antistatic agent is contained in the amount ranging from 0.1 to 20 % by weight of the binder of the coating solution-II, and preferably from 0.5 to 5 %. Namely, the amount of the antistatic agent is generally within the range of from 0.03 to 6 % by weight of both binders of the coating dispersion-I and the coating solution-II, and preferably from 0.1 to 2 %. In this case, such a solvent that the antistatic agent of metal inorganic salt is dissolved in is preferably employed for increasing the dispersibility thereof in the resulting layer. The coating dispersion-I and the coating solution-II are evenly applied onto the surface of a support in simultaneous stage and in the superposed form, in such a manner that the coating dispersion-I is arranged to be placed on the support side to form layers of the coating dispersion and the coating solution. The applying procedure is conducted, for instance, by using a two-hopper type-coating apparatus. The ratio between the coating amount of the dispersion-I and the coating amount of the solution-II varies depending on the characteristics of the aimed radiation image storage panel, the viscosity of the coating dispersion and coating solution, the ratio between the binder and the phosphor, etc. Generally, the ratio therebetween is within the range of from 100 : 1 to 1 : 1 (dispersion -I : solution-II, in volume), preferably from 10 : 1 to 1 : 1. After applying the coating dispersion-I and the coating solution-II to the support, the coating dispersion -I on the support side and the coating solution-II provided thereon are then heated slowly to dryness so as to complete the formation of two layers (namely, a phosphor layer and a protective film). Since both the binders of the coating dispersion-I and the coating solution-II are incompatible with each other, the phosphor layer is formed on the support and further the protective film is formed on the phosphor layer. The phosphor layer together with the protective film can be provided on the support by the methods other than that given in the above. For instance, such two layers are initially prepared on a plane sheet (false support) such as a glass plate, metal plate or plastic sheet by simultaneously applying both the coating dispersion and solution in the superposed form thereonto in the above-mentioned manner, and then the prepared two layers are superposed on the genuine support by pressing or using an adhesive agent. In this case, it is preferable to arrange the coating dispersion-I on the plane sheet side. On the contrary, it is also possible to arrange the coating solution-II on the plane sheet side. Thus, the phosphor layer and the protective film are provided on the support wherein the phosphor layer faces the support. Alternatively, if the phoshor layer is self-supporting, it is not necessary to provide the support. The thickness of the phosphor layer varies depending upon the characteristics of the aimed radiation image storage panel, the nature of the phosphor, the ratio between the binder and the phosphor, etc. Generally, the thickness of the phosphor layer is within the range of from 20 µm to 1 mm, and preferably from 50 to 500 µm. The thickness of the protective film is preferably within the range of from 3 to 20 µm. The ratio between the thichness of the phosphor layer and the thickness of the protective film is preferably within the range of from 100 : 1 to 5 : 1. The support material employed in the present invention can be selected from those employed in the conventional radiographic intensifying screens or those employed in the known radiation image storage panels. Examples of the support material include plastic films such as films of cellulose acetate, polyester, polyethylene terephthalate, polyamide, polyimide, triacetate and polycarbonate; metal sheets such as aluminum foil and aluminum alloy foil; ordinary papers; baryta paper; resin-coated papers; pigment papers containing titanium dioxide or the like; and papers sized with polyvinyl alcohol or the like. From the viewpoint of characteristics of a radiation image storage panel as an information recording material, a plastic film is preferably employed as the support material of the invention. The plastic film may contain a light-absorbing material such as carbon black, or may contain a light-reflecting material such as titanium dioxide. The former is appropriate for preparing a high-sharpness type radiation image storage panel, while the latter is appropriate for preparing a high-sensitivity type radiation image storage panel. In the preparation of a known radiation image storage panel, one or more additional layers are occasionally provided between the support and the phosphor layer, so as to enhance the adhesion between the support and the phosphor layer, or to improve the sensitivity of the panel or the quality of an image (sharpness and graininess) provided thereby. For instance, a subbing layer may be provided by coating a polymer material such as gelatin over the surface of the support on the phosphor layer side. Otherwise, a light-reflecting layer or a light-absorbing layer may be provided by forming a polymer material layer containing a light-reflecting material such as titanium dioxide or a light-absorbing material such as carbon black. In the invention, one or more of these additional layers may be provided on the support. As described in U S Patent Application No. 496,278, the phosphor layer-side surface of the support (or the surface of a subbing layer, light-reflecting layer, or light-absorbing layer in the case that such layers are provided on the phosphor layer) may be provided with protruded and depressed parts for enhancement of the sharpness of the image. The following examples further illustrate the present invention, but these examples are by no means understood to restrict the invention. In those examples, the term "part(s)" means "part(s) by weight", unless specifically mentioned. Example 1 A polyester resin A (Vilon 500, available from Toyobo Co., Ltd.) and a polyester resin B (Vilono 200, available from the same) were dissolved in methyl ethyl ketone to prepare a polyester resin solution. Independently, nitrocellulose (RS-120, available from Daicel Co., Ltd.) was dissolved in methyl ethyl ketone to prepare a nitrocellulose solution. Divalent europium activated barium fluorobromide phosphor particles were dispersed in methyl ethyl ketone. To the dispersion were successively added the polyester resin solution, the nitrocellulose solution, ultramarine blue (pigment; PB-100, available from Daiichi Kasei Co., Ltd.), aliphatic polyisocyanate (Sumidul N-75, available from Sumitomo Bayer Urethane Co., Ltd.) and tricresyl phosphate, and they were mixed by means of a propeller agitator to prepare a homogeneous coating dispersion-I. Subsequently, magnesium bromide (MgBr₂) as an antistatic agent was sufficiently dissolved in methyl ethyl ketone. To the solution were added a polyurethane resin a polyurethane resin (Desmocol 530, available from Sumitomo Bayer Urethane Co., Ltd.), vinyl chloride-vinyl acetate copolymer (UCAR Solution Vinyl VYHH, available from Union Carbide Corp.), nitrocellulose and tricresyl phosphate, and they were mixed by means of a propeller agitator to prepare a homogeneous coating solution-II. The coating dispersion-I and the coating solution-II had the following composition. The coating dispersion-I and the coating solution-II were simultaneously applied in a superposing manner onto a polyethylene terephthalate sheet containing carbon black (support, thickness: 250 µm) placed horizontally on a glass plate using a two-hopper type coating apparatus as shown in Fig. 1. In Fig. 1, the coating dispersion-I was introduced into the right guiding part 2 of the two-hopper type coating apparatus 1, and the coating solution-II was introduced into the left guiding part 3. The width of the openings of the guiding parts 2 and 3 was 0.500 mm and 0.150 mm, respectively. The coating dispersion-I and the coating solution-II were simultaneouly applied in a superposing manner on the support 5, moving the glass plate 4 having the support 5 placed thereon along the direction of indicated arrow 8 at a rate of 1.0 m/min., to form a layer 6 of the coating dispersion-I and a layer 7 of the coating solution-II. After the coating was complete, the support having the coating dispersion-I and the coating solution-II was placed in an oven and heated at a temperature gradually rising from 25 to 100°C for 50 min., to form a phosphor layer having the thickness of approx. 345 µm and a protective film containing the antistatic agent and having the thickness of approx. 7 µm on a support. Thus, a radiation image storage panel consisting of a support, a phosphor layer and a protective film was prepared. Comparison Example 1 The procedure of Example 1 was repeated except for adding no magnesium bromide to the coating solution-II to prepare a radiation image storage panel consisting of a support, a phosphor layer and a protective film. The radiation image storage panels prepared in Example 1 and Comparison Example 1 were evaluated on the surface resistance and the transfering properties according to the following tests. (1) Surface resistance The radiation image storage panel was cut to give a test strip having a size of 110 mm x 110 mm. The test strip was placed on a circular electrode (P-601 type, manufactured by Kawaguchi Electric Co., Ltd.) provided with an insulation resistance tester (EV-40 type super-insulation resistance tester, manufactured by the same) and then the voltage was set up to measure the elecric resistance of the surface of the test strip at a temperature of 23°C and at a humidity of 50 %RH. (2) Transfering properties The radiation image storage panel was repeatedly transferred and piled in a radiation image recording and reproducing apparatus at 100 cycles to measure times at which two panels were simultaneously transferred. The results on the evaluation are shown in Table 1. As is evident from Table 1, the radiation image storage panel of the present invention containing the antistatic agent (Example 1) had the remarkably low surface resistance and was prevented from charging, and as a result the phenomenon of two-panel transfer never occurred to perform the smooth transfering. On the other hand, the radiation image storage panel for comparison containing no antistatic agent (Comparison Example 1) had the high surface resistance and the two-panel transfer occurred at 3 or 4 times. Examples 2 - 11 The procedure of Example 1 was repeated except for using inorganic salts of metals shown in the following Table 2, to prepare radiation image storage panels consisting of a support, a phosphor layer and a protective film. Each of the metal inorganic salts was added to the coating solution-II in the amount of 3 % by weight, except for 2 weight % for lithium chloride and sodium chloride. The radiation image storage panels prepared in Examples 2-11 were evaluated on the above-mentioned surface resistance. The results are set forth in Table 3. As is evident from Table 3, the radiation image storage panels of the present invention containing the antistatic agent (Examples 2-11) had the remarkably low surface resistance and were prevented from charging.

Claims: A radiation image storage panel comprising a support, a phosphor layer which comprises a binder and a stimulable phosphor dispersed therein and a protective film, superposed in this order, characterized in that said protective film contains an antistatic agent of an inorganic salt of a metal. The radiation image storage panel of claim 1, in which said antistatic agent is at least one inorganic salt of a metal selected from the group consisting of LiCl, NaCl, NaBr, NaI, NaNO₃, Na₃PO₄, CsI, MgBr₂, BaBr₂, BaI₂ and AlBr₃. The radiation image storage panel of claim 2, in which said antistatic agent is at least one inorganic salt of a metal selected from the group consisting of LiCl, NaBr, Nal, MgBr₂ and AlBr₃. The radiation image storage panel of claim 1, in which said antistatic agent is contained in the protective film in an amount ranging from 0.1 to 20 % by weight.

Inventors: KITADA AKIRA, YAMAZAKI HISASHI, YAMAZAKI KIKUO, KITADA, AKIRA FUJI PHOTO FILM CO., LTD., YAMAZAKI, HISASHI FUJI PHOTO FILM CO., LTD., YAMAZAKI, KIKUO FUJI PHOTO FILM CO., LTD.
Applicants: FUJI PHOTO FILM CO LTD, FUJI PHOTO FILM CO., LTD.
Application year: 1985
Main classification: G21K4
IPC codes: G21K4