JPH10239498A - Radiosensitizing sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the granularity of a photographic system without loosing the higher sensitivity of a radiosensitizing sheet by a rare earth activated alkali metal fluoride halide phosphor. SOLUTION: A phosphor layer 2 consisting of a rare earth activated alkali earth metal fluoride halide phosphor is provided on a support body 1. As the phosphor particle constituting the phosphor layer 2, a circular rare earth activated alkali earth metal fluoride halide phosphor particle subjected to high- temperature thermal plasma treatment is used. A protecting film 3 is provided on the phosphor layer 2, whereby a radiation intensifying screen 4 is constituted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radiographic intensifying screen used for radiography and the like.

[0002]

2. Description of the Related Art In radiography and the like used for medical diagnosis and industrial nondestructive inspection, a radiographic film is usually used in combination with a radiographic intensifying screen in order to improve the sensitivity of the radiography system. I have. As a radiographic intensifying screen used for X-ray photography or the like, a phosphor intensifying layer and a relatively thin protective film for protecting the phosphor layer are sequentially formed on a support made of paper, plastic, or the like. .

[0003] In recent years, there has been an increasing demand for a reduction in the amount of radiation exposure of a subject in medical diagnosis and the like.
Therefore, for example, in direct X-ray imaging, the radiation exposure of the subject is reduced by using a radiation film or a radiographic intensifying screen with high sensitivity. Such high sensitivity is generally achieved by using a high sensitivity film for the radiation film, and a phosphor having high luminous efficiency is used for the radiation intensifying screen.

[0004] However, the increase in the sensitivity of the radiation film or the radiographic intensifying screen as described above causes the following problems. That is, when using a high-sensitivity film,
Although the sharpness is slightly reduced, the graininess is reduced. On the other hand, when the sensitivity of the radiographic intensifying screen is increased, the reduction in graininess becomes a problem. Here, a rare earth-activated alkaline earth metal fluorinated halide phosphor has attracted attention as a phosphor having high luminous efficiency. However, when such a phosphor is used, a problem of reduction in graininess is particularly problematic. The discrimination ability of a subject in X-ray imaging is related to both graininess and sharpness, and the deterioration of graininess lowers the discrimination ability of a subject having a particularly low contrast.

[0005]

As described above, increasing the sensitivity of a radiographic intensifying screen, that is, using a phosphor having a high luminous efficiency, is effective in reducing the amount of exposure of a subject and improving sharpness. However, there is a problem that the graininess is reduced. In particular, when a rare earth activated alkaline earth metal fluorohalide phosphor is used as the phosphor having high luminous efficiency, there is a problem that the graininess is significantly reduced.

SUMMARY OF THE INVENTION The present invention has been made to address such a problem, and without impairing the enhancement of the sensitivity of a radiographic intensifying screen using a rare earth-activated alkaline earth metal fluorohalide phosphor. It is an object of the present invention to provide a radiographic intensifying screen capable of improving the granularity of an imaging system.

[0007]

Means for Solving the Problems As a result of various studies to achieve the above object, the present inventors have found that a rare earth activated alkaline earth metal fluorinated halide phosphor can be produced by a usual production method. Have become plate-like particles, and it has been found that the plate-like phosphor particles are the main cause of reduction in granularity.

The present invention has been made on the basis of such findings. The radiographic intensifying screen of the present invention is, as described in claim 1, a support and a phosphor formed on the support. In a radiographic intensifying screen comprising a layer and a protective film provided on the phosphor layer, the phosphor layer is composed of rare earth-activated alkaline earth metal fluorohalide phosphor particles having a substantially spherical shape. It is characterized by:

In the radiation intensifying screen of the present invention, substantially spherical rare earth activated alkaline earth metal fluorohalide phosphor particles are used. These substantially spherical phosphor particles are
The light emission of the entire phosphor layer is efficiently and uniformly scattered, and dispersion of the scattered light is suppressed. Therefore, the granularity of the imaging system can be improved. In addition, the sensitivity and sharpness do not impair the inherent properties of the rare earth activated alkaline earth metal fluorohalide phosphor. Therefore, it is possible to satisfy both sensitivity and sharpness and granularity.

[0010]

Embodiments of the present invention will be described below.

FIG. 1 is a sectional view showing the structure of an embodiment of the radiation intensifying screen of the present invention. In FIG. 1, reference numeral 1 denotes a support made of a plastic film, a nonwoven fabric, or the like, and a phosphor layer 2 is provided on one surface of the support 1. A protective film 3 made of a plastic film, a plastic coating film, or the like is provided on the phosphor layer 2.
These constitute a radiation intensifying screen 4, for example, an intensifying screen for X-ray photography.

The above-described phosphor layer 2 is made of a substantially spherical rare earth activated alkaline earth metal fluorohalide phosphor particle. The rare earth activated alkaline earth metal fluorinated halide phosphor is particularly excellent in luminous efficiency. General formula: Ba _1-x M _x FX: yEu (where M is at least one selected from Sr and Ca)
X represents at least one species selected from Cl and Br, and x and y represent 0 ≦ x ≦ 0.5 and 0 <y ≦, respectively.
0.2) is preferably used. Note that x in the above general formula
When the values of y and y are out of the above ranges, the luminous efficiency decreases.

By using the substantially spherical rare earth activated alkaline earth metal fluorohalide phosphor particles for the phosphor layer 2, the phosphor layer 2 can be used without impairing the high luminous efficiency inherent in the phosphor. It is possible to efficiently and uniformly scatter the light emitted from the phosphor particles in the inside, and to suppress the dispersion of the scattered light from the phosphor layer 2, so that the granularity is maintained while maintaining the sensitivity and sharpness. Can be improved.

The average particle diameter of the substantially spherical rare earth activated alkaline earth metal fluorohalide phosphor particles constituting the phosphor layer 2 is preferably in the range of 0.1 to 20 μm. If the average particle size of the phosphor particles is less than 0.1 μm, there is a possibility that a decrease in emission luminance and a decrease in the formability of the phosphor layer 2 may be caused. On the other hand, the average particle size of the phosphor particles is 20 μm
If it exceeds, the sharpness characteristic may be reduced.

The rare earth-activated alkaline earth metal fluorohalide phosphor particles may have a substantially spherical particle shape. The smaller the number of objects and the more uniform the particle size, the more efficiently and uniformly the light emitted from the phosphor particles can be scattered in the phosphor layer 2 as a whole.
For this reason, it is preferable that the ratio of the major axis to the minor axis (aspect ratio: major axis / minor axis) of the substantially spherical phosphor particles is 1.2 or less, and the peripheral length of the projected image of each particle is L (μ).
m), when the actual area of the projected image was ^{A (μm 2), L 2} /4
The shape factor R represented by πA is preferably 1.5 or less.

That is, when the aspect ratio of the substantially spherical phosphor particles exceeds 1.2, the uniformity of the phosphor layer 2 is impaired, the uniformity of scattered light is reduced, and as a result, the graininess may be reduced. is there. Here, the aspect ratio tends to evaluate the sphericity of particles such as ellipsoids low, and is effective as a parameter for evaluating the overall shape of particles, but for example, fine projections and the like are present on the particle surface. Even
The projections themselves have little effect on the aspect ratio.

However, if fine projections or the like are present on the surface of the rare earth activated alkaline earth metal fluorohalide phosphor particles, the uniformity of the scattered light is reduced, so that the aspect ratio of 1.2 or less is satisfied. And the above-mentioned form factor R
Is preferably 1.5 or less. The form factor R described above
For example, as shown in FIG. 2 (a), even if the particles have a high sphericity as a whole shape, a large value (large irregular shape) is obtained when fine projections and the like are present on the surface. On the other hand, as shown in FIG. 2B, if the surface is relatively smooth, even a particle having a slightly lower sphericity has a lower value. That is, that the shape factor R is small means that the particle surface is smooth (partial irregularity is small), and the shape factor R is a parameter effective for evaluating the partial shape of the particle. If the shape factor R exceeds 1.5, the scattered light from the phosphor layer 2 is likely to be dispersed, so that the effect of improving the granularity may be reduced, and the sensitivity and sharpness may be reduced. As described above, the aspect ratio effective for evaluating the overall shape of the particles is 1.2 or less, and the shape factor R effective for determining the presence or absence of the fine protrusions on the particle surface described above is
By forming the phosphor layer 2 with rare earth activated alkaline earth metal fluorohalide phosphor particles having a spherical shape of 1.5 or less, the granularity can be improved while maintaining the sensitivity and sharpness. Becomes

The substantially spherical rare earth activated alkaline earth metal fluorohalide phosphor particles as described above can be produced, for example, as follows. That is, a rare earth activated alkaline earth metal fluorohalide phosphor particle that is a raw material is melted and then rapidly cooled to obtain a substantially spherical rare earth activated alkaline earth metal fluorohalide phosphor particle. Specifically, for example, a method is used in which the raw material phosphor particles are supplied into a high-temperature thermal plasma together with a carrier gas, and after a short time, are taken out of the thermal plasma and rapidly cooled. Here, thermal plasma means a high-temperature ionization state of gas, which can be generated by high-frequency electromagnetic waves of several to several tens of megahertz or gas discharge by DC current. The temperature reaches C.

When melting in a thermal plasma is applied to the preparation of a substantially spherical rare earth activated alkaline earth metal fluorohalide phosphor particle, the thermal plasma is used to avoid evaporation of the phosphor at a high temperature. It is desirable to adjust the output of the raw material, the supply position of the raw material phosphor, and the like, and to put the raw material phosphor around the frame portion whose temperature is lower than that of the high-temperature plasma frame portion at several thousands to tens of thousands of degrees C. And while controlling the conditions of the above-mentioned manufacturing process, and performing shape classification and sieving as needed, a substantially spherical rare earth activated alkaline earth having the above-mentioned shape and particle diameter is performed. Metal fluorohalide phosphor particles are obtained.

The radiation intensifying screen 4 of the above-described embodiment is produced, for example, as follows. That is, first, an approximately spherical rare earth activated alkaline earth metal fluorohalide phosphor particles are mixed with a binder in an appropriate amount, and an organic solvent is added thereto to prepare a phosphor coating solution having an appropriate viscosity.
The phosphor coating solution is applied onto the support 1 by a knife coater, a roll coater, or the like, and dried to form the phosphor layer 2. Some intensifying screens have a structure in which a light reflecting layer, a light absorbing layer, a metal foil layer, and the like are provided between the support 1 and the phosphor layer 2; A light-reflecting layer, a light-absorbing layer, a metal foil layer, and the like may be formed on 1, and a phosphor coating solution may be applied thereon and dried to form the phosphor layer 2.

Examples of the binder used for preparing the phosphor coating solution include nitrified cotton, cellulose acetate, ethyl cellulose, polyvinyl butyral, flocculent polyester, polyvinyl acetate, vinylidene chloride-vinyl chloride copolymer, vinyl chloride-vinyl acetate copolymer, Conventionally used materials such as polyalkyl (meth) acrylate, polycarbonate, polyurethane, cellulose acetate butyrate, and polyvinyl alcohol are exemplified. As the organic solvent, for example, ethanol, methyl ethyl ether, butyl acetate, ethyl acetate, ethyl ether, xylene and the like are used. In addition, a dispersing agent such as phthalic acid and stearic acid and a plasticizer such as triphenyl phosphate and diethyl phthalate can be added to the phosphor coating solution, if necessary.

Examples of the support 1 include cellulose acetate, cellulose propionate, cellulose acetate butyrate, polyesters such as polyethylene terephthalate, polystyrene, polymethyl methacrylate, polyamide, and the like.
A resin formed of a resin such as polyimide, vinyl chloride-vinyl acetate copolymer, or polycarbonate can be used.

Then, by forming a protective film 3 on the above-mentioned phosphor layer 2, the intended radiation intensifying screen 4
Is obtained. As the protective film 3, a transparent resin film such as polyethylene terephthalate, polyethylene, polyvinylidene chloride, or polyamide can be used. The protective film 3 made of these transparent resin films is laminated on the phosphor layer 2. Cellulose acetate, nitrocellulose, cellulose derivatives such as cellulose acetate butyrate, polyvinyl chloride, polyvinyl acetate, vinyl chloride-
A protective film coating solution having a suitable viscosity obtained by dissolving a resin such as a vinyl acetate copolymer, polycarbonate, polyvinyl butyral, polymethyl methacrylate, polyvinyl formal, or polyurethane in a solvent is applied on the phosphor layer 2,
The radiation intensifying screen 4 can also be obtained by drying.

In the radiation intensifying screen 4 of the above-described embodiment, the phosphor layer 2 is made of a rare earth activated alkaline earth metal fluorohalide phosphor having high luminous efficiency, and is derived from the characteristics of the phosphor. After obtaining excellent sensitivity characteristics and sharpness characteristics, by making the particle shape of the phosphor substantially spherical, the granularity can be improved. That is, it is possible to improve the granularity while maintaining excellent sensitivity characteristics and sharpness characteristics. Therefore, for example, when used for medical X-ray photography or the like, good diagnostic performance can be obtained while reducing the amount of X-ray exposure to the subject. Also,
When used for industrial nondestructive inspection, etc., it is possible to improve the inspection accuracy while reducing the X-ray dose.

[0025]

Next, specific examples of the present invention will be described.

Example 1 First, substantially spherical BaFCl: Eu phosphor particles were prepared as rare earth activated alkaline earth metal fluorohalide phosphor particles as follows.

That is, BaF ₂ particles were suspended in an aqueous solution of BaCl ₂ containing BaCl _{2 in} excess of the stoichiometric composition of BaFCl, and thoroughly stirred until BaF ₂ became BaFCl. It was then filtered to remove water and excess BaCl ₂
Was removed. The resulting BaFCl was again suspended in water, and stirred well by adding EuF ₂ and EuCl ₃ suitable amount thereto. This was filtered again, and the obtained powder was dried.

Next, the above-mentioned powder is placed in a nitrogen atmosphere for 7 minutes.
It was fired at 00 ° C. After cooling, the powder is washed with water and BaFCl: E
u phosphor was obtained. Then, the BaFCl: Eu phosphor particles are converted to 4 MHz, 6 kW using argon gas as a carrier gas.
The resultant was supplied to the peripheral portion of the high-frequency plasma of the output, melted, and rapidly cooled to obtain substantially spherical BaFCl: Eu phosphor particles having an average particle diameter of 3.5 μm. When the aspect ratio and the shape factor R of the substantially spherical BaFCl: Eu phosphor particles were evaluated by image processing, the aspect ratio was 1.2 or less and the shape factor R was 1.5 or less.

The above-mentioned substantially spherical BaFCl: Eu phosphor particles (particles) were added to 10 parts by weight of vinyl chloride as a binder.
A phosphor coating solution was prepared by mixing 1 part by weight of a vinyl acetate copolymer and an appropriate amount of ethyl acetate as an organic solvent.
This phosphor coating solution is mixed with carbon black
The phosphor was uniformly coated on a 250 μm polyethylene terephthalate film support with a knife coater so that the dried phosphor coating weight was 50 mg / cm ^2, and dried to form a phosphor layer. A protective film made of a polyethylene terephthalate film having a thickness of 9 μm was laminated on the phosphor layer to produce an intended X-ray intensifying screen (X-ray front intensifying screen). This X-ray intensifying screen was subjected to characteristic evaluation described later.

Comparative Example 1 The procedure of Example 1 was repeated except that the BaFCl: Eu phosphor that had not been subjected to the high-frequency plasma treatment, ie, the BaFCl: Eu phosphor powder after firing was used in the process of producing the phosphor particles of Example 1. In the same manner, an X-ray intensifying screen was prepared and subjected to the characteristic evaluation described later. The BaFCl: Eu phosphor used here was composed of plate-like particles, and its aspect ratio and shape factor R were evaluated by image processing. As a result, the aspect ratio was 2 or more and the shape factor R was 3 or more.

The sensitivity, sharpness, and granularity of each of the X-ray intensifying screens of Example 1 and Comparative Example 1 were measured and evaluated using a regular type film (A, manufactured by Konica Corp.). Table 1 shows the results. The photographic performance of the intensifying screen is the photographic sensitivity, sharpness, and granularity when photographed with a tube phantom of 120 kV through a water phantom with a thickness of 100 mm, and the photographic sensitivity is the intensifying screen of Comparative Example 1. And the sharpness are M at spatial frequency 2 / mm
The TF value was determined, the relative value when the MTF value of the intensifying screen of Comparative Example 1 at the spatial frequency was 100, and the graininess was the relative RMS value at a photographic density of 1.0 and a spatial frequency of 3.12 lines / mm.

[0032]

[Table 1] As is clear from Table 1, the X-ray intensifying screen of Example 1 has significantly improved granularity, and further improved sensitivity and sharpness, as compared with the intensifying screen of Comparative Example 1. You can see that there is.

Example 2 First, substantially spherical BaFBr: Eu phosphor particles were prepared as rare earth activated alkaline earth metal fluorohalide phosphor particles as follows.

That is, BaF ₂ particles were suspended in an aqueous solution of BaBr ₂ containing BaBr _{2 in} excess of the stoichiometric composition of BaFBr, and thoroughly stirred until BaF ₂ became BaFBr. It is then filtered and water and excess BaBr ₂
Was removed. The resulting BaFBr was again suspended in water, and stirred well by adding EuF ₂ and EuBr ₃ suitable amount thereto. This was filtered again, and the obtained powder was dried.

Next, the above-mentioned powder is placed in a nitrogen atmosphere for 7 minutes.
It was fired at 00 ° C. After cooling, the powder is washed with water and BaFBr: E
u phosphor was obtained. The BaFBr: Eu phosphor particles are converted to 4 MHz, 6 kW using argon gas as a carrier gas.
By supplying the high-frequency plasma to the peripheral portion of the output, melting and quenching, substantially spherical BaFBr: Eu phosphor particles having an average particle diameter of 4.0 μm were obtained. When the aspect ratio and shape factor R of the substantially spherical BaFBr: Eu phosphor particles were evaluated by image processing, the aspect ratio was 1.2 or less and the shape factor R was 1.5 or less.

The above-mentioned approximately spherical BaFBr: Eu phosphor particles (particles) were added to 10 parts by weight of vinyl chloride
A phosphor coating solution was prepared by mixing 1 part by weight of a vinyl acetate copolymer and an appropriate amount of ethyl acetate as an organic solvent.
This phosphor coating solution is mixed with carbon black
The phosphor was uniformly coated on a 250 μm polyethylene terephthalate film support with a knife coater so that the dried phosphor coating weight was 50 mg / cm ^2, and dried to form a phosphor layer. A protective film made of a polyethylene terephthalate film having a thickness of 9 μm was laminated on the phosphor layer to produce an intended X-ray intensifying screen (X-ray front intensifying screen). This X-ray intensifying screen was subjected to characteristic evaluation described later.

Comparative Example 2 The procedure of Example 1 was repeated except that the BaFBr: Eu phosphor that had not been subjected to the high-frequency plasma treatment, that is, the BaFBr: Eu phosphor powder after firing was used in the manufacturing process of the phosphor particles of Example 1. In the same manner, an X-ray intensifying screen was prepared and subjected to the characteristic evaluation described later. The BaFBr: Eu phosphor used here was composed of plate-like particles, and its aspect ratio and shape factor R were evaluated by image processing. As a result, the aspect ratio was 2 or more and the shape factor R was 3 or more.

The sensitivity, sharpness, and graininess of each of the X-ray intensifying screens of Example 2 and Comparative Example 2 were measured using a regular type film (A, manufactured by Konica Corporation).
Measurement and evaluation were performed in the same manner as in Example 1. Table 2 shows the results.
Shown in

[0039]

[Table 2] As is clear from Table 2, the X-ray intensifying screen according to Example 2 has significantly improved granularity, and further improved sensitivity and sharpness, as compared with the intensifying screen according to Comparative Example 2. You can see that there is.

[0040]

As described above, according to the radiographic intensifying screen of the present invention, while maintaining the sensitivity characteristics and sharpness characteristics of the rare earth-activated alkaline earth metal fluorinated halide phosphor, the granularity is improved. Can be improved. Therefore, it is possible to obtain a high-quality radiation image with low radiation dose and excellent granularity.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a configuration of an embodiment of a radiographic intensifying screen of the present invention.

FIG. 2 is a view for explaining the shape of substantially spherical phosphor particles used in the radiographic intensifying screen of the present invention.

[Description of Signs] 1... Support 2... Phosphor layer composed of substantially spherical rare earth activated alkaline earth metal fluorohalide phosphor particles 3... Protective film 4.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masaaki Tamaya 1st address, Komukai Toshiba-cho, Saiwai-ku, Kawasaki-shi, Kanagawa Prefecture Inside the Toshiba Research and Development Center Co., Ltd. Nagoya Matsuda, Toshiba R & D Center, Ltd. (72) Inventor: Naotoshi Matsuda 1 Toshiba, Komukai Toshiba-cho, Koyuki-ku, Kawasaki, Kanagawa Prefecture: (72) Miwa Fukasawa, Kawasaki-shi, Kanagawa 1, Komukai Toshiba-cho, Ward Inside Toshiba R & D Center

Claims (4)

[Claims] 1. A radiographic intensifying screen comprising a support, a phosphor layer formed on the support, and a protective film provided on the phosphor layer, wherein the phosphor layer is substantially A radiographic intensifying screen comprising spherical spherical rare earth activated alkaline earth metal fluorohalide phosphor particles. 2. The radiographic intensifying screen according to claim 1, wherein the rare earth activated alkaline earth metal fluorohalide phosphor has a general formula: Ba _1-x M _x FX: yEu (where M is At least one selected from Sr and Ca
X represents at least one species selected from Cl and Br, and x and y represent 0 ≦ x ≦ 0.5 and 0 <y ≦, respectively.
A radiation intensifying screen having a composition substantially represented by the following formula: 3. The radiographic intensifying screen according to claim 1, wherein the ratio of the major axis to the minor axis of the substantially spherical rare earth activated alkaline earth metal fluorohalide phosphor particles is 1.2 or less. Radiation intensifying screen. 4. The radiographic intensifying screen according to claim 1, wherein the substantially spherical rare earth activated alkaline earth metal fluorohalide phosphor particles have a peripheral length of a projected image of each of the particles L,
A radiographic intensifying screen, wherein a shape factor R represented by L ² / 4πA is 1.5 or less, where A is the actual area of the projected image.