JPH0359951B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a radiation intensifying screen (hereinafter abbreviated as "intensifying screen"). More specifically, the present invention relates to an intensifying screen that uses a rare earth oxysulfide phosphor as a phosphor layer and has good photographic image quality, particularly graininess. As is well known, intensifying screens are used for medical diagnosis.
It is used in close contact with X-ray film in order to improve the sensitivity of the imaging system in various fields such as medical radiography such as radiography and industrial radiography for the purpose of non-destructive testing of materials. This intensifying screen basically consists of a support such as paper or plastic and a phosphor layer provided on one side of the support. The phosphor layer is made by dispersing a phosphor that emits high-intensity light when exposed to radiation in a binder resin, and the surface of this phosphor layer (the surface opposite to the support) is generally made of nitrocellulose. They are often protected by a thin transparent protective film such as a polymethacrylate film or a polyethylene terephthalate film. CaWO ₄ has been used as a phosphor for intensifying screens for a long time, but in recent years there has been an increasing demand for reducing the radiation exposure of subjects, and for this reason it has become necessary to improve the sensitivity of the intensifying screen-X-ray film system. In order to increase the photographic sensitivity of the intensifying screen, we used a high-sensitivity type intensifying screen whose phosphor layer uses a phosphor that absorbs more X-rays and has higher light conversion efficiency than the conventional CaWO ₄ phosphor. An intensifying screen was developed. Among them, rare earth oxysulfide phosphors (hereinafter referred to as
An intensifying screen using a phosphor layer (simply referred to as "rare earth oxysulfide phosphor") is widely used as a high-sensitivity type intensifying screen. By the way, it is preferable for an intensifying screen to have high sensitivity to radiation (that is, high light conversion efficiency and high photographic sensitivity) and good photographic image quality such as graininess and sharpness. An intensifying screen using a sulfide phosphor as the phosphor layer has significantly improved sensitivity compared to an intensifying screen using a CaWO ₄ phosphor, but graininess increases (deteriorates).
In addition, there is a slight afterglow, so when high-speed continuous X-ray photography is performed using this intensifying screen, an afterimage will appear on the photograph due to the afterglow of the intensifying screen. , which may cause problems in diagnosis, has been desired to be improved. The present invention has been made in view of the above circumstances, and provides an intensifying screen that improves the graininess of the conventional intensifying screen using a rare earth oxysulfide phosphor as a phosphor layer and further reduces afterglow. The purpose was to provide paper, and various studies were conducted on the relationship between impurities added to rare earth oxysulfide phosphors used as the phosphor layer of intensifying screens and the photographic image quality of the resulting intensifying screens. As a result, terbium (Tb) and neodymium (Nd) were used as the phosphor layer of the intensifying screen.
The present inventors have discovered that the above object can be achieved by using a rare earth oxysulfide phosphor coactivated with holmium (Ho) and/or holmium (Ho), and have completed the present invention. The intensifying screen of the present invention essentially consists of a support and a phosphor layer provided on the support, in which the phosphor layer has a composition formula (Ln _1-xy , Tb _x , R _y ) ₂ O ₂ S (However, Ln is at least one of La, Gd, Y, and Lu, R is at least one of Nd and Ho, and x and y are each 0.001≦x
The number satisfies the following conditions: ≦0.02, 0.001≦y≦0.01, and x≧y. (The same applies hereinafter) It is characterized by being made of a rare earth oxysulfide phosphor represented by The intensifying screen of the present invention has significantly improved graininess when compared with a conventional intensifying screen having a phosphor layer made of a rare earth oxysulfide phosphor activated with terbium only, when compared with the same sensitivity. In addition, afterglow is reduced, there is less deterioration in sharpness, and the sensitivity is significantly higher than that of conventional intensifying screens using CaWO ₄ phosphor. The present invention will be explained in more detail below. The intensifying screen of the present invention is different from the conventional intensifying screen except that a rare earth oxysulfide phosphor represented by the composition formula (Ln _1-xy , Tb _x , R _y ) ₂ O ₂ S is used as the phosphor layer. Manufactured in almost the same way. That is, first, an appropriate amount of (Ln _1-xy , Tb _x , R _y ) ₂ O ₂ S phosphor and a binder resin such as nitrified cotton is mixed, and then an appropriate amount of a solvent is added to obtain a phosphor with an optimum viscosity. A phosphor coating solution is prepared, and this phosphor coating solution is applied onto a support using a roll coater, knife coater, etc., and dried to form a phosphor layer. Some intensifying screens have a structure that has a light-reflecting layer such as a white pigment layer, a light-absorbing layer such as a black pigment layer, or a metal foil layer between the phosphor layer and the support. Accordingly, a light reflecting layer, a light absorbing layer or a metal foil layer is provided on the support in advance, and then (Ln _1-xy ,
A phosphor layer made of Tb _x , R _y ) ₂ O ₂ S phosphor may be formed. Next, if necessary, an appropriate amount of a solvent is added to a resin such as polyvinyl chloride, polyethylene, cellulose acetate, or polyacrylate to obtain a protective film coating solution with an optimal viscosity, and this is applied onto the previously formed phosphor layer. and dries to form a transparent protective film. It goes without saying that the phosphor layer and the transparent protective film may be prepared separately, and then the phosphor layer and the protective film may be laminated in this order on the support and bonded. Generally, the graininess and sharpness of an intensifying screen are contradictory performance factors, and improving the graininess tends to decrease the sharpness, but when manufacturing the intensifying screen of the present invention, the graininess In order to improve the quality and at the same time minimize the decrease in sharpness, the average particle diameter of the (Ln _1-xy , Tb _x , _Ry ) ₂ O ₂ S phosphor used is 2 to 15 μm, more preferably 3 μm. ~10 μm, and the standard deviation value (quartile deviation value) of the particle size distribution
QD) is preferably 0.40 or less, and the coating weight of the phosphor layer is preferably 10 to 100 mg/cm ² in the dry state. Furthermore, in order to further reduce the decrease in sharpness, it is preferable to provide a light absorption layer consisting of a black pigment layer between the support and the phosphor layer, and when a protective film is provided on the phosphor layer, Its thickness is 3-15μm, more preferably 3μm
It is desirable that the thickness be ~10 μm. Figure 1 shows the co-activator (R) of the rare earth oxysulfide phosphor {(Ln _1-xy , Tb _x , R _y ) ₂ O ₂ S} used as the phosphor layer in an intensifying screen. The relationship between the content (y) and the graininess of an intensifying screen using this phosphor is exemplified based on the results of measurement with the photographic sensitivity of each intensifying screen constant, and curves a, b, and c are respectively (Gd _{0.99 -y} , _{Tb 0.01} ,
Nd _y ) ₂ O ₂ S phosphor, ( _{Gd 0.99-y, Tb 0.01 , Ho y ) 2} O ₂ S
Fluorescent material and ₍ Gd _{0.99 -y} , Tb _0.01 , Nd _y/2 , Ho _y/2 )
The case of an intensifying screen using ₂ O ₂ S phosphor is shown. In Figure 1, the vertical axis is the graininess index [G], and the horizontal axis is the rare earth oxysulfide phosphor used in _each intensifying screen {(Gd _{0.99 -y} , Tb _0.01 , R _y ) ₂ The content (y) of the co-activator (R) contained in O ₂ S} is shown.
Here, the granularity index [G] refers to the value of the conventional intensifying screen using a rare earth oxysulfide phosphor {(Gd _0.99 , Tb _0.01 ) ₂ O ₂ S} that does not contain a co-activator (R).
The RMS value is [RMS(o)], and a rare earth oxysulfide phosphor with the same photographic sensitivity and containing a coactivator (R) {(Gd _0.99-y , Tb _0.01 , R _y ) ₂ O ₂ S}
When the RMS value of the intensifying screen using [RMS (y)] is (photo density 1.0, RMS value at spatial frequency 0.5 to 5 lines/mm) [G] = [RMS (o) / [RMS (y)] )] × 100, and the smaller the RMS value, the better the graininess.The larger the value of the graininess index [G] defined here, the better the coactivator (R). This means that the graininess is better than that of a conventional intensifying screen using a rare earth oxysulfide phosphor. As is clear from Figure 1,
By adding a co-activator (R) to the rare earth oxysulfide phosphor used in the phosphor layer, the granularity of the resulting intensifying screen is improved, and moreover, the co-activator (R) Until the addition amount (y) reaches a certain amount, as the y value increases, the graininess of the resulting intensifying screen gradually improves, that is, the y value increases.
Graininess is sufficiently improved when the y value increases to 0.001 or more, and the degree of this improvement increases with the y value, but when the y value exceeds 0.01, no further improvement is obtained as the y value increases. I understand. Figure 2 shows the content (y) of the co-activator (R) in the rare earth oxysulfide phosphor {(Ln _1-xy , Tb _x , R _y ) ₂ O ₂ S} used in the intensifying screen. The relationship between this phosphor and the sharpness of an intensifying screen using this phosphor is exemplified by measuring the photographic sensitivity of each intensifying screen at a constant value, and curves a, b, and c are, respectively, (Gd _0.99-y , Tb _0.01 , Nd _y ) ₂ O ₂ S phosphor,
Enhancement using (Gd _0.99-y , Tb _0.01 , Ho _y ) ₂ O ₂ S phosphor and (Gd _0.99-y , Tb _0.01 , Nd _y/2 , Ho _y/2 ) ₂ O ₂ S phosphor The case of photosensitive paper is shown. In Figure 2, the vertical axis is the sharpness index [M], and the horizontal axis is the rare earth oxysulfide phosphor used in each intensifying screen {(Gd _0.99-y , Tb _0.01 , _Ry ) ₂ O ₂ S} The content (y) of the co-activator (R) contained therein is shown. Here, the sharpness index [M] refers to a rare earth oxysulfide phosphor that does not contain a co-activator (R) {(Gd _0.99 , Tb _0.01 )
₂ O ₂ S} is the MTF value of the conventional intensifying screen [MTF(o)], and a rare earth oxysulfide phosphor { (Gd _0.99-y , Tb _0.01 , _Ry ) ₂ O ₂ S} When the MTF value of an intensifying screen using [MTF(y)] is (all MTF values at a spatial frequency of 2 lines/mm) [ M] = [MTF (y) / [MTF (o)] × 100 The value is defined as: The larger the MTF value, the better the sharpness, so the sharpness index [M] defined here The smaller the value, the more coactivator (R)
This means that the sharpness is worse than that of conventional intensifying screens that use rare earth oxysulfide phosphors that do not contain. As is clear from Figure 2, the sharpness of the intensifying screen using the rare earth oxysulfide phosphor added with the co-activator (R) increases as the content (y) of the co-activator (R) increases. Then, the sharpness gradually decreases, but within a certain content range, the decrease in sharpness is small, and the y value decreases.
Values up to 0.01 are well tolerated. _Figure 3 shows _{the content ( y} ) and the afterglow characteristics of intensifying screens using this phosphor are shown based on the results of measurements made with the photographic sensitivity of each intensifying screen constant; curves a, b, and c are respectively (Gd _0.99-y , Tb _0.01 , Nd _y ) ₂ O ₂ S phosphor, (Gd _0.99-y ,
Tb _0.01 , Ho _y ) ₂ O ₂ S phosphor and (Gd _0.99-y ,
This shows an intensifying screen using Tb _0.01 , Ndy _/2 , Ho _y/2 ) ₂ O ₂ S phosphor. The amount of afterglow on each intensifying screen is determined by irradiating each intensifying screen with X-rays under the same conditions, and after 1 second, placing it in close contact with an X-ray film in a dark place and holding it for 30 minutes. It was determined relatively by measuring the degree of blackening. In Figure 3, the vertical axis shows the afterglow index [L], and the horizontal axis shows the content (y) of the co-activator (R) in the rare earth oxysulfide phosphor used in each intensifying screen. . Here, the afterglow index [L] refers to the rare earth oxysulfide phosphor {(Gd _0.99 ,
The amount of afterglow of a conventional intensifying screen using Tb _0.01 ) ₂ O ₂ S} is taken as [L(o)], and a rare earth oxysulfate with the same photographic sensitivity and containing a co-activator (R) When the amount of afterglow of an intensifying screen using an id phosphor {(Gd _0.99-y , Tb _0.01 , _Ry ) ₂ O ₂ S} is [L(y)], [L] = [L(y)] ] / [L(o)] × 100. Therefore, the smaller the value of [L] defined here, the higher the rare earth oxysulfide fluorescence that does not contain the co-activator (R). This means that the afterglow is shorter than that of conventional intensifying screens that use a body.
As is clear from Fig. 3, by adding a small amount of co-activator (R), there is a greater benefit compared to the conventional intensifying screen using rare earth oxysulfide phosphor that does not contain co-activator (R). The afterglow of the intensifying screen significantly decreases, and when the y value becomes 0.001 or more, the afterglow is hardly recognized. Note that FIGS. 1 to 3 show rare earth oxysulfide phosphors {(Ln _1-xy , Tb _x , _Ry ) ₂ O ₂ S} used as the phosphor layer of the intensifying screen of the present invention. from inside
Although the example is given only for the phosphor where Ln=Gd and x=0.01, if Ln is other than Gd or x is
Rare earth oxysulfide fluorescer used also in the intensifying screen of the present invention using rare earth oxysulfide phosphor {(Ln _1-xy , Tb _x , _Ry ) ₂ O ₂ S} having a value other than 0.01 Co-activator (R) contained in the body
The relationship between the content (y) and the granularity, sharpness, and afterglow characteristics of the obtained intensifying screen is Ln = Gd, x = 0.01
It was confirmed that the trend was almost the same as when using a rare earth oxysulfide phosphor. Tb content (x) of the rare earth oxysulfide phosphor {(Ln _1-xy , Tb _x , R _y ) ₂ O ₂ S} used in the phosphor layer of the intensifying screen of the present invention
From the point of view of sensitivity, is preferably in the range of 0.001≦x≦0.02, similar to conventional rare earth oxysulfide phosphors for intensifying screens that do not contain co-activator (R).
The content (y) of the co-activator (R) is shown in Figures 1 and 2.
As can be seen from the figure and Fig. 3, the graininess of the intensifying screen obtained can be improved by increasing the content (y) of the co-activator (R), but when y exceeds 0.01, the sharpness is significantly reduced. On the other hand, if the co-activator (R) content (y) is low, the sharpness can be prevented from decreasing, but if y is less than 0.001, the graininess will not be improved much and afterglow will be observed. From a practical standpoint, it is preferable that the range is 0.001≦y≦0.01. Although it depends on the content (x) of the activator (Tb), if the content (y) of the co-activator (R) is greater than the content (x) of the activator (Tb), the Since the photographic sensitivity of the intensifying screen significantly decreases, the content (y) of the co-activator (R) should be equal to or less than the content (y) of the activator (Tb) (y≦x). is more preferable. As explained above, the intensifying screen of the present invention is made of CaWO ₄
Significantly more sensitive than conventional intensifying screens using phosphors, and much sharper than conventional intensifying screens for high-sensitivity systems using rare earth oxysulfide phosphors activated with only terbium. The graininess is significantly improved without reducing the graininess, and the afterglow is also reduced compared to conventional ones.
It has great industrial utility value as a high-sensitivity type intensifying screen that can further improve diagnostic performance. Next, the present invention will be explained with reference to examples. Example: Any one of the 13 types of rare earth oxysulfides whose phosphor composition, average particle diameter, and standard deviation value (quartile deviation value QD) are listed in (1) to (13) in the table below. Intensifying screens (1) to (13) were produced in exactly the same manner as described below, except that a phosphor was used. A phosphor coating solution was prepared by mixing 8 parts by weight of rare earth oxysulfide phosphor, 1 part by weight of nitrified cotton, and an organic solvent. This phosphor coating solution is uniformly coated using a knife coater on a polyethylene terephthalate support having a carbon black light-absorbing layer on the surface so that the phosphor coating weight is as described below, and dried to fluoresce. A body layer was prepared. Then,
A protective film coating solution made by dissolving cellulose acetate in a solvent was uniformly applied to the surface of the phosphor layer so that the film thickness after drying was approximately 5 μm, and dried to form a transparent protective film. . On the other hand, for comparison, we used a rare earth oxysulfide phosphor whose phosphor composition, average particle diameter, and standard deviation value (quarter deviation value QD) are listed in (STD) in the table below. The phosphor coating weight is 30
An intensifying screen (STD) was produced in exactly the same manner as above except that the concentration was mg/cm ² . 13 types of intensifying screens obtained as described above (1) ~
(13) and an intensifying screen (STD) manufactured as a comparative example were combined with an orthotype X-ray film and the photographic sensitivity, graininess index [G] and sharpness index [M] were measured. An intensifying screen that gives similar results and has almost the same photographic sensitivity (1)
Looking at items - (3) and (5) - (13), the graininess was better than that of the conventional intensifying screen (STD), and there was less deterioration in sharpness. However, intensifying screen (4)
The graininess was not improved that much, and the third
Afterglow was clearly observed from the L value corresponding to the y value of 0.0005 of curve b in the figure. In the table below, the photographic performance of each intensifying screen is the photographic sensitivity when photographing with X-rays at an X-ray tube voltage of 80 kVp through an 8 cm thick water phantom using orthofilm (RX-OG manufactured by Fujifilm). , graininess index [G], and sharpness index [M], and each display value is shown as the following value. Photographic sensitivity... (Gd _0.99 , Tb _0.01 ) Displays the relative value when the photographic sensitivity of an intensifying screen (STD) with a phosphor layer made of ₂ O ₂ S phosphor is set to 100. Graininess index [G]…Photographic density 1.0, spatial frequency 0.5
Calculate the RMS value of each intensifying screen at ~5.0 lines/mm, and when each RMS value is set to 100 (Gd _0.99 ,
Tb _0.01 ) Expressed as a relative value of the RMS value of an intensifying screen (STD) with a phosphor layer made of ₂ O ₂ S phosphor. Sharpness index [M]...Calculate the MTF value of each intensifying screen at a spatial frequency of 2 lines/mm (Gd _0.99 , Tb _0.01 )
Displayed as a relative value when the MTF value of an intensifying screen (STD) with a phosphor layer made of ₂ O ₂ S phosphor is set to 100.

【table】

【table】 [Brief explanation of drawings]

FIGS. 1, 2, and 3 are graphs showing the graininess characteristics, sharpness characteristics, and afterglow characteristics of the radiation intensifying screen according to the present invention, respectively.

Claims (1)

[Scope of Claims] 1. A radiation intensifying screen essentially consisting of a support and a phosphor layer provided on the support, wherein the phosphor layer has a composition formula (Ln _1-xy , Tb _x , R _y ) ₂ O ₂ S (However, Ln is at least one of La, Gd, Y, and Lu, R is at least one of Nd and Ho, and x and y are each 0.001
A radiation intensifying screen characterized by being made of a rare earth oxysulfide phosphor represented by the following formulas: ≦x≦0.02, 0.001≦y≦0.01, and x≧y. 2 The average particle diameter of the phosphor in the phosphor layer and its standard deviation value (quarterly deviation value) are each 2 to 15 μm.
and 0.40 or less, the radiation intensifying screen according to claim 1. 3 The phosphor coating weight of the phosphor layer is 10 to 100
The radiation intensifying screen according to claim 1 or 2, characterized in that the radiation intensifying screen is mg/cm ² . 4 Claim 1, characterized in that a light absorption layer is provided between the support and the phosphor layer.
The radiation intensifying screen according to item 1, 2 or 3.