X-ray image intensifier and method of manufacturing the same

Abstract: An X-ray image intensifier comprising a vacuum envelope and an input screen having an improved sen­ sitivity and including a substrate disposed on the X-ray input side of the vacuum envelope, a phosphor layer (3) formed on the substrate (1) and a photocathode (6) formed on the phosphor layer (3). The phosphor layer (3) consists of columnar crystals extending in a direc­ tion perpendicular to the substrate surface. The tip portions of the columnar crystals are deformed to close the upper portions of the clearances (2) formed between the columnar crystals.

Description: The invention relates to an X-ray image intensifier, particularly, to an improvement in the input screen of the X-ray image intensifier. Fig. 1A shows the input screen of a conventional X-ray image intensifier. As seen from the drawing, the input screen comprises input substrate 31 having a smooth surface, a first phosphor layer 23 consisting of CsI:Na crystal grains formed on input substrate 31 by vapor deposition under a low degree of vacuum, second phosphor layer 34 consisting of CsI:Na crystal grains grown in a columnar shape on the first phosphor layer, surface layer 35 consisting of CsI:Na phosphor formed on the second phosphor layer 34 by vacuum deposition under a high degree of vacuum, and a photocathode 36. Second phosphor layer 34 consists of columnar CsI crystals grown in a direction substantially perpendicular to the surface of input substrate 31. Columnar crystals have an average diameter of 5 to 50 µm and a length of about 400 µm. The columnar crystals are separated from each other by fine clearance 33. When photocathode 36 is formed directly on the surface of the second phosphor layer 34 consisting of the columnar crystals, photocathode 36 is also divided into fine island-shaped regions. In photocathode 36 of this shape, an electric connection cannot be achieved in a direction parallel to the surface of photocathode 36. It follows that it is impossible to maintain constant the potential of photocathode 36 with increase in the number of photoelectrons emitted from photocathode 36. As a result, the electrooptic uniformity of the X-ray image intensifier is markedly impaired, leading to distortion of the output image or reduction of resolution. To overcome the difficulty, surface layer 35 is formed on second phosphor layer 34, followed by forming photocathode 36 on surface layer 35. Since surface layer 35 has a relatively continuous surface, photocathode 36 formed on surface layer 35 also has a relatively continuous surface, with the result that it is possible to ensure an electric connection in a direction parallel to the surface of photocathode 36. However, clearances 33 formed between the individual columnar crystals in second phosphor layer 34 include relatively large clearances 33, sized about 1 µm, which are distributed over the entire region of second phosphor layer 34, as shown in Fig. 1B. As a result, pin holes 37 corresponding to relatively large clearances 33 are formed in surface layer 35. These pin holes 37 give a detrimental effect to the sensitivity of photocathode 36. Specifically, the material of photocathode 36 is gradually diffused through pin holes 37 into the phosphor layer in the step of forming photocathode 36 which is carried out at such a high temperature as 100°C or more, leading to a low sensitivity of the photocathode formed. The diffusion also takes place even after completion of the step for forming photocathode 36. Accordingly, the sensitivity of the photocathode is gradually lowered, leading to a shortened life of the input screen. It is possible to diminish pin holes 37 and to decrease the number of pin holes 37 by increasing the thickness of surface layer 35. As a result, the sensitivity of photocathode 36 can be improved. However, the increased thickness of surface layer 35 brings about a low resolution of the input screen, leading to a low resolution of the X-ray image intensifier. Under the circumstances, the thickness of surface layer 35 is practically set at about 10 to 30 µm. It should also be noted that photocathode 36 itself has a high electric resistance in some cases depending on the materials of photocathode 36, making it impossible to put the input screen into practical use even if photocathode 36 is formed on surface layer 35 having a relatively continuous surface. In this case, a conductive intermediate layer is formed between surface layer 35 and photocathode 36. The conductive intermediate layer should desirably be highly transparent. An indium oxide film or an indium tin oxide film is known as a desirable material of the conductive intermediate layer. Even in the case of using such a conductive film, however, it is necessary to set the thickness of the intermediate layer at 0.3 µm or less in order to obtain a high enough transmittance (≳ 70%) in the CsI phosphor layer activated by Na. It follows that the use of a conductive intermediate layer is quite incapable of eliminating the pin holes present in the surface layer. Also, it is quite impossible to solve the problem even if the surface layer is formed by vapor deposition of a transparent material other than the phosphor. Japanese Patent Disclosure No. 63-88732 teaches the idea of shaving the surface region of a first CsI phosphor film consisting of completely dispersed phosphor particles, followed by forming a second CsI phosphor layer by vapor deposition on the shaved surface of the first CsI phosphor film so as to provide a continuous phosphor layer surface. However, it is difficult to prevent the pin hole occurrence by the technique of this prior art. As described above, the phosphor layer surface in the input screen of a conventional X-ray image intensifier is not sufficient continuous, but contains a large number of pin holes. The presence of the pin holes makes it difficult to form a photocathode having a high sensitivity and a long life. The present invention is intended to overcome the above-noted problem inherent in the prior art viz. to make the luminance brightness uniform over the entire screen so as to provide an X-ray image intensifier comprising an photocathode having a high sensitivity and a long life and to provide a method of manufacturing the same. According to the present invention, there is provided an X-ray image intensifier comprising a vacuum envelope and an input screen which includes a substrate (1) disposed on the X-ray input side within the vacuum envelope, a phosphor layer (3) formed on the substrate (1), and a photocathode (6) formed on the phosphor layer (3), said phosphor layer (3) consisting of columnar crystals extending in a direction perpendicular to the substrate surface, characterized in that the tip portions of said columnar crystals are deformed by polishing greater in the peripheral portion than in the central portion of the substrate (1) to close the tip portions of the clearances (2) formed between the columnar crystals so as to form a continuous surface of the phosphor layer (3) such that the continuous surface is smoother in the periferal portion than in the central portion. The present invention also Drovides a method of manufacturing an X-ray image intentsifier comprising an input screen, in which the input screen is prepared by the steps of forming a phosphor layer (3) having columnar crystals on a substrate (1) by vapor deposition, forming a photocathode (6) on the phosphor layer (3), characterized by further comprising mechanically deforming by polishing the tip portions (4) of the columnar crystals greater in the peripheral portion than in the central portion of substrate (1) to allow the deformed tip portions (4) to fill the upper portions of the clearances (2) between the columnar crystals, thereby forming a continuous sufrace in the tip portions (4) of the columnar crystals such that the continuous surface is smoother in the peripheral portion than in the central portion. In the present invention, the pin holes in the surface region of the phosphor layer included in the input screen are eliminated, making it possible to prevent diffusion and dissipation of the material forming the photocathode. It follows that the initial sensitivity of the photocathode can be improved. Also, deterioration with time of the photocathode can be prevented in the present invention. This invention can be more fully understood from the following detailed description when taken in conjunction with the drawings, in which: Fig. 1A is a cross sectional view showing in a magnified fashion the gist portion of the input screen of a conventional X-ray image intensifier; Fig. 1B shows the surface condition of the input screen of the conventional X-ray image intensifier shown in Fig. 1A; Fig. 2 is a cross sectional view showing in a magnified fashion the input screen included in an X-ray image intensifier according to one embodiment of the present invention; Fig. 3 is a cross sectional view showing in a magnified fashion the input screen included in an X-ray image intensifier according to another embodiment of the present invention; Figs. 4A and 4B schematically show an apparatus for polishing the input phospher layer included in the X-ray image intensifier of the present invention; and Fig. 5 is a cross sectional view showing in a magnified fashion columnar crystals formed on the input substrate by vapor deposition. The present invention is directed to an improvement in the input screen of an X-ray image intensifier, as described below with reference to the drawings. In the present invention, the input screen comprises substrate 1, phosphor layer 3 formed on substrate 1, and photocathode 6 formed on phosphor layer 3, as shown in Fig. 2. Surface layer 5, which is equal to phosphor layer 3 in the material, can be formed between phosphor layer 3 and photocathode 6. Phosphor layer 3 consists of columnar crystals extending in a direction perpendicular to the substrate surface. As seen from the drawing, columnar clearances 2 are left between the columnar crystals. It is important to note that the tip portions of the columnar crystals are mechanically deformed, with the result that the tip portions of clearances 2 are filled with the deformed tip portions of the columnar crystals so as to form continuous layer 4. Substrate 1 is formed of aluminum or glass, as in the prior art. Phosphor layer 3 is formed of a phosphor for X-ray such as CsI:Na. As shown in Fig. 3, phosphor layers 13 preferably consists of first granular phosphor layer 12a and second columnar phosphor layer formed on first layer 12a. Photocathode 6 may be formed of a compound between Sb and an alkali metal such as (Cs)Na 2 KSb or K 2 CsSb. In the case of using, for example, K 2 CsSb for forming the photocathode, the photocathode itself exhibits a high electrical resistance. In such a case, it is possible to form a conductive intermediate layer between phosphor layer 3 or surface layer 5 and photocathode 6. The intermediate layer can be formed of a highly transparent indium oxide or indium tin oxide. In manufacturing the input sereen, phosphor crystals of, for example, CsI:Na are grown in a columnar form on substrate 1 by vapor deposition. The tip portions of the columnar crystals thus grown are mechanically subjected to plastic deformation so as to form a substantially continuous surface on phosphor layer 3, followed by forming photocathode 6 on phosphor layer 3. The continuous surface is formed by polishing the surface of phosphor layer 3 by using a polishing apparatus. Figs. 4A and 4B collectively show a polishing apparatus. As seen from the drawings, the apparatus comprises turntable 8, polishing tool 11, arm 9 movable in the vertical direction, counterbalancer 20, and shaft 10 supporting arm 9 and movable toward and away from the center of turntable 8. Substrate 1 having phosphor layer 3 formed thereon is fixed to turntable 8. Polishing tool 11 can be moved from the center toward a desired peripheral portion of turntable 8 by moving shaft 10. Further, the pressure applied by polishing tool 11 to the surface of the phosphor layer can be controlled by moving counterbalancer 20. It should be noted in conjunction with the pressure control referred to above that the luminance brightness in the output screen of a conventional X-ray image intensifier is distributed in general such that the luminance brightness is gradually decreased from the central portion toward the periphery even if an input X-ray incident onto the X-ray input screen has a uniform intensity over the entire region including the central and peripheral portions. In order to make the luminance brightness uniform over the entire region of the output screen of the X-ray image intensifier, the pressure applied by the polishing tool to the phosphor layer is made higher in the peripheral portion than in the central portion in the present invention. As a result, the surface region of the phosphor layer is made more smooth in the peripheral portion, leading to an improved sensitivity in the peripheral portion. In the case of applying the polishing, the tip portions of columnar crystals 13a are plastically deformed in one direction in the shape of a hook as shown in Fig. 3. Fine cracks 15 sized 0.1 µm or less may be included in the continuous surface region of the phosphor layer while the plastic deformation treatment described above is applied to the columnar phosphor layer. However, it is possible to close completely the fine cracks 15 by forming surface layer 5 having a thickness of 1 µm or more on surface of phosphor layer 3. Of course, surface layer 5 has a smooth surface, even if viewed microscopically. Additional methods can be employed for forming a smooth surface of the phosphor layer. For example, it is possible to use an apparatus in which a polishing tool itself is rotated or vibrated. Further, a wet polishing method is effective. In this case, a liquid which is incapable of dissolving the phosphor layer such as alcohol solution may be interposed between the polishing tool and the input phosphor screen during the polishing step. The presence of such a liquid serves to lower the friction coefficient between the polishing tool and the input phosphor screen, making it possible to obtain a smooth surface. Still further, polishing may be applied first to fill the pin holes to some extent, followed by impregnating the polishing tool with a small amount of a liquid capable of dissolving CsI such as water or ethyl acetate and subsequently applying a final polishing. In this case, fine cracks sized 0.1 µm or less are not generated in the surface region of the CsI phosphor layer. Since the CsI phosphor layer has a very smooth surface even if viewed microscopically, it is possible to form a photocathode directly on the phosphor layer. Of course, it is possible to form a conductive protective layer about 0.1 µm in thickness on the phosphor layer, followed by forming the photocathode on the protective layer. Reference Examples useful for understanding the manufacture of a phosphor screen according to the present invention, the invention, however, additionally requires a polishing step as defined in the claims. Reference Example 1 CsI:Na phosphor layer 3 was formed by vapor deposition on aluminum substrate 1, as shown in Fig.5. Phosphor layer 3, which was found to have a thickness of 400 µm and to consist of columnar crystals 3a each having a diameter of 5 to 10 µm and tip portion 7, exhibited an excellent resolution. Columnar crystals 3a were separated from each other to provide clearance 2. Under this condition, polishing was applied by using an apparatus as shown in Figs. 4A and 4B. Specifically, input substrate 1 having deposited CsI phosphor layer 3 formed thereon was fixed to turntable 8, and turntable 8 was rotated so as to perform the polishing. In this operation, polishing tool 11 was mounted at the tip of arm 9 so as to push the surface of phosphor layer 3 with an optional pressurizing force. A woven or nonwoven fabric was used as the polishing tool. It is possible to apply the polishing along the curved surface of the input screen from the central portion toward the periphery of phosphor layer 3 by moving arm 9 together with shaft 10. In this experiment, the pressurizing force of the polishing tool was set at 200 g/cm 2 , which is about 50% higher than the critical pressure at which the surface of phosphor layer 3 begins to be deformed. Phosphor layer 3 was gradually deformed to provide a smooth surface by the friction between polishing tool 11 and phosphor layer 3. When the deformation proceeded to provide sufficient continuous layer 4, the frictional force was reduced to 1/2 or less so as to stop further proceeding of the deformation. The tip portions of columnar crystals 3a were found to have been deformed in one direction in the shape of a hook as shown in Fig. 2. Also, fine cracks sized 0.1 µm or less were found in continuous layer 4 thus formed. After the pressurizing step, surface layer 5 consisting of CsI phosphor was formed in a thickness of 3 µm by vapor deposition under high vacuum on continuous layer 4. The average crystal size in the surface layer was about 1.5 times as large as the average diameter of columnar crystals. The positions of the crystal boundaries in the surface layer did not conform with those of the columnar crystals. The surface of surface layer 5 was substantially smooth. Further, photocathode 6 was formed on surface layer 5 so as to prepare an input screen. The X-ray image intensifier comprising the input screen thus prepared exhibited about 50% improvement in sensitivity, compared with the conventional X-ray image intensifier. Also, the resolution was improved from the conventional value of 50 ℓp/cm to 52 ℓp/cm. Further, the MTF value at the spatial frequency of 20 ℓp/cm was improved from the conventional value of 24% to 27% in the X-ray image intensifier of the present invention. Reference Example 2: In the first step, a first phosphor layer consisting of CsI:Na phosphor particles 12a having an average particle size of 10 µm was formed by vapor deposition on input substrate 1 having a smooth surface, as shown in Fig. 3. Then, columnar crystals were grown by vapor deposition with the projected tip portions of crystal particles 12a used as seeds so as to form second phosphor layer. Second phosphor layer, which was 400 µm in thickness and consisted of columnar crystals having a diameter of 5 to 10 µm, exhibited an excellent resolution. A mechanical polishing was applied as in Reference Example 1 to the surface of second phosphor layer 13. After the polishing step, the tip portions of columnar crystals 13a were found to have been deformed in one direction in the shape of a hook as shown in Fig. 3. Further, fine cracks 15 sized 0.1 µm or less were found in continuous layer 14 formed by the polishing treatment. Then, surface layer 16 was formed in a thickness of about 3 µm on continuous layer 14. Surface layer 16 was found substantially smooth. Finally, photocathode 17 was formed on surface layer 16 so as to prepare an input screen. The X-ray image intensifier comprising the input screen thus prepared exhibited about 50% improvement in sensitivity, compared with the conventional X-ray image intensifier. Also, the resolution was improved from the conventional value of 50 ℓp/cm to 52 ℓp/cm. Further, the MTF value at the spatial frequency of 20 ℓp/cm was improved from the conventional value of 24% to 27% in the X-ray image intensifier of the present invention. Reference Example 3 : A surface layer about 1 µm thick was formed on the phosphor layer to which a mechanical polishing had been applied as in Reference Example 1. A transparent material other than the phosphor material, i.e., LiF, NaF, CsF, CaF 2 , MgF 2 or SiO 2 , was used for forming the surface layer. The surface layer was substantially smooth. Then, a photocathode was formed on the surface layer so as to prepare an input screen. The X-ray image intensifier comprising the input screen thus prepared exhibited about 30% improvement in sensitivity, compared with the conventional X-ray image intensifier. Also, the resolution was improved from the conventional value of 50 ℓp/cm to 54 ℓp/cm. Further, the MTF value at the spatial frequency of 20 ℓp/cm was improved from the conventional value of 24% to 30% in the X-ray image intensifier of the present invention. As described above, the input screen included in the X-ray image intensifier of the present invention comprises a phosphor layer having a smooth surface. Since pin holes are not formed in the surface region of the phosphor layer, it is possible to prevent the material constituting the photocathode positioned on the phosphor layer from being diffused or dissipated through the pin holes of the phosphor layer, leading to an improved sensitivity of the photocathode.

Claims: An X-ray image intensifier comprising a vacuum envelope and an input screen which includes a substrate (1) disposed on the X-ray input side within the vacuum envelope, a phosphor layer (3) formed on the substrate (1), and a photocathode (6) formed on the phosphor layer (3), said phosphor layer (3) consisting of columnar crystals extending in a direction perpendicular to the substrate surface, characterized in that the tip portions of said columnar crystals are deformed by polishing greater in the peripheral portion than in the central portion of the substrate (1) to close the tip portions of the clearances (2) formed between the columnar crystals so as to form a continuous surface of the phosphor layer (3) such that the continuous surface is smoother in the periferal portion than in the central portion. The X-ray image intensifier according to claim 1, characterized in that the tip portions (14) of the columnar crystals are deformed in one direction in the shape of a hook. The X-ray image intensifier according to claim 1, characterized in that the tip portions (25) of the columnar crystals are deformed in every direction in the shape of a nail head. A method of manufacturing an X-ray image intentsifier comprising an input screen, in which the input screen is prepared by the steps of forming a phosphor layer (3) having columnar crystals on a substrate (1) by vapor deposition, forming a photocathode (6) on the phosphor layer (3), characterized by further comprising mechanically deforming by polishing the tip portions (4) of the columnar crystals greater in the peripheral portion than in the central portion of substrate (1) to allow the deformed tip portions (4) to fill the upper portions of the clearances (2) between the columnar crystals, thereby forming a continuous surface in the tip portions (4) of the columnar crystals such that the continuous surface is smoother in the peripheral portion than in the central portion. A method according to claim 4, characterized in that a polishing apparatus is used for mechanically deforming.

Inventors: ANNO HIDERO, ONO KATSUHIRO, ANNO, HIDERO, ONO, KATSUHIRO, ANNO, HIDERO C/O PATENT DIVISION, ONO, KATSUHIRO C/O PATENT DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1989
Main classification: H01J29
IPC codes: H01J31, H01J9, H01J29