JPS6131580B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a method of manufacturing an image intensifier tube, particularly an input surface, for converting a radiation image such as an X-ray image into a visible image. An image intensifier tube that converts high-energy radiation such as X-rays into light to create a brighter visible image has an input surface that converts an image caused by the radiation into a photoelectron image, and a photoelectron image emitted from the input surface that converts the photoelectron image into a visible image. It has an output surface that converts to Usually, the input surface uses aluminum as a substrate that easily transmits radiation images, and on this substrate, an alkali halide phosphor layer that efficiently emits light by radiation is formed by vapor deposition, and then the phosphor emits light. It consists of a photocathode made of a material sensitive to ions, such as antimony-cesium. Such an image intensifier tube is required to have uniform brightness (brightness) over the entire area of the screen and good resolution over the entire area as well as the central area. However, there is no conventional input surface that satisfactorily solves the above-mentioned problems. It originates from the characteristics of the image intensifier tube and from the characteristics of the input surface, especially the phosphor layer. That is, for example, in the case of brightness, the radiation source is sufficiently small (can be regarded as a dotted line) and the distance from the image intensifier tube is about 1 meter. Therefore, the intensity of the radiation incident on the image intensifier tube is smaller at the periphery than at the center. Furthermore, since photoelectrons emitted from the photocathode are emitted from the peripheral part from the oblique direction and have a weaker emission intensity per unit area than from the central part, the emission from the output surface is weaker. For these reasons, even if the thickness of the phosphor layer is uniform and the photoelectron emissivity is the same, the brightness in the peripheral area is only about 70% that in the central area. In terms of resolution, the radiation image that passes through the object and enters the input screen has a lower density at the periphery than at the center, and in the phosphor layer, the transmission distance is longer at the periphery than at the center, resulting in light image diffusion. It's also big. Photoelectrons emitted from the photocathode and those emitted from the periphery are strongly deflected and have a long range, so the image formation at the output surface is also weaker than at the center. For these reasons, for example, even if the phosphor layer has a uniform thickness and the input surface has a resolution of 28 p/cm at the center, only 25 p/cm to 22 p/cm can be obtained at the periphery. If the thickness of the phosphor layer in the peripheral area is increased to improve the uniformity of brightness on the input surface, the resolution in the peripheral area will further decrease. There is a paradox in that reducing the film thickness of the film further increases the imbalance in brightness. The present invention relates to a method of manufacturing an input surface for an image intensifier tube that improves brightness uniformity without deteriorating resolution, which has conventionally been impossible for the above-mentioned reasons. FIG. 1 shows the present invention applied to, for example, an X-ray fluorescence multiplier tube that converts an X-ray image into a light image. That is, the X-ray fluorescence multiplier tube consists of an envelope 1 made of glass, an input surface 2, an output surface 3, a focusing electrode 4, an accelerating electrode 5, etc. arranged within the glass envelope 1. X
The ray 6 is irradiated onto the subject 7, and the X-ray image modulated by the X-ray absorption ability of the subject 7 is transmitted through the envelope 1.
A phosphor layer 9 on the input surface 2 emits light. This light passes through the intermediate layer thin film 10, and from the photocathode 11, the photoelectrons 12
emit. The photoelectrons 12 are accelerated by the focusing electrode 5 and reproduce on the output surface 3 an optical image several thousand times brighter than the optical image obtained on the input surface. In this embodiment, the input surface 2 has a structure as shown in FIG. That is, the substrate 21 and this substrate 2
A large number of mosaic structures 22 formed on one surface of 1
(the substrate structure above the position indicated by the dashed-dotted line in the figure) and a phosphor made of cesium iodide or the like deposited on the mosaic structure surface and extending almost perpendicularly to this surface. It consists of a crystal layer 24 and a photocathode 26 formed via an intermediate thin film 25 made of, for example, alumina, which is deposited on the phosphor layer 24. The phosphor layer 24 has phosphor blocks 28 that are optically independent from each other by gaps 27 between the mosaic structures 22 defined by grooves 23. The input surface is drawn from the groove 23 of the substrate 21 and defines a phosphor block 28 by a gap 27 formed in the phosphor layer 24, and the light emitted within the phosphor block 28 is blocked. It is repeatedly reflected inside and taken out to the photocathode side (light induction effect). As a result, light scattering in the phosphor layer 24 is reduced and an input surface with high resolution is obtained. The quality of the resolution depends on the length and width of the voids 27 formed within the phosphor layer 24. If the voids 27 do not disappear midway and remain completely on the surface layer, phosphor blocks are formed that stand up to the surface layer, and even if the photocathode 26 is formed on the top surface, smooth photoelectron emission will not occur. This is a fatal flaw. In the input surface of this embodiment, the thickness of the phosphor layer gradually increases from the center to the periphery, and the phosphor layer at the outermost periphery is the thickest. For example, for a 9 inch input surface, the thickness of the phosphor layer 24 in the center was 150 microns and at the periphery it was 215 microns. In such an input surface, when applied to an X-ray fluorescence multiplier tube, the brightness ratio is 1:0.95, and substantially uniform brightness can be obtained over the entire surface. Furthermore, the decrease in resolution caused by the increase in the thickness of the phosphor layer in the peripheral area is caused by the voids 27 in the phosphor layer 24 that are drawn from the grooves 23 formed on the surface of the substrate 21. By leaving it close to the surface layer, it is improved and leveled with the center. The growth and development of the voids 27 formed within the phosphor layer 24 can be controlled by the deposition conditions for forming the phosphor layer 24. For example, the substrate temperature during vapor deposition is set to 110℃, and cesium iodide is
When deposited at a rate of microns/minute, when the thickness of the central phosphor layer is 150 microns, the central part of the input surface
A resolution of 34 to 37 p/cm can be obtained. On the other hand, for the input surface where the thickness of the central phosphor layer is 220 microns, the substrate temperature during vapor deposition is set at 80°C, and the thickness of the phosphor layer is 7 microns/
When formed at a deposition rate of 31 to 34p/min in the center
A resolution of cm can be obtained. That is, the lower the substrate temperature during vapor deposition, the more likely voids remain in the phosphor layer. Therefore, by setting the evaporation source at a position that provides an appropriate film thickness distribution on the substrate and performing evaporation with the substrate temperature set so that the peripheral area is lower than the central area, brightness can be spread over the entire surface. As a result, it is possible to obtain an input surface for X-ray fluorescence multiplication that is uniform and has almost the same resolution between the center and the periphery. An embodiment of the method for creating an input surface according to the present invention will be described below with reference to FIG. Aluminum substrate 3 with a thickness of 0.5 mm and a diameter of 240 mm
In step 1, the surface on which the phosphor layer will later be formed is treated by an anodic oxidation method. For anodizing, for example, a current of 1 A/dm ² is applied for about 2 hours in a 3% oxalic acid solution. Then, sealing treatment is performed in boiling water for about 2 hours. When this is heat treated at 250℃ or higher, fine grooves 3 with a width of 3 to 5 microns are created.
A surface of a mosaic structure 33 separated by 2 is formed. Next, a cesium iodide phosphor is deposited on the surface of the substrate after the above treatment in a vacuum. The substrate 31 that has been processed is placed on a base 37 with its surface facing downward, and the base 37 is driven and rotated from the outside via the vacuum chamber substrate 36. A substrate heating element 38 is disposed on the upper part of the substrate 31 and is fixed to the top of the bell gear 40. The heating element 38 is made of stainless steel and has a cylindrical shape with a lid having the same curvature as the substrate 31. On the inner surface of the lid, nichrome wires 38 are arranged densely in the center and coarsely in the periphery. An evaporation source filled with cesium iodide 35 is placed in a boat 34 formed by processing tantalum below the substrate 31 , facing the boat 34. A heat shield plate 41 is provided on the upper part of the boat, and the boat 34 is installed at a position where the thickness distribution of the phosphor layer 42 can be obtained so that the luminance is uniform as determined in advance. Evacuate the vacuum chamber and start deposition at a pressure of 10 ^-6 Torr. While rotating the base 37 and the board 31 at a speed of 20 RPM, the nichrome wire 39 is energized to connect the board 31.
When the temperature at the center of the wire is set to 110°C, the temperature at the periphery is maintained at 80°C due to the difference in the winding density of the nichrome wire. In this state, the boat 34 is heated to form a phosphor layer 4 on the substrate 31 while controlling the deposition rate of cesium iodide 35 at a rate of 5 to 10 microns/minute.
form 2. The thickness of the phosphor layer 42 is the thinnest at the center, at 150 microns, and gradually increases to 210 microns at the outermost periphery. In the phosphor layer formed under these conditions, columnar crystals with a diameter of 1 to 4 microns grow on the mosaic structure surface 33 of the substrate 31 , and the voids 43 formed from the grooves 32 of the substrate 31 grow. A phosphor block 44 consisting of a bundle of columnar crystals
is formed substantially perpendicularly from the substrate. Furthermore, an X-ray fluorophore multiplier tube was fabricated in the same manner as before using an input surface with an intermediate layer 25 formed on the surface of the phosphor layer.
The brightness ratio of the peripheral area was 0.94, which was better than the conventional brightness ratio of 0.8 over the entire surface. The resolution was 37p/cm in the center and 34-37p/cm in the periphery. This is because the resolution in the peripheral area is lower compared to the conventional case where the resolution was 37p/cm in the center and 30 to 32p/cm in the peripheral area when the phosphor layer was 210 microns thick from the center to the periphery. This shows that the results have been significantly improved. As described above, the present invention forms a mosaic structure with fine grooves on the substrate surface, and on this mosaic surface, a phosphor made of alkali halide is deposited thickly from the center to the periphery to make the brightness uniform. At the same time, the resulting large drop in resolution in the peripheral area is caused by the temperature applied to the substrate in the vapor deposition process being pulled from the grooves of the substrate, which is low from the center to the peripheral area, causing the phosphor layer to deteriorate. The voids formed inside are made to remain longer at the periphery than at the center to enhance the light guiding effect, prevent a decrease in resolution, and improve uniformity.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing an X-ray fluorescence multiplier tube according to an embodiment of the present invention, FIG. 2 is a sectional view showing a part of the input surface according to the present invention, and FIG. 3 is an input diagram according to the present invention. FIG. 2 is a schematic diagram of a vapor deposition apparatus for explaining a surface manufacturing process. 1... Input surface, 9, 24, 42... Fluorescent layer, 1
0, 25... intermediate layer, 11, 26... photocathode, 21,
31 ...Substrate, 22,33...Mosaic structure of substrate,
23, 32...Groove portion of the substrate.

Claims (1)

[Claims] 1. A substrate, fine grooves that partition a large number of mosaic structures formed on the surface of the substrate, a phosphor layer made of alkali halide deposited on the mosaic structure surface, and a phosphor layer formed directly or on the phosphor layer. In the method for manufacturing an image intensifier tube having an input surface having a photocathode formed through an intermediate thin film, the temperature applied to the front substrate in the phosphor layer vapor deposition step is directed from the center toward the periphery. A method for manufacturing an input surface for an image intensifier tube, characterized in that the thickness of the phosphor layer is increased from the center to the periphery.