JPS5924300A - Scintillation screen for conversion of radiation and manufacture thereof - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a scintillation screen for radiation conversion and a method for manufacturing the same.

Scintillation screens for radiation conversion are known. When a scintillation screen receives X-rays or gamma-rays, it converts them into photons, which can be sensed by a photocathode covering the concave surface of the screen.

The screen that receives X-rays is an X-ray image intensifier tube (X, 1
．． 1. The screen used in the scintillation scanning tube is used in the scintillation scanning tube and receives gamma ray irradiation.

In the known art, scintillation screens for radiation conversion are usually produced by depositing cesium iodide doped with sodium or thallium onto the concave surface of a radiation-transparent metal support, for example made of aluminum. When cesium iodide grows, it naturally forms needle-shaped crystals arranged side by side.

This structure serves to guide the light generated inside the cesium iodide upon receiving radiation. The concave surface of the scintillation screen thus obtained supports a lower layer of the photocathode intended to isolate the screen from the photocathode and/or to improve the surface condition of the concave surface of the screen. A photocathode is then deposited onto the underlying layer.

A considerable number of disadvantages are associated with known scintillation screens, some of which can be listed below.

(1) The concave surface of the scintillation screen obtained is not a mirror surface due to the acicular crystal structure of the scintillation material. It is difficult to completely smooth out this concave surface irregularity even when using the lower layer of the photocathode. The deposited photocathode has a high electrical surface resistance. Above a given incident radiation dose, substantial local variations in potential occur at the surface of the photocathode, resulting in blurring of the electron image. Furthermore, surface irregularities in the photocathode impair its sensitivity. The presence of numerous gaps forming gas pockets near the photosensitive layer leads to a decrease in photon emission efficiency.

(2) The fact that the screen thickness must be limited, since the number of cranks and surface discontinuities increase with increasing screen thickness. This disadvantage is particularly problematic in the case of gamma-ray screens, which require the use of thick scintillation screens.

(3) The fact that a support having a scintillation material deposited on its surface is present inside the scintillation screen. This support transmits most of the incident radiation while still blocking some of it.

The object of the invention is to provide a scintillation screen for radiation conversion that does not have the disadvantages mentioned above.

According to a feature of the present invention, there is provided a scintillation screen for radiation conversion made of a scintillation material having an acicular crystal structure, the screen having a convex surface for receiving radiation and a concave surface having a mirror surface and supporting a photocathode on the surface. A screen is provided that forms a screen.

According to other features of the invention, a step of obtaining a scintillation screen by vapor deposition on a support having a mirror-like convex surface and made of a material having a coefficient of thermal expansion different from that of the scintillation material; and subsequently separating the scintillation screen from the support by simply heating.

The concave surface of the scintillation screen according to the invention has a mirror surface since it is in contact with the convex surface of the support during the deposition of the scintillation material. Therefore, the photocathode deposited on this mirror surface also has a mirror surface. The thickness of the screen can be varied from 2 to 30 microns to several millimeters while maintaining its mirror surface.

Finally, according to the invention, the finished scintillation screen retains no support that was only used for its manufacture.

Other features of the invention will become more apparent from the detailed description given below with reference to the accompanying drawings.

In the accompanying drawings, like reference numerals indicate like elements, but the dimensions and proportions of these elements have not been made identical for clarity of illustration.

As shown in FIG. 1, a cross section of a conventional scintillation screen is obtained by depositing cesium iodide on the concave surface of a thin metal support 1 made of aluminum, for example. This support 1 transmits the scanned X-rays or γ-rays. As the cesium iodide grows, needle-like crystals 2 having tetrahedral crystal edges are obtained, as shown in the circular enlarged view showing the screen structure in more detail. Looking at FIG. 1, it is clear from the enlarged circular view that the concave surface of the scintillation screen thus obtained is highly irregular.

A photocathode underlayer 3 is deposited on the concave surface of the screen in order to isolate the needles 2 from the photocathode and/or to improve the surface condition of the concave surface of the screen. Then,
A photocathode 04 is deposited on the surface of the lower layer 3.

FIG. 2 is a sectional view showing one configuration of the screen according to the present invention.

Looking at the two circular enlarged views in which the screen construction according to the invention is illustrated in more detail, it can be seen that the screen is composed of a scintillation material 2 with an acicular crystal structure. The circular enlarged view on the right shows that the concave surface of the screen is a mirror surface. A photocathode 4 is deposited on the concave surface. If necessary, a photocathode underlayer 3, for example of phosphorus vanadate, can also be deposited between the concave surface and the photocathode.

To obtain the screen according to the invention, the method described below can be used.

A support must be provided that is highly polished but of any desired wall thickness. This support may be made of any material such as glass or metal that has a different coefficient of thermal expansion than the scintillation material used.

A scintillation screen is obtained by depositing scintillation material on the concave surface of the support. After deposition, the screen is separated from the support simply by heating. This separation is possible because the concave surface of the support is a mirror surface.
This is because the support and the scintillation material have different coefficients of thermal expansion.

In this way, a scintillation screen having a mirror-like concave surface as shown in FIG. 2 is obtained. This is because the concave surface of the mirror was in contact with the convex surface of the support during the deposition of the scintillation material. The concave surface of the screen has an optically mirror-polished finish. The diameter of the particles on this concave surface varies within the range of 0.1 microns to 50 microns.

As can be seen from the circular enlarged view on the left side of Figure 2, the convex surface of the screen has a rather irregular surface due to the ends of the needle-like crystals of the scintillation material;
Its convex surface is of no importance since there is no conductive phenomenon.

In fact, this conductive phenomenon is caused by a photocathode deposited on a mirror surface. Therefore, the microstructure as in the conventional case,
As a result of the crater, there is no risk that the layer in contact with the photocathode will be disturbed in its continuity and thus in its conductive phenomena.

Depending on the intended application, the wall thickness of the screen can vary between 2-30 microns to 2-3 millimeters while ensuring a mirror-like concave surface.

If the deposition support is maintained at a low temperature during the deposition process, the scintillation screen will have a finely divided needle-like crystal structure. In this case, the screen can be used in a high-definition X-ray image intensifier tube.

On the other hand, if the deposition support is heated during the deposition process to a temperature range of e.g. be able to.

The deposition support can be made of aluminum, for example.

The scintillation material used is an alkali halide such as cesium iodide doped with sodium or thallium, or doped with thallium.

It may be an alkaline/hydrolytic compound such as potassium iodide. The scintillation material may also be, for example, tungsten, a sulfide or a sulfate of the metal.

It can be seen that by using the method described above, both sides of the scintillation screen thus obtained can be treated as desired thereafter. This treatment may be necessary because, unlike conventional screens, the deposition support does not form part of the finished screen.

In order to increase the mechanical strength of the screen, especially for thin-walled screens with finely separated acicular crystal structures,
To increase the stiffness of the screen, a layer 5 as shown in FIG. 2 can be deposited on the convex side of the screen.

- By way of example, the layer 5 can be made of low-melting glass or enamel or any organic material that can withstand the heating temperatures at which the tube is heated in a furnace. Examples of such materials include epoxy resin, parylene (P-xylene resin), polyimide, and cryolite.

It is also possible to form a layer on the convex surface of the screen that reflects light generated on the screen when radiation is incident. This layer returns all light incident on the convex side of the screen to the photocathode. The layer can be made using any suitable vapor deposited metal such as aluminum or nichrome.

The convex surface of the screen can also be provided with a layer that increases its quantum detection efficiency. This layer consists of barium oxide,
They are made from dense materials with high atomic numbers, such as lead oxide or tungsten oxide, which are deposited in thin films. This type of material is effective in photoemission and ionizing radiation shielding power.

Therefore, one, two or three successive layers can be superimposed on the convex side of the screen. Each of these layers is supposed to perform the following functions. That is, a layer that increases the rigidity of the screen, a layer that reflects light generated on the screen when radiation is incident, and a layer that increases the quantum detection efficiency of the screen. The stacking order of these different layers is variable.

It is possible to use layers made of materials that perform the above-mentioned 2ta function. For example, a layer made of indium or tin
It can perform both the function of reflecting light generated on the screen by the incidence of radiation and the function of increasing the rigidity of the screen. Layers of this type can be obtained by cathodic sputtering, vapor deposition, thermal spraying or any other known method. For the specular concave surface of the screen, the photocathode deposited on its surface has a minimum electrical surface resistance that depends not so much on the scintillation screen but primarily on the photocathode deposited on the screen. have

The concave surface of the screen is covered over the front surface with at least one layer that performs at least one of the following functions:

namely, the function of further reducing the electrical surface resistance of the photocathode, and the function of establishing compatibility from a chemical point of view between the scintillation screen and the photocathode.

As shown in FIG. 2, the underlayer 3 can be deposited on the concave surface of the screen. This underlayer can be made of indium oxide or a thin film metal such as aluminum, which transmits most of the light generated by the scintillator. For example, a photocathode 4 made of cesium 6 or antimony is deposited as a lower layer.

To embed the screen of the invention inside a tube for subsequent use, a support grid 7 can be used which has the same concavity or radius of curvature as the screen, as shown in FIG. This grid is transparent to the incident radiation flux and can be made of nickel or iron, for example.

In FIG. 3, the screen shown comprises a layer 6 which increases the quantum sensing efficiency of the screen and a layer 2 of scintillation material of the acicular crystal type with a highly polished concave surface.
It is composed of a photocathode lower layer 3 and a photocathode 4.

As is clear from FIG. 3, a metal ring 8 is deposited around the concave surface of the screen. A pressure strip 9 is pressed against the ring to provide a connection with the photocathode.

The screen of the invention can also be embedded in a tube without a support grid if the wall thickness is sufficient.

Screens of different configurations according to the invention can be made up of 7 one of the aforementioned layers which can be superimposed in any desired order.
Or by forming a large number of them on one or both sides of the screen.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of a conventional scintillation screen. FIGS. 2 and 3 are cross-sectional views showing two configurations of scintillation screens according to the present invention, respectively. In the drawings: 100. Thin metal support, 200. Needle crystal, 300. Photocathode lower layer, 408. Photocathode, 5.6010 layers, 700. support grid, 880. metal ring, 910. Pressure strip. 8

Claims (1)

[Scope of Claims] (1) A radiation conversion device that converts the irradiation of X-rays or γ-rays into photons, and allows a photocathode covering one of both surfaces to detect the photons. The scintillation screen is made of a scintillation material having an acicular crystal structure, and has a convex surface that is irradiated with radiation and a concave surface that is a mirror surface and supports the photocathode. A scintillation screen featuring (2. The screen according to claim 1, characterized in that the convex surface is covered with at least one layer that performs at least one of the following functions: Increasing the rigidity of the Tal screen. layer, mountain) a layer that reflects light generated on the screen upon receiving radiation; (a layer that increases the quantum detection efficiency of the Cl screen); A screen characterized in that the concave surface is covered with at least one layer that performs at least one of the following functions: (a) a layer that ensures compatibility with the photocathode, a layer) that reduces the surface resistance of the photocathode; Layer to let. (4) The screen according to claim 1, wherein the scintillation material used is an alkali halide such as cesium iodide doped with sodium or thallium, or potassium iodide doped with thallium. A screen characterized by being an alkali halide. (5) The screen according to claim 1, wherein the scintillation material has a structure consisting of finely divided needle-like crystals or a structure consisting of agglomerated needle-like crystals. (6) The screen according to claim 1, wherein the scintillation material has a thickness in the range from 2 to 30 microns to 2 to 3 millimeters. (7) The screen according to claim 1, wherein the mirror-like concave surface includes particles having a diameter in the range of 0.1 microns to 50 microns. (8) In the screen according to claim 2, the above-mentioned layer that increases the rigidity of the screen is made of low melting point glass or enamel, or epoxy resin, parylene, polyimide, cryolite, etc. Screen (9) characterized in that it contains an organic substance.The screen according to claim 2, wherein the layer that reflects light generated on the screen upon incidence of radiation is made of aluminum or nichrome. (10) The screen according to claim 2, wherein the layer for increasing the quantum detection efficiency is made of barium oxide, lead oxide, or tungsten oxide deposited on a thin film. (11) The screen according to claim 3, wherein the layer for reducing the surface resistance of the photocathode is made of indium oxide. (12) A method for manufacturing a screen according to claim 1, which comprises forming a mirror-like convex surface. obtaining a scintillation screen by vapor deposition on a support made of a material having a coefficient of thermal expansion different from that of the scintillation material, and separating the scintillation screen from the support by simply heating after vapor deposition. (13) A method for producing a screen according to claim 12, wherein the support is maintained at a low temperature during vapor deposition, so that finely divided needle-like A method characterized in that a structure consisting of crystals is obtained. (14) A method for producing a screen according to claim 12, wherein the support is heated during vapor deposition, so that a structure consisting of crystals is obtained. A method characterized in that a structure consisting of shaped crystals is obtained.