EP0099285B1 - Scintillative rays conversion screen and process for the manufacture of the same - Google Patents

Description

The present invention relates to a method for manufacturing a scintillator radiation converter screen. It also relates to such a screen.

Radiation converting scintillator screens are well known in the prior art. These screens receive X or y rays and convert them into light photons to which a photocathode is sensitive which covers the concave face of these screens.

When the screen receives X-radiation, it is used in radiological image intensifier tubes or I.I.R. and when the screen receives γ radiation, it is used in scintigraphy tubes.

In the prior art, scintillating radiation converting screens are generally obtained by evaporating cesium iodide doped with sodium or thallium on the concave face of a metal support, for example of aluminum, which is transparent to radiation. . The growth of cesium iodide occurs spontaneously in the form of juxtaposed needles which is a structure conducive to guiding the light created in cesium iodide by the incident radiation. The concave face of the scintillator screen thus obtained receives a photocathode sublayer which is intended to isolate the scintillator screen from the photocathode and / or to improve the surface condition of the concave face of the screen. A photocathode is then deposited on this sublayer.

Document FR-A-2 145 566 discloses a method of manufacturing a layer of scintillator material for a scintillator screen which converts radiation, by evaporation on a support, which may be the concave part of the entry window 12. , glass or metallic, of an intensifying tube of radioscopic images. Cesium iodide may for example be evaporated on a support made of a material having a coefficient of thermal expansion different from that of the scintillator material.

• le fait que la face concave de l'écran scintillateur obtenu n'est pas parfaitement lisse à cause de la structure en aiguilles du matériau scintillateur. Il est difficile de colmater parfaitement les irrégularités de cette face, même en utilisant une sous-couche de photocathode. La photocathode qui est déposée présente une résistance électrique superficielle élevée. Au-delà d'un certain flux de rayonnement incident, des variations locales importantes du potentiel à la surface de la photocathode apparaissent ce qui entraîne une défoca- lisation de l'image électronique. De plus, les irrégularités de surface de la photocathode nuisent à sa sensibilité. La présence de nombreuses crevasses emprisonnant des poches de gaz à proximité de la couche photosensible est à l'origine d'un rendement de photo-émission réduit ;
• le fait qu'on soit tenu à limiter l'épaisseur de l'écran car le nombre de fissures et de discontinuités superficielles augmente avec cette épaisseur. Cet inconvénient est particulièrement gênant dans le cas des écrans destinés à la gammagraphie où des écrans scintillateurs épais sont requis ;
• la présence dans l'écran scintillateur terminé du support sur lequel le matériau scintillateur a été évaporé. Ce support est largement transparent au rayonnement incident mais arrête malgré tout une fraction de ce rayonnement.

The scintillator screens known in the prior art have a certain number of drawbacks, among which we can cite:

• the fact that the concave face of the scintillator screen obtained is not perfectly smooth because of the needle structure of the scintillator material. It is difficult to perfectly seal irregularities on this face, even when using a photocathode undercoat. The photocathode which is deposited has a high surface electrical resistance. Beyond a certain incident radiation flux, significant local variations in the potential at the surface of the photocathode appear, which results in defocusing of the electronic image. In addition, surface irregularities of the photocathode affect its sensitivity. The presence of numerous crevices trapping pockets of gas near the photosensitive layer is at the origin of a reduced photo-emission efficiency;
• the fact that we are required to limit the thickness of the screen because the number of cracks and surface discontinuities increases with this thickness. This drawback is particularly troublesome in the case of screens intended for radiography where thick scintillating screens are required;
• the presence in the finished scintillator screen of the support on which the scintillator material has been evaporated. This support is largely transparent to incident radiation but still stops a fraction of this radiation.

The present invention relates to a scintillator radiation converter screen which does not have the drawbacks stated above.

According to claim 1, the present invention relates to a method of manufacturing a layer of scintillator material for a scintillator radiation converter screen, which receives x or y rays and converts them into light photons, this layer of material being obtained by evaporation on a support made of a material having a coefficient of thermal expansion different from that of the scintillator material, characterized in that the evaporation is carried out on the perfectly smooth convex face of the support and in that after evaporation, the layer of scintillator material is separated from the support by simple heating.

According to claim 4, the present invention also relates to a scintillator radiation converter screen, which receives x-rays and -y, comprising a layer of scintillator material sensitive to x-rays or y and converts them into light photons to which a photocathode is sensitive ( 4) which covers one of the faces of this screen, this layer of scintillator material (2) having a needle structure, having a convex face which receives the radiation and a concave face, on which the photocathode rests, characterized in that the concave face of this layer of scintillator material is as smooth as the convex surface of a support on which it is formed according to the method of one of claims 1 to 3 and comprises grains whose diameter varies from 0.1 to 50 ¡.Lm.

The concave face of the scintillator screen according to the present invention is perfectly smooth because it is this face which is in contact with the convex face of the support during the evaporation of the scintillator material. The photocathode which is deposited on this face therefore has perfectly smooth surfaces. You can vary the thickness. screen size from a few tens of microns to several millimeters while retaining a perfectly smooth concave face. Finally, according to the present invention, the scintillator screen when it is finished no longer comprises the support which was used to its manufacture.

• la figure 1 le schéma d'un écran scintillateur selon l'art antérieur ;
• les figures 2 et 3 deux modes de réalisation d'un écran scintillateur selon l'invention.

Other characteristics and results of the invention will emerge from the following description, given by way of nonlimiting example and illustrated by the appended figures which represent:

• Figure 1 the diagram of a scintillator screen according to the prior art;
• Figures 2 and 3 two embodiments of a scintillator screen according to the invention.

In the different figures, the same references designate the same elements, but for reasons of clarity, the dimensions and proportions of the various elements are not observed.

FIG. 1 represents the diagram seen in section, of a scintillator screen according to the prior art.

This screen is obtained by evaporating cesium iodide on the concave face of a thin metal support 1, made of aluminum for example, which is transparent to X-ray or y to be analyzed. The growth of cesium iodide takes place in the form of needles 2 terminated by tetrahedral crystals which are represented in a box which shows in more detail the structure of the screen. We see in the box of Figure 1 that the concave face of the scintillator screen thus obtained is very irregular. On this concave face, a photocathode 3 sublayer is deposited to chemically isolate the scintillator screen 2 from the photocathode and / or to improve the surface condition of the concave face of the screen. A photocathode 4 is then deposited on the sublayer 3.

FIG. 2 represents the diagram, seen in section, of an embodiment of a screen according to the invention.

On the two boxes which show in more detail the structure of the screen according to the invention, it can be seen that this screen consists of a scintillator material 2 having a structure in needles. The box on the right shows that the concave face of this screen is perfectly smooth. It is on this face that the photocathode 4 is deposited, with, possibly, between this concave face and the photocathode, a sublayer 3 of photocathode, made of phosphovanadates for example.

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

You need a support with a perfectly polished convex face. This support can have any thickness. It can be made of any material, glass or metal, which has a coefficient of thermal expansion different from that of the scintillator material used.

The scintillator screen is obtained by evaporation of the scintillator material on the convex face of the support. After evaporation, the screen is separated from the support by simple heating thanks to the smooth surface of the convex face of the support and thanks to the difference between the coefficients of thermal expansion of the support and of the scintillator material.

A scintillator screen such as that shown in FIG. 2 is thus obtained, the concave face of which is perfectly smooth because it is this face which was in contact with the convex face of the support during the evaporation of the scintillator material. The concave face of the screen has an optical polish. The diameter of the grains on this face varies from 0.1 to 50 micrometers approximately.

As we can see on the left sidebar of figure 2, it is the convex face of the screen which presents a rather irregular surface because of the ends of the needles of the scintillator material but it does not matter because there is no conduction on this face.

Indeed, the conduction is ensured by the photocathode which is deposited on the concave face quite smooth. There is therefore no risk of seeing, as in the prior art, craters of micro-structures interrupt the continuity of the layer in contact with the photocathode and therefore the conduction.

The thickness of the screen can vary, depending on the use, between a few tens of microns and a few millimeters while retaining a perfectly smooth concave face.

If the evaporation support is kept cold during evaporation, the scintillator screen has a structure of finely divided needles. It can then be used in high definition radiological image intensifier tubes.

If, on the other hand, the evaporation support is heated during evaporation, at temperatures ranging from 100 to 600 ° C. for example, a structure in agglomerated needles is obtained which is more monolithic which allows this screen to be used in scintigraphy.

The evaporation support can be made of aluminum for example. The scintillator material used can be an alkali halide, such as cesium iodide doped with sodium or thallium, or such as potassium iodide doped with thallium. It is also possible to use as scintillating material tungstates, sulphides or metallic sulphates for example.

It can be seen that by using the method described above, a scintillator screen is obtained, the two faces of which are accessible for any subsequent treatment desired since the evaporation support is not part of the thermally shielded screen, unlike what happens. in screens according to the prior art.

To increase the mechanical strength of the screen, and particularly in the case of a thin screen in the structure of finely divided needles, a layer 5 can be placed on the convex face of the screen ensuring its rigidity. Such a layer is shown in FIG. 2. This layer ensuring the rigidity of the screen can consist, for example, of a glass or of an enamel with a low melting point, or of any organic substance which withstands the oven temperatures. tube such as for example epoxy resin. parylene, polyimides, or cryolite, for example.

The convex face of the screen can also be provided with a layer reflecting the light produced in the screen by the incident radiation. This layer refers to the entire photocathode the light arriving on the convex face of the screen. This layer can consist, for example, of any evaporated metal such as aluminum or nichrome.

It is also possible to provide the convex face of the screen with a layer increasing the efficiency of quantum detection of the screen. This layer consists of a high density material with a high atomic number, deposited in a thin layer, such as barium oxide, lead or tungsten. This type of material promotes photoelectric emission and the stopping power with respect to ionizing radiation.

It is therefore possible to have on the convex face of the screen one, two or three successive layers, each fulfilling one of the following functions: layer ensuring the rigidity of the screen, layer reflecting the light produced in the screen by the incident radiation, layer increasing the quantum detection efficiency of the screen. The order of the various layers may vary.

It is possible to use layers of materials fulfilling two of the stated functions. For example, a layer of indium or tin can serve both to reflect the light produced in the screen by the incident radiation and also to ensure the rigidity of the screen. Such a layer can be obtained by sputtering, by evaporation, by projection or by any other known method.

As regards the concave face of the screen, this face being perfectly polished, the photocathodes which are deposited there have a minimum surface electrical resistance which no longer depends so much on the scintillator screen but especially on the photocathode which is deposited there.

• couche réduisant encore la résistance électrique superficielle de la photocathode, couche assurant la compatibilité du point de vue chimique entre l'écran scintillateur et la photocathode. On peut déposer sur la face concave de l'écran une sous-couche 3 qui est représentée sur la figure 2. Cette sous-couche peut être constituée d'oxyde d'indium ou d'un métal en couche mince, tel que l'aluminium, qui est largement transparent à la lumière produite par le scintillateur.

The concave face of the screen is generally covered with at least one layer providing at least one of the following functions:

• layer further reducing the surface electrical resistance of the photocathode, layer ensuring compatibility from a chemical point of view between the scintillator screen and the photocathode. One can deposit on the concave face of the screen a sublayer 3 which is represented in FIG. 2. This sublayer can consist of indium oxide or of a metal in a thin layer, such as aluminum, which is largely transparent to the light produced by the scintillator.

On the sub-layer, a photocathode 4 is deposited, consisting for example of cesium and antimony.

To mount the screen according to the invention in the tube where it will be used, one can use a support grid 7 having the same concavity as the screen, as shown in Figure 3. This grid must be transparent to the flow of incident radiation. It can be nickel or iron, for example.

In FIG. 3, the screen shown comprises a layer 6 increasing the efficiency of quantum detection of the screen, a layer 2 of needle scintillator material whose concave face is perfectly polished, a photocathode sublayer 3 and a photocathode 4.

We see in Figure 3, a metal ring 8 is evaporated on the concave face of the screen at the periphery of this face. Pressure tabs 9 are applied to this ring and serve as connections with the photocathode.

The screen according to the invention when it is sufficiently thick can also be mounted in a tube without the use of a support grid.

Various embodiments of the screen according to the invention are obtained by providing one or both sides, or both sides of this screen, with one or more of the various layers mentioned above which can be superimposed in an indifferent order.

Claims (13)

1. Method of fabricating a scintillation material layer for a radiation conversion scintillation screen, receiving the X or γ rays and converting them into light photons, this scintillation material layer (2) being obtained by evaporation on a carrier formed of a material having a thermal expansion coefficient different from that of the scintillation material (2), characterized in that the evaporation is performed on the perfectly smooth convex face of the carrier and in that, after evaporation, the scintillation material layer (2) is separated from the carrier simply by heating.

2. Method according to claim 1, characterized in that the carrier being maintained cool during evaporation, a finely divided needle structure is obtained.

3. Method according to claim 1, characterized in that the-substrate being heated during evaporation, a structure of agglomerated needles is obtained.

4. Radiation conversion scintillation screen receiving X or -y rays, comprising a scintillation material layer which is sensitive to the X or rays and converts them into light photons whereto a photocathode (4) covering one of the faces of this screen is sensitive, this scintillation material layer (2) having a needle structure, a convex face receiving the radiation and a concave face bearing the photocathode (4), characterized in that the concave face of this scintillation material layer is as smooth as the convex surface of a carrier whereupon it is formed by the production method of any of claims 1 to 3, and comprises grains the diameter of which varies from 0.1 to 50 ¡.¡.m.

5. Scintillation screen according to claim 4, characterized in that the convex face of the layer (1) ensuring at least one of the following functions : a layer (5) ensuring the rigidity of the screen, a layer reflecting the light produced in the screen by the incident radiation, a layer (6) increasing the quantum detection efficiency of the screen.

6. Scintillation screen according to claim 4, characterized in that the concave face of the scintillation material layer is covered by at least one layer (3) ensuring at least one of the following functions : a layer ensuring the compatibility with the photocathode, a layer ensuring the reduction of the surface resistance of the photocathode.

7. Scintillation screen according to claim 4, characterized in that the scintillation material (2) used is an alcaline halide, such as cesium iodide doped with sodium or with thallium or such as potassium iodide doped with thallium.

8. Scintillation screen according to claim 4, characterized in that the scintillation material (2) has a structure of finely divided needles or a structure of agglomerated needles.

9. Scintillation screen according to claim 4, characterized in that the scintillation material has a thickness from some ten microns to some millimeters.

10. Scintillation screen according to any of claims 4 to 9, characterized in that the layer ensuring the rigidity of the screen (5) is formed of a glass or an enamel with low melting point or of an organic substance such as epoxy resin, parylene, polyimides, cryolith.

11. Scintillation screen according to any of claims 4 to 10, characterized in that the layer reflecting the light produced in the screen by the incident radiation is of aluminium or of nichrome.

12. Scintillation screen according to any of claims 4 to 11, characterized in that the layer (6) increasing the quantum detection efficiency is formed of a high density material deposited as a thin layer, such as barium, lead or tungsten oxide.

13. Scintillation screen according to any of claims 4 to 12, characterized in that the layer ensuring the reduction of the surface resistance of the photocathode is formed of indium oxide or is a thin metallic layer.

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