EP0325500B1 - Input screen scintillator for a radiological image intensifier tube and its manufacturing method - Google Patents

Description

The present invention relates to an input screen scintillator for an X-ray image intensifier tube. It also relates to a method of manufacturing such a scintillator.

Radiological image intensifier tubes are well known in the art. They make it possible in particular to transform a radiological image into a visible image, generally to ensure medical observation. A detailed description of such tubes is found in particular in two European patent documents EP-A-0 197 597 and EP-A-0 199 426 among which the first further describes a method for absorbing secondary X-rays generated by a scintillator; and the second describes in particular how to improve a photon-electron conversion efficiency.

These tubes include an input screen, an electronic optical system and an observation screen.

The input screen includes a scintillator which converts incident X photons into visible photons which then strike a photocathode, generally constituted by an alkaline antimonide; this photocathode thus excited, generates a flow of electrons. The photocathode is not deposited directly on the scintillator but on an electrically conductive sub-layer which allows the charges of the photocathode material to be reconstituted. This sub-layer can for example consist of alumina, indium oxide or a mixture of these two bodies.

The flow of electrons from the photocathode is then transmitted by the electronic optical system which focuses the electrons and directs them onto an observation screen made up of a phosphor which then emits visible light. This light can then be processed, for example, by a television, cinema or photography system.

The scintillator of the entry screen generally consists of cesium iodide needles formed by vacuum evaporation on a substrate. Evaporation can take place on a cold or hot substrate. The substrate is preferably aluminum. A thickness of cesium iodide is deposited on this substrate, which is generally between 150 and 500 micrometers.

Cesium iodide is naturally deposited in the form of needles 5 to 10 micrometers in diameter. Its refractive index being 1.8, there is a certain optical fiber effect which reduces the lateral scattering of the light generated within the material. We can also cite the case of scintillators of the type as described in European patent document EP-A-0 242 024, type in which the scintillator is divided into a plurality of cells of approximately 100 micrometers in diameter each; of course in such a case, the resolution is lower than that which is obtained with a scintillator formed of needles.

In Figure 1, there is shown schematically, an aluminum substrate 1 carrying a few needles 2 of cesium iodide. The aluminum substrate receives a flow of X photons symbolized by vertical arrows. The figure shows examples of paths followed in cesium iodide needles by the visible radiation created by the incident X photons. The normal paths of these visible rays, which bear the reference 3, cause the production of a light signal at the end of the cesium iodide needles. There is also a lateral scattering of the light carried by the cesium iodide needles, as indicated in the figure by the reference 4. This lateral scattering causes a decrease in the resolution of the tube. Indeed, this resolution depends on the ability of cesium iodide needles to properly channel the light. It also depends on the thickness of the cesium iodide layer. An increase in thickness leads to a deterioration in resolution. But, moreover, the greater the thickness of cesium iodide, the more the X-rays are absorbed. A compromise must therefore be found between the absorption of X-rays and the resolution.

Another factor that plays on the resolution of the tube is the heat treatment that the input screen must undergo during its manufacture. This treatment takes place immediately after the evaporation of the cesium iodide under vacuum. It ensures the luminescence of the screen due to the doping of cesium iodide by sodium or thallium ions for example. This heat treatment consists in bringing the screen to the temperature of approximately 340 ° C, for about an hour, by placing it in an atmosphere of dry air or nitrogen.

During this absolutely compulsory heat treatment, the needles of the scintillator undergo a certain coalescence and agglomerate between them, as has been shown diagrammatically in FIG. 2. This coalescence causes an even greater lateral scattering of light (see the dashed arrows marked 4), and the resolution is deteriorated.

To eliminate the coalescence which occurs during heat treatment, it has been proposed, in the prior art, to produce the scintillator of the input screen by evaporating alternately pure cesium iodide and cesium iodide doped with refractory material. It was hoped that needles thus formed by alternating layers of pure cesium iodide and cesium iodide doped with a refractory material would not come into contact during the heat treatment.

This solution did not achieve the desired result. In addition, another important problem which is to avoid the lateral diffusion of light, is not at all resolved by the alternation of layers of pure cesium iodide and cesium iodide doped with a refractory material.

It has therefore been envisaged, as described in US Pat. No. 4,069,355 published on January 17, 1978, to cover the cesium iodide needles with titanium dioxide or with gadolinium or lanthanum oxysulfide. These deposited materials containing a metal, not under metallic force, but in the form of an oxide or a compound, make it possible to partially solve the problems posed: they avoid the coalescence of the needles and make it possible to slightly reduce the lateral diffusion of the light, however, without this reduction in diffusion causing an appreciable increase in the efficiency of the scintillator.
A European patent application EP-A-0 215 699 teaches to coat the needles in cesium iodide with a refractory material having an optical index close to or lower than that of cesium iodide, in order to avoid the coalescence of the needles and promote the optical fiber effect that these present.

Another unsolved problem, even in the scintillator of the aforementioned patent is that of the electrical conduction which it is desirable to obtain for any layer covering the needles, while avoiding coalescence and lateral light scattering. This conduction is in fact desirable to increase the efficiency of the scintillator by bringing to the same potential the layer covering the needles, the aluminum substrate on which these needles are formed and an annular electrode to which this substrate is connected.

The object of the invention is precisely to remedy these drawbacks by producing a scintillator in which the cesium iodide needles are covered with a material which is good conductor of electricity, avoiding the coalescence of the needles while notably reducing the lateral diffusion. light. These aims are achieved by choosing a material which is a semiconductor or a metal and not a metal oxide.

The invention relates to a scintillator for the entry screen of an X-ray image intensifier tube comprising light-conducting cesium iodide needles formed on an electrically conducting and light-reflecting substrate, each needle being entirely coated on its surfaces. , excluding the surface in contact with the substrate, by a layer of the same material, said layer reflecting at least partially the light circulating in each needle and making an impact on the lateral face of each needle towards the inside of the latter, characterized in that said material is a good conductor of electricity for carrying said layer and the substrate to the same electrical potential, and that said material is a metal or a semiconductor with the exclusion of metal oxides, said layer being in electrical contact with the substrate.

According to another characteristic of the invention, the material is diluted in a resin.

The invention also relates to a method of manufacturing a scintillator according to claim 1, in which the material is a metal, characterized in that it consists in carrying out a direct deposition of said metal on the needles by photochemical decomposition of molecules d '' a metal compound in the gas phase.

According to another characteristic of the invention, the method consists in depositing said material on the needles, by diffusion of this material in solution in an organic solvent or a polymerizable resin, this diffusion being followed by heat treatment.

According to another characteristic of the process, the material is a metal and the process consists in depositing said metal on the needles by thermal decomposition of an organo-metallic compound having previously diffused in the gas phase between the needles.

According to another characteristic of the process, the metal is chosen from a list comprising at least indium, gallium, zinc, tin, lead.

According to another characteristic, the material being a semiconductor, it consists of silicon or germanium.

• les figures 1 et 2 ont déjà été décrites et représentent schématiquement un scintillateur connu dans l'état de la technique,
• la figure 3 représente schématiquement un scintillateur conforme à l'invention,
• la figure 4 est un diagramme qui représente les fonctions de transfert de modulation (FTM), en fonction de la fréquence spatiale des rayonnements reçus par le scintillateur, pour un scintillateur connu dans l'état de la technique et pour le scintillateur de l'invention.

The characteristics and advantages of the invention will emerge more clearly from the description which follows, given with reference to the appended drawings in which:

• FIGS. 1 and 2 have already been described and schematically represent a scintillator known in the state of the art,
• FIG. 3 schematically represents a scintillator according to the invention,
• FIG. 4 is a diagram which represents the modulation transfer functions (FTM), as a function of the spatial frequency of the radiation received by the scintillator, for a scintillator known in the state of the art and for the scintillator of the invention .

The scintillator according to the invention, shown diagrammatically in FIG. 3, comprises, like the known scintillator, a metallic substrate 1 (made of aluminum for example), carrying needles 2 made of cesium iodide. According to the invention, each needle is entirely coated, except its surface in contact with the substrate 1, by a material 5, such as a metal or a semiconductor reflecting the light circulating in the needles, towards the inside of those -this. Light beam paths are shown as an example in this figure, at 6, 7, 8, 9. The needles are coated with the material which is inserted into the interstices between these needles and which acts as an optical barrier, while avoiding the coalescence of these needles.

The material deposited on the needles, which is reflective, metallic or semi-conductive, has a melting point as high as possible so as not to be affected by the heat treatments occurring during manufacture.

The fact that this material is conductive or semiconductor to the exclusion of metal oxides makes it possible to bring to the same potential the layer which covers the needles of cesium iodide as well as the substrate. This makes it possible to reduce the thickness or to remove the conductive sublayers which exist in the image intensifier tubes, between the scintillator and the photocathode; this also makes it possible to increase the efficiency of the scintillator.

The light rays, the paths of which have been shown at 6, 7, 8, 9 in FIG. 4, are channeled inside the cesium iodide needles, thanks to the reflective layer 5 which coats these needles. The angles of incidence of the light rays around the periphery of each needle are such that these rays are reflected inside of them. The angle of incidence of the rays on the exit surface 10 of the scintillator is such that these rays are diffused towards the outside. The material which coats the needles can be a semiconductor such as silicon or germanium, or a metal such as indium, gallium, zinc, tin, lead, etc. In the case where the material is a metal, this metal is in the metallic state, unlike scintillators of the state of the art in which metallic oxysulfides or oxides are used.

According to the method of the invention, in the case where the material is a metal, the deposition of the latter on the needles is carried out by photochemical decomposition of corresponding metal molecules, in the gas phase. For this, after an initial vacuum of the substrate and the cesium iodide needles in an enclosure, silane (SiH₄) diluted in nitrogen. At a temperature which can range from ambient temperature to a temperature in the region of approximately 200 ° C., the silane molecules are destroyed under ultraviolet excitation, possibly in the presence of mercury acting as catalyst. This photochemical action is accompanied by the deposition of the metal on the cesium iodide needles.

According to another embodiment of the process of the invention, whether the material is metallic or semiconductor, it is possible to deposit this material on the needles, by diffusion of this material in solution in a solvent organic or polymerizable resin. This diffusion is followed by a heat treatment which makes it possible to remove the solvent and to leave on the needles a film of polymerized resin containing the reflective material.

According to another embodiment of the process of the invention, when the material is a metal, it is possible to deposit this metal on the needles, by thermal decomposition of an organometallic compound, having previously diffused into gas phase between the needles.

This compound can be of the form MXn, in which M represents the chosen metal and X represents an organic group such as methyl (-CH₃) or ethyl (C₂H₅) or any other organic group containing hydrogen atoms or chlorine atoms.

Les produits gazeux sont généralement de l'hydrogène et des hydrocarbures.The diffusion of the organometallic compound is carried out under vacuum. The scintillator is then heated and the organometallic compound breaks down into a metal, in contact with the needles of the hot scintillator according to the reaction: $$\text{MXn → M + gaseous products.}$$

The gaseous products are generally hydrogen and hydrocarbons.

The process which has just been described makes it possible to deposit the material in a thin layer on an essentially vertical substrate, constituted by the needles of the scintillator. It overcomes the difficulties of carrying out the coating of needles, which mainly arise from the fact that the interstices between these needles have a great length, compared to their diameter. The interstices have in fact a length which is approximately a thousand times greater than their diameter.

The invention makes it possible to achieve the goals mentioned above: it makes it possible to channel the light inside the needles, while making their surface electrically conductive and to increase the efficiency of the scintillator, by eliminating the losses of lateral light .

FIG. 4 is a diagram showing the evolution of the modulation transfer function (FTM), with respect to the spatial frequency F of the received radiation, for a scintillator of the state of the art as represented by the curve 11, - that is to say having no coating around the cesium iodide needles - and for a scintillator according to the invention, with coating by a metal or a semiconductor, as represented by curve 12 We see on this diagram that the transfer function (FTM) is much higher in the case of the scintillator of the invention (curve 12), than in the case of a scintillator according to the prior art (curve 11). The scintillator of the invention therefore has better resolution and a higher modulation transfer function than the scintillators of the prior art.

Claims (8)

1. Input screen scintillator for an X-ray image intensifier tube comprising light-conductive caesium iodide needles (2) formed on an electrically conductive and light-reflecting substrate (1), each needle being entirely coated on its surfaces, with the exception of the surface in contact with the substrate (1), with a layer of one and the same material (5), the said layer at least partially reflecting the light propagating in each needle and incident on the lateral face of each needle towards the inside of the latter, characterized in that the said material is a good conductor of electricity in order to bring the said layer and the substrate to the same electrical potential, and in that the said material is a metal or a semiconductor to the exclusion of metal oxides, the said layer being in electrical contact with the substrate.
2. Scintillator according to Claim 1, characterized in that the said layer consists of the said material diluted in a resin.
3. Scintillator according to Claim 1, characterized in that the said material (5) is a metal chosen from a list comprising at least indium, gallium, zinc, tin and lead.
4. Scintillator according to Claim 1, characterized in that the said material (5) is a semiconductor consisting of silicon or of germanium.
5. Manufacturing process of a scintillator according to Claim 1, wherein the material (5) is a metal, characterized in that it consists in depositing the said metal on the needles by photochemical decomposition of molecules of a compound of the metal in the gaseous phase.
6. Manufacturing process of a scintillator according to Claim 1, wherein the said material (5) is a metal, characterized in that it consists in depositing the said metal on the needles by thermal decomposition of an organometallic compound having previously been diffused between the needles in gaseous phase.
7. Manufacturing process of a scintillator according to Claim 1, characterized in that it consists in depositing the said material (5) on the needles, by diffusion of this material in solution in an organic solvent, this diffusion being followed by a thermal treatment.
8. Manufacturing process of a scintillator according to Claim 2, characterized in that it consists in depositing the said material (5) on the needles by diffusion of this material in solution in a polymerizable resin, this diffusion being followed by a thermal treatment.

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