EP0319080B1 - X-ray image intensifier tube - Google Patents

Description

The invention relates to an X-ray image intensifier tube comprising an entrance window provided with an aluminum substrate which supports a scintillator which transforms, into visible or near visible light radiation, the X-ray which reaches the scintillator through the substrate, the light radiation being converted by a photocathode into a flow of electrons which, using electronic optics, provides a visible image on an output screen, and between the aluminum substrate and the scintillator is interposed a layer absorbing the light radiation emitted by the scintillator towards the aluminum substrate.

Document FR-A-2 515 423 is known which describes an input screen suitable for use in an image intensifier tube with increased resolution power. For this, the colomnial crystals of cesium iodide which constitute the scintillator come from particles of impurities on the surface of the aluminum substrate. A side effect of these surface impurities is to promote the absorption of the light emitted towards the substrate. The guiding action of the light obtained by the columnar crystals being imperfect, this mechanism of absorption of the light by the particles of impurities improves the contrast of the restored image. These particles of impurities must constitute germs for the growth of the columnar crystals. They therefore form islands scattered on the surface of the aluminum substrate made visible by an appropriate chemical treatment. The light emitted by the scintillator towards the substrate is therefore only incidentally absorbed by these particles of impurities whose presence and nature are random.

The problem is therefore to have a tube provided with an input window having a high resolution for the entire surface of the restored image. The performance of the tube must be reproducible and reliable.

In this sense, an invention of the kind described in the preamble is known from document EP-A-0 240 951. It relates to an X-ray image intensifier which notably comprises a film absorbing light. This film is placed on the surface of the aluminum substrate. The characteristics are not exposed in this document but reference is made to document JP-A-56-165251 which indicates that this layer is formed by evaporation of a layer of blackened Al after anodic oxidation of the aluminum substrate. However, this layer is not properly adapted to absorb the green luminescence radiation from cesium iodide. Indeed, its transmission as a function of the wavelength as well as its optical indices are not well suited to solve this problem.

It is therefore necessary to use other materials. The solution to this technical problem consists in that the absorbent layer consists of a material chosen from the following materials: titanium nitride, cadmium sulfide, (Cu, PbI₂). We thus have a selection of materials with optical indices (refraction and extinction) as well as transmission thresholds in wavelength much better suited to the problem posed.

But given the high refractive index of the photocathode material another mechanism helps to weaken the resolution. It is due to the light rays coming from the scintillator which are very far from normal to the surface of the photocathode and which can penetrate into it. To eliminate these light rays which are very far from normal, the invention places a low index layer between the scintillator and the photocathode having a refractive index lower than that of the photocathode. The material of this layer can be chosen from the following materials: MgF₂, cryolite (Na₃AlF₆).

The scintillator is chosen from the following materials: CsI (Na), NaI (Tl), CsI (Tl), CdWO₄, Bi₄Ge₃O₁₂, CaWO₄.

The radiation which is directed towards the substrate being absorbed, it can result therefrom a reduction in luminosity of the tube with a usual photocathode in Cs₃Sb.

A secondary problem is to have a high brightness while retaining the resolution of the tube.

The solution to this secondary problem is to choose the photocathode from the following materials: K₂CsSb, Rb₂CsSb, (SbNa₂K, Cs), this latter material can be of type S20 or S25 according to its spectral response. These names are known to those skilled in the art. All these materials give the tube a long service life.

This produces a tube with increased resolution and high brightness.

To give the tube all its qualities of lifetime, it is desirable to have, between the scintillator and the photocathode, a chemical barrier consisting of a layer chosen from the following materials: Al₂O₃, Si₃N₄, SiO₂. This chemical barrier prevents the sodium contained in the scintillator from migrating to the photocathode.

It is also possible to improve the collection of charges by placing an electrically conductive and optically transparent layer between the photocathode and the chemical barrier. This layer is chosen from the following materials: palladium, aluminum, In₂O₃, SnO₂, ITO (mixture of In₂O₃ at 90% and SnO₂ at 10%).

• figure 1A : un tube intensificateur d'images à rayons X selon l'invention.
• figure 1B : un schéma d'une fenêtre d'entrée d'un tube d'intensificateur d'images à rayons X selon l'invention.
• figure 2 : une courbe de variations du taux de réflexion d'une couche de TiN déposée sur un substrat d'aluminium en fonction de l'épaisseur de la couche de TiN.
• figure 3 : une courbe analogue à celle de la figure 2 pour une couche de CdS.
• figure 4 : un schéma analogue à celui de la figure 1 avec en supplément une couche à bas indice entre le scintillateur et la photocathode.
• figure 5 : un schéma analogue à celui de la figure 1 avec en plus une barrière chimique et une couche conductrice et transparente placée entre le scintillateur et la photocathode.
• figure 6 : trois courbes de variations du rendement photoélectrique pour différentes structures de fenêtre d'entrée en unités arbitraires.

The invention will be better understood using the following figures, given by way of nonlimiting examples, which represent:

• FIG. 1A: an X-ray image intensifier tube according to the invention.
• FIG. 1B: a diagram of an entrance window of an X-ray image intensifier tube according to the invention.
• FIG. 2: a curve of variations in the reflection rate of a layer of TiN deposited on an aluminum substrate as a function of the thickness of the layer of TiN.
• Figure 3: a curve similar to that of Figure 2 for a CdS layer.
• Figure 4: a diagram similar to that of Figure 1 with an additional low index layer between the scintillator and the photocathode.
• Figure 5: a diagram similar to that of Figure 1 with in addition a chemical barrier and a conductive and transparent layer placed between the scintillator and the photocathode.
• FIG. 6: three curves of variations of the photoelectric efficiency for different structures of entry windows in arbitrary units.

The diagrams in Figures 1, 4 and 5 are not represented to scale in order to better highlight the mechanisms involved.

FIG. 1A represents an X-ray image intensifier tube which comprises, at the input, a separation sheet 23 with the vacuum formed of a suitable material, for example titanium. The separation sheet is followed by an entry window 21. The assembly is mounted in a vacuum envelope which further comprises a cylindrical surface 43 with a conical part 45, a terminal anode support 49 and a window outlet tube 20. The tube is provided at its inlet with a mounting ring 22 to which the separation sheet 23 is connected as well as a support 24 for the inlet screen 21. Through an inlet electrode 26 and electrodes 28, 40, 42 the photoelectron beams 52 coming from a photocathode 13 form an image on a luminescent layer 46 which is preferably deposited on an output screen 20 formed of a plate of optical fibers. The electronic image projected onto the output screen generates an optical image in the layer of luminescent material. This optical image is then used in the usual way to be viewed.

In FIG. 1B, the entrance window 21 comprises in order an aluminum substrate 10, an absorbent layer 11, a scintillator 12 and a photocathode 13. The incident X-rays arrive on the structure through the substrate 10 and electrons e- are emitted by photocathode 13. When X-rays are absorbed at a point 50 of the scintillator, visible radiation is emitted. For example, the ray 51 penetrates the photocathode 13 which emits electrons 52. But the same point 50 can emit rays such as the ray 53 in the direction of the substrate 10. In the absence of the absorbent layer 11, the ray 53 is reflected according to the radius 54 and electrons 55 are emitted by the photocathode. Thus the same point 50 produces several emissions of electrons 52, 55 and this results in a resolution defect in the tube.

, avec n l'indice de réfraction et k l'indice d'extinction, sont respectivement pour TiN, n=1,65 et k=0,79 et pour CdS, n=2,5 et k=0,2. Ils sont donnés à titre d'exemple pour une longueur d'onde de 430 nm et varient peu avec la longueur d'onde.According to the invention the rays 53 which are emitted in direction of the substrate are absorbed. However, absorption must occur in a continuous and homogeneous manner over the entire surface of the entry window in order to restore an image of homogeneous quality. The absorption must be as high as possible for the wavelength of the light emitted by the scintillator. The scintillators can be chosen from the following materials: CsI (Na), NaI (Tl), CsI (Tl), CdWO₄, Bi₄Ge₃O₁₂, CaWO₄. For example, for a CsI (Na) scintillator, the wavelength of the light emitted is close to 430 nm. The absorbent layer must allow this radiation to be absorbed. According to the invention, the material can be chosen from the following materials: TiN, CdS, (Cu, PbI₂). Optical indices $\text{n * = n-ik}$

, with n the refractive index and k the extinction index, are respectively for TiN, n = 1.65 and k = 0.79 and for CdS, n = 2.5 and k = 0.2. They are given by way of example for a wavelength of 430 nm and vary little with the wavelength.

FIG. 2 represents the reflection rate of a TiN layer deposited on an aluminum substrate for light emitted at 430 nm by a CsI (Na) scintillator. This rate is represented as a function of the thickness of the absorbent layer. It is noted that the reflection rate becomes less than 10% as soon as the TiN layer reaches a thickness of approximately 50 nm.

A similar situation appears for other materials such as CdS or (Cu, PbI₂). FIG. 3 represents the reflection rate of a layer of CdS deposited on an aluminum substrate for a wavelength of 430 nm as a function of the thickness of the layer. For cadmium sulfide we have an = 2.5 and k = 0.2. Oscillations therefore appear on the curve of FIG. 3. It is therefore possible to determine thicknesses of CdS layers for which the reflection rate is sufficiently unimportant. Thus, if a rate lower than 10% is chosen, the thicknesses of CdS can be chosen in substantially the following ranges: 115 nm to 135 nm, 185 nm at 235 nm, greater than 260 nm for a light emission at 430 nm.

Each scintillator will have a light spectrum centered on its own central wavelength. These light spectra are distributed between substantially 400 nm and substantially 600 nm. The thicknesses of CdS layers are therefore to be determined both according to a predetermined admissible value for the reflection rate and according to the central emission wavelength of the scintillator used. Those skilled in the art by preliminary measurements of the reflection rate as a function of the thickness for the wavelength and the material chosen can thus easily choose the thickness according to the tolerated reflection rate.

FIG. 4 represents an embodiment of the invention which includes an additional layer 19 with a low refractive index placed between the scintillator 12 and the photocathode 13. In fact, if we consider a light ray 60 coming from point 50 , in the absence of the layer 19 it would arrive at point 63 and would enter the photocathode 13 according to a light ray 61 where it would produce electrons. However, point 63 can be quite distant from the radial direction coming from point 50 perpendicular to the curved surface of the photocathode, a direction which is substantially the axial direction of the columnar crystals. The electrons which come from ray 61 will thus contribute to decrease the resolution of the image of the tube. This second cause of a decrease in resolution is corrected using a low index layer 19 having a refractive index lower than that of the scintillator 12, placed between the scintillator 12 and the photocathode 13. Thus the radius 60 strikes the surface of this layer at point 64 and undergoes a total reflection along the radius 62. The light rays which are substantially distant from the axial direction of the columnar crystals are returned and do not participate in the creation of electrons. This layer 19 must have a low absorption so as not to disturb the luminosity. The material of this layer can be chosen from the following materials: MgF₂, cryolite (Na₃AlF₆). In the useful wavelength range which is between approximately 400 nm and 600 nm, the refractive index of MgF₂ is between 1.33 and 1.37 approximately with an extinction index practically zero. The values are substantially similar for the cryolite.

This absorption and this reflection of light which result in an increase in resolution of the tube, are accompanied by a decrease in luminosity of the tube. It may be desirable to increase this brightness. For this it is possible to use other photocathode materials such as: K₂CsSb, Rb₂CsSb, (SbNa₂K, Cs). They have a better photoelectric yield than the usual Cs₃Sb material.

By normalizing the photoelectric efficiency of the structure formed by a photocathode Cs₃Sb associated with a scintillator (CsI, Na) to 1, we obtain a yield of 1.60 with a photocathode (SbNa₂K, Cs) and a yield of 2.32 with a K₂CsSb photocathode. The photocathodes formed from materials K₂CsSb, Rb₂CsSb, or (SbNa₂K, Cs) are therefore well suited to increase the luminous performances which are altered by the absorbent layers of TiN. On the other hand, they make it possible to give the structure a long service life.

But to give such a structure all its potential qualities of long life, it is desirable to interpose a chemical barrier between the scintillator and the photocathode in order to prevent the Na element migrating towards the photocathode during the manufacture of the tube. This chemical barrier consists of a layer chosen from the following materials: Al₂O₃, Si₃N₄, SiO₂.

Since the photocathode material is generally not very conductive, it is possible to ensure a homogeneous distribution of the electrical potential by placing a conductive layer on the photocathode on the side of the scintillator. This conductive layer must also be transparent to allow pass the radiation emitted by the scintillator. If a chemical barrier exists, the conductive and transparent layer is placed between the photocathode and the chemical barrier. The following materials can be used: palladium, aluminum, In₂O₃, SnO₂ or the ITO material which a mixture of In₂O₃ (90%) and SnO₂ (10%).

We then obtain an entry window shown in Figure 5. We find, as in Figure 1, the aluminum substrate 10, the absorbent layer 11, the scintillator 12 and the photocathode 13 which is preferably made of a material with high photoelectric yield. In addition, in contact with the photocathode, is arranged the conductive and transparent layer 18, preceded by the chemical barrier 17. The low index layer 19 shown in FIG. 4 can be associated with the conductive and transparent layer 18 and the barrier chemical 17 shown in FIG. 5. Thus, the low-index layer 19 can be placed between the scintillator 12 and the chemical barrier 17. It can also be placed between the chemical barrier 17 and the conductive and transparent layer 18.

The entry window produced with an absorbent layer, for example made of TiN, and a photocathode with a high photoelectric efficiency, for example made of K₂CsSb, will generally be able to have a photoelectric yield higher than an entry window made without these materials. This is highlighted in FIG. 6 which represents the variations in the photoelectric efficiency Y of a photocathode as a function of the thickness of the scintillator. The thickness of the photocathode is such that the photoelectric efficiency is at its maximum. Curve 31 relates to the Al / TiN / CsI, Na / Al₂O₃ / K₂CsSb structure. Its reflection rate of light emitted by the scintillator and reflected by the substrate is less than 10%. Curve 32 relates to the Al / CsI, Na / Al₂O₃ / K₂CsSb structure. Its reflection rate is around 70%. Curve 31 is below curve 32 because indeed, the absorbed light is lost and does not can generate electrons. Curve 33 relates to the Al / CsI, Na / Cs₃Sb structure. Curve 31 is located above curve 33. This means that an entrance window with an absorbent layer and a photocathode with high photoelectric efficiency can exhibit increased performance compared to a usual structure. Likewise, it can be seen that an input window corresponding to curve 31 can have performances equal to those obtained with an input window corresponding to curve 33, and this with a much smaller scintillator thickness. In Figure 6 we see that this thickness can be reduced from 0.4 to 0.2 mm approximately. This reduction in the thickness of the scintillator also contributes to improving the resolution of the tube. The crystals which constitute the scintillator generally have, over a certain thickness (ten micrometers), dislocations which cause a diffusion of light. The proposed thickness reduction retains a sufficient scintillator thickness so that this dislocated zone only slightly disturbs the mechanisms. However, this reduction in thickness is very advantageous since these X-ray detection tubes require crystals to grow on entry window surfaces of several square decimetres. Such a reduction in thickness results in a significant saving of material and an increased manufacturing yield.

Claims (11)

1. An X-ray image intensifier tube, comprising an entrance window provided with an aluminium substrate which supports a scintillator which transforms X-rays reaching the scintillator through the substrate into visible or nearly visible light radiation which is converted, by way of a photocathode, into a flux of electrons to produce a visible image on an exit screen via electron-optical means, between the aluminium substrate and the scintillator there being provided a layer absorbing the light radiation emitted by the scintillator in the direction of the aluminium substrate, characterized in that the absorbing layer consists of a material chosen from the following materials:
   titanium nitride, cadmium sulphide, (Cu, PbI₂)
2. A tube as claimed in Claim 1, characterized in that a low-index layer, having a refractive index which is lower than that of the photocathode, is inserted between the scintillator and the photocathode.
3. A tube as claimed in Claim 2, characterized in that the material of the low-index layer is chosen from the following materials:
   MgF₂, cryolite (Na₃AlF₆).
4. A tube as claimed in any one of the Claims 1 to 3, characterized in that the titanium nitride layer has a thickness of at least 50 nm.
5. A tube as claimed in Claim 4, characterized in that the thickness is between 75 nm and 120 nm.
6. A tube as claimed in any one of the Claims 1 to 3, characterized in that the cadmium sulphide layer has a thickness in one of the following ranges: between approximately 115 nm and 135 nm, between approximately 185 nm and 235 nm, and more than approximately 260 nm.
7. A tube as claimed in any one of the Claims 1 to 6, characterized in that the photocathode is chosen from the following materials: K₂CsSb, Rb₂CsSb, SbCs₃, (SbNa₂K, Cs).
8. A tube as claimed in any one of the Claims 1 to 7, characterized in that the scintillator is chosen from the following materials: CsI(Na), NaI(Tl), Csl(Tl), CdWO₄, Bi₄Ge₃,O₁₂, CaWO₄
9. A tube as claimed in Claim 8, characterized in that the scintillator made of CsI(Na) has a thickness of between 100 and approximately 1000 micrometers.
10. A tube as claimed in the Claims 7 and 9, characterized in that a chemical barrier is provided between the scintillator and the photocathode, the chemical barrier being chosen from the following materials: Al₂O₃, Si₃N₄, SiO₂.
11. A tube as claimed in Claim 10, characterized in that an electrically conductive and optically transparent layer is deposited between the photocathode and the chemical barrier, which layer is chosen from the following materials: palladium, aluminium, In₂O₃, SnO₂, a mixture of In₂O₃ (90%) and SnO₂ (10%).

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