EP0062553A1 - Image intensifier tube target and image intensifier tube with video output comprising such a target - Google Patents

Abstract

A variable gain IIR target is obtained by using an optical fiber wafer whose face (f1), on the IIR side, has blind holes, containing two types of luminescent grain materials (L1 and L2), of yields different light sources, which are separated by a barrier layer (14). The electron beam from the photocathode of the I.I.R. is subjected to two different acceleration voltages, the lowest acceleration voltage ensuring the excitation of the single luminescent material of lower light output and the highest acceleration voltage ensuring the excitation of the luminescent material of higher light output . Application to I.I.R. operating in radiography and radioscopy.

Description

The present invention relates to an image intensifier tube target. It also relates to image intensifier tubes with video output provided with such a target.

The following description will essentially relate to radiological image intensifier tubes, commonly designated by the initials IIR, but it is understood that the invention also applies to light image intensifier tubes and scintigraphy image intensifier tubes (radiation to).

In I.I.R., we want to have variable gain targets, whose gain, that is to say the number of photons emitted for each electron received by the target, can be multiplied by a factor of about 100. Thus the I.I.R. can, as desired, operate in radiography or radioscopy.

In radiography, the video output signal from the I.I.R. allows viewing on a television screen of the information contained in the X-ray beam reaching the I.I.R. ; the television image is recorded on film or photo. To have a good signal-to-noise ratio, a high dose of X-rays must be sent during the short exposure time. It is therefore necessary to have a low gain target to avoid its saturation.

In radioscopy, we limit ourselves to directly observing the television screen. During the relatively long observation time, a low dose of X-rays is sent. It is therefore necessary to have a high gain target in order to obtain a good image.

We know from French patent application No. 77.05031, published under No. 2,341,939, a target of I.I.R. video output with variable gain.

It is a silicon target of which one of the faces is covered with a luminescent layer 12, itself covered a metallic barrier layer 13.

The electron beam 14 from the cathode of the I.I.R. arrives on the metallic barrier layer which slows it down and lets through only the higher energy electrons. These electrons cause in the luminescent layer the creation of photons which generate charge carriers in the target's silicon. These charge carriers discharge diodes, polarized in reverse, and being on the other face of the target; finally the distribution of the charges on the other face of the target is explored by the electron beam of a shooting tube which provides the video signal.

The gain variation of the target is obtained by varying the acceleration voltage of the beam of the I.I.R. and using the non-linear relationship that exists, for metallic barrier layers, between the penetration of electrons into the barrier layer and the acceleration voltage of the electron beam.

• - la résolution de l'l.I.R. est réduite du fait de l'utilisation de deux couches recouvrant la cible en silicium : la couche-barrière métallique et la couche luminescente ;
• - la présence d'une couche-barrière métallique introduit du bruit et entraîne des défauts de l'image obtenue, comme cela est noté à la page 2, lignes 6 à 21 et à la page 5, lignes 27 à 31 de la demande de brevet citée.

This target with variable gain according to the prior art has the following drawbacks:

• - the resolution of the IR is reduced due to the use of two layers covering the silicon target: the metal barrier layer and the luminescent layer;
• - the presence of a metallic barrier layer introduces noise and causes defects in the image obtained, as noted on page 2, lines 6 to 21 and on page 5, lines 27 to 31 of the application for patent cited.

The present invention relates to a variable gain target which overcomes these drawbacks.

The present invention relates to an image intensifier tube target, this tube comprising means making it possible to subject the electron beam coming from its photocathode to two different acceleration voltages.

The target according to the invention comprises two types of luminescent materials, of different light yields, which receive the impact of the electron beam. Means ensure the excitation of the only luminescent material of lower light output by the electron beam subjected to the lowest acceleration voltage and ensure the excitation of the luminescent material of higher light output by the electron beam subjected to the higher acceleration voltage.

Thus by going from one acceleration voltage to another, there is a variable gain target in large proportions by varying the light output of the two luminescent materials constituting the target. We no longer use a metallic barrier layer which causes noise and image defects.

According to a preferred embodiment of the invention, the two luminescent materials of the target emit light of different wavelength and the target has a suitable optical filter which transmits more of the light emitted by the luminescent material of high luminous efficiency than that emitted by the other luminescent material.

Thus, we obtain a target whose gain can be multiplied by a factor of 100 approximately.

Finally, according to another preferred embodiment of the invention, the two luminescent materials are carried by a wafer of optical fibers and a better resolution is obtained than in the case of the prior art where the target is made of silicon, covered with '' a luminescent layer and a metallic barrier layer.

• - la figure 1, le schéma d'un I.I.R. à sortie vidéo selon l'art antérieur ;
• - la figure 2, le schéma d'un mode de réalisation d'une cible selon l'invention ;
• - les figures 3 et 6 à 8, des schémas montrant, de façon plus détaillée que sur la figure 2, plusieurs modes de réalisation selon l'invention de la face^:de la cible qui reçoit le faisceau d'électrons en provenance de l'I.I.R. ;
• - la figure 4, la variation de la luminance en fonction de la tension d'accélération pour les matériaux luminescents L₁ et L₂ ;
• - la figure 5, les variations du coefficient de transmission du filtre optique 10 en fonction de la longueur d'onde ;
• - les figures 9 et 10, deux modes de réalisation d'un I.I.R. à sortie vidéo comportant une cible selon l'invention.

Other objects, 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 an IIR video output according to the prior art;
• - Figure 2, the diagram of an embodiment of a target according to the invention;
• - Figures 3 and 6 to 8, diagrams showing, in more detail than in Figure 2, several embodiments according to the invention of the face ^: of the target which receives the electron beam from the IIR;
• - Figure 4, the variation of the luminance according to the acceleration voltage for luminescent materials L ₁ and L ₂ ;
• - Figure 5, the variations of the transmission coefficient of the optical filter 10 as a function of the wavelength;
• - Figures 9 and 10, two embodiments of an IIR video output comprising a target 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.

Figure 1 shows the diagram of an I.I.R. with video output which is generally designated by the reference 1.

From left to right in the figure, we first find the I.I.R. then the shooting tube which are contained in the same vacuum chamber 2.

An X-ray beam, after passing through the body to be observed 3, enters the I.I.R. through a window 4.

• - un écran d'entrée, constitué d'un scintillateur 5 et d'une photocathode 6 qui assurent la conversion des rayons X en photons lumineux, puis en photo-électrons ;
• - une optique électronique constituée de grilles g₁, g₂ et g₃ qui assurent la focalisation des électrons et les soumettent à une tension d'accélération ;
• - une anode cônique A ;
• - une cible 7 qui reçoit sur sa face f₁ l'impact du faisceau d'électrons.

The IIR includes:

• - an input screen, consisting of a scintillator 5 and a photocathode 6 which convert X-rays into light photons, then into photo-electrons;
• - an electronic optic made up of g ₁ , g ₂ and g ₃ gates which focus the electrons and subject them to an acceleration voltage;
• - a conical anode A;
• - A target 7 which receives on its face f ₁ the impact of the electron beam.

The other face f ₂ of the target 7 is scanned line after line by an electron beam produced by the cathode K, heated by a filament 8, from the shooting tube. This electron beam is focused and accelerated by grids g ₄ to g ₇ .

Coils, not shown, carry out the concentration and the deflection of the beam.

On target 7, the output video signal S is collected.

FIG. 2 represents the diagram of an embodiment of a target according to the invention.

This target consists of a plate of optical fibers, 2 to 5 mm in length for example.

The face f of this target, that is to say the one placed on the I.I.R. side, is shown in more detail in FIG. 3.

In FIG. 3, it can be seen that each optical fiber of the wafer has a blind hole which is obtained by eliminating, over a depth of 5 μm for example, the core of the fibers 12, without touching their coating 13. This can, for example, to be obtained by selective chemical attack on the two glasses constituting the core and the coating. There are thus obtained blind holes 5 μm deep, 5 μm in diameter, for example, and which are separated by walls of 2 μm for example.

Inside each hole, a layer of luminescent material L ₂ , in grains, of high light output r ₂ , is first deposited, then a barrier layer 14 and another layer of luminescent material L ,, also in grains , but of low light output r ₁ .

Before filling the holes in the manner which has just been exposed, the side walls of each hole are covered with a thin metallic layer 15. It is generally aluminum, evaporated under vacuum, with an appropriate incidence. When the holes are filled, the layer L ₁ is also covered with a thin metallic layer 15.

The IIR comprises means, it is a manual or automatic switching device, which make it possible to subject the electron beam coming from its photocathode to two equal acceleration voltages, V ₁ and V ₂ , equal for example at 10 KV and 30 KV.

We know how to choose the nature, the thickness of the luminescent materials L ₁ , L ₂ and of the barrier layer 14 of FIG. 3 so that only the luminescent material L of lower light output is excited by the electron beam subjected to the lowest acceleration voltage V ₁ , and so that the luminescent material L ₂ with the highest light output is mainly excited by the electron beam subjected to the highest acceleration voltage V ₂ .

Other means enabling this result to be obtained will be presented below; they differ from those presented, for example, by the absence of a barrier layer or by the fact that the luminescent materials are not in grains but in a transparent thin layer, obtained by evaporation under vacuum of their constituent material.

FIG. 4 represents the variations in luminescence L as a function of the acceleration voltage for the materials L ₁ and L ₂ .

The current density of the incident beam being constant, the luminance increases with the acceleration voltage from a threshold value V ₀₁ for L ₁ , V ₀₂ for L ₂ and the growth is faster for L ₂ than for L ₁ .

When an electron beam coming from the photocathode of the IIR tube arrives on the face f of the target 7, it crosses the metal layer 15, then penetrates into the first layer of luminescent material L ₁ .

If this beam is subjected to the lowest acceleration voltage V ₁ , a part, 50% for example, of the beam electrons does not exceed the L layer and the other 50% do not exceed the barrier layer.

The excitation of the L ₁ layer produces light, in a fairly small amount because of the low light output of this layer. The outer surface of layer L ₁ and the walls of each blind hole being covered with the thin metallic layer 15, the light emitted by layer L ₁ of each fiber propagates along the fiber towards the face f ₂ of the target. 7. There is no light scattering and the same resolution as that of the fiber board is maintained.

If the beam is subjected to the highest acceleration voltage V ₂₁ a part 15% for example, electrons of the beam does not exceed the layer L ₁ , another part, 35% for example, does not exceed the barrier layer and the remainder excites the layer L ₂ of high light output.

It is easily understood that for the acceleration voltage V ₂ , the quantity of light emitted is much greater than that which is emitted for the acceleration voltage V ₁ .

For V ₁ = 10 KV and V ₂ = 30 KV, and with a ratio of light yields r ₂ / wrinkle on the order of 5, a gain is obtained which can be multiplied by a ratio of approximately 20.

It is possible to obtain a variable gain which can be multiplied by a ratio approximately 100 by using luminescent materials L ₁ and L ₂ which emit light of different wavelength λ ₁ and λ ₂ , red and green by example, and by using a suitable optical filter which transmits more light emitted by the luminescent material L ₂ with high light output than that emitted by the other luminescent material L ₁ .

The luminescent material in grains L ₁ which emits red light can consist for example of yttrium oxysulfide doped with europium or yttrium oxide doped with europium, with a particle size of less than 1 μm. The luminescent material in grains L ₂ which emits green light can consist, for example, of zinc cadmium sulfide doped with silver, with a particle size of less than 2 μm. The L ₁ layer is a monolayer with a thickness of less than 1 μm and the L ₂ layer has a thickness of 4 μm for example.

In FIG. 5, the variations of the transmission coefficient T of such an optical filter adapted as a function of the wavelength λ have been shown.

We adapt the transmission coefficient T ₁ of the filter for λ ₁ and the transmission coefficient T ₂ of the filter for λ ₂ , to obtain a gain multiplied by a ratio 100, or even more if necessary.

The light emitted by the luminescent materials L _I and L ₂ propagates along the optical fibers to the opposite face f ₂ of the target 7, the structure of which will be examined in FIG. 2.

The face f ₂ of the target 7 is covered with a thin transparent conductive layer 9, which is obtained by evaporation under vacuum. This layer may consist of tin oxide SnO ₂ , indium oxide In ₂ 0 ₃ , cadmium oxide Cd O ₃ , manganese oxide Mn 0, or mixtures of these oxides.

To obtain a gain which can be multiplied by 100, the layer 9 is covered by the adapted optical filter 10.

This filter can be obtained by evaporation of a material in a very thin layer, less than a micron and by acting on the thickness of the layer to modify the transmission, as is known. It is possible to evaporate lutetium diphthalocyanide for example.

On the filter 10, is deposited a conventional photosensitive target 11 of the shooting tube. It can be a continuous photoconductive layer or reverse polarized diodes.

This photoconductive layer may consist of antimony triphide, amorphous selenium, an amorphous compound of selenium tellurium, sulfur and arsenic, or even a layer of lead oxide.

This target is read line after line by the electron beam of the shooting tube.

It is understood that if it is sufficient to obtain a gain which can be multiplied by a ratio approximately 20, it is possible to use luminescent materials emitting light of the same wavelength and the suitable optical filter can be eliminated.

According to another embodiment of the invention, the face f ₂ of the plate can, like the face f ₁ , include blind holes filled with the three layers 9, 10, 11.

Figures 6, 7 and 8 show other embodiments of the face f ₁ of the target. In all of these embodiments, the target consists of a wafer of optical fibers.

In Figure 6, as in Figure 3, each fiber has a blind hole.

Inside each hole, a layer of luminescent material L2, in grains, of high light output r ₂ , is first deposited, then an evaporated layer L of luminescent material, of low light output r ₁ , is deposited.

The evaporated layer L _I can be chosen so that there is no need for a barrier layer sandwiched between layers L ₁ and L2, and that the lowest acceleration voltage V ₁ causes excitation only of the layer L ₁ and the highest acceleration voltage V ₂ causes the excitation of the layer L ₂ .

The evaporated layer L ₁ can also be chosen to have a sufficiently low light output to obtain a gain which is multiplied by approximately 100 when passing from V ₁ to V ₂ and without the need for an adapted optical filter.

As in the case of FIG. 3, to preserve the resolution of the optical fiber wafer, the side walls of the blind holes and the external surface of the layer L ₁ are covered with a thin metallic layer 15.

FIG. 7 represents an embodiment of the face f _i of the target in which the surface of the wafer is covered with two evaporated layers L ₁ and L ₂ of luminescent material, of different light yields. A barrier layer 14, also obtained by vacuum evaporation, can be if necessary interposed between the layers L ₁ and L ₂ . A thin metal layer 15 covers the external surface of the layer L ₁ of low light output.

The use of thin layers L ₁ , L ₂ and 15, obtained by vacuum evaporation of their constituent materials, makes it possible to obtain a target having good resolution without the need to dig the fibers.

In the embodiment of the face f of the target which is shown in Figure 8, the core 12 of the fibers protrudes from the surface of the wafer. This can, for example, be obtained by selective chemical attack of the two glasses constituting the core and the coating, as was the case for obtaining the blind holes of FIGS. 3 and 6, but there, it is the coating of the fibers which is eliminated.

As in the case of FIG. 7, two evaporated layers L ₁ and L ₂ of luminescent material with different light yields are deposited on the surface of each core. A thin metal layer 15 covers the layer L _I and an evaporated barrier layer can be used if necessary.

L ₁ layer can be made of europium doped yttrium oxysulfide and L ₂ layer can be made of terbium doped yttrium oxysulfide. These two layers are deposited in a conventional manner by electron gun.

According to another embodiment of the invention which is not shown in the figures, the target can be formed, no longer by a wafer of optical fibers, but by a semiconductor substrate, made of silicon, for example. The silicon surface is then covered with two layers L ₁ and L ₂ which are preferably evaporated layers of luminescent material and not of luminescent grain material, so as to improve the resolution.

According to another embodiment of the invention, two layers of luminescent materials are no longer used, superimposed and possibly separated by a barrier layer. Two types of luminescent grain materials are used, with different light output, but the grains of the two materials are mixed and the grains of one of the materials are covered with a barrier layer.

Figures 9 and 10 show two embodiments of an I.I.R. with video output comprising a target according to the invention.

Unlike the embodiment of Figure 1, the I.I.R. 20 and the picture tube 21 are located in two separate vacuum chambers.

In FIG. 9, the IIR tube comprises a target 7 such as that which is represented in FIG. 2, which consists of a wafer of optical fibers whose flounder faces f ₂ are covered with several layers L ₁ , L _{2 '} 15 and 9, 10, 11.

Is fixed using a collar 12, in pyroceram sealing for example, the enclosure of the shooting tube on that of the I.I.R ..

This embodiment of the I.I.R. makes it possible to avoid subjecting the shooting tube to the high temperatures necessary for the production of the I.I.R. In addition, one can test the operation of the I.I.R. before adapting the shooting tube.

In Figure 10, the I.I.R. 20 and the shooting tube 21 are coupled by two separate optical fiber plates 22 and 23.

According to a preferred embodiment of the invention the plate 22 of the IIR carries on its left face f ₁ the layers L ₁ , L ₂ and 15 as shown, for example, in Figures 3 and 6 to 8 and the plate 23 of the shooting tube carries on its right face f ₂ the layers 9, 10, 11.

Claims (18)

1. Image intensifier tube target, this tube comprising means allowing the electron beam coming from its photocathode to be subjected to two different acceleration voltages, characterized in that it comprises two types of luminescent materials (L and L ₂ ), of different light efficiency (r ₁ and r ₂ ), which receive the impact of the electron beam, and means (14) ensuring the excitation of the only luminescent material (L ₁ ) of lower efficiency luminous (r ₁ ) by the electron beam subjected to the lowest acceleration voltage (V ₁ ) and ensuring the excitation of the luminescent material (L ₂ ) of higher luminous efficiency (r ₂ ) by the beam of electrons subjected to the highest acceleration voltage (V ₂ ). 2. Target according to claim 1, characterized in that the two luminescent materials (L ₁ and L ₂ ) emit light of different wavelength (λ ₁ and λ ₂ ), and in that it comprises a filter adapted optics (10) which transmits more light emitted by the luminescent material (L ₂ ) of high light output than that emitted by the other luminescent material (L ₁ ). 3. Target according to one of claims 1 or 2, characterized in that the two luminescent materials (L and L ₂ ) are carried by a wafer of optical fibers. 4. Target according to claim 3, characterized in that the two luminescent materials (L ₁ and L ₂ ) are contained in blind holes obtained by digging the core (12) of the fibers, the side walls of these holes being covered with a thin metallic layer (15). 5. Target according to claim .4, characterized in that the blind holes contain two layers of luminescent material in grains (L ₁ and L ₂ ) separated by a barrier layer (14), the layer of luminescent material (L ₂ ) with higher light output being located at the bottom of the holes and the layer of luminescent material (L ₁ ) with lower light output being covered with a thin metallic layer (15). 6. Target according to claim 4, characterized in that the blind holes contain a layer of luminescent material in grains (L ₁ ) of high light output covered with an evaporated layer (L ₂ ) of luminescent material of low light output, this evaporated layer being covered with a thin metallic layer (15). 7. Target according to claim 3, characterized in that the surface of the optical fiber wafer is covered with two evaporated layers of luminescent material (L ₁ and L ₂ ), the first (L ₂ ) of high light output and the second (L ₁ ) of low light output, this second layer (L ₁ ), being covered with a thin metallic layer (15). 8. Target according to claim 3, characterized in that at the surface of the wafer, the core of the fibers (12) projects, the core of the fibers, being covered with two evaporated layers of luminescent material (L ₁ and L ₂ ), the first (L ₂ ) of high light output and the second (L1) of low light output, this second layer (L ₁ ) being covered with a thin metallic layer (15). 9. Target according to one of claims 6, 7 or 8, characterized in that a barrier layer (14) is interposed between the two layers of luminescent materials (L ₁ and L ₂ ). 10. Target according to one of claims 1 or 2, characterized in that the two luminescent materials (L ₁ and L ₂ ) are carried by a semiconductor substrate. 11. Target according to one of claims 3 to 10, characterized in that it comprises, on its face opposite to that covered with luminescent materials (L ₁ , L2), a photosensitive target (11) of shooting tube . 12. Target according to claim 11, characterized in that the optical filter (10) is placed before the photosensitive target (11). 13. Target according to one of claims 11 or 12, characterized in that the luminescent materials are carried by a wafer of optical fibers which has on its face opposite to that covered with luminescent materials, blind holes, each of these blind holes comprising part of the photosensitive target (11) and optionally the optical filter (10). 14. Image intensifier tube with video output, provided with a target according to one of claims 11 to 13, characterized in that it comprises a shooting tube (21) which emits a scanning electron beam the photosensitive target (11). 15. Tube according to claim 14, characterized in that the image intensifier tube (20) and the shooting tube (21) are located in the same vacuum enclosure. 16. Tube according to claim 14, characterized in that the image intensifier tube (20) and the shooting tube (21) are located in two separate vacuum chambers. 17. Image intensifier tube with video output provided with a target according to one of claims 3 to 9, characterized in that the target (7) with an optical fiber plate of the image intensifier (20) is coupled with another optical fiber board (23) to a picture tube (21). 18. Tube according to one of claims 14 to 17, characterized in that it is a radiological image intensifier tube.

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