EP0062553B1 - Image intensifier tube target and image intensifier tube with video output comprising such a target - Google Patents

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 (radiationy).

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 around 100. Thus the I.I.R. can, as desired, operate in radiography or radioscopy.

In radiography, the video output signal from the ILR. 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 observation time, which is relatively long, a low dose of X-rays is sent. It is then necessary to have a high gain target to obtain a good image.

We know from French patent application No. 7,705,031, published under No. 2,341,939, an ILR target. has video output with variable gain.

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

The electron beam 14 from the cathode of the ILR. 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 generates 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 ILR. 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'I.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 del'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 IIR 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 patent application cited.

An image intensifier with a target, which has a luminescent layer on the face directed towards a photocathode and the other face of which is scanned by a detection beam, is known from the article published in "Journal of the electrochemical Society" , Flight. 118, No. 4 (1971) pages 619 to 628.

By French patent application No. 2 356 266, in the name of THOMSON-CSF, penetration screens are known comprising two types of luminescent material having different colors. The electron beam which bombards this screen can be subjected to two different acceleration voltages. Depending on the acceleration voltage of the beam, the screen gives traces of different color.

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

The present invention relates to a variable gain target for an image intensifier tube with video output which receives on one of its faces the impact of an electron beam coming from the photocathode of the intensifier tube, while the other face of the target is scanned by an electron beam produced by the cathode of a shooting tube, the variation of the gain of the target being obtained by means making it possible to subject the electron beam coming from the photocathode to two different acceleration voltages, characterized in that the target comprises, on its face receiving the impact of the electron beam coming from the photocathode of the intensifier tube, two types of luminescent materials, of different light output and of indifferent color, which receive the impact of the electron beam, and means ensuring the excitation of the only luminescent material of lower light output by the electron beam subjected to the lower ac voltage celeration and ensuring the excitation of the luminescent material of higher light output by the electron beam subjected to the highest acceleration voltage.

Thus by passing 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 metal 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 comprises an optical filter suitable which transmits more light emitted by the luminescent material with high light output 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;
• - la 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 as a function of the acceleration voltage for the 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 focalisa- tion des électrons et les soumettent a une tension d'accélération;
• - une anode cônique A;
• - une cible 7 qui reçoit sur sa face f_i 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 _i 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, provide the concentration and 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 fi 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 ₁ , also in grains , but of low light output r ₁ .

Before filling the holes in the manner just described, 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, a thin metallic layer 151a layer L ₁ is also covered.

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 ₁ of lower light output is excited by the electron beam subjected. at 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 of obtaining this result 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 Li and Lz.

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 _i 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 _i , part, 50% for example, of the beam electrons does not exceed the layer L ₁ and the other 50% do not exceed the barrier layer.

The excitation of the L ₁ layer produces light, in a relatively small amount because of the low light output of this layer. The outer surface of the layer L ₁ and the walls of each blind hole being covered with the thin metallic layer 15, the light emitted by the 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 is maintained as that of the fiber board.

If the beam is subjected to the highest acceleration voltage V ₂ , a part 15% for example, of the electrons of the beam does not exceed the layer L ₁ , another part, 35% for example, does not exceed the layer- barrier and the rest excites the L ₂ layer 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 _i = 10 KV and V ₂ = 30 KV, and with a ratio of the light yields r ₂ / r ₁ of 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. for example, and by using a suitable optical filter which transmits more of the light emitted by the luminescent material L ₂ of 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. FIG. 5 shows the variations in the transmission coefficient T of such an optical filter adapted as a function of the wavelength λ.

We adapt the transmission coefficient Ti 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 ₁ 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 can consist of tin oxide. SnO ₂ , indium oxide In ₂ O ₃ , cadmium oxide Cd 0 ₃ , manganese oxide Mn 0, or mixtures of these oxides.

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

This photoconductive layer can consist of antimony trisulfide, amorphous selenium, an amorphous compound of selenium tellurium, sulfur and arsenic, or also a layer of lead oxide.

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

It is understood that if it is sufficient to obtain a gain which can be multiplied by a ratio of 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 wafer 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 L ₂ , in grains, of high luminous efficiency r ₂ , is first deposited, then an evaporated layer Li of luminescent material, of low luminous efficiency r ₁ , is deposited.

The evaporated layer L ₁ can be chosen so that there is no need for a barrier layer interposed between the layers L ₁ and L ₂ , and that the lowest acceleration voltage V ₁ causes excitation only layer L ₁ and the highest acceleration voltage V ₂ causes excitation of 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 going 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 wafer of optical fibers, 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 ₁ of the target in which the surface of the wafer is covered with two evaporated layers L ₁ and L ₂ of luminescent material, with 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 _i , L ₂ and 15, obtained by vacuum evaporation of their constituent materials, makes it possible to obtain a target having good resolution without it being necessary to dig the fibers.

In the embodiment of the face f ₁ 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 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 ₁ and an evaporated barrier layer can be used if necessary.

The Li layer may consist of yttrium oxide or oxysulfide doped with europium and the L ₂ layer may consist of yttrium oxysulfide doped with terbium. 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 faces fi and f ₂ are covered with several layers L ₁ , L ₂ , 15 and 9,10,11.

The enclosure of the shooting tube is fixed on that of the I.I.R using a flange 22, in pyroceram sealing for example.

This embodiment of the IIR avoids subjecting the high temperatures shooting tube necessitated for the _realization of IIR. In addition, you can test the operation of the IIR 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 _i , 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 (17)

1. A variable gain image intensifier target having a video output and receiving by one of its faces (f₁) the impact of an electron beam coming from the photocathode (6) of the intensifier tube (1), whereas the other target face (f₂) is scanned by an electron beam produced by the cathode (K) of a picture tube, the gain variation of the target being obtained by means which submit the electron beam coming from the photocathode (6) to two different acceleration voltages, characterized in that the target comprises, on the face (f₁) receiving the impact of the electron beam coming from the photocathode (6) of the intensifier tube (1), two types of luminescent materials (Li and L₂) having different luminous efficiencies (r₁ and r₂) but equal colours and receiving the impact of the electron beam, and means (14) which ensure the excitation of only the luminescent material (L₁) of lower luminous efficiency (r₁) by the electron beam submitted to the lower acceleration voltage (V_i), whereas these means ensure the excitation of the luminescent material (L₂) of higher luminous efficiency (r₂) by the electron beam submitted to the higher acceleration voltage (V₂).

2. A target according to claim 1, characterized in that the two luminescent materials (L₁ and L₂) emit light having different wavelengths (λ₁ and λ₂), and that it has a matched optical filter (10), which transmits to a greater extent the light emitted by the luminescent material (L₂) with the high luminous efficiency than that emitted by the other luminescent material (L₁).

3. A target according to one of claims 1 or 2, characterized in that the two luminescent materials (L₁ and L₂) are carried by an optical fibre board.

4. A target according to claim 3, characterized in that the two luminescent materials (L₁ and L₂) are contained in blind holes obtained by hollowing out the core (12) of the fibres, the side walls of said holes being covered with a thin metallic coating (15).

5. A target according to claim 4, characterized in that the blind holes contain two granular luminescent material layers (L₁ and L₂) separated by a barrier layer (14), the layer of luminescent material (L₂) having the higher luminous efficiency being located at the bottom of the holes and the layer of luminescent material (L_i) with the lower luminous efficiency being covered by a thin metallic coating (15).

6. A target according to claim 4, characterized in that the blind holes contain a layer of granular luminescent material (L_i) with a high luminescent efficiency covered with an evaporated layer (L₂) of a luminescent material having a low luminous efficiency, said evaporated layer being covered by a thin metallic coating (15).

7. A target according to claim 3, characterized in that the surface of the optical fibre board is covered with two evaporated layers of luminescent material (L_i and L₂), the first layer having a high luminous efficiency and the second a low luminous efficiency, the second layer (L1) being covered by a thin metallic coating (15).

8. A target according to claim 3, characterized- in that the core of the fibres (12) projects from the surface of the board, said fibre core being covered with two evaporated layers of luminescent material (L_i and L₂), the first layer (L₂) having a high luminous efficiency and the second (Li) a low luminous efficency, said second layer (L_i) being covered with a thin metallic coating (15).

9. A target according to one of claims 6, 7 or 8, characterized in that a barrier layer (14) is inserted between the two luminescent material layers (L₁ and L₂).

10. A target according to one of claims 1 or 2, characterized in that the two luminescent materials (Li and L₂) are carried by a semiconductor substrate.

11. A target according to one of claims 3 to 10, characterized in that on its face opposite to that covered by the luminescent materials (Li, L₂) it comprises a photosensitive picture tube target (11).

12. A target according to claim 11, characterized in that the optical filter (10) is placed in front of the photosensitive target (11).

13. A target according to one of claims 11 or 12, characterized in that the luminescent materials are carried by an optical fibre board having blind holes on its face opposite to that covered by the luminescent materials, each of the said holes incorporating part of the photosensitive target (11) and optionally the optical filter (10).

14. An image intensifier tube with video output provided with a target according to one of claims 11 to 13, characterized in that the image intensifier tube (20) and the picture tube (21) are located in the same vacuum enclosure.

15. A tube according to claim 14, characterized in that the image intensifier tube and the picture tube are located in two separate vacuum enclosures.

16. An image intensifier tube with a video output, provided with a target according to one of claims 3 to 9, characterized in that the optical fibre board target (7) of the image intensifier (20) is coupled by means of another optical fibre board (23) to a picture tube. (21).

17. A tube according to one of claims 14 to 16, characterized in that it is a radiological image intensifier tube.

Images