EP0553578B1 - Image intensifier tube with intensity distribution compensation - Google Patents

Description

The present invention relates to an X-ray image intensifier tube according to claim 1.

The radiological image intensifier tubes make it possible to transform a radiological image into a visible image, generally for medical observation.

These tubes are vacuum tubes comprising an input screen, an electronic optical system, and a screen for observing the visible image.

The input screen includes a scintillator which converts incident X photons into visible photons which then excite a photocathode, generally constituted by an alkaline antimonide, for example potassium antimonide doped with cesium. The photocathode thus excited generates a flow of electrons.

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.

In the most recent embodiments, the input screen comprises an aluminum substrate covered by the scintillator, itself covered by an electrically conductive layer and transparent to the light coming from the scintillator, for example made of oxide of indium. The photocathode is deposited on this transparent layer.

X-rays hit the entrance screen on the side of the aluminum substrate and pass through this substrate to reach the material constituting the scintillator.

The light photons produced by the scintillator are emitted a little in all directions. However, in order to increase the resolution of the tube, a substance such as cesium iodide is generally chosen as scintillating material, which has the property of growing in the form of crystals perpendicular to the surface on which they are deposited. The needle crystals thus deposited tend to guide the light perpendicular to the surface, which is favorable for good image resolution.

However, for reasons of electronic optics, the surface of the input screen is not flat but curved; it can be parabolic or hyperbolic for large screens, or more generally in the form of a spherical cap for smaller screens.

It follows from this curvature of the screen that if we illuminate the input screen with a uniform beam of X-rays, the electronic density generated by the screen is not uniform. We can measure, for example, the brightness curve along a diameter of the tube exit screen, for uniform X-ray lighting of the entry screen: the brightness curve represents the light intensity in each point of the diameter of the output screen. We see that this curve is not horizontal; it is generally in the form of an arc of a circle slightly flattened in the center; the brightness of the output screen is maximum towards the center but decreases markedly as one approaches the edges. For small tubes (entry screen 15cm in diameter for example) the reduction in gloss on the edges relative to the center is around 25%. For larger screens (diameter 30 centimeters for example) the reduction reaches 35%.

An object of the invention is to produce an image intensifier tube having a more homogeneous brightness curve, that is to say showing a smaller difference between the brightness in the center and the brightness at the edges for uniform illumination. of the input screen. Another object of the invention is to obtain this better homogeneity of gloss by a simple method and industrially easier to implement than the methods proposed in the prior art.

It can be noted in fact that it has already been proposed in the prior art (EP 0 239 991) to improve the uniformity of the gloss by giving a non-homogeneous distribution to the thickness of the scintillator layer of the screen. entry. However, this is not easy to implement for the following reason: the yield of the scintillator increases and then decreases with the thickness; to have a good yield one must therefore be at the level of the maximum; but then we are on a plateau of the yield curve as a function of the thickness, and it is therefore necessary to vary the thickness very significantly to modify the gloss. We are therefore led to have very strong inhomogeneities in the thickness of the scintillator, which is troublesome industrially, all the more so since the scintillator is deposited in a very thick layer (of the order of 400 micrometers).

It will be noted that it has moreover been proposed in the prior art (EP-A- 0 378 257, according to the preamble of claim 1) to add between the scintillator and the photocathode a selectively absorbent layer, the function of which is to absorb the light wavelengths emitted by the scintillator below a certain wavelength, because these wavelengths are annoying, and allow the preferred wavelengths to pass freely to the photocathode; this layer can have a variable thickness, such that the optical absorption in the center is greater than the absorption on the edges. The greater absorption is due to the greater optical path through this intermediate layer for the light rays coming from the scintillator. To obtain this effect, a thickness varying from 10 to 20 microns is given for the intermediate layer.

It has been found according to the invention that the brightness curve of the intensifier tube can be improved much more easily without affecting the thickness of the scintillator and without adding an optically absorbent layer, and by using rather some very specific properties of the thin transparent layer placed under the photocathode.

It is proposed according to the invention, described in claim 1, to deposit under the photocathode (for an IIR tube: between the scintillator and the photocathode) a thin intermediate layer, of radially variable thickness, made of a transparent material whose presence at this point causes a modification of the emissive properties of the photocathode to an extent related to the thickness of this material.

The invention in fact starts from the following observation made by the Applicant: the photocathode is made of a chemically fairly unstable material which will react with the under layer on which it is deposited; this reaction will modify the emissive properties of the photocathode, and this to an extent related to the thickness of the under layer in the case where this thickness is very thin, that is to say in the case where it does not exceed not a few hundred nanometers.

It has therefore been seen that the interposition of a very thin, even transparent, intermediate layer between the scintillator and the photocathode could have direct consequences on the brightness, and this in close relation with the thickness of this intermediate layer. This effect does not result from a phenomenon of optical absorption, but from a phenomenon of partial chemical deactivation of the photocathode, all the more important as the thickness of the under layer is important if one remains in ranges of thickness of the order of a few hundred nanometers.

The invention therefore proposes to place a very thin intermediate layer of radially variable thickness under the photocathode. This layer is preferably transparent; it is preferably conductive; its thickness is preferably less than a few hundred angstroms; it is preferably made of indium oxide.

This is possible with commonly used photocathodes, potassium antimonide doped with cesium. These photocathodes are very reactive, especially during their deposition because of the high temperature prevailing in the depository. They are strongly reducing and react strongly with rather oxidizing substances.

For example, if a layer of indium oxide In₂O₃ is interposed between the scintillator layer and the photocathode, which has the property of being both conductive and transparent and which is therefore sometimes used as a conductive sublayer before deposition of the photocathode, it has been found according to the invention that the final brightness of the intensifier tube strongly depends on the thickness of the layer of indium oxide. The dependence is much stronger than that which results from the simple (negligible) optical absorption properties of this layer. This is why it is particularly advantageous to give a variable thickness radially to this layer in order to modify the gloss curve at will. This variation in gloss is very probably due to a chemical reaction between the indium oxide and the alkaline antimonide of the photocathode; this reaction tends to decompose an amount of antimonide which is linked to the amount of indium oxide and therefore to the thickness of the indium oxide layer. This chemical reaction takes place during the photocathode deposition phase.

Again, we will put a thicker interlayer thickness in the center, to reduce the efficiency of the photocathode in the center. The order of magnitude of the thicknesses is preferably as follows: approximately 250 angstroms at the edges and 400 angstroms at the center.

• la figure 1 représente une courbe de brillance d'un tube IIR de l'art antérieur;
• la figure 2 représente la structure générale d'un tube IIR de l'art antérieur;
• la figure 3 représente la structure des couches de l'écran d'entrée selon l'invention.
• la figure 4 représente une courbe de brillance compensée selon l'invention.

Other characteristics and advantages of the invention will appear on reading the detailed description which follows and which is given with reference to the appended drawings in which:

• FIG. 1 represents a brightness curve of an IIR tube of the prior art;
• FIG. 2 represents the general structure of an IIR tube of the prior art;
• FIG. 3 represents the structure of the layers of the input screen according to the invention.
• FIG. 4 represents a compensated brightness curve according to the invention.

FIG. 1 shows a conventional brightness curve of an image intensifier tube, taken along a diameter of the output screen: it represents the brightness of a line of dots of the image visible on the screen of output as a function of the distance of these points from the center of the screen, assuming uniform illumination of the input screen. For an IIR tube, the illumination is a uniform beam of X-rays.

On the abscissa we therefore plotted the radial distance from the center, and on the ordinate the brightness of the visible output image. It can be seen that the brightness curve is not at all a horizontal straight line or almost as it might be theoretically desirable; it is rather a kind of arc of a circle flattened towards the center. The difference in gloss between the center and the edges ranges from 25% to 35% depending on the types of tubes and their diameter. In reality, some difference in gloss may be desirable, but not as high as that.

The general structure of a conventional radiological image intensifier is shown in Figure 2. The vacuum tube enclosure contains an EE input screen at the front and an ES output screen at the rear. Electron beam focusing electrodes are provided in the enclosure.

The input screen is most often curved in parabolic or hyperbolic form, with a strong curvature, for reasons of electronic optics, that is to say to make possible a homogeneous focusing of the electrons on the screen of exit. This curvature is one of the causes of the shape of the gloss profile of the tube.

The EE input screen is most often formed by a curved aluminum sheet 10, on which a scintillating layer 12 has been deposited (cesium iodide, several hundred micrometers thick) itself covered with a transparent conductive electrode 14 (most often made of indium oxide In₂O₃) then a photocathode 16 (for example made of potassium and cesium antimonide).

The transparent conductive electrode (14) is intended to uniformly fix the potential of the photocathode.

According to the invention, it is proposed that an intermediate layer between the scintillator layer and the photocathode (and this layer may be the transparent conductive electrode 14 itself) be deposited with a thickness that is radially variable from the center to the edges, this intermediate layer being chosen from a material which modifies the emissive properties of the photocathode as a function of the thickness deposited.

The screen structure which implements the invention in the simplest manner is shown in FIG. 3: the intermediate layer is quite simply the layer of indium oxide 14 serving as transparent conductive electrode under the photocathode. As seen in Figure 3, the thickness varies radially. It is greater (thickness e1) in the center of the screen than at the edges (thickness e2), because it turns out that an increase in thickness of the layer 14 causes a reduction in gloss. We therefore compensate for the excessive curvature of the gloss profile of FIG. 1. The thickness variation is in practice continuous from the center to the edges.

The deposit with variable thickness is carried out in a known manner by evaporation in the presence of a mask which rotates in front of the surface to be covered, the shape of the mask being defined as a function of the thickness profile to be obtained. The thicknesses are a few hundred angstroms.

It has been found that a thickness varying between about 400 angstroms (at the center of the screen) and about 250 angstroms (at the edges) is quite suitable. It is interesting to note that the variation in optical absorption due to this variation in thickness is entirely negligible. And yet the brightness of the screen is compensated to the desired extent (we can easily go from a difference of 25% to a difference of 10% for example between the center and the edges). It therefore seems that it is mainly by reducing the emissivity of the photocathode that the indium oxide layer acts. And the action strongly depends on the thickness of indium oxide.

The choice of other materials than stoichiometric indium oxide In₂O₃ is possible. Partially reduced indium oxide In _x O _y , in thickness of the order of a few hundred angstroms may also be suitable. The thickness variation can be of the same order of magnitude as for stoichiometric indium oxide.

In the case of a visible and non-radiological image intensifier, there is no scintillator and the material such as indium oxide, the thickness of which acts on the final brightness, is deposited on a substrate before depositing the photocathode.

Claims (5)

1. An X-ray image intensifier tube comprising an output screen which supplies a visible image and an input screen which supports a scintillator layer (12), a photocathode layer (16) and an intermediate thin layer (14) between the scintillator layer and the photocathode layer, characterized in that the intermediate thin layer is transparent and is obtained by depositing a material susceptible to react with the photocathode during the manufacturing phase, the thickness of this intermediate layer being variable in radial direction to such an extent that the emission efficiency of the photocathode varies locally in accordance with the thickness of the layer.
2. A tube according to claim 1, characterized in that the intermediate transparent layer is a conducting layer.
3. A tube according to one of claims 1 and 2, characterized in that the material is a metal oxide, in particular stoichiometric indium oxide In₂O₃ or partially reduced indium oxide In_xO_y, the thicknesses being selected at approximately some hundred Å.
4. A tube according to one of claims 1 to 3, characterized in that the thickness of the intermediate layer is greater at the centre than at the borders of the input screen.
5. A tube according to one of claims 1 to 4, characterized in that the photocathode is made from a strongly reducing material and that the intermediate layer is made from a material having an oxidizing tendency.

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