EP0608168A1 - Image conversion tube and method of suppressing parasites in the tube - Google Patents

Abstract

The invention consists of an improvement to image converter tubes which convert the X-ray image (5) supplied on their input screen (3) into a visible image (9). Parasitic glows are developed on the insulators (11, 12, 13) inside these tubes, and the invention makes it possible to suppress these glows by depositing on the insulators a thin layer (14) of a product such as diamond carbon which has a low secondary electron emission ratio. Metal oxides are also suitable. Application to image intensifier tubes. <IMAGE>

Description

The present invention relates to an improvement to image converter tubes: this improvement makes it possible to eliminate stray gleams which may develop on the insulators inside these tubes.

The invention also relates to the method implemented for eliminating these parasitic gleams.

The preliminary reminder of the structure and operation of an image converter tube will make it possible to better understand the nature of the problem posed and that of the solution proposed by the invention. But, in order to be more clear and precise, the explanations as well as those relating to the invention will be based on the nonlimiting example of a radiological image intensifier tube.

Image intensifier tubes are vacuum tubes comprising an input converter, located at the front of the tube, an electronic optical system, and a visible image observation screen located at the rear of the tube, on the side of an exit window of the latter.

In X-ray image intensifier tubes or abbreviated as "IIR tube", the input converter includes a scintillator screen which converts the incident X photons into visible photons.

Figure 1 schematically shows such an image intensifier tube of the radiological type.

The IIR tube comprises an envelope 1 made of glass or metal, one end of which, at the front of the tube, comprises an entry screen 2. This end is closed by an entry window 3 exposed to X-ray radiation.

The second end of the envelope forming the rear of the tube is closed by an exit window 4 transparent to light.

The X-rays are converted into light rays by a scintillator screen 5. The light rays excite a photocathode 6 which in response produces electrons.

The electrons produced by photocathode 6 are accelerated towards the exit window 4 using different electrodes 7, and an anode 8, arranged along a longitudinal axis of the tube and which form the electronic optics system. .

The exit window 4 is formed by a transparent piece of glass which, in the example shown, carries a cathodoluminescent screen or exit screen 9 made of phosphors for example.

The impact of the electrons on the cathodoluminescent screen or output screen makes it possible to reconstruct an image (amplified in luminance) which at the start was formed on the surface of photocathode 6.

The image displayed by the exit screen 9 is visible through the glass piece which constitutes the exit window 4. Generally optical sensor devices (not shown) are arranged outside the tube near the output window 4 to capture this image through it and allow its observation.

But this observation can only be effective if there is no stray light. However, a consequence of the manufacturing process, on the one hand, and of the high voltages of electronic optics, on the other hand, resides in the appearance of gleams on the surface of the insulating parts which support the electrodes. It is easily conceivable that these gleams degrade the radiological image observed, in particular in contrast.

These parasitic gleams come from the fact that the electrical insulation of the electrodes is degraded by the presence of the alkali metals which are deposited on these electrodes, and which favor by field effect an emission of electrons which will charge the insulators.

The invention provides a solution to the drawbacks of the prior art by proposing to limit the electrical charge of the insulators, which is at the origin of the stray gleams. This objective is achieved by covering the surface of the insulators with a thin layer of a product with very low conductivity to limit the leakage current but above all having a low rate of secondary emission of electrons. Diamond carbon is a good example of a substance suitable for these requirements.

More specifically, the invention relates to a radiological image intensifier (IIR) tube comprising, inside a vacuum enclosure, at least one input screen associating a scintillator and a photocathode, which transform the rays. X incidents on the electron scintillator, focused on an output screen by means of electronic optics formed by a plurality of electrodes fixed by means of a plurality of insulating parts, this IIR tube being characterized in that, in view to suppress parasitic gleams which arise in operation on insulators, these are covered with a thin layer of a material having a low rate of secondary emission of electrons, a very low electrical conductivity, and likely to be deposited by a physical or chemical thin film vaporization process.

• - figure 1 : vue en coupe, schématique, d'un tube IIR selon l'art connu ;
• - figure 2 : vue en coupe d'un tube IIR, orientée sur les problèmes d'isolants résolus par l'invention ;
• - figures 3a à 3c : schéma du mécanisme d'apparition des lueurs sur les isolants ;
• - figure 4 : coupe d'un isolant recouvert d'une couche mince selon l'invention.

The invention will be better understood by the presentation of an exemplary embodiment, in conjunction with the appended figures which represent:

• - Figure 1: schematic sectional view of an IIR tube according to the known art;
• - Figure 2: sectional view of an IIR tube, oriented on the insulation problems solved by the invention;
• - Figures 3a to 3c: diagram of the mechanism of appearance of the lights on the insulators;
• - Figure 4: section of an insulator covered with a thin layer according to the invention.

FIG. 1 which has been described previously rapidly exposed the operation of an IIR tube. Figure 2 shows this sectional view, but it is more particularly oriented on electrical insulation. inside.

In order to make the description clearer and more concrete, it will be assumed that this IIR tube has a photocathode 6 made of alkali antimonide, and that it is of the tetrode type, with three grids 71, 72, 73 and an anode 8.

The electrodes are brought to voltages which can go beyond 30 kV for the anode 8 and about 20 kV for the grid 73. The electrodes 71 and 72 are brought to voltages generally not exceeding 1500 V. The primary screen 2 with its photocathode 6 transforms the X-ray radiation into an electron beam which is then focused by the set of electrodes on the secondary screen 4 which transforms it into a bright image. Generally the anode 8 is brought to a fixed voltage, for example 30 kV, while the other electrodes, including in particular the grid 73 can be brought to variable voltages to enlarge the input image on the output screen , creating a zoom effect. The zoom operating mode can lead to operating voltages greater than 20 kV for the electrode 73.

• - d'une part au moyen de cales d'alumine 11 et 12, par exemple, entre les grilles 71, 72 et 73,
• - d'autre part au moyen d'un scellement 13 verre/métal, entre l'enveloppe 1 du tube et les électrodes 8 et 73.

The set of grids 71, 72 and 73, of the anode 8, and of the outlet window 4 form an architectural assembly which is rigidly assembled:

• on the one hand by means of alumina shims 11 and 12, for example, between the grids 71, 72 and 73,
• - on the other hand by means of a glass / metal seal 13, between the envelope 1 of the tube and the electrodes 8 and 73.

Given the high voltages at which the electrodes 73 and the anode 8 can operate, their electrical isolation from the rest of the tube is a delicate problem, but it turns out that the voltage withstand is particularly degraded by the manufacturing process. the photocathode 6 which takes place inside the vacuum tube 1 itself by successive evaporations of its constituent elements. If the evaporation of antimony (Sb) by Joule effect from a crucible inserted on the axis of the tube is directive and makes it possible to avoid significant pollution of the rest of the tube, it is quite different for that of alkalis such as potassium (K), cesium (Cs) or sodium (Na). The evaporation of alkali metals is the result of a hot decomposition of a compound of these metals such as for example a chromate, by the heating by Joule effect of the alkaline generators. The closed geometry of these generators, necessary for the confinement of chromates to optimize decomposition reactions, and their off-center position relative to the axis of the tube make evaporation very poorly directive. The alkalis can even evaporate outside the tube: they are then injected into the tube through a sump. In all cases, this evaporation generates a mist which is deposited everywhere inside the tube.

A part of the vaporized alkalis is deposited on the metal parts of the IIR tube, such as the electrodes 71, 72, 73, while another part of the alkalis is deposited on the insulating parts 11, 12, 13. FIGS. 3a to 3c allow to understand the phenomenon of appearance of gleams on insulators, and consequently to understand the solution provided by the invention.

Or an insulating part 12, made of alumina, which supports and joins two grids 72 and 73 made of stainless steel, for example. In this case, the grid 73 is brought to around 20 kV, the grid 72 to around 1.5 kV and the alumina wedge 12 has been previously polluted by alkalis, as well as metal parts.

The alkalis, deposited on the surface of the internal metal parts of the tube, considerably reduce the work of output of the electrons from the metal, which promotes parasitic emissions of electrons by field effect in places where the electric field is high. In particular, the electric field can be very high in the vicinity of the insulator / low-voltage electrode for reasons of charge of the insulator and proximity of potential sources of electrons.

Thus, in a first emission mechanism shown in FIG. 3a, an incident electron which strikes the alumina wedge 12 causes a multiplier effect and tears off at least two secondary electrons, with the consequence that the wedge 12 is charged with at minus a positive charge. This positive charge attracts, in a second emission mechanism symbolized in FIG. 3b, the electrons which have left the metallic parts by field effect, for example in the vicinity of the insulator / electrode. The electrons thus captured bring back to the previous case and create secondary electrons by multiplier effect. Thus, there is very quickly an avalanche effect, and the emission of electrons by field effect leads - FIG. 3c - to the appearance of gleams on the surface of the insulation bombarded by a mechanism of cathodolu- minescence type. These lights are typically blue on the glass and red on the alumina A1 ₂ 0 ₃ . The lights are generally stable over time although they may vary slightly in position.

The gleams on the surface of the insulators, visible directly from the photocathode or by reflections on the electrodes or the metal walls of the tube, are retransmitted and amplified on the secondary screen 4. The parasitic illumination thus generated disturbs the proper functioning of the IIR tube: glow in the absence of a useful signal and deterioration of the contrast in operation. The large leakage current which may be associated with the presence of the lights is also a source of instability in the supply of the IIR tube to the detriment of the image quality, with loss of resolution.

To improve the electrical insulation and in particular to limit the appearance of gleams on the surface of the insulators, various solutions are known but have performance limitations or remain very expensive.

A first solution is to limit the pos electron emission sensitivities. This solution requires action on the configuration of the parts and their surface condition. Indeed the parasitic emission of electrons by field effect is governed by two parameters: the work of electron output and the microscopic field on the surface of the emission site. If the output work is conditioned by the inevitable presence of alkalis, the microscopic field can be reduced by improving the surface condition and by increasing the radius of curvature of the place at the level of possible emission sites, with decrease in l 'peak effect. The parasitic emission of electrons and therefore the gleams on insulators can therefore be reduced by the introduction of polished and rounded parts, for example at the insulator-metal junctions. These parts are generally expensive and must be handled with care.

A second solution consists in protecting the bombarded insulation by depositing a powdery product. Such a solution consists, for example, of a deposit of chromium oxide, produced using a mixture of chromium oxide powder, water and optionally a binder. Deposited with a brush or a pad, a thick deposit with low adhesion is obtained. This solution, if it makes it possible to eliminate the gleams on the surface of the whitewashed insulation, is a source of particulate pollution in the tube and therefore of appearance defects on the output screen.

Finally, we can optimize the shape of the insulation, using crenellated or conical aluminas. It is an expensive solution and has limited effectiveness given the presence of alkalis in the tube.

• - d'avoir un faible taux d'émission secondaire d'électrons, de sorte que s'il est heurté par un électron, il l'absorbe sans émission secondaire avec mutiplication,
• - d'être homogène, c'est-à-dire non pulvérulent, ou déposé par un procédé dit "de couche mince", avec forte adhérence entre le produit et l'isolant,
• - d'être très peu conducteur pour limiter le courant de fuite dans le tube intensificateur d'image.

According to the invention, the electrical charge of the insulators causing the parasitic glow is limited by a deposit 14 (FIGS. 2 and 4) on these insulators of a product having the main characteristics:

• - to have a low rate of secondary emission of electrons, so that if it is struck by an electron, it absorbs it without secondary emission with multiplication,
• - to be homogeneous, that is to say non-pulverulent, or deposited by a process called "thin layer", with strong adhesion between the product and the insulator,
• - to be very slightly conductive to limit the leakage current in the image intensifier tube.

Such a deposit consists, for example, of an amorphous carbon layer deposited by sputtering or by a chemical process, stimulated by a plasma and known by the English term PECVD (Plasma Enhanced Chemical Vapor Deposition). The PECVD technique makes it possible to obtain a homogeneous, thin, insulating and highly adherent deposit on parts with complex shapes. The deposition consists of a cracking in the substrate surface, in acetylene at low pressure the presence of hydrogen (10- ¹ to 10- ³ torr). To activate the reaction, the substrate is heated to 100 ° C and subjected to a 13.5 MHz HF plasma. This type of thin layer is also known as "diamond carbon" or in English ADLC (Amorphous Diamond Like Carbon).

Diamond carbon is a material known for its low secondary emission factor. This remains below 1 whatever the incident energy of the electrons: the material does not charge whatever the conditions of electron bombardment.

Carbon in the form of graphite is not suitable because it is conductive. Carbon black has been used in vacuum tube technology, but this type of deposit has all the disadvantages of chromium oxide paint: thickness, poor adhesion and therefore the possibility of generating particles in the tube.

The diamond carbon deposited in a thin layer by spraying or by PECVD is perfectly homogeneous and adheres to its support; it does not generate dust like chromium oxide paint.

Carbon deposition by PECVD makes it possible to process a large number of parts simultaneously. A thickness of 1000 A (0.1 f..lm) is enough to gain a factor of 1.5 to 2 on the threshold for the appearance of gleams on the surface of alumina insulators working at voltages up to 40 kV , because diamond carbon is not very conductive and holds very high voltages.

The deposition of amorphous carbon can be done on alumina parts such as the insulators 11 and 12 between the electrodes 72 and 73 for example, or on the glass bulb 13 which allows the grid 73 / anode 8 insulation. The metal parts adjoining such as the ends of the alumina wedges or the metal parts molded in the glass bulb can also be covered, the deposit also being adherent on a metal substrate and is not likely to generate particles during mounting operations in because of its thinness.

FIG. 4 illustrates the invention: an insulating block 12, located between two metal parts such as the electrodes 72 and 73, is covered with a layer 14 of a material having a low secondary emission rate and a low conductivity, deposited using a so-called thin layer technique.

Relative to the insulating block 12, the layer 14 behaves like a shield, to prevent incident electrons from charging the insulator 12, by the secondary emission of electrons.

The invention can be generalized to any other type of insulating material capable of being deposited in a thin layer and having as main characteristic a low secondary emission rate. Mention will be made, for example, of oxides of titanium, tungsten, vanadium, molybdenum, silver, copper or even chromium oxide in a thin layer. In this case, the chromium is deposited for example by spraying cathodic with a sample rotation device to homogenize the deposit, and the deposit is then oxidized.

The invention is specified by the following claims.

Claims (5)

1. Image converter tube comprising, inside a vacuum enclosure (1), at least one input screen (2) associating a scintillator (5) and a photocathode (6), which transform the rays X incidents on the electron scintillator (5), focused on an output screen (4) by means of electronic optics formed by a plurality of electrodes (8, 71, 72, 73) fixed by means of a plurality insulating parts (11, 12, 13), this tube being characterized in that, with a view to eliminating the parasitic gleams which arise in operation on the insulators (11, 12, 13), these are covered with a thin layer (14) of a material having a low secondary electron emission rate, a very low electrical conductivity and capable of being deposited by a physical or chemical vaporization process in a thin layer. 2. Tube according to claim 1, characterized in that said material (14) with a low secondary emission rate is chosen from diamond carbon, or oxides of titanium, tungsten, vanadium, molybdenum, silver, copper, or chromium. 3. Tube according to claim 1, characterized in that said material (14) is deposited in the form of an adhesive layer with a thickness of the order of 1000 Å (0.1 1 micrometer). 4. A method of suppressing parasitic gleams in a tube intensifying radiological images according to claim 1, characterized in that a layer of "diamond carbon" is deposited on the surface of the insulators (11, 12, 13), 100 ° C, by cracking acetylene in the presence of hydrogen, at a pressure between 10 ^-1 and 10 ^-3 torr, by the action of a 13.5 MHz plasma. 5. A method of suppressing parasitic glows in an intensifying tube of radiological images according to claim 1, characterized in that an oxide layer of a metal chosen from titanium, tungsten, vanadium, molybdenum, silver, copper, chromium, is deposited on the surface of the insulators (11, 12, 13) by sputtering, followed by oxidation.

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