EP0608168B2 - Image conversion tube and method of producing such a tube - Google Patents

Description

The present invention relates to an improvement to image converter tubes this improvement eliminates stray lights that can grow on the insulators inside these tubes.

The invention also relates to a method for production of such an image converter tube.

Preliminary reminder of the structure and operation of an image converter tube will allow better understand the nature of the problem posed and that of the solution proposed by the invention But, so to be more clear and precise the explanations as well as those relating to the invention will be based on the example, nonlimiting, of an intensifier tube of radiological images.

Such a convert image tube is known from EP-A-0. 380,147.

Image intensifier tubes are tubes vacuum including 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 outlet window of the latter.

In radiological image intensifier tubes or abbreviated "IIR tube", the input converter has a scintillator screen that converts photons X incidents in visible photons.

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

The IIR tube comprises a glass envelope 1 or of metal, one end of which, at the front of the tube, includes an input screen 2. This end is closed through an entry window 3 exposed to radiation of X photons.

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

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

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

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

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

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

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

These stray lights come from the fact that the insulation the electrodes is degraded by the presence alkali metals that have deposited on these electrodes, and which favor a field effect emission of electrons which will charge the insulators.

The invention aims to limit the electrical charge of the insulators, which is the origin of glows by parasites. In the image converter described in EP-A-0 380 147 there is an entry screen in a vacuum enclosure combining a scintillator and a photocathode which transform X-rays incidents on the electron scintillator focused on an output screen, at by means of electronic optics formed by a plurality of electrodes. Of insulating parts which are part of the enclosure are for this purpose covered a thin layer of chromium oxide. This layer is made by deposit chromium nitrate, by brushing, spraying or immersion followed by heat treatment.

The object of the invention is to provide a converter tube images with such a thin performance layer improved and inexpensive. This objective is reached by the image converter tube according to claim 1 and the method of manufacturing such a tube according to claim 3.

• 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;
• FIGS. 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.

Figure 1 which was described previously exposed quickly the operation of an IIR tube. Figure 2 takes this section view, but it is more particularly oriented towards electrical insulations inside.

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

The electrodes are brought to voltages which can go beyond 30 kV for anode 8 and around 20 kV for 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 X-ray 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. Usually the anode 8 is brought to a fixed voltage, by example 30 kV, while the other electrodes, including in particular the grid 73 can be brought to voltages variables to enlarge the input image on the exit screen, creating a zoom effect. The mode operating mode can lead to voltages operating above 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 unit which is assembled rigidly:

• 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 operate the electrodes 73 and the anode 8, their isolation electrical with respect to the rest of the tube is a problem delicate, but it turns out that the tensile strength is particularly degraded by the manufacturing process of photocathode 6 which is done inside the vacuum tube 1 by successive evaporations of its elements constitutive. If the evaporation of antimony (Sb) by Joule effect from a crucible inserted on the axis of the tube is directive and avoids significant pollution of the rest of the tube, it is quite different for that alkalis such as potassium (K), cesium (Cs) or sodium (N / A). The evaporation of alkali metals is the result hot decomposition of a compound of these metals such as for example a chromate, by the Joule heating of alkaline generators. The closed geometry of these generators, necessary for chromate containment to optimize reactions of decomposition, and their off-center position relative to the axis of the tube makes evaporation very directive. The alkali can even evaporate outside of the tube: they are then injected into the tube through a queusot. In any case, this evaporation generates a mist that settles everywhere inside of the tube.

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

Either an insulating piece 12, made of alumina, which supports and combines two grids 72 and 73 in stainless steel, by example. In this case, the grid 73 is brought to some 20 kV, grid 72 at around 1.5 kV and the alumina shim 12 was previously polluted with alkalis, as well besides metal parts.

Alkalis, deposited on the surface of metal parts tube internals, dramatically decrease the work of the metal's electrons which favors stray electron emissions by field effect where the electric field is high. In particular, the electric field can be very high in the vicinity low voltage insulator / electrode for reasons insulation load and proximity to sources potential 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 metal 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 insulator bombarded by a mechanism of cathodo-luminescence type. These lights are typically blue on the glass and red on the alumina Al ₂ O 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 of 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 illumination parasite 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 flow significant leakage which may be associated with the presence light is also a source of food instability IIR tube at the expense of image quality, with loss of resolution.

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

A first solution is to limit the possibilities electron emission. This solution requires an action on the configuration of the parts and their state of area. Indeed the parasitic emission of electrons by effect of field is governed by two parameters: the work of electron exit and the microscopic field at the surface of the issuing site. If the output job is conditioned by the inevitable presence of alkalis, the field microscopic can be decreased by improving the condition of surface and increasing the radius of curvature of the place at the level of possible emission sites, with reduction of the 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 is to protect the insulation bombarded by a deposit of a powdery product. A such a solution consists for example of an oxide deposit chromium, made using a powder mixture chromium oxide, water and optionally a binder. Deposited with a brush or a pad, a deposit is obtained thick and weak adhesion. This solution if it allows to eliminate the gleams on the surface of the whitewashed insulation, is a source of particulate pollution in the tube and therefore appearance defects on the output screen.

Finally, we can optimize the shape of the insulation, using crenellated or conical aluminas. It is a solution costly and with 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 claimed invention, the electrical charge of the insulators at the origin of the stray gleams 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 insulation,
• to be very little conductive to limit the leakage current in the image intensifier tube.

Such a deposit consists, for example, of a layer of amorphous carbon, deposited by sputtering or by a chemical process, stimulated by a plasma and known under the term PECVD (Plasma Enhanced Chemical Vapor Deposition). The PECVD technique makes it possible to obtain a homogeneous, thin, insulating and very adherent deposit on parts with complex shapes. The deposition consists of cracking, on the surface of the substrate, of acetylene in the presence of hydrogen at low pressure (13.3 to 0.13 Pa = 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 film 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 one remains less than 1 whatever the incident energy of electrons: the material does not charge whatever be the conditions of electronic bombardment.

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

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 treat a large number of parts simultaneously. Thickness 1000 Å (0.1 µm) is enough to gain a factor 1.5 to 2 on the threshold for the appearance of surface lights alumina insulators working at voltages that can go up to 40 kV, because diamond carbon is very not very conductive and holds very high voltages.

The deposition of amorphous carbon can be done on alumina parts such as insulators 11 and 12 between electrodes 72 and 73 for example, or on the bulb glass 13 which allows grid 73 / anode 8 insulation. adjoining metal parts such as end caps alumina shims or molded metal parts in the glass bulb can also be covered, the deposit also adhering to a metal substrate and is not likely to generate particles during assembly operations due to its low thickness.

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 the claimed material with low secondary emission rate and low conductivity, deposited using a so-called technique thin layer.

Relative to the insulating shim 12, the layer 14 is behaves like shielding, to prevent incident electrons charge insulator 12, by emission secondary of electrons.

The invention is specified by the following claims.

Claims (3)

1. Image converter tube including, inside a vacuum chamber (1), at least one input screen (2) combining a scintillator (5) and a photocathode (6) which convert the X-rays incident on the scintillator (5) into electrons which are focused onto an output screen (4) by means of electron optics formed by a plurality of electrodes (8, 71, 72, 73) fixed by means of a plurality of insulating parts (11, 12, 13) which, with a view to eliminating any stray glow which occurs in operation on the insulating parts (11, 12, 13), are covered with a thin film (14) of a material having a low secondary electron emission level of less than or equal to one, whatever the incident energy of the electrons, and a very low electrical conductivity, this film being deposited by a physical or chemical thin-film evaporation or sputtering process, the material being diamond-like carbon.
2. Tube according to Claim 1, characterized in that the said material (14) is deposited in the form of an adherent film with a thickness of the order of 1000 Å (0.1 micrometre).
3. Method of manufacturing a radiological image intensifier tube according to Claim 1, characterized in that the "diamond-like carbon" film is deposited on the surface of the insulating parts (11, 12, 13) which have been heated to 100°C, by cracking acetylene in the presence of hydrogen at a pressure of between 13.3 and 0.13 Pa (10^-1 and 10^-3 torr), under the action of a 13.5 MHz plasma.

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