EP0165851B1 - Radiation image intensifier vacuum container and method of making it - Google Patents

Description

The present invention relates to the structure of the vacuum envelope of radiation image intensifier tubes such as radiological image intensifier tubes, or similar electronic tubes. The present invention also relates to a method of manufacturing these envelopes.

In known manner, the vacuum envelopes of image intensifier tubes are constituted by a central body of revolution, by an inlet window intended for the passage of the radiation to be amplified, said window being connected to one end of the central body and by a transparent exit window connected to the other end of the central body.

Until recently, entrance windows were usually made of glass, which posed few problems of sealing with the central body even when it was made partly of metal, because the glass-metal seals are good known to those skilled in the art. However, the use of glass for entry windows poses a number of problems. Thus, the absorption of radiation, in particular X-rays, in the glass window is variable from 15% to 25% depending on the thickness of the glass used. The thickness of the glass increases with the size of the tube and can vary between 2 and 3.5 mm. In addition, there is a significant diffusion of radiation due to the thickness of the glass.

To overcome these drawbacks, it has been proposed to make the entry window from a metal permeable to the radiation to be transmitted.

Vacuum envelopes have thus been proposed comprising a concave entry window made of titanium or steel. Although this type of window can remain sufficiently thin, therefore not very absorbent or diffusing for the radiation to be transmitted, and nevertheless sufficiently mechanically resistant to withstand pressure differences, it is necessary, due to the concave shape of the window, to lengthen the tube in order to incorporate the convex entry screen into it for the needs of electronic optics.

It has also been proposed to use aluminum or aluminum alloy windows of convex shape. In this case, different techniques are used to achieve the sealing of the window on the central body.

As described in French patent application 2 482 366, the sealing between the window and the central body can be carried out by thermocompression welding. In this case, the convex window made of aluminum or aluminum alloy comprises an annular peripheral flange and the assembly between the window and the body requires either that the body comprises an annular flange perpendicular to the axis of the tube or the use of an L-shaped or S-shaped connection ring. In order to be able to carry out thermocompression welding, the parts to be welded must be in a plane perpendicular to the axis of the tube so as to be able to apply pressure between the two metals or alloys in which the window and the central body are made.

This technique has the disadvantage of increasing the overall diameter of the tube. On the other hand, the thermocompression welding process, in particular when it is used to weld aluminum to an iron alloy such as stainless steel, requires a rise in temperature and a period of contact under pressure which require time. As a result, this process is industrially expensive.

Another solution of the prior art consists in making the window using a convex shaped part made of a material comprising a layer of copper plated on a layer of aluminum in which the layer of copper is removed in the part subjected to the radiation and the aluminum layer is removed at the edge formed by a flat surrounding the convex cap, reserving a localized overlap of the two layers. The copper edge is then welded by electric arc welding along a lip formed on the central metal body which can be made of stainless steel. With this process we find the same problems with the overall diameter of the tube as with thermocompression welding. On the other hand, it is difficult to obtain an industrially manufactured two-layer material which always exhibits the same quality of mutual adhesion with vacuum tightness. In addition, the metal must be removed before welding can be carried out.

The object of the present invention is to provide a new structure of a vacuum envelope for an image intensifier tube comprising an aluminum window which does not have the drawbacks of the structures of the prior art.

The present invention also aims to provide a new vacuum envelope structure for radiation image intensifier tube which is easy and quick to produce.

Consequently, the subject of the present invention is a vacuum envelope for radiation image intensifier tubes or similar electronic tubes of the type comprising a central body and an aluminum or aluminum alloy entry window at one end of the body. central, characterized in that the entry window comprises a peripheral skirt which fits on a ring of the same section as the skirt, made of iron or an iron alloy secured to said end of the body, said skirt being welded so vacuum tight on the ring by magnetic induction welding.

The magnetic induction welding technique has been known for a long time for the welding of small diameter tubes having a good seal against the pressure of various fluids. It is described in particular in French patent No. 1,579,461. However, this technique has never been used in image intensifier tubes.

Indeed, the tubes being of a large diameter, it seemed impossible to a person skilled in the art to be able to produce by magnetic induction a vacuum-tight weld over such a long length. In fact, the applicant has had to solve the numerous leakage problems encountered during the tests by a special form for the window.

• - la figure 1 est une vue en coupe schématique de l'enveloppe sous vide d'un tube intensificateur d'images selon un mode de realisation préférentiel de la présente invention;
• - la figure 2 est une vue en coupe de la partie essentielle du mode de réalisation de la figure 1 avant soudage;
• - la figure 3 est une vue identique à celle de la figure 2 après soudage;
• - la figure 4 est un schéma expliquant le procédé de fabrication de l'enveloppe utilisé dans la présente invention;
• - la figure 5 est un circuit électrique utilisé dans le procédé de la présente invention, et
• - la figure 6 est une courbe fonction du temps du courant induit lors de la décharge du condensateur du circuit de figure 5.

Other characteristics and advantages of the present invention will appear on reading the description of a preferred embodiment made with reference to the attached drawings in which:

• - Figure 1 is a schematic sectional view of the vacuum envelope of an image intensifier tube according to a preferred embodiment of the present invention;
• - Figure 2 is a sectional view of the essential part of the embodiment of Figure 1 before welding;
• - Figure 3 is a view identical to that of Figure 2 after welding;
• - Figure 4 is a diagram explaining the manufacturing process of the envelope used in the present invention;
• FIG. 5 is an electrical circuit used in the method of the present invention, and
• FIG. 6 is a curve as a function of the time of the current induced during the discharge of the capacitor of the circuit of FIG. 5.

In the figures, the same references designate the same elements.

As shown in FIG. 1, the vacuum envelope of the present invention when it is used for an image intensifier tube, essentially comprises a window 1 for entering the radiation to be detected, such as X-rays, a body central 2 of revolution, mainly constituted by a glass cylinder ending in an outlet window 3 forming an integral part of the body 2. In addition, the main elements constituting the intensifier tube are shown diagrammatically inside the envelope. images such as photocathode 4, electrodes 5, 6, 7 for acceleration and focusing and an output screen 8 terminating the last electrode or anode 9.

The entry window 1 is made of aluminum or an aluminum alloy, preferably an aluminum and magnesium alloy such as Ag ₄ MC which is rigid enough to withstand the pressure differences between the outside and inside the tube. The entry window has the shape of a convex cap.

In accordance with the present invention, the entry window 1 comprises a peripheral skirt 10 which fits onto a ring 11 of the same section as the entry window extending the central body. In the embodiment shown, the entry window 1 comprises a peripheral part 1 'folded in a plane parallel to the axis of the tube. The peripheral skirt 10 consists of a substantially T-shaped section ring made of aluminum or aluminum alloy, preferably aluminum, to facilitate welding on the ring 11 which is made of iron or iron alloy, preferably stainless steel as will be explained below. A branch 10 'of the ring is welded to the peripheral part 1'. However, the entry window 1 and the skirt 10 can be formed in one piece when they are made of the same material. As shown in FIG. 2, before being welded to the ring 11, the skirt ends in a flared part 10 "in a cone, the projection towards the inside of the tube, formed by the part" 1 "of the" T "allowing the end of the ring 11 to be supported. In order to be able to carry out the welding using magnetic induction welding in accordance with the invention, the opening angle of the flared part 10" is between 1 and 30 ° relative to the axis of the tube.

On the other hand, as shown in Figure 1, an intermediate ring 12 is provided between the glass part of the central body 2 and the ring 11. This intermediate ring is made of iron or an iron alloy, preferably in a iron-nickel-cobalt alloy such as the Dilver or an iron-nickel alloy such as the Carpenter.

The intermediate ring is provided to facilitate welding on the glass part, in particular when the ring 11 is made of stainless steel. However, it is obvious to those skilled in the art that the two rings 11 and 12 can be formed in one piece when they are made of the same material.

We will now explain, with particular reference to FIGS. 4 to 6, the method used to make a vacuum-tight seal between the ring 11 made of iron, or of an iron alloy preferably made of stainless steel and the aluminum skirt 10. In accordance with the present invention, the sealing is carried out by magnetic induction welding, more particularly by pulse magnetosoldering.

To carry out this welding, the ring 11 is mounted on a mandrel 13 and the flared end of the skirt 10 comes to fit on the ring 11. As shown in FIG. 1, the ring 11 and the skirt 10 at level of the fold both have a folded portion in the form of a notch of identical shape. This folded part gives rigidity to the stainless steel ring 11 and defines the point of rotation of the flared part of the skirt.

On the other hand, the cylindrical skirt 10 is surrounded by a magnetic induction coil L. To carry out a magnetic induction induction pulse, the coil L forms with a capacitance C and the switch 1 an oscillating circuit as shown in the figure 5. Thus, to carry out magnetic induction welding of the flared 10 "end of the aluminum skirt on the stainless steel ring 11, the capacitor C is charged under high voltage and then discharged into the coil L.

The magnetic field which appears in the coil L creates an induction current having the form shown in FIG. 6, in the flared part of the skirt 10 made of aluminum which is a highly conductive material. This results in a mechanical force which has the effect of pressing the flared part of the skirt 10 onto the ring 11 of stainless steel as shown in FIG. 3. The energy released, if it is high enough, drives out the surface oxide aluminum and welds the two materials together due to, it is believed, molecular agitation which then occurs at this level.

It will be recalled that the current induced in the flared part 10 "is limited to the skin thickness. Consequently, the thickness of this flared part 10" has been chosen equal to the skin thickness. It is possible to choose a larger thickness, but in this case, the energy released during magnetic induction welding must be higher.

An example will be given below of the dimensions of the essential parts of the envelope and of the welding parameters for the implementation of the above process.

We want to make the waterproof seal by impulse magnet welding of a window in Ag ₄ MC 0.8 to 1 mm thick, convex, welded to a cylindrical skirt 2 mm thick with a flared portion of 0 , 5 mm thick on a stainless steel ring. The opening angle of the flared part is 7 °.

avec: F: fréquence propre de l'oscillation

• µ: perméabilité de l'air 4 n 10-⁷
• p: résistivité de l'aluminium 2 µΩ cm

si l'on prend τ = 0,5 mm ce qui correspond à l'épaisseur de la partie évasée, on obtient

As recalled above, the current in aluminum is limited to the skin thickness

with: F: natural frequency of oscillation

• µ: air permeability 4 n 10- ⁷
• p: resistivity of aluminum 2 µΩ cm

if we take τ = 0.5 mm which corresponds to the thickness of the flared part, we obtain

avec 0 = n D

• D = 380 mm D: diamètre de l'anneau 11
• d = 3 mm d: ouverture de la partie évasée
• w = 20 mm w: hauteur de la partie évasée
• ce qui donne L = 200 nH
• et C = 300 ₁1F

We can then determine the capacity to discharge.

with 0 = n D

• D = 380 mm D: diameter of the ring 11
• d = 3 mm d: opening of the flared part
• w = 20 mm w: height of the flared part
• which gives L = 200 nH
• and C = 300 ₁ 1F

By charging this capacity under 20 KV, we have an available energy of 60 KJ. If we accept an energy transfer efficiency of 4%, 2, 4 KJ are available for welding. To carry out the welding, the speed of projection of aluminum must reach 400 m / s. Taking into account the mass of the flared part, in this case equal to 32 g, the necessary kinetic energy will be 2.5 KJ. This energy value is compatible with the above value of 2.4 KJ available for welding.

The manufacturing process according to the present invention is a rapid process since it is carried out by instantaneous discharge of a capacitor. It is therefore inexpensive on an industrial level. On the other hand, it makes it possible to seal two cylinders tightly one on the other, which gives a tube of overall diameter reduced compared to the tubes of the prior art. It is also possible with this method to reduce the diameter of the tube for a given field, which results in a reduction in the weight of the tube.

On the other hand, it is obvious to those skilled in the art that the present invention is not limited to image intensifier tubes, but that it can be applied to all electronic tubes having a vacuum enclosure with an aluminum or aluminum alloy window.

Claims (11)

1. An evacuated enclosure for radiation image intensifying tubes or for similar electronic tubes of the type comprising a central member (2) and an entry window (1) of essentially the same diameter as the central member, characterized in that the entry window (1) of aluminum or an alloy thereof comprises a peripheral skirt (10) in the form of a cone which is flared out prior to welding and is provided with a shoulder against which the end of an annulus (11) is caused to bear, said annulus being made of an iron or a ferrous alloy and terminating the central member, said skirt being welded in a vacuum-tight manner on the annulus by magnetic induction welding.

2. The evacuated enclosure as claimed in claim 1, characterized in that the aperture angle of the flared part is comprised between 1 and 30°.

3. The evacuated enclosure as claimed in claim 1 or claim 2, characterized in that the thickness (e) of the flared part is equal to the thickness of the skin used by the current induced in the aluminum or the alloy thereof at the time of magnetic induction welding.

4. The evacuated enclosure as claimed in any one of the claims 1 through 3, characterized in that the entry window is constituted by the window (1) properly so called and by the skirt (10), the window properly so called and the skirt being made of different materials and being made integral with each other in a vacuum-tight manner.

5. The evacuated enclosure as claimed in claim 4, characterized in that the window (1) properly so called is made of an aluminum alloy and the skirt (10) is made of aluminum.

6. The evacuated enclosure as claimed in any one of the claims 1 through 5, characterized in that the annulus (11) of ferrous alloy is made of stainless steel, of an iron-nickel alloy or of an iron-nickel-cobalt alloy.

7. The evacuated enclosure as claimed in any one of the claims 1 through 6, characterized in that it comprises an intermediate annulus (12) between the member (2) and the annulus (11) onto which the window is welded.

8. The evacuated enclosure as claimed in claim 7, characterized in that the intermediate annulus (12) is made of a ferrous alloy such as an iron-nickel alloy or an iron-nickel-cobalt alloy.

9. A process for the manufacture of an evacuated enclosure for radiation image intensifying tubes or similar electronic tubes as claimed in any one of the claims 1 through 8, characterized in that the end (10") of the skirt (10) of aluminum or of an alloy thereof is slipped onto the annulus (11) of iron or of a ferrous alloy and in that the extremity of the skirt is sealed onto the annulus by welding or by magnetic induction while the extremity of the skirt is applied to the annulus and is made integral therewith.

10. The process as claimed in claim 9, characterized in that the magnetic induction welding is performed by an induction coil (L) surrounding the extremity of the skirt, into which a capacitor (C) arranged in an oscillating circuit is discharged.

11. The process as claimed in claim 10, characterized in that the actual oscillation frequency of the oscillating circuit is regulated in order to obtain a thickness of the skin equal to the thickness of the extremity of the skirt.

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