EP1676290A1

EP1676290A1 - Electron gun with a focusing anode, forming a window for said gun and application thereof to irradiation and sterilization

Info

Publication number: EP1676290A1
Application number: EP04805233A
Authority: EP
Inventors: Michel Roche; Philippe Fontcuberta
Original assignee: La Calhene SA; Physique and Industrie
Current assignee: Getinge Life Science France SAS
Priority date: 2003-10-20
Filing date: 2004-10-19
Publication date: 2006-07-05
Also published as: US7800012B2; JP4611993B2; US20070145304A1; FR2861215A1; FR2861215B1; WO2005041241A1; JP2007511039A

Abstract

The invention relates to an electron gun with a focusing anode, forming a window for said gun and the application thereof to irradiation and sterilization. Said gun comprises a vacuum chamber (2) which contains a cathode (6), having an electron-emitter face (8) and an electron-transparent anode (4), formed in one of the chamber walls. The anode is curved in order to resist the pressure difference between the inside and outside of the chamber. The emitter face is also curved and cooperates with the anode to focus the electrons outside of the chamber.

Description

ELECTRON CANON WITH FOCUSING ANODE, FORMING A WINDOW OF THIS CANON, APPLICATION TO IRRADIATION AND STERILIZATION DESCRIPTION

TECHNICAL FIELD The present invention relates to an electron gun whose anode is transparent to electrons and constitutes a window of this gun. It applies in particular: - to the polymerization of products such as paints, varnishes and glues for example, - to the irradiation of surfaces, - to the sterilization of objects, in particular packaging components, such as as stoppers, capsules, bottles, preforms, jars, thermoforming films, sealing films (films used to close off certain containers) and flexible unitary or loop bags for example, - by welding by electron bombardment, food decontamination treatment and - heat treatments such as quenching and amorphization for example. More generally, the invention can be used for all applications of ionization using low energy, provided in focused form, ionization of the kind that can be achieved by laser. STATE OF THE PRIOR ART Reference is made to the following documents:

[1] CA 1 118 180 A, “Process and apparatus for cold-cathode electron-beam generation for sterilization of surfaces and similar applications”, invention of Richard N. Cheever

[2] US 4,721,967 A, "electron gun printer having window sealing conductive plates", invention of Michel Roche.

Document [1] describes an electron gun with a cold cathode. This gun has a conductive window which constitutes the anode of the gun and which the electrons pass through to irradiate the surface of an object or to sterilize it. Document [2] describes a printer comprising an electron gun and several windows formed by curved metal plates, which are transparent to electrons. We therefore know how to obtain an electron beam in the atmosphere, outside the enclosure of the electron gun which generates this beam. As the interior of this enclosure is under vacuum, the window through which the electron beam passes must resist atmospheric pressure. This problem arises particularly when one wants to extract low energy electrons (less than or equal to 500 keV) from the enclosure, since the window must then be very thin. It is necessary then give it a curved shape, for example cylindrical but preferably spherical. However, another problem arises: for certain irradiations by an electron beam, it is interesting that the latter is focused. This is even essential in certain cases where the geometry of the beam is important for irradiating objects properly, for example capsules of milk bottles, or for concentrating the energy of the beam at one point in order to achieve very high power densities. which are required in the case of welding, cutting or surface treatment. But it is well known that, in a conventional electron gun, the focusing elements, like besides the elements of _. deflection and transport of the electron beam, are often very complex and in any case bulky.

PRESENTATION OF THE INVENTION The aim of the present invention is to remedy the above drawbacks. It relates to an electron gun, more particularly a gun capable of supplying a low energy electron beam (less than or equal to 500 keV), this gun comprising a curved window, which is transparent to the electrons, resists pressure atmospheric and serves as both an anode and a focusing electrode. For this focusing, the invention exploits the optical properties of curved surfaces: it uses the curvature that is given to the window anode (so that it resists atmospheric pressure), in cooperation with an equally curved cathode. Specifically, the present invention relates to an electron gun comprising: - a sealed enclosure, intended to be vacuum ("evacuated"), a cathode which is placed in the enclosure and has an emitting face, capable of emitting electrons, an anode constituting a sealed window, formed opposite this emitting face in one of the walls of the enclosure, and capable of letting the electrons emitted by this emitting face, and polarization means ("biasing means ") to establish, between the anode and the cathode, a voltage capable of accelerating these electrons towards the anode, the electrons thus accelerated forming a beam which crosses the anode, this electron gun being characterized in that the anode and the emitting face each have a curvature, the curvature of the anode allowing it to resist a pressure difference between the interior and the exterior of the enclosure and being able to cooperate with the curvature of the emitting face to focus the electron beam outside the enclosure. According to a preferred embodiment of the electron gun object of the invention, the voltage established between the anode and the cathode is capable of communicate to electrons an energy less than or equal to 500 keV. Preferably, the emitting face of the cathode comprises an emitting layer, capable of emitting electrons when it is heated, the electron gun further comprising means for heating the cathode and therefore of this emitting layer. According to a preferred embodiment of the invention, these heating means comprise a filament capable of emitting electrons when it is heated and of bombarding the cathode with these electrons, the cathode and therefore the emitting layer thus being heated by electron bombardment . According to a particular embodiment of the invention, the anode and the emitting face of the cathode form portions of concentric spheres or portions of coaxial cylinders of revolution. The anode preferably comprises a thin metal sheet whose thickness can be less than 50 micrometers. According to a preferred embodiment of the electron gun object of the invention, the polarization means are provided for establishing a pulsed voltage between the anode and the cathode, with a view to an acceleration of the electrons in pulsed mode. In this case, according to a particular embodiment corresponding to the case where the heating means comprise the filament, the biasing means are provided for bringing the cathode to a high voltage drawn negative with respect to the anode, the latter being grounded, and these biasing means comprise: - auxiliary means, capable of supplying a negative pulsed voltage, and - a transformer which is capable of transforming this negative pulsed voltage into the negative pulsed high voltage, this transformer comprising a primary winding, which is connected to the auxiliary means, and a secondary winding which comprises three electrical conductors, two of these conductors being provided for heating the filament and the polarization of this filament relative to the cathode, so that the electrons emitted by the filament reach this cathode, the third conductor being provided to bring the cathode to the high pulsed negative voltage. Preferably, the anode is provided with cooling means. These cooling means preferably comprise means for projecting a gas onto at least part of the periphery of the anode. The present invention also relates to an installation for electronically irradiating at least one object, this installation comprising means for irradiating this object with a focused electron beam, installation in which the means for irradiating comprise the electron gun subject of the invention. The present invention further relates to an installation for electronic sterilization of objects, in particular packaging components, this installation comprising means for irradiating these objects with a focused electron beam, installation in which the irradiation means comprise the electron gun object of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood on reading the description of exemplary embodiments given below, by way of purely indicative and in no way limitative, with reference to the appended drawings, in which: FIG. 1 is a view in schematic longitudinal section of a particular embodiment of the electron gun object of the invention, - Figure 2 shows the variations, as a function of time t, of a high pulsed voltage Va that can be applied to the cathode of the electron gun of Figure 1, to accelerate the electrons emitted by this cathode, - Figure 3 schematically illustrates the acceleration in diode mode that allows this electron gun of Figure 1, Figure 4 is a diagram of means power supply of the electron gun of Figure 1, - Figure 5 schematically illustrates an application of an electron gun according to the invention, to the sterilization of a packaging film llage, - Figure 6 schematically illustrates an application of an electron gun according to the invention, to the sterilization of packaging components, such as capsules or stoppers, and FIGS. 7 and 8 schematically illustrate other applications of the invention, to the treatment of objects whose shapes can be complex.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS The electron gun according to the invention, which is schematically represented in section in FIG. 1, comprises a sealed enclosure 2, which is under vacuum, as well as an anode 4 and a cathode 6. The latter is placed in the enclosure 2 and has an emitting face 8, capable of emitting electrons. The anode 4 is formed in one of the walls of the enclosure, opposite this emitting face 8, and constitutes a sealed window, transparent to electrons. Thus it lets through those which are emitted by the emitting face. The electron gun further comprises electrical supply means 10 making it possible to establish, between the anode 4 and the cathode 6, a voltage Va of acceleration, towards the anode, of the electrons emitted by the cathode. In the example in FIG. 1, the anode 4 is grounded and the voltage Va is a negative pulsed high voltage which is applied to the cathode. The electrons thus accelerated form a beam 12 which passes through the anode 4 and is found outside the enclosure 2, that is to say in the air. According to the invention, the anode 4 and the emitting face 8 each have a curvature. The curvature of the anode allows it to resist the pressure difference between the interior of the enclosure, which is under vacuum, and the exterior of this enclosure, which is at atmospheric pressure. In addition, the curvature of the anode cooperates with that of the emitting face to focus the electron beam 12 outside the enclosure. In the example of FIG. 1, the focusing zone 14, to which we will return later, is punctual or straight. The enclosure 2 seals the vacuum, more specifically the secondary vacuum in the example, and comprises a first metallic part 16, substantially cylindrical, for example made of stainless steel, which is grounded and supports the anode 4 , and a second metallic part 20, substantially annular, which is connected to the cathode 6 and therefore to the negative pulsed high voltage Va. In the example shown, the anode 4 is made from a metal sheet to which we will return later. The edge of this sheet is immobilized between the metal part 16 and a substantially annular part 17. This part 17 is tightened against the metal part 16 by screws 18. On the side of this part 16 and of the part 17, the sealing of the enclosure is obtained by means of a metal seal 19, for example made of indium, which is clamped with the metal sheet, between the latter and the metal part 16, as can be seen in FIG. 1. The metal part 20 is closed by a flange 21 which constitutes the rear wall of the enclosure 2. It is opposite to the wall which carries the anode 4, or front wall. Screws 22 allow the clamp to be tightened

21 against the metal part 20. On the side of this part 20, the enclosure is sealed by means of another metal seal 23, for example made of indium, which is clamped between the metal part 20 and the flange 21, as seen in Figure 1. The gun of Figure 1 is intended to provide a pulsed low energy electron beam, not exceeding 500 keV. As a purely indicative and in no way limitative, a beam of 250 keV is produced, the power of which is 5 k. The cathode 6 comprises a metal part 24, for example made of nickel, the side facing the anode constitutes the side 8 which emits the electrons.

To do this, a thin layer 26 a few tens of millimeters thick has been deposited on this face 8, which emits electrons when it is heated and which for example consists of a mixture of nickel powder and carbonate sintered barium. As a purely indicative and in no way limitative, 5% to 10% of BaCO ₃ by volume is used, which is sintered at 1000 ° C. under a hydrogen atmosphere. Means 28 are provided for heating the part 24 and therefore for the layer 26. In the example, these are means for heating by bombardment. electronic, comprising a filament 30, for example made of tungsten, which emits electrons when heated. Two sealed bushings 34 and 35, of the ceramic-metal type, are welded to the flange 21 and allow the electrical supply of the filament 30 respectively by means of metal rods

36 and 38, as can be seen in FIG. 1. The filament 30, which is opposite part 24, is supported by ceramic screws such as screws 40 and 42, making it possible to electrically isolate the filament from the cathode. This filament is of course continuous but, in Figure 1, only its two ends are visible, the rest extending "behind" the plane of the figure. As can be seen, the cathode comprises another piece 44 of stainless steel, provided with vent holes 46 and fixed, on one side, to piece 24, for example by means of a spot weld 45 of TIG type. , and on the other side, to the flange 21. The part 44 carries the ceramic screws such as the screws 40 and 42, is hollow and crossed by the rods 36 and 38 from which it is electrically insulated by a ceramic ring 48 , as can be seen in FIG. 1. The electrical insulation of the filament 30 with respect to the cathode 6 and therefore with respect to the nickel part 24 makes it possible to apply, between the latter and the filament, a voltage of suitable polarity to bombard and therefore heat the part 24 by the electrons emitted by the filament. In the example, the latter is biased (“biased”) negatively, at -500 V, relative to the cathode. The enclosure 2 comprises a third part 50, substantially in the form of a sleeve, which constitutes an electrical insulator supporting the high voltage and by means of which the parts 16 and 20 of the enclosure are made integral with one of the other. Preferably a ceramic insulator, for example alumina, is used. At the two ends of this insulator, solders 52 of the ceramic-metal type seal the connection between the insulator 50 and the metal parts 16 and 20. In the example, the anode 4 and the face 8 of the piece 24 can respectively form portions of concentric spheres, in which case the zone 14 of focus of the electron beam 12 is punctual, or portions of coaxial cylinders of revolution, in which case this zone is rectilinear (and parallel to the common axis) cylinders, this axis then being perpendicular to the plane of Figure 1). As can be seen in FIG. 1, the electron-emitting layer 26 stops a little before the edge of the part 24 so as to accelerate the electron beam 12 only in an area where the electric field, generated in the accelerator space (that is to say the space between the anode and the cathode) by the application of the voltage Va, is not affected by edge effects. The focusing of the electron beam 12 is essentially carried out by the convergence of the electric field lines in this accelerator space where, preferably, the field hardly exceeds 160 kV / c. As a purely indicative and in no way limitative, a beam of 250 keV is created by providing a distance of 1.5 cm between the anode 4 and the cathode 6, hence an electric field which satisfies the above condition. The anode 4 consists of a thin metal sheet, preferably made of titanium or aluminum. In fact, the lower the energy sought for the electron beam, the thinner this sheet must be. For a beam not exceeding 500 keV, a sheet is preferably used whose thickness is less than 50 micrometers. In the electron gun of Figure 1, one can for example use a titanium sheet, spherical in shape and with a radius of curvature equal to 35 mm, the thickness of which is advantageously between 10 μm and 15 μm. The anode 4, constituting the window of the barrel of FIG. 1, is provided with means 54 allowing air to be projected onto at least part of the periphery of this window, for example over half or even all of this periphery , to cool the window. As can be seen, these means 54 comprise an inlet 56 for compressed air on the desired peripheral part, this inlet 56 being supplied with compressed air by means symbolized by the arrows 58. It is preferable to filter very carefully the air that is sent to the periphery of the anode to prevent dust from being there. Indeed, this dust could stick to the anode where it would be heated by the electron beam and could then cause the anode to pierce. The electron gun of FIG. 1 can also be equipped with suction means (not shown) making it possible to cool the anode with a quasi-controlled atmosphere (for example a nitrogen atmosphere), with a view to d '' avoid the formation of ozone (dangerous gas) during the operation of the electron gun. Furthermore, before operating the barrel of FIG. 1, a vacuum is made in enclosure 2: a secondary vacuum is established there, that is to say a pressure less than or equal to 10 ^"5 Pa. This vacuum can be maintained “statically” in the enclosure, provided that only “ultra-vacuum” techniques are used and that degassing is prolonged at high temperature, for example at 300 ° C, when establishing the secondary vacuum in the enclosure, then placing a getter (not shown) in the enclosure to maintain the secondary vacuum thus obtained. Alternatively, this secondary vacuum can be established in enclosure 2 and then maintain it in a way "Dynamic", for example by means of an ion pump 60. As a purely indicative and in no way limitative, the electron gun of FIG. 1 substantially forms a cylinder 400 mm long and 50 mm in diameter. A preferred embodiment of the electron gun, object of the invention, is based on two principles, namely acceleration in pulsed mode and acceleration in "diode" mode. The electron gun of Figure 1 is an example of this preferred embodiment. With regard to acceleration in pulsed mode, instead of applying a permanent electron acceleration voltage to a gun delivering a low electronic current, for example 10 mA, the electron acceleration voltage is applied for only one small fraction of the time of use of the barrel, preferably 1 thousandth of this time. For example, this voltage is applied for 2 μs with a repetition rate of 500 Hz, but of course the current will have to be 1000 times higher and therefore be 10 A. This has the advantage of reducing the constraints of electrical insulation which are much less severe when the pulse is short (the probability of breakdown ("breakdown") varying as the square root of the time of application of the voltage). This results in a reduction in dimensions and costs both of high voltage generators and of the electron gun. Furthermore, the compactness of this gun has many additional advantages, in particular the reduction in shielding volumes. FIG. 2 shows the variations, as a function of time t, of a negative pulsed high voltage Va which can be applied to the cathode of an electron gun according to the invention, for example the gun of FIG. 1 , to accelerate the electrons emitted by this cathode. We denote by Vm the minimum (negative) value of this voltage Va. Vm is therefore the value of the voltage which is applied to the cathode, only during a fraction of the time of use of the gun and periodically. Regarding mode acceleration

"Diode", this is the simplest possible way of accelerating the electrons: the latter are accelerated between a hot cathode 62 and an anode 64 which are schematically represented in FIG. 3 (and correspond respectively to cathode 6 and at anode 4 of the example in FIG. 1). We also see means 66 for applying the voltage Va to the cathode 62, the anode 64 being grounded. If we add to this acceleration mode the fact that the anode also plays the role of a window necessary for the exit of electrons to the atmosphere, this with a well-controlled "beam span" and adapted to a majority of use cases, we measure the simplicity of the electron gun obtained. In fact, compared to a conventional electron accelerator, the focusing, deflection and transport elements of the electron beam are eliminated, elements which are often very complex and in any case bulky. As already mentioned, the anode has a curvature (in the concave direction for an observer placed on the side of the atmosphere) which is necessary to withstand atmospheric pressure despite the fineness of this anode and which is used also for focusing the electron beam 68 (FIG. 3) directly in the accelerator space and in such a way that the focal zone 70 is outside the electron gun. FIG. 4 schematically illustrates an example of the means 10 for powering the electron gun of FIG. 1. These means 10 make it possible both to apply the high negative pulsed voltage Va to the cathode 6, to polarize the filament 30 by relative to this cathode and to heat this filament, the anode 4 being grounded. These means 10 comprise a transformer 72 which makes it possible to obtain the high negative pulsed voltage. This transformer 72 is essentially characterized by very high electrical insulation, which can advantageously be produced by oil, and by a low leakage inductance. The latter is necessary for obtaining fairly steep rising edges for the output pulse, the duration of these rising edges being by example equal to 1 microsecond, so that the time of application of the high voltage proper on the electron gun can be reduced to the maximum and be for example equal to a few microseconds. The secondary winding 74 of this transformer 72 is wound by means of a cable 76 with three electrical conductors so that one can not only apply the high voltage to the cathode but also, from the ground potential, ensure the heating the filament 30 and applying, between this filament and the cathode 6, a negative voltage Vf allowing the filament to be polarized negatively with respect to the cathode, in order to heat the latter by electron bombardment, at a high temperature, for example of the order 800 ° C. The voltage Vf thus makes it possible to control the temperature of the cathode 6. This temperature itself conditions the emissivity of the cathode. It should be noted that all of these commands, namely the commands for applying the high voltage drawn from the cathode, for heating the filament and for polarizing the filament relative to the cathode, are carried out very simply from the potential of the ground despite the presence of very high voltage pulses. The transformer 72 is controlled by an asymmetrical bridge 80 which is connected to the primary winding 78 of this transformer and designed to supply the latter with a negative pulsed voltage which the transformer converts into a negative pulsed high voltage. This asymmetrical bridge 80 comprises two switching transistors 82 and 84 and two diodes 86 and 88, these diodes and transistors being arranged as seen in FIG. 4. The two diodes 86 and 88 allow the demagnetization of the transformer 72. The two transistors 82 and 84 are preferably IGBT transistors, that is to say bipolar insulated gate transistors. In addition, the transistors 82 and 84 are controlled by means not shown, making it possible to obtain the desired pulsation for the voltage. These means are for example optocoupled "driver" type integrated circuits. The asymmetrical bridge 80 is supplied by a capacitor 90, under a supply voltage which is obtained by rectification of the three-phase sector 92 by means of a Graetz bridge shown diagrammatically by the rectangle 94. As an example, the bridge is supplied asymmetrical 80 by a capacitor whose capacity is worth a few hundred microfarads, under a voltage of the order of 500 V, which is obtained by rectification of the three-phase sector by means of the Graetz bridge. The supply means 10 also include another transformer 96, the primary winding of which is connected to the single-phase sector 98 (220 V-50 Hz). This transformer 96 allows the heating of the filament 30 by means of an alternating current whose frequency is for example 50 Hz, and the intensity 5A, and under a voltage which is worth for example 6 V. The electrical supply means 10 further comprise a generator 100 designed to supply a DC voltage which ensures the temperature control of the cathode 6. This DC voltage can, for example, be adjustable between 100 V and 500 V. The cathode 6 is preferably used in saturated mode. In this case, the density of the current which can be extracted from the accelerator space (space between the cathode and the anode) only depends on the temperature of this cathode. Thus, the current delivered by the electron gun is only controlled by means of this DC voltage. This voltage can optionally be controlled by a servo loop (not shown), from the reading of the current I delivered in a negative high voltage pulse supplied to the cathode. Let us now give details of the secondary winding of the transformer 72. The cable 76, from which this winding is formed, comprises three conductors 102, 104 and 106 which are electrically isolated from each other. The conductors 102 and 104 respectively connect the two terminals of the filament 30 to the two terminals of the secondary winding of the transformer 96. In addition, the generator 100 is mounted between the ground and the end of the conductor 102 which is located on the side of the transformer 96. In addition, the ends of the conductor 106 are respectively connected to the cathode 6 and to the ground. Although the operation in pulsed mode corresponds to a preferred embodiment of the invention, the latter is not limited to such an operation: the cathode can be polarized with respect to the anode of an electron gun conforming to the invention by means of a DC voltage, in order to obtain operation in continuous mode. Similarly, although the use of a hot cathode corresponds to a preferred embodiment of the invention, the latter is not limited to such use: other types of cathodes can be used in a barrel electrons according to the invention, for example a cold cathode, capable of emitting electrons by field effect. In addition, the invention is not limited to the supply of an electron beam of at most 500 keV: higher energies are possible in the invention, by adapting the polarization of the cathode relative to the anode of an electron gun according to the invention. In addition, although the invention is designed for the supply of an electron beam in the air, it goes without saying that an electron gun according to the invention can be used to provide such a beam in a vacuum . We have already mentioned above various applications of the electron gun object of the invention. The latter is particularly suitable for these applications because it can be manufactured in a compact and inexpensive manner and is able to produce a low energy electron beam with a large penetration capacity. Two examples of application of the invention are given in the following with reference to FIGS. 5 and 6. FIG. 5 schematically illustrates an application of the invention to the sterilization of a packaging film 108, for example a thermoforming film or a sealing film. This film 108 is tensioned and moved

(according to arrow FI, from left to right of the figure) on rollers 110, by means not shown, from a reel 112 on which it is wound. As can be seen in the figure, after its unwinding from the reel, the film 108 penetrates and moves in an aseptic enclosure 114 which is put under overpressure by means not shown. An electron gun 116 according to the invention, provided with pumping means 118 and polarization means 120, is provided at the entrance to the aseptic enclosure 114 to sterilize the film 108 by an electron beam 122 supplied by the barrel 116, before the penetration of the film into the enclosure. The barrel is arranged so as to focus the beam on the film 108. Means symbolized by arrows F2 are provided for causing the barrel to move back and forth along the width of the film 108, so that the focused beam sweeps the latter along its width and can therefore scan the entire film taking into account the displacement of the latter along arrow FI, which is perpendicular to arrows F2. One can also use an electron gun with cylindrical focusing to avoid having to move it. In this case, the focal length is a line of length greater than the width of the film being processed. FIG. 6 schematically illustrates another application of the invention to the sterilization of packaging components 124, such as capsules or stoppers for example. These components 124 are pushed by a sterile air jet (symbolized by the arrow F3) and from means not shown, in a vertical pipe 126 in which the components fall by gravity. This pipe 126 is connected to an aseptic enclosure 128 which is put under overpressure by means not shown. Upon their arrival in this enclosure, the components 124 are seized by mechanical means, symbolized by the rectangle 130, and brought by these means to other members, not shown, provided for use of the components in the enclosure. An electron gun 116 according to the invention is also provided, before the enclosure 128, for sterilizing the components 124 before their entry into this enclosure, by means of the focused electron beam 122 supplied by this gun. Several electron guns in accordance with the invention can be coupled to treat, without penetration, the surface of objects whose shapes can be complex. This is schematically illustrated by Figures 7 and 8. We see in Figure 7 three electron guns according to the invention 132a, 132b and 132c, which are placed at 120 ° from each other. The intersection of the electron beams, which are respectively emitted by these guns, covers an area 134 in which an object 136 of complex shape is placed, the surface of which is to be treated by electronic irradiation. As can be seen in FIG. 7, each of the electron guns 132a, 132b or 132c emits a beam 138a, 138b or 138c, the divergence of which, from the corresponding focal zone, is not too great, so as not to not irradiate the other two guns. The electron guns 132a, 132b and 132c are provided with pumping means 140. They are also provided with control means 142 allowing the guns to simultaneously emit pulsed beams of electrons. We see in Figure 8 two electron guns according to the invention 144a and 144b, which are placed opposite one another so as to be able to irradiate an area between these two guns. An object 146 is placed in this zone, approximately equidistant from the two guns, so as to be able to process the two sides of the object respectively by the two electron beams 148a and 148b emitted by the guns. The electron guns 144a and 144b are provided with pumping means 150. They are also provided with control means 152 allowing the guns to simultaneously emit pulsed beams of electrons. These means 152 are activated only when the object 146 is interposed between the two guns, so that one of them is not damaged by the beam emitted by the other and vice versa.

Claims

1. An electron gun comprising: - a sealed enclosure (2), intended to be under vacuum, - a cathode (6) which is placed in the enclosure and has an emitting face (8), capable of emitting electrons, - an anode (4) constituting a sealed window, formed opposite this emitting face in one of the walls of the enclosure, and capable of letting the electrons emitted by this emitting face, and - polarization means (10) to establish, between the anode and the cathode, a voltage capable of accelerating these electrons towards the anode, the electrons thus accelerated forming a beam (12) which crosses the anode, this electron gun being characterized in that the anode (4) and the emitting face (8) each have a curvature, the curvature of the anode allowing it to resist a pressure difference between the inside and the outside of the enclosure and being able to cooperate with the curvature of the emitting face to focus the beam of electrons (12) outside the enclosure.

2. An electron gun according to claim 1, in which the voltage established between the anode (4) and the cathode (6) is able to communicate to the electrons an energy less than or equal to 500 keV.

3. An electron gun according to any one of claims 1 and 2, wherein the emitting face (8) of the cathode (6) comprises an emitting layer (26), capable of emitting electrons when it is heated, the electron gun further comprising means (28) for heating the cathode and therefore of this emitting layer.

4. An electron gun according to claim 3, in which these heating means (28) comprise a filament (30) capable of emitting electrons when it is heated and of bombarding the cathode with these electrons, the cathode and therefore the layer thus being heated by electronic bombardment.

5. An electron gun according to any one of claims 1 to 4, in which the anode (4) and the emitting face (8) of the cathode form portions of concentric spheres or portions of coaxial cylinders of revolution.

6. An electron gun according to any one of claims 1 to 5, wherein the anode (4) comprises a thin metal sheet whose thickness is less than 50 micrometers.

7. electron gun according to any one of claims 1 to 6, wherein the biasing means (10) are provided to establish a pulsed voltage between the anode (4) and the cathode (6), for an acceleration of the electrons in pulsed mode.

8. An electron gun according to claims 4 and 7, in which the polarization means (10) are provided for bringing the cathode (6) to a high pulsed negative voltage with respect to the anode (4), the latter being grounded, and these biasing means comprise: - auxiliary means (80), capable of supplying a negative pulsed voltage, and - a transformer (72) which is capable of transforming this negative pulsed voltage into the negative pulsed high voltage , this transformer comprising a primary winding (78), which is connected to the auxiliary means (80), and a secondary winding (74) which comprises three electrical conductors (102, 104, 106), two of these conductors being provided for heating of the filament (30) and the polarization of this filament relative to the cathode (6), so that the electrons emitted by the filament reach this cathode, the third conductor (106) being provided to bring the cathode to high voltage pu negative.

9. An electron gun according to any one of claims 1 to 8, in which the anode (4) is provided with cooling means (54).

10. An electron gun according to claim 9, in which these cooling means comprise means (56) for projection of a gas onto at least part of the periphery of the anode (4).

11. Installation for electronically irradiating at least one object, this installation comprising means for irradiating this object with a focused electron beam, installation in which the irradiation means comprise the electron gun (116) according to any one of claims 1 to 10.

12. Installation for electronic sterilization of objects, in particular packaging components, this installation comprising means for irradiating these objects with a focused electron beam, installation in which the irradiation means comprise the electron gun ( 116) according to any one of claims 1 to 10.