EP3652772B1 - Compact source for generating ionizing rays - Google Patents

Description

The invention relates to a source for generating ionizing rays and in particular X-rays, an assembly comprising several sources and a method for producing the source.

X-rays have many uses today, especially in imaging and radiotherapy. X-ray imaging is widely used in particular in the medical field, in industry to perform non-destructive testing and in security to detect dangerous objects or materials.

Much progress has been made in making images from X-rays. Originally only photosensitive films were used. Since then, digital detectors have appeared. These detectors, associated with software, allow rapid reconstruction of two-dimensional or three-dimensional images by means of scanners.

On the other hand, since the discovery of X-rays by Rôntgen in 1895, X-ray generators have changed very little. The synchrotrons which appeared after World War II make it possible to generate intense and well-focused radiation. Radiation is due to accelerations or deceleration of charged particles and possibly moving in a magnetic field.

Linear accelerators and x-ray tubes use an accelerated electron beam to bombard a target. The braking of the beam due to the electric fields of the target's nuclei makes it possible to generate X-radiation by braking.

An x-ray tube generally consists of an envelope in which a vacuum is created. The envelope is formed by a metal structure and an electrical insulator made of alumina or glass. In this envelope, are arranged two electrodes. A cathode electrode, brought to a negative potential, is equipped with an electron emitter. A second anode electrode, brought to a positive potential with respect to the first electrode, is associated with a target. The electrons accelerated by the potential difference between the two electrodes, come to produce a continuous spectrum of ionizing rays by braking (bremsstralung) when hit the target. Metal electrodes are necessarily large in size and have large radii of curvature in order to minimize the electric fields on their surface.

Depending on the power of the X-ray tubes, they can be equipped with either a fixed anode or a rotating anode allowing the thermal power to be spread. Fixed anode tubes have a power of a few kilowatts and are used in particular in low power industrial, safety and medical applications. Rotating anode tubes can exceed 100 kilowatts and are mainly used in the medical environment for imaging requiring large X-ray fluxes to improve contrast. By way of example, the diameter of an industrial tube is of the order of 150 mm at 450 kV, 100 mm at 220 kV and 80 mm at 160 kV. The voltage indicated corresponds to the potential difference applied between the two electrodes. For rotating anode medical tubes, the diameter varies from 150 to 300 mm depending on the power to be dissipated on the anode.

The dimensions of known X-ray tubes therefore remain large, of the order of several hundred mm. Imaging systems have seen the emergence of digital detectors with increasingly fast and efficient 3D reconstruction software while at the same time, X-ray tube technologies have hardly evolved for a century and this is a major technological limitation of x-ray imaging systems.

Several factors hinder the miniaturization of today's X-ray tubes.

The electrical insulators must have sufficient dimensions to guarantee good electrical insulation against high voltages from 30 kV to 300 kV. Sintered alumina, often used to produce these insulators, typically has a dielectric strength of the order of 18 MV / m.

The radius of curvature of metal electrodes should not be too small in order to keep a static electric field applied to the surface below an acceptable limit, typically 25 MV / m. Beyond this, the parasitic emissions of electrons by tunnel effect become difficult to control, leading to heating of the walls, unwanted X-ray emissions and micro-discharges. Therefore, for tensions high, as encountered in X-ray tubes, the dimensions of the cathode electrodes are important in order to limit the parasitic emission of electrons.

Thermoinic cathodes are often used in conventional tubes. The dimensions of this type of cathode and their operating temperatures, typically greater than 1000 ° C., generate expansion problems as well as the evaporation of electrically conductive elements such as barium. This makes the miniaturization and integration of this type of cathode in contact with a dielectric insulator difficult.

Surface charge phenomena linked to the Coulomb interaction appear on the surface of the dielectric materials used (alumina or glass) when this surface is in the vicinity of an electron beam. In order to avoid proximity between the electron beam and the surface of the dielectric materials, either an electrostatic screening is carried out using a metal screen placed in front of the dielectric, or the removal of the electron beam from the dielectric . The presence of screens or the distance also tend to increase the dimensions of the x-ray tubes.

The anode forming the target must dissipate significant thermal power. This dissipation can be achieved by circulating a heat transfer fluid or by making a large rotating anode. The need for this dissipation also makes it necessary to increase the dimensions of the X-ray tubes.

Among the emerging technological solutions, the literature describes the use of cold carbon nanotube cathodes in X-ray tube structures, but the solutions currently proposed remain based on conventional X-ray tube structures using a metallic wehnelt. surrounding the cold cathode. This wehnelt is an electrode brought to a high voltage and is always subjected to significant dimensional constraints in order to limit the parasitic emissions of electrons.

The invention aims to overcome all or part of the problems mentioned above by proposing a source of ionizing radiation, for example under in the form of a high voltage diode or triode, the dimensions of which are much smaller than those of conventional x-ray tubes. The principle of generating ionizing radiation remains similar to that implemented in known tubes, namely an electron beam bombarding a target. The electron beam is accelerated between a cathode and an anode between which a potential difference is applied, for example greater than 100 kV. For the same potential difference, the invention makes it possible to significantly reduce the dimensions of the source according to the invention compared to known tubes.

To achieve this goal, the invention provides a source of ionizing radiation comprising a vacuum chamber in which a plug performs several functions.

• une enceinte à vide,
• une cathode pouvant émettre un faisceau d'électrons dans l'enceinte,
• une anode recevant le faisceau d'électrons et comprenant une cible pouvant générer un rayonnement ionisant à partir de l'énergie reçu du faisceau d'électrons,
• une électrode disposée au voisinage de la cathode et permettant de focaliser le faisceau d'électrons,
• un bouchon assurant l'étanchéité de l'enceinte à vide,
• une pièce mécanique réalisée en matériau diélectrique et formant une partie de l'enceinte à vide.

More specifically, the subject of the invention is a source for generating ionizing rays comprising:

• a vacuum chamber,
• a cathode capable of emitting an electron beam in the enclosure,
• an anode receiving the electron beam and comprising a target capable of generating ionizing radiation from the energy received from the electron beam,
• an electrode placed in the vicinity of the cathode and making it possible to focus the electron beam,
• a plug ensuring the tightness of the vacuum chamber,
• a mechanical part made of dielectric material and forming part of the vacuum chamber.

The plug is attached to the mechanical part by means of a conductive solder film used to electrically connect the electrode. An X-ray generating source comprising a vacuum chamber, a cathode, an anode comprising a target and a plug ensuring the vacuum seal and fixed by soldering is described by the patent application published under the no. US2002090053 A1 .

Advantageously, the plug is made of the same dielectric material as the mechanical part.

The solder film is advantageously of revolution about an axis of the electron beam and it forms an equipotential assembly with the electrode.

The plug advantageously comprises at least one electrical connection passing therethrough, making it possible to electrically connect a control of the cathode and brought to a potential different from the solder film.

The plug advantageously forms a coaxial type transmission line whose electrical connection passing through it forms a central conductor of the coaxial line and of which the solder film forms a shielding of the coaxial line.

The stopper advantageously comprises a surface external to the vacuum chamber. The outer surface then comprises several distinct zones which are separately metallized. At least one of these areas is in electrical contact with the at least one electrical connection and another of these areas is in electrical contact with the solder film to ensure the electrical connection of the cathode and the electrode via via the at least one electrical connection and the solder film.

Advantageously, the source comprises a coaxial connector connected to the solder film and to at least one electrical connection, and a cavity located between the coaxial connector and the plug, the cavity being screened by a main electric field of the source.

Advantageously, the mechanical part comprises a surface external to the vacuum chamber having an internal frustoconical shape widening out from the external surface of the stopper. The source further comprises a support having a surface complementary to the internal frustoconical shape of the mechanical part. The complementary surface and the interior frustoconical shape are then configured to convey air trapped between the complementary surface and the interior frustoconical shape during the assembly of the mechanical part in the support towards the cavity.

Advantageously, the cathode emits the electron beam by field effect and the control of the cathode comprises an optoelectronic component electrically connected by the electrical connection passing through the plug.

Advantageously, the mechanical part comprises a cavity in which the cathode is placed. A getter is placed in the cavity, between the cathode and the plug.

• la figure 1 représente schématiquement les principaux éléments d'une source génératrice de rayons X selon l'invention ;
• la figure 2 représente une variante de la source de la figure 1 permettant d'autres modes de raccordement électrique ;
• la figure 3 est une vue partielle et agrandie de la source de la figure 1 autour de sa cathode ;
• les figures 4a et 4b sont des vues partielles et agrandie de la source de la figure 1 autour de son anode selon deux variantes ;
• la figure 5 représente en coupe un mode d'intégration comprenant plusieurs sources conformes à l'invention ;
• les figures 6a, 6b, 6c, 6d et 6e représentent des variantes d'un ensemble comprenant plusieurs sources dans une même enceinte à vide ;
• les figures 7a et 7b représentent plusieurs modes de raccordement électrique d'un ensemble comprenant plusieurs sources.
• les figures 8a, 8b et 8c représentent trois exemples d'ensembles comprenant plusieurs sources conformes à l'invention et pouvant être réalisés suivant les variantes proposées sur les figures 5 ou 6.

The invention will be better understood and other advantages will appear on reading the detailed description of an embodiment given by way of example, description illustrated by the accompanying drawing in which:

• the figure 1 schematically shows the main elements of an X-ray generating source according to the invention;
• the figure 2 represents a variant of the source of figure 1 allowing other methods of electrical connection;
• the figure 3 is a partial and enlarged view of the source of the figure 1 around its cathode;
• the figures 4a and 4b are partial and enlarged views of the source of the figure 1 around its anode according to two variants;
• the figure 5 shows in section an integration mode comprising several sources in accordance with the invention;
• the figures 6a , 6b , 6c, 6d and 6th represent variants of an assembly comprising several sources in the same vacuum chamber;
• the figures 7a and 7b represent several modes of electrical connection of an assembly comprising several sources.
• the figures 8a, 8b and 8c show three examples of assemblies comprising several sources in accordance with the invention and which can be produced according to the variants proposed on the figures 5 Where 6 .

For the sake of clarity, the same elements will bear the same references in the different figures.

The figure 1 shows in section a source 10 generating X-rays. The source 10 comprises a vacuum chamber 12 in which are arranged a cathode 14 and an anode 16. The cathode 14 is intended to emit an electron beam 18 in the chamber 12 in the direction of the anode 16. The anode 16 comprises a target 20 bombarded by the beam 18 and emitting an X-radiation 22 as a function of the energy of the electron beam 18. The beam 18 develops around an axis 19 passing through cathode 14 and anode 16.

X-ray generator tubes conventionally employ a thermionic cathode operating at high temperature, typically around 1000 ° C. This type of cathode is commonly referred to as a hot cathode. This type of cathode composed of a metal matrix or metal oxides emits a flow of electrons caused by the vibrations of the atoms due to thermal energy. However, hot cathodes suffer from several drawbacks, such as a weak temporal dynamic of current control linked to the time constants of thermal processes, the need to use grids located between the cathode and the anode and biased at high voltages in order to be able to control the current. The grids are therefore located in an area of very strong electric fields, they are subjected to high operating temperatures around 1000 ° C. All of these constraints greatly limit the possibilities of integration and lead to large dimensions of the electron gun.

More recently, cathodes operating on the principle of field emission have been developed. These cathodes operate at room temperature and are commonly referred to as cold cathodes. For the most part, they consist of a flat conductive surface provided with structures in relief, on which an electric field is concentrated. These raised structures are emitters of electrons when the field at the top is high enough. The emitters in relief can be formed from carbon nanotubes. Such embodiments are for example described in the patent application published under No. WO 2006/063982 A1 and filed on behalf of the plaintiff. Cold cathodes do not have the drawbacks of hot cathodes and are above all much more compact. In the example shown, the cathode 14 is a cold cathode and therefore emits the electron beam 18 by field effect. The control of the cathode 14 is not shown on the figure 1 . This control can be carried out electrically or optically as also described in the document WO 2006/063982 A1

Under the effect of a potential difference between the cathode 14 and the anode 16, the electron beam 18 is accelerated and strikes the target 20 comprising for example a membrane 20a for example made of diamond or beryllium coated with a thin layer 20b made of an alloy based on a material with a high atomic number such as in particular tungsten or molybdenum. The layer 20b may have a variable thickness, for example between 1 and 12 μm depending on the energy of the electrons of the beam 18. The interaction between the electrons of the electron beam 18 accelerated at high speed and the material of the thin layer 20b allows the production of X-radiation 22. Advantageously, in the example shown, the target 20 forms a window of the vacuum chamber 12. In other words, the target 20 forms part of the wall of the vacuum chamber 12. This arrangement is implemented in particular for a target operating in transmission. For this arrangement, the membrane 20a is formed from a material with a low atomic number, such as diamond or beryllium for its transparency to X-radiation 22. The membrane 20a is configured to ensure, with the anode 16, the vacuum seal. speaker 12.

Alternatively, the target 20, or at least the layer made of a high atomic number alloy, can be placed completely inside the vacuum chamber 12 and the X-radiation leaves the chamber 12 by passing through a window. forming part of the wall of the vacuum chamber 12. This arrangement is implemented in particular for a target operating in reflection. The target is then distinct from the window. The layer in which the X-radiation is produced can be thick. The target can be fixed or rotating allowing a spreading of the thermal power generated during the interaction with the electrons of the beam 18.

Advantageously, it is possible to overcome a significant stress in the electric field level at the surface of the cathode or wehnelt electrode. This constraint is linked to the metallic nature of the interface between the electrode and the vacuum present in the enclosure in which the electron beam propagates. To this end, at the level of the electrode, the metal / vacuum interface is replaced by a dielectric material / vacuum interface which does not allow parasitic emission of electrons by tunneling. It is then possible to accept electric fields much higher than those acceptable with a metal / vacuum interface. Initial internal tests have shown that it is possible to achieve static fields well above 30 MV / m without parasitic emission of electrons. This dielectric / vacuum interface can for example be obtained by replacing the metal electrode, the outer surface of which is subjected to the electric field by an electrode made of a dielectric material, the outer surface of which is subjected to the electric field and the inner surface of which is coated. perfectly adherent conductive deposit ensuring the function of electrostatic wehnelt. It is also possible to cover the outer surface of a metal electrode subjected to the electric field with a dielectric material in order to replace the metal / vacuum interface of the known electrodes with a dielectric / vacuum interface where the electric field is important. This arrangement makes it possible to significantly increase the maximum electric field below which parasitic emissions of electrons do not occur.

The increase in the admissible electric fields allows a miniaturization of the sources of X-rays and more generally of the sources of ionizing radiation.

For this, the source 10 comprises an electrode 24 arranged in the vicinity of the cathode 14 and making it possible to focus the electron beam 18. The electrode 24 forms a wehnelt. In the case of a so-called cold cathode, electrode 24 is placed in contact with the cathode. A cold cathode emits an electron beam by field effect. This type of cathode is for example described in the document WO 2006/063982 A1 filed on behalf of the plaintiff. In the case of a cold cathode, the electrode 24 is placed in contact with the cathode 14. The mechanical part 28 advantageously forms a support for the cathode 14. To ensure the function of wehnelt, the electrode 24 has an essentially shape. convex. The exterior of the concavity of the face 26 is oriented towards the anode 16. Locally at the level of the contact between the cathode 14 and the electrode, the convexity of the electrode 24 may be zero or even slightly reversed.

The electrode 24 is formed of a continuous conductive surface disposed on a concave face 26 of a dielectric material. The concave face 26 of the dielectric material forms a convex face of the electrode 24 facing the anode 16. It is from this convex face of the electrode 24 that strong electric fields develop. In the prior art, a metal / vacuum interface exists on this convex face of the electrode. Consequently, this interface can be the seat of emission of electrons under the effect of the electric field inside the vacuum chamber. This interface of the electrode with the vacuum of the enclosure is eliminated by replacing it with a dielectric / vacuum interface. A dielectric material, not having a free charge, cannot therefore be the seat of a sustained emission of electrons.

It is important to avoid any air gap or vacuum between the electrode 24 and the concave face 26 of the dielectric material. Indeed, in the event of uncertain contact between the electrode 24 and the dielectric material, a very strong amplification of the electric field would appear at the interface and emissions. electrons or the development of a plasma could occur there. For this purpose, the source 10 comprises a mechanical part 28 formed in the dielectric material. One of the faces of the mechanical part 28 is the concave face 26. The electrode 24 is, in this case, constituted by a deposit of a conductive material perfectly adherent to the concave face 26. Different techniques can be implemented to achieve this deposition, such as in particular physical vapor deposition (known in the Anglo-Saxon literature by the acronym PVD for Physical Vapor Deposition) or in chemical phase (CVD) optionally assisted by plasma (PECVD).

Alternatively, it is possible to deposit a dielectric material on the surface of a solid metal electrode. The deposit of dielectric material, adherent to the massive metal electrode, always makes it possible to avoid any air or vacuum gap at the electrode / dielectric material interface. This deposit of dielectric material is chosen to withstand high electric fields, typically greater than 30 MV / m, and to have sufficient flexibility compatible with possible thermal expansions of the massive metal electrode. However, the reverse arrangement implementing the deposition of a conductive material on the internal face of a solid part made of dielectric material has other advantages, in particular that of allowing the use of the mechanical part 28 to fulfill other functions. .

More precisely, the mechanical part 28 may form part of the vacuum chamber 12. This part of the vacuum chamber may even be a predominant part of the vacuum chamber 12. In the example shown, the mechanical part 28 on the one hand forms a support for the cathode 14 and on the other hand a support for the anode 16. The part 28 provides electrical insulation between the anode 16 and the cathode electrode 24.

For the production of the mechanical part 28, the use of conventional dielectric materials such as for example sintered alumina already makes it possible to avoid any metal / vacuum interface. However, the dielectric strength of this type of material, of the order of 18 MV / m, further limits the miniaturization of the source 10. To further miniaturize the source 10, a dielectric material having a dielectric strength greater than 20MV is chosen. / m and advantageously greater than 30 MV / m. The value of the dielectric strength is for example maintained above 30 MV / m in a temperature range between 20 and 200 ° C. Composite ceramics of the nitride type make it possible to fulfill this criterion. Internal tests have shown that a ceramic of this nature can even exceed 60 MV / m.

By miniaturizing the source 10, surface charges can accumulate on an internal face 30 of the vacuum chamber 12, and in particular the internal face of the mechanical part 28, when the electron beam 18 is established. It is useful to be able to drain these charges and for this purpose, the internal face 30 has a surface resistivity measured at ambient temperature of between 1.10 ⁹ Ω.square and 1.10 ¹³ Ω.square and typically close to 1.10 ¹¹ Ω.square. Such resistivity can be obtained by adding at the surface a conductive or semiconductor material compatible with the dielectric material. As a semiconductor material, it is for example possible to deposit silicon on the internal face 30. In order to obtain the correct range of resistivity, for example for a ceramic based on nitride, it is possible to modify its intrinsic properties by adding thereto a few percent (typically less than 10%) of a titanium nitride powder known for its low resistivity properties of the order of 4 .10 ^-3 Ω.m. or semiconductor materials such as silicon carbide SiC.

It is possible to disperse the titanium nitride in the volume of the dielectric material in order to obtain a homogeneous resistivity in the material of the mechanical part 28. Alternatively, it is possible to obtain a resistivity gradient by diffusing titanium nitride. from the internal face 30 by a high temperature heat treatment above 1500 ° C.

The source 10 comprises a plug 32 ensuring the tightness of the vacuum chamber 12. The mechanical part 28 comprises a cavity 34 in which the cathode 14 is disposed. The cavity 34 is delimited by the concave face 26. The plug 32 closes the cavity 34. The electrode 24 comprises two ends 36 and 38 distant along the axis 19. The first end 36 is in contact with the cathode 14 and in electrical continuity therewith. The second end 38 is opposite the first. The mechanical part 28 comprises an internal truncated cone 40 with a circular section arranged around the axis 19 of the beam 18. The truncated cone 40 is located at the level of the second end 38 of the electrode 24. The truncated cone 40 s 'opens in moving away from the cathode 14. The plug 32 comprises a shape complementary to the truncated cone 40 in order to be placed there. The truncated cone 40 ensures the positioning of the plug 32 in the mechanical part 28. The plug 32 can be implemented independently of the production of the electrode 24 in the form of a conductive surface arranged on the concave face 26 of the dielectric material. .

Advantageously, the plug 32 is made of the same dielectric material as the mechanical part 28. This makes it possible to limit possible phenomena of differential thermal expansion between the mechanical part 28 and the plug 32 when using the source 10.

The stopper 32 is for example fixed to the mechanical part 28 by means of a solder film 42 produced in the truncated cone 40 and more generally in an interface zone between the stopper 32 and the mechanical part 28. It is possible to metallize the surfaces intended to be brazed of the plug 32 and of the mechanical part 28 then to carry out the brazing by means of a metal alloy whose melting point is higher than the maximum temperature of use of the source 10. The metallization and the solder film 42 come in electrical continuity with the end 38 of the electrode 24. The frustoconical shape of the metallized interface between the plug 32 and the mechanical part 28 makes it possible to avoid angular shapes that are too pronounced for the. electrode 24 and for the conductive zones extending the electrode 24 in order to limit possible peak effects of the electric field.

Alternatively, it is possible to avoid the metallization of the surfaces by integrating into the brazing alloy an active element which reacts with the material of the plug 32 and that of the mechanical part 28. For ceramics based on nitride, titanium is integrated into the solder alloy. Titanium is a metal reactive with nitrogen and allows to create a strong chemical bond with the ceramic. Other reactive metals can be used such as vanadium, niobium or zirconium.

Advantageously, the solder film 42 is conductive and is used to electrically connect the electrode 24 to a power supply from the source 10. The electrical connection of the electrode 24 by means of the solder film 42 can be implemented for other types of electrodes, in particular metal electrodes coated with a deposit of material dielectric. To strengthen the connection with the electrode 24, it is possible to embed a metal contact in the solder film 42. This contact is advantageous for connecting a solid metal electrode covered with a deposit of dielectric material. The electrical connection of the electrode 24 is provided by this electrical contact. Alternatively, it is possible to partially metallize a surface 43 of the plug 32. The surface 43 is located outside the vacuum chamber 12. The metallization of the surface 43 is in electrical contact with the solder film 42. It is possible to solder to the metallization of the surface 43 a contact which can be electrically connected to a power supply from the source 10.

The solder film 42 extends the shape of revolution of the electrode 24 and in fact contributes to the main function of the electrode 24. This is particularly interesting when the electrode 24 is formed of a conductive surface disposed on the concave face. 26. The solder film 42 extends the conductive surface forming the electrode 24 directly and without discontinuity or angular point moving away from the axis 19. The electrode 24, associated with the solder film 42 when it is conductive, form an equipotential surface which contributes to the focusing of the electron beam 18 and to the setting to the potential of the cathode 14. This makes it possible to minimize the local electric fields in order to make the source 10 more compact.

The face 26 may have locally convex zones, such as for example at its junction with the truncated cone 40. In practice, the face 26 is at least partly concave. The face 26 is generally concave.

On the figure 1 , the source 10 is polarized by means of a high voltage source 50, a negative terminal of which is connected to the electrode 24, for example through the metallization of the solder film 42 and of which a positive terminal is connected to the anode 16. This type of connection is characteristic of a monopolar operation of the source 10 in which the potential of the anode 16 is earthed 52. It is also possible to replace the high voltage source 50 with two high voltage sources 56. and 58 in series to operate source 10 in a bipolar fashion as shown in figure 2 . This type of operation is advantageous in order to simplify the production of the associated high voltage generator. For example in the case of an operating mode in high voltage pulsed at high frequency, it It may be advantageous to lower the absolute voltage by summing the two positive and negative half voltages at the level of the source 10. For this purpose, the high voltage source can include an output transformer driven in half H bridge.

With a source 10 as shown in figure 1 , the bipolar operation can be done by connecting the common point of the generators 56 and 58 to the earth 52. Alternatively, it is also possible to keep the high voltage source 50 floating with respect to the earth 52 as on the figure 2 .

Bipolar operation of a source as described on figure 1 is done by keeping the common point of two high voltage sources connected in series floating. Alternatively, this common point can be used to bias another electrode of the source 10 as shown in Figure figure 2 . In this variant, the source 10 comprises an intermediate electrode 54 dividing the mechanical part 28 into two parts 28a and 28b. The intermediate electrode 54 extends perpendicularly to the axis 19 of the beam 18 and is crossed by the beam 18. The presence of the electrode 54 allows bipolar operation by connecting the electrode 54 to the common point of the two high voltage sources 56 and 58 connected in series. On the figure 2 , the assembly formed by the two high voltage sources 56 and 58 is floating with respect to the earth 52. As shown in the figure. figure 1 , it is also possible to connect to earth 52, one of the electrodes of the source 10, for example the intermediate electrode 54.

The figure 3 is a partial and enlarged view of the source 10 around the cathode 14. The cathode 14 is arranged in the cavity 34 bearing against the end 36 of the electrode 24. A support 60 makes it possible to center the cathode 14 with respect to the electrode 24. The electrode 24 being of revolution about the axis 19, the cathode 14 is therefore centered on the axis 19 allowing it to emit the electron beam 18 along the axis 19. The support 60 comprises a counterbore 61 centered on the axis 19 and in which the cathode 14 is disposed. At its periphery, the support 60 comprises an annular zone 63 centered in the electrode 24. A spring 64 presses on the support 60 so as to hold the cathode 14 resting against the electrode 24. The support 60 is made of an insulating material. Spring 64 may have an electrical function making it possible to route a control signal to the cathode 14. More precisely, the cathode 14 emits the electron beam 18 via a face 65, called the front face and oriented in the direction of the anode 16. The electrical control of the cathode 14 is made by its rear face 66 opposite to the front face 65. The support 60 can comprise an opening 67 with a circular section centered on the axis 19. The opening 67 can be metallized so as to electrically connect the spring 64 and the rear face 66 of cathode 14. The plug 32 can ensure the electrical connection of the control of the cathode 14 by means of a metallized via 68 passing through it and a contact 69 integral with the plug 32. The contact 69 presses on the spring 64 along axis 19 to keep cathode 14 resting against electrode 24. Contact 69 ensures electrical continuity between via 68 and spring 64.

The surface 43 of the plug 32, located outside the vacuum chamber 12, can be metallized in two distinct zones: a zone 43a centered on the axis 19 and a peripheral annular zone 43b around the axis 19. The metallized zone 43a is in electrical continuity with the metallized via 68. The metallized zone 43b is in electrical continuity with the solder film 42. A central contact 70 bears against the zone 43a and a peripheral contact 71 bears against the zone 43b. The two contacts 70 and 71 form a coaxial connector ensuring the electrical connection of the cathode 14 and the electrode 24 via the metallized zones 43a and 43b and via the metallized via 68 and the solder film 42.

The cathode 14 can comprise several distinct emitting zones which can be addressed separately. The rear face 66 then has several separate electrical contact zones. The support 60 and the spring 64 are adapted accordingly. Several contacts similar to contact 69 and several metallized vias similar to via 68 make it possible to connect the different zones of the rear face 66. The surface 43 of the plug 32, the contact 69 as well as the spring 64 are sectored accordingly to provide several zones therein. similar to zone 43a and in electrical continuity with each of the metallized vias.

At least one getter 35 (known in the Anglo-Saxon literature under the name of “getter”) can be arranged in the cavity 34, between the cathode 14 and the plug 32, in order to trap any particle liable to alter the quality of the gas. vacuum of the enclosure 12. The getter 35 generally acts by chemisorption. Zirconium or titanium-based alloys can be used to trap any particles emitted by the various components of the source 10 surrounding the cavity 34. The getter 35 is, in the example shown, fixed to the stopper 32. The getter 35 is made from annular-shaped discs stacked and surrounding the contact 69.

The figure 4a shows an alternative source of ionizing radiation 75 in which an anode 76 replaces the anode 16 described above. The figure 4a is a partial and enlarged view of the source 75 around the anode 76. As for the anode 16, the anode 76 comprises a target 20 bombarded by the beam 18 and emitting an X-ray 22 radiation. anode 16, the anode 76 comprises a cavity 80 into which the electron beam 18 penetrates to reach the target 20. More precisely, the electron beam 18 strikes the target 20 by its internal face 84 carrying the thin layer 20b and emits X-ray radiation 22 through its external face 86. In the example shown, the walls of the cavity 80 have a cylindrical part 88 around the axis 19 extending between two ends 88a and 88b. The end 88a is in contact with the target 20 and the end 88b approaches the cathode 14. The walls of the cavity 80 also have a part 90 in the form of a washer having a hole 89 and closing the cylindrical part at the level of. end 88b. The electron beam 18 enters the cavity 80 through the hole 89 of the part 90.

During the bombardment of the target 20 by the electron beam 18, the rise in temperature of the target 20 can cause molecular degassing of the target 20 which, under the effect of the X radiation 22, is ionized. Ions 91 appearing on the interior face 84 of target 20 can damage the cathode if they return to the accelerating electric field between the anode and the cathode. Advantageously, the walls of the cavity 80 can be used to trap the ions 91. To this end, the walls 88 and 90 of the cavity 80 are electrical conductors and form a Faraday cage vis-à-vis parasitic ions which can be emitted by the target 20 inside the vacuum chamber 12. The ions 91 possibly emitted by the target 20 towards the interior of the vacuum chamber 12 are largely trapped in the cavity 80. Only the hole 89 of the part 90 allows the ions to exit from the cavity 80 and could be accelerated towards the cathode 14. To better trap the ions in the cavity 80, at least one getter 92 is arranged in the Cavity 80. The getter 92 is separate from the walls 88 and 90 of the cavity 80. The getter 92 is a specific component disposed in the cavity 80. Like the getter 35, the getter 92 generally acts by chemisorption. Zirconium or titanium-based alloys can be used to trap the 91 ions emitted.

In addition to the trapping of ions, the walls of the cavity 80 can form a shielding screen against parasitic ionizing radiation 82 generated inside the vacuum chamber 12 and possibly an electrostatic screening of the electric field. generated between the cathode 14 and the anode 76. The X-radiation 22 forms the useful radiation emitted by the source 75. However, stray X-radiation can exit the target 20 through the internal face 84. This stray radiation is unnecessary and undesirable. . Usually, shielding screens opposing this type of parasitic radiation are arranged around the X-ray generators. However, this type of embodiment has a drawback. Indeed, the more the shielding screens are placed far from the X-ray source, that is to say far from the target 20, the more the screens require material surface because of their remoteness. This aspect of the invention proposes to have such screens as close as possible to the parasitic source, which makes it possible to miniaturize them.

The anode 76 and in particular the walls of the cavity 80 are advantageously made of a material with a high atomic number such as for example an alloy based on tungsten or molybdenum in order to stop the parasitic radiation 82. Tungsten or molybdenum have almost no effect of trapping parasitic ions. By making the getter 92 separate from the walls of the cavity 80, this frees up the choice of materials in order to best perform the functions of trapping parasitic ions for the getter 92 and of screen vis-à-vis. parasitic radiation 92 for the walls of the cavity 80 without compromise between the two functions. For this purpose, the getter 92 and the walls of the cavity 80 are made of different materials, each suitable for the function assigned to it. It is the same for the getter 35 vis-à-vis the walls of the cavity 34.

The walls of the cavity 80 surround the electron beam 18 in the vicinity of the target 20.

Advantageously, the walls of the cavity 80 form part of the vacuum chamber 12.

Advantageously, the walls of the cavity 80 are arranged coaxially with the axis 19 so as to be located radially around the axis 19 at a constant distance and therefore as close as possible to the parasitic radiation. At the end 88a, the cylindrical part 88 can partially or totally surround the target 20, thus preventing any parasitic radiation X from escaping from the target 20 radially with respect to the axis 19.

Thus the anode 76 fulfills several functions, its electrical function of course, in addition, a Faraday cage function surrounding parasitic ions which can be emitted by the target 20 inside the vacuum chamber 12, a function of screening against parasitic X radiation and, moreover, a wall of the vacuum chamber 12. By performing several functions by means of a single mechanical part, in this case the anode 76, the source 75 becomes more compact and by weight.

Furthermore, around the cavity 80, it is possible to have at least one magnet or electromagnet 94 making it possible to focus the electron beam 18 towards the target 20. Advantageously, the arrangement of the magnet or electromagnet 94 can be also defined so as to deflect the parasitic ions 91 towards the getter (s) 92 in order to prevent the parasitic ions from leaving the cavity through the hole 89 of the part 90 or at least being deflected with respect to the axis 19 passing through the cathode 14. For this purpose, the magnet or the electromagnet 94 generates a magnetic field B oriented along the axis 19. On the figure 4a , the ions 91 deflected towards the getter 92 follow a trajectory 91a and the ions leaving the cavity 80 follow a trajectory 91b.

The means for trapping the parasitic ions 91 which can be emitted by the target 20 are multiple: faraday cage formed by the walls of the cavity 80, the presence of getter 92 in the cavity 80 and the presence of a magnet or electromagnet 94 for deflect parasitic ions. These means can be implemented independently or in addition to the screening function against parasitic X radiation and the wall function of the vacuum chamber 12.

The anode 76 is advantageously produced in the form of a one-piece mechanical part of revolution about the axis 19. The cavity 80 forms a central tubular part of the anode 76. The magnet or electromagnet 94 is arranged around the cavity 80 in an annular space 95 advantageously located outside the vacuum chamber 12. So that the magnetic flux of the magnet or electromagnet 94 affects the electron beam 18 as well as the ions degassed by the target 20 inside the enclosure 12, the walls of the cavity 80 are made of non-magnetic material. More generally, the entire anode 76 is made from the same material, for example by machining.

The getter 92 is located in the cavity 80 and the magnet or electromagnet 94 is located outside the cavity. Advantageously, a mechanical support 97 of the getter 92 maintains the getter 92 and is made of magnetic material. The support 97 is arranged in the cavity so as to guide the magnetic flux issuing from the magnet or from the electromagnet 94. In the case of an electromagnet 94, it can be formed around a magnetic circuit 99. The support 97 is advantageously arranged in the extension of the magnetic circuit 99. The fact of using the mechanical support 97 to fulfill two functions: the maintenance of the getter 92 and the guiding of a magnetic flux makes it possible to further reduce the dimensions of the anode 76 and therefore from source 75.

At the periphery of the annular space 95, the anode comprises a bearing zone 96 on the mechanical part 28. The bearing zone 96 has for example the form of a flat washer extending perpendicularly to the axis 19. .

On the figure 4a , we define an orthogonal frame of reference X, Y, Z. Z is a direction carried by the axis 19. The field Bz, carried by the axis Z makes it possible to focus the electron beam 18 on the target 20. The size of the electronic spot 18a on target 20 is shown near target 20 in the XY plane. The electronic spot 18a is circular. The size of the X-ray spot 22a emitted by target 20 is also shown near target 20 in the XY plane. The target 20 being perpendicular to the axis 19, the X-ray spot 22a is also circular.

The figure 4b shows a variant of the anode 76 in which a target 21 is inclined with respect to the XY plane perpendicular to the axis 19. This inclination makes it possible to enlarge the surface of the target 20 bombarded by the electron beam 18. By enlarging this surface, the increase of temperature of the target 20 due to the interaction with the electrons is better distributed. When the source 75 is used for imaging, it is useful to keep an X-ray spot 22a as punctual as possible or at least circular as in the variant of the figure 4a . To keep this spot 22a, with an inclined target 21, it is useful to modify the shape of the electronic spot in the XY plane. For the variant of the figure 4b , the electronic spot bears the reference 18b and is represented near the target 21 in its reference XY. The spot is advantageously elliptical in shape. Such a spot shape can be obtained from emitting zones of the cathode distributed in the plane of the cathode in a shape similar to the shape desired for the spot 18b. Alternatively or in addition, it is possible to modify the shape of the section of the electron beam 18 by means of a magnetic field By oriented along the Y axis and for example generated by a quadrupole having windings 98 also located in the annular space 95. The quadrupole forms an active magnetic system generating a magnetic field transverse to the axis 19 making it possible to obtain the expected shape for the electronic spot 18b. For example, for a target inclined with respect to the X direction, the electron beam 18 is spread in the X direction and is concentrated in the Y direction in order to maintain a circular X-ray spot 22a. The active magnetic system can also be driven so as to obtain other forms of electronic spot and possibly other forms of X-ray spot. The active magnetic system is of particular interest when the target 21 is tilted. The active magnetic system can also be used with a target 20 perpendicular to axis 19.

The anodes 16 and 76 in all their variants, can be implemented independently of the realization of the electrode 24 in the form of a conductive surface arranged on the concave face 26 of the dielectric material and independently of the implementation of the plug 32 .

In the variants offered using the figures 1 to 4 , all the components can be assembled by translation of each one along the same axis, in this case the axis 19. This makes it possible to simplify the production of a source according to the invention by automating its manufacture.

More precisely, the mechanical part 28 made of dielectric material and on which various metallizations have been produced, in particular the metallization forming the electrode 24, forms a monolithic support. It is possible to assemble on one side of this support, the cathode 14 and the plug 32. On the other side of this support, it is possible to assemble the anode 16 or 76. The fixing of the anode 16 or 76 and the plug 32 on the mechanical part 28 can be produced by ultra-vacuum brazing. The target 20 or 21 can also be assembled by translation along the axis 19 on the anode 76.

The figure 5 shows two identical sources 75 mounted in the same support 100. This mounting example can be used for mounting more than two sources. This example also applies to sources 10. Sources 10 as represented on the figures 1 and 2 can also be mounted in the support 100. The description of the support 100 and additional parts can apply regardless of the number of sources. The mechanical part 28 advantageously has an outer surface to the vacuum chamber 12 having two frustoconical shapes 102 and 104 extending around the axis 19. The shape 102 is an outer truncated cone widening towards the anode 16. The form 104 is an inner truncated cone widening out from the cathode 14 and more precisely from the outer face 43 of the stopper 32. The two truncated cones 102 and 104 meet on a crown 106 also centered on the. 'axis 19. The ring 106 forms the smallest diameter of the truncated cone 102 and the largest diameter of the truncated cone 104. The ring 106 has for example the shape of a torus portion allowing a connection without sharp angle of the two. truncated cones 102 and 104. The shape of the outer surface of the mechanical part 28 facilitates the positioning of the source 75 in the support 100 which has a complementary surface also having two frustoconical shapes 108 and 110. The truncated cone 108 of support 100 is complementary to the truncated cone 102 of the mechanical part 28. Likewise, the truncated cone 110 of the support 100 is complementary to the truncated cone 104 of the mechanical part 28. The support 100 has a crown 112 complementary to the crown 106 of the mechanical part 28.

In order to avoid any air gap at the high voltage interface between the support 100 and the mechanical part 28, a flexible seal 114, for example based on silicone, is placed between the support 100 and the mechanical part 28 and more. precisely between the trunks of cones and complementary crowns. Advantageously, the truncated cone 108 of the support 100 has an angle at the apex that is more open than that of the truncated cone 102 of the mechanical part 28. Likewise, the truncated cone 110 of the support 100 has an apex angle that is more open than that of the mechanical part 28. truncated cone 104 of the mechanical part 28. The difference in apex angle value between the truncated cones may be less than 1 degree, for example of the order of 0.5 degrees. Thus when mounting the source 75 in its support 100, and more precisely when the seal 114 is crushed between the support 100 and the mechanical part 28, the air can escape from the interface between the rings 106 and 112 on the one hand towards the most flared part of the two truncated cones 102 and 108 in the direction of the anode 16 and on the other hand towards the narrower part of the two truncated cones 104 and 110 in the direction of the cathode 14 and more precisely in the direction of the stopper 32. The air located between the two truncated cones 102 and 108 escapes to the ambient air and the air located between two truncated cones 104 and 110 escapes towards the stopper 32. In order to prevent the trapped air from being subjected to a strong electric field, the source 75 and its support 100 are configured so that the air located between two truncated cones 104 and 110 escapes inside. of the coaxial link formed by the two contacts 70 and 71 and supplying the cathode 14. To do this, the external contact 7 1 supplying the electrode 24 comes into contact with the metallized zone 43b by means of a spring 116 allowing functional play between the contact 71 and the stopper 32. In addition, the stopper 32 can include an annular groove 118 separating the two metallized zones 43a and 43b. Thus the air escaping between the truncated cones 104 and 110 crosses the functional clearance between the contact 71 and the plug 32 to reach a cavity 120 located between the contacts 70 and 71. This cavity 120 is protected from the strong electric field because being located inside the coaxial contact 71. In other words, the cavity 120 is screened from the main electric field of the source 10, electric field due to the potential difference between the anode 16 and the cathode electrode 24.

After mounting the mechanical part 28 equipped with its cathode 14 and its anode 76, a closure plate 130 can ensure the maintenance of the mechanical part 28, equipped with its cathode 14 and its anode 76, in the support 100. The plate 130 can be made of a conductive material or include a metallized face to ensure the electrical connection of the anode 76. The plate 130 can allow the cooling of the anode 76. The cooling can be provided for conduction by means of a contact. between the anode 76 and for example the cylindrical part 88 of the cavity 80 of the anode 76. To reinforce this cooling, it is possible to provide a channel 132 disposed in the plate 130 and surrounding the cylindrical part 88. A heat transfer fluid circulates in channel 132 to cool the anode 76.

On the figure 5 , the sources 75 all have separate mechanical parts 28. The figure 6a shows a variant of a multi-source assembly 150 in which a mechanical part 152 common to several sources 75, four in the example shown, fulfills all the functions of the mechanical part 28. The vacuum chamber 153 is common to the different sources. 75. The support 152 is advantageously formed from a dielectric material in which, for each of the sources 75, a concave face 26 is produced. For each of the sources, an electrode 24 (not shown) is placed on the corresponding concave face 26. In order not to overload the figure, the cathodes 14 of the different sources 75 are not shown.

In the variant of the figure 6a , the anodes of all the sources 75 are advantageously common and together bear the reference 154. To facilitate their production, the anodes comprise a plate 156 in contact with the mechanical part 152 and pierced with 4 holes 158 each allowing the passage of a beam. of electrons 18 coming from each of the cathodes of the sources 75. The plate 156 fulfills, for each of the sources 75, the function of the part 90 described above. Above each orifice 158, there are disposed a cavity 80 limited by its wall 88 and a target 20. Alternatively, it is possible to keep separate anodes which makes it possible to dissociate their electrical connection.

The figure 6b shows another variant of a multi-source assembly 160 in which a mechanical part 162 is also common to several sources, the respective cathodes 14 of which are aligned on an axis 164 passing through each of the cathodes 14. The axis 164 is perpendicular to the axis 19 of each of the sources. An electrode 166 making it possible to focus the electron beams emitted by the various cathodes 14 is common to all the cathodes 14. The variant of the figure 6b makes it possible to further reduce the distance separating two neighboring sources.

In the example shown, the mechanical part 162 is made of dielectric material and comprises a concave face 168 disposed in the vicinity of the various cathodes 14. The electrode 166 is formed of a conductive surface disposed on the concave face 168. The electrode 166 fulfills all the functions of the electrode 24 described above.

Alternatively, it is possible to implement an electrode common to several sources in the form of a metal electrode without the presence of dielectric material, ie having a metal / vacuum interface. Likewise, the cathodes can be thermionic. In this embodiment, the common metal electrode forms the support for the different cathodes of the different sources. Since this electrode is of large size, it is advantageous to connect it to the mass of the generator of the multi-source assembly. The anode or anodes are then connected to one or more positive potentials of the generator.

The multi-source assembly 160 can include a plug 170 common to all the sources. The stopper 170 can fulfill all the functions of the stopper 32 described above. The plug 170 can in particular be fixed to the mechanical part 162 by means of a conductive solder film 172 used to electrically connect the electrode 166.

As in the variant of the figure 6a , the multi-source assembly 160 can include an anode 174 common to the different sources. The anode 174 is similar to the anode 154 of the variant of the figure 6a . The anode 174 comprises a plate 176 fulfilling all the functions of the plate 156 described using the figure 6a . To avoid overloading the figure 6b , for the anode 174, only the plate 176 is shown.

On the figure 6b , the axis 164 is rectilinear. It is also possible to arrange the cathodes on a curved axis, such as for example an arc of circle as shown on the figure 6c making it possible to focus the X-rays 22 from all the sources at a point situated at the center of the arc of a circle. Other forms of curved axis, in particular a parabolic curve, also allow the focusing of the X-rays at a point. The curved axis remains locally perpendicular to each of the axes 19 around which the electron beam from each source develops.

The arrangement of the cathodes 14 on an axis makes it possible to obtain sources distributed in one direction. It is also possible to produce a multi-source assembly in which the cathodes are distributed along several concurrent axes. It is for example possible to arrange the sources along several curved axes, each made in a plane and the planes being intersecting. As an example, as shown in the figure 6d , it is for example possible to have several axes 180 and 182 distributed over a parabolic surface of revolution 184. This makes it possible to focus the X-rays 22 from all the sources at the focus of the parabolic surface. On the figure 6e , the various axes 190, 192 and 194 on which the various cathodes 14 of the multi-source assembly are distributed are mutually parallel.

The figures 7a and 7b represent two embodiments of the power supply of the assembly shown in the figure 6a . The figures 7a and 7b are shown in section in a plane passing through several axes 19 of different sources 75. Two sources appear on the diagram. figure 7a , and three sources on the figure 7b . It is understood that the description of the multi-source assembly 150 can be implemented whatever the number of sources 75 or possibly 10.

In these two embodiments, the anodes 114 are common to all the sources 75 of the assembly 150 and their potential is the same, for example that of the earth 52. The control of each of the sources 10 can be distinct in the two. embodiments. On the figure7a , two high voltage sources V1 and V2 separately supply the electrodes 24 of each of the sources 10. The insulating nature of the mechanical part 152 makes it possible to separate the two high voltage sources V1 and V2 which can for example be pulsed at two different energies. Likewise separate current sources I1 and I2 each control the different cathodes 14.

In the embodiment of the figure 7b , the electrodes 24 of all the sources 75 are interconnected for example by means of a metallization produced on the mechanical part 152. A high voltage source V _Common supplies all the electrodes 24. The various cathodes 14 are controlled by sources. current I1 and I2 separated. The power supply of the multi-source assembly described using the figure 7b is well suited to the variant described using the figures 6b , 6d and 6th .

The figures 8a, 8b and 8c represent several examples of sets for generating ionizing rays each comprising several sources 10 or 75. In these various examples, the support, as described with the aid of the figure 5 is common to all the sources 10. A high voltage connector 140 provides power to the various sources 10. A control connector 142 makes it possible to connect each of the assemblies to a control module, not shown and configured to switch each of the sources 10 according to a predetermined sequence.

On the figure 8a the support 144 has an arcuate shape and the different sources 10 are aligned on the arcuate shape. This type of arrangement is for example useful in a medical scanner in order to avoid moving the X-ray source around the patient. The different sources 10 each in turn emit X-rays. The scanner also comprises a radiation detector and a module making it possible to reconstitute a 3-dimensional image from the information picked up by the detector. In order not to overload the figure, the detector and the reconstitution module are not shown. On the figure 8b , the support 146 and the sources 10 follow a straight line segment. On the figure 8c , the support 148 has the shape of a plate and the sources are distributed in two directions on the support 148. For the sets for generating ionizing rays shown in the figures. figures 8a and 8b , the variant of figure 6b is particularly interesting. This variant makes it possible to reduce the pitch between the different sources.

Claims (10)

1. A source (10) for generating ionizing radiation, comprising:

   • a vacuum chamber (12);

   • a cathode (14) that is able to transmit an electron beam (18) into the chamber (12);

   • an anode (16) that receives the electron beam and that comprises a target (20) that is able to generate ionizing radiation (22) from the energy received from the electron beam (18);

   • an electrode (24) that is placed in the vicinity of the cathode (14) and that allows the electron beam (18) to be focused;

   • a stopper (32; 170) ensuring the seal tightness of the vacuum chamber (12),

   characterized in that the source (10) further comprises a mechanical part (28) that is made of dielectric material and that forms a portion of the vacuum chamber, and
   in that the stopper (32; 170) is fastened to the mechanical part (28) by means of a conductive brazing film (42) that is used to electrically connect the electrode (24).
2. The source according to claim 1, characterized in that the stopper (32; 170) is made from the same dielectric material as the mechanical part (28).
3. The source according to one of the preceding claims, characterized in that the brazing film (42) is rotatable about an axis (19) of the electron beam (18) and in that the brazing film (42) forms an equipotential assembly together with the electrode (24).
4. The source according to one of the preceding claims, characterized in that the stopper (32; 170) comprises at least one electrical connection (68) passing therethrough, allowing a means for controlling the cathode (14) to be electrically connected, and brought to a potential different from the brazing film (42).
5. The source according to claim 4, characterized in that the stopper (32; 170) forms a coaxial-type transmission line, the electrical connection (68) of the stopper through which it passes, forms a central conductor of the coaxial line and the brazing film (42) of the stopper forms a shield of the coaxial line.
6. The source according to one of claims 4 or 5, characterized in that the stopper (32; 170) comprises a surface (43) exterior to the vacuum chamber (12), in that the exterior surface (43) comprises multiple separate zones (43a, 43b) that are metallized separately, in that at least one of these zones (43a) are in electrical contact with the at least one electrical connection (68) and in that another of these zones (43b) is in electrical contact with the brazing film (42), in order to ensure the electrical connection of the cathode (14) and of the electrode (24) by way of the at least one electrical connection (68) and of the brazing film (42).
7. The source according to one of claims 4 to 6, characterized in that it comprises a coaxial connector (70, 71) connected to the brazing film (42) and to the at least one electrical connection (68), and a cavity (118, 120) located between the coaxial connector (70, 71) and the stopper (32; 170), the cavity (118, 120) being shielded from a main electric field of the source (10).
8. The source according to claims 6 and 7, characterized in that the mechanical part (28) comprises a surface exterior to the vacuum chamber (12) having an interior frustoconical shape (104) that flares from the external surface (43) of the stopper (32; 170), in that the source (10) further comprises a holder (100) having a surface (110) that is complementary to the interior frustoconical shape (104) of the mechanical part (28) and in that the complementary surface (110) and the interior frustoconical shape (104) are configured to convey air trapped between the complementary surface (110) and the interior frustoconical shape (104) when the mechanical part (28) is mounted in the holder (100) toward the cavity (118, 120).
9. The source according to one of claims 4 to 8, characterized in that the cathode (14) transmits the electron beam (18) via field effect and in that the means for controlling the cathode (14) comprises an optoelectronic component that is electrically connected via the electrical connection (68) passing through the stopper (32; 170).
10. The source according to one of the preceding claims, characterized in that the mechanical part (28) comprises a cavity (34) in which the cathode (14) is placed and in that a getter (35) is placed in the cavity (34), between the cathode (14) and the stopper (32; 170).

Images