EP3652774A1

EP3652774A1 - Compact, ionising ray-generating source, assembly comprising a plurality of sources and method for producing the source

Info

Publication number: EP3652774A1
Application number: EP18736949.1A
Authority: EP
Inventors: Pascal Ponard
Original assignee: Thales SA
Current assignee: Thales SA
Priority date: 2017-07-11
Filing date: 2018-07-11
Publication date: 2020-05-20
Also published as: AU2018298826A1; JP2020526866A; IL271798A; WO2019011997A1; KR20200024213A; TW201909228A; SG11201912210RA; US20200194213A1; FR3069099B1; FR3069099A1; CN110870037A

Abstract

The invention relates to a source generating ionising rays, and in particular X-rays, an assembly comprising a plurality of sources and a method for producing the source. The source comprises: · a vacuum chamber (12), · a cathode capable of emitting an electron beam (18) in the vacuum chamber (12), the electron beam (18) developing around an axis (19), and · an anode (76) receiving the electron beam (18) and comprising a target (20) capable of generating ionising radiation (22) from the energy received from the electron beam (18), the ionising radiation (22) being generated towards the outside of the vacuum chamber (12); wherein the anode (76) comprises a cavity (80) in which the electron beam (18) is intended to penetrate to reach the target (20), and the walls (88, 90) of the cavity (80) form a Faraday cage surrounding parasitic ions (91) that can be emitted by the target (20) inside the vacuum chamber (12); at least one getter (92), separate from the walls (88, 90) of the cavity (80) and intended to trap the parasitic ions (91), is arranged in the cavity (80).

Description

Source generating compact ionizing radiation, set comprising several sources and method of producing the source

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

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

The realization of images from X-rays has progressed a lot. Originally only photosensitive films were used. Since then, digital detectors have appeared. These detectors, associated with software, allow the rapid reconstruction of image in two dimensions or in three dimensions by means of scanners.

On the other hand, since the discovery of X-rays by Roentgen in 1895, X-ray generators have evolved very little. The synchrotrons that emerged after the Second World War generate intense and well-focused radiation. The 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 bombarding a target. The braking of the beam of the electric fields of the nuclei of the target can generate by braking X-radiation.

An X-ray tube generally consists of an envelope in which the vacuum is produced. The envelope is formed of a metal structure and an electrical insulator made of alumina or glass. In this envelope, two electrodes are arranged. 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 radiation by braking (bremsstralung) when hit the target. Metal electrodes are necessarily large and have large radii of curvature to minimize electric fields on their surface.

Depending on the power of X-ray tubes, these can be equipped with either a fixed anode or a rotating anode for spreading thermal power. Fixed anode tubes have a power of a few kilowatts and are particularly used in industrial, safety and medical applications of low power. Rotating anode tubes can exceed 100 kilowatts and are mainly used in the medical field for imaging requiring significant X-ray fluxes to improve contrast. For 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 1 60 kV. The indicated voltage corresponds to the potential difference applied between the two electrodes. For medical tubes with rotating anodes, 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 thus remain important, of the order of several hundred mm. Imaging systems have seen the appearance of digital detectors with 3D reconstruction software faster and more efficient while at the same time, the technologies of X-ray tubes have hardly evolved for a century and this is a major technological limitation of X-ray imaging systems.

Several factors hinder the miniaturization of the current X-ray tubes.

Electrical insulators must be of sufficient size to ensure good electrical isolation from high voltages of 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 the metal electrodes should not be too low to maintain a static electric field applied to the surface below an acceptable limit, typically 25 MV / m. Beyond parasitic electron emissions by tunnel effect become difficult to control resulting in wall heating, unwanted X-ray emissions and micro discharges. Because of this, for As seen 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 problems of expansion 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 difficult dielectric insulator.

Surface charge phenomena related 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 the proximity between the electron beam and the surface of the dielectric materials, 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 remoteness also tend to increase the dimensions of the X-ray tubes.

The anode forming the target must dissipate a significant thermal power. This dissipation can be achieved by circulating a coolant or by producing a large rotating anode. The need for this dissipation also requires increasing the dimensions of the X-ray tubes.

Among the emerging technological solutions, the literature describes the use of cold cathodes with carbon nanotubes in X-ray tube structures, but the solutions currently proposed remain based on conventional structures of X-ray tubes using a metallic wehnelt. surrounding the cold cathode. This wehnelt is an electrode carried at a high voltage and is always subject to significant dimensional constraints to limit the parasitic emission 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 as a diode or a high voltage triode whose dimensions are much smaller than those of conventional X-ray tubes. The principle of ionizing radiation generation remains similar to that implemented in the known tubes, namely an electron beam bombarding a target. The electron beam is accelerated between a cathode and an anode between which is applied a potential difference, 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 with respect to the known tubes.

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

More specifically, the subject of the invention is an ionizing radiation generating source comprising:

· A vacuum chamber,

A cathode capable of emitting an electron beam into the vacuum chamber, the electron beam developing around an axis,

An anode receiving the electron beam and comprising a target capable of generating ionizing radiation from the energy received from the electron beam, the ionizing radiation being generated towards the outside of the vacuum chamber;

the anode comprises a cavity in which the electron beam is intended to penetrate to reach the target and the walls of the cavity form a faraday cage surrounding parasitic ions that can be emitted by the target inside the enclosure empty; at least one separator separate from the walls of the cavity and intended to trap the parasitic ions is placed in the cavity.

The sorbeur is advantageously made of a material different from that of the cavity.

The source advantageously comprises at least one magnet or electromagnet surrounding the cavity. The walls of the cavity are then made of non-magnetic material.

The source advantageously comprises a mechanical support ensuring the maintenance of the sorbeur and made of magnetic material. The mechanical support is disposed in the cavity so as to guide the magnetic flux from the magnet or the electromagnet.

The at least one magnet or electromagnet is advantageously arranged to deflect parasitic ions to the at least one sorbeur.

At least one of the walls of the cavity advantageously forms a wall of the vacuum chamber.

The walls of the cavity are advantageously arranged coaxially with the axis.

The walls of the cavity advantageously comprise a cylindrical portion about the axis extending between the target and a washer-shaped portion having a hole and closing the cylindrical portion. The electron beam then enters the cavity through the hole of the part.

The source advantageously comprises a mechanical part made of dielectric material and forming a wall of the vacuum chamber. The anode is sealed to the mechanical part.

The target may be inclined relative to a plane perpendicular to the axis.

The source advantageously comprises an active magnetic system generating a magnetic field transverse to the axis in the cavity and configured to modify the shape of an electron spot formed by the electron beam on the target.

The walls of the cavity advantageously form a screen shield against parasitic ionizing radiation generated inside the vacuum chamber.

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

Figure 1 shows schematically the main elements of an X-ray generating source according to the invention;

FIG. 2 represents a variant of the source of FIG. 1 allowing other modes of electrical connection;

Figure 3 is a partial and enlarged view of the source of Figure 1 around its cathode; Figures 4a and 4b are partial and enlarged views of the source of Figure 1 around its anode according to two variants;

FIG. 5 represents in section an integration mode comprising several sources in accordance with the invention;

FIGS. 6a, 6b, 6c, 6d and 6e show variants of an assembly comprising several sources in the same vacuum enclosure;

Figures 7a and 7b show several electrical connection modes of a set comprising several sources.

FIGS. 8a, 8b and 8c show three examples of assemblies comprising several sources in accordance with the invention and which can be made according to the variants proposed in FIG. 5 or 6.

For the sake of clarity, the same elements will bear the same references in the different figures. FIG. 1 is a sectional view of an X-ray generating source 10. The source 10 comprises a vacuum chamber 12 in which a cathode 14 and anode 1 6 are disposed. The cathode 14 is intended to emit an electron beam 18 into the enclosure 12 towards the anode 16. The anode 16 comprises a target 20 bombarded by the beam 18 and emitting an X-radiation 22 depending on the energy of the electron beam 18. The beam 18 develops around an axis 19 passing through the cathode 14 and the anode 1 6.

X-ray generating tubes conventionally implement a thermionic cathode operating at high temperature, typically around 1000 ° C. This type of cathode is commonly called hot cathode. This type of cathode composed of a metal matrix or metal oxides emits a flow of electrons caused by the vibrations of atoms due to thermal energy. However, the hot cathodes suffer from several disadvantages, such as a weak temporal dynamics of the current control related to the time constants of the thermal processes, the need to use grids located between the cathode and the anode and polarized at high voltages in order to to be able to control the current. The grids are therefore located in an area of very high electric fields, they are subjected to high operating temperatures around 1000 ° C. All these constraints greatly limit the possibilities of integration and lead to significant 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. Most of them consist of a conductive flat surface with relief structures, on which an electric field is concentrated. These relief structures are emitters of electrons when the field at the top is sufficiently high. The emitters in relief can be formed of carbon nanotubes. Such embodiments are for example described in the patent application published under No. WO 2006/063982 A1 and filed in the name of the applicant. Cold cathodes do not have the disadvantages of hot cathodes and are especially much more compact. In the example shown, the cathode 14 is a cold cathode and thus emits the electron beam 18 by field effect. The control of the cathode 14 is not shown in FIG. This control can be performed electrically or optically as also described in WO 2006/063982 A1.

Under the effect of a potential difference between the cathode 14 and the anode 1 6, 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 high atomic number material such as in particular tungsten or molybdenum. The layer 20b may have a variable thickness, for example between 1 and 12 μηι 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-rays 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 enclosure 12. This provision is particularly implemented for a target operating in transmission. For this arrangement, the membrane 20a is formed of a low atomic number material, such as diamond or beryllium for its X-ray transparency 22. The membrane 20a is configured to ensure with the anode 1 6, the seal at empty of the enclosure 12. Alternatively, the target 20, or at least the layer made of an alloy with a high atomic number, can be placed completely inside the vacuum chamber 12 and the X radiation emerges from the chamber 12 while passing through a window forming part of the wall of the vacuum chamber 12. This provision is particularly implemented for a target operating in reflection. The target is then distinct from the window. The layer in which X-radiation is produced can be thick. The target may be fixed or rotating allowing a spread of the thermal power generated during the interaction with the electrons of the beam 18.

Advantageously, it is possible to exceed a significant stress level electric field on the surface of the cathode electrode or wehnelt. This constraint is related to the metallic nature of the interface between the electrode and the vacuum present in the chamber in which the electron beam propagates. For this purpose, at the electrode, the metal / vacuum interface is replaced by a dielectric / vacuum material interface which does not allow parasitic emission of electrons by tunnel effect. It is then possible to accept electric fields well above those acceptable with a metal / vacuum interface. Initial internal tests have shown that it is possible to reach 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 whose external surface is subjected to the electric field by an electrode made of a dielectric material whose external surface is subjected to the electric field and whose internal surface 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 by a dielectric material to replace the metal / vacuum interface of the known electrodes by a dielectric / vacuum interface where the electric field is important. This arrangement makes it possible to significantly increase the maximum electric field below which the parasitic emission of electrons does not occur.

The increase of the admissible electric fields allows a miniaturization of the X-ray sources and more generally the sources of ionizing radiations. For this, the source 10 comprises an electrode 24 disposed in the vicinity of the cathode 14 and for focusing the electron beam 18. The electrode 24 forms a wehnelt. In the case of a so-called cold cathode, the electrode 24 is disposed in contact with the cathode. A cold cathode emits an electron beam by field effect. This type of cathode is for example described in WO 2006/063982 A1 filed in the name of the applicant. In the case of a cold cathode, the electrode 24 is disposed in contact with the cathode 14. The mechanical part 28 advantageously forms a support for the cathode 14. 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 1 6. To ensure the wehnelt function, the electrode 24 has a substantially convex shape. The outside of the concavity of the face 26 is oriented towards the anode 1 6. Locally at the contact between the cathode 14 and the electrode, the convexity of the electrode 24 may be zero or slightly reversed.

It is from this convex face of the electrode 24 that significant electric fields develop. In the prior art, a metal / vacuum interface exists on this convex face of the electrode. As a result, this interface can be the seat of electron emission under the effect of the electric field inside the vacuum chamber. This interface of the electrode with the vacuum of the enclosure is removed by replacing it with a dielectric / vacuum interface. A dielectric material, having no free charge, can therefore be seat of a sustained electron emission.

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 case 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 electron emissions 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. Various techniques can be implemented to achieve this deposit, such as in particular the physical vapor deposition (known in the literature Anglo-Saxon under the acronym PVD for Physical Vapor Deposition) or in chemical phase (CVD) possibly assisted by plasma (PECVD).

Alternatively, it is possible to perform a deposition of dielectric material on the surface of a massive metal electrode. The deposition of dielectric material, adherent to the solid metal electrode, always avoids any air or vacuum gap at the electrode / dielectric material interface. This deposition of dielectric material is chosen to withstand high electric fields, typically greater than 30 MV / m, and to have a sufficient flexibility compatible with any thermal expansion of the massive metal electrode. However, the opposite arrangement implementing the deposition of a conductive material on the internal face of a solid piece 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 can form a part of the vacuum chamber 12. This part of the vacuum chamber can even be a preponderant 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 the electrical isolation between the anode 1 6 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, one chooses a dielectric material having a dielectric strength greater than 20MV / 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. In-house tests have shown that a ceramic of this nature even allowed to 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 inner face 30 has a surface resistivity measured at room temperature of between 1 .10 ⁹ Ω. square and 1 .10 ¹³ Ω. square and typically close to 1 .10 ¹¹ Ω. square. Such a resistivity can be obtained by the addition on the surface of a conductive or semiconductive material compatible with the dielectric material. As a semiconductor material, it is for example possible to deposit silicon on the inner face 30. In order to obtain the right resistivity range, for example for a nitride-based ceramic, it is possible to modify its intrinsic properties by adding 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 Qm or semiconductor materials such as silicon carbide SiC .

It is possible to disperse the titanium nitride in the volume of the dielectric material 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 inner face 30 by a high temperature heat treatment greater than 1500 ° C.

The source 10 comprises a plug 32 sealing the vacuum chamber 12. The mechanical part 28 comprises a cavity 34 in which is disposed the cathode 14. The cavity 34 is delimited by the concave face 26. The cap 32 closes the cavity 34. The electrode 24 comprises two ends 36 and 38 spaced 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 to the first. The mechanical part 28 comprises an inner cone trunk 40 with circular section disposed around the axis 19 of the beam 18. The truncated cone 40 is located at the second end 38 of the electrode 24. The truncated cone 40 s opens up away from the cathode 14. The plug 32 comprises a shape complementary to the truncated cone 40 to be disposed therein. 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 embodiment of the electrode 24 in the form of a conductive surface disposed 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 any differential thermal expansion phenomena between the mechanical part 28 and the plug 32 when using the source 10.

The plug 32 is for example fixed to the mechanical part 28 by means of a solder film 42 made in the truncated cone 40 and more generally in an interface zone between the plug 32 and the mechanical part 28. It is possible to metallizing the surfaces to be brazed of the plug 32 and the mechanical part 28 and then to achieve the solder by means of a metal alloy whose melting point is greater than the maximum temperature of use of the source 10. The metallization and the solder film 42 are electrically continuous 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 too pronounced for the electrode 24 and for the conductive areas extending the electrode 24 to limit possible peak effects of the electric field.

Alternatively, it is possible to avoid the metallization of the surfaces by integrating the solder alloy an active element that reacts with the material of the plug 32 and that of the mechanical part 28. For nitride-based ceramics, the titanium is embedded in the solder alloy. Titanium is a metal reactive with nitrogen and can create a strong chemical bond with the ceramic. Other reactive metals may 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 supply of the source 10. The electrical connection of the electrode 24 by means of the solder film 42 may be used for other types of electrodes, in particular metal electrodes covered with a deposit of dielectric material. To strengthen the connection with the electrode 24, it is possible to embed in the solder film 42 a metal contact. 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 cap 32. The surface 43 is located outside the vacuum chamber 12. The metallization of the surface 43 is in electrical contact with the braze film 42. It is possible to braze on the metallization of the surface 43 a contact that can be connected electrically to a supply of the source 10.

The solder film 42 extends the revolution form of the electrode 24 and contributes to the main function of the electrode 24. This is particularly advantageous 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 away from the axis 19. The electrode 24, associated with the solder film 42 when it is conductive, form an equipotential surface that contributes to the focusing of the electron beam 18 and to the potential of the cathode 14. This minimizes the local electric fields to reduce the compactness of the source 10.

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 partially concave. The face 26 is generally concave.

In FIG. 1, the source 10 is biased 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. at the anode 1 6. This type of connection is characteristic of a monopolar operation of the source 10 in which the potential of the anode 1 6 is grounded 52. It is also possible to replace the high voltage source 50 by two high voltage sources 56 and 58 in series to operate the source 10 bipolar manner as shown in Figure 2. This type of operation is useful for simplifying the realization of the associated high voltage generator. For example, in the case of a mode of operation in high voltage pulsed at high frequency, it may be advantageous to lower the absolute voltage by summing the two half positive and negative voltages at the source 10. For this purpose, the High voltage source may include an output transformer driven in half bridge H.

With a source 10 as shown in FIG. 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 in FIG.

Bipolar operation of a source as described in Figure 1 is made by keeping floating the common point of two high voltage sources connected in series. Alternatively, this common point can be used to bias another electrode of the source 10 as shown in Figure 2. In this variant, the source 10 comprises an intermediate electrode 54 splitting into two parts 28a and 28b the mechanical part 28. The intermediate electrode 54 extends perpendicular to the axis 19 of the beam 18 and is traversed 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. In FIG. 2, the assembly formed by the two high-voltage sources 56 and 58 is floating relative to earth 52. As shown in FIG. 1, it is also possible to connect one of the electrodes to earth 52. of the source 10, for example the intermediate electrode 54.

FIG. 3 is a partial and enlarged view of the source 10 around the cathode 14. The cathode 14 is disposed in the cavity 34 bearing against the end 36 of the electrode 24. A support 60 makes it possible to center the cathode 14 relative to the electrode 24. The electrode 24 being of revolution about the axis 19, the cathode 14 is centered on the axis 19 allowing it to emit the electron beam 18 along the axis 19. The support 60 comprises a countersink 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 of to hold the cathode 14 against the electrode 24. The support 60 is made of insulating material. The spring 64 may have an electrical function for conveying a control signal to the cathode 14. More specifically, the cathode 14 emits the electron beam 18 by a face 65, said front face and oriented toward the anode 1 6. The electrical control of the cathode 14 is by its rear face 66 opposite to the front face 65. The support 60 may comprise an opening 67 with a circular section centered on the axis 19. The opening 67 may be metallized with way to electrically connect the spring 64 and the rear face 66 of the cathode 14. The plug 32 can provide the electrical connection of the control of the cathode 14 by means of a metallized via 68 therethrough and a contact 69 secured to the plug 32 The contact 69 presses the spring 64 along the axis 19 to hold the cathode 14 against the electrode 24. The contact 69 ensures electrical continuity between the via 68 and the spring 64.

The surface 43 of the plug 32, situated outside the vacuum chamber 12, can be metallized into 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 metallic 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 area 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 may comprise several separately addressable transmitting zones. 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 the contact 69 and several metallic 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 and the spring 64 are sectored accordingly to provide for several zones. similar to the zone 43a and electrical continuity with each metallized vias.

At least one sorbiner 35 (known in the English literature as the "getter") may be placed in the cavity 34, between the cathode 14 and the stopper 32, in order to trap any particle that may alter the quality of the vacuum of the chamber 12. The sorbeur 35 is generally chemisorption. Alloys based on zirconium or titanium can be used to trap any particles emitted by the various components of the source 10 surrounding the cavity 34. The sorber 35 is, in the example shown, fixed to the plug 32. The sorber 35 is made from stacked annular discs surrounding the contact 69. FIG. 4a represents a variant of ionizing radiation source 75 in which an anode 76 replaces the anode 16 described above. FIG. 4a is a partial and enlarged view of the source 75 around the anode 76. As for the anode 1 6, the anode 76 comprises a target 20 bombarded by the beam 18 and emitting X-ray radiation 22. At the Unlike the anode 16, the anode 76 comprises a cavity 80 in 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-radiation 22 by its outer face 86. In the example shown, the walls of the cavity 80 have a cylindrical portion 88 about the axis 19 extending between two ends 88a and 88b. The end 88a is in contact with the target 20 and the end 88b is close to the cathode 14. The walls of the cavity 80 also have a portion 90 in the form of a washer having a hole 89 and closing the cylindrical portion at the level of the end 88b. The electron beam 18 enters the cavity 80 through the hole 89 of the portion 90.

Upon bombardment of the target 20 with the electron beam 18, the temperature rise of the target 20 may result in molecular outgassing of the target 20 which, under the effect of X-radiation 22, is ionized. Ions 91 appearing on the inner face 84 of the 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. For this purpose, the walls 88 and 90 of the cavity 80 are electrical conductors and form a faraday cage vis-à-vis parasitic ions that 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 leave the cavity 80 and could be accelerated to the cathode 14. To better trap the ions in the cavity 80, at least one sorbeur 92 is disposed in the cavity 80. The sorbeur 92 is distinct from the walls 88 and 90 of the cavity 80. The sorbeur 92 is a component Specifically disposed in the cavity 80. Like the sorbeur 35, the sorbeur 92 generally acts by chemisorption. Alloys based on zirconium or titanium can be used to trap the emitted 91 ions. In addition to ion trapping, the walls of the cavity 80 may form a shielding shield with respect to parasitic ionizing radiation 82 generated inside the vacuum enclosure 12 and possibly electrostatic shielding 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, a parasitic X-ray can emerge from the target 20 via the internal face 84. This parasitic radiation is unnecessary and undesirable. . Usually, shielding screens opposing this type of unwanted radiation are arranged around the X-ray generators. This type of embodiment, however, has a disadvantage. Indeed, the more shielding screens are disposed 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 closer to the parasitic source, which allows them to be miniaturized.

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, in a tungsten or molybdenum-based alloy in order to stop the parasitic radiation 82. Tungsten or molybdenum have virtually no effect of trapping parasitic ions. By making the separator 92 distinctly from the walls of the cavity 80, this makes it possible to free the choice of materials in order to best ensure the parasitic ion trapping functions for the sorbeur 92 and the vis-à-vis screen parasitic radiation 92 for the walls of the cavity 80 without compromise between the two functions. For this purpose, the sorbeur 92 and the walls of the cavity 80 are made of different materials each adapted to the function assigned to it. It is the same for the sorbeur 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 enclosure 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 closer to the parasitic radiation. At the level from the end 88a, the cylindrical portion 88 may partially or completely 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 performs several functions, its electrical function of course, in addition, a faraday cage function surrounding parasitic ions that can be emitted by the target 20 inside the vacuum chamber 12, a function of shielding against the parasitic X-radiation and, moreover, a wall of the vacuum chamber 12. By filling 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 disposition of the magnet or electromagnet 94 may be also defined so as to deflect the parasitic ions 91 to the or the sorbeurs 92 in order to prevent the parasitic ions can not leave the cavity through the hole 89 of the part 90 or at least be 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. In FIG. 4a, the ions 91 deflected towards the sorber 92 follow a trajectory 91 a and the ions leaving the cavity 80 follow a trajectory 91 b.

The means for trapping the parasitic ions 91 that can be emitted by the target 20, are multiple: faraday cage formed by the walls of the cavity 80, presence of sorbers 92 in the cavity 80 and presence of a magnet or electromagnet 94 for divert the parasitic ions. These means can be implemented independently or in addition to the parasitic X-ray shielding function and the wall function of the vacuum enclosure 12.

The anode 76 is advantageously made 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. For the magnetic flux of the magnet or electromagnet 94 affects the electron beam 18 and the ions degassed by the target 20 to the inside the enclosure 12, the walls of the cavity 80 are made of non-magnetic material. More generally, the entire anode 76 is made of the same material, for example by machining.

The sorbeur 92 is located in the cavity 80 and the magnet or the electromagnet 94 is located outside the cavity. Advantageously a mechanical support 97 of the sorbeur 92 ensures the maintenance of the sorbeur 92 and is made of magnetic material. The support 97 is disposed in the cavity so as to guide the magnetic flux coming from the magnet or the electromagnet 94. In the case of an electromagnet 94, it can be formed around a magnetic circuit 99. 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 sorbeur 92 and the guidance 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 shape of a flat washer extending perpendicularly to the axis 19 .

In FIG. 4a, an orthogonal coordinate system X, Y, Z is defined. Z is a direction carried by the axis 19. The field Bz carried by the Z axis makes it possible to focus the electron beam 18 on the target 20 The size of the electronic spot 18a on the target 20 is shown near the target 20 in the XY plane. The electronic spot 18a is circular. The size of the X-ray spot 22a emitted by the target 20 is also shown near the target 20 in the XY plane. Since the target 20 is perpendicular to the axis 19, the X-ray spot 22a is also circular.

FIG. 4b represents 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 beam of Electrons 18. By enlarging this surface, the temperature increase of the target 20 due to the interaction with the electrons is better distributed. When the source 75 is implemented 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 Figure 4a. To preserve 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 Figure 4b, the electronic spot is marked 18b and is shown near the target 21 in its XY mark. The spot is advantageously of elliptical shape. Such a spot shape can be obtained from cathode emitting zones distributed in the cathode plane in a shape similar to the desired shape for 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 to obtain the expected shape for the electronic spot 18b. For example, for a target inclined to the X direction, the electron beam 18 is spread in the X direction and is concentrated in the Y direction to maintain a circular X-ray spot 22a. The active magnetic system can also be controlled 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 inclined. The active magnetic system may also be employed with a target perpendicular to the axis 19.

The anodes 1 6 and 76 in all their variants can be implemented independently of the embodiment of the electrode 24 in the form of a conductive surface disposed on the concave face 26 of the dielectric material and independently of the implementation of the plug 32.

In the variants proposed with reference to FIGS. 1 to 4, all the components can be assembled by translating each along a same axis, in this case the axis 19. This makes it possible to simplify the production of a source that conforms to FIGS. the invention by automating its manufacture.

More specifically, the mechanical part 28 made of dielectric material and on which different metallizations have been made, 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 1 6 or 76. The fixing of the anode 1 6 or 76 and the plug 32 on the mechanical part 28 can be performed by ultra-vacuum brazing. The target 20 or 21 can also be assembled by translation along the axis 19 on the anode 76.

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 the sources 10. The sources 10 as represented in FIGS. 1 and 2 can also be mounted in the support 100. The description of the support 100 and the complementary parts can be applied 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 form 102 is an outer truncated cone flaring towards the anode 1 6. The shape 104 is an inner truncated cone flaring from the cathode 14 and more precisely from the outer face 43 of the plug 32. The two truncated cones 102 and 104 meet on a ring 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 a shape of a torus portion allowing a connection without sharp angle of two truncated cones 102 and 104. The shape of the outer surface of the mechanical part 28 facilitates the establishment of the source 75 in the support 100 which has a complementary surface also having two frustoconical shapes 108 and 1 10. The tr onc cone 108 of the support 100 is complementary to the truncated cone 102 of the mechanical part 28. Similarly, the truncated cone 1 10 of the support 100 is complementary to the truncated cone 104 of the mechanical part 28. The support 100 has a crown 1 12 complementary to the ring 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 1 14, for example based on silicone, is arranged 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 a more open apex angle than that of the truncated cone 102 of the mechanical part 28. Similarly, the truncated cone 1 10 of the support 100 has a more open aperture angle than that of the truncated cone 104 of the mechanical part 28. The difference in the angle value at the apex between the truncated cones may be less than 1 degree, for example of the order of 0.5 degree. Thus during the mounting of the source 75 in its support 100, and more precisely when the seal January 14 is crushed between the support 100 and the mechanical part 28, the air can escape from the interface between the rings 106. and 1 12 on the one hand towards the widest part of the two truncated cones 102 and 108 in the direction of the anode 1 6 and on the other hand towards the tighter part of the two truncated cones 104 and 1 10 in direction of the cathode 14 and more precisely towards the plug 32. The air located between the two truncated cones 102 and 108 escapes to the ambient air and the air between two truncated cones 104 and 1 10 s escapes towards the plug 32. In order to prevent trapped air from being subjected to a large electric field, the source 75 and its support 100 are configured so that the air situated between two truncated cones 104 and 110 s escapes inside the coaxial connection formed by the two contacts 70 and 71 and feeding the cathode 14 To do this, the external contact 71 providing power to the electrode 24 comes into contact with the metallized zone 43b by means of a spring 1 1 6 allowing a functional clearance between the contact 71 and the plug 32. In addition the plug 32 may comprise an annular groove 1 18 separating the two metallized zones 43a and 43b. Thus the air escaping between the truncated cones 104 and 1 10 passes through 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 important electric field because it is located inside the coaxial contact 71. In other words, the cavity 120 is shielded from the main electric field of the source 10, the electric field of the potential difference between the anode 16 and the cathode electrode 24.

After assembly of 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. plate 130 can be made of conductive material or comprise 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 ensured for conduction by means of contact between the anode 76 and for example the cylindrical portion 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 portion 88 A coolant circulates in the channel 132 to cool the anode 76.

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

In the variant of FIG. 6a, the anodes of all the sources 75 are advantageously common and together bear the reference 154. To facilitate their realization, the anodes comprise a plate 156 in contact with the mechanical part 152 and pierced with 4 holes 158 enabling each passing an electron beam 18 from each of the cathodes of the sources 75. The plate 156 fills, for each of the sources 75, the function of the portion 90 described above. Above each orifice 158 are disposed a cavity 80 bounded by its wall 88 and a target 20. Alternatively, it is possible to keep separate anodes, which makes it possible to separate their electrical connection.

FIG. 6b represents another variant of a multi-source assembly 1 60 in which a mechanical part 1 62 is also common to several sources whose respective cathodes 14 are aligned on an axis 164 passing through each of the cathodes 14. The axis 1 64 is perpendicular to the axis 19 of each of the sources. An electrode 1 66 for focusing the electron beams emitted by the different cathodes 14 is common to all the cathodes 14. The variant of Figure 6b further reduces the distance between two neighboring sources. In the example shown, the mechanical part 1 62 is made of dielectric material and comprises a concave face 1 68 disposed in the vicinity of the different cathodes 14. The electrode 166 is formed of a conductive surface disposed on the concave face 1 68. The electrode 1 66 fulfills all the functions of the electrode 24 described above.

Alternatively, it is possible to implement a common electrode with several sources in the form of a metal electrode without the presence of dielectric material, that is to say having a metal / vacuum interface. Similarly, the cathodes may be thermo-nic. 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 generator ground of the multi-source assembly. The anode (s) are then connected to one or more positive potentials of the generator.

The multi-source set 1 60 may comprise a plug 170 common to all sources. The plug 170 can fulfill all the functions of the cap 32 described above. The plug 170 may in particular be fixed to the mechanical part 1 62 by means of a conductive solder film 172 used to electrically connect the electrode 1 66.

As in the variant of FIG. 6a, the multi-source set

160 may include an anode 174 common to different sources. The anode 174 is similar to the anode 154 of the variant of Figure 6a. The anode 174 comprises a plate 176 fulfilling all the functions of the plate 156 described with reference to FIG. 6a. To avoid overloading Figure 6b, for the anode 174, only the plate 176 is shown.

In Figure 6b, the axis 1 64 is rectilinear. It is also possible to arrange the cathodes on a curved axis, such as for example a circular arc as shown in FIG. 6c, which makes it possible to focus the X-rays 22 of 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 make it possible to focus the X-rays at a point. The curved axis remains locally perpendicular to each of the axes 19 around which develops the electron beam of each source.

The arrangement of the cathodes 14 on an axis makes it possible to obtain sources distributed in one direction. It is also possible to make a multi-source assembly in which the cathodes are distributed along several intersecting axes. It is for example possible to arrange the sources according to several curved axes, each realized in a plane and the planes being secant. By way of example, as shown in FIG. 6d, it is possible, for example, 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 sources to the focus of the parabolic surface. In Figure 6e, the various axes 190, 192 and 194 on which are distributed the different cathodes 14 of the multi-source assembly are parallel to each other.

Figures 7a and 7b show two embodiments of the power supply of the assembly shown in Figure 6a. Figures 7a and 7b are shown in section in a plane passing through several axes 19 of different sources 75. Two sources appear in Figure 7a, and three sources in Figure 7b. It is understood that the description of the multi-source set 150 can be implemented regardless of 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. In FIG. 7a, two high voltage sources V1 and V2 supply the electrodes 24 of each of the sources 10 separately. 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 with two energy sources. different. Similarly, separate current sources 11 and 12 each provide control of the different cathodes 14.

In the embodiment of FIG. 7b, the electrodes 24 of all the sources 75 are interconnected for example by means of a metallization carried out on the mechanical part 152. A high voltage source V _Co mmun supplies all the electrodes 24. control of the different cathodes 14 remains assured by separate current sources 11 and 12. The power supply of the multi-source assembly described using the Figure 7b is well adapted to the variant described with the aid of Figures 6b, 6d and 6e.

FIGS. 8a, 8b and 8c represent several examples of sets of ionizing radiation generators each comprising several sources 10 or 75. In these various examples, the support, as described using FIG. 5, is common to all Sources 10. A high voltage connector 140 allows the supply of the different sources 10. A control connector 142 makes it possible to connect each of the sets to a control module that is not shown and configured to switch each of the sources 10 in a predetermined sequence.

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

Claims

1. An ionizing radiation generator source comprising:

A vacuum chamber (12),

A cathode (14) capable of emitting an electron beam (18) into the vacuum chamber (12), the electron beam (18) developing around an axis (19),

An anode (76) receiving the electron beam (18) and comprising a target (20; 21) capable of generating ionizing radiation (22) from the energy received from the electron beam (18); ionizing radiation (22) being generated towards the outside of the vacuum enclosure (12),

characterized in that the anode (76) comprises a cavity (80) in which the electron beam (18) is intended to penetrate to reach the target (20) and in that the walls (88, 90) of the cavity (80) form a faraday cage surrounding parasitic ions (91) emitted by the target (20) inside the vacuum chamber (12) and in that at least one sorbeur (92) separate from the walls (88, 90) of the cavity (80) and for trapping the parasitic ions (91) is disposed in the cavity (80).

2. Source according to claim 1, characterized in that the sorbeur (92) is made of a material different from that of the cavity (80).

3. Source according to one of the preceding claims, characterized in that it comprises at least one magnet or electromagnet (94) surrounding the cavity (80), and in that the walls of the cavity (80) are made of non-magnetic material.

4. Source according to claims 2 and 3, characterized in that it comprises a mechanical support (97) ensuring the maintenance of the sorbeur (92) and made of magnetic material and in that the mechanical support (97) is disposed in the cavity (80) to guide the magnetic flux from the magnet or electromagnet (94).

5. Source according to claims 2 and 3 or claim 4, characterized in that the at least one magnet or electromagnet (94) is arranged to deflect parasitic ions to the at least one sorbeur (92).

6. Source according to one of the preceding claims, characterized in that at least one of the walls (88) of the cavity (80) forms a wall of the vacuum chamber (12).

7. Source according to one of the preceding claims, characterized in that the walls (88, 90) of the cavity (80) are arranged coaxially with the axis (19).

8. Source according to one of the preceding claims, characterized in that the walls of the cavity (80) comprise a cylindrical portion (88) about the axis (19) extending between the target (20) and a portion Washer (90) having a hole (89) and closing the cylindrical portion (88), and in that the electron beam (18) enters the cavity (80) through the hole (89) of the portion (90).

9. Source according to one of the preceding claims, characterized in that it comprises a mechanical part (28) made of dielectric material and forming a wall of the vacuum chamber (12) and in that the anode (76) ) is sealingly attached to the mechanical part (28).

10. Source according to one of the preceding claims, characterized in that the target (21) is inclined relative to a plane perpendicular to the axis (19).

1 1. Source according to one of the preceding claims, characterized in that it comprises an active magnetic system (98) generating a transverse magnetic field (By) to the axis (19) in the cavity (80) and configured to modify the shape an electron spot (18) formed by the electron beam (18) on the target (20; 21).

12. Source according to one of the preceding claims, characterized in that the walls (88, 90) of the cavity (80) form a shielding screen vis-à-vis parasitic ionizing radiation (82) generated inside. of the vacuum chamber (12).