Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J35/00—X-ray tubes
  • H01J35/02—Details
  • H01J35/04—Electrodes ; Mutual position thereof; Constructional adaptations therefor
  • H01J35/06—Cathodes
  • H01J35/065—Field emission, photo emission or secondary emission cathodes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J35/00—X-ray tubes
  • H01J35/02—Details
  • H01J35/14—Arrangements for concentrating, focusing, or directing the cathode ray
  • H01J35/147—Spot size control

Definitions

• the present invention relates to an X-ray tube comprising a microtip electron source ("microtips").
• the invention is particularly applicable to analysis by absorption of X-rays through thin objects or thin layers, for example to make radiographic observations of thin objects with very good resolution insofar as the source of X-rays (which is part of the tube and which is the place from which the X-rays are emitted) is extremely well defined, i.e. with sharp edges and / or an intensity controlled over the entire area emission zone, this emission zone can be small or on the contrary very large.
• the invention also makes it possible to radiograph liquids circulating in buried pipes of very small dimensions and of small thickness.
• the invention also applies to the medical field and in particular to mammography from a point source of X-rays.
• the invention also applies to analysis by X-ray fluorescence.
• This sample can then be known more precisely than with a conventional X-ray tube.
• thermo ion usually a hot tungsten filament
• the current extracted from the filament strikes the anode (on a more or less well-defined surface depending on the configurations and the focusing means with which the tube is provided), which generates X-rays at the places of impact.
• the anode can be brought to a high voltage and the filament to a potential close to the ground or the anode can be to the ground and the filament negatively polarized. Only the difference in potential counts.
• the anode is grounded and the filament polarized negatively, the anode is more easily cooled (hydraulically) to dissipate the heat dissipated by the electrons penetrating into the material of the target (anode) since the potential of this target is OV, i.e. is equal to the potential of the water discharged through pipes.
• a tube X has a diode structure.
• More complex tubes may include, in addition to the anode and the filament, an intermediate grid, the role of which is indicated below.
• the filament being hot (and therefore likely to emit electrons), the potential of the grid is sufficiently close to that of the filament so that the cloud of electrons emitted by the filament remains blocked in the area between this filament and the grid .
• the sudden increase in the potential of this grid allows the electron cloud to be extracted from this area and reach the anode by crossing the grid.
• This grid therefore acts as a sort of "electron valve”.
• the configuration is then that of a diode (the electric field results from the potential difference which exists between the anode and the needles).
• the electrons leave a certain zone of the cathode and reach the anode on a zone whose surface is delimited.
• the configuration of the anode-cathode assembly is best defined by calculating the trajectories of the electrons in the region between the anode and the cathode using the formulas of electronic optics.
• the shape of the filaments does not always allow an impact of predetermined shape on the anode and therefore the source of X-rays, the extension of which corresponds to the impact zone of the electrons, suffers from this. default.
• X spots of reduced or better defined size are generated. If we use, for example, the electron beam of an electron microscope (of submicron diameter) and that we direct this beam towards a target, we have the equivalent of an X-ray tube to microfocus circular in shape.
• Such an electron microscope used as an X-ray tube generally has an electron gun fitted with magnetic and electrostatic lenses in order to focus the electron beam on a reduced area.
• Microtips are also known for their use in flat screens or in certain instruments such as pressure gauges.
• Cathodes with a large-area matrix structure that use microtips are also known and their use inside flat screens as sources of electrons to produce visible light by cathodoluminescence is also known. It is also known from the American patent application of Cha-Mei Tang et al, serial number 201.963, of February 25, 1994, that an X-ray tube could include a microtip cathode and electrostatic focusing means which are integrated on the cathode itself. Such a structure does not make it possible to obtain an extended emissive area, well delimited with an intensity controlled over the entire area.
• filament X-ray tubes do not make it possible to define any shape of the X-ray source, that is to say the area of the tube from which the X-rays are emitted, in a precise and controllable.
• the object of the present invention is to remedy these drawbacks.
• At least one electron source one zone of which, called the first zone, is intended to emit electrons
• At least one anode one zone of which, called the second zone, is intended to emit X-rays under the impact of these electrons, and
• this X-ray tube being characterized in that the source of electrons is a source of electrons with at least one microtip and with an extraction grid and in that the means for guiding the electrons are magnetic guiding means capable of creating a magnetic field which is homogeneous (i.e. - say which has a direction and an intensity substantially constant or slowly variable spatially) at least in the volume between the first and second zones, the vector characteristics (intensity, direction) of this field being such that the second zone is homothetic of the first zone.
• the source of electrons is a source of electrons with at least one microtip and with an extraction grid
• the means for guiding the electrons are magnetic guiding means capable of creating a magnetic field which is homogeneous (i.e. - say which has a direction and an intensity substantially constant or slowly variable spatially) at least in the volume between the first and second zones, the vector characteristics (intensity, direction) of this field being such that the second zone is homothetic of the first zone.
• the invention makes it possible to obtain a source of X-ray radiation (second zone) having the shape, the intensity distribution (number of X photons emitted per second and per unit area) or the uniformity of emission which one wish by choosing judiciously the magnetic field (for example, parallel to the mean direction of propagation of the electrons) and the shape of the emissive cathode (first zone).
• the combination
• a microtip source whose geometry and distribution of the microtips in the source are adapted to the geometry and the distribution of the desired X-ray radiation
• magnetic guide means including intensity and direction are adapted to the homothetic reproduction (identical or lower or higher) of the electron emission area both spatially and in intensity, allows to obtain an X-ray tube perfectly defined in intensity and geometry.
• the intensity obtained can be variable or spatially constant.
• the intensity of the magnetic field must be greater or equal to a threshold beyond which there is always an electron beam whose envelopes of the trajectories are parallel.
• the X-ray tube object of the invention has the following advantages in particular compared to a conventional X-ray tube using an electron-emitting filament:
• the microtip cathode (s ) can be installed in front of this anode.
• the cathode can be pulsed (the duration of the pulses can be much less than 1 ⁇ s and even reach lOOps) and this possibility of drawing the cathode is accompanied by extremely flexible electronics, not affecting high voltage circuits.
• the tube can work with a battery.
• the area irradiated by electrons can be uniformly (which is not the case with a filament); the X-ray source is therefore uniform (or has controlled uniformity) and the edges of an emissive area of large area are sharp.
• the electron source can comprise a single microtip or a plurality of microtips depending on the geometry and the intensity desired for the X-ray emitting area.
• the X-ray tube comprises a plurality of electron sources, an X-ray emitting area corresponding to each electron source.
• the tube which is the subject of the invention may comprise a single anode or a plurality of anodes, each anode then being associated with at least one microtip.
• the electron source can be drawn to obtain X-ray pulses.
• the X-ray tube which is the subject of the invention may further comprise an electrically conductive grid which is arranged between the electron source and each anode, this grid being polarized so as to prevent ions from reaching the electron source and to avoid the formation of electric arcs between this source of electrons and each anode.
• the magnetic guide means of the tube which is the subject of the invention may comprise one or more magnets or Helmholtz coils or both one or more magnets and Helmholtz coils.
• Figure 1 is a schematic view of a particular embodiment of the object X-ray tube of the invention, in which the electron source comprises only a single microtip,
• FIG. 2 is a schematic view of another particular embodiment in which the source of electrons comprises a plurality of microtips
• Figure 3 is a schematic view of another particular embodiment in which there is a plurality of anodes
• Figure 4 is a schematic view of another particular embodiment in which the anode is formed on the tube window
• Figure 5 schematically illustrates means for regulating the electron source of an X-ray tube according to the invention.
• a magnetic field is used whose intensity can range from a few hundredths of a tesla to a few tenths of tesla for example, this magnetic field being, in the case of an identical reproduction of the electron-emitting area, parallel to the median trajectory of the electron beam.
• the insertion can use a divergent or convergent field to reproduce in an enlarged or narrowed manner said electron-emitting area.
• the trajectories of the electrons are wound around the direction of the magnetic field with a radius whose value is inversely proportional to the intensity of this magnetic field.
• the average trajectories of the electrons are then substantially parallel and practically do not diverge.
• spot in which the electron beam meets the anode is then identical to the area of the source which emits the electrons if we assume that the anode is placed perpendicular to the electron beam.
• the shape of the emissive zone of the electron source (cathode) is thus reproduced on the anode and the X-ray source therefore rigorously has this same shape.
• the X-ray emission density depends on the density of the incident current which itself depends on the density of the microtips on the cathode and on the current emitted by each microtip.
• a more complex magnetic configuration can possibly concentrate the electron beam more, instead of simply preventing its divergence.
• the "spot" formed on the anode may have an even smaller size.
• the area which emits X-rays has a shape similar to that of the area which emits electrons if the angle of incidence of the electrons on the anode is not taken into account (when that -this is different from 90 °). This can be corrected by giving the sending area of electrons a shape such that once projected on the anode the spot obtained has the desired shape.
• this anode is thick, the only X photons that can be used are those that are emitted outside the anode.
• an X-ray tube is provided with a window made of a material chosen so as to be as little absorbent as possible with respect to X-rays so that they these can pass through this window and exit the tube or as thin as possible to limit the absorption (one can for example use a membrane of nanometric thickness in Si 3 N 4 or Sic).
• This window also maintains the tightness of the enclosure of each X-ray tube, enclosure in which a sufficiently low pressure is created by means (not shown in FIGS. 1 to 4) (for example of the order of 10 " 8 hPa or less) to keep the X-ray tube working properly and sustainably.
• the X-ray tube is itself under vacuum (for example in the case of an electron microscope) and this window is then deleted or else it only plays the role of optical filter or pollution filter and the X-rays produced then propagate in the vacuum and irradiate a sample also placed in the vacuum.
• FIG. 1 is a schematic view of a first example of the X-ray tube of the invention.
• the X-ray tube schematically represented in this FIG. 1 comprises, in a vacuum enclosure 2, an electron source 4 comprising a single microtip 6, made of an electron emitting material and formed on a suitable substrate 8, thus an integrated extraction grid 16, the source preferably being produced by microelectronics techniques.
• Means are provided for bringing this anode 10 to a high positive voltage with respect to the microtip 6.
• the X-ray tube of Figure 1 also includes coils of Helmoltz 12 preferably placed outside of the enclosure 2 (the latter being made of a non-magnetic material) and intended to create a magnetic field B which is substantially parallel to the axis Z of the microtip and which is homogeneous in the volume between the microtip and the anode 10, this volume being delimited by dashed lines t; in figure 1.
• Helmoltz 12 preferably placed outside of the enclosure 2 (the latter being made of a non-magnetic material) and intended to create a magnetic field B which is substantially parallel to the axis Z of the microtip and which is homogeneous in the volume between the microtip and the anode 10, this volume being delimited by dashed lines t; in figure 1.
• one or more magnets can be used to create this magnetic field and this magnet or these magnets can or can be placed in the enclosure 2 or outside of it.
• the voltage applied between the anode and the microtip can be of the order of +5 kV to +50 kV.
• An electron beam is then emitted by the microtip 6 along the Z axis, in the direction of the anode 10, thanks to the application of a voltage on the extraction grid 16.
• the microtip 6 is capable of emit a current of the order of 100 ⁇ A.
• This electron beam is guided while being concentrated on the anode by the magnetic field B.
• This magnetic field is of the order of a few tenths of Teslas.
• the electron emitting area is of the order of 1 ⁇ m 2 or even less.
• the size of the electronic spot on the anode is also of the order of 1 ⁇ m 2 or even less with more intense magnetic fields.
• X-rays are thus generated (having the reference X in FIGS. 1 to 4) from a microfocus FI whose size is of the order of 1 ⁇ m 2 .
• the enclosure 2 is closed by a beryllium window 14.
• this cathode does not screen or shade the X-rays emitted.
• an intermediate grid 17, which has great transparency vis-à-vis the electrons emitted by the microtip 6, is arranged between the source 4 and the anode 10, in the vicinity from source 4, on the path of the electron beam, a few millimeters from source 4.
• This grid 17 is for example made of a conductive material and perforated to 90% to let the electrons pass.
• this grid 17 is brought (by means not shown) to a potential greater than that of the extraction grid 16. It can either be much less than that of the anode, for example of the order of 200 V to 500 V or, if the grid is extremely transparent to electrons, slightly higher than that of the anode in order to prevent the positive ions produced on the anode by the electronic impact from going up to the cathode.
• FIG. 2 A second example of the X-ray tube of the present invention is schematically represented in FIG. 2.
• the X-ray tube of Figure 2 is similar to that of Figure 1 except that, in the case of Figure 2, the electron source 4 includes several microtips 6 which are formed on the substrate 8 and whose axes Z are substantially parallel.
• the anode 10 is also placed opposite these microtips.
• the magnet or the coils of Helmotz 12 are also provided to create the homogeneous magnetic field B in the volume between the source 4 and the anode 10, this volume being delimited by the phantom lines t which we see in the figure 2.
• This magnetic field is substantially parallel to the Z axes of the microtips.
• the magnetic field B guides the electrons emitted by these microtips so that the average trajectory of the electrons is substantially parallel to this magnetic field B in the volume delimited by the dashed lines t.
• Means not shown also make it possible to positively polarize the anode 10 relative to the microtips 6, for example at a voltage of the order of +10 kV, and to bring the grid 17 to a potential higher than that of the grids 16 but much lower than that of anode 10 or slightly higher.
• the substrate has for example a surface of the order of 100 ⁇ m 2 to 1 mm 2 and comprises for example 100 to 1000 microtips distributed over a surface area equal to 100 ⁇ m 2 and making it possible to obtain an electronic current of the order from 1 mA to 10 mA. If the space charge of the electron beam is not taken into account, the magnetic guidance makes it possible to obtain an electronic spot F2 on the anode 10 having the same size as the area occupied by the microtips of the cathode 4 (in not taking into account the inclination of the anode 10 relative to the electron beam).
• This inclination of the anode in the X-ray tube of Figure 2 (as well as in the X-ray tube of Figure 1) is intended to send a large amount of X-rays towards the window in beryllium 14.
• X-ray tubes can therefore be produced in accordance with the invention, the X-ray emitting area of which has exactly the dimensions and shape desired for the intended application, the distribution of the intensity of the X-ray emitting area being a function the distribution of the emission intensity of the first zone.
• the X-ray tube according to the invention which is schematically represented in FIG. 3, differs from that of FIG. 1 in that it comprises, in addition to the anode 10, another anode 18 which is placed next to the anode 10 and an additional microtip 6a placed on the substrate 8, opposite this other anode 18. In this example, there are therefore two electron emitting zones and two X-ray emitting zones.
• the anodes 10 and 18 are inclined in the same way with respect to the electron beams, as seen in FIG. 3, to each send a large amount of X-rays to the window 14.
• the X-ray emitting zones F3 and F4 respectively located on the anodes, are homothetic of the two electron emitting zones
• microtip (respectively to a microtip or to a set of microtips).
• the anode 10 it is for example possible for the anode 10 to emit X-rays whose wavelength does not make it possible to highlight particles 20 contained in a sample 22 situated outside the X-ray tube, opposite of window 14, a detector 24 being placed following this sample 22 (which is thus included between window 14 and detector 24) and ensuring that the anode 18 emits X-rays whose wavelength instead allows to highlight these particles.
• the tube according to the invention which is schematically represented in FIG. 4, also comprises an enclosure 2 under vacuum closed by a window
• microtip cathode 4 opposite which is placed a grid 17 transparent to the electrons emitted by the microtips 6.
• the X-ray tube of Figure 4 also includes an anode 10 grounded and constituted for example by a layer of tungsten which is deposited on the beryllium window.
• Polarization means 28 are provided for bringing the microtips formed on an appropriate substrate 8 to a negative voltage with respect to the extraction grid 16 and means 29 provided for bringing the whole of the cathode to a high negative voltage with respect to that of the anode.
• the anode 10 formed on the window 14 is placed opposite the grid 16 and the microtips 6 and this anode is substantially parallel to the substrate 8 and to the grid 16.
• the X-ray tube of FIG. 4 also includes a magnet 30 external to the enclosure 2 and intended to create a magnetic field B perpendicular to the anode, homogeneous in the volume comprised between the source 4 and the anode 10 and provided to focus the electrons emitted by the microtips on this anode.
• the anode 10 When the anode 10 is struck by the electrons emitted by the microtips it emits X-rays which pass through the beryllium window 14.
• FIG. 4 also shows a sample screen 34 partially opaque to X-rays, provided with an opening 36 and placed between the window 14 and the space detector 32, the X-rays thus passing through this opening 36 before reaching the detector.
• This example highlights the concept of planar radiography with an extended X source: only the regions of low absorption (symbolized by the hole 36 allow the X-rays detected by the two-dimensional detector 32 to pass through.
• the X-ray tube in Figure 4 has an extended, defined focal point F5 (X-ray emitting area) by magnetic guidance, this focal point having a uniformity which can be constant or controlled.
• F5 X-ray emitting area
• this area F5 which emits X-rays can have a surface of several tens of cm 2 .
• the zone F5 of FIG. 4 which emits the X-rays, has rigorously the same degree of extension as the zone emitting the electrons (all of the microtips) although a few millimeters separate the cathode with microtips 4 from the anode 10.
• microtip cathode of an X-ray tube could be given any shape, for example the shape of a "P".
• the X-ray emitting area would then also have the shape of a "P", something that cannot be achieved with a conventional X-ray tube, using an electron-emitting filament or a thermionic cathode.
• An X-ray tube according to the invention can be drawn.
• microtip cathode having a matrix structure and successively control the different rows of this microtip cathode, which also corresponds to an operation in pulsed mode of the X-ray tube comprising this cathode with structure matrix.
• an aluminum or magnesium plate or a thin layer of tungsten formed by evaporation on a heat conducting support in order to evacuate the latter
• the material of the anode is chosen in the periodic classification of the elements according to the application.
• the window 14 which closes the vacuum chamber 2 is thick enough to ensure the seal but thin enough not to absorb too much X-rays emitted when the X-ray tube is operating.
• nanometric thickness membranes can be used.
• This window may have a honeycomb structure ensuring both rigidity and vacuum tightness and the transmission of X-rays thanks to the thinnest thicknesses.
• the thickness of this window depends on the diameter of the latter and can be of the order of 100 ⁇ m or less in places and in the case of membranes it can be a few hundred nanometers. If desired, one can have in this enclosure 2 a getter type element to maintain a very low pressure.
• This layer 38 itself rests on a substrate 40, for example made of silicon.
• anode 10 of the X-ray tube as well as means 44 for applying a suitable positive variable voltage to the grid 16 relative to the microtip 6 and means 46 for applying a suitable positive high voltage to the anode 10 relative to the microtip.
• a resistor 48 of value r is mounted between ground and the negative terminal of the means 46 for applying the high voltage to the anode.
• the regulation system comprises an operational amplifier 50 which controls the voltage application means 44 as a function of a value of reference voltage R set by the users and of the voltage image of the current flowing in the resistor 48.
• the electrons entering the anode 10 correspond to a current of intensity i.
• This voltage V is sent to the operational amplifier 50 and the latter compares this voltage V to the reference voltage R corresponding to the current desired by the user.

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• X-Ray Techniques (AREA)

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• 1997
  • 1997-06-13 FR FR9707342A patent/FR2764731A1/fr not_active Withdrawn
• 1998
  • 1998-06-12 WO PCT/FR1998/001236 patent/WO1998057349A1/fr not_active Ceased
  • 1998-06-12 US US09/445,445 patent/US6259765B1/en not_active Expired - Fee Related
  • 1998-06-12 EP EP98930850A patent/EP0988645A1/de not_active Withdrawn

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