EP1747570A1 - Tube a rayons x pour emissions de doses elevees - Google Patents

Tube a rayons x pour emissions de doses elevees

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Publication number
EP1747570A1
EP1747570A1 EP04741611A EP04741611A EP1747570A1 EP 1747570 A1 EP1747570 A1 EP 1747570A1 EP 04741611 A EP04741611 A EP 04741611A EP 04741611 A EP04741611 A EP 04741611A EP 1747570 A1 EP1747570 A1 EP 1747570A1
Authority
EP
European Patent Office
Prior art keywords
ray tube
cold cathode
anode
electron
cathode
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP04741611A
Other languages
German (de)
English (en)
Inventor
Kurt Holm
Lars-Ola Nilsson
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Comet Holding AG
Original Assignee
Comet Holding AG
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Comet Holding AG filed Critical Comet Holding AG
Publication of EP1747570A1 publication Critical patent/EP1747570A1/fr
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J33/00—Discharge tubes with provision for emergence of electrons or ions from the vessel; Lenard tubes
    • 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
    • H01J2235/00—X-ray tubes
    • H01J2235/06—Cathode assembly
    • H01J2235/068—Multi-cathode assembly
    • 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
    • H01J35/08—Anodes; Anti cathodes
    • H01J35/112—Non-rotating anodes
    • H01J35/116—Transmissive anodes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J35/00—X-ray tubes
    • H01J35/02—Details
    • H01J35/16—Vessels; Containers; Shields associated therewith
    • H01J35/18—Windows
    • H01J35/186—Windows used as targets or X-ray converters

Definitions

  • the present invention relates to an X-ray tube and an electron gun for high dose rates with an electron (e ' ) emitting cathode, in particular for the large-area irradiation of 5 objects with various geometries and the use of the X-ray tube for sterilization, and the use of the electron gun for sterilization, for drying of ink or polymer crosslinking. It is known that X-ray and electron radiation are increasingly used in the sterilization of blood plasma, medical instruments,
  • Industrial applications also include drying ink and polymer crosslinking with electrons (e " ) in an energy range of 80-300keV.
  • the aim is to achieve the highest possible dose rate. This allows the irradiation time to be shortened considerably, which is a great advantage shorter throughput time and thus cost reduction.
  • the achievable dose rate differs between X-ray emitters and electron cartridges. In the range up to 1 MV acceleration voltage, only 1% of the electron energy is converted into X-rays to generate the X-rays. In turn, less of these are used in standard X-ray tubes for geometric reasons
  • Electrons have a low penetration depth and are therefore only suitable for the sterilization of surfaces.
  • materials can also be Inside are sterilized, such as blood plasma, while accepting the poorer efficiency.
  • the dose rate per area is determined by the distance of the object from the focal spot of the radiation source and the amount of radiation that is generated in the focal spot. This amount of radiation in turn is limited by the thermal energy that has to be dissipated by cooling the focal spot so that the material in the focal spot does not melt.
  • the specific dose rate of a conventional X-ray source is severely limited by these two factors.
  • the object to be irradiated In order to achieve a high dose rate, the object to be irradiated must come as close as possible to the radiation source. It may also be necessary that the focal spot of the radiator is as large as possible so that the specific load in the focal spot does not melt the target. With electron emitters, the object must also come as close as possible to the radiation source, otherwise the electrons will lose too much energy on the way in the air. With an optimized design of the exit window from the electron emitter, a relatively small proportion of the beam power is lost in the anode (target) and thus a considerably higher dose rate is obtained with electron irradiation than with X-ray radiation.
  • thermionic electron sources have usually been used as radiation sources.
  • the thermionic electron source can be heated either directly or indirectly and releases electrons (e " ) into the vacuum of the radiator if the annealing temperature is sufficient.
  • the heated sources can be manufactured relatively reliably, robustly and inexpensively, they suffer from some weaknesses.
  • the heating power of the cathode is usually only about 1 - 5% of the radiator power, precautions must be taken for cooling in the cathode range in the case of high-current electron sources.
  • the generator must provide the heating power at a high potential, which means a high expenditure and a susceptibility to faults. Since the thermionic electron sources have a high current density, they are not arranged in terms of area but rather rather in a punctiform manner. This makes it more difficult to evenly irradiate complicated geometries. Thermionic electron sources are operated at high temperatures at which the emitting material is already evaporating. This limits the lifespan of such sources. Because of the power supply and possibly cooling, it is difficult to build thermionic electron sources so that they are transparent to X-rays. As a result, the geometrical possibilities for irradiation are further restricted.
  • an radiation source is required which achieves a high dose rate and also enables the shape of the radiation source to be adapted to the shape of the objects to be irradiated and, in particular, a simultaneous irradiation of large quantities of these objects to be irradiated .
  • the integration of a cold cathode into the radiation device according to the invention, for example in an X-ray tube or an electron gun, is decisive for the economy of the radiation method according to the invention.
  • Electrons (e " ) are bound inside a solid by a potential barrier.
  • the potential barrier also called work function 0, is typically 4.5 eV (electron volts) for conventional tungsten filaments.
  • A is a pre-factor to adapt experimentally determined currents and sqrt 0 - is the square root of the work function 0.
  • sqrt 0 - is the square root of the work function 0.
  • Fig. 2a Field strength relationship according to the formula of Fowler and Nordheim is illustrated in Fig. 2a.
  • at the emission site is then several thousand volts per micrometer.
  • the voltage required for this is technically feasible.
  • a high density of field-increasing structures must be brought into the electrical field. Until almost 30 years ago, this was hardly possible.
  • microstructure processes have been developed, with which a density of up to 10 8 emitting microtips / cm 2 can be achieved.
  • a lithographically structured cathode is also shown schematically in FIG. 2 and usually consists of micrometer-sized metal tips, for example made of molibdene, and is known from the technology of flat screens.
  • the process of producing microtips with micrometer precision is complex and expensive. For this reason, research results in the mid-1990s on cold emission of thin carbon films at extremely low applied electrical field strengths caused a great deal of excitement. At first it was assumed that exceptionally deep work functions 0 of approx. 0.1 to a few eV were responsible for this.
  • Carbon which also has sharp structures on the surface, as described, for example, in the patents US 6087765 B1 and US 6593683 B1.
  • a carbon-containing gas mixture e.g. methane, acetylene etc.
  • an evacuated reactor vacuum recipient
  • H 2 (hydrogen), N 2 (nitrogen) etc. admixed introduced.
  • a microwave plasma is ignited or the substrate is brought to 600 ° to 900 ° Celsius.
  • a transition metal (nickel, cobalt, iron, etc.) is brought onto the substrate in the form of small clusters, ie a few nanometers to micrometers in size. Carbon nanotubes can grow on these clusters.
  • a transition metal nickel, cobalt, iron, etc.
  • Called "cathodic are” an arc discharge, at currents I of about 80A, is ignited between two graphite electrodes in a helium atmosphere. After the discharge there are nanotubes in the carbon soot that can be used after a cleaning procedure. For example, this can also be done So-called laser ablation processes are used. Laser is shot at a graphite target. There are also nanotubes in the soot. By adding transition metals to the graphite target, single-walled nanotube chips can be created. There are a number of other production processes or variants of the above-mentioned Generally, there is a limited influence on defect rates in the tube, geometry of the tube, scrap etc. has. This is due to the fact that little is known about the growth mechanisms.
  • the adhesion of the tubes is often very poor and in the electric field they can be torn away in the direction of the anode due to their charge (field-induced emitter destruction).
  • the carbon nanotubes can on the one hand ignite electrical discharges and on the other hand the emission performance deteriorates over time. Indeed it is currently
  • Emitter density (F) J f (ß) dß [cm "2 ]
  • Current density (F) I f (ß) l (ß, F) dß [Acrn 2 ]
  • l (ß, F) is the current of a single emitter in Dependence on the external electric field F and the geometric field elevation It has been shown that f (ß) of a typical cold cathode with carbon nanotubes has an exponential dependence on ß, f (ß) ⁇ exp (kß) due to the relatively small number of efficient field-increasing structures in a higher ß-range (> 400) therefore only contribute about 0.01% of all potential emitters to the current, the rest of the emitters have too low field-increasing values and therefore remain passive, since the field F ⁇ is less than 2 V / nm is, see equation 2.
  • the most efficient emitters with a percentage of 0.01% deliver current at a low (ie with a small voltage difference) applied field F, but since the number of these is so small, the total current density remains low Try that from the outside placed field F to increase, so that the less efficient emitter for Contributing electricity inevitably leads to electrical discharges or, above all, to current-induced emitter destruction of the most efficient emitters.
  • ballast resistors are known and is already used for microtips. If one or more emitters in series with a resistor, for example in the form of a resistance layer, are switched, the emission deviates from the typical Fowler Nordheim behavior. The larger the geometrically excessive field F
  • the object of the invention is therefore to overcome the disadvantages of thermionic radiation sources shown above and to provide an irradiation device with X-rays or electron beams using a high-dose radiator with low power losses Propose cathode with which objects of various geometries and in large quantities can be irradiated simultaneously.
  • an X-ray emitter is to be proposed which enables a dose rate that is several times higher than conventional X-ray emitters.
  • the percentage of energy converted into and usable in X-rays should be increased and a uniform distribution of the X-rays should be obtained with respect to the surface to be irradiated and the depth of the material.
  • an X-ray tube comprises a cathode which emits electrons (e-) into a vacuumed interior and a target designed as an anode for generating X-rays (y) of high dose rate, the cathode comprising at least one Cold cathode, based on an electron (e-) emitting material with a field-increasing structure.
  • the field-high structures can include, for example, carbon nanotubes, coral-like carbon, metal tips, silicon tips, diamond tips and / or diamond powder.
  • the field-increasing structures advantageously emit electrons (e ' ) even at room temperature.
  • the hot cathodes known as thermionic electron sources, they do not require any heating power in order to release electrons (e ' ) into the vacuum.
  • Field-increasing structures that can be integrated on the surface of the cathode cause a cold emission of electrons (e " ) by amplifying an externally applied electrical field.
  • the functioning of the cold cathodes is based on the fact that an externally applied electric field is exaggerated in the case of pointed structures, so that high electrical fields, typically in the order of 2000 to 4000 volts per micrometer, arise, for example the anode can be small or the same compared to the electron-emitting surface of the cathode Size ratio be formed.
  • One advantage of this invention is that the electron emission takes place at room temperature and thus the device for heating the emitter is omitted. There is also no cooling of the immediate surroundings of the emitters. The lifespan of the emitters should be mentioned as a further advantage. Since the emitter is operated at room temperature, there is no aging by evaporation of the emitter material.
  • the cold cathode comprises at least one carrier layer for holding the electron (e-) emitting material, the emission surface of the cold cathode being essentially defined by the shape of the carrier layer.
  • the geometry and spatial arrangement of the cold cathode and / or the emission surface of the cold cathode is determined by the shape of the carrier layer.
  • the geometry of the radiation unit can be easily adapted to the requirements of the radiation method.
  • the ratio of the area of the cold cathode to the layer depth is large.
  • the cathode is suitable for large-area irradiation devices.
  • the shape and size of the irradiation space of the X-ray tube is determined by the surface area and / or spatial arrangement of the cold cathode and / or the anode.
  • the carrier layer comprises a matrix with embedded carbon nanotubes and / or coral-like structured carbon.
  • One advantage of this embodiment is that it becomes very economical for large-area emitter devices. Carbon nanotubes are commercially available and coral-like structured carbon can be applied inexpensively over a large area. Because of its strong covalent bonds, carbon is also more resistant than metal micro-tips to ion bombardment and electrical discharges. Carbon is able to cope with large emissions.
  • the first comprises
  • Carrier layer of the cold cathode at least one substrate with ceramic material or glass.
  • the carrier material is inexpensive, malleable and suitable for vacuum.
  • the attenuation of X-rays by these materials is relatively low.
  • the carrier layer comprises at least one resistance layer and / or interconnect layer.
  • the emission current can be distributed evenly over the cathode surface. The specific power can thus be optimally distributed to the anode, and local overheating is thereby avoided.
  • the conductor track layer comprises an evaporated copper layer.
  • the copper has good electrical and heat-dissipating properties.
  • Other metals can also be used to advantage.
  • the x-ray tube is as
  • Anode high cylinder formed with a coaxial cathode high cylinder inside This variant has the advantage that e.g. the material to be irradiated can be attached to the inside of the hollow cathode cylinder (the radiation goes to the inside - reflector).
  • the x-ray tube is as
  • This variant has the advantage that, for example the material to be irradiated can be placed inside the anode hollow liner (the radiation goes inside - transmission radiator).
  • the x-ray tube is designed as an anode high cylinder with a coaxial high cathode cylinder inside.
  • This version has among other things the advantage that e.g. the material to be irradiated can be attached outside the hollow anode cylinder (the radiation goes outside - transmission radiator).
  • the x-ray tube is designed as an anode high cylinder with a coaxial high cathode cylinder outside the anode.
  • This variant has the advantage that e.g. the material to be irradiated can be attached outside the cathode hollow cylinder (the radiation goes outwards - reflectors).
  • the cross section of the cold cathode and / or anode is designed as a full circle, segment of a circle, annulus, triangle, square, polygon or any definable polygon.
  • the length of this arrangement is in principle arbitrary.
  • One advantage of this variant is that the spotlight arrangement can be assembled modularly.
  • the electron (e -) emitting material is arranged on the carrier layer at a defined distance next to one another and / or one behind the other and / or adjacent. This has advantages in terms of production technology, among other things, since the extraction grid can be built more easily in flat geometries. A large number of such radiator modules can thus be assembled into a complex geometry of the radiator arrangement.
  • the cold cathode is for
  • X-ray radiation (y) is transparent or essentially transparent.
  • An advantage of this embodiment variant is, inter alia, that a rear or transmission radiator arrangement can be built without special cooling devices for the cold cathode (apart from air convection).
  • at least one extraction grid is arranged between the cold cathode and the anode. Between cold cathode and Extraction grid, for example, an electrical insulator can be arranged.
  • One advantage of this embodiment variant is that the distance between the extraction grid and the cold emitter can be kept constant over the emission area. The local variation in the emission intensity can thus be reduced. Under certain circumstances, the use of an extraction grid can also serve as protection against ion bombardment and electrical discharges.
  • the anode has at least one coolant layer (KM), the coolant layer (KM) comprising a liquid coolant (KM) and / or a gaseous coolant (KM).
  • the coolant layer (KM) comprising a liquid coolant (KM) and / or a gaseous coolant (KM).
  • the anode can withstand a higher specific electron intensity. A higher dose rate can thus be achieved.
  • the present invention also relates to a method for sterilization and / or radiation using an X-ray tube according to the invention and to an electron gun of the same type. Variants of the present invention are described below with the aid of examples. The examples of the designs are illustrated by the following figures:
  • FIG. 1 shows an X-ray tube with a thermionic electron source according to the prior art. Electrons (e " ) are emitted by a cathode 30 and X-rays are emitted by an anode 20 through a window 301.
  • FIG. 2 shows a cathode emitting cold electrons (e " ); a lithographically structured cathode with metal tips as field-increasing structures of the prior art is shown schematically.
  • FIG. 3 shows a cross section of an embodiment of an x-ray tube according to the invention in a hollow cylindrical shape, in particular the cross section through the hollow cylindrical cold cathode anode arrangement and the radiation chamber, likewise formed, is shown schematically.
  • a uniform 4 ⁇ r gamma radiation can be achieved inside the cathode hollow cylinder 31.
  • the material to be irradiated can be attached inside the anode hollow cylinder 31. This guarantees a uniform radiation of the object from all sides, which would otherwise hardly be possible.
  • FIG. 4 shows the cross section of a cold cathode with carbon nanotubes with an extraction grid in a so-called triode configuration of the electrodes.
  • 5a shows the cross section of a transmission radiator arrangement in variable electrode geometry with a modular composition
  • a large number of such transmission radiator arrangements can advantageously be assembled in a modular manner.
  • the extent of the transmission radiator arrangement can be freely selected in the longitudinal direction, perpendicular to the paper plane.
  • 5b shows the cross section of a transmission radiator arrangement according to FIG. 5a, with a special case of the dimensioning of the cold cathodes and anode radii, the cathode and anode being arranged in parallel or essentially in parallel.
  • Fig. ⁇ a shows the cross section of a reflector arrangement in variable electrode geometry with modular cold cathodes as Elektro ⁇ en sources in a partial circle segment.
  • the carrier layer of the cathode and the cold cathode are essentially transparent to X-rays.
  • the extent of the reflector arrangement can be freely selected in the longitudinal direction, perpendicular to the paper plane.
  • FIG. 6b shows the cross section of a retroreflective arrangement according to FIG. 6a, with a special case of dimensioning the cold cathodes and anode radii, the cathode and anode being arranged in parallel or essentially in parallel.
  • Fig. 7 shows an electron transmission emitter with a modular
  • FIG. 1 schematically shows an architecture of such a conventional X-ray tube 10 of the prior art.
  • Electrons e " are accelerated by an electron emitter, ie a cathode 30, usually a hot tungsten filament, emitted by a high voltage applied to a target s, X-rays y being emitted by the target, ie the anode 20, through a window 301.
  • the x-ray radiation y passes through a window 301 into the outside space and is used for radiation purposes.
  • FIG. 2 schematically shows a known lithographically structured cold cathode 22 from the prior art.
  • a conductor track layer 2020 is evaporated onto an inexpensive carrier 201, for example a ceramic substrate, and a resistance layer 203 is also applied to this.
  • Metal tips 70a made of molybdenum are applied to the resistance layer 203 as field-increasing structures 70, also called (electron) emitters.
  • the metal tips 70a are spaced apart by insulators 60 arranged laterally adjacent to one another. Spaced apart in height, ie upwards from the resistance layer 203, a gate 80, also called a grid, is positively applied to the surface of the insulators 60.
  • An electric field F (not shown) is applied between the metal tips 70a and the gate 80, which in the function of an extraction grid consists of a metallic material.
  • Gate 80 is electrically (insulated) and spatially separated from both resistive layer 203 and metal tips 70a and typically has an opening of a few micrometers.
  • FIG. 3 shows in cross section the diagram of an X-ray tube 11 which, in a preferred embodiment, is made up of a hollow cylindrical cold cathode 21 and a hollow cylindrical array 31 which are arranged coaxially to one another.
  • the common center axis of both hollow cylinders runs, as can be seen in the cross section of FIG. 3, through the common center point MP.
  • the cold cathode 21 of the x-ray tube 11 is shown in cross-section on an outer full circle with the radius r1 with respect to the center MP.
  • the cold cathode surface as drawn out and shown enlarged in the diagram of section A, has a matrix with embedded carbon nanotubes 71a as field-increasing structures. Electrons (e " ) are emitted from the carbon nanotubes into the vacuumized interior 40 of the X-ray tube 11 already at room temperature as a result of an external electric field F (not shown). These electrons (e " ) thus strike the target on the anode side in an accelerated manner 31 and are known to cause the emission of X-rays ( ⁇ ).
  • the x-ray radiation ( ⁇ ) is emitted on all sides due to the arrangement of the anode 31 with a smaller radius r2 with respect to the center MP in a radiation chamber 90, which is likewise hollow cylindrical.
  • a transmission radiator with a diode configuration of the electrodes 21, 31 is formed in the illustration shown in FIG. 3.
  • the full high voltage between the cold electron (e ") is located on emissive cathode 21 and the anode 31, in contrast to the other arrangement, no extraction grid is arranged here.
  • the carrier material (not shown) of the cold cathode 21 consists, for example, of an inexpensive ceramic substrate, which already closes off the X-ray tube 11 to the outside from the outside
  • the carrier substrate is optionally metallized on the outside with a further layer or comprises a further housing wall, not shown, made of metal or a polymeric material.
  • a further layer or comprises a further housing wall, not shown, made of metal or a polymeric material As shown in Fig. 3, when using cold electron (e " ) emitting cathodes 21 is only cooling the anode surface necessary. The cooling can be carried out with a liquid or gaseous coolant KM, such as water, oil or air.
  • the schematically illustrated coolant has a radius r3 (r3 smaller than r2) starting from the center MP of the common center axis of the anode 31 and cold cathode 21 5, encloses together with the anode 31 the likewise hollow-cylindrical radiation chamber 90.
  • material for the anode 31 is known to use a metal with a high atomic number, for example tungsten.
  • Fig. 4 shows in schematic cross section the arrangement of a cold cathode 23 with extraction grid 80; the anode associated with the emitter arrangement is not shown.
  • a carrier material 201 e.g. a cost
  • a layer with conductor tracks 202 is first evaporated.
  • the conductor track layer 202 serves to control the individual field-increasing structures 71 on the surface of the cathode 23.
  • a resistance layer 203 is arranged in series with the field-increasing structures 71.
  • This resistance layer 203 serves, according to the third solution already described above, to improve the current density and emitter density as a Baiast resistance.
  • the layers 201, 202, 203 are essentially transparent to X-rays and also resistant to the radiation. That means liability, or electrical
  • the emitter destruction due to the lack of liability was a further problem of the emitter recognized on the cathode surface.
  • the emitter can be destroyed more by current and field-induced destruction than by ion bombardment or electrical discharges.
  • the lack of adhesion of the emitter can have an adverse effect on the long-term stability of the emitter performance.
  • measures must be taken to keep the long-term stability of the radiator power constant; this is done by increasing the extraction voltage as a function of time.
  • an arrangement of the electrodes in a triode configuration as shown in the diagram in FIG. 4, is particularly advantageous.
  • the extraction voltage (not shown) is applied between the gate 80 and the cold cathode 23 and is typically 10 to 10000 volts depending on the geometry of the field-increasing structures 71 and the distance between the cathode surface and the gate 80; the latter indicated by arrow d.
  • the field-increasing structures 71 are less exposed to ion bombardment and, above all, less to the possible high-voltage electrical discharges.
  • the spatial and electrical separation of the gate 80 from the surface of the cold cathode 23 is associated with additional effort and thus also with additional costs.
  • the electrical / spatial separation takes place with an insulator 60, the height or thickness of which corresponds to the distance (arrow d) from the gate 80 to the surface of the cold cathode 23.
  • the insulators / placeholders 60 for example like the cold cathode 23 itself, can also be flat and have the shape of a perforated glass or ceramic plate, for example.
  • Each placeholder 60 thus consists, for example, of a glass rod, which is inexpensive, in particular if the cold cathode is formed over a large area.
  • gate 80 also called extraction grid
  • a metal can be evaporated onto the end face of insulators 60 facing away from the cathode surface.
  • a metal grid with variable hole spacing indicated by arrow c in the cross section of FIG.
  • gate 60 can also be used as gate 60.
  • great value must be placed on the geometry of the gate 80 (arrows a - d).
  • the arrows b and c already mentioned define the perforation pattern or the lattice web opening of the insulator 60
  • arrow d determines the distance from the cathode surface to the gate 80
  • arrow a defines the distance between two insulators 60.
  • the grid web width (arrow b) must be as small as possible and the grid web opening (arrow c) should be as large as possible. While the grid web opening (arrow c) cannot be dimensioned arbitrarily large, since otherwise the externally applied electrical field F (not shown) at the emitter location becomes too small, the grid web width (arrow b) must be dimensioned sufficiently large so that the grid-shaped Gate 80 is not deformed too much due to the electrostatic attraction. For the latter reason, it can furthermore be advantageous if a separate placeholder / insulator 60 is arranged below each grid web 80a. As a result, the distance between two insulators (arrow a) is the same as the grating bar opening (arrow c).
  • the following value ranges can be assumed, for example: (i) distance between two insulators (arrow a) 0.01 to 2 mm; (ii) grid web width (arrow b) 0.01 to 0.2 mm; (iii) grid web opening (arrow c) 0.01 to 0.3 mm; (iv) Distance of the cathode surface to the gate (arrow d) 0.01 to 2 mm.
  • a typical extraction voltage of several thousand volts must be used. This significantly increases the power losses at gate 80.
  • FIG. 5a shows a transmission radiator arrangement with a modular cold cathode 24 consisting of several cold cathodes. denmodulen 25 and an Andede 32 in an arbitrarily definable circle segment for use according to the invention in an X-ray tube.
  • a plurality of cold cathode modules 25 are arranged on the outer pitch circle section with the outer radius r1 essentially at the same distance.
  • the cold cathode modules 25 have on their surface field-increasing structures (not shown), which at room temperature already release electrons (e " ) into the vacuumized interior 40 of the X-ray tube.
  • the cold cathode modules 25 can be equipped according to the variant in FIG. 4. The electrons (e " ) meet
  • anode-side target 32 accelerates onto an anode-side target 32.
  • this will cause x-ray radiation ( ⁇ ) from target 32, e.g. emitted into the radiation room 90.
  • the anode-side target 32 is also arranged on a pitch circle section, but with a smaller radius r2 with respect to the center point MP.
  • anode-side target 32 form a circular ring section, which, besides the radii r1 and r2, can be variably defined by the lateral limitation and thus by the legs of the angle ⁇ drawn in broken lines.
  • the angle ⁇ is dimensioned at 360 °, an omnidirectional transmission radiator arrangement is produced analogously to FIG. 3, the corresponding
  • the angle of between 0 and 360 ° can be defined in the arrangement of cold cathode modules 25 and the anode 32 according to FIG. 5a, and the radii r1 or r2 in any case
  • the difference between the outer cold cathode radius r1 and the inner target radius r2 is the acceleration distance of the electrons e " and thus the one to be vacuumized, for example
  • r3 (with r3 chosen smaller than r1 and r2) is schematic with a wider radius indicated a layer with coolant KM.
  • 5b also shows a transmission radiator arrangement with a modular cold cathode 24.
  • the modular cold cathode 24 and the anode-side target 32 with coolant layer KM are parallel or essentially parallel arranged to each other.
  • emitter devices of several devices X-ray tubes, electron cartridges
  • four emitters each with an angle of 90 ° or only two emitters with a high radius of curvature r or a combination of the aforementioned emitter arrangements can be put together.
  • These radiator arrangements can be constructed either according to the diode configuration already described in FIG. 3, ie without an extraction grid, or according to FIG. 4 in a triode configuration, ie with an extraction grid.
  • the individual cold cathode modules 24 are put together without any space-increasing structures applied to their surface over their entire surface, this also results in an essentially full-surface emission surface of the assembled cold cathode modules 25.
  • the cold cathode modules 25 are assembled with any definable lateral, front and rear sections, as shown in FIG. 5a and 5b is indicated in cross section in the manner of pearl strings, network-like structures of the surface of the modular cold cathodes 25 arise, which can be defined as desired, the mesh structure depends on the shape of the individual cold cathodes 25 used or the cold cathode modules 24 that can be put together and their arrangement.
  • anode-side target is also possible analogous to the modular design of the cold cathode 24, but for cost reasons, as shown in FIGS. 3, 5a, 5b, the anode is formed in one piece, which is the case, for example, when used in X-ray tubes with a hollow cylindrical design of the cold cathode and anode or in the electron emitter with an essentially plane-parallel arrangement of the cold cathode and anode is easy to implement in terms of production technology.
  • 6a shows, analogous to FIG. 5a, an arrangement of a modular cold cathode 24 and anode, likewise in an arbitrarily definable partial circle segment. In contrast to FIG.
  • FIG. 6a a rear reflector arrangement is constructed in FIG. 6a, a material which is transparent to X-ray radiation ( ⁇ ) being used for the modular cold cathode 24 and the individual cold cathode modules 25 pointing to an inner circular ring with the radius r1 (towards the radiation chamber 90
  • ⁇ X-ray radiation
  • the anode 32 with the cooling layer KM with the radius r3 is arranged on an outer circular ring with the radius r2.
  • the radius r1 is smaller than the radius r2 and this in turn is dimensioned smaller than the radius r3.
  • the modular cold cathode 24 When using such an arrangement, for example in an X-ray tube, the modular cold cathode 24 emits electrons e ′′ at room temperature, which are accelerated in the vacuum-sealed interior 40 and hit the target 32, thereby again causing X-ray radiation ( ⁇ ) from the anode-side target 32 into the Irradiation room 90 is emitted.
  • the X-ray radiation ( ⁇ ) passes through the cathode material which is transparent to X-ray radiation ( ⁇ ) on the cold cathode side into the irradiation room 90, which in this arrangement is enclosed by the cold cathode 24.
  • Fig. 7 shows schematically an electron transmission radiator 12 with a modular cold cathode 24, in an arrangement analogous to Fig. 5b, for the use of the electron gun 12. In an electron gun, therefore, the anode-side material is permeable to electron beams, which is indicated in Fig. 7.
  • anode foil with a supporting grid is to be used in particular.
  • the supporting grid webs 33a are visible in Fig. 7.
  • the thickness of the anode film 33 is typically 3 - 200 ⁇ m.
  • the combination of the anode foil 33 with a support grid absorbs a portion of the incident electrons (e " ), in particular the support grid itself.
  • the above-mentioned proposed surface radiators and omnidirectional radiator arrangements or transmission and retroreflector arrangements as well as the conventional radiator arrangement in X-ray radiography can be constructed with a modular cold cathode and a correspondingly arranged anode. All of the methods mentioned above are suitable for applying the field-increasing structures to the surface of the cold cathode, which essentially represents the emission surface for the electrons.
  • the modular assembly of individual cold cathode elements and of radiator segments constructed from them is particularly suitable for the large-scale formation of flat and curved emission surfaces or irradiation surfaces.
  • radiator This makes it possible to build up any desired geometries of the irradiation room and to arrange a radiator around any geometry of an irradiation object; high-dose radiators can be arranged in a large area and in a definable manner in the surface or in the room.
  • the cold cathode in addition to a high dose rate, can be produced economically, in particular when the field-increasing structures are applied over the entire area, the cold cathode has in particular low thermal losses and requires no additional cooling due to its emission at room temperature, by using either for X-rays transparent cathode material or cathode material not transparent to X-rays is possible to form a retroreflector or a transmission radiator.
  • the anode is designed so that all incident electrons are absorbed and used to generate X-rays, and in the second case, the anode is made out specifies that the electrons essentially penetrate the anode and can be used directly for irradiation.

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  • Cold Cathode And The Manufacture (AREA)
  • X-Ray Techniques (AREA)

Abstract

L'invention concerne un tube à rayons X (11) qui comprend une cathode, qui émet des électrons (e-) vers une chambre intérieure (40) dans laquelle le vide a été fait, ainsi qu'une cible (31, 32), se présentant sous la forme d'une anode, pour générer des doses élevées de rayons X (η). La cathode est constituée d'au moins une cathode froide (21, 22, 23) à base d'une matière émettant des électrons (e-) et présentant une structure de surélévation de champ (70). En particulier, l'invention concerne un tube à rayons X (11) pourvu d'une cathode froide (21, 22, 23) qui comprend au moins une couche de support (201) soutenant la matière émettant des électrons (e-), la surface d'émission de ladite cathode froide (21, 22, 23) étant définie par la forme de la couche de support (201).
EP04741611A 2004-05-19 2004-05-19 Tube a rayons x pour emissions de doses elevees Withdrawn EP1747570A1 (fr)

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