EP1747570A1

EP1747570A1 - High-dose x-ray tube

Info

Publication number: EP1747570A1
Application number: EP04741611A
Authority: EP
Inventors: Kurt Holm; Lars-Ola Nilsson
Original assignee: Comet Holding AG
Current assignee: Comet Holding AG
Priority date: 2004-05-19
Filing date: 2004-05-19
Publication date: 2007-01-31
Also published as: WO2005117058A1; US20080267354A1; JP2007538359A

Abstract

The invention relates to an X-ray tube (11) which comprises a cathode emitting electrons (e-) into an interior chamber (40) that is subjected to a vacuum, and a target (31, 32), configured as an anode, for generating high-dose X-radiation (η). The cathode comprises at least one cold cathode (21, 22, 23) based on an electron (e-) emitting material having a field-enhancing structure (70). The invention especially relates to an X-ray tube (11) having a cold cathode (21, 22, 23) that comprises at least one support layer (201) for supporting the electron (e-) emitting material. The emission area of the cold cathode (21, 22, 23) is defined by the shape of the support layer (201).

Description

X-ray tube for high dose rates

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,

10 packaging material for food and medicine, food, e.g. Use vegetables, etc. This is advantageously done with X-rays or electron beams, since isotopes as radiation sources are dangerous and difficult to handle, and because alternative, e.g. chemical sterilization processes, either uneconomical or unusable for legal reasons

15 are. Industrial applications also include drying ink and polymer crosslinking with electrons (e ^" ) in an energy range of 80-300keV. In all applications, 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.

20 Basically, 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

25 10% used for radiation. This results in a very low efficiency when converting electrical power into dose rate. When using electron cartridges, however, it can be assumed that at least 50% of the electron energy can also be used for sterilization. About 50% of the energy is lost in the exit window. X-ray tubes and

30 Elektroπenkanoneπ also differ in their application. Electrons have a low penetration depth and are therefore only suitable for the sterilization of surfaces. With X-rays, on the other hand, materials can also be Inside are sterilized, such as blood plasma, while accepting the poorer efficiency. For irradiation with X-rays, 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. 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.

So far, 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. Although the heated sources can be manufactured relatively reliably, robustly and inexpensively, they suffer from some weaknesses.

Although 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. In addition, 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.

In many applications of X-ray radiation sources, for example for sterilization, 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.

The functioning of cold cathodes will be described in more detail below. 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. With thermionic electron emission from the filament of the cathode, the electrons (e ^" ) sufficient energy to overcome the potential barrier against the vacuum. The current density J of thermal emission that can be achieved is J = aT ² exρ (-0 / kT)

according to the so-called Richardson's formula; where a is the Richardson constant, T the temperature and K the Bolzmann constant.

From Richardson's formula it follows that lowering the work function 0 favors the thermal emission and therefore attempts are made to work with other emitter materials such as tantalum, BaO, thorium etc. Due to the deeper work function 0 and lower temperatures consequently, lower evaporation rates and longer service life of the hot cathodes are used. Nevertheless, due to the high temperatures T that are required (> 1000 ° Kelvin), the thermal emission has a high heating power requirement and thus high energy consumption. This is easily possible in conventional applications of the X-ray tube with a small filament. The tension generators can deliver the required power with currents of 5A at 8V applied voltage via the filament. As soon as more than three filaments are connected in series, an upper power limit of the generators is currently reached. In contrast to thermal emission, the cold emission

Potential barrier deformed by an external electric field F, and takes on a triangular shape of height 0 with the thickness x = 0 / eFι, e is the charge of the electron (e ^" ) and Fι is the local electric Field at the emission location If the barrier becomes sufficiently thin, ie if 0 / eFι ≤ 2 nm, the electrons (e ^" ) can tunnel through the barrier and get into a vacuum; this is called cold emission or field emission. Very large field strengths F in the order of magnitude of 2 to 4000 V / μm are required locally at the emission location in order to cause an electron emission. The flow of a cold-emitter can be näheruπgsweise with a simplified formula of Fowler and Nordheim in 1928 expressed as: l = (1.5e-6 A (F |) ^2/0) exp (1O.4 / sqrt (0)) exp ( -6.44e7 0 ^{1 5} / F,)

A is a pre-factor to adapt experimentally determined currents and sqrt 0 - is the square root of the work function 0. The typically very steep curve of the characteristic of the current

Field strength relationship according to the formula of Fowler and Nordheim is illustrated in Fig. 2a. The locally excessive field strength F | at the emission site is then several thousand volts per micrometer. Such high electric fields F are achieved by increasing the geometric field ß. If an electrically conductive object with a high length-to-width ratio ß is brought into an electrical field F, an electrical field increase in the object will occur due to the geometrically induced charge shift in the object Peak take place. If this object has the height h and the radius of curvature r, the result is approximately ß = h / r. In a first approximation, the greatly exaggerated electrical field F can then be expressed by the following equation F, = ßF where F is the external electrical field. If, for example, the field-increasing structures have dimensions of h = 1000 nm and r = 1 nm (this is possible, for example, when using carbon nanotubes as field-increasing structures), an electrical field increase F and thus cold emission of electrons at an applied voltage are obtained causes the field F applied from the outside, typically the field F applied from the outside being a few volts per micrometer and the electrical field increase F a few volts per nanometer. The voltage required for this is technically feasible. In order to achieve a sufficiently high current density in a cold cathode, a high density of field-increasing structures must be brought into the electrical field. Until almost 30 years ago, this was hardly possible. In the past few decades, however, various microstructure processes have been developed, with which a density of up to 10 ⁸ emitting microtips / cm ² can be achieved. Such 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. Today, with a few exceptions, it is generally accepted scientifically that these carbon films can efficiently donate electrons, not because the work function 0 is low, but because they too have structures that exceed the field. These structures can either be on the surface or within in a matrix surrounded by isolating sp ₃ phases. Sp ₃ is the strong covalent bond in an electrically insulating diamond. For example, thin carbon films grown in gas phase can have micrometer-sized, graphite-like sp ₂ phases at the grain boundaries between insulating diamond-like sp ₃ carbon. Since the applied electric field can penetrate this matrix, the graphitic sρ ₂ phases act as field-increasing structures. Carbon nanotubes are suitable for the use of field-increasing structures, as described, for example, in US Pat. No. 5726524 B1, but other types of carbon are also commercially attractive, such as coral-like

Carbon, which also has sharp structures on the surface, as described, for example, in the patents US 6087765 B1 and US 6593683 B1. In order to produce the carbon structures, the usual methods of the prior art are conceivable. For example, when separating from the gas phase (Chemical Vapor Deposition - CVD), a carbon-containing gas mixture (e.g. methane, acetylene etc.) in an evacuated reactor (vacuum recipient), often with H ₂ (hydrogen), N ₂ (nitrogen) etc. admixed, introduced. Then either a microwave plasma is ignited or the substrate is brought to 600 ° to 900 ° Celsius. In both cases, depending on the deposition parameters, different carbon structures grow on the substrate. Catalytic growth is also often used. 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. In the method, in English

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. An important reason why the use of carbon nanotubes and other specifically structured carbon and its modifications as a cold emitter is particularly attractive according to the invention is the potential for cheap, large-area cathode disposition. But there are other reasons why the use of carbon is interesting. Due to the strong covalent bonds in carbon, cold emitters made of carbon are less sensitive to destruction than, for example, vapor-deposited molybdenum tips or etched silicon tips. The atoms do not migrate in the high-voltage field and have less tendency to explode, such as metal tips. It can be said that at the moment the production processes for carbon nanotubes as electron emitters are not yet 100% mature. The adhesion of the tubes, for example in the case of catalytically grown 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

Long-term stability of the tubes unsatisfactory and efforts are constantly being made to improve the adhesion. Another problem with the use of carbon-based cold emitters relates to the limited emission current density of a large number of parallel-emitting carbon structures on a flat surface. Actually, there are typically more than 10 ° potential emitters per cm ^{2 of} a typical nanotube thin film layer on average. A well contacted nanotube should be able to easily transport a current of up to 10 μA (theoretically even into the mA range). So that makes current densities of 10 ³ A / cm ² or more. Nevertheless, the experimental values show that current densities of 1 to 100 mA / cm ² and emitter densities of 10 ⁴ to 10 ⁵ emitters per cm ^{2 are achieved} for electrical field strengths F of around 5-10V / μm (for higher field strengths F disadvantageously begin between Anode-cathode electrical discharges). There are two main explanations for this. On the one hand, a very high density of structures is disadvantageous for the field increase. With a very short emitter-emitter distance, electrostatic shielding occurs, which leads to a lowering of the geometrically excessive field F 1. On the other hand, a typical cold emitter film with carbon

Nanotubes, a stochastic distribution of field-increasing structures. In all experimentally examined cases, this leads to a spatially stochastic distribution β (x, y) of the field-increasing structures on a cold cathode surface. Accordingly, a statistical ß distribution can be defined as follows f (ß) = dn / dß where dn is the number of field-increasing structures per area in a small interval ß to (ß + dß). f (ß) is a measure of quality, or the efficiency of a cold cathode and gives a quantitative description of the emitter and current density

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 Fι 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.

In principle, three approaches are known in the prior art to improve the current and emitter density. First of all, controlled growth attempts to control the emitter-emitter distance or the emitter geometry (height-radius of curvature ratio). This approach is known as the so-called "self-organization" of the field-increasing structures. In this way, the electrostatic shielding that occurs between the emitters can be largely switched off or reduced. The geometrically excessive electrical field Fι thus increases. Secondly, attempts are made through controlled growth, f ( The exponential behavior of f (ß) of a typical carbon cold cathode seems to be intrinsic, but by increasing the slope of the straight line of f (ß) a lot of emitters are brought in a high ß-range. Thus, these emitters will also contribute to increasing the current density. As a third solution, the use of ballast resistors is 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 more the current deviates from the F-N characteristic.

This effect is used to suppress the current of the most efficient emitters. That sounds paradoxical, but it prevents current-induced emitter destruction of the strongest emitters and the external electric field F can thus be increased. Emitters with a lower ß can thus also contribute to the current density in the electrical field F 1 thereby increased, see equation 5. Since these occur in very large quantities due to the exponential behavior of f (ß), the total current density of the cathode increases.

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. In particular, an X-ray emitter is to be proposed which enables a dose rate that is several times higher than conventional X-ray emitters. Likewise, 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. Furthermore, the proposed device should also enable inexpensive radiation, in particular for the sterilization of various objects, and the drying of ink and polymer crosslinking, in particular on an industrial scale. According to the present invention, these goals are achieved in particular by the elements of the independent claims. Further advantageous embodiments also emerge from the dependent claims and from the description. In particular, these objectives are achieved by the invention in that 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. In contrast to 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. 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. X-ray tubes with transparent for X-rays

Cathodes and / or anodes are therefore difficult or impossible to manufacture in the prior art. In one embodiment variant, 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. One advantage of this variant is that almost any geometric arrangement can be realized. In another embodiment variant, 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. One advantage of this embodiment variant is that the geometry of the radiation unit can be easily adapted to the requirements of the radiation method.

In a further embodiment variant, the ratio of the area of the cold cathode to the layer depth is large. One advantage of this variant is that the cathode is suitable for large-area irradiation devices.

In yet another embodiment variant, 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. One advantage of this embodiment variant is that the material to be irradiated can be irradiated from all sides at the same time. In an embodiment variant, 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. In another embodiment variant, the first comprises

Carrier layer of the cold cathode at least one substrate with ceramic material or glass. One advantage of this design variant is that the carrier material is inexpensive, malleable and suitable for vacuum. In addition, the attenuation of X-rays by these materials is relatively low. In one embodiment variant, the carrier layer comprises at least one resistance layer and / or interconnect layer. One advantage of this variant is that 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.

In a further embodiment variant, the conductor track layer comprises an evaporated copper layer. One advantage of this variant is that the copper has good electrical and heat-dissipating properties. Other metals can also be used to advantage. In one embodiment, 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). In another embodiment variant, the x-ray tube is as

Anode high cylinder formed with a coaxial cathode high cylinder outside the anode. 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).

In another embodiment variant, 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). In a further embodiment, 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). In another embodiment, 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.

In one embodiment variant, 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. In one embodiment, 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). In one embodiment, 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. In another embodiment variant, the anode has at least one coolant layer (KM), the coolant layer (KM) comprising a liquid coolant (KM) and / or a gaseous coolant (KM). One advantage of this embodiment variant is that the anode can withstand a higher specific electron intensity. A higher dose rate can thus be achieved. At this point it should be noted that in addition to the X-ray tube according to the invention, 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.

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. Thus, for example, a uniform 4τr 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.

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

Cold cathodes as electron sources in a partial circle segment. In order to achieve a 4ττ gamma radiation, see also FIG. 3, 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.βa shows the cross section of a reflector arrangement in variable electrode geometry with modular cold cathodes as Elektroπen 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

Cold cathode, in an arrangement analogous to Fig. 5b. Figure 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. This means that when the electrons e ^' strike the target, x-ray radiation y is generated in the focal spot that is created. The x-ray radiation y passes through a window 301 into the outside space and is used for radiation purposes. Only a small part of the radiation generated on the target 200 reaches it For geometrical reasons, most of the radiation is absorbed in the tube itself, so that depending on the size of the object, a certain irradiation distance must be selected in order to completely irradiate the object. In conventional arrangements, typically only about 10% of the radiation in the half-space of the target surface can be used ur 1 shows a radiation window 301 with an opening of 50 °. 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. The voltage difference between gate 80 and emitters 70a is typically less than 100 volts. For applications in flat screens, for example, groups (pixels) of ten to five several hundred such micro tips 70a must be able to be controlled in parallel. This is not absolutely necessary for X-ray tubes. 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 (γ). The x-ray radiation (γ) 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. Using a carrier material for the cold cathode 21 that is not transparent to X-ray radiation (γ), a transmission radiator with a diode configuration of the electrodes 21, 31 is formed in the illustration shown in FIG. 3. Thus, 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. While the field-enhancing structures, in particular the carbon nanotube 71 in a matrix 71 a are embedded on the cold cathode surface, see enlargement A, 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

X-ray tube room and encloses both the vacuumized interior 40 and the radiation room 90 in the manner of a double wall. In further embodiments, not shown, 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. 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. As material for the anode 31 is known to use a metal with a high atomic number, for example tungsten. In the embodiment of the cathode surfaces described in FIG. 3, the carbon nanotubes 71 or other used field-exceeding

10 structures 70 exposed to ion bombardment. Residual gases (even in low concentrations) can be ionized in the electron beam. You can thus receive energies corresponding to the fully applied cathode / anode voltage (not shown) when it hits the cold cathode 21. However, due to the strong atomic bond of the carbon nanotubes

15 71 these endure the ion bombardment to a certain extent. The construction of an x-ray tube 11 with an omnidirectional transmission arrangement according to FIG. 3 of cold cathode 21 and anode 31 is of particular interest for cost reasons, because the radiator arrangement can be produced simply and inexpensively without an extraction grid or gate.

20 In particular, application of the entire area of the structure 71 which overlaps the field to a ceramic substrate is easily possible in terms of production technology.

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. On a carrier material 201, e.g. a cost

25 favorable ceramic substrate, 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. Between the conductor track layer 202 and the field-increasing structure 71, a resistance layer 203 is arranged in series with the field-increasing structures 71.

30 brought. 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

35 properties are long-term stable. As already mentioned, the emitter destruction due to the lack of liability was a further problem of the emitter recognized on the cathode surface. Under certain circumstances, the emitter can be destroyed more by current and field-induced destruction than by ion bombardment or electrical discharges. However, the lack of adhesion of the emitter can have an adverse effect on the long-term stability of the emitter performance. As a result, 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. In order to ensure a temporally variable extraction voltage, 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. In the embodiment shown in FIG. 4, 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. As gate 80 (also called extraction grid), for example, a metal can be evaporated onto the end face of insulators 60 facing away from the cathode surface. Furthermore, a metal grid with variable hole spacing, indicated by arrow c in the cross section of FIG. 4 and with a variable web width, indicated by arrow b in the cross section of FIG. 4, can also be used as gate 60. 4, 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, and arrow a defines the distance between two insulators 60. Through the above The specified values of the design (arrows a to d) determine the power losses and the extraction voltage. The larger the shielding area of the gate 80 against the cathode 23, the area for the placeholders / insulators 60 being eliminated, the greater the losses at the gate 80. In an optimized design, 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). In a first design of the cold cathode 23 according to FIG. 4, 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. For large values of the distance from the surface of the cathode 23 to the gate 80 (arrow d), a typical extraction voltage of several thousand volts must be used. This significantly increases the power losses at gate 80. At a distance of the gate 80 from the surface of the cathodes 23, which is, for example, a few dozen μm, electrical extraction voltage up to a few hundred volts is generally sufficient, but the risk of a short circuit of a cathode not defined by lithography is relatively high. Thus, in the design of the cold cathode 23, a compromise between the distances mentioned, indicated by the arrows a, b, c and d, must be made. It is therefore of further advantage to produce the cathode 23 using a lithographic process, using defined gate, insulator and emitter areas in the micrometer range.

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. As schematically shown, 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. Alternatively, the cold cathode modules 25 can be equipped according to the variant in FIG. 4. The electrons (e ^" ) meet

10 accelerates onto an anode-side target 32. As is known, this will cause x-ray radiation (γ) 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. The modular cold cathode 24 and that

15 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 α drawn in broken lines. When the angle α is dimensioned at 360 °, an omnidirectional transmission radiator arrangement is produced analogously to FIG. 3, the corresponding

20 chende number of cold cathode modules 25 on the outer ring, without spacing, is to be arranged. In principle, not only can the emission surface of the cold cathodes 24 be assembled modularly, but also several transmission radiator arrangements as shown in FIG. 5a, for example four partial circle segment arrangements with an angle of α =

25 90 ° with the same outer cold cathode radius r1 and inner aπode radius r2 to be assembled into a circular transmission radiator arrangement analogous to FIG. 3. In principle, 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

30 larger than zero μm, whereby in a use of this arrangement, for example analogously to FIG. 3 in an X-ray tube with a transmission emitter arrangement, 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

35 interior 40 and the radius r2 determines the irradiation area. On the surface of the anode-side target 32 towards the irradiation space 90, r3 (with r3 chosen smaller than r1 and r2) is schematic with a wider radius indicated a layer with coolant KM. As already mentioned, when constructing a transmission radiator with cold cathodes, it is advantageous that cooling is only necessary on the anode side. 5b also shows a transmission radiator arrangement with a modular cold cathode 24. When the radii r1, r2 and r3 are dimensioned towards infinity and the angle cc towards 0 °, which is a special case of the dimensioning according to FIG. 5a, the modular cold cathode 24 and the anode-side target 32 with coolant layer KM are parallel or essentially parallel arranged to each other. With this arrangement, emitter devices of several devices (X-ray tubes, electron cartridges) can be implemented in combination. For example, 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. If 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. When 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. In principle, a modular construction of the 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. 5a, however, a rear reflector arrangement is constructed in FIG. 6a, a material which is transparent to X-ray radiation (γ) 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 For example, as shown in an X-ray tube according to FIG. 3) and the anode 32 with the cooling layer KM with the radius r3 is arranged on an outer circular ring with the radius r2. Thus the radius r1 is smaller than the radius r2 and this in turn is dimensioned smaller than the radius r3. 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 (γ) from the anode-side target 32 into the Irradiation room 90 is emitted. The X-ray radiation (γ) passes through the cathode material which is transparent to X-ray radiation (γ) 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. When using an electron transmission lamp arrangement with a modular design auter

Cold cathode 24 on the side of the anode 33, either by air convection, water or other special cooling, removes the lost heat. In order to make the anode 33 transparent to electrons (e ^" ), a thin anode foil with a supporting grid is to be used in particular. The supporting grid webs 33a are visible in Fig. 7. At energies in the range of 80-300 kV, the thickness of the anode film 33 is typically 3 - 200 μm. 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. Provided that the film is sufficiently thin as described above and the ratio of the web width increases Grid opening of the support grid sufficiently small is, the power loss is relatively small in the transmission window and averages less than 30% of the incident power.

In principle, 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. 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. At this point it should be noted that there can be four basic arrangements in the structure of the radiator: 1. Cathode inside, anode outside, radiation inside (reflector)

2. Cathode outside, anode inside, radiation inside (transmission radiator)

3. cathode inside, anode outside, radiation outside transmission radiator)

4. Cathode outside, anode inside, radiation outside (reflector).

While all arrangements for X-ray emitters are possible, only arrangements as transmission emitters are possible for electron guns, in which a transparent anode always allows the passage of the electrons from the vacuum space. The advantages of the invention are summarized below: in addition to a high dose rate, the cold cathode 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 combination of the use of field-overlapping structures for a cold cathode and a defined cold cathode geometry by means of structures overlapping the field in a layer structure on a carrier layer, the defined formation of further functional layers and in particular the defined geometry of the contact areas between the carrier layer and (e ^" ) -emitting layer enable the structure in particular a large-area or modular, assembled cold cathode and, with the appropriate design of the anode, a large irradiation area of freely definable shape. Partial irradiation on the object is also possible, for example through the defined arrangement of individual cold cathode modules. The advantages listed above apply to both an X-ray source and an electron gun. In the first case, 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.

Claims

claims

1. X-ray tube (11) with a cathode, which emits electrons (e-) into a vacuumized interior (40), and a target (31, 32) designed as an anode for generating X-ray radiation (y) high dose, characterized in that the cathode comprises at least one cold cathode (21, 22, 23) based on an electron (e -) emitting material with a field-increasing structure (70).

2. X-ray tube (11) according to claim 1, characterized in that the cold cathode (21, 22, 23) at least one carrier layer (201) for

Holding the electron (e -) - emitting material comprises, wherein the emission surface of the cold cathode (21, 22, 23) is essentially defined by the shape of the carrier layer (201).

3. X-ray tube (11) according to one of claims 1 or 2, characterized in that the geometry and spatial arrangement of the cold cathode (21, 22,

23) and / or the emission surface of the cold cathode (21, 22, 23) is determined by the shape of the carrier layer.

4. X-ray tube (11) according to one of claims 1 to 3, characterized in that the ratio of the area of the cold cathode (21, 22, 23) to the layer depth is large.

5. X-ray tube (11) according to any one of claims 1 to 4, characterized in that the shape and size of the radiation chamber (), by the surface expansion and / or spatial arrangement of the cold cathode (21, 22, 23) and / or the anode (31, 32) 90) of the X-ray tube (11) is determined.

6. X-ray tube (11) according to one of claims 1 to 5, characterized in that the field-increasing structures (70) comprise carbon nanotubes (71).

7. X-ray tube (11) according to claim 6, characterized in that the field-increasing structures (70) comprise coral-like carbon.

8. X-ray tube (11) according to one of claims 1 to 5, characterized in that the field-enhancing structures (70) have metal tips

(70a).

9. X-ray tube (11) according to one of claims 1 to 5, characterized in that the field-increasing structures (70) comprise silicon tips.

10. X-ray tube (11) according to one of claims 1 to 5, characterized in that the field-increasing structures (70) comprise diamond tips and / or diamond powder and / or diamond-like carbon matrices of sp ₂ and sp ₃ bound carbon.

11. X-ray tube (11) according to one of claims 1 to 10, characterized in that the carrier layer is a matrix with embedded

Includes carbon nanotubes and / or coral-like carbon.

12. X-ray tube (11) according to one of claims 1 to 11, characterized in that the first carrier layer (201) of the cold cathode (21, 22, 23) comprises at least one substrate with ceramic material.

13. X-ray tube (11) according to one of claims 1 to 12, characterized in that the carrier layer (201) comprises at least one resistance layer (203) and / or interconnect layer (202).

14. X-ray tube (11) according to claim 13, characterized in that the conductor track layer (202) comprises a vapor-deposited copper layer.

15. X-ray tube (11) according to one of claims 13 to 14, characterized in that at least one electron (e -) - emitting layer of the carrier layer and at least one resistance layer (203) are connected in series.

16. X-ray tube (11) according to one of claims 1 to 15, characterized in that the cross-sectional geometry of the cold cathode (21, 22, 23) and / or anode as a full circle, segment of a circle, annulus, triangle, square, polygon or any definable Polygon is formed.

17. ^* X-ray tube (11) according to one of claims 1 to 16, characterized in that 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.

18. X-ray tube (11) according to one of claims 1 to 17, characterized in that the cold cathode (21, 22, 23, 24) for X-ray radiation (y) is transparent or substantially transparent.

19. X-ray tube (11) according to one of claims 1 to 18, characterized in that the cold cathode (21, 22, 23, 24) closes the vacuum-sealed interior (40) or the radiation chamber (90) to the outside.

20. X-ray tube (11) according to one of claims 1 to 19, characterized in that at least one extraction grid (80) is arranged between the cold cathode (23) and the anode (31, 32).

21. X-ray tube (11) according to claim 19, characterized in that an electrical insulator (60) is arranged between the cold cathode (23) and the extraction grid (80).

22. X-ray tube (11) according to one of claims 1 to 21, characterized in that the anode (31, 32) has at least one coolant layer (KM), the coolant layer (KM) being a liquid coolant (KM), and / or includes a gaseous coolant (KM).

23. A method for sterilizing and / or irradiating food and / or medication, and / or blood plasma and / or packaging materials and / or instruments and / or killing bacteria, beetles, vermin, characterized in that an X-ray tube (11) according to one of claims 1 to 22 is used.

24. Electron gun with an electron beam arrangement which has an electron (e -) emitting cold cathode (21, 22, 23, 24) and an anode (33), a high-dose electron beam being generated, characterized in that the cold cathode is the characteristic one Features at least one of claims 1 to 22.

25. Electron gun according to claim 24, characterized in that the anode (33) is designed to be permeable to the electrons (e-).

26. Electron gun according to one of claims 24 or 25, characterized in that the anode (33) comprises a very thin film with a thickness between 6 to 200 microns with a support grid.

27. Electron cartridge according to one of claims 24 to 26, characterized in that the anode (33) is cooled by air convection, by thermal conduction and / or by a liquid cooling medium.

28. Process for the irradiation and / or drying of ink or polymer crossling of plastics, characterized in that a

Electron gun according to one of claims 24 to 27 is used.