Patent Application
20080267354
This invention relates to an X-ray tube and an electron beam gun for high dose rates, with an electron (e−)-emitting cathode, in particular for large-surface irradiation of objects having diverse geometry, and for use of the X-ray tube for sterilization, as well as to the use of the electron beam gun for sterilization, for drying ink or respectively polymer crosslinking.
In the sterilization of blood plasma, medical instruments, packaging materials for medicine and food, such as e.g. vegetables, etc., using roentgen irradiation and electron irradiation is increasingly common. This takes place preferably using X-rays or electron beams since isotopes as radiation sources are dangerous and difficult to handle, and since alternative, e.g. chemical, sterilization methods are either not cost-efficient, or they may not be used for reasons of law. Industrial applications also include the drying of ink and polymer crosslinking with electrons (e−) in an energy range of 80-300 keV. In all applications, one aims for as high a dose rate as possible. The irradiation time can thereby be significantly shortened, which means a shorter throughput time and, with that, a reduction in costs.
The achievable dose rate differs fundamentally between X-ray emitters and electron beam guns. In the range up to 1 MV acceleration voltage, only 1% of the electron energy is converted into X-radiation for generation of X-rays. Of this, in turn, in standard X-ray tubes, less than 10% is used for irradiation, for reasons of geometry. There results from this a very small efficiency factor in the conversion of the electrical power into dose rate. When using electron beam guns, on the other hand, it can be assumed that at least 50% of the electron energy can also be used for sterilization. About 50% of the energy gets lost in the exit window. X-ray tubes and electron beam guns also differ in their application. Electrons have a low depth of penetration, and are thus suitable for sterilization of surfaces only. Using X-rays, however, materials can also be sterilized on the inside, but one must accept a poorer degree of efficiency.
For irradiation using X-rays, the dose rate per surface is determined by the distance of the object from the focal point of the radiation source and by the quantity of radiation generated at the focal point. This radiation amount is limited, for its part, by the thermal energy which must be dissipated or transported away through cooling of the focal point so that the material in the focal point does not melt. The specific dose rate of a conventional X-ray emitter is greatly limited by these two factors. To achieve a high dose rate, therefore, the object to be irradiated must come as close as possible to the radiation source. It can be necessary furthermore for the focal point of the emitter to be as large as possible so that the specific exposure at the focal point does not cause the target to melt.
With electron emitters, the object must also come as close as possible to the radiation source, since otherwise the electrons lose too much energy on the stretch of path through the air. With an optimized design of the exit window of the electron emitter, a relatively minimal portion of the radiating power gets lost in the anode (target), and thus a significantly higher dose rate is achieved with electron irradiation than with X-ray irradiation.
Thermionic electron sources have usually been used up to now as radiation sources. The thermionic electron source may be heated either directly or indirectly, and, with sufficient thermionic temperature, emit electrons (e−) directly into the vacuum of the emitter. Although the heated sources can be produced in a relatively reliable, robust and economical way, they suffer from some weaknesses.
Even though, as a rule, the heat output of the cathode amounts to only about 1-5% of the emitter power, measures for cooling in the cathode region are nevertheless to be taken with high current electron sources. Moreover the generator has to provide the heating power at high potential, which means high cost and a vulnerability to malfunctions. Since the thermionic electron sources have a high current density, they cannot be arranged surface-wise, but instead more point-wise. It is thus more difficult to irradiate evenly complex geometries as well. Thermionic electron sources are operated at high temperatures, at which the emitting material volatilizes already. Thus the service life of such sources is limited. Owing to current supply lines and possibly cooling, it is difficult to construct thermionic electron sources in such a way that they are transparent for X-rays. The geometric possibilities in irradiation are thus further limited.
With many applications of X-ray emitting sources, for example for sterilization purposes, a radiation source is required that achieves a high dose rate, and also makes possible an adaptation of the form of the radiation source to the shape of the respective object to be irradiated, and at the same time makes possible in particular irradiation of large quantities of these objects to be irradiated. Decisive for the cost-effectiveness of the irradiation method according to the invention is the integration of a cold cathode in the inventive irradiation device, for example in an X-ray tube or an electron beam gun.
The mode of operation of cold cathodes is to be described more closely in the following. Electrons (e−) are bound inside a solid body through a potential barrier. The potential barrier, also called work function ø, typically lies at 4.5 eV (electron volt) for conventional spiral-wound filaments of tungsten. With thermionic electron emission from the spiral-wound filament of the cathode, the electrons (e−) receive sufficient energy to overcome the potential barrier toward the vacuum. The thus achievable current density J of the thermoemission is
J=aT
2exp(−ø/kT)
according to the so-called Richardson equation; a is thereby the Richardson constant, T the temperature and K the Bolzmann constant.
It follows from the Richardson equation that a lowering of the work function ø favors the thermoemission, and one tries therefore to work with other emitter materials such as tantalum, BaO, thorium, etc. Through the lower work function ø, it is possible to work at lower temperatures and consequently with lower volatilization rates and longer service life for the hot cathodes. Nevertheless the thermoemission has a high heating capacity requirement, owing to the high temperatures T which are needed (>1000° Kelvin), and thus high energy consumption. In conventional applications of the X-ray tube with a small filament, this is no problem. The voltage generators can additionally supply via the filament the required power with currents of 5 A at 8V impressed voltage. An upper capacity limit for the generators is reached at the present time as soon as more than three filaments are connected in series.
In contrast to thermoemission, with cold emission the potential barrier is deformed by an externally applied electrical field F, and assumes in first approximation a triangular form of height ø with thickness x=ø/eFl, e being thereby the charge of the electron (e−) and Fl being the local electrical field at the place of emission. If the barrier becomes sufficiently thin, i.e. when ø/eFl≦2 nm, the electrons (e−) are able to tunnel through the barrier and reach the vacuum; this is called cold emission or also field emission. To bring about an electron emission, very high field strengths Fl on the order of magnitude of 2-4000 V/μm are needed locally at the place of emission.
The current of a cold emitter may be expressed approximately using a simplified formula by Fowler and Nordheim 1928:
I=(1.5e−6 A(Fl)2/ø)exp(10.4/sqrt(ø))exp(−6.44e7ø1.5/Fl)
A is thereby a prefactor to adapt experimentally determined currents, and sqrt ø—is the square root of the work function ø.
The typically very steep course of the characteristic curve of the current-field strength relation according to the formula of Fowler and Nordheim is illustrated in
Fl=βF
whereby F is the externally applied electrical field. If, for example, the field-enhancing structures have dimensions of h=1000 nm and r=1 nm (this is possible e.g. with use of carbon nanotubes as field-enhancing structures, according to the invention), an electrical field enhancement is achieved, and thus cold emission of electrons with an impressed voltage, which voltage causes the externally applied field F, whereby the externally applied field F typically amounts to a few volts per micrometer, and the electrical field enhancement Fl amounts to a few volts per nanometer. The voltage necessary therefor is absolutely achievable technically.
To achieve a sufficiently high current density with a cold cathode, a high density of field-enhancing structures must be brought into the electrical field. Until just 30 years ago, this was hardly possible. In the last few decades, however, various microstructure methods have been developed, by means of which a density of up to 108 emitting microtips/cm2 can be achieved. Such a lithographically structured cathode is schematically represented in
The method of producing microtips with micrometer precision is elaborate and expensive. For this reason, the research results in the mid 1990s on cold emission of thin carbon films at extremely low applied electrical field intensities gave cause for a lot of excitement. At first it was assumed that responsible therefor were exceptionally low work functions ø of about 0.1 to a few eV. Today, with few exceptions, it is generally scientifically accepted that these carbon films are able to emit electrons efficiently, not because the work function ø is low, but because they also have field-enhancing structures. These structures may be located either on the surface or inside, in a matrix surrounded by insulating sp3 phases. Designated as sp3 is the strong covalent bonding in an electrically insulating diamond. For example, thin carbon films, grown in gas phase, can have micrometer-size, graphite-like sp2 phases at the grain boundaries between insulating diamond-like sp3 carbon. Since the electrical field applied can penetrate into this matrix, the graphite sp2 phases act as field-enhancing structures. Carbon nanotubes are suitable for use as field-enhancing structures, such as described in the U.S. Pat. No. 5,726,524 B1, for example, but also other carbon types are commercially attractive, such as, for instance, coral-like carbon, which likewise has sharp structures on the surface, such as described in the U.S. Pat. No. 6,087,765 B1 and U.S. Pat. No. 6,593,683 B1.
The usual methods of the state of the art are conceivable for producing the carbon structures. For example, with precipitation out of the gas phase (Chemical Vapor Deposition—CVD), a carbonaceous gas mixture (for example, methane, acetylene, etc.) can be conducted into an evacuated reactor (vacuum recipient), often with H2 (hydrogen), N2 (nitrogen), etc., added. After that, either a microwave plasma is ignited, or the substrate heated to 600° to 900° Celsius. In both cases, dependent upon deposition parameters, different carbon structures grow on the substrate. Often catalytic growth is also used. A transition metal (nickel, cobalt, iron, etc) is thereby put on the substrate in the form of small clusters, i.e. only a few nanometers to micrometers in size. Carbon nanotubes are able to grow on these clusters. In the method referred to as “cathodic arc” in English, an arc discharge is ignited between two graphite electrodes in a helium atmosphere at current strength I of about ca. 80 A. After the discharge, found in the carbon black are nanotubes, which can be used after a cleaning procedure. The so-called laser ablation method can also be used, for example. Using laser, shots are thereby fired at a graphite target. Nanotubes are likewise found in the carbon black. By adding transition metals in the graphite target, single-walled nanotubes can be created. There exists a range of other production methods, or variants of the above-mentioned. It generally applies that one has a limited influence upon rates of defect in tubes, geometry of the tubes, rejects, etc. This has to do with the growth mechanisms being in fact little understood so far.
An important reason why, according to the invention, the use of carbon nanotubes in particular and other specifically structured carbon and its modifications as cold emitters is attractive, is the potential for cost-effective, large-area configuration of cathodes. But there are also other reasons why the use of carbon is interesting. Owing to the strong covalent bonds in carbon, cold emitters made of carbon are less prone to destruction than, for example, vapor-deposited molybdenum tips or etched silicon tips. The atoms in the high voltage field do not migrate, and have less the tendency to explode, as do metal tips, for instance.
It can be said that at the moment the methods of production of carbon nanotubes as electron emitters are not yet fully developed. Often the bonding of the tubes, for example with catalytically grown tubes, is very poor, and in the electrical field these tubes can be pulled off in the direction of the anode, owing to their charge (field-induced emitter destruction). The carbon nanotubes can thus, on the one hand, ignite electrical discharges, and, on the other hand, the emission performance becomes poorer over time. In fact, long-term stability of the tubes is unsatisfactory at the moment, and work is continuously being done to improve the bonding.
Another problem with 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 plane. Actually, there typically are more than 108 potential emitters per cm2 on the average in a typical nanotube thin film layer. A well contacted nanotube should be able to transport without any difficulty a current up to 10 μA (theoretically even up into the mA range). That makes for current densities of 103 A/cm2 or more. Nevertheless the experimental values show that current densities of 1 to 100 mA/cm2 and emitter densities of 104 to 105 emitters per cm2, are achieved for electrical field strengths F of about 5-10V/μm (for higher field strengths, electrical discharges begin between anode-cathode in a disadvantageous way).
There are basically two explanations for this. On the one hand, a very high density of structures is disadvantageous for the field enhancement. With very short emitter-emitter spacing, an electrostatic shielding occurs, leading to a lowering of the geometrically enhanced field Fl.
On the other hand, a typical cold emitter film with carbon nanotubes has a stochastic distribution of field-enhancing structures. This leads in all experimentally studied cases to a spatially stochastic distribution β(x,y) of the field-enhancing structures on a cold cathode surface. Thus a statistical β-distribution may be defined as follows
f(β)=dn/dβ
dn being the number of field-enhancing structures per surface area in a small interval β to (β+dβ). f(β) is a measure of the performance or the efficiency of a cold cathode, and gives a quantitative description of the emitter-density and currency density via
emitter density(F)=∫f(β)dβ[cm−2]
current density(F)=∫f(β)I(β,F)dβ[Acm−2]
I(β,F) is the current of a single emitter as a function of the externally applied electrical field F, and the geometric field enhancement. It has been shown that f(β) for a typical cold cathode with carbon nanotubes has an exponential function of β, f(β)˜exp(kβ).
Owing to the relatively minimal number of efficiently field-enhancing structures in a higher β-range (>400), only a fraction of about 0.01% of all potential emitters consequently contributes to the current. The remainder of the emitters has too low a field enhancement value, and remains therefore passive, since the field Fl is smaller than 2 V/nm, see equation 2. The most efficient emitters, with a percentage share of 0.01%, do supply current at a low (i.e. with minimal voltage difference) applied field F, but since the number of these emitters is so minimal, the overall current density remains slight. An attempt to increase the externally applied field F, so that the less efficient emitters also contribute to the current, leads unfailingly to electrical discharges or above all to current-induced emitter destruction of the most efficient emitters.
In principle, known in the state of the art are three approaches for improving the current density and emitter density. As a first approach, an attempt is made through controlled growth to control the emitter-emitter spacing or respectively the emitter geometry (height-radius of curvature ratio). This approach is known as the so-called “self organization” of the field-enhancing structures. The electrostatic shielding arising between the emitters can thereby be eliminated or reduced to a large degree. The geometrically enhanced electrical field Fl thus increases. As the second approach, an attempt is made to control and to manipulate f(β) through controlled growth. The exponential behavior of f(β) of a typical carbon cold cathode seems to be intrinsic, but by greatly increasing the gradient of the straight line of f(β), a larger number of emitters are brought into a high β range. Thus these emitters will also contribute to increasing the current density. As a third approach, the use of ballast resistors is known, and is applied already to microtips. If one or more emitters are connected in series to a resistor, e.g. in the form of a resistive film or layer, the emission deviates from the typical Fowler-Nordheim behavior. The larger the geometrically enhanced field Fl becomes, the more the current deviates from the F-N characteristic line.
This effect is exploited to suppress the current of the most efficient emitters. That sounds paradoxical, but in this way a current-induced emitter destruction of the most powerful emitters is prevented, and the electrical field applied from outside can thus be increased. In the thereby increased electrical field, also emitters with slighter β can thus contribute to the current density, see equation 5. Since these emitters occur in very large quantities, owing to the exponential behavior of f(β), the overall current density of the cathodes increases.
The object of the invention is therefore to overcome the above-identified drawbacks of thermionic radiation sources, and to propose an irradiation device with X-rays or electron beams using a high dose emitter with minimal power losses of the cathode, with the aim of diverse geometry and of being able to irradiate in large quantities at the same time. In particular an X-ray emitter should be proposed that enables a dose rate many times higher than conventional X-ray emitters. The percentage of usable energy converted into X-rays should also be increased, and a uniform distribution of the X-rays with respect to the surface to be irradiated and the depth of the material should be obtained. Furthermore the proposed device should also make possible a cost-effective irradiation, in particular for sterilization of different objects and the drying of ink and polymer crosslinking, especially on an industrial scale.
These objects are achieved according to the present invention in particular through the elements of the independent claims. Further advantageous embodiments follow moreover from the dependent claims and from the specification.
In particular these objects are achieved through the invention in that an X-ray tube is constructed with a cathode, which emits electrons (e−) in an interior chamber that is under vacuum, and a target, configured as an anode, for generating high dose X-radiation (γ), the cathode comprising at least one cold cathode, based on an electron (e−) emitting material with field-enhancing structures, the cathode and the anode being designed as a first and a second closed hollow body, and one hollow body being formed inside the other hollow body, and the cathode and/or the anode comprising a material substantially transparent for X-radiation (γ). An advantage of this invention is, among others, that e.g. the material to be irradiated can be placed inside the X-ray tube, whereby a concentration of the X rays and a considerably more powerful and more even irradiation can be achieved. A further advantage of this embodiment variant is, among others, that both a reflector or transmission emitter configuration can be built without special cooling devices for the cold cathode (except air convection).
In an embodiment variant, the field-enhancing structures may include e.g. carbon nanotubes, coral-like carbon, metal tips, silicon tips, diamond tips and/or diamond dust. Advantageously the field-enhancing structures emit electrons (e−) already at room temperature. In contrast to the hot cathodes known as thermionic electron sources, they do not require any heating capacity in order to emit electrons (e−) in the vacuum. Field-enhancing structures able to be integrated on the surface of the cathode bring about a cold emission of electrons (e−) through intensification of an externally applied electrical field. The mode of operation of the cold cathodes in based on an externally applied electrical field being enhanced on structures of pointed design, so that high electrical fields are created, typically e.g. on the order of magnitude of 2000-4000 volts per micrometer. In relation to the electron-emitting surface of the cathode, the anode may be designed e.g. small or in the same proportion. This embodiment variant has the advantage, among others, that the electron emission takes place at room temperature, and thus the device for heating the emitter is omitted. Furthermore the cooling of the immediate vicinity of the emitter is done away with. As another advantage, the service life of the emitter should be mentioned. Since the emitter is operated at room temperature, no deterioration takes place through ablation of the emitter material. Owing to current supply lines and possibly cooling, it is difficult to construct thermionic electron sources such that they are transparent for X-rays. The geometric possibilities for irradiation are thereby further restricted. X-ray tubes with cathodes and/or anodes transparent for X-radiation are therefore not able to be produced in the state of the art, or only with difficulty.
In a further embodiment variant, the X-ray tube is designed as cathode hollow cylinder with a coaxial anode hollow cylinder inside. This embodiment variant has the advantage, among others, that e.g. the material to be irradiated is able to be placed inside the anode hollow cylinder and can be irradiated directly through the X-radiation (the radiation goes inward—transmission emitter).
In still another embodiment variant, the X-ray tube is designed as an anode hollow cylinder with a coaxial cathode hollow cylinder inside. This embodiment variant has the advantage, among others, that e.g. the material to be irradiated is able to be placed inside the cathode hollow cylinder and be irradiated with a transparent cathode (the radiation goes inward—reflector).
In a further embodiment variant, the cold cathode comprises at least one support layer for holding the electron (e−) emitting material, the emission surface of the cold cathode being defined substantially by the shape of the support layer. One advantage of this embodiment variant is, among others, that almost any desired geometric configurations may be achieved.
In another embodiment variant, geometry and spatial configuration of the emission surface of the cold cathode is determined by the shaping of the support layer. One advantage of this embodiment variant is, among others, that the geometry of the irradiation unit may be adapted in a simple way to the requirements of the irradiation method.
In a further embodiment variant, the ratio of the surface of the cold cathode to the layer depth is large. One advantage of this embodiment, among others, is that the cathode is suitable for large-surface irradiation devices.
In still another embodiment variant, the shape and size of the radiation chamber of the X-ray tube is determined by the surface area and/or spatial configuration of the cold cathode and/or of the anode. One advantage of this embodiment is, among others, that the material to be irradiated can be irradiated from all sides simultaneously.
In an embodiment variant, the support layer comprises a matrix with embedded carbon nanotubes and/or carbon structured in a coral-like way. An advantage of this embodiment variant is, among others, that it becomes very economical for large-surface emitter devices. Carbon nanotubes are commercially available, and carbon structured in a coral-like way allows itself to be applied economically in a large-surface way. Owing to its strong covalent bonds, moreover, carbon is more resistant than metal tips to ion bombardment and electrical discharges. Carbon is able to withstand large emission currents.
In another embodiment variant, the first support layer of the cold cathode comprises at least one substrate with ceramic material or glass. One advantage of this embodiment variant is, among others, that the support material is cheaper, shapeable, and vacuum-suitable. Moreover, the weakening of the X-rays by these materials is relatively slight.
In an embodiment variant, the support layer comprises at least one resistive layer and/or conductor path layer. One advantage of this embodiment is, among others, that the emission current is able to be distributed evenly over the cathode surface. Thus the specific power is able to be distributed optimally on the anode, and local overheating thereby avoided.
In a further embodiment variant, the conductor path layer comprises a vapor-deposited copper layer. An advantage of this embodiment variant is, among others, that the copper has good electrical and heat-transfer characteristics. Other metals can likewise be employed to advantage.
In an embodiment variant, the electron (e−) emitting material on the support layer is disposed with a defined spacing side-by-side, back-to-back and/or adjacently. This has advantages, among others, related to production technique, since the extraction grid allows itself to be more easily constructed in flat geometries. Thus a multiplicity of such emitter modules may be assembled into a complex geometry for the emitter configuration.
In another embodiment variant, the cold cathode and/or the anode are constructed from at least two independent segments. The number of these segments and thus the length of this configuration is in principle selectable as desired. An advantage of this embodiment variant is, among others, that the emitter configuration allows itself to be assembled in a modular way.
In an embodiment variant, at least one extraction grid is disposed between cold cathode and anode. An electrical insulator, for example, can be disposed between cold cathode and extraction grid. One advantage of this embodiment variant is, among others, that the spacing extraction grid-cold emitter can be kept constant over the emission surface. The local variation in emission intensity can thereby be reduced. The use of an extraction grid can also possibly 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 and/or a gaseous coolant (KM). One advantage of this embodiment variant is, among others, that the anode can withstand a higher specific electron intensity. Thus a higher dose rate can be achieved.
It should be stated here that, besides the X-ray tube according to the invention, the present invention also relates to a method for sterilization and/or irradiation by means of an X-ray tube according to the invention as well as to a compatible electron beam gun.
Embodiment variants of the present invention will be described in the following with reference to examples. The examples of the embodiments are illustrated by the following figures:
An evenly distributed 4π-gamma radiation, for example, can thereby be achieved inside the cathode hollow cylinder 31. The material to be irradiated can be placed inside the anode hollow cylinder 31. This ensures a uniform irradiation of the object from all sides, which would otherwise be hardly possible.
a shows the cross section of a transmission emitter configuration in variable electrode geometry with modularly assembled cold cathodes as electron sources in a circle segment portion. To achieve a 4π-gamma radiation (see also
b shows the cross section of a transmission emitter configuration according to
a shows the cross section of a reflector configuration in variable electrode geometry with modularly assembled cold cathodes as electron sources in a circle segment portion. The support layer of the cathode and the cold cathode are substantially transparent for X-radiation. The prolongation of the reflector configuration in the longitudinal direction, perpendicular to the paper plane, may be selected freely.
b shows the cross section of a reflector configuration according to
In a first design of the cold cathode 23 according to
With large values for the spacing of the surface of the cathode 23 to the gate 80 (arrow d), a typical extraction voltage of several thousand volts must be worked with. The power losses at the gate 80 increase significantly as a result. With a spacing of the gate 80 from the surface of the cathode 23 amounting to some dozen μm, for instance, electrical extraction voltage of up to some hundred volts generally suffices, but, on the other hand, the risk of a short circuit of a not lithographically defined cathode is relatively great. Thus, in the design of the cold cathode 23, a compromise must be made with respect to said spacings, indicated by the arrows a, b, c and d. It is thus of further advantage to produce the cathode 23 using a lithographic method, defined gate, insulator and emitter surfaces in the micrometer range being used.
a shows a transmission emitter configuration with a modularly assembled cold cathode 24 consisting of a multiplicity of cold cathode modules 25 and an anode 32 in any definable circle segment for use according to the invention in an X-ray tube. Disposed on the outer segment section with the outer radius r1 are a multiplicity of cold cathode modules 25 with substantially the same spacing, as shown schematically. The cold cathode modules 25 have on their surface field-enhancing structures (not shown) which emit already at room temperature electrons (e−) in the interior chamber under vacuum 40 of the X-ray tube. Alternatively, the cold cathode module 25 could be equipped according to the embodiment variant in
Shown in
In a way comparable to
In principle, the above-mentioned proposed surface emitters and round emitter configurations or respectively transmission emitter and reflector configurations as well as the conventional emitter configuration in roentgen radiography can be constructed with a modularly assembled cold cathode and a correspondingly disposed anode. For applying the field-enhancing structures on the surface of the cold cathode, which substantially represents the emission surface for the electrons, all methods mentioned further above are suitable. The modular assembly of individual cold cathode elements as well as of emitter segments constructed therefrom is especially suitable for the large-surface design of flat and curved emission surfaces or irradiation surfaces. This way the construction of any desired geometry for the radiation chamber is possible as well as the arrangement of an emitter around any geometry for an irradiation object; high dose emitters can be disposed on the surface area, or respectively in the space, in an especially large-surface and definable way. It should be stated here that four basic configurations for the design of the emitter are possible:
1. Cathode inside, anode outside, radiation inward (reflector)
2. Cathode outside, anode inside, radiation inward (transmission emitter)
3. Cathode inside, anode outside, radiation outward (transmission emitter)
4. Cathode outside, anode inside, radiation outward (reflector).
While other configurations for the X-ray emitter are possible, only the configurations as transmission emitter may be considered for electron beam guns, in which a transparent anode always makes possible the passage of the electrons out of the vacuum space.
The advantages of the invention may be summed up as follows: besides a high dose rate, the cold cathode can be economically produced, in particular with overall application of the field-enhancing structures; the cold cathode has especially minimal thermal losses, and requires no additional cooling owing to its emission at room temperature; construction of a reflector or a transmission emitter is possible by using either cathode material transparent for X rays or cathode material not transparent for X rays. The combination of use of field-enhancing structures for a cold cathode and a defined cold cathode geometry by means of field-enhancing structures specifically in layer formation on a support layer, the defined development of further functional layers, and in particular the defined geometry of the contact surfaces between support layer and (e−)-emitting layer make <sic. makes> possible the construction in particular of a large-area-type or respectively modularly assembled cold cathode, and, with corresponding design of the anode, a large radiation chamber of freely definable shape. Partial irradiation on the object is likewise possible, e.g. through defined arrangement of individual cold cathode modules.
The advantages enumerated above apply for an X-ray emitter as well as for an electron beam gun. In the first case, the anode is designed such that all impinging electrons are absorbed and are used for generation of X rays. In the second case, the anode is designed such that the electrons substantially penetrate the anode, and can be used directly for irradiation.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003-0911/03 | May 2003 | CH | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP04/50866 | 5/19/2004 | WO | 00 | 11/16/2006 |
