EP2283508A1 - Radiation source and method for generating x-ray radiation - Google Patents

Abstract

In a radiation source and a method for the generation of X-radiation, a liquid is arranged in a liquid line, the liquid being completely surrounded by the liquid line in the direction of an evacuated chamber. A portion of the liquid line is permeable to an electron beam such that the electron beam extending through the chamber is able to enter via the liquid line so as to interact with the liquid in an interaction zone for the generation of X-radiation. The radiation source ensures a good dissipation of heat from the interaction zone and prevents liquid from entering the chamber.

Description

 Radiation source and method for generating X-radiation

The invention relates to a radiation source for generating X-radiation. Furthermore, the invention relates to a method for generating X-radiation.

The non-destructive testing of objects using X-ray computed tomography requires the use of high-energy X-ray sources to inspect objects with high transmission lengths or high densities. In known X-ray sources, a solid body is used as the X-ray target, which is strongly heated and thermally stressed by the bombardment with an electron beam in the interaction zone designated as the focal spot. The heat generated in the focal spot is difficult to remove from the solid. The thermal load of the X-ray target thus limits the achievable output power of the X-ray radiation. In particular, to obtain a good image quality with a short exposure time, however, X-ray sources with a high output power of the X-radiation are required.

From WO 02/1 1 499 Al an X-ray source is known in which a liquid jet is used as the X-ray target. The liquid jet is generated by means of a nozzle and collected by a suction pipe again. Between the nozzle and the suction tube, the liquid jet moves freely in an evacuated chamber. The liquid jet is bombarded with an electron beam to generate X-rays. The fact that the X-ray target is designed as a liquid jet, the resulting heat in the focal spot can be better dissipated compared to a solid. The achievable output of the X-radiation is higher compared to X-ray sources with a solid body as the X-ray target. The disadvantage is that if the liquid jet is heated too much, the vapor pressure of the liquid jet can rise to such an extent that it is no longer completely sucked off, but a portion of the liquid jet evaporates and deposits on the inner walls of the evacuated chamber. The function and reliability of the X-ray source is thereby compromised.

The invention is therefore based on the object to provide a radiation source for generating high-energy X-ray radiation, which allows a high degree of heat removal from the interaction zone and at the same time ensures full function and high reliability of the radiation source.

This object is achieved by a radiation source for generating X-radiation with the features of claim 1. Because the liquid acting as X-ray target is completely surrounded by the liquid line in the direction of the evacuable chamber, the liquid is completely separated from the chamber, so that no liquid can escape from the liquid line and be deposited in the chamber. The heat removal due to the liquid flow from the interaction zone is not affected by the liquid line. In order for the electrons of the electron beam to lose as little as possible of their kinetic energy as they enter the liquid line, at least the part of the liquid line through which the electrons enter the liquid line is essentially permeable or transparent to the electron beam. Due to the permeable formation of the electrons do not interact with the liquid line, so that the electron beam with the liquid without a Energy loss can interact within the fluid line and form the interaction zone. The radiation source according to the invention thus enables good heat removal from the interaction zone due to the liquid acting as X-ray target, at the same time preventing the escape of liquid from the liquid line and thus avoiding impairment of the function and reliability of the radiation source. If required, the electron beam generating unit can be operated with a high acceleration voltage, in particular with more than 500 kV, in particular with more than 1 MV and in particular with more than 3 MV, so that a correspondingly high-energy X-ray radiation is generated which preferably radiates in the electron beam direction becomes. As liquids, liquid metals, such as mercury, or liquids with metallic microparticles can be used.

An embodiment of the target unit according to claim 2 enables a simple provision of high-energy X-radiation. The liquid provided by means of the target unit serves as a so-called transmission X-ray target. The generated X-radiation is emitted essentially in the electron beam direction. For this purpose, the liquid line is permeable or transparent to the X-ray radiation on the side opposite to the part which is permeable to the electron beam, so that it can emerge from the liquid line essentially without loss of energy in the electron beam direction. With increasing accelerating voltage or energy of the electrons, the intensity ratio of X-ray radiation generated in the electron beam direction increases relative to X-ray radiation generated against the electron beam direction. At high acceleration voltages from about 1 MV or high energies of electrons from about 1 MeV is the X-radiation generated substantially in the electron beam direction, which is used in the target unit. The liquid disposed in the liquid line thus forms a transmission X-ray target. The efficiency in the generation of high-energy X-radiation is substantially higher in the transmission X-ray target according to the invention than in a reflection X-ray target, in which the X-radiation is generated substantially opposite to the electron beam direction. Accordingly, the electron beam generation unit can be operated with an acceleration voltage of at least 500 kV, in particular of at least 1 MV, and in particular of at least 3 MV.

A liquid line according to claim 3 is largely permeable to the electrons of the electron beam. The permeability increases with decreasing atomic number of the chemical elements of the material. As materials, for example, compounds of berrylium, carbon, oxygen, aluminum and / or silicon can be used. As the material, for example, carbon in the form of graphite or diamond can be used. Furthermore, glassy compounds of carbon can be used, which are available, for example, under the trade name Sigradur. The materials can be ceramics. Decisive for a high permeability or transparency is that all chemical elements of the material have an atomic number of at most 14.

A material according to claim 4 is both permeable and stable.

An embodiment of the fluid conduit according to claim 5 increases the transmission of the electron beam. The smaller the dimension in the electron beam direction, the larger the transmittance. Equal- In time, the permeable part of the liquid line has sufficient stability to absorb the forces due to the pressure difference between the pressures inside and outside the liquid line. Since the stability also decreases with decreasing dimension in the electron beam direction, the dimension must be chosen such that there is sufficient transmittance and stability of the transmissive part at the same time.

An embodiment of the fluid conduit according to claim 6 increases the stability of the permeable part. The smaller the dimension of the transmissive part transverse to the electron beam direction, the higher its stability. Preferably, the dimension of the transmissive part transverse to the electron beam direction is at most as large as the cross section of the electron beam. In particular, when the target unit is formed to generate the X-ray in the electron beam direction and the liquid acts as a transmission X-ray target, a small dimension across the electron beam direction is possible since the generated X-radiation need not leave the liquid conduit again through the transmissive member.

An entrance window according to claim 7 allows in a simple manner to form a part of the liquid line permeable. In particular, the entrance window and the remaining conduit wall of the fluid conduit can be made of different materials. The entrance window and / or the conduit wall are preferably made of a heat and corrosion resistant metal. The conduit wall preferably has a wall thickness in the range of 0.5 mm to 50 mm, in particular in the range of 1.0 mm to 20 mm, and in particular in the range of 2 mm to 10 mm. The conduit wall is opposite to the entrance window Side preferably designed such that the generated X-ray radiation can emerge from the liquid line substantially unattenuated.

An embodiment of the fluid conduit of claim 8 minimizes the pressure differential between the evacuated chamber and the fluid at the location of the permeable portion of the fluid conduit or entrance window. Reducing the internal cross-sectional area in the landing section increases the velocity of the fluid flowing through it, reducing the static pressure according to the Bernoulli equation. The transition section between the feed section and the impact section can in principle be tapered as desired. For example, the taper may be symmetric in all directions or asymmetrical in at least one selected direction.

A transition section according to claim 9 prevents the formation of a turbulent flow.

By forming the liquid line according to claim 10, the size of the interaction zone designated as the focal surface can be kept small. The smaller the inner dimension of the impact section, the smaller the focal spot. The smaller the focal spot, the higher the achievable image quality.

A liquid pump according to claim 11 improves the heat removal from the interaction zone. The pressure and the velocity of the liquid in the liquid line are adjustable by means of the liquid pump. A cooling unit according to claim 12 ensures that the temperature of the liquid can be kept permanently constant. The liquid is preferably pumped after the bombardment with the electron beam through a heat exchanger serving as a cooling unit.

A liquid according to claim 13 ensures a good ratio of the generation of X-radiation to the generation of heat. This ratio improves with increasing atomic number of the chemical elements of the liquid. Mercury as a liquid has proven itself in practice for generating X-ray radiation.

An X-ray computer tomograph according to claim 14 allows the investigation of objects with high transmission lengths and / or high densities with good image quality.

A further object of the present invention is to provide a method for generating high-energy X-ray radiation which to a great extent permits heat removal from the interaction zone and at the same time ensures unrestricted and reliable generation of X-ray radiation.

This object is achieved by a method for generating X-radiation with the features of claim 15. The advantages of the method according to the invention correspond to the already described advantages of the radiation source according to the invention.

Further advantages, details and features of the invention will become apparent from the following description of an embodiment with reference to the drawing. Show it: 1 is a schematic representation of a radiation source for generating X-ray radiation with a liquid arranged in a liquid line and acting as X-ray target, and

Fig. 2 is a schematic representation of the liquid line in the region of an entrance window for an electron beam.

A radiation source 1 has an evacuated chamber 3 for generating high-energy x-ray radiation 2. At a first end 4 of the evacuated chamber 3, an electron beam generating unit 5 is arranged. The electron beam generating unit 5 serves to generate an electron beam 6 which extends in an electron beam direction 7 in the chamber 3. The electron beam generation unit 5 is operable to accelerate the electrons forming the electron beam 6 with a maximum acceleration voltage U _B of 160 kV to 24 MV, in particular 500 kV to 24 MV, in particular 1 MV to 24 MV, and especially 3 MV to 24 MV , Alternatively, the upper limit for the acceleration voltage may be 18 MV. The Elektronenstrahlerzeugungs- unit 5 is designed as a linear accelerator (LINAC), in which the electrons are generated via Glühemission and in several stages in an evacuated tube, the so-called waveguide accelerated. For smaller acceleration voltages U _B , the electron beam generating unit 5 can alternatively also be designed as an X-ray tube.

The radiation source 1 has a target unit 8 which serves to provide an X-ray target 9. The X-ray target 9 is formed as a liquid and is hereinafter referred to as liquid 9. The liquid 9 is arranged in a closed liquid line 10 which extends transversely to the electron beam direction 7 at a second end 11 of the chamber 3 and closes the chamber 3. The liquid 9 is thus completely surrounded by the liquid line 10 in the direction of the chamber 3.

To generate a flow of the liquid 9 in a flow direction 12, the target unit 8 has a liquid pump 13. Starting from the liquid pump 13, the liquid line 10 is subdivided into a feed section 14, a funnel-shaped tapering first transition section 15, an impact section 16, a funnel-shaped expanding second transition section 17 and a discharge section 18. The impact section 16 is arranged at the second end 11 of the chamber 3 in the middle of this, so that the electron beam 6 strikes the liquid line 10 in the impact section 16. For cooling the liquid 9, a cooling unit 19 designed as a heat exchanger 19 is arranged in the liquid line 10 in the feed section 14.

In the landing section 16, a part 20 of the liquid line 10 for the electron beam 6 is so permeable or transparent that it can enter the liquid line 10 through the permeable part 20 substantially without the loss of kinetic energy. The permeable part 20 of the liquid line 10 is formed as a separate inlet window, which is tightly arranged in a recess 21 of a liquid line 10 forming the conduit wall 22. By interaction of the electron beam 6 and the liquid 9, an interaction zone 23 designated as a focal spot for generating the X-radiation 2 can thus be generated within the liquid line 10. Of the permeable part of the liquid line 10 is hereinafter referred to as entrance window 20.

The entrance window 20 consists of a material of one or more chemical elements, each having an atomic number of at most 14. Preferably, the material of the entrance window 20 is beryllium, diamond or aluminum. These materials have a high permeability to the electron beam 6 due to their atomic numbers. In principle, the conduit wall 22 can consist of any desired material and, in particular, does not need to be permeable to the electron beam 6.

The entrance window 20 has in the electron beam direction 7 a dimension D of at most 1000 .mu.m, in particular of at most 100 .mu.m, and in particular of at most 10 .mu.m. The smaller the thickness dimension D, the greater the transparency of the entrance window 20 for the electron beam 6. If the dimension D of the entrance window 20 is at most 1000 .mu.m, this is preferably in the range of 10 .mu.m to 1000 .mu.m, in particular in the range of 20 microns to 800 microns, and in particular in the range of 50 microns to 500 microns. As a result, a high stability of the entrance window 20 is ensured at the same time.

Transverse to the electron beam direction 7, the entrance window 20 has a dimension H of at most 2000 .mu.m, in particular of at most 1000 .mu.m, and in particular of at most 500 .mu.m. The entrance window 20 may be circular or square. In a circular design, the dimension H denotes the diameter. In a square design, the dimension H denotes the side length. Preferably, the dimension H corresponds to the diameter of the electron beam 6.

In the feed section 14 and in the discharge section 18, the liquid line 10 has a first inner cross-sectional area Aj. Correspondingly, the liquid line 10 has a second inner cross-sectional area A ₂ in the impact section 16 comprising the entrance window 20. The inner cross-sectional areas Ai and A ₂ are indicated in Fig. 2. The entrance window 20 is formed flush with the conduit wall 22 toward an inner side of the liquid passage 10, so that the second inner cross-sectional area A ₂ in the entire impact portion 16 is constant. The ratio Aj / A ₂ of the first inner cross-sectional area Ai to the second inner cross-sectional area A ₂ is greater than 1, in particular greater than 10, and in particular greater than 100. The larger the ratio Ai / A ₂ , the lower is the equation of Bernoulli the static pressure of the liquid 9 on the entrance window 20 radially outward. The impact section 16 has along the electron beam direction 7 an internal dimension B of at most 5000 μm, in particular of at most 1000 μm, and in particular of at most 100 μm. The smaller the inner dimension B, the smaller the interaction zone 23, whereby the image quality of the X-ray images that can be generated by means of the X-ray radiation 2 is improved.

In order to obtain a high efficiency from the generation of X-radiation 2 compared to the generation of heat, the liquid 9 consists of a material having at least one chemical element, wherein the at least one chemical element has an atomic number of at least 50. If the liquid 9 is made of a material having a plurality of chemical elements, then each chemical element has an atomic number of at least 50. Preferably, the material of the liquid 9 Mercury. The efficiency of the generation of X-radiation 2 against the generation of heat increases linearly with the atomic number of the material of the liquid 9.

The radiation source 1 is surrounded by a - in Fig. 1 only indicated - lead shield 24. The lead shield 24 has an exit window 25 for the generated X-radiation 2 in the region of the impact section 16. The radiation source 1 is, for example, part of an X-ray computer tomograph for nondestructive testing of industrial objects.

Hereinafter, the generation of X-ray radiation 2 by means of the radiation source 1 will be described. By means of the electron beam generating unit 5, electrons are generated by thermal emission, which are accelerated by the acceleration voltage U _B and form the electron beam 6. The electron beam 6 passes through the chamber 3 in the electron beam direction 7 and impinges on the liquid line 10 in the impingement section 16. Because the entrance window 20 is substantially transparent to the electrons of the electron beam 6, the electron beam 6 can pass through the liquid line 10 into the liquid line 10 Enter liquid 9. In the interaction zone 23, the electron beam 6 and the liquid 9 cooperate in a known manner, so that X-ray radiation 2 is emitted, which is emitted substantially in the electron beam direction 7 and leaves the radiation source 1 through the exit window 25. Since the entrance window 20 is permeable and has a small dimension D in the electron beam direction 7, the electrons of the electron beam 6 hardly lose any kinetic energy upon entry into the liquid line 9. Characterized in that the second inner cross-sectional area A _{2 is} significantly smaller than the first inner cross-sectional area A ₁ , the pressure difference between the liquid 9 and the chamber 3 at the location of the entrance window 20 minimized. In addition, a high stability of the liquid line 10 in the impingement section 16 is ensured by the fact that the entrance window 20 is at most as large as the diameter of the electron beam 6.

The liquid 9 is continuously pumped by the liquid pump 13 through the liquid line 10, so that the heat generated in the interaction zone 23 heat is dissipated by an exchange of the liquid 9 in the interaction zone 23. The liquid 9 heated in the interaction zone 23 is pumped by means of the liquid pump 13 through the cooling unit 19, whereby the supplied heat is dissipated again and the liquid 9 has no increased temperature when it flows through the interaction zone 23 again. Because the liquid 9 is completely surrounded by the liquid line 10 towards the chamber 3, no liquid 9 can evaporate in the interaction zone 23 and leave the liquid line 10. This ensures unrestricted function and high reliability of the radiation source 1. The pressure of the liquid 9 and the flow rate of the liquid 9 can be adjusted by means of the liquid pump 13. Because the first transition section 15 tapers in a funnel shape in the flow direction 12, a laminar flow is ensured at the transition between the feed section 14 and the impact section 16.

Characterized in that the liquid 9 is constantly replaced in the interaction zone 23 and circulating in the liquid line 10, a thermal destruction of the acting as X-ray target liquid 9 is not possible. The heat removal from the interaction zone 23 is in the equal to solids improved, whereby an increased output power of the X-ray radiation 2 is possible. In addition, by an appropriate choice of the dimension H of the entrance window 20 and the inner dimension B of the Auftreffabschnitts 16, the size of the interaction zone 23, d. h the size of the focal spot, be affected. Accordingly, a small size of the focal spot can be achieved, thereby enabling a high image quality of the X-ray images generated by the X-ray 2. In particular, this can avoid typical problems in the examination of objects with high transmission lengths and / or of strongly absorbing materials such as edge blurring, poor detail recognition and large error detection limits of inclusions.

Due to the high acceleration voltage U _B, the liquid 9 acts as a transmission X-ray target, so that the generated X-radiation 2 is emitted substantially in the electron beam direction 7. This is extremely efficient since, with increasing acceleration voltage U _B, the intensity ratio of X-radiation 2 generated in the electron beam direction 7 increases with respect to X-radiation 2 generated against the electron beam direction 7. At acceleration voltages U _β from about 1 MV, substantially all of the X-radiation 2 is generated in the electron beam direction 7. At least on the side opposite the entrance window 20, ie, for example, in the entire impact section 16, the conduit wall 22 is designed such that the generated X-radiation 2 can emerge from the fluid conduit 9 substantially unattenuated in the electron beam direction 7.

Claims

claims 1. Radiation source for generating X-radiation with an evacuated chamber (3), an electron beam generation unit (5) for generating an electron beam (6) running in an electron beam direction (7) and in the chamber (3), - A target unit (8), this one transversely to the electron beam direction (7) extending liquid line (10) having a liquid disposed therein (9), wherein by interaction of the electron beam (6) and the liquid (9) an interaction zone (23 ) is generated for generating X - radiation (2), - wherein the liquid (9) from the liquid line (10) in Direction of the chamber (3) is completely surrounded, and wherein at least a part (20) of the liquid line (10) for the electron beam (6) is designed so permeable, that the interaction zone (23) within the liquid keitsleitung (10) can be generated , 2. Radiation source according to claim 1, characterized in that the Target unit (8) is formed such that the X-ray radiation (2) is substantially emitable in the electron beam direction (7). 3. Radiation source according to claim 1 or 2, characterized in that the permeable part (20) of the liquid line (10) consists of a material of one or more chemical elements, each having an atomic number of at most 14. 4. Radiation source according to claim 3, characterized in that the material is at least one of the group beryllium, diamond and aluminum. 5. Radiation source according to one of claims 1 to 4, characterized in that the permeable part (20) of the liquid line (10) in the electron beam direction (7) has a dimension (D) in the range of 10 .mu.m to 1000 .mu.m, in particular in the range of 20 microns to 800 microns, and in particular in the range of 50 microns to 500 microns. 6. Radiation source according to one of claims 1 to 5, characterized in that the permeable part (20) of the liquid line (10) transversely to the electron beam direction (7) has a dimension (H) of at most 2000 .mu.m, in particular of at most 1000 .mu.m, and in particular of at most 500 μm. 7. Radiation source according to one of claims 1 to 6, characterized in that the permeable part (20) is formed as an entrance window which is arranged in a recess (21) of a conduit wall (22) of the liquid line (10) dense. 8. Radiation source according to one of claims 1 to 7, characterized in that the liquid line (10) comprises a feed section (14) having a first inner cross-sectional area (Ai) and a the permeable part (20) of the liquid line (10) comprising impact portion (16). having a second inner cross-sectional area (A ₂ ), wherein a ratio of first inner cross-sectional area (Ai) to second inner area cross-sectional area (A ₂ ) greater than 1, in particular greater than 10, and in particular greater than 100, is. 9. radiation source according to claim 8, characterized in that between the feed section (14) and the impact section (16) has a funnel-shaped extending transition section (15) is formed. 10. Radiation source according to one of claims 1 to 9, characterized in that the liquid line (10) has a striking portion (16) along the electron beam direction (7) has an inner dimension (B) of at most 5000 microns, in particular of at most 1000 microns, and in particular of at most 100 μm. 11. Radiation source according to one of claims 1 to 10, character- ized in that the target unit (8) comprises a liquid pump (13). 12. Radiation source according to one of claims 1 to 1 1, characterized in that the target unit (8) has a cooling unit (19) for cooling the liquid (9). 13. Radiation source according to one of claims 1 to 12, characterized in that the liquid (9) consists of a material of one or more chemical elements, each having a number of ordinates of at least 50. 14. X-ray computer tomograph with a radiation source according to one of claims 1 to 13. 15. A method of generating X-radiation comprising the steps of: Generating an electron beam (6) running in an electron beam direction (7) and in an evacuated chamber (3), - guiding one of a liquid line (10) in the direction of Chamber (3) completely surrounded liquid (9) transversely to the electron beam direction (7) through the chamber (3), - Introducing the electron beam (6) into the liquid (9) through at least a part (20) of the liquid line (10), which is permeable to the electron beam (6), and - Producing an interaction zone (23) within the liquid line (10), in which the electron beam (6) and the liquid (9) for generating X-radiation (2) cooperate.