EP2301042A2

EP2301042A2 - X-ray target and a method for producing x-rays

Info

Publication number: EP2301042A2
Application number: EP09777336A
Authority: EP
Inventors: Norman Uhlmann; Frank Sukowski; Frank Nachtrab; Petra-Maria Kessling
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 2008-07-29
Filing date: 2009-07-21
Publication date: 2011-03-30
Anticipated expiration: 2029-07-21
Also published as: EP2301042B1; DE102008035210A1; ATE545936T1; DE102008035210B4; WO2010012403A2; PL2301042T3; WO2010012403A3

Abstract

An X-ray target (110) for producing X-radiation (120) by way of an electron beam (130) striking the X-ray target (110) has a cross-sectional area (A) perpendicular to the electron beam (130) and a longitudinal extent (L) parallel to the electron beam (130), wherein the electron beam (130) can be directed onto the cross-sectional area (A) and A 2.

Description

X-ray target and a method for generating X-rays

description

Embodiments of the present invention relate to an X-ray target for generating X-ray radiation by an electron beam and to a method for generating X-ray radiation. Further exemplary embodiments relate to an X-ray target for linear accelerators (LINACs), betatrons and x-ray tubes.

For generating X-rays, a collimated beam of accelerated electrons (or other charged particles) is usually directed to a target (X-ray target). Due to the interaction of the accelerated electrons with the target material, the X-radiation is generated in a region called the focal spot. For example, the X-rays are generated as bremsstrahlung during this process of sudden deceleration of the high-energy charged particles.

X-rays are not only of outstanding importance in medical technology, but are also used extensively in materials testing, for example to detect defects in materials. The examination of objects or even materials with high transmission lengths (eg freight containers, motor vehicles, etc.) requires the use of a high-energy and intensive X-ray source. Linear accelerators (LINACs), which operate with excitation energies in the megaelectron volt range (MeV range), are frequently used for this field of application.

In order to achieve the highest possible fault detection, the image quality of the entire imaging system should be optimized, the system comprising, for example, a radiation source, manipulators and a detector can. An achievable resolution is currently limited less by the detector side of the system, but rather by the radiation source. Namely, the minimum focal spot size of the X-ray source directly limits the maximum applicable magnification.

Therefore, in the development of novel targets, the focal spot size for high energy X-ray sources should be minimized to allow an increase in the imaging performance of such imaging systems.

In a linear accelerator, electrons are generated via glow emission and accelerated in an evacuated waveguide to energies in a range of usually 3 MeV to 24 MeV. At the end of the waveguide is either directly the X-ray target or a deflection magnet, which directs the electrons to the target. The X-ray target usually has a copper window closing the vacuum and a tungsten layer, wherein the layer thickness of the tungsten layer may for example comprise several hundred micrometers. The heat generated is dissipated via thermal contact with the housing (radiator head).

For example, the X-rays are produced by the Bremsstrahlung effect (sudden deceleration of the electrons due to interaction with the atoms in the X-ray target), and the source size of the resulting X-rays, which is the focal spot size, depends on several factors. These factors include, for example:

(a) Distribution of the sites of impact of the electrons on the target surface;

(b) scattering of the electrons in the target; and (c) scattering of the generated (X-ray) radiation in the target or in the exit window.

Conventional linear accelerators achieve a focal spot size of 2 mm to 4 mm. In contrast, in conventional X-ray tubes, focal spot sizes are indeed achievable in the order of micrometers - but only in an energy range of a few kiloelectronvolt (keV) and not, as with linear accelerators of usually a few megaelectronvolts. Therefore, conventional x-ray tubes are unsuitable for many applications.

With conventional target materials and target geometries of X-ray linear accelerators used, for example, in NDT, the X-ray targets are sufficiently thick for the electrons to deliver all their energy to the material. These targets, by their thickness and the resulting large interaction zones of the electrons create an undesirably large focal spot. As a result, the typical problems such as edge blurring, poor detail recognition and high defect detection limits occur, for example, when examining highly absorbent materials and large objects.

Another alternative method for generating X-radiation is the use of an X-ray target in a circulating electron beam (electron storage ring or synchrotron). An example of this is described in "Novel X-ray source based on a tabletop Synchrotron and Its Unique Features", Nuclear Instruments and Methods in Physics Research B 199 (2003), p 509-516, the disclosed X-ray targets for linear accelerators being inefficient are and were therefore excluded.

Based on this prior art, there is thus a desire to provide an X-ray target that has a small X-ray target Focal spot at high radiation energy while minimizing the heat input into the target.

This object is achieved by an X-ray target according to claim 1 and a method according to claim 16.

The core idea of the present invention is to provide an X-ray target for generating X-ray radiation, wherein the X-ray radiation is generated by an incident electron beam in the X-ray target and the X-ray target has an elongate extent. The elongate extent can be defined, for example, by the X-ray target having a cross-sectional area A perpendicular to the electron beam and a longitudinal extent L parallel or along the electron beam, the cross-sectional area A being smaller than the square of the longitudinal extent L and the electron beam being directable to the cross-sectional area , As an alternative to this definition, the elongate extent can be defined such that a maximum extent of the X-ray target along the electron beam is greater than a maximum extent of the X-ray target perpendicular to the electron beam.

In other words, the idea of the present invention is thus to select a target geometry which has a sufficiently large extent along the direction of the electron beam in order to achieve a correspondingly high efficiency. At the same time, only a limited space should be available in the directions perpendicular to the electron beam for the generation of bremsstrahlung by the electrons. This has the consequence that the possible focal spot size is limited upwards by the selected lateral extent. This means at the same time that the focal spot size can be suitably adjusted via the lateral extent (cross-sectional area) - for example below a predetermined value (eg less than 2 mm or less than 1 mm). For this purpose, instead of a flat transmission target, as used for example in conventional X-ray targets, exemplary embodiments have a wire-like or rod-shaped X-ray target which has a heavy material (with a high atomic number, such as tungsten) and can be introduced into the electron beam in the linear accelerator. The electron beam in this case runs essentially parallel to the longitudinal axis of the wire / rod. This X-ray target may optionally be cylindrical or parallelepiped, wherein the longitudinal axis is again arranged parallel to the electron beam direction.

In further embodiments, the X-ray target is fixed by threads on a frame, wherein the threads optionally comprise a material with a low atomic number. For example, the dimensioning of the filaments can be chosen such that an interaction of the charged particles (for example, the electrons) with the wires can be suppressed as much as possible. Optionally, the wires may further have a high thermal conductivity.

Embodiments of the present invention also include a method for generating X-radiation by means of an X-ray target, which has a cross-sectional area A and a longitudinal extent L with A <L ² . The method comprises directing the electron beam onto the cross-sectional area A and deflecting the electrons after the generation of the X-ray radiation by means of a magnetic field.

For example, the electrons of the electron beam can strike the X-ray target after passing through a straight-line or circular acceleration path and, after leaving the X-ray target, the deflection is effected by the magnetic field. Embodiments of the present invention have a number of advantages over conventional x-ray targets. For example, a focal spot size is achieved which, in contrast to conventional targets, is not predetermined by the extent (perpendicular to the propagation direction) of the electron beam, but by the diameter (cross-sectional area) of the wire piece. This limitation of the focal spot size has, for example, the following cause. The electrons have a very low mass compared to the solid-state atoms, which means that the electrons in the solid undergo a large scattering angle. In a conventional area target, where the extent of the cross-sectional area is significantly greater than the longitudinal extent, the Bremsstrahlung is thus generated in regions that are located in part very far from the original beam entry point. This Bremsstrahlung generated far from the Einstrahlachse leads to an enlargement of the focal spot. This widening of the focal spot is avoided by embodiments of the present invention, which use, for example, a wire target, wherein the focal spot size can be set flexibly, for example by the wire diameter.

A second advantage of exemplary embodiments is that active cooling of the target can be dispensed with. The energy of the accelerated electrons is released in the solid state through two different processes: the radiation braking process and the impact braking process. The radiation braking process is responsible for the generation of bremsstrahlung. In this case, little or no energy is given off in the form of heat to the target. In the impact braking process, no or hardly any Bremsstrahlung is generated instead, but the energy of the electrons is released in the form of heat to the target. The interaction cross sections and thus the probabilities of both processes are energy dependent. The likelihood of radiation generation is high at high electron energies and decreases with decreasing kinetic energy Electrons off. Thus the electrons deposit more and more energy in the form of heat in the material, the slower they become (lower energy). It is therefore advantageous that the electrons leave the target after a few interactions and do not interact further. The number of interactions is limited by the small volume of the target in the embodiments. The remaining energy of the electrons can optionally be converted into heat in an absorber block, wherein the absorber block is formed in order to dissipate the resulting heat well.

Embodiments of the present invention will be explained in more detail below with reference to the drawings. Show it:

1 is a schematic representation of a Röntgentar- gets according to embodiments of the present invention;

Fig. 2 is a schematic representation of a conventional X-ray target;

3a shows spatial representations of X-ray targets according to further and 3b rer embodiments;

4 shows a schematic illustration of an attachment of the X-ray target; and

5 shows a cross-sectional view through a linear accelerator with an X-ray target according to exemplary embodiments.

With regard to the following description, it should be noted that in the different embodiments, identical or equivalent functional elements have the same reference numerals and thus the description of these Functional elements in the various embodiments are interchangeable.

1 shows an X-ray target 110 which serves to generate an X-radiation 120 through an electron beam 130 impinging on the X-ray target 110, wherein the X-ray target 110 has a cross-sectional area A perpendicular to the electron beam 130 and has a longitudinal extent L along the electron beam 130. The electron beam 130 can be directed onto the cross-sectional area A, so that the electron beam 130 penetrates through the cross-sectional area A into the x-ray target 110. The x-ray target 110 has an elongate shape that can be defined by the cross-sectional area A being smaller than the square of the longitudinal extent L (A <L ² ).

The X-ray 120 is caused in an origin region R by the impact of the electrons e ^~ in the electron beam 130 on atoms of the X-ray target 110 (by the Bremsstrahlung effect), wherein the generation of the X-rays is particularly efficient when the electrons have a high energy, whereas At low energy, lattice vibration of the crystal atoms in the X-ray target increases (ie heat is generated).

Fig. 1 also shows a focal spot B which marks the region in which X-ray radiation is generated. For almost all applications, it makes sense to produce a focal spot B which is as small as possible, so that the X-ray source can be regarded as almost point-like. This purpose is also served by the wire-shaped configuration of the X-ray target 110, which has the consequence that the region R within which the X-radiation 120 is generated has a maximum extent perpendicular to the incident electron beam 130, which is given by the cross-sectional area A. Since this lateral extent of the region R is limited, thus the focal spot B is limited. After the X-radiation has been generated, the electrons leave the X-ray target 110 again and are deflected to an electron absorber (electron trap).

FIG. 2 shows a conventional X-ray target which, in comparison with the exemplary embodiment shown in FIG. 1, has a significantly larger region R 1 within which the X-radiation 120 is produced. Consequently, the focal spot Bl, on which the X-ray radiation 120 leaves the conventional X-ray target, is also significantly larger. The conventional X-ray target has a planar design, so that the cross-sectional area A, which the electron beam 130 impinges on the X-ray target, is significantly larger than the longitudinal extent L, which runs parallel to the electron beam 130. As a result of the planar design of the conventional X-ray target, the X-ray origin region R 1 in the directions perpendicular to the direction of propagation of the electrons is significantly larger than is the case in embodiments of the present invention (see FIG. 1). One of the causes of this expansion is the strong scattering of the electrons at the lattice atoms.

Furthermore, in the case of the conventional X-ray target, the deceleration of the electrons takes place completely within the X-ray target or in a substrate arranged behind it. In comparison, the thermal conversion of the kinetic carried out in embodiments of the present invention provide energy of the electrons is not in the X-ray target 110 or in an adjacent thereto substrate but the electrons e ^~ generally leave after generating the X-ray radiation 120, the X-ray target 110 again and continue to move within the X-ray tube or linear accelerator to be trapped, for example, after being deflected by a magnetic field in the electron trap (absorber block), where they release the kinetic energy in the form of heat. FIGS. 3a, 3b show possible embodiments of the cross-sectional area A.

3a shows a cuboid configuration of the X-ray target 110, wherein, according to exemplary embodiments, the longitudinal extent L is greater than the maximum extent D of the cross-sectional area A (eg greater than a maximum diameter of the cross-sectional area A).

3b shows a further embodiment in which the cross-sectional area A is circular, so that the X-ray target 110 has a cylindrical shape, wherein in turn the longitudinal extent L is greater than the diameter or the maximum extent D of the cross-sectional area A.

3a and 3b show only two examples in the design of the cross-sectional area A, wherein further design possibilities (eg an oval or hexagonal) are possible.

In further embodiments, it is also possible that the cross-sectional area A does not remain constant over the longitudinal extent L, but that, for example, the cross-sectional area A decreases or increases in the direction of the longitudinal extent L.

4 shows an exemplary embodiment for attachment of the X-ray target 110 by means of threads 210 and 230, which fix the X-ray target 110 in the direction perpendicular to the electron beam 130. For this purpose, near the cross-sectional area A, four threads 210a, 210b, 210c, 21Od are fastened to the X-ray target 110, which in turn are in turn connected to a ring holder 220 (frame). In the same way, four further threads 230a, 230b, 230c, 23Od are spanned between the X-ray target 110 and a further frame 240 on the side of the X-ray target 110 opposite the cross-sectional surface A. The four threads 210 and For example, four further threads 230 can contact the X-ray target 110 at an angular distance of 90 ° to each other. In further embodiments, the number of threads 210, 230 may be changed - for example, three threads each may be arranged at an angular distance of 120 ° to each other. Optionally, even more threads can be used to improve, for example, the thermal conductivity.

The attachment or stabilization of the X-ray target 110, as shown in FIG. 4, is thus effected by thin filaments of a solid material having, for example, a low atomic number Z (lower than that of the X-ray target 110). In addition, the filaments 210, 230 may have a high melting point and a high thermal conductivity. Carbon fiber could be taken as an exemplary material for this. The positioning of the threads 210, 230 should be finely adjustable, as is possible, for example, by fixing the X-ray target 110 to the outer frame 220, 240. The outer frames 220, 240 may then be arranged, for example, such that the X-ray target 110 in the electron beam 130 is adjusted in such a way that the electron beam 130 is aligned with the cross-sectional area A.

Accelerated electrons, which do not interact with the X-ray target 110, should not impinge on the beam exit window for the X-radiation 120, but instead be directed by a magnetic field onto a so-called beam dump (absorber block). This avoids the formation of a diffuse X-ray background.

Embodiments of the present invention are particularly advantageous for use in a linear accelerator.

FIG. 5 shows an exemplary embodiment of a linear accelerator having an X-ray target 110 according to the embodiment of FIG. has examples. The linear accelerator in FIG. 5 has a housing 310 which hermetically seals an evacuated cavity 312 so that a vacuum can be formed therein. The X-ray target 110 can in turn be fastened to the frames 220 and 240 by means of the threads 210 and 230 and is adjusted in such a way that the electron beam 130 strikes the cross-sectional area A. The linear accelerator furthermore has an electron source 320 (eg heating wire) and, on an opposite side thereof, an X-ray exit window 410 (radiation exit window), through which the resulting X-radiation 120 exits.

The electron source 320 is contacted via two terminals 322 and 324, and the electrons released are accelerated in a (straight) acceleration path 420 by acceleration electrodes 332a-d and 334a-d. For this purpose, an alternating voltage can be applied to the acceleration electrodes 332 and 334, for example, which generates an alternating polarity between the acceleration electrodes 332 and 334 along the acceleration path 420, so that the electrons are accelerated. For example, a positive potential may initially be present at the first acceleration electrodes 332a, 334a, a negative potential at the second acceleration electrodes 332b, 334b, a positive potential at the third acceleration electrodes 332c, 334c, and a negative potential at the fourth acceleration electrodes 332d, 334d issue. This polarity then changes with the frequency of the applied AC voltage. As the distance between adjacent acceleration electrodes 332, 334 increases along the acceleration path 420, this causes accelerated movement of the electrons.

The number of acceleration electrodes 332, 334 can be selected differently in further embodiments, wherein not only the number along the acceleration section 420 can be varied, but at one given location perpendicular to the propagation may be more than the two acceleration electrodes 332 and 334 are arranged. In addition, the acceleration electrodes 332 and 334 may also be formed cylindrically around the electron beam 130 (such that the acceleration electrodes 332a and 334a belong to one and the same electrode, for example).

After the electrons have been accelerated in the acceleration section 420, the electron beam 130 strikes the X-ray target 110. In the X-ray target 110, the kinetic energy of the electrons is converted into X-radiation 120, which in turn leaves the X-ray target 110 mainly in one direction of the electron beam 130 (due to the conservation of momentum assuming that there is no or hardly any momentum transfer to the X-ray target 110). After passing through a deflection region 430, the X-radiation 120 leaves the radiation exit window 410 of the housing 310.

As described above, the electrons do not completely emit their energy within the X-ray target 110, but in turn exit the X-ray target 110 and are deflected in the deflection region 430 by a magnetic field. The magnetic field is aligned perpendicular to the direction of propagation of the electrons, so that the electrons do not pass the beam exit window 410, but meet an absorber block 440 where they convert their residual kinetic energy into thermal energy. The electrons in the absorber block 440 are finally removed via a port 450.

In summary, embodiments of the present invention include a wire-type x-ray target 110 for generating x-ray radiation 120 by bombardment of accelerated electrons. The wire-type X-ray target can be defined, for example, in that the cross-section A is small in comparison to the longitudinal extent L, wherein the wire-type X-ray target 110 also can be configured cylindrical or cuboid. The diameter or the maximum extent D of the cross-sectional area A of the X-ray target 110 may be, for example, less than 3 mm or less than 1 mm or in a range between 0.05 mm and 3 mm or in a range between 0.1 and 1 mm. The cross-sectional area A may thus be, for example, less than 0.01 mm ² or less than 1 mm ² or in a range between 0.01 and 1 mm ² . The longitudinal extent L may for example be in a range between 0.5 and 20 mm or in a range between 1 and 10 mm. Apart from using the X-ray target 110 within a linear accelerator, the X-ray target may also be used in conventional X-ray tubes or betatrons.

The outstanding advantages of the X-ray target 110 according to the invention lie, on the one hand, in the realization of a very small focal spot B, which results in a clear improvement of the X-ray image (greater sharpness). A further advantage of exemplary embodiments is that the electrons within the X-ray target generate only the X-ray radiation, but the thermal conversion of the residual energy of the electrons does not take place in the X-ray target 110 or in a substrate in contact with it Generate the X-ray 120 left the X-ray target 110 again and fed by means of a magnetic field, for example, an absorber block 440 and there is the thermal conversion of the residual energy of the electrons.

Further conceivable materials for the X-ray target 110 would be, for example: molybdenum (Mo), rhodium (Rh), tungsten (W), rhenium (Re), platinum (Pt), gold (Au). The threads 220, 240 may include, for example, the following materials: carbon fiber, beryllium, graphite, silicon carbide, etc.

Claims

claims

1. X-ray target (110) for generating an X-radiation (120) by an electron beam (130) striking the X-ray target (110), comprising:

a longitudinal extent (L) parallel to the electron beam (130); and

a cross-sectional area (A) perpendicular to the electron beam (130), wherein the cross-sectional area (A) has a maximum extension (D) ranging between 0.1 mm and 1 mm and 2 times the maximum extension (D ) is smaller than the longitudinal extension (L),

wherein the electron beam (130) is directable to the cross-sectional area (A) and A <L ² and the X-ray target (110) comprises molybdenum or rhodium or gold or tungsten or rhenium or platinum.

2. X-ray target (110) according to claim 1, wherein the cross-sectional area (A) is selected such that the X-radiation (120) emanates from a focal spot (B) which is smaller than a predetermined size.

3. X-ray target (110) according to claim 1 or claim 2, wherein the cross-sectional area (A) has a maximum extension (D) and the m-times the maximum extent (D) is smaller than the longitudinal extent (L), where m = 3, 4 or 10.

4. X-ray target (HO) according to one of the preceding claims, wherein the cross-sectional area (A) has a circular or rectangular shape.

5. X-ray target (110) according to any one of the preceding claims, wherein the longitudinal extent (L) is in a range between 1 mm and 10 mm.

6. X-ray target (110) according to one of the preceding claims, which can be fixed via threads (210) to a frame (220) and via further threads (230) to a further frame (240).

7. X-ray target (110) according to claim 6, in which the threads (210) or the further threads (230) comprise a material having an atomic number (Z) of less than 40.

8. X-ray target (110) according to claim 7, wherein the material comprises carbon.

9. Use of an X-ray target (110) according to one of claims 1 to 8 in a linear accelerator.

10. Linear accelerator with:

a housing (310);

an electron source (320) for releasing electrons;

a beam exit window (410);

a rectilinear acceleration path (420) for accelerating the released electrons to form an electron beam (130); and

an X-ray target (110) with a cross-sectional area (A) perpendicular to the electron beam (130), wherein the electron beam (130) can be directed to the cross-sectional area (A) and A <L ² , wherein the x-ray target (110) is disposed in the electron beam (130) such that the released electrons strike the cross-sectional area (A) and the generated x-ray radiation (120) exits the housing (310) through the radiation exit window (410).

11. The linear accelerator of claim 10, wherein the X-ray target (110) is disposed in the housing (310) such that the released electrons pass a deflection region (430) after irradiation of the X-radiation (120), the deflection region (430). is formed to deflect the released electrons by means of a magnetic field on an absorber block (440).

12. A linear accelerator according to claim 11, wherein the absorber block (440) is adapted to convert the residual energy of the released electrons into heat and dissipate the heat.

13. A method for generating an X-ray radiation (120) by means of an X-ray target (110) having a cross-sectional area (A) and a longitudinal extent (L) with A <L ² , wherein the cross-sectional area (A) a maximum extent (D), the is in a range between 0.1 mm and 1 mm, and 2 times the maximum extent (D) is smaller than the longitudinal extent (L) and the X-ray target (110) molybdenum or rhodium or gold or tungsten or rhenium or PIa - tin, with the following steps:

Directing the electron beam (130) onto the cross-sectional area (A) so that the X-radiation (120) is generated as Bremsstrahlung; and

Deflecting the electrons after X-ray generation by means of a magnetic field.

14. The method of claim 13, wherein the electrons are deflected onto an absorber block (440) and the absorber block (440) is configured to convert a remaining energy of the electron beam (130) into heat.

15. The method of claim 13 or claim 14, wherein the cross-sectional area (A) is selected so that a focal spot (B), from which the X-radiation (120) emanates, falls below a predetermined size.