X-RAY SOURCE AND SYSTEM AND METHOD FOR GENERATING X-RAY RADIATION

Abstract

The invention relates to an X-ray source (10) comprising at least one waveguide (30) for X-ray radiation, wherein the at least one waveguide (30) has a core (32) and a casing (40) surrounding the core (32), and wherein at least one part of the waveguide (30) is designed to emit X-ray radiation (50), if the part of the waveguide (30) is bombarded with electrons (52). The invention also relates to a system for generating X-ray radiation comprising an X-ray source of this type, and a method for generating X-ray radiation by means of an X-ray source of this type or a system of this type.

Claims

1. An X-ray source (10) comprising at least one waveguide (30) for X-rays, wherein the at least one waveguide (30) has a core (32) and a casing (40) surrounding the core (32), and wherein at least one part of the waveguide (30) is adapted to emit X-ray radiation (50) if the part of the waveguide (30) is bombarded with electrons (52).

2. The X-ray source (10) as claimed in claim 1, wherein the core (32) has a first core portion (34) and a second core portion (36), wherein the part of the waveguide (30) includes the first core portion (34).

3. The X-ray source (10) as claimed in claim 2, wherein the first core portion (34) is thinner than the second core portion (36), and/or wherein the first core portion (34) has a smaller volume than the second core portion (36).

4. The X-ray source (10) as claimed in claim 2, wherein the first core portion (34) has a different material to the second core portion (36), and/or wherein the material of the first core portion (34) is a metal, preferably a transition metal, or a metal alloy comprising the metal, and/or wherein the material of the second core portion (36) is a non-metal.

5. The X-ray source (10) as claimed in claim 2, wherein the material of the first core portion (34) has elements with a first atomic number and the material of the second core portion (36) has elements with a second atomic number, wherein the first atomic number is different from the second atomic number, wherein the first atomic number is in particular greater than the second atomic number, wherein the first atomic number is preferably at least 16 or at least 22, and/or wherein the second atomic number is preferably not more than 15 or not more than 6.

6. The X-ray source (10) as claimed in claim 2, wherein the material of the first core portion (34) comprises one or more elements from the following group: cobalt, copper, molybdenum, nickel, chromium, iron, silver, tantalum, platinum, gold, tungsten, and/or wherein the material of the casing (40) comprises one or more elements from the following group: cobalt, copper, molybdenum, nickel, chromium, iron, silver, tantalum, platinum, gold, tungsten, and/or wherein the material of the second core portion (36) comprises one or more elements from the following group: carbon, boron, beryllium, aluminum, magnesium, silicon.

7. The X-ray source (10) as claimed in claim 2, wherein the material of the first core portion (34), the material of the second core portion (36) and/or the material of the casing (40) each has a refractive index for X-ray radiation (50) for the real part of which the following formula applies: n=1δ, wherein δ is the decrement for the first or second core portion (36), by which the real part of the respective refractive index for X-ray radiation (50) with a photon energy of 10 keV differs from 1.

8. The X-ray source (10) as claimed in claim 7, wherein the value of the decrement δ of the material of the first core portion (34) is greater than the decrement δ of the material of the second core portion (36) by at least 20%, at least 50% or at least 100% of the value of the decrement of the material of the second core portion, and/or wherein the decrement δ of the material of the first core portion (34) and/or of the material of the casing (40) is at least 1×10.sup.−7, at least 5×10.sup.−7, at least 1×10.sup.−6 or at least 5×10.sup.−6, and/or wherein the decrement δ of the material of the second core portion (36) is not more than 5×10.sup.−5, not more than 3×10.sup.−5, not more than 1×10.sup.−5 or not more than 5×10.sup.−6.

9. The X-ray source (10) as claimed in claim 2, wherein the thickness of the first core portion (34) is not more than 50%, not more than 30%, preferably not more than 15%, of the thickness of the second core portion (36), wherein the first core portion (34) is preferably not more than 15 nm or not more than 10 nm thick, and/or wherein the thickness of the second core portion (36) is preferably from 10 nm to 400 nm, most preferably from 20 nm to 200 nm.

10. The X-ray source (10) as claimed in claim 2, wherein the first core portion (34) is arranged spaced apart relative to the casing (40), and/or wherein the first core portion (34), when viewed in a cross-sectional plane of the waveguide (30), is arranged in the middle of the second core portion (36), and/or wherein the casing (40) is thicker on one side of the core (32) than on the other side of the core (32).

11. The X-ray source (10) as claimed in claim 2, wherein the at least one waveguide (30) is a two-dimensional waveguide with a substantially circular, oval, polygonal, in particular rectangular or square, cross-section, or wherein the at least one waveguide (30) is a one-dimensional waveguide, the core (32) and casing (40) of which are in the form of layers.

12. The X-ray source (10) as claimed in claim 2, wherein the number of waveguides (30) is at least two, wherein the waveguides (30) are preferably arranged parallel to one another.

13. The X-ray source (10) as claimed in claim 2, wherein the part of the waveguide (30) includes at least one part of the casing (40), in particular the entire casing (40), and/or wherein the part of the waveguide (30) includes the core (32).

14. A system for generating X-ray radiation (50), comprising a vacuum chamber, an X-ray source (10) as claimed in claim 1 arranged in the vacuum chamber, and an electron source arranged in the vacuum chamber, which electron source is adapted to emit electrons (52) into the vacuum and radiate them onto the X-ray source (10).

15. A method for generating X-ray radiation (50), comprising the following steps: providing an X-ray source (10) as claimed in claim 1, and irradiating at least the part of the waveguide (30) of the X-ray source (10) that is adapted to emit the X-ray radiation with synchrotron radiation, with ions, in particular with high-energy ions, with laser pulses, in particular with ultra-short and/or focused laser pulses, in order to generate the X-ray radiation (50), and/or bombarding at least the part of the waveguide (30) of the X-ray source (10) that is adapted to emit the X-ray radiation with electrons (52) in order to generate the X-ray radiation (50).

Description

[0032] Preferred embodiments of an X-ray source and of a system for generating X-ray radiation will now be explained in greater detail with reference to the accompanying schematic drawings, wherein

[0033] FIG. 1 shows a first embodiment of an X-ray source in a schematic partially cross-sectional view;

[0034] FIG. 2 shows the X-ray source of FIG. 1 in perspective in a measuring setup for characterizing the emission properties thereof;

[0035] FIG. 3a shows a curve of the value of the decrement δ over the cross-section of the X-ray source of FIG. 1;

[0036] FIG. 3b shows a diagram of the intensity of the X-ray radiation over the angle of elevation θ.sub.f for the X-ray source of FIG. 1;

[0037] FIG. 4 shows multiple diagrams of the measured and simulated intensity of the X-ray radiation over the angle of elevation θ.sub.f for the X-ray source of FIG. 1 at different positions of the bombardment with electrons;

[0038] FIG. 5 shows the X-ray source of FIG. 1 on irradiation of X-ray radiation in the form of plane waves for X-ray fluorescence at different angle of elevations θ.sub.PW;

[0039] FIGS. 6a and 6b show simulation results for the X-ray fluorescence intensity distribution in the X-ray source of FIG. 1 on irradiation at different angle of elevations θ.sub.PW;

[0040] FIGS. 7a and 7b show a second embodiment of an X-ray source in a perspective detail view and a perspective overall view, wherein this X-ray source has a plurality of one-dimensional waveguides;

[0041] FIG. 8 shows measurement and simulation results for the X-ray fluorescence intensity distribution in the X-ray source of FIG. 7a/7b with a plurality of waveguides on irradiation of focused synchrotron radiation at different angle of elevations θ.sub.f;

[0042] FIG. 9 shows a measurement result for the energy distribution of the X-ray radiation of the X-ray source according to FIG. 7a/b in dependence on the angle of elevation θ.sub.f on bombardment of the X-ray source with electrons;

[0043] FIG. 10 shows measurement results for the intensity distribution of the X-ray radiation in a third embodiment of an X-ray source on bombardment of the X-ray source with electrons at different distances from the exit of the waveguide;

[0044] FIGS. 11a and 11b show a fourth embodiment of an X-ray source with a plurality of two-dimensional waveguides in perspective partial views; and

[0045] FIG. 12 shows a fifth embodiment of an X-ray source with a one-dimensional waveguide, wherein the X-ray source is in the form of a rotating anode.

[0046] FIGS. 1 and 2 show an X-ray source 10, which in this variant has a substrate 20 and a waveguide 30, carried by the substrate 20, for X-rays. The waveguide 30 comprises a core 32 having a first core portion 34 and a second core portion 36, and a casing 40 which surrounds the core 32 at least in some portions. As is apparent from FIG. 2, the waveguide 30 is a one-dimensional waveguide. Accordingly, the casing 40 is a layer formed directly on the substrate 20. A first portion 41 of the casing 40 is formed as a layer on the substrate 20. On a side of the first portion 41 opposite the substrate 20, a first part 37 of the second core portion 36 is formed, likewise as a layer. The first core portion 34 is formed as a layer on the first part 37 of the second core portion 36. In a transverse direction y perpendicular to the longitudinal axis A, which extends in the longitudinal direction z, of the waveguide 30, a second part 38 of the second core portion 36 covers the first core portion 34, and a second portion 42 of the casing 40 in turn covers the second part 38 of the second core portion 36. The layers are in each case in contact with one another (preferably substantially over the entire surface). FIG. 1 shows the waveguide 30 in a longitudinal section, containing the longitudinal axis A, along the plane E shown in FIG. 2.

[0047] The substrate is in the present case a silicon wafer, but it can alternatively be produced from a different material which is suitable for carrying an X-ray waveguide. The first portion 41 of the casing 40 is a copper layer about 40 nm thick, the first part 37 and the second part 38 of the second core portion 36 are each a carbon layer (here for example DLC, diamond-like carbon) about 20 nm thick, the first core portion 34 is a cobalt layer about 2 nm thick, the second portion 42 of the casing is a copper layer about 5 nm thick. However, there are suitable as the material of the first core portion 34 and/or of the casing 40 also other metals, in particular transition metals, or metal alloys comprising the metal in question. Similarly, there are suitable as the material of the second core portion 36 also other non-metals, in particular semiconductors. The first core portion 34 is thus thinner in the transverse direction y than any of the other layers. In particular, the first core portion 34 is thinner than the second core portion 36. The first portion 41 of the casing 40, on the other hand, is thicker than the second portion of the casing 42, in order on the one hand to ensure that the boundary roughness between the first portion 41 of the casing 40 and the first part 37 of the second core portion 36 is low for the purpose of improved total reflection at the casing 40. On the other hand, in this setup electrons 52 are able to pass relatively easily into the core 32 of the waveguide 30 when they are irradiated, as shown in FIG. 1, transversely to the waveguide 30 in the negative y-direction. As a result, a comparatively intensive X-ray emission from the X-ray source 10 is achieved.

[0048] In the case of an X-ray photon energy considered here by way of example of 10 keV, the value of the decrement δ of the material of the first core portion 34 is between the value of the decrement δ of the material of the casing 40 (or of at least one of the portions 41 and 42) and the value of the decrement δ of the material of the second core portion 36 (or of at least one of the parts 37 and 38). It is thereby preferred that the value of the decrement δ of the material of the casing 40 (or of at least one of the portions 41 and 42) is greater than the value of the decrement δ of the material of the second core portion 36 (or of at least one of the parts 37 and 38), so that the development of the modes in the waveguide 30 is disrupted as little as possible. For the materials used here for the casing, the first core portion and the second core portion, the following decrement values apply in the case of the above-mentioned X-ray photon energy: copper 1.62×10.sup.−5; carbon (amorphous) 4.57×10.sup.−6; cobalt 1.67×10.sup.−5 (see FIG. 3a).

[0049] The waveguide 30 of FIGS. 1 and 2 is, as explained above, a one-dimensional waveguide. A modification (not shown in the figures) of this X-ray source 10 of FIG. 1 has a two-dimensional waveguide, the core and casing of which are configured to be substantially (circular-)ring-shaped in cross-section perpendicular to the longitudinal axis A. In the longitudinal section containing the longitudinal axis A, this modified X-ray source has the appearance as shown in FIG. 1. In this respect, the explanation given in relation to the X-ray source 10 with a one-dimensional waveguide 30 here applies analogously to the modified X-ray source with a two-dimensional waveguide.

[0050] It is shown schematically in FIG. 1 that the electrons 52 propagate substantially in the negative y-direction before they strike the X-ray source 10. The electron beam is thereby focused on part of the first core portion 34. As shown in FIG. 1, there are thus excited, in addition to the waveguide fundamental mode 60 (m=0), in particular waveguide modes 61, 62 with mode numbers m=1 and m=2, respectively. By measuring the X-ray intensity by means of a semiconductor spectrometer 64 having an entrance slit 66, the angle-of-elevation-, θ.sub.f-, dependent intensity distribution of the X-ray radiation shown in FIG. 3b can be determined. Such a measurement can be performed, for example, by focusing electrons with an energy of 35 keV from an electron source for X-ray microtomography (here: an electron source from the X-ray source MetalJet® D2 from Excillum AB, Kista, Sweden) by means of an electron optics (here the electron optics from the same X-ray source MetalJet® D2 from Excillum AB, Kista, Sweden) onto a spot approximately 10 μm in size at a distance Δz of about 1 mm from the exit-side end 54 of the waveguide 30 along the longitudinal axis A onto the grounded X-ray source 10. The X-ray anode of the MetalJet® D2 is thereby de facto replaced by the X-ray source 10.

[0051] X-ray radiation can thereby be generated in the first core portion 34 of the waveguide 30 and/or in the casing 40, in particular in the second portion 42 of the casing 40, and coupled directly into the core 32 of the waveguide 30. FIG. 3b clearly shows that a plurality of waveguide modes are excited. In particular, in the case of the X-ray source 10 of FIG. 1, a fundamental mode (m=0) with an intensity maximum 70 is excited at θ.sub.f≈5 mrad and the mode m=2 with an intensity maximum 71 is excited at θ.sub.f≈7 mrad. It is noted that the X-ray radiation not only leaves the waveguide 30 at its end 54 on the exit side in the longitudinal direction z, but also, as indicated in FIG. 1, passes in the form of an evanescent wave (underlying the absorption by the material of the second portion 42 of the casing 40) through the second portion 42 of the casing 40 and exits the waveguide 30 on the side of the second portion 42 of the casing 40 that is opposite the core 32.

[0052] FIG. 4 shows four diagrams with measurement and simulation results, from which it is apparent that the dependence of the intensity distribution of the X-ray radiation on the angle of elevation varies with the distance Δz and is additionally dependent on whether the X-ray radiation originates from the casing made of copper or from the first core portion made of cobalt. It is also apparent from the diagrams that simulation results are in agreement with corresponding measurement results. In particular, the top left diagram of FIG. 4 shows the measured emission of the Kα- and Kβ-transitions of the material of the first core portion 34 on electron bombardment (curve 82) and on excitation by means of X-ray or synchrotron radiation (curve 84), together with the corresponding simulation (curve 86). The top right diagram shows the measured emission of the Kα-line of the material of the casing 40 on electron bombardment (curve 88) and on excitation by means of X-ray or synchrotron radiation (curve 90), as well as the corresponding simulation (curve 92). The local intensity maxima correspond to the modes (cobalt: only linear modes (m=0; m=2); copper: linear and non-linear modes). The bottom two diagrams of FIG. 4 show the measured and the calculated intensity distribution of the X-ray emission from the thin cobalt layer (first core portion 34) for a distance Δz of 35 μm and 350 μm. Here too, the measurements confirm the simulation results.

[0053] The excitation of the modes and the propagation thereof in the X-ray source, in particular in the waveguide, can be calculated by means of finite difference simulation on the basis of the reciprocity theorem. The finite difference simulation can be carried out as described in the scientific publication of L. Melchior and T. Salditt, “Finite difference methods for stationary and time-dependent x-ray propagation”, Opt. Express, 25: 32090, 2017, the disclosure of which relating to the finite difference simulation is incorporated herein by reference. As is shown in FIG. 5, this simulation proceeds from a planar wave 94 irradiated at an angle of elevation θ.sub.PW. The internal field distribution of the X-ray radiation in the plane E is shown in FIG. 6a for irradiation at different angles of elevation θ.sub.PW. The probability distribution for the exit of an X-ray photon emitted at a specific point from the X-ray source at a corresponding angle of elevation θ.sub.PW can be seen.

[0054] An X-ray source 10 with a plurality of one-dimensional waveguides 30 is shown in FIGS. 7a and 7b. Each waveguide 30 can have any, in particular all, of the features of the waveguide 30 of the X-ray source 10. The waveguides 30 are positioned in the form of a waveguide stack on the substrate 20. Adjoining waveguides 30 can thereby divide a portion of the casing in the region of the boundary between them. That is to say, a core 32 of a second waveguide 30 can directly adjoin the second portion 42 of the casing 40 of a first waveguide 30 adjacent to the substrate. The materials of the X-ray source 10 of FIG. 7 can be the materials of the X-ray source 10 of FIG. 1. Alternatively, nickel, for example, can be used instead of copper, and iron, for example, can be used instead of cobalt. In this case, the following preferred layer sequence on the silicon substrate is obtained: [Ni (about 10 nm)|C (about 24.5 nm)|Fe (about 1 nm)|C (about 24.5 nm)].sub.n, wherein n is the number of waveguides. The value n can be at least 2. In the case of the X-ray source 10 of FIG. 7, n=50.

[0055] For the X-ray source 10 with the preferred layer sequence mentioned above, the X-ray fluorescence intensity distribution on irradiation of focused synchrotron radiation at different angles of elevation Of is shown in FIG. 8. Diagram a) of FIG. 8 shows a distribution of the iron K fluorescence on a MÖNCH3 detector (from Paul Scherrer Institut, Villigen, Switzerland; see M. Ramilli et al., “Measurements with MÖNCH, a 25 μm pixel pitch hybrid pixel detector”, J. Instrum., 12: C01071-001071, 2017, the disclosure of which relating to the MÖNCH detector is incorporated herein by reference). There are shown in this distribution intensity in particular peaks and modeling as a function of the exit angle (angle of elevation) Of. Diagram b) shows the correspondingly summed intensity distribution as a function of the exit angle (angle of elevation). The dependence of the intensity distribution over the exit angle on the distance Δz is shown in diagram c). Finally, diagram d) shows a clear agreement of the measurement results with corresponding simulation results based on the reciprocity theorem. As is apparent from FIG. 9, there is emitted by means of the X-ray sources 10 disclosed herein, of which the X-ray source 10 of FIG. 7 with the preferred layer sequence is representative, on bombardment of the X-ray source 10 with electrons, not only characteristic radiation (in FIG. 9: Fe Kα radiation 96, Ni Kα radiation 97, Ni Kβ radiation 98), but also bremsstrahlung radiation 99.

[0056] In an X-ray source 10 with a different, likewise preferred layer sequence on the silicon substrate of [Mo (about 25 nm)|C (about 16 nm)|Mo (about 1 nm)|C (about 16 nm)].sub.n, with the above-mentioned values for n (here for example 30), the dependence of the molybdenum fluorescence intensity generated by electron bombardment on the distance Δz between the location of the irradiation and the exit-side end 54 is depicted in FIG. 10. The depicted intensity distribution is corrected in respect of the self-absorption of the emitted fluorescence by the substrate. It is clear that the intensity decreases significantly as the distance Δz increases.

[0057] An X-ray source 10 with a plurality of two-dimensional waveguides 30 is shown in FIGS. 11a and 11b, wherein the first core portion has in each case been omitted for the sake of clarity. Each of the two-dimensional waveguides 30 can here have any, in particular all, of the features of the waveguide 30 from the X-ray source 10. The two-dimensional waveguides 30 can, as is shown in the figures, be formed periodically within a portion having an optionally substantially hexagonal base area in the transverse plane (the x-y plane). The waveguides 30 can be formed substantially cylindrically symmetrically in the substrate 20 and/or can be arranged at substantially equal distances from one another. It is additionally shown in FIGS. 11a and 11b that the electrons 52 can be irradiated onto the X-ray source 10 in the longitudinal direction (along the axis z). The X-ray radiation 50 leaves the X-ray source on the exit side likewise in the longitudinal direction.

[0058] A further variant of an X-ray source 10 with a one-dimensional waveguide 30, here in the form of a rotating anode, is depicted in FIG. 12. The electrons here preferably strike the waveguide parallel to the axis of rotation of the rotating anode. Here too, the first core portion has been omitted for the sake of clarity. The waveguide 30 of the X-ray source of FIG. 12 can have any, in particular all, of the features of the waveguide 30 of the X-ray source 10. As a result of the rotation of the X-ray source 10, the location in the coordinate system of the rotating X-ray source 10 at which the electrons 52 bombard the first core portion 34 migrates along a circular path, so that larger electron streams can advantageously be used and correspondingly higher X-ray intensities can be achieved.

[0059] The X-ray sources described herein are adapted to emit radiation in one or more angle ranges with dimensions below about 10 mrad. The efficiency of the generation of the X-ray radiation is substantially higher in the case of X-ray sources according to the invention than in the case of conventional systems for generating X-ray radiation, in which the X-ray radiation is generated outside the waveguide and then coupled into a waveguide. The X-ray sources according to the present invention are therefore distinguished not only by a small and compact construction but also by high brilliance. With the X-ray sources proposed herein, the photon yield in a phase space volume which is defined by the exit surface (source surface) and the solid angle of the radiation of the waveguide modes can be increased by a factor of from 10 to 100 for one-dimensional waveguides and from 100 to 10,000 for two-dimensional waveguides. The X-ray source according to the invention accordingly has a comparatively high phase space density and coherence. Therefore, the present invention makes it possible to perform in the laboratory many different X-ray analyses (for example by means of X-ray microtomography) for which synchrotron sources were hitherto required.