X-ray apparatus with deflectable electron beam

Abstract

An x-ray apparatus (1), has an electron beam source (2), a target (4), onto which the electron beam (3) is directed to form a focal spot (5; 5a, 5b) on the target (4), x-ray optics (6) for collecting x-rays emitted from the focal spot (5; 5a, 5b) to form an x-ray beam (8) and a sample position (9) at which the x-ray beam (8) is directed. The x-ray apparatus (1) further includes an electrostatic or electromagnetic electron beam deflection device (10) suitable for moving the focal spot (5; 5a, 5b) on the target (4). The extension of the focal spot (5; 5a, 5b) in any direction (x, y, z) is at least a factor of 1.5 smaller than the extension of the target (4). An x-ray apparatus is thereby provided with simplified alignment of the x-ray optics with respect to a microfocus x-ray source.

Claims

1. An x-ray apparatus comprising: an electron beam source, emitting an electron beam; a target onto which the electron beam is directed, the electron beam thereby forming a focal spot on the target, wherein the target has a curved target surface; x-ray optics having a focus and structured to collect x-rays emitted from the focal spot, thereby forming an x-ray beam; a sample position to which the x-ray beam is directed; an electrostatic or electromagnetic electron beam deflection device, the deflection device being disposed, structured and dimensioned to move the focal spot on the target in a plane in which the target surface is curved between a first position in which the electron beam is substantially perpendicular to the curved target surface and a second position in which the electron beam is incident on the curved target surface at a flat angle, the focal spot thereby having a size which is smaller at least by a factor F=1.5 than a size of the target, wherein, for the first position, a photon flux density is maximized and, for the second position, a photon flux is maximized.

2. The apparatus of claim 1, wherein the target is a liquid metal jet target.

3. The apparatus of claim 2, wherein, in a direction transverse to a liquid metal jet target propagation direction and transverse to a propagation direction of the electron beam, an extension of the focal spot is smaller at least by a factor FT=2 than an extension of the liquid metal jet target.

4. The apparatus of claim 3, wherein FT=5.

5. The apparatus of claim 1, wherein said curved surface has a radius of curvature R, with 0<R10 mm or with 0<R1 mm.

6. The apparatus of claim 1, further comprising an electrostatic or electromagnetic electron beam focusing device, suitable for changing a spot area of the focal spot at least by a factor FS=2 or FS=5.

7. The apparatus of claim 6, wherein the electron beam focusing device comprises one or more electromagnetic coils and/or one or more charged electrodes.

8. The apparatus of claim 1, wherein the electron beam deflection device is suitable for moving the focal spot on the target by at least a distance D=50 m or D=200 m.

9. The apparatus of claim 1, wherein the electron beam deflection device is suitable for deflecting the electron beam in two independent directions perpendicular to a propagation direction of the electron beam.

10. The apparatus of claim 9, wherein the independent directions are perpendicular to each other.

11. The apparatus of claim 1, wherein the electron beam deflection device comprises one or more electromagnetic coils and/or one or more charged electrodes.

12. The apparatus of claim 1, wherein the x-ray optics comprises a multilayer mirror, a Montel mirror, a Gbel mirror or mirror having a single reflective surface curved with respect to both a sagittal and a meridional direction of incident x-rays and/or capillary x-ray optics.

13. The apparatus of claim 1, wherein the factor F=2 or F=5.

14. The apparatus of claim 1, wherein the x-ray optics is positioned to collect x-rays emitted from the focal spot at essentially 90 with respect to a propagation direction of the electron beam hitting the target.

15. A method for aligning an x-ray apparatus, the x-ray apparatus having an electron beam source emitting an electron beam, a target onto which the electron beam is directed, thus forming a focal spot on that target and x-ray optics for collecting x-rays from a focus thereof, the method comprising the step of: moving the focal spot on the target by deflecting the electron beam with an electric and/or magnetic field until the focal spot overlaps the focus of the x-ray optics.

16. A method for aligning an x-ray apparatus, the x-ray apparatus having an electron beam source emitting an electron beam, a target having a curved target surface onto which the electron beam is directed, thereby forming a focal spot on that target and x-ray optics for collecting x-rays from a focus of those x-ray optics, the method comprising the steps of: a) directing the electron beam to a first position on the target in which the electron beam is substantially perpendicular to the curved target surface; and b) moving the focal spot on the target in a plane in which the target surface is curved from the first position of step a) to a second position on the target at which the electron beam is incident on the curved target surface at a flat angle, the electron beam thereby being deflected using an electric and/or magnetic field and/or a spot area of the focal spot being altered by changing a focusing of the electron beam using an electric and/or magnetic field, wherein a photon flux density of an x-ray beam formed by the x-ray optics is maximized in step a) and a photon flux of the x-ray beam is maximized in step b).

17. The method of claim 16, wherein the curved target surface has a radius of curvature R, with 0<R1 mm.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1a shows schematically an embodiment of an inventive X-ray apparatus, with an electron beam deflection device for moving a focal spot of an electron beam on a flat target, with an undeflected electron beam;

(2) FIG. 1b shows the embodiment of FIG. 1a, with deflected electron beam;

(3) FIG. 1c shows schematically a second embodiment of an inventive X-ray apparatus, with an additional electron beam focusing device for adjusting the size of the focal spot;

(4) FIG. 2a shows a schematic cross section of a curved target, hit basically perpendicularly by an electron beam much narrower than the target;

(5) FIG. 2b shows a schematic cross section of the curved target of FIG. 2a, hit at a flat angle by the electron beam;

(6) FIG. 3a shows a schematic drawing of parts of an inventive X-ray apparatus, with a liquid metal jet target;

(7) FIG. 3b shows schematically another embodiment of an inventive X-ray device, with an electron beam deflection device for moving a focal spot of an electron beam on a curved target;

(8) FIG. 3c shows schematically an X-ray device with an electron beam focusing device for adjusting the size of the focal spot on a curved target in accordance with the invention;

(9) FIG. 4a shows schematically a front view of a circular focal spot on a liquid metal jet target, in accordance with the invention;

(10) FIG. 4b shows schematically a cross-section of the target of FIG. 4a through the focal spot;

(11) FIG. 4c shows schematically a front view of an ellipsoidal focal spot on a liquid metal jet target, in accordance with the invention;

(12) FIG. 5 shows schematically another embodiment of an inventive X-ray device, with capillary optics;

(13) FIG. 6 shows a diagram illustrating the ratio of mean X-ray intensities at alignment positions with the flux density maximized and the flux maximized, for different sample sizes, measured on an inventive X-ray apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENT

(14) Overview of the Invention

(15) The invention proposes an X-ray apparatus with an X-ray source, in particular a microfocus X-ray source, which allows for a continuous variation of the position of the electron beam on the target, in particular a liquid metal jet target, preferably in two directions. In other words, the position of the focal spot of the electron beam is variable. To alter the spot position, the electron beam can be deflected by applying an electric and/or magnetic field to the electron beam.

(16) As an advantage of the variable spot position, it is possible to align the X-ray source and a subsequent X-ray optics in a fast and comfortable way. In the state of the art, the alignment is done only mechanically. Due to the backlash of the mechanics in and/or at the optics housing it is difficult and time consuming to optimize the alignment (which is done by increasing the photon flux of the primary beam). However, by varying the spot position on the target, the relative position of the X-ray optics and the focal spot can be changed and thus optimized, in particular such that the photon flux or the photon flux density is maximized. As the spot position is not varied mechanically, but electro-magnetically via electrodes or coils (e.g. in the source), this alignment procedure is very reproducible with an accuracy in the m-range.

(17) Preferably, the target has a curved surface, for example wherein the target is of liquid metal jet type, what is preferred for the invention. By moving the electron beam perpendicular to the flow direction of the jet, the projected size of the X-ray emission area can be changed continuously. A combination of said microfocus X-ray source with curved (in particular elliptical or parabolic shape) multilayer mirrors allows to tailor the size, shape, divergence and intensity of the X-ray beam at the sample position. These properties of the X-ray beam may be changed continuously, allowing to adapt the X-ray beam to the needs of the experiment without the need of swapping optics. The optimization of the X-ray beam properties further results in an improved data quality and a shortened measurement time.

(18) When the electron beam is positioned close to the center position of the jet, the take-off angle of the X-ray beam is small, and X-ray self-absorption in the target is high, resulting in a small apparent source size with reduced integral flux, but increased flux density (flux density maximization). This small FWHM size of the X-ray source is the optimum X-ray beam condition for analyzing small samples; using focusing optics most of the photons are in the center of the X-ray beam hitting the small sample. By this, a diffracted intensity from the sample is maximized and the background noise is reduced, as the amount of photons that do not hit the sample, but just contribute to the background noise, is low.

(19) When shifting the electron beam away from the center towards the edge of the liquid metal jet target, the take-off angle is increased, enlarging the apparent spot size and reducing the self-absorption in the metal jet target. Consequently, using focusing optics the FWHM of the X-ray beam is increased and the peak intensity (flux density) is decreased (Flux maximization). Compared to the flux density maximization, the integral flux is now increased, as the self-absorption of the generated X-ray photons in the jet is reduced by placing the electron beam closer to the edge of the jet. This is the optimum condition for analyzing larger samples. It should be noted that by changing the position of a typical focal spot on a typical jet, the integral intensity can be changed by about 20%, and the flux density can be changed by about 50% with ease, compare FIG. 6. For this diagram, at different sample sizes, the flux and the flux density were maximized each, and the ratio of the mean fluxes incident on the respective sample diameter was determined at these alignment positions. According to the results, depending on the sample diameter used in the respective experiment, either a flux density or a flux optimized alignment is preferable.

(20) Preferably, the inventive X-ray apparatus is further capable of changing the size of the focal spot of the electron beam on the target by changing the focusing of the electron beam (variable spot size). In other words, the electron beam is widened or narrowed by electromagnetic means. This way the (microfocus) X-ray source is capable of changing the e-beam spot size on the metal jet target. It was found that the electron power density can be increased when the e-beam spot size decreases, without overheating the target. This can be used to increase the photon flux density, at the expense of integral photon flux. Small e-beam spots will result in small apparent X-ray spot sizes, advantageous for smaller samples, while larger e-beam spots will allow larger X-ray spot sizes at higher X-ray flux, advantageous for larger samples. Together with X-ray optics, this enables the system to control the size of the X-ray spot size on the sample position, the divergence of the X-ray beam and the integral flux downstream the X-ray optics.

(21) Description of Inventive Experimental Setups Shown in the Figures

(22) FIG. 1a shows schematically an embodiment of an inventive X-ray apparatus 1. An electron beam source 2 emits an electron beam 3. The electron beam 3 hits a target 4, here of a solid and flat type. A typical solid target material for use with the invention is copper. The area where the electron beam 3 hits the target 4 is called a focal spot 5. At the focal spot 5, X-rays are generated.

(23) X-ray optics 6, here of Montel type with two graded multilayer mirrors in a side by side orthogonal configuration, within an optic housing 6a, collect X-rays from a focus 7 of the X-ray optics 6 (compare focal length f.sub.1 on the entry side) and its close vicinity, thus forming an X-ray beam 8 directed to a sample position 9, where a sample to be investigated (not shown) is located. Note that the X-rays are collected at an angle of about 90 with respect to the electron beam propagation direction (here negative z). Beyond the sample position 9, an X-ray detector (not shown) is located. In the example shown, the X-ray beam 8 is focused at the sample position 9 (compare focal length f.sub.2 on the exit side); however it is also possible to parallelize (or otherwise shape) the X-ray beam 8 by means of the X-ray optics 6, in accordance with the invention.

(24) In the configuration shown, with the electron beam 3 being undeflected (i.e. propagating linearly), the focus 7 of the X-ray optics 6 deviates slightly from the focal spot 5 of the electron beam 3. Accordingly, only a small percentage of the X-rays generated at the target 4 or its focal spot 5, respectively, is collected by the X-ray optics 6.

(25) In order to increase the percentage of collected X-rays, the electron beam 3 may be deflected by means of an electron beam deflection device 10, here comprising a pair of charged electrodes (alternatively or in addition, the electron beam can be deflected by a magnetic field, generated by an electromagnetic coil). The deflection device 10 can deflect (shift) the electron beam 3 continuously in two orthogonal directions x, y perpendicular to its propagation direction z by adjusting a control voltage at the electrodes (or alternatively or in addition, adjusting a current at electromagnetic coils). In the embodiment shown, the defection device 10 is separate from the electron beam source 2; however, the deflection device 10 may also be integrated into the electron beam source 2.

(26) In FIG. 1b, the electron beam deflection device 10 has been activated in order to move the focal spot 5 on the target 4. After proper adjustment of the deflection device 10, namely slightly moving the focal spot 5 over a distance D basically in negative y direction or deflecting the electron beam 3 by a small angle to the right, respectively, the focal spot 5 overlaps with the focus 7 of the X-ray optics 6. Thus a high percentage of the X-rays generated at the focal spot 5 may be collected by the X-ray optics 6 and directed to the sample position 9. Note that the optimum position of the focal spot 5 is typically found by maximizing the photon flux or the photon flux density downstream the X-ray optics 6, such as at the sample position.

(27) It should be noted that the width of the electron beam as well as the width of the X-ray beam is shown enlarged in the figures, in order to increase comprehensibility. A typical distance D over which the focal spot 5 can be moved on the target 4 is about 200 m.

(28) FIG. 1c illustrates a variant of the embodiment of the X-ray apparatus 1 of FIG. 1a, but comprising an electron beam focusing device 11 (here comprising an electromagnetic coil assembly) in addition to the deflection device 10. Note that the focusing device 11 may be integrated into the deflection device 10 and/or into the electron beam source 2, if desired. The electron beam focusing device 11 allows to change the focusing of the electron beam 3, i.e. the width of the electron beam 3 on the target 4, by changing the electric currents through the coils of the coil assembly. By this means, the area of the focal spot can be adjusted directly.

(29) In the figure, the solid lines of the strongly narrowing electron beam 3 belong to a focal spot 5a with a small focal spot area Aa, whereas the dashed lines of the electron beam 3 only slightly narrowing belong to a focal spot 5b with a rather large focal spot area Ab; note that the areas Aa, Ab are shown in a projection each. Typically, the focusing device 11 allows an area change by a factor of up to five. By altering the focusing of the electron beam 3, some properties of the X-ray beam 8 at the sample position 9 can be altered, such as the beam divergence or the integral photon flux, without changing the electron beam power.

(30) FIGS. 2a and 2b illustrate focal spots 5a, 5b on a curved target 4 for an electron beam 3 for different positions of the focal spots 5a, 5b on the target 4. The figures show cross-sections through the target 4, here a circular liquid metal jet propagating in the x direction with a radius of curvature R, in a plane (yz-plane) including the electron beam propagation direction (negative z) and perpendicular to the jet propagation direction x. In this plane, the target surface 12 is curved. The electron beam 3 can be moved at least within this plane, i.e. here basically in y direction.

(31) If the electron beam 3 hits the target 4 basically perpendicular to the curved target surface 12 (compare angle of about 80), as shown in FIG. 2a, the apparent focal spot size SZ in z direction is rather small, in particular smaller than the focal spot size SY in y direction. Since the X-rays originate from a small area, a high photon flux density can be achieved. On the other hand, if the electron beam 3 hits the curved target surface 12 under a relatively flat angle (compare angle of about 35), as shown in FIG. 2b, the apparent focal spot size SZ in z direction is rather large, in particular larger than the focal spot size SY in y direction.

(32) In the configuration of FIG. 2a, with a small focal spot size SZ in z direction, self-absorption of X-rays is rather large when the X-ray beam is taken on the left side in y direction (i.e. perpendicular to the electron beam propagation direction): X-rays generated on the right hand side of the focal spot 5a have to pass much target material before leaving the target 4. In contrast, in the configuration of FIG. 2b, with a large focal spot size SZ in z direction, self-absorption is relatively weak: Even X-rays generated on the right hand side of the focal spot 5b have to pass only few target material before leaving the target 4. Accordingly, the latter configuration yields a high integral photon flux.

(33) Preferably, an inventive X-ray apparatus is switchable between the two configurations of FIG. 2a and FIG. 2b electrically, for a quick change of X-ray beam characteristics between different measurements. Towards this end, a switching element (schematically indicated in the drawing of FIGS. 2a and 2b with a double arrow) is disposed, structured and dimensioned to switch the apparatus between two operation modes. In the first operation mode, the photon flux is maximized and in the second operation mode, a photon flux density is maximized.

(34) In the example shown, the diameter 2*R of the target 4 (representing its extension both in y and z) is more than a factor F of F=5 larger than both SY and SZ for the two shown configurations.

(35) FIG. 3a shows schematically parts of an embodiment of an inventive X-ray apparatus 1, wherein the electron beam 3 emitted by an electron beam source 2 passes through an electron beam deflection device 10 (or a combined electron beam deflection and focusing device), suitable for deflecting the electron beam 3 in x and y direction, and hits a liquid metal jet target 4 at a focal spot 5 where X-rays are generated.

(36) A continuous stream of liquid metal (for example consisting of gallium) is pumped through a circuit 13 by means of a pump 14 and directed via a nozzle 15 into a funnel type recovery unit 16; between the nozzle 15 and the recovery unit 16, the free metal stream constitutes the jet type target 4. If needed, the circuit 13 includes a tempering stage for heating and/or cooling the metal within the circuit 13 (not shown). Note that the jet has typically a diameter of about 50-250 m, whereas the electron beam diameter is typically 100 m or less. Marked with a dashed box are the parts of the X-ray apparatus 1 which should be located in a vacuum chamber 17; in particular, the electron beam 3 should only propagate inside the vacuum chamber 17.

(37) FIG. 3b illustrates an embodiment of an inventive X-ray apparatus 1 similar to the one shown in FIG. 1a, but with a curved target 4, namely a liquid metal jet target 4 (as shown in FIG. 3a, for example). The jet propagates in x direction, i.e. perpendicular to the electron beam 3 and the X-ray beam 8. By means of the electron beam deflection device 10 (or a combined electron beam deflection and focusing device) the electron beam 3 can be deflected in x and y direction.

(38) FIG. 3c shows an X-ray apparatus 1 also similar to the one shown in FIG. 1a, again with a liquid metal jet target 4. Here the size of the focal spot 5 can be changed by means of an electron beam focusing device 11. By changing the focal spot size (in x- and/or y-direction), the properties of the X-ray beam 8 downstream the X-ray optics 6 can be altered, in particular to obtain desired properties at the sample position 9. In particular, the properties of the X-ray beam 8 can be altered such that alternatively a maximum photon flux or a maximum photon flux density of the X-ray beam 8 can be obtained, in accordance with the invention.

(39) FIG. 4a and FIG. 4b illustrate in more detail a focal spot 5 of an electron beam on a target 4, here a liquid metal jet target, and their extension proportions in accordance with the invention. FIG. 4a shows a front view perpendicular to the z direction in which the electron beam propagates; FIG. 4b shows a cross-section in the plane perpendicular to the jet propagation direction x.

(40) The size (or extension) SX of the focal spot 5 in x direction is here more than five times smaller than the size (or extension) TX of the target 4 in x direction (Note that typically, the jet is a some tens of mm in x direction, which is the direction in which the jet propagates). In the example shown, the size (or extension) SY of the focal spot 5 is about 3 times smaller than the size (or extension) TY of the target 4 in y direction. The size (or extension) SZ of the focal spot in z direction (resulting from the propagation depth of electrons in the target material) is about 5 times smaller than the size (or extension) TZ of the target 4 in z direction here. So all in all, for all directions (x, y, z), the size of the focal spot 5 is at least about a factor F, with F=3, times smaller than the size of the target 4. Note that in accordance with the invention, a factor F=1.5 is sufficient, but a factor F=2 is preferred, and a factor F=5 is particularly preferred.

(41) FIG. 4c illustrates a focal spot 5 of elliptical shape. Here, too, the sizes SX, SY (and SZ, not shown) are at least about a factor F, with F=3, times smaller than the corresponding size TX, TY (and TZ, not shown) of the target 4. The target 4 is here of liquid metal jet type again. An elliptical electron beam may be the preferred choice because it can produce a circular X-ray spot (apparent focal spot) when viewed along the y direction (at an 90 angle with respect to the electron beam and the metal jet propagation direction); towards this direction the X-ray optics is placed then, receiving an X-ray beam with circular cross-section.

(42) FIG. 5 illustrates another embodiment of an inventive X-ray apparatus 1, similar to the one shown in FIG. 1a, but with capillary optics used as X-ray optics 6 for directing the X-ray beam 8 to the sample position 9. The capillary optics include one or more hollow, bent tubes (capillaries), at the internal surfaces of which total reflection of the x-rays occurs, so the X-rays may be guided by means of the capillaries (not shown in detail). The target 4 is of liquid metal jet type.

(43) In summary, the present invention proposes to align the focal spot of an electron beam and the focus of X-ray optics by deflecting the electron beam, thus allowing to do without mechanical fine alignment of the X-ray optics in an X-ray apparatus. Furthermore this invention allows to change the maximized X-ray beam properties downstream the X-ray optics by controlling shape and position of the focal spot on the target, in particular a target with a curved surface.