Liquid jet target X-ray source

Abstract

An x-ray source is provided that includes a target generator configured to generate a liquid jet having an elongated cross-section with a major axis and a minor axis, an electron source configured to generate an electron beam configured to interact with the liquid jet in an interaction region to generate x-ray radiation, and an x-ray transmissive window configured to transmit the x-ray radiation generated in the interaction region, the x-ray transmissive window positioned to extract the x-ray radiation at an angle α relative to the major axis, and the target generator configured to generate the liquid jet such that the liquid jet has a thickness in the interaction region along a direction of propagation of the electron beam that is less than an electron penetration depth of the electron beam in the liquid jet. Corresponding methods for generating x-ray radiation are also provided.

Description

Technical Field

[0001] The present invention relates to a liquid jet target X-ray source and related methods.

Background Art

[0002] Liquid jet target X-ray sources are generally known in the art. An electron beam is directed at a liquid jet of target material, and X-ray radiation is generated when the electron beam impinges on the target. Various shapes for the electron beam cross-section and the liquid jet target have been studied. For example, WO2019 / 106145 discloses an X-ray source comprising a liquid target former configured to shape the liquid target to have a non-circular cross-section with respect to the flow axis of the liquid target. Generally, such liquid targets can have an oval or elliptical cross-sectional shape. The cross-section can even be elongated enough that the surface of the liquid target where the electron beam impinges can be seen as substantially flat. In extreme cases, such a liquid target can be referred to as a liquid curtain. Using such a shape, a wider collision surface for the electron beam can be used without the need to increase the flow rate of the liquid target, which facilitates the use of more than one electron beam on the same liquid jet.

Summary of the Invention

[0003] Below, reference will be made to the X-ray absorption length, which is defined as the distance over which the X-ray beam is reduced by a factor of 1 / e due to absorption in the target material. <00000​​​​​​​​​​​​​​Here, d is in microns (μm), E0 is the energy of the arriving electron measured in keV units, and ρ is g / cm 3 This is the density of the target material measured in units.

[0007] The present invention provides a liquid jet target X-ray source that uses a liquid jet target having an elongated, preferably convex, cross-section with a thickness in the direction of electron beam propagation that is smaller than the electron penetration depth into the target. Preferably, the elongation (eccentricity) of the liquid jet cross-section is significant enough that the electron impact surface can be considered substantially flat. By having a thickness smaller than the electron penetration depth, the apparent X-ray spot will be partially determined by the thickness of the target jet along the direction of electron beam propagation. The liquid jet can propagate freely into the surrounding environment, at least at the impact surface. The material of the liquid jet can therefore be exposed to the environment in the chamber of the X-ray source. The liquid jet is preferably a liquid metal jet. The liquid metal can be an alloy. Examples of metals suitable for use in the present invention are In, Sn, Pb, Bi, and Ga. As is generally known in the art, the use of a liquid metal target offers several advantages over other techniques. For example, since the target is continuously regenerated and is already in a liquid state, any problems associated with permanent target damage are eliminated. Such liquid metal targets can therefore support higher electron beam power and, consequently, provide an increased X-ray flux compared to other types of X-ray sources.

[0008] The electron beam is typically generated using an accelerating voltage of at least 10 kV. In some implementations, the accelerating voltage can exceed at least 50 kV, or even 100 kV. The output of the electron beam can be at least 38 W, for example, at least 50 W or at least 100 W.

[0009] While the cross-section of each conceivable liquid jet target is not necessarily elliptical, the cross-section can still be described as having a major axis along the maximum dimension and a minor axis along the minimum dimension. The major axis thus extends from edge to edge (vertex to vertex) of the elongated cross-section, and the minor axis extends from face to face.

[0010] To understand the principles of the present invention in general terms, one can even assume that the cross-section of the target jet is a rectangle with sides corresponding to the major and minor axes, respectively. However, it should be understood that liquid jet targets in actual implementations may have an elliptical or at least a convex cross-section without sharp corners. Other cross-sectional shapes are also conceivable, such as a substantially flat portion connecting two round segments at the edge, i.e., similar to a 2D dumbbell shape. The cross-sectional shape of the target jet can still be characterized by one major axis and one minor axis. Generally, and even in the case of more specific shapes, the major axis can be defined as being along the maximum dimension of the target cross-section, and the minor axis can be defined as the perpendicular bisector to the major axis.

[0011] In the first embodiment, the X-ray source is configured such that the electron beam collides with the target at a distance from the edge (or apex) of the target in a direction perpendicular to the long axis. For example, the electron beam, as measured by the full width at half maximum, is at least the X-ray absorption length within the target material from the edge / apex of the liquid target. The generated X-ray emission is captured by reflection at an angle to the long axis of the target jet, typically such that the apparent X-ray spot along the extraction angle is smaller than the extension of the electron beam perpendicular to the long axis.

[0012] In a second embodiment, the X-ray source is configured such that the electron beam collides with the target at its edge or apex, with the center of the electron beam located less than the X-ray absorption length from the edge / apex of the target jet. The generated X-ray emission can then be extracted from the X-ray source in a direction parallel to the long axis of the target jet without being bothered by excessive reabsorption. As can be understood, with respect to an extraction angle parallel to the long axis of the target jet cross section, the apparent X-ray spot has an extension in a direction across the target jet thickness equal to the thickness of the target, i.e., the size of the X-ray spot will be determined in one dimension by the target jet thickness along the short axis of the target jet.

[0013] In the third embodiment, the generated X-ray emission is extracted by transmission, i.e., generally in the direction of electron beam propagation. The achievable spot size of the X-ray emission is limited by the scattering of the electron beam within the target, which results in the gradual spreading of the electron beam as it penetrates the target material. Therefore, the thinner the target is in the direction of electron beam propagation, the smaller the spreading of the electron beam, and consequently, the smaller the X-ray spot size. In this embodiment, the electron beam may collide with the target at its edge / vertex or at a distance from the edge / vertex.

[0014] Within the scope of the present invention, several modifications and variations are possible. In particular, X-ray sources having more than one target or more than one electron beam are conceivable within the scope of the present invention. Furthermore, the types of X-ray sources described herein may be advantageously combined with X-ray optics and / or detectors tailored to specific applications, as exemplified by medical diagnostics, non-destructive testing, lithography, crystal analysis, microscopy, materials science, microscopic surface physics, protein structure determination by X-ray diffraction, X-ray spectroscopy (XPS), limiting-size small-angle X-ray scattering (CD-SAXS), wide-angle X-ray scattering (WAXS), and X-ray fluorescence (XRF).

[0015] In the following embodiments for carrying out the invention, reference is made to the accompanying drawings.

Brief Description of the Drawings

[0016] [Figure 1a] Schematically shows a liquid jet X-ray source. [Figure 1b] Schematically shows a liquid jet X-ray source provided with a magnetic field generator for shaping a liquid jet. [Figure 2] Illustrates a first embodiment of the present invention. [Figure 3] Illustrates a second embodiment of the present invention. [Figure 4] It is a graph illustrating how the spot size and X-ray beam can change according to the extraction angle. [Figure 5] Illustrates a third embodiment of the present invention. [Figure 6] Shows typical X-ray absorption lengths and electron penetration depths for Ga and In. [Figure 7] Schematically illustrates various target jet cross-sectional shapes. [Figure 8] Illustrates a method according to the present invention.

Embodiments for Carrying out the Invention

[0017] The X-ray source according to the present invention is schematically shown in Fig. 1a. The electron beam 100 is generated from an electron source 102 such as an electron gun provided with a high voltage cathode, for example, and the liquid jet target 104 is provided from a target generator 106. The electron beam 100 is directed towards the collision portion of the liquid target 104 such that the electron beam 100 interacts with the liquid target 104 to generate X-ray radiation 108. The liquid target 104 is preferably collected and returned to the target generator 106 by a pump 110 such as a high pressure pump adapted to raise the pressure to at least 10 bar, preferably up to at least 50 bar, for generating the liquid target 104.

[0018] The liquid target 104 can be formed by a target generator 106 using a nozzle through which a fluid such as, for example, a liquid metal or a liquid alloy is ejected to form the liquid target 104. It should be understood that an X-ray source comprising a plurality of liquid targets and / or a plurality of electron beams is possible within the scope of the concept of the present invention. In order to generate a liquid jet with a high propagation speed, the pressure used to eject the liquid (e.g., a metal or an alloy) through the nozzle can be at least 50 bar, or at least 100 bar, or at least 200 bar. A high-pressure pump, or in some cases a two-stage pump configuration, is used to recirculate the liquid and raise the pressure to the desired level before the liquid is ejected through the nozzle.

[0019] Still referring to FIG. 1a, the X-ray source can comprise an X-ray window (not shown) configured to allow the X-ray radiation generated from the interaction of the electron beam 100 and the liquid target 104 to pass through. The X-ray window can be arranged substantially perpendicular to the direction of travel of the electron beam.

[0020] Referring now to FIG. 1b, a magnetic field generator 103 is shown in relation to the target generator 106 and the liquid target 104. The magnetic field generator 103 and the liquid target 104 can be provided in an X-ray source that can be configured similarly to the X-ray source discussed in relation to FIG. 1a. The magnetic field generator 103 can extend further along the flow axis, and it should be understood that the arrangement of the magnetic field generator 103 shown is only an example among several different configurations. In this example, the magnetic field generator 103 can comprise a plurality of elements for generating a magnetic field for modifying or shaping the cross-section of the liquid target 104. Examples of such means can include, for example, electromagnets, which can be arranged on different sides of the path of the liquid target 104 so as to affect the shape of the liquid target 104.

[0021] A liquid jet target X-ray source is provided, configured such that an electron beam collides with a liquid jet target to produce X-ray emission, the liquid jet having an elongated cross-section. Generally, it is preferable that the electron beam collides with the target jet along the minor axis of the elongated cross-section of the target jet. The X-ray emission is produced in an interaction region defined by the extension of the electron beam and the penetration of the electron beam into the target material. The electron beam may have an elliptical cross-section having a major axis (referred to herein as width) perpendicular to the direction of travel of the liquid jet and a minor axis (height) perpendicular to the direction of travel of the liquid jet. The interaction region will thus have a cross-section defined by the width and height of the electron beam cross-section. The width of the liquid jet may be at least 500 μm, for example, at least 1000 μm.

[0022] In prior art systems where the thickness of the target jet is greater than the electron penetration depth into the target material, the X-ray emission will be generated in a region of the target limited by the electron penetration depth. If the target jet is thinner than the electron penetration depth along the electron beam propagation direction, according to embodiments of the present invention, the interaction region will be determined by the thickness of the target jet. The size of the X-ray spot along any extraction angle, i.e., in any extraction direction, will be the projection of the interaction region in that direction.

[0023] For efficient extraction of X-ray radiation, however, the generated X-ray radiation should not be excessively reabsorbed by the target. The distance from any point in the interaction region to the surface of the target jet along the extraction direction should therefore be less than the X-ray absorption length. However, it should be understood that the X-ray absorption length does not define any abrupt cutoff, but there is a gradual decrease in the X-ray beam. For this reason, the X-ray absorption length is used as a convenient measure of characteristic X-ray reabsorption.

[0024] In the first embodiment, referring to Figure 2, the X-ray source is configured such that the electron beam collides with the target at a distance from the edge or apex of the target. For example, the electron beam, as measured by the full width at half maximum, is at least the X-ray absorption length within the target material from the edge / apex of the liquid jet target. The generated X-ray emission is extracted by reflection at an angle to the long axis of the target jet. For extraction angles perpendicular to the long axis, i.e., along the short axis of the target jet cross section, the effective X-ray spot size will be determined by the extension of the electron beam. For non-perpendicular extraction angles, the effective spot size will be determined by the geometric projection of the interaction region along that direction and by the electron penetration depth or the target thickness (whichever is shorter). For a given extraction angle, the effective spot size of the X-ray emission can therefore be reduced by making the target jet thickness shorter than the electron penetration depth. Assuming that the X-ray emission is extracted at an angle α with respect to the long axis of the target jet, and noting that only the emission crossing distances less than the X-ray absorption length in the target contributes to the X-ray spot size, the effective spot size can generally be expressed as follows:

[0025]

number

[0026] Here, S eff λ is the effective spot size of the X-ray emission along the extraction angle α with respect to the long axis of the target jet, w is the width of the electron beam when it collides with the target jet, λ is the X-ray absorption length, t is the thickness of the target in the direction of electron beam propagation, and d is the electron penetration depth. effThe X-ray spot size in a dimension orthogonal to is determined by the height of the electron beam (i.e., the size of the electron beam along the direction of propagation of the target jet), and will be equal to the height of the electron beam if the X-ray emission is extracted in a direction orthogonal to the direction of propagation of the target jet. As is clear from (2) above, when the extraction angle is zero, the spot size will be zero. However, if reabsorption becomes significant, the total X-ray flux will also decrease. This can be understood as the effective penetration depth, i.e., only electrons that have penetrated to a depth less than the projection of the absorption length (λ sin α) will be able to generate X-ray emission that contributes to the extracted X-ray beam. In embodiments of the present invention, the thickness of the target jet in the direction of electron beam propagation is less than the electron penetration depth, i.e., t <dである。

[0027] In the second embodiment, referring to Figure 3, the interaction region is located at or near the edge / vertex of the target jet. The X-ray source according to the second embodiment is configured such that the electron beam collides with the target at its edge or vertex, which typically means that the center of the electron beam is less than the X-ray absorption length from one edge / vertex of the target jet. The generated X-ray emission can then be extracted from the X-ray source in a direction parallel to the long axis of the target jet cross-section without being affected by excessive reabsorption. For extraction angles close to the short axis, this embodiment will produce an X-ray spot similar to that of the first embodiment described above. However, for extraction angles along or close to the long axis of the target jet cross-section, this second embodiment may provide a smaller X-ray spot while maintaining the X-ray flux. Different combinations of electron beam width, electron penetration depth, jet thickness, and X-ray absorption length will yield different characteristics. In this embodiment, the extraction angle parallel to the long axis of the target cross-section will produce an X-ray spot size determined by the maximum thickness of the portion of the target jet exposed to the electron beam (provided it is less than the electron penetration depth). In the perpendicular direction, i.e., along the direction of target jet propagation, the X-ray spot size will be determined by electron beam focusing along with electron beam scattering within the target material. Extraction of X-ray emission parallel to the long axis of the target jet cross-section, i.e., from the edges / apex of the target jet, is not affected by reabsorption, so the spot size will be independent of the X-ray absorption length. Thus, the effective spot size can generally be expressed as follows:

[0028]

number

[0029] Here, to repeat, α is the extraction angle with respect to the long axis of the target jet cross-section, w is the width of the electron beam when it collides with the target jet, t is the thickness of the target in the propagation direction of the electron beam, and in an embodiment of the present invention, it is considered that t < d.

[0030] In the second embodiment, X-ray emission will also occur when the extraction angle is zero (i.e., when the extraction angle is parallel to the long axis). If the respective projections of the absorption lengths are longer than the width and depth of the interaction region, respectively, the total X-ray flux will be independent of the extraction angle for the first approximation.

[0031] Figure 4 shows a graph illustrating how the spot size and X-ray flux can vary as a function of the extraction angle for the first embodiment ("surface emitter") and the second embodiment ("edge emitter"). In the example shown, the electron beam spot size is set to 4×1 units (width and height respectively), the jet thickness is set to 1 unit (shorter than the electron penetration depth according to the present invention), while the absorption length is set to 4 units. The apparent spot size is calculated according to the above expressions. The total X-ray flux is calculated as the volume of a part of the interaction region contributing to the emitted X-ray emission. If it is desirable to obtain an X-ray spot symmetric with respect to the edge emitter, the extraction angle along the long axis of the target jet would preferably give an apparent spot size of 1×1 unit and a total X-ray flux of 4 units. For the surface emitter, an angle of about 7 degrees gives the corresponding symmetric spot and a total light flux of about 2 units. The preferred extraction angle for this embodiment can generally be about 3 - 10 degrees, or at least less than about 20 degrees.

[0032] The third embodiment, referring to Figure 5, utilizes a transmission target shape, which means that the X-ray emission emitted in the direction of the electron beam is utilized. Typically, a circular electron beam spot would be used in the third embodiment. As electrons penetrate the target jet, they will be scattered in such a way that the width of the electron beam widens, resulting in an expansion of the effective X-ray spot. Assuming a point-like electron beam, the width at the depth inside the target material corresponding to the electron penetration depth can be approximated as follows:

[0033]

number

[0034] Here, y is the width expressed in microns (μm). From this, the electron is tan from the direction of the incoming electron beam. -1 The electrons are distributed within a cone or frustum with an apex angle of (0.077 / (2x0.1)), and the effective or apparent spot size will increase as the electrons penetrate the target material. The effective or apparent spot size can be written as follows:

[0035]

number

[0036] Here, S eff w is the effective spot size of the X-ray emission, w is the width of the electron beam when it strikes the target jet, t is the thickness of the target in the direction of electron beam propagation, and d is the electron penetration depth. Here, it was assumed that the thickness of the target was less than the X-ray absorption length.

[0037] If the target is thinner than the electron penetration depth in the direction of electron beam propagation, this will limit the amount of scattering that can occur, thereby limiting the spreading and thus the effective spot size. The spreading of the X-ray spot will therefore be smaller than the width y, given by equation (4), which would have resulted if the target were thicker than the electron penetration depth. On the other hand, there will also be less target material available for electrons to interact with, which will reduce the amount of X-ray radiation produced compared to a thicker target (which is common for transmission targets). As an example, consider a situation where the electron penetration depth is comparable to the electron beam spot size. In this case, as electrons penetrate the target, the spot will spread by approximately 80%. Further reducing the electron beam spot will result in a larger relative change in spot size. Therefore, limiting electron scattering by making the target thinner is important for achieving small spot sizes for transmission targets.

[0038] The electron penetration depth depends on the electron energy and material properties, as shown by equation (1) above. Similarly, the X-ray absorption length depends on the energy of the X-ray emission and the material properties. X-ray absorption is an inherently nonlinear process in that the discreteness of the electron excitation energy will cause jumps in the absorption spectrum. To illustrate this, Figure 6 shows typical X-ray absorption lengths for Ga and In. X-ray absorption is conventionally quantified by the mass decay coefficient μ / ρ, where μ is the linear decay coefficient and ρ is the density. The densities of Ga and In are 5.9 and 7.31 g / cm³, respectively, for the plots shown in Figure 6. 3 It was set to [value]. For comparison, the figure also shows the electron penetration depth calculated using equation (1).

[0039] In general, in embodiments where the X-ray spot size is determined by the target jet thickness, the problem of providing an X-ray spot of consistent size is translated into the problem of providing a liquid jet of consistent thickness. Target jets with non-circular cross-sections can be generated, for example, using a nozzle with a non-circular opening, as described in WO2019 / 106145 referenced above. Typical nozzle openings are rectangles with rounded corners and aspect ratios, for example, 1:2 or 1:4. Alternatively, or in addition, liquid jet targets can be formed using a magnetic field generator configured to generate a magnetic field that shapes the liquid target into a desired cross-sectional shape. Figure 7 illustrates some possible cross-sectional shapes. While it is preferable to have a target cross-section that is essentially elliptical, as shown in Figure 7(a), the target may have a dumbbell-shaped cross-section, as shown in Figure 7(b), or even an asymmetrical cross-section, as schematically shown in Figure 7(c).

[0040] In embodiments of the present invention, the thickness of the target jet in the electron beam propagation direction may be in the range of 5 to 150 μm, and the width of the target jet is typically in the range of 200 to 500 μm or greater. The width of the target jet is not critical to the function and in some implementations it may have a width of at least 2000 or 3000 μm. The exemplary cross-sectional shape of the target jet in the interaction region is elliptical and, for an electron energy of about 100 keV, may be about 300 μm along the major axis (i.e. width) and about 10 μm along the minor axis (i.e. thickness), as shown in Figure 6.

[0041] A corresponding method 800 is schematically illustrated in Figure 8. In step 801, a liquid jet having an elongated cross-section with a long axis and a short axis is provided. In step 802, an electron beam is provided that interacts with the liquid jet in an interaction region to produce X-ray emission. In step 803, the produced X-ray emission is extracted at an angle α with respect to the long axis. According to embodiments of the present invention, the liquid jet has a thickness less than the electron penetration depth of the electron beam in the liquid jet, along the propagation direction of the electron beam. The following is a direct reproduction of the claims as originally filed. [1] A target generator configured to generate a liquid jet having an elongated cross-section with a long axis and a short axis, An electron source configured to generate an electron beam configured to interact with the liquid jet in an interaction region to generate X-ray emission, An X-ray source comprising an X-ray transmission window configured to transmit X-ray radiation generated in the interaction region, wherein the X-ray transmission window is positioned to extract X-ray radiation at an angle α with respect to the long axis, The target generator is configured to generate the liquid jet such that, in the interaction region, the liquid jet has a thickness less than the electron penetration depth of the electron beam in the liquid jet, along the propagation direction of the electron beam. [2] The X-ray source according to [1], wherein the electron source and the target generator are configured such that the electron beam collides with the liquid jet substantially perpendicular to the long axis. [3] The electron source and the target generator are configured such that the electron beam collides with the liquid jet at a distance shorter than the X-ray absorption length from the edge of the liquid jet, An X-ray source as described in [1] or [2], wherein α is less than 20 degrees. [4] The X-ray transmission window is positioned downstream of the liquid jet with respect to the electron beam, The X-ray source described in [1] or [2], wherein α is approximately 90 degrees. [5] The electron source and the target generator are configured such that the electron beam collides with the liquid jet at a distance from the edge of the liquid jet that is longer than the X-ray absorption length, The X-ray transmission window is positioned upstream of the liquid jet with respect to the electron beam. An X-ray source as described in [1] or [2], wherein α is less than 20 degrees and greater than 0 degrees. [6] The X-ray source described in [5], where α is 3 to 10 degrees. [7] The X-ray source according to [3], wherein α is less than 10 degrees, preferably less than 5 degrees, most preferably about 0 degrees. [8] The X-ray source according to any one of [1] to [7], wherein the target generator is configured to generate a target jet having a thickness of 5 to 150 μm, preferably 5 to 25 μm, along the propagation direction of the electron beam. [9] A method for generating X-ray radiation, To provide a liquid jet having an elongated cross-section with a long axis and a short axis, To provide an electron beam that interacts with the liquid jet in an interaction region to generate X-ray emission, The system includes extracting X-ray radiation at an angle α with respect to the aforementioned major axis, The method wherein the liquid jet has a thickness in the interaction region that is less than the electron penetration depth of the electron beam in the liquid jet, along the propagation direction of the electron beam.

[10] The electron beam collides with the liquid jet at a distance shorter than the X-ray absorption length from the edge of the liquid jet, and α is less than 20 degrees. The method described in [9].

[11] X-ray radiation is extracted downstream from the liquid jet relative to the electron beam, α is approximately 90 degrees, as described in [9].

[12] The electron beam collides with the liquid jet at a distance from the edge of the liquid jet that is longer than the X-ray absorption length, The X-ray emission is extracted upstream from the liquid jet relative to the electron beam. The method described in [9], where α is less than 20 degrees and greater than 0 degrees.

[13] The method described in

[12] , where α is 3 to 10 degrees.

[14] The method according to

[10] , wherein α is less than 10 degrees, preferably less than 5 degrees, most preferably about 0 degrees.

[15] The method according to any one of [9] to

[14] , wherein the liquid jet has a thickness of 5 to 150 μm, preferably 5 to 25 μm, along the propagation direction of the electron beam.

Claims

1. A target generator configured to generate a liquid jet having an elongated cross-section with a long axis and a short axis, An electron source configured to generate an electron beam configured to interact with the liquid jet in an interaction region to generate X-ray emission, An X-ray source comprising an X-ray transmission window configured to transmit X-ray radiation generated in the interaction region, wherein the X-ray transmission window is positioned to extract X-ray radiation at an angle α with respect to the long axis, The target generator is configured to generate the liquid jet such that, in the interaction region, the liquid jet has a thickness less than the electron penetration depth of the electron beam in the liquid jet, along the propagation direction of the electron beam.

2. The X-ray source according to claim 1, wherein the electron source and the target generator are configured such that the electron beam collides with the liquid jet substantially perpendicular to the long axis.

3. The electron source and the target generator are configured such that the electron beam collides with the liquid jet at a distance shorter than the X-ray absorption length from the apex of the liquid jet. The X-ray source according to claim 1 or 2, wherein α is less than 20 degrees.

4. The X-ray transmission window is positioned downstream of the liquid jet with respect to the electron beam. The X-ray source according to claim 1 or 2, wherein α is approximately 90 degrees.

5. The electron source and the target generator are configured such that the electron beam collides with the liquid jet at a distance longer than the X-ray absorption length from the apex of the liquid jet. The X-ray transmission window is positioned upstream of the liquid jet with respect to the electron beam. The X-ray source according to claim 1 or 2, wherein α is less than 20 degrees and greater than 0 degrees.

6. The X-ray source according to claim 5, wherein α is 3 to 10 degrees.

7. The X-ray source according to claim 3, wherein α is less than 10 degrees.

8. The X-ray source according to claim 1 or 2, wherein the target generator is configured to generate the liquid jet having a thickness of 5 to 150 μm along the propagation direction of the electron beam.

9. A method for generating X-ray radiation, To provide a liquid jet having an elongated cross-section with a long axis and a short axis, To provide an electron beam that interacts with the liquid jet in an interaction region to generate X-ray emission, The system includes extracting X-ray radiation at an angle α with respect to the aforementioned major axis, The method wherein the liquid jet has a thickness in the interaction region that is less than the electron penetration depth of the electron beam in the liquid jet, along the propagation direction of the electron beam.

10. The electron beam collides with the liquid jet at a distance shorter than the X-ray absorption length from the apex of the liquid jet, and α is less than 20 degrees. The method according to claim 9.

11. The X-ray emission is extracted downstream from the liquid jet relative to the electron beam. The method according to claim 9, wherein α is approximately 90 degrees.

12. The electron beam collides with the liquid jet at a distance longer than the X-ray absorption length from the apex of the liquid jet. The X-ray emission is extracted upstream from the liquid jet relative to the electron beam. The method according to claim 9, wherein α is less than 20 degrees and greater than 0 degrees.

13. The method according to claim 12, wherein α is 3 to 10 degrees.

14. The method according to claim 10, wherein α is less than 10 degrees.

15. The method according to claim 9, wherein the liquid jet has a thickness of 5 to 150 μm along the propagation direction of the electron beam.

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