X-ray source having a liquid film target

Abstract

An X-ray source is provided, comprising an electron source configured to provide an electron beam; a jet generator configured to provide a freely propagating liquid metal jet; an impact surface arranged so that the liquid metal jet can impact on the impact surface and form a liquid metal film; wherein the electron beam is arranged to impact the liquid metal film. The X-ray source further comprises an exit window arranged to transmit X-ray radiation generated by interaction between the electron beam and the liquid metal film, wherein the impact surface is formed on the exit window. A corresponding method for generating X-ray radiation is also provided.

Description

[0001] X-RAY SOURCE HAVING A LIQUID FILM TARGET

[0002] Technical field

[0003] The present invention relates to electron-impact X-ray sources, in which X-ray radiation is generated by interactions between an electron beam and a target material.

[0004] Background

[0005] Electron-impact X-ray sources are generally known in the art. In this kind of sources, X-ray radiation is generated by interactions between an electron beam and a target material. By focusing the electron beam onto the target, an X-ray source having a fairly small spot size can be obtained. A small spot size is often advantageous because it enables high-resolution X-ray imaging, for example.

[0006] In many applications, both high power and high brightness are sought, which means that X-ray radiation should be generated using a high-power electron beam focused to a small spot size. The local power loading of the target then becomes a limiting factor. To address the issue of power loading, some X-ray sources use a liquid target that is flowing through the region where the electron beam is focused to generate X-ray radiation, a typical example being liquid-metal-jet X-ray sources.

[0007] There is a desire in the art of X-ray sources, however, for still higher brightness and still higher powers.

[0008] The over-arching desire for high brightness may be seen from two sides, either the desire is to have more photons from a given spot, implying higher power, or else the desire is to have a given photon flux from a smaller spot, implying a denser electron beam. For spot sizes of the order of tens of micrometers, liquid metal jet sources have proven to be able to provide considerably higher brightness than solid target sources. However, the minimum spot size is limited by scattering of electrons within the liquid jet. For spot sizes below one micrometer, solid transmission targets seem to be state of the art. Here, the scattering of electrons is not a limiting factor since the target consists of a thin film that a majority of the electrons penetrate. However, the brightness may be limited by the thermal load that the target is able to withstand. The acceptable thermal load for a transmissiontarget is proportional to the diameter of the spot. This is because cooling becomes less efficient for larger spot sizes where heat conduction is mainly in an axial direction for the interior of the spot. This means that for a transmission target, the available X-ray brightness decreases as the spot size is increased. Thus, there is a range of powers and spot sizes where neither a liquid metal jet source, nor a solid target transmission source seems to be the preferred choice.

[0009] US 2007 / 258563 Al discloses an anode module for a liquid metal anode X-ray source. The anode module is designed to provide a high yield of X-ray radiation by arranging an exit window at a non-normal angle relative to the electron beam. The idea behind such non-normal angle for the exit window is to extract X-ray radiation at an angle at which the emitted X-ray distribution has a maximum. For an electron energy of 500 keV, this angle is stated to be 15°. The liquid metal is carried in a line system that has a constricting channel in a region of focus. The constricting channel has a height parallel to the direction of incidence of the electron beam and perpendicular to the direction of flow of the liquid metal to, on the one hand, provide a maximum conversion of the electron energy into X-ray energy and, on the other hand, avoid to disproportionally attenuate the produced X-ray radiation by self-absorption in the liquid metal. A typical channel height of 200 pm is mentioned. A typical focus length for the electron beam is said to be 5 mm, giving an effective focus size of about lxl mm2. Such focus size is said to be useful in, for example, customs and security applications including CT-supported luggage inspection and examination of castings such as wheel rim weld seams.

[0010] Summary

[0011] The present invention is based on the recognition that the minimum spot size obtainable in a liquid-target electron-impact X-ray source is limited by diffusion of electrons within the liquid target, which has the effect of making the spot size larger. It is therefore proposed herein an X-ray source and a related method for generating X-ray radiation, wherein X-ray radiation is generated by interactions between an electron beam and a target in the form of a liquid metal film flowing over a surface. By having a liquid metal film as the target for the electron beam, the X-ray spot formed by the impinging electron beam can be made smaller compared to thicker targets (such as a regular liquid jet that is freely propagating) because the electrondiffusion in the film target and the ensuing broadening effect that is has on the X-ray spot is reduced. Compared to a liquid-jet target, the proposed liquid film target also allows a shorter working distance between the X-ray spot and the sample or X-ray optics (i.e., the sample or the X-ray optics, as the case may be, can be placed closer to the X-ray spot and thereby collect radiation from a larger solid angle and thus more of the available X-ray radiation). In comparison to an X-ray source having a solid target, a liquid film target allows higher power loading due to the constant flow of target material. When the flowing liquid metal film is created by letting a freely propagating liquid metal jet impact on a slanted (relative to the propagation direction of the liquid jet) surface a thin and fast flowing film is provided. The liquid film may have a thickness below 100 pm, such as below 75 pm or below 50 pm. A typical film thickness may be about 10 micrometers or several tens of micrometers. This is advantageous in that a thin film enables a small diameter of the resulting X-ray spot and the fast flow enables high thermal loading. Creating a flowing liquid film by moving, e.g. rotating a member supporting the liquid inherently implies accelerating mechanical parts thus limiting the practically accessible thickness- flow speed combinations attainable.

[0012] In general terms, the principles behind the present invention can be summarized as follows.

[0013] A freely propagating liquid metal jet is generated. The liquid metal jet can be generated in the same or in a similar manner to jets formed in known liquid-jet X-ray sources.

[0014] The liquid metal jet is directed onto a smooth and preferably flat surface of an impact member arranged at a slanted angle relative to the propagation direction of the liquid metal jet. The surface of the impact member is sometimes herein referred to as the impact surface. The impact member may preferably comprise an X-ray transparent material having suitable thermal properties, such as sapphire, diamond, or the like.

[0015] After impact on the surface of the impact member, the jet spreads out into a thin liquid metal film.

[0016] An electron beam is aimed upon the liquid metal film such that X-ray radiation is generated. According to the present invention, the impact surface is formed on the exit window, and the impact member may then conveniently be attached toor constitute the exit window for the generated X-ray radiation, provided that the impact member is made from an X-ray transparent material.

[0017] While the present invention relates to transmission-type X-ray sources, in which the impact member is made from an X-ray transparent material, the principles are not limited to such implementations. The impact member used for forming the liquid film could indeed be located inside the X-ray source and the generated X-ray radiation can be used in reflection mode through a standard exit window, i.e., as a reflection-type X-ray source.

[0018] In such reflection-type implementations, it may also be preferred to direct the electron beam to a location on the liquid film downstream (with respect to the liquid flow) from the impact member that forms the film, i.e., at a location where the liquid film has separated from the surface of the impact member.

[0019] As will be understood, it is advantageous to have a liquid metal film of predictable thickness and with a predictable surface onto which the electron beam is directed. Hence, in implementations of the present invention, the surface of the impact member used to form the film is conveniently made with a smoothness that is sufficient to provide a desired predictability for the liquid metal film in this regard. A particular smoothness is not essential, however, and it will be straightforward for one of ordinary skill in the art to select an appropriate smoothness for a particular implementation after having read and understood this disclosure.

[0020] Further, although the invention is primarily described herein using examples where the surface of the impact member is flat, it is of course also possible to use curved surfaces that may be concave, convex, parabolic, cylindrical, spherical, etc. For example, curved surfaces of the impact member could be used to make sure that the liquid film does not separate ("bounce up") from the surface or, contrarily, to promote such separation, as the case may be.

[0021] The present disclosure provides an X-ray source comprising an electron source configured to provide an electron beam; a jet generator configured to provide a freely propagating liquid metal jet; an impact surface arranged so that the freely propagating liquid metal jet can impact on the impact surface and form a liquid metal film; wherein the electron beam is arranged to impact the liquid metal film. The X-ray source further comprises an exit window arranged to transmit X-ray radiation generated by interaction between the electron beam and the liquid metalfilm. The impact surface may be provided at an oblique angle relative to the liquid metal jet. According to the invention, the impact surface is formed on the exit window. Further, the electron beam may be arranged to impact the liquid metal film at a point where the film has separated from the impact surface. The exit window may be flat and oriented such that, during operation of the X-ray source, a normal direction thereto is perpendicular to the gravitational field. An electron optic system of the X-ray source may be arranged to shape a cross section of the electron beam to have an elongated shape with an aspect ratio of at least 1:7 at a point where the electron beam impacts the liquid metal film. The X-ray source may further comprise a sensor configured to measure a quantity indicative of interactions between the electron beam and the liquid metal film or the liquid metal jet. The sensor may be, for example, a backscatter electron detector, a sensor arranged to measure absorbed current at the impact surface, or an X-ray detector. Further, the impact surface may comprise a supplemental target layer in which X-ray radiation is generated by interaction with the electron beam. Conveniently, the X-ray radiation generated in the liquid metal film and that generated in the supplemental target layer may then have different X-ray spectra.

[0022] In some embodiments, the exit window is flat and oriented such that, during operation of the X-ray source, a normal direction thereto is parallel to the gravitational field. This provides a suitable geometry for X-ray inspection of, for example, electronic components transported on a conveyor belt underneath the X-ray source.

[0023] A corresponding method for generating X-ray radiation is also provided. Several modifications and variations are possible within the scope of the invention. In particular, radiation sources comprising more than one liquid metal jet, or more than one electron beam are conceivable within the scope of the present inventive concept. Furthermore, X-ray sources of the type described herein may advantageously be combined with X-ray optics and / or detectors tailored to specific applications exemplified by, but not limited to, medical diagnosis, non-destructive testing, lithography, crystal analysis, microscopy, laminography, materials science, microscopy surface physics, protein structure determination by X-ray diffraction, X-ray photo spectroscopy (XPS), critical dimension small angle X-ray scattering (CD-SAXS), and X-ray fluorescence (XRF).Brief

[0024]

[0025] of the

[0026]

[0027] In the following detailed description, reference is made to the accompanying drawings, on which:

[0028] Fig. la schematically illustrates a transmission type X-ray source in which the impact member for the liquid metal jet is formed as an exit window for X-ray radiation from the X-ray source;

[0029] Fig. lb schematically shows an enlarged view of the impact member shown in Fig. la;

[0030] Fig. lc schematically shows the impact member and exit window oriented with its normal direction parallel to the gravitational field, positioned above a conveyor belt for objects to be inspected;

[0031] Fig. 2a schematically illustrates a reflection type X-ray source in which the impact member for the liquid metal jet is located inside the X-ray source away from the exit window;

[0032] Fig. 2b schematically shows an enlarged view of the impact member shown in Fig. 2a;

[0033] Fig. 3a schematically illustrates, in a horizontal plan view, a reflection type X-ray source similar to that shown in Figs. 2a and 2b, but where the electron beam is directed to a portion of the liquid metal film that has separated from the impact member;

[0034] Fig. 3b schematically illustrates the X-ray source shown in Fig. 3a, but in a vertical plan view;

[0035] Fig. 4 is a graph showing an example of the liquid metal film thickness as a function of position along the surface of the impact member;

[0036] Fig. 5 is a graph showing an example of the liquid metal film speed as a function of position along the surface of the impact member;

[0037] Fig. 6 illustrates a method according to the principles disclosed herein.

[0038] In the drawings, like parts are designated by like reference numerals throughout unless otherwise stated.Detailed

[0039]

[0040] As an example of a challenge faced in the prior art, reference can be made to a solid-target electron-impact X-ray source. For a 10 pm diameter spot size in a typical prior art solid-target X-ray source using a tungsten target layer on a diamond substrate, the electron beam power is limited to about 10 W due to thermal loading of the target. Hence, while such a solid target may provide a small spot size, the power is limited. On the other hand, in an X-ray source using a liquid jet as the target, much higher electron beam powers can be used, e.g., more than 100 W, such as 250 W or even 1000 W and above. However, even if the electron beam is focused to a 10 pm spot on the liquid-jet target, the diffusion / spread of the electrons in the target causes the actual X-ray spot to be much larger than that. In the prior art, there has thus been a trade-off between small spot sizes on the one hand, and high powers on the other hand.

[0041] The present invention provides an X-ray source having both the liquid target advantages in terms of power loading, and the solid target advantages in terms of attainable spot sizes.

[0042] Implementations of the present invention will be discussed in more detail below.

[0043] To provide an understanding of the advantages offered by embodiments of the present invention, a general introduction will first be given.

[0044] Fig. 4 is a graph illustrating the thickness of the liquid film, shown as t in pm, as a function of distance, shown as r in mm, from the position at which the liquid jet hits the impact surface, and Fig. 5 is a graph illustrating the flow speed of the liquid film, shown as v in m / s, as a function of distance (same as in Fig. 4) from the position at which the liquid jet hits the impact surface. The values shown in Figs. 4 and 5 indicate that a reasonable speed-to-thickness combination can be obtained about a millimeter downstream from where the jet hits the impact surface. A thinner film can be obtained if a jet of smaller diameter is used, but at the cost of the film speed slowing down more rapidly.

[0045] As an example of what can be attained using the present invention, consider an implementation where the electron beam is focused to a 5 pm spot on the liquid metal film at a location 1 mm downstream (in the flow direction of the liquid metal) from where the liquid metal hits the surface of the impact member. At that point,the liquid metal film may have a thickness of about 10 ^m and a flow speed of about 75 m / s. The electron beam can then be allowed to have a power of several hundred watts, e.g. about 250 W (possibly depending on which target material that is used), without overheating the target liquid. Hence, the present invention may provide almost an order of magnitude higher powers for the same spot size compared to solid target sources even when taking into account that target materials having lower X-ray generation efficiency than tungsten are used.

[0046] Further, it is contemplated that the surface of the impact member, upon which the liquid metal jet is directed to form the film, may itself be provided with a supplemental target layer to enhance the X-ray generation. Thereby, the liquid metal will provide cooling of the supplemental target layer, and a higher X-ray yield will be achieved since electrons that penetrate the liquid metal film will contribute to further X-ray generation in the supplemental target layer. Generation of X-ray radiation both in the liquid metal film and in the supplemental target layer will also provide a broader spectrum for the generated radiation. The supplemental target layer may be metallic, ceramic, or semiconducting. The supplemental target layer may, for example, comprise a metal, such as tungsten; a carbide, such as tungsten carbide; a nitride, such as gallium nitride; or the like. The supplemental target layer may comprise a desired element to provide for a certain X-ray wavelength, and this element may be provided in a compound selected for its adhesion towards the impact member.

[0047] The impact angle, i.e. the angle between the liquid metal jet and the normal of the impact surface, may be adjustable to adjust the shape and / or extent of the film formed on the impact surface. The impact angle may be adjusted either by adjusting the direction of the liquid metal jet or by adjusting the angle of the impact surface relative to other components of the X-ray source. The X-ray take-off angle, i.e. the angle between the impact surface and the exit window, may be adjusted in accordance with the electron beam spot. Typically, an apparent circular spot is preferred and hence the take-off angle should preferably match the aspect ratio of the electron beam spot.

[0048] Embodiments of the inventive X-ray source are transmission-type X-ray sources having the impact surface as part of the exit window. At least two advantages may then be attained. First, the electron beam spot size is not limited byelectron diffusion in the target because of the thin target and, second, short source-to-object distances are enabled since the X-ray radiation is generated close to the exit window.

[0049] To achieve the smallest possible electron beam spot sizes in a transmission geometry, a magnetic focus lens with a yoke extending close to the target may be employed. To implement the present invention, a nozzle (where the jet would be created) may be placed either inside or outside of such a yoke. Assembly of the exit window / impact surface should consider requirements both for the outer side and for the inner side. To enable a short distance to an object or sample, the outer side (i.e. the side facing the ambient atmosphere) should not be obstructed. The inner side (i.e. the side facing the jet and the electron beam) should allow for the liquid metal from the jet to flow off the exit window / impact surface and be collected for circulation and re-use. A further challenge is posed when starting and stopping the jet since this may be associated with jet instability leading to spatter. It has furthermore been noticed that prolonged exposure to an unstable jet (e.g. a jet breaking up into droplets) may cause damage to the impact surface. To protect other parts of the system from such spatter, a jet shield may be employed during startup and shutdown of the jet. Special care may be required where the liquid metal leaves the exit window to prevent spatter as well as wear. It may be preferable to ensure that the flow velocity of the liquid metal film is sufficiently low at this point.

[0050] The power load that a liquid film, created on an impact surface, may withstand is to a large extent governed by the flow speed of the film at the point where the electron beam impacts the film. The speed of the film reduces with distance away from point of jet impact (see Fig. 5). Thus, it may be advantageous to be able to detect the jet position with sufficient accuracy. One way of doing this may be to scan the electron beam over the jet and measure a quantity indicative of interactions between the jet and the electron beam. Preferably, the nozzle and the impact surface are both directly connected to an electrical ground potential, thus measuring absorbed current is not a preferred option. Instead, backscatter electrons may be measured with a suitable sensor. An alternative may be to measure X-ray radiation generated by interactions between the electron beam and the jet. An alternative, potentially beneficial, work point may be defined as where the filmattains a minimum thickness. This will typically be at some distance away from the point of jet impact upon the impact surface (see Fig. 4). The location of this point may be assumed to be at a fixed distance from where the jet impacts, or it may be detected by measuring a quantity indicative of interactions between the electron beam and the liquid metal film formed on the impact surface. It may be preferable to measure a quantity indicative of interactions between the electron beam and the liquid metal jet, since the edges of the jet may be sharper than those of the liquid metal film, and from such measurements deduce information about the liquid metal film.

[0051] The development of a liquid film after jet impact is characterized by liquid flowing radially outwards from the jet impact region. Close to the impact region the velocity profile within the film is not fully developed, i.e. the viscous drag caused by the impact surface does not affect the flow over the entire cross section. In this region the thickness of the liquid film will decrease to preserve flow continuity. When the velocity profile is fully developed, the viscous drag will decrease the radial flow velocity, and the thickness of the film will increase with increasing distance from the impact region. As the flow progresses, an internal pressure gradient may build up resulting in a hydraulic jump manifested as a rim of liquid beyond which a laminar flow ensues. Observation of the hydraulic jump requires that the impact surface is sufficiently wide so that the liquid does not leave the surface before the rim has been able to form. Thus, there is a region where the flow velocity is comparatively large, and the film thickness is comparatively small. This is the preferred region to operate within since the small thickness implies low levels of electron scattering and corresponding broadening of the electron beam while the large flow velocity implies good cooling capacity. Following impact of the liquid jet onto the surface of the impact member, a liquid film will generally be formed in all directions along the surface of the impact member. However, the flow velocity will not be the same in all radial directions from the point of impact of the liquid jet. As will be understood, directions along the impact surface which are downstream, i.e. having a component parallel with the impact direction of the jet, will have higher flow velocities than directions along the impact surface which are upstream, i.e. having a component anti-parallel with the impact direction of the jet. The maximum velocity of the liquid film will be obtained in a direction along the surface of theimpact member where the projection of the jet impact velocity is the largest. Thus, it is preferred to locate the interaction region by directing the electron beam directly downstream of the jet impact point. From continuity of flow and conservation of energy it is evident that only film thicknesses below half the jet radius may be considered in this context.

[0052] A typical jet diameter is in the sub-millimeter range, such as less than 500 pm, such as about 200 pm. The impact velocity of the jet is typically more than 50 m / s, such as more than 100 m / s, and typically less than 1000 m / s. The liquid metal jet may comprise a liquid metal or a liquid metal alloy. The liquid metal jet may comprise at least one of the elements gallium, indium, tin, lead, or bismuth. The liquid film preferably has a thickness below 100 pm, e.g. below 75 pm or below 50 pm. A typical film thickness may be about 10 pm.

[0053] Figs, la and lb illustrate an embodiment of the present invention implemented as a transmission-type X-ray source. Here, the liquid metal film is formed on a flat impact surface that is part of an exit window allowing radiation to be emitted from the X-ray source. Fig. la schematically shows an X-ray source 100 comprising a low-pressure chamber (or vacuum chamber) enclosed by an outer enclosure 102. Within the enclosure 102, there is provided an electron source 104 configured to provide an electron beam that can be directed to a target to generate X-ray radiation. The X-ray source 100 also comprises a jet generator 106 configured to provide a freely propagating liquid metal jet of target material. In the illustrated example, the liquid metal jet is directed towards an exit window 108 through which generated X-ray radiation can be output from the enclosure 102. The exit window 108 also functions as an impact member for the liquid metal jet. Hence, when the liquid metal jet impacts upon this impact member (here, the exit window 108), a liquid metal film is formed. The X-ray source is configured to focus the electron beam generated by the electron source 104 onto the liquid metal film of target material to thereby generate X-ray radiation. A backscatter sensor 118 may be arranged to detect electrons scattered from the liquid metal film and / or the exit window 108. Data from this sensor may be used to estimate the thickness of the liquid metal film. Provided the electron beam is scanned over the liquid metal film and a signal from the backscatter sensor is monitored, the extent of the liquid metal film on the exit window may be estimated. Alternatively, a current caused byelectrons absorbed in the exit window 108 may be measured. Furthermore, the backscatter probability depends on the thickness of the liquid metal film, and thus the amount of backscattered electrons, or conversely the current in the exit window, may be used as an indication of film thickness. The amount of X-ray radiation generated by interactions between the electron beam and the liquid metal film also depends on the film thickness, accordingly measurements with an X-ray detector may be used to get an indication on the extent and the thickness of the liquid metal film. The liquid target material is collected in a liquid collector 110 to be re-circulated back to the jet generator 106 using a recirculation system including, for example, a pump 112 and a conduit 114. X-ray radiation generated by the interaction between the electron beam and the target film is emitted from the X-ray source through the exit window 108 as illustrated by the dashed lines at 116 in Fig. la. Fig. lb illustrates a close-up of the region around the exit window 108 where the liquid metal jet 107 impacts the exit window thus forming the liquid metal film 109, flowing downward in the figure as indicated by the arrow at 109. The electron beam 105 is directed towards the liquid metal film formed on the exit window and X-ray radiation 116 is created by interactions between the electrons comprised in the electron beam and the material of the liquid metal film. An interaction region, where X-ray radiation is generated, is thus defined by the extent of the electron beam and the thickness of the liquid metal film.

[0054] Fig. lc illustrates another embodiment, in which the exit window is flat and oriented such that a normal direction thereto is parallel to the gravitational field. Such geometry may be particularly useful for inspection implementations, where objects (e.g., electronic components 150) to be inspected are transported on a conveyor belt 152 underneath the X-ray source.

[0055] Figs. 2a and 2b illustrate an alternative implemented as a reflection-type X-ray source 200. Here, the liquid metal film is formed on a flat surface of an impact member that is separate from the exit window. For parts that are common between Fig. la and Fig 2a, the same reference numerals are used. For this implementation, an impact member 120 is provided that is separate from the exit window 108. Fig.

[0056] 2b illustrates a close-up of the region around the impact member 120 where the liquid metal jet 107 impacts to form the liquid metal film 109. The electron beam 105 is directed towards the liquid metal film at a point where it flows over theimpact member. X-ray radiation will be created by interactions between the electrons and the liquid metal film, to be emitted from the source through exit window 108. As seen in figure 2b the liquid metal film will flow off of the impact member to be collected by jet receiver 110.

[0057] Hence, a liquid metal film target X-ray source may also be implemented as a reflection-type X-ray source, e.g., as illustrated in Figs. 2a and 2b. For this kind of X-ray sources, the source-to-object distances will generally be longer than for transmission-type sources (e.g., as illustrated in Figs, la and lb), although the electron beam spot size advantage is still achieved. Further, it may provide an opportunity to have a wide flat target surface where a comparatively long line focus of the electron beam may be used. The aspect ratio of the line focus may be at least 1:7 such as at least 1:10.

[0058] Figs. 3a and 3b illustrate different views of an implementation where the electron beam impacts the liquid metal film at a location where it has left the impact surface. This embodiment is similar to the one illustrated in Figs. 2a and 2b, the difference being that the electron beam is arranged to impact the liquid metal film when it has left the impact surface and is flowing freely. The impact member 120 is further arranged so that there is line of sight from the electron source 104 to an electron beam dump 132. The electron beam dump 132 is used to collect electrons that have penetrated or passed by the free-flowing liquid metal film 130. By having the electron beam dump at a floating electrical potential and connecting an ammeter 134 between the electron beam dump 132 and electrical ground 136, a current indicative of the number of electrons absorbed or scattered by the free-flowing liquid metal film 130 may be obtained. This may be used as a measure of film thickness. By scanning the electron beam over the free-flowing liquid metal film and monitoring changes to the measured current a measure of film width may be obtained. This arrangement is an example of a sensor arranged to measure electrons downstream of the liquid metal film.

[0059] As a general note, in embodiments of the present invention, to prolong the lifetime of the exit window / impact surface, the point of impact for the jet may be moved after some predetermined time (or some other condition) and the electron beam would then be moved in accordance with the jet. An alternative embodiment may comprise moving the impact member to provide a virgin location for jet impact.Fig. 6 is a flow chart illustrating a method according to the principles disclosed herein. The method comprises the actions of forming a liquid metal jet 601; forming an electron beam 602; forming a liquid metal film by impacting the liquid metal jet on an impact surface 603; directing the electron beam towards an interaction region on the liquid metal film 604; and generating X-ray radiation by interactions between the electron beam and the liquid metal film 605. As will be understood, the actions are not necessarily performed in this order. For example, the electron beam can be formed after forming the liquid metal film. In some implementations, the electron beam is shaped to have an elongated cross section with an aspect ratio of at least 1:7 when impacting the liquid metal film. Further, the method may include measuring a quantity indicative of an interaction between the electron beam and the liquid metal film; estimating a thickness of the liquid metal film based on the measured quantity; and adjusting the interaction region to achieve a desired thickness of the liquid metal film. The liquid metal film can be formed, for example, by impacting the liquid metal jet under an oblique angle onto an impact surface, which may be flat or curved. The electron beam may then be made to impact the liquid metal film either at a point where the film is flowing over the impact surface or at a point where the liquid metal film has separated from the impact surface.

Claims

CLAIMS1. An X-ray source, comprisingan electron source configured to provide an electron beam;a jet generator configured to provide a freely propagating liquid metal jet; an impact surface arranged so that the freely propagating liquid metal jet can impact on the impact surface and form a liquid metal film;wherein the electron beam is arranged to impact the liquid metal film; the X-ray source further comprising an exit window arranged to transmit X-ray radiation generated by interaction between the electron beam and the liquid metal film, wherein the impact surface is formed on the exit window.

2. The X-ray source of claim 1, wherein the impact surface is provided at an oblique angle to the liquid metal jet.

3. The X-ray source of claim 1 or 2, wherein the liquid metal film has a thickness, in a direction parallel to the electron beam, of less than 100 pm, and preferably less than 75 pm or less than 50 pm, such as about 10 pm.

4. The X-ray source of any one of the preceding claims, wherein the exit window is flat and oriented such that, during operation of the X-ray source, a normal direction thereto is parallel to the gravitational field.

5. The X-ray source of any one of claims 1-3, wherein the exit window is flat and oriented such that, during operation of the X-ray source, a normal direction thereto is perpendicular to the gravitational field.

6. The X-ray source of any one of claims 1-5, further comprising an electron optic system arranged to shape a cross section of the electron beam to have an elongated shape with an aspect ratio of at least 1:7 at a point where the electron beam impacts the liquid metal film.

7. The X-ray source of any one of claims 1-6, further comprising a sensor configured to measure a quantity indicative of interactions between the electron beam and the liquid metal film or the liquid metal jet.

8. The X-ray source of claim 7, wherein the sensor is one of:a backscatter electron detector;a sensor arranged to measure absorbed current at the impact surface; and an X-ray detector.

9. The X-ray source of any one of claims 1-8, wherein the impact surface comprises a supplemental target layer in which X-ray radiation is generated by interaction with the electron beam.

10. The X-ray source of claim 9, wherein the X-ray radiation generated in the liquid metal film and the X-ray radiation generated in the supplemental target layer have different X-ray spectra.

11. The X-ray source of claim 9 or 10, wherein the supplemental target layer comprises tungsten.

12. The X-ray source of any one of claims 1-11, wherein the impact surface is comprised on an X-ray transparent substrate.

13. A method for generating X-ray radiation in an X-ray source, comprising:forming a freely propagating liquid metal jet;forming an electron beam;forming a liquid metal film by impacting the freely propagating liquid metal jet on an impact surface, wherein the impact surface is formed on an exit window of the X-ray source;directing the electron beam towards an interaction region on the liquid metal film; andgenerating X-ray radiation by interactions between the electron beam and the liquid metal film.1714. The method of claim 13, further comprising:shaping the electron beam to have an elongated cross section with an aspect ratio of at least 1:7 when impacting the liquid metal film.

15. The method of claim 13 or 14, further comprising:measuring a quantity indicative of an interaction between the electron beam and the liquid metal film;estimating a thickness of the liquid metal film based on the measured quantity; andadjusting the interaction region to achieve a desired thickness of the liquid metal film.

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