WO2025237970A1

WO2025237970A1 - X-ray window

Info

Publication number: WO2025237970A1
Application number: PCT/EP2025/063019
Authority: WO
Inventors: Ulf LUNDSTRÖM
Original assignee: Excillum AB
Current assignee: Excillum AB
Priority date: 2024-05-17
Filing date: 2025-05-13
Publication date: 2025-11-20
Anticipated expiration: 2026-11-17
Also published as: EP4651173A1

Abstract

A liquid metal target X-ray source comprising a protection element for preventing deposition of contaminants on an X-ray transparent window separating a reduced-pressure region enclosing an X-ray source is disclosed. The protection element comprises a first portion configured to transmit X-ray radiation and to be heated by absorbing electrons scattered from the liquid metal target; and a second portion configured to be attached to a holder adjacent the X-ray transparent window. The liquid target comprises a first metal that generates X-ray radiation upon impact thereupon of an electron beam. The first portion and the second portion are made from materials having substantially the same coefficient of thermal expansion, and the second portion has an extension transverse to the first portion. The X-ray source is configured to maintain the first portion of the protection element above a first temperature, and to maintain a pressure within the reduced-pressure region below a vapor pressure of the first metal at the first temperature.

Description

X-RAY WINDOW

Technical field

The present disclosure relates to X-ray sources, and more particularly to arrangements for an X-ray transparent window.

Background

In electron-impact X-ray sources, X-ray radiation is generated from interaction between an electron beam and a target material. Generated X-ray radiation exits from the X-ray source through a window, which separates a low- pressure environment (often referred to as a vacuum chamber or vacuum enclosure) inside the source from outside pressure or from another chamber containing a sample under study.

A dual window configuration where an outer window provides for integrity between the vacuum enclosure and the ambient atmosphere, and an inner window provides self-cleaning protection against metal vapor deposition on (the inside of) the outer window is known from WO 2010/083854. A typical example comprises an outer window made from beryllium and an inner window made from a carbon foil. The carbon foil is typically heated by passing current through the foil although implementations where heating is provided by electrons scattered from a liquid metal jet target have also been proposed. In practice, providing a heating current to such carbon foil entails a lot of considerations and comes with a whole set of failure modes.

Solid target X-ray sources may also exhibit problems related to evaporated target material being deposited on the exit window. This process is expected to be considerably slower than for liquid metal jet sources since the latter operate considerably closer to the boiling point of the target material. Furthermore, it may not be preferable to introduce a carbon foil heated to such a high temperature that deposited solid target material is evaporated since this may correspond to a temperature that the carbon foil may not withstand. Instead, the carbon foil may be included as a disposable part that is discarded and replaced with a new one once the amount of deposited target material absorbs more X-ray radiation than deemed acceptable.

Hence, there is a need for improved ways of protecting the X-ray exit window from contaminants.

Summary

While it may be tempting to utilize backscattered electrons as the source of heat, it turns out that just replacing ohmic heating with backscatter heating leads to a new set of problems. An inner window embodied as a carbon foil needs to be fixated and traditional techniques, e.g. clamping the foil between titanium pieces, are prone to failures where the carbon foil cracks and/or the titanium pieces corrode due to interactions with gallium (a common target material in liquid-jet X- ray sources) at elevated temperatures. To avoid cracks, the foil should be supported by a material with a similar, preferably the same, coefficient of thermal expansion. To avoid corrosion, the support structure should preferably be non-metallic since gallium is prone to form intermetallic compounds with other metals especially at elevated temperatures. Previous proposals on how to address issues related to thermal expansion comprise inserting at least an edge of the foil into a slit in a reservoir, where it makes contact with an electrically conducting liquid.

The present disclosure provides improved arrangements for X-ray windows. Embodiments of the present invention are particularly useful for liquid metal jet X- ray sources, in which generation of debris in the vacuum enclosure is often more pronounced than in solid target X-ray sources. A secondary window element in the form of a protection element is proposed. The protection element has the purpose of preventing deposition of contaminants on the primary window element, i.e. the X-ray transparent window element separating a reduced-pressure region inside the X-ray source from the outside. The protection element comprises a first portion configured to transmit X-ray radiation and to be heated by absorbing electrons, and a second portion configured to be attached to a holder or frame adjacent to the primary window element, i.e. adjacent to the X-ray transparent window. The first portion and the second portion are comprised of materials having substantially the same coefficient of thermal expansion, and the second portion has an extension transverse to the first portion. The protection element is provided in an X-ray source, wherein the X-ray source further comprises an electron source for providing an electron beam; and a liquid target generator arranged for providing a liquid target comprising a first metal for generating X-ray radiation in an interaction region upon impact of the electron beam on the target. The first portion of the protection element is configured to be heated by absorbing electrons scattered from the interaction region. Electrons scattered from the interaction region may be the only source for heating the protection element. The X-ray source is configured to maintain the first portion of the protection element above a first temperature, and to maintain a pressure within the reduced-pressure region that is below a vapor pressure of the first metal at the first temperature. The pressure within the reduced- pressure region may, for example, be below 10^-2 mbar and the first temperature may, for example, be at least 400 K. The first metal may be an element selected from the group consisting of gallium, indium, tin, bismuth, lead, mercury, cesium, and rubidium. For implementations where the first metal is gallium, the first temperature is preferably 1300 K.

Preferably, the first portion and the second portion are made from the same material and may constitute a single monolithic part. A preferred material for the protection element is carbon.

It may be advantageous to provide the protection element as a monolithic part with an interface towards the rest of the window arrangement that allows assembly and disassembly without specific tooling or the like.

A dual window as disclosed herein may also be used to protect the exit window from excessive heat and/or potentially enabling a thinner exit window. Depending on the X-ray energies and window materials involved, a dual window set up may actually increase the total X-ray flux in cases where the thickness of the exit window has a lower limit, e.g., as set by thermal considerations, which results in less undesired X-ray absorption.

When designing an X-ray source comprising a window arrangement according to the present invention, several factors should be taken into account. The primary window element, i.e. the one facing the ambient atmosphere, should provide a vacuum tight seal and be transparent to X-ray radiation. Furthermore, it is desirable that the window material does not oxidate. Although the secondary window element, i.e., the one facing the liquid metal jet, will absorb a large fraction of the energy carried by backscattered electrons also the primary window element will be subject to some thermal load during operation. It is thus preferable that the material in the primary window has good heat conduction properties and is also electrically conducting to avoid charge build up. A preferred choice of material for the primary window is beryllium, a metal with an atomic number of four thus being a good conductor for both electrical current and heat, and also highly transparent to X-ray radiation. Beryllium does however form intermetallic compounds with gallium, especially so at elevated temperatures. Thus, the secondary window element should preferably be made from some other material, preferably a non-metal to avoid problems with gallium-induced corrosion.

Carbon is the element with the lowest atomic number among the non-metals that are in a solid state at room temperature and ambient pressure. While being a non-metal, carbon is still an electrical conductor and may also be provided in a form where it has a suitable thermal conductivity. Carbon is thus a particularly preferred material for the secondary window element. To minimize (or at least reduce) the amount of X-ray radiation absorbed in the carbon layer comprising the secondary window element it should be made comparatively thin, although not too thin since the backscattered electrons may in that case predominantly penetrate the window element instead of being absorbed. The design intent is to have a window element that achieves a sufficiently high temperature by absorbing electron so as to avoid accumulation of metal vapor and/or spatter while being transparent to X-ray radiation.

The primary window element and the secondary window element could in principle be assembled in a common frame. However, this may result in thermally induced stress in the secondary window element, and the stress may ultimately lead to failure of the window element. Instead, the secondary window element preferably comprises a first portion that is transparent to X-ray radiation and a second portion that supports the first portion and is assembled in the frame. The material in the first portion may generally comprise elements with low atomic numbers, i.e., less than 30, to improve X-ray transparency. Furthermore, the thickness of the first portion may be selected to achieve a desired temperature during operation, wherein said temperature is set to avoid damage to the first portion while still ensuring that deposited metal vapor does not accumulate over time. The first and the second portion may be made from electrically conducting materials to avoid charge buildup. The first portion and the second portion may be made from the same material or two materials with the same coefficient of thermal expansion to avoid thermally induced stress. The first portion and the second portion may be made from non-metals to avoid formation of intermetallic compounds with metal debris originating from the liquid metal target (typically gallium or an alloy comprising gallium). The first portion and the second portion may be provided as one monolithic piece. A preferred material choice for the secondary window element is carbon, which has low atomic number and hence high X-ray transparency, is electrically and thermally conducting, and is non-metallic.

In some implementations, the X-ray source may comprise a cooling arrangement in contact with the second portion of the protection element, wherein the cooling arrangement is maintained at a second temperature, such as below 300 K, during operation of the X-ray source.

Typically, the X-ray source may be configured to maintain the first portion of the protection element above the first temperature, and the pressure within the reduced-pressure region below a vapor pressure of the first metal at the first temperature during the time when source is operational, i.e. when X-ray radiation is generated. The X-ray source may also be configured to maintain the first portion of the protection element above the first temperature intermittently, e.g. when the power transmitted by the electron beam is above a limiting value.

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, 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 of the

In the detailed description below, reference is made to the accompanying drawings, on which:

Fig. la schematically shows a first implementation of a protection element according to the principles disclosed herein, attached to a frame or holder in an X- ray source;

Fig. lb schematically shows a second implementation, where a bushing is provided at the attachment between the protection element and the frame/holder;

Figs. 2a and 2b schematically show implementations in which the second portion has a slanted angle with respect to the first portion;

Figs. 2c-e schematically show examples of how the protection element can be installed from the outside of the X-ray enclosure;

Figs. 3a and 3b show the geometry used for describing a heuristic model of the protection element;

Fig. 4 is a graph showing calculated temperatures of the protection element for different electron beam powers; and

Fig. 5 schematically shows an X-ray source provided with the protection element.

Detailed

To investigate the limitations of the principles disclosed herein, a simplified heuristic physical model may be of some use. The geometry assumed for the calculations below is schematically shown in Figs. 3a and 3b, which schematically show a side view and a top view, respectively, of a protection element according to the principles disclosed herein. The protection element comprises a first portion in the form of a circular foil 30 with radius Ro and thickness t suspended on a second portion in the form of a cylindrical shell 32 with inner radius Ro, outer radius Ri and length L. The foil 30 and the cylindrical shell 32 are assumed to be made from the same material characterized by a thermal conductivity A; and a maximum allowable temperature Tmcix- Tmax typically represents the maximum temperature that the material can withstand while maintaining its physical integrity and may e.g. be determined according to the vapor pressure of the foil material. The protection element is subjected to electron irradiation (generally from the above in the view of Fig. 3a) with a power density p. Impacting electrons are absorbed in the material of the protection element over a characteristic length 2 and thereby heats the foil 30 and the top part of the cylindrical shell 32 to a temperature T. The part of the cylindrical shell opposite to the circular foil is in thermal contact with a cooling arrangement kept at a constant temperature T_a, effectively giving the cylindrical shell a temperature gradient from T at a first side where the foil is located to T_a at the opposite side where the cooling arrangement is located, as illustrated in Fig. 3a. It is assumed that the circular foil 30 is cooled by radiation from both sides (i.e. upwards and downwards in the view of Fig. 3a). It is further assumed that the foil 30 is sufficiently thin so that the temperature T within the foil can be considered homogeneous. The cylindrical shell 32 is assumed to be cooled by radiation through the end surface in contact with the foil at the first side and through heat conduction along the shell's axial direction towards the cooling arrangement. The power deposited in the protection element by the incoming electrons may be written as

The power removed from the protection element is the sum of the radiated power and the conducted power; it may be written as where the first term describes radiated power upwards (in the view of Fig. 3a) from the annular end surface of the cylindrical shell, the second term describes radiated power both upwards and downwards from the foil, and the third term describes conducted power along the cylindrical shell downwards. By equating the incoming and outgoing power, a fourth order equation for T may be obtained.

For cases where the thickness of the foil t is large in comparison with the absorption length 2, the exponential in the expression above may be approximated to zero. By applying this approximation, and by some re-arrangement, Equation (3) may be somewhat simplified to

To achieve the intended effect, i.e. preventing condensation of metal vapor on the (upper) foil surface, the temperature of the foil should preferably be above some limiting temperature T_min, as imposed by the constituents of the expected contaminants, at the power density (p_m/n) corresponding to the minimum power setting allowed by the system. T_m/n may e.g. be selected according to the vapor pressure of the material, e.g., a liquid metal, used for the target in the X-ray source. As mentioned above, there is also a maximum allowed temperature Tmax of the foil, above which it will deteriorate. Thus, for the maximum power allowed by the system, resulting in power density p_m x at the foil, the temperature of the foil should preferably be below the limiting temperature Tmax. In all practical cases T_m/n and Tmax are both larger than T_a. By inserting the prescribed power densities and temperatures in Equation (4) a set of equations for the length L and the squared ratio between the Ro and Ri may be obtained. By solving this set of equations, the following expressions are obtained where

Since Ri is by definition larger than Ro, a must be smaller than 2 (corresponding to a vanishingly thin cylindrical shell) and larger than 1 (corresponding to an infinitely thick cylindrical shell). These limiting values for a may be interpreted as limits on the maximum power density, and from Equation (7) the limits may be written as

Equation (8b) gives the upper limit on the allowable power density for which this geometry may be realized, the limiting value corresponding to a= 2, i.e. a vanishingly thin cylindrical shell where Ri equals Ro. The lower limit for the allowable power density given by Equation (8a) corresponds to the limit where a= l, i.e. an infinitely thick cylindrical shell (or the uninteresting case of no foil i.e. Ro = 0).

A lower limit on the power density may be inferred from Equation (6), where the denominator needs to be positive for L to correspond to a physical length. This limit may be understood as the minimum power density required for this type of geometry to be able to attain the prescribed minimum foil temperature (the limit in this case, i.e. when the denominator approaches zero, corresponds to an infinite length of the cylindrical shell). A lower power density may imply that metal vapor will deposit on the foil contrary to the design intent. Hence where the last step follows from the lower limit on a, i.e. 1. The minimum power density may in this simple model be chosen arbitrarily large where the calculated L will, according to Equation (6), approach zero as p_min increases. In reality the cooling capacity will limit how much power that may be absorbed by the protective element without causing it to overheat.

A more relevant measure than power density of backscattered electrons at the protection element, at least from an X-ray source perspective, may be the power of the electron beam impacting on the target. To convert electron beam power to backscatter electron power density, it may be reasonably assumed that about 30% of the electron beam power will scatter off the target. These scattered electrons will be distributed over about half of the unit sphere; the other half being occupied by the target. If the electron beam power is denoted by P and the distance between the target and the protection element is denoted by d the power density may be written as

The geometry calculated above according to Equations (5)-(7) corresponds to the ideal case where the minimum required temperature is attained for the minimum allowed power density and the maximum allowed temperature is attained for the maximum allowed power density. To have some design margins it is preferable that the minimum required temperature is attained at a power density slightly below the intended minimum. By replacing T_m/n with a slightly larger value than what is required and Tmax with a slightly lower value than what can be allowed in Equations (5)-(7) a more conservative geometry may be obtained. It turns out that to be on the safe side in most practical situations, the ratio in Equation (5) should be seen as an upper limit, whereas the length according to Equation (6) may have to be adjusted up or down depending on the power density range designed for.

As an example, consider a protection element comprised of carbon graphite with a thermal conductivity of 5 W/Km. The temperature required for carbon to evaporate at low pressures is about 2800 K, thus this value may be used as Tmax. A typical constituent in liquid metal jets used in X-ray sources is gallium. The temperature required for gallium to evaporate at low pressure is about 1300 K so this value may be used for T_m/n. With a distance of 5 mm between the target and the protection element and a controlled temperature T_a = 300 K the lower limit for the electron beam power calculated from Equations (9) and (10) in this example is about 85 W. To calculate the upper limit for the electron beam power from Equations (8b) and (10) a value for the minimum applied power density is needed. Taking the just calculated lower limit gives a corresponding upper limit of 3400 W. To be able to have a solution to Equation (4) with a physically realizable geometry, the maximum electron beam power must also fulfill the condition imposed by Equation (8a). In the present case this means that the maximum electron beam power must be at least 1800 W. Note that the limits imposed by Equation (8a) and (8b) will grow with increasing minimum power density. Assuming a desired lower limit on the electron beam power of 250 W gives an allowed range for the maximum electron beam power from 2200 W to 3900 W. If 2500 W is used as design criterion Equation (5) - (7) yields a geometry where Ro/Ri is about 0.40 and L is about 14 mm. The radius of the foil part of the protection element may be selected to achieve a desired X-ray beam divergence angle, and with an exemplary value of Ro = 2 mm the outer radius of the cylindrical shell Ri would in this case be about 5 mm. Using this geometry and solving Equation (4) for a range of power densities, the corresponding foil temperature has been calculated and plotted in Fig. 4 as a function of electron beam power (with the help of Equation (12)). Horizontal dotted lines mark the minimum (1300 K) and maximum (2800 K) temperature, whereas dashed vertical lines mark the minimum (250 W) and maximum (2500 W) electron beam power.

A first exemplary protection element 10 for preventing deposition of contaminants on an X-ray transparent window 22 in an X-ray source is schematically shown in Fig. la. The X-ray transparent window 22 separates a reduced-pressure region 24 from the outside of the X-ray source. The protection element 10 comprises a first portion 12 configured to transmit X-ray radiation and to be heated by absorbing electrons. The electrons heating the protection element may be scattered from an interaction region 20 in the X-ray source at which an electron beam interacts with a target material to generate X-ray radiation. The protection element 10 further comprises a second portion 14 that projects from the first portion 12 to have an extension transverse thereto. The first portion 12 and the second portion 14 are made from materials having substantially the same coefficient of thermal expansion, and preferably the first portion 12 and the second portion 14 are made from the same material, such as carbon. Most preferably, the protection element 10 is a monolithic element comprising the first portion 12 and the second portion 14. Adjacent to the X-ray transparent window 22 of the X-ray source, there is provided a holder/frame 26 to which the second portion 14 of the protection element 10 is removably attached.

The protection element may have a cylindrical shape with a closed end forming the first portion 12 and a side wall forming the second portion 14. Depending on the implementation, the second portion 14 may have an extension transverse to the first portion 12between 5 mm and 50 mm, and the first portion 12 may have a thickness of several tens of micrometers, such as 50 pm to 250 pm, e.g. about 100 pm. As discussed above, the temperature achieved in the first portion 12 of the protection element depends, inter alia, on the separation between the interaction region 20 and the first portion 12, and also on the cooling capacity through the holder 26 and the power of the electron beam incident on the target material in the interaction region 20.

A second exemplary protection element is schematically shown in Fig. lb. The example of Fig. lb is similar to that described above and shown in Fig. la, except that a bushing 28 is provided between the second portion 14 of the protection element 10 and the holder 26. The bushing 28 may prevent contaminants from leaking between the protection element 10 and the holder 26 towards the X- ray transparent window 22. The bushing 28 may also be designed to facilitate thermal contact between the protection element and the holder/frame.

In the examples of Figs, la and lb, the second portion projects from the first portion at a right angle. In some implementations, however, it may be preferred to have the second portion 14 project from the first portion 12 at a slanted angle. In such implementations, the protection element may have the form of a truncated cone or pyramid, as schematically shown in Figs. 2a and 2b. An advantage of having the second portion at a slanted angle may be that more X-ray radiation from the interaction region 20 can be extracted from the X-ray source. As will be understood, the slanted second portion can be made to conform to the angle defined by the diameter of the exit window 22 and the distance therefrom to the interaction region 20, so that the protection element does not reduce the emitted X-ray flux.

Some implementations of the protection element 10 are configured to be installed in an X-ray source from the outside of the X-ray enclosure. To this end, the holder 26 for the protection element may conveniently be provided with a shape and size that fits into a corresponding recess in the enclosure. Various alternatives of such implementations are shown in Figs. 2c-e. The holder 26 may, for example, be attached to the enclosure using screws (not shown) and one or more bushings 29 may be provided at the interface between the holder and the enclosure to ensure a vacuum seal, if required.

In some embodiments, as illustrated in Fig. 2e, it may be preferred to provide an opening 27 in the second portion of the protection element, wherein the opening 27 allows passage of gas in order to ensure that the space between the first portion as the region 24. This may be advantageous, for example, if the protection element 10 is mounted at atmospheric pressure (e.g., the vacuum enclosure may need to be opened in order to install the protection element) and the pressure is subsequently reduced to establish the required low pressure in region 24. Advantageously the opening 27 is arranged so that there in no line of sight between the opening and the interaction region 20 so as to prevent contaminants created during the interaction from reaching the opening. Furthermore, the holder 26 and the corresponding recess in the enclosure may be configured so that the opening is connected to the reduced pressure region 24 through a labyrinth-like seal enabling passage of air molecules and preventing passage of metal vapor.

An X-ray source 50 comprising the protection element 10 is schematically shown in Fig. 5. The X-ray source comprises an electron source 52 for providing an electron beam 54 directed towards a target 56. The region in which the electron beam 54 interacts with the target 56 constitutes the interaction region 57 corresponding to interaction region 20 as shown in Figs, la to 2e. The X-ray source further comprises an enclosure 51 (typically comprising a material that blocks X-ray radiation, such as tungsten) that encloses a reduced-pressure region in which the target 56 (and thus the interaction region) is located. The enclosure may be evacuated and sealed during the manufacturing of the X-ray source, or it may comprise a port (not shown) where a vacuum pump (not shown) may be connected so that the enclosure may be evacuated at any desired time during its service life. The enclosure may be continuously evacuated during operation to maintain a sufficiently low pressure. An X-ray transparent window 55, preferably made from beryllium, is provided in the enclosure 51 through which generated X-ray radiation can be output from the X-ray source 50. The protection element 10 is arranged between the interaction region 57 and the window 55. Electrons scattered from the interaction region are shown as wavy arrows in Fig. 5. Some of the scattered electrons will be absorbed by the protection element 10 and thereby elevate the temperature of at least the first portion of the protection element. The location of the protection element between the window 55 and the interaction region 57 is such that it protects the window 55 from contaminants present inside the enclosure 51, e.g. caused by evaporation or splatter from the target 56. The temperature of the first portion of the protection element attained by absorbing scattered electrons is such that deposition of contaminants on the surface of the protection element is reduced or even eliminated. Typically, in order for a sufficient number of scattered electrons to reach the protection element, at least some part of (the first portion of) the protection element is located no more than 50 mm from the interaction region 57. For example, the distance from the protection element to the interaction region is no more than 20 mm, and preferably no more than 10 mm.

The principles disclosed herein are particularly advantageous for liquid-jet target X-ray sources. A preferred material for the liquid target-jet is gallium, and in this case the X-ray source is preferably configured to maintain the first portion of the protection element at a temperature of at least 1300 K.

Further, the protection element is preferably made from carbon, and in this case the X-ray source may be configured to maintain the first portion of the protection element at a temperature of no more than 2800 K.

A holder or frame 58 is provided adjacent to the window 55, to which the protection element 10 is removably attached.

Conclusion

A liquid-metal target X-ray source has been disclosed, comprising a protection element arranged between an interaction region located in a reduced- pressure region, where X-ray radiation is to be generated by interaction between the target and an electron beam, and an X-ray transparent window, through which generated X-ray radiation is to be output. The protection element prevents deposition of contaminants from the liquid target on the X-ray transparent window. The protection element is configured to attain an increased temperature by absorbing electrons scattered from the interaction region between the electron beam and the target. A first portion of the protection element, typically configured as a disc or foil portion in the form of an end cap of an open cylinder, is configured to be heated by absorbing the electrons while being transparent to X-ray radiation. A second portion of the protection element has an extension transverse to the first portion and is configured to be attached, preferably removably attached, to a holder or frame of the X-ray source. The X-ray source is configured to maintain the first portion of the protection element above a first temperature, and to maintain a pressure within the reduced-pressure region below a vapor pressure of the first metal at the first temperature.

Claims

1. An X-ray source, comprising: an electron source for providing an electron beam; a liquid target generator arranged for providing a liquid target comprising a first metal for generating X-ray radiation in an interaction region upon impact of the electron beam on the target; an enclosure enclosing a reduced-pressure region, wherein the interaction region is located in the reduced-pressure region; an X-ray transparent window in the enclosure through which generated X-ray radiation can be output from the X-ray source; and a protection element arranged between the interaction region and the X-ray transparent window to prevent deposition of contaminants from the liquid target on the X-ray transparent window; wherein the protection element comprises: a first portion configured to transmit X-ray radiation and to be heated by absorbing electrons scattered from the interaction region; and a second portion configured to be attached to a holder adjacent the X-ray transparent window; wherein the first portion and the second portion are made from materials having substantially the same coefficient of thermal expansion; wherein the second portion has an extension transverse to the first portion; and wherein the X-ray source is configured to maintain the first portion of the protection element above a first temperature; and to maintain a pressure within the reduced-pressure region below a vapor pressure of the first metal at the first temperature.

2. The X-ray source of claim 1, wherein the pressure within the reduced- pressure region is below 10^-2 mbar.

3. The X-ray source of claim 1 or 2, wherein the first metal is an element selected from the group consisting of gallium, indium, tin, bismuth, lead, mercury, cesium, and rubidium.

4. The X-ray source of any one of the preceding claims, wherein the first temperature is at least 400 K.

5. The X-ray source of claim 1 or 2, wherein the first metal is gallium, and the first temperature is 1300 K.

6. The X-ray source of any one of the preceding claims, wherein the first portion and the second portion are comprised of the same material.

7. The X-ray source of any one of the preceding claims, wherein the protection element is a monolithic element comprising the first portion and the second portion.

8. The X-ray source of claim 6 or 7, wherein the protection element is made from a material having an atomic number less than 30.

9. The X-ray source of claim 8, wherein the protection element is made from carbon, and wherein the first temperature is no more than 2800 K.

10. The X-ray source of any one of the preceding claims, wherein the protection element has a cylindrical shape having a closed end forming the first portion and having a side wall forming the second portion.

11. The X-ray source of any one of the preceding claims, wherein the extension of the second portion transverse to the first portion is between 5 mm and 50 mm.

12. The X-ray source of any one of the preceding claims, wherein the first portion has a thickness of 50 pm to 250 pm, such as about 100 pm.

13. The X-ray source of any one of the preceding claims, wherein at least some part of the first portion of the protection element is located no more than 50 mm from the interaction region.

14. The X-ray source of any one of the preceding claims, wherein at least some part of the first portion of the protection element is located no more than 20 mm from the interaction region, preferably no more than 10 mm from the interaction region.

15. The X-ray source of any one of the preceding claims, wherein the X-ray transparent window comprises beryllium.

16. The X-ray source of any one of the preceding claims, wherein the protection element is removably attached to the holder.

17. The X-ray source of any one of the preceding claims, comprising a cooling arrangement in contact with the second portion of the protection element, wherein the cooling arrangement is maintained at a second temperature during operation of the X-ray source.

18. The X-ray source of claim 17, wherein the second temperature is below 300 K.

19. The X-ray source of any one of the preceding claims, wherein electrons scattered from the interaction region is the only source for heating the protection element.