Characterization of electron beam

Abstract

To provide an improvement in electron beam impact liquid metal jet X-ray sources.SOLUTION: A method includes: directing an electron beam to an interference region; stopping the electron beam at an electron beam dump 124 connected to ground potential for receiving the electron beam after the electron beam has traversed the interference region; scanning at least part of the interference region with the electron beam; measuring X-ray radiation generated by interaction between the electron beam and the electron beam dump during the scanning to obtain an X-ray profile; and calculating an electron beam characteristic based on the X-ray profile.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The invention disclosed herein generally relates to characterization of the electron beam in a liquid metal jet X-ray source. [Background technology]

[0002] X-ray radiation can be generated by impinging an electron beam on a target material. X-ray radiation can be generated as bremsstrahlung or characteristic radiation from the target material. The performance of an X-ray source depends, among other things, on the characteristics of the focal spot of X-ray radiation generated by the interaction between the electron beam and the target. In general, there is a striving for higher brightness and smaller focal spot of X-ray radiation, which requires improved control of the electron beam and its interaction with the target. In particular, several attempts have been made to more accurately determine and control the spot size and shape of the electron beam impinging on the target.

[0003] WO2012 / 087238 discloses a technique for determining and controlling the width of an electron beam at the point of interaction with a target. This prior art involves the use of a sensor with a charge-sensitive area. The measurement of the width of the electron beam is performed by deflecting the electron beam over the sensor area while an electronic target is present and partially obscures the sensor area. Since the electronic target obscures or partially obscures part of the sensor area, the recorded sensor signal shows a transition between the minimum attenuation of the beam (unobscured sensor area) and the maximum attenuation (behind the target). The beam width can be derived from this information, in particular from the width of the transition. However, for measurement purposes, the sensor area cannot be electrically grounded. This technique therefore involves challenges such as how to avoid short circuits and arcing at the sensor edges. Summary of the Invention

[0004] The present invention provides an improvement to electron beam impingement liquid metal jet X-ray sources and is based on the idea of ​​characterizing the electron beam by measuring the X-ray radiation generated by the electron beam. More specifically, the X-ray radiation generated can be measured while scanning the electron beam such that an X-ray profile is obtained. One or more properties of the electron beam can then be calculated based on the obtained X-ray profile.

[0005] Thus, in an embodiment of the invention, the characteristics of the electron beam are determined through detection of X-ray radiation generated from the electron beam during its scan. Measurement of the current through the part of the X-ray source that the electron beam impinges on after crossing the interference region (the "electron dump") is not required, so this part can be connected to ground (i.e., electrically grounded). By relying on X-ray measurements instead of current measurements, the electron beam dump does not need to be electrically insulated from its surroundings, thereby eliminating the risk of short circuits due to, for example, the deposition of droplets of material from a liquid jet at the edge of the electron dump. Furthermore, image distortions due to the deposition of droplets on the electron dump surface can be significantly reduced, since such deposited materials are typically either transparent to X-rays or act as another X-ray source depending on the configuration.

[0006] One characteristic of the electron beam of interest is its cross-sectional expansion (width). The width or cross-sectional expansion of the electron beam can be appropriately defined as the full width at half maximum (FWHM). This is sometimes referred to as the "spot size" of the beam. The width of the electron beam in the interference region where it impinges on the liquid metal jet target is an important factor affecting the x-ray generation process. An embodiment of the present invention can be used to determine the width of the electron beam in the interference region by using a liquid metal jet as an obscuring object over which the electron beam is scanned, or by scanning the electron beam over the interference region and having it pass through an aperture before being detected. In the latter case, the width of the electron beam is determined at the aperture, and then the width in the interference region is determined mathematically through a direct geometric transformation. Other characteristics of the electron beam can include, for example, the intensity profile and alignment.

[0007] In some embodiments of the invention, the liquid metal jet is used as an object that obscures the electron beam from the electron beam dump. The electron beam is scanned between a first position where the electron beam impinges on the electron beam dump not obscured by the liquid metal jet, a second position where the liquid metal jet is maximally obscured from the electron beam dump, and a suitable set of intermediate positions. The X-ray radiation generated by the interaction between the electron beam and the electron beam dump is measured during the scan to obtain an X-ray profile that maps the scan position to the X-ray radiation generated, i.e. the X-ray profile can be considered as a function of the deflection setting during the scan. Thus, a transition between the unobscured and obscured positions can be identified, the width of such a transition corresponding to the width of the electron beam measured at the liquid metal jet. As will be appreciated, the width determined with respect to the scan position can be easily converted to units of length if the displacement of the electron beam in the liquid metal jet is known for each scan position.

[0008] In embodiments where the electron beam is scanned over the liquid metal jet, the distance required to move the beam from one side of the jet to the other can be taken as a measure of the width of the liquid metal jet itself. Furthermore, the position of the liquid metal jet can be obtained from the position where the electron beam is obscured by the liquid metal jet. The variation in the width and / or position of the liquid metal jet can be considered as an indication of the stability of the process generating the liquid metal jet.

[0009] In some embodiments, the scan can be performed between a first position where at least half of the electron beam passes through a first side of the liquid metal jet before impacting the electron beam dump, and a second position where at least half of the electron beam passes through a second side of the liquid metal jet before impacting the electron beam dump. The width of the electron beam can then be extracted from the change in the x-ray radiation generated as the electron beam is scanned from the first side to the other. In this way, the electron beam width can be measured in excess of the liquid metal jet width.

[0010] In some embodiments, obscuring objects other than a liquid metal jet are used. A variety of obscuring objects can be used, provided they absorb and / or reflect the electrons so that they do not reach the electron beam dump.

[0011] In other embodiments, the X-ray profile is determined not only from the X-ray radiation generated by the interaction between the electron beam and the electron beam dump, but also from the interaction between the electron beam and the liquid metal jet itself. In such embodiments, the electron beam dump serves primarily as a feature for dumping charge. The X-ray radiation generated by the interaction between the electron beam and the liquid metal jet during the scanning of the electron beam is measured with an X-ray detector. As will be appreciated, the X-ray detector can detect the X-ray radiation when the electron beam strikes the liquid metal jet, but does not detect the X-ray radiation when the electron beam does not strike the liquid metal jet. At a certain scanning position of the electron beam, the occurrence of the X-ray radiation is maximum, and similar to what has been described above, the width of the electron beam can be determined from the relationship between the scanning position and the detected X-ray radiation, i.e., from the X-ray profile.

[0012] In some embodiments, the generated X-ray radiation passes through a pinhole before being detected by the X-ray detector. Such use of a pinhole provides imaging capabilities that can be used to determine properties such as the cross-sectional expansion of the electron beam.

[0013] Therefore, there is provided a method and a device as set forth in the independent claims. The dependent claims define advantageous embodiments of the invention. [Brief description of the drawings]

[0014] Embodiments of the disclosed invention are described in the following detailed description with reference to the accompanying drawings. [Figure 1] FIG. 1 is a flow chart illustrating a method according to the present invention. [Diagram 2] FIG. 2 is a schematic perspective view of a liquid metal jet X-ray source according to some embodiments of the present invention. [Diagram 3] FIG. 3 shows diagrammatically a first embodiment of a liquid metal jet X-ray source according to the invention. [Figure 4]FIG. 4 shows diagrammatically a second embodiment of a liquid metal jet X-ray source according to the invention. [Diagram 5] FIG. 5 shows diagrammatically a third embodiment of a liquid metal jet X-ray source according to the invention. [Figure 6] FIG. 6 shows a schematic representation of a fourth embodiment of a liquid metal jet X-ray source according to the invention.

[0015] In the drawings, corresponding features are designated by the same reference numerals throughout. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] An embodiment of the present invention provides for characterization of an electron beam used for generation of X-ray radiation in a liquid metal jet X-ray source. To characterize the electron beam, the electron beam is scanned over an obscuring object and the X-ray radiation generated during such scan is detected. The obscuring object can be an aperture, a liquid metal jet, or any reference object.

[0017] From a general perspective, it may be preferable to perform measurements at the focal plane of the electron beam, since the focal plane is typically where the electron beam interacts with the liquid metal jet target to generate x-rays during operation. However, it is also contemplated that measurements may be performed at some other plane along the electron beam and mathematically transformed so that the results reflect the conditions at the focal plane. In some embodiments, such a transformation could possibly be established as part of a factory calibration procedure.

[0018] In a preferred embodiment of the present invention, characterization of the electron beam is performed using an obscuring object along the path of the electron beam. When the electron beam is scanned in at least a portion of the interference region, the path of the electron beam is at least partially crossed by an obscuring object for some scanning directions, while the electron beam is not obscured for other scanning directions. Electron beam characteristics, such as the cross-sectional size or shape of the electron beam, can thus be obtained based on how much the electron beam is obscured for different scanning directions, which is then estimated by measuring the X-ray radiation generated during the scanning of the electron beam.

[0019] The obscuring object may be placed in various locations. For example, an aperture may be provided in front of the electron beam dump such that only electrons passing through the aperture are detected at said beam dump. This approach is useful in embodiments where the measured x-ray radiation is generated by interaction between the electron beam and the electron beam dump. Alternatively or additionally, the obscuring object may be placed in the interference region such that it intersects the electron beam at or near where it interacts with the liquid metal jet during operation of the x-ray source.

[0020] In some preferred embodiments, the liquid metal jet itself is used as the obscuring object, i.e. the edge along which the electron beam is scanned, and the measured X-ray radiation may be the radiation generated by the interaction between the electron beam and the liquid metal jet.

[0021] In other embodiments, the reference object is inserted into the beam path of the electron beam when measurements are made and then removed prior to normal operation of the X-ray source. Such a reference object may provide edges in more than one direction, thus facilitating measurements of, for example, electron beam astigmatism. Measurements performed using a reference object are typically part of a factory calibration or maintenance procedure.

[0022] Similar measurements can also be made "in situ" for diagnostic purposes. Several quantities can then be determined and compared to pre-set limits without calculating the actual properties of the electron beam. If the diagnostic measurements indicate that the electron beam is out of specification, the system can adjust the settings of the electron-optical system until the measured quantities are within limits, or alternatively alert the operator that maintenance is required to achieve system specifications.

[0023] A general introduction is first provided with reference to Figure 2, which is a schematic perspective view of a liquid metal jet x-ray source 200 according to some embodiments of the present invention. The illustrated x-ray source 200 utilizes a liquid metal jet 210 as a target for an electron beam. It should be noted that some of the illustrated features of the x-ray source 200 are included only as possible examples and may not necessarily be present or required for the operation of all embodiments.

[0024] The X-ray source 200 includes an electron source 214, 246 and a liquid jet generator 208 configured to form a liquid jet 210 that serves as an electron target. The components of the X-ray source 200 are disposed within an airtight housing 242. However, some components, such as a power supply 244 and a controller 247, may be disposed outside the airtight housing 242. It is also contemplated that various electronic optical components that operate by electromagnetic interaction may be disposed outside the housing 242 if the housing does not shield electromagnetic fields to a significant extent (e.g., austenitic stainless steel).

[0025] The electron source generally comprises a cathode 214 powered by a power supply 244 and includes an electron emitter 246, e.g., a thermionic, thermal field, or cold field charged particle source. Typically, the electron energy may range from about 5 keV to about 500 keV. The electron beam from the electron source is accelerated towards an acceleration aperture 248, at which point the electron beam enters an electron optical system including an arrangement of alignment plates 250, lenses 252, and deflection plates 254. Variable properties of the alignment plates 250, lenses 252, and deflection plates 254 can be controlled by signals provided by a controller 247. In the illustrated example, the deflection plates 250 and alignment plates 254 are operable to accelerate the electron beam in at least two transverse directions. After initial calibration, the alignment plate 250 is typically maintained at a constant setting throughout the working cycle of the X-ray source 200, while the deflection plate 254 is used to dynamically scan or adjust the electron spot position during use of the X-ray source 200. The controllable properties of the lens 252 include their respective focusing powers (i.e., focal lengths). Although FIG. 2 depicts the alignment, focusing, and deflection means symbolically in a manner that suggests they are of the electrostatic type, the invention can be equally well embodied using electromagnetic devices, or a mixture of electrostatic and electromagnetic electro-optical components. The X-ray source 200 can also include a stigmator coil 253 that can provide adjustment of the cross-sectional shape of the electron spot.

[0026] Downstream of the electron optical system, the outgoing electron beam I2 intersects with the liquid metal jet 210 in an interference region 212. This is where x-ray generation can occur. X-ray radiation may be directed out of the housing 242 in a direction that is not coincident with the electron beam propagation direction. Any portion of the electron beam I2 that continues to pass through the interference region 212 may reach an electron beam dump 228 that is electrically connected to ground. As shown, the electron beam dump 228 may be located a distance D away from the interference region 212 so as not to interfere with normal operation of the x-ray source 200. An aperture (not shown in FIG. 2) may be provided that is positioned such that electrons that pass through the aperture strike the electron beam dump 228 while electrons that do not pass through the aperture do not strike it.

[0027] FIG. 1 illustrates a method according to the present invention.

[0028] A method according to the present invention for characterizing an electron beam in a liquid metal jet X-ray source includes steps S110 of providing an electron beam and directing the electron beam toward an interference region, S120 of providing an electron beam dump connected to ground potential to receive the electron beam after it traverses the interference region, S130 of scanning the electron beam over at least a portion of the interference region, S140 of measuring X-ray radiation generated during the scan to obtain an X-ray profile, and S150 of calculating electron beam properties based on the X-ray profile.

[0029] In a preferred embodiment, step S140 of measuring the X-ray radiation generated during the scan includes measuring the X-ray radiation generated by the interaction between the electron beam and the electron beam dump. For example, the cross-sectional expansion (width) of the electron beam can be determined by scanning the electron beam across the interference region, thereby scanning across the electron beam dump, and simultaneously measuring the generated X-ray radiation. An aperture may be provided such that X-ray radiation is generated only at the electron beam dump by any portion of the electron beam passing through the aperture. For example, an aperture can be provided as illustrated in FIG. 3, in which case only electrons that reach the electron beam dump surface 124 contribute to the detected X-ray radiation. By correlating the direction of the electron beam (e.g., in terms of the voltages applied to the corresponding deflection plates) to the detected X-ray radiation, an X-ray profile is obtained that can be used to calculate the cross-sectional expansion of the electron beam in the scan direction. By scanning the electron beam in two or more directions across the aperture, the total cross-sectional expansion of the electron beam can be calculated.

[0030] Alternatively, rather than relying on an aperture to limit the amount of electrons reaching the electron beam dump, an object can be provided that partially intersects the path of the electron beam. Any object that absorbs and / or reflects electrons can be used. In this context, it may be preferable to use a liquid metal jet target for this purpose. The electron beam is then scanned across the obstructing object, which acts as a kind of reverse aperture in the sense that it prevents the electrons from reaching the electron beam dump. When the electron beam strikes the electron beam dump that is not obscured by the object, a maximum amount of X-ray radiation is generated. As the electron beam is scanned over the object, it is partially obscured and the amount of X-ray radiation generated at the electron beam dump decreases until the electron beam is maximally obscured by the object. Again, an X-ray profile is obtained that can be used to calculate the cross-sectional expansion of the electron beam.

[0031] In another embodiment, the method involves measuring X-ray radiation generated by an interaction between the electron beam and the liquid metal jet target during a scan of the electron beam.

[0032] In a liquid metal jet X-ray source as shown diagrammatically in Figures 4 to 6, in which one or more X-ray detectors are arranged to detect X-ray radiation generated by interaction between the electron beam and the liquid metal jet, the step S140 of measuring the X-ray radiation generated during the scan to obtain an X-ray profile therefore includes measuring the X-ray radiation generated by interaction between the electron beam and the liquid metal jet.

[0033] The effect of self-absorption of X-ray radiation in the liquid metal jet target can be reduced by using two X-ray detectors 128a and 128b arranged on either side of the interference region, as shown diagrammatically in FIG. 5. Step S140 of measuring the X-ray radiation generated during the scan to obtain an X-ray profile can include taking into account the sum of the X-ray radiation detected at the two detectors, for example by summing the outputs from the two detectors. If the two detectors are arranged symmetrically on either side of the interference region, the sum of the outputs compensates for any self-absorption induced asymmetry in the X-ray profile as recorded by one of the detectors, since a correspondingly higher or lower level of X-ray radiation will be detected at the other detector. If the detectors are not arranged symmetrically, appropriate weights can be applied to the respective outputs before summing.

[0034] The effect of self-absorption of the X-ray radiation in the liquid metal jet can be further reduced by measuring the generated X-ray radiation from the same side where the electron beam strikes the liquid metal jet, as shown diagrammatically in Fig. 6. In such a setup, there is no self-absorption between the interference region and the X-ray detector that affects the generated X-ray radiation. This may be particularly useful for alignment purposes, as the detector "sees" where the electron beam strikes the liquid metal jet. The X-ray detector may be equipped with imaging capabilities, for example by including a CCD array, and to improve imaging, a pinhole may be provided between the CCD array and the interference region where the electron beam strikes the liquid metal jet.

[0035] For example, an X-ray detector including a CCD array and pinhole similar to that described above may also be useful in a setup such as that shown in Figure 4. The X-ray detector then "sees" the X-ray radiation that is produced.

[0036] Alternatively, some other object capable of generating X-ray radiation upon electron impact may be placed in the interference region during characterization, and in such an embodiment the liquid metal jet may not be present during characterization.

[0037] FIG. 3 shows a schematic of a liquid metal jet X-ray source 300 according to a first embodiment of the invention. The X-ray source 300 comprises an electron source / cathode 110 that emits electrons towards an anode 114. An accelerating potential 112 can be applied between the cathode 110 and the anode 114 to accelerate the emitted electrons. Downstream of the anode 114, one or more alignment coils 116 are arranged for alignment of the electron beam. One or more focusing lenses 118 and deflection plates 120 are also arranged along the electron beam path to focus and direct the electron beam towards an interference region where the electron beam can interact with a liquid metal jet target 122. During normal operation, useful X-ray radiation is generated by the interaction between the electron beam and the liquid metal jet 122 in the interference region. The X-ray source 300 also comprises an electron beam dump 124 against which electrons that have passed through the interference region impinge. The electron beam dump 124 is electrically grounded so that electrons impinging thereon are discarded, i.e. dumped.

[0038] In the embodiment of FIG. 3, the electron beam dump 124 is positioned such that X-ray radiation is generated when struck by electrons. An X-ray detector 128 is provided to detect X-ray radiation generated from the electron beam dump 124. The detector 128 may be positioned to detect only X-ray radiation generated from the electron beam dump (e.g., not radiation generated from the interaction between the electron beam and the liquid metal jet 122). In such a configuration, alignment and focusing procedures can be performed in a similar manner to a conventional electron beam dump in which the current through the beam dump is measured, for example as described in the above-mentioned WO 2012 / 087238. However, since the X-ray radiation generated from the beam dump 124 is used in the procedure rather than the current through the beam dump, it is not necessary to maintain the beam dump 124 at a particular potential. In contrast, in an embodiment of the present invention, the electron beam dump 124 is electrically grounded, as shown at 126 in FIG. 3. Thus, for example, if metal droplets were to land on the edge of beam dump 124, no harmful short circuits would occur, and the deposited metal droplets would also generate x-rays upon electron impact, and thus would not impair functionality.

[0039] In some embodiments, the design can be optimized in the sense that the material of the electron beam dump 124 provides a similar cross section for x-ray generation for all relevant orientations of the electron beam. One embodiment can include, for example, a flat surface positioned at an appropriate angle relative to the impingement direction of the electron beam. In other embodiments, the electron beam dump 124 can include a cylindrical surface, with the radius of the cylinder being large compared to the distance the electron beam traverses the surface during a scan of the electron beam across the aperture of the electron beam dump.

[0040] Preferably, the electron beam dump is equipped with a suitable cooling arrangement to handle the heat load associated with the impingement of the electron beam.

[0041] In an embodiment of the present invention, the electron beam dump 124 is electrically grounded. This effectively prevents charge build-up in the beam dump 124 and avoids the prior art problems of short circuits between the beam dump and other parts of the arrangement. It is noted, however, that the electron beam dump 124 does not have to be consistently connected to earth. It is conceivable that the grounding could be intermittently activated, for example, when a threshold potential in the beam dump is reached, to dump the accumulated charge, optionally through a suitable current limiting arrangement, for example a resistor, to earth. However, a preferred embodiment has the electron beam dump 124 consistently connected to an electrical ground such that the electron beam dump is maintained at ground potential. It is conceivable within the scope of the present invention to create a virtual ground potential for the housing and the electron beam dump, i.e. these components can be actively held at a specific potential that is not necessarily equal to zero. This type of embodiment can have design advantages in some circumstances, but the general concept of the present invention is not affected.

[0042] Any suitable type of detector may be used for the x-ray detector 128, for example, a cadmium telluride (CdTe) diode in a tungsten (W) housing.

[0043] 3, an X-ray profile during a scan of the electron beam is obtained by measuring X-ray radiation generated by the interaction between the electron beam and the electron beam dump 124. Optionally, an object such as the liquid metal jet target 122 can be present to partially obscure the electron beam during the scan. The X-ray detector 128 is positioned to detect only the X-ray radiation from the electron beam dump and not to detect radiation generated from the interaction between the electron beam and the liquid metal jet or any other part of the system, such as a housing or aperture positioned between the interference region and the electron beam dump.

[0044] Other embodiments may also rely on the detection of X-ray radiation generated by the interaction between the electron beam and the liquid metal jet of the source 100, or between the electron beam and a reference object placed in the electron beam path. FIG. 4 shows an embodiment of an X-ray source 400 in which an X-ray detector 128 is arranged to detect X-rays generated in the interference region. The X-ray sensor for detecting X-rays generated in the interference region is preferably a second sensor dedicated for this purpose. The X-ray detector 128 is conveniently arranged outside the vacuum chamber of the X-ray source and can detect X-ray radiation through an X-ray transparent window. A typical X-ray source according to the invention can comprise one or more X-ray transparent windows or ports through which the generated X-ray radiation is extracted. The detector 128 can be conveniently arranged at one such port. The detector 128 can thereby detect X-ray radiation when the electron beam strikes a liquid metal target (or a suitably placed reference object), but not when the electron beam does not strike a target. Thereby, by sampling the X-ray detector 128 while the electron beam is scanned over the target, an X-ray intensity profile can be obtained which can be used to infer properties such as the cross-sectional dimensions of the electron beam. It is contemplated that self-absorption in the target may result in a somewhat distorted measurement of the profile, but this can be compensated for by subtracting a varying background or by using only X-ray radiation of energies such that self-absorption in the target is reduced, e.g., by detecting only X-ray radiation having energies well above the X-ray absorption limit of the target material.

[0045] 5 shows diagrammatically another embodiment of a liquid metal jet X-ray source 500 according to the invention, where self-absorption is compensated for by using two X-ray detectors 128a, 128b, each positioned at a different angle relative to the liquid metal jet 122. A compensated measurement can thus be obtained by considering the sum of the X-ray radiation detected by the two detectors.

[0046] 6 shows a schematic representation of yet another embodiment in which the X-ray detector 128 is positioned within the line of sight from the interference region but below (or above) the electron beam. As long as the X-ray detector has a sufficiently narrow field of view, any radiation generated from electrons impinging on the electron beam dump 124 can be prevented from being detected by the X-ray detector 128. Such positioning of the X-ray detector can reduce artifacts caused by self-absorption in the target.

[0047] It will be appreciated that the detector arrangements shown in Figures 3 to 6 may also be combined. For example, an X-ray source and / or corresponding method involving measurement of X-ray radiation generated by interaction between the electron beam and an electron beam dump as shown in Figure 3 may be combined (i.e., supplemented) with measurement of X-ray radiation generated by interaction between the electron beam and a liquid metal jet or another obscuring object as shown in any of Figures 4 to 6. It is also conceivable to have an implementation in which X-ray radiation generated by interaction between the electron beam and an electron beam dump is not taken into account when determining the X-ray profile, and thus relies only on one or more of the detection schemes described with reference to Figures 4 to 6.

[0048] In various embodiments of the invention, an x-ray profile is obtained by scanning the electron beam across either a liquid metal jet, a reference object, an aperture, or the like, and the resulting x-ray profile can be used when calibrating or adjusting the focus of the electron beam. A second sensor, e.g., a sensor that detects backscattered electrons, can be used to align the electron beam along the optical axis of the system. However, the use of such a backscatter sensor is less advantageous for non-flat targets, since the backscatter coefficient changes as the electron beam is scanned across the target.

[0049] In cases where metal droplets deposit somewhere between the target and the X-ray detector, there may be a reduction in the amount of X-ray radiation reaching the detector, but this does not impair functionality and an X-ray intensity profile can still be obtained that can be used to determine the electron beam width, albeit at a slightly reduced intensity.

[0050] In case an X-ray detector is used that has sufficient imaging capabilities, for example by being provided with a pinhole and / or including a CCD array, the spread of the focal spot in a direction substantially parallel to the liquid metal jet can be obtained by scanning the electron beam along that direction while detecting the amount of radiation that reaches the detector. The distance the electron beam spot has to be moved for the X-ray signal to go from full signal to zero, or some other appropriately defined limit, corresponds to the beam spot size.

[0051] In the above-described embodiments, the detection of X-ray radiation is direct (e.g., by using a diode-based detector). However, the detection of X-ray radiation may also be indirect, by first converting the X-ray radiation to radiation having a lower frequency and then detecting the lower frequency radiation (e.g., using a scintillator and a detector for visible light). In all embodiments, it is preferable to shield or position the X-ray detector so that only radiation from the intended source is detected. As described above, such shielding can be achieved by CdTe diodes located inside the W casing. In a preferred embodiment, the X-ray detector has a CdTe diode located at an appropriate depth inside the W cylinder to shield it from unwanted X-ray radiation. Other types of collimators that limit the field of view of the X-ray detector are considered within the scope of the present invention.

[0052] In summary, embodiments of the present invention provide a method for determining properties such as the cross-sectional expansion of an electron beam. An electron beam is directed at an interference region. After passing through the interference region, the electron beam strikes an electron beam dump and the charge is dumped to electrical ground. The electron beam is scanned over at least a portion of the interference region and X-ray radiation generated during the scan is measured to obtain an X-ray profile relating the measured X-ray radiation to the electron beam direction. Electron beam properties such as the cross-sectional expansion of the electron beam are then calculated based on the generated X-ray profile.

[0053] In some embodiments, the X-ray radiation is generated by the interaction between the electron beam and the beam dump, where the electron beam passes through an aperture before reaching the electron beam dump. Only the portion of the electron beam that passes through the aperture can reach the electron beam dump and thus contribute to the generation of the X-ray radiation. The X-ray profile can therefore be used to calculate the cross-sectional expansion of the electron beam. It is also contemplated that the aperture may be embodied as an extension of the electron beam dump itself. In other embodiments, the aperture is embodied as an opening in the wall of the liquid metal jet X-ray source, as shown diagrammatically in the accompanying drawings. As will be appreciated, in embodiments utilizing such an aperture, the electron beam needs to be scanned over a sufficiently large angle to reach the edge of the aperture.

[0054] In another embodiment, an object is provided that partially intersects the path of the electron beam during scanning. Such an object can take many different forms, so long as it has properties that absorb and / or reflect electrons such that fewer electrons reach the electron beam dump when the object partially intersects the path of the electron beam. The object that intersects the path may be a liquid metal jet that is present in the interference region.

[0055] Although some exemplary embodiments have been described herein, those skilled in the art are not limited to these examples when implementing embodiments of the present invention. On the contrary, many modifications and variations are possible within the scope of the appended claims. In particular, X-ray sources with two or more targets or two or more electron beams can be considered within the scope of the inventive concept. Furthermore, X-ray sources of the type described herein can be advantageously combined with X-ray optics and / or detectors tailored to specific applications, exemplified but not limited to medical diagnostics, non-destructive testing, lithography, crystallography, microscopy, materials science, surface physics, protein structure determination by X-ray diffraction, X-ray photospectroscopy (XPS), critical dimension small angle X-ray scattering (CD-SAXS), and X-ray fluorescence. After reading and understanding this disclosure in conjunction with the accompanying drawings, those skilled in the art will be able to implement various embodiments.

Claims

1. 1. A method for characterizing an electron beam in a liquid metal jet x-ray source, comprising: providing the electron beam and directing the electron beam toward an interference region; providing an electron beam dump connected to ground potential for receiving the electron beam after it has traversed the interference region; scanning the electron beam over at least a portion of the interference region; measuring X-ray radiation generated by an interaction between the electron beam and the electron beam dump during the scan to obtain an X-ray profile; and calculating electron beam properties based on the x-ray profile.

2. The method of claim 1 , further comprising providing an object that partially intersects a path of the electron beam during the scan, where the object absorbs and / or reflects electrons.

3. The method of claim 2 , wherein the object is a liquid metal jet present in the interference region.

4. The method of claim 3 , further comprising calculating properties of the liquid metal jet based on the x-ray profile.

5. 5. The method of claim 2, wherein the step of measuring X-ray radiation to obtain an X-ray profile further comprises measuring X-ray radiation generated by an interaction between the electron beam and the object.

6. 5. The method of claim 1, further comprising providing an aperture between the interference region and the electron beam dump, the aperture being positioned such that only electrons passing through the aperture contribute to x-ray radiation measured during the scan.

7. The method of claim 6 , wherein scanning the electron beam across the interference region comprises scanning the electron beam across the aperture.

8. 6. The method of claim 5, further comprising providing a pinhole, and wherein measuring a profile of X-ray radiation generated during the scan comprises detecting X-ray radiation that passes through the pinhole.

9. 1. A liquid metal jet X-ray source comprising: an electron source positioned to provide an electron beam and direct the electron beam towards an interference region; an electron beam dump connected to a ground potential and positioned to receive the electron beam after it has traversed the interference region; a scanning device capable of scanning the electron beam over at least a portion of the interference region; an x-ray sensor positioned and arranged to detect x-ray radiation generated by interaction between the electron beam and the electron beam dump; A liquid metal jet x-ray source comprising: a scanning device and a circuit operably connected to the x-ray sensor, the circuit configured to determine an x-ray profile during a scan of the electron beam.

10. The liquid metal jet x-ray source of claim 9 , wherein the circuitry is further configured to calculate a characteristic of the electron beam based on the x-ray profile.

11. 11. A liquid metal jet X-ray source as claimed in claim 9 or 10, further comprising a reference object removably mounted to partially intersect a path of the electron beam during the scan, the reference object absorbing and / or reflecting electrons.

12. 12. The liquid metal jet X-ray source of claim 9, further comprising a collimator for limiting the field of view of the X-ray sensor.

13. 13. A liquid metal jet X-ray source as claimed in any one of claims 9 to 12, further comprising an aperture between the interference region and the electron beam dump, the aperture being positioned such that only electrons passing through the aperture contribute to the X-ray radiation measured during the scan.

14. 14. A liquid metal jet x-ray source according to any one of claims 9 to 13, further comprising a second x-ray sensor positioned and arranged to detect x-ray radiation generated in the interference region.

15. 15. The liquid metal jet x-ray source of claim 14, further comprising a pinhole positioned and arranged such that x-ray radiation detected by the second x-ray sensor passes through the pinhole.

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