Methods and configurations for X-ray sources

Abstract

An X-ray source is provided comprising an electron source for providing an electron beam, the electron source comprising a cathode, a Wehnelt, and an anode, an electron optical arrangement configured to deflect and focus the electron beam towards a target for generation of X-ray radiation, an arrangement for determining a quantity indicative of a width of the electron beam, and a controller operatively connected to the electron source, the electron optical arrangement, and the arrangement for determining the quantity indicative of the width of the electron beam, the controller being configured to: calculate a quantity dependent on a divergence of the electron beam at an entrance to the electron optical arrangement based on the quantity indicative of the width of the electron beam; and apply a bias voltage to the Wehnelt such that the quantity dependent on the divergence of the electron beam at an entrance to the electron optical arrangement is adjusted towards a desired value. Corresponding methods are also provided.

Description

[Technical field]

[0001] The present disclosure relates to X-ray sources and methods therein. [Background technology]

[0002] X-ray radiation may be generated by impinging an electron beam on a target material. X-ray radiation may occur as bremsstrahlung or characteristic radiation emission 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] A Wehnelt cylinder or grid cap, or Wehnelt for short, is an electrode in an electron emission unit used to control an electron beam generated by emission of electrons from a cathode (especially from a thermionic cathode). The use of a Wehnelt is generally known in the art, for example as disclosed in US2017 / 0148605. Summary of the Invention

[0004] The present disclosure relates to an x-ray source and related method for using a Wehnelt grid bias to compensate for cathode aging, and the inventive principles presented herein extend the useful life of a cathode by using a Wehnelt grid bias to compensate for such cathode aging.

[0005] A typical cathode for use in an electron gun of an X-ray source comprises a lanthanum hexaboride (LaB6) crystal embedded in a carbon guard ring. To emit electrons, the crystal is heated, which causes a slow evaporation of the crystal. Thus, after continued use, the end face of the crystal may recede into the carbon embedding and the diameter of the end face may decrease. Both of these effects result in a reduced electron emission angle and, consequently, a drift in the X-ray spot performance.

[0006] The present disclosure proposes to compensate for this by applying and / or adjusting the bias voltage to the Wehnelt, thereby widening the electron emission angle so that the X-ray spot performance is further maintained. The ageing of the cathode can be monitored, for example, by measuring quantities indicative of the electron beam width, e.g. the electron beam spot size or the cross-sectional intensity distribution of the electron beam, for at least two different focus settings, and fitting a model of the electron optical system to the results. From this, quantities depending on the spread of the electron beam at the entrance of the electron optical arrangement (related to the electron emission angle at the cathode), such as the focus angle (convergence angle) towards the target, the minimum achievable spot size, the size of the electron beam at the focusing lens plane, or the spread of the electron beam spread itself can be extracted. With knowledge of how the grid bias affects the emission angle from the cathode and the Wehnelt (or equivalently, the spread at the entrance of the electron optical arrangement), and thereby of quantities depending on the emission angle, appropriate compensations can be applied, thus extending the operating life of the cathode.

[0007] Measuring only the electron beam spot size (or any other quantity related to the cross-sectional intensity distribution of the electron beam) for one focus setting typically does not provide enough information to extract, for example, the electron emission angle from the cathode or the convergence angle as a measure of the divergence at the entrance of the electron optics. The approach proposed here is to make at least two measurements of a quantity indicative of the electron beam width and to calculate from these measurements a quantity that depends on the electron beam divergence at the entrance of the electron optics arrangement. If the actual emission angles from the cathode and the Wehnelt are of interest, it is possible to fit a model of the system to the (at least) two measurements, which is advantageous in that different contributions to the observed spot size change can be separated.

[0008] However, in some implementations, the need to determine a quantity indicative of the electron beam width at two different focus settings may be avoided. For example, the spread of the electron beam may be estimated from a measurement of how much of the electron beam is able to pass through the aperture. This may be determined by comparing the total current emitted from the cathode with how much of this total current passes through the aperture. In this case, the ratio of the current through the aperture to the total emission current may be taken as an indication of the electron beam spread. As will be appreciated, excessive spread may have the effect that fewer than all electrons emitted from the cathode are able to pass through the aperture; the fewer electrons that pass through the aperture, the greater the spread. Similarly, if substantially all electrons emitted from the cathode are able to pass through the aperture, this may be an indication that the spread from the cathode (or, correspondingly, the electron beam spread at the entrance to the electron optics) is too small and that a grid bias should be applied to the Wehnelt to increase the spread.

[0009] Another option for determining a quantity indicative of the electron beam width is to directly measure the electron beam width at a known distance from the cathode upstream of the electron optical arrangement, i.e. before the electron beam is shaped or influenced by the optical system. This can be done, for example, by providing an aperture in the region between the cathode and the electron optical system and moving the electron beam across the aperture while measuring the amount of electrons that reach an electronic sensor downstream of the aperture. The electronic sensor can be located, for example, downstream of the target. In this implementation, moving the electron beam across the aperture can be done, for example, by lateral movement of the cathode.

[0010] The measurement of the cross-sectional intensity distribution of the electron beam (e.g., electron beam spot size) can be performed by scanning the electron beam over an edge separating two regions with different abilities to reflect and / or absorb electrons and recording a number of values ​​of a quantity indicative of the intensity of the electron beam, where the multiple values ​​are recorded for a number of electron beam positions. The two regions can be provided as different parts of a structured target, e.g., a tungsten layer on a diamond substrate. In another alternative, the two regions can be provided by a target, such that the electron beam can be deflected to a position at least partially outside the target, e.g., a liquid metal jet. The two regions can also be provided as dedicated calibration objects that can be inserted into the electron beam path, or as apertures included as part of the electron optical arrangement.

[0011] The present disclosure therefore provides a method in an X-ray source comprising an electron source for providing an electron beam and an electron optical arrangement for deflecting and focusing the electron beam towards a target for generating X-ray radiation, the electron source comprising a cathode, a Wehnelt and an anode, determining a quantity indicative of a width of the electron beam, calculating a quantity dependent on the electron beam divergence at an entrance of the electron optical arrangement (i.e. where the electron beam enters the electron optical arrangement) based on the determined quantity indicative of the width of the electron beam, and applying or adjusting a bias voltage to the Wehnelt such that the quantity dependent on the electron beam divergence at the entrance of the electron optical arrangement is adjusted towards a desired value.

[0012] In general, it is preferred that the bias voltage applied to the Wehnelt be such that the electron beam spot size on the target approaches the desired value, i.e., such that the desired electron spot can be obtained using the electron optical arrangement.

[0013] The step of applying a bias voltage to the Wehnelt may be performed by applying or varying a grid bias voltage within a range of + / - 10 kV relative to the cathode.

[0014] The anode may be implemented by an anode aperture located between the cathode and the electron optical arrangement. Alternatively, the target itself may constitute the anode.

[0015] Since it is practically difficult to have direct access to the cathode and the Wehnelt during operation of the X-ray source, the quantity indicative of the width of the electron beam is preferably determined at a position downstream of the electron optical arrangement, which also provides for the calculation of a quantity dependent on the electron beam divergence at the entrance of the electron optical arrangement, taking into account the influence of the electron optical arrangement on the electron beam.

[0016] The determination of the quantity indicative of the width of the electron beam is preferably performed multiple times during the operation of the X-ray source. Each time the quantity is determined and the quantity dependent on the electron beam divergence at the entrance of the electron optical arrangement is calculated, the bias voltage applied to the Wehnelt can be updated to compensate for the aging of the cathode so that the desired focusing of the electron beam at the target is maintained over time. Such updates can be performed, for example, on request by the operator, due to a trigger from one or more monitoring systems in the X-ray source, or at predetermined occasions during the useful life of a particular cathode. The predetermined occasions can be, for example, regular intervals or intervals that become shorter and shorter towards the end of the expected life of the cathode.

[0017] In another aspect, an X-ray source is provided that includes an electron source for providing an electron beam, the electron source including a cathode, a Wehnelt, and an anode. The X-ray source further includes an electron optical arrangement configured to deflect and focus the electron beam toward a target for generation of X-ray radiation, an arrangement for determining a quantity indicative of a width of the electron beam, and a controller operatively connected to the electron source, the electron optical arrangement, and the arrangement for determining the quantity indicative of the width of the electron beam. The controller is configured to calculate a quantity dependent on the electron beam divergence at an entrance to the electron optical arrangement based on the determination of the quantity indicative of the electron beam width, and to apply or adjust a bias voltage to the Wehnelt such that the quantity dependent on the electron beam divergence at an entrance to the electron optical arrangement is adjusted towards a desired value.

[0018] The present invention contemplates different types of x-ray generating targets. The target may comprise a solid target, such as, for example, a solid reflective target or a solid transmissive target. The solid target may be provided as a stationary or moving (e.g., rotating anode) target. In other implementations, the target may comprise a liquid target, such as a liquid jet target (e.g., a liquid metal jet target).

[0019] Several modifications and variations are possible within the scope of the invention. In particular, X-ray sources with more than one target or more than one electron beam are considered within the scope of the inventive concept. Furthermore, X-ray sources of the type described herein may be advantageously combined with X-ray optics and / or detectors tailored to specific applications, exemplified by, but not limited to, medical diagnostics, non-destructive testing, lithography, crystallography, microscopy, materials science, microscopy surface physics, protein structure determination by X-ray diffraction, X-ray photospectroscopy (XPS), critical dimension small angle X-ray scattering (CD-SAXS), wide angle X-ray scattering (WAXS), and X-ray fluorescence (XRF).

[0020] In the context of the present invention, the determination of the cross-sectional intensity distribution of the electron beam should be understood in a broad sense. In general, it is not necessary to obtain a complete intensity distribution providing the intensity at all points of the electron beam cross section. Rather, the determination of the cross-sectional intensity distribution can be limited to determining, for example, the full width at half maximum (FWHM) of the electron beam, the spot size of the electron beam focus, or any other quantity related to the cross-sectional intensity distribution that can be used to calculate any quantity that depends on the divergence of the electron beam at the entrance of the electron optical arrangement, for example the convergence angle of the electron beam towards the target.

[0021] The following detailed description is presented with reference to the accompanying drawings. [Brief description of the drawings]

[0022] [Figure 1] FIG. 1 shows a schematic diagram of an X-ray source with parts and components relevant to the present disclosure. [Diagram 2] 2a and 2b show how the cross-sectional intensity distribution of the electron beam can be determined at two different focus settings. [Diagram 3] Figures 3a-d show how cathode aging can cause changes in the electron beam distribution. [Figure 4] FIG. 4 is a flow diagram outlining a method in accordance with the principles disclosed herein. [Diagram 5]FIG. 5 shows a schematic of an electron source provided with an aperture for determining the electron beam width. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] 1 shows diagrammatically an X-ray source 100 according to the principles disclosed herein. The X-ray source comprises an electron source 110 for providing an electron beam, an electron optical arrangement 120 and an arrangement 130 for determining a cross-sectional intensity distribution of the electron beam (or, more generally, an arrangement for determining a quantity indicative of the width of the electron beam).

[0024] The illustrated electron source 110 comprises an electron emitter 111 embedded in a carbon guard ring 112, a Wehnelt grid 113, and an anode 114. The electron emitter 111 and the carbon guard ring 112 may be collectively referred to herein as a cathode. As shown in FIG. acc is applied between the cathode and the anode during operation to accelerate the emitted electrons and form an electron beam. Typical values ​​of the acceleration voltage can be 10-1000 kV. Additionally, a grid voltage V grid may be applied between the cathode and the Wehnelt to aid in shaping the emitted electron beam. The grid voltage may generally be within + / - 10 kV relative to the cathode. Note that it is not excluded that the grid voltage may be zero, i.e., the Wehnelt having the same potential as the cathode. The grid voltage will be adjusted during the operational life of the cathode according to the principles disclosed herein.

[0025] The electron optical arrangement 120 may include various elements for shaping and focusing the electron beam onto the target 140 for generation of X-ray radiation. In the illustrated example, the electron optical arrangement includes alignment coils 121 for aligning the electron beam along a desired beam path, stigmator coils 122 for introducing a desired astigmatism into the electron beam (which may be zero if a symmetric electron spot is desired), focusing coils 123 for focusing the electron beam, and deflector 124 for lateral movement of the focused electron beam.

[0026] In the illustrated example, the arrangement 130 for determining the cross-sectional intensity distribution of the electron beam comprises an electron blocking member or stop 131 and an electron sensor 132. To determine the cross-sectional intensity distribution, the electron beam is scanned across the edge defined by the blocking member 131 using the deflector 124. Thus, the amount of electrons reaching the electron sensor 132 depends on how much of the beam cross-section is blocked by the blocking member 131. If the entire electron beam passes through the blocking member and reaches the sensor 132, a maximum intensity will be detected. As the electron beam is scanned over the edge of the blocking member 131, less and less electrons will reach the sensor 132, and thus the electron beam cross-sectional intensity distribution can be determined from the deflection angle and the corresponding amount of electrons detected by the sensor 132 at different deflection angles. In this case, from the sensor signals at different deflection angles, it is possible to determine various characteristics of the electron beam, such as the FWHM. It is noted that there is a known relationship between the deflection angle and the voltage applied to the deflector 124, which makes the determination easy.

[0027] The X-ray source also includes a control device 150 operably connected to the electron source 110, the electron optical arrangement 120, and the arrangement 130 for determining a cross-sectional intensity distribution of the electron beam. The operative connections of the control device are indicated diagrammatically by arrows in Figure 1.

[0028] In order to calculate quantities that depend on the divergence of the electron beam at the entrance of the electron optical arrangement 120, such as the convergence angle of the electron beam (the angle at which the electrons converge towards the focal point), the cross-sectional intensity distribution of the electron beam (e.g. the width of the electron beam) can be determined for at least two different focus settings. This is shown diagrammatically in Fig. 2a and Fig. 2b, which show the position of the electron blocking member 131 relative to the electron beam focal point for two different focus settings. To determine the cross-sectional intensity distribution, the electron beam can be scanned over the edge of the electron stop 131 as described above. The distribution / width determined at the edge of the stop 131 will depend on the focus setting, i.e. the position of the electron beam focal point shown at 200 in Fig. 2a and Fig. 2b. In the first focus setting shown in Fig. 2a, the focal point 200 is relatively close to the electron stop and therefore the distribution / width w1 will be determined at a position relatively close to the focal point. In the second focus setting shown in Fig. 2b, the focal point 200 is relatively far from the electron stop and therefore the distribution / width w2 will be determined at a position relatively far from the focal point. From these two measurements of the electron beam distribution / width, the convergence or focusing angle of the electron beam towards the focal point can be determined.

[0029] In the illustrated example, the arrangement for determining the cross-sectional intensity distribution of the electron beam includes an electron blocking member 131 and a sensor 132. Alternatively, the electron blocking member 131 may be constituted by the target 140 itself, in which case the sensor 132 may be a target current sensor that senses the current accumulated in the target by the electron beam. Furthermore, the sensor 132 may be an electron backscatter sensor that senses electrons scattered from the blocking member 131. Another possibility is to use an X-ray sensor that senses X-ray radiation generated when the electron beam hits the blocking member 131 (X-ray sensors may be particularly useful when the blocking member 131 is constituted by the target 140).

[0030] As described herein, the focus angle will depend on the angle at which electrons are emitted from the cathode and the Wehnelt. Thus, if a focus angle is determined that is not optimal for interacting with the target to generate x-ray radiation, a bias voltage can be applied to the Wehnelt such that the focus angle is adjusted toward the desired value.

[0031] Figures 3a-d show the gradual aging of a cathode diagrammatically, where Figure 3a represents a newly installed cathode and Figures 3b-d show the electron emitter 111 gradually retracting into the carbon guard ring 112 as the cathode ages. Electron emission is shown diagrammatically by dotted lines in Figure 3. A desired emission angle from the cathode can be represented by the diagram of Figure 3a. As will be appreciated, to obtain this desired electron emission angle even for the electron emitter retracted into the carbon guard ring, as shown in Figures 3b-d, a grid voltage can be applied to the Wehnelt 113 in accordance with the principles disclosed herein.

[0032] Typically, the bias voltage applied to the Wehnelt to increase the emission angle from the cathode will be positive with respect to the cathode voltage, however, as shown in Figure 3d, broadening of the electron beam can also be achieved using a negative bias voltage to increase the spread downstream of the crossover if the cathode is recessed into the carbon guard ring to the extent that the crossover is obtained.

[0033] FIG. 4 is a flow chart outlining a method according to the principles disclosed herein. The method is carried out in an X-ray source comprising an electron source for providing an electron beam and an electron optical arrangement for deflecting and focusing the electron beam towards a target for generation of X-ray radiation. The electron source comprises a cathode, a Wehnelt, and an anode. The method comprises a step S410 of determining a quantity indicative of the width of the electron beam, a step S420 of calculating a quantity dependent on the divergence of the electron beam at the entrance of the electron optical arrangement based on the determined quantity indicative of the width of the electron beam, and a step S430 of applying a bias voltage to the Wehnelt such that the quantity dependent on the divergence of the electron beam at the entrance of the electron optical arrangement is adjusted towards a desired value. Steps S410-S430 may be conveniently repeated multiple times during the useful life of a particular cathode to compensate for aging of the cathode. The method may be triggered manually by an operator or automatically. The automatic triggering may be based on various inputs, such as a secondary indication that the desired focusing at the target is no longer achieved, or a predetermined occasion during the useful life of the cathode. The method may also be triggered, for example, when the X-ray source is turned on or when the system is calibrated.

[0034] The quantity indicative of the width of the electron beam may be the cross-sectional intensity distribution of the electron beam or may be the width of the electron beam at a given percentage of the maximum intensity of the electron beam, e.g. the FWHM. Conveniently, the quantity indicative of the width may be determined downstream of the electron optical arrangement, e.g. by determining the cross-sectional intensity distribution or the width of the electron beam.

[0035] A preferred method of determining a quantity indicative of the electron beam width is to scan an electron beam over an edge separating two regions with different abilities to reflect and / or absorb electrons and detect the electrons as a function of the electron beam position relative to the edge. For example, the electrons may be detected as a function of the electron beam position relative to the edge using an electron backscatter sensor, a sensor that detects electrons downstream of the target position, or using a target current sensor that senses the current delivered to the target by the electron beam. It is also possible to use an x-ray sensor that detects x-ray radiation generated during the scanning of the electron beam.

[0036] FIG. 5 shows a schematic representation of an alternative implementation of an X-ray source. As in the implementation described above, the electron source 110 comprises an electron emitter 111 embedded in a carbon guard ring 112, a Wehnelt grid 113 and an anode 114. In this example, an aperture 510 is provided upstream of the electron optics arrangement 120 (not shown in FIG. 5). Thus, electrons emitted from the cathodes 111, 112 will reach the aperture 510 without being affected by the electron optics 120. The spread of the emitted electrons can then be determined by laterally moving the cathodes 111, 112 (and the Wehnelt 113) and detecting the electrons that can pass through the aperture 510. The electrons that pass through the aperture 510 can be detected, for example, using a downstream electron sensor. The lateral movement of the cathodes 111, 112 as indicated by the up and down arrows in FIG. 5 will have the effect that a different amount of electrons can pass through the aperture 510 for each position of the cathodes. Thus, the spread of the emitted electrons may in this case be determined based on the distance between the cathodes 111, 112 and the aperture 510. It should be appreciated that the aperture 510 may be removable such that it may be inserted into the electron beam path for measurement purposes and removed from the electron beam path once the measurement is completed. Furthermore, the aperture 510 does not necessarily have to be a separate element; instead, an opening in the anode 114 may be used for this purpose.

[0037] If the electron beam is wide compared to the aperture, the electron beam width at the plane of the aperture may be determined by the aperture itself. The electron beam spread may be estimated by measuring the electron beam current passing through the aperture and calculating what percentage of the current emitted from the cathode this corresponds to. The reduced spread of the electron beam from the cathode and Wehnelt will correspond to a narrower electron beam reaching the anode, allowing a larger percentage of the emission current to pass through the aperture. By adjusting the Wehnelt bias so that the percentage of the emission current passing through the aperture is kept close to its initial value, the spread of the electron beam will also remain close to its initial value, resulting in a more stable performance of the x-ray source over time (i.e., reduced effects from cathode aging as described herein).

[0038] According to the principles disclosed herein, various quantities depending on the divergence of the electron beam at the entrance of the electron optical arrangement can be used to control the grid bias applied to the Wehnelt. For example, the convergence angle of the electron beam towards the focal spot can be calculated based on the measured beam characteristics and a grid bias can be applied to the Wehnelt so that the desired convergence angle is obtained. Similarly, the grid bias can be controlled based on the minimum achievable electron beam spot size, the electron beam diameter (or width or area) at the focal plane. It can even be considered to determine the width of the electron beam upstream of the electron optical arrangement and thereby obtain the divergence of the electron beam directly at the entrance of the electron optical arrangement.

Claims

1. an electron source for providing an electron beam, said electron source comprising a cathode, a Wehnelt, and an anode; an electron optical arrangement configured to deflect and focus the electron beam towards a target for generation of X-ray radiation; an arrangement for determining a quantity indicative of a width of the electron beam; a controller operatively connected to the electron source, the electron optical arrangement, and the arrangement for determining a quantity indicative of a width of the electron beam, the controller comprising: - calculating a quantity dependent on the divergence of the electron beam at the entrance of the electron optical arrangement based on a quantity indicative of the width of the electron beam; applying a bias voltage to the Wehnelt such that the quantity dependent on the divergence of the electron beam at the entrance of the electron optical arrangement is adjusted towards a desired value; 4. The method of claim 3, An X-ray source comprising:

2. 2. The x-ray source of claim 1, wherein the quantity indicative of the width of the electron beam is a cross-sectional intensity distribution of the electron beam or a width of the electron beam at a predetermined percentage of maximum intensity, such as a full width at half maximum of the electron beam.

3. 2. The X-ray source of claim 1, wherein the arrangement for determining a quantity indicative of a width of the electron beam is configured to determine a cross-sectional intensity distribution or a width of the electron beam at a position downstream of the electron optical arrangement.

4. The arrangement for determining a quantity indicative of a width of the electron beam comprises: scanning the electron beam over an edge separating two regions having different abilities to reflect and / or absorb electrons; detecting a quantity indicative of an intensity of at least a portion of the electron beam as a function of electron beam position relative to the edge using a sensor; The X-ray source according to any one of claims 1 to 3, configured to:

5. The sensor includes: Electron backscatter sensor, Target current sensor, a sensor for detecting electrons downstream of the target; X-ray sensor, 5. The X-ray source of claim 4, selected from the group consisting of:

6. 6. An X-ray source according to claim 1, wherein the arrangement for determining a quantity indicative of a width of the electron beam comprises an aperture and means for measuring a proportion of the current emitted from the cathode that passes through the aperture.

7. 7. An X-ray source according to claim 1, wherein the control device is configured to calculate the quantity depending on a divergence of the electron beam at an entrance of the electron optical arrangement based on a quantity indicative of a width of the electron beam determined for at least two different focus settings of the electron optical arrangement.

8. The cathode is LaB ６ An X-ray source according to any one of claims 1 to 7, comprising a crystal.

9. A quantity that depends on the divergence of the electron beam at the entrance of the electron optical arrangement is The convergence angle, The spot size and the diameter, width, or area of ​​the electron beam at the focusing lens plane; Diffusion and 9. The X-ray source according to claim 1, wherein the X-ray source is one or more of:

10. 1. A method in an X-ray source comprising an electron source for providing an electron beam and an electron optical arrangement for deflecting and focusing the electron beam towards a target for generating X-ray radiation, the electron source comprising a cathode, a Wehnelt, and an anode, the method comprising: determining a quantity indicative of a width of the electron beam; - calculating a quantity dependent on the divergence of the electron beam at the entrance of the electron optical arrangement based on the determined quantity indicative of the width of the electron beam; applying a bias voltage to the Wehnelt such that a quantity dependent on the divergence of the electron beam at the entrance of the electron optical arrangement is adjusted towards a desired value; A method comprising:

11. 11. The method of claim 10, wherein the quantity indicative of a width of the electron beam is a cross-sectional intensity distribution of the electron beam or a width of the electron beam at a predetermined percentage of maximum intensity, such as a full width at half maximum of the electron beam.

12. The method of claim 10 or 11, wherein determining a quantity indicative of a width of the electron beam comprises determining a cross-sectional intensity distribution or a width of the electron beam at a position downstream of the electron optical arrangement.

13. Determining a quantity indicative of a width of the electron beam comprises: scanning the electron beam over an edge separating two regions having different abilities to reflect and / or absorb electrons; detecting a quantity indicative of an intensity of at least a portion of the electron beam as a function of electron beam position relative to the edge; The method according to any one of claims 10 to 12, comprising:

14. A method according to any one of claims 10 to 13, wherein the bias voltage is within + / - 10 kV relative to the cathode.

15. The quantity dependent on the divergence of the electron beam at the entrance of the electron optical arrangement is The convergence angle, The spot size and the diameter, width, or area of ​​the electron beam at the focusing lens plane; Diffusion and The method according to any one of claims 10 to 14, wherein the method is one or more of:

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