X-RAY TUBE WITH FLEXIBLE INTENSITY ADJUSTMENT

Abstract

An x-ray tube includes a thermionic cathode generating an electron beam propagating from the cathode to a target along a beam axis. The x-ray tube has apertures in the form of a control electrode with a first aperture opening, a focusing electrode with a second aperture opening and a beam shaping electrode with a third aperture opening. The first aperture opening is smaller than the emission surface and has a contour rotationally symmetric with respect to the beam axis. The second aperture opening is larger than the first aperture opening and has a contour rotationally symmetric with respect to the beam axis. The third aperture opening has a contour which is aligned with an xy plane and non-rotationally symmetric with respect to the beam axis. The X-ray tube has a simple structure for generating an electron beam where the number of electrons can be varied easily over a wide range.

Claims

1. An x-ray tube, comprising: a thermionic cathode having a flat electron emission surface; a plurality of electrostatic electrodes; and a target; wherein the x-ray tube is designed for generating an electron beam propagating from the cathode to the target along a beam axis running along a z direction and for generating a microfocus spot on the target; wherein the target is configured as a target anode; and wherein the x-ray tube comprises: a control electrode, located in z direction in front of the flat electron emission surface, and configured as an aperture with a first aperture opening smaller than the emission surface, the first aperture opening having a contour which is rotationally symmetric with respect to the beam axis; a focusing electrode, located in z direction in front of the control electrode, and configured as an aperture with a second aperture opening larger than the first aperture opening, the second aperture opening having a contour which is rotationally symmetric with respect to the beam axis; a beam shaping electrode, located in z direction in front of the focusing electrode and before the target anode, and configured as an aperture with a third aperture opening, the third aperture opening having a contour which is aligned with an xy plane and non-rotationally symmetric with respect to the beam axis; with x, y, z forming a Cartesian coordinate system.

2. The x-ray tube according to claim 1, wherein the third aperture opening is of an elliptical shape, and the third aperture opening has a major axis aligned with the y direction.

3. The x-ray tube according to claim 2, wherein the target anode has a flat target surface, and the flat target surface is inclined with respect to the y direction, inclined with respect to the z direction, and is parallel to the x direction.

4. The x-ray tube according to claim 2, wherein the beam shaping electrode is adapted to shape the electron beam into a line focus on the anode target with an aspect ratio in x:y smaller than 1:5.

5. An x-ray tube according to claim 1, wherein the x-ray tube comprises a control connection configured for independently applying a control voltage to the control electrode, and a focusing connection configured for independently applying a focusing voltage to the focusing electrode.

6. The x-ray tube according to claim 1, wherein the thermionic cathode and the beam shaping electrode are electrically connected.

7. The x-ray tube according to claim 1, wherein the x-ray tube comprises a beam shaping connection for independently applying a beam shaping voltage to the beam shaping electrode.

8. The x-ray tube according to claim 1, wherein the control electrode is adapted to mask a portion P of the electron beam originating from the thermionic cathode, with P50%.

9. The x-ray tube according to claim 1, wherein for a first distance DIST1 of the control electrode from the emission surface and a second distance DIST2 of the focusing electrode from the control electrode, the following applies: DIST21.5*DIST1, wherein 100 mDIST1400 m and/or 200 mDIST21000 m.

10. The x-ray tube according to claim 1, wherein for a first diameter DIA1 of the first aperture opening of the control electrode and a second diameter DIA2 of the second aperture of the focusing electrode, the following applies: DIA23*DIA1, wherein 0.2 mmDIA11.0 mm and/or 0.6 mmDIA23.0 mm.

11. The x-ray tube according to claim 1, wherein for a third distance DIST3 of the beam shaping electrode from the focusing electrode and a second distance DIST2 of the focusing electrode from the control electrode, the following applies: DIST34*DIST2; and/or, wherein for a largest diameter of the third aperture opening of the beam shaping electrode, called third diameter DIA3 in the following, and a second diameter DIA2 of the second aperture of the focusing electrode, the following applies: DIA36*DIA2.

12. The x-ray tube according to claim 11, wherein 3 mmDIST350 mm, and/or, 6 mmDIA325 mm.

13. The x-ray tube according to claim 1, wherein the x-ray tube encloses an evacuated interior space, in which the thermionic cathode, the target anode, the control electrode, the focusing electrode and the beam shaping electrode are located.

14. The x-ray tube according to claim 1, wherein the target anode comprises a diamond heat spreader.

15. The x-ray tube according to claim 14, wherein the diamond heat spreader is composed of isotopically enriched 12C with a purity >99.5% or isotopically enriched 13C with a purity >99.5%.

16. The x-ray tube according to claim 1, wherein the emission cathode is a dispenser cathode, with the dispenser cathode comprising a powder compact containing a matrix of tungsten grains embedding BaO, CaO and Al2O3, and the dispenser cathode comprising an indirect heating.

17. A method for operating the x-ray tube according to claim 1, wherein a cathode potential PC is applied to the emission cathode, a first potential P1 is applied to the control electrode, a second potential P2 is applied to the focusing electrode, a third potential P3 is applied to the beam shaping electrode, and an anode potential PA is applied to the target anode, wherein an electron beam is generated at the emission cathode and propagates to the target anode, and x-rays are emitted from the target anode in the region of a beam spot of the electron beam on the target anode, wherein P1PC>0, P2>PC, and PAPC>+5 kV, wherein P1PC:=PDC1, with +10VPDC1+200V, and wherein P2PC:=PDC2, with +100VPDC2+800V.

18. The method according to claim 17, wherein the emission cathode is grounded with PC at or near zero, and wherein PA is at a high positive potential with respect to ground.

19. The method according to claim 17, wherein the target anode is grounded with PA at zero, and wherein PC is at a high negative potential with respect to ground.

20. The method according to claim 17, wherein P2P1>0, wherein P2P1:=PD12, with +100VPD12+600V.

21. The method according to claim 17, wherein PC=P3.

22. The method according to claim 17, wherein P3P2<0, and wherein P3P2:=PD23, with 100VPD23800V.

23. The method according to claim 17, wherein the method includes an intensity adjustment of the x-ray tube in order to vary the number of electrons in the electron beam, wherein the intensity adjustment includes changing of the potential PC and/or P1 and/or P2, wherein at least at the beginning of the intensity adjustment and at the end of the intensity adjustment there holds PDC1=Poly(PDC2), where Poly(PDC2) is a polynomial of second order of PDC2, wherein at least at the beginning of the intensity adjustment and at the end of the intensity adjustment, the size of the focal spot of the electron beam on the target anode is the same.

24. The method according to claim 23, wherein there holds PDC1=Poly(PDC2), during the entire intensity adjustment, wherein the size of a focal spot of the electron beam on the target anode is kept unchanged during the entire intensity adjustment, wherein during the intensity adjustment, PC is kept constant and P1 and P2 are changed concurrently.

25. The method according to claim 16, wherein the method includes a focus adjustment, wherein the focus adjustment includes varying the second potential P2 at the focusing electrode until a desired spot size of the beam spot of the electron beam at the target anode is achieved, where the desired spot size is between the minimum spot size and 2 the minimum spot size.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0068] FIG. 1 schematically shows an exemplary first embodiment of an X-ray tube according to the invention;

[0069] FIG. 2 schematically shows a cross-section through an electrode arrangement of the X-ray tube of FIG. 1;

[0070] FIG. 3 shows a schematic top view of a target anode of the X-ray tube in operation;

[0071] FIG. 4 schematically shows a second embodiment of an X-ray tube according to the invention;

[0072] FIG. 5 shows a diagram illustrating the dependence of voltage differences PDC1 and PDC2 of an X-ray tube according to the invention during an adjustment process of the intensity of the X-ray radiation from the X-ray tube in an example; and

[0073] FIG. 6 shows a diagram illustrating the intensity of X-ray radiation emitted from an X-ray tube according to the invention compared to an intensity of X-ray radiation emitted from a conventional X-ray tube in an example.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0074] FIG. 1 schematically shows an exemplary embodiment of an X-ray tube 1 according to the invention in a longitudinal cross-section with an inner space 2, the inner space 2 being evacuated and enclosed by a housing 3. The X-ray tube 1 is provided with a cathode 4 which is used for thermal emission of electrons. In operation of the X-ray tube 1, the electrons, after emission from the cathode 4, flow in the form of an electron beam 5 to a target 6 which is configured as a target anode 7. For this purpose, a positive electrical potential is applied to the target anode 7 with respect to the cathode 4.

[0075] The electrons in the electron beam 5 produce X-rays 8 when they hit the target anode 7. At least some of the X-rays 8 exit the X-ray tube 1 through a beryllium window 9 in the housing 3.

[0076] The cathode 4 and the target anode 7 are arranged in the inner space 2 of the X-ray tube 1. The propagation direction of the electron beam 5 is along the direction of a z-axis of a reference system R in the form of a Cartesian coordinate system with an x-axis, a y-axis and the z-axis, compare electron beam axis 12. The x-axis of the reference system R is symbolized by a cross; it is directed perpendicular to the plane of drawing. Note that for better understanding, part of the X-ray tube 1 is shown with a little tilt, deviating from the longitudinal cross-section (see also below).

[0077] The cathode 4 is connected to an external electrical power source (not shown) via two electrical conductors 10a, 10b so that a heating current can flow through the cathode 4 to heat the cathode 4 and thereby cause an emission of electrons from the cathode 4. The cathode 4 is at potential PC, which is ground potential here. The cathode 4 has a flat surface 11 from which the electrons can be thermally emitted, such that the density distribution of the emitted electrons is highly homogeneous. The highly homogeneous density distribution of the emitted electrons contributes to a density distribution of electrons in the electron beam 5 that is highly homogeneous in the xy-plane, too. Furthermore, the cathode 4 is formed as a dispenser cathode 13, which allows the cathode 4 to be operated at a lower operating temperature and with a longer lifetime.

[0078] A control electrode 14 is arranged next to the cathode 4 in the direction of the z-axis at a distance DIST1 in order to enhance electron emission from the cathode 4 and to accelerate the electrons towards the target anode 7. During operation of the X-ray tube 1, a positive potential P1 is applied to the control electrode 14 with respect to the cathode 4 by a control connection 15a; i.e., the potential difference PDC1 between the potential P1 at the control electrode and the potential PC at the cathode is positive. In the embodiment shown in FIG. 1, the potential P1 at the control electrode can be P1=50V and the distance DIST1 can be DIST1=250 m. The potential P1 of the control electrode 14 accelerates electrons from the cathode 4 towards the control electrode 14. The control electrode 14 is formed as an aperture 16 with a first aperture opening 17, wherein the first aperture opening 17 has a first diameter DIA1 and is smaller than the emission surface 11 of the cathode. Therefore, a considerable part P of the emitted electrons are blocked at the first aperture 17, here about P=50%. The size and shape (in particular, the edges) of the cathode 4 do not affect the electron beam or its homogeneity of intensity distribution. In the embodiment shown in FIG. 1, the first diameter DIA1 can be DIA1=0.6 mm. The first aperture opening 17 has rotational symmetry around the beam axis 12 of the electron beam 5. This causes a symmetrical distribution of electrons passing through the first aperture opening 17 with respect to the beam axis 12 of the electron beam 5. Via adjusting the first potential P1, the number of electrons in the electron beam 5 can be adjusted.

[0079] To focus the electron beam 5 in the direction of the target anode 7, a focusing electrode 18 is arranged next to the control electrode 14 in the z-direction at a distance DIST2. The focusing electrode 18 is designed as an aperture 19 with a second aperture opening 20 which, like the first aperture opening 17, has a rotational symmetry about the beam axis 12 of the electron beam 5. During operation of the X-ray tube 1, a potential P2 is applied to the focusing electrode 18 which is positive with respect to the potential P1 of the control electrode 14 (i.e., P2>P1) which further accelerates the electrons in the electron beam 5 in the direction of the target anode 7. The potential is applied to the focusing electrode 18 by a focusing connection 15b. In the embodiment shown in FIG. 1, the potential P2 at the focusing electrode 18 can be P2=200V and the distance DIST2 can be DIST2=600 m. Through its potential P2, the focusing electrode 18 also has some influence on the number of electrons extracted from the cathode 4. However, this influence is typically less than the influence of the potential P1 of the control electrode 14, which is located closer to the cathode 4.

[0080] The electron beam 5 is widened on its way from the control electrode 14 to the focusing electrode 18, wherein the electron beam 5 has a cross-section that is rotationally symmetrical around the beam axis. To avoid a reduction in the number of electrons in the electron beam 5 as it passes through the second aperture opening 20, a diameter DIA2 of the second aperture opening 20 is larger than the diameter DIA1 of the first aperture opening 17. In the embodiment shown in FIG. 1, the diameter DIA2 can be DIA2=2 mm. During operation of the X-ray tube 1, the electrons that have passed through the first aperture opening 17 can pass through the second aperture opening 20, without or only an insignificant amount of electrons being removed from the electron beam 5 by the focusing electrode 18, even if the electron beam 5 between the control electrode 14 and the focusing electrode 18 has a relatively large divergence.

[0081] A beam shaping electrode 21 is arranged next to the focusing electrode 18 in the z-direction at a distance DIST3. The beam shaping electrode 21 is designed as an aperture 22 with a third aperture opening 23, the third aperture opening 23 having an elliptical shape and extending in the xy-plane perpendicular to the beam axis 12. With its longest extension, the third aperture opening 23 extends in the direction of the y-axis of the reference system R. In other words, the y-axis of the reference system R is parallel to the major axis of the third aperture opening 23. In particular, the longest extension of the third aperture opening 23 is referred to as the third diameter DIA3. In the embodiment shown in FIG. 1, the distance DIST3 can be DIST3=2 cm and the third diameter DIA3 can be DIA3=20 mm. Perpendicular to its longest extension, the third aperture opening 23 extends in the direction of the x-axis of the reference system R. During operation of the X-ray tube 1, the beam shaping electrode 21 is at potential P3 which is negative with respect to the potential P2 at the focusing electrode 18. The potential P3 is applied to the beam shaping electrode 21 by a beam shaping connection 15c. In the embodiment shown, the beam shaping connection 15c and the electrical conductor 10b are both connected to the grounded housing 3, and accordingly, the same potential (here ground potential) is applied to the beam shaping electrode 21 as to the cathode 4.

[0082] The electron beam 5 still widens between the focusing electrode 18 and the beam shaping electrode 21, but the divergence of the electron beam 5 becomes smaller again towards the target anode 7. The electron beam 5 reaches its maximum width at or near the beam shaping electrode 21.

[0083] In operation of the X-ray tube 1, the diameter of the electron beam 5 is sufficiently large that the elliptical shape of the third aperture opening 23 causes the cross-section of the electron beam 5 to be deformed from a cross-section with the aforementioned rotational symmetry into an elliptical cross-section.

[0084] The target anode 7 is located next to the beam shaping electrode 21 in the z-direction at a distance DIST4. In the embodiment shown in FIG. 1, the distance DIST4 can be DIST4=5 cm. During operation of the X-ray tube 1, the cross-section of the electron beam 5 is narrowed between the beam shaping electrode 21 and the target anode 7, the potential PA of the target anode 7 being positive with respect to the potentials P1, P2, P3 at the other electrodes (in particular, with respect to the potential at the beam shaping electrode 21). The electron beam 5 is focused on the target anode 7 and forms a focal spot 24 on the target anode 7. In the embodiment shown in FIG. 1, the potential PA at the target anode 7 can be PA=+10 kV.

[0085] In order to better dissipate the heat generated when the electrons hit the target anode 7, the target anode 7 has a layer with high thermal conductivity in the form of a diamond heat spreader 25. On the diamond heat spreader 25 there is deposited a target cover layer 33, here of Molybdenum. Electrons from the electron beam 5 impinging on the target anode 7 in the focal spot 24 cause the X-rays 8 to be emitted from the target anode 7 or its target cover layer 33, respectively. In this process, the electron beam 5 retains its elliptical cross-section, which the electron beam 5 has obtained through the beam shaping electrode 21. Therefore, the focal spot 24 on the target anode 7 also has an elliptical cross-section. In the example shown, the target surface 31 of the target anode 7 is perpendicular to the electron beam axis 12, compare the angle of inclination IA of 90 here. A (central) emission direction ED of the X-ray radiation 8 passing through the beryllium window 9 has a take-off angle TOA of about 12 with the target surface 31. Between the electron beam axis 12 and the emission direction ED of the X-rays 8 here results an angle of about 78. Seen in the (central) emission direction ED of the X-rays 8, the elliptical focal 24 spot appears as a circular focal spot, resulting in comparatively simple optical properties of the X-rays 8 in this emission direction ED. In addition, the elliptical focal spot 24 causes a lower heat load on the target anode 7 as compared to a circular spot.

[0086] Advantageously, the current density of the electron beam 5 (and thus the intensity of the X-rays 8) in the X-ray tube 1 according to the invention is determined by the first potential P1 at the control electrode 14 and the second potential P2 at the focusing electrode 18, but not or only slightly by the difference of the potentials PC and PA at the target anode 7 and the cathode 4.

[0087] The cathode 4 and the electrodes 14, 18 and 21 are shown in FIG. 1 slightly inclined with respect to the x-axis of the reference system R to indicate their shape.

[0088] FIG. 2 schematically shows a cross-section through the electrode arrangement of the X-ray tube 1 of FIG. 1. During operation of the X-ray tube 1, a potential P1 is applied to the control electrode 14 which is positive with respect to the potential PC at the cathode 4. As a result, equipotential surfaces 32 of electric field lines between the cathode 4 and the control electrode 14 form an electrostatic first lens 27a between the cathode 4 and the control electrode 14. This first lens 27a has a domed part 28a pointing in the direction of the control electrode 14. Through the first lens 27a, the electron emission at the cathode 4 and the movement of the electrons near the beam axis 12 can easily be controlled, and electrons are accelerated towards the target anode 7. This can be used to influence the number of electrons passing the first aperture opening 17 during operation of the X-ray tube 1. The shape of the first lens 27a can be changed precisely and quickly by the potentials P1 and PC at the control electrode 14 and the cathode 4.

[0089] During operation of the X-ray tube 1, the potential P2 at the focusing electrode 18 is positive compared to the potential P1 at the control electrode 14. Thus the equipotential surfaces 32 of electric field lines between the focusing electrode 18 and the control electrode 14 form an electrostatic second lens 27b between the focusing electrode 18 and the control electrode 14, wherein the electrons are accelerated towards the focusing electrode 18. The second electrostatic lens 27b is rotationally symmetric around the beam axis 12 of the electron beam 5 and has a domed part 28b pointing towards the focusing electrode 18. In operation of the X-ray tube 1, this causes the electron beam 5 to widen in the direction of the focusing electrode 18. The shape of the second lens 27b can be adjusted quickly and accurately by changing the potentials P1 and P2 at the control electrode 14 and the focusing electrode 18.

[0090] Further, during operation of the X-ray tube 1 the potential at the beam shaping electrode 21 is negative compared to the potential at the focusing electrode 18 and the potential at the target anode 7, and an electrostatic third lens 27c is formed by equipotential surfaces 32 of electric field lines between the focusing electrode 18 and the beam shaping electrode 21. The equipotential lines 32 of the third electrostatic lens 27c are shaped by the potentials at the focussing electrode 18, the beam shaping electrode 21 and the anode 7. Typically, however, the electrostatic third lens 27c between the beam shaping electrode 21 and the focusing electrode 18 is hardly affected by the potential at the focusing electrode 18, making the control of the electron beam 5 between the focusing electrode 18 and the beam shaping electrode 21 easier. Most of the third lens 27c has a domed part 28c pointing in the direction of the beam shaping electrode 21. The electron beam 5 is further widened between the focusing electrode 18 and the beam shaping electrode 21, wherein the beam divergence decreases with increasing proximity to the beam shaping electrode 21. The electron beam 5 usually reaches its maximum cross section near or at the beam shaping electrode 21. Advantageously, the shape of the third lens 27c between the focusing electrode 18 and the beam shaping electrode 21 can be changed flexibly by changing the potential at the target anode 7 or the beam shaping electrode 21 (or the focusing electrode 18). Thereby, the shape and size of the electron beam 5 and its maximum cross section can be altered if desired.

[0091] In operation of the X-ray tube 1, a potential PA is applied to the target anode 7 which is positive compared to the potential P3 at the beam shaping electrode 21. Thus an electrostatic fourth lens 27d is formed by equipotential surfaces 32 of electric field lines between the target anode 7 and the beam shaping electrode 21 between the beam shaping electrode 21 and the target anode 7. This lens 27d has a domed part 28d pointing towards the beam shaping electrode 21, wherein the electrons are accelerated towards the target anode 7. The electron beam 5 is focused on the target surface 31 of the target anode 7 by the fourth lens 27d. Because of the non-rotationally symmetric shape of the third aperture opening 23 of the beam shaping electrode 21, the fourth lens 27d between the beam shaping electrode 21 and the target anode 7 can act as a stigmator in that this lens 27d deforms the cross-section of the electron beam 5 from a rotationally symmetric cross-section to a non-rotationally symmetric cross-section (see also FIG. 3).

[0092] Advantageously, the fourth lens 27d between the target anode 7 and the beam shaping electrode 21 can be flexibly changed by adjusting the potential at the target anode 7 or the beam shaping electrode 21, if need may be, which affects the focusing of the electron beam 5 on the target anode 7. Furthermore, the electrostatic fourth lens 27d between the beam shaping electrode 21 and the target anode 7 is usually hardly affected by the potential at the focussing electrode 18, facilitating the control of the focusing of the electron beam 5 between the target anode 7 and the beam shaping electrode 21. However note that in practice, for changing the focus of the electron beam 5 on the target anode 7, the second potential P2 is adjusted often together with the first potential P1, and P3 and PA are kept constant.

[0093] FIG. 3 shows a top view of the target anode 7 during operation of the X-ray tube 1 (see FIG. 1) in a direction perpendicular to the surface of the target anode 7. The electron beam 5 (see FIG. 1) generates the focal spot 24 on the target anode 7, and the focal spot 24 has an elliptical shape due to the elongated shape of the beam shaping electrode 21. The largest longitudinal extension of the focal spot 24 runs in the y-direction. The ratio of the major semi-axis 29 between the center C and the vertex V.sub.1 of the ellipse in the y-direction to the minor semi-axis 30 between the center C and the vertex V.sub.2 in the x-direction is about 1:7 in the example shown. When seen from emission direction ED (which is inclined somewhat to the plane of drawing in FIG. 3, here about 12, see TOA in FIG. 1), the focal spot 24 appears basically circular in projection (not shown in FIG. 3).

[0094] FIG. 4 schematically shows a second embodiment of an X-ray tube 1 according to the invention. The second embodiment of the X-ray tube 1 is similar to the embodiment shown in FIG. 1, so only the major differences are discussed here. In the second embodiment of FIG. 2, the flat target surface 31 is inclined with respect to the z direction or the electron beam axis 12 with an angle of inclination IA of about 78. In other words, the flat target surface 31 deviates by about 12 from a perpendicular orientation (that would be aligned with the xy-plane) with respect to the y-axis and z-axis. The take-off angle TOA between the plane of the flat target surface 31 and the emission direction ED of the X-rays 8 is again about 12. The emission direction ED of the X-rays 8 is perpendicular to the z-direction or the electron beam axis 12. The inclination of the target surface 31 with respect to the electron beam axis 12 also contributes to the elliptical shape of the focal spot 24 on the target surface 31 here.

[0095] Furthermore, the beam shaping connection 15c of the beam shaping electrode 21 is led out of the housing 3 of the X-ray tube 1 to be connected to an external voltage source (not shown). This allows the beam shaping electrode 21 to have a potential P3 that is individually set, in particular, different from the potential PC of the cathode 4.

[0096] FIG. 5 shows a diagram illustrating an example of an adjustment process of the intensity of the X-ray radiation 8 from the X-ray tube 1 (see FIG. 1) according to the invention, in which the current density of the electron beam 5 has been changed while the size of the focal spot 24 on the target anode 7 has been kept constant. The first potential P1 was set to different voltage values, and in each case, the second potential P2 was adjusted until the same (minimum size) focal spot was achieved. PC was kept constant (at ground potential). In FIG. 5, the potential difference between the control electrode 14 and the cathode 4 (PDC1=P1PC) is plotted as a function of the potential difference between the focusing electrode 18 and the cathode 4 (PDC2=P2PC) for the different setups.

[0097] In FIG. 5, the voltage between the control electrode 14 and the cathode 4 (PDC1) is plotted on the ordinate and the voltage between the focusing electrode 18 and the cathode 4 (PDC2) is plotted on the abscissa. The thick black dots show experimental values for the voltage PDC2 and the corresponding voltage PDC1 when PDC2 changes from about 190 volts to about 630 volts. The values for PDC1 change from about 5 volts to about 70 volts during this process. The dashed line shows a calculated dependence of PDC1 on PDC2 under a fitting procedure, where the dependence is modelled by a second order polynomial fit:

PDC1=0.0003(PDC2).sup.20.0924PDC2+13.78.

[0098] FIG. 5 shows that a sharp focus of the electron beam can be maintained in good approximation when a relationship between PDC1 and PDC2 is kept, with the relationship being modelled by a second order polynomial. So the intensity of the X-ray radiation (resp. the electron beam intensity) can be changed (set) via P1, and when adjusting P2 according to the relationship above concurrently, the focusing and more specifically the size of the focal spot does not change. This substantially simplifies the operation of the tube.

[0099] FIG. 6 concerns the intensity of an X-ray radiation 8 emerging from an X-ray tube 1 (see FIG. 1) according to the invention compared to an intensity of an X-ray radiation emerging from a conventional X-ray tube equipped with a wound tungsten filament as a cathode. The anodes in both X-ray tubes 1 comprise molybdenum as a target cover layer, and the anodes in both X-ray tubes 1 are arranged on a diamond heat spreader 25.

[0100] In the experimental setup, the X-rays 8 from the respective X-ray tube are reflected at a Montel mirror, pass through a pinhole positioned at the image focus of the Montel mirror, and then hit a photodiode. Pinholes with different diameters are used for the measurement. The photon flux in the X-ray radiation 8 can be determined from the current of the photodiode. Then, using the diameter of the respective pinhole, the intensity of the X-ray radiation hitting the photodiode can be determined. Both tubes were operated at conditions where the maximum intensity could be obtained without significant degradation over time (on the order of 6 months or more).

[0101] In FIG. 6, the intensity of the X-ray radiation 8 from the respective X-ray tube 1 (measured in number of photons per second and per mm.sup.2) is plotted on the ordinate and the diameter of the respective pinhole (measured in mm) is plotted on the abscissa. The intensity of the X-ray radiation 8 from the X-ray tube 1 according to the invention is indicated with a solid line, with the measured values shown as triangles. The intensity of the X-ray radiation from the X-ray tube according to the prior art is indicated with a dashed line, with the measured values shown as diamonds. For both X-ray tubes, the intensity curves resemble 2D Gaussian functions, indicating that the tube focal spot is of 2D Gaussian type. The maximum values of the intensity can be determined from very small diameters of the pinholes close to zero.

[0102] As illustrated in FIG. 6, the maximum intensity of the X-ray radiation 8 emerging from the X-ray tube 1 according to the invention is about a factor of 2 higher than the maximum intensity of the X-ray radiation emerging from the conventional X-ray tube. The intensity of the X-ray radiation of the conventional X-ray tube is limited, in particular, by the thermal load capacity of the tungsten filament. In contrast, in the X-ray tube 1 according to the invention, the current density of the electron beam and thus the intensity of the emitted X-ray radiation 8 can be controlled to a large extent by the control electrode 14 and the focusing electrode 18 (see FIG. 1). Further, the flat cathode with high homogeneity of electron emission results in less burden to the target anode. As a result, a higher intensity of the emitted X-ray radiation 8 can be effected in X-ray tubes according to the invention than in the conventional X-ray tubes with tungsten filaments.

LIST OF REFERENCE SIGNS

[0103] 1 X-ray tube [0104] 2 inner space [0105] 3 housing [0106] 4 cathode [0107] 5 electron beam [0108] 6 target [0109] 7 target anode [0110] 8 X-rays [0111] 9 beryllium window [0112] 10a,b electrical conductors [0113] 11 flat emission surface [0114] 12 beam axis [0115] 13 dispenser cathode [0116] 14 control electrode [0117] 15a-c control connections [0118] 16 aperture (control electrode) [0119] 17 first aperture opening [0120] 18 focusing electrode [0121] 19 aperture (focusing electrode) [0122] 20 second aperture opening [0123] 21 beam shaping electrode [0124] 22 aperture (beam shaping electrode) [0125] 23 third aperture opening [0126] 24 focal spot [0127] 25 diamond heat spreader [0128] 27a-d electrostatic lenses [0129] 28a-d domed parts of lenses [0130] 29 major semi axis [0131] 30 minor semi axis [0132] 31 target surface [0133] 32 equipotential lines [0134] 33 target cover layer [0135] R reference system [0136] ED emission direction of X-rays [0137] IA angle of inclination [0138] TOA take-off angle [0139] angle of emission direction of X-rays versus electron beam axis