Patent Number: 
Section: claims

1. An X-ray generation tube comprising:a target for generating an X-ray through irradiation with an electron beam;an electron source provided opposed to the target; anda grid electrode having multiple electron passage apertures,wherein the grid electrode is disposed between the target and the electron source so that a part of a source-side electron beam emitted from the electron source passes through the multiple electron passage apertures and irradiates the target,wherein the source-side electron beam shows a current density distribution,wherein the grid electrode has an aperture ratio distribution,wherein a region of the source-side electron beam, in which a current density is largest, is aligned with a region of the grid electrode, in which an aperture ratio is smallest,wherein the grid electrode causes the part of the source-side electron beam to pass through the multiple electron passage apertures so as to form a target-side electron beam on the target side of the grid electrode, andwherein a current density at a beam center of the target-side electron beam is lower than a current density at a beam center of the source-side electron beam. 2. The X-ray generation tube according to claim 1, wherein when the source-side electron beam shows the current density distribution, the electron beam shows an irradiation density distribution on the target in a beam diameter direction of the source-side electron beam. 3. The X-ray generation tube according to claim 1, wherein the grid electrode has the aperture ratio distribution in a beam diameter direction. 4. The X-ray generation tube according to claim 1, wherein the aperture ratio distribution comprises at least one of a surface density distribution of the multiple electron passage apertures or an aperture area distribution of the multiple electron passage apertures. 5. The X-ray generation tube according to claim 4, wherein the aperture ratio distribution is formed by the surface density distribution of the multiple electron passage apertures and the aperture area distribution of the multiple electron passage apertures. 6. The X-ray generation tube according to claim 1, wherein the grid electrode further comprises an extraction electrode of the electron source. 7. The X-ray generation tube according to claim 1, wherein the electron source comprises an impregnated hot cathode. 8. The X-ray generation tube according to claim 1, wherein the target comprises a transmission type target including a target layer disposed on a side opposed to the grid electrode, and a transmissive substrate for supporting the target layer. 9. The X-ray generation tube according to claim 8, wherein the transmissive substrate comprises a diamond substrate. 10. The X-ray generation tube according to claim 9, wherein the diamond substrate has a substrate thickness of 500 μm to 2 mm. 11. The X-ray generation tube according to claim 9, wherein the diamond substrate comprises one of polycrystal diamond and single crystal diamond. 12. The X-ray generation tube according to claim 8, wherein the target layer contains at least a metal selected from the group consisting of tantalum, tungsten, molybdenum, silver, gold, and rhenium. 13. The X-ray generation tube according to claim 8, wherein a thickness of the target layer is 1 μm or more to 12 μm or less. 14. An X-ray generation device comprising:the X-ray generation tube according to claim 1;a tube voltage circuit to be electrically connected to each of the target and the electron source, so as to output a tube voltage to be applied between the target and the electron source; anda grid potential circuit for defining a voltage between the grid electrode and the target. 15. An X-ray imaging system comprising:the X-ray generation device according to claim 14; andan X-ray detector for detecting an X-ray which is emitted from the X-ray generation device and passes through an object. 16. An X-ray generation tube comprising:a target for generating an X-ray through irradiation with an electron beam;an electron source provided opposed to the target;a grid electrode having multiple electron passage apertures; anda focusing lens electrode,wherein the grid electrode is disposed between the target and the electron source so that a part of a source-side electron beam emitted from the electron source passes through the multiple electron passage apertures and irradiates the target,wherein the source-side electron beam shows a current density distribution,wherein the grid electrode has an aperture ratio distribution,wherein a region of the source-side electron beam, in which a current density is largest, is aligned with a region of the grid electrode, in which an aperture ratio is smallest,wherein the focusing lens electrode focuses the source-side electron beam,wherein the focusing lens electrode defines, between the focusing lens electrode and the target, a crossover that is a virtual point at which a beam diameter of the target-side electron beam becomes smallest,wherein the focusing lens electrode further defines a crossover conjugate point at a position conjugate to the crossover,wherein the focusing lens electrode further defines, on the electron source side, a focus center conjugate point at a position conjugate to a focus center on the target, andwherein the grid electrode is disposed at a position from the crossover conjugate point to the focus center conjugate point. 17. The X-ray generation tube according to claim 16, wherein when the source-side electron beam shows the current density distribution, the electron beam shows an irradiation density distribution on the target in a beam diameter direction of the source-side electron beam. 18. The X-ray generation tube according to claim 16, wherein the grid electrode has the aperture ratio distribution in a beam diameter direction. 19. The X-ray generation tube according to claim 16, wherein the aperture ratio distribution comprises at least one of a surface density distribution of the multiple electron passage apertures or an aperture area distribution of the multiple electron passage apertures. 20. The X-ray generation tube according to claim 19, wherein the aperture ratio distribution is formed by the surface density distribution of the multiple electron passage apertures and the aperture area distribution of the multiple electron passage apertures. 21. The X-ray generation tube according to claim 16, wherein the grid electrode further comprises an extraction electrode of the electron source. 22. The X-ray generation tube according to claim 16, wherein the grid electrode is positioned so as to be overlapped with the focus center conjugate point. 23. The X-ray generation tube according to claim 16, wherein the electron source comprises an impregnated hot cathode. 24. An X-ray generation device comprising:the X-ray generation tube according to claim 16;a tube voltage circuit to be electrically connected to each of the target and the electron source, so as to output a tube voltage to be applied between the target and the electron source; anda grid potential circuit for defining a voltage between the grid electrode and the target. 25. An X-ray imaging system comprising:the X-ray generation device according to claim 24; andan X-ray detector for detecting an X-ray which is emitted from the X-ray generation device and passes through an object.