X-ray tube electron sources

1. An electron source for an X-ray scanner comprising

an emitter support block having a first side and a second side;

an electron-emitting region formed on the emitter support block and arranged to emit electrons;

an electrical connector arranged to connect a source of electric current to the electron-emitting region;

at least one focusing element arranged to focus the electrons towards an anode and having a lower portion extending parallel to said first or second side and a curved upper portion extending upward from the lower portion, beyond the electron-emitting region, and curving back toward the electron-emitting region;

an extraction grid between said emitter support block and the at least one focusing element, wherein the at least one focusing element is held at an electrical potential that is negative with respect to the extraction grid; and

heating means arranged to heat the support block.

2. An electron source according to claim 1 wherein the support block has a plurality of electron-emitting regions formed on it, the electron-emitting regions being electrically insulated from each other.

3. An electron source according to claim 1 wherein the electron-emitting region comprises a layer of conductive material applied to the support block.

4. An electron source according to claim 3 wherein the electron-emitting region further comprises a layer of electron emitting material extending over the layer of conductive material.

5. An electron source according to claim 4 wherein the electron emitting material is a metal oxide.

6. An electron source according to claim 1 wherein the electron-emitting region is located on one side of the support block and the electrical connecter is arranged to extend from said one side to an opposite side of the support block.

7. An electron source according to claim 1 further comprising a control means arranged to control the relative electrical potential between the grid and the electron-emitting region to control extraction of electrons from the electron-emitting region.

8. An electron source according to claim 7 wherein the grid includes an extraction region through which electrons can pass, and a shielding region arranged to intercept heat radiated from the support block or the heating means.

9. An electron source for an X-ray scanner comprising

an emitter arranged to emit electrons, wherein the emitter has a first side and a second side;

an electrical connector arranged to connect a source of electric current to the emitter;

heating means arranged to heat the emitter;

an extraction grid;

control means arranged to control the relative electrical potential between the grid and the emitter to control extraction of electrons from the emitter wherein the grid includes an extraction region through which electrons can pass;

at least one focusing element arranged to focus the electrons towards an anode and having a lower portion extending parallel to said first or second side and a curved upper portion extending upward from the lower portion, beyond the extraction grid, and curving back toward the extraction grid, wherein the at least one focusing element is held at an electrical potential that is negative with respect to the extraction grid; and

a shielding region arranged to intercept heat radiated from the emitter or the heating means.

10. An electron source according to claim 9 wherein the extraction grid is formed from a sheet of electrically conductive material, with apertures formed therein to form the extraction region.

11. An electron source according to claim 9 wherein the extraction grid is arranged to extend over a plurality of electron-emitting regions, and has a plurality of extraction regions each associated with one of the electron-emitting regions, and blocking regions between the extraction regions arranged to block electrons thereby at least partially to focus the electrons.

12. An electron source according to claim 9 wherein the shielding region is arranged to extend past at least one side of a support block thereby partially enclosing the support block.

13. An electron source according to claim 9 wherein the at least one focusing element is arranged to define a focusing aperture above the electron-emitting region, and to extend past at least one side of the support element thereby partially enclosing the support block.

14. An electron source according to claim 9 wherein the at least one focusing element is formed from at least one sheet of material.

15. An electron source according to claim 9 wherein the heat shielding means forms part of a rigid support structure for the support block.