X-ray tube emitter

1. An emitter assembly for a closed x-ray tube, comprising:

an emitter body formed of a low work function material, the emitter body having a major surface and a secondary surface;

wherein the major surface is adapted for emission of electrons in a direction from the low work function material and wherein the emitter assembly is adapted to reduce an emission current density emitted from the secondary surface of the emitter body, as compared to the major surface, wherein the secondary surface extends laterally from the major surface opposite from the direction of the emission of electrons.

2. The emitter assembly of claim 1 , further comprising a cathode head; wherein the emitter body is disposed in a surface of the cathode head to define a gap of less than 0.5 millimeter between the major surface of the emitter body and the surface of the cathode head, such that the emitter body does not contact the surface of the cathode head.

3. The emitter assembly of claim 1 , wherein the low work function material comprises lanthanum hexaboride or cerium hexaboride.

4. The emitter assembly of claim 1 , wherein the low work function material comprises rhenium boride or cerium rhenium boride, a lanthanide crystal material, a rare earth metal boride, hafnium carbide or zirconium carbide, yttrium oxide, a tungsten thorium or tungsten lanthanum oxide, a tungsten zirconium oxide or other Schottkey emitter, or a dispenser cathode material.

5. The emitter assembly of claim 1 , wherein the low work function material has a work function less than 4.0 electron volts (eV).

6. The emitter assembly of claim 5 , further comprising a thermal decoupling between the shield and the side surface of the emitter body, wherein an outer surface of the shield operates at a lower temperature than the side surface of the emitter body.

7. The emitter assembly of claim 1 , wherein the secondary surface comprises a side surface extending from the major surface of the emitter body and means for reducing an emission current density along the side surface as compared to the major surface.

8. The emitter assembly of claim 7 , wherein means for reducing an emission current density along the side surface comprises a shield on the side surface that includes a shield material having a work function higher than the low work function material on the side surface of the emitter body.

9. The emitter assembly of claim 7 , wherein the shield material comprises carbon, graphite, tungsten, rhenium, or platinum.

10. The emitter assembly of claim 7 , wherein the shield material is disposed on the emitter body in direct contact with the side surface, with a thickness of less than one millimeter.

11. The emitter assembly of claim 1 , wherein the secondary surface extends from the major surface at an acute angle.

12. The emitter assembly of claim 1 , further comprising a fixture disposed about the emitter body, wherein the secondary surface comprises a side surface extending from the major surface along a side of the emitter body, and the fixture is configured to modulate an electric field strength to reduce an emission current density along the side surface.

13. The emitter assembly of claim 1 , wherein the low work function material is configured to emit electrons at a temperature below 1500° C.

14. A cathode assembly for a closed x-ray tube, comprising:

a cathode head;

an emitter at least partially disposed in the cathode head, the emitter comprising an emitter body coupled to a base and having a major surface adapted for emission of electrons, wherein the emitter body is formed of a low work function material; and

means for reducing an emission current density emitted from a side surface of the emitter body, as compared to the major surface, wherein the side surface extends laterally from the major surface away from an anode.

15. The cathode assembly of claim 14 , wherein a gap defines a distance between the emitter body and the cathode head and the distance is less than 0.5 millimeter (mm).

16. The cathode assembly of claim 14 , wherein the low work function material has a work function less than 4.0 electron volts (eV).

17. The cathode assembly of claim 14 , further comprising a second emitter element disposed adjacent the emitter body along an anode-facing surface of the cathode head, wherein the second emitter element is formed of a transition metal.

18. The cathode assembly of claim 17 , wherein the second emitter element comprises a tungsten filament.

19. The cathode assembly of claim 14 , wherein the emitter is rotationally symmetric about an axis and an opening in the cathode head is rotationally symmetric about the axis.

20. The cathode assembly of claim 19 , wherein the axis extends perpendicularly to the major surface of the emitter body.

21. The cathode assembly of claim 14 , wherein the major surface of the emitter is a non-planar surface.

22. A closed x-ray tube comprising the cathode assembly of claim 14 , and further comprising an anode adapted to emit x-rays responsive to impingement of the electrons emitted from the major surface of the emitter, wherein the anode is configured as a transmissive target or a reflective target.

23. A method comprising:

providing an x-ray tube;

providing a cathode in the x-ray tube including:

seating a base of an emitter in a cathode head, the emitter comprising an emitter body formed of a low work function material having a major surface adapted for emitting electrons; and

spacing the emitter body from a perimeter of an aperture in an anode-facing surface of the cathode head;

providing an anode in the x-ray tube;

wherein the anode and cathode are sealed within the x-ray tube.

24. The method of claim 23 , wherein providing the cathode further comprises:

aligning the emitter body within the aperture in an anode-facing surface of the cathode head.

25. The method of claim 23 , wherein providing the cathode further comprises:

engaging the emitter with an alignment feature in the cathode head;

wherein the major surface of the emitter body is aligned with respect to the anode for operation of the x-ray tube, responsive to engaging the emitter with the alignment feature.