Neutron Generator

• US 20090135982A1
• Filed: 11/28/2007
• Published: 05/28/2009
• Est. Priority Date: 11/28/2007
• Status: Active Grant

• Current Assignee: Schlumberger Technology Corporation (Schlumberger NV)
• Original Assignee: Schlumberger Technology Corporation (Schlumberger NV)
• Inventors: Groves, Joel L.
• Granted Patent: US 9,001,956 B2
• Time in Patent Office: Days
• US Class Current: 376/113
• CPC Class Codes: B82Y 99/00 Subject matter not provided for in other groups of this subclass
• CPC Class Codes: G01V 5/10 using neutron sources neutron generating tubes H05H5/00; neutron sources using isotopes G21G4/00
• CPC Class Codes: H01J 2201/30469 Carbon nanotubes (CNTs)
• CPC Class Codes: H05H 3/06 Generating neutron beams targets for producing nuclear reactions H05H6/00; neutron sources G21G4/02
• CPC Class Codes: Y10S 977/963 Miscellaneous

Abstract

A neutron generator includes an ion source disposed in a pressurized environment containing an ionizable gas. The ion source includes a substrate with a bundle of carbon nanotubes extending therefrom. The ends of the nanotubes are spaced from a grid. Ion source voltage supply circuitry supplies a positive voltage potential between the substrate and the grid of the ion source to cause ionization of the ionizable gas and emission of ions through the grid. An ion accelerator section is disposed between the ion source and a target. The ion accelerator section accelerates ions that pass through the grid towards the target such that collisions of the ions with the target cause the target to generate and emit neutrons therefrom. The ion source, accelerator section and target are housed in a sealed tube and preferably the carbon nanotubes of the bundle are highly ordered with at least 10⁶ carbon nanotubes per cm² that extend in a direction substantially parallel to the central axis of the tube. The neutron generator provides gas ionization at much higher atomic to molecular ratio that the prior art, which allows for small compact size designs suitable for logging tools that are used in space-constrained downhole environments.

Claims

1. A neutron generator comprising:

an ion source disposed in a pressurized environment containing an ionizable gas, the ion source including a grid and a substrate with a bundle of carbon nanotubes extending therefrom to ends that are spaced from said grid;

ion source voltage supply means, operably coupled to said ion source, for supplying a positive voltage potential between said substrate and said grid to cause ionization of said ionizable gas and emission of ions through said grid;

a target; and

an accelerator section disposed between said ion source and said target that accelerates ions that pass through said grid towards said target such that collisions of ions with said target cause said target to generate and emit neutrons therefrom.

2. A neutron generator according to claim 1, further comprising:

power supply means, operably coupled to at least said target, for supplying to said target voltage signals that accelerate ions toward said target.

3. A neutron generator according to claim 1, further comprising:

means for producing the ionizable gas.

4. A neutron generator according to claim 3, wherein:

said means for producing the ionizable gas includes a filament and a current source that heats said filament.

5. A neutron generator according to claim 1, wherein:

said ion source, accelerator section and target are disposed within a sealed chamber defined by a glass or ceramic tubular housing having a central axis, and said carbon nanotubes of said ion source extend in a direction substantially parallel to said central axis.

6. A neutron generator according to claim 1, wherein:

said bundle of carbon nanotubes of said ion source is realized by a highly ordered array of parallel carbon nanotubes.

7. A neutron generator according to claim 1, wherein:

said bundle of carbon nanotubes of said ion source comprise single-walled carbon nanotubes, multi-walled carbon nanotubes, or a combination thereof.

8. A neutron generator according to claim 1, wherein:

said bundle of carbon nanotubes include at least 10⁶ carbon nanotubes per cm².

9. A neutron generator according to claim 8, wherein:

said ion source voltage supply means supplies a positive voltage potential between said substrate and said grid on the order of 1 kV.

10. A neutron generator according to claim 9, wherein:

the ends of said bundle of carbon nanotubes are spaced from said grid by a gap that is less than 1 mm.

11. A neutron generator according to claim 10, wherein:

said ion source is capable of creating an ion current of at least 100 μ

A at a gas pressure of 1 mTorr.

12. A neutron generator according to claim 2, wherein:

said accelerator section includes an extracting electrode and said power supply means supplies to said extracting electrode a high positive potential voltage signal relative to said target.

13. A neutron generator according to claim 2, wherein:

said accelerator section includes a suppressor electrode and said power supply means supplies to said suppressor electrode a negative potential voltage signal relative to said target.

15. A neutron generator according to claim 1, wherein:

the ionizable gas comprises at least one hydrogen isotope.

16. A neutron generator according to claim 5, wherein:

said at least one hydrogen isotope is selected from the group including deuterium (H²), tritium (H³), and a combination thereof.

17. A logging tool for investigating earth formations surrounding a borehole, comprising a sonde incorporating at least one radiation detector and a neutron generator according to claim 1.

14. A neutron generator according to claim L wherein:

said accelerator section is realized by applying a high positive voltage potential signal to said grid relative to said target.