FIELD-IONIZATION NEUTRON GENERATOR

• US 20180049305A1
• Filed: 04/16/2015
• Published: 02/15/2018
• Est. Priority Date: 04/16/2015
• Status: Active Grant

• Current Assignee: Halliburton Energy Services Incorporated (Halliburton Company)
• Original Assignee: Halliburton Energy Services Incorporated (Halliburton Company)
• Inventors: Navarro-Sorroche, Juan, Guo, Weijun
• Granted Patent: US 10,455,684 B2
• Time in Patent Office: Days
• CPC Class Codes: G01V 5/102 the neutron source being of the pulsed type
• CPC Class Codes: H05H 3/06 Generating neutron beams targets for producing nuclear reactions H05H6/00; neutron sources G21G4/02
• CPC Class Codes: H05H 6/00 Targets for producing nuclear reactions supports for targets or objects to be irradiated G21K5/08 ; preparation of tritium C01B4/00; targets, e.g. pellets for fusion reactions by laser or charged particles beam injection H05H1/22
• CPC Class Codes: H05H 6/005 Polarised targets polarising devices, e.g. for obtaining a polarised ion beam G21K1/16

Abstract

Described herein are neutron generators that employ direct field ionization of ionizable fusion gases, as well as well-logging tools and methods that utilize such neutron generators. In various embodiments, the neutron generator includes a cylindrical field-ionization structure distributed around the inner surface of a tubular housing, and a cylindrical ion-accelerating grid disposed about the longitudinal axis concentrically to the field-ionization structure. Ions generated by the field-ionization structure may accumulate inside the ion-accelerating grid, from which they can be axially extracted and accelerated towards a fusion target. Additional tools, systems, and methods are disclosed.

Claims

1. A neutron generator comprising:

a closed housing containing an ionizable gas, the housing defining first and second neutron-generator portions along a longitudinal axis thereof;

a cylindrical field-ionization structure arranged along an inner surface of the housing in the first portion, the field-ionization structure being configured to ionize the ionizable gas upon application of an electrical voltage to the structure;

a cylindrical accelerating grid placed about the longitudinal axis in the first portion, the accelerating grid being configured, when operating, to cause accumulation of ions in the grid; and

a target cathode disposed at an end of the second portion that is distal to the first portion, the target cathode being held, during operation, at a negative electrical potential relative to the accelerating grid to extract ions from the grid and accelerate them towards the target cathode to thereby cause fusion reactions between the ions and target atoms contained in the target cathode.

2. The neutron generator of claim 1, wherein the field-ionization array comprises:

an array of nanotips disposed on a cylindrical substrate; and

a field-ionization grid disposed concentrically interior to the cylindrical substrate and aligned with the array of nanotips such that the nanotips are located at openings in the grid.

3. The neutron generator of claim 2, wherein a distance between the substrate and the field-ionization grid is smaller than one micrometer.

4. The neutron generator of claim 1, wherein the housing has an outer diameter of less than 1.5 inches.

5. The neutron generator of claim 4, capable of achieving a neutron yield from the fusion reactions of at least 10⁹ neutrons per second.

6. The neutron generator of claim 1, wherein the field-ionization structure is configured to produce, when operating, ionized gas having at least 50% monatomic ions.

7. The neutron generator of claim 1, wherein application of a voltage pulse to the field-ionization structure creates a fusion-neutron pulse within less than one microsecond thereafter.

8. The neutron generator of claim 1, wherein the ionizable gas comprises at least one of deuterium and tritium.

9. A well-logging tool comprising:

a pulsed neutron generator comprising, contained within a closed housing defining first and second neutron-generator portions along a longitudinal axis thereof;

a cylindrical field-ionization structure arranged along an inner surface of the housing in the first portion, the field-ionization structure being configured to ionize, upon application of an electrical voltage thereto, an ionizable gas contained within the housing;

a cylindrical accelerating grid placed about the longitudinal axis in the first portion, the accelerating grid being configured, when operating, to cause accumulation of ions in the grid; and

a target cathode disposed at and end of the second portion that is distal to the first portion, the target cathode being held, during operation, at a negative electrical potential relative to the accelerating grid to thereby extract ions from the grid and accelerate them towards the target cathode, fusion reactions between the ions and target atoms contained in the target cathode causing generation of neutrons; and

a radiation detector configured to detect radiation resulting from interaction of the neutrons with materials outside the well-logging tool.

10. The tool of claim 9, further comprising circuitry configured to at least one of control operation of the neutron generator or the radiation detector, or process data indicative of the radiation detected by the detector.

11. The tool of claim 9, wherein the field-ionization array of the pulsed neutron generator comprises

an array of nanotips disposed on a cylindrical substrate;

and

a field-ionization grid disposed concentrically interior to the cylindrical substrate and aligned with the array of nanotips such that the nanotips are located at openings in the grid.

12. The tool of claim 9, wherein a diameter of the tool is less than two inches.

13. The tool of claim 12, wherein an outer diameter of the neutron generator is less than 1.5 inches.

14. The tool of claim 13, wherein the neutron generator is capable of achieving a neutron yield from the fusion reactions of at least 10⁹ neutrons per second.

15. The tool of claim 9, wherein the field-ionization structure is configured to produce, when operating, ionized gas having at least 50% monatomic ions.

16. The tool of claim 9, wherein the neutron generator is configured to cause creation of a fusion-neutron pulse within less than one microsecond following application of a voltage pulse to the field-ionization structure.

17. A method comprising:

disposing a tool in a borehole, the tool comprising a neutron generator including;

a closed housing containing an ionizable gas, the housing defining first and second neutron-generator portions along a longitudinal axis thereof;

a cylindrical field-ionization structure, arranged along an inner surface of the housing in the first portion, for ionizing the ionizable gas upon application of an electrical voltage thereto;

a cylindrical accelerating grid, placed about the longitudinal axis in the first portion, for causing accumulation of ions in the grid; and

a target cathode, disposed at an end of the second portion that is distal to the first portion and held at a negative electrical potential relative to the accelerating grid, for extracting ions from the accelerating grid and accelerating them towards the target cathode to thereby cause fusion reactions between the ions and target atoms contained in the target cathode; and

operating the neutron generator in a pulsed mode to cause generation of neutron pulses by fusion of the ionized gas with the target atoms.

18. The method of claim 17, wherein operating the neuron generator comprises applying a pulsed voltage to the cylindrical field-ionization structure.

19. The method of claim 18, wherein the applied pulsed voltage is less than one thousand Volts.

20. The method of claim 17, further comprising measuring radiation resulting from interaction of the neutrons with materials in or surrounding the borehole.

21. The method of claim 17, wherein disposing the tool in the borehole comprises suspending the tool from a wireline.

22. The method of claim 17, wherein disposing the tool in the borehole comprises disposing a drill string including the tool in the borehole.