Field emission ion source neutron generator

• US 10,408,968 B2
• Filed: 12/31/2013
• Issued: 09/10/2019
• Est. Priority Date: 12/31/2013
• 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
• Primary Examiner(s): Porta, David P
• Assistant Examiner(s): Gutierrez, Gisselle
• Application Number:
  US14/914,560
• Publication Number:
  US 20160216400A1
• Time in Patent Office: 2,079 Days
• Field of Search: 250253
• CPC Class Codes: G01V 5/10 using neutron sources neutron generating tubes H05H5/00; neutron sources using isotopes G21G4/00
• CPC Class Codes: G21G 4/02 Neutron sources
• CPC Class Codes: H05H 3/06 Generating neutron beams targets for producing nuclear reactions H05H6/00; neutron sources G21G4/02

Abstract

A well logging tool includes a neutron generator having an ion source cathode comprising a nano-structure field emission (FE) array for producing a charged particle current routed through an ionizable gas, thus generating ions by electron impact ionization of the gas. The nanostructures can be provided by bundles of silicon nanotips grown on a substrate. The FE array can be provided on an axially facing substrate located co-axially in an elongate housing, with the charged particle current directed from the centrally located FE array to a co-axial annular anode and having axial and radial directional components.

Claims

1. A neutron generator comprising:

a housing defining an ion source chamber containing an ionizable gas;

an anode in the ion source chamber, wherein the anode is radially offset from a longitudinal axis of the ion source chamber; and

a field emitter device comprising a field emitter array and an embedded extraction grid, the field emitter array comprising a plurality of field electron emitters, the field electron emitters comprising a plurality of nanotips, the embedded extraction grid comprising a plurality of openings corresponding to positions of the plurality of nanotips and each nanotip extending beyond a surface of the extraction grid thru a corresponding opening of the plurality of openings, the embedded extraction grid configured to control electron emissions by the field electron emitters based at least in part on a voltage applied to the embedded extraction grid; and

wherein the neutron generator is configured so that a flow of electrons produced by the field electron emitters is routed through the ionizable gas to produce ions independent of a magnetic field from a magnet, and

wherein the field emitter array is configured to supply the flow electrons large enough to enable sub-microsecond ignition of the ionizable gas.

2. The neutron generator of claim 1, wherein each nanotip comprising an elongate filament which is aligned with the longitudinal axis of the ion source chamber.

3. The neutron generator of claim 1, wherein the housing is elongate.

4. The neutron generator of claim 1, further comprising a substrate with a substrate surface that is perpendicular to the longitudinal axis-of the ion source chamber, wherein the field emitter array is attached to the substrate.

5. The neutron generator of claim 1, wherein the anode comprises an annular anode element that is co-axial with the longitudinal axis of the ion source chamber.

6. The neutron generator of claim 4, wherein the substrate surface is substantially circular.

7. The neutron generator of claim 1, further comprising a control arrangement to apply an ion source voltage pulse between a field emitter array substrate and the anode to accelerate emitted electrons in a radial path toward the anode and ionize the ionizable gas by electron impact.

8. The neutron generator of claim 7, wherein the ion source voltage pulse has an amplitude of 500 V or less.

9. The neutron generator of claim 8, wherein a turn-on/turn-off delay for an ion source comprising the field emitter array and the anode is less than 1 μ

s.

10. The neutron generator of claim 1, wherein the electron current is greater than or equal to 1 A/cm².

11. The neutron generator of claim 1, wherein the field electron emitters comprise multiple nanotips each having a length between 0.2 and 100 nanometers and wherein the field electron emitters have a nanotip density between 10,000 and 1,000,000 nanotips per square centimeter.

12. The neutron generator of claim 1, further comprising:

a target rod; and

a frustoconical insulator extending between the ion source chamber and the target rod.

13. A method comprising:

lowering a neutron generator into a borehole, wherein the neutron generator comprises a housing that defines an ion source chamber, and wherein the neutron generator further comprises a field emitter device comprising a field emitter array and an embedded extraction grid inside of the housing, the field emitter array comprising a plurality of field electron emitters, the field electron emitters comprising a plurality of nanotips, the embedded extraction grid comprising a plurality of openings corresponding to positions of the plurality of nanotips and each nanotip extending beyond a surface of the extraction grid thru a corresponding opening of the plurality of openings, the embedded extraction grid configured to control electron emissions by the field electron emitters based at least in part on a voltage applied to the embedded extraction grid;

delivering an ionizable gas to the ion source chamber;

applying an ion source voltage pulse between the field emitter array and an anode in the ion source chamber, wherein the anode is radially offset from a longitudinal axis of the ion source chamber; and

producing an electric field that attracts electrons emitted from the field emitter array towards the anode through the ionizable gas independent of a magnetic field from a magnet, wherein the electrons emitted from the field emitter array provide sub-microsecond ignition of the ionizable gas.

14. The method of claim 13, wherein the ion source voltage pulse has an amplitude of 500 V or less.

15. The method of claim 13, wherein a turn-on/turn-off delay for ionization of the ionizable gas in the ion source chamber is less than 1 μ

s.

16. An apparatus comprising:

a body; and

a neutron generator housed by the body, wherein the neutron generator comprises,

a housing that defines an ion source chamber containing an ionizable gas;

an anode in the ion source chamber, wherein the anode is radially offset from a longitudinal axis of the ion source chamber; and

a field emitter device comprising a field emitter array and an embedded extraction grid, the field emitter array comprising a plurality of field electron emitters, the field electron emitters comprising a plurality of nanotips, the embedded extraction grid comprising a plurality of openings corresponding to positions of the plurality of nanotips and each nanotip extending beyond a surface of the extraction grid thru a corresponding opening of the plurality of openings, the embedded extraction grid configured to control electron emissions by the field electron emitters based at least in part on a voltage applied to the embedded extraction grid;

wherein an electron current produced by the field electron emitters is routed through the ionizable gas independent of a magnetic field from a magnet, and

wherein the field emitter array is configured to supply a flow of electrons extracted by the embedded extraction grid that is large enough to enable sub-microsecond ignition of the ionizable gas.

17. The apparatus of claim 16, further comprising a substrate with a substrate surface that is perpendicular to the longitudinal axis of the housing, wherein the field emitter array is attached to the substrate.

18. The apparatus of claim 16, wherein the anode comprises an annular anode element that is co-axial with the longitudinal axis of the housing.

19. The apparatus of claim 16, wherein each nanotip comprising an elongate filament which is aligned with the longitudinal axis of the housing.

20. The apparatus of claim 16, further comprising a control arrangement to apply an ion source voltage pulse between the field emitter array and the anode.

21. The apparatus of claim 16, wherein the field electron emitters comprise multiple nanotips each having a length between 0.2 and 100 nanometers and wherein the field electron emitters have a nanotip density between 10,000 and 1,000,000 nanotips per square centimeter.