Patent Number: 
Section: claims

1. A method of detecting a presence of a material comprising an actinide in a container, comprising:a) locating at least one neutron detector capable of distinguishing neutron energy such that said detector views said container at a first angle relative to a first incident neutron beam;b) illuminating at least a portion of said container with said first incident neutron beam;c) detecting, in at least one said neutron detector capable of distinguishing neutron energy, viewing said container at the first angle, a plurality of prompt neutrons produced by a fission interaction of said first incident neutron beam with at least a portion of said container;d) for at least one of said detected prompt neutrons, determining an energy of said detected prompt neutron; ande) based upon the determined energy of a predetermined quantity of said detected prompt neutrons exceeding a predetermined value, determining that the material comprising the actinide is present in the container. 2. The method of claim 1, wherein determining the energy of said detected neutron comprises measuring a time of flight of said detected neutron. 3. The method of claim 1, wherein determining the energy of said detected neutron comprises analyzing a quantity of energy deposited in at least one of said neutron detectors. 4. The method of claim 1, wherein the first incident neutron beam comprises thermal neutrons with energies below the predetermined value. 5. The method of claim 4, further comprising locating at least one filter to absorb a plurality of thermal neutrons that otherwise would be incident on the at least one neutron detector. 6. The method of claim 4, wherein the at least one neutron detector is not sensitive to neutrons below a pre-determined minimum neutron detector threshold energy. 7. The method of claim 6, wherein the pre-determined minimum neutron detector threshold energy is above an energy of a thermal neutron. 8. The method of claim 4, further comprising,f) determining a neutron energy distribution in at least one of said neutron detectors; andg) based upon said determined neutron energy distribution in at least one said neutron detector, identifying an actinide nuclear species present in the container. 9. The method of claim 1, wherein the first incident neutron beam comprises neutrons with a first predetermined incident neutron energy greater than thermal neutron energies. 10. The method of claim 9, wherein the predetermined value is greater than the first predetermined incident neutron energy. 11. The method of claim 9, further comprising:f) illuminating at least a portion of said container with a second incident neutron beam, said second incident neutron beam comprising neutrons with a second predetermined incident neutron energy greater than thermal neutron energies;g) detecting, in at least one said neutron detector viewing said container at the first angle, a plurality of neutrons produced by an interaction of said second incident neutron beam with at least a portion of said container;h) for at least some of the plurality of said detected neutrons, determining an energy of said detected neutron;i) for each of the first predetermined incident neutron energy and the second predetermined incident neutron energy, determining a neutron energy distribution in at least one said neutron detector; andj) based upon comparing said determined neutron energy distributions in at least one said neutron detector, resulting from said incident neutron beams, confirming that the material comprising the actinide is present in the container if said neutron energy distributions change by no more than a predetermined amount as a function of incident neutron energy. 12. The method of claim 9, further comprising:f) locating at least one neutron detector such that said detector views said container at a second angle relative to the first incident neutron beam;g) detecting, in at least one said neutron detector viewing said container at the second angle, a second plurality of neutrons produced by the interaction of said first incident neutron beam with at least a portion of said container;h) illuminating at least a portion of said container with a second incident neutron beam, said second incident neutron beam comprising thermal neutrons with energies below the predetermined value;i) detecting, in at least one said neutron detector viewing said container at the first angle, a third plurality of neutrons produced by an interaction of said second incident neutron beam with at least a portion of said container;j) detecting, in at least one said neutron detector viewing said container at the second angle, a fourth plurality of neutrons produced by the interaction of said second incident neutron beam with at least a portion of said container;k) based at least in part upon the detection of said neutrons produced by the interaction of said second incident neutron beam with at least a portion of said container, in at least one said neutron detector viewing said container at the first angle and in at least one said neutron detector viewing said container at the second angle, determining a total absorption as a function of neutron energy for neutrons between the container and said detectors;l) based at least in part upon said total absorption as a function of neutron energy, and at least in part upon the detection of said neutrons produced by the interaction of said first incident neutron beam with at least a portion of said container, in at least one said neutron detector viewing said container at the first angle and in at least one said neutron detector viewing said container at the second angle, identifying an actinide nuclear species present in the container. 13. The method of claim 12, wherein the identification of the actinide nuclear species present is based at least in part upon an anisotropic angular distribution, of the neutrons produced by the interaction of said first incident neutron beam with at least a portion of said container, between neutron detectors viewing the container at said first and second angles. 14. The method of claim 12, wherein the identification of the actinide nuclear species present is based at least in part upon the energy distributions, of the neutrons produced by the interaction of said first incident neutron beam with at least a portion of said container, in neutron detectors viewing the container at said first and second angles. 15. The method of claim 9, further comprising:f) locating at least one neutron detector such that said detector views said container at a second angle relative to the first incident neutron beam;g) detecting, in at least one said neutron detector viewing said container at the second angle, a second plurality of neutrons produced by the interaction of said first incident neutron beam with at least a portion of said container;h) for each of the first angle and the second angle, determining neutron energy distributions in at least one of said neutron detectors viewing said container at said angle;i) based at least in part upon the energy distributions, in said neutron detectors viewing said container at the first angle and in said neutron detectors viewing said container at the second angle, identifying an actinide nuclear species present in the container. 16. The method of claim 9, further comprising:f) illuminating at least a portion of said container with a second incident neutron beam, said second incident neutron beam comprising neutrons with a second predetermined incident neutron energy greater than thermal neutron energies;g) detecting, in at least one said neutron detector viewing said container at the first angle, a second plurality of neutrons produced by an interaction of said second incident neutron beam with at least a portion of said container;h) for at least one of said detected neutrons, determining an energy of said detected neutron;i) for each of the first predetermined incident neutron energy and the second predetermined incident neutron energy, determining a fission neutron yield in at least one said neutron detector; andj) based upon comparing said determined fission neutron yields in at least one said neutron detector, resulting from said incident neutron beams, identifying an actinide nuclear species present in the container. 17. The method of claim 9, further comprising:f) locating at least one neutron detector such that said detector views said container at a second angle relative to the first incident neutron beam;g) detecting, in at least one said neutron detector viewing said container at the second angle, a second plurality of neutrons produced by the interaction of said first incident neutron beam with at least a portion of said container;h) illuminating at least a portion of said container with a second incident neutron beam, said second incident neutron beam comprising neutrons with a second predetermined incident neutron energy greater than thermal neutron energies;i) detecting, in at least one said neutron detector viewing said container at the first angle, a third plurality of neutrons produced by an interaction of said second incident neutron beam with at least a portion of said container;j) detecting, in at least one said neutron detector viewing said container at the second angle, a fourth plurality of neutrons produced by the interaction of said second incident neutron beam with at least a portion of said container;k) for each of the first angle and the second angle, for each said incident neutron energy, determining a neutron energy distribution in at least one of said neutron detectors viewing said container at said angle;l) based at least in part upon the energy distributions, in said neutron detectors viewing said container at the first angle and in said neutron detectors viewing said container at the second angle, at each incident neutron energy, identifying an actinide nuclear species present in the container. 18. The method of claim 1, further comprising,f) for a plurality of subportions of the container illuminated by the first incident neutron beam, illuminating, with a second incident neutron beam said subportion of the container,g) for at least one said subportion, detecting, in at least one said neutron detector viewing said container at the first angle, a second plurality of neutrons produced by an interaction of said second incident neutron beam with said subportion of the container;h) for at least one said subportion, for at least one of said detected neutrons, determining an energy of said detected neutron; andi) for at least one said subportion, based upon the determined energy of a predetermined quantity of said detected neutrons exceeding a second predetermined value, determining that the material comprising the actinide is present in said subportion of the container. 19. The method of claim 18, wherein the second incident neutron beam is collimated. 20. The method of claim 18, wherein the second incident neutron beam is the first incident neutron beam.