Patent Number: 
Section: claims

1. A system for detecting the presence of fissionable material, the system comprising:a source of radiation that is switchable between a screening mode and a verification mode, the source configured to:produce, in the screening mode, a first type of radiation having a first energy and a second type of radiation having a second energy, the second energy being higher than the first energy,direct, in the screening mode, the first type of radiation and the second type radiation toward a physical region, such that the first type of radiation propagates towards the physical region in a first direction and the second type of radiation propagates towards the physical region in a second direction that is substantially parallel to the first direction;produce, in the verification mode, a third type of radiation, anddirect, in the verification mode, the third type of radiation toward the physical region, the third type of radiation being sufficient to induce fission in a fissionable material;a sensor system comprising:a sensor configured to sense radiation comprising the first energy and the second energy from the physical region, anda sensor configured to sense a fission product; anda processor operable to:access data sensed by the sensor configured to sense radiation comprising the first energy and the second energy,determine, for the physical region represented by the accessed data, an absorption of the first type of radiation and the second type of radiation,determine whether the physical region is a region of interest based on the absorption, andcause the source of radiation to switch from the screening mode to the verification mode when the physical region is a region of interest. 2. The system of claim 1, wherein the first type of radiation is x-ray radiation, and the second type of radiation is x-ray radiation. 3. The system of claim 2, wherein to determine whether the physical region is a region of interest based on the absorption, the processor is operable to determine an effective atomic number of the physical region. 4. The system of claim 1, wherein the third type of radiation is x-ray radiation having an energy that is higher than the energy of the first energy. 5. The system of claim 4, wherein the first type of x-ray radiation has an energy spectrum with a maximum energy of 6 MeV, the second type of x-ray radiation has an energy spectrum with a maximum energy of 9 MeV, and the third type of x-ray radiation has an energy spectrum with a maximum energy of 10 MeV. 6. The system of claim 1, wherein the first type of radiation, the second type of radiation, and the third type of radiation are the same type of radiation. 7. The system of claim 1, wherein the first type of radiation, the second type of radiation, and the third type of radiation are different types of radiation. 8. The system of claim 1, further comprising a track configured to:support the source, andenable the source to move along the track with respect to the physical region. 9. The system of claim 8, wherein the source and the sensor system move concurrently with respect to the physical region. 10. The system of claim 9, wherein the physical region is a region within a larger region, the source moves with respect to the larger region during the screening mode, and the physical region is determined to be a region of interest, and further comprising moving the source to the physical region during the verification mode. 11. The system of claim 1, further comprising a photo-neutron conversion target configured to produce, in response to interaction with the third type of radiation, a neutron of sufficient energy to cause fission in a fissionable material. 12. The system of claim 11, wherein the photo-neutron conversion target is made of beryllium, deuterium, or lithium. 13. The system of claim 11, wherein the conversion target is between the source and the physical region. 14. The system of claim 13, wherein the conversion target is coupled to the source. 15. The system of claim 1, wherein the source of radiation comprises a first source of radiation and a second source of radiation that is separate from the first source of radiation, the first source of radiation producing the first type of radiation and the second type of radiation in the screening mode, and the second source of radiation producing the third type of radiation in the verification mode. 16. The system of claim 1, wherein the first type of radiation, the second type of radiation, and the third type of radiation are produced by a single source of radiation that is configured to operate in multiple modes, including the screening mode and the verification mode. 17. A method of detecting the presence of fissionable material, the method comprising:directing, from an imaging system in a screening mode, a first type of radiation towards a physical region, the first type of radiation having a first energy;directing, from the imaging system in the screening mode, a second type of radiation towards the physical region, the second type of radiation having a second energy that is higher than the first energy, and the second type of radiation propagating in a direction that is substantially parallel to a direction of propagation of the first type of radiation;determining an absorption characteristic of the physical region based on an absorption of the first type of radiation and the second type of radiation by the physical region;determining, from the absorption characteristic, whether the physical region is a region of interest;switching the imaging system from the screening mode to a verification mode in response to determining that the physical region is a region of interest;directing, from the imaging system in the verification mode, a third type of radiation toward the physical region, the third type of radiation being sufficient to induce fission in a fissionable material; anddetermining whether a fissionable material is present in the physical region based on an interaction between the third radiation and the physical region. 18. The method of claim 17, wherein the first type of radiation is x-ray radiation, the second type of radiation is x-ray radiation, and the third type of radiation is a photon or a neutron. 19. The method of claim 17, further comprising detecting radiation from a fission product emitted from the physical region after the source of the third type of radiation is turned off. 20. The method of claim 17, wherein the absorption characteristic comprises an effective atomic number. 21. The method of claim 17, wherein the physical region is a region of interest, and further comprising:moving the imaging system during the screening mode, andmoving the imaging system to the physical region at the beginning of the verification mode. 22. The method of claim 17, further comprising identifying fissionable material. 23. An imaging system for discriminating fissionable materials from among other high-effective atomic number materials, the imaging system comprising:a source configured to:produce dual-energy x-ray radiation sufficient to cause fission in fissionable materials, the dual-energy x-ray radiation comprising a first beam of energy at a first energy and a second beam of energy at a second energy, anddirect the dual-energy x-ray radiation sufficient to cause fission in fissionable materials towards a physical region, the first beam of energy propagating in a direction that is substantially parallel to a direction of propagation of the second beam of energy;a sensor configured to sense x-ray radiation and a product of fission from the physical region;a processor configured to:determine an absorption of the dual-energy x-ray radiation by the physical region based on the sensed x-ray radiation, anddetermine whether the physical region includes fissionable material based on the presence of a product of fission. 24. The system of claim 1, wherein the processor is further operable to cause the sensor system to switch between the screening mode and the verification mode when the physical region is a region of interest. 25. The system of claim 1, wherein the third type of radiation comprises a neutral particle. 26. The system of claim 1, wherein both the sensor configured to sense radiation comprising the first energy and the second energy and the sensor configured to sense a fission product are mounted on a single gantry. 27. The system of claim 1, wherein the third type of radiation propagates along a direction that is substantially parallel to the first direction. 28. The method of claim 17, further comprising:determining a characteristic of the region of interest,modifying the third type of radiation based on the characteristic, andscanning the region of interest with the modified third type of radiation. 29. The method of claim 28, wherein the characteristic of the region of interest comprises an effective atomic number, and modifying the third type of radiation comprises selecting the third type of radiation based on the effective atomic number. 30. The method of claim 29, wherein selecting the third type of radiation based on the effective atomic number comprises selecting one of a neutron probe or a photon probe. 31. The method of claim 17, further comprising presenting a perceivable indication when a fissionable material is present.