Patent Number: 
Section: claims

1. A method for identifying fissile material within an interrogated vessel, comprising:casting an incident photon beam from an electron beam accelerator through the interrogated vessel on the fissile material;detecting an emerging photon beam within an energy range from about 1 MeV to about 50 MeV from the fissile material with an array of fission-fragment detectors, a first set of scintillator paddles, and a second set of scintillator paddles, wherein the array of fission-fragment detectors, the first set of scintillator paddles, and the second set of scintillator paddles (a) are arranged sequentially in a direct path of the emerging photon beam such that each receives the emerging photon beam, and (b) are sensitive to different ranges of photon beam energy;obtaining a first signal from the array of fission-fragment detectors, a second signal from the first set of scintillator paddles, and a third signal from the second set of scintillator paddles, each signal indicative of photon yield within the different ranges of photon beam energy; anddetermining a photon energy regime of the emerging photon beam through identification of a drop in photon yield in at least one of the three signals, the determined photon energy regime identifying the fissile material within the interrogated vessel. 2. The method of claim 1, wherein said identifying comprises determining a range of an atomic number of a material in a container. 3. The method of claim 1, wherein detecting the emerging photon beam from the material with the array of fission-fragment detectors comprises detecting an energy range of the emerging photon beam in a range between about 10 MeV to 20 MeV. 4. The method of claim 1, wherein detecting the emerging photon beam from the material with the first set of scintillator paddles comprises detecting an energy range of the emerging photon beam in a range up to about 6 MeV. 5. The method of claim 1, wherein detecting the emerging photon beam from the material with the second set of scintillator paddles comprises detecting an energy range of the emerging photon beam exceeding about 6 MeV. 6. The method of claim 1, further comprising creating a photon distribution energy curve using a combination of the first signal from the array of fission-fragment detectors, the second signal from the first set of scintillator paddles, and the third signal from the second set of scintillator paddles. 7. The method of claim 1, wherein casting an incident photon beam from the electron beam accelerator comprises directing an electron beam onto a radiator for producing a photon beam through bremsstrahlung process. 8. The method of claim 1, further comprising producing electron positron pairs with a convertor coupled to the second set of scintillator paddles. 9. The method of claim 8, further comprising detecting an energy range of the electron positron pairs exceeding about 6 MeV. 10. The method of claim 1, wherein the array of fission fragment detectors is sensitive to a range of photon beam energy between about 10 MeV and 20 MeV, the first set of scintillator paddles is sensitive to a range of photon beam energy up to about 6 MeV, and the second set of scintillator paddles is sensitive to a range of photon beam energy above about 6 MeV. 11. The method of claim 10, wherein the first and second set of scintillator paddles comprise plastic scintillator paddles. 12. The method of claim 1, wherein the array of fission fragment detectors, the first set of scintillator paddles, and the second set of scintillator paddles are sensitive to different, but overlapping ranges of photon beam energy.