Patent Number: 
Section: claims

1. A method of interrogating a container, comprising the steps of:a) irradiating the container for approximately 30 seconds with neutrons having energies between about 2.45 and 14 MeV or with a gamma ray beam having an energy of at least 10 MeV;b) stopping the irradiating;c) after stopping the irradiating, counting β-delayed gamma rays having an energy range between about 3 and 6 MeV for approximately 30 seconds;d) making a first plot comprising number of β-delayed gamma rays counted in step c as a function of β-delayed gamma ray energy to produce an observed energy spectrum;e) making a second plot of the total number of β-delayed gamma rays counted in a portion of the energy range between about 4 and 6 MeV as a function of time in order to determine an effective half-life;f) comparing the observed energy spectrum with known energy spectra produced by fission products of U-235; andg) concluding that there is U-235 in the container when:the observed energy spectrum and the known energy spectra have the same overall shape; andthe observed effective half-life is approximately 20 to 30 seconds. 2. The method of claim 1 wherein the neutrons comprise D-D neutrons. 3. The method of claim 1 wherein the neutrons comprise D-T neutrons. 4. The method of claim 1 wherein a plastic or liquid scintillation detector is used for the counting step. 5. The method of claim 1 wherein, in step g, the observed energy spectrum and the known energy spectra having the same overall shape comprises having the same overall shape wherein the number of β-delayed gamma rays decreases as the energy increases at energies greater than approximately 3 MeV. 6. The method of claim 1 wherein:the container has dimensions of approximately 8 feet by 40 feet by 8.5 feet and is made of steel;the container holds at least 500 grams of U-235;the neutrons have an energy of approximately 14 MeV;the neutrons have a flux of approximately 3.8×104 neutrons/cm2 sec at a distance of approximately 15 feet from the container;detectors surrounding the container on at least three sides are used for the counting; andat least 350 β-delayed gamma rays with energies above 3 MeV are counted.