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 Pu-239; and g) concluding that there is Pu-239 in the container when: the observed energy spectrum and the known energy spectra have the same overall shape; and the observed effective half-life is approximately 20 to 30 seconds.