Measurements Of Short-Lived Isomers From Photofission As A Method Of Active Interrogation For Special Nuclear Materials

Isomeric fission products, such as Te-134m (T-1/2 = 164 ns) and Xe-136m (T-1/2 = 2.95 mu s), are proposed as a signature of special nuclear materials in active interrogation applications. To test this conjecture, monoenergetic 9, 11, and 13 MeV photons from the HI gamma S facility are used to induce fission of U-235, U-238, and Pu-239 targets. The de-excitation gamma rays of the isomers are measured using time-gated spectroscopy with high-purity germanium detectors. The Te-134m and Xe-136m isomers are detected and identified by the energy and decay half-life of their characteristic gamma-ray transitions. The ratio of yields for these two signature gamma rays, corresponding to Te-134m/(Xe-136m + I-136), is found to be strongly correlated with the identity of the fissioning nuclei. These results show that fission-product isomers may be used in active interrogation to detect and identify special nuclear materials, even providing information on the isotopic enrichment. The feasibility of an active interrogation scenario using a bremsstrahlung beam is discussed.