Measurements of Short-Lived Fission Isomers

Fission yields of the short lived isomers $^{134m}$Te ($T_{1/2}=162$ ns) and $^{136m}$Xe ($T_{1/2}=2.95$ $\mu$s) were measured for $^{235}$U and $^{238}$U. The isomers were detected by the $\gamma$ rays associated with the decay of the isomeric states using high-purity germanium detectors. Fission was induced using both monoenergetic $\gamma$ rays and neutrons. At TUNL's High-Intensity Gamma-ray Source (HI$\gamma$S), $\gamma$ rays of 9 and 11 MeV were produced . Monoenergetic 8 MeV neutrons were produced at TUNL's tandem accelerator laboratory. Both beams were pulsed to allow for precise time-gated spectroscopy of both prompt and delayed $\gamma$ rays following fission. This technique offers a non-destructive probe of special nuclear materials that is sensitive to the isotopic identity of the fissile material.