Fission product yield measurements at HIγS

Fission is complex nuclear process, with one of the key observables being the fission product yields (FPYs). Photofission offers a unique probe to help test theoretical models. A TUNL-LLNL-LANL collaboration is performing precision measurements of FPYs using monoenergetic γ-ray beams produced at the high intensity γ-ray source, HIγS. Fission chambers are used to measure the γ-ray flux and high-purity germanium detectors quantify the decaying fission products. The fission products are identified by their characteristic γ-rays and tracked across several half-lifes. Our current measurements span fission products with half-lifes ranging from minutes to 60 days. Future work includes the addition of a rapid transit system, which will extend our measurements to include fission products with half-lifes <10 s. Measurements of even shorter-lived fission products were made using the pulsing of the HIγS beam. This was done for the isomeric fission products ^134mTe (T_1/2 = 162 ns) and ^136mXe (T_1/2 = 2.95 μs). The ratio of the yields ^134mTe/^136mXe was found to be strongly dependent of the identity of the fissioning nuclei, indicating that these fission isomers may be used for active interrogation to detect and identify special nuclear materials. Additionally, measurements of the isomeric to ground state yields can provide information on the angular momentum of the fragments.