Preliminary Report on the Population of the 235U T1/2=25-Minute Isomer by the (Nu,Nu'Gamma)Reaction

The population cross section of the T{sub 1/2} = 25-minute, E{sub x} = 76.8-keV isomer in {sup 235}U via the {sup 235}U(n,n{prime}{gamma}) reaction has been estimated in the E{sub n} = 2.1-20-MeV range. Gamma rays populating both isomer and ground states were detected using the GEANIE spectrometer at the LANSCE/WNR ''white-source'' neutron facility. Partial {gamma}-ray cross sections were obtained as a function of incident neutron energy, using {gamma}-ray spectroscopy and the time-of-flight technique. A correction for unobserved transitions was applied to the measured partial cross sections using the Hauser-Feshbach code GNASH to produce population cross sections for the isomer- and ground-state levels. The deduced isomer population cross section at E{sub n} = 2.1 MeV is 1.1(1) b, and the isomer-to-ground state population ratio decreases from 0.9 to 0.06 over the E{sub n} = 2.1-20-MeV range. The details of the measurement and recommendations to improve the current results are discussed.