To the continuum and beyond: Structure of U nuclei

An experiment was performed at the 88-inch cyclotron at LBNL to investigate the structure of uranium isotopes and concurrently test the so-called surrogate ratio method. A 28 MeV proton beam was used to bombard ²³⁶U and ²³⁸U targets and the outgoing light ions were detected using the STARS silicon telescope allowing isotopic assignments and the excitation energy of the compound nucleus to be measured. A fission detector was placed at backward angles to give particle-fission coincidences, while the six clover germanium detectors of the LIBERACE array were used for particle-γ coincidences. The (p,d) reaction channels on ²³⁶U and ²³⁸U targets were used as a surrogate to measure the σ(²³⁴U(n,f))/σ(²³⁶U(n,f)) cross section ratio. The results give reasonable agreement with literature values over an equivalent neutron energy range between 0 MeV and 6 MeV. Structure results in ²³⁵U include a new (3/2⁻) level at 1035 keV, that is tentatively assigned as the 3/2⁻[501] Nilsson state. The analogue 3/2⁻[501] state in ²³⁷U may be associated with a previously observed level at 1201 keV, whose spin/parity is restricted to J^π = 3/2⁻ on the basis of newly observed decays to the ground band.