Scissors resonance in the quasicontinuum of Th, Pa, and U isotopes

The γ-ray strength function in the quasicontinuum has been measured for 231-233Th, 232,233Pa, and 237-239U using the Oslo method. All eight nuclei show a pronounced increase in γ strength at ωSR≈2.4 MeV, which is interpreted as the low-energy M1 scissors resonance (SR). The total strength is found to be BSR=9-11μN2 when integrated over the 1-4 MeV γ-energy region. The SR displays a double-hump structure that is theoretically not understood. Our results are compared with data from (γ, γ′) experiments and theoretical sum-rule estimates for a nuclear rigid-body moment of inertia. © 2014 American Physical Society.