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

The gamma-ray strength function in the quasicontinuum has been measured for Th231-233, Pa-232,Pa-233, and U237-239 using the Oslo method. All eight nuclei show a pronounced increase in gamma strength at omega(SR) approximate to 2.4 MeV, which is interpreted as the low-energy M1 scissors resonance (SR). The total strength is found to be B-SR = 9-11 mu(2)(N) when integrated over the 1-4 MeV gamma-energy region. The SR displays a double-hump structure that is theoretically not understood. Our results are compared with data from (gamma, gamma') experiments and theoretical sum-rule estimates for a nuclear rigid-body moment of inertia.