The low-energy dipole structure of 232Th , 236U and 238U actinide nuclei

Abstract

In this study, $\ensuremath I^{\pi} = 1^{+}$ and $\ensuremath I^{\pi} = 1^{-}$ dipole mode excitations are systematically investigated within the rotational and translational + Galilean invariant quasiparticle random-phase approximation for 232Th , 236U , and 238U actinide nuclei. It is shown that the investigated nuclei reach a B(M1) strength structure, which corresponds to the scissors mode. The calculated mean excitation energies as well as the summed B(M1) value of the scissors mode excitations are consistent with the available experimental data. The results of calculations indicate large differences to the rare-earth nuclei as is the case for the experiment: a doubling of the observed dipole strengths and a shift of the energy centroid to the lower energies by about 800keV. The calculations indicate the presence of a few prominent negative-parity $\ensuremath K^{\pi} = 1^{-}$ states in the 2.0-4.0MeV energy interval. The occurrence of the negative-parity dipole states with the rather high B(E1) value less than 4MeV shows the necessity of explicit parity measurements for the correct determination of the scissors mode strength in 232Th , 236U , and 238U isotopes.