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

Abstract

In this study, I-pi = 1(+) and I-pi = 1(-) dipole mode excitations are systematically investigated within the rotational and translational + Galilean invariant quasiparticle random-phase approximation for Th-232, U-236, and U-238 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 800 keV. The calculations indicate the presence of a few prominent negative-parity K-pi = 1(-) states in the 2.0-4.0 MeV energy interval. The occurrence of the negative-parity dipole states with the rather high B(E1) value less than 4 MeV shows the necessity of explicit parity measurements for the correct determination of the scissors mode strength in Th-232, U-236, and U-238 isotopes.