Dipole response of U-238 to polarized photons below the neutron separation energy

Nuclear resonance fluorescence experiments were carried out at the High-Intensity gamma-ray Source facility at Triangle Universities Nuclear Laboratory to characterize the low-energy dipole structure of U-238 using 100% linearly polarized photon beams from 2.0 to 6.2 MeV. 113 transitions corresponding to de-excitations to the ground state in U-238 were observed and the energy, spin, parity, integrated cross section, reduced width, and branching ratio were determined for each of these identified levels. The total E1 gamma-ray interaction cross section was calculated and it was deduced that the observed concentration of low-lying E1 transitions were excited from the low-energy tail of the giant dipole resonance and were not a pygmy dipole resonance. Comparisons were made between quasiparticle random-phase approximation calculations and the experimentally observed strength. The observed and predicted M1 strength agreed well with each other. However, there was no similar agreement for the E1 strength.