Valence-shell Dependence of the Pygmy Dipole Resonance: E1 Strength Difference in Cr50,54

Background: The low-lying electric dipole strength provides insights into the parameters of the nuclear equation of state via its connection with the pygmy dipole resonance and nuclear neutron skin thickness. Purpose: The aim was to complement the systematic of the pygmy dipole resonance and first study its behavior across the N = 28 neutron shell closure. Methods: Photon-scattering cross sections of states of Cr-50,Cr-54 were measured up to an excitation energy of 9.7 MeV via the nuclear resonance fluorescence method using gamma-ray beams from bremsstrahlung and Compton backscattering. Results: Transitions strengths, spin and parity quantum number, and average branching ratios for 55 excited states, 44 of which were observed for the first time, were determined. The comparison between the total observed strengths of the isotopes Cr-50,Cr-52,Cr-54 shows a significant increase above the shell closure. Conclusions: The evolution of the pygmy dipole resonance is heavily influenced by the shell structure.