Low-lying electric dipole $γ$-continuum for the unstable $^{62,64}$Fe nuclei: Strength evolution with neutron number

The γ -ray emission from the nuclei $^{62,64}$Fe following Coulomb excitation at bombarding energy of 400-440 AMeV was measured with special focus on E 1 transitions in the energy region 4-8 MeV. The unstable neutron-rich nuclei $^{62,64}$Fe were produced at the FAIR-GSI laboratories and selected with the FRS spectrometer. The γ decay was detected with AGATA. From the measured γ -ray spectra the summed E 1 strength is extracted and compared to microscopic quasi-particle phonon model calculations. The trend of the E 1 strength with increasing neutron number is found to be fairly well reproduced with calculations that assume a rather complex structure of the 1$^−$ states (three-phonon states) inducing a strong fragmentation of the E 1 nuclear response below the neutron binding energy.