Low-energy limit of the radiative dipole strength in nuclei

We explain the low-energy anomaly reported in several experimental studies of the radiative dipole strength functions in medium-mass nuclei. These strength functions at very low gamma energies correspond to the gamma transitions between very close nuclear excited states in the quasicontinuum and attract an increasing interest because of their substantial astrophysical impact. We show that the low-energy enhancement of the strength functions in highly excited compound nuclei is explained by nucleonic transitions from the thermally unblocked single-quasiparticle states to the single-(quasi) particle continuum. The case of radiative dipole strength functions at the nuclear excitation energies typical for the thermal neutron capture is illustrated for Mo-94,Mo-96,Mo-98 and Sn-116,Sn-122 in comparison to available data.