Approaching the pygmy dipole resonance in Sn isotopes with the Oslo method

The γ-ray strength functions (GSF) and nuclear level densities (NLD) below the neutron separation energy were extracted for 111−113,116−122,124 Sn from particle-γ coincidence data with the Oslo method in a model consistent way. Combined with results of a recent systematic Coulomb excitation study, the Oslo GSFs were used to extract bulk characteristics of the low-lying electric dipole response, commonly associated with the pygmy dipole resonance (PDR). A resonance-like peak on top of the tail of the isovector giant dipole resonance was observed in all of the isotopes. It is centered at 8 MeV and exhausts ≈2% of the classical Thomas-Reiche-Kuhn (TRK) sum. For the heaviest isotopes, the data suggest also a second peak located at ≈6.5 MeV, which might potentially be the isovector component of the PDR. At variance with microscopic calculations, no clear increase of the total low-lying E 1 strength with neutron excess was observed in the experimental data. The results also suggest that association of the full low-lying electric dipole strength with the PDR, especially in its neutron skin interpretation, might be unfounded.