Evolution of the pygmy dipole resonance in Sn isotopes

Nuclear level density and γ-ray strength functions of 121,122Sn below the neutron separation energy are extracted with the Oslo method using the (³He,³He{}'{γ}$) and ($³He$,${α}{γ}$) reactions. The level densities of $121,122Sn$ display steplike structures, interpreted as signatures of neutron pair breaking. An enhancement in both strength functions, compared to standard models for radiative strength, is observed in our measurements for E_γ5.2 MeV. This enhancement is compatible with pygmy resonances centered at ≈8.4(1) and ≈8.6(2) MeV, respectively, and with integrated strengths corresponding to ≈1.8_-5⁺¹% of the classical Thomas-Reiche-Kuhn sum rule. Similar resonances were also seen in ^116-119Sn. Experimental neutron-capture cross reactions are well reproduced by our pygmy resonance predictions, while standard strength models are less successful. The evolution as a function of neutron number of the pygmy resonance in ^116-122Sn is described as a clear increase of centroid energy from 8.0(1) to 8.6(2) MeV, but with no observable difference in integrated strengths.