Evolution of the pygmy dipole resonance in Sn isotopes

Nuclear level density and gamma-ray strength functions of Sn-121,Sn-122 below the neutron separation energy are extracted with the Oslo method using the (He-3, He-3'gamma) and (He-3,alpha gamma) reactions. The level densities of Sn-121,Sn-122 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-gamma greater than or similar to 5.2 MeV. This enhancement is compatible with pygmy resonances centered at approximate to 8.4(1) and approximate to 8.6(2) MeV, respectively, and with integrated strengths corresponding to approximate to 1.8(-5)(+1)% of the classical Thomas-Reiche-Kuhn sum rule. Similar resonances were also seen in Sn116-119. 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 Sn116-122 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.