Evolution of the pygmy dipole resonance in Sn isotopes

Nuclear level density and {gamma}-ray strength functions of {sup 121,122}Sn below the neutron separation energy are extracted with the Oslo method using the ({sup 3}He,{sup 3}He{sup '{gamma}}) and ({sup 3}He,{alpha}{gamma}) reactions. The level densities of {sup 121,122}Sn 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{sub {gamma}} > or approx. 5.2 MeV. This enhancement is compatible with pygmy resonances centered at {approx_equal}8.4(1) and {approx_equal}8.6(2) MeV, respectively, and with integrated strengths corresponding to {approx_equal}1.8{sub -5}{sup +1}% of the classical Thomas-Reiche-Kuhn sum rule. Similar resonances were also seen in {sup 116-119}Sn. 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 {sup 116-122}Sn 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.