Evolution of the pygmy dipole resonance in Sn isotopes

Abstract

Nuclear level density and ${\hat{I}}^3$-ray strength functions of ${}^{121,122}$Sn below the neutron separation energy are extracted with the Oslo method using the (${}^3$He,${}^3$He${}^{\prime }{\hat{I}}^3$) and (${}^3$He,$\hat{I}\pm {\hat{I}}^3$) reactions. The level densities of ${}^{121,122}$Sn display step-like 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 $E_{\hat{I}}^3\gtrsim 5.2$ MeV. This enhancement is compatible with pygmy resonances centered at $\approx 8.4(1)$ and $\approx 8.6(2)$ MeV, respectively, and with integrated strengths corresponding to $\approx 1.8_{-5}^{+1}$ of the classical Thomas-Reiche-Kuhn sum rule. Similar resonances were also seen in ${}^{116-119}$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 ${}^{116-122}$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.