Evolution of the pygmy dipole resonance in Sn isotopes

Abstract

Nuclear level density and $\ensuremath{\gamma}$-ray strength functions of $^{121,122}\mathrm{Sn}$ below the neutron separation energy are extracted with the Oslo method using the ($^{3}\mathrm{He}$,$^{3}\mathrm{He}$${}^{'}\ensuremath{\gamma}$) and ($^{3}\mathrm{He}$,$\ensuremath{\alpha}\ensuremath{\gamma}$) reactions. The level densities of $^{121,122}\mathrm{Sn}$ 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 ${E}_{\ensuremath{\gamma}}\ensuremath{\gtrsim}5.2$ MeV. This enhancement is compatible with pygmy resonances centered at $\ensuremath{\approx}8.4(1)$ and $\ensuremath{\approx}8.6(2)$ MeV, respectively, and with integrated strengths corresponding to $\ensuremath{\approx}1.{8}_{\ensuremath{-}5}^{+1}%$ of the classical Thomas-Reiche-Kuhn sum rule. Similar resonances were also seen in $^{116\ensuremath{-}119}\mathrm{Sn}$. 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 $^{116\ensuremath{-}122}\mathrm{Sn}$ 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.