Evolution of the pygmy dipole resonance in Sn isotopes

Abstract

Nuclear level density and $\gamma$-ray strength functions of ${}^{121,122}$Sn below the neutron separation energy are extracted with the Oslo method using the (${}^3$He,${}^3$He${}^{\prime }\gamma$) and (${}^3$He,$\alpha \gamma$) 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 $E_{\gamma }\gtrsim 5.2$ 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.

References in corpus (13)

• Exotic modes of excitation in atomic nuclei far from stability
• Extraction of level density and gamma strength function from primary gamma spectra
• Investigation of Pygmy Dipole Resonances in the Tin Region
• Isospin Character of the Pygmy Dipole Resonance in 124Sn
• Analysis of possible systematic errors in the Oslo method
• Dipole states in stable and unstable nuclei
• Odd-even staggering of binding energies as a consequence of pairing and mean-field effects
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• Gamma-ray strength function and pygmy resonance in rare earth nuclei
• Level densities and $\gamma$-ray strength functions in Sn isotopes
• Evidence for the pair-breaking process in 116,117Sn
• Microscopic description of the pygmy and giant electric dipole resonances in stable Ca isotopes
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