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arXiv · 1105.3599

Evolution of the pygmy dipole resonance in Sn isotopes

H. K. Toft·A. C. Larsen·A. Bürger·M. Guttormsen·A. Görgen·H. T. Nyhus·T. Renstrøm·S. Siem·G. M. Tveten·A. Voinov
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Abstract

Nuclear level density and $γ$-ray strength functions of $^{121,122}$Sn below the neutron separation energy are extracted with the Oslo method using the ($^3$He,$^3$He$^\primeγ$) and ($^3$He,$αγ$) 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_γ\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 $\approx1.8^{+1}_{-5}%$ 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.

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H. K. Toft, A. C. Larsen, A. Bürger, M. Guttormsen, A. Görgen, H. T. Nyhus, T. Renstrøm, S. Siem, G. M. Tveten, A. Voinov. 2011-05-18. Evolution of the pygmy dipole resonance in Sn isotopes. https://doi.org/10.1103/physrevc.83.044320

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