Evolution of the pygmy dipole resonance in Sn isotopes

TL;DR

This study investigates the evolution of pygmy dipole resonances in Sn isotopes by measuring gamma-ray strength functions and level densities, revealing resonance enhancements and their systematic energy shift with neutron number.

Contribution

The paper provides new experimental data on pygmy resonances in Sn isotopes and demonstrates their evolution with neutron number, improving understanding of nuclear structure and radiative strength models.

Findings

Resonance enhancements observed at ~8.4-8.6 MeV in Sn isotopes.

Centroid energy of pygmy resonances increases with neutron number.

Standard models underestimate the resonance strengths.

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 $\approx1.8^{+1}_{-5}%$ 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.