Structure of the pygmy dipole resonance in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mrow/><mml:mn>124</mml:mn></mml:msup></mml:math>Sn

Background: In atomic nuclei, a concentration of electric dipole strength around the particle threshold, commonly denoted as pygmy dipole resonance, may have a significant impact on nuclear structure properties and astrophysical scenarios. A clear identification of these states and the structure of this resonance is still under discussion.Purpose: We present an experimental and theoretical study of the isospin character of the pygmy dipole resonance and investigation of a splitting of the electric dipole strength previously observed in experiments on $N=82$ nuclei.Method: The pygmy dipole resonance has been studied in the semi-magic $Z=50$ nucleus ${}^{124}$Sn by means of the $(\ensuremath{\alpha},{\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{\gamma})$ coincidence method at ${E}_{\ensuremath{\alpha}}=136\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$ using the Big-Bite Spectrometer at the Kernfysisch Versneller Instituut in Groningen, The Netherlands.Results: A splitting of the low-energy part of the electric dipole strength was identified in ${}^{124}$Sn by comparing the differential cross sections measured in $(\ensuremath{\alpha},{\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{\gamma})$ to results stemming from $(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})$ photon-scattering experiments. While an energetically lower-lying group of states is observed in both kinds of experiments, a higher-lying group of states is only excited in the $(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})$ reaction. In addition, theoretical calculations using the self-consistent relativistic quasiparticle time-blocking approximation and the quasiparticle-phonon model have been performed. Both calculations show a qualitative agreement with the experimental data and predict a low-lying isoscalar component that is dominated by neutron-skin oscillations as expected for the pygmy dipole resonance. Furthermore, the states at higher energies show a pronounced isovector component and a different radial dependence of the corresponding transition densities as expected for the tail of the giant dipole resonance.Conclusions: An experimental signature of the neutron-skin oscillation of the pygmy dipole resonance has been corroborated. The combination of the presented reactions might make it possible to identify states of this resonance.

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