Isospin Character of the Pygmy Dipole Resonance in ${}^{124}\mathrm{Sn}$

The pygmy dipole resonance has been studied in the proton-magic nucleus ${}^{124}\mathrm{Sn}$ with the ($\alpha$, ${\alpha }^{\prime }\gamma$) coincidence method at $E_{\alpha }=136\mathrm{MeV}$. The comparison with results of photon-scattering experiments reveals a splitting into two components with different structure: one group of states which is excited in ($\alpha$, ${\alpha }^{\prime }\gamma$) as well as in ($\gamma$, ${\gamma }^{\prime }$) reactions and a group of states at higher energies which is only excited in ($\gamma$, ${\gamma }^{\prime }$) reactions. Calculations with the self-consistent relativistic quasiparticle time-blocking approximation and the quasiparticle phonon model are in qualitative agreement with the experimental results and predict a low-lying isoscalar component dominated by neutron-skin oscillations and a higher-lying more isovector component on the tail of the giant dipole resonance.

Figure 1

Summed $\gamma$-ray signals of all HPGe detectors after applying the $\alpha \mathrm{\text{−}}\gamma$-coincidence condition for the ground-state decays. The PDR is clearly visible at energies above 5 MeV.

Figure 2

(a) Singles cross section for the excitation of the $J^{\pi }=1^{-}$ states in ${}^{124}\mathrm{Sn}$ obtained in the ($\alpha$, ${\alpha }^{\prime }\gamma$) coincidence experiment. The solid line shows the energy-dependent experimental sensitivity limit. (b) $B(E1)\uparrow$ strength distribution measured with the ($\gamma$, ${\gamma }^{\prime }$) reaction. The middle column shows the QPM transition probabilities in ${}^{124}\mathrm{Sn}$ for the isoscalar (c) and electromagnetic (d) dipole operators. The RQTBA strength functions in ${}^{124}\mathrm{Sn}$ for the isoscalar and electromagnetic dipole operators are shown in (e) and (f), respectively.

Figure 3

(a) Differential cross section obtained from the ${}^{124}\mathrm{Sn}(\alpha ,{\alpha }^{\prime }\gamma )$ experiment integrated to bins with a width of 100 keV. (b) Energy integrated cross section measured in ${}^{124}\mathrm{Sn}(\gamma ,{\gamma }^{\prime })$ integrated to bins with a width of 100 keV.

Figure 4

Electromagnetic $I_1(r)$ (top) and isoscalar $I_3(r)$ (bottom) radial integrals for two particular RQTBA states—solid lines. Contribution from neutrons (protons) is shown by dashed (dot-dashed) lines. Vertical dotted line is plotted at the nuclear surface $R_0=1.24A^{1/3}$. See text for details.