Fine structure of the pygmy quadrupole resonance in ${}^{112,114}$Sn isotopes

Abstract

The electric quadrupole response in ${}^{112,114}$Sn isotopes is investigated by energy-density functional (EDF) and three-phonon quasiparticle-phonon model (QPM) theory with special emphasis on 2${}^{+}$ excitations located above the first collective quadrupole state and below 5 MeV. Additional quadrupole strength clustering as a sequence of states similar to the recently observed pygmy quadrupole resonance in ${}^{124}$Sn is found. The spectral distributions and transition densities of these 2${}^{+}$ states show special features being compatible with oscillations of a neutron skin against the isospin-symmetric nuclear core. Furthermore, two new ($p$, $p^{\prime }\gamma$) Doppler-shift attenuation (DSA) coincidence experiments were performed at the SONIC@HORUS setup. Quadrupole states with excitation energies up to 4.2 MeV were populated in ${}^{112,114}$Sn. Lifetimes and branching ratios were measured allowing for the determination of the reduced quadrupole transition strengths to the ground state. A stringent comparison of the new data to EDF+QPM theory in ${}^{112}$Sn and ${}^{114}$Sn isotopes hints at the occurrence of a low-energy quadrupole mode of unique character which could be interpreted as pygmy quadrupole resonance.

Comments: 6 pages, 4 figures, 1 table