Abstract
The electric quadrupole response in Sn-112,Sn-114 isotopes is
investigated by energy-density functional (EDF) and three-phonon
quasiparticle-phonon model (QPM) theory with special emphasis on
electric quadrupole excitations located above the first collective 2(+)
state and below 5 MeV. Additional quadrupole strength clustering as a
sequence of states similar to the recently observed pygmy quadrupole
resonance in Sn-124 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 (p, p' gamma) Doppler-shift attenuation
(DSA) coincidence experiments were performed at the SONIC@HORUS setup.
2(+) states with excitation energies up to 4.2 MeV were populated in
Sn-112,Sn-114. Lifetimes and branching ratios were measured allowing for
the determination of E2 transition strengths to the ground state. A
stringent comparison of the new data to EDF+QPM theory in Sn-112 and
Sn-114 isotopes hints at the occurrence of a low-energy quadrupole mode
of unique character which could be interpreted as pygmy quadrupole
resonance. (C) 2019 Elsevier B.V. All rights reserved.
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