A pygmy quadrupole resonance in the stable Sn isotopes
Abstract: An extensive experimental study of the recently predicted pygmy quadrupole res- onance (PQR) in the stable even-even Sn isotopes [1] will be presented. In this study, (α, αâ²Î³) and (γ, γâ²) experiments were performed on 124Sn [2] as well as life- time measurements in 112,114Sn using the recently established (p, pâ²Î³) Doppler-shift attenuation (DSA) coincidence technique [3]. In all experiments, J Ï = 2+ states be- low an excitation energy of 5 MeV were populated. The E2 strength integrated over the full transition densities could be extracted from the (γ, γâ²) and the (p, pâ²Î³) DSA experiments, while the (α, αâ²Î³) experiment at the chosen kinematics strongly favors the excitation of surface modes because of the strong α-particle absorption in the nuclear interior. The excitation of such modes is in accordance with the quadrupole- type oscillation of the neutron skin predicted by a microscopic approach based on self-consistent density functional theory and the quasiparticle-phonon model (QPM). The newly determined γ-decay branching ratios hint at a non-statistical character of the E2 strength, as it has also been recently pointed out for the case of the pygmy dipole resonance (PDR). This allows us to distinguish between PQR-type and multiphonon excitations and, consequently, supports the recent first experi- mental indications of a PQR in 124Sn [2, 4].