The pygmy quadrupole resonance and neutron-skin modes in Sn-124

We present an extensive experimental study of the recently predicted pygmy quadrupole resonance (PQR) in Sn isotopes, where complementary probes were used. In this study, (alpha,alpha 'gamma) and (gamma,gamma ') experiments were performed on Sn-124. In both reactions, J(pi)=2(+) states below an excitation energy of 5 MeV were populated. The E2 strength integrated over the full transition densities could be extracted from the (gamma,gamma ') experiment, while the (alpha, alpha 'gamma) experiment at the chosen kinematics strongly favors the excitation of surface modes because of the strong alpha-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 gamma-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 experimental indications of a PQR in Sn-124. (C) 2015 The Authors. Published by Elsevier B.V