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

Abstract 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, ( α , α ′ γ ) and ( γ , γ ′ ) experiments were performed on 124 Sn. In both reactions, J π = 2 + states below an excitation energy of 5 MeV were populated. The E 2 strength integrated over the full transition densities could be extracted from the ( γ , γ ′ ) experiment, while the ( α , α ′ γ ) 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 E 2 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 124 Sn.