Open Access

Mark-Christoph Spieker, Nadja Tsoneva, Vera Derya, Janis Endres, Deniz Savran, Muhsin N. Harakeh, Sotirios Harissopulos, Rolf-Dietmar Herzberg, Anastasios Lagoyannis, Horst Lenske, Norbert Pietralla, Lucia Popescu, Marcus Scheck, Friederike Schlüter, Kerstin Sonnabend, Victor Ionuț Stoica, Heinrich Johannes Wörtche, Andreas Zilges

• 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 124Sn. In both reactions, Jπ = 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 (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 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 124Sn.