Quadrupole and octupole collectivity in doubly-magic ¹³²Sn

The vibrational first 2₁⁺ and 3₁^- states of the doubly-magic nucleus ¹³²Sn were excited via safe Coulomb excitation (CE) employing the recently commissioned HIE-ISOLDE accelerator at CERN in conjunction with the highly efficient MINIBALL array. The ¹³²Sn ions were accelerated to an energy of 5.5 MeV/nucleon and impinged on a ²⁰⁶Pb target. Dexciting γ rays from the first excited states of the target and the projectile nucleus were recorded in coincidence with scattered particles. The optimized beam energy, the high-energy resolution and good efficiency of the HPGe spectrometer provide a beneficial combination to master the challenging measurement characterized by small CE cross sections and excitation energies above 4 MeV. The reduced transition strengths were determined for 0_g.s.⁺ → 2₁⁺, 0_g.s.⁺ → 3₁^-, and 2₁⁺ → 3₁^- in ¹³²Sn. In the past first preliminary results for the B(E2;0_g.s.⁺ → 2₁⁺) value were obtained with an efficient BaF2 array at ORNL. The results on excited collective states in ¹³²Sn provide crucial information on cross shell configurations that are expected to be dominated by a strong proton contribution. State-of-the-art large-scale shell model calculations and new mean field predictions are compared to the results.