Quadrupole collectivity in ⁴²Ca from low-energy Coulomb excitation with AGATA

A Coulomb-excitation experiment to study electromagnetic properties of 42Ca was performed using a 170-MeV calcium beam from the TANDEM XPU facility at INFN Laboratori Nazionali di Legnaro. γ rays from excited states in 42Ca were measured with the AGATA spectrometer. The magnitudes and relative signs of ten E2 matrix elements coupling six low-lying states in 42Ca, including the diagonal E2 matrix elements of 2+1 and 2+2 states, were determined using the least-squares code gosia. The obtained set of reduced E2 matrix elements was analyzed using the quadrupole sum rule method and yielded overall quadrupole deformation for 0+1,2 and 2+1,2 states, as well as triaxiality for 0+1,2 states, establishing the coexistence of a weakly deformed ground-state band and highly deformed slightly triaxial sideband in 42Ca. The experimental results were compared with the state-of-the-art large-scale shell-model and beyond-mean-field calculations, which reproduce well the general picture of shape coexistence in 42Ca.