First Measurement of the Quadrupole Moment of the 2₁^+ State in ^110Sn

The Sn isotopic chain, exhibiting double shell closures at 100 Sn and 132 Sn , is a key testing ground for theoretical models of the atomic nucleus. It was originally predicted that the transitional matrix elements between the first 2+ state and the 0+ ground state for the even-even isotopes in this chain should show a simple dependence on the neutron number. This prediction was, however, disproven experimentally in some of the first experiments with postaccelerated radioactive beams, a situation that has remained unresolved ever since. Subsequent theoretical work has suggested that the explanation can be found in proton excitations across the 𝑍 =50 shell gap, with an accompanying experimental signature that the first excited 2+ state in 110 Sn should have a distinct oblate shape. In this Letter, we present the first measurements of the spectroscopic quadrupole moment of the 2+1 state, 𝐵⁡(𝐸⁢2;4+1→2+1) and 𝐵⁡(𝐸⁢2;4+2→2+1) values for 110 Sn , as well as the 𝐵⁡(𝐸⁢2;2+1→0+1) value with significantly improved precision compared to previous results. From the same experiment, half-lives of the 2+1 and 4+1 states were measured using the Doppler shift attenuation method. Our combined result, 𝑄⁡(2+1)=20⁢(8) efm2 for 110 Sn , is the largest positive value known among the Sn isotopes, indicating an oblate shape of the state by more than 2⁢𝜎. Comparison of the 𝐸⁢2 transition strengths and quadrupole moments with recent shell model calculations are presented.