First Measurement of the Quadrupole Moment of the 2_{1}^{+} State in ^{110}Sn

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(E2;4_{1}^{+}→2_{1}^{+}) and B(E2;4_{2}^{+}→2_{1}^{+}) values for ^{110}Sn, as well as the B(E2;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)  efm^{2} for ^{110}Sn, is the largest positive value known among the Sn isotopes, indicating an oblate shape of the state by more than 2σ. Comparison of the E2 transition strengths and quadrupole moments with recent shell model calculations are presented.