Enhanced collectivity in 12Be

Electromagnetic quadrupole transition strength is a sensitive probe of the evolution of the structure of nuclei, particularly the competition between collectivity and magicity. We have performed a new lifetime measurement of the $2_1^{+}$ state of ¹²Be to study the interplay of these phenomena. The lifetime was measured with the Doppler Shift Attenuation Method using the γ-ray detector GRETINA. Excited states of ¹²Be were produced via inelastic scattering at 55 MeV/nucleon, using several different targets to control for systematic uncertainties in the stopping powers. The lifetime is determined to be $\tau =1.38\pm 0.10\text{(stat)}\pm 0.19\text{(sys)}$ ps, which is about half the previously reported value at twice the precision. The reduced transition strength deduced from this result is $B(E2;2_1^{+}\to 0_1^{+})=14.2\pm 1.0\text{(stat)}\pm 2.0\text{(sys)}{e}^2{\text{fm}}^4$, which supports the quenching of the $N=8$ shell gap in ¹²Be.