Enhanced collectivity in 12 Be

Abstract 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 12 Be 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 12 Be 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 τ = 1.38 ± 0.10 (stat) ± 0.19 (sys) ps, which is about half the previously reported value at twice the precision. The reduced transition strength deduced from this result is B ( E 2 ; 2 1 + → 0 1 + ) = 14.2 ± 1.0 (stat) ± 2.0 (sys)  e 2 fm 4 , which supports the quenching of the N = 8 shell gap in 12 Be.