First lifetime measurement of 21+ state in 12Be

The lifetime of the 21+ state at 2.1 MeV in 12Be has been measured using inelastic scattering of a 12Be beam at 43 MeV/nucleon with a gold target. Through the Doppler shift attenuation method, the mean-life of the 21+ state has been determined as 2.5±0.7 (stat) ±0.3 (syst) ps, which gives a B(E2;21+→0g.s.+) value of 4.9±1.3±0.5 in Weisskopf units. The result shows a large quadrupole strength in the ground state transition, providing further evidence on the disappearance of the N=8 magic number. The B(E2;21+→0g.s.+) value together with the deformation length measured by proton inelastic scattering yields a neutron quadrupole matrix element two times larger than those for 14C and 16O.