Anomalous B(E2) transition of neutron-rich isotopes 16,18C

The electric quadrupole transition from the first 2+ state to the ground 0+ state in 18C was studied through a lifetime measurement by a recoil shadow method. The measured mean lifetime was 18.9±0.9±4.4(syst) ps, corresponding to a B(E2;21+ → 0g.s+) value of 4.3±0.2±1.0 e2 fm4. The mean lifetime of the first 2+ state in 16C was remeasured to be 18.3±1.4±4.8 ps, about four times shorter than the value reported previously. The discrepancy between the two results was explained by incorporating the γ-ray angular distribution measured in this work into the previous measurement. The anomalous B(E2) observed in 16C clearly persists in 18C, since both B(E2) values of 16C and of 18C are four times smaller than the value expected from the global systematics of B(E2). (Y. Kazumata)