Lifetime measurements of first excited states in C16,18

The electric quadrupole transition from the first 2+ state to the ground 0+ state in C18 was studied through a lifetime measurement by an upgraded recoil shadow method applied to inelastically scattered radioactive C18 nuclei. The measured mean lifetime is 18.9±0.9(stat)±4.4(syst) ps, corresponding to a B(E2;21+→0g.s.+) value of 4.3±0.2±1.0e2 fm4, or about 1.5 Weisskopf units. The mean lifetime of the first 2+ state in C16 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. These transition strengths are hindered compared to the empirical transition strengths, indicating that the anomalous hindrance observed in C16 persists in C18.