Observation/confirmation of hindered E2 strength in 18C/16C

We have measured the lifetime of the first excited 2(+) state in 18 C using an upgraded recoil shadow method to determine the electric quadrupole transition. The measured mean lifetime is 18.9 +/- 0.9 (stat) +/- 4.4 (syst) ps, corresponding to B(E2; 2(1)(+) -> 0(gs)(+)) = 4.3 +/- 0.2 +/- 1.0 e(2) fm(4), or about 1.5 Weisskopf units. The mean lifetime of the first 2(+) state in C-16 was remeasured to be 18.3 +/- 1.4 +/- 4.8 ps, about four times shorter than the value reported previously. The discrepancy is explained by incorporating the gamma-ray angular distribution obtained in this work into the previous measurement. The observed transition strengths in C-16,C-18 are hindered compared to the empirical values, indicating that the anomalous E2 strength observed in C-16 persists in C-18.