  The electric quadrupole transition from the first 2+ state to the ground 0+
state in 18C was studied through lifetime measurement by an upgraded recoil
shadow method applied to inelastically scattered radioactive 18C nuclei. The
measured mean lifetime is 18.9 +/- 0.9 (stat) +/- 4.4 (syst) ps, corresponding
to a B(E2;2+ -> 0+) value of 4.3 +/- 0.2 +/- 1.0 e2fm4, or about 1.5 Weisskopf
units. The mean lifetime of the first 2+ state in 16C was remeasured to be 18.0
+/- 1.6 +/- 4.7 ps, about four times shorter than the value reported
previously. The discrepancy between the two results was resolved by
incorporating the gamma-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 16C persists in 18C.
