Lifetime measurements of first excited states in ^{16,18}C

The electric quadrupole transition from the first 2(+) state to the ground 0(+) state in C-18 was studied through a lifetime measurement by an upgraded recoil shadow method applied to inelastically scattered radioactive C-18 nuclei. The measured mean lifetime is 18.9 +/- 0.9(stat) +/- 4.4(syst) ps, corresponding to a B(E2;2(1)(+)-> 0(g.s.)(+)) value of 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 between the two results was explained 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 C-16 persists in C-18.