Lifetime Measurement of the $21+$ state in $20C$

The mean lifetime of the $21+$ state in the near drip‐line nuclide $20C$ was measured for the first time using the Recoil Distance Method with intermediate energy radioactive beams via a knockout reaction. The measured value of $τ21+ = 9.8±2.8(stat)−1.1+0.5(syst)$ ps gives a reduced transition probability of $B(E2;21+→0g.s.+) = 7.51.7+3.0(stat) e2fm4.$ This value is in good agreement with shell model calculations using a p—sd shell model space and isospin‐dependent (N—Z) effective charges.