Excited-state transition-rate measurements in 18C.

Excited states in 18C were populated by the one-proton knockout reaction of an intermediate energy radioactive 19N beam. The lifetime of the first 2+ state was measured with the KöIn/NSCL plunger via the recoil distance method to be τ(21+) = 22.4 ± 0.9(stat)-2.2+3.3(syst) ps, which corresponds to a reduced quadrupole transition strength of B(E2; 21+ → 01+ = 3.64-0.14+0.15(stat)-0.47+0.40(syst) e²fm4. In addition, an upper limit on the lifetime of a higher-lying state feeding the 21+ state was measured to be τ < 4.6 ps. The results are compared to large-scale ab initio no-core shell model calculations using two accurate nucleon-nucleon interactions and the importance-truncation scheme. The comparison provides strong evidence that the inclusion of three-body forces is needed to describe the low-lying excited-state properties of this A = 18 system. [ABSTRACT FROM AUTHOR]