Excited-state transition-rate measurements in18C

Abstract

Excited states in ${}^{18}$C were populated by the one-proton knockout reaction of an intermediate energy radioactive ${}^{19}$N beam. The lifetime of the first ${2}^{+}$ state was measured with the K\"oln/NSCL plunger via the recoil distance method to be $\ensuremath{\tau}({2}_{1}^{+})=22.4\ifmmode\pm\else\textpm\fi{}0.9{(\mathrm{stat})}_{\ensuremath{-}2.2}^{+3.3}(\mathrm{syst})$ ps, which corresponds to a reduced quadrupole transition strength of $B(E2;{2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})=3.{64}_{\ensuremath{-}0.14}^{+0.15}{(\mathrm{stat})}_{\ensuremath{-}0.47}^{+0.40}(\mathrm{syst})$ e${}^{2}$fm${}^{4}$. In addition, an upper limit on the lifetime of a higher-lying state feeding the ${2}_{1}^{+}$ state was measured to be $\ensuremath{\tau}<4.6$ 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.