We have measured the longitudinal momentum distribution of ${}^{18}\mathrm{C}$, ${}^{17}\mathrm{C}$, and ${}^{16}\mathrm{C}$ after the one-neutron breakup of ${}_{}{}^{19}{}_{}{}^{}C$, ${}^{18}\mathrm{C}$, and ${}^{17}\mathrm{C}$, respectively, using the A1200 fragment separator as an energy-loss spectrometer. The observed narrow width of $44.3\pm 5.9\mathrm{MeV}/c$ for the ${}^{18}\mathrm{C}$ fragments indicates that ${}^{19}\mathrm{C}$ is a new example of a one-neutron halo, so far the heaviest ever observed. The consequences of the obtained results on the structure of the three isotopes ${}^{19,18,17}\mathrm{C}$ are discussed together with shell-model calculations.

• Received 27 December 1994

DOI:https://doi.org/10.1103/PhysRevLett.74.3569