Observation of a Large Reaction Cross Section in the Drip-Line Nucleus 22C

Reaction cross sections ({sigma}{sub R}) for {sup 19}C, {sup 20}C and the drip-line nucleus {sup 22}C on a liquid hydrogen target have been measured at around 40A MeV by a transmission method. A large enhancement of {sigma}{sub R} for {sup 22}C compared to those for neighboring C isotopes was observed. Using a finite-range Glauber calculation under an optical-limit approximation the rms matter radius of {sup 22}C was deduced to be 5.4+-0.9 fm. It does not follow the systematic behavior of radii in carbon isotopes with N<=14, suggesting a neutron halo. It was found by an analysis based on a few-body Glauber calculation that the two-valence neutrons in {sup 22}C preferentially occupy the 1s{sub 1/2} orbital.