Reaction cross sections (σR) for C19, C20 and the drip-line nucleus C22 on a liquid hydrogen target have been measured at around 40AMeV by a transmission method. A large enhancement of σR for C22 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 C22 was deduced to be 5.4±0.9fm. 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 C22 preferentially occupy the 1s1/2 orbital.
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