One- and two-neutron removal reactions from the most neutron-rich carbon isotopes

The structure of ^19,20,22C has been investigated using high-energy (around 240 MeV/nucleon) one- and two-neutron removal reactions on a carbon target. Measurements were made of the inclusive cross sections and momentum distributions for the charged residues. Narrow momentum distributions were observed for one-neutron removal from ¹⁹C and ²⁰C and two-neutron removal from ²²C. Two-neutron removal from ²⁰C resulted in a relatively broad momentum distribution. The results are compared with eikonal-model calculations combined with shell-model structure information. The neutron removal cross sections and associated momentum distributions are calculated for transitions to both the particle-bound and particle-unbound final states. The calculations take into account the population of the mass A-1 reaction residues ^A-1C and, following one-neutron emission after one-neutron removal, the mass A-2 two-neutron removal residues ^A-2C. The smaller contributions of direct two-neutron removal, that populate the ^A-2C residues in a single step, are also computed. The data and calculations are shown to be in good overall agreement and consistent with the predicted shell-model ground-state configurations and one-neutron overlaps with low-lying states in ^18-21C. These suggest significant νs_1/2² valence neutron configurations in both ²⁰C and ²²C. The results for ²²C strongly support the picture of ²²C as a two-neutron halo nucleus with a dominant νs_1/2² ground-state configuration.