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

The structure of C-19,C-20,C-22 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 C-19 and C-20 and two-neutron removal from C-22. Two-neutron removal from C-20 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 CA-1 and, following one-neutron emission after one-neutron removal, the mass A - 2 two-neutron removal residues CA-2. The smaller contributions of direct two-neutron removal, that populate the CA-2 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 C18-21. These suggest significant nu s(1/2)(2) valence neutron configurations in both C-20 and C-22. The results for C-22 strongly support the picture of C-22 as a two-neutron halo nucleus with a dominant nu s(1/2)(2) ground-state configuration.