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

Abstract

The structure of 19,20,22 C 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 19 C and 20 C and two-neutron removal from 22 C. Two-neutron removal from 20 C 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-1 C and, following one-neutron emission after one-neutron removal, the mass A -2 two-neutron removal residues A-2 C. The smaller contributions of direct two-neutron removal, that populate the A-2 C 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 18-21 C. These suggest significant s 2 1/2 valence neutron configurations in both 20 C and 22 C. The results for 22 C strongly support the picture of 22 C as a two-neutron halo nucleus with a dominant s 2 1/2 ground-state configuration.