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

Abstract

Reactions that involve the fast removal (knockout) of one and two neutrons from the neutron-rich carbon isotopes, $^{15\ensuremath{-}19}\mathrm{C}$, by light nuclear targets are studied within an eikonal reaction model. Shell model calculations are used to describe the excitation energy spectra and the structures of the carbon isotopes. The calculated one-neutron knockout cross sections from the $^{A}\mathrm{C}$ isotopes, to particle-bound configurations of the $^{A\ensuremath{-}1}\mathrm{C}$ residues, are in agreement with the available experimental data. The two-neutron removal cross sections, producing $^{A\ensuremath{-}2}\mathrm{C}$ residues, receive contributions from both the direct, single-step two-neutron knockout and the indirect mechanism, involving single-neutron removal strength to neutron-unbound excited states in the $^{A\ensuremath{-}1}\mathrm{C}$ system followed by neutron emission. The latter two-step reaction mechanism is shown to be dominant. The empirical odd-even staggering of the single-neutron separation energies along the carbon isotopic chain is reflected in the two-neutron removal data. This staggering and the magnitudes of the two-neutron removal cross sections are reproduced qualitatively by the theoretical calculations.