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, 15−19 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 C isotopes, to particle-bound configurations of the A−1 C residues, are in agreement with the available experimental data. The two-neutron removal cross sections, producing A−2 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−1 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.

One- and two-neutron removal reactions from19,20C with a proton target

“…For the ( 19 C, 18 C) reaction, the theoretical inclusive σ −1n is consistent with the observed σ −1n . This consistency is also shown in the reaction with the Be target [13]. However, for the ( 20 C, 19 C) reaction, the theoretical inclusive σ −1n is larger than the observed σ −1n by the factor of five.…”

“…where the C 2 S are the shell model spectroscopic factors and σ sp is the single-particle cross section calculated using the eikonal model assuming unit spectroscopic factor [13]. The quantum numbers of the removed neutron are denoted by nlj and S N eff is the effective separation energy of the neutron for N the given final state.…”

“…Since some nuclei in C isotopes ( 15,17,19 C) have relatively small one-neutron separation energies (S 1n ), the halo structure for the nuclei has been investigated. Interaction cross sections (σ I ), which are defined as the sum of reaction cross section for the change of proton and/or neutron number in the incident nucleus, have been measured at relativistic energies (around 950 A MeV) for [9][10][11][12][13][14][15][16][17][18][19][20] C [3].…”

“…[11,12]. The p // distributions for one and two-neutron removal reactions for [15][16][17][18][19] C with a C target have been theoretically investigated using an eikonal reaction model [13].…”

Direct reactions with exotic nuclei

“…The two neutron transfer has been widely used in both direct and indirect kinematics and is known to be sensitive to configuration transitions, such as shape transitions [17] or pairing in light [18,19] or heavy nuclei [20]. Two-nucleon knockout has been more recently introduced to probe correlations [21,22] and a dedicated formalism has been developed showing a strong sensitivity to two-body correlations of the intermediate-energy inclusive knockout [23,24]. At variance with transfer, the two-nucleon knockout probes only the spatial correlations.…”