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

N. Kobayashi et al.
Phys. Rev. C 86, 054604 – Published 5 November 2012
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  • INTRODUCTION
  • EXPERIMENTAL SETUP AND DETAILS
  • EXPERIMENTAL RESULTS
  • REACTION ANALYSIS
  • ANALYSIS AND DISCUSSION
  • CONCLUSIONS
  • ACKNOWLEDGEMENTS
    • References

    Abstract

    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 νs1/22 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 νs1/22 ground-state configuration.

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    • Received 30 November 2011

    DOI:https://doi.org/10.1103/PhysRevC.86.054604

    ©2012 American Physical Society

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    Vol. 86, Iss. 5 — November 2012

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    Images

    • Figure 1
      Figure 1
      A schematic view of the layout of BigRIPS (F0–F8) and zero-degree spectrometer (ZDS) (F8–F11), where F1 through F11 represent each focal plane. The secondary carbon target was mounted at the achromatic focal plane F8. The 18,19,20C residues were identified using the ZDS.Reuse & Permissions
    • Figure 2
      Figure 2
      Particle-identification spectrum of (a) the 22C beam by BigRIPS, and (b) the residues in the ZDS after selecting 22C ions before the secondary target.Reuse & Permissions
    • Figure 3
      Figure 3
      Measured inclusive parallel momentum distribution of 18C, following one-neutron removal from 19C on a carbon target at 243 MeV/nucleon compared to the theoretical calculations. The solid line includes contributions from the 23+ and 31+ shell-model states of 18C, assumed bound (see also Table ). The dashed line shows the results when assuming that the 23+ and 31+ states are unbound. Here, and in Figs. 4, 5, 6, 7, the theoretical distributions have been convoluted with the experimental resolution and normalized to the measured inclusive cross section.Reuse & Permissions
    • Figure 4
      Figure 4
      Measured inclusive parallel momentum distribution of 19C, following one-neutron removal from 20C on a carbon target at 241 MeV/nucleon compared to the theoretical calculations. The solid curve assumes that only the 1/2+ shell-model ground-state transition (2s1/2 neutron removal) is bound. The long-dashed, short-dashed, and dotted-dashed curves result if one assumes that 0.5, 1.0, or 1.5 units of 1d5/2 spectroscopic strength also lead to bound final states.Reuse & Permissions
    • Figure 5
      Figure 5
      Measured inclusive parallel momentum distribution of 18C, following two-neutron removal from 20C on a carbon target at 241 MeV/nucleon compared to the theoretical calculations. The theoretical curves are the weighted sum of the exclusive calculations of the unbound 19C states (see text). Recoil effects associated with the neutron emission are included, assuming the most important contributions come from states with ɛ* of 1.0 MeV (dashed curve) and 2.0 MeV (solid curve).Reuse & Permissions
    • Figure 6
      Figure 6
      Measured inclusive parallel momentum distribution of 20C, following two-neutron removal from 22C on a carbon target at 240 MeV/nucleon compared to the theoretical calculations. The solid curve is the weighted sum of the exclusive calculations for the unbound 21C states (see text). The dashed and dotted-dashed curves show the contributions from knockout via the 1/21+ and 5/21+ unbound 21C intermediate states, respectively. The recoil broadening arising from neutron emission from these unbound intermediate states is folded in.Reuse & Permissions
    • Figure 7
      Figure 7
      Measured inclusive parallel momentum distribution of 20C, following two-neutron removal from 22C on a carbon target at 240 MeV/nucleon compared to the theoretical calculations. The theoretical curves are the inclusive cross sections calculated assuming 22C two-neutron separation energies S2n(22C) = 0.40 MeV (solid curve), 0.70 MeV (dashed curve), and 1.20 MeV (dotted-dashed curve). The curves include the recoil broadening arising from the neutron decay of the unbound 21C intermediate states.Reuse & Permissions
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