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Two-neutron transfer reaction mechanisms in 12C(6He,4He)14C using a realistic three-body 6He model

D. Smalley et al.
Phys. Rev. C 89, 024602 – Published 6 February 2014
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  • INTRODUCTION
  • EXPERIMENTAL SETUP
  • THEORY
  • RESULTS AND DISCUSSION
  • CONCLUSION
  • ACKNOWLEDGEMENTS
    • References

    Abstract

    The reaction mechanisms of the two-neutron transfer reaction 12C(6He,4He) have been studied at Elab=30 MeV at the TRIUMF ISAC-II facility using the Silicon Highly-segmented Array for Reactions and Coulex (SHARC) charged-particle detector array. Optical potential parameters have been extracted from the analysis of the elastic scattering angular distribution. The new potential has been applied to the study of the transfer angular distribution to the 22+ 8.32 MeV state in 14C, using a realistic three-body 6He model and advanced shell-model calculations for the carbon structure, allowing to calculate the relative contributions of the simultaneous and sequential two-neutron transfer. The reaction model provides a good description of the 30-MeV data set and shows that the simultaneous process is the dominant transfer mechanism. Sensitivity tests of optical potential parameters show that the final results can be considerably affected by the choice of optical potentials. A reanalysis of data measured previously at Elab=18 MeV, however, is not as well described by the same reaction model, suggesting that one needs to include higher-order effects in the reaction mechanism.

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    • Received 3 December 2013

    DOI:https://doi.org/10.1103/PhysRevC.89.024602

    ©2014 American Physical Society

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    Vol. 89, Iss. 2 — February 2014

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    Images

    • Figure 1
      Figure 1

      Laboratory angular coverage of SHARC for this experiment. The areas of detection are the downstream end cap (DCD) at 7∘<θlab<27∘, the downstream box (DBx) at 32∘<θlab<71∘, the upstream box (UBx) at 103∘<θlab<145∘, and the partial upstream end cap (UCD) at 148∘<θlab<172∘. One of the DCD detectors was single-sided, hence the single row of pixels along the θlab direction at 135∘<ϕlab<140∘, which depicts the center ϕlab angle used for reconstruction (the coverage was 100∘<ϕlab<175∘).

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    • Figure 2
      Figure 2

      Particle identification for the DCD. The ΔE-E plot shows the energy loss in the thin 80-μm detector (ΔE) plotted against the energy loss in the thick 1-mm detector (E). The nuclei unambiguously identified are shown in the figure.

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    • Figure 3
      Figure 3

      The level scheme adopted for this two-neutron transfer study. The solid line shows the pathway for simultaneous transfer. The dotted lines show the multiple pathways taken into account in the sequential transfer. The total transfer calculation then takes the coherent sum of all the pathways. The true level scheme is shown in (a) and the modified level scheme which was used is depicted in (b).

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    • Figure 4
      Figure 4

      The elastic and inelastic scattering energy vs. θlab kinematics. The DCD used the straightforward particle ID of the 6He ejectiles, while the DBx required coincident detection considerations (see text for details). The kinematic curves for 6He and 12C are shown in black and red, respectively, with strong population of the 0+ ground state and the 2+ 4.4 MeV state of 12C. The slight mismatch in energy of the 12C expected kinematics is due to increased energy loss from the higher Z and straggling in the carbon target.

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    • Figure 5
      Figure 5

      The elastic scattering angular distribution. The top figure shows 6He + 12C elastic scattering at Elab=30 MeV. The dotted red line is the theoretical cross section calculated using the optical model parameters for the 6Li + 12C at 30 MeV [39], while the solid line is the theoretical cross section calculated using the optical model parameters of 6He + 12C obtained from the fit to the current data set. The bottom figure shows the 6Li + 12C elastic scattering at Elab=30 MeV with the dotted red line being the optical model parameters for the 6Li + 12C at 30 MeV [39].

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    • Figure 6
      Figure 6

      The inelastic scattering to the 2+ 4.4 MeV state. An independent model of the transfer was performed with the elastic scattering optical potential described above (see text for details).

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    • Figure 7
      Figure 7

      The energy excitation spectrum for the two-neutron transfer in the DCD. Separation of the 22+ 8.32 MeV of 14C is clearly observed. A high density of bound states from 6 to 7.3 MeV is observed, but cannot be separated without 4He-γ coincidences. The 8.32 MeV state is located 200 keV above the neutron separation energy (Sn=8.1 MeV).

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    • Figure 8
      Figure 8

      Comparison of the angular distribution of the 12C(6He,4He)14C 2+ Ex=8.32 MeV state at Elab=30 MeV with the current model. The dotted red line is the simultaneous two-neutron transfer accounting for the three-body nature of 6He, the dashed blue line is the sequential two-step transfer accounting for the structure of 13C, and 14C and the solid black line is the coherent sum of the simultaneous and sequential transfer. For comparison, the dot-dashed black line is the simple dineutron model. See text for details.

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    • Figure 9
      Figure 9

      Comparison of the angular distribution of the 12C(6He,4He)14C 2+ Ex=8.32 MeV state at Elab=18 MeV with the current model. Refer to Fig. 8 for the description of the lines and the text for details.

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