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Spectroscopy of Na28: Shell evolution toward the drip line

A. Lepailleur et al.
Phys. Rev. C 92, 054309 – Published 16 November 2015
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  • INTRODUCTION
  • β-DECAY EXPERIMENT
  • IN-BEAM γ-RAY SPECTROSCOPY
  • DISCUSSION
  • SUMMARY
  • ACKNOWLEDGMENTS
    • References

    Abstract

    Excited states have been studied in Na28 using the β-decay of implanted Ne28 ions at the Grand Accélérateur National d'Ions Lourds/LISE as well as the in-beam γ-ray spectroscopy at the National Superconducting Cyclotron Laboratory/S800 facility. New states of positive (Jπ=3+,4+) and negative (Jπ=1−–5−) parity are proposed. The former arise from the coupling between 0d5/2 protons and 0d3/2 neutrons, while the latter are attributable to couplings of 0d5/2 protons with 1p3/2 or 0f7/2 neutrons. While the relative energies between the Jπ=1+–4+ states are well reproduced with the USDA interaction in the N=17 isotones, a progressive shift in the ground-state binding energy (by about 500 keV) is observed between F26 and Al30. This points to a possible change in the proton-neutron 0d5/2−0d3/2 effective interaction when moving from stability to the drip line. The presence of Jπ=1−–4− negative-parity states around 1.5 MeV as well as of a candidate for a Jπ=5− state around 2.5 MeV give further support to the collapse of the N=20 gap and to the inversion between the neutron 0f7/2 and 1p3/2 levels below Z=12. These features are discussed in the framework of shell-model and energy-density-functional calculations, leading to predicted negative-parity states in the low-energy spectra of the F26 and O25 nuclei.

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    • Received 26 March 2015
    • Revised 15 May 2015

    DOI:https://doi.org/10.1103/PhysRevC.92.054309

    ©2015 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Beta decayNuclear spin & parity
    1. Techniques
    Gamma ray spectroscopyNuclear density functional theoryShell model
    Nuclear Physics

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    Vol. 92, Iss. 5 — November 2015

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    Images

    • Figure 1
      Figure 1

      Neutron effective single-particle energies (ESPE) of the N=20 isotones between Z=8 and Z=20 based on the work of Ref. [14]. The slope of the lines corresponds to the strength of the proton-neutron monopole interactions. The red line, which has the largest slope, shows the effect of the strong d5/2−d3/2 proton-neutron monopole interaction when the proton d5/2 is filled.

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    • Figure 2
      Figure 2

      (Top) Schematic view of the experimental setup for the study of the β decay of Ne28. (Bottom) Identification of the nuclei produced in the experiment and implanted in the DSSSD through their energy loss and time of flight. The color code indicates the number of identified ions.

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    • Figure 3
      Figure 3

      β-gated γ-ray spectra obtained up to 40 ms (top spectrum) and between 120 and 1000 ms (bottom spectrum) after the implantation of a Ne28 nucleus. Identified γ rays are shown with different symbols. The new ones at 564(1) and 636(1) keV attributed to the decay of Ne28 are visible on the left-hand side of the top spectrum. Above 2 MeV the scale of the y axis of the spectrum changes, the number of counts should be read from the scale on the right.

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    • Figure 4
      Figure 4

      Experimental (top, blue) and simulated (bottom, red) β-gated γ-ray spectrum between 2.7 and 3.7 MeV for a time window of 40 ms. The γ line at 2714 keV is observed from the present experiment with a total intensity of 0.9%. The doublet of states of 3082 and 3087 keV, whose respective branching ratios of 1.3(3)% and 4.0(6)% were determined in Ref. [37], originate from the decay of Na28 to Mg28. The simulation reproduces remarkably well the amplitude of the three experimental peaks, as well as the background contribution using measured feeding intensities and decay branching ratios. It also shows how many counts would have been observed if the levels at 3286 and 3512 keV (and their corresponding γ transitions) existed with β feedings of 3σ weaker than the values of 1.3(3)% and 0.9(2)%, respectively, proposed in Ref. [33].

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    • Figure 5
      Figure 5

      β-gated γ-ray spectrum in coincidence with the 564(1)-keV transition.

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    • Figure 6
      Figure 6

      (Top) Schematic view of the experimental setup for the in-beam γ-ray spectroscopy of Na28 produced in a double-step fragmentation reaction. (Bottom) Event-by-event particle identification plot for reaction residues from the Mg31 secondary beam. The plot shows the energy loss versus the time of flight measured between scintillators before and after the target, corrected for the different trajectories of the ions determined by means of the two sets of drift chambers. The circle indicates the Mg30 one-neutron removal reaction residues as well as Na28.

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    • Figure 7
      Figure 7

      Doppler-reconstructed γ-ray energy spectrum obtained from the in-beam γ-decay spectroscopic study of Na28. The solid green line is the response obtained from the geant4 simulation of individual transitions (blue lines) and a continuous background (orange line) are considered. The inset shows the high-energy part of the spectrum with in particular the doublet of transitions at 2605 and 2650 keV.

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    • Figure 8
      Figure 8

      Doppler-reconstructed γ-ray energy spectrum gated on the 55-keV transition from the first excited state in Na28 in two different energy ranges. The blue line (top spectrum in each panel) shows the total spectrum for comparison. While most of the transitions are observed in coincidence with the 55 keV γ ray, the transitions of 1255 and 1481 keV are for instance clearly missing in the γ-gated spectrum.

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    • Figure 9
      Figure 9

      γ−γ coincidence spectra of Na28 obtained during the in-beam experiment when gating on several γ rays [from top to bottom 633, 403, 858, and (633,1298) keV].

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    • Figure 10
      Figure 10

      Experimental level scheme of Na28 derived by adding information from the β decay of Ne28 (left, blue) and from the in-beam γ-ray spectroscopy of Na28 (right, green). The thickness of each arrow represents the relative intensity of the transitions, normalized to the 2063-keV transition for the β decay and to the 636-keV transition for the in-beam experiment, respectively. The middle part shows transitions, among which the 2063-keV one, that were observed in the two experimental studies. The very right part displays shell-model predictions for negative-parity states using the WBA-M interaction, together with positive-parity states from the predictions of the USDA interaction. The energies and uncertainties of the levels are derived from the β decay when possible (Table 1), from the in-beam experiment (Table 2), or from a combination of the two experiments otherwise. Tentative spin assignments for the levels at 1749 and 1792 keV are discussed in the text.

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    • Figure 11
      Figure 11

      Comparison between the experimental binding energies of the Jπ=1+–4+ states in Al30 [29, 43] (red), Na28 [33, 43] and present work (green), and F26 [28, 31, 32, 44] (blue) and the USDA shell-model predictions [38] (in black, labeled with the ground state binding energy value for each nucleus). Experimental binding energies in keV are referenced to the calculated ground-state binding energy of each nucleus. Experimental binding energy uncertainties for the Al30,Na28, and F26 ground states are 14, 10, and about 130 keV, respectively.

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    • Figure 12
      Figure 12

      Effective single-particle energies of the 0d3/2,0f7/2, and 1p3/2 orbits calculated for nuclei with an N=16 core using two Skyrme functionals that implements tensor forces (solid line, Skxtb) or not (dashed line, Skx). The down bending of the unbound 1p3/2 orbital observed when reaching the drip line is attributable to the halolike structure of the nuclei. The results for the configuration-interaction (CI) model with the WBA-M interaction are shown for comparison with the solid circles at Z=8 and Z=14.

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