One-proton and one-neutron knockout reactions from $N=Z=28\phantom{\rule{0.16em}{0ex}}^{56}\mathrm{Ni}$ to the $A=55$ mirror pair $^{55}\mathrm{Co}$ and $^{55}\mathrm{Ni}$

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One-proton and one-neutron knockout reactions from $N = Z = 28 {}^{56}{Ni}$ to the $A = 55$ mirror pair ${}^{55}{Co}$ and ${}^{55}{Ni}$

M. Spieker, A. Gade, D. Weisshaar, B. A. Brown, J. A. Tostevin, B. Longfellow, P. Adrich, D. Bazin, M. A. Bentley, J. R. Brown, C. M. Campbell, C. Aa. Diget, B. Elman, T. Glasmacher, M. Hill, B. Pritychenko, A. Ratkiewicz, and D. Rhodes

Phys. Rev. C 99, 051304(R) – Published 22 May 2019

Abstract

We present a high-resolution in-beam $γ$ -ray spectroscopy study of excited states in the mirror nuclei ${}^{55}{Co}$ and ${}^{55}{Ni}$ following one-nucleon knockout from a projectile beam of ${}^{56}{Ni}$ . The newly determined partial cross sections and the $γ$ -decay properties of excited states provide a test of state-of-the-art nuclear structure models and probe mirror symmetry in unique ways. The new experimental data are compared to large-scale shell-model calculations in the full $p f$ space which include charge-dependent contributions. A mirror asymmetry for the partial cross sections leading to the two lowest $3 / 2^{-}$ states in the $A = 55$ mirror pair was identified as well as a significant difference in the $E 1$ decays from the $1 / 2_{1}^{+}$ state to the same two $3 / 2^{-}$ states. The mirror asymmetry in the partial cross sections cannot be reconciled with the present shell-model picture or small mixing introduced in a two-state model. The observed mirror asymmetry in the $E 1$ decay pattern, however, points at stronger mixing between the two lowest $3 / 2^{-}$ states in ${}^{55}{Co}$ than in its mirror ${}^{55}{Ni}$ .

Received 25 March 2019

DOI:https://doi.org/10.1103/PhysRevC.99.051304

Physics Subject Headings (PhySH)

Direct reactions Nuclear reactions Nuclear structure & decays Shell model Unstable nuclei induced nuclear reactions

Nuclear Physics

Authors & Affiliations

M. Spieker^1,*, A. Gade^1,2, D. Weisshaar¹, B. A. Brown^1,2, J. A. Tostevin³, B. Longfellow^1,2, P. Adrich^1,†, D. Bazin^1,2, M. A. Bentley⁴, J. R. Brown⁴, C. M. Campbell^1,2,‡, C. Aa. Diget⁴, B. Elman^1,2, T. Glasmacher^1,2, M. Hill^1,2, B. Pritychenko⁵, A. Ratkiewicz^1,2,§, and D. Rhodes^1,2

¹National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824, USA
²Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA
³Department of Physics, Faculty of Engineering and Physical Sciences, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom
⁴Department of Physics, University of York, Heslington, York YO10 5DD, United Kingdom
⁵National Nuclear Data Center, Brookhaven National Laboratory, Upton, New York 11973, USA

^*spieker@nscl.msu.edu
^†Present address: National Centre for Nuclear Research, Otwock, Poland.
^‡Present address: Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
^§Present address: Lawrence Livermore National Laboratory, Livermore, California, 94550, USA.

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Vol. 99, Iss. 5 — May 2019

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Figure 1
In-beam $γ$ -ray singles spectra for ${}^{55}{Ni}$ (top) and ${}^{55}{Co}$ (bottom) in black compared to $γ$ -ray spectra obtained from a geant4 simulation (red). Observed transitions are marked with dashed vertical lines and their corresponding transition energies. Also shown are the fit residuals $Δ$ (red) in combination with the $1 σ$ confidence level (black) in the lower panels. The background structures between 400 and 800 keV, seen on top of the smooth background, are caused by $γ$ rays emitted from stopped components and taken into account in the simulation.
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Figure 2
Level scheme observed for ${}^{55}{Co}$ and ${}^{55}{Ni}$ . All transitions visible in Fig. 1 with the exception of the 1352 keV ( ${}^{55}{Co}$ ) transition are placed. The color coding is the same as in Fig. 3. The bottom panel shows the calculated MED for the two $3 / 2^{-}$ and the $5 / 2_{1}^{-}$ , $1 / 2_{2}^{-}$ , $7 / 2_{2}^{-}$ , and $(9 / 2^{-}, 11 / 2^{-})$ states (solid circles) as well as the $1 / 2^{+}$ and $3 / 2^{+}$ states (solid diamonds) in comparison to the shell-model results (open circles, $p f$ states only). Even though the transition intensities seen in Fig. 1 are comparable, the MED for the excited $J^{π} = 5 / 2_{1}^{-}, 1 / 2_{2}^{-}, 7 / 2_{2}^{-}$ , and $(9 / 2^{-}, 11 / 2^{-})$ states are only tentatively assigned and, thus, shown in parentheses. Except for the $3 / 2^{+}$ state discussed in the text, $J^{π}$ assignments were adopted from Refs. [27, 36, 42].
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Figure 3
Partial cross sections $σ_{part}$ determined for (a) ${}^{55}{Ni}$ and (b) ${}^{55}{Co}$ in comparison to (c), (d) the theoretical cross sections. Only states predicted with $σ_{part} \geq 0.05$ mb are presented. The spectroscopic factors and excitation energies predicted for the $1 / 2^{+}$ and $3 / 2^{+}$ state have been taken from Ref. [36]. The location of the $3 / 2_{2}^{-}$ state is indicated by a red cross ( $σ_{part} \approx 0.01$ mb). In addition, the partial cross sections relative to the inclusive cross section $σ_{inc}$ are shown, see second axis. Only statistical uncertainties are given. No reduction factor $R_{s}$ has been applied for the comparison. See text for further details.
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Figure 4
Parallel momentum distributions measured for the $J^{π} = 3 / 2^{+}$ and $(9 / 2^{-}, 11 / 2^{-})$ states in ${}^{55}{Ni}$ . For the $J^{π} = 3 / 2^{+}$ state, the predictions of the eikonal theory calculated at a mid-target energy of 85.9 MeV/u and folded with the momentum distribution of the unreacted beam passing through the target are shown with dashed lines. The parallel momentum distribution of the 2882 keV state was obtained by gating on the high-energy part of the doublet seen in Fig. 1. For both distributions, background contributions were subtracted. Very similar distributions were observed for ${}^{55}{Co}$ . See text for further discussion.
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Physical Review C

covering nuclear physics

One-proton and one-neutron knockout reactions from $N = Z = 28 {}^{56}{Ni}$ to the $A = 55$ mirror pair ${}^{55}{Co}$ and ${}^{55}{Ni}$

M. Spieker, A. Gade, D. Weisshaar, B. A. Brown, J. A. Tostevin, B. Longfellow, P. Adrich, D. Bazin, M. A. Bentley, J. R. Brown, C. M. Campbell, C. Aa. Diget, B. Elman, T. Glasmacher, M. Hill, B. Pritychenko, A. Ratkiewicz, and D. Rhodes

Phys. Rev. C 99, 051304(R) – Published 22 May 2019

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Physical Review C

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One-proton and one-neutron knockout reactions from N=Z=28Ni56 to the A=55 mirror pair Co55 and Ni55

M. Spieker, A. Gade, D. Weisshaar, B. A. Brown, J. A. Tostevin, B. Longfellow, P. Adrich, D. Bazin, M. A. Bentley, J. R. Brown, C. M. Campbell, C. Aa. Diget, B. Elman, T. Glasmacher, M. Hill, B. Pritychenko, A. Ratkiewicz, and D. Rhodes

Phys. Rev. C 99, 051304(R) – Published 22 May 2019

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One-proton and one-neutron knockout reactions from $N = Z = 28 {}^{56}{Ni}$ to the $A = 55$ mirror pair ${}^{55}{Co}$ and ${}^{55}{Ni}$