Multistep Coulomb excitation of $^{64}\mathrm{Ni}$: Shape coexistence and nature of low-spin excitations

Multistep Coulomb excitation of ${}^{64}{Ni}$ : Shape coexistence and nature of low-spin excitations

D. Little, A. D. Ayangeakaa, R. V. F. Janssens, S. Zhu, Y. Tsunoda, T. Otsuka, B. A. Brown, M. P. Carpenter, A. Gade, D. Rhodes, C. R. Hoffman, F. G. Kondev, T. Lauritsen, D. Seweryniak, J. Wu, J. Henderson, C. Y. Wu, P. Chowdhury, P. C. Bender, A. M. Forney, and W. B. Walters

Phys. Rev. C 106, 044313 – Published 14 October 2022

Abstract

The structure of ${}^{64}{Ni}$ , the heaviest stable Ni isotope, has been investigated via high-statistics, multistep safe Coulomb excitation to search for shape coexistence, a phenomenon recently observed in neutron-rich ${}^{66}{Ni}$ and ${}^{70}{Ni}$ as well as in doubly magic, $N = 40, {}^{68}{Ni}$ . The study was motivated by recent, state-of-the-art Monte Carlo shell-model calculations (MCSM), where a Hamiltonian with effective interactions incorporating the monopole tensor force predicts the existence of shape coexistence, also in the lower-mass ${}^{62, 64}{Ni}$ isotopes. A set of transition and static $E 2$ matrix elements for both yrast and near-yrast structures was extracted from the differential Coulomb excitation cross sections. From comparisons between the new results and MCSM as well as other shell-model calculations, a clearer picture of the structure of ${}^{64}{Ni}$ emerges. Specifically, the low-spin states are shown to be dominated by proton and neutron excitations mainly within the $f p$ shell, with minimal contribution from the $g_{9 / 2}$ shape-driving neutron orbital. The agreement between experimental data and MCSM results indicates a small oblate deformation for the $0_{2}^{+}$ level and a spherical shape for the $0_{3}^{+}$ state. In addition, the small upper limit determined for the $B (E 2)$ probability of a transition associated with the decay of the recently observed 3463-keV, $0_{4}^{+}$ state agrees with its proposed assignment to a prolate shape, herewith providing first evidence for triple shape coexistence in a stable Ni isotope.

Received 30 June 2022
Accepted 28 September 2022

DOI:https://doi.org/10.1103/PhysRevC.106.044313

Physics Subject Headings (PhySH)

Coulomb excitation Electromagnetic transitions Nuclear structure & decays Nucleon distribution Shell model

59 ≤ A ≤ 89

Nuclear Physics

Authors & Affiliations

D. Little^1,2, A. D. Ayangeakaa ^1,2,*, R. V. F. Janssens ^1,2,†, S. Zhu^3,‡, Y. Tsunoda ^4,5, T. Otsuka^6,7,8, B. A. Brown^9,10, M. P. Carpenter ¹¹, A. Gade^9,10, D. Rhodes^9,10,§, C. R. Hoffman¹¹, F. G. Kondev ¹¹, T. Lauritsen¹¹, D. Seweryniak¹¹, J. Wu¹¹, J. Henderson¹², C. Y. Wu ¹², P. Chowdhury¹³, P. C. Bender ¹³, A. M. Forney ^14,¶, and W. B. Walters ¹⁴

¹Department of Physics and Astronomy, University of North Carolina Chapel Hill, Chapel Hill, North Carolina 27599, USA
²Triangle Universities Nuclear Laboratory, Duke University, Durham, North Carolina 27708, USA
³National Nuclear Data Center, Brookhaven National Laboratory, Upton, New York 11973, USA
⁴Center for Computational Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan
⁵Center for Nuclear Study, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
⁶Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
⁷RIKEN Nishina Center, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
⁸KU Leuven, Instituut voor Kern- en Stralingsfysica, 3000 Leuven, Belgium
⁹National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824, USA
¹⁰Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA
¹¹Physics Division, Argonne National Laboratory, Argonne, Illinois 60439, USA
¹²Lawrence Livermore National Laboratory, Livermore, California 94550, USA
¹³Department of Physics, University of Massachusetts Lowell, Lowell, Massachusetts 01854, USA
¹⁴Department of Chemistry and Biochemistry, University of Maryland College Park, College Park, Maryland 20742, USA

^*ayangeak@unc.edu
^†rvfj@email.unc.edu
^‡Deceased.
^§Present address: TRIUMF, 4004 Wesbrook Mall, Vancouver, Canada BC V6T 2A3.
^¶Present address: Sensors and Electromagnetic Applications Laboratory, Georgia Tech Research Institute, Atlanta, Georgia 30318.

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Vol. 106, Iss. 4 — October 2022

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Physical Review C

covering nuclear physics

Multistep Coulomb excitation of ${}^{64}{Ni}$ : Shape coexistence and nature of low-spin excitations

D. Little, A. D. Ayangeakaa, R. V. F. Janssens, S. Zhu, Y. Tsunoda, T. Otsuka, B. A. Brown, M. P. Carpenter, A. Gade, D. Rhodes, C. R. Hoffman, F. G. Kondev, T. Lauritsen, D. Seweryniak, J. Wu, J. Henderson, C. Y. Wu, P. Chowdhury, P. C. Bender, A. M. Forney, and W. B. Walters

Phys. Rev. C 106, 044313 – Published 14 October 2022

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Physical Review C

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Multistep Coulomb excitation of Ni64: Shape coexistence and nature of low-spin excitations

D. Little, A. D. Ayangeakaa, R. V. F. Janssens, S. Zhu, Y. Tsunoda, T. Otsuka, B. A. Brown, M. P. Carpenter, A. Gade, D. Rhodes, C. R. Hoffman, F. G. Kondev, T. Lauritsen, D. Seweryniak, J. Wu, J. Henderson, C. Y. Wu, P. Chowdhury, P. C. Bender, A. M. Forney, and W. B. Walters

Phys. Rev. C 106, 044313 – Published 14 October 2022

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Multistep Coulomb excitation of ${}^{64}{Ni}$ : Shape coexistence and nature of low-spin excitations