1. Persistence of the Z=28 shell gap in A=75 isobars: Identification of a possible (1/2−) μs isomer in Co75 and β decay to Ni75
- Author
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Hiroki Nishibata, A. I. Morales, Hirofumi Watanabe, Shin-Ichiro Nishimura, Megumi Niikura, Daisuke Suzuki, H. S. Jung, V. Werner, Ryo Taniuchi, D. Sohler, D. R. Napoli, Jan Taprogge, T. Isobe, N. Kurz, I. Kojouharov, Toshiyuki Sumikama, Jinguang Wu, P. A. Söderström, Ayumi Yagi, I. Matea, P. Morfouace, P. R. John, S. Franchoo, Shigeru Kubono, S. Escrig, F. Naqvi, Alfredo Poves, Atsuko Odahara, K. Matsui, Giuseppe Lorusso, H. Baba, Hiroyoshi Sakurai, Zhengyu Xu, Z. H. Li, E. Sahin, P. Doornenbal, D. Mengoni, Zs. Vajta, H. Schaffner, K. Yoshinaga, I. Stefan, F. Browne, and Guillaume Gey
- Subjects
Physics ,Proton ,010308 nuclear & particles physics ,Center (category theory) ,Nuclear structure ,Nuclear isomer ,7. Clean energy ,01 natural sciences ,Spectral line ,Nuclear physics ,Nucleosynthesis ,Excited state ,0103 physical sciences ,Isobar ,010306 general physics - Abstract
Background: The evolution of shell structure around doubly-magic exotic nuclei is of great interest in nuclear physics and astrophysics. In the `south-west' region of $^{78}$Ni, the development of deformation might trigger a major shift in our understanding of explosive nucleosynthesis. To this end, new spectroscopic information on key close-lying nuclei is very valuable. Purpose: We intend to measure the isomeric and $\beta$ decay of $^{75}$Co, with one proton- and two neutron-holes relative to $^{78}$Ni, to access new nuclear structure information in $^{75}$Co and its $\beta$-decay daughters $^{75}$Ni and $^{74}$Ni. Methods: The nucleus $^{75}$Co is produced in relativistic in-flight fission reactions of $^{238}$U at the Radioactive Ion Beam Factory (RIBF) in the RIKEN Nishina Center. Its isomeric and $\beta$ decay are studied exploiting the BigRIPS and EURICA setups. Results: We obtain partial $\beta$-decay spectra for $^{75}$Ni and $^{74}$Ni, and report a new isomeric transition in $^{75}$Co. The energy ($E_{\gamma}=1914(2)$ keV) and half-life ($t_{1/2}=13(6)$ $\mu$s) of the delayed $\gamma$ ray lend support for the existence of a $J^{\pi}=(1/2^-)$ isomeric state at 1914(2) keV. A comparison with PFSDG-U shell-model calculations provides good account for the observed states in $^{75}$Ni, but the first calculated $1/2^-$ level in $^{75}$Co, a prolate $K=1/2$ state, is predicted about 1 MeV below the observed $(1/2^-)$ level. Conclusions: The spherical-like structure of the lowest-lying excited states in $^{75}$Ni is proved. In the case of $^{75}$Co, the results suggest that the dominance of the spherical configurations over the deformed ones might be stronger than expected below $^{78}$Ni. Further experimental efforts to discern the nature of the $J^{\pi}=(1/2^-)$ isomer are necessary.
- Published
- 2021
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