Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 51 (2018)

Persistence of the Z = 28 shell gap around 78 Ni: first spectroscopy of79Cu†L. Olivier,∗1 S. Franchoo,∗1 M. Niikura,∗2 Z. Vajta,∗3 D. Sohler,∗3 P. Doornenbal,∗4 A. Obertelli,∗4,∗5Y. Tsunoda,∗6 T. Otsuka,∗2,∗4,∗6 G. Authelet,∗5 H. Baba,∗4 D. Calvet,∗5 F. Château,∗5 A. Corsi,∗5 A. Delbart,∗5J.-M. Gheller,∗5 A. Gillibert,∗5 T. Isobe,∗4 V. Lapoux,∗5 M. Matsushita,∗7 S. Momiyama,∗2 T. Motobayashi,∗4H. Otsu,∗4 C. Péron,∗5 A. Peyaud,∗5 E. Pollacco,∗5 J.-Y. Roussé,∗5 H. Sakurai,∗2,∗4 C. Santamaria,∗4,∗5M. Sasano,∗4 Y. Shiga,∗4,∗8 S. Takeuchi,∗4 R. Taniuchi,∗2,∗4 T. Uesaka,∗4 H. Wang,∗4 K. Yoneda,∗4 F. Browne,∗9L. Chung,∗10 Z. Dombradi,∗3 F. Flavigny,∗1 F. Giacoppo,∗11 A. Gottardo,∗1 K. Hadyńska-Kle˛ k,∗11 Z. Korkulu,∗3S. Koyama,∗2 Y. Kubota,∗4,∗7 J. Lee,∗12 M. Lettmann,∗13 C. Louchart,∗13 R. Lozeva,∗14 K. Matsui,∗2,∗4T. Miyazaki,∗2 S. Nishimura,∗4 K. Ogata,∗15 S. Ota,∗7 Z. Patel,∗16 E. Sahin,∗11 C. Shand,∗16 P.-A. Söderström,∗4I. Stefan,∗1 D. Steppenbeck,∗7 T. Sumikama,∗17,∗4 D. Suzuki,∗1 V. Werner,∗13 J. Wu,∗4,∗18 and Z. Xu∗12sity of 12 pnA was sent on a 3-mm thick 9 Be target.
The secondary 80 Zn beam, with an average intensityof 260 particles per second, was selected in the BigRips
separator. The isotopes before and after the secondary
target placed at the end of BigRips were identiﬁed
event-by-event in the BigRips and ZeroDegree spectrometers, respectively, with the tof-Bρ-∆E method.
The detector set-up installed between the two spectrometers was composed of the Minos device mounted
inside the Dali-2 γ-ray multidetector. Minos consistedof a liquid-hydrogen target of 102 mm long surrounded
by a cylindrical time-projection chamber (TPC). The79
Cu nucleus was produced mainly through proton
knock-out from the incoming 80 Zn isotopes, the (p, 3p)
channel contributing with 8%. The emitted protons
were tracked in the TPC, while the beam trajectory
was given by two parallel-plate avalanche counters before the target. The interaction vertex was reconstructed with 95% eﬃciency and 5-mm uncertainty
FWHM along the beam axis. The Dali-2 array, comprising 186 NaI scintillator crystals, yielded a photopeak eﬃciency with add-back of 27% and an energy
resolution of σ = 45 keV for a 1 MeV transition emitted at 250 MeV per nucleon.
We carried out the ﬁrst spectroscopy of 79 Cu andcompared the results with MCSM calculations. The
calculations show the restoration of the single-particle
nature of the low-lying states, which is supported by
the experiment. There is no signiﬁcant knock-out feeding to the excited states below 2.2 MeV, indicating that
the Z = 28 gap remains large. The ability to describe
the 79 Cu nucleus as a valence proton outside a 78 Ni
core presents us with indirect evidence of the magic
character of the latter.

The shell model constitutes one of the main building blocks of our understanding of nuclear structure.
Its robustness is well proven for nuclei close to the valley of stability, where it successfully predicts and explains the occurrence of magic numbers. However, themagic numbers are not universal throughout the nuclear chart and their evolution far from stability, observed experimentally over the last decades, has generated much interest. With 28 protons and 50 neutrons,
the 78 Ni nucleus is expected to be one of the mostneutron-rich doubly magic nuclei. Up to now, no evidence has been found for the disappearance of the shellclosures at Z = 28 and N = 50, even if recent studies hint at a possible weakening of the N = 50 magic
number below 78 Ni. The half-life of 78 Ni was determined at 122.2(5.1) ms, rather suggesting a survival ofmagicity, and calculations predict a ﬁrst excited statein 78 Ni above 2 MeV. But so far no information on thespectroscopy of 78 Ni is available and the behavior ofthe πf7/2 orbital, of primary importance as one of theorbitals deﬁning the Z = 28 gap, is elusive. Access tothis hole state is possible through proton transfer orknock-out reactions.
In our experiment performed at the Radioactive Isotope Beam Factory (RIBF), a 238 U beam with an energy of 345 MeV per nucleon and an average inten†

Condensed from Phys. Rev. Lett. 119, 192501 (2017)
IPN, CNRS and University of Paris-Saclay∗2 Department of Physics, University of Tokyo∗3 Institute for Nuclear Research, MTA Atomki∗4 RIKEN Nishina Center∗5 Irfu, CEA and University of Paris-Saclay∗6 CNS, University of Tokyo∗7 CNS, University of Tokyo∗8 Department of Physics, Rikkyo University∗9 School of Computing Engineering and Mathematics, University of Brighton∗10 INST, Vinatom∗11 Department of Physics, University of Oslo
∗12 Department of Physics, University of Hong Kong
∗13 IKP, Technical University of Darmstadt∗14 IPHC and University of Strasbourg∗15 RCNP, Osaka University∗16 Department of Physics, University of Surrey∗17 Department of Physics, Tohoku University∗18 State Key Laboratory of Nuclear Physics and Technology,
University of Peking∗1-1-