Ⅰ. HIGHLIGHTS OF THE YEAR78

RIKEN Accel. Prog. Rep. 53

Ni revealed as a doubly magic stronghold against nucleardeformation†

R. Taniuchi,∗1,∗2 C. Santamaria,∗3,∗2 P. Doornenbal,∗2 A. Obertelli,∗3,∗2,∗4 and K. Yoneda∗2for SEASTAR2014 Collaboration

†∗1
∗2
∗3
∗4Counts / 80 keV

The doubly magic character of 78 Ni was investigated
for the ﬁrst time via the in-beam γ-ray spectroscopic
technique at the RIBF. Vital to the experiment wasa high intensity 238 U primary beam, accelerated bythe RIBF’s cyclotron complex and followed by the BigRIPS spectrometer1) for the production of the secondary cocktail beams. BigRIPS was centered on 79 Cu
and 80 Zn ions, which are bound by one and two protons
more than the target isotope 78 Ni. Excited states of78
Ni were populated by one- and two-proton knockout
reactions, respectively. To produce the excited statesin suﬃcient numbers, a new secondary target system,MINOS,2) was implemented. It consisted of a 10cm long liquid hydrogen target surrounded by a timeprojection chamber to trace outgoing protons following
the proton knockout reactions. Reaction residues weretagged by the ZeroDegree spectrometer1) installed behind MINOS. De-excitation γ rays emitted in prompt
coincidence with the secondary reactions were detectedby the large volume γ-ray scintillator array DALI2.3)
The γ-ray spectra of the (p, 2p) and (p, 3p) reactions
are shown in Fig. 1. The energies of the peaks indicatedin the ﬁgure were deduced by maximum-likelihood ﬁts
with simulated response functions. As a result, a clear
high-energy γ-ray transition observed at 2,600(33) keV
in the (p, 2p) reaction illustrates the persistence of thedoubly magic nature of 78 Ni. However, another transition at a similar energy, 2,910(43) keV observed in the(p, 3p) channel, questions the stability of the protonand neutron shell closures.
The obtained levels of the excited states have been
compared with several state-of-the-art calculations.
Two large-scale shell-model calculations, LSSM andMCSM, employing phenomenological interactions well
reproduced the experimentally observed transitions
just below 3 MeV as two excited 2+ states feeding the ground state independently. These theoretical predictions interpret the experimental result as
78
Ni having two coexisting shapes, spherical in theground-state band and prolate for the excited band,respectively. Other theoretical approaches, such as
the QRPA, Coupled-Cluster, and In-Medium SRG calculations, could solely reproduce the ﬁrst 2+ state at
2,600 keV as spherical shape. The absence of the second 2+ state found at 2,910 keV is understood as due

80

a 79Cu(p,2p)78NiMγ < 6583(10)60
1,103(14)40
1,540(30) 2,110(50) 2,600(33)Counts / 80 keV

200b 80Zn(p,3p)78Ni
Mγ < 41,067(17)202,910(43)

100

1,000

2,000

3,000

4,000

5,000
Energy (keV)

Fig. 1. Doppler-corrected γ-ray spectra of 78 Ni for the(p, 2p) and (p, 3p) channels. These histograms arefitted with simulated response functions (light bluelines) and double-exponential background curves (bluedashed line). The labels indicate the energies of therespective peaks. Solid response function lines had significance levels above 3σ, while the dotted ones did not.
The magenta line in each panel shows the summed curveof the simulated functions.

to the limitations of these models such that they do
not incorporate the deformed, or collective phenomena. This study suggests that the neutron-rich isotope
78
Ni cannot be envisaged as a simple doubly closedshell nucleus, but rather having a competing deformed
structure that prevails for more neutron-rich Ni isotopes and N = 50 isotones.
References

1) T. Kubo et al., Prog. Theor. Exp. Phys. 2012, 3C003(2012).
2) A. Obertelli et al., Eur. Phys. J. A 50, 8 (2014).
3) S. Takeuchi et al., Nucl. Instrum. Methods Phys. Res.
A 763, 596 (2014).

Condensed from the article in Nature 569, 53–58 (2019)
Department of Physics, The University of Tokyo
RIKEN Nishina Center
IRFU, CEA, Université Paris-Saclay
Institut für Kernphysik, Technische Universität Darmstadt-3-