Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 51 (2018)

Shell evolution beyond Z = 28 and N = 50: spectroscopy of 81–84 Zn†
C. M. Shand,∗1 Zs. Podolyák,∗1 M. Górska,∗2 P. Doornenbal,∗3 A. Obertelli,∗3,∗4 F. Nowacki,∗5 T. Otsuka,∗6K. Sieja,∗5 J. A. Tostevin,∗1 and Y. Tsunoda∗6 for the SEASTAR collaboration
The Shell Evolution and Search for Two-plus states
At the RIBF (SEASTAR) experimental campaigns
were conducted at the Radioactive Isotope Beam Factory (RIBF). For the experiments a 238 U primary beam
was accelerated to 345 MeV/nucleon and subsequently
impinged onto a 3 mm thick 9 Be production target atthe entrance of the BigRIPS separator. Secondary ﬁssion beams of interest were then selected within BigRIPS using the Bρ-∆E-Bρ technique. The resultspresented here on neutron-rich Zn isotopes were obtained from settings centered on 79 Cu and 85 Ga in theﬁrst (2014) and second (2015) SEASTAR campaigns,
respectively.

ium isotopes present a typical case. Shape evolution
proceeds from a seniority level pattern in N = 82
semi-magic 144 Sm, to a vibrational pattern at N =86 in 148 Sm, and ﬁnally a rotational one at N = 92
in 154 Sm. At N = 84 146 Sm provides the transition
between the seniority and vibrational schemes. In thecase of Zn isotopes, with only two protons outside theZ = 28 shell, the situation is rather diﬀerent. As deduced from the present experiment for the ﬁrst time,
the proton-neutron correlations are strong enough fora rapid change from the semi-magic structure at N =50 to a collective structure at N = 52. This is partly
due to the weak Z = 28 sub-magic structure, whichis a consequence of the repulsive nature of the tensor
force between the proton f7/2 and the fully occupied
neutron g9/2 orbits.
The experimental results were compared to threestate-of-the- art shell-model calculations (see Fig. 2),
considering diﬀerent model spaces. They all correctlypredict that the 82, 84 Zn isotopes exhibit collective-likecharacter. The good agreement between experimentand theory suggests that breaking the 78 Ni core provides a signiﬁcant contribution to low-lying states beyond Z = 28 and N = 50.

Fig. 1. Doppler corrected γ-ray spectrum of 84 Zn.

The incoming ions were impinged on the liquid H2target of the MINOS device, while the resulting γ rays
were detected with the DALI2 high-eﬃciency NaI(Tl)array. Low-lying excited states in the neutron-rich
81–84
Zn isotopes have been investigated. The 4+
1 state+82
84
and4statesinZn(seeFig. 1)
in Zn and the 2+
1
1
were observed for the ﬁrst time. In addition, γ-ray
transition were identiﬁed in odd-mass 81, 83 Zn. Themain experimental conclusion of the work is that themagicity is conﬁned to neutron number N = 50 only,
as indicated by the increased R4/2 = E(4+ )/E(2+ ) ratios in 82, 84 Zn when compared to than in the neutronmagic 80 Zn nucleus.
A magic or semi-magic core can be distorted as valence nucleons are added to a closed shell. The samar†

∗1
∗2
∗3
∗4
∗5
∗6Fig. 2. Systematics of R4/2 = E(4+ )/E(2+ ) for Zn isotopes. The filled symbols are from this work. The
results of the Ni78-II,1) A3DA-m,2) and PFSDG-U3)shell-model calculations are also indicated. The line at
R4/2 = 2 indicates the vibrational limit.

References
Condensed from the article in Phys. Lett. B 737, 492 (2017)
Department of Physics, University of Surrey
GSI
RIKEN Nishina Center
IRFU, CEA, Saclay
IPHC, CNRS/IN2P3, Orsay
Center for Nuclear Study, University of Tokyo

1) K. Sieja, T. R. Rodriguez, K. Kolos, D. Verney, Phys.
Rev. C 88, 034327 (2013).
2) Y. Tsunoda, T. Otsuka, N. Shimizu, M. Honma, Y. Utsuno, Phys. Rev. C 89, 031301 (2014).
3) F. Nowacki, A. Poves, E. Caurier, B. Bounthong, Phys.
Rev. Lett. 117, 272501 (2016).

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