RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-1. Nuclear Physics

Shape coexistence along N = 40 studied with isomer and beta decay
F. Recchia,∗1 S. Riccetto,∗2 K. Wimmer,∗3,∗4 S.M. Lenzi,∗1 T. Davinson,∗5 A. Estrade,∗6,∗4 C.J. Griﬃn,∗5
S. Nishimura,∗4 V. Phong,∗4 P.-A. Söderström,∗4 O. Aktas,∗7 M. Alaqueel,∗8,∗9 T. Ando,∗3 H. Baba,∗4
S. Bae,∗10 S. Choi,∗10 P. Doornenbal,∗4 J. Ha,∗10,∗4 L. Harkness-Brennan,∗8 T. Isobe,∗4 P. John,∗1D. Kahl,∗5 G. Kiss,∗4 M. Labiche,∗11 K. Matsui,∗4 S. Momiyama,∗3 D. Napoli,∗12 M. Niikura,∗3 C. Nita,∗13Y. Saito,∗4 H. Sakurai,∗4,∗3 P. Schrock,∗3 T. Sumikama,∗4 C. Stahl,∗14 V. Werner,∗14 and W. Witt∗14
In 68 Ni the presence of a high-lying 2+ state withsmall transition probability to the ground state is a
result of the N = 40 harmonic oscillator shell gap between the f p shell and the g9/2 orbital. This shell gap
is reduced as protons are removed in Fe and Cr isotopes1) . Collective behavior is caused by quadrupole
correlations which favour energetically the deformed
intruder states involving the neutron g9/2 and d5/2 orbitals and proton excitations across the Z=28 subshellgap2) leading to rather low-lying ﬁrst 2+ states andlarge B(E2) values.
Limited experimental data is still available for thelow spin states of the region of deformation that develops south of 68 Ni. The trend of the ratio E4+ /E2+
towards N = 40 in Cr isotopes suggests a transition
from spherical (at N = 32) to deformed shapes, thatapproach better the gamma-unstable regime than theaxially deformed one, while Fe isotopes lie at the O(6)limit from N = 30 to N = 423) . To better understandthe structure of these nuclei, the knowledge of otherstates at low excitation energy is needed.
The large diﬀerence in angular momentum between
the p1/2 , f5/2 and d5/2 , g9/2 orbitals around the Fermi
surface in N ≈ 40 nuclei leads to the occurrence ofseveral isomeric states. In the Cr and Ti nuclei with
N = 39 and N = 41 similar conﬁgurations should also
lead to long-lived states. Observation of isomers at N
= 39 will allow us to draw conclusions on the location and evolution of intruder orbitals towards 60 Ca.
Theoretical and experimental investigations show thatthe collective behavior observed in 64 Cr, with its small
E(2+ ) energy and large B(E2) value, is restored approaching 60 Ca2,4) .
In this report we present some preliminary experimental results. Several new gamma transitions deexciting isomeric states, as well as states populated inFig. 1. Ions produced, identiﬁed and implanted into theAIDA active stopper for (top) the 64 Cr setting, (bottom) the 60 Ti setting.

the beta decay have been identiﬁed for the ﬁrst time.
The determination of beta decay half-lives in this region is of relevance for a better understanding of ther-process nucleosynthesis.
A high intensity 238 U beam provided by the RIKEN
Nishina Center Accelerator Complex impinging on a
Be target was used to produce the nuclides of interest in in-ﬂight ﬁssion. In the experiment the EURICA
gamma-ray array surrounded the implantation detector AIDA into which the fragments of interest wereimplanted. The fragments were identiﬁed using the BigRIPS separator employing the ΔE-ToF-Bρ method.
Figure 1 shows the particle identiﬁcation plots of thefragments using this technique for the 60 Ti and the64
Cr settings. The analysis of the experimental dataon the β-decays in 64 Cr region and the isomers decay
is in progress.

∗1

Department of Physics and Astronomy, University andINFN Padova∗2 Department of Physics, University and INFN Perugia∗3 Department of Physics, University of Tokyo∗4 RIKEN Nishina Center∗5 School of Physics and Astronomy, University of Edinburgh∗6 Department of Physics, Central Michigan University∗7 Department of Physics, KTH Stockholm∗8 Oliver Lodge Laboratory, The University of Liverpool∗9 Imam Muhammad Ibn Saud Islamic University of Riyadh∗10 Department of Physics, Seoul National University∗11 STFC Daresbury Laboratory∗12 INFN Legnaro (Padova)∗13 IFIN Horia Hulubei Bucharest∗14 Institut für Kernphysik, Technische Universität Darmstadt

References
1) Y. Tsunoda et al. J. Phys. Conf. Ser. 445, 012028(2013).
2) S.M.Lenzi et al. Phys.Rev. C 82, 054301 (2010).
3) J. Kotila and S.M. Lenzi, Phys.Rev. C 89, 064304(2014).
4) A. Gade et al., Phys. Rev. Lett. 112 (2014) 112503.

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