I. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Mapping of a new deformation region around 62 Ti†
S. Michimasa,∗1 M. Kobayashi,∗1 Y. Kiyokawa,∗1 S. Ota,∗1 R. Yokoyama,∗2 D. Nishimura,∗3 D. S. Ahn,∗4
H. Baba,∗4 G. P. A. Berg,∗5 M. Dozono,∗1 N. Fukuda,∗4 T. Furuno,∗6 E. Ideguchi,∗7 N. Inabe,∗4 T. Kawabata,∗6
S. Kawase,∗8 K. Kisamori,∗1 K. Kobayashi,∗9 T. Kubo,∗10,∗11 Y. Kubota,∗4 C. S. Lee,∗1 M. Matsushita,∗1H. Miya,∗1 A. Mizukami,∗12 H. Nagakura,∗9 H. Oikawa,∗12 H. Sakai,∗4 Y. Shimizu,∗4 A. Stolz,∗11 H. Suzuki,∗4M. Takaki,∗1 H. Takeda,∗4 S. Takeuchi,∗13 H. Tokieda,∗1 T. Uesaka,∗6 K. Yako,∗1 Y. Yamaguchi,∗1Y. Yanagisawa,∗4 K. Yoshida,∗2 and S. Shimoura∗1Condensed from the article in Phys. Rev. Lett. 125, 122501(2020)∗1 Center for Nuclear Study, The University of Tokyo∗2 Dept. of Physics & Astronomy, University of Tennessee∗3 Dept. of Physics, Tokyo City University∗4 RIKEN Nishina Center∗5 Dept. of Physics, University of Notre Dame
∗6 Dept. of Physics, Kyoto University∗7 RCNP, Osaka University∗8 Dept. of Advanced Energy Engineering Sciences, Kyushu University∗9 Dept. of Physics, Rikkyo University∗10 FRIB, MSU∗11 NSCL, MSU∗12 Dept. of Physics, Tokyo University of Science∗13 Dept. of Physics, Tokyo Institute of Technology
†

14LZU

12S2n (MeV)

The mass of atomic nuclei is a fundamental quantity
as it reﬂects the sum of all interactions within the nucleus, which is a quantum many-body system comprisedof two kinds of fermions, protons and neutrons. Changes
in the shell structures of nuclei far from stability can bedirectly probed by mass measurements.
In the neutron-rich Cr and Ti region, the shell evolution around the ls-closed neutron number of 40 has
attracted considerable attention in recent years. The
onset of island of inversion (IoI), which was discoveredaround 32 Mg1) for the ﬁrst time, was theoretically predicted along the N = 40 isotones.2) The IoI is well characterized by the emergence of the Jahn-Teller (JT) stabilization,3,4) which is promoted by conﬁguration mixing
on the Fermi surface. The goal of the present experiment was to conﬁrm the presence or absence of thiseﬀect through the ﬁrst mass measurements of neutronrich Ti isotopes around N = 40.
The experiment was performed at the RI Beam Factory (RIBF) at RIKEN, which is operated by RIKEN
Nishina Center and Center for Nuclear Study, University of Tokyo. The masses were measured directly by using the TOF-Bρ technique. Neutron-rich isotopes wereproduced by fragmentation of a 70 Zn primary beam at345 MeV/nucleon in a 9 Be target. The fragments wereseparated by the BigRIPS separator,5) and transported
in the High-Resolution Beamline to the SHARAQ spectrometer.6)
Figure 1 shows the present results of the twoneutron separation energy (S2n ) of Sc, Ti, and V isotopes, together with theoretical S2n systematics obtained from the macroscopic-microscopic WeizsäckerSkyrme-type formula with treatments of two radial basis
functions corrections (LZU).9) This model largely reproduces the S2n trends including the present results. How-V

1023

8226TiSc

2143234
36
3840
Neutron Number NFig. 1. Two-neutron separation energies (S2n ) of Sc, Ti, andV isotopes around N = 40. Closed symbols indicate values determined from the present experimental masses,
and open symbols are literature values.7,8) . Solid lines
connect isotopes, and dashed lines show theoretical predictions by the LZU model.9)

ever, it is obvious that the model underestimates the S2nvalues in 61, 62 Ti isotopes. The results, therefore, conﬁrm that 62 Ti becomes very stable.
Since the behavior of the mass surface is similar tothat in the N = 20 IoI, it is reasonably demonstrated
that the JT stabilization arises in the vicinity of 62 Ti.
The theory4) suggests that this enhancement of the JTstabilization around 62 Ti is caused by conﬁguration mixing among neutron f pgd orbitals enhanced by the degeneracy of the neutron orbitals.
References

1) E. K. Warburton, J. A. Becker, B. A. Brown, Phys. Rev.
C 41, 1147 (1990).
2) B. A. Brown, Prog. Part. Nucl. Phys. 47, 517 (2001).
3) H. A. Jahn, E. Teller, Proc. Poy. Soc. A161, 220 (1937).
4) P. -G. Reinhard, E. W. Otten, Nucl. Phys. A420, 173(1984).
5) T. Kubo, Nucl. Instrum. Methods Phys. Res. B 204, 97(2003).
6) T. Uesaka et al., Prog. Theor. Exp. Phys. 2012, 03C007(2012).
7) M. Wang et al., Chin. Phys. C 41, 030003 (2017).
8) Z. Meisel et al., Phys. Rev. C 101, 052801(R) (2020).
9) N. -N. Ma et al., Chin. Phys. C 43, 044105 (2019).

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