I. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Three quasiparticle isomers in odd-even 159, 161 Pm: Calling for
modiﬁed spin-orbit interaction for the neutron-rich region†
R. Yokoyama,∗1,∗2 E. Ideguchi,∗3,∗8 G. Simpson,∗4 Mn. Tanaka,∗3 Y. Sun,∗5 C. -J. Lv,∗5 Y. -X. Liu,∗6L. -J. Wang,∗7 S. Nishimura,∗8 P. Doornenbal,∗8 G. Lorusso,∗8 P. -A. Söderström,∗8 Z. Y. Xu,∗11,∗2 J. Wu,∗10,∗8T. Sumikama,∗8 N. Aoi,∗3,∗8 H. Baba,∗8 F. L. Bello-Garrote,∗12 G. Benzoni,∗13 F. Browne,∗14,∗8 R. Daido,∗15
Y. Fang,∗15 N. Fukuda,∗8 A. Gottardo,∗16,∗17 G. Gey,∗18,∗8 S. Go,∗1 S. Inabe,∗8 T. Isobe,∗8 D. Kameda,∗8K. Kobayashi,∗19 M. Kobayashi,∗1 I. Kojouharov,∗20 T. Komatsubara,∗21,∗22 T. Kubo,∗8 N. Kurz,∗20 I. Kuzi,∗23
Z. Li,∗10 M. Matsushita,∗1 S. Michimasa,∗1 C. B. Moon,∗24 H. Nishibata,∗15 I. Nishizuka,∗9 A. Odahara,∗15Z. Patel,∗8,∗25 S. Rice,∗8,∗25 E. Sahin,∗12 H. Sakurai,∗8,∗11 H. Schaﬀner,∗26,∗8 L. Sinclair,∗26,∗8 H. Suzuki,∗8H. Takeda,∗8 J. Taprogge,∗27,∗28 Zs. Vajta,∗23 H. Watanabe,∗29,∗8 and A. Yagi∗15†

Condensed from the article in Phys. Rev. C 104, L021303(2021)∗1 Center for Nuclear Study, University of Tokyo∗2 Department of Physics and Astronomy, University of Tennessee, Knoxville∗3 Research Center for Nuclear Physics, Osaka University∗4 LPSC, Université Grenoble-Alpes, CNRS/IN2P3∗5 School of Physics and Astronomy, Shanghai Jiao Tong University∗6 School of Physics, Huzhou University∗7 School of Physical Science and Technology, Southwest University, Chongqing∗8 RIKEN Nishina Center∗9 Department of Physics, Tohoku University∗10 Department of Physics, Peking University∗11 Department of Physics, University of Tokyo∗12 Department of Physics, University of Oslo
∗13 INFN, Sezione di Milano∗14 School of Computing Engineering and Mathematics, University of Brighton∗15 Department of Physics, Osaka University∗16 Dipartimento di Fisica dell Universitá degli studi di Padova∗17 INFN, Laboratori Nazionali di Legnaro∗18 Institut Laue-Langevin, Grenoble∗19 Department of Physics, Rikkyo University∗20 GSI Helmholtzzentrum für Schwerionenforschung GmbH
∗21 Department of Physics, University of Tsukuba∗22 RISP, Institute for Basic Science, Daejeon∗23 Institute for Nuclear Research (ATOMKI)∗24 Department of Display Engineering, Hoseo University∗25 Department of Physics, University of Surrey∗26 Department of Physics, University of York∗27 Instituto de Estructura de la Materia, CSIC
∗28 Departamento de Fı́sica Teórica, Universidad Autónoma deMadrid∗29 IRCNPC, Beihang University- S8 -(a) 159Pm(b) 161Pm
p5/2[532]
n7/2[633]
n1/2[521]

2500

Excitation energy (keV)

Excitation energy (keV)

Nuclear properties of neutron-rich rare-earth nuclei
at Z ≈ 60 could be the possible key to answer oneof the longstanding astrophysical questions: the formation of the A ≈ 160 (rare-earth) peak observed in theelemental abundance distribution. From the nuclearstructure perspective, the formation of a peak in elemental abundance originates in extra stability of localnuclei. For example, in r-process, the neutron magic
number N = 82 and N = 126 are responsible for theprominent abundance peaks at A ≈ 130 and A ≈ 195,respectively. Mumpower et al. identiﬁed that the nuclear properties of nuclei at N ≈ 100 are critical to
the rare-earth peak formation.1) In the case of the deformed rare-earth nuclei, a large shell gap between Nilsson single-particle orbitals stabilizes the nuclear shape

2000
1500
1000
500

(17/2+)exp.

PSM PSM
17/2+ 13/2+
p5/2[532]
n7/2[633]
n5/2[523]
6201/2-[521]
7/2+[633]

2500
2000
1500
1000
500

98

5/2-[523]
proton neutronp5/2[532]
n7/2[633]
n1/2[521](13/2+)
exp.

PSM
17/2+

p5/2[532]
PSM n7/2[633]13/2+
n5/2[523]
100

1/2-[521]
7/2+[633]6205/2-[523]proton neutron

Fig. 1. Excitation energies of the isomers in (a) 159 Pm and(b) 161 Pm compared with the PSM calculation with themodiﬁed Nilsson parameters.

at large deformation. Several theoretical works havepredicted the existence of the deformed shell gaps atN ≈ 100.2) Recently, a 6− ν(5/2[523] ⊗ 7/2[633]) state
was discovered in 162 Gd98 ,3) indicating the large shell
gap at N = 98.
The spectroscopic study on neutron-rich rare-earth
nuclei was performed at RIBF using the in-ﬂight ﬁssion of 238 U86+ beam. The delayed γ rays weremeasured using a Ge detector array, EURICA.4) Accordingly, we discovered new quasi-particle isomers in
159, 161
Pm98, 100 . The 3-qp isomers involve one quasiproton plus two quasi-neutrons near the Fermi surface
as shown in Fig. 1. A calculation employing the projected shell model (PSM)5) was performed to understand the isomers. The observation of the (17/2+) isomers cannot be explained by the PSM calculation withtraditional Nilsson parameters. We demonstrate thatto explain the observation, the strength of the spinorbit interaction needs to be changed according to theneutron number in exotic nuclei, as suggested by Liu etal.6) Furthermore, we also conﬁrmed that the reported
deformed shell gap at N = 983) is present both in eveneven and odd-mass nuclei.
References
1) M. R. Mumpower et al., Phys. Rev. C 86, 035803 (2012).
2) S. K. Ghorui et al., Phys. Rev. C 85, 064327 (2012).
3) D. J. Hartley et al., Phys. Rev. Lett. 120, 182502 (2018).
4) P. A. Söderström et al., Nucl. Instrum. Methods Phys.
Res. B 317, 649 (2013).
5) Y. Sun, Phys. Scr. 91, 043005 (2016).
6) Y. X. Liu et al., J. Phys. G 47, 055108 (2020).

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