九州大学-研究者情報 [西畑洸希 (助教) 理学研究院物理学部門]

1.	H.Nishibata, K.Tajiri, T.Shimoda, A.Odahara, S.Morimoto, S.Kanaya, A.Yagi, H.Kanaoka, M.R.Pearson, C.D.P.Levy, M.Kimura, N.Tsunoda, T.Otsuka, Structure of the neutron-rich nucleus 30Mg, Physical Review C, https://doi.org/10.1103/PhysRevC.102.054327, 102, 054327-1-054327-14, 2020.11, 本研究は中性子過剰なMg-30の励起状態の構造をスピン偏極したNa-30原子核のベータ崩壊を用いるという独自の手法を用いて明らかにした。この研究では、Mg-30核の励起状態において球形、変形状態、三軸対称な振動状態など様々な構造が共存していることを初めて突き止めた。.
2.	Y. Ichikawa, H. Nishibata, Y. Tsunoda, A. Takamine, K. Imamura, T. Fujita, T. Sato, S. Momiyama, Y. Shimizu, D. S. Ahn, K. Asahi, H. Baba, D. L. Balabanski, F. Boulay, J. M. Daugas, T. Egami, N. Fukuda, C. Funayama, T. Furukawa, G. Georgiev, A. Gladkov, N. Inabe, Y. Ishibashi, T. Kawaguchi, T. Kawamura, Y. Kobayashi, S. Kojima, A. Kusoglu, I. Mukul, M. Niikura, T. Nishizaka, A. Odahara, Y. Ohtomo, T. Otsuka, D. Ralet, G. S. Simpson, T. Sumikama, H. Suzuki, H. Takeda, L. C. Tao, Y. Togano, D. Tominaga, H. Ueno, H. Yamazaki, X. F. Yang, Interplay between nuclear shell evolution and shape deformation revealed by the magnetic moment of ⁷⁵ Cu, Nature Physics, 10.1038/s41567-018-0410-7, 15, 4, 321-325, 2019.04, [URL], Exotic nuclei are characterized by having a number of neutrons (or protons) in excess relative to stable nuclei. Their shell structure, which represents single-particle motion in a nucleus ^1,2 , may vary due to nuclear force and excess neutrons ^3–6 , in a phenomenon called shell evolution ⁷ . This effect could be counterbalanced by collective modes causing deformations of the nuclear surface ⁸ . Here, we study the interplay between shell evolution and shape deformation by focusing on the magnetic moment of an isomeric state of the neutron-rich nucleus ⁷⁵ Cu. We measure the magnetic moment using highly spin-controlled rare-isotope beams and achieve large spin alignment via a two-step reaction scheme ⁹ that incorporates an angular-momentum-selecting nucleon removal. By combining our experiments with numerical simulations of many-fermion correlations, we find that the low-lying states in ⁷⁵ Cu are, to a large extent, of single-particle nature on top of a correlated ⁷⁴ Ni core. We elucidate the crucial role of shell evolution even in the presence of the collective mode, and within the same framework we consider whether and how the double magicity of the ⁷⁸ Ni nucleus is restored, which is also of keen interest from the perspective of nucleosynthesis in explosive stellar processes..
3.	H. Nishibata, S. Kanaya, T. Shimoda, A. Odahara, S. Morimoto, A. Yagi, H. Kanaoka, M. R. Pearson, C. D.P. Levy, M. Kimura, N. Tsunoda, T. Otsuka, Structure of Mg 31 Shape coexistence revealed by β-γ Spectroscopy with spin-polarized Na 31, Physical Review C, 10.1103/PhysRevC.99.024322, 99, 2, 2019.02, [URL], The level structure of Mg31, which is located close to the region of the N = 20 "island of inversion" in the nuclear chart, has been studied by β-γ spectroscopy with spin-polarized Na31. In Mg31, shape coexistence is expected as a result of subtle competition between the spherical mean field and the nuclear correlation which favors deformed configurations. In the present work, our unique method utilizing the anisotropic β decay of spin-polarized Na31 enables us to firmly assign the spins of all positive-parity excited levels in Mg31 below the neutron separation energy at 2.3 MeV. Furthermore, by constructing a very detailed decay scheme, including two newly found levels, the spins of negative-parity levels are restricted. The examination of the spectroscopic information shows that the deformed rotational bands with Kπ=1/2+ and 1/2-, which have very similar structures to those observed in a higher excitation energy region of Mg25, appear as the ground-state and low-lying bands, respectively, in Mg31. The experimental levels of Mg31 are compared, on the level-by-level basis, with two types of theoretical calculations. These are, first, the antisymmetrized molecular dynamics (AMD) plus generator coordinate method (GCM) and, second, the shell model with the EEdf1 interaction, which is microscopically derived from chiral effective field theory. It is understood that 8 levels among the experimental 11 levels are the members of four types of largely deformed rotational bands and 2 levels are of spherical nature. The 1/2+ 2.244-MeV level is successfully reproduced by the shell-model calculation with a dominant 4p4h configuration. The present work clearly demonstrates that various structures coexist in a low excitation energy region of Mg31..
4.	H. Nishibata, T. Shimoda, A. Odahara, S. Morimoto, S. Kanaya, A. Yagi, H. Kanaoka, M. R. Pearson, C. D.P. Levy, M. Kimura, Shape coexistence in the N = 19 neutron-rich nucleus ³¹Mg explored by β–γ spectroscopy of spin-polarized ³¹Na, Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 10.1016/j.physletb.2017.01.049, 767, 81-85, 2017.04, [URL], The structure of excited states in the neutron-rich nucleus ³¹Mg, which is in the region of the “island of inversion” associated with the neutron magic number N=20, is studied by β–γ spectroscopy of spin-polarized ³¹Na. Among the ³¹Mg levels below the one neutron separation energy of 2.3 MeV, the spin values of all five positive-parity levels are unambiguously determined by observing the anisotropic β decay. Two rotational bands with K^π=1/2⁺ and 1/2⁻ are proposed based on the spins and energies of the levels. Comparison on a level-by-level basis is performed between the experimental results and theoretical calculations by the antisymmetrized molecular dynamics (AMD) plus generator coordinate method (GCM). It is found that various nuclear structures coexist in the low excitation energy region in ³¹Mg..
5.	H. Nishibata, R. Leguillon, A. Odahara, T. Shimoda, C. M. Petrache, Y. Ito, J. Takatsu, K. Tajiri, N. Hamatani, R. Yokoyama, E. Ideguchi, H. Watanabe, Y. Wakabayashi, K. Yoshinaga, T. Suzuki, S. Nishimura, D. Beaumel, G. Lehaut, D. Guinet, P. Desesquelles, D. Curien, K. Higashiyama, N. Yoshinaga, High-spin states in la 136 and possible structure change in the N=79 region, Physical Review C - Nuclear Physics, 10.1103/PhysRevC.91.054305, 91, 5, 2015.05, [URL], High-spin states in the odd-odd nucleus La136, which is located close to the β-stability line, have been investigated in the radioactive-beam-induced fusion-evaporation reaction Sn124(N17,5n). The use of the radioactive beam enabled a highly sensitive and successful search for a new isomer [14+,T1/2=187(27) ns] in La136. In the A=130-140 mass region, no such long-lived isomer has been observed at high spin in odd-odd nuclei. The La136 level scheme was revised, incorporating the 14+ isomer and six new levels. The results were compared with pair-truncated shell model (PTSM) calculations which successfully explain the level structure of the πh11/2 - νh11/2-1 bands in La132 and La134. The isomerism of the 14+ state was investigated also by a collective model, the cranked Nilsson-Strutinsky (CNS) model, which explains various high-spin structures in the medium-heavy mass region. It is suggested that a new type of collective structure is induced in the PTSM model by the increase of the number of πg7/2 pairs, and/or in the CNS model by the configuration change associated with the shape change in La136..

主要学会発表等

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学会活動

所属学会名

日本物理学会

その他の研究活動

海外渡航状況, 海外での教育研究歴

TRIUMF, Canada, 2020.10～2020.11.

受賞

核物理談話会新人賞, 核物理談話会, 2018.10.

研究資金

科学研究費補助金の採択状況(文部科学省、日本学術振興会)

2023年度～2025年度, 基盤研究(C), 代表, 偏極核ベータ核分光法という独自の手法を用いた”逆転の島”境界原子核の構造研究.

2020年度～2022年度, 若手研究, 代表, 新奇手法による中性子過剰核の構造研究：”逆転の島”境界原子核の完全理解に向けて.

2018年度～2022年度, 国際共同研究強化(B), 分担, 中性子超過剰核の構造~最新技術を統合して探る新奇な核構造.

2018年度～2019年度, 若手研究, 代表, 中性子過剰核における新奇手法を用いた構造解明.

2016年度～2018年度, 特別研究員奨励費, 代表, 原子核のスピン分布制御という新手法を用いた中性子超過剰核の構造の研究.

2013年度～2015年度, 特別研究員奨励費, 代表, 中性子過剰核の特異な構造を通じた核子間対相互作用の解明.

日本学術振興会への採択状況(科学研究費補助金以外)

2016年度～2018年度, 特別研究員, 代表, 原子核のスピン分布制御という新手法を用いた中性子超過剰核の構造の研究.

2013年度～2015年度, 特別研究員, 代表, 中性子過剰核の特異な構造を通じた核子間対相互作用の解明.

九大関連コンテンツ

pure2017年10月2日から、「九州大学研究者情報」を補完するデータベースとして、Elsevier社の「Pure」による研究業績の公開を開始しました。

QIR　九州大学学術情報リポジトリシステム情報科学研究院

理学部・理学研究院


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