「ダブルマジック」の希少同位体酸素28は中性子過剰の不安定性を克服できない(‘Doubly magic’ rare isotope oxygen-28 can’t overcome its neutron-rich instability)

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2023-08-30 オークリッジ国立研究所(ORNL)

◆同位体は異なる中性子数を持つ元素の原子で、気候変動の追跡から医学研究まで多岐にわたる用途がある。特に希少な同位体は、加速器施設で生成され、核構造の理論検証や未使用同位体の研究に利用される。東京工業大学を中心に37機関の研究者が共同で、中性子が非常に豊富な酸素28(28O)を調査。その結果、28Oが非常に中性子豊富な領域で結合同位体を得るか、不安定性が支配するかという核物理学の問いに答えた。
◆研究により、酸素28は非結合同位体であることが判明しました。また、コンピュータモデリングとシミュレーションを通じて、極端な条件下での核の振る舞いや新たな物理学の理解を追求しています。

<関連情報>

28Oの初観測 First observation of 28O

Y. Kondo,N. L. Achouri,H. Al Falou,L. Atar,T. Aumann,H. Baba,K. Boretzky,C. Caesar,D. Calvet,H. Chae,N. Chiga,A. Corsi,F. Delaunay,A. Delbart,Q. Deshayes,Zs. Dombrádi,C. A. Douma,A. Ekström,Z. Elekes,C. Forssén,I. Gašparić,J.-M. Gheller,J. Gibelin,A. Gillibert,G. Hagen,M. N. Harakeh,A. Hirayama,C. R. Hoffman,M. Holl,A. Horvat,Á. Horváth,J. W. Hwang,T. Isobe,W. G. Jiang,J. Kahlbow,N. Kalantar-Nayestanaki,S. Kawase,S. Kim,K. Kisamori,T. Kobayashi,D. Körper,S. Koyama,I. Kuti,V. Lapoux,S. Lindberg,F. M. Marqués,S. Masuoka,J. Mayer,K. Miki,T. Murakami,M. Najafi,T. Nakamura,K. Nakano,N. Nakatsuka,T. Nilsson,A. Obertelli,K. Ogata,F. de Oliveira Santos,N. A. Orr,H. Otsu,T. Otsuka,T. Ozaki,V. Panin,T. Papenbrock,S. Paschalis,A. Revel,D. Rossi,A. T. Saito,T. Y. Saito,M. Sasano,H. Sato,Y. Satou,H. Scheit,F. Schindler,P. Schrock,M. Shikata,N. Shimizu,Y. Shimizu,H. Simon,D. Sohler,O. Sorlin,L. Stuhl,Z. H. Sun,S. Takeuchi,M. Tanaka,M. Thoennessen,H. Törnqvist,Y. Togano,T. Tomai,J. Tscheuschner,J. Tsubota,N. Tsunoda,T. Uesaka,Y. Utsuno,I. Vernon,H. Wang,Z. Yang,M. Yasuda,K. Yoneda & S. Yoshida
Nature  Published:30 August 2023
DOI:https://doi.org/10.1038/s41586-023-06352-6

「ダブルマジック」の希少同位体酸素28は中性子過剰の不安定性を克服できない(‘Doubly magic’ rare isotope oxygen-28 can’t overcome its neutron-rich instability)

Abstract

Subjecting a physical system to extreme conditions is one of the means often used to obtain a better understanding and deeper insight into its organization and structure. In the case of the atomic nucleus, one such approach is to investigate isotopes that have very different neutron-to-proton (N/Z) ratios than in stable nuclei. Light, neutron-rich isotopes exhibit the most asymmetric N/Z ratios and those lying beyond the limits of binding, which undergo spontaneous neutron emission and exist only as very short-lived resonances (about 10−21 s), provide the most stringent tests of modern nuclear-structure theories. Here we report on the first observation of 28O and 27O through their decay into 24O and four and three neutrons, respectively. The 28O nucleus is of particular interest as, with the Z = 8 and N = 20 magic numbers1,2, it is expected in the standard shell-model picture of nuclear structure to be one of a relatively small number of so-called ‘doubly magic’ nuclei. Both 27O and 28O were found to exist as narrow, low-lying resonances and their decay energies are compared here to the results of sophisticated theoretical modelling, including a large-scale shell-model calculation and a newly developed statistical approach. In both cases, the underlying nuclear interactions were derived from effective field theories of quantum chromodynamics. Finally, it is shown that the cross-section for the production of 28O from a 29F beam is consistent with it not exhibiting a closed N = 20 shell structure.

1701物理及び化学
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