「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)

2023-08-30 オークリッジ国立研究所(ORNL)

＜関連情報＞

• https://www.ornl.gov/news/doubly-magic-rare-isotope-oxygen-28-cant-overcome-its-neutron-rich-instability
• https://www.nature.com/articles/s41586-023-06352-6

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

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 numbers^1,2, 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.