新しい「核分裂」の発見！ 99番元素アインスタイニウムが導く元素の世界～超重元素の存在限界と宇宙での元素合成の理解へ～

2025-05-15 日本原子力研究開発機構,近畿大学,東北大学,九州大学

＜関連情報＞

• https://www.jaea.go.jp/02/press2025/p25051501/
• https://journals.aps.org/prc/abstract/10.1103/PhysRevC.111.044609

4He + 254Es反応で生成する258 Mdにおける質量対称核分裂モードと非対称核分裂モードの競合 Competition between mass-symmetric and asymmetric fission modes in 258Md produced in the 4He+254Es reaction

K. Nishio, K. Hirose, H. Makii, R. Orlandi, K. R. Kean, K. Tsukada, A. Toyoshima, M. Asai, T. K. Sato et al.
Physical Review C  Published 21 April, 2025
DOI: https://doi.org/10.1103/PhysRevC.111.044609

Abstract

Earlier studies of spontaneous fission of heavy actinides found that the fission-fragment mass distribution suddenly changes from predominantly asymmetric to symmetric, when the mass number of the fissioning nucleus becomes larger than c=257. We have measured for the first time fission-fragment mass and total-kinetic-energy (TKE) distributions of the excited compound nucleus ²⁵⁸Md* (=101), populated in the reaction ⁴He+²⁵⁴Es, using the rare radioactive target isotope ²⁵⁴Es with a short half-life, T_1/2=275.7d. The fission of ²⁵⁸Md* can be interpreted as involving three fission modes: one mass-asymmetric and two mass-symmetric fission modes, the latter two are manifested by different TKE values. In this assumption, symmetric and asymmetric fission modes have nearly similar yields at the excitation energy of 15.0 MeV. The spectrum of 18.0 MeV can be explained as due to the increase of asymmetric fission mode.