セリウム近傍の中性子過剰な新しい原子核７種を発見～理研の飛行分離方式による新同位元素発見数が200種超え～

2026-01-27 理化学研究所

核図表における理研で発見された新同位元素

＜関連情報＞

• https://www.riken.jp/press/2026/20260127_1/index.html
• https://journals.jps.jp/doi/10.7566/JPSJ.95.024202

同位体フロンティアの拡大：理化学研究所RIビームファクトリーで7つの新しい中性子過剰希土類同位体を観測 Expanding the Isotopic Frontier: Seven New Neutron-Rich Rare-Earth Isotopes Observed at RIKEN RI Beam Factory

Toshiyuki Sumikama, Naoki Fukuda, Toshiyuki Kubo, Hiroshi Suzuki, Hiroyuki Takeda, Naohito Inabe,…
Journal of the Physical Society of Japan  Published: January 27, 2026
DOI:https://doi.org/10.7566/JPSJ.95.024202

Abstract

An experiment conducted at the RIKEN Nishina Center RI Beam Factory, using a ²³⁸U beam with an energy of 345 MeV/nucleon, led to the discovery of seven new neutron-rich isotopes in the rare-earth region: ¹⁵²Cs, ¹⁵⁵Ba, ¹⁵⁸La, ¹⁵⁹Ce, ¹⁶⁰Ce, ¹⁷³Gd, and ¹⁷⁵Tb. These isotopes were produced through in-flight fission of a ²³⁸U beam and were subsequently separated and identified in-flight using the BigRIPS fragment separator. For the identification, the mass-to-charge ratio and atomic number of the isotopes were determined by measuring the time-of-flight, magnetic rigidity, and energy loss. The observations of all seven new isotopes were statistically confirmed based on a significance test with p-values. The observed counts for the new isotopes were consistent with a systematic decrease in production cross sections with increasing mass number, supporting their discovery.