Klein, O. (English spelling) KleinO

...There, together with D. Koster, he measured the energy levels of rare earth elements and elements with atomic numbers close to or less than that of them by X-ray spectroscopy, experimentally proving the interpretation of rare earth elements in Bohr's atomic structure theory, and then developed a method of X-ray elemental analysis for G. Hevesy that was useful for the quantitative determination of hafnium in zirconium ore. In 1927, when his Ralph Ørsted Foundation scholarship, which he had received for three years after the deadline for his study abroad in Japan, ended, he left Copenhagen for a time, but went to Hamburg to study quantum mechanics, a new theory that had begun to emerge the previous year. In 1928, he returned to Copenhagen and, together with Oskar Klein (1894-1977) of Sweden, derived the Klein-Nishina formula, which gives the cross section of Compton scattering of electromagnetic waves by electrons. At the end of the same year, he returned to Japan via the United States, and in 1931 he became head of the laboratory at the RIKEN Institute, where he nurtured a large school of theoretical physics, atomic nuclei, and cosmic rays research that included Shin'ichiro Tomonaga and Ryokichi Sagane. …

*Some of the terminology that mentions "Klein, O." is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…そこでD.コスターとともに希土類とその前後の原子番号の諸元素のエネルギー準位をX線分光学的に測定，ボーアの原子構造理論の希土類の解釈を実験的に証明し，次いでG.ヘベシーのためにジルコニウム鉱石中のハフニウムの定量などに役だつX線元素分析の方法を開発した。日本からの留学期限後3年間受けていたラルフ･エルステッド財団の奨学金も終わった27年，いったんコペンハーゲンを離れたが，その前年勃興しはじめた新理論，量子力学を学ぶためハンブルクに留学，28年コペンハーゲンに戻ってスウェーデンのクラインOskar Klein(1894‐1977)とともに，電子による電磁波のコンプトン散乱の断面積を与えるクライン=仁科の式を導出した。その年の暮れにアメリカ経由で帰国，31年理化学研究所で研究室主任となり，理論物理学，原子核および宇宙線の研究の分野で，朝永振一郎，嵯峨根遼吉らを擁する大きな学派を育てた。…

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