Proc Jpn Acad Ser B Phys Biol Sci. 2011 July 25; 87(7): 371–376.
The discoveries of uranium 237 and symmetric fission — From the archival papers of Nishina and Kimura
Editor: Toshimitsu YAMAZAKI
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Shortly before the Second World War time, Nishina reported on a series of prominent nuclear physical and radiochemical studies in collaboration with Kimura. They artificially produced 231Th, a member of the natural actinium series of nuclides, by bombarding thorium with fast neutrons. This resulted in the discovery of 237U, a new isotope of uranium, by bombarding uranium with fast neutrons, and confirmed that 237U disintegrates into element 93 with a mass number of 237.”