The dawn of radiochemistry in Japan

In the early history of radiochemistry in Japan Yoshio Nishina and Kenjiro Kimura were internationally known for their discovery of symmetric nuclear fission of uranium. They also succeeded in getting U-237, which is an (n+1) series (neptunium series) nuclide, from U-238 by fast neutron irradiation. Kimura was an excellent leader of the radiochemical analysis of Bikini ashes in 1954. The results of that analysis contributed to the improvement of environmental radioactivity in the world. A tragic story of Nobuo Yamada working at the Radium Institute is related. The start of radiochemistry symposia in Japan is described.

The discoveries of uranium 237 and symmetric fission - from the archival papers of Nishina and Kimura

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 (231)Th, a member of the natural actinium series of nuclides, by bombarding thorium with fast neutrons. This resulted in the discovery of (237)U, a new isotope of uranium, by bombarding uranium with fast neutrons, and confirmed that (237)U disintegrates into element 93 with a mass number of 237. They also identified the isotopes of several middle-weighted elements produced by the symmetric fission of uranium. In this review article, the highlights of their work are briefly summarized along with some explanatory commentaries.