Yoshio Nishina and two cyclotrons

ABSTRACT Yoshio Nishina is often honored as ““a father of modern physics in Japan.”” By performing multiple roles as a competent researcher, a formidable teacher, and a shrewd organizer, he not only made a great contribution to the emergence of a research network that produced two Nobel prize winners in physics but also raised the level of Japanese physics overall. Among Nishina's many contributions to the Japanese physics community, the construction and operation of two cyclotrons during the 1930s and 1940s were the most celebrated. In this paper I try to answer the following questions: why did Nishina start the construction of two cyclotron in the mid-1930s?; how did he secure the necessary financial support?; what were the original objectives of the machines, and how were they were actually used?; what difficulties did he meet and overcome in the construction and running of the cyclotrons?; how significant was the Berkeley connection in contributing to the construction and operation of the cyclotrons?; why did Nishina skip the construction of the medium size cyclotron (30––40 inches) and move directly from a small (26-inch) to a large one (60-inch)?; and how much did the cyclotron project influence the future path of Japanese physics? I argue that Nishina's two cyclotrons, especially the larger one, should be considered as successful examples of reverse engineering, a hallmark of Japanese technology in the interwar period.