Birth of Cosmic-ray Studies in Japan(The Centenary of the Discovery of Cosmic Rays)

Abstract

Systematic studies of cosmic rays in Japan were started when the Nishina Laboratory was founded in Riken in 1931, after Nishina came back from the Niels Bohr Institute in Copenhagen. He spent a great deal of effort to spread modern physics in Japan, which later established the strong group of theoretical physicists including Yukawa, Tomonaga, Sakata and others. Nishina constructed a cloud chamber with a strong magnetic field of 1.7 Tesla, which lead to the success of precise measurements of mass of mesons in 1937. His laboratory concerned with comprehensive cosmic-ray studies including the deepest under ground experiment at Shimizu Tunnel (3,000 mwe), continuous observation of cosmic ray intensity by constructing Nishina Type Ionization Chambers, observations of Latitude Effect, Pressure and Temperature effects and other efforts. Here description is made for the Cosmic ray research in our country, started from Nishina laboratory, developed later at each time by introducing new concept and also by exploring new fields.