抄録
Before the fall on the earth, iron and stone meteorites have been irradiated in space by so-called Galactic "cosmic rays", high energy protons and neutrons, and contain various species of stable and radioactive nuclides. The first discovery of the products was that of Helium-3, 3He, which is a stable isotope of helium and it is up to about 30% of He-4 contents in iron meteorites. The He-3 consists of direct products of He-3 and also those by an indirect path via H-3 which is produced primarily and decayed into He-3. Therefore just after the fall the radioactivity of tritium can be observed. When continuous irradiation has continued for longer than the half-life of the radioactive nuclides. the activities will saturate and reach to a radioactive equilibrium, and the level of the decay rate becomes equal to the production rate in space. In this article we may observe the historical developments of studies in this field. Some datailed descriptions will be given especially in the examples of unexpected developments and the importance of related faint information.
