2001 年 35 巻 1 号 p. 13-25
Activities of cosmogenic nuclides have been numerously reported for extraterrestrial material, meteorites (especially chondrites). In addition to noble gases, cosmogenic radionuclides (e.g., 22Na, 26Al, 46Sc, 53Mn, 54Mn and 60Co) have also kept important records of their history during last ten million years. For example, low 60Co activity observed in most chondrites (<30 dpm/kg) suggests that their preatmospheric radii are less than 〜30 cm, and activities of 22Na and 26Al reflect various irradiation conditions by cosmic-ray such as shielding effect, exposure age and cosmic-ray flux with time and space. Terrestrial age (Antarctic meteorites; H group, <0.4×106y; L, LL,<106y estimated from 26Al, 36Cl and 14C activities) as well as exposure age (H, 3 -40×106y; L, LL, 3 -50×106y) shows different distributions between H- and L-, LL-group chondrites, which have possibly preserved the information about breakage of parent body and so on. The combined data of exposure age and activities of cosmogenic nuclides also give constraints on the complex history of chondrites (e.g., multi-stage irradiation as a result of fragmentation) until the collision with the Earth. Recently, with the progress of nondestructive γ-ray techniques, activities of relatively short-lived nuclides such as 46Sc, 22Na implied the irradiation conditions just before fall to the Earth. In this paper, "the evolution history of chondrites after separation from the parent body" is represented from activities of cosmogenic radionuclides in meteorites including recently fallen ones.
