Abstract
Excesses of 80 Kr and 82 Kr, produced by secondary neutron capture effects of 79 Br and 81 Br, were observed in meteorites. Epithermal neutron flux, Jn (30—300 eV), and fast neutron flux, Jn( > 5 MeV), were dete rmined according to reactions of 79 Br(n, γβ ) 80 Kr, 81 Br(n, γβ ) 82 Kr, and 24 Mg(n, α ) 21 Ne, respectively. Cosmogenic noble gases of several ordinary chondrites fell in China indicate a positive relationship between Jn (30—300 eV)/Jn ( > 5 MeV) ratio and Jn (30—300 eV). This suggests large pre-atmospheric sizes of the meteorites, and a conside rable fraction of the secondary neutrons were slowed down to epithermal energy. According to its relationship with Jn (30—300 eV)/Jn ( > 5 M eV) ratio, the pre-atmospheric minimum radii and masses of several Chinese chondrites were estimated as below: Zhaodong: 60 cm, 3200 kg; Laochengzhen: 23 cm, 490 kg; Suizhou: 31 cm, 450 kg; Xi Ujimgin: 22 cm, 160 kg; Dongtai: 21 cm, 140 kg.
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