Cosmic Ray Induced 53Mn in Yamato-7301(j), -7305(k) and -7304(m) Meteorites

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Cosmic ray-induced long-lived nuclide, ^<53>(Mn) (t_<1/2>=3.7×(10)^6y) has been determined by the neutron activation analysis in Yamato-7301 (j), -7305 (k), and -7304 (m) meteorites, and the contents were found to be 102±6,357±19 and 417±22dpm ^<53>(Mn)/kg Fe respectively. The content found in Yamato-7304 is in good agreement with other chondrites, and the slightly lower value in Yamato-7305,which is also common in others, may be due to some minor effects. In contrast, the content of ^<53>(Mn) in Yamato-7301 is remarkably low, lower than that of any other stone meteorites of relatively long exposure age so far studied. Several possible mechanisms are discussed to explain the puzzling ^<53>(Mn) activity in Yamato-7301 : 1) Long terrestrial age of about 7 m. y., and the decay of ^<53>(Mn). 2) Heavy preatomospheric shielding of more than 70cm, and low production rate of ^<53>(Mn). 3) Multistage irradiation history, and undersaturation of ^<53>(Mn). 4) A combined mechanism of the above two or three factors.

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