| Summary: | P(論文) As part of a program to compile statistical information on the cosmic ray exposure history of meteorites, cosmogenic ^<53>(Mn) (t_<1/2>=3.7×(10)^6y) was determined in twenty-one Yamato meteorites. Yamato-74116 and -74193 contain especially low amounts of ^<53>(Mn) suggesting short cosmic ray exposure ages. The ^<53>(Mn) activity level of Yamato-74192 is 578±24 dpm/kg (Fe+1/3 Ni) which is the highest value we have ever measured in a chondrite to date. Although the recovered mass of Yamato-74159 was small, its ^<53>(Mn) content indicates a large preatmospheric mass. Two histograms, which illustrate the frequencies of the ^<53>(Mn) contents of chondrites from Antarctica and those of chondrites from other parts of the world, show similar statistical distributions. This indicates that the terrestrial ages of Antarctic meteorites are generally short (<1 my) in comparison to the ^<53>Mn half-life. departmental bulletin paper
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