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Mass Spectrometries Study of Rare Gas Compositions and Neutron Capture Effects in Yamato-74191 (L 3) Chondrite

Nobuo Takaoka * and Keisuke Nagao **
Department of Physics, Faculty of Science, Osaka University,
Osaka 660, Japan
Z. Naturforsch. 35ft, 29—36 (1980);

[received December 5, 1979]

[Dedicated to Prof. H. Hintenberger on the occasion of his 70 th birthday]

The unequilibrated hypersthene chondrite Yamato-74191 was studied mass spectrometrically for rare gases released at various temperatures. Cosmogenic gases dominate in He and Ne. The meteorite contains large amounts of trapped Ar, Kr and Xe, and radiogenic ^Ar and 129Xe. Cosmic-ray irradiation and K-Ar ages were determined. In addition to spallogenic components of Kr and Xe, isotopic excesses of ^Kr, 82Kr, 128Xe and 126 Xe relative to AVCC-Kr and -Xe were found. The ratio of ^Kr-exoess to 8aKr-excess is 2.66 after correction for spallogenic Kr. A correlation between 128Xe/1MXe and 129Xe/182Xe was found The i^Xe/i^Xe ratio for trapped Xe in Yamato-74191 was determined as 1.12 ± 0.29 with the correlation plot. The excesses found in Yamato-74191 are best explained by epithermal neutron capture on Br and I, and by the 127I(n, 2n/7) 126Xe reaction. Using neutron-produced ^Kr, the neutron slowingdown density was estimated to be 0.14 ± 0.03 cm-8 sec-1. A minimum mass and a preatmospheric radius was estimated to be 470 kg and 32 cm, respectively.

1* Introduction