Calculation of Cosmic-ray Ages in the Iron Meteorites `Carbo' and `Treysa'

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IN a recent communication1, Dr. R. R. Marshall estimates the cosmic-ray age of the iron meteorite `Carbo' to be 0.69 × 109 yr. His calculation was based upon the following experimental results of ours2 and Barr's3. We showed that the relative isotopic abundances of potassium from a sample of the iron meteorite `Carbo' were markedly different from those of terrestrial potassium. Considerable enrichment of the isotopes of masses 40 and 41 was found (39Km = 79.5 per cent, 40Km = 4.8 per cent, and 41Km = 15.7 per cent as compared to 93.08, 0.012 and 6.91 per cent respectively for ordinary terrestrial potassium). Evidence was given that a considerable amount of this meteoritic potassium-nearly all the potassium-40 and more than half the potassium-41-is of cosmogenic origin, that is, it is a reaction product of the spallation of mainly iron nuclides, induced by high-energy particles from cosmic radiation. Barr3 measured the cross-sections for the production of fifty-nine radioactive isotopes formed by 5.7-BeV. proton bombardment of copper. From his summary of measured and interpolated independent cross-sections, Marshall calculated the total cross-sections for the production of the potassium isotopes to be σ39 = 14.8 mb., σ40 = 9.4 mb., and σ41 = 14.7 mb.

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