Calculation of a ‘Cosmic Ray Age’ for the Iron Meteorite ‘Carbo’

Abstract

THE potassium of the iron meteorite ‘Carbo’, compared with ordinary potassium, is substantially enriched in the isotopes potassium-40 and -41, as shown in the recent isotopic analysis by Voshage and Hintenberger¹. At least 15 per cent of these isotopes in their sample of ‘Carbo’ appear to have been produced through the action of cosmic rays. Stoenner and Zähringer² found that the concentration of potassium may vary considerably within an iron meteorite. Their analyses were based on the detection of potassium-41 and the assumption that the potassium had the same isotopic composition as terrestrial potassium. If, however, their value of 0.031 p.p.m. of potassium from one sample is typical of ‘Carbo’ and is corrected to 0.012 p.p.m. in order to fit the new isotopic analysis, the potassium produced by spallation is at least 4.6 × 10⁻¹¹ mole/gm. The total ion beam in the Voshage and Hintenberger analysis indicated that the concentration of potassium-40 alone is even greater than 1.8 × 10⁻¹² mole/gm. The amount of potassium produced by spallation is probably about 10⁻¹¹ mole/gm. This figure is quite compatible with that of 1.1 × 10⁻¹⁰ mole/gm. for the helium-3 content² because the cross-section for the formation of potassium, including the contribution from argon-39 and calcium-41, seems to be³ about 40 mb., and the measured cross-section for the production of helium-3 (with hydrogen-3) by means of 3 BeV. protons on iron is approximately⁴ 340 mb. The neon also formed by spallation has a concentration⁵ of 5.8 × 10⁻¹² mole/gm., which is comparable with that indicated for the potassium.