Measurement of Li 6, He 3, and H 3 in meteorites and its relation to cosmic radiation

Abstract

The He 3 and Li 6 contents of six iron and five stone meteorites are measured by neutron activation. Both the iron and the stone meteorites have large He 3 contents ranging from 0.13 × 10 −6 c.c./g to 9 × 10 −6 c.c./g. The stone meteorites have Li 6 contents ranging from 1.1 × 10 16 Li 6 atoms/g to 6.2 × 10 16 Li 6 atoms/g. The iron meteorites have less than 10 14 Li 6 atoms/g. There is no uniform relation between the Li 6 contents of the meteorites and their He 3 contents, indicating that the He 3 contents of both the stone meteorites and the iron meteorites are due to spallation caused by high-energy costnic rays rather than slow neutrons and that the average slow-neutron flux has been less than 1 neutron/cm 2 sec. The tritium contents of the iron meteorites. Norfork (1918), Para de Minas (1934), and Charcas (1804) are measured. A tritium content of (2.7 ± 0.3) × 10 6 H 3 atoms/g at its time of fall is indicated for the Norfork meteorite and of (5.9 ± 1.2) × 10 5 H 3 atoms at its time of fall is indicated for the Para de Minas meteorite. There is less than 1.2 × 10 4 H 3 atoms/g in Charcas. The tritium and helium-3 content of Norfork combine to give it a cosmic-ray age of 0.9 × 10 9 years or greater. The tritium and helium-3 content of Para do Minas combine to give it a cosmic-ray age of 1.7 × 10 9 years or greater. This indicates that if iron meteorites resulted from the breakup of a larger mass, the breakup occurred 0.9 × 10 9 or more years ago. Small H 3 contents but negligible helium-3 contents are found in terrestrial iron samples. The tritium content of the iron meteorites compared with that produced in iron by high-energy proton bombardment gives the average flux of cosmic rays bombarding the meteorite in extraterrestrial space. This flux as determined from the Norfork meteorite, equals the 1954 cosmic-ray flux which does not contain the low energy cutoff; the flux determined from the Para de Minas meteorite is a factor of 3 to 6 lower. This indicates that Para de Minas, which is a 112-kg meteorite, was shielded from cosmic radiation by material ablated during its passage through the atmosphere, or was irradiated by a smaller cosmic-ray flux than Norfork.