Terrestrial Age of Nineteen Stony Meteorites derived from their Radiocarbon Content

Abstract

THE effects of cosmic radiation on meteorites have been studied thoroughly during the past 15 yr^1,2. ¹⁴C is chiefly produced by three nuclear reactions: ¹⁴N(n,p)¹⁴C, ¹⁶O(p,3p)¹⁴C and ¹⁶O(n,2pn)¹⁴C. The first reaction occurs in the Earth's atmosphere^3–4 and provides the basis for the well known radiocarbon dating method. The ¹⁴N(n,p)¹⁴C reaction has its largest cross-sections (σ=1.7 bar) in the thermal neutron energy region. The ¹⁶O spallation reactions (σ=2 mbar) require energies above 10 MeV. The air mass of 1 kg cm⁻² above the surface of the Earth dissipates most of the energy of the primary cosmic rays. Therefore, ¹⁴C production in rocks on the Earth's surface is negligible. The surface of an object in space, however, is exposed to the full flux of the primary cosmic radiation. Meteorites, therefore, contain a measurable amount of high-energy spallation products. Meteorites contain very little nitrogen, and hence the ¹⁴C production is dominated by ¹⁶O spallation. The equilibrium ¹⁴C activity in stony meteorites per unit weight has a different value from that of the terrestrial atmosphere. The approximate average values for the ¹⁴C production rate are 120 ¹⁴C atoms min⁻¹ cm² at the Earth's surface (containing 1 kg of air) and 60 ¹⁴C atoms min⁻¹ kg of meteorite⁵.