Cosmic-ray-produced iron 60 in Odessa meteorite

Abstract

It has been assumed that the long-lived radionuclide Fe60(t1/2 ∼ 1.0 · 105 yr) was produced by cosmic radiation in meteorites. Until now its detection was prevented (1) because its concentration is expected to be quite small [Arnold et al., 1961] and (2) because the specific activity of the sample is low. We have measured this nuclide in a large sample of Odessa iron meteorite by counting the daughter radionuclide Co60. Our sample was the Fe fraction from 2.5 kg of Odessa processed by Honda et al. [1961] about five years ago. Since most of the meteoritic cobalt had been previously removed, the specific activity of the daughter radionuclide Co60, which accumulated during the five-year storage period, was expected to be quite high. Most of the Fe was in the form of FeCl3 in aqueous solution. Total Fe was estimated by EDTA titration [Honda and Arnold, 1961]. After adding cobalt carrier (47.5 mg Co) the solution was made 8 molar in HCl by passing HCl gas. Fe was removed by extraction with isopropyl ether. The aqueous phase, containing cobalt, was bluish green. Some nickel was found in it, apparently owing to incompleteness in the mechanical separation of the aqueous and ether layers. The Ni-Co mixture was reduced to a small volume and was passed through a 115-ml Dowex-1 column at 9 molar HCl. The total Ni in the sample was estimated as 726 mg by EDTA titration. Cobalt was stripped with 4 molar HCl. After CuS and Fe(OH)3 scavengings the metal was precipitated as CoS by adding (NH4)2S in the ammoniacal solution. The CoS was ignited to Co3O4 at 900°C. It was mounted on a cylindrical copper holder and counted on a low-level beta counter.