Cosmic-ray produced 59Ni and its astrophysical and geophysical implications

Abstract

The processes for the production of 59Ni in meteorites and in micrometeorites have been studied. For large bodies, 59Ni nuclei are produced by galactic cosmic rays through (i) the spallation reaction of 60Ni, 63Cu, etc., and (ii) radiative capture of secondary neutrons slowed down in meteorites: 58Ni(n, γ) 59Ni. For small bodies, the contribution of solar cosmic rays is important. The main contribution comes from the (α, n) reaction, induced by solar cosmic-ray α particles: 59Fe(α, n)59Ni, and the secondary contribution is due to solar cosmic-ray protons: 60Ni(p, pn)59Ni and 59Co(p, n)59Ni. From the use of solar cosmic-ray data, the 59Ni activity in micrometeorites has been calculated. Under conditions of radioactive equilibrium, the 59Ni activity is 220 d.p.m./g Ni, if 93% of the micrometeorites are composed of stone and 7% of iron. The contribution of solar cosmic-ray protons is 15 d.p.m./g Ni, giving a total of 235 d.p.m./g Ni. Experimental information in the 59Ni sedimentation rate will determine the fallout rate of extraterrestrial nickel–iron. Measurement of the 59Ni activity in pelagic clay, dated by the Th–Io method will provide information concerning the variation of solar activity, as represented by the solar cosmic-ray intensity, over the last few hundred thousand years.