Cosmoradiogenic Chronologies of Nucleosynthesis.

Abstract

New methods for using radioactive decays to date the time of nucleosynthesis in our Galaxy are presented in this work. The methods depend upon a comparison of the cosmoradiogenic abundances of daughter species with the abundances of their radioactive parents. The decays which can provide useful rmation of this type are Re187-Os187, U235-Pb207, and U238-Pb206. It will be shown that the amount of enrichment of the daughter products by radioactive decay throughout the history of our Galaxy is calculable. This calculation differs from previous radioactive chronologies which have concentrated on relative abundances of the radioactive species themselves, U235, U238, and Th232. Definitive numerical calculations at the present time are forestalled by uncertainties in several key measurable quantities, particularly neutron-capture cross-sections and solar abundances. Tentative calculations based on probable values of these quantities indicate that galactic nucleosynthesis began at least 5 X 10^9 years before the formation of the solar system, and perhaps considerably earlier. Measurements capable of reducing the uncertainty of these methods are emphasized.