Some heavy elements (with atomic number A > 69) are produced by the 'rapid' (r)-process of nucleosynthesis, where lighter elements are bombarded with a massive flux of neutrons^{1, 2, 3, 4, 5, 6, 7, 8}. Although this is characteristic of supernovae and neutron star mergers, uncertainties in where the r-process occurs persist because stellar models are too crude to allow precise quantification of this phenomenon. As a result, there are many uncertainties and assumptions in the models used to calculate the production ratios of actinides (like uranium-238 and thorium-232). Current estimates of the U/Th production ratio range from 0.4 to 0.7. Here I show that the U/Th abundance ratio in meteorites⁹ can be used, in conjunction with observations of low-metallicity stars in the halo of the Milky Way^{10, 11, 12}, to determine the U/Th production ratio very precisely . This value can be used in future studies to constrain the possible nuclear mass formulae used in r-process calculations^{5, 6}, to help determine the source of Galactic cosmic rays, and to date circumstellar grains⁵. I also estimate the age of the Milky Way ( in a way that is independent of the uncertainties associated with fluctuations in the microwave background¹³ or models of stellar evolution^{14, 15}.

1. Origins Laboratory, Department of the Geophysical Sciences, Enrico Fermi Institute, and Chicago Center for Cosmochemistry, The University of Chicago, 5734 South Ellis Avenue, Chicago Illinois 60637, USA

Correspondence to: Nicolas Dauphas¹ Correspondence and requests for materials should be addressed to N.D. (Email: dauphas@uchicago.edu).

Received 17 February 2005; Accepted 14 April 2005

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