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The Astrophysical Journal


The American Astronomical Society


The nucleosynthesis of heavy stable nuclei by successive captures of neutrons in a weak neutron flux (the s-process) is terminated at large atomic weight by alpha decay to isotopes of lead. We analyze the details of this termination and calculate the abundances of lead and bismuth isotopes synthesized by the exposure of lighter nuclei to neutrons. We demonstrate that the results are nearly insensitive to the poorly known neutron-capture cross-section of Pb368. The continuous distribution of integrated neutron exposures responsible for the observed abundances has an anomalously large high-exposure component. This result suggests that the bulk of the lead has been synthesized by special astrophysical circumstances rather than by a smooth extension of the circumstances attending the synthesis for A < 200.