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The r-process is a nucleosynthesis process that occurs in core-collapse supernovae and is responsible for the creation of approximately half of the neutron-rich heavier than iron. The process entails a succession of rapid neutron captures (hence the name r-process) by heavy seed nuclei, typically 56Fe or other more neutron-rich heavy isotopes.
The process thus terminates in bismuth, the heaviest "stable" element, and polonium, the first non-primordial element after bismuth. (Bismuth is actually slightly radioactive, but with a half-life so long—a billion times the present age of the universe—that it is effectively stable over the lifetime of any existing star.)
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