At sufficiently large neutron density, successive neutron captures outrun relevant beta decays and produce very neutron-rich nuclei. Decay toward stability afterward yields many heavy isotopes, including some actinides. Neutron-rich ejecta from neutron star mergers provide an established site; particular magnetically driven collapse outflows are additional candidates. This does not imply that every ordinary core-collapse supernova produces a complete heavy-element pattern.
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