= Solution
Neutron-rich material ejected dynamically or in remnant-disk outflows can undergo the <r-process>: successive neutron captures, followed by beta decays, make heavy nuclei. The yield depends on the amount ejected, its entropy, expansion and <electron> fraction; not all outflow is equally neutron rich. Radioactive products can power a <kilonova>. A neutron-star–black-hole event contributes if matter escapes capture. An isolated <binary black hole> merger has no analogous reservoir of neutron-rich baryonic material, so it contributes essentially no such <nucleosynthesis>.
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