s-Process, r-Process & the Origin of Heavy Elements: Structures & Vocabulary

Many nuclei heavier than iron are built primarily by neutron capture rather than energy-producing fusion. The slow neutron-capture process, or s-process, proceeds slowly enough for unstable nuclei to beta-decay between captures, while the rapid r-process occurs in environments with neutron fluxes so intense that many neutrons are captured before beta decay.

A neutron carries no electric charge, so it can enter a nucleus without overcoming the strong Coulomb repulsion faced by charged particles. In the s-process, capture and beta decay alternate along a path near stable isotopes. In the r-process, repeated rapid captures push nuclei to very neutron-rich states before a cascade of decays.

Element and isotope abundance patterns in stars show characteristic s- and r-process signatures. Observations of kilonovae associated with neutron-star mergers demonstrate production of heavy r-process material, while evolved giant stars show enrichment expected from s-process nucleosynthesis.

The s-process operates mainly in evolved stars with moderate neutron fluxes and follows paths near stable nuclei. The r-process requires extreme neutron-rich conditions, producing very heavy nuclei and characteristic abundance peaks after radioactive decay.