s-Process, r-Process & the Origin of Heavy Elements: Physical Mechanisms

The s-process operates mainly in evolved stars where moderate neutron sources expose seed nuclei over long timescales. The r-process requires extreme neutron-rich ejecta and produces nuclei far from stability that later decay toward stable heavy isotopes. The two pathways populate different portions of the chart of nuclides.

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.

Neutrons avoid the Coulomb barrier that inhibits charged-particle capture. The competition between neutron-capture rate and beta-decay lifetime determines the pathway. In r-process conditions, capture outruns decay; in s-process conditions, decay often occurs before another neutron arrives.