Rprocess

The rapid neutron capture process, or r-process, is a nucleosynthesis mechanism that builds elements heavier than iron. In environments with very high neutron flux, nuclei capture neutrons faster than they can beta-decay. When neutron exposure ends, the neutron-rich nuclei beta-decay toward stability, yielding the observed heavy-element abundance pattern.

The r-process path proceeds through very neutron-rich isotopes near the neutron drip line. Its outcome depends on neutron density, temperature, and exposure time; fission of the heaviest nuclei can recycle material, shaping the final abundances.

Astrophysical sites include neutron star mergers, where neutron-rich ejecta naturally arise; GW170817 and its kilonova provided strong evidence for r-process synthesis. Some rare core-collapse supernovae have also been proposed. The relative contributions depend on electron fraction (Ye), entropy, and expansion timescales.

Solar-system r-process abundances show peaks near A ≈ 130 and 195, reflecting a robust main pattern. Metal-poor stars preserve this signature, indicating early r-process enrichment. Kilonovae reveal spectral features of freshly synthesized lanthanides and actinides.

Nuclear physics uncertainties remain substantial: many r-process nuclei lie far from stability, leaving rates, half-lives, and fission properties uncertain. Experiments and models aim to constrain these quantities.

Compared with the slow s-process in asymptotic giant branch stars, the r-process accounts for roughly half of the elements heavier than iron.