sprocess

The s-process, or slow neutron capture process, is a nucleosynthesis channel in which atomic nuclei capture neutrons more slowly than they beta-decay. As a result, the reaction path follows nuclei close to the valley of beta stability, producing stable isotopes up to lead and bismuth. It accounts for a substantial portion of the heavy-element isotopes found in the solar system and is a major contributor to the abundances of elements heavier than iron.

Mechanism and path: In the s-process, each neutron capture is typically followed by beta decay before another neutron is captured, so the path stays near stable isotopes. Branching points occur at nuclei where beta-decay half-lives are comparable to neutron-capture times, influencing the final distribution of isotopes.

Sites and neutron sources: The main s-process occurs in asymptotic giant branch stars of low- to intermediate-mass, during He-shell flashes. The principal neutron source is 13C(α,n)16O in the He intershell during the interpulse period, with a secondary contribution from 22Ne(α,n)25Mg in hotter conditions of more massive AGB stars. A separate, weaker channel—the weak s-process—occurs in massive stars during core He burning and subsequent C burning, powered mainly by 22Ne(α,n)25Mg. The process produces heavy elements up to the first and second peaks in the solar abundance pattern and, in the most metal-poor environments, can enrich heavier Pb and Bi.

Abundance patterns and observations: The s-process yields characteristic abundance peaks at neutron numbers N≈50 (Sr, Y, Zr), N≈82 (Ba, La, Ce), and N≈126 (Pb, Bi). Its efficiency depends on metallicity: lower metallicity increases the neutron-to-seed ratio, favoring heavier elements like lead, while higher metallicity enhances production of the light s-process peaks. Evidence comes from solar-system abundances, spectra of s-process–enhanced stars, and isotopic signatures in presolar grains, all constraining models of AGB and massive-star evolution.