S-process

The S-process proceeds when neutron captures are slower than beta decay along the valley of beta stability. A seed nucleus, commonly an iron-group isotope, captures a neutron to form a heavier isotope; if that isotope is unstable, it beta-decays and increases its atomic number before another capture occurs.¹ The path therefore follows mostly stable or long-lived nuclei rather than the extremely neutron-rich route of the r-process. “Slow” refers to the competition between reaction and decay rates, not necessarily to a uniform stellar timescale.

Neutron exposure—the time-integrated neutron flux—largely controls how far material moves toward heavier nuclei. Cross sections vary strongly with isotope and neutron energy, producing peaks near the magic neutron numbers 50, 82, and 126, where closed nuclear shells make further captures less likely.

Low-mass asymptotic giant branch stars are the principal site of the main S-process component. During thermal pulses, helium-shell burning creates carbon-rich material, while partial proton mixing can form a carbon-13 pocket; the reaction ¹³C(α,n)¹⁶O then supplies a relatively low neutron density over an extended period.² A second source, ²²Ne(α,n)²⁵Mg, becomes important at higher temperatures during pulses and in more massive AGB stars.

Massive stars produce a weak S-process during core helium burning and shell carbon burning, mainly making nuclei up to the strontium–yttrium–zirconium region.¹ The resulting abundance pattern depends on metallicity: fewer iron seeds at low metallicity can favor larger neutron exposure and enhance production near barium and lead.

The S-process is identified through characteristic abundance patterns rather than a single element. Its products include nuclei of strontium, zirconium, barium, lanthanum, cerium, lead, and bismuth, although several elements also receive contributions from the r-process and other mechanisms.¹ Solar abundances are commonly decomposed into estimated S-process and r-process components, a method that guides models but is not a direct separation of individual atoms.

Evidence comes from several independent sources: heavy-element enrichments in AGB stars, the compositions of stellar spectra, and isotopic anomalies in presolar silicon-carbide grains from meteorites. Technetium is especially informative because its radioactive isotopes have stellar-timescale half-lives; technetium-rich giant stars provide direct evidence that neutron-capture products can be freshly made and mixed to the surface.²

Small branch points in the S-process preserve information about stellar conditions. If an unstable nucleus captures a neutron before beta decay, the path branches, and the resulting isotope ratios can diagnose neutron density, temperature, and convective mixing.¹ Branching-sensitive isotopes therefore carry more environmental information than elemental abundances alone.

The process is not a single universal pattern: nuclear uncertainties in neutron-capture cross sections, the size and formation of the ¹³C pocket, stellar mass loss, and the treatment of convection all affect predictions. Some nuclei, called branching-point or neutron-magic nuclei, act as bottlenecks. Laboratory measurements of capture reactions and evaluated nuclear-data libraries are consequently central to interpreting stellar observations.³⁴

Neutron exposure

The time-integrated neutron flux experienced by stellar material; it summarizes the cumulative opportunity for neutron captures.

Branching point

An unstable nucleus at which neutron capture and beta decay compete, allowing alternative S-process paths.

13C pocket

A proton-enriched layer in some AGB-star models where carbon-13 forms and later supplies neutrons through alpha capture.

Magic number

A nuclear shell-closure number of protons or neutrons associated with unusually stable nuclei and reduced capture flow.