sprocessus

The sprocessus, often written as the s-process, or slow neutron capture process, is a mode of stellar nucleosynthesis in which atomic nuclei capture neutrons at a rate slow enough that beta decay can occur before another capture. As neutrons are added, the nucleus moves to higher atomic numbers along the path of beta stability, favoring stable isotopes and producing heavier elements up to bismuth.

Astrophysical sites for the sprocessus include asymptotic giant branch stars (low- to intermediate-mass stars) where neutron captures occur during thermal pulses, and massive stars where the process operates during helium-core and carbon-shell burning. These environments give rise to a main s-process component (in AGB stars) and a weaker component (in massive stars).

Neutron sources driving the sprocessus differ by site. In low-mass AGB stars, the 13C(alpha,n)16O reaction in the so-called 13C pocket provides neutrons at relatively modest intensities. In more massive stars, the 22Ne(alpha,n)25Mg reaction becomes important at higher temperatures during convective burning. Neutron densities are comparatively modest, typically ranging from about 10^7 to 10^11 neutrons per cubic centimeter, with exposure over timescales of thousands of years in stars.

The resulting abundance pattern features peaks near neutron magic numbers and extends to lead and bismuth. Nuclear data, including cross sections and branching points, determine the flow through the network. Evidence comes from solar-system abundances, spectroscopic measurements of stars, and isotopic signatures in presolar grains.

Compared with the r-process, which operates under intense neutron flux in explosive environments, the sprocessus builds heavy elements by gradual neutron capture and beta decay, contributing substantially to the Galaxy’s chemical evolution.