s-process

The s-process (or slow neutron-capture process) is a nucleosynthesis process in stars consisting of neutron capture followed by beta decay, resulting in a nucleus with an mass number (A) one more and an atomic number (Z) one more (or one less). Many of the elements heavier than iron result from this process. The word "slow" designates neutron capture at a sufficiently slow rate that a nucleus generally subsequently experiences beta decay before it captures yet another neutron. The s-process is presumed to occur in some asymptotic giant branch (AGB) stars formed with some metallicity (from its ancestor stars) that includes some iron.

If many free neutrons are present and neutron capture is occurring so often that additional captures generally occur before any beta decay, the neutron capture is termed the r-process (rapid neutron-capture process), which produces some elements not produced by the s-process.