CoachNeutron capture is the absorption of a free neutron by a nucleus, . In the s process (slow), the captures are slower than most beta decays: the time between two captures on one nucleus is hundreds to thousands of years, so an unstable nucleus nearly always decays before it can capture, and the chain of captures and decays follows the valley of stability. In the r process (rapid), the neutron density is so high that captures are far faster than the decays: the nuclei are driven to very neutron-rich isotopes, far from stability, in a few seconds, and decay back to stability once the neutrons run out.
