A type of nuclear reaction that has been proposed to explain the formation of stable, neutron-rich isotopes of certain heavy elements including tin, cadmium, indium, and antimony. It involves the capture of neutrons by elements such as iron sufficiently slowly (in contrast to the r-process) for the resulting isotope, if unstable, to have time to decay to a stable isotope before it captures another neutron. This requires a relatively weak flux of neutrons, as found in red giants between shells of nuclear burning.