Successive Neutron capture of several neutrons by a parent nuclide can occur by either the s-process (slow) or the r-process (rapid), depending upon the flux density of the neutrons (i.e. the average amount of time before successive neutron capture events). If the amount of time before another neutron capture event can occur is longer than the mean decay time, then the daughter nuclide will probably decay before the next neutron capture event can occur. This is the s-process, the slow process. If, on the other hand, the neutron flux is high, then the daughter nuclide has in-sufficient time to decay before the next neutron capture event occurs. This is the r-process, or rapid process.

The two processes result in a different set of nuclides being generated by neutron capture events. Both processes result in nucleo-synthesis. (The n-process, neutron process, is a mixture of s- and r-processes and is capable of synthesizing a wide variety of heavy elements outside supernovae).

With these processes, nucleo-synthesis is capable of producing almost every nuclide on the neutron rich side of the stability line, with the exception of a few stable proton-rich nuclides which cannot be produced by either the s-process or the r-process, such as ruthenium-96, which has an isotopic abundance of 5.5%. In this case, the inverse beta decaying technetium-96 and the stable molybdenum-96 screens the production of ruthenium-96. Such nuclides are probably produced by p-processes (proton capture). These stable proton-rich nuclides are in islands on the upper part of the Segre chart.