Mechanisms of slow neutron capture. Image source: commons.wikimedia.org

Slow neutron capture by nuclei in massive stars. In the s-process, one starts with existing iron-group nuclei. Therefore, it would only be expected to take place in second-generation stars that collapsed out of the residue of a previous supernova explosion. The flux of neutrons is small enough that rate of neutron capture by atomic nuclei is slow relative to the rate of radioactive beta-decay. These neutrons come from various reactions in the He-burning region of a red giant star. Hundreds to thousands of years may pass between successive neutron captures. In this situation, a seed nucleus will slowly capture neutrons, for example 56Fe → 57Fe → 58Fe → 59Fe, followed by 59Fe → 59Co (β decay). This process builds nuclei by climbing the line of stability, until 208Pb and 209Bi are reached. Beyond this point, no nuclei are stable enough to allow neutron capture to operate so that actinides cannot be synthesized by the s-process.

The s-process is one of three nucleosynthesis processes that also includes the r-process and the rp-process.

Some or all content above used with permission from J. H. Wittke.