R process

The rapid neutron capture process or r-process is a set of reactions in nuclear astrophysics that are responsible for the creation (nucleosynthesis) of approximately half the atomic nuclei heavier than iron.

This process entails a succession of rapid neutron captures (hence the name) by heavy seed nuclei, typically beginning with ⁵⁶Fe. The captures must be rapid in the sense that the nucleus does not have time to undergo radioactive decay before another neutron arrives to be captured. The r-process therefore occurs in locations where there is a high flux of free neutrons. These include the material ejected from a core-collapse supernovae (as part of supernova nucleosynthesis) and neutron-rich matter thrown off from a neutron star merger. The relative contributions of these, and other, sources to the astrophysical abundance of r-process elements is a matter of ongoing research.

The r-process also occurs to a minor extent in thermonuclear weapon explosions. This led to the historical discovery of the elements einsteinium (element 99) and fermium (element 100) in nuclear weapon fallout.

The r-process is distinguished from the s-process, the other predominant mechanism for the production of heavy elements, which is nucleosynthesis by means of slow captures of neutrons. The s-process primarily occurs within stars, particularly AGB stars, where the neutron flux is sufficient to cause reactions but too low for the r-process. The s-process is secondary, meaning that it requires pre-existing heavy isotopes as seed nuclei to be converted into other heavy nuclei. Taken together, the r- and s-processes account for the majority of abundance evolution of elements heavier than iron.

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