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The r-process (rapid neutron-capture process) is a nucleosynthesis process consisting of repeated neutron captures more frequent than beta decay (because many neutrons are being produced and/or beta decay is being suppressed), resulting in synthesis of isotopes with a mass number more than one higher, while remaining with the same atomic number. When conditions change so the rate of neutron captures no longer exceeds that of beta decays, the latter repeat until the nucleus reaches a stable configuration, but the resulting abundances of elements and isotopes are not what they would otherwise be. The r-process explains the abundances of some elements, such as germanium, xenon, platinum and other elements heavier than iron and nickel.
If fewer free neutrons are present and beta decay generally occurs before any subsequent neutron capture, the neutron capture is termed the s-process (slow neutron-capture process), which produces some elements not produced by the r-process.
The process is presumed to occur in core collapse supernovae due to models but observational evidence has only been indirect, via the success of such models in explaining other observable phenomena. More-direct evidence would be spectral lines from the particular radioactive elements that an r-process would produce, that have light curves indicating they are disappearing and/or the analogous growing appearance of the elements that would result from their subsequent beta decay. The first observation of this was in the optical counterpart of the GW detection GW170817, which was a type of neutron star merger termed a kilonova. The possibility of the r-process occurring in kilonovae had already been theorized.
Neutron capture requires free neutrons, which have a limited lifetime (a mean lifetime of about 880 seconds). In events that produce such free neutrons, the ratio of neutrons to all nucleons at each point in space and time is of interest. Often, the complementary ratio of protons to all nucleons is what is specified in calculations, and often its equivalent (presuming overall neutrality), the ratio of electrons to nucleons, which is termed (for this purpose) the electron fraction (Ye). The r-process is more likely when this ratio is small, and basically won't occur if this ratio is 0.5 or more.
The spectral signatures of elements attributed to the r-process can be used to identify and age old stars. Very old (high redshift?) galaxies have been observed with r-process-produced elements but not s-process-produced elements, suggesting that at an early time, the r-process was working but the s-process was not. Individual stars with similar characteristics are presumed to be of similar age.
The weak r-process refers to a limited r-process, specified as that producing nuclei with mass numbers of no more than 130. Other (i.e., not weak) r-process is sometimes called the strong r-process (or occasionally the main r-process), which produces mass numbers above 130 and larger atomic numbers: lanthanides are considered products of the strong r-process.