User:ComplexRational/R-process
From Wikipedia, the free encyclopedia
The rapid neutron-capture process, or so-called r-process, is a series of successive nuclear reactions in nuclear astrophysics that is responsible for the creation (nucleosynthesis) of approximately half the abundances of the atomic nuclei heavier than iron. The r-process entails a succession of rapid neutron captures (hence the name) by one or more seed nuclei, typically beginning with nuclei near 56Fe that are the heaviest produced in stars. Captures must be rapid enough such that another neutron may be captured before beta decay has sufficient time to occur. The sequence of captures is halted only when the increasingly neutron-rich nuclei cannot physically retain another neutron as they approach the neutron drip line, or at peaks of abundance at magic numbers where binding energy and stability increase.[1] The r-process therefore can occur only in locations where there exists a high density of free neutrons, on the order of 1024 free neutrons per cm3.[2][3] This amounts to almost a gram of free neutrons in every cubic centimeter, an astonishing number requiring extreme locations with temperatures of about one billion degrees. The r-process is known to occur in core-collapse supernovae (as part of supernova nucleosynthesis)[4] or decompression of neutron-star matter thrown off by a binary neutron star merger,[5] though the relative contributions of these sources to the astrophysical abundance of r-process elements is a matter of ongoing research.[6]
This is not a Wikipedia article: It is an individual user's work-in-progress page, and may be incomplete and/or unreliable. |
The r-process contrasts with the s-process, the other predominant mechanism for the production of heavy elements, in which neutron captures occur at a much slower rate. Together, the r- and s-processes account for almost the entire abundance of chemical elements heavier than iron. The s-process primarily occurs within ordinary stars, particularly AGB stars, where the neutron flux is sufficient to cause neutron captures to recur in accumulated heavy seed nuclei in intervals longer than beta decay half-lives. The r-process, however, requires many neutron captures to occur faster than beta-decay,[3] and no sufficient quantities of heavy seed nuclei are thus created in s-process nucleosynthesis to contribute to the production of significant amounts of heavy elements. Furthermore, as the r-process generates its own seed nuclei and can occur much farther from the valley of stability, it is entirely responsible for the production of the most neutron-rich isotopes of each element as well as all elements heavier than bismuth.[3] Hence, in heavy element formation, the s-process is secondary to the r-process.
A limited r-process-like series of neutron captures occurs to a minor extent in thermonuclear weapon explosions. These led to the discovery of the elements einsteinium (element 99) and fermium (element 100) in nuclear weapon fallout. The historical challenge has been to locate physical settings appropriate for their time scales; thus, it remains open whether the r-process can artificially be induced and if it is capable of producing elements heavier than fermium.