r-process
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In nuclear astrophysics, the rapid neutron-capture process, also known as the r-process, is a set of nuclear reactions that is responsible for the creation of approximately half of the atomic nuclei heavier than iron, the "heavy elements", with the other half produced largely by the s-process. The r-process synthesizes the more neutron-rich of the stable isotopes of even elements, and those separated from the beta-stable isotopes by those that are not often have very low s-process yields and are considered r-only nuclei; the heaviest isotopes of most even elements from zinc to mercury fall into this category. Abundance peaks for the r-process occur near mass numbers A = 82 (elements Se, Br, and Kr), A = 130 (elements Te, I, and Xe) and A = 196 (elements Os, Ir, and Pt). Further, all the elements heavier than bismuth, including natural thorium and uranium (and other actinides) must ultimately originate in an r-process nucleus.
The r-process entails a succession of rapid neutron captures (hence the name) by one or more heavy seed nuclei, typically beginning with nuclei in the abundance peak centered on 56Fe. The captures must be rapid in the sense that the nuclei must not have time to undergo radioactive decay (typically via β− decay) before another neutron arrives to be captured. This sequence can continue up to the limit of stability of the increasingly neutron-rich nuclei (the neutron drip line) to physically retain neutrons as governed by the short range nuclear force. The r-process therefore must occur in locations where there exists a high density of free neutrons. At some time following the neutron captures, the nucleus beta-decays back to the line of stability (just as with fission products) resulting in a stable isotope of the same mass number A, and normally the most neutron-rich of those.
Early studies theorized that 1024 free neutrons per cm3 would be required, for temperatures of about 1 GK, in order to match the waiting points, at which no more neutrons can be captured, with the mass numbers of the abundance peaks for r-process nuclei.[1] This amounts to almost a gram of free neutrons in every cubic centimeter, an astonishing number requiring extreme locations. Traditionally this suggested the material ejected from the re-expanded core of a core-collapse supernova, as part of supernova nucleosynthesis,[2] or decompression of neutron star matter thrown off by a binary neutron star merger in a kilonova.[3] The relative contribution of each of these sources to the astrophysical abundance of r-process elements is a matter of ongoing research as of 2018.[4]
An r-process-like series of neutron captures (on uranium-238 normally) occurs to a minor extent in thermonuclear weapon explosions, and can be enhanced by purposeful design. The elements einsteinium (element 99) and fermium (element 100) in nuclear weapon fallout, and in general this neutron capture results in isotopes as heavy as A = 257.
The r-process contrasts with the s-process, the other predominant mechanism for the production of heavy elements, which is nucleosynthesis by means of slow captures of neutrons. In general, isotopes involved in the s-process have half-lives long enough to enable their study in laboratory experiments, but this is not typically true for isotopes involved in the r-process.[5] The s-process primarily occurs within ordinary stars, particularly AGB stars, where the neutron flux is sufficient to cause neutron captures to recur every 10–100 years, much too slow for the r-process, which requires up to 100 captures per second. The s-process is secondary, meaning that it requires pre-existing heavy isotopes as seed nuclei to be converted into other heavy nuclei by a slow sequence of captures of free neutrons. The r-process scenarios create their own seed nuclei, so they might proceed in massive stars that contain no heavy seed nuclei. Taken together, the r- and s-processes account for almost the entire abundance of chemical elements heavier than iron. The historical challenge has been to locate physical settings appropriate to their time scales.
- ^ Burbidge, E. M.; Burbidge, G. R.; Fowler, W. A.; Hoyle, F. (1957). "Synthesis of the Elements in Stars". Reviews of Modern Physics. 29 (4): 547–650. Bibcode:1957RvMP...29..547B. doi:10.1103/RevModPhys.29.547.
- ^ Thielemann, F.-K.; et al. (2011). "What are the astrophysical sites for the r-process and the production of heavy elements?". Progress in Particle and Nuclear Physics. 66 (2): 346–353. Bibcode:2011PrPNP..66..346T. doi:10.1016/j.ppnp.2011.01.032. S2CID 119412716.
- ^ Kasen, D.; Metzger, B.; Barnes, J.; Quataert, E.; Ramirez-Ruiz, E. (2017). "Origin of the heavy elements in binary neutron-star mergers from a gravitational-wave event". Nature. 551 (7678): 80–84. arXiv:1710.05463. Bibcode:2017Natur.551...80K. doi:10.1038/nature24453. PMID 29094687.
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Frebel, A.; Beers, T. C. (2018). "The formation of the heaviest elements". Physics Today. 71 (1): 30–37. arXiv:1801.01190. Bibcode:2018PhT....71a..30F. doi:10.1063/pt.3.3815.
Nuclear physicists are still working to model the r-process, and astrophysicists need to estimate the frequency of neutron-star mergers to assess whether r-process heavy-element production solely or at least significantly takes place in the merger environment.
- ^ Cowan, John J.; Thielemann, Friedrich-Karl Thielemann (2004). "R-Process Nucleosynthesis in Supernovae" (PDF). Physics Today. 57 (10): 47–54. Bibcode:2004PhT....57j..47C. doi:10.1063/1.1825268.