R-process

The r-process is a nucleosynthesis process that occurs in core-collapse supernovae (see also supernova nucleosynthesis) and is responsible for the creation of approximately half of the neutron-rich atomic nuclei heavier than iron. The process entails a succession of rapid neutron captures (hence the name r-process) by heavy seed nuclei, typically ⁵⁶Fe or other more neutron-rich heavy isotopes.

The other predominant mechanism for the production of heavy elements in the universe (and in the Solar System) is the s-process, which is nucleosynthesis by means of slow captures of neutrons, primarily occurring in AGB stars. The s-process is secondary, meaning that it requires preexisting heavy isotopes as seed nuclei to be converted into other heavy nuclei. Taken together, these two processes account for a majority of galactic chemical evolution of elements heavier than iron.

The r-process occurs to a slight extent in thermonuclear weapon explosions, and was responsible for the historical discovery of the elements einsteinium (element 99) and fermium (element 100).

The need for some kind of rapid capture of neutrons was seen from the relative abundances of isotopes of heavy elements given in a newly published table of abundances by Hans Suess and Harold Urey in 1956. Radioactive isotopes must capture another neutron faster than they can undergo beta decay in order to create abundance peaks at germanium, xenon, and platinum. According to the nuclear shell model, radioactive nuclei that would decay into isotopes of these elements have closed neutron shells near the neutron drip line, where more neutrons cannot be added. Those abundance peaks created by rapid neutron capture implied that other nuclei could be accounted for by such a process. That process of rapid neutron capture in neutron-rich isotopes is called the r-process. A table apportioning the heavy isotopes phenomenologically between s-process and r-process was published in the famous B²FH review paper in 1957, which named that process and outlined the physics that guides it. B²FH also elaborated the theory of stellar nucleosynthesis and set substantial frame-work for contemporary nuclear astrophysics.

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