Nucleosynthesis – r-s-p-Processes
Channel: Dominik Hezel
Date: 02/09/2019

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Elements with mass-numbers above Fe are produced in collisions of neutrons with an element’s nucleus. Supernova explosions, for example, free enormous amounts of neutrons. When many collisions occur, e.g., in an environment of high neutron density, individual nuclei can be hit by neutrons in rapid succession, which the nuclei can then capture. This produces nuclides of increasing mass. This is called the r-process (rapid). This process can produce super-heavy nuclides that are unstable and usually quickly decay via beta^- reaction. This decay results in nuclides of higher atomic numbers. If collisions are comparatively sparse, neutron capture is less frequent. This is called the s-process (slow). Super-heavy nuclides are not formed, as the first super-heavy radioactive nuclide in most cases decays more quickly than the next super-heavy nuclide is formed in the s-process. In cases stable nuclides cannot be formed by the s-process, as an unstable nuclide with short half-life is positioned  directly before it. This so protected stable nuclide can then only be formed in the r-process. The r- and s-process nuclide abundances of each so-formed element is known from model calculations. A few nuclides cannot be formed by any of these two processes. These are always the lightest nuclides of an element, that cannot be reached by an r- or s-process reaction chain. Their formation is yet still not entirely clear, and is often collectively called p-process. This name refers to a possible process in which protons are added to the nucleus. Another possibility is the (gamma,xn)-reaction (gamma capture, loss of x neutrons) and subsequent beta^+ decay.