Since the s-process starts with existing iron-group nuclei, it is only expected to take place in second-generation stars that have collapsed out of the residue of previous supernova explosions. A flux of neutrons is required, and it is most likely that these neutrons come from various reactions in the helium-burning region of a red giant. The seed isotope Z, A, from the iron region absorbs a neutron, changing from A to A + 1. If the new isotope is stable, it can absorb another neutron, going to A + 2. If it is unstable, it is assumed that the neutron capture rate is low enough that the nuclide has plenty of time to decay to Z + 1 by beta emission before the next capture. The same neutron-absorption process is then repeated for Z + 1. Thus, the nuclides produced lie in the "valley of beta stability" of the chart of the nuclides.
Related category STELLAR ASTROPHYSICS
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