The r- and s-processes.The horizontal axis indicates increasing proton number Z, and the vertical axis indicates increasing neutron number N. In an r-process, a neutron is captured, and in an s-process the capture is followed by a beta decay.The +n next to an arrow indicates neutron capture.The e~ next to an arrow indicates a beta decay.
(Z, A + 1) will beta decay before it can capture another neutron:
If the neutron capture is rapid we call the sequence of reactions an r-process (Fig. 9.7). The nucleus (Z, A+1) will capture another neutron before it beta decays:
In either case, the resulting nucleus can either beta decay or capture a neutron, depending on the relative rates. When we have a string of nuclei for which neutron capture is favorable, the r-process allows the buildup of neutron rich nuclei. This will go on until so many neutrons are added that a beta decay breaks the chain. The r- and s- processes can explain the abundances of many of the heavier nuclei. (It should be noted that these are not equilibrium processes.)
The various nuclear processes that we have discussed are responsible for the presence of the heavy elements around us. We will see in later chapters how this material is spread into interstellar space. The net result is to produce the abundances shown in Fig. 9.8. Nuclear physics determines which elements are the most abundant.
When we discuss stellar structure in the next section, we will treat the nuclear physics as something that is known. We assume that once we know the composition of some region and the temperature, we can specify which nuclear reactions are important. Moreover, we assume that we know how the reaction rates depend on temperature. This is a very important point. We have already seen in this
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