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, A = 130 and A = 196. Further, all the elements heavier than bismuth, including natural thorium and uranium must ultimately originate in an r-process nucleus.