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 by the p-process and s -process. The r -process usually synthesizes the most neutron-rich stable isotopes of each heavy element. The r -process can typically synthesize the heaviest four isotopes of every heavy element; of these, the heavier two are called r-only nuclei because they are created exclusively via the r -process. Abundance peaks for the r -process occur near mass numbers A = 82 (elements Se, Br, and Kr), A = 130 (elements Te, I, and Xe) and A = 196 (elements Os, Ir, and Pt).