Supernova Nucleosynthesis - The R-process

The R-process

During supernova nucleosynthesis, the r process (r for rapid) creates very neutron-rich heavy isotopes, which decay after the event to the first stable isotope, thereby creating the neutron-rich stable isotopes of all heavy elements. This neutron capture process occurs in high neutron density with high temperature conditions. In the r process, any heavy nuclei are bombarded with a large neutron flux to form highly unstable neutron rich nuclei which very rapidly undergo beta decay to form more stable nuclei with higher atomic number and the same atomic weight. The neutron flux is astonishingly high, about 1022 neutrons per square centimeter per second. First calculation of a dynamic r process, showing the evolution of calculated results with time, also suggested that the r process abundances are a superposition of differing neutron fluences. Small fluence produces the first r process abundance peak near atomic weight A=130 but no actinides, whereas large fluence produces the actinides uranium and thorium but no longer contains the A=130 abundance peak. These processes occur in a fraction of a second to a few seconds, depending on details. Hundreds of subsequent papers published have utilized this time-dependent approach. Interestingly, the only modern nearby supernova, 1987A, has not revealed r process enrichments. Modern thinking is that the r process yield may be ejected from some supernovae but swallowed up in others as part of the residual neutron star or black hole.