Mechanisms of rapid neutron capture. Image Source: www.asc.ohio-state.edu

Rapid (hence “r”) absorption of neutrons by atoms when the neutron flux is very high (~1022 neutrons per cm2/s) and the temperature is very high (T > 109 K). These conditions are hypothesized to occur during a supernova explosion/collapse or neutron star mergers. The time between neutron captures is much shorter than the average b decay half-life (on the order of 0.1 to 1 seconds) of these neutron-rich nuclei. Capture moves the nucleus toward “neutron drip line” where the probability for absorbing a new neutron is overwhelmed by the probability that a neutron will be knocked off by photodisintegration. This balance point defines the (n, γ) ↔ (γ, n) equilibrium. The path of nucleosynthesis moves up along a line somewhere between the valley of stability and the neutron drip line (the offset depending on conditions such as temperature, neutron flux, and photon flux) until finally fission blocks the chain in the actinide region. Nuclei with “magic” neutron numbers serve as bottlenecks to nuclei climbing the r-process path. For example, 130Cd is an isotope with the A = 82 magic number, but the heaviest stable isotope of cadmium is 116Cd with 14 fewer neutrons.

If the neutron source only lasts for a short time, highly unstable nuclei will be left on the r-process path, with many stuck at the “magic” bottlenecks. These undergo b decay back to the line of stability. In our example, 130Cd would eventually decay to 130Te, the most abundant isotope of tellurium. Since β decay reduces the number of neutrons, abundance peaks show up at lower neutron number than the s-process peaks.

In some cases, the r-process may be fast enough to break through the region of α-instability beyond 208Pb. The stable actinides may be produced directly from a neutron-rich precursor, or from α-decay of even heavier elements.

The r-process is one of three nucleosynthesis processes that also includes the s-process and the rp-process.

Some or all content above used with permission from J. H. Wittke.