The effect of fission cycling on r-process nucleosynthesis in neutron star mergers

Believed to be born from the Big Bang, the universe is made up of nearly 98% hydrogen and helium, with the remaining 2% being heavier elements from complex nucleosynthesis processes. However, due to the necessary extreme conditions, the mechanism behind the production of heavier, neutron-rich isotopes is not well understood. Neutron star mergers (NSMs) are expected to be the most promising astrophysical sites, as evidenced by the GW170817 NSM event where kilonova (gamma-ray bursts) were observed; this is indicative of r-process nucleosynthesis. To simulate the r-process in the conditions of a NSM, a nuclear physics library known as NucNet was used; afterwards, the calculation was compared to the isotopic and elemental abundances observed in our solar system. Fission reactions, including beta-delayed, neutron-induced, and spontaneous, were also implemented for exotic nuclides using theoretical nuclear models from GEF and TALYS to account for material that is cycled down to a lower mass. Adjusting parameters and adding fission reactions improved the results in the A = 120-192 / Z = 50-77 range, demonstrating not only that NSMs promote the r-process but also that fission cycling must be included to accurately simulate the mechanism.