Dependence of (n, γ)-(γ, n) equilibrium r-process abundances on nuclear physics properties

In most r-process expansions, the dominant nuclear evolution occurs in an (n, gamma )-(gamma , n) equilibrium in which nuclei rapidly exchange neutrons but change charge much more slowly by beta decay. Freeze-out from this equilibrium shapes the final abundances but does not significantly alter the overall global abundance pattern; therefore, it is important to understand the details of (n, gamma )-(gamma , n) equilibrium both because it is the main evolution phase that determines the final abundance pattern and because it is the starting point for the freeze-out. Through use of a simple but realistic phenomenological nuclear physics model, we show that isotopic abundances versus neutron number in (n, gamma )-(gamma , n) equilibrium are well approximated as Gaussians. Nuclear pairing causes isotopic abundances to alternate between two Gaussians, and shell effects cause the isotopic abundances to shift from one Gaussian to another when the neutron number crosses a magic number. More complex neutronseparation energy curves versus mass number can be generated by adding a spike function to a linearly declining curve. In such a case, the equilibrium abundance curve jumps from one Gaussian to another for each added spike. Insights from our model can help shed light on how detailed theoretical or experimental nuclear data affect r-process nucleosynthesis during the (n, gamma )-(gamma , n) equilibrium phase.

Automated summary

This passage discusses the nuclear evolution and abundance pattern formation during r-process expansions. The isotopic abundances in equilibrium are approximated as Gaussians, and are influenced by nuclear pairing and shell effects. Understanding these details is crucial for studying r-process nucleosynthesis.