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,γ)-(γ,n) equilibrium in which nuclei rapidly exchange neutrons but change charge much more slowly by beta decay. Freezeout 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,γ)-(γ,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,γ)-(γ,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. Nuclear physicists can use insights from our model to understand how their detailed theoretical or experimental nuclear data affect r-process nucleosynthesis during the (n,γ)-(γ,n) equilibrium phase. We have also prepared a number of Jupyter notebooks that allow the user to study (n,γ)-(γ,n) equilibrium isotopic abundances with either our simple nuclear physics model or more realistic ones(https://github.com/mengkel/ng-gn-abundances).