Low-energy 17O(n,g)18O reaction within the microscopic potential model and its role for the weak r-process

TL;DR

This work advances the microscopic calculation of the $^{17}\mathrm{O}(n,\gamma)^{18}\mathrm{O}$ cross section using a Skyrme-Hartree-Fock potential model to describe direct and resonant $E1$ captures into bound states below the $\alpha$-decay threshold. By applying two Skyrme forces (SLy4, SkP) and adjusting bound-state depths and spectroscopic factors, the authors obtain cross sections that diverge notably from standard libraries, particularly at low energies where sub-threshold resonances dominate. Incorporating the new cross section into large-scale reaction networks reveals that, under a focused set of $Y_e$ and $S_0$ conditions, $^{17}$O$(n,\gamma)$ can significantly influence the production of the first $r$-process peak elements (e.g., Sr, Kr), via shifts in the reaction-flow pathways between $\alpha$-process and neutron-capture regimes. The findings highlight the importance of accurate, nucleus-specific data for light isotopes in modeling heavy-element nucleosynthesis and motivate updates to reaction-rate libraries used in astrophysical simulations.

Abstract

The neutron radiative capture reaction $^{17}$O(n,$γ$)18O plays a pivotal role in both nuclear structure studies and astrophysical nucleosynthesis, particularly in the formation of elements during hydrostatic and explosive stellar environments. We calculated the $^{17}$O(n,$γ$)$^{18}$O cross section within the Skyrme Hartree-Fock potential model and analyzed electric dipole E1 transitions to both positive and negative-parity states below the alpha-decay threshold in $^{18}$O. Our cross sections are significantly different from the data available in commonly used libraries. We further investigate the impact of the new calculated cross section on weak r-process nucleosynthesis using large-scale reaction network calculations across a wide range of electron fractions and entropies. Our results show that the $^{17}$O(n, $γ$)$^{18}$O reaction rate significantly influences the production of first r-process peak elements, such as strontium, under specific astrophysical conditions. This study highlights the importance of accurate nuclear dat$ for light isotopes in modeling heavy-element synthesis and provides updated reaction rates for future nucleosynthesis simulations.

Figures (12)

• Figure 1: The structure of $^{18}\mathrm{O}$ and $E1$ transitions. The black dashed arrows indicate the transitions $J_s^- \to J^+_b$, while the blue dashed arrows indicate the transitions $J_s^+ \to J^-_b$. The energies are in MeV.
• Figure 2: The partial-wave analysis for the $E1$ transitions from $J^-_s \to J^+_b$ in $^{17}\mathrm{O}(n,\gamma)^{18}\mathrm{O}$ with SLy4 interaction.
• Figure 3: The partial-wave analysis for the $E1$ transitions from $J^+_s \to J^-_b$ in $^{17}\mathrm{O}(n,\gamma)^{18}\mathrm{O}$ with SLy4 interaction. A resonance is observed at low energy near the threshold, attributed to the scattering $d_{3/2}$ wave.
• Figure 4: Same as Figure \ref{['fig:SLy4_minus']} but for SkP interaction. No resonance is observed above the threshold, the resonance is located below the threshold.
• Figure 5: Cross section of the $^{17}\mathrm{O}(n,\gamma)^{18}\mathrm{O}$ reaction using the SLy4 interaction. The results are compared with those from Ref. zhang2022. The resonance is located near the resonances analyzed in Ref. zhang2022.