The role of near neutron drip-line nuclei in the $r$-process

Abstract

The role of near neutron-drip-line nuclei in the rapid neutron-capture process ($r$-process) is studied with the classical $r$-process model. Simulations under different astrophysical conditions ($T$, $n_n$) show that $r$-process paths approach the neutron-drip line under low-temperature and high-neutron-density conditions. A sensitivity study reveals that variations in the nuclear masses of these exotic nuclei significantly impact the abundances of superheavy nuclei, and lead to obvious abundance variations in the $A=110-125$, $A=175-185$, and $A=200-205$ regions. By contrast, the $r$-process rare-earth peak and the $A=130,195$ peaks remain largely unaffected. The nuclei that obviously impact $r$-process abundances are mainly distributed in the region of $25\leq Z\leq 90$ and $50\leq N\leq 180$, with the nuclei around neutron magic numbers found to be particularly important for the $r$-process, even in the near-neutron-drip-line region.

Figures (7)

• Figure 1: $r$-process nucleosynthesis paths under different astrophysical conditions, characterized by temperature ($T$) and neutron number density ($n_n$).
• Figure 2: The $r$-process abundances under different astrophysical conditions, characterized by temperature ($T$) and neutron number density ($n_n$), with a fixed irradiation time of $\tau=850~{\rm ms}$.
• Figure 3: The averaged distances between $r$-process paths and the neutron-drip line under different astrophysical conditions, characterized by neutron number density $n_n$ and temperature $T$. Each $r$-process path is divided into three segments based on neutron number $N$: $N\leq82$ (light nuclei), $82\leq N\leq126$ (medium nuclei), and $N\geq126$ (heavy nuclei). The black lines denote the astrophysical conditions that first enable the $r$-process paths to reach the neutron-drip line.
• Figure 4: (a) $r$-process paths and (b) $r$-process abundances under four typical sets of astrophysical conditions. The neutron number densities are $n_n = {10}^{25} \mathrm{~cm}^{-3}$, $n_n = {10}^{27} \mathrm{~cm}^{-3}$, $n_n = {10}^{28} \mathrm{~cm}^{-3}$, and $n_n = {10}^{30} \mathrm{~cm}^{-3}$, with fixed irradiation time $\tau = 850~{\rm ms}$ and temperature $T = 1.5~{\rm GK}$.
• Figure 5: Illustrations of the different nuclear collections varied in the sensitivity studies: set 1, sets 1 to 5, sets 1 to 10, and sets 1 to 20. Each collection consists of neutron-rich nuclei classified by their distance relative to the neutron-drip line of their respective isotopic chains. The $r$-process path is obtained by weighted superposition of the four paths in Fig. \ref{['fig4']} (a). The inset subplot shows the detailed partitioning of the 20 most neutron-rich nuclei in each isotope chain into each collection.