Nucleosynthesis at the termination point of the $s$ process

Stellar cross sections of importance with respect to the termination of the $s$-process reaction chain have been determined for the two cases, ${}^{208}\mathrm{Pb}(\mathit{n},\gamma ){}^{209}\mathrm{Pb}$ and ${}^{209}\mathrm{Bi}(\mathit{n},\gamma ){}^{210}\mathrm{Bi}^g$, yielding $kT=30\mathrm{keV}$ values of $⟨\sigma v⟩∕v_T=0.31\pm 0.2\mathrm{mb}$ and $2.54\pm 0.14\mathrm{mb}$, respectively. The measurements were carried out by activation of Pb and Bi samples in a quasistellar neutron spectrum using gold as a cross section standard. With this technique the uncertainties reported in previous works could be considerably reduced. The measurements are complemented by a discussion of the recycling at the termination point of the $s$-process neutron capture chain in a $3M_{☉}$ and $[\mathrm{Fe}∕\mathrm{H}]=-1.3$ asymptotic giant branch star. At this metallicity, AGB stars give rise to the maximum production of $s$-process lead. The sensitivity of the isotopic lead abundances is discussed with respect to the remaining cross section uncertainties. The information obtained in this work is also of relevance for an assessment of the $\alpha$ activity due to a buildup of ${}^{210}\mathrm{Po}$ in $\mathrm{Pb}∕\mathrm{Bi}$ cooled fast reactor systems.

Figure 1

The reaction network at the end of the $s$-process path. Stable isotopes are indicated with shaded boxes. Note, that the recycling effect via the $\alpha$-unstable polonium isotopes is weak. Most of the reaction flow is trapped at ${}^{208}\mathrm{Pb}$ and ${}^{209}\mathrm{Bi}$ because of the small $(\mathit{n},\gamma )$ cross sections of these neutron magic nuclei. Because the isomeric state in ${}^{210}\mathrm{Bi}$ is quickly depopulated to the short-lived ground state by the hot photon bath at $s$-process temperatures, its $\alpha$-decay and neutron capture contributions are negligible.

Figure 2

Decay schemes of ${}^{209}\mathrm{Pb}$ and ${}^{210}\mathrm{Bi}^g$ illustrating the features of the present activations.

Figure 3

Sketch of the activation setup at the Van de Graaff accelerator.

Figure 4

Schematic setup of the $4\pi$ electron spectrometer.

Figure 5

The ${\beta }^{-}$ spectra taken with samples Pb4 (top) and Bi3 (bottom).

Figure 6

The ${}^{210}\mathrm{Bi}$ activity vs time fits perfectly to the well-known half-life, excluding significant background from activation of unexpected sample impurities.

Figure 7

Production factors for the $\mathrm{Tl}\text{−}\mathrm{Pb}\text{−}\mathrm{Bi}$ isotopes during the 25th thermal pulse in a $3M_{☉}$ star with $[\mathrm{Fe}∕\mathrm{H}]=-1.3$. The data are normalized to the final ${}^{204}\mathrm{Pb}$ abundance.