New measurement of neutron capture resonances in ${}^{209}\mathrm{Bi}$

The neutron capture cross section of ${}^{209}\mathrm{Bi}$ has been measured at the CERN n_TOF facility by employing the pulse-height-weighting technique. Improvements over previous measurements are mainly because of an optimized detection system, which led to a practically negligible neutron sensitivity. Additional experimental sources of systematic error, such as the electronic threshold in the detectors, summing of γ-rays, internal electron conversion, and the isomeric state in bismuth, have been taken into account. γ-Ray absorption effects inside the sample have been corrected by employing a nonpolynomial weighting function. Because ${}^{209}\mathrm{Bi}$ is the last stable isotope in the reaction path of the stellar $s$-process, the Maxwellian averaged capture cross section is important for the recycling of the reaction flow by α decays. In the relevant stellar range of thermal energies between $\mathit{kT}=5$ and 8 keV our new capture rate is about 16% higher than the presently accepted value used for nucleosynthesis calculations. At this low temperature an important part of the heavy Pb-Bi isotopes are supposed to be synthesized by the $s$-process in the He shells of low mass, thermally pulsing asymptotic giant branch stars. With the improved set of cross sections we obtain an $s$-process fraction of 19±3% of the solar bismuth abundance, resulting in an $r$-process residual of 81±3%. The present ($n,\gamma$) cross-section measurement is also of relevance for the design of accelerator driven systems based on a liquid metal Pb/Bi spallation target.

Figure 1

The $s$-process reaction network terminates at ${}^{209}\mathrm{Bi}$. Shaded boxes designate stable isotopes. Decays from the α-unstable ${}^{210,211}\mathrm{Po}$ isotopes are represented with dashed lines. Quantities below the isotopic symbols show natural abundances or half-lives.

Figure 2

The geometry of the experimental setup used in the GEANT4 simulation of the weighting functions.

Figure 3

Comparison of experimental (dashed) and simulated (solid) pulse height spectra of ${}^{210}\mathrm{Bi}$ (left) and ${}^{198}\mathrm{Au}$ (right) illustrating the quality of the simulations.

Figure 4

$R$-matrix analysis of the first two resonances in bismuth. The dashed line corresponds to the yield calculated with the resonance parameters quoted by the ENDF/B-VI.8 evaluation.

Figure 5

Comparison of the radiative capture kernels derived from the measurements at n_TOF, GELINA, and ORNL. The capture area shown at 2.3 keV for ORNL corresponds to an estimation reported in Ref. [6]. The capture area shown at ∼12 keV corresponds to the sum of the three resonances in that region.