Neutron Capture Cross Section of Unstable ${}^{63}\mathrm{Ni}$: Implications for Stellar Nucleosynthesis

The ${}^{63}\mathrm{Ni}(n,\gamma )$ cross section has been measured for the first time at the neutron time-of-flight facility n_TOF at CERN from thermal neutron energies up to 200 keV. In total, capture kernels of 12 (new) resonances were determined. Maxwellian averaged cross sections were calculated for thermal energies from $kT=5–100\mathrm{keV}$ with uncertainties around 20%. Stellar model calculations for a $25M_{\odot }$ star show that the new data have a significant effect on the $s$-process production of ${}^{63}\mathrm{Cu}$, ${}^{64}\mathrm{Ni}$, and ${}^{64}\mathrm{Zn}$ in massive stars, allowing stronger constraints on the Cu yields from explosive nucleosynthesis in the subsequent supernova.

Figure 1

The $s$-process reaction path in the Ni-Cu-Zn region during He core burning (dashed lines) and C shell burning (solid lines).

Figure 2

Capture yield of the ${}^{63}\mathrm{Ni}$ sample (black) compared to the empty PEEK container and to the spectrum obtained with a pure ${}^{62}\mathrm{Ni}$ sample. The background from the activity of the sample and from ambient radiation is almost negligible at keV energies. The capture yield of the ${}^{62}\mathrm{Ni}$ sample was scaled for the ${}^{62}\mathrm{Ni}$ mass present in the ${}^{63}\mathrm{Ni}$ sample. The first resonance at 203 eV (marked by an arrow) is assigned to a small impurity of ${}^{59}\mathrm{Ni}$ in the sample.

Figure 3

(a) Final isotopic $s$-process distributions using the new measured ${}^{63}\mathrm{Ni}$ MACS (red circles) and the MACS quoted by KADoNiS (blue squares) [17]. The distribution is normalized to solar system abundances. (b) Ratio of the two distributions in (a), zoomed in the Ni-Cu-Zn mass region. Isotopes of the same element are connected by solid lines.