${}^{63}\mathrm{Ni}(n,\gamma )$ cross sections measured with DANCE

The neutron capture cross section of the $s$-process branch nucleus ${}^{63}\mathrm{Ni}$ affects the abundances of other nuclei in its region, especially ${}^{63}\mathrm{Cu}$ and ${}^{64}\mathrm{Zn}$. In order to determine the energy-dependent neutron capture cross section in the astrophysical energy region, an experiment at the Los Alamos National Laboratory has been performed using the calorimetric $4\pi {\mathrm{BaF}}_2$ array DANCE. The ($n,\gamma$) cross section of ${}^{63}\mathrm{Ni}$ has been determined relative to the well-known ${}^{197}\mathrm{Au}$ standard with uncertainties below 15%. Various ${}^{63}\mathrm{Ni}$ resonances have been identified based on the $Q$ value. Furthermore, the $s$-process sensitivity of the new values was analyzed with the new network calculation tool NETZ.

Figure 1

The $s$-process path in the nickel region, which branches at ${}^{63}\mathrm{Ni}$ and proceeds either to ${}^{64}\mathrm{Ni}$ or ${}^{63}\mathrm{Cu}$. The branching ratio depends on stellar temperatures, neutron densities, and the neutron capture CS of ${}^{63}\mathrm{Ni}$.

Figure 2

A sketch of the DANCE array. Almost half of the crystals are blended out to give an insight. Colors distinguish between different crystal shapes, and the blue disk indicates the neutron absorber.

Figure 3

2D plot of $E_{\mathrm{f}}$ versus $E_{\mathrm{s}}$. Signals from alphas separate in the encircled region.

Figure 4

The neutron flux at DANCE measured with the ${}^6\mathrm{Li}$ monitor. The absorption features have their origin in neutron captures in the material of beam line components, Al and Mn in this case.

Figure 5

A two-dimensional map of all detected capture events with $M_{\mathrm{cl}}>2$ and with $E_{\mathrm{n}}$ on the $y$ axis and $E_{\mathrm{tot}}$ on the $x$ axis. Neutron capture resonances appear as horizontal stripes. For the most prominent resonances pileup effects are visible but reach only relatively low count numbers. The red box indicates the cut on $E_{\mathrm{tot}}$ used for data selection.

Figure 6

The detected capture events per electronvolt between 100 eV and 500 keV with an $E_{\mathrm{tot}}$ cut from 9.2 to 9.7 MeV. ${}^{63}\mathrm{Ni}$ resonances are labeled with their energies with black font color. Backgrounds from other isotopes and absorption features from beamline components (red labels) are denoted as well.

Figure 7

The ratio of ${}^{63}\mathrm{Ni}$ events versus ${}^{62}\mathrm{Ni}$ events as a function of cluster multiplicity. ${}^{62}\mathrm{Ni}$ dominates at low multiplicities, which can be used for data selection and thus, background reduction.

Figure 8

Comparison of the measured cluster multiplicity distribution (red) with the simulation (blue). The measured distributions of the strongest resonance is depicted with black symbols.

Figure 9

The energy dependence of the correction factor for scattering in the sample ${\kappa }_{\mathrm{scatter}}$.

Figure 10

(a) The neutron capture cross section of ${}^{63}\mathrm{Ni}$ as a function of the neutron energy. The gray-colored peak at 200 eV was identified as a ${}^{59}\mathrm{Ni}$ resonance. (b) The MACS data are shown as a red line, the errors are depicted as a gray area. For comparison, data from the KADoNiS database (black) and from the n_TOF Collaboration (blue circles) published in Ref. [9] are plotted as well.

Figure 11

The time development of the neutron density $n_{\mathrm{n}}$ and the integrated neutron exposure ${\tau }_{\mathrm{n}}$ for one thermal pulse in a TP-AGB star. During the interpulse phase, the $s$ process is driven by the ${}^{13}\mathrm{C}$($\alpha ,n$) reaction in the ${}^{13}\mathrm{C}$ pocket. On the right side the short pulse phase is visible, where the ${}^{22}\mathrm{Ne}$($\alpha ,n$) reaction provides the neutrons for the $s$ process.

Figure 12

(a) The time development of the neutron density $n_{\mathrm{n}}$ and the integrated neutron exposure ${\tau }_{\mathrm{n}}$ for the He-core burning phase in massive stars. (b) The C-shell burning phase. In both cases the ${}^{22}\mathrm{Ne}$($\alpha ,n$) reaction is the neutron source. The runtime of the $s$ process during the C-shell burning is much shorter, but the neutron density is higher by orders of magnitude.

Figure 13

(a), (b) The relative production of the isotopes in the vicinity of ${}^{63}\mathrm{Ni}$ and lower panels for the weak $s$ process and the main $s$ process, respectively. (c), (d) Relative net yields for the elements from Ni to As for both scenarios.