Experimental neutron capture data of ${}^{58}$Ni from the CERN n_TOF facility

The ${}^{58}$Ni($n,\gamma$) cross section has been measured at the neutron time of flight facility n_TOF at CERN, in the energy range from 27 meV up to 400 keV. In total, 51 resonances have been analyzed up to 122 keV. Maxwellian averaged cross sections (MACS) have been calculated for stellar temperatures of $kT=5–100$ keV with uncertainties of less than 6%, showing fair agreement with recent experimental and evaluated data up to $kT$ = 50 keV. The MACS extracted in the present work at 30 keV is $34.2\pm 0.6_{\mathrm{stat}}\pm 1.8_{\mathrm{sys}}$ mb, in agreement with latest results and evaluations, but 12% lower relative to the recent KADoNIS compilation of astrophysical cross sections. When included in models of the $s$-process nucleosynthesis in massive stars, this change results in a 60% increase of the abundance of ${}^{58}$Ni, with a negligible propagation on heavier isotopes. The reason is that, using both the old or the new MACS, ${}^{58}$Ni is efficiently depleted by neutron captures.

Figure 1

Top panel (a): Total experimental yield of ${}^{58}$Ni (1) together with the background from an empty sample holder (2), the ambient background (3), and the background from neutrons scattered off the sample (the neutron background) (4). Bottom panel (b): Experimental yields within the resonance-dominated region between 5 and 400 keV.

Figure 2

Capture yield of ${}^{58}$Ni—corrected for an empty frame, ambient background, and the neutron background—compared with the yield calculated from ENDF/B-VII.0 and ENDF/B-VII.1 resonance parameters. The n_TOF resonances are mostly consistent with ENDF/B-VII.1 data.

Figure 3

Top panel (a): Cross section from 27 meV to 10 keV neutron energy, compared to the sammy fit with two negative-energy resonances. Bottom panel (b): Comparison between the global sammy fit and an independent 1/$v$-fit to the data between 27 meV and 50 meV.

Figure 4

Examples of sammy fits of several resonances in the capture yield of ${}^{58}$Ni.

Figure 5

Top panel (a): Kernel ratios n_TOF vs ENDF/B-VII.0 and ENDF/B-VII.1. The weighted means of ratios are also indicated. Bottom panel (b): Distribution of kernel ratios from top panel.

Figure 6

Measured yield compared to the yield simulated by sesh code. While the two plots designated as n_TOF and n_TOF${}^{*}$ show the same data, the n_TOF${}^{*}$ plot is shown in 50 times coarser binning in order to reproduce the average yield behavior and to facilitate the visual comparison with sesh results.

Figure 7

MACS values from several sources [6, 31, 32, 33, 34] relative to the n_TOF data.

Figure 8

Compilation of MACS over the temperature range of astrophysical importance. The shaded region represents the uncertainty range for the n_TOF data.