${}^{62}\mathrm{Ni}$($n,\gamma$) and ${}^{63}\mathrm{Ni}$($n,\gamma$) cross sections measured at the n_TOF facility at CERN

The cross section of the ${}^{62}\mathrm{Ni}$($n,\gamma$) reaction was measured with the time-of-flight technique at the neutron time-of-flight facility n_TOF at CERN. Capture kernels of 42 resonances were analyzed up to 200 keV neutron energy and Maxwellian averaged cross sections (MACS) from $kT=$ 5–100 keV were calculated. With a total uncertainty of $4.5$%, the stellar cross section is in excellent agreement with the the KADoNiS compilation at $kT=30$ keV, while being systematically lower up to a factor of 1.6 at higher stellar temperatures. The cross section of the ${}^{63}\mathrm{Ni}$($n,\gamma$) reaction was measured for the first time at n_TOF. We determined unresolved cross sections from 10 to 270 keV with a systematic uncertainty of 17%. These results provide fundamental constraints on $s$-process production of heavier species, especially the production of Cu in massive stars, which serve as the dominant source of Cu in the solar system.

Figure 1

Capture yield of ${}^{62}\mathrm{Ni}$ using water (black) and borated water (red) as moderator. The addition of boron yields a significant reduction of the photon-induced background in the keV region.

Figure 2

(Top) Capture yield of ${}^{62}\mathrm{Ni}$ (black, solid line) compared with backgrounds from neutron reactions in surrounding materials (pink, solid line, measured with empty sample holder) and ambient background (blue, shaded line). While the ambient background is 2 orders of magnitude smaller than the signal over the whole energy range, the empty background plays a crucial role in the higher keV range. (Bottom) Zoom into the neutron energy region from 6 to 100 keV.

Figure 3

Capture yield of ${}^{63}\mathrm{Ni}$ (green) compared with backgrounds from neutron reactions on ${}^{62}\mathrm{Ni}$ in the sample (black) and with surrounding materials (pink, shaded line, measured with empty sample container), and ambient background (black, shaded line). The spectrum recorded with the ${}^{62}\mathrm{Ni}$ sample was scaled to the areal density of ${}^{62}\mathrm{Ni}$ in the ${}^{63}\mathrm{Ni}$ sample.

Figure 4

(a)–(d) Examples for resonances fitted with the program sammy [40]. The dots are the measured data; the line represents the result of the resonance fit. Panel (a) shows the fit of the 4.6-keV resonance which was analyzed using the spectra obtained with the ${}^{63}\mathrm{Ni}$ sample. The data in panels (b)–(d) are from the first ${}^{62}\mathrm{Ni}$ campaign.

Figure 5

Accumulated number of levels as a function of neutron energy. The black dots represent the data; the red line is a linear fit from 0 to 80 keV.

Figure 6

Maxwellian averaged cross sections from 5 to 100 keV compared to previous measurements (Alpizar-Vicente et al. [19], Nassar et al. [20], Dillmann et al. [22], and Tomyo et al. [18]). The results obtained with data from the jendl-4.0 evaluation (dashed line, [27]) and the recommended MACS values of the kadonis compilation (solid line, [51]) are included as well.

Figure 7

(Top) Final isotopic $s$-process abundances between Fe and Ga normalized to solar system abundances. The red circles represent the abundances using the ${}^{62}\mathrm{Ni}$($n,\gamma$) MACS of this work and the ${}^{63}\mathrm{Ni}$($n,\gamma$) MACS reported in Ref. [15]. This distribution is compared to the results using the recommended MACS of ${}^{62}\mathrm{Ni}$ and ${}^{63}\mathrm{Ni}$ of the kadonis compilation [51]. As a reference, the overabundance of ${}^{16}$O is shown as a black continuous line, divided and multiplied by 2 (black dashed lines). (Bottom) Ratio between the abundances using the new cross sections and the abundances using kadonis cross sections.