Measurement of the ${}^{72}\mathrm{Ge}\mathrm{(}n,\gamma )$ cross section over a wide neutron energy range at the CERN n_TOF facility

The ${}^{72}\mathrm{Ge}(n,\gamma )$ cross section was measured for neutron energies up to $300\mathrm{keV}$ at the neutron time-of-flight facility $\mathrm{n}\_\mathrm{TOF}$ (CERN), Geneva, for the first time covering energies relevant to heavy-element synthesis in stars. The measurement was performed at the high-resolution beamline EAR-1, using an isotopically enriched ${}^{72}\mathrm{Ge}\mathrm{O}_2$ sample. The prompt capture $\gamma$ rays were detected with four liquid scintillation detectors, optimized for low neutron sensitivity. We determined resonance capture kernels up to a neutron energy of $43\mathrm{keV}$, and averaged cross sections from 43 to $300\mathrm{keV}$. Maxwellian-averaged cross section values were calculated from $kT=5$ to $100\mathrm{keV}$, with uncertainties between $3.2\%$ and $7.1\%$. The new results significantly reduce uncertainties of abundances produced in the slow neutron capture process in massive stars.

Figure 1

${}^{72}\mathrm{Ge}$ sample spectrum compared to backgrounds due to ambient radioactivity (Beam off) and neutron reactions on the sample holder (Empty holder).

Figure 2

Examples of resonance fits of the neutron capture yield using the $R$-matrix code sammy [39]. The data are compared to the yield calculated from resonance parameters listed in the ENDF/B-VIII.0 evaluation [33]. The bottom panel shows an example of resonances measured at $\mathrm{n}\_\mathrm{TOF}$ for the first time.

Figure 3

Averaged cross sections with statistical uncertainties from 43 to $300\mathrm{keV}$ neutron energy compared to ENDF/B-VIII.0 [33] and TENDL-2017 [17, 44]. See text for details.

Figure 4

Maxwellian-averaged cross sections of this work $(\mathrm{n}\_\mathrm{TOF})$ from $kT=5$ to $100\mathrm{keV}$ compared to MACS values recommended in the latest version of the database KADoNiS-$\mathrm{v}1.0$ [16].

Figure 5

Abundances produced in the $s$ process for a 25 solar mass star using the new cross sections determined at $\mathrm{n}\_\mathrm{TOF}$. Abundances are normalized to results using the ${}^{72}\mathrm{Ge}(n,\gamma )$ MACSs recommended in KADoNiS-$\mathrm{v}1.0$ [16]. Isotopes of the same elements are connected by thin solid lines. Panel (a) shows abundances after He core burning. The shaded areas represent abundance variations when taking into account uncertainties of KADoNiS (blue) or $\mathrm{n}\_\mathrm{TOF}$ (red) cross sections. Panel (b) shows abundances after the later C shell burning phase.