Measurement of the ${}^{70}\mathrm{Ge}(n,\gamma )$ cross section up to 300 keV at the CERN n_TOF facility

Neutron capture data on intermediate mass nuclei are of key importance to nucleosynthesis in the weak component of the slow neutron capture processes, which occurs in massive stars. The $(n,\gamma )$ cross section on ${}^{70}\mathrm{Ge}$, which is mainly produced in the $s$ process, was measured at the neutron time-of-flight facility n_TOF at CERN. Resonance capture kernels were determined up to 40 keV neutron energy and average cross sections up to 300 keV. Stellar cross sections were calculated from $kT=5$ keV to $kT=100$ keV and are in very good agreement with a previous measurement by Walter and Beer (1985) and recent evaluations. Average cross sections are in agreement with Walter and Beer (1985) over most of the neutron energy range covered, while they are systematically smaller for neutron energies above 150 keV. We have calculated isotopic abundances produced in $s$-process environments in a 25 solar mass star for two initial metallicities (below solar and close to solar). While the low metallicity model reproduces best the solar system germanium isotopic abundances, the close to solar model shows a good global match to solar system abundances in the range of mass numbers $A=60–80$.

Figure 1

Main nucleosynthesis path of the $s$ process in massive stars during He core burning. Solid boxes represent stable, dashed boxes represent unstable isotopes.

Figure 2

Plot of the weighted Ge spectrum compared to empty sample holder and ambient background.

Figure 3

$\mathrm{Ge}+$filter spectrum (ambient and empty sample holder $+$ filter spectrum subtracted), and background determined from filter dip minima.

Figure 4

(a)–(d) Examples for some sammy fits of the experimental capture yield.

Figure 5

Neutron capture cross section with statistical uncertainties in the unresolved resonance region from 25 to 300 keV. The data obtained in this work are compared to experimental results by Walter and Beer [15] and the ENDF/B-VIII evaluation [18].

Figure 6

Abundances produced in a $25M_{\odot }$ star for two initial metallicities $z$, normalized to solar system abundances for which contributions from the main $s$ process and $p$ process have already been subtracted. The distribution has been normalized to ${}^{70}\mathrm{Ge}$. $s$-only isotopes are shown as full circles and isotopes of the same element are connected by solid lines. The $z=0.006$ metallicity model produces germanium isotopic abundances close to solar proportions, while the $z=0.01$ model provides a better global fit for $A=60–80$.