Abstract
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 $^{70}$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 being 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 between mass numbers A=60-80.
Highlights
Background subtractionThe background B(En) consists of several components that need to be corrected for
The neutron time-of-flight spectra were converted to neutron energy using the relativistic relation
The empty sample holder contributes to background over the entire neutron energy range, while the ambient activity, which is constant in time, only contributes at low neutron energies
Summary
The elemental abundances above Fe are mainly produced by two distinct neutron capture processes in stars and stellar explosions, the slow neutron capture process (s process) [1] and the rapid neutron capture process (r process) [2], while only about 1% of heavy element abundances is produced by charged particle and photoinduced reactions (p process) [3]. In massive stars (>8 solar masses M ), the s process occurs in two different burning stages, towards the end of He core burning at temperatures of about 0.3 GK (1 GK = 109 K), and later during carbon shell burning, when temperatures reach 1 GK [4,5,6,7,8] This so-called weak component of the s process produces dominantly elements between Fe and Zr, owed to the relatively low neutron exposures reached. There are few experimental data available on this reaction in the keV neutron energy range; Walter and Beer [15] measured cross sections from 3 to 240 keV and calculated MACSs in the range kT = 20–50 keV. Evaluated 70Ge(n, γ ) cross sections recommended in major nuclear data libraries such as ENDF/B-VIII [18] and JENDL-4.0 [19] use experimental data by Maletski et al and transmission data on natural germanium by Harvey and Hockaday for neutron energies below 14 keV, while for higher neutron energies evaluated cross sections are based on statistical parameters extracted from data at lower neutron energies
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