Measurement of neutron-capture cross sections of 70,72Ge using the DANCE facility

Background: Approximately half of all atomic nuclei heavier than iron are synthesized by the slow neutroncapture process. The weak component of this process is not well understood and the reaction rates of each isotope in the s-process path affect nucleosynthesis abundances downstream. Purpose: To measure the neutron-capture cross sections of two weak s-process nuclei, 70,72Ge, using the neutron time-of-flight technique. Measuring the capture cross sections for isotopes in this region of the chart of nuclides has proven challenging due to dominant scattering cross sections. Method: Samples consisted of pellets made of pressed enriched metallic powders. The 70,72Ge neutron-capture cross sections were measured as a function of neutron energy using the Detector for Advanced Neutron Capture Results: Neutron-capture cross sections were measured from 10 eV to 1 MeV. These are the first measurements for 70,72Ge between 300 keV and 1 MeV neutron energy. Maxwellian-averaged cross sections were calculated in the astrophysically relevant neutron energy range (5 keV kT 100 keV). Their value at kT = 30 keV was found to be 89 +/- 11 mb for 70Ge and 58 +/- 5 mb for 72Ge. Both values are in agreement with recent time-of-flight measurements at n_TOF (neutron Time-Of-Flight facility at the European Organization for Nuclear Research). Conclusions: The average cross section results from this work for 70Ge show minor (<1 Sigma) disagreement with a recent measurement by the n_TOF collaboration at higher neutron energies. This corresponds to the neutron energy region that had previously never been measured (>300 keV). Two reaction library databases underestimate the 72Ge average cross section below 30 keV according to n_TOF and DANCE. This is likely due to capture resonances that are missing from the theoretical cross sections in the databases that were identified in both time-of-flight measurements. Additionally, a rudimentary analysis of the impact of both cross section measurements on stellar nucleosynthesis abundances using the NETZ nucleosynthesis tool is presented.

Automated summary

The neutron capture cross sections of weak s-process nuclei 70,72Ge were measured using the neutron time-of-flight technique. The results showed minor disagreement with previous measurements for 70Ge at higher neutron energies, while both 70Ge and 72Ge values were in agreement with recent time-of-flight measurements. Two reaction library databases underestimated the average cross section of 72Ge at lower energies. The impact of these cross section measurements on stellar nucleosynthesis abundances was also analyzed.