Measurement of neutron-capture cross sections of Ge70,72 using the DANCE facility

Abstract

Background: Approximately half of all atomic nuclei heavier than iron are synthesized by the slow neutron-capture 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, $^{70,72}\mathrm{Ge}$, 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 $^{70,72}\mathrm{Ge}$ neutron-capture cross sections were measured as a function of neutron energy using the Detector for Advanced Neutron Capture Experiments at Los Alamos National Laboratory.Results: Neutron-capture cross sections were measured from 10 eV to 1 MeV. These are the first measurements for $^{70,72}\mathrm{Ge}$ between 300 keV and 1 MeV neutron energy. Maxwellian-averaged cross sections were calculated in the astrophysically relevant neutron energy range (5 keV $\ensuremath{\le}kT\ensuremath{\le}$ 100 keV). Their value at $kT=30$ keV was found to be $89\ifmmode\pm\else\textpm\fi{}11$ mb for $^{70}\mathrm{Ge}$ and $58\ifmmode\pm\else\textpm\fi{}5$ mb for $^{72}\mathrm{Ge}$. 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 $^{70}\mathrm{Ge}$ show minor ($<1\ensuremath{\sigma}$) 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 $^{72}\mathrm{Ge}$ 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.