Fast-neutron activation of long-lived isotopes in enriched Ge

Abstract

We measured the production of $^{57}\mathrm{Co}$, $^{54}\mathrm{Mn}$, $^{68}\mathrm{Ge}$, $^{65}\mathrm{Zn}$, and $^{60}\mathrm{Co}$ in a sample of Ge enriched in isotope $76$ due to high-energy neutron interactions. These isotopes, especially $^{68}\mathrm{Ge}$, are critical in understanding background in Ge detectors used for double $\ensuremath{\beta}$ decay experiments. They are produced by cosmogenic-neutron interactions in the detectors while they reside on the Earth's surface. These production rates were measured at neutron energies of a few hundred MeV. We compared the measured production to that predicted by cross-section calculations based on CEM03.02. The cross-section calculations overpredict our measurements by approximately a factor of $3$ depending on isotope. We then use the measured cosmic-ray neutron flux, our measurements, and the CEM03.02 cross sections to predict the cosmogenic production rate of these isotopes. The uncertainty in extrapolating the cross-section model to higher energies dominates the total uncertainty in the cosmogenic production rate.