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

We measured the production of Co57, Mn54, Ge68, Zn65, and Co60 in a sample of Ge enriched in isotope 76 due to high-energy neutron interactions. These isotopes, especially Ge68, are critical in understanding background in Ge detectors used for double β 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.