  We report a comprehensive study of cosmogenic activation of germanium used
for tonne-scale rare event search experiments. The germanium exposure to cosmic
rays on the Earth's surface are simulated with and without a shielding
container using Geant4 for a given cosmic muon, neutron, and proton energy
spectrum. The production rates of various radioactive isotopes are obtained for
different sources separately. We find that fast neutron induced interactions
dominate the production rate of cosmogenic activation. Geant4-based simulation
results are compared with the calculation of ACTIVIA and the available
experimental data. A reasonable agreement between Geant4 simulations and
several experimental data sets is presented. We predict that cosmogenic
activation of germanium can set limits to the sensitivity of the next
generation of tonne-scale experiments.
