Study on cosmogenic radioactive production in germanium as a background for future rare event search experiments

Rare event search experiments are one of the most important topics in the field of fundamental physics, and high-purity germanium (HPGe) detectors with an ultra-low radioactive background are frequently used for such experiments. However, cosmogenic activation contaminates germanium crystals during transport and storage. In this study, we investigated the movable shielding containers of HPGe crystals using Geant4 and CRY Monte Carlo simulations. The production rates of 68Ge, 65Zn, 60Co, 55Fe, and 3H were obtained individually for different types of cosmic rays. The validity of the simulation was confirmed through a comparison with the available experimental data. Based on this simulation, we found that the interactions induced by neutrons contribute to approximately 90% of the production rate of cosmogenic activation. In addition, by adding an optimized shielding structure, the production rates of cosmogenic radionuclides are reduced by about one order of magnitude. Our results show that it is feasible to use a shielding container to reduce the cosmogenic radioactivity produced during the transport and storage of high-purity germanium on the ground.