  A study on cosmogenic activation in germanium was carried out to evaluate the
cosmogenic background level of natural and $^{70}$Ge depleted germanium
detectors. The production rates of long-lived radionuclides were calculated
with Geant4 and CRY. Results were validated by comparing the simulated and
experimental spectra of CDEX-1B detector. Based on the validated codes, the
cosmogenic background level was predicted for further tonne-scale CDEX
experiment. The suppression of cosmogenic background level could be achieved by
underground germanium crystal growth and high-purity germanium detector
fabrication to reach the sensitivity requirement for direct detection of dark
matter. With the low cosmogenic background, new physics channels, such as solar
neutrino research and neutrinoless double-beta decay experiments, were opened
and the corresponding simulations and evaluations were carried out.
