  High-purity germanium detectors are well suited to analysis the radioactivity
of samples. In order to reduce the environmental background, low-activity lead
and oxygen free copper are installed outside of the probe to shield gammas,
outmost is a plastic scintillator to veto the cosmic rays, and an anti-Compton
detector can improve the Peak-to-Compton ratio. Using the GEANT4 tools and
taking into account a detailed description of the detector, we optimize the
sizes of the detectors to reach the design indexes. A group of experimental
data from a HPGe spectrometer in using were used to compare with the
simulation. As to new HPGe Detector simulation, considering the different
thickness of BGO crystals and anti-coincidence efficiency, the simulation
results show that the optimal thickness is 5.5cm, and the Peak-to-Compton ratio
of 40K is raised to 1000 when the anti-coincidence efficiency is 0.85. As the
background simulation, 15 cm oxygen-free copper plus 10 cm lead can reduce the
environmental gamma rays to 0.0024 cps/100 cm3 Ge (50keV~2.8MeV), which is
about 10-5 of environmental background.
