A method of efficiency calculation and coincidence-summing correction of a germanium detector by using MCNP code is presented. Modeling of the detector geometry is described in detail and differences between the simulated and measured spectra are discussed. Standard point sources traceable to NIST were used to measure the full-energy peak and total efficiencies. A /sup 60/Co point source was placed in five positions above the detector from 0.6 cm to 14.2 cm. For the 1173 keV and 1332 keV gamma rays from /sup 60/Co, their spectra were simulated by MCNP separately. Subsequently, these spectra were combined according to their coincidence relationship to form the simulated /sup 60/Co spectrum. Cs-137 was used to compare a simple measured spectrum with the MCNP simulation. The simulated full energy peak efficiency for non-coincidence gamma rays agreed with the measured value to within 2%, but the simulated total efficiency is about 8% lower for 662 keV. The calculated coincidence summing factors for 1173 and 1332 keV are about 3% lower than the measured values at the closest geometry for a point source due to the underestimation of the total efficiency. This technique will be used in the efficiency calibration of Ge detectors to avoid the difficulties of preparing standard sources.
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