Efficiency calculation and coincidence summing correction for germanium detectors by Monte Carlo simulation

A method is presented for efficiency calculation and coincidence-summing correction of high-purity germanium (HPGe) detector spectra by using Monte Carlo N-particle transport (MCNP) code. This technique will be used in the efficiency calibration of HPGe detectors to reduce the number of standard sources to be prepared. Modeling of the detector geometry is described in detail, and differences between the simulated and measured spectra are discussed. Standard point sources traceable to the National Institute for Science and Technology were used to measure the full-energy peak and total efficiencies. The simulated full-energy peak efficiency for noncoincidence ¹³⁷Cs gamma rays agreed with the measured value to within 2%, but the simulated total efficiency is about 8% lower than the measured value for 662 keV. A ⁶⁰Co point source was placed in five positions above along the center line of the detector from 0.6 to 14.2 cm. For the 1173- and 1332-keV gamma rays from ⁶⁰Co, their spectra were simulated using MCNP separately. Subsequently, these spectra were combined according to their coincidence relationship to form the simulated ⁶⁰Co spectrum. 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.