EVALUATION OF MONTE-CARLO SIMULATION OF PHOTON-COUNTING EFFICIENCY FOR GERMANIUM DETECTORS

The reliability of calculating the full-energy peak counting efficienc y of germanium detectors by Monte Carlo simulation was evaluated by co mparing MCNP-4 code results with measurements over a range of conditio ns, The measurements were performed for two detectors of 20% and 110% nominal efficiencies with a point source at two distances and with fou r different volume sources, including a reentrant beaker, placed on th e end caps, The radionuclides were National Institute of Standards and Technology standard sources that emit photons between energies of 42. 8 and 1,596.4 keV. Each detector was modeled in detail with respect to the dimensions of the detection volume and attenuation material in th e can, dead layer, and crystal holder foil. The measurements of the 42 .8-keV photon were used to check dimensions and to change slightly the value of the dead-layer thickness so that the simulation agreed with measurements at this energy, After this adjustment, the average ratio of simulation results to measurements for 13 photon energies above 42. 8 keV was between 0.97 and 1.03 for all sets of point and volume sourc e comparisons. Ratios at individual energies were between 0.92 and 1.0 6 for the point source and between 0.94 and 1.09 for volume sources, T he observed differences were consistent with the estimated standard de viations of simulation and of measurement, which ranged from 1 to 6% a nd typically were 2 to 3% except for larger uncertainties at low energ ies, On the basis of this comparison, simulation with the MCNP-4 code is equivalent to measurement with photon standards if the detector and source configuration can be modeled within a tenth of a millimeter.