Precise modelling of coaxial germanium detectors in preparation for a mathematical calibration

In preparation for the planned mathematical efficiency calibration of a whole-body counter, models of the four HPGe detectors have to be developed as input for Monte Carlo simulations. The results for two of the detectors are presented here. With the initial detector models, based on data provided by the manufacturer, calculated efficiencies for various photon energies and measuring geometries were 10–20% too high compared with the measured efficiencies. To obtain more reliable information about the constructional features, scanning measurements with collimated radiation were performed at the front window and the sides of each detector. Using the information from these measurements, the initial detector model was improved by comparing the measured efficiencies with efficiencies calculated by the Monte Carlo code MCNP 4C2 and correcting the model by an iterative process if necessary. Finally, the quality of the detector models was verified by measurements with uncollimated point sources, which were placed in 77 different positions on a 10 cm×10 cm grid on the stretcher of the whole-body counter. For all photon energies and all source positions the deviation between measured and simulated efficiencies was found to be less than 3%.