Validation of a Monte Carlo efficiency calibration procedure for a partial body counter system with a voxel model of the LLNL torso phantom.

Virtual models of real phantoms used with Monte Carlo methods facilitate the calibration and other studies associated with whole-body and partial-body counting systems. In this investigation, a voxel model of an LLNL torso phantom, available physically in the in vivo laboratory at KIT, was created from computed tomography scans. Series of measurements with a high-purity germanium detector and the real torso phantom, loaded with different radioactive organs, have been carried out. Computer simulations of these measurement setups were performed with the aid of MCNPX, using a coarsened voxel phantom and a validated model of the germanium detector. The results of simulations were compared with data from the measurements and an agreement within the uncertainties was found. The voxel model could therefore be validated. The results of the simulations were then used to quantify the activity of (241)Am impurities detected in the liver loaded with (239)Pu.