A Monte Carlo-based knee phantom for in vivo measurements of 241Am in bone

Determination of internal contamination of 241Am can be done by direct counting of gamma emission using a Whole Body Counter. Due to the strong attenuation of the low-energy photons, it is advised to perform the measurement on bones surrounded by a thin layer of tissue. In vivo measurements are performed at CIEMAT using a system of four Low-Energy germanium (LE Ge) detectors calibrated with realistic anthropomorphic phantoms. As an alternative, Monte Carlo techniques are applied on voxel phantoms based on tomographic images to avoid the need of different physical phantoms for different radionuclides and organs. This technique is employed to study the convenience of americium measurements in the knee for the evaluation of the deposition in the whole skeleton. The spatial distribution of the photon fluence through a cylinder along the axis of the leg has been calculated to determine the best counting geometry. The detection efficiency is then calculated and the results are compared with those obtained using the physical phantom to validate the proposed method.