A MCNP-based calibration method and a voxel phantom for in vivo monitoring of 241Am in skull

Whole body counter (WBC) facilities are currently used for assessment of internal radionuclide body burdens by directly measuring the radiation emitted from the body. Previous calibration of the detection devices requires the use of specific anthropomorphic phantoms. This paper describes the MCNP-based Monte Carlo technique developed for calibration of the germanium detectors (Canberra LE Ge) used in the CIEMAT WBC for in vivo measurements of 241Am in skull. The proposed method can also be applied for in vivo counting of different radionuclides distributed in other anatomical regions as well as for other detectors. A computer software was developed to automatically generate the input files for the MCNP code starting from any segmented human anatomy data. A specific model of a human head for the assessment of 241Am was built based on the tomographic phantom VOXELMAN of Yale University. The germanium detectors were carefully modelled from data provided by the manufacturer. This numerical technique has been applied to investigate the best counting geometry and the uncertainty due to improper positioning of the detectors.