Mathematical Calibration of a Whole-Body Counter

This paper describes the mathematical efficiency calibration of the whole-body counter at Forschungszentrum Jülich using Monte Carlo codes and mathematical anthropomorphic phantoms. As a prerequisite for the Monte Carlo simulations a precise model of the 4 high-purity germanium (HPGe) detectors had to be developed. Because the technical data provided by the manufacturer were not sufficient for this purpose, more reliable information about the constructional features was obtained by scanning measurements with collimated radiation at the front window and the sides of each detector. Efficiency calibrations were performed for many different combinations of body height, weight and photon energy. The required efficiency values for the whole energy range of interest (50 keV − 2 MeV) were simulated with the Monte Carlo codes MCNP 4C2 and MCNP 5 using a detailed geometrical model of the detectors and various Medical Internal Radiation Dose (MIRD) phantoms representing adults and children. The dependence of the efficiency on height and weight of the measured person is discussed. The calculated efficiency curves based on mathematical MIRD phantoms were compared with conventional calibration results from measurements of bottle phantoms and brick phantoms. Generally, good agreement was found between the simulated and experimental results. In a sensitivity analysis simulations with MIRD phantoms were used to analyse the influence of possible sources of error (such as an incorrect position of the measured person or the detectors) on the accuracy of whole-body counting results.