Development and characterisation of a head calibration phantom for in vivo measurements of actinides

The investigation of actinides' internal contamination in human body makes use of a variety of techniques. In large scale screening the technique of "in vivo" evaluation of bone 241Am burden via the determination of the nuclide activity in the skull is often used. For this purpose, adequate calibration procedures and standard phantoms are needed. The present paper summarises the studies and technical procedures followed for the development of a calibration phantom based on a commercial Alderson angiographic head in which a set of 24 241Am point sources were embedded. A theoretical study was first carried out, at the ENEA Institute for Radiation Protection, using the MCNP4-B Monte Carlo code to determine the point source distribution that closely approximates a homogeneous bone contamination. The numerical models were also used to evaluate the resulting degree of approximation. The point sources were prepared at the ENEA National Metrology Institute for ionising radiation quantities and were traceable to the Italian national standard of radionuclide activity. The sources were prepared by quantitatively dispensing a liquid solution onto a plastic disc. The activity of each source was checked by gamma-ray spectrometry and the reproducibility of the activity values was determined. Each source was then placed in the optimum position in the skull, given by the Monte Carlo modelling, by a precision mechanical device. The phantom was finally used to calibrate a whole body counter operating at the ENEA Institute for Radiation Protection. The paper reports the main theoretical and experimental aspects of this work, and also discusses the results of the first calibrations.