EFFECT OF RADIONUCLIDE DISTRIBUTIONS ON LUNG COUNTING EFFICIENCY

The Human Monitoring Laboratory which acts as Canada's National Calibr ation Reference Centre for in vivo monitoring, recently acquired a new lung counting system comprised of four 70 mm diameter and 30 mm thick germanium detectors. Prior to purchasing the system, calibrations wer e performed with an identical evaluation detector. The work was carrie d out using activity homogeneously distributed in lung sets which were placed in the torso phantom that was originally developed at Lawrence Livermore National Laboratory, USA. Results of the cross-talk and het erogeneously loaded lungs indicated that detectors used for low energy photon emitters should be calibrated individually instead of as an ar ray, as is often the case. It was clear that no activity, particularly at 17 keV (Pu-239), contributed to the counts of a detector which was not directly above this activity and that to use a summed array of de tectors with a distributed source (i.e. two lungs) could lead to some very large errors.