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.