239Pu/241Am calibration of scintillation detectors using anthropomorphic phantoms

The direct in-vivo assessment of insoluble 239Pu and 241Am compounds in human lungs is only possible by external counting of their low-energy photons using special detectors with high response probability and low background. Owing to the high body absorption of the radiation to be monitored, the calibration factors had to be determined as a function of chest-wall tissue thickness between the lungs and detectors. The examinations were performed with the aid of an anthropomorphic phantom being nearly tissue-equivalent in the energy range from 10 to 100 keV and steplessly variable as to the absorption power. As detectors a 1 mm thin NaI(Tl) single crystal as well as a phoswich system consisting of a 2 mm thin CsI(Tl) crystal coupled to a 51 mm thick NaI(Tl) anti-Compton scintillator were used. The calibration results obtained with both these systems were in good agreement, however, the background of the phoswich was lower by a factor of 2.5. In normal subjects the minimum detectable 239Pu activities were found to be 14 nCi for the single crystal detector and 9 nCi for the phoswich. As regards the phoswich, a further reduction of the detection limit would likely be achieved by enlarging the active area of the detector up to 350 cm2. (author)