Activity ratios of thorium daughters in vivo

A computerized method of least squares has been used to analyze the 228Ac and 212Pb-212Bi and daughter γ-ray spectra obtained in vivo from 133 former workers at a thorium refinery. In addition, the exhalation rate of 220Rn was determined for each subject and expressed as pCi of emanating 224Ra. This value was added to the 212Pb value determined from the γ-ray measurements to obtain the total 224Ra present, and the ratio of 224Ra to 228Ac was calculated. Values of the ratio ranged from 0.52 +- 0.32 to 2.1 +- 1.7, with a weighted mean of 0.92 +- 0.17. However, it appears that the ratio observed in a given case is characteristic for that case alone; the computed mean value may not be meaningful. The least squares fitting procedure and the overall calibration of the counting system were validated by measurements of 224Ra in the lungs of one subject postmortem, compared with results obtained from the same subject in vivo. 6 references, 5 figures