In Vivo Gamma Counting As a Measurement of Uranium in the Human Lung

A direct, in vivo method is reported for the measurement of uranium within the human lung, utilizing the detection by scintillation spectroscopy of the gamma radiation accompanying alpha decay of the uranium. The in vivo gamma counting facility includes a low background counting chamber with 8-in. thick iron walls, NaI scintillation detector, and a multichannel pulse-height analyzer. To achieve a quantitative estimate of uranium concentration, two techniques for estimating the normal, non-uranium gamma activity at the characteristic uranium gamma energies have been developed. Calibrations have been derived from measurements of samples in phantoms simulating the human chest. The 186 keV gamma line from U235 is the most suitable for measurement, but if secular equilibrium is assumed, the 90 keV lines of the uranium daughters may be used. The ratio of the 90 keV line to the 186 keV line may be used as an indication of U235 abundance. Quantities as low as 50 per cent of the maximum permissible body burden of any of the isotopic enrichments investigated can be measured.