In vivo gamma measurements at very low levels with 4Π liquid scintillation detectors

A large 4Π liquid scintillation counter for the rapid measurement of total internal gamma contamination of the human body at concentrations 1000 times less than the maximum permissible levels is described. A counting time of only 100 seconds per subject is required to obtain a sensitivity of 5 × 10^-10 curies for 1.5 mev gamma rays. Because of the high counting efficiency and the great stability of operation, the magnitude of counter background is of comparatively little importance so that elaborate shielding precautions are not required. Energy resolution is sufficient to distinguish between gamma rays of markedly different energy, but not adequate for the identification of individual nuclides. The operating characteristics of the counter are discussed including the spectrum and stability of background, energy resolution, and the spectra and limits of sensitivity for nuclides of interest in connection with nuclear power stations and uranium mining operations.