Gamma-ray spectrometry of humans at the University of Utah.

A human total-body counter was designed and built with two 20 X 10 cm NaI (Tl) crystals suspended over an "isoresponse surface" upon which the subject reclines. This surface is curved from head to knee and from left to right, so that a gamma-ray emitting object is detected with equal efficiency when placed anywhere upon it. The positioner and detectors are housed in a low background enclosure constructed of steel 31 cm thick with a graded inner lining of lead + cadmium + copper. Calibration of the system was accomplished by administering trace amounts of various radionuclides to 48 human subjects of various sizes, ranging in age from 4--80 years. Counting rates per retained muCi at 0.53, 0.66, 1.53, and 2.75 MeV (83Rb, 137Cs, 42K, and 24Na) were determined as a function of body size and were compared with counting rates per muCi of corresponding emitters centered in a polyethylene cylinder of radius 10.3 cm. Limits of detection, corresponding to three times the standard deviation of a 50 min background, were 170 nCi 90Sr (via Bremsstrahlung X-rays), 0.78 nCi 131I, 0.48 nCi 83Rb, 0.52 nCi 137 Cs, 4.9 nCi 40K (or 5.8 g of natural potassium) and 1.7 nCi 222Rn.