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The problem of measuring gamma activity at the natural levels, particularly in people and foodstuffs, is an extremely important one because of the necessity of monitoring both fallout from nuclear weapons tests and the disposal of reactor wastes. This problem will also be encountered because of the necessity for minimizing radiation in clinical and industrial tracer applications. Two principal techniques have been developed for this purpose which are applicable to in vivo studies: the NaI crystal spectrometer in a special, low-activity shield, and the large 4Â¿ liquid scintillation counter. Both have sensitivities which permit the detection of gamma activity in the human body at levels a factor of 10 to 100 below the natural K-40 concentration. The choice between the two systems depends largely on the particular application, since they are comparable in terms of ultimate sensitivity and cost. The liquid scintillator is the method of choice used for routine studies involving large numbers of samples in which the identity of the activity is known. This is the case, for example, in studies of the gamma activity of people and foodstuffs due to fallout because of the large discrimination factors in biological systems against elements not closely resembling nutritional essentials. Natural K-40 and Cs-137 are normally the only gamma activities present, and the energy resolution of the liquid scintillator permits their simultaneous determination. Because of the constancy and predictability of the K-40 level, additional hard gamma activity (e.g.