The Natural Radiation Dose to Indigenous Rodents on the Morro Do Ferro, Brazil

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Abstract
The feasibility of studying the dose from natural radioactivity received by indigenous rodents in remote areas has been demonstrated on the Morro do Ferro, a large hill in Minas Gerais, Brazil. The abnormally high ambient gamma radiation, 1 m above the ground, ranges from 0.05 to 3.2 mR/hr, and is due to the presence of thorium in the soils and rocks. Estimates were made of the dose from external gamma radiation, internally deposited 226Ra and 228Ra, and inhalation of 220Rn, 222Rn and their daughter products. The techniques used to obtain these data in the field and laboratory are described. External gamma exposure was measured by releasing trapped rodents into which fluoroglass rod dosimeters had been implanted subcutaneously. The dosimeters were removed after retrapping about 5 weeks later and indicated that the average dose over a several week period ranged from 0.15 to 0.76 mrad/hr, equivalent to 1.3–6.7 rad/yr. The bone dose from 228Ra has been estimated by radiochemical analysis, the preliminary results of which indicate concentrations of 0.7 to 2.8 pc/g bone ash leading to a dose in the order of 3 rem/yr. Estimates of the dose from inhalation of the radioactive noble gases were made using field measurements of the 222Rn and 220Rn content of air aspirated from rodent burrows. It was found that the 220Rn concentrations vary from near normal levels to 10−4 μc/ml. The problems involved in calculating the dose to the lung and other organs from 220Rn and 222Rn inhalation are discussed and preliminary data are given. Using alternative methods of calculation, the dose to the lung tissues is estimated to be in the range of 3 × 103 to 3 × 104 rem/yr.

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