Assessment of the Radiation Dose Due to Fall-Out

The testing of nuclear weapons during the past decade has resulted in environmental radioactive contamination of world-wide proportions. The degree of contamination varies with the geographical location, depending primarily upon the position of the area relative to the testing sites, the amount of rainfall, and the latitude (1). The variation in fall-out deposition with latitude is illustrated in Figure 1. This is the distribution pattern for fall-out coming from the stratosphere, which is the primary source of deposition. Of the many radioelements produced in the fission process, Sr90 and Cs137 have been examined most thoroughly because of their high fission yield, long radioactive half-lives, and chemical properties that readily permit entry into the biosphere, including man himself. Many factors relevant to the processes whereby these radioisotopes enter the food chain and are deposited in the human body have been qualitatively assessed (2, 3). The human body burdens of Sr90 and Cs137 have been measured as a function of time, and thus the radiation doses arising from such internal deposition may be calculated (4, 5). A second source of radiation from fall-out consists in gamma-emitting fission products external to man, which are capable of delivering a dose often many fold greater than that due to Sr90 and Cs137 combined. This presentation will be concerned with this external dose, its measurement, its variation in time, and its magnitude compared with the dose from natural background radiation. Measurements of gamma-ray-emitting fission products have been made periodically on soils collected at the Argonne National Laboratory site since the spring of 1957. The measuring technic employed was gamma-ray spectrometry of soil samples of finite size (1–2 kg.), with a 5 × 4-in. NaI crystal and an Argonne Type 256 channel analyzer (6). The soil was arranged geometrically, by means of a stainless steel container, in a 1-inch layer around and over the crystal. Background reduction, essential for the detection of the quantities of activity involved (approximately 10−10 curie of Cs137/kg. of soil), was effected through the use of thick steel shielding around the sample and detector. Soil sampling was done to a depth of 6 inches, which includes essentially all fission products present in a given area. Two gamma-ray spectra of soil taken with a NaI crystal are shown in Figure 2. The one from deep soil is free of any fission activity and contains only Th, U, and K40. The other, from soil to a depth of 6 inches collected in July 1959, indicates the presence of two peaks not evident in the first case. One peak is due to Zr95-Nb95 and the other to Ru103, 106. The concentration of fission and natural radioactivity in soil was determined by the use of sources of known activity incorporated in mock soil (Na3PO4). This method presupposes knowledge of the various isotopes present in the soil.