INFLUENCE OF SOIL AND BUILDINGS ON OUTDOOR GAMMA DOSE RATES IN S??O PAULO, BRAZIL

Abstract

This work analyzes the influence of the most abundant natural gamma emitters in soil (226Ra, 232Th, and 40K) on the total outdoor gamma dose rate in the city of São Paulo, Brazil. A new method is introduced to determine gamma dose rates due to soil 1 m above the ground through measurements performed deep in the soil. This allows evaluation of the soil component even in places where the measurement at 1 m height is influenced by other sources (mainly the presence of buildings). The methodology was tested in non-urbanized areas by comparing direct dose rate measurements in air with those deep in soil. In addition, high-resolution gamma ray spectrometry of soil samples collected throughout the city was used to determine the natural radionuclide concentrations, allowing the comparison with the in-situ dose rate results. Measurements deep in soil followed a log-normal distribution. The fitted geometric mean (median) and geometric standard deviation of the soil contribution to the ambient dose equivalent rate at 1 m height were, respectively, 80.9(6) and 0.642(4) nSv h(-1). Compared to previous data, these values show that buildings enhance about 35% the outdoor gamma dose rate expected only from soil. The specific activities of 226Ra, 232Th, and 40K in dry soil, given by their medians, were, respectively, 41, 75, and 176 Bq k(-1). These results reveal that the terrestrial gamma dose rates in São Paulo are higher than the world average, a fact that can be attributed to high thorium concentration. Direct measurements of dose rates were compared to the corresponding values determined from radionuclide concentrations in soil. Good agreement between methods was found.