ASSESSMENT OF THE EQUILIBRIUM OF TH-228 AND RA-228 BY GAMMA-RAY SPECTROMETRY IN MANGROVE SOILS

摘要

The mangrove environment consists of soil containing high organic matter, characterized by the influence of continental and oceanic waters, relatively high concentrations of salts and exuberant vegetation. Mangroves also present high dynamics of chemical compound cycling, in which soils become quite relevant due to the influence of continental and oceanic sediments. Taking into account the different sources and transport of radionuclides within the mangrove environment, this study focused at the radioactive equilibrium of thorium-232 series radionuclides, especially the Ra-228 and thorium-228. For this, soil samples were collected in the crown projection of tree species from two mangroves located in the municipalities of Olinda/Recife and Rio Formoso, Pernambuco State, Brazil. The material was oven-dried and milled and test portions of 38 g were transferred to cylindrical vials and sealed. After 30 days, the natural radioactivity was measured during 80,000 seconds using High Resolution Gamma-Ray Spectrometry with a germanium detector of 2.2 keV of resolution at the 1,332 keV Co-60 photopeak. For Ra-228 determination, 911 keV gamma-ray line (Ac-228) were employed, while 238 keV (Pb-212) and 727 keV (Bi-212) were considered for estimating activity concentrations of Th-228. As a result, Ra-228 and Th 228 were in equilibrium since the activity concentrations (ranging from 35 Bq/kg to 50 Bq/kg) were not significantly different at the 95% confidence level. The results have confirmed that, even for environments of high dynamics such as mangroves, radioactive equilibrium of Th-228 and Ra-228 was kept, corroborating Th-232 determination in mangrove soils.