Distribuição de radionuclídeos em um afloramento de sedimentos permianos da formação Iratí na Bacia do Paraná
Data
2008-02-28
Autores
Ferreira, Ademar de Oliveira
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Resumo
The objective of this work is to study natural radionuclides in sedimentary rocks. The concentration of them reflects the origin of the sediments, the deposicional environment as well as some mineralogical characteristics of the rock matrix, and also more recent events as weathering and erosion. Using gama ray high resolution spectrometry, the profile of activity concentration of the natural radionuclídeos was assessed for 226Ra, 238U, 232Th e 40K in rocks of the Irati Formation belonging to Paraná Sedimentary Basin. The samples were collected at a limestone abandoned mine, in the city of Sapopema, (PR). 24 samples were collected, eleven from the exposed vertical profile with approximately 5.50 m, whose stratigraphy is represented by an alternation among decimetrics layers of limestones, bituminous shales, and some rhythmits layers (milimetrics sheets of limestone and bituminous shales), 9 repetitions of a sample to study the variability, and three rigolits samples in sequential apprenticeships of weathering. Each sample was dried in the sun during about 48 hours, broken, drizzled in a sieve of 4 mesh and put, in a cylindrical container. The measures were accomplished using a Germanium Hiper Pure detector (HPGe) with relative efficiency of 66%, connected to a standard spectrometry electronic chain. The measured concentrations of activity of 238U were smaller for the limestones (17.80 ± 0.09 Bq.Kg-1), larger for the bituminous shales (125.5 ± 2.6 Bq.Kg-1), with enrichment of uranium in the sample (200), 548 ± 16 Bq.Kg-1, upper part of the column), and intermediate for the rhythmits (23.0 ± 1,3 Bq.Kg-1). The ratio eTh/K obtained for the studied profiles has equivalent values, indicating similar mineralogical characteristics for the limestones, bituminous shales, rhythmits and studied rigolits. On the other hand, to the ratio eTh/eU showed that two of the three regolits samples belong to oxidizer atmospheres, favoring the leaching of uranium, what can be causing the enrichment of the sample (200), once these soils are above the column. The average measured doses were 16 nGy.h-1 for the limestones and rhythmits, 79.5 nGy.h-1 for the bituminous shales and 329.5 nGy.h-1 for the enriched bituminous shales of the top of the column. These results supply data to estimate the contribution of these products for the radiation dose received by the public in general, besides of database for mineralogical and geophysical studies of the Paraná Basin.
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Espectrometria gama, Rochas sedimentares, Radiação ambiental