Radioactivity of rocks from the geological formations belonging to the Tibagi River hydrographic basin

This work is a study of the 40K and the 238U and 232Th series radioactivity in rocks measured with high resolution gamma ray spectrometry. The rocks were taken from the geologic formations in the region of the Tibagi river hydrographic basin. The course of this river cuts through the Paleozoic and Mesozoic stratigraphic sequences of the Parana sedimentary basin. In order to take into account the background radiation attenuation by the samples, a technique was developed that eliminated the need to measure a blank sample. The effects of the radiation's self-attenuation in the sample matrix were taken into account by using a gamma ray direct transmission method. The results for 87 rock samples, taken from 14 distinct formations, and their corresponding radioactivity variations are presented and discussed according to the possible geological processes from which they originated. Among the most discussed results are: an outcrop that profiles shale, limestone and rhythmite in the Irati Formation; a sandstone and siltstone sequence from the Rio do Rasto Formation; and a profile sampled in a coal mine located in the Rio Bonito Formation. The calculations of the rocks' contributions to the outdoor gamma radiation dose rate agree with the values presented by other authors for similar rocks. The highest dose values were obtained from felsic rocks (rhyolite of the Castro group, 129.8 ± 3.7 nGy.h-1, and Cunhaporanga granite, 167 ± 37 nGy.h-1). The other highest values correspond to the shale rocks from the Irati Formation (109 ± 16 nGy.h-1) and the siltic shale rocks from the Ponta Grossa Formation (107.9 ± 0.7 nGy.h-1). The most recent geological formations presented the lowest dose values (e.g. the Botucatu sandstone, 3.3 ± 0.6 nGy.h-1). The average value for sedimentary rocks from seven other formations is equal to 59 ± 26 nGy.h-1. The Rio Bonito Formation presented the highest dose value (334 ± 193 nGy.h-1) mainly due to the anomalous 226Ra concentration in the samples. The Raeq parameter and the activity concentration index for environmental radioactivity monitoring were calculated for rocks used as building materials. Only one sample reached a Raeq value higher than 370 Bq.kg-1. In relation to the activity concentration index ali results are lower than two, satisfying the most rigorous dose criterion (0.3 mSv.a-1) for materials with restricted use (e.g. tiles and coverings). When materials used in bulk amounts are considered (e.g. bricks, sand, gravel), the Castro group and the Cunhaporanga granite samples do not satisfy the less rigorous dose criterion (1 mSv.a-1). This indicates that there is a radiological protection concern when these materials are used as concrete gravel. This result warrants a more detailed analysis using dose assessments based in the scenarios where these rocks are typically used. (author)