| Abstrakt: |
Naturally occurring radioisotopes are present in different concentrations in sedimentary rocks, reflecting the origin of the sediments, the depositional environment, and more recent events such as weathering and erosion. Using a high-resolution γ-ray spectrometry methodology, sedimentary rocks were measured to assess the concentration activities of the natural radioisotopes. The surveyed rocks are from the Irati formation in the Paraná sedimentary basin, which are exposed by an abandoned, open-pit limestone mine, in the city of Sapopema, southern Brazil. The exposed vertical profile is 5 m, and its stratigraphy is represented by an alternation of limestone and bituminous shale (layers being a few decimeters thick), and some millimeter rhythm layers with limestone and bituminous shale laminas. Eleven samples were collected along this profile, each of them dried in the open air during 48 hours, sieved through 4 mm mesh and sealed in cylindrical recipients. Measurements were accomplished using a 66% relative efficiency HPGE detector connected to a standard gamma ray spectrometry electronic chain. The detector efficiency in the range of 60 to 1800 keV was carried out with the certified IAEA-385 sediment sample. The Lower Limit of Detection (LLD) to the system is 2.40 Bq·kg-1 for 226Ra, 1.84 Bq·kg-1 for 232Th and 4.20 Bq·kg-1 for 40K. Activity concentrations were determined for 226Ra (from 16.22 to 151.55 Bq·kg-1), 232Th (from 2.93 to 56.12 Bq·kg-1) and 40K (from 38.45 to 644.63 Bq·kg-1). The layers enriched with organic matter presented the higher values of activity. The measured concentrations of the natural radioisotopes were lower for limestone samples (average values and respective deviations were 22.81±0.22 Bq·kg-1 for 226Ra, 4.21±0.07 Bq·kg-1 for 232Th, and 50.11±0.82 Bq·kg-1 for 40K). Higher concentrations were measured for the bituminous shale samples (average values and respective deviations were 108.10±12.17 Bq·kg-1 for 226Ra, 43.69±0.30 Bq·kg-1 for 232Th, and 465.82±3.99 Bq·kg-1 for 40K). The concentrations were intermediate for the rhythmite samples (average values and respective deviations were 50.69±1.09 Bq·kg-1 for 226Ra, 7.63±0.21 Bq·kg-1 for 232Th, and 85.96±2.47 Bq·kg-1 for 40K). As the analyzed rocks are raw materials for the ceramic, cement and soil correction compound industries, the results of this work furnish data to estimate the contribution of these products to the general public's radiation exposure. [ABSTRACT FROM AUTHOR] |
