ISSSD 2019 October 7 to 11th, 2019. Zacatecas, Zac. Mexico. _________________________________________________________________________________ Cs-137 activity concentration in the soil of Alagoas State, Brazil Rogério Filgueiras1, Ricardo Garcêz1*, Leandro da Silva1, José Lopes2, Dejanira Lauria3, Fernando Ribeiro3, Eduardo P. Viglio4 & Ademir X da Silva1,5 1Department of Nuclear Engineering, Federal University of Rio de Janeiro, Rio de Janeiro, RJ, BRA 2Institute of Physics, Federal University of Bahia, Salvador, Ba, BRA 3Institute of Radioprotection and Dosimetry, Rio de Janeiro, RJ, BRA 4Geological Survey of Brazil, Rio de Janeiro, BRA 5Polytechnic School of the Federal University of Rio de Janeiro, Rio de Janeiro, BRA *Email: [email protected] Abstract Alagoas is a state of Brazil Northeast, which has an area of approximately 27779 km², borders the states of Sergipe and Bahia to the south and with Pernambuco to the north. The Cs-137 is an artificial radionuclide and is present in the whole planet mainly due to the tests and nuclear accidents. A total of 41 samples were collected by the CPRM (Geological Service of Brazil) under the PGAGEM BRASIL Project, in the first 25 cm of soil, spaced approximately 25 to 30 km apart, forming a regular mesh. The samples were identified and georeferenced with a GPS. From 2 to 3 kg of soil were collected per sample, then dried, homogenized, sieved and dried in the oven at 40ºC. Finally, they were packed in polyethylene pots of 300 ml and sealed to achieve the secular equilibrium. Samples were analyzed using the gamma spectrometry technique using hyperpure germanium detectors (HPGe), which have a relative efficiency of 20, 40, 45 and 100%. The activity concentration of the samples and their respective uncertainties were determined with the aid of Genie2000 software. The counting time used was 60,000 seconds. Among the 41 collected samples, 6 presented activity concentration for cesium 137, with activity concentration ranging from 0.3 to 1.1 Bq/kg, with a geometric mean of 0.53 Bq/kg. The samples were found in the high rainfall regions of the State, on the Borborema Plateau and one near the state capital: Maceió. Keywords: Cs-137; Activity Concentration; soil; HPGe. ______________________________________________________________________________ 233 Proccedings of the ISSSD 2019 Volume 1 ISSSD 2019 October 7 to 11th, 2019. Zacatecas, Zac. Mexico. _________________________________________________________________________________ 1.- INTRODUCTION Soils may change due to anthropogenic activities, increasing exposure to radiation in individuals beyond natural radiation. Among artificial radionuclides, 137Cs are one of the most important for radiological protection (UNSCEAR, 1982). 137Cs is an artificial radionuclide that is produced through the fission process of 235U and 239Pu. The dispersion of 137Cs on the planet is due to an atmospheric process called fallout, where smaller particles containing 137Cs are suspended for a long period and settle away from the release site. However, the distribution of 137Cs across the planet is not uniform, approximately 25% of all 137Cs released into the atmosphere have been deposited in the southern hemisphere (UNSCEAR, 2008). Local weather conditions have a significant impact on the deposition and transport of this radionuclide. Rainier sites tend to accumulate more 137Cs, ie the spatial variability of 137Cs fallout seems to be strongly related to rainfall rates and annual rainfall distribution patterns (DAVIES, 1963). The Alagoas State is located between latitudes -8 and -10, has an area of approximately 27,779.343 km² (IBGE, 2002), located in the northeast region of Brazil. Its rainfall regime is directly related to the large-scale atmospheric and ocean circulation configurations on the tropics, but it is also subject to the influence of mesoscale systems such as the convective complexes and the sea and terrestrial breezes that influence all sub- regions, besides the orographic circulations and small convective cells that constitute the microscale phenomena (EMBRAPA, 2012). Figure 1 shows the rainfall distribution in the state. The Alagoas relief is divided into: coastal plain (east), plateau (north) and depression (midwest). The local relief stands out the Borborema Plateau that separates the east from the midwest, has peaks over 1000 m, and is responsible for establishing the differences between the east and the midwest of the state. The objective of this paper is to calculate the 137Cs activity concentration in soils in the Alagoas State and to compare the results found with other regions of Brazil. And as secondary objectives, the article discusses the factors that may influence the increase of 137Cs concentration in the studied soils: relief, rainfall, latitude, soil type, and the accident at the Fukushima Nuclear Power Plant, which is the most recent nuclear accident. ______________________________________________________________________________ 234 Proccedings of the ISSSD 2019 Volume 1 ISSSD 2019 October 7 to 11th, 2019. Zacatecas, Zac. Mexico. _________________________________________________________________________________ Figure 1.- Average annual rainfall (EMBRAPA, 2012). 2.- Materials and Methods 2.1.- Sampling For this research 41 samples were collected in an approximate distribution of 25 to 30 km from each other, where the collection points were established by intersecting longitude and latitude lines forming a regular mesh, as shown in Figure 2. The collection points were georeferenced through GPS. The samples were collected in the first 25 cm of soil, from 2 to 3 kg of soil. They were then labeled, numbered and field sieved in the 60 mesh fraction (nylon sieve). 2.2.- Sample preparation The samples were again sieved in the laboratory now with 230 mesh screen, ie a sieve with a maximum particle size of 63 µm was used for the soil samples, and dried in the oven at 40ºC. Each sample was recorded on a field form containing the location and sampling parameters. The characteristics of the sampling site were passed on to the data matrix, such as: rainfall close to collection; vegetation type and geographical coordinates. ______________________________________________________________________________ 235 Proccedings of the ISSSD 2019 Volume 1 ISSSD 2019 October 7 to 11th, 2019. Zacatecas, Zac. Mexico. _________________________________________________________________________________ Figure 2.- Distribution of sample collection sites in the Alagoas State. Approximately 300g of soil sample was packed in 300 ml polyethylene pots, sealed and lined in foil and allowed to stand for at least 30 days to achieve radioactive secular equilibrium. 2.2.- Instrumentation The samples activity concentrations were measured by gamma spectrometry on hyperpure germanium (HPGe) detectors designed by Canberra Inc. (Meriden, CT, USA). HPGe detectors (gc2020, well, gx45a and gx100) and their respective electronic devices (HV sources, amplifiers, multichannel analyzer) from the Laboratory of Environmental Radiometry, the Division of Environmental and Occupational Radioprotection (DIRAD), the Institute of Radioprotection and Dosimetry (IRD / CNEN) were used to measure the specific activity of 137Cs in soil samples. These detectors have individual shielding made of 15 cm thick old lead and 2 mm copper layer, also produced by Canberra Inc. The detectors used for the measurements have a relative efficiency of 20%, 25%, 40% and 100%. ______________________________________________________________________________ 236 Proccedings of the ISSSD 2019 Volume 1 ISSSD 2019 October 7 to 11th, 2019. Zacatecas, Zac. Mexico. _________________________________________________________________________________ The 137Cs specific activity was measured by means of the 661.6 keV gamma phototypes. The counting time used for the samples was 60,000 seconds, and the system background was measured by the time of 230,000 seconds. Detector efficiency calibration was performed with a standard nitric acid solution (HNO3) containing several radionuclides (57Co, 109Cd, 203Hg, 139Ce, 210Pb and 241Am), provided by the National Laboratory of Ionizing Radiation Metrology (LNMRI) belonging to the IRD / CNEN. LNMRI is the Brazilian radiation metrology laboratory, tracked by the Bureau des Poids et Measures (BIPM) in France. Detectors energy calibration used in the present study was routinely performed using a 152Eu source. In order to ensure measurement quality, a certified soil standard (089 / ERA), provided by the National Institute of Standards and Technology (NIST), was analyzed as a blind sample during sample measurement. 3.- RESULTS Among the 41 samples analyzed, only 6 presented 137Cs above the minimum detectable activity (MDA), which corresponds to 15% of the soil samples. Activity concentration values are shown in Table 1. Table 1.- Activity concentration values of 137Cs above MDA. 137Cs Uncertainty MDA Sample (Bq/kg) (Bq/kg) (Bq/kg) 3 0.46 0.08 <0.32 11 0.47 0.10 <0.41 13 0.30 0.08 <0.24 14 1.13 0.46 <0.71 22 0.66 0.20 <0.59 24 0.46 0.10 <0.43 MDA ranged from 0.22 to 3.39 Bq/kg, and the latter was considered as an outlier by the Rosner method performed by ProUCL 5.1. ______________________________________________________________________________ 237 Proccedings of the ISSSD 2019 Volume 1 ISSSD 2019 October 7 to 11th, 2019. Zacatecas, Zac. Mexico. _________________________________________________________________________________ For the statistical analysis of the results obtained in this article,