2017 International Nuclear Atlantic Conference - INAC 2017 Belo Horizonte, MG, , October 22-27, 2017 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR – ABEN

NATURAL RADIOACTIVITY IN ROCKS FROM PARAÍBA SERTÃO, BRAZIL

Kennedy F. R. Damascena1, José A. dos Santos Júnior1, Romilton dos S. Amaral1, Jairo D. Bezerra1, Lino V. Rojas1,3, Nilson V. da S. Medeiros1, Alberto A. da Silva1,2, Josineide M. do N. Santos1, Otávio P. dos Santos Júnior4

1 Department Nuclear Energy – Study Group for Radioecology (RAE) Federal University of (UFPE) Av. Prof. Luiz Freire, 1000 - 50740-540 - Recife, PE [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected]

2 Federal Institute of Education, Science and Technology of Pernambuco (IFPE) Fazenda Sapé, s/n, Zona Rural, 55560-000 - Barreiros, PE [email protected]

3 Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN) Calle 30 #502, Playa, La Habana, Cuba. ZIP Code 11300 [email protected]

4 Federal Institute of Education, Science and Technology of Pernambuco (IFPE) Propriedade Terra Preta, s/n - Zona Rural, 55600-000 - Vitória de Santo Antão, PE [email protected]

ABSTRACT

Northeastern Brazil is a region with a large number of natural radioactive occurrences. Monitoring studies carried out over the last 30 years have identified a hundred anomalous points, especially in the State of Paraíba, more specifically the region of Seridó Ocidental Paraibano, geologically characterized by the presence of rocky outcrops with radioactive materials associated with granites and pegmatites. Regions with differentiated levels of natural radioactivity and, consequently, greater radioecological relevance, have been the constant object of radiometric and dosimetric studies. Considering their relevance, the present study aimed to evaluate the levels of natural radioactivity in rocks located in the Riacho da Serra and Serra dos Porcos, previously unmonitored, located in the municipalities of São José do Sabugi and Santa Luzia, in Paraíba, Northeast of Brazil. The radiometric evaluation was performed by measuring the specific activities of U-238, Th-232 and K-40 in rock samples using a high resolution gamma spectrometry system. The mean specific activities of U-238, Th-232 and K-40 were: 2562.30 ± 672.22; 180.68 ± 672.22 and 1374.13 ± 36.90 Bq/kg, respectively. The monitored radionuclides presented high values of specific activity, being 1.6; 4.1 and 71.2 times higher than the mean values for the earth's crust

1. INTRODUCTION

Natural radioactivity, responsible for more than 70% of the effective environmental doses, comes from cosmic sources, characterized by cosmogenic radionuclides produced in the atmosphere and from terrestrial sources, such as primordial radionuclides 238U, 232Th and their decay products, in addition to 40K, accounting for about 85% of human external gamma exposure levels [1,2].

Because they have remarkable radioecological relevance, it is necessary to know the distribution of these radionuclides in the environment and, consequently, the levels of radioactivity associated with them. Monitoring makes it possible to measure levels of exposure to humans and ensures compliance with the principles of radiation protection.

Some areas are well known due to high levels of natural radioactivity, as in the cases of Kerala, India and Ramsar, Iran [3]. In Brazil, studies to monitor areas with such characteristics have been carried out since the beginning of the 20th century [4] and have been conducted over the years in a large number of localities. Special attention have been paid to Poços de Caldas and Araxá, in and Guarapari, in the coastal area of the State of Espírito Santo [5].

Since the 1970s, the Brazilian Northeast was the object of a radiometric monitoring study, promoted by the National Nuclear Energy Commission (CNEN) through the use of aerogamaespectrometry, allowing the discovery of the existence of numerous occurrences of U and Th [6]. Since then, a considerable number of studies have been developed aiming at assessing the radiometric characteristics of the region with greater detail [7,8,9,10,11].

Seridó and Borborema regions, in the State of Paraiba, are the areas with great radiometric relevance. They present a high occurrence of rocks containing associated radioactive minerals, such as pegmatites, granites and gneisses, originating from separation of the African and South American continents, as well as secondary minerals of U (uraninite, meta- autunite, phosphoraninite, beta-uranophane, gadolinite, etc.). Due to the presence of such geological peculiarities, some monitoring studies have been recorded in the region [12,13,14].

Taking into account the elements presented above, the present study aimed at monitoring the levels of 238U, 232Th and their descendants, in addition to 40K, present in rock samples collected in the municipalities of São José do Sabugi and Santa Luzia, both in the state of Paraíba. In those localities is reported the existence of radioactive occurrences [6]. In order to achieve this objective, high-resolution gamma-ray spectrometry was used for its numerous operational advantages and for being quite efficient in the measurement of such radionuclides in environmental samples [15]. From the data obtained in this research, it is expected to elaborate the radiometric and dosimetric profile of the monitored localities.

2. MATERIAL AND METHOD

2.1 Study area

The study area is located in the municipalities of São José do Sabugi and Santa Luzia, State of Paraiba. It includes the localities called Riacho da Serra and Serra dos Porcos, with a geographical location between latitudes 06º 49 '38.5' 'and 06º 52' 30.6 "S and longitudes 36º 49 '55,6" and 36º 53 '33.4' W (datum SIRGAS2000).

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The municipalities have a total area of approximately 662 km2 and a population of 19,500 inhabitants. In the municipality of Santa Luzia, 91% of the population resides in the urban area, unlike the municipality of São José do Sabugi, where the level of occupation in urban area is around 64%. In these municipalities, the primary sector is the main support to the economy, whose activities are concentrated in , with an average participation of approximately 62.5%. [16].

The study area is located in the Borborema Province, characterized by granite-gnassic- migmatitic massifs normally structured in the general N-NE direction, and covering formations of the groups Calcialcalina Suite, Seridó, Ecuador, Várzea Alegre Suite and Serra dos Quintos Complex, consisting mainly of granites, biotite-shale, metarithmites, orthogneisses, shales and gneisses, which normally present radionuclides in their composition [17,18,19].

2.2. Sample collection and treatment.

Prior to sample collection, the study area was visited to locate the outcrops and verify the existing levels of natural radioactivity. The areas that presented differentiated levels were defined as sampling points. Measurements to define these areas were done using a portable gamma radiation detector of the discriminator type.

At the sampling points, the material was collected (approximately 3 kg of sample at each point), neglecting the rocks that presented the weathering action. Considering the two localities monitored in the present study (Riacho da Serra and Serra dos Porcos), 16 samples were collected.

The initial preparation of the environmental samples was carried out in partnership with the Mineral Technology Laboratory (LTM), belonging to the Department of Mining Engineering (DEMINAS) of the Federal University of Pernambuco (UFPE), where the granulometry of the collected samples was reduced to values ≤ 1.0 mm, using jaw crushers and roller mills.

After the initial preparation, the samples were homogenized, quarteted and conditioned in polyethylene containers with an approximate capacity of 100 cm³, hermetically sealed. Thereafter, a minimum period of 40 days was waited for the secular radioactive equillibrium to be established for the short half-lives (214Bi and 228Ac) radionuclides, to be analyzed in a high resolution gamma spectrometry system.

2.3. Sample analysis

After the condition of secular radioactive equillibrium was reached, the samples were measured in a high resolution gamma spectrometry system with standardized counting geometry. The containers, with 100 g of sample, were positioned at 0 cm from the detector and measured during 86,400 seconds.

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2.3.1 Gamma spectrometry system.

A high resolution gamma spectrometry system, with an HPGe detector covered by a Be window was used for analysis of samples. The germanium cristal have cylindrical and coaxial format, external diameter equal to 4.54 cm and active volume of 41.1 cm³. To attenuate the external radiation, the detector was surrounded by a Pb shield whose approximate thickness is 6.5 cm, containing a thin inner surface composed of layers of Cu and Cd. The detector was mounted in a cryostat and was cooled with nitrogen liquid.

The system also includes a digital Multichannel Analyser, EAGLE Plus model and the Genie- 2k® software that allows data acquisition, storage, display and analysis of acquired γ spectra.

2.3.2 Energy calibration

The energy calibration of the measurement system was performed using sealed standard sources of 241Am, 133Ba, 137Cs, 22Na and 60Co (obtained from IRD and IAEA). The sources were positioned directly over the detector and analyzed individually for a period of 1800 seconds. After the acquisitions, energy/channel tuning was performed using the capabilities of Canberra® Genie-2k software, based on the technical document of the International Atomic Energy Agency TECDOC-619 [20].

2.3.3 Efficiency calibration

The energy efficiency curve was constructed using secondary standards of 133Ba, 137Cs, 152Eu and 241Am, whose initial activities were certified by IRD. The preparation of these standards was carried out in the facilities of the Laboratory of Radioecology and Environmental Control (LARCA in portuguese) of the Department of Nuclear Energy of the Federal University of Pernambuco. Standards sources were prepared using 100 g of rock with density similar to the samples collected, doped with the aforementioned radioactive materials. Standards sources were measured for 86,400 seconds.

2.4 Specific Activity calculations

The calculation of the specific activities of 238U, 232Th and 40K for the analyzed samples was performed through equation 1 [21]. N A= Ei (1) ε × Iγ × t × m

Where: A represents the activity concentration determined for the radionuclide under -1 consideration (Bq.kg ); NEi corresponds to the total net area of the photopeak for the chosen energy (counts); ε is the counting efficiency, considering the specific energy; t is the count time (s); Iγ is the probability of gamma emission and m is the mass of the analyzed sample (kg).

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3. RESULTS AND DISCUSSION

Table 1 shows the specific activities of the natural radionuclides (238U, 232Th and 40K) determined in the rock samples collected in the study areas. Considering as the first criterion of comparison the mean values, 238U presented the highest levels, followed by 40K and 232Th. For the 238U, the specific activities ranged from 202.04 to 6.382.04 Bq.kg-1, with an average value of 2,562.30 Bq.kg-1. As for 40K, activities were recorded between 716.24 to 1.604.01 Bq.kg-1, with an average of 1,374.13 Bq.kg-1. The 232Th showed activities between 114.36 to 355.41 Bq.kg-1 and mean of 180.68 Bq.kg-1.

The heterogeneous characteristic of the data presented in Table 1, suggests the heterogeneous distribution for the analyzed radionuclides, although located in the same rock formation, something not uncommon to be observed when evaluating environmental samples of this type of matrix.

The occurrence of data with high variations in its values makes it difficult to perform an accurate diagnosis of the region studied, in radiometric terms. In order to verify the type of distribution of the data obtained in the study area, the statistical test of normality of Shapiro- Wilk was applied, which showed a tendency of normality, for a confidence level of 95%, for the data coming from the study area.

A comparative study was carried out between the data obtained in the present study and those data considered as reference in the earth's crust [22], in addition to similar studies performed in other countries [15, 23, 24] (Table 2). The arithmetic mean of the set of activity values was used as the reference parameter, considering that the other studies used this parameter to describe the overall behavior of the data.

Table 1: Specific activity for 238U, 232Th e 40K in samples from Santa Luzia and São José do Sabugi municipalities, Paraíba

Geographical coordinates Specific Activity (Bq.kg-1) Code S W 238U 232Th 40K 1 06o 52' 19,3'' 36o 53' 33,4'' 3.970,92 ± 822,94 145,42 ± 53,11 1497,60 ± 38,69 2 06o 52' 18,1'' 36o 53' 32,1'' 2.054,37 ± 478,97 242,36 ± 74,58 1094,22 ± 33,08 3 06o 52' 17,6'' 36o 53' 32,4'' 4.802,20 ± 1.243,69 160,91 ± 13,85 1178,39 ± 34,33 4 06o 52' 16,3'' 36o 53' 33,2'' 3.037,65 ± 839,09 147,80 ± 52,95 1222,86 ± 34,97 5 06o 52' 16,8'' 36o 53' 33,2'' 6.382,04 ± 1.776,49 189,83 ± 20,17 1588,12 ± 39,85 6 06o 52' 15,4'' 36o 53' 31,7'' 202,04 ± 86,23 148,48 ± 14,63 1604,01 ± 40,05 7 06o 52' 15,2'' 36o 53' 31,2'' 462,42 ± 106,98 164,47 ± 26,26 1715,17 ± 41,41 8 06o 52' 30,6'' 36o 50' 14,3'' 1.851,11 ± 479,08 114,36 ± 15,23 1535,72 ± 39,19 9 06o 49' 42,8'' 36o 49' 55,6'' 1.879,08 ± 572,56 194,86 ± 23,32 1427,72 ± 37,78 10 06o 49' 42,0'' 36o 49' 56,4'' 4.654,56 ± 867,15 183,61 ± 71,12 1384,84 ± 37,21 11 06o 49' 44,6'' 36o 49' 57,3'' 4.357,40 ± 968,12 182,79 ± 20,61 1154,57 ± 33,98 12 06o 49' 42,9'' 36o 49' 58,7'' 2.035,02 ± 703,23 167,15 ± 68,87 1521,42 ± 39,00

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13 06o 49' 42,9'' 36o 49' 59,2'' 522,99 ± 202,23 159,02 ± 71,59 1559,54 ± 39,49 14 06o 49' 41,4'' 36o 49' 59,8'' 682,90 ± 204,91 116,63 ± 49,34 1294,32 ± 35,98 15 06o 49' 38,6'' 36o 50' 04,8'' 917,18 ± 320,56 355,41 ± 162,68 1491,25 ± 38,62 16 06o 49' 44,9'' 36o 49' 55,7'' 3.184,85 ± 1083,21 217,82 ± 85,47 716,24 ± 26,76 Mean ± Standard Deviation 2.562,30 ± 672,22 180,68 ± 51,48 1374,13 ± 36,90

Table 2: Comparison between the results of the present research and some other studies

Specific Activity, mean value (Bq.kg-1) Regions Reference 238U 232Th 40K Paraíba, Northeast Brazil 2.562,30 180,68 1.374,13 Present study Crosta Terrestre 36 44 850 [22] Nigéria 26,99 61,91 91,7 [15] Egito 46,39 65,76 1186,45 [23] Turquia - 215,5 1248,6 [24]

The comparative study presented in Table 2 shows that the radionuclides analyzed in the present study presented high values of average specific activities, reinforcing the relevance of the obtained data.

When compared to the mean values for the earth's crust, the specific activities obtained for the 238U, 232Th and 40K in our study was about 71.2; 4.1 and 1.6 times higher. In relation to the other reports used for comparation, the data obtained for the study area showed average specific activity values that varied between 1.1 and 94.9 times higher, being the greater variation found when compared to the levels of 238U present in the northeast of Nigeria. The exception, however, was observed in relation to 232Th, where the average specific activity was lower than that recorded in Turkey.

The geological composition of the study area, located in the Seridó geosynclinal area, formed by granitic-gnamic-migmatitic masses, orthogneisses and pegmatites, usually associated with radioactive minerals, mainly U, can be considered as a justification for the high levels found for the radionuclides analyzed in the present study [6].

4. CONCLUSIONS

The results obtained showed that the study area has high levels of specific activities for the 238U, 232Th and 40K. These results justify the accomplishment of future analyzes that guarantee the quantification of the corresponding doses, allowing the detailed elaboration of a profile radiometric and dosimetric measurements of the monitored localities.

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ACKNOWLEDGMENTS

The authors like to thank the Coordination of Improvement of Higher Level Personnel (CAPES in Portuguese) for the grant. Also thanks the National Council for Scientific and Technological Development (CNPq in Portuguese) for the financial assistance, the Federal University of Pernambuco (UFPE) through the Laboratory of Mineral Technology (LTM), belonging to the Department of Mining Engineering (DEMINAS); and the Laboratories of the Department of Nuclear Energy: Laboratory of Radioecology and Environmental Control (LARCA), Laboratory of Radiochemistry and Nuclear Analysis (LABRAN) and Laboratory of Metrology of Ionizing Radiation (LMRI), for the infrastructure provided.

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