A MULTIELEMENT ANALYSIS OF NATURAL RADIONUCLIDES IN AN AREA OF PERNAMBUCO - BRAZIL

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1 2007 International Nuclear Atlantic Conference - INAC 2007 Santos, SP, Brazil, September 30 to October 5, 2007 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: A MULTIELEMENT ANALYSIS OF NATURAL RADIONUCLIDES IN AN AREA OF PERNAMBUCO - BRAZIL José Araújo dos Santos Júnior 1,2, Jorge João Ricardo Ferreira Cardoso 1,3, Romilton dos Santos Amaral 1, Rômulo Simões Cezar Menezes 1,4 Cleomacio Miguel da Silva 1, 5, Edilson Accioly Rocha 1,6 and Dilmo Marques da Silva Leotério 1 1 Grupo de Radioecologia (RAE) - Departamento de Energia Nuclear - Universidade Federal de Pernambuco Av. Professor Luiz Freire, 1000, Cidade Universitária Recife, PE jaraujo@ufpe.br, romilton@ufpe.br 2 Departamento de Química - Universidade Federal Rural de Pernambuco Rua Dom Manoel de Medeiros, s/n, Dois Irmãos Recife, PE junior28_santos@yahoo.com.br 3 Departamento de Geologia - Universidade Federal de Pernambuco Av. Acadêmico Hélio Ramos, s/n, Cidade Universitária Recife, PE jcardoso@ufpe.br 4 Grupo de Fertilidade de Solos - Departamento de Energia Nuclear - Universidade Federal de Pernambuco Av. Professor Luiz Freire, 1000, Cidade Universitária Recife, PE rmenezes@ufpe.br 5 Departamento de Física - Universidade Federal Rural de Pernambuco Rua Dom Manoel de Medeiros, s/n, Dois Irmãos Recife, PE cleomacio@hotmail.com 6 Centro Federal de Educação Tecnológica de Pernambuco Av. Professor Luiz Freire, 500, Cidade Universitária Recife, PE edilsonaccioly@cefetpe.br ABSTRACT Levels of natural radioactivity are the major cause of external exposure to gamma radiation. Based on this parameter, natural radioactivity of 238 U, 232 Th and 40 K were evaluated in a semiarid region of Pernambuco, Brazil, with the aim of obtaining radiometric monitoring of agricultural soils. For the analyses, the gamma spectrometry was utilized with a hyperpure germanium detector with an intrinsic efficiency of 27.7%. Seventy eight samples of soil that were collected in an area of approximately 55,500 m 2, around an anomaly of uranium in rock, were analyzed. The activities of 238 U were evaluated by the 63 kev photopeak of 234 Th, that of 232 Th by the 338, 911 and 969 kev peak of 228 Ac, and that of 40 K by its 1,461 kev peak. The mean of the activities for these radionuclides were 91 Bq.kg -1 (median 88 Bq.kg -1 ) for the 238 U, 228 Bq.kg -1 (median 215 Bq.kg -1 ) for the 232 Th and 1,827 Bq.kg -1 (median 1,662 Bq.kg -1 ) for the 40 K. The anomalous rock did not present neither associated thorium nor potassium (values below the limit of detection: < 10 Bq.kg -1 of 232 Th and < 62 Bq.kg -1 of 40 K), thus giving the characteristic of another natural radioactivity source. The obtained data allow to take important conclusions about the distribution of the radioactivity levels, to establish radioecological parameters.

2 1. INTRODUCTION The radioactivity of Earth s crust is derived from three major categories of radionuclides: primordial, secondary and cosmogenic. Among the primordial radionuclides, 235 U, 238 U, 87 Rb and 40 K are responsible for the greatest contribution of external radioactivity exposure, compared to other sources of radioactivity [1]. This exposure occurs mainly due to gamma radiation, which depend primarily on the geological characteristics and on the concentration of uranium, thorium and potassium of the parent material that originates the soils of each region of the world [2, 3]. In general, high levels of radiation are associated to acidic rocks, while sedimentary rocks present low levels of radiation. Besides that, some shists and phosphatic rocks may contain relatively high levels of radionuclides [2]. Regarding mobility, uranium presents intermediate mobility in oxidizing environments, while thorium presents very little mobility. It is important to point that the mobility of the radionuclides can be influenced by the chemical and physical processes that cause rock weathering. Therefore, the geochemical characteristics of the rocks combined with the effects of time and metamorphism, originate significant differences in the profile of natural radionuclide concentrations and, consequently, in the concentration of uranium, thorium and potassium in different types of rocks [3]. Uranium is found in nature mainly as oxides, the most important of them being uraninite (UO 2 ) and pitchblende (U 3 O 8 ). 238 U is found in the Earth s lithosphere in an average concentration of approximately 2.8 μg.g -1 [1], varying from 0.5 to 5 μg.g -1 [4]. The mean activity of 238 U in soils is approximately 25 Bq.kg -1 [5]. 232 Th is the precursor of one of the main radioactive decay series. The world average of 232 Th specific activity in soils is around 40 Bq.kg -1 [6], and the average concentration of this radionuclide is 13 μg.g -1 [7]. In rocks, the concentration of 232 Th varies from 1.6 to 20 μg.g -1, with an average of 8.5 μg.g -1 in the Earth s continental crust [8], reaching concentrations up to 4 times greater than uranium concentrations in acidic rocks. The main minerals that contain thorium are thorite and thorianite. However, even though monazite is a Ce mineral, it contains high concentrations of Th (up to 33%), wherever Th is associated to rare earth elements. Potassium is an essential element to the human metabolism. It is evenly distributed in the human body and is present in all living cells, particularly in muscular tissues. The fraction of 40 K in realtion to total K is of % [1, 9]. This radionuclide is responsible for 98% of the gamma radiation emission from all primordial radionuclides present in the lithosphere [10]. The world average specific concentration of 40 K is 370 Bq.kg -1, varying from 100 to 700 Bq.kg -1 [11]. Based on the above discussion, it is very important to identify and study areas with natural radioactivity anomalies in order to implement measures of radioprotection. Therefore, the objective of the present study was to characterize the levels of 238 U, 232 Th and 40 K in an agricultural area in the semi-arid region of Pernambuco, Brazil, using high resolution gamma spectrometry analyses.

3 2. MATERIALS AND METHODS 2.1. Study area and sample collection The study area is in the Ageste region of Pernambuco State, in Northeastern Brazil, located at 08º S and 36º W (Figure 1). The area was chosen because it was identified as having an uranium anomaly in an amphibolium calcium-silicate rock, according to previous studies conducted in this site [12]. This anomaly, combined with the agricultural and livestock activities observed in this site, which consist primarily of the cultivation prickly pear (Opuntia fícus-indica) for dairy cattle forage, made it essential to monitoring of the radionuclide concentration in the area around the anomaly. The area comprises about 55,500 m 2, where seventy eight soil samples were collected. The sampling was done in a grid with a 25 m distance between sampling points in the whole area. Due to the climate type, the samples were collected from the C horizon at a depth of approximately 40 cm and taken to the laboratory, where thay were dried, sieved (63 μm), homogeneized, divided in subsamples and 200 g of each sample was stored in sealed poliethylene vials. Figure 1. Location of the study area. Mapping software: MapSource - Garmin. - Study area. Considering that 238 U and 232 Th analyses are done indirectly, i.e., they are a function of the secular radioactive equilibrium, the gamma counting was performed after a storage time of 40 days.

4 2.2. Measuring system The measuring system is comprised of a hyperpure germanium detector (HPGe), Canberra, BeGe (Broad Energy Germanium), with 4.54 cm of external diameter and active volume of 41.1 cm 3. The detector presents a resolution of 1.77 kev for the energy of 1,332 kev of 60 Co, intrinsic relative efficiency of 27.7% and is coupled to a pre-amplifier model 2002 CSL MCA (multichannel analyzer) with 8,192 channels and software Genie Canberra. In order to reduce the effects of BG interference, the detector is involved in a Pb shield of approximately 6.5 cm thickness and an internal tin brass Determination of activities The activities of each radinuclide were calculated using the following equation: C A = ε t γ m (1) where A is the activity (Bq.kg -1 ) of the radionuclide in the sample; C is the net total peak area for the energy of the radionuclide (countings); ε is the counting efficiency for the specific energy being considered; t is the time of counting (s); γ is the gamma abundancy of the radionuclide; and m is the mass of the sample (kg). The counting efficiency was calculated using an 152 Eu Standard, which was diluted in a soil matrix with density similar to the density of the soil samples of the present study, in order to reduce the interferences due to self-absorption. The correction fo the curve to low energy levels was done using an statistical software. For the calculation of the 238 U activities, the gamma photopeak of 234 Th with energy of 63 kev was used. The activity of 232 Th was calculated using the average value of the activities obtained for the photopeaks of 338, 911 and 969 kev of 228 Ac, while the 40 K was directly analyzed by its gamma photopeak of 1,461 kev. 3. RESULTS Figure 2 present the correlations between the radionuclides and the increasing distribution of the specific activities of the seventy eight soil samples of the present study. Figures 2a and 2b show that practically there is no correlation between 238 U and 232 Th neither between 238 U and 40 K. However, Figure 2c shows good correlation between 232 Th and 40 K (r = 0.97). Figure 2d shows the activities obtained for 238 U, 232 Th and 40 K, which are presented from the lowest to the highest values. It can be observed that the values for 40 K are much greater than for the other elements. It is important to notice that concentrations of 232 Th are greater than those of 238 U, even though the study area has an anomaly of uranium in a bedrock. The activities of 238 U varied from 22 to 268 Bq.kg -1 (arithmetic mean: 91 Bq.kg -1 ; median: 88 Bq.kg -1 ); 232 Th varied from 77 to 433 Bq.kg -1 (arithmetic mean: 228 Bq.kg -1 ; median: 215

5 Bq.kg -1 ) and 40 K varied from 541 to 3,572 Bq.kg -1 (arithmetic mean: 1,827 Bq.kg -1 ; median: 1,662 Bq.kg -1 ). The mean activities for 238 U, 232 Th and 40 K found in the present study are 3.6, 5.7 and 4.9 times greater than the mean concentration of the Earth s crust worldwide. 232 Th presents the highest concentration factor, obtained by dividing the activity obaserved by the global mean activity. Th Activity (Bq.kg -1 ) Statistical parameters: Y = 178, ,53408 * X R = 0,25132 DP= 88,86118 N = 78 P = 0, a U-238 K Activity (Bq.kg -1 ) Statistical parameters: Y = 1528, ,23051 * X R = 0,19849 DP= 689,13009 N = 78 P = 0, U-238 b Ativity (Bq.kg -1 ) Statistical parameters: Y = 127, ,45383 * X R = 0,97326 DP= 161,50666 N = 78 P = <0, U-238 Th-232 K-40 K Activity (Bq.kg -1 ) Th-232 c 500 d Samples by increasing activities Figura 2. Correlations between a: 238 U- 232 Th; b: 238 U- 40 K; c: 232 Th- 40 K e d: Distribution of increasing activities of 238 U, 232 Th and 40 K in the study area. 4. CONCLUSIONS It was obseverd that 40 K is the predominat radionuclided present in the studied area, followed by 232 Th, when compared to the activity of 238 U, which is present in lesser amounts. The activities obtained for uranium are characterized by the presence of this radionuclide in the anomalous rock. Sources of thorium and potassium were not observed in this rock. Only the concentrations of 232 Th and 40 K present high correlation coefficient, showing that both originate from the same natural source, which is not the amphibolium calcium-silicate rock. Very probably, the source for these radionuclides is the granite, which is very abundant in the study site.

6 ACKNOWLEDGMENTS The authors thank the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and the PROPESQ (Federal University of Pernambuco) for the financial support. REFERENCES 1. EISENBUD, M.; GESELL, T., Environmental Radioactivity: from natural, industrial, and military sources, Academic Press, New York (1997) 2. TZORTZIS, M.; TSERTOS, H., Determination of thorium, uranium and potassium elemental concentrations in surface soils in Cyprus, Journal of Environmental Radioactivity, 77, pp (2004). 3. ANJOS, R. M.; VEIGA, R.; SOARES, T.; SANTOS, A. M. A.; AGUIAR, J. G.; FRASCÁ, M. H. B. O.; BRAGE, J. A. P.; UZÊDA, D.; MANGIA, L.; FACURE, A.; MOSQUERA, B.; CARVALHO, C.; GOMES, P. R. S., Natural radionuclide distribution in Brazilian commercial granites, Radiation Measurements, 39, pp (2005). 4. HICKOX, W. H., Uranium: Public health goals for chemicals in drinking water, Agency Secretary: California Environmental Protection Agency (2001). 5. DIMOVA, N.; KINOVA, L.; VELEVA, B.; SLAVCHEV, B., Radiochemical procedures for determination of naturally occurred uranium isotopes in environmental samples, Geology and Geophysics, 46, pp (2003). 6. PAPP, Z.; DEZSO, Z.; DARÓCZY, S., Measurement of the radioactivity of 238 U, 232 Th, 226 Ra, 137 Cs e 40 K in soil using direct Ge(Li) γ-ray spectrometry, Journal of Radioanalytical and Nuclear Chemistry, 222, pp (1997). 7. ROSE, A. W.; HAWKES, H. E.; WEBB, J. S., Geochemistry in mineral exploration,. 2 ed., Academic Press, New York (1979). 8. KRAUSKOPF, B. K., Introduction to Geochemistry, McGraw-Hill, Tokyo (1979). 9. PEIXOTO, C. M.; AULER, L. M. L. A.; FONSECA, R. C. M., Interferência do K-40 na atividade beta total em amostras ambientais, Anais do X ENFIR, Águas de Lindóia, SP, 1995, Vol. 1., 1995, Águas de Lindóia, SP, pp (1995). 10. PIRES, A. C. B., Identificação geofísica de áreas de alteração hidrotermal, Crixás - Guarinos, Goiás, Revista Brasileira de Geociências, 25, pp (1995). 11. MCAULAY, I. R.; MORAN, D., Natural radioactivity in soil in the Republic of Ireland, Radiation Protection Dosimetry, 24, pp (1988). 12. SANTOS JÚNIOR, J. A., CARDOSO, J. J. R. F., SILVA, C. M., SILVEIRA, S. V., AMARAL, R. S. Determination of radionuclides in the environment using gammaspectrometry, Journal of Radioanalytical and Nuclear Chemistry, 269, pp , (2006).