NATURAL RADIOACTIVITY FROM THE SOIL OF SÃO BERNARDO DO CAMPO

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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: NATURAL RADIOACTIVITY FROM THE SOIL OF SÃO BERNARDO DO CAMPO M. M. Matsumoto 1, M. A. G. Silveira 1, N. H. Medina 2 and N. K. Umisedo 2. 1 Centro Universitário da FEI Av. Humberto Alencar de Castelo Branco São Bernardo do Campo, SP marcos.m.matsumoto@gmail.com shila@if.usp.br 2 Instituto de Física da Universidade de São Paulo Travessa R da Rua do Matão São Paulo, SP medina@if.usp.br numisedo@if.usp.br ABSTRACT We have studied the distribution of natural activity in the soil of the city of São Bernardo do Campo, Brazil using gamma-ray spectroscopy techniques. The main contribution of the radiation dose is due to the isotope 40 K, with smaller contributions from the elements of the series of 238 U and 232 Th. The results indicate that the dose in all of the studied areas is under the average international dose for external exposure to gamma rays (0.48 msv/yr) proceeding from natural terrestrial elements. 1. INTRODUCTION The study of the distribution of natural radioactivity in Brazilian soil, as well as in air and in the water, supply information about its variation in different regions of the country. Unfortunately, there is no systematic study of radionuclide distribution in Brazilian soil due to its continental dimensions. For South America, there are only a few measurements reported for 40 K (650 Bq/kg) in Argentinian soil [1]. For external exposure of terrestrial gamma-radiation in Latin America there are values only for Chile and Paraguay, with absorbed doses of 51 and 46 ngy/h, respectively. Brazil has the sixth largest uranium reserve in the world and there are many areas with high background radiation, but only few of them are reported, such as Guarapari (ES), Poços de Caldas (MG), Araxá (MG ) and Goiás [1]. The doses absorbed in air vary in Guarapari from 90 to 170 ngy/h in the downtown area and 90 to ngy/h on the beaches. The high doses are due to the concentration of monazitic sands. The values for the state of Goiás are 110 to 1300 ngy/h and for the cities Poços de Caldas and Araxá, in the state of Minas Gerais, the dose varies from 340 to 2800 ngy/h. For the case of the cities from the estate of Minas Gerais the radiation is from the volcanic soil. Natural radiation comes from cosmic and terrestrial sources, while artificial radiation is produced by human beings. The Earth is continuously bombarded by particles proceeding from outer space (muons, protons, photons, electrons, neutrons and pions), known generically as cosmic rays, that arrive at the terrestrial atmosphere, in general, with high energy. The amount of cosmic radiation varies with altitude, with a higher concentration of muons at sea level, which contributes to the equivalent dose [1]. Another component of natural radiation

2 comes from radionuclides in the soil, such as potassium, uranium and thorium. The gamma radiation from potassium is emitted by the 40 K isotope, which decays mainly via -decay to the stable nucleus 40 Ca. The isotope 40 K also decays by electron capture emitting gamma rays with an energy of 1460 kev to the stable nucleus 40 Ar. The lifetime of this isotope is of the order of 10 9 years. The isotopes 232 Th and 238 U are part of two radioactive series where a parent nucleus decays to daughter nucleus, also radioactive. This process continues until the production of the stable nucleus 208 Pb. The study of the distribution of natural soil radioactivity distribution would supply valuable information for civil construction, since it could inform against construction in areas with high radioactive element concentration. Monazite, for example, is a mineral found in sand and soil which contains high concentration of thorium and uranium. Regions rich in these minerals usually show high radiation activities and the use of such soils, sands or rocks, mixed with construction materials, could increase the local radiation dose [2]. The knowledge of the radiation distribution is important also for the prevention of diseases, which may be related to long time radiation exposure. The average international effective radiation dose per capita is 2.4 msv. In this value is included the doses due to the cosmic radiation, internal and external gamma radiation and those due to radon gas. There are Brazilian regions where the background radiation is very high, with values up to 10 msv/yr [1]; for example, in the cities: Pitinga (AM), Caetité (BA), Buena (RJ), Poços de Caldas (MG), and Guarapari (ES) the radiation doses varies between 3.5 to 10mSv/yr [4]. In recent years several works related to radiation doses in Brazilian regions have been published. R. M. Anjos et al. studied natural gamma radiation along the southern Brazilian coast [3]. This research group investigated also, in detail, gamma radiation from granites and marble, which are used commonly in Brazilian construction [4]. There are no specific works about populational doses in Brazil. Nevertheless, some work, focused on the city of São Paulo, have contributed to give information of the soil gamma-ray dose [5,6,7] and the contribution of radon inside houses [8]. In 2005, a study on the exposure due to X-ray radiography was published [9]. Gamma-ray spectroscopy has been used also to verify the contribution of fertilizer to the radioactive dose, since there is transfer of the radioactive elements from the soil to the plant which will be consumed by the population [10]. Human beings have always been exposed to ionizing radiations. Radionuclides may be present in different compartments of the ecosystem and reach human beings by means of external or internal irradiation, e.g. radon gas inhalation, and ingestion of water or foodstuff with radionuclides. The study of the distribution of these radionuclides is very important for the understanding of the radiological implications relative to the exposure of the human body to gamma radiation and, particularly, to the knowledge of which radioactive elements are found in a specific geographic region. Since the mapping of the natural radioactivity distribution in Brazil is still very scarce we have started the study with the natural radioactivity from the soil of the city of São Bernardo do Campo, São Paulo, Brazil. 2. EXPERIMENTAL PROCEDURES

3 We have collected three samples in each of five sites in São Bernardo do Campo, São Paulo, Brazil: a) Parque Estoril, which is near the Billings dam, on the border of the industrialized region and the beginning of the protected area of the Serra do Mar. b) Praça Samuel Sabatini, in the central downtown area. c) Praça Giovani Breda, which is a heavily industrialized region with very intense traffic. d) Parque Municipal Engenheiro Salvador Arena and Ginásio Poliesportivo. These last two areas are intensely used by the local population. The samples were collected at three different depths 10, 25 and 50 cm. The samples were dried in a stove at about 100 o C for 24 hours, and, after sifting materials larger than 1.2(5) mm in diameter, the samples were packed in plastic boxes. The samples were stored for a period of approximately of 28 days, in order to reach secular equilibrium. We have used gamma spectroscopy techniques with a 60% GeHP detector inside a 15 cm thick lead shield to collect the emitted gamma rays. A 60 Co source was used for energy calibration. We have measured also the activities from 40 K, 232 Th and 238 U standard sources for normalization purposes. 3. RESULTS The sample activities are extracted directly from the gamma-ray spectra, since we know the activity from standard sources. To calculate the intensity of the gamma-rays we have taken into account the gamma-rays from the background. In Table I the results of the activities in different locations and at various depths in São Bernardo do Campo are shown. From these results it is possible to observe that the isotope 40 K contributes more to the total activity than all the other radioactive isotopes. The activity of 40 K varied from (16.19 ± 0.04) Bq/kg to ( ± 0.64) Bq/kg and with an average value of (160 ± 25) Bq/kg; for 238 U from (27.61 ± 0.09) Bq/kg to (63.67 ± 0.17) Bq/kg, with an average value of (51 ± 11) Bq/kg and for 232 Th from (46.28 ± 0,14) Bq/kg to (83.65 ± 0.22) Bq/kg, with an average value of (68 ± 12) Bq/kg. The values for the superficial soil in Praça Giovani Breda and Praça Sabatini show values too low with respect to the concentration of 40 K. In fact, these values do not represent the real soil of these places once the superficial soil was transported from other places for landscaping purposes. These values were not taken into account in the average value. To calculate the absorbed dose due to the incident gamma radiation from external sources (external exposure) we need to take into account the conversion coefficients from Bq/Kg to the dose in ngy/h [1]. The coeficients are [ngy/h]/[bq/kg] for 40 K, [ngy/h]/[bq/kg] for the nuclides in the 238 U series and [ngy/h]/[bq/kg] for the 232 Th nuclide series. To transform the absorbed dose in air into effective dose we have taken into account another conversion factor for external terrestrial gamma radiation which is 0.7Sv/Gy [1]. The average effective doses are 0.053(18) msy/yr for 40 K, 0.018(4) msy/yr for 238 U and 0.025(4) msy/yr for 232 Th. Table I: Soil activities, in Bq/kg, for different locations in São Bernardo do Campo,

4 Brazil, at various depths. Location 40 K (Bq /kg) 238 U (Bq/ kg) 232 Th (Bq /kg) Parque do Estoril 10 cm ± ± ± cm ± ± ± cm ± ± ± 0.20 Parque Salvador Arena 10 cm ± ± ± cm ± ± ± cm ± ± ± 0.19 Ginásio Poliesportivo 10 cm ± ± ± cm ± ± ± cm ± ± ± 0.20 Praça Samuel Sabatini 10 cm ± ± ± cm ± ± ± cm ± ± ± 0.23 Praça Giovani Breda 10 cm ± ± ± cm ± ± ± cm ± ± ± CONCLUSIONS We have studied the distribution of natural activity from the soil of the city São Bernardo do Campo, São Paulo. The results: 0.053(8) msy/yr for 40 K, 0.018(4) msy/yr for 238 U and 0.025(4) msy/yr for 232 Th, indicate that for all the studied areas, the measured dose is under the average international value for a dose due to external exposure to gamma rays (0.48 msv/yr) proceeding from natural terrestrial elements. REFERENCES [1] UNSCEAR. Sources and Effects of Ionizing Radiation. United Nations Scientific Committee on the Effects of Atomic Radiation, United Nations, New York, [2] R. M. Anjos et al., Radioecology teaching: evaluation of the background radiation levels from areas with high concentrations of radionuclides in soil. Eur. J. Phys. 25 (2),

5 [3] R. Veiga et al., Measurement of natural radioactivity in Brazilian beach sands. Radiation Measurement 41, [4] R. M. Anjos et al., Natural radionuclide distribution in the Brazilian comercial granites. Radiation Measurement 39, [5] F. H. M. Medeiros., Radiação gama no solo da cidade de São Paulo e implicações dosimétricas. Tese de Mestrado. IFUSP. [6] R. E. R Oliveira., Doses Populacionais de Radiação Ionizante na Cidade de São Paulo: medidas de dose gama a céu aberto. Dissertação de mestrado. IFUSP. [7] S. M. Otsubo., Contribuição da radiação gama em ambientes fechados para a dose populacional na cidade de São Paulo. Dissertação de mestrado. IFUSP. [8] A. A Da Silva., Radônio e filhos em residências na cidade de São Paulo. Tese de Doutorado. IFUSP. [9] M. B. Freitas, Panorama das exposições médicas em radiologia convencional no Estado de São Paulo. Tese de Doutorado. IFUSP. [10] N. K. Umisedo, Tese de doutorado, Faculdade de Saúde Pública, USP.