|Year : 2011 | Volume
| Issue : 3 | Page : 178-184
Determination of natural radioactivity in beach sand in the extreme south of Bahia, Brazil, using gamma spectrometry
Danilo C Vasconcelos1, Claubia Pereira1, Arno H Oliveira1, Talita O Santos1, Zildete Rocha2, Maria Ângela de B. C. Menezes2
1 Departamento de Engenharia Nuclear, Escola de Engenharia, Universidade Federal de Minas Gerais, PCA 1 - Anexo Engenharia, Av. Antônio Carlos, 6627 Campus UFMG, CEP 31.270-901, Brazil
2 Centro de Desenvolvimento da Tecnologia Nuclear / Comissão Nacional de Energia Nuclear (CDTN/CNEN), Laboratório de Trítio, Laboratório de Ativação Neutrônica, Caixa Postal 941, CEP 30.123-970, Belo Horizonte, Minas Gerais, Brazil
|Date of Web Publication||27-Sep-2012|
Departamento de Engenharia Nuclear, Escola de Engenharia, Universidade Federal de Minas Gerais, PCA 1 - Anexo Engenharia, Av. Antônio Carlos, 6627 Campus UFMG, CEP 31.270-901
Source of Support: None, Conflict of Interest: None
The natural radionuclides activity concentrations in beach sand of the extreme south of Bahia, Brazil, was measured by Gamma Spectrometry. The Radium Equivalent Activity, the external hazard index, the absorbed dose rate and the annual effective dose were assessed and compared with internationally published values for external dose and activity concentrations. The activity concentrations of 226 Ra, 232 Th and 40 K in beach sand ranged from 8.4 to 8,300 Bqkg -1 with a mean value of 910 Bq.kg -1 , from 21 to 18,450 Bqkg -1 with a mean value of 2,220 Bqkg -1 and from 3.4 to 3,110 Bqkg -1 with a mean value of 352 Bqkg -1 , respectively. The results indicate that the absorbed dose rates range from 21 to 14,450 nGyh -1 with mean value of 1,792 nGy.h -1 . The highest value of gamma dose rates among the studied beaches were found in Cumuruxatiba (14,450 nGyh -1 ). The annual effective dose range between 0.03 and 17.70 mSvy -1 , with the mean value of 2.20 mSvy -1 . In four studied beaches, the assessed outdoor annual effective doses are above the worldwide average of 0.07 mSvy -1 as reported by the United Nations Scientific Committee on the Effects of Atomic Radiation. Especially in the area of black sands, a big part of Cumuruxatiba beach, whose annual effective dose of 17.70 mSvy -1 is much higher than worldwide average.
Keywords: Annual effective dose, gamma ray spectrometry, natural radioactivity, radiation hazard indexes
|How to cite this article:|
Vasconcelos DC, Pereira C, Oliveira AH, Santos TO, Rocha Z, de B. C. Menezes MÂ. Determination of natural radioactivity in beach sand in the extreme south of Bahia, Brazil, using gamma spectrometry. Radiat Prot Environ 2011;34:178-84
|How to cite this URL:|
Vasconcelos DC, Pereira C, Oliveira AH, Santos TO, Rocha Z, de B. C. Menezes MÂ. Determination of natural radioactivity in beach sand in the extreme south of Bahia, Brazil, using gamma spectrometry. Radiat Prot Environ [serial online] 2011 [cited 2020 Mar 31];34:178-84. Available from: http://www.rpe.org.in/text.asp?2011/34/3/178/101714
| 1. Introduction|| |
Human beings have always been exposed to natural radiation, which is mainly due to the activity concentration of primordial radionuclides 238 U( 226 Ra) series, 232 Th series and 40 K that are present in the earth's crust, in building materials and in air, water and foods and in the human body itself.  These exposures may vary depending on the local geology of each region in the world. The knowledge of distribution of the primordial radionuclides is an important role for peoples avoid long exposure.
Radionuclides are classified into four groups according to their origins: (1) cosmic-ray produced nuclides (such as 7 Be and 14 C); (2) artificially-produced nuclides (such as 137 Cs and 90 Sr); (3) primordial isotopes (such as 238 U and 40 K) and (4) natural decay products (such as 226 Ra and 222 Rn). The three naturally occurring radioactive decay chains include the 238 U, 235 U, and 232 Th, which decay through a series of radioactive elements up to stable Pb isotopes.
The study of the distribution of primordial radionuclides allows the understanding of the radiological implication of these elements due to the g-ray exposure of the body and irradiation of lung tissue from inhalation of radon and its daughters.  In particular, it also is important to assess the radiation hazards arising due to the use of soil or sand samples in the construction of dwellings. Therefore, the assessment of gamma radiation dose from natural sources has significant importance as natural radiation and is the largest contributor to the external dose of the world population. 
These dose rates vary depending upon the concentration of the natural radionuclides, 238 U, 232 Th, their daughter products and 40 K, present in soil, sands and rocks, which depend on the local geology of each region in the world. In order to estimate the possible radiological hazards to human health, special attention has been given in the last two decades to low-level exposure arising from members of the uranium and thorium decay chains and by 40 K in soils. Some investigations were made in developed countries but very few have been made in developing nations.  In certain beaches of Brazil there are areas which are well known for their high background radiation. , However, Cumuruxatiba, in Bahia state, is a place which has a large amount of monazite sand (4,500T according to Mezhari, 2005)  and there are no measurements on the natural radioactivity.
The present work investigates the concentrations of radioisotopes such as 232 Th, 226 Ra, and 40 K, in beach sand samples from the extreme south of Bahia state, an area in Brazil which hasn´t been studied very much and estimates the radiological hazard. The radium equivalent activity, the external hazard index, the absorbed dose rate and the effective dose rates were estimated and compared with internationally published values for high background radiation area.
| 2. Materials and Methods|| |
Beach sand samples were collected in Porto Seguro (16°26'S 39°03'W), Arraial D'Ajuda (16°29'S 39°04'W), Trancoso (16°35'S 39°05'W), CaraÍva (16°48'S 39°08'W), Cumuruxatiba (17°07'S 39°11'W), Prado (17°20'S 39°13'W), Alcobaça (17°33'S 39°11'W), and Caravelas (17°43'S 39°10'W) from May 2008 to January 2009. The samples were collected from a depth of 10 cm. Each sample was collected from an area corresponding to one m 2 and was homogenized in situ, and this sand mixture, weighing approximately 1.5 kg, was considered a representative of the profile. [Figure 1] shows the studied area. For each local, about 12-15 samples were measured. The investigated area, Cumuruxatiba, has beaches with black and non-black sands. Thus, samples were collected in both locations.
|Figure 1: Map showing the region in the extreme south of Bahia state, in Brazil and the locations of the studied beaches|
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The samples were dried for about 48 hours in an oven at 60°C. The analytical techniques used for the gamma emitters isotopes 214 Bi ( 226 Ra), 228 Ac ( 232 Th) and 40 K was gamma spectrometry. The 226 Ra and 232 Th concentrations were indirectly determined through the 214 Bi determination after the radioactive equilibrium was established in the samples. [Table 1] shows the technique used for each radionuclide, the specific gamma line used and the detection limits that were calculated according to the procedures and the experimental conditions. 
|Table 1: Analytical techniques and detection limits for the radionuclides|
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Energy calibration was performed in the energy range by using point sources 0.1 to 2 MeV made by Institute of Isotopes Co., Ltd, Budapest, Hungary. The following photon emissions and radionuclides were used: 241 Am, 133 Ba, 152 Eu, 137 Cs and 60 Co. The efficiency calibration curve was determined by using a standard solution made by the National Institute of Standards and Technology (NIST). Each sand sample, of approximately 1.0 kg, was counted for 86,400 s. The same geometry was used for both the samples and the reference materials. All samples were measured in a low background spectrometry system inside a special counting room, shielded by water all around, over and below.
2.2 Analytical technique - Gamma spectrometry
The specific activities of the radionuclides 226 Ra and 40 K in the collected samples were determined in a specially reduced background detection system, consisting of a hyper-pure germanium detector (HPGe), coaxial geometry, 15% relative efficiency and 1.66 keV FWHM resolution at 1,332.5 keV g-rays for 60 Co. The data were treated with Genie 2000 software. The samples, after being dried and weighed, were transferred to Marinelli beakers (500 mL). Each sample was hermetically and carefully sealed to prevent the escape of gaseous 222 Rn and 220 Rn. It was kept aside for 30 days to reach radioactive equilibrium. After that, all samples were submitted to gamma spectrometry. The samples were counted for 24 hours in order to obtain statistically small uncertainty for the areas of the gamma ray peak.
Following the spectrum analysis, the activity concentration in units of Becquerel per kilogram for each nuclide was calculated based on equation 1:
where N Ei is the net peak area at energy E i of radionuclide i; t the counting time in seconds; m is the mass of samples in kg; eEi and gEi are detection efficiency and emission probability of gamma ray, respectively.
2.3 Dose calculation
The total air absorbed dose rate (nGy h -1 ) 1 m above the ground due to the specific activities of 226 Ra, 232 Th and 40 K (Bq kg -1 ) was calculated by using equation 2: ,
where D is the absorbed dose rate, A K , A Ra and A Th are the activity concentrations for 40 K, 226 Ra and 232 Th respectively.
To estimate the annual effective dose rates, the conversion coefficient from absorbed dose in air to effective dose (0.7 Sv Gy -1 ) and outdoor occupancy factor (0.2) proposed by  was used. The effective dose rate in units of mSv y -1 was calculated by following equation 3:
where D is the calculated dose rate (in nGy h -1 ), T is the outdoor occupancy time (24 h × 365.25 days × 0.2 = 1,753 h y -1 ), and F is the conversion factor (0.7 Sv Gy -1 ).
2.4 Radiation hazard indexes
According to Veiga et al, 2006,  sand beach minerals, rejected light sands and sea beach soils can be used in industries and building constructions. Although its use is not convenient, the fisherman community in lives houses built with sand and soil available in the coastal area. Therefore, it is important to measure the g-ray radiation hazards indexes, due to the presence of radionuclides such as 40 K, 226 Ra ( 238 U daughter) and 232 Th. It is important to assess the gamma ray radiation hazards of sand to human health and analyze if the use of sand in building construction is safe or unsafe.
When comparing the specific activity of samples containing different amounts of 226 Ra, 232 Th and 40 K, it is usually introduced the term radium equivalent activity (Ra eq ). It can is defined as a single quantity that represents the combined specific activities of 226 Ra, 232 Th and 40 K and is developed as a numerical indicator of an external dose to public and internal dose due to radon and its daughters. The value of 370 Bq kg -1 is the maximum value allowed for public dose considerations according to Beretka and Mathew, 1985.  Equation 4 was used to calculate the radium equivalent activity:
where A Ra , A Th and A K are the specific activities of 226 Ra, 232 Th and 40 K in Bq kg -1 respectively, the use of the constants 1.43 and 0.077 are explained by Beretka and Mathew, 1985. 
According to Kumar et al, 1999,  there are variations in the radium equivalent activities of different materials and within the same type of materials as well. The results may be important from the point of view of selecting suitable materials to be used in building construction. Large variations in radium equilibrium activities may suggest that it is advisable to monitor the radioactivity levels of materials from a new source before adopting it to be used as a building material.
The external hazard index (H ex ) is a radiation hazard index defined by Beretka and Mathew, 1985,  to evaluate the indoor radiation dose rate due to the external exposure to g-radiation from the natural radionuclides in the construction building materials of dwellings. This index value must be less than unity to keep the radiation hazard insignificant, i.e. the radiation exposure due to the radioactivity from construction materials was limited to 1.0 mSv y -1 . For limiting the radiation dose to this value,  proposed the following conservative model based on infinitely thick walls without windows and doors to serve as a criterion, based on equation 5:
where A Ra , A Th and A K are the specific activities of 226 Ra, 232 Th and 40 K in Bq kg -1 respectively. The maximum value of H ex equal to unity corresponds to the upper limit of Ra eq (370 Bq kg -1 ).
Another radiation hazard index called the representative level index I yr was defined in equation 6: ,
where A Ra , A Th and A K are the specific activities of 226 Ra, 232 Th and 40 K in Bq kg -1 respectively. This index was used to estimate the level of gamma-radiation hazard associated with the natural radionuclides in specific building materials.
| 3. Results and Discussions|| |
The activity concentrations of radionuclides measured in beach sand samples from the extreme south of Bahia, Brazil, are shown in [Table 2]. The naturally occurring 226 Ra, 232 Th and 40 K in beach sand ranged from 8.4 to 8,300, 21 to 18,450, 3.4 to 3,110 Bq kg -1 , with a mean value of 910 ± 25, 2,220 ± 50, and 352 ± 25 Bq kg -1 respectively.
As [Table 2] shows, the values of 226 Ra and 232 Th in Cumuruxatiba, Trancoso, Caraíva, Arraial D'Ajuda and Alcobaça are higher than the range of the corresponding typical world values. The values of 232 Th in Caravelas, 226 Ra in Prado and 40 K in Cumuruxatiba also are higher than the range of the corresponding typical world values.
[Table 2] also lists the statistical data (arithmetic mean with standard deviation, geometric mean, geometric std. dev., skewness, kurtosis co-efficient and the type of frequency distribution) for these radionuclides in the sand samples. The values of skewness and kurtosis co-efficients that can be observed for all the three radionuclides, are not closer to the null value, indicating the non-existence of normal distribution.
[Table 3] presents the results obtained for the radium equivalent activity (Ra eq ), the representative level index (I yr ), external hazard index (H ex ), the total absorbed dose rate in air due to gamma radiation (D), as well as the outdoor annual effective dose rate (H E ) assessment for beach sand samples.
|Table 3: Characteristics of the radioactivity in sand samples collected from extremesouth of Bahia, Brazil|
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The values of Ra eq , I yr and H ex in Cumuruxatiba (non-black and black sands), Alcobaça and Caraíva are higher than the limit set in the Organization for Economic Cooperation and Development (OECD) which reports 370 Bq kg -1 , 1 Bq kg -1 and 1. ,,, It suggests that the use of this sand is unsafe for constructing buildings. Trancoso has only value exceeded for I yr .
As observed in [Table 3], the values of the absorbed dose rates in Alcobaça and Cumuruxatiba are higher than the estimate of average global primordial radiation of 59 nGy h -1 and than the world range (10-200 nGy h -1 ).  The results show that the absorbed dose rates range from 21-14,450 nGy h -1 with mean value of 1,792nGy h -1 . The highest value of gamma dose rates among the studied beaches was found in Cumuruxatiba (14,450nGy h -1 ). The dose rate in Areia Preta sample from Guarapari, for example, ranged from 75 to 14,400 nGy h -1 ,  which is one of the most widely known high background radiation areas in the world. The largest contribution from natural radionuclides in Cumuruxatiba (black and non-black sands) and Alcobaça to the absorbed dose rate in air is due to 232 Th, about 75%, 89% and 85%, respectively. 226 Ra contributes about 25%, 11% and 14%, respectively. The Contribution of 40 K is negligible, except for the sand samples from Cumuruxatiba, where 40 K contributed with 1% of the absorbed dose rate in air, which was equivalent to 111 nGy h -1 .
The outdoor annual effective dose was determined as recommended by UNSCEAR, 2000.  In studied locations, the annual effective dose varied between 0.03 and 17.70 mSv y -1 , with a mean of 2.20 mSv y -1 . Values of Alcobaça, Trancoso, Caraíva and Cumuruxatiba (non-black and black sands) are higher than the worldwide average for outdoor annual effective dose, which is 0.07 mSv y -1 , as reported by UNSCEAR, 2000.  It indicates the presence of significant amounts of monazite and zirconite sands in these beach sand samples, and may be it is the reason of the black colorations of the sand of Cumuruxatiba and for high concentration of radionuclides.
In [Table 4], a summary of the result on natural gamma radioactivity levels derived from similar investigation conducted in some of world regions and around the Atlantic coast of Brazil was presented. Activity concentrations of 232 Th and 226 Ra from Guarapari are higher than the averages from Cumuruxatiba. However, activity concentrations of 40 K from Cumuruxatiba are higher than the other regions. Values of total air absorbed dose rate (D) and annual effective dose (H E ) from Cumuruxatiba (black sand) are higher than the values from Orissa, India, which has a high background radiation area.
|Table 4: Summary of activity concentrations, radiation hazard indexes and dose rates ofnatural radioisotopes in soil and sand samples in some of the world regions (-) notdetermined|
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| 4. Conclusions|| |
The analytical results confirmed that the samples of the studied area contain three important radioactive isotopes ( 226 Ra, 232 Th and 40 K). The values of the activity concentrations of these isotopes in Cumuruxatiba (7810, 17,770 and 2,650 Bq kg -1 , respectively for, black sands) were higher than other studied locations and than the typical world value as well the values of the total air absorbed dose rate of Alcobaça (330 nGy h -1 ), Cumuruxatiba black (14,450 nGy h -1 ) were higher than the estimate of the average global primordial radiation of 59 nGy h -1 and above the world range (10-200 nGy h -1 ). Values of outdoor annual effective dose in Alcobaça, Trancoso, Caraíva and Cumuruxatiba are higher than the worldwide average for outdoor annual effective dose, which is 0.07 mSv y -1 . The values of Ra eq , I yr and H ex in Caraíva, Alcobaça and Cumuruxatiba (non-black and black sands) are higher than the acceptable limits that are 370 Bq kg -1 , 1 Bq kg -1 and 1 Bq kg -1 . The results of this work are important alerts for the local people to avoid the use of these sands to construct the dwellings. These sands are unsafe for use in building constructions. Furthermore, the use of such sands are not suitable for building construction due to their radiological and technical proprieties. Additionally, this work may be used in order to help with the development of reference levels of natural radioactivity along the coast of Brazil.
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[Table 1], [Table 2], [Table 3], [Table 4]
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