Analysis of the 40K levels in soil using gamma spectrometry

Abstracts

Potassium-40 was determined in soil in an area with 40,000 m² situated in the western State of Pernambuco, Brazil. For radiometric measurements, the gamma spectrometry method with a high purity germanium (HPGe) detector was used. Sampling of 78 soil samples has been performed at intervals of 25 m. The specific activities of 40K were calculated based on the photopeak of 1.46 MeV. Values from 541 to 3,572 Bq kg-1 were obtained (mean of 1,827 Bq kg-1). These values allowed the determination of the elemental concentrations as well as the absorbed dose rates in air, 1 m above the ground. The values varied from 1.7 to 11.5% (mean of 6%) and from 23.4 to 154.3 nGy h-1 (mean of 79 nGy h-1), respectively.

40K in soil; 40K gamma spectrometry; 40K in environmental radiological protection

A radioatividade do 40K foi determinada em solo em uma área de aproximadamente 40.000 m² localizada no Estado de Pernambuco, Brasil. Para as medidas radiométricas foi usado o método de espectrometria gama com detector de germânio de alta pureza (HPGe). A amostragem perfaz um total de setenta e oito amostras de solo coletadas em intervalos de 25 m. As atividades específicas do 40K foram calculadas com base no fotopico gama de 1,46 MeV. Foram obtidos valores que variaram de 541 a 3.572 Bq kg-1 (média aritmética de 1.827 Bq kg-1). Com base nestes cálculos, obteve-se concentrações elementares e doses absorvidas no ar a 1 m, derivadas da contribuição do 40K no solo, onde os valores obtidos variaram respectivamente de 1,7 a 11,5% (média 6%) e 23,4 a 154,3 nGy h-1 (média 79 nGy h-1).

RADIOPROTECTION

Analysis of the ⁴⁰K levels in soil using gamma spectrometry

José Araújo dos Santos JúniorI, ^* ; Jorge João Ricardo Ferreira Cardoso^{I, II}; Cleomacio Miguel da Silva^I; Suêldo Vita Silveira^I; Romilton dos Santos Amaral^I

^IGrupo de Estudos em Radioproteção e Radioecologia (GERAR); Department of Nuclear Energy; Federal University of Pernambuco; Av. Prof. Luiz Freire, 1000; Cidade Universitária; 50740-540; [e-mail]; Recife - PE - Brasil

^IIDepartment of Geology; Federal University of Pernambuco; Av. Acadêmico Hélio Ramos, s/n; Cidade Universitária; 50740-530; [e-mail]; Recife - PE - Brasil

ABSTRACT

Potassium-40 was determined in soil in an area with 40,000 m² situated in the western State of Pernambuco, Brazil. For radiometric measurements, the gamma spectrometry method with a high purity germanium (HPGe) detector was used. Sampling of 78 soil samples has been performed at intervals of 25 m. The specific activities of ⁴⁰K were calculated based on the photopeak of 1.46 MeV. Values from 541 to 3,572 Bq kg^-1 were obtained (mean of 1,827 Bq kg^-1). These values allowed the determination of the elemental concentrations as well as the absorbed dose rates in air, 1 m above the ground. The values varied from 1.7 to 11.5% (mean of 6%) and from 23.4 to 154.3 nGy h^-1 (mean of 79 nGy h^-1), respectively.

Key words:⁴⁰K in soil; ⁴⁰K gamma spectrometry; ⁴⁰K in environmental radiological protection.

RESUMO

A radioatividade do ⁴⁰K foi determinada em solo em uma área de aproximadamente 40.000 m² localizada no Estado de Pernambuco, Brasil. Para as medidas radiométricas foi usado o método de espectrometria gama com detector de germânio de alta pureza (HPGe). A amostragem perfaz um total de setenta e oito amostras de solo coletadas em intervalos de 25 m. As atividades específicas do ⁴⁰K foram calculadas com base no fotopico gama de 1,46 MeV. Foram obtidos valores que variaram de 541 a 3.572 Bq kg^-1 (média aritmética de 1.827 Bq kg^-1). Com base nestes cálculos, obteve-se concentrações elementares e doses absorvidas no ar a 1 m, derivadas da contribuição do ⁴⁰K no solo, onde os valores obtidos variaram respectivamente de 1,7 a 11,5% (média 6%) e 23,4 a 154,3 nGy h^-1 (média 79 nGy h^-1).

INTRODUCTION

The brazilian Northeast region, despite its interesting geological structure (Malanca et al., 1993), has not been sufficiently explored in terms of environmental radiological studies. Since 1974, the Brazilian Nuclear Enterprise (NUCLEBRAS) performed geological and radiometric mapping studies in that region, where a small anomalous area of uranium was discovered in the Pernambuco sylvan. The first radiometric-geological studies occurred after an accord between NUCLEBRAS and the Federal University of Pernambuco (UFPE) in 1975 (Magalhães, 1977).

Natural environment radionuclides are responsible for the most of the external exposures of gamma radiation. This contribution is mainly due to the radionuclides of the natural series ²³⁸U, ²³⁵U, and ²³²Th, followed by ⁴⁰K, universally present in the Earth (Shenber, 1997; Tzortzis; Tsertos, 2004). The world mean specific concentration of ⁴⁰K (activity per unit soil mass) is 370 Bq kg^-1, varying from 100 to 700 Bqkg^-1 (Mcaulay; Moran, 1988). Radioactivity levels of the environment depend on geological aspects, mainly on rocks and soil, where they are found in varying concentrations (Tzortzis; Tsertos, 2004).

The average absorbed dose rate in air at a 1 m distance from the ⁴⁰K source is 15.54 nGy h^-1 (Selvasekarapandian et al., 2000). The United Nations Scientific Committee on the Effects of Atomic Radiation, UNSCEAR (1993), establish that, the world mean dose from natural radiation sources of normal areas is estimated to be 2.4 mSva^-1, while that for all man-made sources, including medical exposure, is about 0.8 mSv.a^-1 (Ghiassi-Nejad et al., 2001). Thus, 75% of the radiation dose received by humanity is come from natural radiation sources. Based on these radiation levels, one can certify that the knowledge of primordial radionuclides, such ²³⁸U, ²³²Th, ⁴⁰K and ⁸⁷Rb, is an important pre-requisite for evaluation of the rate of exposure and the absorbed dose by the population (Kannan et al., 2002). This knowledge ensures radiological control, since a predominant part of the environmental radiation is found in the upper soil layer (Kannan et al., 2002).

Potassium is an essential element of human metabolism and can be found in all living cells, mainly in the muscular tissue. Natural potassium is composed of three isotopes: ³⁹K, ⁴⁰K and ⁴¹K, of which only ⁴⁰K is radioactive. This radionuclide occurs at the approximate abundance of 0.012% in relation to the whole mass of potassium (Pires, 1995). ⁴⁰K has a half-life of 1.28 x 10⁹ years and is a beta and gamma emitter (89% and 11% of its radiation, respectively) with respective energies of 1.3 MeV and 1.46 MeV (Peixoto et al., 1995; Erdtmann; Soyka, 1979). The relevance of the potassium-40 study is mainly due to its long halflife, its importance for living organisms with a uniform distribution, and because it is responsible for 98% of the gamma emission of the primordial radionuclides present in the earth. The present work is important because there is no reference regarding the area concerning the ⁴⁰K concentrations in the soil, which were found to be high.

MATERIALS AND METHODS

Sampling and preparation

The soil samples were collected in an area of approximately 40,000 m², located in the Sylvan region of the state of Pernambuco, Brazil (Fig.1).

The sampling was carried out at 25 m intervals, and the samples were collected from the C horizon (average depth of 35 cm), mainly due to the kind of climate. In the laboratory, the samples were dried, sieved at a granulometry lower than 63 µm and homogenized. Each sample of 200 grams was stored in standardized polyethylene containers.

Measure of the radioactivity

The activities of the ⁴⁰K in the soil were determined by gamma spectrometry HPGe of 4.54 cm of diameter, active volume of 41.1 cm³, and efficiency of 27%, coupled to a Canberra MCA (multi-channel analyzer) with 8,192 channels. This system presented a resolution of 1.77 keV in 1,332 keV for ⁶⁰Co. To reduce the background effect, the detector was enveloped by nearly 6.5 cm lead covering. The measurement time for each sample was standardized at 43,200 seconds (12 hours). The counting efficiency of ⁴⁰K in the energy of approximately 1.46 MeV was determined with ¹⁵²Eu, multi-gamma emitting, with energies varying from 39 keV to 2 MeV, certified by the Institute for Radioprotection and Dosimetry (IRD), Rio de Janeiro, Brazil. Based on the calculations of the counting efficiency of the ¹⁵²Eu standard, it was possible to calculate the specific activities of the potassium 40, with the energy 1.46 MeV, based on equation 1 (Tzortzis et al., 2003):

where A is the specific activity of the radionuclide (Bq kg^-1); C is the liquid count; e the counting efficiency; g the percentage of gamma emission probability of the radionuclide under consideration; t the counting time (s) and m the mass of the sample (kg). Using the above equation, it was possible to calculate the specific activities of the ⁴⁰K in the study area through the energy of 1.46 MeV. For the detection limit calculation (LD), equation 2 was used, where the factor 4.66 corresponded to one level of confidence of 95% (Silva; Mazzilli, 2005).

The parameters of equation 2 for the calculation of the LD were applied to the measurement of the background in the photopeak of 1.46 MeV of the ⁴⁰K.

Calculation of elemental concentrations of ⁴⁰K

Through the measurement of the activities of potassium 40, calculated from its gamma emission in the photopeak of 1.46 MeV (equation 1), it was possible to calculate the elemental concentrations reported in units of µg.g^-1, using equation 3 (Tzortzis; Tsertos, 2004).

where F_E is the fraction of element E in the sample, M_E, l _E, f _A,E and A_E are the atomic mass (kg mol^-1), the radioactivity decay constant (s^-1), the fraction atomic abundance in nature and the measured activity concentration (Bq kg^-1), respectively, of the corresponding radionuclide considered (⁴⁰K), N_A is Avogadro’s number (6.023 x 10²³ atoms mol^-1), and C is a constant with a value of 100 for K. These elemental concentrations are presented in percentages for potassium.

Calculation of the absorved dose in air

Equation 4, published in UNSCEAR (1988), was used to calculate the absorbed dose rate (D) in outdoor air at one meter above ground, proceeding from the gamma emissions of ⁴⁰K (Shenber, 1997; Selvasekarapandian et al., 2000; Kannan et al., 2002):

where C_K is the specific activity of ⁴⁰K (Bq kg^-1) and 10^-8 is the conversion factor from the absorbed dose (D).

RESULTS AND DISCUSSION

The Figs. 2, 3 and 4 show the frequency distributions based on the calculations of the specific activities, elemental concentrations and absorbed dose rates in air 1 m above the ground surface, respectively, for the ⁴⁰K in the soil of the Sylvan region of the state of Pernambuco, Brazil. The non-parametric test of Kolmogorov-Smirnov at a reliability level of 95%, showed that the best distribution of these data for all the tests is the log-normal one, with asymmetry on the right.

The map of the activity distribution of the ⁴⁰K in the study area is shown in Fig. 5. The highest values obtained for the ⁴⁰K activities were determined in NE portion of the area, probably a result of the presence of granite and, consequently, higher elemental concentrations and absorbed doses.

The specific activities of ⁴⁰K in soil varied from 541 to 3,572 Bq kg^-1 (mean 1,827 Bq kg^-1), with a detection limit of approximately 62 Bqkg^-1.

Table 1 shows the statistical parameters obtained for ⁴⁰K in soil of the study area.

The elementary concentrations of ⁴⁰K varied from 1.7% to 11.5%, with a mean of approximately 6%. The worldwide average for the elemental concentration of ⁴⁰K in soil is 1.3%. The UNSCEAR 2000 report (Tzortzis; Tsertos, 2004), presents fc varying from 1.3 to 9, approximately (fc mean about 5). The absorbed doses in air 1 m above the surface were calculated using equation 4 UNSCEAR report (Shenber, 1997; Selvasekarapandian et al., 2000; Kannan et al., 2002), where values varying from 23.4 to 154.3 nGy h^-1, with a mean of 79 nGy h^-1, were obtained.

The elemental concentration of the ⁴⁰K in soil samples from work conducted in various countries (Tzortzis; Tsertos, 2004) is presented in Table 2.

When the values for the concentrations of ⁴⁰K in the present study are compared with those obtained in other regions of the world, it can be seen that the ratio of the variations of the present study to those of Italy, Spain, India, Jordan and Cyprus are 0.9-4.6; 8.5-2.2; 8.5-23; 17-11.5; > 1.7-6, respectively. Thus, only the minimum concentration of Italy (1.9) is higher than the minimum concentration in the present study (1.7). All the other elemental concentrations are lower.

The specific activities and absorbed doses in the air one meter above the ground for ⁴⁰K obtained from the study, and the work conducted in other regions, according to Tzortzis and Tsertos (2004) can be seen in Table 3.

CONCLUSIONS

The results show that gamma spectrometry is an important tool for environmental monitoring of ⁴⁰K. By this method, the distribution the elemental concentration, as well as the absorbed dose in air, one meter above the ground surface, could be determined. The measured activities in the studied area are five fold higher than the world mean, and, consequently, the elemental concentrations and absorbed dose are five fold higher than the world average. The highest concentrations were found in the NE region of the area.

ACKNOWLEDGEMENTS

The authors thank the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the scholarship; to Professor Suêldo Vita da Silveira (through FADE - Fundação de Apoio ao Desenvolvimento da Universidade Federal de Pernambuco) for the financial support; and Professor Carlos Adolpho M. Baltar for making the laboratory under his management available to us (Laboratory of Mineral Technology of the Federal University of Pernambuco).

Received: July 05, 2005;

Revised: July 14, 2005;

Accepted: August 01, 2005.

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*

Author for correspondence

Publication Dates

• Publication in this collection
  09 Dec 2005
• Date of issue
  Oct 2005

History

• Accepted
  01 Aug 2005
• Received
  05 July 2005
• Reviewed
  14 July 2005