Radiation Protection and Environment

ORIGINAL ARTICLE
Year
: 2012  |  Volume : 35  |  Issue : 1  |  Page : 4--8

The derived radiological parameters associated with beach mineral sand deposits prevail in the south west coastal regions of India


Deva Sigamony Deva Jayanthi1, Chavarachirayil Govindan Manian2, Subramania Pillai Perumal3,  
1 Department of Physics, Women's Christian College, Nagercoil, India
2 Environmental Assessment Division, BARC, Mumbai, India
3 Department of Physics and Research Centre, S.T. Hindu College, Nagercoil, India

Correspondence Address:
Deva Sigamony Deva Jayanthi
Department of Physics, WomenSQs Christian College, Nagercoil
India

Abstract

The distribution of natural radioactivity in beach sand samples collected from naturally high background radiation areas of south west coast of Tamil Nadu in India is studied by gamma spectrometry. The activity concentration of radionuclide collected from the study area ranges from 18.9 ± 4.6 Bq/kg to 260.5 ± 29.4 Bq/kg for 226 Ra, 534.5 ± 34.0 Bq/kg to 2961.4 ± 33.7 Bq/kg for 232 Th, and 40.6 ± 7.2 Bq/kg to 148.7 ± 17.9 Bq/kg for 40 K. The measured activity of 226 Ra and 232 Th in soil samples collected from the study area is higher than the activity of 40 K. The external hazard index, internal hazard index, absorbed dose rate, and annual effective dose equivalent are calculated and tabulated. It is found that all the evaluated values are higher than the safe limit. The annual effective dose of radiation ranges from 0.41 mSv to 2.35 mSv due to naturally occurring radionuclide.



How to cite this article:
Deva Jayanthi D, Manian CG, Perumal SP. The derived radiological parameters associated with beach mineral sand deposits prevail in the south west coastal regions of India.Radiat Prot Environ 2012;35:4-8


How to cite this URL:
Deva Jayanthi D, Manian CG, Perumal SP. The derived radiological parameters associated with beach mineral sand deposits prevail in the south west coastal regions of India. Radiat Prot Environ [serial online] 2012 [cited 2020 Jun 6 ];35:4-8
Available from: http://www.rpe.org.in/text.asp?2012/35/1/4/111402


Full Text

 Introduction



Radiation is present in every environment of the earth's surface. The greatest contribution to mankind's exposure comes from natural terrestrial gamma radiation. [1],[2] Naturally occurring radioactive materials in soil are one of the components of external gamma ray exposure. Soil is an important environmental material used as raw materials for streets and play grounds, and for land filling. If they are present in building raw materials, those naturally occurring radionuclides add to the indoor exposure. The external radiation exposure is caused mainly by the activity concentrations of natural radionuclides of uranium and thorium series and natural potassium. These primordial radionuclides have long half-lives, that they survived since their formation and decaying to attain the stable state and produce ionizing radiation in various degrees. [3] Beach sediments are mineral deposits formed through weathering and erosion of either igneous or metamorphic rocks. It is necessary to quantify the distribution of radionuclides in the marine constituents to assess radiological impacts of the radionuclides on human health. Also natural radioactivity measurements are necessary not only to assess its radiological impacts but also because it acts as excellent biochemical and geochemical tracers in the environment. Only nuclides with half-lives comparable with the age of the earth or their corresponding decay products such as 238 U, 232 Th, and 40 K existing in terrestrial materials are of great interest. Major sources of radioactivity in the environment are the elements of uranium, thorium, and potassium. Among these elements, uranium is the most significant source of radioactivity and decay to form radium. [4] Major variations in the concentration of radioactive minerals in soil and sand are found in number of countries such as China, Brazil, and India. In India, there are quite a few monazite sand bearing placer deposits causing natural high background radiation areas (NHBRAs) along its long coastal line. Ullal in Karnataka, [5] Kalpakkam in Tamil Nadu, [6] coastal parts of Tamil Nadu and Kerala State, and the south west coast of India are known for HBRAs. [7],[8] The south west coast is one of the very few NHBRAs of the world where significant quantities of monazite minerals are present. Beach sands are weather resistant remainder of geological formations, which may have come to their place after transported by wind, rivers, and glaciers and are deposited on the beaches by the action of waves and currents. Human beings are exposed to the outdoor natural terrestrial radiation that originates predominantly from the upper 30 cm of the soil. [9] terrestrial radiation level varies from place to place depending upon the variation of radionuclide concentration in soil. The presence of monazite deposits on the coastal areas of Kerala and Tamil Nadu is due to the weathering of rocks in Nilgiris hills and Western ghats. This work reports the activity concentrations of natural radionuclides such as 226 Ra, 232 Th, and 40 K in soil samples and the associated radiation dose level in the ambient air prevailing in the region. The main objective of this work is to evaluate radiological hazard indices due to natural radioactivity associated with the soil by calculating the internal and external hazard indices, absorbed dose rate in air, and annual effective dose.

 Materials and Methods



Sample preparation

The study area selected for this investigation is a part of south west coast of Tamil Nadu in India [Figure 1]. Study area of south west coast of Tamil Nadu in India shows the geographic location of the study area in Tamil Nadu in the map of India. Fifty beach sand samples were collected 5 m away from the water line from the study area (i.e., five samples from each place). About 1 kg of the sand sample was collected from each location. The samples were first air-dried for 2 or 3 days, and then dried in an oven at 100-110°C for about 24 h. The dried samples were sieved through sieve of 2 mm mesh size. The homogenized samples were placed in an air-tight PVC container for about a month to ensure equilibrium between 226 Ra and its daughter products before being taken for gamma ray spectrometric analysis. [10] The concentration of primordial radionuclide in the sample was determined by using high efficiency NaI (Tl) detector coupled with a multi-channel analyzer. The spectrometer was calibrated using different standards. The activity of 40 K was evaluated from the 1461 keV photopeak, activity of 226 Ra from 1764 keV gamma line of 214 Bi, and that of 232 Th from the 2614 keV gamma line of 208 Tl. [11] Each sample was counted for 10,000 s to reduce statistical uncertainty. The background spectrum was recorded immediately after or before the sample counting. The activity of each sample was determined using the total net counts under the selected photopeaks after subtracting appropriate background counts and applying appropriate factors for photopeak efficiency, gamma intensity of the radionuclides, and weight/volume of the sample. [1] From the measured activities of 226 Ra, 232 Th, and 40 K, the following calculations were done.{Figure 1}

Calculations of radiological parameters

To arrive at a safe conclusion on the health impact of an environment, it is important to assess the gamma radiation hazards to human associated with the soil used for filling yards and buildings. This is done by calculating the different radiation hazard indices.

The distribution of 226 Ra, 232 Th, and 40 K in soil is not uniform. Uniformity with respect to exposure to radiation has been defined in terms of radium equivalent activity (Ra eq ) in Bq/kg to compare the specific activity of materials containing different amounts of 226 Ra, 232 Th, and 40 K. It is calculated through the following relation: [12]

[INLINE:1]

where C Ra , C Th , and C k are the activity concentrations of 226 Ra, 232 Th, and 40 K in Bq/kg, respectively. While defining Ra eq activity, it has been assumed that 370 Bq/kg 226 Ra, 259 Bq/kg 232 Th, or 4810 Bq/kg 40 K produce the same gamma dose rate.

A widely used hazard index (reflecting external exposure) called the external hazard index H ex is defined as follows: [12]

[INLINE:2]

In addition to external hazard index, radon and its short-lived progeny are also hazardous to the respiratory organs. The internal exposure to radon and its daughter progenies are quantified by the internal hazard index Hin, which is given by the equation: [12]

[INLINE:3]

For the safe use of a material in the construction of dwellings, index H in should be less than unity.

The absorbed dose rates (D) due to gamma radiation in air at 1 m above the ground surface, assuming uniform distribution of the naturally occurring radionuclides ( 226 Ra, 232 Th, and 40 K), have been calculated based on the guidelines provided by UNSCEAR. [13] It has been assumed that the contributions from other naturally occurring radionuclides are insignificant. Therefore, D can be calculated using the relation:

[INLINE:4]

Finally, in order to estimate the annual effective dose, a value of 0.7 Sv/Gy was used for the conversion coefficient from absorbed dose in air to effective dose received by adults and 0.2 for outdoor occupancy factor, implying that 20% of time is spent in outdoors. Effective dose exceeding the dose criterion of 1 mSv/y should be taken into account in terms of radiation protection.

[INLINE:5]

 Results and Discussions



From the activity concentrations of 226 Ra, 232 Th, and 40 K of the collected soil samples, the Ra eq is calculated and the results are presented in [Table 1]. The activity concentration of radionuclide collected from the study area ranges from 18.9 ± 4.6 Bq/kg to 260.5 ± 29.4 Bq/kg for 226 Ra, 534.5 ± 34.0 Bq/kg to 2961.4 ± 33.7 Bq/kg for 232 Th, and 40.6 ± 7.2 Bq/kg to 148.7 ± 17.9 Bq/kg for 40 K. The Ra eq is calculated and the value ranges from 793.2 Bq/kg to 4501.0 Bq/kg, which is very much higher than the recommended safe limit (370 Bq/kg) by Organization for Economic Cooperation and Development. [14] The high values of radionuclides recorded in the collected samples are due to the presence of elevated levels of monazite content in the soil.{Table 1}

[Table 2] gives the dose rate, annual effective dose, external hazard index, and internal radiation hazard in soil samples collected from the study area. The external hazard index values range from 2.1 to 11.9, internal hazard index ranges from 1.8 to 12.7, and the absorbed dose in air ranges from 336 to 1912 nGy/h. All the calculated values are high in Melmidalam village and low in Mela Vaniyakudy village. Next to Melmidalam, the higher values are observed in Kurumpanai village. The absorbed gamma dose rate in air in the study area varied from 336.6 nGy/h to 1912.5 nGy/h. These values are compared with other values obtained within India and are presented in [Table 3]. The annual effective dose equivalent varied from 0.4 mSv to 2.35 mSv. The annual effective dose distribution in the study area is shown in [Figure 2]. Graphical representation of annual effective dose in the study area. The calculated values of external hazard index, internal hazard index, and the absorbed dose in air are more than the world average and the recommended safe limit.{Figure 2}{Table 2}{Table 3}

 Correlation Study



Correlation is a statistical measure for finding the degree of relationship between two or more variables. Two variables are said to be correlated if a change in one variable leads to a change in the other variable.

[Figure 3] shows the correlation between 226 Ra and 232 Th activity concentrations computed from the soil samples collected from the study area. There exists a strong correlation between 226 Ra and 232 Th for the samples in all sampling places. The correlation coefficient between these two radionuclides is found to be r = 0.945. Similar findings were also reported in beach sand samples of Kalpakkam. [6]{Figure 3}

[Figure 4] shows the correlation between 232 Th concentration and the absorbed dose in air. There exists a very good correlation between 232 Th and the absorbed dose in air for all the sampling villages. The value of correlation coefficient is r = 0.9996.{Figure 4}

[Figure 5] shows the correlation between 226 Ra and the absorbed dose in air. There exists a good correlation between 226 Ra and the absorbed dose in air. The value of correlation coefficient is r = 0.954.{Figure 5}

 Conclusion



The natural radioactivity levels of 226 Ra, 232 Th, and 40 K have been measured in soil samples of south west coastal regions of India using gamma ray spectrometry. The activity profiles of the radionuclides have clearly showed high activity along the study area. For the soil samples collected from different coastal villages, the Ra eq and the radiation hazard indices were calculated to assess the radiological hazards from the soil. All the calculated parameters are higher than the recommended safe level. Thus, the result of this work is an important alert for the local people to avoid the use of these sands for construction of dwellings.[16]

References

1Senthilkumar B, Dhavamani V, Ramkumar S, Philominathan P. Measurement of gamma radiation levels in soil samples from Thanjavur using gamma-ray spectrometry and estimation of population exposure. J Med Phys 2010;35:48-53.
2Agbalagba EO, Onoja RA. Evaluation of natural radioactivity in soil, sediment and water samples of Niger Delta (Biseni) flood plain lakes, Nigeria. J Environ Radioact 2011;102:667-71.
3Mohanty AK, Sengupta D, Das SK, Vijayan V, Saha SK, et al. Natural radioactivity in the newly discovered high background radiation area on the eastern coast of Orissa, India. Radiat Meas 2004;38:153-65.
4Simsek C. Assessment of natural radioactivity in aquifer medium bearing uranium ores in Koprubasi, Turkey. Environ Geol 2008;55:1637-46.
5Radhakrishna AP, Somashekarappa HM, Narayana Y, Siddappa K. A new natural background radiation area on the southwest coast of India. Health Phys 1993;65:390-5.
6Kannan V, Rajan MP, Iyenga MA, Ramesh R. Distribution of natural and anthropogenic radionuclides in soil and beach sand samples of Kalpakkam (India) using hyper pure germanium (HPGe) gamma ray spectrometry. Appl Radiat Isot 2002;57:109-19.
7Mishra UC. Exposure due to the high background radiation and radioactive springs around the world. In: Proceedings of the international conference on high level natural radiation areas. Ramsar, Iran: IAEA Publication Series, IAEA, Vienna; 1993. p. 29.
8Sunta CM. 1993 A review of the studies of high background areas of the southwest coast of India. In: Sohrabi M, Ahmad JU, Durrani SA, editors. Proceedings of the international conference of high levels of natural radiations, Rasmar; 1990.
9Chilkasawa K, Ishil T, Sugiyama H. Terrestrial gamma radiation in kochi prefecture, Japan. J Health Sci 2001;47:362-72.
10Ramasamy V, Murugesan S, Mullainathan S. Gamma ray spectrometric analysis of primordial radionuclides in sediments of Cauvery river in Tamilnadu, India. Ecologica 2004;2:83-8.
11Quindós LS, Fernández PL, Soto J, Ródenas C, Gómez J. Natural radioactivity in Spanish soils. Health Phys 1994;66:194-200.
12Beretka J, Matthew PJ. Natural radioactivity of Australian building materials, industrial wastes and by-products. Health Phys 1985;48:87-95.
13UNSCEAR. Sources and effects of ionizing radiation. Report of the United Nations scientific committee on the effects of atomic radiation to the general assembly, with scientific annexes; 1993.
14Organization for economic co-operation and Development (OECD). Exposure to radiation from the natural radioactivity in building materials. Report by a group experts of the OECD nuclear energy agency, Paris, France; May, 1979.
15Sunta CM, David M, Abani MC, Basu AS, Nambi KS. Analysis of dosimetry data of high natural radioactivity areas of southwest coast of India. In: Vohra KG, editor. The Natural Radiation Environment. India: Wiley Eastern Limited; 1982:35-42.
16Paul AC, Pillai PM, Haridasan P, Radhakrishnan S, Krishnamony S. Population exposure to airborne thorium at the high natural radiation areas in India. J Environ Radioact 1998;40:251-9.