International Journal of Pure and Applied Physics. ISSN 0973-1776 Volume 13, Number 1 (2017), pp. 209-214 © Research India Publications http://www.ripublication.com HETEROGENEITY OF RADIOACTIVITY IN SOIL FROM VARKALA, KERALA S.R. Soniya, S. Monica, A.K. Visnu Prasad , P.J. Jojo* Center for Advanced Research in Physical Sciences, Department of Physics, Fatima Mata National College (Autonomous), Kollam, India * Corresponding author: E-mail:[email protected] Abstract measured specific activity in soil samples show a wide range of activity with certain radioactive There exist monazite sand-bearing placer pockets in the region. The results indicate the deposits causing high background radiation necessity of an in depth study in the region. along its long coastline of Kerala and Tamil Nadu in India. The extent and effects of the Keywords: Gamma ray spectrometer, Specific radioactivity in this region is well studied and Activity, Radium equivalent activity, Gamma are available in literature. Other regions along absorbed dose rate. the coast are assumed to have normal radiation levels. Still, it has been observed that certain regions along the coast have regions of high INTRODUCTION radioactivity with a highly heterogeneous nature of distribution. We have made a brief Radionuclides occurring naturally in building investigation to study the extend and diversity of materials are sources of external and internal levels in one of the coastal thaluk, which is radiation exposure in dwellings, besides gamma otherwise known as normal background region. radiation and cosmic radiation. External The gamma spectrometric measurement was radiation exposure is caused by the gamma used for the estimation of natural radioactivity in radiation originating from members of the 40 soil samples collected in a systematic way from uranium and thorium decay chain and from k. Varkala, which is an important tourist Internal radiation exposure, mainly affecting the destination in Kerala. Radium equivalent respiratory tract, is due to the short lived activity (Raeq) and the radiological parameters products of radon which are exhaled from such as internal and external hazard indices, building materials in room air [1]. The content of absorbed dose rate, indoor and outdoor annual the natural radionuclides in building materials is effective dose, Representative Level index were caused by the factors such as geological origin, also determined from the estimated value of the composition of soil, density, porosity, content of specific activity of 226Ra, 232Th and40K.The water in soil, diffusion rate, rate of emanation 209 International Journal of Pure and Applied Physics. ISSN 0973-1776 Volume 13, Number 1 (2017), pp. 209-214 © Research India Publications http://www.ripublication.com and exhalation etc.[2]. Radon can move freely impermeable airtight can with capacity of from the place of its origin through cracks in 305cm3 for more than 30 days to reach secular walls. Radon transportation is mainly due to equilibrium were the rate of decay of the diffusion and forced flow [3].This transportation daughter becomes equal to that of the parent. All of radon resulted in radiological risk to human samples were analyzed using a gamma health, which mainly depends on the factors spectrometer with NaI(Tl) based detector. The such as the level of radon and duration of samples were counted for 10000seconds. The exposure. The worldwide average annual spectrum was stored in a PC based multichannel effective dose for natural sources is 2.4mSv of analyzer. Radiometric measurements were which 1.1mSv is due to basic background carried out for the determination of radiation and 1.3 mSv is due to exposure to radionuclides present in the samples of building radon [4]. This makes the importance of materials. investigating natural radiation from building 1. Determination of natural radioactivity materials. The activity concentrations of the The present studies give an insight of the radionuclides 226Ra, 232Th and 40K for the activity concentration of the natural radioactive samples were determined using the equation. nuclides namely, 226Ra, 232Th and 40K of brick and granite, each of five samples. The material, 퐴푐푡푖푣푖푡푦(퐵푞) brick is of great interest in construction industry 푐푝푠 × 100 × 100 = since the raw material is easily available. The 퐵퐼 × 퐸푓푓 granite is not only used as a basic construction material but also as a decoration material. The Where cps is the net count per second; BI is wide use of these materials paves the way for the branching intensity and Eff is the testing the NORM levels. The gamma efficiency of the detector. spectroscopic measurement technique is adopted 2. Radium equivalent activity (Raeq) for the measurement. Radium equivalent activity is an index that has been introduced to evaluate the specific activities of 226Ra, 232Th and 40K by a single quantity [6]. It is generally defined METHODOLOGY as A total of 12 samples of soils are collected from 푅푎푒푞 = 퐶푅푎 + 1.43 퐶푇ℎ + 0.077퐶푘 Varkala Taluk for the measurement of natural Where CRa, CTh and Ck are activities of radiation. These samples were crushed to get 226Ra, 232Th and 40K respectively in Bq/Kg. fine powder and moisture content is completely The radium equivalent activity is defined on o removed by heating at 110 C in an oven. The the assumption that 10Bq/Kg of 226Ra, 7 sample is then homogenized and sealed in radon Bq/kg of 232Th and 130 Bq/kg of 40K 210 International Journal of Pure and Applied Physics. ISSN 0973-1776 Volume 13, Number 1 (2017), pp. 209-214 © Research India Publications http://www.ripublication.com produce the same gamma ray dose rates [7]. 5. Annual effective dose rate The maximum value of radium equivalent The annual effective dose rate is must be less than 370Bq/Kg for the safe determined by considering the conversion limit [8]. coefficient from absorbed dose in air to effective dose 0.7 SvGy-1 and the indoor 3. Estimation of absorbed dose rate occupancy factor of 0.8 and the outdoor The absorbed dose rates (D) in air at occupancy factor of 0.2 proposed by above the ground surface for the uniform UNSCEAR 2000. The annual effective dose distribution of radionuclides(226Ra, 232Th and was calculated from the equation, 40K) was calculated using the following equation [9], Indoor(mSvy−1) −1 −1 −1 퐷(nGyh ) = (0.462 퐶푅푎 + = D(nGyh ) × 8766 hy −1 0.621 퐶푇ℎ + 0.0417 퐶퐾 × 0.8 × 0.7(SvGy ) Where the numerical values 0.462, 0.621 × 10−6 and 0.417 are the dose conversion factors Outdoor(mSvy−1) for converting activity concentrations of = D(nGyh−1) × 8766 hy−1 226 232 40 Ra, Th and K into doses. × 0.2 × 0.7(SvGy−1) × 10−6 4. External and internal hazard index Where D is the Absorbed dose rate in nGyh- The value of external hazard index 1. should be less than or equal to unity for the safe use of building materials, which 6. Radioactivity level index corresponds to the upper limit of Raeq The radioactivity level index is used to 370Bq/Kg for limiting the dose from represent the γ radiation hazards associated -1 building materials to 1.5mGyy . External with the natural radio nuclide. The hazard index can be calculated using the representative level of Iγ was obtained by the equation [6], equation [11], 퐶푅푎 퐶푇ℎ 퐶퐾 퐶 퐶 퐶 퐻푒푥 = + + ≤ 1 퐼훾 = 푅푎 + 푇ℎ + 퐾 370 259 4810 150 100 1500 222 Internal exposure to Rn and its radioactive progeny is controlled by the internal hazard 7. Alpha index index (Hin) and is obtained by the equation The index is used for the assessment of [10], for the safe use of a material in the internal hazard due to the radon inhalation construction of dwellings internal hazard originating from building materials and is index should be less than unity. defined by the equation [12], 퐶 퐶 퐶 퐻 = 푅푎 + 푇ℎ + 퐾 ≤ 1 푐푅푎 푛 185 259 4810 퐼훼 = 200 211 International Journal of Pure and Applied Physics. ISSN 0973-1776 Volume 13, Number 1 (2017), pp. 209-214 © Research India Publications http://www.ripublication.com 232 Where CRa is the activity concentration of Bq/Kg from the below detectable level, Th radium in and its recommended limit is varies from 13.34 Bq/Kg to a maximum of 30.27 200Bq/Kg. Hence for the safe use of Bq/kg and 40K varies from 65.99 Bq/Kg to a building materials the value of Iα chosen to maximum level of 81.64 Bq/Kg. In the case be less than unity. granite, the detectable level of radium is noticed only in black with red coloured granite from the selected samples. The concentration of RESULT AND DISCUSSION 232Th and 40K samples varies from 4.82 to 8.75Bq/Kg and 119.18 to 147.92Bq/Kg The concentrations of the radionuclides present respectively. From the table it is clear that Raeq in the collected samples are summarized in the values of the soil samples ranges from table 1. In the selected soil sample, the activity 31.16Bq/Kg to 68.30Bq/Kg is less than the concentrations for 226Ra varies up to 20.19 maximum admissible value of 370Bq/Kg. 226 232 40 Soil Ra(Bq/Kg) Th(Bq/Kg) K(Bq/Kg) Raeq(Bq/Kg) Sample 1 18.51 28.15 83.52 65.19 Sample 2 7.03 14.84 66.82 33.39 Sample 3 4.80 14.34 76.06 31.16 Sample 4 BDL 30.27 78.38 49.32 Sample 5 13.98 22.20 65.99 50.80 Sample 6 BDL 26.11 76.98 43.26 Sample 7 20.19 29.12 84.07 68.30 Sample 8 9.22 22.58 71.06 46.98 Sample 9 5.00 19 119.18 41.34 Sample 10 6.8 17.35 78.34 37.64 Sample 11 7.9 15.91 73.55 36.31 Sample 12 BDL 24.58 81.92 41.45 Table 1: Activity concentrations of 226Ra, 232Th, 40k and Radium equivalent activity Table 2 presents the radiological risk index, alpha index.