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ORIGINAL ARTICLE
Year : 2020  |  Volume : 43  |  Issue : 2  |  Page : 82-87  

Measurement of natural radioactivity of building materials used in the southern valley region of Manipur, India


1 Department of Radiation Oncology, Regional Institute of Medical Sciences, Imphal, Manipur, India
2 Department of Physics, Oriental College (Autonomous), Manipur University, Imphal, Manipur, India
3 Department of Physics, Manipur University, Imphal, Manipur, India

Date of Submission30-Mar-2020
Date of Decision09-Apr-2020
Date of Acceptance02-May-2020
Date of Web Publication27-Aug-2020

Correspondence Address:
Dr. B Arunkumar Sharma
Department of Radiation Oncology, Regional Institute of Medical Sciences, Imphal - 795 004, Manipur
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_12_20

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  Abstract 


A study of natural radionuclides of different types of houses was conducted at 493 houses in the southern valley region of Manipur, India. The average annual effective doses of gamma radiation level in indoor and outdoor were estimated from the measurements using Micro R-survey meter and found as 1.25 ± 0.10 (range: 1.02–1.38) mSv/y and 0.76 ± 0.08 (range: 0.58–0.99) mSv/y for reinforced cement concrete (RCC) houses, followed by 1.07 ± 0.09 (range: 0.95–1.19) mSv/y and 0.76 ± 0.08 (range: 0.67–0.95) mSv/y for Adobe laid earthen houses, 1.05 ± 0.09 (range: 0.78–1.24) mSv/y and 0.74 ± 0.08 (range: 0.58–0.91) mSv/y for Assam-type (AT) brick houses, 0.83 ± 0.09 (range: 0.40–1.07) mSv/y and 0.72 ± 0.08 (range: 0.23–0.97) mSv/y for AT mud houses and 0.73 ± 0.08 (range: 0.66–0.82) mSv/y and 0.74 ± 0.07 (range: 0.64–0.82) mSv/y for AT katcha houses. The activity concentration226Ra,232Th, and40K of building materials was evaluated using the gamma ray spectrometry and found average value as 42.4 (range: 23.0–69.9) Bq/kg, 111.3 (range: 23.8–214.5) Bq/kg, and 1369.0 (range: 995.7–2284.6) Bq/kg. Moreover, the annual effective dose conceived from the building material was found as 1.1 mSv/y from Portland cement, 1.4 mSv/y from sand, and 1.5 mSv/y from brick. Seasonal variations of indoor effective dose rates were observed in RCC houses; the highest dose rate was observed during the premonsoon season followed by postmonsoon and least with monsoon season.

Keywords: Building materials, effective dose, natural radioactivity, seasonal effect


How to cite this article:
Sharma B A, Singh S N, Singh N S, Singh H N. Measurement of natural radioactivity of building materials used in the southern valley region of Manipur, India. Radiat Prot Environ 2020;43:82-7

How to cite this URL:
Sharma B A, Singh S N, Singh N S, Singh H N. Measurement of natural radioactivity of building materials used in the southern valley region of Manipur, India. Radiat Prot Environ [serial online] 2020 [cited 2020 Oct 30];43:82-7. Available from: https://www.rpe.org.in/text.asp?2020/43/2/82/293619




  Introduction Top


Natural radionuclides are found present in our environment and the knowledge of this radionuclides concentration level and their distribution in the environment has become a focus of much attention in assessing the human risk from the radiation exposure. Most of the general population spends their time (about 80%) in indoors, and there is a high chance of getting both external as well as internal radiation exposure from natural radionuclide present in the building materials.[1],[2] All building materials used for constructing our houses such as concrete, brick, sand, marble, granite, limestone, and gypsum mainly contain natural radionuclides such as Uranium (238 U) and Thorium (232 Th) and their decay products and radioactive Potassium (40 K).[3],[4] In238 U series, 98.5% of the radiological effects are produced by radium and its daughter products, so radium and its decay chain segment are radiologically very important.[3] Radon, thoron, and their progenies contribute maximum natural radiation dose to the occupational workers and general public.[4] These radionuclide gases present in the ambient air as well as in the indoor atmosphere.[5] It is also expected to be more concentrated in indoor than outdoor. Exhalation of radon and thoron is associated with the presence of226 Ra and232 Th. The natural radioactivity in soil sample or building material is usually determined from226 Ra,232 Th, and40 K contents. The average indoor-effective dose of worldwide due to gamma rays from building materials is estimated to be about 0.4 mSv/y.[6] Studies on building materials have reported that granite and phosphogypsum enhance indoor absorbed dose rate up to five times than the dose criterion,[7] whereas building materials collected from Yan'an, China may be used safely as the construction materials.[8] Hence, the assessment of the level of natural radioactivity in building materials thus becomes important to evaluate any possible risk to human health and develop any precautionary measures in using building materials.

The study area, namely Thoubal and Bishnupur is in the southern side of the state and are the only remaining valley districts next to Imphal districts, Manipur, India. It is in the north-eastern part of the country. Thoubal district is having an area of 514 km2 with total population of 420,517 (2011 census), whereas Bishnupur district is having an area of 496 km2 with a population of 240,363 (2011 census).


  Materials and Methods Top


Measurement of gamma radiation dose rate (survey meter)

Indoor and outdoor background gamma radiation of different types of houses was measured by using NaI (Tl) scintillator-based micro-Roentgen Survey Meter (SM), manufactured by Nucleonix Systems Pvt. Ltd., Hyderabad, India, having a sensitivity of 1 μR/h. It is measured by keeping the SM at a height of about 1 m distance from the ground surface and wall. Repeated measurements not less than ten times for each spot were taken. The average value for all these ten measurements is assumed to be the dose rate for that particular area.[9] Four hundred and ninety-three houses from the two districts were selected for the evaluation of natural gamma radiation dose rates.

Radiological analysis of building materials

A total of nine samples of building materials from the study area were collected and crushed into small sizes. The samples were subjected to dry in a hot air oven at 110°C for 24 h. Then, ground into fine powder, homogenized, and sieved through a mesh size of 0.45 mm,[10] approximately 250 g of this homogeneous fine mesh of each sample material was then packed inside a plastic container with predefined geometry (dimension of cylindrical plastic container: 3” diameter x 5” height), weighed, and properly sealed to restrict the escape of radon gas from the packed. The processed samples were then stored carefully for a period of 4 weeks to achieve the equilibrium condition of226 Ra and232 Th along with their respective daughter nuclides.[9]

A 3” x 3” NaI(Tl) scintillation detector-based gamma spectrometer was employed with adequate shielding (about 10 cm lead). The efficiency calibration of the gamma ray spectrometer was made using different energy peaks covering the range up to ≈2000 keV. Measurements were performed using calibrated standard source samples, which contain a known activity of gamma ray emitters radionuclides, namely133 Ba (356.1 keV),137 Cs (661.6 keV),60 Co (1173 KeV and 1332 KeV), and226 Ra (1764.5 keV).[11] All samples were subjected to gamma spectral analysis with a counting time of 36,000 s.[12] The activity concentration of226 Ra was determined from the average activity concentration obtained from the prominent gamma lines of214 Bi (1.76 MeV) and214 Pb (0.35 MeV) and that of232 Th was estimated from the average concentration obtained from the gamma lines of212 Bi (0.73 MeV),228 Ac (0.91 MeV), and208 Tl (2.61 MeV) respectively; however,40 K was evaluated from its own gamma photo-peak (1.46 MeV).[10],[13],[14]

The activity concentrations of226 Ra,232 Th series, and40 K were calculated using the following equation;[15]



Where, N = the net gamma counting rate (counts per se cond), ε = the detector efficiency of the specific gamma-ray, β = the absolute transition probability of gamma decay and M = the mass of the sample (kg).

The relative concentration and distribution of226 Ra,232 Th, and40 K are not uniform in the environment as well as in building materials. Radium equivalent (Raeq) is most commonly used single quantity to represent natural radioactivity associated to those materials containing226 Ra,232 Th, and40 K. Estimation of the activity was made on the assumption that 370.0 Bq/kg of226 Ra, 259.0 Bq/kg of232 Th, and 4810.0 Bq/kg of40 K produce similar gamma dose rate as mentioned by the following equation[16]



Where, ARa, ATh, and AK are the specific activities of226 Ra,232 Th, and40 K in Bq/kg, respectively.

Activity concentration index or gamma index “I,” is defined to examine the applicability of using building materials in construction. It is defined by the following expression[17] as



This is a simple criterion of the applicability of a building material.

Adsorbed dose rate (D): The absorbed dose rate due to gamma radiation in air for a standard room dimensions and for common building materials with naturally occurring radionuclides226 Ra,232 Th and40 K was calculated using the guidelines given in EC report 112[17] as



The annual effective dose (Deff) calculated by applying the dose conversion factor of 0.7 Sv/Gy from the absorbed dose in air received by an adult and a value of 0.8 as an indoor occupancy factor.[6] The annual effective dose (Deff) due to gamma radiation from building materials was evaluated as




  Results and Discussion Top


The natural background gamma radiation levels of 493 different houses of this southern valley region were measured using SM. These houses may be classified into five different types based on their building materials utilized, which are derived from soils and rocks available in and around the valley region of Manipur state. The classified houses are (i) reinforced cement concrete (RCC) house, (ii) Assam type (AT) katcha house, (iii) AT mud house, (iv) AT brick house, and (v) adobe laid (AL) earthen house (an indigenous – traditional house of Manipur) so called as Meitei Yumjao.[18] The variation of annual effective dose of gamma radiation level in outdoor as well as indoor of these five different types of houses are shown in [Figure 1], and details of measured data with error are given in [Table 1]. It shows that average annual effective dose in indoor of RCC house is 1.3 mSv, followed by AT brick and AL earthen houses with a value of 1.1 mSv and least with AT Katcha and AT mud houses, with values less than unity. Indoor effective dose rate of RCC house is slightly higher than the world average exposure of 0.48 mSv/y.[19] Whereas, average annual-effective dose rate of outdoor of these houses is almost same with a value of (0.74±0.02)mSv/y.
Figure 1: Effective dose rate of outdoor and indoor for different types of houses

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Table 1: Annual effective dose of different types of house in the southern valley region of Manipur

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The specific radioactivity values of226 Ra,232 Th, and40 K measured in the building materials are shown in [Table 2]. All the portland cements (three numbers) and sand-3 (one number) are imported from the neighbouring states. The highest values for specific activity of226 Ra,232 Th, and40 K are 69.9 Bq/kg (sand-2 and Soil[20]), 214.5 Bq/kg (sand-3), and 2284.6 Bq/kg (cement-3), respectively. The average specific activity of226 Ra,232 Th, and40 K is 42.4 Bq/kg (range: 23.0–69.9), 111.3 Bq/kg (range: 23.8–214.5), and 1369.0 Bq/kg (range: 995.7–2284.6). The average value specific activity of226 Ra is slightly higher than world average value 35 Bq/kg, whereas average value of232 Th and40 K is moderately higher than corresponding worldwide average values of 30 Bq/kg and 400 Bq/kg, respectively.[6] Radium equivalent (Raeqv), Gamma index (I), adsorbed dose rate (D), and effective dose (Deff) are given in [Table 2]. Raeqv of the building material samples ranged from 213.0 Bq/kg (sand-1) to 470.7 Bq/kg (sand-3). Raeqv of two samples, namely sand-3 (470.7 Bq/kg) and brick-2 (437.8 Bq/kg) are found to be higher than the criterion limit of 370 Bq/kg.[21] However, the average value of Raeqv obtained from the samples of building materials is 307.0 Bq/kg, which is less than the recommended value (370 Bq/kg) and as such does not pose a radiological hazard when used for construction of buildings. [Figure 2] shows the Raeqv with different kinds of building materials of the southern valley region of Manipur.
Table 2: Radiological parameters of building materials used in the southern valley region of Manipur

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Figure 2: Absorbed dose rate for various building materials

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Gamma index dealing with the assessment of excess gamma radiation arising from building materials ranged from 0.8 (sand-1) to 1.7 (sand-3) with an average value of 1.1, which is marginally higher than the recommended value of unity.[17]

The absorbed dose rate due to gamma radiations in air for a standard room dimensions and for common building materials with naturally occurring radio nuclides226 Ra,232 Th, and40 K ranged from 191.9 nGy/h (sand-1) to 400.4 nGy/h (sand-3) with average value of 270.9 nGy/h, which is higher than world average (populated weighted) indoor absorbed gamma dose rate of 84 nGy/h.[6]

The annual effective dose conceived from building materials were found as 1.1 mSv/y from portland cement, 1.4 mSv/y from sand, and 1.5 mSv/y from brick. Moreover, Deff due to gamma radiation from building materials ranges from 0.9 mSv (sand-1) to 2.0 mSv (sand-3) with average value of 1.3 mSv.

The SM measurements were made in twenty numbers of RCC houses in this southern valley region of Manipur for a period of about 1 year. During the survey, three seasons, namely, premonsoons (March–May) associated with hot and dry season, monsoons (June–September), a high rainfall with lower temperature period, and postmonsoons (October–November) associated with less rainfall and low temperature, were recorded as shown in [Table 3]. The average annual effective dose rates of the three seasons were found as 1.30 mSv/y (range: 1.24–1.40) of premonsoon season, followed by 1.17 mSv/y (range: 1.03–1.22) of monson season, and 1.23 mSv/y (range: 1.03–1.35) of postmonsoon season [Figure 3]. The average effective dose rates of the three seasons are significantly different from each other at 5% level of probability when analyzed with statistical t-test; indicating a significant seasonal variation of indoor effective dose rate of RCC houses similar with other reported cases.[22],[23]
Table 3: Seasonal variation of effective dose rate (indoor) of reinforced cement concrete houses in the southern valley region of Manipur

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Figure 3: Seasonal variation of effective dose rate (indoor) of reinforced cement concrete houses in southern valley region of Manipur

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  Conclusions Top


A study of natural radioactivity of different types of houses available in the southern valley region of Manipur was conducted. Indoor effective dose rate of RCC houses (1.3 mSv/y) is observed slightly higher than the AT brick and AL earthen houses with a value of 1.1 mSv/y and least for AT katcha and AT mud houses The activity concentration of common building materials in these regions was found as 42.4 (range: 23.0–69.9) Bq/kg for226 Ra, 111.3 (range: 23.8–214.5) Bq/kg for232 Th, and 1369.0 (range: 995.7–2284.6) Bq/kg for40 K. The effective dose rate in building is observed depending not only on its radioactive contaminants but also on climate, particularly on the thermal and rainfall regimes of a region.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

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