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Study of Radon and Thoron exhalation from soil samples of different grain sizes
Author’s Accepted Manuscript Study of Radon and Thoron exhalation from soil samples of different grain sizes N. Chitra, B. Danalakshmi, D. Supriya, I. Vijayalakshmi, S. Bala Sundar, K. Sivasubramanian, R. Baskaran, M.T. Jose www.elsevier.com/locate/apradiso
PII: DOI: Reference:
S0969-8043(17)30783-2 https://doi.org/10.1016/j.apradiso.2017.12.017 ARI8200
To appear in: Applied Radiation and Isotopes Received date: 29 June 2017 Revised date: 28 November 2017 Accepted date: 18 December 2017 Cite this article as: N. Chitra, B. Danalakshmi, D. Supriya, I. Vijayalakshmi, S. Bala Sundar, K. Sivasubramanian, R. Baskaran and M.T. Jose, Study of Radon and Thoron exhalation from soil samples of different grain sizes, Applied Radiation and Isotopes, https://doi.org/10.1016/j.apradiso.2017.12.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Study of Radon and Thoron exhalation from soil samples of different grain sizes. N.Chitra*1,2, B. Danalakshmi1, D. Supriya1, I. Vijayalakshmi1, S. Bala Sundar1, K. Sivasubramanian1, R.Baskaran1,2 and M.T. Jose1,2 1Radiological Safety Division, Indira Gandhi Center for Atomic Research, 2Homi Bhabha National Institute, Kalpakkam-603102, Tamilnadu, India *Corresponding Author: [email protected] Abstract The exhalation of radon (222Rn) and thoron (220Rn) from a porous matrix depends on the emanation of them from the grains by the recoil effect. The emanation factor is a quantitative estimate of the emanation phenomenon. The present study is to investigate the effect of grain size of the soil matrix on the emanation factor. Soil samples from three different locations were fractionated into different grain size categories ranging from <0.1 to 2mm. The emanation factors of each of the grain size range were estimated by measuring the mass exhalation rates of radon and thoron and the activity concentrations of 226Ra
and 232Th. The emanation factor was found to increase with decrease in grain size. This effect was
made evident by keeping the parent radium concentration constant for all grain size fractions. The governing factor is the specific surface area of the soil samples which increases with decrease in grain size Key words: Radon, thoron, exhalation, emanation, emanation factor, grain size. 1.0 Introduction The earth's crust and most common building materials contain trace amounts of decay to radon
222Rn
and thoron
220Rn,
238U
and232Th which
respectively. The radon gas molecules diffuse out of the ground
through pore spaces in rocks and soils and mix with the atmosphere. Inhalation of radon and its daughters can cause a significant health hazard when they are present in enhanced levels in enclosed indoor environments such as human dwellings if they are poorly ventilated. It has been observed that radon is the second most important cause of lung cancer, after smoking [1]. Epidemiological studies have provided convincing evidence of an association between indoor radon exposure and lung cancer, even at relatively low radon levels commonly found in residential buildings [2]. Radon exhalation from inside the room surfaces and infiltration of Radon from external atmosphere coupled with air exchange rate inside a room govern the concentration of Radon inside a dwelling. The indoor radon levels is largely dictated by its concentration in soil gas in the surroundings. The primary mechanism for indoor radon entry is by convective flow through cracks and gaps in the basement. Based on studies evolving the models for radon entry and experimental observations , it is now understood that the predominant source for indoor radon concentration is soil.[3,4].This poses the problem of quantifying the radon potential of any given soil 1
which needs knowledge of radon generation rate in the pore space of the soil matrix. These studies are applicable for thoron as well. The Thoron issues have special significance in the high background radiation areas (HBRAs) located on thorium rich soil in Brazil, China and India. The results of the present study contributes to the current understanding of the emanation phenomenon from a porous matrix. A detailed account is given below. Exhalation of radon from materials depends primarily on radon emanation rate from the grains and afterwards on the microstructure of the material as well as on some of the parameters like diffusion, advection, absorption and adsorption affecting the physical transport of radon in the material. Recoil of 222Rn
atom after decay of
226Ra
largely governs the emanation process by which the
222Rn
atoms gets
released into the pore space of the matrix [5] .To give a quantitative explanation of 222Rn emanation rate, a parameter called 222Rn emanation factor is introduced, which is defined as the ratio of 222Rn atoms that reach out of grain into pore volume to that of the total
222Rn
atoms that are produced in the sample
matrix. The emanation phenomenon has been extensively studied in the past. Nazaroff has compiled the experimental findings of emanation factor of soil from different sources. The studies indicate an approximate range of 0.05-0.7 for soil [6]. Analysis of the emanation process for uniformly distributed radium in soil grains leads to much lower emanation factors than are generally observed [7,8]. Several hypotheses have been advanced to account for the large discrepancy between measured emanation coefficients and those predicted by the recoil theory. One such hypothesis is that the distribution of radium within the grain is not uniform. It is concentrated in the surface layers. [9,10]. As an offshoot, analytical and modelling studies have been done on radon emanation from the grain structure of a porous matrix so that the experimental observations can be explained and the effect of moisture, grain size and the radium distribution on the emanation factor can be studied [11,12,13,14,15]. These modeling studies have explored a wide grain size range starting from nm to mm. The observations indicate that for sizes below 100 m, the emanation factor increases with grain size due to increase in the pore space and reaches a constant value thereafter. Such a trend was observed assuming radium distribution confined to the grain surface. As far as experimental studies are concerned, Akihiro Sakoda et. al. have done a comprehensive review of earlier studies on the factors affecting emanation factor for mineral , soil, mill tailing and flyash samples and have organized such data [10]. Referring to the studies on different grain sizes of soil in particular[16,17,18,19]the emanation factor of 222Rn does not exhibit a definite trend with the increase in grain size. This needs investigation. In these cases, the parent 226Ra activity concentration varies for different grain sizes .Owing to this intervening factor , such experimental studies unlike the theoretical studies ,could not explicitly bring out the effect of grain size on the emanation factor. In view of this, the present experimental study focuses particularly on emanation of
222Rn
and
220Rn
from soil
2
matrix of grain sizes in the range of 50 - 2000m and an attempt has been made to remove the interference created due to varying parent radium content. 2.0 Theoretical Background The release of radon to the surrounding air from a porous matrix is a two step process. First, the emanation of radon to the pore system and secondly, the transport through the pores to the surface of the body. The driving forces in the transport can be pressure or concentration gradients. A simplified formulation of radon diffusion equation for a porous matrix is given [20, 21, 22, 23]. Consider a porous matrix such as soil, in which radon is continuously released to the pore volume of the matrix due to the emanation from the grains containing
226Ra.
Let S be the radon activity released into
unit volume of the pore space per unit time by the above process. Assuming that radon is transported in the pore volume by the process of diffusion through the pores, one may define a local flux density Fp representing the activity of radon crossing per unit pore area per unit time. Let Cp be the pore space radon concentration (i.e. radon activity / volume of pore space) at any point at a given time. The general diffusion equation is obtained by a limiting process of the time rate of change of radon activity in an infinitesimal pore volume arising due to the difference between the generation rate and losses due to leakage rate and radioactive decay (with decay constant λ). This is given by:
In general, Fp is composed of two components (i) pressure driven flux and (ii) gradient driven diffusive flux. Generally in most situations, the pressure difference sustained between the pore space and the atmosphere is sufficiently small to cause significant pressure induced flux as compared to diffusion driven flux and hence the former is neglected. For a one‐dimensional system, the diffusion flux is expressed in terms of the concentration gradient by applying Fick’s law of diffusion as follows:
Where, D is the radon diffusion coefficient in the pore space in the soil matrix. This yields the following diffusion equation:
3
Where R is the radium content of the matrix (i.e. Radium activity in the soil grain / mass of the matrix), ρb is the bulk density of the matrix ( i.e. mass of the matrix / bulk volume of the matrix), E is the radon emanation factor and nT is the total porosity of the matrix (i.e. total pore volume/ bulk volume) Emanation factor E, is defined as the fraction of radon atoms generated that escape the solid phase in which they are formed and become free to migrate through the bulk medium. While the flux Fp above refers to unit pore area, the exhalation flux at the soil surface refers to the bulk area that includes both pore area and the area covered by the solid materials. Upon assuming the fractional pore area at the surface to be the same as the fractional pore volume of the bulk of the soil matrix, the exhalation flux density may be related to the pore space concentration as follows:
Solving the steady state diffusion equation by applying boundary conditions, we obtain the expression for surface flux ,
F= R E Ls
(6)
Ls is the diffusion length given by √
The radon mass exhalation rate, Jm (Bq kg‐1 h‐1), is defined as the radon activity released per unit time from unit mass of the matrix. It is generally measured using the closed chamber technique. If the thickness of the sample is far less ( ~ 10 times) than the radon diffusion length in the matrix, then the radon mass exhalation rate may be considered as the radon production rate in the pore volume of the matrix due to emanation from the grain. Typically, radon diffusion length in soil is about 1 m and samples used for measurement are limited to about 0.1 m. Hence in all practical cases, the measured radon mass exhalation rate from a soil sample may be considered as the radon production rate in the pore volume. Mathematically, it is expressed as:
Studies have been done to obtain emanation factor of a matrix from the mass exhalation rate and the radium content [22, 24]. 3. Methods and Materials: 3.1 Sampling and Sieving 4
The soil samples were collected from three different locations. The locations were chosen such that they represent different ranges of 226Ra and 232Th activity distributions .This was based on survey done as part of previous studies[25,26] . The samples were collected from the surface (up to 5cm depth) and ovendried at a temperature of about 105C . The samples were sieved with different mesh sizes and sorted out into different grain size ranges. One of the samples was coarse and was categorized into four ranges >2mm, 0.8- 2mm , 0.4-0.8mm and <0.4 mm .The other two soil samples were finer. They were categorized into five ranges >0.8mm, 0.4-0.8mm, 0.2-0.4mm, 0.1-0.2mm and < 0.1mm. Each of these samples of different grain sizes were analyzed for activity concentrations of
226
Ra and 232Th and the estimation of
radon and thoron mass exhalation rates. 3.2 Estimation of activity concentrations of
226 Ra
and 232Th
The soil samples were packed in airtight 250 ml PVC containers and sealed. The sealing is done to prevent the escape of the gaseous Radon and Thoron that would be formed. They were stored for 30 days to enable 226Ra / 232Th and their progenies to attain equilibrium. The samples were then counted for 10,000 seconds in a HPGe detector based system equipped with PC based MCA. The efficiency of the system was determined using IAEA reference standards. The activity concentrations of
226Ra
and
232Th
were
measured through gamma rays of their progenies. For the present study, the gamma rays chosen for 238U and 232Th were 1764 keV (214Bi) and 2614 keV (208Tl) respectively. The net peak area was obtained from the respective photo peaks. The activity concentrations, A( Bq/kg) of 226 Ra and
232
A= NE / (t γ ε M)
Th were calculated using the following equation (8)
where NE is the net peak area of the peak at energy E , ε is the detection efficiency at energy E, t is the counting live time ,γ is the gamma ray yield per disintegration of the radionuclide and M is the mass of the sample in kg. 3.3 Estimation of radon /thoron exhalation rates The rate of radon/thoron exhalation from the soil samples were estimated using the AlphaGuard (Saphymo) radon/thoron monitor. The sample is placed and sealed in a SS chamber with inlet/outlet nozzles. The chamber is then connected in a closed loop configuration to the online radon/thoron monitor. The configuration is such that, the instrument draws air from the chamber through an inlet filter into the measurement chamber. The air is then returned to the chamber from the AlphaGuard outlet. This arrangement is according to the standard closed chamber technique [23, 27, 28, 29].
5
The radon/thoron concentration inside the exhalation chamber increases exponentially and tends to an equilibrium value Taking the initial radon/thoron concentration as zero, and assuming there is no appreciable leak from the chamber, the exhalation rate, Jm from the sample is expressed as
where C is the radon /thoron concentration at time t (Bq m-3), is the decay constant (s-1), V is the effective air volume(m3) and M is the sample’s mass (Kg) For equilibrium condition, this equation reduces to = C0 V/ M
(10)
where C0 is the equilibrium radon/thoron concentration. Any minute leak, if present, will add to the value to give effective . It takes several days for radon to reach the equilibrium value. This difficulty is overcome by restricting the observation period to the initial linear portion of the build –up curve which satisfies the condition t << T1/2 and E is estimated from the slope of C versus t plot. Jm = Slope × V/ M
(11)
Hence, in this case, the exhalation rate does not depend on the value. In case of thoron, equilibrium condition is attained within minutes. Hence, equation (10) holds good for the observation period. Hence, thoron exhalation rate is estimated from the equilibrium concentration, C0. A minute leak does not affect the measurement since it is negligible compared to the for thoron. The thoron diffusion length in soil Ls is small compared to the height of the sample in the chamber. From the experiment thoron surface exhalation rate is obtained. The mass exhalation rate is calculated as follows [30],
3.4 Analysis of Specific Surface Area. The specific surface area of the soil samples of different grain sizes was measured using the Surface Area Analyzer Smart Sorb 92/93 which is based on the standard BET method.[31,32].In this method gas mixture 6
(approximately 30% nitrogen and 70% helium) continuously flows over the sample. The sample is dipped in liquid nitrogen. At liquid nitrogen temperature (-198°C) nitrogen in this flow gets adsorbed on the surface and forms a mono layer on the surface. This adsorbed nitrogen is allowed to desorb by bringing the sample to room temperature. This desorbed nitrogen is proportional to the surface area .The quantity of gas is measured with the help of Thermal Conductivity Detector and is then integrated with the help of electronic circuit in terms of counts The instrument is then calibrated by injecting known quantity of nitrogen. Prior to the measurement with the soil samples , the instrument was calibrated with alumina standard. (Al2O3). 4.0 Results and Discussion: Soil samples were collected from three different locations .The radon mass exhalation rates , the
226Ra
activities ,thoron mass exhalation rates and the 232 Th activities of the samples of different grain sizes of soil-1 ,soil-2 and soil 3 are given in Tables 1,2 and 3 respectively. Their respective emanation factors are calculated using equation (7). These results indicate that the radon and thoron emanation factors for Soil -1 ,Soil-2 and Soil-3 do not show any notable trend by varying grain size. There is a drop in both radon and thoron emanation factors for grain size fractions 0.1-0.2 mm and <0.1mm.Another observation is that the radium and thorium content shoots up for these two grain size fractions. This is on par with the results of earlier experimental studies [16, 17, 18, 19 ] wherein the parent
226Ra
and
232Th
activity was
varying for different grain size samples considered for the study. Similar observation is also noticed in the present study. The
226Ra
and
232Th
activity concentrations are higher for lower grain size samples .The
fine beach sand containing monazite mineral is present in the soil, particularly in coastal areas [14] . The proportion of monazite sand is more in the grain sizes below 0.2mm.
Table-1 Radon mass exhalation rates, 226Ra, content and radon emanation factors, Thoron mass exhalation rates, content and thoron emanation factors for Soil-1
Size range (mm)
Radon mass ex. Rate Jm (mBq /kg/h)
>2 0.8-2 0.4-0.8 <0.4
11 ± 2 10 ± 1 12 ± 1 15 ± 1
Radon Ra Emanation content factor (Bq/kg) E 226
BDL BDL 21 ± 10 12 ± 9
Minimum detectable activity –
0.08 ±0.04 0.16±0.11
Thoron surface Ex. Rate Js (Bq /m2/h) 2519 ± 540 3930 ± 370 3535 ± 520 6452 ± 830
Thoron mass ex. Rate Jm (Bq /kg/h) 199 ± 43 311 ± 30 279 ± 42 511 ± 66
Th Content (Bq/kg)
Thoron Emanation factor E
27 ± 13 33 ±16 26 ±14 87 ±16
0.16 ± 0.08 0.20 ± 0.09 0.24 ±0.13 0.13 ± 0.03
232
232
Th
226
Ra- 8.75 Bq/kg
7
Minimum detectable activity 232Th – 15.5 Bq/kg
Table-2 Radon mass exhalation rates, 226Ra, content and radon emanation factors, thoron mass exhalation rates, content and thoron emanation factors for Soil-2
Size range (mm) 0.4-0.8 0.2-0.4 0.1-0.2 <0.1
Radon mass ex. Rate Jm (mBq /kg/h) 13 ± 1 19 ± 1 70 ± 3 90 ± 5
226
Ra content (Bq/kg) BDL 44 ± 11 660 ±31 249 ± 23
Thoron surface Ex. Rate Js (Bq /m2/h)
Radon Emanation factor E 0.06 ±0.01 0.02 ±0.001 0.05 ±0.005
11023 ± 910 35250 ± 1900 88330 ± 5500 94900 ± 3300
Thoron 232 mass ex. Th Rate Jm Content (Bq /kg/h) (Bq/kg) 874± 72 118± 15 2799 ± 150 393 ±24 7016 ± 430 4615 ±67 7534 ± 260 2016 ±52
232
Th
Emanation factor E 0.16 ±0.02 0.16 ±0.01 0.03 ±0.001 0.08 ±0.003
Table-3 Radon mass exhalation rates, 226Ra, content and emanation factors , thoron mass exhalation rates, 232Th content and thoron emanation factors for Soil-3
Size range (mm) >0.8 0.4-0.8 0.2-0.4 0.1-0.2 <0.1
Radon mass ex. Rate Jm (mBq /kg/h) 11 ± 1 13 ± 1 18 ± 1 42 ± 3 41 ± 2
226
Ra content (Bq/kg) BDL 14 ± 8 13 ±8 26 ±12 40 ±15
Radon Emanation factor E 0.13 ± 0.07 0.18 ± 0.10 0.21 ± 0.09 0.14 ± 0.05
Thoron surface Ex. Rate Js (Bq /m2/h) 2328 ± 120 3909 ± 140 5596 ± 890 6527 ± 1000 9034 ± 1000
Thoron mass ex. Rate Jm (Bq /kg/h) 184 ± 10 309 ± 11 444 ± 71 512 ± 85 714 ± 82
232
Th Content (Bq/kg) 35 ±12 51 ±13 66 ±15 215 ±25 204 ±27
Thoron Emanation factor E 0.11 ± 0.03 0.13 ± 0.03 0.15 ± 0.04 0.05 ± 0.01 0.08 ± 0.01
As discussed in section – 1, the observed emanation factor values of grain sizes above 100 m rules out uniform radium distribution in the grain. Regarding the grain size effect on the emanation factor, the following reasoning is applicable. Equation (7) explicitly implies that mass exhalation rate is governed by two factors namely the 226Ra /232Th content and the emanation factor. Any increase in the exhalation rate is contributed by both emanation factor and cannot be clearly demarcated when the
226Ra
226Ra
/232Th content. The contribution from the two factors
content is high, since the exhalation rate measured is the
bound exhalation rate. This is due to the retarding effect of back diffusion phenomenon [27, 29, 33, 34 ] 8
]. If the 226Ra /232Th content is kept constant for samples of different grain sizes the effect of grain size on emanation factor can be solely studied. To achieve this, for samples of higher radium content (grain size <0.2mm), the chosen sample mass ( and hence thickness) is small ,thereby preventing back diffusion. It is a well known fact that as the grain size gets larger, the surface to volume ratio of the grains decreases. The decreasing grain size results in increase in the specific surface area. Hence the emanation power should increase. This is observed and reported in a few other studies. [35, 36, 37]. The specific surface area for each grain size range is measured as given in section 3.3. For each grain size, the
measurement was repeated for five times and the average value of specific surface area is reported with standard deviation in Table-4. The results obtained concur with the values reported by studies done on soil samples with grain size ranging from 20-2000 m [38, 39, 40]. Table-4 Specific surface Area of soil samples of different grain sizes.
Size range (mm)
Specific 2
Surface area (m /g)
0.4 - 0.8
3.71 ± 0.29
0.2 - 0.4
4.25 ± 0.71
0.1 - 0.2
6.38 ± 0.45
<0.1
7.86 ± 0.47
On this basis, the present study attempts to fix the parent 226Ra activity for samples of different grain size so that the influence of specific surface area on the emanation of radon can be observed. Soil sample -2 has significant
226Ra
concentration and hence considered for this study. The 226Ra activity was fixed (35
Bq) for samples of different grain sizes (0.2-0.4, 0.1-0.2, <0.1mm) by suitably varying the sample mass with reference to the available
226Ra
activity concentration (Bq Kg-1) values. The results obtained are
given in Table-5 and depicted in Fig-2.The study shows that emanation factor decreases with increase in grain size. This study is not effective for the case of thoron, since the exhalation rate also depends on the sample thickness. This is because of the low diffusion lengths of thoron [30]. Hence , the study was not done for thoron. Table-5 Emanation factors of different grain sizes of Soil-2 Size Range(mm) 0.2-0.4
Mass of sample (g) 790
Radium Activity (Bq)
Radon exhalation rate (mBq h-1 )
Emanation factor
35 ± 9
15.07 ± 1
0.05 ± 0.01
9
0.1-0.2 <0.1
53 140
35 ± 2 35 ±3
22.21 ± 2 33.18 ± 2
0.08 ± 0.01 0.12 ± 0.01
With the decrease in grain size, the porosity decreases. But this does not lower the emanation. This can be explained on the basis of emanation modeling studies done elsewhere. (36). If radon atoms that escape from a Ra-bearing grain collide with a neighboring grain, they will be embedded in the grain .A pore gap of at least 53 m is needed to stop recoil atoms within the pore. A grain size of about 20 m starts to provide the mean pore-gap length of 53 m. Hence, for ranges in the present study porosity does not affect emanation.
Specific surface area vs grain size 9
7
2
Specific surface area (m /g)
8
6 5 4 3 2 1 0 <0.1
0.1 - 0.2
0.2 - 0.4
0.4 - 0.8
Grain size(mm)
Fig-1- Specific surface area of the samples decreases with increase in grain size
Emanation factor vs grain size 0.14
0.12
Emanation factor
0.10
0.08
0.06
0.04
0.02
0.00 <0.1
0.1-0.2
0.2-0.4
Grain size(mm)
Fig-2- Emanation factor decreases with increase in grain size
5.0 Conclusion:
10
Soil samples from three different locations were analyzed to study the effect of grain size on the emanation factor .The radon and thoron mass exhalation rates were estimated for different grain sizes ranging from <0.1mm to 2mm.The radium and thorium content were also estimated . These results indicate that the radon
and thoron emanation factors do not show any notable trend by varying grain size. There is a drop in both radon and thoron emanation factors for grain size fractions 0.1-0.2 mm and <0.1mm.In an attempt to investigate this observation and clearly bring out the effect of grain size on emanation factor, the activity concentration of radium-226 for all the grain sizes was fixed and the study was repeated. It was observed that the emanation factor increases with decrease in grain size .The specific surface area of the different grain size fractions was analyzed and the observation further emphasizes that emanation, being a surface phenomenon, is more pronounced for the lower grain size fractions due to higher specific surface area.
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27. Petropoulos N. P. M.J. Anagnostakis, S.E. Simopoulos (2001) Building materials radon exhalation rate: ERRICA intercomparision exercise results. Science of the Total Environment 272 (1-3) 109118 28. Jing Chen, Naureen M. Rahman, Ibrahim Abu Atiya (2010) Radon exhalation from building materials for decorative use Journal of Environmental Radioactivity 101 (4), 317-322 29. S. Stoulos ∗, M. Manolopoulou, C. Papastefanou (2003) Assessment of natural radiation exposure and radon exhalation from building materials in Greece Journal of Environmental Radioactivity 69 225–240 30. P.Ujic, I.Celikovic´, A.Kandic, Z.Žunic (2008) Standardization and difficulties of the thoron exhalation rate measurements using an accumulation chamber Radiation Measurements 43 1396 – 1401 31. G.Fager lund.(1973) Determination of specific surface area by the BET method. Materials and structures, 6, 239-245 32. Kenneth Sing(2001) The use of nitrogen adsorption for the characterization of porous materials . Colloids and surfaces A :Physico chemical and Engineering aspects ,187-188 ,3-9 33. I. Lo´pez-Coto , J.L.Mas , J.P.Bolivar, R.Garcı´-Tenorio(2009) A short-time method to measure the radon potential of porous materials Applied Radiation and Isotopes 67 133–138 34. J.L. Guti!errez*, M.Garcıa-Talavera, V.Pen , J.C. Nalda, M. Voytchev, R. L!opez(2004) Radon emanation measurements using silicon photodiode detectors Applied Radiation and Isotopes 60 583–587 B 35. R. Shweikani, T.G. Giaddui and S.A.Durrani, The effect of soil parameters on the radon concentration values in the environment Radiation Measurements Vol.25, Nos. 1-4 581-584. 36. Akihiro Sakoda , Yuu Ishimori , Katsumi Hanamoto , Takahiro Kataoka , Atsushi Kawabe Kiyonori Yamaoka (2010) Experimental and modeling studies of grain size and moisture content effects on radon emanation . Radiation Measurements 45 204–210 37. Y. Shirom,, M. Hosoda, T. Ishikawa, S. K. Sahoo, S. Tokonami and M. Furukawa (2015), Estimation of radon emanation coefficient for representative soils in Okinawa, Japan Radiation Protection Dosimetry Vol. 167, No. 1–3, 147–150 13
38. S.A.Parry, M.E.Hodson, E.H.Oelkers , S.J.Kemp (2011) Is silt the most influential soil grain size fraction? Applied Geochemistry 26 119–122 39. Christian feller,' Elisabeth Schouller, Fabien Thomas, James Rouiller and Adrien J. Herbillon.(1992)N2-bet specific surface areas of some low activity clay soils and their relationships with secondary constituents and organic matter contents ,Soil Science ,153, 293299 40. Colin J. Whitfield and Carolyn Reid (2013)Predicting surface area of coarse-textured soils: Implications for weathering rates .Canadian. Journal of Soil Science ,93, 621-630
14
Highlights
To investigate the effect of grain size of the soil matrix on the emanation factor. The grain size ranges considered for the study - <0.1mm , 0.1-0.2 mm, 0.2-0.4mm, 0.4-0.8 mm, 0.82mm ,>2mm. The emanation factors of each of the grain size range were estimated by measuring the mass exhalation rates of radon and thoron and the activity concentrations of 226Ra and 232Th. It was observed as expected that the specific surface area of the sample increases with decrease in grain size It was inferred that the emanation factor increases with increase in the specific surface area However this trend could not be observed in the grain sizes below 0.2mm because of the intervening high activity concentrations of the parent radium and thorium An attempt was made to fix the activity concentration of radium-226 for all the grain sizes and the study was repeated to explicitly bring out the effect of grain size.
15
PII: DOI: Reference:
S0969-8043(17)30783-2 https://doi.org/10.1016/j.apradiso.2017.12.017 ARI8200
To appear in: Applied Radiation and Isotopes Received date: 29 June 2017 Revised date: 28 November 2017 Accepted date: 18 December 2017 Cite this article as: N. Chitra, B. Danalakshmi, D. Supriya, I. Vijayalakshmi, S. Bala Sundar, K. Sivasubramanian, R. Baskaran and M.T. Jose, Study of Radon and Thoron exhalation from soil samples of different grain sizes, Applied Radiation and Isotopes, https://doi.org/10.1016/j.apradiso.2017.12.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Study of Radon and Thoron exhalation from soil samples of different grain sizes. N.Chitra*1,2, B. Danalakshmi1, D. Supriya1, I. Vijayalakshmi1, S. Bala Sundar1, K. Sivasubramanian1, R.Baskaran1,2 and M.T. Jose1,2 1Radiological Safety Division, Indira Gandhi Center for Atomic Research, 2Homi Bhabha National Institute, Kalpakkam-603102, Tamilnadu, India *Corresponding Author: [email protected] Abstract The exhalation of radon (222Rn) and thoron (220Rn) from a porous matrix depends on the emanation of them from the grains by the recoil effect. The emanation factor is a quantitative estimate of the emanation phenomenon. The present study is to investigate the effect of grain size of the soil matrix on the emanation factor. Soil samples from three different locations were fractionated into different grain size categories ranging from <0.1 to 2mm. The emanation factors of each of the grain size range were estimated by measuring the mass exhalation rates of radon and thoron and the activity concentrations of 226Ra
and 232Th. The emanation factor was found to increase with decrease in grain size. This effect was
made evident by keeping the parent radium concentration constant for all grain size fractions. The governing factor is the specific surface area of the soil samples which increases with decrease in grain size Key words: Radon, thoron, exhalation, emanation, emanation factor, grain size. 1.0 Introduction The earth's crust and most common building materials contain trace amounts of decay to radon
222Rn
and thoron
220Rn,
238U
and232Th which
respectively. The radon gas molecules diffuse out of the ground
through pore spaces in rocks and soils and mix with the atmosphere. Inhalation of radon and its daughters can cause a significant health hazard when they are present in enhanced levels in enclosed indoor environments such as human dwellings if they are poorly ventilated. It has been observed that radon is the second most important cause of lung cancer, after smoking [1]. Epidemiological studies have provided convincing evidence of an association between indoor radon exposure and lung cancer, even at relatively low radon levels commonly found in residential buildings [2]. Radon exhalation from inside the room surfaces and infiltration of Radon from external atmosphere coupled with air exchange rate inside a room govern the concentration of Radon inside a dwelling. The indoor radon levels is largely dictated by its concentration in soil gas in the surroundings. The primary mechanism for indoor radon entry is by convective flow through cracks and gaps in the basement. Based on studies evolving the models for radon entry and experimental observations , it is now understood that the predominant source for indoor radon concentration is soil.[3,4].This poses the problem of quantifying the radon potential of any given soil 1
which needs knowledge of radon generation rate in the pore space of the soil matrix. These studies are applicable for thoron as well. The Thoron issues have special significance in the high background radiation areas (HBRAs) located on thorium rich soil in Brazil, China and India. The results of the present study contributes to the current understanding of the emanation phenomenon from a porous matrix. A detailed account is given below. Exhalation of radon from materials depends primarily on radon emanation rate from the grains and afterwards on the microstructure of the material as well as on some of the parameters like diffusion, advection, absorption and adsorption affecting the physical transport of radon in the material. Recoil of 222Rn
atom after decay of
226Ra
largely governs the emanation process by which the
222Rn
atoms gets
released into the pore space of the matrix [5] .To give a quantitative explanation of 222Rn emanation rate, a parameter called 222Rn emanation factor is introduced, which is defined as the ratio of 222Rn atoms that reach out of grain into pore volume to that of the total
222Rn
atoms that are produced in the sample
matrix. The emanation phenomenon has been extensively studied in the past. Nazaroff has compiled the experimental findings of emanation factor of soil from different sources. The studies indicate an approximate range of 0.05-0.7 for soil [6]. Analysis of the emanation process for uniformly distributed radium in soil grains leads to much lower emanation factors than are generally observed [7,8]. Several hypotheses have been advanced to account for the large discrepancy between measured emanation coefficients and those predicted by the recoil theory. One such hypothesis is that the distribution of radium within the grain is not uniform. It is concentrated in the surface layers. [9,10]. As an offshoot, analytical and modelling studies have been done on radon emanation from the grain structure of a porous matrix so that the experimental observations can be explained and the effect of moisture, grain size and the radium distribution on the emanation factor can be studied [11,12,13,14,15]. These modeling studies have explored a wide grain size range starting from nm to mm. The observations indicate that for sizes below 100 m, the emanation factor increases with grain size due to increase in the pore space and reaches a constant value thereafter. Such a trend was observed assuming radium distribution confined to the grain surface. As far as experimental studies are concerned, Akihiro Sakoda et. al. have done a comprehensive review of earlier studies on the factors affecting emanation factor for mineral , soil, mill tailing and flyash samples and have organized such data [10]. Referring to the studies on different grain sizes of soil in particular[16,17,18,19]the emanation factor of 222Rn does not exhibit a definite trend with the increase in grain size. This needs investigation. In these cases, the parent 226Ra activity concentration varies for different grain sizes .Owing to this intervening factor , such experimental studies unlike the theoretical studies ,could not explicitly bring out the effect of grain size on the emanation factor. In view of this, the present experimental study focuses particularly on emanation of
222Rn
and
220Rn
from soil
2
matrix of grain sizes in the range of 50 - 2000m and an attempt has been made to remove the interference created due to varying parent radium content. 2.0 Theoretical Background The release of radon to the surrounding air from a porous matrix is a two step process. First, the emanation of radon to the pore system and secondly, the transport through the pores to the surface of the body. The driving forces in the transport can be pressure or concentration gradients. A simplified formulation of radon diffusion equation for a porous matrix is given [20, 21, 22, 23]. Consider a porous matrix such as soil, in which radon is continuously released to the pore volume of the matrix due to the emanation from the grains containing
226Ra.
Let S be the radon activity released into
unit volume of the pore space per unit time by the above process. Assuming that radon is transported in the pore volume by the process of diffusion through the pores, one may define a local flux density Fp representing the activity of radon crossing per unit pore area per unit time. Let Cp be the pore space radon concentration (i.e. radon activity / volume of pore space) at any point at a given time. The general diffusion equation is obtained by a limiting process of the time rate of change of radon activity in an infinitesimal pore volume arising due to the difference between the generation rate and losses due to leakage rate and radioactive decay (with decay constant λ). This is given by:
In general, Fp is composed of two components (i) pressure driven flux and (ii) gradient driven diffusive flux. Generally in most situations, the pressure difference sustained between the pore space and the atmosphere is sufficiently small to cause significant pressure induced flux as compared to diffusion driven flux and hence the former is neglected. For a one‐dimensional system, the diffusion flux is expressed in terms of the concentration gradient by applying Fick’s law of diffusion as follows:
Where, D is the radon diffusion coefficient in the pore space in the soil matrix. This yields the following diffusion equation:
3
Where R is the radium content of the matrix (i.e. Radium activity in the soil grain / mass of the matrix), ρb is the bulk density of the matrix ( i.e. mass of the matrix / bulk volume of the matrix), E is the radon emanation factor and nT is the total porosity of the matrix (i.e. total pore volume/ bulk volume) Emanation factor E, is defined as the fraction of radon atoms generated that escape the solid phase in which they are formed and become free to migrate through the bulk medium. While the flux Fp above refers to unit pore area, the exhalation flux at the soil surface refers to the bulk area that includes both pore area and the area covered by the solid materials. Upon assuming the fractional pore area at the surface to be the same as the fractional pore volume of the bulk of the soil matrix, the exhalation flux density may be related to the pore space concentration as follows:
Solving the steady state diffusion equation by applying boundary conditions, we obtain the expression for surface flux ,
F= R E Ls
(6)
Ls is the diffusion length given by √
The radon mass exhalation rate, Jm (Bq kg‐1 h‐1), is defined as the radon activity released per unit time from unit mass of the matrix. It is generally measured using the closed chamber technique. If the thickness of the sample is far less ( ~ 10 times) than the radon diffusion length in the matrix, then the radon mass exhalation rate may be considered as the radon production rate in the pore volume of the matrix due to emanation from the grain. Typically, radon diffusion length in soil is about 1 m and samples used for measurement are limited to about 0.1 m. Hence in all practical cases, the measured radon mass exhalation rate from a soil sample may be considered as the radon production rate in the pore volume. Mathematically, it is expressed as:
Studies have been done to obtain emanation factor of a matrix from the mass exhalation rate and the radium content [22, 24]. 3. Methods and Materials: 3.1 Sampling and Sieving 4
The soil samples were collected from three different locations. The locations were chosen such that they represent different ranges of 226Ra and 232Th activity distributions .This was based on survey done as part of previous studies[25,26] . The samples were collected from the surface (up to 5cm depth) and ovendried at a temperature of about 105C . The samples were sieved with different mesh sizes and sorted out into different grain size ranges. One of the samples was coarse and was categorized into four ranges >2mm, 0.8- 2mm , 0.4-0.8mm and <0.4 mm .The other two soil samples were finer. They were categorized into five ranges >0.8mm, 0.4-0.8mm, 0.2-0.4mm, 0.1-0.2mm and < 0.1mm. Each of these samples of different grain sizes were analyzed for activity concentrations of
226
Ra and 232Th and the estimation of
radon and thoron mass exhalation rates. 3.2 Estimation of activity concentrations of
226 Ra
and 232Th
The soil samples were packed in airtight 250 ml PVC containers and sealed. The sealing is done to prevent the escape of the gaseous Radon and Thoron that would be formed. They were stored for 30 days to enable 226Ra / 232Th and their progenies to attain equilibrium. The samples were then counted for 10,000 seconds in a HPGe detector based system equipped with PC based MCA. The efficiency of the system was determined using IAEA reference standards. The activity concentrations of
226Ra
and
232Th
were
measured through gamma rays of their progenies. For the present study, the gamma rays chosen for 238U and 232Th were 1764 keV (214Bi) and 2614 keV (208Tl) respectively. The net peak area was obtained from the respective photo peaks. The activity concentrations, A( Bq/kg) of 226 Ra and
232
A= NE / (t γ ε M)
Th were calculated using the following equation (8)
where NE is the net peak area of the peak at energy E , ε is the detection efficiency at energy E, t is the counting live time ,γ is the gamma ray yield per disintegration of the radionuclide and M is the mass of the sample in kg. 3.3 Estimation of radon /thoron exhalation rates The rate of radon/thoron exhalation from the soil samples were estimated using the AlphaGuard (Saphymo) radon/thoron monitor. The sample is placed and sealed in a SS chamber with inlet/outlet nozzles. The chamber is then connected in a closed loop configuration to the online radon/thoron monitor. The configuration is such that, the instrument draws air from the chamber through an inlet filter into the measurement chamber. The air is then returned to the chamber from the AlphaGuard outlet. This arrangement is according to the standard closed chamber technique [23, 27, 28, 29].
5
The radon/thoron concentration inside the exhalation chamber increases exponentially and tends to an equilibrium value Taking the initial radon/thoron concentration as zero, and assuming there is no appreciable leak from the chamber, the exhalation rate, Jm from the sample is expressed as
where C is the radon /thoron concentration at time t (Bq m-3), is the decay constant (s-1), V is the effective air volume(m3) and M is the sample’s mass (Kg) For equilibrium condition, this equation reduces to = C0 V/ M
(10)
where C0 is the equilibrium radon/thoron concentration. Any minute leak, if present, will add to the value to give effective . It takes several days for radon to reach the equilibrium value. This difficulty is overcome by restricting the observation period to the initial linear portion of the build –up curve which satisfies the condition t << T1/2 and E is estimated from the slope of C versus t plot. Jm = Slope × V/ M
(11)
Hence, in this case, the exhalation rate does not depend on the value. In case of thoron, equilibrium condition is attained within minutes. Hence, equation (10) holds good for the observation period. Hence, thoron exhalation rate is estimated from the equilibrium concentration, C0. A minute leak does not affect the measurement since it is negligible compared to the for thoron. The thoron diffusion length in soil Ls is small compared to the height of the sample in the chamber. From the experiment thoron surface exhalation rate is obtained. The mass exhalation rate is calculated as follows [30],
3.4 Analysis of Specific Surface Area. The specific surface area of the soil samples of different grain sizes was measured using the Surface Area Analyzer Smart Sorb 92/93 which is based on the standard BET method.[31,32].In this method gas mixture 6
(approximately 30% nitrogen and 70% helium) continuously flows over the sample. The sample is dipped in liquid nitrogen. At liquid nitrogen temperature (-198°C) nitrogen in this flow gets adsorbed on the surface and forms a mono layer on the surface. This adsorbed nitrogen is allowed to desorb by bringing the sample to room temperature. This desorbed nitrogen is proportional to the surface area .The quantity of gas is measured with the help of Thermal Conductivity Detector and is then integrated with the help of electronic circuit in terms of counts The instrument is then calibrated by injecting known quantity of nitrogen. Prior to the measurement with the soil samples , the instrument was calibrated with alumina standard. (Al2O3). 4.0 Results and Discussion: Soil samples were collected from three different locations .The radon mass exhalation rates , the
226Ra
activities ,thoron mass exhalation rates and the 232 Th activities of the samples of different grain sizes of soil-1 ,soil-2 and soil 3 are given in Tables 1,2 and 3 respectively. Their respective emanation factors are calculated using equation (7). These results indicate that the radon and thoron emanation factors for Soil -1 ,Soil-2 and Soil-3 do not show any notable trend by varying grain size. There is a drop in both radon and thoron emanation factors for grain size fractions 0.1-0.2 mm and <0.1mm.Another observation is that the radium and thorium content shoots up for these two grain size fractions. This is on par with the results of earlier experimental studies [16, 17, 18, 19 ] wherein the parent
226Ra
and
232Th
activity was
varying for different grain size samples considered for the study. Similar observation is also noticed in the present study. The
226Ra
and
232Th
activity concentrations are higher for lower grain size samples .The
fine beach sand containing monazite mineral is present in the soil, particularly in coastal areas [14] . The proportion of monazite sand is more in the grain sizes below 0.2mm.
Table-1 Radon mass exhalation rates, 226Ra, content and radon emanation factors, Thoron mass exhalation rates, content and thoron emanation factors for Soil-1
Size range (mm)
Radon mass ex. Rate Jm (mBq /kg/h)
>2 0.8-2 0.4-0.8 <0.4
11 ± 2 10 ± 1 12 ± 1 15 ± 1
Radon Ra Emanation content factor (Bq/kg) E 226
BDL BDL 21 ± 10 12 ± 9
Minimum detectable activity –
0.08 ±0.04 0.16±0.11
Thoron surface Ex. Rate Js (Bq /m2/h) 2519 ± 540 3930 ± 370 3535 ± 520 6452 ± 830
Thoron mass ex. Rate Jm (Bq /kg/h) 199 ± 43 311 ± 30 279 ± 42 511 ± 66
Th Content (Bq/kg)
Thoron Emanation factor E
27 ± 13 33 ±16 26 ±14 87 ±16
0.16 ± 0.08 0.20 ± 0.09 0.24 ±0.13 0.13 ± 0.03
232
232
Th
226
Ra- 8.75 Bq/kg
7
Minimum detectable activity 232Th – 15.5 Bq/kg
Table-2 Radon mass exhalation rates, 226Ra, content and radon emanation factors, thoron mass exhalation rates, content and thoron emanation factors for Soil-2
Size range (mm) 0.4-0.8 0.2-0.4 0.1-0.2 <0.1
Radon mass ex. Rate Jm (mBq /kg/h) 13 ± 1 19 ± 1 70 ± 3 90 ± 5
226
Ra content (Bq/kg) BDL 44 ± 11 660 ±31 249 ± 23
Thoron surface Ex. Rate Js (Bq /m2/h)
Radon Emanation factor E 0.06 ±0.01 0.02 ±0.001 0.05 ±0.005
11023 ± 910 35250 ± 1900 88330 ± 5500 94900 ± 3300
Thoron 232 mass ex. Th Rate Jm Content (Bq /kg/h) (Bq/kg) 874± 72 118± 15 2799 ± 150 393 ±24 7016 ± 430 4615 ±67 7534 ± 260 2016 ±52
232
Th
Emanation factor E 0.16 ±0.02 0.16 ±0.01 0.03 ±0.001 0.08 ±0.003
Table-3 Radon mass exhalation rates, 226Ra, content and emanation factors , thoron mass exhalation rates, 232Th content and thoron emanation factors for Soil-3
Size range (mm) >0.8 0.4-0.8 0.2-0.4 0.1-0.2 <0.1
Radon mass ex. Rate Jm (mBq /kg/h) 11 ± 1 13 ± 1 18 ± 1 42 ± 3 41 ± 2
226
Ra content (Bq/kg) BDL 14 ± 8 13 ±8 26 ±12 40 ±15
Radon Emanation factor E 0.13 ± 0.07 0.18 ± 0.10 0.21 ± 0.09 0.14 ± 0.05
Thoron surface Ex. Rate Js (Bq /m2/h) 2328 ± 120 3909 ± 140 5596 ± 890 6527 ± 1000 9034 ± 1000
Thoron mass ex. Rate Jm (Bq /kg/h) 184 ± 10 309 ± 11 444 ± 71 512 ± 85 714 ± 82
232
Th Content (Bq/kg) 35 ±12 51 ±13 66 ±15 215 ±25 204 ±27
Thoron Emanation factor E 0.11 ± 0.03 0.13 ± 0.03 0.15 ± 0.04 0.05 ± 0.01 0.08 ± 0.01
As discussed in section – 1, the observed emanation factor values of grain sizes above 100 m rules out uniform radium distribution in the grain. Regarding the grain size effect on the emanation factor, the following reasoning is applicable. Equation (7) explicitly implies that mass exhalation rate is governed by two factors namely the 226Ra /232Th content and the emanation factor. Any increase in the exhalation rate is contributed by both emanation factor and cannot be clearly demarcated when the
226Ra
226Ra
/232Th content. The contribution from the two factors
content is high, since the exhalation rate measured is the
bound exhalation rate. This is due to the retarding effect of back diffusion phenomenon [27, 29, 33, 34 ] 8
]. If the 226Ra /232Th content is kept constant for samples of different grain sizes the effect of grain size on emanation factor can be solely studied. To achieve this, for samples of higher radium content (grain size <0.2mm), the chosen sample mass ( and hence thickness) is small ,thereby preventing back diffusion. It is a well known fact that as the grain size gets larger, the surface to volume ratio of the grains decreases. The decreasing grain size results in increase in the specific surface area. Hence the emanation power should increase. This is observed and reported in a few other studies. [35, 36, 37]. The specific surface area for each grain size range is measured as given in section 3.3. For each grain size, the
measurement was repeated for five times and the average value of specific surface area is reported with standard deviation in Table-4. The results obtained concur with the values reported by studies done on soil samples with grain size ranging from 20-2000 m [38, 39, 40]. Table-4 Specific surface Area of soil samples of different grain sizes.
Size range (mm)
Specific 2
Surface area (m /g)
0.4 - 0.8
3.71 ± 0.29
0.2 - 0.4
4.25 ± 0.71
0.1 - 0.2
6.38 ± 0.45
<0.1
7.86 ± 0.47
On this basis, the present study attempts to fix the parent 226Ra activity for samples of different grain size so that the influence of specific surface area on the emanation of radon can be observed. Soil sample -2 has significant
226Ra
concentration and hence considered for this study. The 226Ra activity was fixed (35
Bq) for samples of different grain sizes (0.2-0.4, 0.1-0.2, <0.1mm) by suitably varying the sample mass with reference to the available
226Ra
activity concentration (Bq Kg-1) values. The results obtained are
given in Table-5 and depicted in Fig-2.The study shows that emanation factor decreases with increase in grain size. This study is not effective for the case of thoron, since the exhalation rate also depends on the sample thickness. This is because of the low diffusion lengths of thoron [30]. Hence , the study was not done for thoron. Table-5 Emanation factors of different grain sizes of Soil-2 Size Range(mm) 0.2-0.4
Mass of sample (g) 790
Radium Activity (Bq)
Radon exhalation rate (mBq h-1 )
Emanation factor
35 ± 9
15.07 ± 1
0.05 ± 0.01
9
0.1-0.2 <0.1
53 140
35 ± 2 35 ±3
22.21 ± 2 33.18 ± 2
0.08 ± 0.01 0.12 ± 0.01
With the decrease in grain size, the porosity decreases. But this does not lower the emanation. This can be explained on the basis of emanation modeling studies done elsewhere. (36). If radon atoms that escape from a Ra-bearing grain collide with a neighboring grain, they will be embedded in the grain .A pore gap of at least 53 m is needed to stop recoil atoms within the pore. A grain size of about 20 m starts to provide the mean pore-gap length of 53 m. Hence, for ranges in the present study porosity does not affect emanation.
Specific surface area vs grain size 9
7
2
Specific surface area (m /g)
8
6 5 4 3 2 1 0 <0.1
0.1 - 0.2
0.2 - 0.4
0.4 - 0.8
Grain size(mm)
Fig-1- Specific surface area of the samples decreases with increase in grain size
Emanation factor vs grain size 0.14
0.12
Emanation factor
0.10
0.08
0.06
0.04
0.02
0.00 <0.1
0.1-0.2
0.2-0.4
Grain size(mm)
Fig-2- Emanation factor decreases with increase in grain size
5.0 Conclusion:
10
Soil samples from three different locations were analyzed to study the effect of grain size on the emanation factor .The radon and thoron mass exhalation rates were estimated for different grain sizes ranging from <0.1mm to 2mm.The radium and thorium content were also estimated . These results indicate that the radon
and thoron emanation factors do not show any notable trend by varying grain size. There is a drop in both radon and thoron emanation factors for grain size fractions 0.1-0.2 mm and <0.1mm.In an attempt to investigate this observation and clearly bring out the effect of grain size on emanation factor, the activity concentration of radium-226 for all the grain sizes was fixed and the study was repeated. It was observed that the emanation factor increases with decrease in grain size .The specific surface area of the different grain size fractions was analyzed and the observation further emphasizes that emanation, being a surface phenomenon, is more pronounced for the lower grain size fractions due to higher specific surface area.
6.0 References: 1. World Health Organization (WHO), 2009. Handbook on Indoor Radon: a Public Health Perspective. WHO Press, Geneva. 2. B.K. Sahoo, B.K. Sapra, S.D. Kanse, J.J. Gaware, Y.S. Mayya , (2013) .A new pin-hole discriminated 222Rn/220Rn passive measurement device with single entry face Radiation Measurements 1-9 3. A.J.Gadgil (1992) Models of Radon Entry. Radiation Protection Dosimetry 45,373-379 4. A.V.Nero, W.W Nazaroff (1984).Radiation Protection Dosimetry 7, 23-29 5.
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Highlights
To investigate the effect of grain size of the soil matrix on the emanation factor. The grain size ranges considered for the study - <0.1mm , 0.1-0.2 mm, 0.2-0.4mm, 0.4-0.8 mm, 0.82mm ,>2mm. The emanation factors of each of the grain size range were estimated by measuring the mass exhalation rates of radon and thoron and the activity concentrations of 226Ra and 232Th. It was observed as expected that the specific surface area of the sample increases with decrease in grain size It was inferred that the emanation factor increases with increase in the specific surface area However this trend could not be observed in the grain sizes below 0.2mm because of the intervening high activity concentrations of the parent radium and thorium An attempt was made to fix the activity concentration of radium-226 for all the grain sizes and the study was repeated to explicitly bring out the effect of grain size.
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