Natural radioactivity and radon emanation coefficient in the soil of Ninh Son region, Vietnam

Accepted Manuscript Natural radioactivity and radon emanation coefficient in the soil of Ninh Son region, Vietnam Huynh Nguyen Phong Thu, Nguyen Van Thang, Truong Thi Hong Loan, Nguyen Van Dong, Le Cong Hao PII:

S0883-2927(19)30074-5

DOI:

https://doi.org/10.1016/j.apgeochem.2019.03.019

Reference:

AG 4313

To appear in:

Applied Geochemistry

Received Date: 1 November 2018 Revised Date:

16 March 2019

Accepted Date: 18 March 2019

Please cite this article as: Phong Thu, H.N., Van Thang, N., Hong Loan, T.T., Van Dong, N., Hao, L.C., Natural radioactivity and radon emanation coefficient in the soil of Ninh Son region, Vietnam, Applied Geochemistry (2019), doi: https://doi.org/10.1016/j.apgeochem.2019.03.019. 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 proof before it is published in its final 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.

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Huynh Nguyen Phong Thu1,2, Nguyen Van Thang1, Truong Thi Hong Loan1,2, Nguyen

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Van Dong3, and Le Cong Hao1,2*

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Natural radioactivity and radon emanation coefficient in the soil of Ninh Son region, Vietnam

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campus Linh Trung Ward, Thu Duc District, Ho Chi Minh City, Vietnam.

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Engineering Physics, University of Science, VNU-HCM, 227 Nguyen Van Cu Street,

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District 5, Ho Chi Minh City, Vietnam

Nuclear Technique Laboratory, University of Science, VNU-HCM, Linh Trung

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Department of Nuclear Physics and Nuclear Engineering, Faculty of Physics and

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VNU-HCM, 227 Nguyen Van Cu Street, District 5, Ho Chi Minh City, Vietnam

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*Corresponding author: Le Cong Hao

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E-mail address: [email protected].

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Department of Analytical Chemistry, Faculty of Chemistry, University of Science,

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Abstract The natural radioactivity (238U, 226Ra, 232Th and 40K) and radon emanation coefficient

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for 57 soil samples belonging to alluvial, red, forest surface, slip-debris, metamorphic and

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sandy soil of the Ninh Son region in Ninh Thuan province have been determined. The

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soil gas radon was measured by in-situ with RAD7 radon monitor coupled with a soil gas

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probe while activity concentrations of

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238

U,

226

HPGe gamma-ray spectrometry system. The 226

Ra,

226

232

Th, and

238

Ra/

238

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40

K were measured by an

U disequilibrium occurred in the

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soil samples and a great majority of the

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concentrations of

226

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average concentrations in soils published by UNSCEAR 2008. The gamma dose rate

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ranged from 55±2 to 248±7 nGy.h-1 with an average of 130±4 nGy.h-1 which is greater

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than the world value. Strong positive correlations were recorded between 238U and 226Ra,

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232

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alteration processes were proposed to be dominated reasons for the

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disequilibrium occurred in the soil samples. Most of the radon in soil gas samples are

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considered “normal risk” or low radon index. The mean values of the emanation

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coefficient for alluvial, red, forest surface, slip-debris, metamorphic and sandy soil were

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found to be 0.51±0.03, 0.40±0.02, 0.36±0.02, 0.30±0.02, 0.26±0.02 and 0.15±0.01,

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respectively. Radon emanation was found to be an inverse function of grain size for grain

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sizes larger than 0.1 mm in diameter and independent on the radium content of the soil

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sample.

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Keywords

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Soil, natural radioactivity, radon concentration, emanation coefficient, HPGe, and RAD7.

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Introduction

Ra,

232

Th and

238

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Ra/ U values lie above 1. Average activity

K are significantly higher than the worldwide

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Th, and

40

U, and

226

Ra and

222

Rn. The results of weathering and 226

Ra/238U

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Th and

Ra,

232

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Natural radiation makes up approximately 80% of the human effective dose in a

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year (IAEA, 1996). Primordial radionuclides, the long-lived radionuclides left over from

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when the earth was created, are the major contributors to our radiation environment.

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Radionuclides in soils,

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terrestrial natural radiation (UNSCEAR, 2008, Kovács et al., 2013; Bala et al., 2014,

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Forkapic et al., 2017; Taher et al., 2018, Bangotra et al., 2018). The spatial distributions

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of these radionuclides depend on the nature of the parent rock and soil (Jakhu et al., 2017;

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Ribeiro et at., 2018). The radionuclides can transfer from soil to man in various pathways

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which constantly exposes on the population and can reach hazardous radiological levels

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(Srodka, 2012; Jakhu et al., 2017; Forkapic at al., 2017). In radiation protection point of

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view, the background of the natural radiation levels in a local environment is necessary

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for a better understanding of human exposure from natural (Navas et al., 2011).

U,

226

Ra,

232

Th, and

40

K are the major contributors of outdoor

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Public exposure to natural ionizing radiation is mostly due to radon. The estimated

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value of annual exposure to the various components of natural radiation shows that 222Rn

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contribution constitutes as much as 50 % of the overall radiation dose (UNSCEAR,

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2000). Radon is a naturally occurring radioactive isotope of 238U series. Unlike the others

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mentioned above, radon is the radioactive inert gas and have sufficient half-life (3.82

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days) for radon to exhale out from the solid materials and enter the atmosphere. The

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inhalation of radon and its short-lived daughters (218Po, 214Pb,

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the dwelling is one of the radiation risks for the population. Radon gas can arrive the

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indoors from different sources such as soil or rock under or surrounding the buildings,

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building materials, water supplies, natural gas and outdoor air (European Commission,

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1995). Among them, at least 80% of the radon emitted into the atmosphere comes from

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the top few meters of the ground (Abumurad et al., 2001).

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Bi,

214

Po, and

210

Po) in

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The releases of radon from soil to the atmosphere include three processes (IAEA,

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2013): The Emanation: radon atoms formed from the decay of radium released from the

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grains into the pore space between the grains. The fraction of radon atoms formed from

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the decay of radium escaped from the radium-bearing grains into the interstitial space of

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the grains is called the radon emanation coefficient. The transport: diffusion and

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convection of radon atoms between the grains through the soil matrix to the ground

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surface. The exhalation: radon atoms transported to the ground surface exhale to the

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atmosphere.

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Radon atoms located within solid grains are not easily released into the atmosphere,

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due to their very low diffusion ability in solids (IAEA, 2013). However, if they are

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located in the space between the soil particles, they can completely diffuse to the soil

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surface and enter the atmosphere as soil gas. Therefore, in the study, we evaluated the

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radon emanation coefficient, an important factor controlling radon concentration in soil

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and radon exhalation rate from the soil surface.

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The emanation coefficient depends on

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mineralogy (IAEA, 2013).

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how many radium atoms are close enough to the surface of the grains to allow the radon

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to escape into the intergranular space. Soil moisture content can decide how many radon

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atoms retained in the pore space after escaping from the soil grains instead of burying

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themselves in adjacent particles. The typical ranges of radon recoil from radium decay in

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water (77 nm) are much less than in air (53 µm) (IAEA, 2013). Therefore, water in the

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soil is more effective at stopping radon atoms within the pore space instead burying

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themselves in adjacent grains. This may increase the radon emanation in the soil samples

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with high water content. However, some works in the world have suggested that the

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radon emanation coefficient remains nearly constant with increasing moisture up to

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saturation (Strong et al., 1982; Bossew, 2003; Breitner et al., 2010).

Ra distribution, particle size, moisture and

Ra distribution, particle size, and mineralogy determine

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226

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Around the world, several authors have been studying direct correlations between

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uranium, radium, radon in soil gas, and indoor radon concentrations (Vaupotic et al.,

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2002; Kitto, 2005; Kovács et al., 2013; Mahur et al., 2013; Forkapic at al., 2017). In

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Vietnam, there have not been many scientific studies on the level of natural radioactivity

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in Ninh Son region. The region is surrounded by many magma rocks. Around the district,

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many granitic and igneous granitoid bodies exist. According to some studies (Moura et

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al., 2011, Papadopoulos et al., 2013, Tositti et al., 2016), the radioactivity of magmatic

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rocks is generally high. The present work aims to conduct the measurements of natural

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radioactivity (238U,

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coefficient in some types of soil in Ninh Son region and discusses in detail in some

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correlations as well as the factors that affect them. This study to be made in this area is

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important for not only the health of the people living in this region but also for tourism

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from all over the world.

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226

Ra,

232

Th and

40

K), radon concentration, and radon emanation

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Experimental

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Description of the site Ninh Thuan is a coastal province in the South-Central Coast of Vietnam where it

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located from 11°18'14" to 12°09'15" North and 108°09'08" to 109°14'25" East. The

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topography of Ninh Thuan is typical for the South-Central Coast in that high mountains

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are located not only near the western border to the Central Highlands but also near the

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coast and the mountain area occupies over 60% of the province. Ninh Thuan has a

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typically tropical monsoon climate, characterized by hot dry, strong wind and strong

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evaporation. It has two distinct seasons; the rainy season usually starts in September and

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ends in November while the dry season lasts from December to August next year. The

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mean temperature in a year is about 26-270C, the annual average rainfall of 700-800 mm

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and humidity is around 75-77%. Water resources are distributed unevenly, mainly located

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in the North and center of the province. Underground water in the province is only one-

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third of the national average. Soil samples were collected in Ninh Son district, Ninh

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Thuan province. Ninh Son district is located in the northwest of Ninh Thuan province.

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The total natural area accounts for 23% of the total natural area of the province.

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Soil sampling

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All sampling locations are shown on the map of Fig. 1. In this study, 57 soil samples

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were collected from 57 different locations throughout Ninh Son district, Ninh Thuan

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province. Samples were taken during summer 2017 with AMS professional soil sampling

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kits. The soil samples were collected at the depth of 0.2 - 0.3 m from the soil profile to

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obtain undisturbed and pure soil samples. During the collecting, gravels and pebbles were

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removed from the soil samples. About 1.5 kg of soil samples from three holes (separated

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by 50 cm) at each location were collected in a polyethylene bag then transported to the

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laboratory where they were dried at 105°C for at least 1 day in an electric oven. The dried

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samples were crushed by using a mortar and pestle then a 0.2 mm sieve size was used to

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obtain homogeneous samples. Finally, about 150 grams of the samples were placed in

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metal cylinder beakers and sealed off for at least 30 days at room temperature before

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proceeding to the measurement to establish secular radioactive equilibrium between 226Ra

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and their decay products.

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Soil classification During different natural weathering processes, elements in the soil get transferred to

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surface and subsurface water levels and contaminate it to different extents (Jakhu et al.,

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2017). The surveyed soil samples can be divided into six types and their organic matter

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contents were determined using Eq (6).

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Slip-debris soil: This type develops on rocky outcrops. The soil has yellow grey,

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brown grey or brown-red in color. It develops on the saprolite weathering shell and be

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characteristic of the mountainous region. The soil contains a lot of raw materials,

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minerals (such as hydromorphone, kaolinite, montmorillonite, and iron glue have high

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absorption and exchange capacities), less clay and organic matter. Organic content ranges

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from 1 to 3% in the samples. On average, soil fraction with particles of less than 0.1 mm

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in diameter is approximately 55%.

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Sandy soil: This type of soil usually forms along rivers and springs. Part of that is the

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sand formed due to prolonged drought conditions of the climate and human activities,

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called desertification. The composition of sand varies, depending on the local rock

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sources and conditions. However, the majority of sand is dominantly composed of silicate

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minerals or silicate rock fragments. Silica is the most common mineral resistant to

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weathering due to its chemical inertness and considerable hardness. Sandy soil primarily

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contains particles with a diameter of between 0.1- and 1-mm. Soil fraction with particles

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of less than 0.1 mm in diameter is approximately 20%. Organic content is less than 1% in

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the samples.

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Alluvial soil: This type of soil is formed from young sediments, Holocene sediments,

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originating from rivers, lakes, springs, etc. Therefore, the mechanical components of the

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soil are usually light and mixed with many durable minerals. Grain sizes of the soil

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samples are usually small (less than 0.1 mm). The organic matter content in the soil is

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quite high. It ranges from 8 to 15% in the samples.

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Red soil: The red soil is generally derived from crystalline rock. In high temperature

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and humidity conditions, the strong weathering process creates this soil layer. The rain is

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washed away with soluble bases, simultaneously, accumulation of iron and aluminum

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oxides produce the red color in the soil layer. Organic content ranges from 1 to 4% in the

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samples. Soil fraction with particles of less than 0.1 mm in diameter is approximately

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65%. Metamorphic soil: The Metamorphic soil is made from the weathering of all rocks in a

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long time that was originally sedimentary such as schist, slate, or gneiss. The soil type is

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relevant to plant wet rice. Organic content ranges from 3 to 6% in the samples. Soil

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fraction with particles of less than 0.1 mm in diameter is approximately 58%.

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Forest surface soil: Mountainous forests occupy quite a large area in the district. The

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soil formation has been influenced by forest vegetation including roots, branches, leaves

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of trees, dwelling organisms. Like other soils, forest soils have developed from

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geological parent materials in various topographic positions interacting with climates and

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organisms. Most of the surface soil samples collected do not contain any rocks. Organic

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content ranges from 15 to 20% in the samples. Soil fraction with particles of less than 0.1

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mm in diameter is approximately 60%.

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Measurement of 238U, 226Ra, 232Th, and 40K concentrations

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The activity concentration of primordial radionuclides

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232

Th,

238

U,

226

Ra and

40

K in

the collected samples were analyzed by gamma spectrometry with high purity germanium

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(HPGe) detector of Canberra, with a closed-end coaxial geometry. The HPGe-detector

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was coupled to a computer-based multi-channel analyzer card system, which could

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determine the area under the characteristic peak of energy by using Genie 2000 software.

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For the measurement of low-level radioactivity, the counting system has a well-shielding

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arrangement with 4-inch thick low-background lead. The shielding aims to reduce the

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background from the environment affecting the results. Efficiency and resolution of the

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counting system are 35% and 1.8 keV, respectively at the 1332.5 keV peak of 60Co. The

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energy calibration of the system was done using a standard mixture of gamma-emitting

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isotopes (241Am, 137Cs, 54Mn, 57Co, 60Co, 22Na, and 65Zn). The absolute efficiency of the

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system was performed using ISOCS/LabSOCS mathematical calibration software in

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build with Monte Carlo simulation which has been archived good results in some tests

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with the reference materials (IAEA-434, IAEA-RGU-1, IAEA-RGTh-1, and IAEA-

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RGK-1) and IAEA proficiency testing program in 2015, 2016, and 2017. All samples

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were counted for 36,000s to archive the gamma spectrum with good statistics. The γ-ray

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of

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Th (63.4 keV) was used for the purpose of determining the activity of

activity concentration of 212

232

Th was determined through isotopes

U. The

Ac (338.3 keV, 911.2

200

keV),

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activity concentrations of

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1764.5 keV (214Bi); 295.2 keV and 351.9 keV (214Pb) were used. The activity

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concentration of

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activity concentrations of radioisotopes were determined using the well-known relation

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given in Eq (1). A=

Ra, the γ-ray lines 609.3 keV, 1120.3 keV, 1238.1 keV,

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Tl (583.2 keV, 2614.3 keV). In order to determine the

K was measured directly from the 1460.8 keV γ-ray energy. The

CPS ε γ × Iγ × W

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Pb (238.6 keV) and

208

228

238

(1)

Where A (Bq.kg-1) is activity concentration of the considered radioisotope, CPS is the

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net counts per second of the experimental samples, W (kg) is the weight of the sample, εγ

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is the absolute gamma peak detection efficiency and Iγ is emission probability of the

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corresponding gamma-ray energy.

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Radiation Hazard Parameters

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Natural radionuclides distribution in the soil samples is not non-uniform. The activity

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levels of 226Ra, 232Th, and 40K in the samples can be evaluated by means of a common

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radiological index called radium equivalent activity. Radium equivalent activity was

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defined as that activity concentration of a radionuclide equivalent to 370 Bq.kg-1 of 226Ra,

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which gives outdoors an external effective dose rate of 1.5 mGy (1 mSv) per year

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(UNSCEAR, 2000). The radium equivalent activity is defined as for Eq (2) (Beretka et

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al., 1985; UNSCEAR, 2000).

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Ra eq =CRa +0.077CK +1.43CTh

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(2)

Based on the activity concentrations of

40

K,

238

U and

232

Th, in the soil, outdoor

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gamma-ray exposure rate in air at a one-meter height above the ground due to natural

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radionuclides in soils was calculated by using Eq (3) (UNSCEAR, 2000).

D ( nGyh -1 ) =0.462C Ra +0.604C Th +0.0417C K

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(3)

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Where, CRa, CTh, and CK are respectively the activity concentrations of radionuclides

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226

Ra, 232Th and 40K existing in the soil in Bq.kg-1.

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The annual effective dose, E (mSv.y-1) in air 1 m above the ground due to outdoor

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external exposure from soil can be calculated using Eq (4) (UNSCEAR, 2000).

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E=D×O f ×8760×0.7×10 -6

(4)

In this equation, Of is the occupation factor, which is the fraction of the year for which a

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hypothetical member of the public is exposed outdoors. The suggested value of Of by

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UNSCEAR (2000) is 0.2. The factor 0.7 Sv Gy−1 is the conversion factor from the

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absorbed dose in the air to the effective dose received by adults at a height 1 m above the

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ground surface (UNSCEAR, 2000). The value of 8760 is the time for one year.

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Measurement of radon concentration

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For measurement of in situ radon concentration, a RAD7 detector with a stainless-steel

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soil gas probe was used. At each site, first look for locations where the soil is uniform

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and generally free of rocks was conducted then the stainless-steel probe with holes near

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the tip was inserted in the soil at depth required for sampling after removing the pilot rod

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in the ground for the probe which was done before (Durridge Co, 2017). The probe was

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then connected to the RAD7 detector through the desiccant tube and inert filters for

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sucking the soil gas from the underground soil. The soil gas was pumped through the

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RAD7 chamber at a flow rate of ~0.5 dm3.min−1. Three hours counting time for all

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sampling points had been taken. The radon in the RAD7 chamber decays, producing

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detectable alpha emitting progenies, particularly the polonium isotopes (218Po, 214Po). The

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RAD7 detector then converts alpha radiation directly to an electric signal and has the

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possibility of determining electronically the energy of each particle, so it is possible to

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instantaneously distinguish between old and new radon, radon from thoron, and the signal

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from noise (Durridge Co, 2017).

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Determination of radon emanation coefficient

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According to some studies and empirical surveys (Bossew, 2003, Sakoda et al., 2011),

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the mean water content in collected soil samples should be 10% due to the emanation

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coefficient increases as water content increases and be saturated at 10% in moisture. In

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this study, depending on sample weight, the radon emanation coefficient of all soil

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samples is identified at 10% of moisture content by adding some distilled radon-free

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water (226Ra-free). For a study of the radon emanation coefficient, from 300 to 500 grams 9

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(10% of moisture content) of collected soil samples were stored in a closed metal

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cylindrical container for 15 days at 27 – 28oC in temperature. The volume of the

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container is 1.3 dm3. The emanation coefficient is calculated by Eq (5) (IAEA, 2013, Thu

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et al., 2018).

E=

CRn × V k × (1-e-λt ) × M × CRa

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(5)

Where, E is the radon emanation coefficient; CRn (Bq.m-3) is the radon concentration

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in the stored samples obtained by RAD7; V (m3) is the effective volume of the sampling

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container; k is the correction factor for both escape or leakage of radon in storing time; λ

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is decay constant of radon; M is the total mass of the sample in the container; t is time for

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storing the sample and CRa (Bq.kg-1) is the radium activity content.

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The k value was determined by using a Standard Reference Material (SRM) capsule of 226

NIST. The SRM capsule contained

Ra with an activity of approximately 5 Bq. The k

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value is the ratio between the activity measured by the equipment and the activity

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provided by the manufacturer. The evaluation of the correction factor was found to be

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0.81±0.03 (Thu et al., 2018).

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Determination of grain size and organic matter content of the soil samples

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According to some studies carried out by Breitner et al., 2008 and Sakoda et al., 2010,

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the radon emanation coefficient is almost only increased when the particle size of the

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sample is less than 0.1 mm in diameter. It means that almost small particles can make

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differences in the radon emanation coefficient between the samples. Therefore, in order

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to evaluate the correlation between grain sizes with radon emanation coefficient, the

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number of grains contained diameter less than 0.1 mm is determined. The sieve analysis

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was performed to determine the distribution of the particles. The soil sample (~200

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grams) was sieved by using a 0.1 mm sieve. The weight of soil from the bottom of the

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sieve (passing mass) was determined and compared to the initial sample weight.

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Soil organic matter content (expressed as a percentage) is the ratio of the mass of

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organic matter to the mass of the dry soil solids in a soil sample. Organic matter content

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was calculated by heating the dried soil sample at 550°C during 4 hours and measuring

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the weight loss once the oven temperature had dropped to 150°C (Hoogsteen et al.,

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2015). Organic matter content (OM) was then determined by Eq (6).

OM=

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W-Wh Wh

(6)

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after heating and cooling to 150°C.

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Results and discussion

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Where, W is the weight of the dried soil sample and Wh is the weight of the sample

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40

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Table 1S (Supplementary Material) shows activity concentrations of 232Th, 238U, 226Ra

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and

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concentrations of the soil samples vary in the study area due to the differences in

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geological structures among different areas. The activity concentrations of

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226

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493±16 and 1625±50 Bq.kg-1, with a mean of 95±6, 55±7, 60±3 and 1073±34 Bq.kg-1,

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respectively. The mean values of particular soil types are higher than their world averages

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of 45 Bq.kg-1 (232Th), 33 Bq.kg-1 (238U), 32 Bq.kg-1 (226Ra) and 412 Bq.kg-1 (40K)

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(UNSCEAR, 2008). Among the soil types, soil slips- debris type shows the highest

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values in average activity concentrations of

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almost no significant differences in other soil types. According to Table 1S

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(Supplementary Material), radium equivalent activity in the six soil types ranges from

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114±6 to 533±20 Bq.kg-1 for all points where sandy, alluvial, red, metamorphic and forest

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surface soil samples resulted in the radium equivalent activity values lower as compared

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with the maximum value of 370 Bq.kg-1 which corresponds to an effective dose of 1 mSv

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for the general public (UNSCEAR, 2000). It should be noted that the values of nine

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samples (S5, S20, S24, S25, S31, S51, S54, S56, and S57) in the soil slips- debris type

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are in excess of the maximum value, 370 Bq.kg-1. Other points like S4, S7, S14, S18,

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S22, S32, S34, S35, S37, S41, S42, S50, and S53 have also high radium equivalent

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activity, between 295±10 and 358±16 Bq.kg-1. This type of soil grows on the exposed

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rock, contains many minerals and iron glue with many coarse grains (cracked gravels)

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that they cause high concentrations of natural radionuclides in the samples. This finding

232

Th,

238

U,

K vary between 30±3 and 206±11, 13±6 and 161±14, 25±2 and 193±9,

232

Th,

226

Ra, and

238

U. These values have

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K radionuclides for a total of 57 soil samples. The radionuclide activity

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implied that using the type of soils in the area as building material might present a

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significant radiological health risk. Based on the activity concentrations of 226Ra,

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232

Th and 40K in the soil, the calculated

values of the outdoor gamma-ray exposure rate in air at a one-meter height above the

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ground are also presented in Table 1S (Supplementary Material). The outdoor gamma

317

dose in air varied from 55±2 nGy.h-1 and 248±7 nGy.h-1 with an average of 130±4 nGy.h-

318

1

319

population-weighted mean value of 58 nGy h−1 for the regular area given by UNSCEAR

320

(2008). For particular soil types, different mean values of outdoor gamma dose in the air

321

are also higher than that given by UNSCEAR (2008). The high values of the

322

concentration and outdoor gamma dose in the air are associated with the presence of the

323

granitic and gratinoid bodies in the study area. The mean value of annual effective

324

external dose is 159±5 µSv.y-1, which varies between 67±2 and 304±9 µSv.y-1. The dose

325

is within of the world ranges of 10–430 µSv.y-1 (UNSCEAR, 2000).

RI PT

315

M AN U

SC

for all soil types. The recorded mean value is over 2.2 times than the reported

326

Table 1, 2, 3, 4, 5 and 6 express correlations of radionuclide contents in 6 soils. Our 238

328

obtained with Pearson’s test for soil slips- debris and sandy soil (0.51), for red soil (0.42)

329

and metamorphic soil (0.63). Poor positive correlations are found in forest surface soil

330

and alluvial soil. Similarly, poor correlations between

331

obtained soil slips- debris (0.33) and forest surface soil (-0.22). Mean correlations

332

between them are found in red soil (0.62), alluvial soil (0.58) and metamorphic soil

333

(0.40). The good correlation is obtained in sandy soil (0.75). There is almost no

334

correlation between 238U and 232Th in metamorphic, alluvial and forest surface soil. Those

335

have a moderate correlation in soil slips- debris (0.53), red (0.57) and sandy soil (0.43). It

336

can be clearly seen from the observed data in Table 1S (Supplementary Material) that the

337

232

338

correlation or independence between contents of radionuclides in soils may be justified

339

by the radioactive disequilibrium generated under soil weathering and different behavior

340

for these radioisotopes in the environment.

341

solution (Schon, 2015). It cannot be transported by water during the weathering process

342

and thus, it is concentrated in its parent rock and from here it is transported in the form of

Th and

Ra contents are

226

Ra contents are

AC C

EP

TE D

findings show those mean positive correlations between

232

U and

226

327

Th activity in soils is higher than those of

232

12

226

Ra and

238

U. The difference and

Th is very stable and will not dissolve in a

ACCEPTED MANUSCRIPT

343

colloidal suspension (Jakhu et al., 2017). Due to physical bond to the surface of colloids

344

(Nodar et al, 2018), 232Th can be thus found in sedimentary rocks, coarse sandstones and

345

gravel deposits (Kovács et al., 2013). This is one of the reasons for the high

346

concentration in the soil slips- debris as compared with the world average of 45 Bq.kg-1

347

(UNSCEAR, 2000). Following results of weathering and alteration processes, 238U forms

348

soluble salts, which are transported in the sea and river water (Schon, 2015) while

349

from the host rocks can be transported and deposited as loess, silt placers and tertiary soil

350

(IAEA, 2014). This may be the cause of the poor correlations of radioactivity between

351

238

U and

352

232

Th and

353

soil in the areas near mountains, this soil can be accreted or washed under the impact of

354

rain. Therefore, the correlations between the contents of the radionuclides were found to

355

be very weak.

U and

232

RI PT

238

Ra (0.15),

Th

226

Ra

Th (-0.12) in alluvial soil, while the radioactivity of

SC

226

232

226

M AN U

Ra has to moderate correlation (0.6) in this soil. Forest surface is a type of

356

A study carried out by Stenkin et al. 2017 (Stenkin and Shchegolev, 2017) has

357

proposed a way to assume cosmic rays could take part in process of soil activation where

358

they transform long-lived nuclei of thorium (232Th) to nuclei with shorter lifetime (230Th)

359

through specific nuclear reactions. Finally, they can lead to the production of

360

high altitudes with integral cosmic ray flux accumulated for a long period (thousands of

361

years). In addition, after examining the correlations between 232Th and 226Ra in all the six

362

soil types, the effect of the cosmic radiation on the equilibrium state was supported to be

363

a minor reason for a

364

(Supplementary Material) shows the soil of high concentrations displayed

365

disequilibrium (226Ra/238U of 0.7–4.1).

366

concentrations for some samples indicating a great majority of the

367

above 1. It seems to be that the higher the

368

ratio was observed in the soil. However, this is not true for the sample of S18 whereas the

369

low level of

370

means that the geochemical processes and the effect of the soil water (Kovács et al.,

371

2013) have a great effect on the ratio. Especially, the alpha recoil is mainly responsible

372

for the disequilibrium phenomenon (Suksi et al., 2006). The chemical behavior of these

373

radioisotopes is also one of the most important factors which influence the concentration

TE D

Ra at

Ra/238U disequilibrium occurred in the soil. Indeed, Table 1S

EP

226

AC C 226

226

Ra and

238

226

226

Ra/238U

Ra concentrations were greater than

226

226

U

Ra/238U values lie

Ra concentration, the higher the

U linked to the highest value of 4.1 for

13

226

238

226

Ra/238U

Ra/238U ratio. This

ACCEPTED MANUSCRIPT

of them in the soil (Nodar et al, 2018). Indeed, potassium seems to be more or less

375

unaffected by these processes (Schon, 2015) and that is why the radioactivity of

376

potassium is high in most of the samples. The difference in potassium content among the

377

samples is due to the different origin of the soil samples where the samples were not

378

collected on areas of cultivation. However, the high radioactivity of potassium also may

379

be due to the different farm practices involving in improving the soil fertility by using fly

380

ash in appropriate combination with organic matter and chemical fertilizer of neighboring

381

farms. This is possibly the second reason for the difference in potassium content at

382

different sampling sites.

SC

RI PT

374

Radon concentrations of 57 samples are presented in Table 2S (Supplementary

384

Material) and they are between 2.8±0.1 kBq.m-3 and 72.6±1.0 kBq.m-3. Particularly,

385

mean values in slips-debris, sandy, alluvial, red, metamorphic and forest surface soil are

386

29.0±0.6, 4.7±0.1, 17.9±0.5, 13.6±0.4, 15.8±0.5 and 9.7±0.2 kBq.m-3, respectively. There

387

are limits for radon release rates to the atmosphere but no apparent limits for radon in

388

soil-gas were found in the literature. However, some assessments of risks from radon

389

have proposed in recent years (Cinelli et al., 2015, Gruber et al. 2013). According to the

390

Sweden Criteria (Lara et al., 2015), soils showing radon concentrations in soil gas below

391

10 kBq.m-3 are considered “low risk”, while radon concentrations in soil gas between 10

392

and 50 kBq.m-3 are classified as “normal risk” and require protective actions in

393

dwellings. If soils show concentrations above 50 kBq.m-3 are classified as “high risk” and

394

require buildings with safety criteria against radon (Lara et al., 2015). Our data results

395

show that there are approximately 67% (38 samples) between 10 until 50 kBq.m-3

396

considered “normal risk”, and 9% (5 samples) presented concentrations greater than 50

397

kBq.m-3, classified as “high risk” areas.

TE D

EP

AC C

398

M AN U

383

Another way to define the radon potential is the Radon Index (RI) based on

399

multivariate cross-tabulation (EPA., 1993, Gruber et al., 2013, Barnet et al. 2008). Based

400

on soil gas radon and permeability data, Cinelli et al., 2015 have used a classification

401

reported in a study carried out by Barnet et al. 2008 to assess the radon risk. On the basis

402

of the recorded measurements of the weight percentage of the fine fraction (<100 µm),

403

the soils of the studied area were predicted as high, medium and low permeable soils

404

according to particle size analysis method (Barnet et al. 2008). According to the

14

ACCEPTED MANUSCRIPT

classification table carried out by Barnet et al. 2008, the medium permeable soils with the

406

mean value of 29.0±0.6 kBq.m-3 resulted in medium RI for the soil slips-debris type while

407

medium permeable soils with the mean values lower than 20 kBq.m-3 resulted in low RI

408

for the red, metamorphic and forest surface soil samples. The high and low permeable

409

soils resulted in the same low RI belong to the sandy and alluvial soil samples,

410

respectively. This observation is similar to the earlier observation which most of the

411

samples are considered “normal risk” or low radon index.

RI PT

405

The emanation coefficient in the soil samples was also subjected for our investigations

413

due to the most important factor which influences the radon emanation is the grain size of

414

the soil sample (IAEA, 2013). Table 2S (Supplementary Material) showed the radon

415

emanation coefficient levels and it ranged from 0.08±0.01 to 0.55±0.03 with a mean

416

value of 0.30±0.02 in all the soil samples. The range of the values is also in good

417

agreement with the results of some other studies (Bossew, 2003, Breitner et al., 2008,

418

Sakoda et al., 2011, Thu et al., 2018). Particularly, the highest mean radon emanation

419

coefficient in alluvial soil is 0.51±0.03 where almost particles in the alluvial soil samples

420

are less than 0.1 mm in diameter. The second highest average radon emanation is

421

0.40±0.02 which belongs to the red soil. The emanation in forest surface soil is the third

422

highest (0.36±0.02). The mean values in slips-debris soil and metamorphic soil samples

423

are 0.30±0.02 and 0.26±0.02, respectively. As a consequence, sandy soil has the lowest

424

mean radon emanation coefficient (0.15±0.01). First of all, the obtained results can be

425

explained by the fact that the only ~ 20% of weight of the sandy soil is created by the

426

grains less than 0.1 mm while it is ~ 100% for alluvial soil, 65% for red soil, 60% for

427

forest surface soil, 58% for metamorphic soil and in the slip-debris soil, it is ~ 55%. Fig.

428

2 shows the linear relationship between the percentage of soil particles for less than 0.1

429

mm in diameter and radon emanation coefficient. Owning a high positive correlation

430

coefficient (R = 0.81) and P values below 0.050, the radon emanation coefficient tends to

431

increase with decreasing the grain sizes. Therefore, these grains could be responsible for

432

the lowest emanation coefficient for sandy soil and the highest values of emanation

433

coefficient for alluvial soil (Thu et al., 2018). However, this is not the only cause of the

434

difference in the emanation between the samples. The radon emanation fraction in the red

435

and forest surface soil is greater than that in the metamorphic soil although the fraction of

AC C

EP

TE D

M AN U

SC

412

15

ACCEPTED MANUSCRIPT

particle size less than 0.1 mm is not a significant difference between the types of soil. The

437

reason for this observation may be due to the accumulation of iron oxides in the red soil

438

and high organic content in the forest surface soil. Indeed, iron is common soil

439

weathering products important to radon generation and compounds of iron form as fine-

440

grained particles and surface coatings (IAEA, 2014). The slightly larger values of

441

emanation in forest surface soil samples for organic matter as compared to the

442

metamorphic soil seems reasonable since the recoil range of the

443

30–50 nm in solid materials and the recoil range and indirect recoil should be much

444

greater in the non-crystalline organic matter (Greeman et al., 1995). The correlations between

222

Rn atom is typically

SC

445

RI PT

436

226

Ra content, radon emanation and radon concentration of

the soils are presented in Table 1, 2, 3, 4, 5 and 6. Pearson’s correlation coefficient was

447

used to express the correlation between

448

soil gas and expected high positive correlations were found. Following that the

449

correlation coefficients in soil slips- debris, sandy, red, metamorphic and forest surface

450

soil are 0.78, 0.88, 0.86, 0.64 and 0.68, respectively. The recorded results also show that

451

some soil samples having high

452

radon. However, the positive correlation between

453

concentration in the alluvial soil was not found. This can be explained by that fact that

454

radon concentration in the soil gas strongly depends on the emanation (IAEA, 2013).

455

This hypothesis was then investigated by checking the relationship between radon

456

concentration and emanation.

457

quantities in soil slips- debris, alluvial, red, metamorphic and forest surface soil were

458

found to be 0.30, 0.51, 0.69, 0.47 and 0.24, respectively. The negative correlation was

459

found for sandy soil where the correlation coefficient between

460

concentration in sandy soil is very high. Due to the very poor radon emanation in sandy

461

soil,

462

From these observations, we then confirm that the concentration of radon in soil gas

463

depends mainly on radon emanation and diffusion (IAEA, 2013). These quantities do not

464

only depend on lithology, morphology and grain size but are also affected by hydro-

465

meteorological conditions (Kovács et al., 2013). Therefore, excellent correlations could

466

not be expected under in situ experiment conditions. In general, the highest radon

226

M AN U

446

226

Ra content in soil and radon concentration in

Ra contents are likely to have relatively high levels of 226

TE D

Ra radioactivity and radon

AC C

EP

The Pearson’s correlation coefficients between two these

226

226

Ra content and radon

Ra content in soil gas becomes the main factor controlling radon concentration.

16

ACCEPTED MANUSCRIPT

467

concentrations were found in the slips-debris soil and the radon concentrations in the

468

different soils varied widely.

469

The correlation coefficient between radium content and radon emanation coefficient in

470

soils are presented in Table 1, 2, 3, 4, 5 and 6. There is no significant relationship

471

between the

472

metamorphic and forest surface soil). This finding may be explained by the fact that the

473

emanation coefficient is affected by some parameters such as radium distribution, particle

474

size, and shape, moisture content and mineralogy in the soil. The results of the present

475

study indicate that radon emanation is not dependent on the radium content of the soil

476

sample. Radium has been found to adsorb onto oxidized Fe phases (IAEA, 2014) which

477

makes radium more concentrated on the surface of the soil grains. Therefore, any oxide-

478

hydroxides of iron in the red soil samples will help bring radium to the surface of the soil

479

particles and this makes the correlation coefficient between radium content and the

480

emanation coefficient reaches quite good value (0.64).

226

M AN U

SC

RI PT

Ra and radon emanation in 5 soils (soil slips- debris, sandy, alluvial,

481

Conclusions

482

The six main soil types, collected at 57 points in Ninh Son region, were analyzed for

483

40

484

radon monitor coupled with a soil gas probe. The mean activity concentrations of

485

radionuclides are higher than mean values published by UNSCEAR 2008. The greatest

486

activity concentrations of

487

samples while the other types of soil have no significant difference in the contents of

488

these isotopes. The 232Th activity in soils is higher than those of 226Ra and 238U and this is

489

consistent with the flexibility of the radionuclides under weathering and alteration

490

processes. The recorded mean value is over 2.2 times than the reported population-

491

weighted mean value of 60 nGy h−1 for the regular area given by UNSCEAR which

492

implied that using the type of soils in the area as building material might present a

493

significant radiological health risk. The results of weathering, alteration processes,

494

chemical behavior of the radioisotopes and soil activations by cosmic rays were proposed

495

to explain the

496

Criteria and the Radon Index (RI) showed that the radon in soil gas samples is considered

497

“normal risk” or low radon index. The mean radon emanation coefficient in alluvial soil

238

U,

232

Th and

226

Ra using HpGe gamma spectrometry, and for

TE D

K,

K,

232

Th,

238

U, and

226

Rn using Rad-7

Ra are recorded in soil slip-debris

AC C

EP

40

222

226

Ra/238U disequilibrium occurred in the soil samples. Both the Sweden

17

ACCEPTED MANUSCRIPT

498

is greatest while the lowest value is recorded in sandy soil. There is no significant

499

relationship between the

500

metamorphic and forest surface soil. The radon emanation coefficient was found to

501

depend on oxide-hydroxides of iron and organic matter content in the soil and on particle

502

size (0.1 mm in diameter) but independent on the radium content of the soil sample.

Ra and radon emanation in soil slips- debris, sandy, alluvial,

RI PT

226

Acknowledgments

504

This research was conducted on instruments at the Nuclear Technique Laboratory

505

(NTLab), University of Science, Vietnam National University Ho Chi Minh City (VNU-

506

HCM), Vietnam. The authors would like to thank Mr. Vu Ngoc Ba for his

507

for assistance with measurements using HPGe gamma-ray spectrometry system. We also

508

thank the reviewers, English proofreaders and editors for their thorough review and

509

highly appreciated comments and suggestions, which significantly contributed to

510

improving the quality of this manuscript.

511

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Table captions:

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Table 1: Pearson’s correlation matrix of radionuclide contents and emanation coefficient in soil slips- debris Table 2: Pearson’s correlation matrix of radionuclide contents and emanation coefficient in sandy soil Table 3: Pearson’s correlation matrix of radionuclide contents and emanation coefficient in alluvial soil Table 4: Pearson’s correlation matrix of radionuclide contents and emanation coefficient in red soil Table 5: Pearson’s correlation matrix of radionuclide contents and emanation coefficient in metamorphic soil

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Table 6: Pearson’s correlation matrix of radionuclide contents and emanation coefficient in forest surface

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soil

ACCEPTED MANUSCRIPT Table 1: Pearson’s correlation matrix of radionuclide contents and emanation coefficient in soil slips- debris 238

232

Ra content (Bq.kg-1)

40

Th content (Bq.kg-1)

K content (Bq.kg-1)

Emanation Radon coefficient concentration

U content (Bq.kg-1)

1.0000

0.5088

0.5261

-0.2083

0.2823

0.5133

226

Ra content (Bq.kg-1)

0.5088

1.0000

0.3287

-0.0377

0.0185

0.7813

232

Th content (Bq.kg-1)

0.5261

0.3287

1.0000

0.0066

-

-

K content (Bq.kg-1)

-0.2083

-0.0377

0.0066

Emanation coefficient

0.2823

0.0185

-

Radon concentration

0.5133

0.7813

-

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226

U content (Bq.kg-1)

Variables

1.0000

-

-

-

1.0000

0.2975

-

0.2975

1.0000

238

226

232

Ra content (Bq.kg-1)

40

Th content (Bq.kg-1)

K content (Bq.kg-1)

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U content (Bq.kg-1)

Variables

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Table 2: Pearson’s correlation matrix of radionuclide contents and emanation coefficient in sandy soil Emanation Radon coefficient concentration

U content (Bq.kg-1)

1.0000

0.5141

0.4288

0.4234

-0.3863

0.4108

226

-1

Ra content (Bq.kg )

0.5141

1.0000

0.7464

0.5709

-0.7889

0.8784

232

Th content (Bq.kg-1)

0.4288

0.7464

1.0000

0.8923

-

-

K content (Bq.kg-1)

0.4234

0.5709

0.8923

1.0000

-

-

Emanation coefficient

-0.3863

-0.7889

-

-

1.0000

-0.4659

Radon concentration

0.4108

0.8784

-

-

-0.4659

1.0000

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Table 3: Pearson’s correlation matrix of radionuclide contents and emanation coefficient in alluvial soil 238

Ra content (Bq.kg-1)

232

Th content (Bq.kg-1)

40

K content (Bq.kg-1)

Emanation coefficient

Radon concentration

U content (Bq.kg-1)

1.0000

0.1470

-0.1235

0.2354

-0.0050

0.1492

226

Ra content (Bq.kg-1)

0.1470

1.0000

0.5823

0.5944

-0.5654

-0.1541

232

Th content (Bq.kg-1)

-0.1235

0.5823

1.0000

0.4332

-

-

K content (Bq.kg-1)

0.2354

0.5944

0.4332

1.0000

-

-

Emanation coefficient

-0.0050

-0.5654

-

-

1.0000

0.5143

Radon concentration

0.1492

-0.1541

-

-

0.5143

1.0000

40

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Ra content (Bq.kg-1)

232

Th content (Bq.kg-1)

40

K content (Bq.kg-1)

Emanation Radon coefficient concentration

1.0000

0.4206

0.5736

-0.0371

0.2955

0.1911

226

Ra content (Bq.kg-1)

0.4206

1.0000

0.6205

-0.0789

0.6403

0.8579

232

Th content (Bq.kg-1)

0.5736

0.6205

1.0000

-0.3784

0.4049

-

K content (Bq.kg-1)

-0.0371

-0.0789

-0.3784

1.0000

-

-

Emanation coefficient

0.2955

0.6403

-

-

1.0000

0.6937

Radon concentration

0.1911

0.8579

-

-

0.6937

1.0000

40

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U content (Bq.kg-1)

SC

238

226

U content (Bq.kg-1)

Variables

Table 5: Pearson’s correlation matrix of radionuclide contents and emanation coefficient in metamorphic soil U content (Bq.kg-1)

Ra content (Bq.kg-1)

232

Th content (Bq.kg-1)

40

K content (Bq.kg-1)

Emanation Radon coefficient concentration

U content (Bq.kg-1)

1.0000

0.6281

0.2091

-0.0203

0.2338

0.4078

226

Ra content (Bq.kg-1)

0.6281

1.0000

0.3961

-0.0424

0.2733

0.6398

232

Th content (Bq.kg-1)

0.2091

0.3961

1.0000

0.0072

-

-

K content (Bq.kg-1)

-0.0203

-0.0424

0.0072

1.0000

-

-

Emanation coefficient

0.2338

0.2733

-

-

1.0000

0.4705

Radon concentration

0.4078

0.6398

-

-

0.4705

1.0000

40

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Variables

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Table 6: Pearson’s correlation matrix of radionuclide contents and emanation coefficient in forest surface

U content (Bq.kg-1)

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Ra content (Bq.kg-1)

soil 232

Th content (Bq.kg-1)

40

K content (Bq.kg-1)

Emanation coefficient

Radon concentration

238

U content (Bq.kg-1)

1.0000

0.3028

0.0771

-0.3544

0.2169

0.4991

226

-1

Ra content (Bq.kg )

0.3028

1.0000

-0.2251

0.3134

0.0758

0.6750

232

Th content (Bq.kg-1)

0.0771

-0.2251

1.0000

0.4728

-

-

K content (Bq.kg-1)

-0.3544

0.3134

0.4728

1.0000

-

-

Emanation coefficient

0.2169

0.0758

-

-

1.0000

0.2420

Radon concentration

0.4991

0.6750

-

-

0.2420

1.0000

40

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Figure captions: Fig. 1: Location map of the sampling points.

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Fig. 2: Correlation between radon emanation coefficient and soil grain size.

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Fig.1: Location map of the sampling point

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0.1

0.0 20

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Rn emanation coefficient

0.5

Pearson Product Moment Correlation R = 0.81 P-value <<0.05

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Amount of soil particles for less than 0.1 mm in diameter (%)

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Fig. 2: Correlation between radon emanation coefficient and soil grain size

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ACCEPTED MANUSCRIPT Highlights Natural radioactivity and radon emanation coefficient in soil were determined.

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The 226Ra/238U disequilibrium implied specific geochemical processes.

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Correlations among radionuclides contents and soil characteristics were identified

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A novel method for determining radon emanation coefficient in LAB was described.

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Radon emanation coefficient depends on grain sizes and is independent on the radium content.

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