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 Table of Contents  
ORIGINAL ARTICLE
Year : 2019  |  Volume : 10  |  Issue : 1  |  Page : 66-71

Investigation of radon concentrations and effective radium content in soils and dwellings of Wolaita Sodo Town, Ethiopia


1 Department of Physics, College of Natural Science, Wollo University, Wollo, Ethiopia
2 Department of Geology, School of Earth Sciences, Bahir Dar University, Bahir Dar, Ethiopia
3 Department of Physics, College of Natural and Computational Science, Wolaita Sodo University, Wolaita Sodo, Ethiopia

Date of Web Publication22-May-2019

Correspondence Address:
Mr. Lingerew Nebere
Department of Geology, School of Earth Sciences, Bahir Dar University, Bahir Dar
Ethiopia
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jrcr.jrcr_20_18

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  Abstract 


Background: People in the world have always been exposed continuously to natural radionuclides originated from the Earth's crust. Radionuclide of terrestrial origin like radon is the most abundant in the environment and the second leading cancer risk next to cigarette. Objectives: The purpose of this study was to investigate the radon concentration and effective radium content in soil samples and dwellings of Wolaita Sodo town, Ethiopia. Materials and Methods: A total of 35 soil samples and 24 records in dwellings were collected. The radon concentrations and effective radium content in soil samples and dwellings were estimated using LR-115 plastic track detector and alpha spectroscopy, respectively. Results: The result shows the concentrations of radon gas and effective radium content in soil samples ranges from 74.74 to 436.97 Bq/m3 with an average value of 221.4 Bq/m3 and 89.30–522.07 Bq/kg with an average value of 264.54 Bq/kg respectively. While radon in dwellings ranges from 30.78 to 708.12 Bq/m3 with an average value of 236.72 Bq/m3. Conclusion: Generally, the average radon concentration both in soil samples and dwellings of the area was higher than the recommended level by the International Commission on Radiation Protection.

Keywords: Alpha spectroscopy, inhalation dose, LR-115, radium content, radon concentration


How to cite this article:
Maregu N, Nebere L, Abeje N, Dessalegn B, Yibka T. Investigation of radon concentrations and effective radium content in soils and dwellings of Wolaita Sodo Town, Ethiopia. J Radiat Cancer Res 2019;10:66-71

How to cite this URL:
Maregu N, Nebere L, Abeje N, Dessalegn B, Yibka T. Investigation of radon concentrations and effective radium content in soils and dwellings of Wolaita Sodo Town, Ethiopia. J Radiat Cancer Res [serial online] 2019 [cited 2019 Dec 7];10:66-71. Available from: http://www.journalrcr.org/text.asp?2019/10/1/66/258714




  Introduction Top


Minerals containing uranium (U) and thorium (Th) within soils and rocks are the primary sources of terrestrial radiation.[1] The radon produced in soils or rocks, due to the presence of the parent nuclei, migrates freely through pore spaces and weak zones.[2] The concentration level of radon (222 Rn) within the environment mainly depends on the local geology.[2] It diluted to harmless concentrations when it surfaces in open air, but when it enters an enclosed space, it can sometimes accumulate to relatively high concentrations.[3],[4],[5],[6] It is the decay products of uranium (238 U) series; uranium (238 U) decays to radium (226 Ra) which decays to radon (222 Rn).

Soils and rocks are the main components of buildings,[1] and these materials either may contain long half-life radioelements which are able to produce radon gas or serve as a passage of radon to indoors. Therefore, people are mostly exposed to radiation via soils and building materials,[7] by inhalation and ingestion of radon progenies. When radon gas is inhaled, densely ionizing alpha particles emitted by short-lived decay products of radon (218 Po and 214 Po) cause lung cancer [5],[6],[7],[8],[9] and cause stomach cancer when it is ingested through foods.[5],[9],[10]

As evidenced from different literatures, the radiological effect of radon gas takes the upper hand relative to other radiation effects.[11],[12],[13] It is the first for nonsmokers causing lung cancer in the world.[6] This study is conducted on a particular area, Wolaita Sodo, Ethiopia. It is located [Figure 1] in the Southern Nations, Nationalities, and Peoples Region of Ethiopia at elevation ranges from 1200 to 2100 m above sea level. It has highly undulated topography. The Eastern side of the zone has relatively low altitude, and it is part of the Main Ethiopian Rift, whereas its Western side is part of Omo Basin having irregular topography.
Figure 1: Location map of the study area

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The objective of this study is to determine the effective radium content and radon concentration in soil samples and dwellings and also to assess the radiological effects. Some part of the area of study is located within the Main Ethiopian Rift where existence of week zones is probably high. Thus, the radon gas flow easily and increase the exhalation rate.

Therefore, the area needs detail study of the radiological effect of natural radioactive elements and their decay products. This study will be very important in the view of radiation protection and reference for future studies.


  Materials and Methods Top


The content of natural radioactive nuclides, i.e., uranium and thorium in subsurface rocks, and the suitability of subsurface materials for the passage of radium decay products increase the concentration of radon (222 Rn) in soil and thereby dwellings.[14] Therefore, the concentration of radon gas from subsurface material (i.e., soils) can be measured based on the effective radium content,[6],[15]

Thirty-five soil samples were collected randomly from different places within the study area, and each sample was collected from an average depth of 30 cm from the surface. The stones and roots were removed from the samples, and each sample was dried and grinded in an oven at a temperature of 110°C for 24 h to remove all the moisture content. After grinding, samples were sieved to obtain the fine quality of them. It was placed at the bottom of a leak-proof plastic can (“can techniques”) of 10 cm height and 3.5 cm radius, which is kept closed for 1 month to get equilibrium between radium and radon progeny. After that, soil samples were exposed to LR-115 plastic track detector, 2 cm × 2 cm size, which is fastening on the inside of the mouth of the can face to the sample to register the alpha radiation. The detector would record all the tracks of alpha particles resulting from the decay of radon in the whole volume of the can. The concentration of radon increases with time t elapsed according to the relation;



where CRn is the concentration of radon, CRa is the effective radium content of the given sample (Bq/kg), λRn is the decay constant for radon (hour − 1), T is the actual exposure time (hour).

Therefore, the effective radium content is the source term for radon release into the nearby environment.[16],[17]

After the exposure period, the detectors were etched for 120 min in 2.5 N NaOH at 60o C. Therefore, alpha tracks were counted using optical microscope. A plastic track detector, however, measures the time integral of equation (1), i.e., the total number of alpha-disintegration in unit volume of the can with sensitivity K, during the exposure time t. Therefore, the track density observed is given by the equation;[18]



Where ρ is the track density, K is the sensitivity factor, and its value is 0.0312 tracks m -2/day/Bq/m 3, te is effective time, and cRa is the effective radium content. The effective exposure time is calculated using the following equation,[19]



The effective radium content cRa can be calculated using the following equation;[11]



Where h is the distance between detector and surface of the sample, A is the area cross-section of the cylindrical can, and M is the mass of the sample.

The mass and surface exhalation rate of radon gas have been estimated based on equation (5) and (6);[20],[21],[22]





Where λRa is the decay constant for radium, λRn is the decay constant of radon, Te is effective exposure time.

From the measured radon concentration in soil samples and dwellings, the inhalation dose has been calculated in (mSv.y −1) using equation (7);[23]



Where n is a constant which is equal to 0.009 (mSv.m 3/Bq.y).

Moreover, to assess the exposure level, the alpha indices were determined using equation (8).[24]



The measurements of indoor radon were performed using alpha spectroscopy detection method, Corentium digital radon detector, in 24 dwellings for 3 months. The detector can record the tracks of alpha particles emitted by radon and its short-lived decay products present in the ambient air, typically 218 Po and 214 Po which generally attach them to the aerosols.


  Results and Discussion Top


Radon concentration in soil samples and dwellings

The measured values of radon concentration in 35 soil samples and 24 dwellings collected in Wolaita Sodo town, Ethiopia, are tabulated in [Table 1] and [Table 2]. As shown from the table, the radon concentration in soil samples has a wide range of variation. It ranges from 74.74 to 436.97 Bq/m 3 with an average value of 221.418 Bq/m 3. About 51.42% of the soil sample radon concentration are higher than the world action level of 200 Bq/m 3.[25] The variation in concentration of Radon in dwellings is wide too. It ranges from 30.78 to 708.12 Bq/m 3 with an average value of 236.72 Bq/m 3. This wide range is perhaps the result of many factors such as living style, construction materials, ventilation, environmental conditions, and difference in concentration of radioactive elements in soils,[12],[13] and as a result usually, average concentrations in soil samples are less than the concentration of radon in dwellings.[26] About 50% of the dwellings radon concentration are above the world action level of 200Bq/m 3, and it is also higher than the worldwide average of 39 Bq/m 3.[24]
Table 1: Radon concentration, effective radium content, mass, and surface exhalation rate of soil samples

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Table 2: Indoor radon concentration, inhalation dose, and alpha index of the dwellings of Wolaita Sodo town, Ethiopia

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In [Table 3] and [Table 4], a comparison of radon concentration in soil samples and dwellings of this work with other countries is presented. From which it can be observed that soil sample radon concentration is less than studies made at other countries, i.e., India and Iraq [Table 3]. However, the indoor radon concentration of this study is comparably less than the study made by Khan et al.[30] and greater than the study made by Maghraby et al.[5],[13]
Table 3: Comparison of radon concentration in soil samples from Wolaita Sodo town, Ethiopia and previous works from other countries

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Table 4: Comparison of radon concentration in dwellings from Wolaita Sodo town, Ethiopia and previous works from other countries

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Effective radium content and radon exhalation rates

The effective exposure time of soil samples, sensitivity factor, mass of the sample and dimension of the cylindrical have been used to compute the effective radium content and the radon exhalation rates of soil samples, and the results are given in [Table 1]. Effective radium content of soil samples ranges from 89.30 to 522.07 Bq/kg with an average value of 264.54 Bq/kg, radon mass exhalation ranges from 1.1 to 9.9 Bq/kg 2/day with an average value of 4.76 Bq/kg 2/day, and radon surface exhalation ranges from 1.3 to 7.8 Bq/m 2/day with an average value of 3.97 Bq/m/day [Figure 2].
Figure 2: Radon mass and surface exhalation rates

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In [Table 5], a comparison of effective radium content in soil samples with other countries is presented. The effective radium content of this study is higher than other previous works.[28],[31]
Table 5: Comparison of effective radium content value in soil samples from Wolaita Sodo town, Ethiopia, with those reported from different countries

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Inhalation dose and alpha index

As recommended worldwide,[15],[25] average value of inhalation dose is 0.4 mSv/y. The inhalation dose computed from soil samples is found to be greater than this average value. It ranges from 0.067 to 2.67 mSv/y with an average value of 0.99 mSv/y [Figure 3]. The calculated indoor inhalation dose is also greater than the recommended value. As compared to the inhalation dose resulted from soil samples, the indoor inhalation dose is higher. Perhaps, the high value of indoor inhalation dose may be due to the effect of construction materials, subsurface natural radon concentration, and the living style of the community.
Figure 3: Indoor radon inhalation dose (a), soil sample radon inhalation dose (b)

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  Conclusion Top


Effective radium content and radon concentration in soil samples and dwellings of Wolaita Sodo town, Ethiopia, has been investigated for the first time. The average radon concentration both in soil samples and dwellings of the area was higher than the recommended level by the International Commission on Radiation Protection. Thus, the inhalation dose is also greater than the worldwide recommended value. Dwellings radon concentration was higher than radon concentration in soil samples. This may be due to the effect of construction materials and availability of ventilation in addition to the content of radionuclides in the subsurface. The results highlight that the need to consider the effect of natural radioactive nuclides in the area.

Financial support and sponsorship

This research was conducted with the support of Wolaita Sodo University. We acknowledge Wolaita Sodo zone soil laboratory staffs for their support during laboratory work. Authors would like to thank the anonymous reviewer and the editor for their reviews and comments.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

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

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]



 

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