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ORIGINAL ARTICLE
Year : 2020  |  Volume : 43  |  Issue : 3  |  Page : 179-184  

Radium content and radon exhalation rates in Egyptian soil samples using active and passive techniques


Department of Security Sciences, Nuclear Security Program, Naif Arab University for Security Sciences, Riyadh, KSA; Nuclear Power Plants Authority, Cairo, Egypt

Date of Submission24-Jul-2020
Date of Decision05-Oct-2020
Date of Acceptance23-Nov-2020
Date of Web Publication6-Jan-2021

Correspondence Address:
Ahmed Saad Omar
Department of Security Sciences, Nuclear Security Program, Naif Arab University for Security Sciences, Riyadh

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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_39_20

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  Abstract 


In this study, we look at radium content in soil samples collected from different locations in Egypt which have been measured using active gamma ray spectrometry with high-purity germanium (HPGe) detector and the passive sealed cup technique with LR-115 nuclear track detectors. Furthermore, the radon exhalation rates (mass and area) were measured using passive technique with LR-115 detectors. This investigation was undertaken to evaluate the possible health risks posed by the elements in question. Radium content values were found to vary from 20.83 to 47.57 Bq/kg with an average value of 32.46 ± 7.75 Bq/kg and 17.30 to 42.70 Bq/kg with an average 29.15 ± 6.75 Bq/kg using HPGe and LR-115 detectors, respectively. Area (surface) exhalation rate values were found to vary from 2.88 × 10-6 to 8.53 × 10-6 Bq/m2/h with an average value of 5.75 × 10-6 Bq /m2/h. Mass exhalation rate values were found to vary from 42.9 × 10-9 to 128 × 10-9 Bq/kg/h with an average value of 78.7 × 10-9 Bq/kg/h. All the results obtained in this particular study were found to be less than their corresponding world limits. Overall, the present results have revealed that radium content and both area and mass exhalation rates in the studied area do not pose a risk to human health. The results were compared nationally and with various other countries.

Keywords: Egypt, exhalation rates, HPGe detector, LR-115 detectors, radium content, soil


How to cite this article:
Omar AS. Radium content and radon exhalation rates in Egyptian soil samples using active and passive techniques. Radiat Prot Environ 2020;43:179-84

How to cite this URL:
Omar AS. Radium content and radon exhalation rates in Egyptian soil samples using active and passive techniques. Radiat Prot Environ [serial online] 2020 [cited 2021 Apr 18];43:179-84. Available from: https://www.rpe.org.in/text.asp?2020/43/3/179/306280




  Introduction Top


Radium (226Ra) and radon (222Rn) mainly come from naturally occurring uranium (238U), which is present in all types of rocks, soil, building materials, and ground water. The radium content of a sample contributes to the level of environmental radon as radon is produced from 226Ra through α-decay. Higher values of 226Ra in soil contribute significantly to the enhancement of indoor radon. The main contributors to indoor radon concentrations are soil gas emanating from the ground beneath a dwelling and the materials from which the dwelling is constructed.[1] Entry of radon into a dwelling from the soil is influenced by a number of parameters. They include the concentration of radon in the soil gas itself, soil moisture, soil permeability, and environmental and meteorological conditions in the vicinity of the dwelling.[2]

The dose due to inhaled radon and its progeny accounts for more than 50% of the total radiation dose to the public from natural sources.[3] Radon is assumed to be an important cause of lung cancer after smoking.[4]

Radon exhalation from soils may enable the estimation of indoor level in case of using it in manufacturing of building materials. Radon exhalation is a complex phenomenon depending on a number of parameters such as radium content in sand, sand morphology, sand moisture, sand grain size, temperature, atmospheric pressure, and rainfall.[5] Exhalation designates the escape of radon from a material to the atmosphere. In the sand, radon molecules can escape from grain of sand by diffusion or recoil into the sand pores; this process is called emanation. The number of radon atoms released per unit area per unit time from the material is termed as exhalation rate.[6],[7]

The importance of this study is to determine the level of radium in soil samples collected from 12 cities in Egypt, shown in [Figure 1], using the active γ-spectrometry with High-purity Germanium HPGe detector and the passive with LR-115 detectors in a sealed cup. There are many published studies on the concentrations of radium in soil in Egypt. However, this study is characterized by measurements for the first time from regions of El-Kargha, Marsa Matroh, and Siwa Oasis.
Figure 1: The map of Egypt shown the studied regions

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  Materials and Methods Top


A total of 72 soil samples were collected from twelve cities in Egypt, 6 samples from each region, as shown in [Figure 1]. Measurement of radium content in the soil samples collected from the study locations were carried out using active as well as passive techniques. High-purity germanium (HPGe) detector was used as active technique, whereas LR-115 detector based on sealed cup method was used as passive technique. In addition to this, the passive technique was used to calculate radon exhalation rates from these soil samples.

Active technique

The samples were collected to a depth of 5 cm using a coring tool that was thoroughly cleaned and dried before each sample was collected. The samples were brought to the laboratory for further processing and were processed using standard procedures. The samples were kept in an air circulated hot air oven to remove the moisture content in the samples. The samples were then stored in 1 L Marinelli beakers for a period of time not less than 1 month for secular equilibrium of naturally radioactive decay series, where the decay rate of the daughters became equal to that of the parents.[8] Marinelli beakers 1 L were placed in a high-purity germanium (HPGe) detector coupled to 8192 multichannel CANBERA analyzer for data acquisition. The used detector is P type and has an efficiency of 30% and measured for a counting time of 18 h. Measurements were repeated triple times to minimize standard error. For efficiency calibration, 226Ra point source in two geometry (Top and side) was used to give a broad spectrum (186 keV up to 2.45 MeV). The activity of 226Ra was evaluated from the γ–ray lines 295.21, 351.9 keV (214Pb), 609.3,1120, and 1764.1 keV (214Bi). This spectral analysis was performed with the aid of the computer software Genie 2000 (Genie 2000 Basic Spectroscopy Software, CANBERRA). The system was calibrated for energy calibration using Cs-137 (661.76 keV), Co-60 (1173.21,1332 keV), and Ba-133 (356, 284, 276.61 keV) sources. The environmental γ-ray background of the laboratory was determined using an empty Marinelli beaker under identical measured conditions. The detector is surrounded by a lead shield to reduce the background of the system.

Passive technique

In the present investigation, “sealed cup technique” was used to study the radium content and radon exhalation rates.[1],[5],[9],[10] Different surface soil samples were collected from twelve cities in Egypt as shown in [Figure 1]. The samples were dried and saved at room temperature to 1 mm grain size, each sample divided into five equal volumes, weighed, and then placed in cylindrical containers made of aluminum. Each sample container was capped tightly to an inverted cylindrical cup of 3.5 cm radius and 11 cm height. A piece of LR-115 type II (Kodak Pathé, France) detector with area 1.5 cm2 was fixed at the top center of the inverted cup. The experimental arrangement is shown in [Figure 2]. The cups were left undisturbed at room temperature for a 3 month exposure time. During this period, α-particles from the decay of radon bombarded the LR-115 detector inside the inverted cup through the α-decay of radium contents of the samples. This device is called radon only device due to the configuration of the measuring cup and the chemical etching conditions of the irradiated detectors. First, concerning 222Rn and its daughters, LR-115 detector is free from self-plate out effect. Thus, it does not record tracks due to self-plating daughters, α-particles of energy 6 MeV (218Po) and 7.69 MeV (214Po).[1],[7],[11],[12],[13] Second, concerning 220Rn and its daughters, the distance of the LR-115 detector to the top surface of the soil sample causes negligible contribution of the 220Rn gas to the track density because it decays before reaching the effective volume of the detector. The distribution of 220Rn atoms exhaling from soil surface decreases exponentially with distance inside the measuring cup. It is known that the diffusion length of 220Rn atoms is approximately 2–3 cm in air compared to the diffusion length of 222Rn atoms witch approximately 239 cm. Thus, more than 90% of the exhaled 222Rn atoms could reach the detector. The first 220Rn progeny (216Po) behaves as its progenitor due to its very short half-life. The rest of the 220Rn progeny are almost completely deposited on the inert surface of the measuring cup due to plate-out effect.[1],[12],[14] After the irradiation period, the bombarded detectors were collected and chemically etched in 2.5 M NaOH solution at 60°C for duration of 2 h. The etching was carried out to reduce the thickness of the LR-115 detectors to about 5 μm.[10],[12] Subsequently, α-tracks were counted using an optical microscope. The diffusion cups were calibrated at the National Institute for Measurements and Standards, Cairo Egypt. During the calibration process, the cups were placed inside a radon chamber with a concentration of 17.4 ± 0.5 kBq/m3 for different exposure times.
Figure 2: Schematic diagram of the passive radon device

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The radon activity concentration (CRn) is calculated using the equation: [9,11-13]



Where η the track density (tracks cm-2), η the sensitivity factor of LR-115 detector (tracks cm-2/d/Bq/m3), and t the exposure time (d). The value of η depends on the height and radius of the measuring cylinder cup.[12] An effective equilibrium (about 98%) for radon members of the decay series is reached in about 30 days.[15] Once the radioactive equilibrium is established, one may use the radon alpha analyses for the determination of steady-state activity of radium. The radium content (CRa) is calculated using the following equations:[1],[5],[6],[7],[9],[12],[16],[17]





Where γRn is the radon decay constant (d-1), h is the distance between the detector and the top of the soil samples (m), A is the area of cross section of the cylindrical cup (m2), M is the mass of the soil samples (kg), and Te is effective exposure time given by:



The mass exhalation rate (EM) of the sample release of the radon can be calculated using the expression:[5]



Where ηRa is radium decay constant (d-1).

The surface exhalation rate (EA) of the sample for release of radon can be calculated using the expression:[5]




  Results and Discussion Top


The calibration coefficient for LR-115 nuclear trach detector obtained from the calibration experiment is 0.036±0.006 a-tracks cm-2 d-1 per Bq m-3 of radon. -tracks cm-2 d-1 per Bq m-3. This value is in good agreement with that reported by other investigators.[1],[7],[11],[12],[13]

[Table 1] represents the values of radium content for surface soil samples collected from 12 cities in Egypt. Radium content values were found to vary from 20.83 to 47.57 Bq/kg with an average value of 32.46 ± 7.75 Bq/kg and 17.30 to 42.70 Bq/kg with an average 29.15 ± 6.75 Bq/kg using HPGe and LR-115 detectors, respectively. The obtained results are within the range (5–64 Bq/kg) of UNSCEAR 2000 survey of natural radionuclide content in Egyptian soil.[3]

As also shown in [Table 1], a comparison between the passive and active techniques used was made and found a ratio between them at approximately 0.91, which shows a pleasant agreement between the two techniques. From these results, we can conclude that the passive sealed cup technique with LR-115 detector is suitable for radium content and radon exhalation rate measurements. Moreover, this passive technique has many advantages such as its simplicity and cost-effectiveness and its ability to be used for a long integrated time to measure low radon concentration materials.
Table 1: Active and passive measurement of radium content in different surface soil samples in Egypt

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The average value of radium content obtained from this study is in an agreement with the world value of 35 Bq/kg as reported in UNSCEAR 2000[3] and much lower than the maximum permissible value of 370 Bq/kg for building materials recommended by the OECD 1979[18] and the UNSCRAE 2000.[3] Thus, results revealed that the surface soil samples analyzed are radiologically safe and can be used as building materials without posing a significant radiological threat to the population.

[Table 2] shows the radon exhalation rates (mass and surface) from soil samples collected from different locations in Egypt. Area (surface) exhalation rate values were found to vary from 2.88 × 10-6 to 8.53 × 10-6 Bq/m2/h with an average value of 5.75 × 10-6 Bq/m2/h. Mass exhalation rate values were found to vary from 42.9 × 10-9 to 128 × 10-9 Bq/kg/h with an average value of 78.7 × 10-9 Bq/kg/h. It can be observed from the results that the radon exhalation rate varied appreciably from one sample to another. The highest radium concentration in soil was found at Aswan. The mass exhalation rate and surface exhalation rate in soil were found to be high at Aswan. This is due to the fact that the area is rich in granite stones and the presence of many phosphate fertilizer factories.[19] The variation may be due to the geological condition of location and geochemical process in soil.[20] Hence, it is clear that there is a positive correlation between mass exhalation and surface exhalation rates of radon with radium content in soils samples. The surface radon exhalation in these samples was found to be greatly lower than the average world value of 57.6 Bq/m2/h.[21]
Table 2: Radon exhalation rates (mass and surface) in soil samples from different locations in Egypt

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[Table 3] shows the radium content (CRa) in surface soil samples from this study in comparison to these values in Egypt and other countries using different techniques. The [Table 3] shows the compatibility of the values from this study with the measured values in Egypt and in many regions of the world except in Cameroon,[18] Ethiopia,[19] India,[20] and Malaysia.[21] This is because these regions are characterized by a high level of natural background radiation.
Table 3: Comparison of current results of radium content values with research published for surface soil samples in Egypt and other countries

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  Conclusions Top


Radium content and radon exhalation rates (both the mass and surface exhalation rates) have been measured successfully using HPGe detector and LR-115 detectors using the sealed cup technique. The sealed cup technique is a passive and convenient useful tool for determining the radon exhalation rates as well as the radium contents in some surface soil samples from twelve cities in Egypt. The results revealed that the studied regions are safe as far as the health hazards of radium are concerned. It is possible to establish a national database of surface soil samples using low-cost passive techniques with large-scale screening measurements. The measured values were compared nationally and with various other countries.

Financial support and sponsorship

Nil.

Conflict of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

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