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Radon exhalation from Libyan soil samples measured with the SSNTD technique
Applied Radiation and Isotopes 72 (2013) 163–168
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Radon exhalation from Libyan soil samples measured with the SSNTD technique A.F. Saad n,1, R.M. Abdallah, N.A. Hussein Physics Department, Faculty of Science, University of Benghazi, Benghazi, Libya
H I G H L I G H T S c c c c
Radon exhalation was measured in Libyan soil samples by the SSNTD technique. The results indicate mostly normal levels of radon concentration. The mean radon concentration is 220.3 7 7.4 and 325.5 7 10.9 Bq m 3 for Benghazi and Al-Marj cities, respectively. We evaluated radon exhalation rates, the radium content and annual effective doses.
a r t i c l e i n f o
abstract
Article history: Received 12 June 2012 Received in revised form 28 September 2012 Accepted 5 November 2012 Available online 17 November 2012
Radon concentrations in soil samples collected from the cities of Benghazi and Al-Marj, located in northeastern Libya, were measured using the sealed-can technique based on the CR-39 SSNTDs. Mass and areal radon exhalation rates, radium content and radon concentration contribute to indoor radon, and annual effective doses were determined. The results indicate mostly normal rates, but there were some higher levels of radon concentration and emanation in samples collected from Al-Marj and one sample from Benghazi. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Radon exhalation rates Radium content CR-39 NTDs Can technique Soil Annual effective dose
1. Introduction The radioactive elements found naturally on earth can be classified into three series: 238U, 235U and 232Th. These radioactive, toxic elements are found in traces in almost all types of soil, rock, construction materials, water and air. People are exposed to ionizing radiation from natural sources because of the occurrence of natural radioactive elements in the earth’s environment. 222Rn is present in nature as the only gas found in the natural 238U radioactive decay series. It is a direct progeny of 226 Ra, which is the heaviest gaseous element and has a half-life of 3.82 days. Two other isotopes of radon exist in nature; they are members of the 232Th and 235U natural radioactive decay series, 220 Rn (thoron) and 219Rn (actinon), respectively. In a closed system, radon, in about 27 days (i.e. about seven half-lives of n
Corresponding author. Tel.: þ218 92 8128608; fax: þ218 61 2229617. E-mail address: [email protected] (A.F. Saad). 1 On leave from: Physics Department, Faculty of Science, Zagazig University, Zagazig, Egypt. 0969-8043/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2012.11.006
radon), comes into secular equilibrium with its long-lived (T1/2 ¼ 1600 years) immediate precursor, 226Ra. Radon emanation is linked with the presence of radium and its ultimate source, uranium, in the earth. These radioactive elements occur in virtually all soils, rocks, water and air; their concentrations vary with the geology of specific regions. Radium, a decay product of uranium in the naturally-occurring uranium series, contained in material lying close to the surface of the earth can generate a high radon exhalation rate, which pose hazards due to high radon exposure. Radon concentration in soil varies greatly from one site to another. Once the radon atoms are formed by the decay of the parent 226Ra, they move either by diffusion or by transport mechanisms or by both Mogro-campero and Flaischer (1977). Radon concentrations in soil pores at depth are directly dependent upon the radium content of the soil and soil parameters. Exhalation designates the escape of radon into the atmosphere from the rock and soil surfaces around the globe. Radon exhalation from soil, in particular, contributes to the indoor radiation environment. In this regard, studies on soil and building materials
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made of soil and rock have been carried out in different parts of the world, and much data is available in the literature. This literature on exhalation of radon from soil and building materials has now become quite voluminous and has brought new impetus to the field of radon physics (Ching-Jiang et al., 1993; Al-Jarallah et al., 2001; Al-Jarallah, 2001; Vukotich et al., 2002; Saad et al., 2002; Kumar et al., 2003, 2005, 2008; Duenas et al., 2007; Mudd, 2008; Singh et al., 2008; Rafique et al., 2011; Appleton et al., 2011). However, not much investigation has been carried out on radon exhalation from soil and building materials in the northeastern region of Libya (Saad et al., 2010). Consequently, we have proposed a plan to provide a wide spectrum of systematic study to determine conclusively and clearly the role of environmental radioactivity in that area of our country. For this purpose, in the present study, the method of measuring radon exhalation called the sealed-can technique (Fleischer et al., 1980; Abu-Jarad et al., 1980; Somogyi et al., 1984) based on CR-39 NTDs was used. Soil radon measurements were carried out in the districts of two cities in northeastern Libya. We should mention that this is the first time that such work has been carried out in this study area. The annual effective doses based on radon concentrations of soil were also determined to assess the health hazard to inhabitants in their dwellings.
2. Geography and geology of the study area The studied area included the cities of Benghazi and Al-Marj, both located in the northeastern region of Libya. Benghazi is the second largest city in Libya after the capital Tripoli. Benghazi is positioned on the Mediterranean sea between latitudes 20.71 and 32.121N as shown in Fig. 1a. The city is laid out in a radial pattern with the city’s lake at its center. Benghazi’s soil consists mainly of lithosol, reddish brownarid and salinesaland solonchoks located in certain areas of the city and its surroundings. The reddish brownarid has thicknesses ranging from a few centimeters to about one meter outside the valley area in the south of the city, and the thickness increases to 6–7 m in the valley area southeast of the city. The lithosol occurs sporadically, but it is found mainly at the foot of the hills; it is easily affected by erosion. The salinesaland solonchoks are found along the western coastal area and are characterized by high percentages of dissolved salts. Al-Marj is one of the oldest cities in Libya. It is situated in the green mountain area on the northeast side of the country at latitudes of 20.831 and 32.51N as shown in Fig. 1b. Al-Marj is in a plain area surrounded by hills on all sides. It is known for its fertile land rich in iron, and the inhabitants engage in agriculture and stock breeding. Al-Marj’s soil is characterized by the ferrosiallitic known as ‘‘Terra Rosa’’, which is rich in iron oxides and has less than 15% calcium carbonates. The ‘‘Terra Rosa’’ is located at depths of 40–60 cm and is capable of retaining water. There is also ‘‘the valley soil’’ found on the banks of valleys in Al-Marj basin. This type of soil varies in characteristics according surface topography. It is deep in the valley precipitation area and is affected by erosion when it rains. This soil is soft and deep and covers an area of about 7% of the Al-Marj area.
3. Materials and methods 50 soil samples were collected from Benghazi and Al-Marj, 25 from each (Fig. 1). The samples were all collected in summer 2009 and were dried in an oven at a temperature of 10571 1C for 24 h to remove moisture and then sieved through a 1 mm-mesh sieve. Each sample (0.5 kg) was placed in a cylindrical stainless-steel container of radius 7.35 cm, length 14.8 cm and volume 2.513 10 3 m3.
The dried soil was in the form of a porous, powdery material. It was gently pressed to form a disk-like shape, which permitted radon to diffuse out of this host material with a high degree of homogeneity. The thickness of each disk-like sample in the emanation container was about 2 cm. The emanation container was sealed tightly to prevent the escape of radon and was kept sealed for 3 months to obtain good statistics, after which it was ready for a-particle measurements using a CR-39 nuclear track detector (NTD). The concentration and exhalation rate of radon can be made using CR-39 detectors because of their ability to register tracks at different levels of registration sensitivity. The CR-39 detectors used in this investigation were purchased from Intercast Europe Co., Parama, Italy, in the form of small sheets, which were cut into 1.5 1.5 cm2 pieces. The emanation container was sealed with silicone and stored for 90 days. The film was then exposed to radon and its daughters in the chamber for a known period of time. The exposure of the detectors, followed by etching of the tracks in the film, provided the concentrations of radon and its daughters. The etching of the CR-39 detectors was carried out with 6.25 M NaOH at 70 1C for 8 h. After etching, the detectors were washed in distilled water and dried with fresh air. We counted the tracks with an optical microscope at 400 magnification. The number of tracks in 100 fields was scanned for each detector to determine the track density per cm2. A calibration factor of 0.239 70.008 tracks cm 2(Bq m 3) 1, obtained from an earlier calibration of the CR-39 track detector (Saad, 2008), was used to compute the radon activity from the track density.
4. Theoretical considerations 4.1. Calculation of radon exhalation rates The radon level of a soil sample placed in an emanation container can be monitored with a passive radon dosimeter based on the CR-39 solid-state nuclear track detector SSNTD. The radon concentration in the emanation container was measured hourly for a long period (90 days). The radon concentration in the closed can reached secular equilibrium within several half-lives of radon. It should be noted that the radon exhalation rate is constant, whereas the radon concentration reaches a maximum value that depends on the exhalation rate. The passive radon detector can be used to measure the accumulated activity and then to evaluate the average radon exhalation rate from the soil using the following equations (Fleischer and Mogro-campero, 1978; Khan et al., 1992; Kumar et al., 2005, 2008; Mahur et al., 2008a, 2008b; Singh et al., 2008; Rafique et al., 2011): EA ¼
C Rn lV A½T þ 1=lRn elRn T 1 Þ
ð1Þ
EM ¼
C Rn lV M½T þ 1=lRn elRn T 1 Þ
ð2Þ
where EM is the mass exhalation rate (mBq kg 1 h 1), EA is the areal exhalation rate (mBq m 2h 1), CRn is the integrated radon exposure (Bq m 3), A is the total surface area of the soil sample from which radon is exhaled (m2), V is the empty volume of the emanation container or can (m 3), M is the mass of the soil sample (kg), l is the decay constant of radon (h 1), and T is the time since sealing (h). 4.2. Estimation of the effective radium content The effective radium content in the soil samples was determined using a track detection method (Somogyi, 1986). Once radioactive equilibrium between radium and radon is established,
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Fig. 1. Map showing the study areas in (a) Benghazi and (b) Al-Marj. The numbers refer to the locations where samples were collected, as listed in Tables 1 and 2.
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one may use the analysis of the radon alpha activity for the estimation of the steady-state activity concentration of radium. After the sealing of the can, the activity concentration of radon begins to increase with time T as follows:
internal surface area and volume of the room, respectively, and the air exchange rate lV is 0.5 h 1. The annual effective dose equivalent Ep is then related to the average radon concentration CRn and is given by the following formula:
C Rn ¼ C Ra ð1elRn T Þ
8760nFC Rn EP WLM y1 ¼ 170 3700
ð3Þ
where CRa is the effective radium content of the soil sample. This effective radium content of a specimen in a sealed-can can be estimated as: C Ra Bq kg1 ¼ r=kT e hA=M ð4Þ where h is the distance between the detector and the top of the solid sample in m, r is the counted track density, k is the calibration factor of the CR-39 track detector, and Te denotes the effective exposure time given by T e ¼ T1=lRn ð1elRn T Þ
ð5Þ
ð7Þ
where, CRn is in Bq m 3, n is the fraction of time spent indoors, F is the equilibrium factor, 8760 is the number of hours per year, and 170 is the number of hours per working month. The values of n¼0.8 and F¼0.42 were used to calculate Ep. The effective dose equivalents from radon exposure were estimated by using a conversion factor of 6.3 mSv/WLM (International Commission on Radiological Protection (ICRP), 1987).
5. Results and discussion
The exposure time in these measurements was 90 days. 4.3. Calculation of annual effective dose The risk of lung cancer from domestic exposure due to radon and its daughters can be computed directly from the effective dose equivalents. The radiation hazards due to radon and its daughters are calculated from the radon exhalation rate of soil samples. The contribution of indoor radon concentration from soil can be calculated from the following formula (Nazaroff and Nero, 1988; Mahur et al., 2008a, 2008b; Saad et al., 2010): C Rn ¼
Ex Sr V r lv
ð6Þ
where CRn is the radon concentration in soil contributing to indoor radon (Bq m 3), EX is the radon exhalation rate (Bq m 2 h 1), Vr is the room volume (m 3) and lV is the air exchange rate (h 1). In these calculations, the maximum radon concentration from building materials was assessed by assuming the room to be a cavity with the ratio Sr/Vr ¼2.0 m 1, where Sr and Vr are the
In the current investigation, all houses are built on the earth’s crust, which is composed basically of soil and rocks, and most of the houses in the studied area are built of blocks made of sand and cement as well as gravel aggregates. Tables 1 and 2 show the values of radon activity, the radon exhalation rate in terms of area and mass, the radium equivalent content, the radon concentration contribution to indoor radon and the annual effective dose equivalent for soil samples collected from Benghazi and Al-Marj. The variations (minimum, maximum, range and mean values) of radon concentrations (Bq kg 1) and radon exhalation rates in terms of area and mass, together with the statistical uncertainty (1s) and standard deviation (SD), are presented in Table 3. Radon activity concentrations in soil samples collected from Benghazi are found to vary from 31.171.0 to 469.0 Bq m 3, with a mean, standard deviation and range of 220.3, 105.6 and 437.8 Bq m 3, respectively. Those from Al-Marj vary from 59.37 2.0 to 515.8717.3 Bq m 3, with a mean, standard deviation and range of 325.5, 117.3 and 456.5 Bq m 3, respectively. The radon exhalation rate in Benghazi varies from 30.6071.02 to
Table 1 Measured radon concentration, radon exhalation rates, radon concentration contribution to indoor radon, annual effective dose equivalent, and radium equivalent content of soil samples collected from different sites in Benghazi, Libya. B stands for Benghazi. Sample number/ Benghazi
Radon concentration (Bq m 3)
Areal radon exhalation rate (mBq m 2 h 1)
Mass radon exhalation rate (mBq kg 1 h 1)
Radon concentration contribute to indoor radon (Bq m 3 h)
Annual effective dose equivalent (lsv y 1)
Radium equivalent content (Bq kg 1)
1B 2B 3B 4B 5B 6B 7B 8B 9B 10B 11B 12B 13B 14B 15B 16B 17B 18B 19B 20B 21B 22B 23B 24B 25B
236.57 7.9 186.77 6.3 54.87 1.8 115.27 3.9 64.37 2.2 266.57 8.9 279.57 9.4 89.17 3.0 148.47 5.0 248.37 8.3 469.07 15.7 113.97 3.8 31.17 1.0 296.47 9.9 287.77 9.6 373.56 7 12.5 281.21 7 9.4 309.17 10.4 122.27 4.1 268.17 9.0 255.47 8.6 230.57 7.7 239.47 8.0 241.07 8.1 298.77 10.0
232.47 7.8 183.57 6.4 53.87 1.8 113.37 3.8 63.27 2.1 261.97 8.8 274.87 9.2 87.67 2.9 145.97 4.9 244.07 8.2 460.97 15.4 112.07 3.8 30.67 1.0 291.47 9.8 282.87 9.5 367.27 12.3 276.47 9.3 303.87 10.2 120.17 4.0 263.57 8.8 251.07 8.4 226.67 7.6 235.37 7.9 236.97 7.9 293.67 9.8
8.777 0.29 6.927 0.23 2.037 0.07 04.277 00.14 02.397 00.08 09.887 00.33 10.377 00.35 03.317 00.11 05.507 00.18 09.217 00.31 17.407 00.58 04.237 00.14 01.167 00.04 11.007 00.37 10.677 00.36 13.867 00.46 10.437 00.35 11.467 00.38 04.537 00.15 09.957 00.33 09.477 00.32 08.557 00.29 08.887 00.30 08.947 00.30 11.087 00.37
0.937 0.03 0.737 0.02 0.227 0.01 0.457 0.02 0.257 0.01 1.057 0.04 1.107 0.04 0.357 0.01 0.587 0.02 0.987 0.03 1.847 0.06 0.457 0.02 0.127 0.01 1.177 0.04 1.137 0.04 1.477 0.05 1.127 0.04 1.227 0.04 0.487 0.02 1.057 0.04 1.007 0.03 0.917 0.03 0.947 0.03 0.957 0.03 1.187 0.04
27.417 0.92 21.647 0.72 6.357 0.21 13.367 0.45 7.467 0.25 30.897 1.03 32.47 1.09 10.337 0.35 17.27 0.58 28.787 0.96 54.367 1.82 13.27 0.44 3.617 0.12 34.367 1.15 33.357 1.12 43.37 1.45 32.67 1.09 35.827 1.20 14.167 0.47 31.087 1.04 29.67 0.99 26.727 0.89 27.757 0.93 27.947 0.94 34.637 1.16
11.67 0.4 9.27 0.3 2.77 0.1 5.77 0.2 3.27 0.1 13.17 0.4 13.77 0.5 4.47 0.2 7.37 0.2 12.27 0.4 23.07 0.8 5.67 0.2 1.57 0.1 14.57 0.5 14.17 0.5 18.37 0.6 13.87 0.5 15.27 0.5 5.07 0.1 13.27 0.4 12.57 0.4 11.37 0.4 11.87 0.4 11.87 0.4 14.77 0.5
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Table 2 Measured radon concentration, radon exhalation rates, radon concentration contribution to indoor radon, annual effective dose equivalent, and radium equivalent content of soil samples collected from different sites in Al-Marj, Libya. M stands for Al-Marj. Sample Radon number/ concentration Al-Marj (Bq m 3)
Areal radon exhalation rate (mBq m 2 h 1)
Mass radon exhalation rate (mBq kg 1 h 1)
Radon concentration contribution to indoor radon(Bq m 3 h)
Annual effective dose equivalent (lsv y 1)
Radium equivalent content (Bq kg 1)
1M 2M 3M 4M 5M 6M 7M 8M 9M 10M 11M 12M 13M 14M 15M 16M 17M 18M 19M 20M 21M 22M 23M 24M 25M
333.28 711.16 190.10 706.36 333.77 711.14 275.58 709.19 058.25 702.95 269.54 709.02 249.14 708.34 429.15 714.37 393.79 713.18 457.76 715.32 506.94 716.97 393.68 713.18 289.86 709.70 368.52 712.34 408.07 713.66 361.10 712.09 333.05 711.15 061.08 702.05 142.67 704.78 393.85 713.18 441.33 714.77 324.26 710.85 306.99 710.27 253.39 78.48 423.65 714.18
12.587 00.42 07.177 00.24 12.567 00.42 10.367 00.35 02.207 00.74 10.177 00.34 09.407 00.32 16.207 00.54 14.867 00.50 17.277 00.58 19.137 00.64 14.867 00.50 10.947 00.37 13.917 00.47 15.407 00.52 13.637 00.46 12.577 00.42 02.317 00.08 05.387 00.18 14.867 00.50 16.667 00.56 12.247 00.41 11.597 00.39 09.567 00.32 60.997 00.54
1.337 0.05 0.767 0.03 1.337 0.05 1.107 0.04 0.237 0.01 1.087 0.04 0.107 0.00 1.727 0.06 1.587 0.05 1.837 0.06 2.037 0.07 1.587 0.05 1.167 0.04 1.477 0.05 1.637 0.06 1.447 0.05 1.337 0.05 0.247 0.01 0.577 0.02 1.587 0.05 1.777 0.06 1.307 0.04 1.237 0.04 1.017 0.03 1.707 0.06
39.317 1.32 22.427 0.75 39.257 1.31 32.387 1.31 6.877 0.23 31.797 1.06 29.387 0.98 50.617 1.69 46.447 1.56 53.997 1.81 59.777 2.00 46.437 1.55 34.197 1.14 43.467 1.46 48.137 1.61 42.597 1.43 39.287 1.32 7.207 0.24 16.837 0.56 46.457 1.56 52.057 1.74 38.247 1.28 36.217 1.21 29.887 1.00 49.967 1.67
16.67 0.6 9.57 0.3 16.67 0.6 13.77 0.5 2.97 0.1 13.57 0.5 12.47 0.4 21.47 0.7 19.77 0.7 22.97 0.8 25.37 0.9 19.77 0.7 14.57 0.5 18.47 0.6 20.47 0.7 18.07 0.6 16.67 0.6 3.107 0.1 7.107 0.2 19.77 0.7 22.07 0.7 16.27 0.5 15.37 0.5 12.77 0.4 21.27 0.7
339.17 11.4 193.47 6.5 338.67 11.3 279.47 9.4 59.37 2.0 274.27 9.2 253.57 8.5 436.67 14.6 400.77 13.4 465.77 15.6 515.87 17.3 400.57 13.4 294.97 9.9 375.07 12.6 415.27 13.9 367.47 12.3 338.97 11.3 62.17 2.1 145.27 4.9 400.77 13.4 449.07 15.0 329.97 11.0 312.37 10.5 257.87 8.6 431.07 14.4
Table 3 Variations in concentration of the radon activity and radon exhalation rates, in terms of area and mass, of soil samples collected from Benghazi and Al-Marj.
Minimum Maximum Range Mean Standard deviation
Radon activity concentration (Bq m 3)
Areal radon exhalation rate Ea (mBq m 2 h 1)
Mass radon exhalation rate Em (mBq kg 1 h 1)
Benghazi
Al-Marj
Benghazi
Al-Marj
Benghazi
Al-Marj
31.17 1.0 469.07 15.7 437.9 7 14.7 220.37 7.4 103.47 3.5
59.3 7 2.0 515.8 7 17.3 456.5 7 15.3 325.5 7 10.9 144.9 7 4.9
30.6 7 1.0 460.9 7 15.4 430.3 7 14.4 216.5 7 7.3 101.7 7 3.4
58.3 7 2.0 506.9 7 17.0 448.6 7 15.2 320.07 10.7 113.0 7 3.8
1.27 0.1 17.47 0.6 16.27 0.5 8.27 0.3 3.87 0.1
2.27 0.1 61.07 2.0 58.8 7 2.0 13.9 7 0.5 10.57 0.4
460.94715.43 mBq m 2 h 1, with a mean of 216.49 mBq m 2 h 1, whereas in Al-Marj it varies from 58.2572.95 to 506.94716.97 mBq m 2 h 1, with a mean of 316.5 mBq m 2 h 1. The radon exhalation rate in terms of mass in Benghazi varies from 1.1670.04 to 17.4070.58 mBq kg 1 h 1, with a mean of 8.08 mBq kg 1 h 1, whereas in Al-Marj it varies from 2.2070.74 to 19.1370.64 mBq kg 1 h 1, with a mean of 12.07 mBq kg 1 h 1. Also, the radium activity in Benghazi varies from 1.570.05 to 23.070.77 Bq kg 1, whereas in Al-Marj it varies from 2.970.1 to 25.370.9 Bq kg 1. It can be seen from the results that the radon concentration varies significantly among samples from both cities. The differences are due to the large radium and uranium contents of some of the samples, which result in higher exhalation rates. In general, the radon concentrations of soil samples from Al-Marj are higher than those of samples from Benghazi. Thus radon exhalation rates of Al-Marj samples are found to be higher than those of Benghazi samples. However, they are far smaller than the values recorded in the report issued by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) (2000). As previously mentioned, it is expected that exhalation of radon should depend on the uranium and radium concentrations in these samples, although it also depends on many other factors, such as permeability, porosity, density, texture and grain size.
The relationship between the geological features of the areas under study and indoor radon levels has been extensively studied (Otton and Duval, 1990; Klingel and Kemski, 2001; Singh et al., 2010; Mihci et al., 2010). These studies were done because soil has been found to be the most important source of indoor radon (Brown et al., 1993; Durrani and Ilic, 1997; Singh et al., 2010). Unfortunately, there is no published data of indoor radon in our study area. We can only estimate the contribution of radon from soil to indoor radon without considering the use of insulator materials to reduce the radon activity levels. In soil samples collected from Benghazi district, the radon concentration contribution to indoor radon was found to vary from 0.1270.01 to 1.8470.06 Bq m 3 h, while in Al-Marj district it varies from 0.2370.01 to 2.0370.07 Bq m 3 h. The results obtained for Benghazi soil samples show a maximum radon concentration of 468.97715.7 Bq m 3 for sample 11B, which belongs to the salinesaland solomchoks type. Sample 16B, of the reddish brownarid type, has the second-largest value, 373.567 12.5 Bq m 3. The third-largest radon value is for sample 18B, also of the reddish brownarid type, and equals 309.05710.4 Bq m 3. Results from Al-Marj show a maximum radon concentration of 515.8717.3 Bq m 3 in sample 11M. The next-five-largest radon concentrations are 465.7715.6, 449.0715.0, 436.6714.6,
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431.0714.4 and 415.2713.93 Bq m 3 for samples 10M, 21M, 8M, 25M and 15M, respectively. All of the above samples are categorized as ferrosiallitic soil. The Radium Equivalent Content (REC) for all samples from both Benghazi and Al-Marj were calculated using Eq. (4), and the results are shown in Tables 1 and 2. A range of RECs from 1.5 to 23.0 Bq kg 1 and an average of 10.81 Bq kg 1 were obtained for Benghazi. For Al-Marj, the range was 2.9–25.3 Bq kg 1, and the average was 15.96 Bq kg 1. The REC values in the current investigation are lower than the allowed maximum value of 370 Bq kg 1 (OECD, 1979). The dose received by the inhabitants of the cities from soil can be found using annual effective dose equivalents, which in turn were calculated from internal radon concentration values of all soil samples using Eq. (7). The results are displayed in the sixth column of Tables 1 and 2 for Benghazi and Al-Marj, respectively. It can be seen that for soil samples collected from Benghazi, the annual effective dose equivalent is in the range 3.61– 54.36 mSv y 1, with an average of 25.53 mSv y 1. The annual effective dose equivalent for Al-Marj is in the range 6.87– 59.77 mSv y 1, with an average of 37.72 mSv y 1. The values of annual effective dose do not exceed limits recommended by the European Commission (EC) (1999).
6. Conclusions
The measurements indicate normal levels of radon exhaled from
soil samples collected from Benghazi and Al-Marj in northeastern Libya. The mean values of radon concentrations and mass and areal radon exhalation rates of soil samples from Benghazi are found to be significantly lower than those from Al-Marj. In general, the current results are within the world-wide range of values found in soil, and this range is within the safe limits given in the report issued by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) (2000). Some high values of radon concentration are observed in soil samples from Al-Marj. For this reason, the use of soil from Benghazi in agriculture and fabrication of building materials from that soil is much more desirable than the use of soil from Al-Marj. There is the same concern that radium content and annual effective dose equivalents are within the safe limits given in the reports issued by OECD (1979), European Commission (EC) (1999) and United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) (2000). Finally, our results clearly show that both cities under investigation are safe as far as the health hazards of radon are concerned.
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Radioact. 99, 288–315. Nazaroff, W.W., Nero, A.V., 1988. Radon and its Decay Products in Indoor Air. A Wiley-Interscience Publication, New York. OECD, 1979. Exposure to Radiation from the Natural Radioactivity in Building Materials. Report by a Group of Experts of the OECD, Nuclear Energy Agency, Paris, France. Otton, J.K., Duval, J.S., 1990. Geologic controls on indoor radon in the Pacific Northwest. In: Proceedings of the International Symposium on Radon and Radon Reduction Technology vol. III Preprints: US E.P.A. Report EPA/600/9-90/005c, (unpaginated). Rafique, M., Ur-Rahman, S., Mahmood, T., Rahman, S., Matiullah, Ur, Rehman, Shafi, 2011. Radon exhalation rate from soil, sand, bricks, and sedimentary samples collected from Azad Kashmir, Pakistan. Russ. Geol. Geophys. 52, 450–457. Saad, A.F., Abdalla, Y.K., Hussein, N.A., Elyaseery, I.S., 2010. Radon exhalation rate from building materials used on the Garyounis University campus, Benghazi, Libya. Turk. J. Eng. Environ. 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Contents lists available at SciVerse ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Radon exhalation from Libyan soil samples measured with the SSNTD technique A.F. Saad n,1, R.M. Abdallah, N.A. Hussein Physics Department, Faculty of Science, University of Benghazi, Benghazi, Libya
H I G H L I G H T S c c c c
Radon exhalation was measured in Libyan soil samples by the SSNTD technique. The results indicate mostly normal levels of radon concentration. The mean radon concentration is 220.3 7 7.4 and 325.5 7 10.9 Bq m 3 for Benghazi and Al-Marj cities, respectively. We evaluated radon exhalation rates, the radium content and annual effective doses.
a r t i c l e i n f o
abstract
Article history: Received 12 June 2012 Received in revised form 28 September 2012 Accepted 5 November 2012 Available online 17 November 2012
Radon concentrations in soil samples collected from the cities of Benghazi and Al-Marj, located in northeastern Libya, were measured using the sealed-can technique based on the CR-39 SSNTDs. Mass and areal radon exhalation rates, radium content and radon concentration contribute to indoor radon, and annual effective doses were determined. The results indicate mostly normal rates, but there were some higher levels of radon concentration and emanation in samples collected from Al-Marj and one sample from Benghazi. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Radon exhalation rates Radium content CR-39 NTDs Can technique Soil Annual effective dose
1. Introduction The radioactive elements found naturally on earth can be classified into three series: 238U, 235U and 232Th. These radioactive, toxic elements are found in traces in almost all types of soil, rock, construction materials, water and air. People are exposed to ionizing radiation from natural sources because of the occurrence of natural radioactive elements in the earth’s environment. 222Rn is present in nature as the only gas found in the natural 238U radioactive decay series. It is a direct progeny of 226 Ra, which is the heaviest gaseous element and has a half-life of 3.82 days. Two other isotopes of radon exist in nature; they are members of the 232Th and 235U natural radioactive decay series, 220 Rn (thoron) and 219Rn (actinon), respectively. In a closed system, radon, in about 27 days (i.e. about seven half-lives of n
Corresponding author. Tel.: þ218 92 8128608; fax: þ218 61 2229617. E-mail address: [email protected] (A.F. Saad). 1 On leave from: Physics Department, Faculty of Science, Zagazig University, Zagazig, Egypt. 0969-8043/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2012.11.006
radon), comes into secular equilibrium with its long-lived (T1/2 ¼ 1600 years) immediate precursor, 226Ra. Radon emanation is linked with the presence of radium and its ultimate source, uranium, in the earth. These radioactive elements occur in virtually all soils, rocks, water and air; their concentrations vary with the geology of specific regions. Radium, a decay product of uranium in the naturally-occurring uranium series, contained in material lying close to the surface of the earth can generate a high radon exhalation rate, which pose hazards due to high radon exposure. Radon concentration in soil varies greatly from one site to another. Once the radon atoms are formed by the decay of the parent 226Ra, they move either by diffusion or by transport mechanisms or by both Mogro-campero and Flaischer (1977). Radon concentrations in soil pores at depth are directly dependent upon the radium content of the soil and soil parameters. Exhalation designates the escape of radon into the atmosphere from the rock and soil surfaces around the globe. Radon exhalation from soil, in particular, contributes to the indoor radiation environment. In this regard, studies on soil and building materials
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made of soil and rock have been carried out in different parts of the world, and much data is available in the literature. This literature on exhalation of radon from soil and building materials has now become quite voluminous and has brought new impetus to the field of radon physics (Ching-Jiang et al., 1993; Al-Jarallah et al., 2001; Al-Jarallah, 2001; Vukotich et al., 2002; Saad et al., 2002; Kumar et al., 2003, 2005, 2008; Duenas et al., 2007; Mudd, 2008; Singh et al., 2008; Rafique et al., 2011; Appleton et al., 2011). However, not much investigation has been carried out on radon exhalation from soil and building materials in the northeastern region of Libya (Saad et al., 2010). Consequently, we have proposed a plan to provide a wide spectrum of systematic study to determine conclusively and clearly the role of environmental radioactivity in that area of our country. For this purpose, in the present study, the method of measuring radon exhalation called the sealed-can technique (Fleischer et al., 1980; Abu-Jarad et al., 1980; Somogyi et al., 1984) based on CR-39 NTDs was used. Soil radon measurements were carried out in the districts of two cities in northeastern Libya. We should mention that this is the first time that such work has been carried out in this study area. The annual effective doses based on radon concentrations of soil were also determined to assess the health hazard to inhabitants in their dwellings.
2. Geography and geology of the study area The studied area included the cities of Benghazi and Al-Marj, both located in the northeastern region of Libya. Benghazi is the second largest city in Libya after the capital Tripoli. Benghazi is positioned on the Mediterranean sea between latitudes 20.71 and 32.121N as shown in Fig. 1a. The city is laid out in a radial pattern with the city’s lake at its center. Benghazi’s soil consists mainly of lithosol, reddish brownarid and salinesaland solonchoks located in certain areas of the city and its surroundings. The reddish brownarid has thicknesses ranging from a few centimeters to about one meter outside the valley area in the south of the city, and the thickness increases to 6–7 m in the valley area southeast of the city. The lithosol occurs sporadically, but it is found mainly at the foot of the hills; it is easily affected by erosion. The salinesaland solonchoks are found along the western coastal area and are characterized by high percentages of dissolved salts. Al-Marj is one of the oldest cities in Libya. It is situated in the green mountain area on the northeast side of the country at latitudes of 20.831 and 32.51N as shown in Fig. 1b. Al-Marj is in a plain area surrounded by hills on all sides. It is known for its fertile land rich in iron, and the inhabitants engage in agriculture and stock breeding. Al-Marj’s soil is characterized by the ferrosiallitic known as ‘‘Terra Rosa’’, which is rich in iron oxides and has less than 15% calcium carbonates. The ‘‘Terra Rosa’’ is located at depths of 40–60 cm and is capable of retaining water. There is also ‘‘the valley soil’’ found on the banks of valleys in Al-Marj basin. This type of soil varies in characteristics according surface topography. It is deep in the valley precipitation area and is affected by erosion when it rains. This soil is soft and deep and covers an area of about 7% of the Al-Marj area.
3. Materials and methods 50 soil samples were collected from Benghazi and Al-Marj, 25 from each (Fig. 1). The samples were all collected in summer 2009 and were dried in an oven at a temperature of 10571 1C for 24 h to remove moisture and then sieved through a 1 mm-mesh sieve. Each sample (0.5 kg) was placed in a cylindrical stainless-steel container of radius 7.35 cm, length 14.8 cm and volume 2.513 10 3 m3.
The dried soil was in the form of a porous, powdery material. It was gently pressed to form a disk-like shape, which permitted radon to diffuse out of this host material with a high degree of homogeneity. The thickness of each disk-like sample in the emanation container was about 2 cm. The emanation container was sealed tightly to prevent the escape of radon and was kept sealed for 3 months to obtain good statistics, after which it was ready for a-particle measurements using a CR-39 nuclear track detector (NTD). The concentration and exhalation rate of radon can be made using CR-39 detectors because of their ability to register tracks at different levels of registration sensitivity. The CR-39 detectors used in this investigation were purchased from Intercast Europe Co., Parama, Italy, in the form of small sheets, which were cut into 1.5 1.5 cm2 pieces. The emanation container was sealed with silicone and stored for 90 days. The film was then exposed to radon and its daughters in the chamber for a known period of time. The exposure of the detectors, followed by etching of the tracks in the film, provided the concentrations of radon and its daughters. The etching of the CR-39 detectors was carried out with 6.25 M NaOH at 70 1C for 8 h. After etching, the detectors were washed in distilled water and dried with fresh air. We counted the tracks with an optical microscope at 400 magnification. The number of tracks in 100 fields was scanned for each detector to determine the track density per cm2. A calibration factor of 0.239 70.008 tracks cm 2(Bq m 3) 1, obtained from an earlier calibration of the CR-39 track detector (Saad, 2008), was used to compute the radon activity from the track density.
4. Theoretical considerations 4.1. Calculation of radon exhalation rates The radon level of a soil sample placed in an emanation container can be monitored with a passive radon dosimeter based on the CR-39 solid-state nuclear track detector SSNTD. The radon concentration in the emanation container was measured hourly for a long period (90 days). The radon concentration in the closed can reached secular equilibrium within several half-lives of radon. It should be noted that the radon exhalation rate is constant, whereas the radon concentration reaches a maximum value that depends on the exhalation rate. The passive radon detector can be used to measure the accumulated activity and then to evaluate the average radon exhalation rate from the soil using the following equations (Fleischer and Mogro-campero, 1978; Khan et al., 1992; Kumar et al., 2005, 2008; Mahur et al., 2008a, 2008b; Singh et al., 2008; Rafique et al., 2011): EA ¼
C Rn lV A½T þ 1=lRn elRn T 1 Þ
ð1Þ
EM ¼
C Rn lV M½T þ 1=lRn elRn T 1 Þ
ð2Þ
where EM is the mass exhalation rate (mBq kg 1 h 1), EA is the areal exhalation rate (mBq m 2h 1), CRn is the integrated radon exposure (Bq m 3), A is the total surface area of the soil sample from which radon is exhaled (m2), V is the empty volume of the emanation container or can (m 3), M is the mass of the soil sample (kg), l is the decay constant of radon (h 1), and T is the time since sealing (h). 4.2. Estimation of the effective radium content The effective radium content in the soil samples was determined using a track detection method (Somogyi, 1986). Once radioactive equilibrium between radium and radon is established,
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165
Fig. 1. Map showing the study areas in (a) Benghazi and (b) Al-Marj. The numbers refer to the locations where samples were collected, as listed in Tables 1 and 2.
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one may use the analysis of the radon alpha activity for the estimation of the steady-state activity concentration of radium. After the sealing of the can, the activity concentration of radon begins to increase with time T as follows:
internal surface area and volume of the room, respectively, and the air exchange rate lV is 0.5 h 1. The annual effective dose equivalent Ep is then related to the average radon concentration CRn and is given by the following formula:
C Rn ¼ C Ra ð1elRn T Þ
8760nFC Rn EP WLM y1 ¼ 170 3700
ð3Þ
where CRa is the effective radium content of the soil sample. This effective radium content of a specimen in a sealed-can can be estimated as: C Ra Bq kg1 ¼ r=kT e hA=M ð4Þ where h is the distance between the detector and the top of the solid sample in m, r is the counted track density, k is the calibration factor of the CR-39 track detector, and Te denotes the effective exposure time given by T e ¼ T1=lRn ð1elRn T Þ
ð5Þ
ð7Þ
where, CRn is in Bq m 3, n is the fraction of time spent indoors, F is the equilibrium factor, 8760 is the number of hours per year, and 170 is the number of hours per working month. The values of n¼0.8 and F¼0.42 were used to calculate Ep. The effective dose equivalents from radon exposure were estimated by using a conversion factor of 6.3 mSv/WLM (International Commission on Radiological Protection (ICRP), 1987).
5. Results and discussion
The exposure time in these measurements was 90 days. 4.3. Calculation of annual effective dose The risk of lung cancer from domestic exposure due to radon and its daughters can be computed directly from the effective dose equivalents. The radiation hazards due to radon and its daughters are calculated from the radon exhalation rate of soil samples. The contribution of indoor radon concentration from soil can be calculated from the following formula (Nazaroff and Nero, 1988; Mahur et al., 2008a, 2008b; Saad et al., 2010): C Rn ¼
Ex Sr V r lv
ð6Þ
where CRn is the radon concentration in soil contributing to indoor radon (Bq m 3), EX is the radon exhalation rate (Bq m 2 h 1), Vr is the room volume (m 3) and lV is the air exchange rate (h 1). In these calculations, the maximum radon concentration from building materials was assessed by assuming the room to be a cavity with the ratio Sr/Vr ¼2.0 m 1, where Sr and Vr are the
In the current investigation, all houses are built on the earth’s crust, which is composed basically of soil and rocks, and most of the houses in the studied area are built of blocks made of sand and cement as well as gravel aggregates. Tables 1 and 2 show the values of radon activity, the radon exhalation rate in terms of area and mass, the radium equivalent content, the radon concentration contribution to indoor radon and the annual effective dose equivalent for soil samples collected from Benghazi and Al-Marj. The variations (minimum, maximum, range and mean values) of radon concentrations (Bq kg 1) and radon exhalation rates in terms of area and mass, together with the statistical uncertainty (1s) and standard deviation (SD), are presented in Table 3. Radon activity concentrations in soil samples collected from Benghazi are found to vary from 31.171.0 to 469.0 Bq m 3, with a mean, standard deviation and range of 220.3, 105.6 and 437.8 Bq m 3, respectively. Those from Al-Marj vary from 59.37 2.0 to 515.8717.3 Bq m 3, with a mean, standard deviation and range of 325.5, 117.3 and 456.5 Bq m 3, respectively. The radon exhalation rate in Benghazi varies from 30.6071.02 to
Table 1 Measured radon concentration, radon exhalation rates, radon concentration contribution to indoor radon, annual effective dose equivalent, and radium equivalent content of soil samples collected from different sites in Benghazi, Libya. B stands for Benghazi. Sample number/ Benghazi
Radon concentration (Bq m 3)
Areal radon exhalation rate (mBq m 2 h 1)
Mass radon exhalation rate (mBq kg 1 h 1)
Radon concentration contribute to indoor radon (Bq m 3 h)
Annual effective dose equivalent (lsv y 1)
Radium equivalent content (Bq kg 1)
1B 2B 3B 4B 5B 6B 7B 8B 9B 10B 11B 12B 13B 14B 15B 16B 17B 18B 19B 20B 21B 22B 23B 24B 25B
236.57 7.9 186.77 6.3 54.87 1.8 115.27 3.9 64.37 2.2 266.57 8.9 279.57 9.4 89.17 3.0 148.47 5.0 248.37 8.3 469.07 15.7 113.97 3.8 31.17 1.0 296.47 9.9 287.77 9.6 373.56 7 12.5 281.21 7 9.4 309.17 10.4 122.27 4.1 268.17 9.0 255.47 8.6 230.57 7.7 239.47 8.0 241.07 8.1 298.77 10.0
232.47 7.8 183.57 6.4 53.87 1.8 113.37 3.8 63.27 2.1 261.97 8.8 274.87 9.2 87.67 2.9 145.97 4.9 244.07 8.2 460.97 15.4 112.07 3.8 30.67 1.0 291.47 9.8 282.87 9.5 367.27 12.3 276.47 9.3 303.87 10.2 120.17 4.0 263.57 8.8 251.07 8.4 226.67 7.6 235.37 7.9 236.97 7.9 293.67 9.8
8.777 0.29 6.927 0.23 2.037 0.07 04.277 00.14 02.397 00.08 09.887 00.33 10.377 00.35 03.317 00.11 05.507 00.18 09.217 00.31 17.407 00.58 04.237 00.14 01.167 00.04 11.007 00.37 10.677 00.36 13.867 00.46 10.437 00.35 11.467 00.38 04.537 00.15 09.957 00.33 09.477 00.32 08.557 00.29 08.887 00.30 08.947 00.30 11.087 00.37
0.937 0.03 0.737 0.02 0.227 0.01 0.457 0.02 0.257 0.01 1.057 0.04 1.107 0.04 0.357 0.01 0.587 0.02 0.987 0.03 1.847 0.06 0.457 0.02 0.127 0.01 1.177 0.04 1.137 0.04 1.477 0.05 1.127 0.04 1.227 0.04 0.487 0.02 1.057 0.04 1.007 0.03 0.917 0.03 0.947 0.03 0.957 0.03 1.187 0.04
27.417 0.92 21.647 0.72 6.357 0.21 13.367 0.45 7.467 0.25 30.897 1.03 32.47 1.09 10.337 0.35 17.27 0.58 28.787 0.96 54.367 1.82 13.27 0.44 3.617 0.12 34.367 1.15 33.357 1.12 43.37 1.45 32.67 1.09 35.827 1.20 14.167 0.47 31.087 1.04 29.67 0.99 26.727 0.89 27.757 0.93 27.947 0.94 34.637 1.16
11.67 0.4 9.27 0.3 2.77 0.1 5.77 0.2 3.27 0.1 13.17 0.4 13.77 0.5 4.47 0.2 7.37 0.2 12.27 0.4 23.07 0.8 5.67 0.2 1.57 0.1 14.57 0.5 14.17 0.5 18.37 0.6 13.87 0.5 15.27 0.5 5.07 0.1 13.27 0.4 12.57 0.4 11.37 0.4 11.87 0.4 11.87 0.4 14.77 0.5
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Table 2 Measured radon concentration, radon exhalation rates, radon concentration contribution to indoor radon, annual effective dose equivalent, and radium equivalent content of soil samples collected from different sites in Al-Marj, Libya. M stands for Al-Marj. Sample Radon number/ concentration Al-Marj (Bq m 3)
Areal radon exhalation rate (mBq m 2 h 1)
Mass radon exhalation rate (mBq kg 1 h 1)
Radon concentration contribution to indoor radon(Bq m 3 h)
Annual effective dose equivalent (lsv y 1)
Radium equivalent content (Bq kg 1)
1M 2M 3M 4M 5M 6M 7M 8M 9M 10M 11M 12M 13M 14M 15M 16M 17M 18M 19M 20M 21M 22M 23M 24M 25M
333.28 711.16 190.10 706.36 333.77 711.14 275.58 709.19 058.25 702.95 269.54 709.02 249.14 708.34 429.15 714.37 393.79 713.18 457.76 715.32 506.94 716.97 393.68 713.18 289.86 709.70 368.52 712.34 408.07 713.66 361.10 712.09 333.05 711.15 061.08 702.05 142.67 704.78 393.85 713.18 441.33 714.77 324.26 710.85 306.99 710.27 253.39 78.48 423.65 714.18
12.587 00.42 07.177 00.24 12.567 00.42 10.367 00.35 02.207 00.74 10.177 00.34 09.407 00.32 16.207 00.54 14.867 00.50 17.277 00.58 19.137 00.64 14.867 00.50 10.947 00.37 13.917 00.47 15.407 00.52 13.637 00.46 12.577 00.42 02.317 00.08 05.387 00.18 14.867 00.50 16.667 00.56 12.247 00.41 11.597 00.39 09.567 00.32 60.997 00.54
1.337 0.05 0.767 0.03 1.337 0.05 1.107 0.04 0.237 0.01 1.087 0.04 0.107 0.00 1.727 0.06 1.587 0.05 1.837 0.06 2.037 0.07 1.587 0.05 1.167 0.04 1.477 0.05 1.637 0.06 1.447 0.05 1.337 0.05 0.247 0.01 0.577 0.02 1.587 0.05 1.777 0.06 1.307 0.04 1.237 0.04 1.017 0.03 1.707 0.06
39.317 1.32 22.427 0.75 39.257 1.31 32.387 1.31 6.877 0.23 31.797 1.06 29.387 0.98 50.617 1.69 46.447 1.56 53.997 1.81 59.777 2.00 46.437 1.55 34.197 1.14 43.467 1.46 48.137 1.61 42.597 1.43 39.287 1.32 7.207 0.24 16.837 0.56 46.457 1.56 52.057 1.74 38.247 1.28 36.217 1.21 29.887 1.00 49.967 1.67
16.67 0.6 9.57 0.3 16.67 0.6 13.77 0.5 2.97 0.1 13.57 0.5 12.47 0.4 21.47 0.7 19.77 0.7 22.97 0.8 25.37 0.9 19.77 0.7 14.57 0.5 18.47 0.6 20.47 0.7 18.07 0.6 16.67 0.6 3.107 0.1 7.107 0.2 19.77 0.7 22.07 0.7 16.27 0.5 15.37 0.5 12.77 0.4 21.27 0.7
339.17 11.4 193.47 6.5 338.67 11.3 279.47 9.4 59.37 2.0 274.27 9.2 253.57 8.5 436.67 14.6 400.77 13.4 465.77 15.6 515.87 17.3 400.57 13.4 294.97 9.9 375.07 12.6 415.27 13.9 367.47 12.3 338.97 11.3 62.17 2.1 145.27 4.9 400.77 13.4 449.07 15.0 329.97 11.0 312.37 10.5 257.87 8.6 431.07 14.4
Table 3 Variations in concentration of the radon activity and radon exhalation rates, in terms of area and mass, of soil samples collected from Benghazi and Al-Marj.
Minimum Maximum Range Mean Standard deviation
Radon activity concentration (Bq m 3)
Areal radon exhalation rate Ea (mBq m 2 h 1)
Mass radon exhalation rate Em (mBq kg 1 h 1)
Benghazi
Al-Marj
Benghazi
Al-Marj
Benghazi
Al-Marj
31.17 1.0 469.07 15.7 437.9 7 14.7 220.37 7.4 103.47 3.5
59.3 7 2.0 515.8 7 17.3 456.5 7 15.3 325.5 7 10.9 144.9 7 4.9
30.6 7 1.0 460.9 7 15.4 430.3 7 14.4 216.5 7 7.3 101.7 7 3.4
58.3 7 2.0 506.9 7 17.0 448.6 7 15.2 320.07 10.7 113.0 7 3.8
1.27 0.1 17.47 0.6 16.27 0.5 8.27 0.3 3.87 0.1
2.27 0.1 61.07 2.0 58.8 7 2.0 13.9 7 0.5 10.57 0.4
460.94715.43 mBq m 2 h 1, with a mean of 216.49 mBq m 2 h 1, whereas in Al-Marj it varies from 58.2572.95 to 506.94716.97 mBq m 2 h 1, with a mean of 316.5 mBq m 2 h 1. The radon exhalation rate in terms of mass in Benghazi varies from 1.1670.04 to 17.4070.58 mBq kg 1 h 1, with a mean of 8.08 mBq kg 1 h 1, whereas in Al-Marj it varies from 2.2070.74 to 19.1370.64 mBq kg 1 h 1, with a mean of 12.07 mBq kg 1 h 1. Also, the radium activity in Benghazi varies from 1.570.05 to 23.070.77 Bq kg 1, whereas in Al-Marj it varies from 2.970.1 to 25.370.9 Bq kg 1. It can be seen from the results that the radon concentration varies significantly among samples from both cities. The differences are due to the large radium and uranium contents of some of the samples, which result in higher exhalation rates. In general, the radon concentrations of soil samples from Al-Marj are higher than those of samples from Benghazi. Thus radon exhalation rates of Al-Marj samples are found to be higher than those of Benghazi samples. However, they are far smaller than the values recorded in the report issued by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) (2000). As previously mentioned, it is expected that exhalation of radon should depend on the uranium and radium concentrations in these samples, although it also depends on many other factors, such as permeability, porosity, density, texture and grain size.
The relationship between the geological features of the areas under study and indoor radon levels has been extensively studied (Otton and Duval, 1990; Klingel and Kemski, 2001; Singh et al., 2010; Mihci et al., 2010). These studies were done because soil has been found to be the most important source of indoor radon (Brown et al., 1993; Durrani and Ilic, 1997; Singh et al., 2010). Unfortunately, there is no published data of indoor radon in our study area. We can only estimate the contribution of radon from soil to indoor radon without considering the use of insulator materials to reduce the radon activity levels. In soil samples collected from Benghazi district, the radon concentration contribution to indoor radon was found to vary from 0.1270.01 to 1.8470.06 Bq m 3 h, while in Al-Marj district it varies from 0.2370.01 to 2.0370.07 Bq m 3 h. The results obtained for Benghazi soil samples show a maximum radon concentration of 468.97715.7 Bq m 3 for sample 11B, which belongs to the salinesaland solomchoks type. Sample 16B, of the reddish brownarid type, has the second-largest value, 373.567 12.5 Bq m 3. The third-largest radon value is for sample 18B, also of the reddish brownarid type, and equals 309.05710.4 Bq m 3. Results from Al-Marj show a maximum radon concentration of 515.8717.3 Bq m 3 in sample 11M. The next-five-largest radon concentrations are 465.7715.6, 449.0715.0, 436.6714.6,
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431.0714.4 and 415.2713.93 Bq m 3 for samples 10M, 21M, 8M, 25M and 15M, respectively. All of the above samples are categorized as ferrosiallitic soil. The Radium Equivalent Content (REC) for all samples from both Benghazi and Al-Marj were calculated using Eq. (4), and the results are shown in Tables 1 and 2. A range of RECs from 1.5 to 23.0 Bq kg 1 and an average of 10.81 Bq kg 1 were obtained for Benghazi. For Al-Marj, the range was 2.9–25.3 Bq kg 1, and the average was 15.96 Bq kg 1. The REC values in the current investigation are lower than the allowed maximum value of 370 Bq kg 1 (OECD, 1979). The dose received by the inhabitants of the cities from soil can be found using annual effective dose equivalents, which in turn were calculated from internal radon concentration values of all soil samples using Eq. (7). The results are displayed in the sixth column of Tables 1 and 2 for Benghazi and Al-Marj, respectively. It can be seen that for soil samples collected from Benghazi, the annual effective dose equivalent is in the range 3.61– 54.36 mSv y 1, with an average of 25.53 mSv y 1. The annual effective dose equivalent for Al-Marj is in the range 6.87– 59.77 mSv y 1, with an average of 37.72 mSv y 1. The values of annual effective dose do not exceed limits recommended by the European Commission (EC) (1999).
6. Conclusions
The measurements indicate normal levels of radon exhaled from
soil samples collected from Benghazi and Al-Marj in northeastern Libya. The mean values of radon concentrations and mass and areal radon exhalation rates of soil samples from Benghazi are found to be significantly lower than those from Al-Marj. In general, the current results are within the world-wide range of values found in soil, and this range is within the safe limits given in the report issued by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) (2000). Some high values of radon concentration are observed in soil samples from Al-Marj. For this reason, the use of soil from Benghazi in agriculture and fabrication of building materials from that soil is much more desirable than the use of soil from Al-Marj. There is the same concern that radium content and annual effective dose equivalents are within the safe limits given in the reports issued by OECD (1979), European Commission (EC) (1999) and United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) (2000). Finally, our results clearly show that both cities under investigation are safe as far as the health hazards of radon are concerned.
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