Measurement of Radium Concentration and Radon Exhalation Rates of Soil Samples Collected from Selected Area of Aden Governorate, Yemen, Using Plastic Track Detectors

Radon exhalation rate from soil is one of the most important factors for evaluation of the environmental radon level. Solid State Nuclear Track Detectors (CR-39) has been widely used for the study of different aspects of radon emission from soil and others. In this paper, we are presenting the results of radon concentration, its exhalation rate and radium content in soil samples collected from different locations of northern part of Aden governorate, south of Yemen. The outdoor radon levels concentrations were found to vary from 264.59Bq m -3 to 539.72Bq m -3 with a mean value of 369.29Bq m -3 . The radon exhalation rate in terms of area and in terms of mass were found to vary from 460.08 mBq.m -2 .h -1 to 938.47mBq.m -2 .h -1 with a mean value of 642.12mBq.m -2 .h -1 and from 25.99mBq.m -2 .h -1 to 53.02mBqkg -1 .h -1 with a mean value of 36.28 mBqkg -1 .h -1 . The results of effective radium concentrations in these sites were found to vary from 3.44 Bqkg -1 to 7.01 Bqkg -1 with a mean value of 4.80Bqkg -1 . The working level varied from 4.54mWL to 9.26mWL with a mean value of 6.34mWL. The radon annual effective dose was varied from 0.049mSvy -1 to 0.100mSvy -1 with a mean value of (0.068±0.18) mSvy -1 for outdoor and from 0.20mSvy -1 to 0.40mSvy -1 with a mean value of 0.27mSvy -1 for indoor. The results indicate that the soil is quite safe for occupancies and to be used as building materials.


Introduction
Terrestrial radiation is due to various radioactive nuclides that are present in soil, water, air and their abundance changes depending on the geological and geographical features of region. The external exposure is caused by gamma rays emitted mainly by radionuclides of uranium 238 U and thorium 232 Th decay series as well as potassium 40 K. The internal exposure is caused by radon ( 222 Rn and 220 Rn) and its short-lived decay products. Radon is an alpha emitter that may be easily inhaled and its descendants may be deposited in tissues of the respiratory tract [1].
Radon is a natural inert radioactive tasteless and odorless gas, whose density is 7.5 times higher than that of air. Radon gas and its radioactive isotopes have special attention among the other naturally radioactive materials, because it has the largest amount of total annual effective dose to humane. There are two important natural occurring isotopes of radon; 222 Rn, a direct product of 226 Ra in the 238 U series and 220 Rn a direct product of 224 Ra in the 232 Th series. Because radon has relatively long half-life enabling it to migrate quit significant distance before decaying and can be found in everywhere [2].
Radon comes from the natural decay of uranium that is found in nearly all soils. It appears mainly by diffusion processes from the point of origin following α-decay of 226 Ra in underground soil and rocks. It typically moves up through the ground to the air above and into homes through cracks and other holes in the foundation [3].
Radon is emitted from the ground and enters a home through cracks in walls, basement floors, foundations and other openings. Because radon comes from rock and soil, it can be found anywhere. Exposure to limited concentrations, like those found outdoors, is impossible to avoid. However, when radon gets trapped indoors, it may build up to dangerous concentrations, [4]. Long term exposures to radon via inhalation in closed rooms or caves or open air saturated with Radon gas is the cause for about 10% of all deaths from lung cancer [5]. The emission of radon per unit area per unit time is called exhalation rate and depends upon: (a) radium concentration in the material which in turn depends on the uranium concentration in the material, (b) emanation factor of radon from the material, (c) porosity and density of the material, and (d) diffusion coefficient of radon in the material [6]. Radon gas ionizes the ambient atmospheres both indoor and outdoor. The exhalation of radon from soil involves two mechanisms, the emanation and transport. These mechanisms are affected by many factors including the properties of the soil [7]. CR-39 polymer track detector is one of the most common polymer detectors that belong to SSNTDs. CR-39 polymer track detector (Poly-ally diglycol carbonate) is used in a wide range of different scientific and industrial technological applications such as radiological experiments, neutrons spectroscopy and radon dosimetry [8].
In the present study, we have used solid state nuclear track detectors (CR-39) for the analysis of radium content and radon exhalation rates in twenty six of soil samples collected from various locations in Aden city which is located in the South of Yemen on the Gulf of den. The location of this city has been determined using the Global Positioning System (GPS): Latitude: 12°49'.468"N., Longitude: 44°51'.708"E. The map of studied area is shown in Figure 1.

Materials and Methods
Radium content and radon exhalation rates in the soil samples of study area were measured by following the sealed can technique [9]. The experimental set-up is shown in Figure 2. Solid State Nuclear Track Detectors (SSNTD) with sheet thickness 300 was used in this study, which is usually known as CR-39 [10]. Total of 26 soil samples were collected by the grab sampling method from different places of Aden governorate for the measurement. A dried and sieved sample (250g) was placed at the bottom of a cylindrical sealed can of 11cm height and 7cm diameter. The dosimeters were stored (closed) for one month to reach secular equilibrium between radium and radon. After this period, CR-39 plastic detector (1cm × 1cm), which was previously fixed by adhesive tape to the inside surface of a second identical cover was mounted quickly and closed the chamber [11]. Plastic can was closed well by its cover and was left for sixty days as exposure time, closed can techniques produced in Figure 1. CR-39 polymer detector registers alpha particles which emitted by decay of radium to radon gas as tracks. After the exposure time, CR-39 detectors were assembled from cans and chemically etched in NaOH solution 6.25 M at 70°C to enlarge and appear the alpha tracks through time equal 8 hour [12,13]. After etching the CR-39 detectors were washed in distilled water and then dipped for few minutes in a 5% acetic acid solution and washed again with distilled water and finally air dried. The track density was determined by using optical microscope at 640× magnification which calibrated before usages [14]. The background of CR-39 track detector was counted by optical microscope and subtracted from the count of all detectors [15,4].

Theoretical Consideration
The tracks density (ρ) of the samples is measured using Where ρ: track density (track/mm 2 ), ∑ Ni = Total of the tracks, ! = Total number of field counted, " = Area of the field of view. The track density ρ (in track/cm 2 ) is related to the radon activity concentration A Rn (in Bq/m 3 ) and the exposure time T by the formula: [17] C $% &Bq. cm +, -= . / 0 (2) Where: η: is the sensitivity factor of CR-39 plastic track detector, the value of η is 6.59×10 −2 1 2 3 4 5 .6 −2 .7 3 8 −1 /9 : . −3 The calculated tracking density was converted to radon activity in Bq⋅m −3 using a calibration factor (k), which is obtained from the date sheet of the manufacturer (Bq⋅m −3 )⋅(tracks⋅cm −2 ⋅day −1 ) −1 , where every track recorded per one cm −2 per day on the CR-39 detector is equivalent to 15.28 Bq m −3 [18,19]. This is due to exposure to a variety of activities derived from radon gas and its progenies. The activity concentration of radon begins to increase with time T, after the closing of the can, according to the relation: [20,11] C $% &Bqcm +, -= C $< =1 − @ +A BC D E Where: C Ra (in Bq⋅kg −1 ) is the radium activity density of the sample and F G ! is the decay constant of 222 Rn (7.56 × 10 −3 h −1 ). Then, the tracks in the detector are used to measure the time-integrated value of the density of the track, as per expression (2). The total number of alpha disintegrations in unit volume of the can with a sensitivity η during the exposure time T, hence the track density observed is given by in equation (4): [21] ρ&Tr. cm +H -= η C $% T J Where 1 @ denotes, by definition, the effective exposure time given by equation (5): [22] Where T is time of exposure a day, hence the effective time calculated from Equation (5) was 24.5 days.
Radium concentration of the samples is calculated from the relations: [11,23] C $< &M-&Bq. kg + -= C $% Q R S T Where M is the mass of the soil sample in kg, A is the area of cross-section of the can in m 2 ; h is the distance between the detector and top surface of the soil sample in meter.
The radon exhalation rate in terms of mass of the samples is calculated from the relation: [24,25] Where: E M is the radon exhalation rate in terms of mass and M is the mass of the sample (kg).
The radon exhalation rate in terms of area of the samples is calculated from the relations; Where: E A is the radon exhalation rate in terms of mass, V refers to the volume of the cup (m 3 ), T refers to the time of exposure (hour), and A is the area of cross-section of the cup (38.46×10 -4 m 2 ) [24,25].
The annual effective dose equivalent, E, due to the activity in the soil was calculated using the following equation: Where C Rn is in Bq.m -3 ; n is the fraction of time spend n = 0.2 for outdoor and n = 0.8 for indoor, F is the equilibrium factor which equal to 0.42 as suggested by UNSCEAR [1], the number 8760 is refer to the time of hours per year and 170 is the number of hours per working month. For radon gas exposure in the present work, the effective dose equivalents were estimated by using a conversion factor of 6.3mSv/WLM [2,26].
The working levels of the samples is calculated from the relations: [ Where, C Rn is radon concentration in Bqm -3 and F is the equilibrium factor for radon has been taken as 0.42 as suggested by [3,29]. Table 1 list the track density and radon concentrations, mass exhalation rate and area exhalation rate, in samples taken from selected area of northern part of Aden governorate, South of Yemen. The radon concentration varies from (264.59-539.72) Bq/m 3 with a mean value of (369.29±15.55) Bq/m 3 . It is notices that, radon concentration in the studied area are within the recommended limits of ICRP and closed or less than the nearby countries [30,31]. The radon exhalation rate in terms of area and in terms of mass varies from (460.08-938.47) mBq.m -2 .h -1 with a mean value of (642.12±27.04) mBq.m -2 .h -1 and (25.99-53.02) mBq.kg -1 .h -1 with a mean value of (36.28±4.14) mBq.kg -1 .h -1 . Radon exhalation rates observed in the current study are also quite below the world average of 57,600 mBq⋅m -2 ⋅h -1 (0.016Bq⋅m 2 ⋅s -1 ) and hence will not cause any health hazards to residents [32,11]. It is notice from table 3 that the effective radium content of soil sample varies from (3.44-7.01) Bqkg -1 with a mean value of 4.80 Bqkg -1 . The results of exhalation rates and effective radium content in soil sample are in agreement with the results of A. Subber et al. [2,19], whom used the same method for measurement of those parameters in soil samples. The radium content is lower than the permitted value of 370 Bq.Kg -1 as recommended by Organization for Economic Cooperation and Development (OECD) [33]. The calculated values of working level (WL), the outdoor and indoor annual effective dose were given by table 3. The working level varied from 4.54mWL to 9.26mWL with a mean value of 6.34mWL. The radon annual effective dose was varied from 0.049mSvy -1 to 0.100mSvy -1 with a mean value of 0.068mSvy -1 for outdoor and from 0.20 mSvy -1 to 0.40mSvy -1 with a mean value of 0.27mSvy -1 for indoor. The outdoor and indoor annual effective dose lower than the normal background level of 1.10mSvy -1 as quoted by [1]. Thus, results reveal that the area is safe as far the health hazard effects. Figures 3 and 4 have shown the distribution of Activity Concentration of radon and radium content in the different soil samples of Aden governorate, south of Yemen. While figures 5 and 6 have shown the distribution values of mass exhalation rates and surface exhalation rates of radon for all soil samples, respectively, These figures and Tables 2  and 3 show that the value of radium content and radon exhalation rates for soil samples are less than the values reported by many researchers [1, 2, and 5].

Conclusions
The main aim of this study was to determine radon concentration and exhalation rate in the soil samples using solid state nuclear track detectors. The measured values of radon concentration and exhalation rate generally in the recommended limit by the world wide safe limit given by UNSCEAR 2000. Most of the indoor radon values lie in the range of action levels from 200 to 600 Bqm -3 [34,35]. The average values of radon concentrations in the soil samples are lower than the recommended limit 800 Bqm -3 which, reported by WHO, 1993. The effective radium distribution in the soil samples varies from point to point in the same district. Its value is found to be much below the hazard limit. The obtained results can be used as reference information to assess any changes in the radioactive background level due to geological processes in the investigated area. Finally, our results clearly show that the area under investigation is safe as far as the health hazards of radon are concerned.

Conflicts of Interest
The authors declare no conflict of interest.