Determination of Radium and Radon Exhalation Rate as a Function of Soil Depth of Duhok Province - Iraq

ABSTRACT Radon is a naturally occurring radioactive gas; generated during the decay process of radium, when alpha particles start to be emitted, turning the radium into radon. The rate of radon gas that escapes from the soil into the atmosphere is called radon exhalation rate. In this study, radon, radium, and both radon surface and mass exhalation rates were measured for 40 samples of soil at four sampling depths (10, 20, 30, and 40 cm) in three districts of Duhok province. For the radon measurement, alpha-sensitive RAD7 detector was used. While radium concentration measured by well type NaI (Tl) detector. Analysis, shows radon surface exhalation rate vary from 24 ± 10.7 to 57 ± 2.5 Bq.m−2h−1 with an average value of 38.7 ± 8.9 Bq.m−2h−1. While mass exhalation rate vary from 1.2 ± 0.9 to 6.7 ± 1.6 Bq.kg−1.h−1 with an average of 4.2 Bq.kg−1.h−1. Furthermore, the results showed that radon exhalation rate and radon concentrations in soil have direct proportion to soil depths. Overall, radon concentration, radium content and both surface and mass exhalation rate in all sample points present a good correlation and less than global mean average recommendation [30].


Introduction
Radon ( 222 Rn) is a naturally radioactive inert gas produced after the radioactive disintegration of its parent radium 226 Ra as part of the 238 U decay series (Ismail & Jaafar, 2010;Kim et al., 2018). Radon generated in rock and soil, with a half-life of 3.8 days. It decays primarily via alpha particle and produce polonium isotopes ( 218 Po and 214 Po) (Korkmaz et al., 2017). Generation and migration of radon in porous materials, involve emanation, advection, diffusion, and exhalation (Yanchao et al., 2020). Radon exhalation rate and the concentrations of 222 Rn in soil within a few meters of the ground surface are the most two significant parameters used in measuring rate of radon that enter into pore spaces (Yanchao et al., 2020).
Radon exhalation rate changes according to various composition of the soil. 222 Rn concentration and exhalation depend on geophysical, hydrogeological, and geochemical phenomena of the region (Suresh et al., 2020). Geophysical parameters related to the characters of soil can be 226 Ra content which is a direct source of radon, soil grain size, the internal structure of soil, mineralization type, porosity and permeability of soil, and emanation coefficient (Ayman & Tayseer, 2019;Azeez et al., 2021).
In the last years, more attention has been paid to radon exhalation determination from soil, rock, building materials, and water worldwide (Abo-Elmagd et al., 2018;Amin, 2015;Xie et al., 2015). The radon exhalation rate is defined as the rate of radon that escapes from the soil into the atmosphere, and it can be measured by exhalation of radon gas per unit mass of soil or per unit area of the surface (Soares et al., 2020). Analyses of radon exhalation rate in different materials helps to study the health risk related to radon (Sudhir et al., 2016).
The present study aims to measure radon and radium concentrations in different depths of soils collected from three districts of Duhok province. Then determining radon exhalation rate in dry soil by measuring concentration of radon, in order to analyze a correlation between radon, radium, and radon exhalation rate and establish a data base for future work in Duhok province.

Study area
This study has been done on province of Duhok, located in the north-west of Iraq. It has borderline with Syria from the west, Turkey from the north, Nineveh from the south, and Erbil city from the east. The study area is divided officially into four districts. The altitudes are quite different, ranging from 300 to more than 1300 meter above the sea level with three different northern, northeastern and western types of wind. The geographical locations of the study points are shown and tabulated in the Figure 1.
The geological composition formed of red beds of silt, limestone, and hard clay. Conglomerate and regain is existed within the low folded area. The composition of the soil throughout investigation area is mostly characterized by thick sedimentary cover, broad synclines and well-marked folds of asymmetrical anticlines, very low permeability, and moisture of sediment of depth from (surface to 10 cm) of 6-7% (Abdulah & Ramadhan, 2011).

Sample collection and preparation
A total of 40 soil samples were carefully gathered in 10 locations of Duhok governorate. Four measurement points were taken in Duhok district, three in Sumel and three points in Amadi district. At each sample point soil was taken in depths 10 cm, 20 cm, 30 cm, and 40 cm. Measurements were taken in July (dry season). In each point an area of about 50 cm×50 cm was remarked and accurately cleared of any sediment or debris depending on a standard methodology (Abdulah & Ramadhan, 2011).
It was found that radon exhalation rate has a strong dependence on the moisture and temperature of the soil (Soares et al., 2020). Radon exhalation rate had a tendency to increase when the soil moisture content below 8%, While it decrease steadily when the soil moisture content exceeded 8% (Maeng et al., 2019;Soares et al., 2020). The theory is as follows: Water works to promote radon exhalation up to a particular moisture level. Water keeps more radon if the water content exceeds a certain level, and consequently, exhalation is then restrained (Hosoda et al., 2008). Much research has been conducted concerning the relationship between the soil radon exhalation rates and the soil moisture content (Maeng et al., 2019) and effect of rainfall (Masahiro et al., 2007). In many studies radon exhalation rate in summer season is higher than any other seasons because of the variation in soil moisture, which evidently affect the seasonal variation in soil air radon concentration. (Hosoda et al., 2010;Maeng et al., 2019;O.B. Modibo et al., 2011). Therefore, this research adopts dry soil to eliminate the effect of moisture content and rainfall. Thus, the collected samples of soil dried at room temperature for four days then dried in an oven at 110°C for one hour. Then the dried samples were sieved through 250 mm mesh to obtain a homogeneous powder (Amin et al., 2018).
In preparation to measure 222 Rn, a mass of 125 g from each of the soil samples were stored in a cup of cylindrical plastic tube. Each plastic tube contain of two valves, the dimension and the valves of the plastic containers shown in Figure 2. These plastic tubes stored for one month to acquire secular equilibrium between 222 Rn and its progeny (Ismail & Jaafar, 2010).
While to estimate concentration of 226 Ra in soil, smaller tubes of about 13 cm 3 ( Figure 3) were used. These tubes were filled with soil and also stored for one month to acquire secular equilibrium between 226 Ra and its progeny.

Radon activity measurement
In this work the solid-state detector RAD7 was used to measure 222 Rn concentration in soil. The benefit of this detector is the measurement of 222 Rn and 220 Rn concentration separately and having short measurement time. The measurement set up is shown in Figure 4; it consists of a cylindrical plastic tube, RAD7 Professional Electronic Radon Detector and a vinyl tube that contained desiccant (CaSO 4 ). When plastic tube connected to the closed loop, both valves of the plastic tube are opened. The accumulated radon gas will pass from the plastic tube to the desiccant, after that to the inlet filter of RAD7. The air is leaving from the outlet of RAD7. Inside RAD7 air is decayed and detects alpha particle that emitted from polonium isotopes. Then alpha technique is used by RAD7 which convert alpha particle right away to electrical sign. Also this detector capable to separate the different between electrical pulses generated from 218 Po and 214 Po with energies 6 MeV and 7.69 MeV, respectively.
The experiment is done in a dry condition, with humidity less than 8% (by purging RAD7 sometimes before starting the test). Then RAD7 is setup onto four cycle mode, each mode about one hour. Finally, the data were transferred from RAD7 to computer. Then, the obtained spectrum was analyses by Capture software.
For quality assurance and tracking the background radiation, radon concentration is measured for a blank container following the same manner as for the soil samples in term of starts, stops and time of exposure.

Estimation of 222 Rn exhalation rate
After the determination of radon concentration, surface and mass exhalation rate of 222 Rn can be calculated using equations 1 and 2 (Amin et al., 2018).
Where: λis the radon decay constant (0.00756 h −1 ), T is the exposure time in hours, V is the volume (0.00104 m 3 ) (the difference between the volume of the container and the volume of the sample), and A is the surface area of the sample (0.014 m 2 ), M is the mass of the soil.

Radium concentration measurement
In the same time, the activity per unit mass of 226 Ra is performed from the examined samples by a gammaray spectrometry system consisting of a well-type 3"×3" NaI (TI) scintillation detector. For limiting and reducing background radiation, the detector is surrounded by 4π geometry lead shield with thickness 6 cm and additional shield of 2 mm of electrolytic copper with an upper opening to change the source.
In the center of lower segment of lead shield has a small hole about 5 cm in diameter to catch the detector inside the shield. While, photomultiplier tube was wrapped by a thin plastic sheet to decrease the noise of electrical signal created by the shield and avoid direct shield-detector contact. Detector arrangement parameters are shown in Figure 5. The spectrum is analyzed by using MAESTRO Multi Channel Analyzer (MCA) application software. Energy, resolution, and efficiency calibration of the detector had been calibrated using gamma standard radioactive sources. Channel number convert to energy scale by using photo-peak of the standard International Atomic Energy Agency (IAEA) radioactive sources 155 Eu, 22 Na, and 60 Co that emit gamma-ray energy of 105.3, 511, 1173, and 1332.49 keV. Figure 6 shows the relation between channel number and energy spectrum of standard sources. After identification of Photopeak energies with the corresponding channel numbers of standard sources, the same multi sources are used to determine the efficiency (ε) of the detector (Figure 7), by using the following equation (Ramadhan & Abdullah, 2018): Where, N is the net peak area obtained for each Photopeak, A is the activity value of each radioisotope source at the time of the measurement, t is the time during which the spectrum was acquired, and fγ is the probability of gamma disintegration of the radionuclide; the energy, activity, and probabilities emission of gamma of each standard source shown in Table 1. After NaI(TI) was calibrated intern of energy resolution and efficiency, soil sources spectrum were analyzed. Based on the γ-ray spectrometer counts (N) at

Results and discussion
The activity concentrations of 222 Rn, 226 Ra, and surface and mass radon exhalation rate Ex and E M for different depths of soil samples from Duhok governorate were determined and represented in Table 2. The mean soil depth activity of 222 Rn and 226 Ra in Duhok district ranged from 246 ± 41 to 382 ± 17 Bq. m −3 and 25 ± 3.4 to 48 ± 2.6 Bq.kg −1 , with an average of 305 ± 44 Bq.m −3 and 33 ± 5 Bq.kg −1 respectively. The mean soil depth activity of 222 Rn in Somel district ranged from 252 ± 63 to 294 ± 11 Bq.m −3 with an average of 270 ± 13 Bq.m −3 and of 226 Ra 20 ± 5 to 29 ± 4 Bq.kg −1 with an average of 25 ± 2.3 Bq.kg −1 , respectively. Furthermore, the minimum and maximum mean of radon concentration in each sample point in Amedi district are 163 ± 72 to 221 ± 58 Bq. m −3 , respectively with a mean of 196 ± 17 Bq.m −3 While radium content ranged from 15 ± 6 to 23 ± 6 Bq.kg −1 with a mean of 19.3 ± 2 Bq.kg −1 .
It can be noted from the results that the concentration of 222 Rn and 226 Ra in soil samples varied appreciably from one points to another. Maximum radon concentration and radium content were found at Duhok District (points 3) while minimum recorded at Amedi district (point 8) about 80 km to the north-east from point 3. The variation may be due to the geochemical process in soil or geological condition of locations (El-Araby et al., 2019;Jayasheelan et al., 2013;Shashikumar et al., 2008). Many researchers found a positive correlation between radon concentration and radium concentration (Jayasheelan et al., 2013;Shashikumar et al., 2008). In the present study the same trend is observed as shown in Figure 8. A positive correlation has been observed between radon and radium concentration with correlation coefficients of 0.98. This means that, radium content in soil is the main source of radon.  Furthermore, the outcomes show that the activity concentrations of radon and radium content in the investigated soil samples were found to be directly correlated with the soil depth (10 cm -40 cm), except the point 3 (in this point radon concentration in depth 20 cm is more than that in 30 and 40 cm). Figure 9 shows the variation of 222 Rn concentration as a function of soil depths in three different districts of Duhok provinces. The outcomes suggest that the maximum soil gas radon observed at depth 40 cm and minimum at 10 cm. The obtained results from the present study indicate that the concentration of radon gas in soil increases with the soil depth. This is because of the variation of radium content, soil grain size, and soil porosity. Present results show behavior which agrees with that findings of many other researchers (Amin et al., 2018;Antonopoulos-Domis et al., 2009;El-Zohry et al., 2017;Kaliprasad & Naryana, 2018). Figure 10 illustrates the correlations between exhalation rate of radon and average concentration of radon and radium in each soil samples while Figure 11 shows the relation between surface and mass radon exhalation rate. Statistical t-test demonstrates that a strong positive correlation has been observed between radon exhalation rate with radon concentration (R 2 = 1). In the same manner as 222 Rn and 226 Ra, the highest value of E x and E M recorded in point 3 while lowest value measured in point 8. This is because there is a direct relationship between level of radon and its exhalation (Amin et al., 2018;El-Araby et al., 2019;Shashikumar et al., 2008). 222 Rn exhalation rate depend on radioactive disintegration of 226 Ra to produce radon, the moisture condition of the soil in the vicinity of the escaped radon atom, the direction of recoil of radon gas in the grain and its diffusion in the pore space (Elzain, 2015a). Hence, it is clear that there is a positive relation between radon exhalation with the concentrations of both radon and radium in soil (El-Araby et al., 2019;Elzain, 2015b). Finally, Figure 12 shows the average radon exhalation rate intern of area at each sample points.  The results of this study are broadly in agreement with the research carried out throughout the world. Table 3 shows comparison between present work and other local study (Ismail & Jaafar, 2010 (Abbasb et al., 2020). As a whole, the study has confirmed that soil gas radon and 226 Ra content in the study area and consequently the associated radon exhalation does not expose the human health to associated risk.

Conclusion
Radium and Radon exhalation rates for soil have been measured in three districts of Duhok province by alpha and gamma spectroscopy. A strong correlation was observed between radon gas in soil and radon surface and mass exhalation rate. From this study, it is clear that, there is a positive correlation between radon exhalation with the concentrations of radon and radium in soil. Furthermore, the present study indicate that the soilgas radon increases with soil depth which might be due to the radium content, soil grain size, and soil porosity. The results show that the value of radon concentration, radium content, and radon exhalation rates are less than the mean world value.

Disclosure statement
No potential conflict of interest was reported by the author(s).  (Dehghani et al., 2013)