Study of the Parameters Affecting Radon Gas Flux from the Stream Sediments at Seila Area, South Eastern Desert, Egypt

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1 Study of the Parameters Affecting Radon Gas Flux from the Stream Sediments at Seila Area, South Eastern Desert, Egypt Y.A. Abdel-Razek a, M.S. Masoud a, M.Y. Hanafi ab and M.S. El-Nagdy b. a Nuclear Materials Authority, Cairo, Egypt. b Physics Department, Faculty of Science, Helwan University, Cairo, Egypt. Received: 11/3/2014 Accepted: 25/3/2014 ABRACT Fifty one locations distributed on two sites at Seila area, South Eastern Desert of Egypt were chosen to study the effect of the local controlling parameters, namely the activity concentration of 238 U, density and porosity of the stream sediments on the radon gas flux at this area. The average value of the activity concentration of 238 U in the stream sediments at the first site is Bq/kg while it is Bq/kg at the second site. These values are higher than the worldwide average of 33 Bq/kg. The average value of the radon flux from the stream sediments into the atmosphere at both sites at the studied area is much lower than the worldwide value of Bq m -2. It is essential for reliable radon measurements inside the stream sediments for any application that the porosity of these sediments should be studied to estimate the suitable hole depth. Accordingly, this study was carried out in this study. It is concluded that the passive measurements of radon flux are sensitive to the local parameters while the active measurements reflect the diurnal variations reliably. Key words: Radon gas flux / Sediments / Density / Porosity. INTRODUCTION Gabal El Seila area, South Eastern Desert of Egypt is located between latitudes N and longitudes E (Fig.1) at a distance of about 22 km southwest of Abu Ramad City (1). The younger granite at Gabal El Seila area is represented by Gabal Qash Amir, Gabal El Seila, are and isolated granite stocks. These rocks affected by East rth East West South West (ENE- WSW) shear zone and sub-parallel fault system dipping 50º-70º to the south and extending about 9km. with thicknesses between 2 to 40m. The ENE-WSW trend was intersected by the N-S sinestral strike slip and dip slip fault systems. Shear zones and fault systems filled with quartz veins, fine granite and basic dykes. The ENE-WSW shear zone display radioactive anomally along the sheared fine granite and basic dykes. Generally, these rock is pale pink slightly leucocratic, medium to coarse grained, cavernous and exfoliated monzogranite. These rocks are U-bearing minerals such as biotite, zircon and muscovite. Radon gas 222 Rn belongs to the radioactive uranium series and occurs in pores of the stream sediments in varying concentrations. 222 Rn and its progeny are responsible for about two thirds of the exposure of the world population to ionizing radiation from natural sources (2). Radon in the outdoor atmosphere mainly comes from the stream sediments, but some other secondary sources such as ocean, ground water, natural gas etc. also occur (3). The atmospheric concentrations of radon at ground level are governed by radium content (uranium content) and the radon emanation coefficients (4). In addition, 222 Rn concentration in stream sediments can strongly be affected by the geological structure (faults and volcanoes) (5). 222 Rn has also been used to determine diffusion coefficients (6&7). However, the 222 Rn release value is in itself an important parameter, as it is directly related to the presence or absence of seismic activity (8) and also to the effects of underground nuclear explosions. Corresponding author m.nuc2012@gmail.com 14

2 The characteristics of the stream sediments itself are important, including the amount of parent 226 Ra present, the porosity and permeability of stream sediments and the degree of water saturation. The present work was undertaken with three principal goals: 1- Calculations of the flux of 222 Rn from the stream sediments of Seila area. 2- Studying the factors responsible for the variation in radon flux from one site to the other. and 3- Comparing flux values of 222 Rn measured by different methods. Methods, Techniques and Experimental Results Field Works: EXPERIMENTAL Fifty one locations distributed in grid patterns over two sites at Seila area, (Fig. 1) were selected in two sites. The first site has an area of 400x150m 2 and includes 15 locations divided to five profiles (Fig. 2), while the second site has an area of 1.25x1km 2 and includes 36 locations divided to six profiles (Fig. 3). These locations were chosen to measure the uranium content in the stream sediments and the radon gas conentration inside these sediments. Fig. (1): Geological map of Seila area, south Eastern Desert, Egypt showing the two studied sites, modified after (1). 51

3 Fig. (2): Fifteen locations distributed in a grid pattern over site 1. Fig. (3): Thirty six locations distributed in a grid pattern over site 2. At each location, a pit of 40cm depth and 10cm diameter was dug. A radon chamber was put into the hole with its bottom facing the bottom of the hole as will be described below. The pit was covered with a wood sheet of 0.3x20x20cm 3. About 2kg sample from the sediments withdrawn from the pit at rondom locations at Seila area were taken to the laboratory for further investigations. Measurements of uranium content in the stream sediments: A portable RS-230 Gamma-Ray Spectrometer (1024 channels) was used to determine the uranium content in the stream sediments at this location. The RS-230 depends on a 6.3 in 3 (103 cm 3 ) higher density Bismuth Germanate Oxide (BGO) detector. Direct readings of the uranium content in (ppm) were recorded (22). The uranium activity concentration was obtained using the conversion factor 51

4 of (12.35 Bq/kg) 238 U/1 ppm of uranium, (9). The activity concentration of 238 U in the stream sediments in Bq/kg over the two sites at Seila area is represented in Table1 and Table 2. It must be mentioned that the age of the granitic rocks that originate the stream sediments at Seila area is about 600Ma. This allows the assumption that the activity concentration of 238 U is in equilibrium with that of 226 Ra in the U-238 radioactive series and replaces it wherever needed. Table (1): Activity concentration of uranium A U at site (1) Profile 1 profile 2 profile 3 profile 4 profile Table (2): Activity concentration of uranium A U at site (2) Profile 1 profile 2 profile 3 Profile 4 profile 5 profile 6 L. AU no Measurements of radon in the air of the stream sediments Active method After 24h, a hole of 5mm was made in the wood sheet covering the pit to insert a probe into the air of the pit. The probe was followed by a drier column and a valve. This valve was connected to a previously evacuated Lucas cell. The valve was opened so that a sample of the air inside the pit can be collected in the Lucas cell. After 3h, radon concentration inside the Lucas cell was determined using a suitable counter EDA (RDA-200). Radon concentration C Rn in the stream sediments at each of the chosen locations was then calculated using the following relation, (10) : where is the counts per minute for the sample, the counts per minute for background, E (cpm/bq) the efficiency of the system determined for each cell and (cm 3 ) is the volume of counting cell. However, the three hours needed for radon measurements by the described active method limit the number of radon samples that can be collected during the mission such that only some representative locations were chosen for radon measurements as shown in Table 3. (1) 51

5 Table (3): The radon concentrations C Rn (Bq m -3 ) in the stream sediments at representative locations at Seila area. Bq m -3 Bq m -3 Site (1) Site (2) Passive method: The technique used in this work is based on CR-39 nuclear track detectors of 1mm thickness (Pershore Mouldings, England). The passive radon dosimeter geometry consists of a closed chamber into which radon diffuses (11). It is made from plastic cup of 12cm height and 10cm diameter delivered to the Egyptian Nuclear Materials Authority (ENMA) by the International Atomic Energy Authority (IAEA). The cup has a hole at its bottom covered with a filter on the internal side. A 1x1cm 2 slide of CR-39 is fixed to the down side of the cap of the cup. This design of the chamber ensures that the aerosol particles and radon decay products are deposited on the filter allowing only radon gas to diffuse through it into the volume of the chamber (12). The cups were left inside the pits for an exposure time of t=30d. At the end of the exposure time, the radon dosimeters were collected and the detectors were removed, and then treated using etching solution NaOH with 6.25N in water bath at 70±1 C for 8h. Then, the CR-39 detectors were washed with distilled water and dried. The tracks produced were counted using optical microscope with 400x magnification. The radon concentrations were calculated using the equation (13). C Rn = ρ/kt e (2) Where represents the track density (tracks.cm -2 ) resulting on the surface of CR-39, the calibration factor K (tracks.cm -2.d -1 /Bq.m -3 ) and (d) is the effective exposure time that can be calculated from the following equation: T e=t-1/λ Rn(1-exp(-λ Rnt)) (3) where t is the exposure time CR-39 detector that equal 30 days. 51

6 Table (4): The radon concentration C Rn (Bq.m -3 ) in the first site at Seila area Profile 1 profile 2 profile 3 profile 4 profile 5 Bq.m Bq.m Bq.m Bq.m Bq.m Table (5): The radon concentration C Rn (Bq.m -3 ) in the second site at Seila area. Profile 1 profile 2 profile 3 Profile 4 profile 5 profile 6 Bq.m -3 Bq.m -3 Bq.m -3 Bq.m -3 Bq.m Bq.m Missed Missed Laboratory Works Determination of the density of the stream sediments: A representative portion from each dry sample is added to 500cm 3 placed in a graduated cylinder of volume 1000cm 3. The difference in weight is represented by W(g) while the difference in the volume of the sample was taken from the cylinder graduation an represented by V(cm 3 ). The density of rocks originating the studied stream sediments is obtained as follows: ρ s= W/V (4) The density ρ s (kg/m 3 ) of some random samples at the two sites at Seila area is represented in Table 6 and Table 7. Determination of the porosity of the stream sediments: A representative portion from each dry sample is weighed in a graduated cylinder, W1 (g). Water is added slowly inside the cylinder while shaking thoroughly to reach the minimum volume of the sediments, V (cm 3 ). The volume of the sample was taken from the cylinder graduation. This volume represents the complete compactness of the sediment inside the cylinder and simulates the natural compactness. If there is an excess of water, it must be removed by absorption using the edge of a filter paper and the sample is weighed, W2 (g). Knowing that, the density of water is nearly 1 (g/cm 3 ), therefore, the porosity of the dry sediments at each location can be determined by dividing the water weight, (W2-W1), by the volume of the sample V, (14&15). The porosity ε of some random samples at the two sites at Seila area is represented in Table 6 and Table 7. ε = (5) 59

7 Table (6): Density ρ s (kg.m -3 ) and porosity ɛ of the stream sediments in the first site at Seila area. L. no. ρs ɛ % (kg.m -3 ) Table (7): Density ρ s (kg.m -3 ) and porosity ɛ of the stream sediments in the second site at Seila area. L. no. ρs ɛ % (kg.m -3 ) Estimation of the diffusion length of radon at Seila area: The diffusion path of radon is characterized by its diffusion length (L), which is defined as the distance over which the radon diffuses through the medium during its mean life time. The diffusion length of Rn can be calculated as follows, (16) : L= (6) where D e : is the pore diffusion coefficient of radon gas in the medium, (cm 2 /s). λ Rn : is the decay constant of 222 Rn = , : is the porosity of the studied sediments. Assuming that the fluid filling the pore spaces in the studied medium is only air, the diffusion coefficient of the medium is calculated as follows, (16) : D e = 0.66 ε D air (7) where D air: is the radon diffusion coefficient in air (0.1 cm 2 /s). In this study, the average value of the diffusion length of the stream sediments at Seila area according to the calculated porosity and diffusion coefficient equals to (177 cm). 02

8 Radon flux density from the stream sediments Mathematical radon flux density using 238 U in the stream sediments: Considering the properties of the studied stream sediments; uranium content A U, density ρ, porosity ε and the diffusion length L which were determined above, we can calculate the flux density of radon the gas ( ) from the surface of the sediments out to the surrounding air at the chosen locations from the following equation, (17) : where: A U : is the activity concentration of 238 U in the sands (Bq kg -1 ), f : is the emanation fraction for the stream sediments =0.35 (18), : is the stream sediments density kg/m 3, 8) Table (8): Radon flux density J D (Bq. m -2. ) from the stream sediments in the first site at Seila area. profile 1 profile 2 profile 3 profile 4 profile 5 L. no. J L. no. J L. no. J L. no. J L. no. J Table (9): The radon flux density J D (Bq. m -2. ) from the stream sediments in the second site at Seila area. profile 1 profile 2 profile 3 profile 4 profile 5 profile 6 L. no. J L. no. J L. no. J L. no. J L. no. J L. no. J Radon flux from the stream sediments air by active method: Radon concentration C Rn determined by Eqn.(1) can be used to calculate the radon flux density of the studied stream sediments as follows: Where is volume of the hole, the surface area of the pit (m 2 ) and T is the radon accumulation time in the pit (24h). (9) 05

9 Table (10): The obtained radon flux density J (Bq. m -2. ) from the stream sediments at representitave locations at Seila area J J (Bq m-2 s-1) (Bq m-2 s-1) Site (1) Site (2) Radon flux from the stream sediments air by passive method: For the passive method, the term (1-exp( T)) was employed previously to obtain the effective time using equation (3), accordingly, equation (9) reduces to the following formula: where C Rn is the radon gas concentration in the stream sediments determined by passive method using Eqn. (2). (10) Table (11): The radon flux density J (Bq.m -2. ) at the first site at Seila area. profile 1 profile 2 profile 3 profile 4 profile 5 L. J L. J L. J L. J L. J

10 Table (12): The radon flux density J D (Bq.m -2. ) at the first site at Seila area profile 1 profile 2 profile 3 profile 4 profile 5 Profile 6 L. J Missed L. J Missed L. J DISCUSSION Activity concentration of 238 U in the stream sediments at Seila area: L. J From Table1& Table 2, the average value of the activity concentration of 238 U in the stream sediments at the first site is 41.68±12 Bq/kg while it is 37.34±10 Bq/kg at the second site. These values are higher than the worldwide average of 33 Bq/kg, (17). High uranium contents in the studied area are related to the surrounding granitic rocks which have elevated uranium content, (1). However, the distribution of uranium activity concentration is scattered over the two sites and no trend of its variation. Figs. (4 and 5) represent the variation of the activity concentration of uranium averaged over the studied profiles at Seila area. L. J L. J Fig.(4): Activity concentration (Bq/kg) of 238 U in the stream sediments at site 1. Fig.(5): Activity concentration (Bq/kg) of 238 U in the stream sediments at site 2. Radon gas concentration in the stream sediments: From Tables (4 and 5) the average value of radon gas concentration obtained by passive method, CR-39, in the stream sediments at the first site at Seila area is slightly higher than that in the stream sediments at the second site. This difference is consistent with the slight differences in the values of the controlling parameters; activity concentration of 238 U, density s and porosity ε of the studied stream sediments at the two sites. 02

11 From Table (3) the average value of radon concentration, obtained by the active method, in the stream sediments at the second site is only 60% of the average value at the first site. Referring to the fact that the active measurements at the first site were achieved at 9:00-12:00 am and the measurements at the second site were achieved at 12:00-3:00 pm, this valuable difference reflects the diurnal variation of the radon concentration in the sediments which may vary in an order of magnitude with minimum at noon and maximum at midnight, (19&20). This concludes that the passive measurements of radon concentration are sensitive to the local parameters while the active measurements reflect the diurnal variations reliably. This is because concentrations of radon vary widely with the environmental conditions which are constant in the short interval of time in the order of minutes and alter according to the diurnal variations while the passive technique as an integrating one over a long time period of time gives results that correlate well with the local factors (21). In contrast with uranium content, Figs. (6 and 7) clarify a trend of increasing radon concentrations in the stream sediments at the eastern parts of site 1 and 2. Fig. (2) shows that the eastern part at the first site is located near the EW shear zone while the eastern part at the second site is parallel to a SN fault. These geologic features may be responsible for the high values of radon concentration in the stream sediments at the eastern parts. Fig.(6): Radon concentration (Bq/m 3 ) in the stream sediments at site 1. Fig.(7): Radon concentration (Bq/m 3 ) in the stream sediments at site 2. Radon flux from the stream sediments: From Table 8 and Table 9, the average value of the radon flux (J) obtained mathematically by Equation (8) from the stream sediments into the atmosphere at both sites at Seila area is much lower than the worldwide value of Also the values of (J) obtained by both active, Table (10), and passive methods, Tables (11 and 12), are below the reported average value. From Table 11 and Table 12, the average value of radon flux obtained by the passive measurements of radon concentration in the stream sediments at both sites is an order of magnitude lower than the value obtained mathematically. This may be attributed to the geometry of the measurements. However, Equation (8) calculates the radon flux assuming a semi-infinite source. This condition is satisfied at a pit depth of one diffusion length which equals one meter at a porosity value of From the field works described above, the average depth of the holes at the studied area is 40 cm. accordingly, it may be recommended for reliable radon measurements inside the stream sediments for any application that the porosity of these sediments should be studied to estimate the suitable pit depth. 02

12 Effect of the studied parameters on radon flux: From Equation (8), the parameters that may affect the radon gas flux from the stream sediments into the surrounding atmosphere are; the activity concentration of 238 U, density and porosity of the sediments. Uranium content: The average values of uranium activity concentration change from the first site to the second only by 10%. This percent is reflected in the average values of radon flux obtained mathematically or by passive methods. On the other hand, by comparing the average of radon flux obtained by active methods at the first site to that obtained at the second site declares that the diurnal variation of radon flux is the dominant factor. Geometry and porosity: From Table 11 and Table 12 the average value of radon flux obtained by the passive measurements of radon concentration in the stream sediments at both sites is an order of magnitude lower than the value obtained mathematically. This may be attributed to the geometry of the measurements. However, Equation (8) calculates the radon flux assuming a semi-infinite source. This condition is satisfied at a pit depth of one diffusion length which L equals one meter at a porosity ε of 0.2. This study obtained an average value of ε=0.25 for the first site and an average value of ε=0.23 for the second site and accordingly a higher value of the diffusion length L=1.77m. From the field works described above, the average depth of the holes at the studied area is 40 cm. Accordingly, it may be recommended for reliable radon measurements inside the stream sediments for any application that the porosity of these sediments should be studied to estimate the suitable pit depth. Equation (8) suggests that the value of radon flux J decreases with higher values of the porosity ε. However, it is clear from Table 6 and Table 7 that the higher values of ε are recognized at the eastern part of both the studied sites at Seila area which should decrease the values of J at these parts. Inversely, the eastern parts recorded the higher values of radon flux J at both sites. This arises from the geological features that affect the radon concentration at these parts as mentioned above. Density: The average value of the density of the stream sediments from the first site is 2730 kg/m 3 with relative error of 2.4% while the average value at the second site is 2700 kg/m 3 with relative error of 1.3%. The average density of the samples from the whole area is 2700 kg/m 3 with relative error of 3%. Indeed, this slight difference in the density values of the stream sediments at the studied sites can't be recognized when using Equation (8) to obtain radon flux J since some other quantities e.g. A U and have higher differences of 10% and 11% respectively. CONCULSION The average values of the activity concentration of 238 U are higher than the worldwide average value while, that of the radon flux from the stream sediments into the atmosphere at both sites at Seila area is much lower than the worldwide value. It may be recommended for reliable radon measurements inside the stream sediments for any application that the porosity of these sediments should be studied to estimate the suitable pit depth. It may be concluded that the passive measurements of radon flux are sensitive to the local parameters while the active measurements reflect the diurnal variations reliably. 01

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