Determination of Radon Flux and Radium Content in Some Egyptian ...

Tenth Radiation Physics & Protection Conference, 27-30 November 2010, Nasr City - Cairo, Egypt

EG1100467 Determination of Radon Flux and Radium Content in Some Egyptian Recent Sediments Using Can Technique Y. A. Abdel-Razek Department of Medical and Radiation Researches, ResearchDevision, NMA, Cairo, Egypt, P.O. Box 530 El Maadi. [email protected] ABSTRACT Laboratory Can Technique was employed to estimate the radon flux and the radium content in the recent sediments collected from Baltim area at the northern cost of Egypt, Wadi Nugrus and Wadi Rimarim areas at the Southeastern Desert of Egypt. Ra-226 contents in the stream sediments at Wadi Nugrus and Wadi Rimarim show a typical variation which is much closer to that of older granites, which have low U-238 and Ra-226 contents. The obtained average values of the radon flux and radium content of the studied sediments were 0.545 (mBq.m-2.s-1) and 20.38 (Bq.kg-1), 0.84 (mBq.m-2.s-1) and 31.46 (Bq.kg-1) and 0.36 (mBq.m-2.s-1) and 13.42 (Bq.kg-1) at Baltim area, Wadi Nugrus and Wadi Rimarim areas respectively. These values are lower than the reported worldwide values which indicates the safe use of these sediments as building materials. Can Technique is reliable to estimate the radium content in solid samples. Key Words: Can Technique/ Radon Flux/ Sediments/ Radium Content. INTRODUCTION Uranium is present naturally in virtually all soil, rock and water. The dominant isotope, uranium-238, forms a long series of decay products that include the key radionuclides radium-226, and radon-222. Radon is the decay product of radium in the naturally occurring uranium series. As an inert gas, radon can move freely through the soil from its source to a distance which is determined by many factors such as rate of diffusion, effective permeability of the soil and its own half-life. Being a natural alpha emitter, radon can be detected by an alpha sensitive detector. It has been established that radon is a causative agent of lung cancer when present in higher concentrations(1). Radium is a solid radioactive element under ordinary conditions of temperature and pressure. Radium-226 nuclide decays to a radon-222 nuclide emitting an α-particle followed by a γ-radiation. It is the concentration of radium that governs how many radon atoms are formed. How many of the produced radon atoms leave i.e. emanate from the mineral grain or matter and enter the pore spaces, depends on; where the radium atoms are situated in the grain, the permeability of the grain, the texture and size of the grain, temperature and pressure(2-4). Since radium is present at relatively low levels in the natural environment, everyone has some level of exposure to its radiation. However, individuals may be exposed to higher levels of radium and its associated external gamma radiation if they live in an area

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where there is an elevated level of radium in soil. In addition, radium is particularly hazardous because it continually produces radon, which can diffuse into nearby homes. The measurements of radon thus necessitate the need for radium estimation in the parent source for public health risk evaluation. Numerous studies of Rn-222 concentrations in the atmosphere have been conducted over the last decades, both for investigations of atmospheric processes and for radiation protection(5-7). In comparison, much less work has been performed on the source term, i.e. Rn-222 emanation from the ground surface, particularly in the tropical regions(8-10). Radium content and radon emanation studies have been carried out in Hamirpur, Kullu and Una districts of Himachal Pradesh(11-13). Some radiation hazard indices of the recent sediments at various areas in Egypt were investigated because of the high potential of being used as building materials(14-16 ). In the present investigations, radon flux and radium concentration have been estimated for the samples of the recent sediments collected previously from three studied locations at Egypt. EXPERIMENTAL METHODS Samples of the recent sediments were collected from different areas at Egypt, i.e., Baltim (6 samples), Fig. (1), Wadi Nugrus (9 samples), Fig. (2) and Wadi Rimarim (5 samples), Fig (3), areas. To measure the radon flux from the studied samples, laboratory “Can Technique” was used(17-18). The dried samples from different locations were sieved through a 100 mesh seive. The fine powder, about 400 gm of the sample was placed at the bottom of a cylindrical emanation chamber (7 liter plastic bucket), which was then closed for a period of one month in order to get equilibrium between radium and radon. After one month, CN-85 plastic detector (1cmx1cm) which was previously fixed by adhesive tape to the inside surface of the bucket cap and covered with a cardboard, is set free using a string passing through the cap to register α-particles emitted from the surrounding nuclides of radon gas and its decay products, Fig. (4). The detector is hanged at a certain distance (about 15 cm) from the surface of the material to avoid registering α-particles emitted from thoron gas. The equilibrium inside the bucket had been kept and the detector is exposed for another month. After the second month, the detectors were taken out, etched (2.5 N NaOH at 60°C for 3h) and counted using an optical microscope at 400x magnification for alpha particle tracks under standard conditions(19). The radon flux is calculated by using equation(17-18): J=ρVλ/KA

(1)

where J is the radon flux in terms of area (Bq.m-2.s-1); ρ is the track density on the surface of the detector (tracks.cm-2.d-1); V is the effective volume of the bucket (=7x10-3 m3); λ is the decay constant of radon (=2.1x10-6 s-1); K is the calibration factor of CN-85 detector (= 0.2838 tracks.cm-2.d-1 per Bq.m-3)(19) and A is the area of the base of the bucket (=0.0314 m2).

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Fig. (1): Map of Baltim area showing the locations of the studied recent sediments, modified after Abel-Razek et. al., 2009(14).

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Fig.(3): Location of Wadi Rimarim At SED, Egypt a) and distribution of the chosen samples b). After Bakhit (2010)(16).

Fig. (4): Configuration of the emanation chamber. Radon flux of the recent sediments from Baltim area obtained by the laboratory "Can Technique" in this study is compared with the flux measured locally at the same sites using the "Active Technique"(14). To estimate the radium content in the samples, the “Can Technique” developed by Somogyi(21) is used. The effective radium content in the soil samples is calculated by the relation(21) :

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CRa = ρ V / K M

(2)

where CRa is the effective radium-226 content in the sample (Bq.kg-1); M is the mass of the soil sample (400 g) and K and V are as defined above. To obtain the radium content in the six samples collected from baltim area by γspectrometry, about 300-350g from each sample was packed in a plastic container, sealed well and stored for about 30 days before analysis. This allowed the in-growth of uranium decay products, prevented the escape of radiogenic gas 222Rn and allowed secular equilibrium between 238U and its decay products. After attainment of secular equilibrium, each of the prepared samples was measured in the laboratory for their U, Th, Ra and K contents using a high efficiency multichannel analyzer of γ-ray spectrometer (NaI detector). Each sample was counted for 1000s. Ra-226 was measured from the γ-ray photon emitted by 214Pb (327390keV). The activity concentration of a sample containing 1 ppm by weight of radium is 11.1 (Bq/kg). This technique was carried out at laboratory of γ-ray spectrometry of the Egyptian Nuclear Materials Authority (ENMA)(22). Its probable measurement error was about 10%. The radium content measured by γ-spectrometry technique for Baltim samples in this study or previously for Wadi Nugrus(15) and Wadi Rimarim(16) is compared with that obtained by “Can Technique.” RESULTS AND DISSCUSSION Table (1) shows the radon flux (J, mBq.m-2.s-1) and the radium contents (CRa, Bq.kg-1) of the recent sediments collected from a) Baltim area at the northern coast of Egypt, b) Wadi Nugrus at the Southeastern Desert (SED) of Egypt and c) Wadi Rimarim SED, Egypt. The table also shows the radon flux obtained by Active Technique at the same locations at Baltim area(14) and the radium contents in all samples obtained by γ-spectrometery(14-16). Table (1): Radon Flux J (mBq.m-2.s-1) and radium content CRa (Bq.kg-1) of the recent sediments collected from different locations at a) Baltim area, b) Wadi Nugrus and c) Wadi Rimarim. J (mBq.m-2.s-1) Sample CRa (Bq.kg-1) code Laboratory Local Can Technique γ-spectrometry a) Baltim Area 1.01 60 37.74 44.4 1 0.284 22 10.61 11.1 5 0.363 5 13.56 11.1 7 0.757 32 28.31 22.2 8 0.536 51 20.05 22.2 9 0.322 34 12.03 11.1 18 Ave. 0.545±0.28 34 20.38±10.73 20.35±12.97 b) Wadi Nugrus 0.83 31.14 33.3 1 0.70 26.18 22.2 2 0.61 22.88 22.2 3

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0.56 4 0.73 5 6 1.72 1.11 7 0.74 8 0.57 9 Ave. 0.84±0.37 c) Wadi Rimarim 0.32 R1 0.37 R5 0.27 R6 0.35 R7 R8 0.49 Ave. 0.36±0.08

20.99 27.36 64.28 41.51 27.60 21.23 31.46±13.81

22.2 22.2 55.5 44.4 22.2 22.2 29.6±12.65

11.79 13.80 10.26 13.09 18.16 13.42±2.98

11.1 11.1 11.1 22.2 13.88±4.81

From the table, it is clear that the radon flux measured locally by the Active Technique at Baltim area is almost twice order of magnitude when compared with those estimated by laboratory Can Technique. This reflects the enormous difference in the geometry between the two techniques as the local measurements are classified to have semi-infinite geometry while the laboratory Can Technique has finite geometry. The stream sediments collected from Wadi Nugrus have high values of the radon flux while the stream sediments collected from Wadi Rimarim and beach sands collected from Baltim area have low values. As for the radium which is the source for radon gas, Table (1) shows that Wadi Nugrus has the highest radium content while Wadi Rimarim has the lowest radium content. It must be stressed that the U-238 content and accordingly Ra-226 content in the stream sediments at a certain location are the weighted averages of these contents in the different surrounding parent rocks and the contents in the stream sediments coming to this location by drainage from other higher elevated wadis or tributaries. Therefore, these averages are affected by the different masses of the contributing rocks(16). Referring to the fact that the dominant rock masses at the Southeastern Desert of Egypt are older granites, U-238 and Ra-226 contents in the stream sediments at Wadi Nugrus and Wadi Rimarim show a typical variation which is much closer to that of older rocks, which have low U-238 and Ra-226 contents than that of younger rocks, which have relatively higher contents. However, the sample coded (6) at Wadi Nugrus has the highest Ra-226 content. Figure (2) clarifies that this sample was collected from the exit of Wadi Abu Rusheid, a tributary of Wadi Nugrus, which has varieties of rock type including younger porphyritic biotite monzogranites, biotite monzogranites and two mica monzogranites which have higher U-238 and Ra-226 contents(23). Wadi Rimarim has the lowest Ra-226 content. From Fig. (3), Wadi Rimarim is located at the coastal part of SED, Egypt. The lower Ra-226 in the stream sediments at this wadi may be ascribed to the dilution by mixing the sediments from the different wadis with the sediments originated from older parent rocks of lower Ra-226 content through its long passage towards the coastal plain. The low values of Ra-226 content in the recent sediments at Baltim area at

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the northern coast of Egypt arises from the dilution of radium due the water movement at this area. However, the averages of the radon flux obtained by "Can Technique" for the recent sediments collected from different areas in Egypt is much lower than the worldwide average; 16 (mBq.m-2.s-1)(24) while the averages of the radium content in the studied sediments lie within the Egyptian range obtained by UNSCEAR; 5-64 (Bq.kg-1)(25). This recommends the safe use of these sediments as building materials. Figure (5) plots the radium content (Bq.kg-1) in all samples of the stream sediments collected from the different areas determined by γ-spectrometry against those determined by the Can Technique. The correlation between the two methods is excellent which assures the reliability of using Can Technique to determine the radium content in the solid samples. CONCLUSION Ra-226 contents in the stream sediments at Wadi Nugrus and Wadi Rimarim show a typical variation which is much closer to that of older granites, which have low U-238 and Ra-226 contents. Local estimation of radon flux at Baltim area is almost twice order of magnitude of the laboratory estimation of the radon flux of the stream sediments collected from the same area due the different measurement geometry. The obtained values of radon flux and radium content of the recent sediments collected from the different areas at Egypt indicate the safe use of these sediments as building materials. The Can Technique is reliable to estimate the radium content in solid samples. 70

Radium content (Bq/kg)

Can Technique

60

2

R = 0.9133

50 y = 0.9869x + 1.1888 40 30 20 10 0 0

10

20 30 40 Gamma Specrometry

50

60

Fig. (5): Comparison between the radium content determined by γ-Spectrometry Technique and by Can Technique. REFERENCES

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