Natural radioactivity measurements in building

Journal of Environmental Radioactivity 99 (2008) 1279–1288

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Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad

Natural radioactivity measurements in building materials in Southern Lebanon M.A. Kobeissi a, O. El Samad a, K. Zahraman a, S. Milky b, F. Bahsoun b, K.M. Abumurad c, * a

Lebanese Atomic Energy Commission, National Council for Scientific Research, P.O. Box 11-8281, Beirut, Lebanon Department of Physics, The Lebanese University, Faculty of Sciences (I), Hadeth, Beirut, Lebanon c Department of Physics, Yarmouk University, P.O. Box 566, Irbid 21163, Jordan b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 October 2007 Received in revised form 18 February 2008 Accepted 7 March 2008 Available online 28 April 2008

Using g-spectroscopy and CR-39 detector, concentration C of naturally occurring radioactive nuclides 226 Ra, 222Rn, 214Bi, 228Ac, 212Pb, 212Bi and 40K, has been measured in sand, cement, gravel, gypsum, and paint, which are used as building materials in Lebanon. Sand samples were collected from 10 different sandbank locations in the southern part of the country. Gravel samples of different types and forms were collected from several quarries. White and gray cement fabricated by Shaka Co. were obtained. g-Spectroscopy measurements in sand gave Ra concentration ranging from 4.2 0.4 to 60.8 2.2 Bq kg1 and Ra concentration equivalents from 8.8 1.0 to 74.3 9.2 Bq kg1. The highest Ra concentration was in gray and white cement having the values 73.2 3.0 and 76.3 3.0 Bq kg1, respectively. Gravel results showed Ra concentration between 20.2 1.0 and 31.7 1.4 Bq kg1 with an average of 27.5 1.3 Bq kg1. Radon concentration in paint was determined by CR-39 detector. In sand, the average 222Rn concentration ranged between 291 69 and 1774 339 Bq m3 among the sandbanks with a total average value of 704 139 Bq m3. For gravel, the range was found to be from 52 9 to 3077 370 Bq m3 with an average value of 608 85 Bq m3. Aerial and mass exhalation rates of 222Rn were also calculated and found to be between 44 7 and 2226 267 mBq m2 h1, and between 0.40 0.07 and 20.0 0.3 mBq kg1 h1, respectively. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: g-Spectroscopy CR-39 Building materials 222 Rn Radionuclides Rn exhalation rates Lebanon

1. Introduction It is well known that radioactive nuclides in the uranium and thorium decay chains do occur with varying degrees of concentration in the earth’s crust. While radioactive nuclides such as radium (226Ra), radon (222Rn) (here called Rn) and bismuth (214Bi) are the product in the decay chain of uranium (238U), other radioactive nuclides, such as actinium (228Ac), bismuth (212Bi) and lead (212Pb) do occur in the decay chain of the thorium element (232Th). In addition, the radionuclide 40K does also occur in construction materials. These radioactive elements can be found almost in all types of soils, such as rocks, granite, sand, cement and gypsum from which building materials are produced. Rn, as an emitter of a-particles with energy 5.48 MeV, is the most important and unique radioactive element. This noble gas is chemically inert and it can move through the earth and structural materials with a half-life of 3.82 d to reach the outdoor atmosphere and the indoor spaces (Nazaroff and Nero, 1988). 226Ra, with a half-life of 1600 years, is contained in the above-mentioned building materials and decays to 222Rn emitting a-particles followed by g-rays radiation. It is the * Corresponding author. Tel.: þ962 2 7211111x2313, 2300; fax: þ962 2 7211117. E-mail address: [email protected] (K.M. Abumurad). 0265-931X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2008.03.007

concentration of 226Ra, which determines the number of 222Rn atoms formed in any building material. Radon exhalation rate from construction materials varies with its type and origin. 238U content can also vary among granite types. Furthermore, the amount of the produced Rn atoms depends on the location of the Ra atoms in the grain and on the texture, size, and permeability and emanation power of the grains. Also porosity of the material, temperature and pressure play an effective role in the radon emanation into the air. These parameters define the sources and origin of Rn gas (Nazaroff and Nero, 1988). It is believed that radon contribution forms more than 50% of the dose equivalent received through inhalation by the general population from all sources of radiation naturally occurring and man-made ones (Von Philipsborn, 2003). It is known that exposure of population to high concentration of radon gas and its progenies for a long period of time, such as in the case in the uranium exploration, can lead to pathological effects like respiratory functional changes and the occurrence of lung cancer (BEIR, 1999). Due to such health risks to humans through exposure to and inhalation of radon, studies of radon concentration and its effects on human health should be made in order to reduce such risks and knowledge of the exhalation rate from the surface of the construction materials enables one to estimate their contribution to the radon input into

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the indoor space. Thus, measurements of 226Ra content and Rn concentration and its progenies are necessary for the estimation of health hazard for public. Such measurements of Rn concentration and its exhalation rate from construction materials have been the subject of many worldwide studies by numerous researchers (AbuJarad et al., 1980; Mustonen, 1984; Savidou et al., 1996; Durrani and Ilic’, 1997; Al-Jarallah, 2001; Al-Jarallah et al., 2001; Ismail et al., 1997; El-Amri et al., 2003). In addition, intensive investigations and measurements of radioactive nuclides’ concentrations produced in the 232Th decay chain in building materials have been undertaken in many countries (Sajo’-Bohus et al., 1999). In this paper, a study is presented on radium concentration CRa and on Ra equivalent concentration CRaeq and its progenies, especially radon concentration in building materials used in high rise buildings as well as in residential homes in Lebanon. This study is done to identify the radioisotopes and their activities’ concentrations in these materials and to estimate the corresponding natural exposure rate. Such research activity has never been undertaken in Lebanon and the present study on radon and other radioactive nuclides in building materials is a preliminary work and forms part of a pioneering, and long term program to study naturally occurring radioactive materials in the environment throughout the country. The results will be compared with those obtained in other parts of the world. The two methodologies used in the present work were the can technique using CR-39 solid state nuclear track detector and the g-ray spectroscopy method.

CR-39 were enclosed in glass jar containers which can be sealed, as it will be described below. In this case, the radon aerial and mass exhalation rates, EA and EM, can be calculated from the following equations (Abu-Jarad, 1988; Khan et al., 1992; Maged and Borham, 1997; Sharma et al., 2003): EA ¼ ðCx TV lÞ=ðATcÞ;

(7)

where Tc ¼ [T þ (1/l)(exp(lT ) 1)] and T is the exposure time (in hours) of the CR-39 detector to Rn gas emanated from the sample, Cx is the radon concentration (Bq m3), V is the effective volume of Rn gas in the jar chamber (m3) and A is the cross-sectional area (m2) of the soil surface in the jar. In addition, the mass exhalation rate, EM, is given by EM ¼ ðCx TVlÞ=ðMTcÞ;

(8)

where M is the mass of the sample in the jar. Since 226Ra being the parent of 222Rn, the can technique, which was described and developed by Fleischer et al. (1965, 1975) and later by Somogyi (1990), has been used. The effective Ra concentration, CRa, in soil samples was calculated using the following expression: CRa ¼ ðrx hAÞ=ðKTcMÞ;

(9)

where rx and h are tracks density on the CR-39 detector and the distance between the sample surface and the detector CR-39, respectively. M and Tc are defined above while K is the sensitivity factor [tracks cm2 d1(Bq m3)1]. In this experiment and from the calibration used in this study, the value K ¼ 0.187 [tracks cm2 d1 (Bq m3)1] has been obtained.

2. Theoretical aspects 3. Experimental procedure and experimental settings For a dry solid grained soil, such as sand, through which radon migrates only by diffusion, the activity concentration in the pores is described by mass conservation equation (Nazaroff and Nero, 1988) vCðzÞ=vt ¼ De V2 CðzÞ lCðzÞ þ G;

(1)

where z is the depth from which Rn diffuses, l is the decay constant of radon (0.181 d1 or 2.1 106 s1) and G is the volumetric radon generation rate in the soil pores (Bq m3 s1) and is given by G ¼ f rs CRag l½ð1 3Þ=3;

(2)

where f is the emanation fraction (or emanation power), rs is the solid sand grain particles density, CRag is the 226Ra activity concentration in the sand Bq kg1 and 3 is the soil (sand) porosity. It is assumed that the effective diffusion coefficient, De, is constant. For steady state diffusion of radon the surface of the sample has zero concentration, and then a solution of Eq. (1) can be given at depth z by CðzÞ ¼ CN ½1 expðz=lÞ;

(3)

1/2

where l ¼ (De/l) is the diffusion length in the sand, with a typical value ranging from 100 to 140 cm, and CN is the radon concentration at large z (z [ l ) with CN related to G by G ¼ CNl. Thus we get from Eq. (2), the expression for f as f ¼ CN = rs CRag ½ð1 3Þ=3 (4) from which f can be obtained if other parameters are known. For small value of the sand depth z, as is the case in the present experiment, we obtain an approximation for C(z) as CðzÞ ¼ ðCN =lÞz

(5)

3.1. Collections of the samples 3.1.1. The case of the sand, gravel, cement, gypsum and paint Samples of sand of different types were collected from 10 different locations of sandbanks. From each sandbank (location) surface, we took samples from 2 to 4 different site points (here we call them sites). In collecting the samples, the exposed part of sand to the air was removed in order to make sure that the samples are localized and not assembled from different sites. The sand samples were mostly dry. The geographical area of collecting is bounded by Damascus highway connecting Beirut to the Lebanese-Syrian borders at Massna’ and the international borders with Syria in the east, Palestine and Israel in the south and the Mediterranean coast in the west as shown in Fig. 1. The map shows the locations of collection which are marked by the letters S. In Table 1, the names of the locations are coded for later tabulations. As for the gravel, the collection of the samples was obtained from several different quarries in coarse, crushed, finely crushed and powder forms as they are used in the construction industry for building dwellings. Locations of the quarries are marked on the map by the letters G. The cement samples were obtained from a contracting firm and were originally fabricated by the Lebanese cement company Shaka. Gypsum was obtained from a hardware store company dealing with building materials, while the paint samples of several types were taken from well sealed metal cans supplied by a paint dealer. 3.2. Density and porosity measurements of sand, cement and gypsum

and an exhalation rate given by vCðzÞ=vz ¼ El ¼ ðCN =lÞ;

(6)

which is constant for small depth of sand. In the can technique used in this experiment, the samples (sand, cement, gypsum, powdered gravel and paint) and the detectors

Measurements of the porosity 3, solid particles density rs and dry density rd of cement, gypsum and the different types of sand were performed. We have used a hydraulic pressure method and water method. In the second method and for the sand, which was relatively pure of soil dirt, we have measured the void volume Vv


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Fig. 1. The map of Lebanon shows, in the southern area, the locations of sandbanks (indicated by letters S) and the sites of the gravel quarries (indicated by letters G).

(space) by pouring a known initial volume Vi and mass M of sand from each type into a 100 cc distilled water contained in a graduated glass tube. By this way, we obtained the core volume Vs (solid volume) of the sand from the increase in the volume in the graduated tube and subtracted it from the initial bulk volume, thus obtaining Vv in the sample material. From these measurements, 3, rs and rd were calculated, which are related by the equation rd ¼ rs(1 3). Both methods, hydraulic and volumic measurements in water, gave the same consistent results. The

water method is valid, provided that sand sample does not contain soluble soil. 3.3. The g-spectroscopy technique 3.3.1. Procedural setting of the spectrometer Sand, gray and white cement, gravel and gypsum samples have been used for the measurements of the radionuclides contained in their grains. The collected samples were weighted and

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Table 1 Shows the names and codes of locations and the specific concentration values (Bq kg1) of radionuclides obtained for different sand samples from several sandbank locations in Lebanon by using g-ray spectrometry measurements Location name

Location code

40

DeirAl-ashayer Naba’ Alsafa Al-barouk Aital-fakhar WadyAl aishyeh DhahrAl aishyeh Dhohor Alaishyeh Al-Reehan Al-katran Al-khardaly

S1 4 sites S2 2 sites S3 3 sites S4 4 sites S5 2 sites S6 One site S7 2 sites S8 3 sites S9 2 sites S10 2 sites

3.4 0.6 2.6 0.6 2.5 0.3 13.6 0.6 1.1 0.3 11.2 0.8 7.8 0.6 1.0 0.3 2.4 0.6 1.6 0.6

Range Av. tot.

1.0–13.6 4.7 0.5

226

K

214

228

8.5 0.9 60.8 2.2 9.6 0.9 16.9 0.8 12.9 1.0 BDL BDL 10.1 0.9 4.2 0.4 7.2 0.6

5.1 0.2 26.6 0.9 5.1 0.2 8.7 0.3 5.7 0.3 6.0 0.3 4.7 0.2 4.3 0.2 5.2 0.3 4.3–26.6

4.2–60.8 13.0 0.8

4.3–26.6 7.8 0.3

Ra

Bi

homogenized. The homogenized samples, each having a volume of 500 cc, were put into a 1 l size well sealed containers (similar to those of the standard), stored a month to get a secular equilibrium and counted with two sets of g-spectroscopy system, each consists of high purity coaxial germanium detector; the first one of 30% and the second of 40% relative efficiencies, and a Desktop Inspector from Canberra. The full width at half maximum (FWHM) of the two systems were found to be 1.85 and 2.0 keV at 1332 keV of 60Co, respectively. The second g-system was shielded by a cylindrical chamber of 10 cm thick lead and 0.5 cm of copper in order to lower the background. The linearity of the detector was checked with a 152Eu point source. The detectors were energy calibrated with standard multigamma reference source and the efficiency calibration was performed. The curves were obtained by fitting the experimental efficiencies for each sample density and the efficiency curves were corrected for attenuation and self-absorption. The counting time for each sample was 36 h, which is sufficient to measure almost any radioactivity level. The spectra obtained by the second detector (40% efficiency) were analyzed off-line with the Genie-2000 analysis program from Canberra. The activity per unit dry mass Bq kg1 of each sample was evaluated.

212

212

238

4.6 0.2 9.3 0.3 4.8 0.2 9.3 0.3 6.3 0.3 6.3 0.3 5.7 0.2 4.0 0.2 5.3 0.2 6.2 0.3

4.5 0.1 9.6 0.3 4.4 0.2 8.9 0.3 6.3 0.2 6.7 0.2 5.9 0.2 3.8 0.1 5.5 0.2 6.2 0.2

2.9 0.2 5.5 0.4 2.8 0.2 5.9 0.4 3.9 0.3 4.6 0.3 3.9 0.3 2.0 0.1 3.7 0.3 4.3 0.3

0.5 0.1 3.7 0.2 0.6 0.1 1.0 0.1 0.8 0.1 0.9 0.1 0.7 0.1 0.6 0.1 0.7 0.1 0.9 0.1

15.3 1.2 74.3 9.3 16.7 1.5 31.2 3.0 21.9 1.9 9.9 1.3 8.8 1.0 15.9 1.5 11.9 1.1 16.2 1.7

4.0–9.3 6.2 0.2

3.8–9.6 6.2 0.2

2.0–5.9 4.0 0.3

0.5–3.7 1.0 0.1

8.8–74.3 22.9 2.1

Ac

Pb

Bi

U

CRaeq

was equipped with a flat rubber ring and strong elastic clamp, which allowed tight closing of the jar space. The interior of the jar has a square base with the dimensions 11.5 11.5 cm2 ¼ 132.25 cm2 as cross-sectional area. Its measured total volume is 2130 cm3. The dosimeter cup was calibrated earlier at Birmingham University, Physics Department and two recent additional calibrations have been performed in Germany; the first at the radon laboratory at Regensburg University and the second at the Federal Office for Radiation Protection in Berlin. All these calibrations gave almost the same calibration factor. This factor was derived from the second calibration using the following formula: Cx ¼ ðCo to rx Þ=ðro tx Þ;

(10) 3

where Co, ro, and to are the concentration (35,363 Bq m ), tracks density (25.4 103 tracks cm2) and the calibration exposure time (92 h), respectively. Cx, rx, and tx (2568 h) are the concentration, tracks density and exposure time of the measured 222Rn, respectively. From these quantities, the following calibration factor was derived as

Cx

Bq m3 ¼ 4:98 102 rx :

(11)

3.4. The solid state nuclear track detector method 3.4.1. The dosimeter The second technique used in this study is based on the utilization of a high quality CR-39 nuclear track detector, of thickness 500 mm, fabricated by Pershore Page Moulding, Ltd., UK. The geometry and dimensions of the measuring system as a dosimeter are shown in Fig. 2. It consists of a plastic cup, on whose bottom a 1.5 1.5 cm2 CR-39 detector has been fixed at the center by a double-stick tape and a pressure glass jar container. The jar cover

11.5 cm H = 16 cm

Sample

Sponge

Z

Glass Jar

CR-39

h = 5cm

Dosimeter Cup

Fig. 2. Schematic diagram shows detailed structure of the CR-39 dosimeter and the dosimeter jar used for radon measurements.

3.4.2. Measurement of Cx in sand For the measurements of radon concentrations, the same parts and types of the sand samples corresponding to those used in the g-measurements were employed and distributed into the jars’ containers. Two groups of dosimeter jars were arranged. One group of jars was prepared to measure Cx at constant and equal masses of sand from each site within the location of a sandbank and for all the locations. We took few sites in each location. For the second group of the dosimeter jars, the sand samples of the same type in a site were distributed in jars at varying heights z (thickness) so that measurement of 222Rn concentration can be taken in terms of these heights and weights. The lid of the cup dosimeter was facing the sand sample. The jars containing the monitoring dosimeters were tightly closed and left for 107 d as exposure time. Subsequently, the detectors were removed and etched in 7 N KOH solution bath for 8 h at a temperature of 70 C. The detectors were then rinsed thoroughly with distilled water and cleaned in an ultrasound distilled water bath for 15 min to detach any possible closing of the tracks’ openings by the solution residuals. The detectors were dried by fine paper tissues. The exposed area of each detector was scanned manually under a binocular optical microscope. The scanning of the area was performed sufficiently by counting tracks in a number of 40 field views making sure that a large


countable tracks per field were taken for each magnifying power, in order to obtain an average tracks density in each detector and to minimize the uncertainty due to counting errors. Standard deviations were found to be 10–20%. Radon concentrations were obtained using Eq. (11). 3.4.3. Measurement of Cx in cement, gypsum, gravel and paint For these materials the same procedure as for the sand was followed, using the glass jar containers. For gray and white cement, each jar contains the same mass of 1250 g. As for the gravel, measurements for coarse, crushed and powdered gravel having the same mass of 1500 g were taken. The etching and the exposure time was the same as for the sand. For the measurement of Cx in paint, 1 l of paint was taken from its sealed can and set into the dosimeter jar. The exposure time was 101 d and the same etching procedure mentioned above has been used.

4. Results and discussion 4.1. g-Measurements in the sand case The specific activities were expressed as concentrations of nuclides in Bq kg1 dry mass of sample with 95% confidence limit. Determination of 232Th was performed via its daughters 228Ac and 212 Pb at the g-rays’ energies 911.6 and 238.6 keV, respectively, and concentration of 212Bi, 214Bi and 226Ra were determined at the g-energies to be 727, 609.3 and 186.2 keV, respectively. Concentration of 40K was determined via the g-energy as 1460.8 keV. Ra concentration equivalent CRaeq was determined by the following expression (Kumar et al., 2003): CRaeq ¼ CRa þ 1:43CTh þ 0:077CK ;

(12)

where, CRa, CTh and CK are the specific activity concentrations (Bq kg1) of 226Ra, 232Th (228Ac) and 40K, respectively. Table 1 presents the results of the measurements of naturally radionuclides and shows the concentration distribution of Ra among the sites and consequently among the sandbanks’ locations, where site S2 has the highest mean value level of 60.8 2.2 Bq kg1. In some sites (sites S6 and S7), the concentration was below the detection level (