Determination of the natural radioactivity in Qatarian building materials using high-resolution gamma-ray spectrometry

Nuclear Instruments and Methods in Physics Research A 652 (2011) 915–919

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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima

Determination of the natural radioactivity in Qatarian building materials using high-resolution gamma-ray spectrometry Huda Al-Sulaiti a,b,n, N. Alkhomashi c, N. Al-Dahan a,d, M. Al-Dosari b, D.A. Bradley a, S. Bukhari e, M. Matthews f, P.H. Regan a, T. Santawamaitre a a

Centre for Nuclear and Radiation Physics, Department of Physics, University of Surrey, Guildford, GU2 7XH, UK Radiation Protection and Chemicals Department, Ministry of Environment, P.O. Box: 7634, Doha, Qatar c King Abdulaziz University of Science and Technology (KACST), P.O. Box 6086, Riyadh 11442, Saudi Arabia d Department of Physics, College of Science, University of Kerbala, Kerbala, Iraq e Information Systems Management Department, Ministry of Environment, P.O. Box: 7634, Doha, Qatar f Centre of Environmental Health Engineering, Department of Civil Engineering, University of Surrey, Guildford, GU2 7XH, UK b

a r t i c l e i n f o

abstract

Available online 12 January 2011

This study is aimed at the determination of the activity concentrations of naturally occurring and technically enhanced levels of radiation in building materials used across the State of Qatar. Samples from a range of common building materials, including Qatarian cement, Saudi cement, white cement, sand and washed sand, have been analyzed, in addition to other samples of cement’s raw materials and additives collected from the main suppliers in Qatar. In order to establish the activity concentrations associated with the 235,8U and 232Th natural decay chains and 40K, the samples have been studied using a high-resolution, low-background gamma-ray spectrometry set-up. Details of the sample preparation and the gamma-ray spectroscopic analysis techniques are presented, together with the preliminary results of the activity concentrations associated with the naturally occurring radionuclide chains for the building materials collected across the Qatarian peninsula. & 2011 Elsevier B.V. All rights reserved.

Keywords: HPGe detector Activity concentrations Gamma spectrometry Building materials NORM 226 Ra 232Th 40 K

1. Introduction Construction raw materials are products derived from rock, soil and industrial additives such as the byproduct of phosphoric acid manufacture (phosphogypsum) and fly ash from power stations. In addition to the naturally occurring radioactivity in the soil and rock these building material additives also contain trace amounts of natural radionuclides. Since individuals typically spend 80% of their time indoors, knowledge of the natural radioactivity levels in building materials is an important issue in the assessment of overall human exposure to natural radiation associated with 226Ra and 232Th (and their decay progeny) and the primordial radionuclide 40K [1,2]. Such studies are also used to set national standards in the light of global recommendations. The radionuclides present in the building materials are responsible for external and internal radiation exposures of individuals living in dwellings that vary depending on the geology of the region and the geochemical characteristics of the building materials. Internal exposure arises following the inhalation of alpha particles emitted from the short-

lived radionuclides radon (222Rn, the daughter product of 226Ra) and thoron (220Rn, the daughter product of 224Ra). External exposure is due to gamma radiation from the radionuclides present in the building materials. This information also helps to assess the level of potential radiological hazard to humans caused by the use of specific building materials. The current research aims to determine the radiation levels in the building materials used in the State of Qatar. This will contribute to the establishment of Qatarian national standards for the levels of natural radioactivities in building materials by determining the Gamma Dose Rate (D), Radium Equivalent (Raeq), External Hazard Index (Hex), Internal Hazard Index (Hin) and Annual Effective Dose Equivalent (AEDE) for individuals living in domestic dwellings and also in the workplace. This paper presents measurements from 20 samples of building materials collected from dwellings in Doha and discusses the techniques used in the analysis of the experimental results. This is part of a larger project aimed at providing a complete radiological map of Qatar [3].

2. Experimental arrangements n Corresponding author at: Centre for Nuclear and Radiation Physics, Department of Physics, University of Surrey, Guildford, GU2 7XH, UK. Tel.: + 44 1483 686602; fax: + 44 1483 686781. E-mail addresses: [email protected], [email protected] (H. Al-Sulaiti).

0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.01.020

2.1. Sample preparation Fifteen samples of four different commonly used building materials in the State of Qatar (such as Saudi cement, white cement,

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sand and washed sand) were collected directly from houses and buildings under construction. Other samples from the Qatarian cement, raw materials used as main constituents and samples from the minor additives of the Qatarian cement were collected from the main cement factories in the State of Qatar and from dwellings under construction. The samples were then filled into labeled polyethylene bags, sealed, packed in a box and shipped to the UK. The final sample preparation and all the high-resolution gammaray spectrometry measurements used to determine the sample radioactivities were performed in the low-background radiation detection laboratories of the Center for Nuclear and Radiation Physics, at the University of Surrey. Prior to final measurements in the laboratories, the collected samples were placed in a drying oven. The drying temperature was set to 60 1C for 24 h to ensure that any significant moisture was removed from the samples. The samples were then weighed and transferred to 550 ml labeled marinelli beakers and stored. The sealed samples were then kept for about one month in order to establish radioactive secular equilibrium between 222Rn and 220Rn and their respective parent nuclei, 226Ra and 224Ra. 2.2. Detector calibration

238 U decay chain, such as 214Pb and 214Bi, were also used to estimate the activity concentration of 226Ra. The activity concentration of 232Th was determined using gamma-ray transitions associated with the decay of 228Ac, 212Pb and 208Tl. The gammaray peaks associated with decays from 40K and 137Cs at 1461 and 662 keV, respectively, were used to determine the activity concentrations for these nuclei. Background contributions were subtracted from the peak areas for the measured samples using the GF3 data analysis package [4]. The activity concentration of the radionuclide found in the soil samples was determined using Eq. (1), and expressed in Bq/kg:

Ac ¼ Cnet =geðEg Þm

ð1Þ

where Cnet are the net peak counts, g is absolute gamma decay intensity for the specific energy photopeak, e(Eg) is the absolute photopeak efficiency of the germanium detector at this energy and m is the mass of the sample in kg. The total external absorbed dose rates (D) in indoor air due to gamma rays emitted by the 214Pb and 214Bi progeny of 226Ra, 232 Th decay chain and 40K at 1 m above the ground level can be calculated by Eq. (2): X Ax Cx ð2Þ D¼ x

The building material samples were analyzed using a highresolution, low-background gamma-ray spectrometry system based on a coaxial hyper-pure germanium detector (HPGe). Each spectrum was acquired for a counting time of 86,400 s (i.e. 24 h). The absolute photopeak efficiency calibration of the system was carried out using standard sources of 226Ra, 232Th, 152Eu and NG3 (a mixed source containing 241Am, 57Co, 60Co, 85Sr, 88Y, 109Cd, 137 Cs, 139Ce and 203Hg). The sources were placed surrounding the germanium detector with the radionuclides dispersed in gel matrices within marinelli beakers of geometries identical to that of the evaluated samples. The source activities of 226Ra, 232Th and 152 Eu were 3.10, 1.08 and 3.02 kBq, respectively for active volumes of 550 ml each and were spread homogenously in gel matrices of densities 1.1, 1.1 and 1.6 g/cm3, respectively. These were taken to be representatives of the density of the samples measured and therefore, corrections for gamma-ray self-attenuation within the samples are accounted for in the initial efficiency measurements using these marinelli-housed sources. The calibration spectra were also acquired for 86,400 s each. A range of discrete gamma-ray energies from 0.059 MeV (from the decay of 241 Am) up to 2.614 MeV (from the decay of 208Tl) were covered using these standard sources. 2.3. Measurements of activity concentration A marinelli beaker with the same geometry filled with deionized water was used on a weekly basis during the measurements period to determine the background spectrum observed by the germanium detector. The counting time of the ambient background spectrum was also 86,400 s. The building materials samples were placed, in their marinelli beakers, directly on to the front face of the detector. The counting geometry of the samples and the standard sources used for efficiency calibration were kept constant. A wide range of different gamma-ray energy transition lines ranging from 100 keV up to 2.614 MeV, associated with the decay products of the 235,8U and 232Th decay chains were then analyzed independently to obtain more statistically significant overall results. These data were combined under the assumption of secular equilibrium of the radionuclides within these samples. The initial activity concentration of 226Ra present in the samples was estimated using the g-ray transition at 186 keV. Several transitions from decays of shorter-lived radionuclides in the

where Ax (Bq/kg) is the mean activity of 226Ra, 232Th or 40K, and Cx (in units of nGy/h/Bq/kg) are the corresponding dose conversion coefficients that transform the specific activities into absorbed dose. In UNSCEAR 1993 and 2000 [2,5] these conversion coefficients were determined by the Monte Carlo simulation using the standard room model. Since this model is same as the typical Qatarian room geometry that is 4 m  5 m  2.8 m and thickness of walls, floor and ceiling and density of the structures are 20 cm and a density of 2350 kg m  3 (concrete), the dose conversion coefficients used in the calculation of 226Ra, 232Th and 40K were 0.92, 1.1 and 0.08, respectively. The published maximal admissible dose rate is 55 nGy/h. Radium equivalent activity (Raeq) is used to assess the hazards associated with materials that contain 226Ra, 232Th and 40K in Bq/kg [2]. This equivalent is calculated with the assumption that 370 Bq/kg 226Ra or 260 Bq/kg 232Th or 4810 Bq/kg 40K produce the same g dose rate. The Raeq of the sample in (Bq/kg) can be determined using the following equation [6]: Raeq ¼ ðAK 0:077Þ þðAU Þ þðATh 1:43Þ

ð3Þ

The published maximal admissible Raeq is 370 Bq/kg [2]. The external hazard index is an evaluation of the hazard of the natural gamma radiation [7]. The prime objective of this index is to limit the radiation dose to the admissible dose equivalent limit of 1 mSv/y [8,9]. This index can be evaluated using the following equation: Hex ¼ ðAU =370Þ þ ðATh =259Þ þ ðAK =4810Þ r 1

ð4Þ

This model did not take into consideration the wall thickness and the existence of doors and windows. Hewamanna et al. [10] modified the standard room model in order to take into account the effect of these parameters. The external hazard index equation becomes in this case: Hex ¼ ðAU =740Þ þ ðATh =520Þ þðAK =9620Þ r1

ð5Þ

The external hazard index should be below unity for the radiation hazard to be negligible. Inhalation of alpha particles emitted from the short-lived radionuclides radon (222Rn, the daughter product of 226Ra) and thoron (220Rn, the daughter product of 224Ra) is also hazardous to the respiratory organs. This hazard can be quantified by the internal hazard index (Hin) [11,12], which is given by the

H. Al-Sulaiti et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 915–919

materials samples under investigation with those obtained from published studies for other Arabian countries in Bq/kg. The Qatari and Saudi cement samples contain higher 226Ra and 232Th than sand, washed sand and white cement samples. Consequently these samples have higher radium equivalent activities, which in turn would produce higher absorbed dose rates in indoor air due to gamma rays emitted by the 226Ra, 232Th decay chain and 40 K to inhabitants at 1 m above the ground level in Qatarian dwellings. The results obtained in the current work are lower than other published data but lie within the world ranges for building materials of 50, 50 and 500 (Bq/kg) for 226Ra, 232Th and 40 K, respectively [5]. The levels obtained in the current work for the 226Ra and 232Th decay chains and the 40K radioisotope were comparable with the level obtained for soil from our previous study [3]. Such agreement might be expected since cement’s raw materials (limestone, sand, shale and clay) are usually mined

following equation: Hin ¼ ðAU =185Þ þðATh =259Þ þ ðAK =4810Þ r1

ð6Þ

The internal hazard index should be below unity in order to provide safe levels of radon and its short-lived daughters for the respiratory organs of individuals living in the dwellings. In order to estimate the annual effective dose rate in air the conversion coefficient from absorbed dose in air to effective dose received by an adult has to be taken into consideration. This value is published in UNSCEAR 2000 and 1993 [2,5], to be 0.7 Sv/Gy for environmental exposure to gamma rays of moderate energy. The indoor occupancy factor is about 0.8 [2]. The annual effective dose equivalent is given by the following equation [13]: AEDEðmSv=yÞ ¼ DðnGy=h8760ðh=yÞ  0:8  0:7ðSv=GyÞ  103

917

ð7Þ

The world average annual effective dose equivalent (AEDE) from indoor terrestrial gamma radiation is 0.460 mSv/year [2].

50

3. Results Activity Concentration (Bq/Kg)

45

30 25 20 15 226

Ra Pb 214 Bi

10

214

0 0

500

1000

1500

2000

2500

3000

Energy (keV)

Th 93 Bi 609

Pb 87

234

Pb 75

214

214

Pb 351

400

214

Pb 295

Fig. 2. Individual activity concentrations of the observed gamma-ray transitions from 226Ra, 214Pb and 214Bi from the 238U series in sample QC2. The horizontal line corresponds to the weighted mean of these individual data points.

Bi 1238

Bi 1377

214

214

214

1200

214

Bi 1847

Bi 1764 214

Bi 1729 214

1000

214

600

214

228

Bi 1120

400

Bi 1280

Ac 969

208

228

Tl 583

Ac 338

214

Bi 935

Ac 911

Tl 860

228 208

Tl 1592

208 214

DE

Bi 1509

x3.7

K 1460

20

800

40

40

214

Bi 769

Bi 728

30 0

Counts

200

214

214

SE

Counts

60

35

200

200 0

40

5

214

212

Ra 186

400

226

Counts

600

Pb 242

Pb 238

Fig. 1 is the background-subtracted spectrum for Saudi cement sample no. L007 and clearly shows transitions associated with decays from the 226Ra, 214Pb and 214Bi isotopes associated with the 238U decay chain, as well as 228Ac and 208Tl decay lines from the 232Th series. Figs. 2 and 3 show the measured activity concentrations for Qatarian cement sample QC2 highlighting the data from the transitions assuming equilibrium decays of 226Ra, 214Pb and 214 Bi in the 238U series and from 228Ac, 212Pb and 208Tl decay associated with the 232Th series. The mean activity concentrations of the radionuclides 226Ra, 232Th and 40K in this sample were found to be 23.4 70.3, 12.1 70.2 and 159 74 Bq/kg. These compare to the worldwide population-weighted mean concentrations of the radionuclides 226Ra, 232Th and 40K in building materials of 50, 50 and 500 Bq/kg, respectively [5]. Table 1 summarizes the results of the mean activity concentration and mean radium equivalent of Saudi and Qatari building

1800

2000

208

Tl 2614

Bi 2203

10

1600

214

Counts

0

0 2200

2400 Energy (keV)

2600

Fig. 1. Background-subtracted gamma-ray spectrum associated with Saudi cement sample L007. Lines from the

2800 238

U and

232

Th decay series are clearly identified.

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locally and represent the local soil components. The mean values of radium equivalent activities of Saudi cement samples ranged from 28 to 56 Bq/kg, while the Qatari cement samples ranged from 51 to 53 Bq/kg; both are well below the published maximal

18

Activity Concentration (Bq/Kg)

16

admissible Raeq of 370 Bq/kg [2], which corresponds to an annual effective dose of 1 mSv. From Table 2 it can be observed that the mean values of annual effective dose due to terrestrial gamma radiation indoors from cement are in the range 0.127–0.252 mSv, taking into account typical geometries of rooms in most Qatarian houses. The mean value is found to be less than the average external annual effective dose of 0.410 mSv from natural indoor radiation sources of terrestrial origin evaluated by UNSCEAR 2000 [2]. The values of Hin and Hex for the building materials samples were also found to be below unity. Table 3 shows values of activity concentration of raw materials samples used as main constituents and samples from the minor

14 Table 3 Weighted mean values of activity concentration of raw materials samples used as main constituents and samples from the minor additives of the Qatarian cement compared to the mean activity concentration for those obtained from other published studies in Bq/kg.

12

10

228 212 208

8 0

Ac

Sample

Mean activity concentration (Bq/kg)

Pb 226

Tl

500

1000

1500

2000

2500

3000

Energy (keV) Fig. 3. Individual activity concentrations and weighted mean (dotted line) calculated from the observed gamma-ray transitions from 228Ac, 212Pb and 208Tl decay lines associated with the 232Th series in cement QC2.

Ra

23.5 70.3 15.3 70.3 11.5 70.2

Clinker Kiln feed Gypsum

232

40

9.77 0.2 6.87 0.2 2.57 0.1

185 7 8 139 7 5 72 7 3

Th

235

K

U

1.4 70.4 0.77 0.2 0.47 0.2

Clinker (Turkey, 2008) [18] 28.3 713.3 15.9 73.2 219.0 7 45.2 – Gypsum (Turkey, 2008) [18] 10.8 712.2 3.67 2.7 44.5 7 23.2 – Gypsum (India, 2003) [19] 8.3 – 26.7 –

Table 1 Mean activity concentration of building materials samples and mean radium equivalent of Saudi and Qatari building materials samples with those obtained in other Arabian countries in Bq/kg. Material

No. of samples

Mean activity concentration (Bq/kg) 226

Ra Mean 7S.D. Range

232 Th Mean 7S.D. Range

40 K Mean7 S.D. Range

Mean 7S.D. Range

8.07 1.7 (4–11) 12.2 7 0.2 (12–13) 3.34 7 0.05 (3–4) 3.67 0.2 (3–4) 4.97 0.5 (4–6)

81 7 3 (39–178) 158.8 7 4.3 (155–162) 225.5 7 6.1 (216–235) 228.2 7 7.7 (210–242) 62.9 7 22.6 (26–87)

40.6 77.4 (28–56) 52.35 71.13 (51–53) 33.72 70.78 (32–35) 34.5 71.2 (31–37) 30.3 71.2 (28–32)

9a 27 11.23 33 50

79.7 422 265.12 337 500

92.2 112 79.7 151 370

Saudi cement Qatari cement Sand Washed sand White cement

4 2 4 4 3

23.5 7 3.6 23.4 7 0.6 13.2 7 0.3 13.5 7 0.5 18.9 7 0.5

Cement (Lebanon, 2008) [14] Cement (Algeria, 2001) [15] Cement (Jordan, 1998) [16] Cement (Egypt, 2006) [17] Worldwide value [5]

– 12 25 85

73.2 41 43.21 78 50

a

This value was estimated from

Mean Raeq (Bq/kg)

(17–31) (23–24) (12–14) (12–14) (17–22)

228

Ac gamma lines only.

Table 2 Mean values and standard deviation of dose in air, annual effective dose equivalent of inhabitants at 1 m above the ground level in Qatarian dwellings and the corresponding external and internal hazard indices due to building materials. Material

D in air (nGy/h)

Annual effective dose (mSv)

Mean7 S.D. Range

Mean 7S.D. Range

Saudi cement Qatari cement Sand Washed sand White cement

36.9 7 6.9 (26–51) 48 71 (47–49) 33.8 7 0.8 (32–35) 34.5 7 1.2 (31–37) 27.8 7 0.8 (26–28)

0.181 7 0.034 0.236 7 0.005 0.166 7 0.004 0.169 7 0.006 0.137 7 0.004

Worldwide value [2]

55

0.410

a

The upper limit.

Hazard index Hin Mean 7S.D. Range

Hex Mean7 S.D. Range (0.127–0.252) (0.232–0.239) (0.159–0.172) (0.156–0.180) (0.130–0.140)

0.0567 0.010 0.0727 0.002 0.0487 0.001 0.0497 0.002 0.0427 0.001 1a

(0.039–0.076) (0.071–0.073) (0.046–0.049) (0.045–0.052) (0.039–0.043)

0.175 7 0.029 0.209 7 0.005 0.131 7 0.003 0.134 7 0.005 0.134 7 0.008 1a

(0.125–0.226) (0.205–0.212) (0.126–0.136) (0.123–0.142) (0.125–0.146)

H. Al-Sulaiti et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 915–919

additives of the Qatarian cement with those obtained from other published data in Bq/kg. The activity concentration of 226Ra, 232Th and 40K of raw materials of Qatarian cement are comparable with published values of other countries.

4. Conclusions The measured level of natural radioactivity in the present study for the Saudi and Qatari building materials from the 20 investigated samples used in the State of Qatar is lower than other published data but lies within the world ranges for building materials of 50, 50 and 500 (Bq/kg) for 226Ra, 232Th and 40K, respectively [5]. It can be concluded as well from the results obtained in the current work that the mean values of radium equivalent activities of Saudi and Qatari building materials samples are all well below the recommended safety limit of 370 Bq/kg [2]. Therefore, the use of these building materials in construction of Qatarian dwellings is considered to be safe for human habitation. Absorbed dose and annual effective dose for individuals living in domestic dwellings were all found below the worldwide average. The external and internal hazard indices were also found to be less than unity, which indicates a low dose. In view of the above results, these building materials are quite safe to be used for residential construction in the State of Qatar. Since only few samples for each type of building materials have been investigated in the current work it is suggested that more samples from other areas in the State of Qatar and from cement’s raw materials and additives are to be collected and investigated in a further study in order to get more representative values for the level of naturally occurring and technically enhanced radioactive materials in construction materials, which will be useful for establishing a national standard on radioactivity of building materials in the State of Qatar.

Acknowledgments This work is funded by the Ministry of Environment, Doha-Qatar. Huda Abdulrahman AL-Sulaiti would like to acknowledge the minister of environment, H.E. Abdulla bin Mubarak Al-

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Moadhadi for his continuous support. HA would also like to thank Qatar National Cement Company for providing the samples. Nawras Al-Dahan would like to acknowledge the Ministry of Higher Education and Scientific Research, University of Kerbala, College of Science, Department of Physics. TS acknowledge receipt of PhD studentships from the Royal Thai Government.

References [1] O. Brigido Flores, A. Montalvan Estrada, R. Rosa Suarez, J.Tomas Zerquera, et al., Journal of Environmental Radioactivity 99 (12) (2008) 1834. [2] UNSCEAR, Effects of Atomic Radiation to the General Assembly, in United Nations Scientific Committee on the Effect of Atomic Radiation, United Nations, New York, 2000. [3] H. Al-Sulaiti, P.H. Regan, D.A. Bradley, D. Malain, et al., Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 619 (1–3) (2010) 427. [4] D.C. Radford, Nuclear Instruments and Methods in Physics Research Section 361 (1995) 297. [5] UNSCEAR, Exposure from natural sources of radiation, in United Nations Scientific Committee on the Effect of Atomic Radiation, United Nations, New York, 1993. [6] UNSCEAR, Ionising radiation: sources and biological effect, in United Nations Scientific Committee on the Effect of Atomic Radiation, United Nations, New York, 1982, ISBN: 9211422426. [7] Noorddin Ibrahim, Journal of Environmental Radioactivity 43 (3) (1999) 255. [8] ICRP 60, Recommendations of the International Commission on Radiological Protection, in ICRP Publication 60, Pergamon Press Annals of the ICRP, Oxford, UK, 1990. [9] Ibrahim F. Al-Hamarneh, Mohammad I. Awadallah, Radiation Measurements 44 (1) (2009) 102. [10] R. Hewamanna, C.S. Sumithrarachchi, P. Mahawatte, H.L.C. Nanayakkara, et al., Applied Radiation and Isotopes 54 (2) (2001) 365. [11] J. Beretka, P.J. Mathew, Health Physics 48 (1985) 87. [12] Lu Xinwei, Radiation Measurements 40 (1) (2005) 94. [13] UNSCEAR, Sources, Effect and Risk of Ionising Radiation, in United Nations Scientific Committee on the Effect of Atomic Radiation, United Nations, New York, 1988, ISBN: 92-1-142143-8. [14] M.A. Kobeissi, O. El Samad, K. Zahraman, S. Milky, et al., Journal of Environmental Radioactivity 99 (8) (2008) 1279. [15] D. Amrani, M. Tahtat, Applied Radiation and Isotopes 54 (4) (2001) 687. [16] N. Ahmad, A.J.A. Hussein, Aslam, Journal of Environmental Radioactivity 41 (2) (1998) 127. [17] E.M. El Afifi, M.A. Hilal, S.M. Khalifa, H.F. Aly, Radiation Measurements 41 (5) (2006) 627. [18] S. Turhan, Journal of Environmental Radioactivity 99 (2) (2008) 404. [19] Ajay Kumar, Mukesh Kumar, Baldev Singh, Surinder Singh, Radiation Measurements 36 (1–6) (2003) 465.