ENVIRONMENTAL PHYSICS

ASSESSMENT OF NATURAL RADIOACTIVITY LEVELS AND RADIATION HAZARDS FOR BUILDING MATERIALS USED IN QASSIM AREA, SAUDI ARABIA A. EL-TAHER1,2 1

Physics Department, Faculty of Science, Qassim University, Buraydah, 51452, Saudi Arabia Physics Department, Faculty of Science, Al-Azher University, Assuit Branch, 71542 Assuit, Egypt E-mail: [email protected]

2

Received July 3, 2011

Building materials are one of the potential sources of indoor radioactivity because of the naturally occuring radionuclides in them. External as well as internal exposures are the two pathways of radiation dose imparted to the human beings from the building materials. Natural radioactivity levels of 35 samples of natural and manufactured building materials used in Qassim area, Saudi Arabia have been investigated by using gamma spectrometer with NaI(Tl) detector. The samples were collected from local market and construction sites. From the measured γ-ray spectra, activity concentrations were determined. The activity ranged from 12.7 ±3.4 to 38.4 ±4.4 Bq kg-1 for 226Ra, from 13.2±0.7 to 49.2±2.3 Bq kg-1 for 232Th and from 64±3 to 340±6.7 Bq kg-1 40K. The activities are compared with available reported data from other countries and with the world average value for soils. The radium equivalent activity Raeq, the external hazard index Hex and the absorbed dose rate in air D in each sample was evaluated to assess the radiation hazard for people living in dwelling made of materials studied. All building materials have shwon Raeq ranged from 39.64 to 122.71Bq kg-1. These values are lower than the limit of 370 Bq kg-1 adopted by OECD the Organization for Economic Cooperation and Development. The absorbed dose rate in indoor air are lower than the international recommended values of 55 n Gy h-1 for all test samples. All the materials examined are acceptable for use as building materials as defined by the OECD criterion. Key words: natural radioactivity, building materials, radiation hazard parameters.

INTRODUCTION

All building raw materials and products derived from rock and soil contain various amounts of mainly natural radionuclides of the uranium (238U) and thorium (232Th) series, and the radioactive isotope of potassium (40K). In the 238U series, the decay chain segment starting from radium (226Ra) is radiologically the most important and, therefore, reference is often made to 226Ra instead of 238U. These radionuclides are sources of the external and the internal radiation exposures in dwellings. The external exposure is caused by direct gamma radiation while the Rom. Journ. Phys., Vol. 57, Nos. 3–4, P. 726–735, Bucharest, 2012

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inhalation of radioactive inert gases radon (222Rn, a daughter product of 226Ra) and thoron (220Rn, a daughter product of 224Ra), and their short-lived secondary products lead to the internal exposure of the respiratory tract to alpha particles. The specific activities of 226Ra, 232Th and 40K in the building raw materials and products mainly depend on geological and geographical conditions as well as geochemical characteristics of those materials [1]. The radiological impact from the natural radioactivity is due to radiation exposure of the body by gamma-rays and irradiation of lung tissues from inhalation of radon and its progeny. From the natural risk point of view, it is necessary to know the dose limits of public exposure and to measure the natural environmental radiation level provided by ground, air, water, foods, building interiors, etc., to estimate human exposure to natural radiation sources [2]. Low level gamma-ray spectrometry is suitable for both qualitative and quantitative determinations of gamma-ray-emitting nuclides in the environment. The concentration of radioelements in building materials and its components are important in assessing population exposures, as most individuals spend 80% of their time indoors. The average indoor absorbed dose rate in air from terrestrial sources of radioactivity is estimated to be 70 nGyh-1 [3]. Great attention has been paid to determining radionuclide concentrations in building materials in many countries [4-13]. In Saudi Arabia the information about the radioactivity of building materials is limited therefore, it is important to study: 1) Assess natural radioactivity (226Ra, 232 Th and 40K) in building materials used in Saudi Arabia by using γ-ray spectrometry, 2) Calculate the radiological parameters (radium equivalent activity Raeq, external hazard index Hex and absorbed dose rate) which is related to the external γ-dose rate to assess the radiological hazards to human health and for checking its quality in general and knowing its effect on the environment, 3) The measured activity concentrations for these natural radio-nuclides were compared with the reported data for other countries. The data obtained are essential for development of standards and guidelines concerning the use and management of building materials. SAMPLING AND SAMPLE PREPARATION

A total of 35 samples of natural and manufactured building materials used in Qassim area, Saudi Arabia have been collected from local market and construction sites. The sample each about 1 kg in weight were dried in an oven at about 105°C to ensure that moisture is completely removed. The samples were crushed, homogenized, and sieved through a 200 mesh, which is the optimum size enriched in heavy minerals. Weighted samples were placed in polyethylene beaker, of 350 cm3 volume each. The beakers were completely sealed for 4 weeks to reach secular equilibrium where the rate of decay of the daughters becomes equal to that of the parent [14-16]. This step is necessary to ensure that radon gas confined within the volume and the decay products will also remain in the sample.

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INSTRUMENTATION AND CALIBRAT

Radioactivity measurements were performed by gamma ray spectrometer, employing a scintillation detector 3"x 3". Its hermitically seald assembly which includes a high-resolution NaI (Tl) crystal, photomultiplier tube, an internal magnetic light shield, an aluminum housing and a 14 pin connector coupled to PC - MCA Canberra Accuspes. It has the following specifications: 1 – Resolution 7.5% specified at the 662 keV peak of 137Cs. 2 – Window Alumminum 0.5mm thick, density 147 mg/cm2. 3 – Reflector oxide; 1.6mm thick; density 88 mg/cm2. 4 – Magnetic / light shield-conetic lined steel. 5 – Operating voltage positive 902V.dc The efficiency calibration curve was made using different energy peaks covering the range up to ≈2000 keV. Measurements were performed with calibrated source samples, which contain a known activity of one or more gamma-ray emitters of the radionuclides 60Co (1173.2 and 1332.5 keV), 133Ba (356.1keV), 137 Cs (661.9 keV) and 226Ra (1764.49 keV). With certified accuracies of < 2 % supplied by PTB Braunschweig, Germany. We used for calculating the absolute efficiency the eq.(1) Eff =

100 ⋅ N p I γ ⋅ TOC ⋅ ABOC

(1)

With: Np= net peak area (count/ S) at Eγ , Iγ = intensity of emitted γ-ray(%), TOC = time of counting (S), and ABOC = activity (Bq) of the standard source at beginning of counting (BOC). ABOC was calculated by eq. (2) ABOC = ADOC · exp (-λ⋅(BOC-DOC))

(2)

Where ADOC is the activity (Bq) of the standard source at date of calibration DOC, and λ(s-1) is the decay constant. Daily efficiency and energy calibrations for each sample measurement were carried out to maintain the quality of the measurements. (BOC – DOC) defined as elapsed time between initial calibration and moment of measurement. CALCULATION OF ACTIVITY

Calculations of count rates for each detected photopeak and radiological concentrations (activity per mass unit or specific activity) of detected radionuclides depend on the establishment of secular equilibrium in the samples. The 232Th concentration was determined from the average concentrations of 212Pb (238.6 keV) and 228Ac (911.1 keV) in the samples, and that of 226Ra was determined from the average concentrations of the 214Pb (351.9 keV) and 214Bi (609.3 and 1764.5 keV) decay products [17-21].

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The activity concentration in Bqkg-1 (A) in the environmental samples was obtained as follows: A =

Np e ×η × m

(3)

Where Np = net count rate (cps), measured count rate minus background count rate, e is the abundance of the γ-line in a radionuclide, η is the measured efficiency for each gamma-line observed for the same number of channels either for the sample or the calibration source, and m the mass of the sample in kilograms. RESULTS AND DISCUSSION

Cement is an important construction material for houses and buildings in urban areas of Saudi Arabia. It is used for blocks and concrete manufacturing as well as for plastering the buildings walls, which made of bricks. However, detailed information of the specific activities of 226Ra, 232Th and 40K in cement and other building materials used in Saudi Arabia is not available in literature. This study is a continuation of our ongoing project related to the measurement of specific activity of 238U (226Ra), 232Th and 40K in environmental samples from Saudi Arabia using gamma-ray spectrometric technique and estimation of the gamma dose rate from these radionuclides. The average of measured activity values together with their respective standared deviation (SD) of the above natural radionuclides are presented in Table 1. As can be seen from Table 1, the activity concentrations of 226 Ra, of sand, gravels, marble, limestone, gypsum and granite were lower than that of the world average for soil 35 Bq kg-1 [22]. The activity levels of cement samples were slightly higher than the world average value of soil. For all building materials under investigation the measured thorium activities were higher than the radium activities. The observed thorium-232 activities were in the range 13.2- 49.2 Bq kg-1 while the observed potassium-40 activities were in the range 64-340 Bq kg-1. Table 2 compares the reported values of the radionuclides activities for selected building materials, obtained in other countries with those determined in this study. As shown from the table, the radioactivity in building materials varied from one country to another. It was important to point out that these values were not the representative values for the countries mentioned but for the regions from where the samples were collected. Radium, thorium and potassium are not uniformly distributed in soil or rocks, from which building materials are derived, but the radioactivity varies, often greatly, over a distance of some meters. The measured values of radium and thorium contents show only the average radioactivity in building materials used in Qassim area.

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Table 1 The average activity concentrations of 226Ra, 232Th and 40K in Bq.kg-1 for building materials used in Qassim area Saudi Arabia Ra-226

Th-232

K-40

Portland cement

Type of Material

38.4 ±3.8

45.3±1.2

86±4

Sand

12.3 ±1.4

19.7±1.5

260±5

Gravels

14.7±1.3

24.2±1.7

195±6

Limestone

28.6±4.2

49.2±2.5

66±3.6

Gypsum

33.28±4.7

47.2±2.8

88±4.4

Granite

23±1.4

30.0±0.4

340±6.7

Marble

12.7 ±3.4

13.2±1.4

64±3

Table 2 Comparison between the activity concentrations of our building materials with that of other countries of the World Radioactivity concentration (Bq/ kg) Materials Countries Portland Cement

Sand

Cameron Finland Norway Sweden U.K. Hong Kong Brazil Bangladesh Malaysia Bangladesh Algeria Italy Zambia India Egypt Egypt K.S.A Hong Kong Brazil Zambia Kuwait Jordan Bangladesh India Malaysia Egypt

226

Ra Mean ±Std 27±4 44 30 55 22 19.2 61.7 62.3±9.7 51 ± 1.0 29.7 41 ± 7 38 ± 14 23 ± 2 37 31.3 ± 3.6 36.6 ± 4.4 38.4 ±3.8 24.3 14.3 25 ± 1.0 7.4 ± 0.6 25.1 14.1± 2.3 43.7 60 ± 3 9.2 ± 2.5

232

Th Mean ± Std 15±1 26 18 47 18 18.9 58.5 59.4±7.4 23 ± 1.0 54.3 27±3 22 ± 14 32 ± 3 24.1 11.1 ± 1.1 43.2 ± 2.2 45.3±1.2 27.1 18 26 ± 2.0 7.2 ± 0.3 14.6 25.0± 4.3 64.4 13 ± 2 3.3 ± 1.3

40

K Mean ± Std 277±16 241 241 241 155 127 564 329.0±22.4 832 ± 69 523 422±3 218 ± 248 134 ± 13 432.2 48.6 ± 4 82 ± 4.1 86±4 841 807 714 ± 12 360 ± 14 188.1 158.4±31.4 455.8 750 ± 53 47.3 ± 9

(Ra)eq References Mean ± Std 70 (23) n.m. (24) n.m. (24) n.m. (24) n.m. (24) n.m. (25) 188.8 (26) 172.8±19.8 (27) 188 ± 27 (28) 148 (29) 112±8.2 (30) 92 ± 60 (31) 79 ± 11 (32) 104.7 (33) 50.9 (34) 103 (2) 108 Present work N.D. (25) 102.2 (26) 117 ± 12 (32) 45.4 (35) n.m. (33) 62.1±8.0 (27) 170.8 (33) 136 ± 33 (28) 16.6 (34)

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Table 2 (continued)

Gravels

Limestone

Gypsum

Granite

Marble

Algeria Egypt K.S.A Australia Brazil Nederland U.S.A Pakistan Egypt K.S.A Brazil Bangladesh Egypt Algeria Italy Egypt K.S.A Finland U.K. Brazil Kuwait Bangladesh Italy Egypt K.S.A Taiwan Greece India Hong Kong Brazil France Egypt K.S.A Algeria Italy Egypt K.S.A

12 ± 1.0 11.7 ± 1.4 12.3 ±2.3 13.9 10.3 9.7 33.3 24.8 13.4 ±1.2 14.7±1.3 24.3 68 20.4 ±2.8 16 ± 3 11 27.8 ±1.4 28.6±4.2 7 22 6.3 2.81 ±0.4 254±28. 6.0 ± 5.0 31.7 ±4.6 33.28±4.7 42 67 82 202 48.6 90 18 ± 1.4 23±1.6 23 ± 2 4.0 ± 12 15.9± 3.3 12.7 ±3.4

7.0 ± 1.0 18.8 ±1.5 24.2±1.3 14.8 N.D 12.6 33.3 9.9 23 ±1.3 24.2± 1.7 7.00 107 4.4 ± 0.8 13 ±2 2.0 ± 2.0 46.6 ±2.3 49.2±2.5 2 7 N.D. 0.55 ±0.1 21.4±2.8 2.0 ± 2.0 55 ± 2.7 47.2±2.8 73 95 112 140 288.2 80 24 ± 1.3 30.0±0.4 18 ± 2 0.9±3.6 12.3±0.7 13.2±1.4

74 ± 7.0 248 ± 5 195±4 171 933 140 14.8 51.3 193 ±4 195±6 205 1660 19.3 ± 2 36 ±3 22 ± 3 66 ±3.3 66±3.6 n.m. n.m. 18.1 17.4 294.3±27.7 12 ± 11 116 88±4.4 1055 1200 1908 1030 1335 1200 350±4 340±6.7 310±3 16±20 60±3 64±3.6

28 ± 7.1 57.65 60.35 48.2 82.1 38.5 82 42.9 62.4 65.7 50.1 349 25.4 37 ± 4.7 14 ± 11 79.85 83 n.m. n.m. 18.1 17.4 294.3±27.7 12 ± 11 116 107 Nm Nm N.m. N.m. 56.3.6 58.98 61.92 73±4.1 6.0±7.0 37.76 39.46

(30) (2) Present work (35) (26) (36) (37) (38) (2) Present work (26) (29) (34) ( 30) (31) (2) Present work (24) (24) (24) (33) (27) (31) (2) Present work (39) (40) (41) (25) (26) (43) (2) Present work ( 30) (31) (2) Present work

ESTIMATION OF EXPOSURE RISK

In this study, the radiological parameters such as indices of radium equivalent activity, external hazard index, and indoor absorbed gamma dose rate were calculated to estimate the exposure risk for building materials used in Qassim area in Saudi Arabia. Table 3 presents the average values of absorbed dose rate, radium equivalent activity and external hazard index.

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Table 3 The average values of radiation hazard parameters for Building materials used in Qassim area, Saudi Arabia Type of material Portland cement Sand Gravels Limestone Gypsum Granite Marble

Raeq (Bq kg-1) 108.23 59.88 65.51 83.84 122.71 61.89 39.64

Dose rate (nGy h-1) 48.94 29.49 37.64 37.01 54.27 25.08 18.17

Hex 0.19 0.17 0.28 0.17 0.24 0.17 0.07

ABSORBED DOSE RATE

Conversion factors to transform specific activities AK, ARa and ATh of K, Ra and Th, respectively, in absorbed dose rate at 1m above the ground (in nGy h-1 by Bq kg-1) are calculated by Monte Carlo method [42] as D (nGy h-1) = 0.0417 AK + 0.462 ARa + 0.604 ATh 226

232

(4) 40

where ARa, ATh and AK are the specific activities of Ra, Th and K in Bqkg-1. From Table 3, the average values of calculated absorbed dose rates in samples under investigation are ranged between 18.17 to 54.27 nGyh-1 and found to be comparable to the world average of 55 nGyh-1 [43]. RADIUM EQUIVALENT ACTIVITY

The natural radioactivity of building materials is usually determined from Ra, 232Th and 40K contents. As Radium and its daughter products produce 98.5 % of the radiological effects of the Uranium series, the contribution from the 238U has been replaced with the decay product 226Ra. Radium equivalent activity is an index that has been introduced to represent the specific activities of 226Ra, 232Th and 40K by a single quantity, which takes into account the radiation hazards associated with them. This first index can be calculated according to [44] as 226

Raeq = ARa + 1.43ATh + 0.077AK,

(5)

where ARa, ATh and AK are the specific activities of 226Ra, 232Th and 40K in Bqkg-1, respectively. The Raeq is related to the external γ-dose and internal dose due to radon and its daughters. The radium equivalent activity (Raeq) values for all building materials under investigation ranged from 39.64 to 122.71 Bq kg-1. These values are less than 370 Bq kg-1, which are acceptable for safe use [43].

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EXTERNAL HAZARD INDEX

The external hazard index is obtained from Raeq expression through the supposition that its maximum value allowed (equal to unity) corresponds to the upper limit of Raeq (370 Bq kg-1). This index value must be less than unity in order to keep the radiation hazard insignificant; i.e. the radiation exposure due to the radioactivity from construction materials is limited to 1.0 mSv y-1. Then, the external hazard index can be defined as

H ex =

ARa ATh A + + k ≤1 370 259 4810

(6)

where ARa, ATh and AK are the specific activities of 226Ra, 232Th and 40K in Bq kg-1, respectively. The calculated values of external hazard index obtained in this study ranged from 0.07 to 0.28. Since these values are lower than unity, we can say that the radiation hazard is insignificant for the population living in the investigated area. From the results it is evident that there are considerable variations in the Raeq of the different materials and also within the same type of materials originating from different areas. This fact is important from the point of view of selecting suitable materials for use in building and construction especially concerning those which have large variations in their activities. Large variation in radium equivalent activities may suggest that it is advisable to monitor the radioactivity levels of materials from a new source before adopting it for use as a building material [25]. The recommended maximum levels of radium equivalents for building materials to be used for homes is < 370 Bq kg-1 and for industries is 370-740 Bq kg-1. All the materials examined are acceptable for use as building materials as defined by the OECD criterion [45]. CONCLUSION

Materials derived from rock and soil contain mainly natural radioisotopes of the uranium-238 and thorium-232 series and radioactive isotope of potassium-40. Gamma ray spectrometry is powerful experimental tool in studying natural radioactivity and determining elemental concentration in various building materials. The radium-equivalent activities obtained for the building materials in this study were below the criterion limit of γ-radiation dose (370 Bq kg-1) adopted by the OECD, 1979 criterion. Therefore, the use of these materials in construction of dwellings is considered to be safe for inhabitants. The obtained results show that the most majority of the building materials used in Qassim area have the exemption level, thus they can be exempted from all controls concerning their radioactivity. Thus from the radiation safety, these materials are below the recommended limits for their gamma dose rates, therefore, they can be used for all kinds of republic buildings. Acknowledgements. The author thanks Deanship of Scientific Research at Qassim University for financial support for this work through financed grant made by Saudi Basic Industries Corporation (SABIC).

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