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ORIGINAL ARTICLE
Year : 2013  |  Volume : 36  |  Issue : 3  |  Page : 99-105  

Dose assessment for natural radioactivity resulting from tiling granite rocks


1 Department of Siting and Environmental, Nuclear and Radiological Regulatory Authority, Cairo, Egypt
2 Ahmed El Zommer, Cairo, Egypt
3 Department of Nuclear Law, Nuclear and Radiological Regulatory Authority, Cairo, Egypt

Date of Web Publication28-Jul-2014

Correspondence Address:
Kh A Allam
Nuclear and Radiological Regulatory Authority, 3 Ahmed El Zommer, Cairo
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.137471

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  Abstract 

The gamma radiation in samples of a variety of natural tiling granite rocks from different quarries located in South Sinai, Egypt used in the building industry is measured, using high-resolution g-ray spectroscopy. The samples pulverized, sealed in plastic Marinelli beakers, are analyzed in the laboratory with an accumulating time between 18 and 24 h each. Activity concentrations are determined for 238 U (from 18 to 361 Bq/kg), 232 Th (range from 20 to 316 Bq/kg) and 40 K (from 499 to 3089 Bq/kg). The total absorbed dose rates in air ranged from 42 to 440 nGy/h. The external hazard index ranged (from 0.23 to 2.49), the internal hazard index ranged (from 0.28 to 3.38), and the activity utilization index ranged (from 0.69 to 5.90). Applying dose criteria recently recommended by the European Union for superficial materials, 25 of the samples meet the exemption dose limit of 0.3 mSv/year, two of them meet the upper dose limit of 1 mSv/year and only one clearly exceeds this limit.

Keywords: Environmental radioactivity, granite radioactivity, hazard index, high-purity germanium detector


How to cite this article:
Allam KA, Ramadan AA, Taha A. Dose assessment for natural radioactivity resulting from tiling granite rocks. Radiat Prot Environ 2013;36:99-105

How to cite this URL:
Allam KA, Ramadan AA, Taha A. Dose assessment for natural radioactivity resulting from tiling granite rocks. Radiat Prot Environ [serial online] 2013 [cited 2020 Oct 27];36:99-105. Available from: https://www.rpe.org.in/text.asp?2013/36/3/99/137471


  Introduction Top


Granites are the most abundant plutonic rocks of mountain belts and continental shield areas. They occur in great batholiths that may occupy thousands of square kilometers and are usually closely associated with quartz Mennonite, granodiorite, diorite, and gabbro. They are extremely durable and scratch resistant; their hardness lends themselves for the stone to be mechanically polished to a high gloss finish. Their variety of colors and unique heat and scratch resistant properties make them ideal for use as a work-surface, flooring or external and internal cladding. They mainly consist of coarse grains of quartz, potassium (K) feldspar and sodium feldspar. Other common minerals that granites consist of mica and hornblende. Typical granite chemically composed of 75% silica, 12% aluminum, <5% potassium oxide, <5% soda, as well as by lime, iron, magnesia, and titania in smaller quantities.

In terms of natural radioactivity, granites exhibit an enhanced elemental concentration of uranium (U) and thorium (Th) compared with the very low abundance of these elements observed in the mantle and the crust of the Earth. Geologists provide an explanation of this behavior in the course of partial melting and fractional crystallization of magma, which enables U and Th to be concentrated in the liquid phase and become incorporated into the more silica-rich products. For that reason, igneous rocks of granitic composition are strongly enriched in U and Th (on an average 5 ppm of U and 15 ppm of Th), compared with rocks of basaltic or ultramafic composition (1 ppm of U). [1],[2]

A gamma-ray spectrometer based on high-purity germanium (HpGe) detector was used to determine the concentration of natural radionuclides in an extensive selection of Egyptian samples. The annual effective dose rates and the gamma activity concentration index will be evaluated and compared to the average worldwide exposure limits represented in UNSCEAR 2000 and to the dose criteria recommended European Commission (EC, 1999), respectively. [3],[4] In addition, the correlations between Th, U and K will be shown in this paper with an aim to correlate the petrographic characteristics of commercial granites with their corresponding dose rates for natural radioactivity.


  Materials and methods Top


Sample collection and preparation

In this study, 30 samples of different types of the main commercial granites used in Egypt were collected directly from mining and quarries in three Egyptian locations in Sinai: Wadi Umadawy, Wadi Mandar and Wadi Lethy as shown in [Figure 1] and listed in [Table 1]. Each sample is crushed to small pieces and grinded. Then the samples were weighed and packed in Marinelli-type beaker (100 and 1000 ml capacities according to the available sample amounts) to be analyzed using gamma spectrometers. Samples were carefully sealed and stored for more than 4 weeks for secular equilibrium. The samples were analyzed in the geometries used during the procedure of efficiency determination.
Figure 1: Sampling locations

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Table 1: Samples province, rock type and activity concentration in Bq.kg−1


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Calibration and measurements by gamma ray spectrometry

The gamma ray spectra of the prepared samples were measured for at least 82,000 s using a typical high resolution gamma spectrometer based on a coaxial type shielded HpGe detector, with a relative photo peak efficiency of 35% and energy resolution of 1.9 keV full width at half maximum for the 1332 keV gamma ray line of 60 Co. The spectrum was collected and analyzed using computer software called Genie 2000 software made by Canberra Industries Inc, USA. The activity of 40 K was measured directly via its 1461 (10.7%) keV peak of the gamma ray spectra. To determine the activity concentration of 226 Ra, the average value of gamma ray lines 295.1 (19.2%) and 351.9 (37.1%) keV from 214 Pb to 609.3 (46.1%) and 1764.5 (15.9%) keV gamma rays from 214 Bi are used. Activity concentration of 232 Th is determined using the average value of gamma ray lines 238.6 (43.6%) keV from 212 Pb, 338.4 (12%), 911.1 (29%) and 968.9 (17.4%) keV from 228 Ac, 583.1 (86%) and 2614 keV from 208 Tl. The detector was calibrated for the efficiency using 226 Ra point source to first produce a relative efficiency curve followed by standardization using KCl as a standard solution (Farouk and Al Soraya, 1980). [5] Quality control and quality assurance of the measurements using International Atomic Energy Agency (IAEA) reference materials (Soil6, IAEA-326). In addition, duplicate samples were added to insure the analyses consistency of the measurements. Blank samples were added to eliminate the cross-contamination occurrence in the samples.


  Results and discussion Top


Specific radioactivity

The distribution of natural radionuclides in granite samples is presented in [Table 1]. The activity concentrations of 238 U ranged from 18 to 361 Bq/kg, 232 Th from 20 to 316 Bq/kg and 40 K from 499 to 3089 Bq/kg). From the 30 samples measured studied in this work, the maximum activity value of 238 U was in the sample (7) (syanogranite) 361 Bq/kg and the minimum value was in the sample (28) (granodiorite) 18 Bq/kg. While for 232 Th the maximum level was observed in sample (24) (syanogranite) 361 Bq/kg and the minimum level in sample (28) (granodiorite) 20 Bq/kg. The value of 40 K ranged from 499 Bq/kg in the sample (28) (granodiorite) to 3089 Bq/kg in the sample (24) (syanogranite). Syanogranite appears to present the highest concentrations of all the elements investigated, reaching levels of 361 Bq/kg for 238 U, 316 Bq/kg for 232 Th, and 3089 Bq/kg for 40 K. All measured samples except of sample (28), show concentrations of 40 K above the value of 1000 Bq/kg. In addition, only four samples appear to present concentration of 232 Th and 238 U higher than 150 Bq/kg [Table 1].

The measured activity concentrations of 232 Th, 238 U and 40 K can be converted into total elemental concentrations of U, Th (in ppm) and of the K (in percent), respectively. [6] The extracted values for the elemental concentration are for U (from 1.5 to 29.1 ppm), for Th (range from 5.0 to 79.0 ppm), and for K (from 1.52 to 9.42). The results are summarized in [Figure 2].
Figure 2: Total elemental concentrations of uranium, thorium (in ppm) and of potassium (in percent)

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Radium equivalent activity (Ra eq )

The distribution of natural radionuclides in the samples under investigation is not uniform. Therefore, a common radiological index has been used to evaluate the actual activity level 226 Ra, 232 Th and 40 K in the samples and the radiation hazards associated with these radionuclides. [7] This index usually known as a radium equivalent activity. [7]



Where ARa , ATh and AK are the specific activities of 226 Ra, 232 Th and 40 K respectively in Bq/kg. In the definition of radium equivalent, it is assumed that 10 Bq/kg of 226 Ra, 7 Bq/kg of 232 Th and 130 Bq/kg 0f 40 K produce an equal gamma ray dose rate. [8],[9]

The values of calculated Ra eq for collected samples are shown [Table 2]. The calculated Ra eq values range from 84.8 to 922.9 Bq/kg with an average of 374.2 Bq/kg. In this study, there are 19 samples found to be lower than the criterion limit of 370 Bq/kg (Nuclear Energy Agency [NEA] OECD (1979). [10]
Table 2: The calculated dose rate, annual effective dose HE (mSv/year), utilization index, external hazard index, internal hazard index, concentration index IC and elemental concentration (ppm)


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Dose rates (D), annual effective doses and activity utilization index (I u )

If naturally occurring radioactive nuclides uniformly distributed in sample environment, dose rates, D, in units of nGy/h can calculate by the following formula: [11]



The total absorbed dose rates calculated from the concentrations of the nuclides of the 238 U and 232 Th series, and of 40 K, range from 41.9 to 440.3 nGy/h. In round terms and for full utilization, 21 of the samples exhibit dose rates that range from 100 to 250 nGy/h, four samples exhibit dose rates under the typical limit of 100 nGy/h and only five of the samples exhibit values over the limit of 250 nGy/h.

The analytical results for the total absorbed dose rates in the air for each of the measured samples and for various fractional masses indicated are also given in [Table 2]. For comparison, measurements in former Czechoslovakia, in houses with outside walls containing uraniferous coal slag, gave values approaching 1000 nGy/h, [12] while measurements in a granite region of the United Kingdom, where some of the houses are made of local stone, gave 100 nGy/h. [13]

The relative contribution to total absorbed dose due to 238 U ranges from 13% for sample no. (19, 20) to 39% for sample no (7), due to 232 Th ranges from 14% for sample no.(27) to 40% for sample no (24), and due to 40 K ranges from 22% for sample no (7) to 72% for sample no (27).

In order to estimate the annual effective doses, one has to take into account the conversion coefficient from absorbed dose in air to effective dose and the indoor occupancy factor. In the UNSCEAR reports (2000), a value of 0.7 Sv/year is used for the conversion coefficient from absorbed dose in air to effective dose received by adults, and 0.8 for the indoor occupancy factor, implying that 20% of time is spent outdoors, on average, around the world. [14] The effective dose rate indoors, HE , in units of mSv/year, calculated by the following formula:



where D is the calculated dose rate (in nGy/h), T is the indoor occupancy time (0.8 × 24 h × 365.25 d = 7008 h/year), and F is the conversion factor (0.7 × 10−6 mSv/Gy).

The annual effective doses indoors estimated, using Equation 5. Four examined samples are below or equal the average worldwide exposure of external terrestrial radiation (indoors) ∼0.41 mSv/year due to natural sources (UNSCEAR 2000 Report). [4] Twenty-two examined samples (73.3%) are meet the IAEA exemption criteria for natural sources 1 mSv/year (General Safety Requirements Part 3, 2011).

In order to facilitate the calculation of dose rates in air from different combinations of the three radionuclides in building materials and by applying the appropriate conversion factors, an activity utilization index (IU ) is constructed that is given by the following formula:



Where AU, ATh and AK are the actual values of the activities per unit mass (Bq/kg) of 238 U, 232 Th, and 40 K in the building materials considered; fU, fTh and fK are the fractional contributions to the total dose rate in air due to gamma radiation from the actual concentrations of these radionuclides. In the NEA 1979 Report, typical activities per unit mass of 232 Th, 238 U, and 40 K in building materials. [10]

The activity utilization index estimated using the fractional contribution to the dose rate from the three radionuclides for the 30 samples are presented in [Table 2]. As the activity concentration of the three radionuclides ( 232 Th, 238 U, 40 K) and their corresponding fractional contribution to the total dose rate vary from sample to sample, the activity utilization index ranges from 0.7 for sample 28-5.9 for sample 24. It should be noted that, 29 of the measured samples exhibit an activity utilization index that ranges from 1.5 to 5.9 and only one samples show values under that range (≤1.0).

Radiation hazard indices

In order to measure the hazards one can define radiation hazard indices (Beretka and Mathew (1985) Mathew (1985) (a) the external radiation hazard, Hex and (b) internal radiation hazard, Hin , as follows: [7]

External radiation hazard (H ex )

The external hazard index is another criterion to assess the radiological suitability of a material. It is defined as follows:



where ARa, ATh and AK are the actual values of the activities per unit mass (Bq/kg) of 238 U, 232 Th, and 40 K in the building materials considered: It is observed from [Table 2] that the mean value 1.0 of Hex is equal the criterion value (1). [Figure 3] shows different kind of building materials with Hex .
Figure 3: Internal and external hazard index for each sample

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Internal radiation hazard (H in )

The internal hazard index is a criterion for index radiation hazard. In addition to gamma rays, 222 Rn plays an important role for internal exposure in a room. Effectively, the radio toxicity of 238 U is increased by a factor of two to allow for the contribution from 222 Rn and its short lived progeny. The internal exposure due to radon and its daughter products is quantified by the internal hazard index Hin . It's given by the following formula:



The internal hazard index is defined to reduce the acceptable maximum concentration of 226 Ra to half the value appropriate to external exposures alone. For the safe use of materials in the construction of dwellings, the following criterion was proposed by Krieger Hin ≤ 1. The mean value of Hin is determined to be 1.3, which is bigger than one which indicates that the internal hazards are bigger than the critical value. [Figure 3] shows the different kinds of building materials with internal hazards.

Finally, according to most recent regulations and especially the recommendation No. 112 issued by the European Union in 1999 (European Commission [EC], 1999), [3] building materials should be exempted from all restrictions concerning their radioactivity, if the excessive gamma radiation due to those materials causes the increase of the annual effective dose received by an individual with a maximum value of 0.3 mSv. [3] Effective doses exceeding the dose criterion of 1 mSv/year should be taken into account in terms of radiation protection. It is therefore recommended that controls should be based on a dose range of 0.3-1 mSv/year, which is the building material gamma dose contribution to the dose received outdoors.

In order to examine whether a building material meets these two dose criteria, the following gamma activity concentration index (IC ):



where AU , ATh , AK are the Th, U and K activity concentrations (Bq/kg) in the building material, respectively. For superficial and other building materials with restricted fractional mass usage, such as those studied in this work, the exemption dose criterion (0.3 mSv/year) corresponds to an activity concentration index IC ≤ 2, while the dose criterion of 1 mSv/year is met for I C ≤ 6. [3] This approach has initially been developed by the Radiation Protection Authorities in the Nordic Countries (Nordic, 2000) and is generally accepted by the EU member states and many other countries. [14]

Based on the activity concentration index calculated according to Equation 4, 26 of the samples exhibit I C ≤ 2, four of them, with I C ≅ 2.9-3.4, show values 2 ≤ I C ≤ 6, there is no sample exceeds clearly the dose limit of 1 mSv/year. It should be noted that quite similar conclusions are drawn by considering the effective dose rates calculated according to Equations 4 and 5.

As shown in [Table 2], the results for the activity utilization index, the total absorbed dose rate in air due to gamma radiation, and the indoor effective dose assessment for a specific fractional mass for the 30 "granite" samples presented.


  Conclusions Top


Exploitation of high-resolution g-ray spectroscopy provides a sensitive experimental tool in studying natural radioactivity and determining radionuclide concentrations and dose rates in various rock types. Most of the tiling rock "granite" samples studied in this work reveal high values for the activity and radionuclide concentrations of Th, U and K, thus contributing to the high-absorbed dose rates in the air. In general, the calculated values are distinctly higher than the corresponding population-weighted (world-averaged) ones, they lie outside the typical range of variability reported values from world-wide areas due to terrestrial gamma radiation, given in the recent UNSCEAR 2008 Report. [15]

In addition, according to the dose criteria recommended by the European Union (EC, 1999), 26 of the samples meet the exemption dose limit of 0.3 mSv/year, and four of them meet the upper dose limit of 1 mSv/year. [2] From radiological protection considerations, use of the granites is acceptable for their restricted utilization as superficial materials like tiles, boards, etc.

 
  References Top

1.Faure G. Principles of Isotope Geology. 2 nd ed. USA: John Wiley and Sons; 1986.  Back to cited text no. 1
    
2.Me`nager MT, Heath MJ, Ivanovich M, Montjotin C, Barillon CR, Camp J, et al. Migration of uranium from uranium-mineralized fractures into the rock matrix in granite: Implications for radionuclide transport around a radioactive waste repository. Fourth International Conference of Chemistry and Migration Behavior of Actinides and Fission Products in the Geosphere (Migration, 1993), Charleston, USA, 12-17, December 1993. Radiochimica Acta 1993;66/67:47-83.  Back to cited text no. 2
    
3.European Commission Report on "Radiological Protection Principles concerning the Natural Radioactivity of Building Materials". Radiation Protection, 1999; 112.  Back to cited text no. 3
    
4.UNSCEAR. Sources and Effects of Ionizing Radiation. Report to General Assembly, with Scientific Annexes. New York: United Nations; 2000.  Back to cited text no. 4
    
5.Farouk M A, Al-Soraya A M. 226 Ra as a standard source for efficiency calibration of Ge (Li) detector. Nucl Inst Methods 1982;200:593-5 .  Back to cited text no. 5
    
6.Tzortzis M, Tsertos H, Christofides S, Christodoulides G. Gamma-ray measurements of naturally occurring radioactive samples from cyprus characteristic geological rocks. Preprint UCY-PHY-02/02 (physics/0212099). Radiat Meas 2003. [In press].  Back to cited text no. 6
    
7.Beretka J, Matthew PJ. Natural radioactivity of Australian building materials, industrial wastes and by-products. Health Phys 1985;48:87-95.  Back to cited text no. 7
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8.Krisiuk EM, Tarasov SI, Shamov VP, Shalak NI, Lisachenko EP, Gomelsky LG. A Study on Radioactivity in Building Materials. Leningrad: Research Institute for Radiation Hygiene; 1971.  Back to cited text no. 8
    
9.Stranden E. Some aspects on radioactivity of building materials. Phys Norv 1976;8:167-73.  Back to cited text no. 9
    
10.Nuclear Energy Agency (NEA). Exposure to radiation from the natural radioactivity in building materials. Report by NEA Group of Experts, OECD, Paris; 1979.  Back to cited text no. 10
    
11.Kohshi C, Takao I, Hideo S. Terrestrial gamma radiation in Koshi prefecture, Japan. J Health Sci 2001;47:362-72.  Back to cited text no. 11
    
12.Thomas J, Hulka J, Salava J. New houses with high radiation exposure levels. In: Proceedings of the International Conference on High Levels of Natural Radiation, Ramsar, 1990. Vienna: IAEA; 1993.  Back to cited text no. 12
    
13.Wrixon AD, Green BM, Lomas PR, Miles JC, Cliff KD, Francis EA, et al. Natural Radiation Exposure in UK Dwellings. NRPB-R190 . London: National Radiological Protection Board; 1988.  Back to cited text no. 13
    
14.Nordic, Åkerblom G, Mjönes L, Annanmäki M, Magnusson S, Strand T, Ulbak K. Naturally Occurring Radioactivity in the Nordic Countries-Recommendations. The radiation protection authorities in Denmark, Finland, Iceland, Norway and Sweden ; 2000.  Back to cited text no. 14
    
15.UNSCEAR. Sources and Effects of Ionizing Radiation. Report to General Assembly, with Scientific Annexes. New York: United Nations; 2008.  Back to cited text no. 15
    


    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2]


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