Measurement of radioactivity in building materials in Serbia

Abstract

This work presents a comprehensive study of natural radioactivity in 720 building materials imported in Serbia in 2012. Radioactivity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in the studied samples range from <5 to 4,938, <3 to 763 and <10 to 3,192 Bq kg⁻¹, respectively. The maximum values of ²²⁶Ra and ²³²Th activity concentrations were found in zirconium mineral, while the highest ⁴⁰K activity concentration was in the feldspar. Based on the obtained radionuclide concentrations, radium equivalent activity, air absorbed dose rate, annual effective dose, external and internal hazard indices, gamma and alpha index due to radon inhalation were evaluated to assess the potential radiological hazard associated with these building materials.

Introduction

All building materials contain various amounts of natural radioactive nuclides. Materials derived from rock and soil contain mainly natural radionuclides of the uranium (²³⁸U) and thorium (²³²Th) series, and the radioactive isotope of potassium (⁴⁰K). In the uranium series, the decay chain segment starting from radium (²²⁶Ra) is radiologically the most important because the health effects of ²³⁸U and the precursors of ²²⁶Ra are negligible. It is well known that ²²⁶Ra precursors are mostly alpha and beta emitter, while ²²⁶Ra progeny have many gamma lines and therefore the reference is often made to radium instead of uranium. The world-wide average concentrations of radium, thorium and potassium in the earth’s crust are about 40 Bq kg⁻¹, 40 Bq kg⁻¹ and 400 Bq kg⁻¹, respectively [1]. Radioactive isotopes in building materials can increase both external and internal radioactive exposures of humans. Knowledge of the level of natural radioactivity in building materials is therefore important to assess the possible radiological hazards to human health and to develop standards and guidelines for the use and management of these materials. The worldwide average specific activity in the soil are given as follows: ²²⁶Ra (32 Bq kg⁻¹), ²³²Th (45 Bq kg⁻¹) and ⁴⁰K (420 Bq kg⁻¹), and in building materials: ²²⁶Ra (50 Bq kg⁻¹), ²³²Th (50 Bq kg⁻¹) and ⁴⁰K (500 Bq kg⁻¹) [2].

The purpose of setting controls on the radioactivity of building materials is to limit the radiation exposure due to materials with enhanced or elevated levels of natural radionuclides. The doses to the public should be kept as low as reasonably achievable. However, since small exposures from building materials are ubiquitous, controls should be based on exposures above typical levels and their normal variations.

Equations for alpha, gamma and hazard indices calculations.

By measuring the activity of radionuclides in building materials of unknown origin, or materials with addition of industrial waste, we were able to determine the compliance with regulations (that limit the maximum concentration of ²²⁶Ra, ²³²Th and ⁴⁰K). Gamma index for construction materials is calculated as follows [3]:

$${I_\gamma } = \frac{{{C_{Ra}}(Bq\,k{g^{ - 1}})}}{{MPC(Ra)}} + \frac{{{C_{Th}}(Bq\,k{g^{ - 1}})}}{{MPC(Th)}} + \frac{{{C_K}(Bq\,k{g^{ - 1}})}}{{MPC(K)}}$$

(1)

where C _Ra, C _Th and C _K are concentration and MPC(Ra), MPC(Th) and MPC(K) are maximum permissible concentration of ²²⁶Ra, ²³²Th and ⁴⁰K respectively. Values of maximum permissible concentration depend on particular use of construction materials. These values for building materials used for interior, exterior and in civil engineering construction as a base for roads, playgrounds and other civil engineering construction (under the overlay layer) are presented in Table 1.

Table 1 Maximum permissible concentration for ²²⁶Ra, ²³²Th and ⁴⁰K in buildings material used for interior, exterior and in civil engineering construction

Gamma index must be less than 1 if material is to be used in high construction for interior. If the values obtained for the gamma index recalculated through the Eq. (1) meet the requirements for interior (Article 13 in [3]), then the material can also be used for exterior and civil engineering construction [3]. If the criterion for interior is not satisfied, then the calculation should be done according to Articles 14 and 15 [3].

The excess alpha radiation due to the radon inhalation originating from the building materials is assessed through alpha index which has to be less than 1 and is calculated according to the following equation [4]:

$$ I_{\text{a}} = \frac{{C({}^{226}Ra)}}{200} \le 1 $$

(2)

where the recommended limit concentration of ²²⁶Ra is 200 Bq kg⁻¹ for which I _α = 1.

The radium equivalent activity, Ra _eq, the absorbed gamma dose rate, $\dot{D}$, the annual effective dose, D _E, the external and internal hazard index, H _ex and H _int, were evaluated to assess the radiation hazard for people living in dwellings.

Radium equivalent activity, Ra _eq, can be calculated as follows [5, 6]:

$$ Ra_{\text{eq}} = C_{\text{Ra}} + 1.43 \cdot C_{\text{Th}} + 0.077 \cdot C_{\text{K}} $$

(3)

The limit for Ra _eq is 370 Bq kg⁻¹ set by the Organization for Economic Cooperation and Development (OECD 1979) report [7].

The absorbed gamma dose rate in indoor air, $\dot{D}$, and the annual effective dose, D _E, was calculated using the following equations [5]:

$$ \dot{D} = 0.92 \cdot C_{\text{Ra}} + 1.1 \cdot C_{\text{Th}} + 0.08 \cdot C_{\text{K}} $$

(4)

$$ D_{\text{E}} = \mathop {0.7}\limits^{{}} \cdot 7000 \cdot { \dot {D}}$$

(5)

The external hazard index H _ex is defined as [5]:

$$ H_{\text{ex}} = \frac{{C_{\text{Ra}} }}{370} + \frac{{C_{\text{Th}} }}{259} + \frac{{C_{\text{K}} }}{4810} $$

(6)

In addition to the external radiation, radon and its short lived products are also hazardous to the respiratory organs. The internal exposure to radon and its daughter products is quantified by the internal hazard index H _ex which is given by the following equation [4]:

$$ H_{\text{in}} = \frac{{C_{\text{Ra}} }}{185} + \frac{{C_{\text{Th}} }}{259} + \frac{{C_{\text{K}} }}{4810} $$

(7)

The value of these indices must be less than 1 in order to keep the radiation hazard insignificant.

Materials and methods

Before the measurements, the samples were crushed, sieved (mesh size 2 mm) to obtain homogeneity and placed in the plastic Marinelli beakers of 500 cm³. The density of the samples ranged from 0.8 to 1.2 g cm⁻³.

Two HPGe p-type detectors with relative efficiencies of 18 and 50 % and one n-type detector with relative efficiency of 20 % were used. Calibration of detectors was performed using silicone resin matrix spiked with a series of radionuclides (²⁴¹Am, ¹⁰⁹Cd, ¹³⁹Ce, ⁵⁷Co, ⁶⁰Co, ²⁰³Hg, ⁸⁸Y, ¹¹³Sn, ⁸⁵Sr and ¹³⁷Cs) with total activity of 41.48 kBq on the day thirty first August 2012 (Czech Metrological Institute, Praha, 9031-OL-420/12, type CBSS 2). The calibration was performed in the 500 cm³ Marinelli beaker geometry, too. For efficiency calibration of samples having different density compared to resin, two additional secondary reference materials were produced in accordance with IAEA recommendations [8]. These are charcoal and sand in Marinelli beaker, spiked with standard radioactive solution ER X 9031-OL-426/12 issued by Czech Metrological Institute, Inspectorate for Ionizing Radiation. The radioactive solution contained following radionuclides: ²⁴¹Am, ¹⁰⁹Cd, ¹³⁹Ce, ⁵⁷Co, ⁶⁰Co, ¹³⁷Cs, ²⁰³Hg, ¹¹³Sn, ⁸⁵Sr and ⁸⁸Y, with the energies that span from 59 to 1,898 keV with total activity of 1,342 Bq at reference date 31.08.2012.

The spectra were analyzed using the program GENIE 2,000. The activity of ²²⁶Ra and ²³²Th was determined by their decay products: ²¹⁴Bi (609, 1,120 and 1,764 keV), ²¹⁴Pb (295 and 352 keV) and ²²⁸Ac (338 and 911 keV), respectively. The activities of ⁴⁰K and ¹³⁷Cs were determined from its 1,460 and 661 keV γ-energy, respectively. The background spectrum was recorded regularly after or before the sample counting.

Relative measurement uncertainty for the experimental values was calculated according to the following Equation:

$$ u(\varepsilon ) = \sqrt {(\delta A)^{2} + (\delta N)^{2} + (\delta M)^{2} + (\delta P)^{2} } $$

(8)

where δΑ represents relative uncertainty of the radioactive solution given by the manufacturer, δΝ is the relative counting uncertainty, δΜ is the uncertainty introduced in the process of production of the secondary reference material and δP represent other components such as the sample position and “run to run” uncertainty. δΑ + δΜ is estimated to be approximately 2–3 %, while δP is estimated to 2 %. Relative measurement uncertainty u(ɛ) for all energies did not exceed 10 % at 2σ level of confidence.

Results

The building materials were gathered from different regions of Serbia or imported from other countries and analyzed by gamma spectrometry to quantify radioactivity concentrations using high purity germanium detector. The largest quantity of imported samples were cement, gypsum, granite, stone, clay, ceramic tiles and zirconium minerals.

The range (minimum and maximum) and average concentration of ²²⁶Ra, ²³²Th and ⁴⁰K measured in 720 building material in 2012 are presented in Tables 2 and 3. The building materials which fulfilled 100 % criterion for interior are presented in Table 2, while the results for other building materials are presented in Table 3. All materials presented in Table 3 except zirconium mineral and granite fulfilled criterion for exterior. For the granite 4 % samples obtained certificate for using in civil engineering construction as a base for roads, playgrounds and other civil engineering construction (under the overlay layer), while 1 % of granite material were rejected. According to Serbian regulations [3] gamma index is applied to any building material and its additives. Zirconium mineral is an additive and its index has to be calculated too. As the zirconium mineral is used as component in the recipe for the production of ceramics, therefore a disclaimer from the client was obtained, stating the purpose of using the zirconium mineral and the percentage in which it will be used in the production process. The zirconium mineral is used as component in concentration not above 3 % [9] and in that case all zirconium mineral samples obtained certificate for interior.

Table 2 The concentration of ²²⁶Ra, ²³²Th and ⁴⁰K in building material which fulfilled 100 % criterion for interior

Table 3 The concentration of ²²⁶Ra, ²³²Th and ⁴⁰K in building material which not fulfilled criterion for interior

The radium equivalent activity, Ra _eq, the absorbed gamma dose rate, $\dot{D}$, the annual effective dose, D _E, the external and internal hazard index, H _ex and H _int, are presented in Table 4. The results for zirconium mineral in Table 4 are omitted because the zirconium mineral is used only as a small component of some building materials.

Table 4 The radium equivalent activity, the absorbed gamma dose rate, the annual effective dose and external and internal hazard index for building materials

Conclusion

²²⁶Ra, ²³²Th and ⁴⁰K activity concentration were measured in building materials using gamma spectrometry. The highest ²²⁶Ra and ²³²Th activity concentrations were found in zirconium mineral, 4,938 and 763 Bq kg^-1, respectively. The maximum ⁴⁰K activity concentration was 3,192 Bq kg⁻¹ in the feldspar. The radium equivalent activities ranged from 10 to 355 Bq kg⁻¹ in all building materials except granite in which the radium equivalent activity maximum was up to 879 Bq kg⁻¹. The alpha index was from <0.03 to 0.79 in all samples except gypsum and granite, where maximum values were 1.3 and 2.15, respectively.

According to Serbian regulations gamma index is applied to any building material and its additives. The present study shows that the measured bentonite, cement, dolomite, gypsum, clay, talk, limestone and marble fulfilled 100 % criterion for dwelling construction. The measurement in other construction materials, such as feldspar, granite, stone, kaolinite, silica sand, sand and tiles showed that only 80 % samples fulfilled the criterion for dwelling construction, 18.5 % samples meet the criteria for the exterior usage and the rest of 1.4 % samples could be used in civil engineering construction as a base for roads, playgrounds and other civil engineering construction (under the overlay layer) and 0.1 % materials were rejected. Some material, for example zirconium mineral, which is used as component in small concentration, also obtained certificate for dwelling construction, although the all alpha, gamma and hazard indices were very high.

The results of activity concentration measurements in building materials used in construction industry in Serbia allows the authors to conclude that almost all measured samples could be safely used in building constructions.