Natural radioactivity in imported ceramic tiles used in Serbia #

Ceramic tiles are one of the commonly used decorative building materials. Body of ceramic tiles is a mixture of different raw materials including clays, quartz materials and feldspat, and may be glazed or left unglazed. Due to the presence of zircon in the glaze, ceramic tiles can show natural radioactivity concentration significantly higher than the average values for building materials. This study presents a summary of results obtained by a survey which was consisted of measurements of activity concentrations of natural radionuclides in imported ceramic tile samples used in Serbia using a gamma spectrometer with HPGe detector. Based on the obtained concentrations, gamma index, radium equivalent activity, the indoor absorbed dose rate and the corresponding annual effective dose were evaluated to assess the potential radiological hazard associated with these building materials.


I. Introduction
All building materials contain various amounts of natural radioactive nuclides.Materials derived from rock and soil contain mainly natural radionuclides of the uranium ( 238 U) and thorium ( 232 Th) series, and the radioactive isotope of potassium ( 40 K).In the uranium series, the decay chain segment starting from radium ( 226 Ra) is radiologically the most important and, therefore, reference is often made to radium instead of uranium.The knowledge of the natural radioactivity of building materials is important for the determination of population exposure to radiations, as most of the people spend 80% of their time indoors.High levels of radioactivity in construction materials can increase external and internal indoor exposure [1].
Ceramic tiles are one of the commonly used decorative building materials: they are made of a mixture of earthly materials that has been pressed into shape and fired at high temperature.They can be glazed or left unglazed.Based on the fact that the zircon is a common opacifying constituent of glazes applied to ceramic tiles and sanitary ware and is also used as an opacifier in porcelain tiles by incorporation directly into the mixture used for forming the body of the tile, ceramic tiles can show natural radioactivity concentration significantly higher than the average values for building materials [2-6].
In this study, results of the activity concentrations of 238 U, 235 U, 232 Th, 226 Ra and 40 K measured in 16 samples of ceramic tiles, that are commercially available, are presented.Also, radium equivalent activity, the gamma-index, the indoor gamma absorbed dose rate and the annual effective dose due to external exposure were estimated to assess the potential radiological hazards associated with these building materials.

II. Materials and methods
Samples of 16 different ceramic tiles have been collected from local store.The investigated ceramic tiles (floor and wall) were imported from Croatia, Italy, Ukraine and Spain.The samples were crushed, sieved (mesh size 2 mm) and placed into the plastic box of 100 g.All samples were left for four weeks to reach radioactive equilibrium [7] and measured after that.The samples were counted using a high purity germanium detector (HPGe) with relative efficiency of 23 % and energy resolution of 1.8 keV for the 1332 keV 60 Co peak.Geometric efficiency for Marinelli beaker was determined by a reference soil material (Czech Metrological Institute, Prague, 9031-OL-420/12, with total activity of 41.48 kBq on the day 31.08.2012., ( 241 Am, 109 Cd, 139 Ce, 57 Co, 60 Co, 203 Hg, 88 Y, 113 Sn, 85 Sr 137 Cs).The spectra were analysed using the program called GENIE 2000.The activity of 226 Ra and 232 Th was determined by their decay products: 214 Bi (609 keV, 1120 keV and also 1764 keV), 214 Pb (295 keV and 352 keV) and 228 Ac (338 keV and 911 keV), respectively.The activities of 40 K were determined from its 1460 keV γ-energy. 235U was determined via 186 keV corrected for 226 Ra. 238 U was determined via 234 Th (63 keV) or by 234 Pa (t 1/2 = 1.17 min, 1000 keV).Counting time interval was 60 000 s.The combined measurement uncertainty of the results was calculated at the 95% level of confidence (k = 2).
The specific activity, A, of the radionuclides in the samples was calculated using the following equation: (1)   where N is count of the sample (imp), t -counting time [s], P γ -probability of gamma decay [%], e f -detector efficiency [%] and m -the mass of the sample [kg].Minimum detectable activity was calculated by the equation: (2)   where LLD=2.71+4.65√B is the detection limit, B is background.

III. Results and discussion
Activity concentration of radionuclides detected in investigated ceramic tiles samples by gamma spectrometry is presented in Table 1.The activity concentration of 226 Ra has been found to be varying from 61 to 150 Bq/kg in floor ceramic tiles, while in wall tiles the activity concentration of 226 Ra varies from 70 to 135 Bq/kg.The activity concentration of 232 Th ranges from 53 Bq/ kg to 72 Bq/kg and 50 Bq/kg to 101 Bq/kg in floor and wall tiles, respectively.The activity concentration of 40 K lies between 560 and 1030 Bq/kg for floor tiles and between 590 and 1070 Bq/kg for wall tiles. 235U has concentrations between 2.8 and 4.0 Bq/kg (floor tiles) and between 2.8 and 6.4 Bq/kg (wall tiles).For 238 U detected concentrations were in range from 43 to 114 Bq/kg (for floor tiles) and in range from 43 to 143 Bq/kg (for wall tiles).As it is shown in this Table 1, the radioactivity in the examined samples varies greatly depending on their region of origin.The worldwide average specific activity in the building materials is given as follows: 226 Ra (50 Bq/kg), 232 Th (50 Bq/kg) and 40 K (500 Bq/kg) [8].
The higher natural activity concentrations found in ceramic tiles can be explained through the amount of zircon added.For example uranium and thorium atoms are easily incorporated in the crystalline structure of the zircon; furthermore, zircon ores undergo an enrichment during sand processing which produces almost pure zirconium silicate.For these reasons zircon minerals, used in cermic industry, are usually included in the category of sources of technologically enhanced natural radioactivity [9].Keeping in mind that the zircon is one component in the recipe for the production of ceramics it can be concluded that there are no legal restrictions in terms of radioactivity in circulation and use of the investigated materials only if 3% of the zircon is used [10].Table 2 presents comparison with literature.For all investigated samples gamma index, I, was calculated using the following equation [15]: (3)   where Max(Ra), Max(Th) and Max(K) present maximum limit of radionuclide content of 226 Ra, 232 Th and 40 K in Bq/kg in the appropriate application (values are given in Table 3).Gamma index must be less than 1.If the values obtained for the gamma index recalculated through the equation (3) meet the requirements for interior, then the material can be used for exterior and low construction.Calculated values for gamma index are presented in Table 4.As it can be seen from Table 4, for three floor tiles from Italy this value exceed 1 for interior (values are 1.29, 1.00 and 1.13), but for exterior these values are 0.79, 0.65 and 0.72, so these tiles can be used for exterior and low construction.Also for two wall tiles from Spain and one wall tile from Croatia gamma indexes for interior are 1.05, 1.03 and 1.25, while for exterior they are 0.66, 0.66 and 0.82.Gamma index in these samples are higher than 1 because of higher concentration of 226 Ra.For the rest 10 samples gamma index for interior is less than 1.
The absorbed dose rate in indoor air, Ḋ, was calculated using the equation ( 5) [18].This equation is used to calculate the dose rate of material used in floor, ceiling and walls (all structures). (5) The estimated indoor gamma dose rate values for all investigated samples are shown in Table 4.It ranges from 0.16 to 0.29 μGy/h.
To estimate the annual effective dose (D E ), one has to take into account the conversion factor from absorbed dose in air to effective dose.In the recent UNSCEAR 2010 reports [8], a value of 0.7 Sv/Gy is used as for the conversion factor to convert absorbed dose in air to effective dose received by adults.The annual exposure time used is 7000 h.The annual effective dose in units of mSv was estimated using the following equation: (6)   The obtained results for annual effective dose are presented in Table 4 and show that these values, ranged from 0.80 to 1.43 mSv.

IV. Conclusions
For each sample of floor and wall ceramic tile imported from different countries investigated in this study, the specific activity of 226 Ra, 232 Th, 40 K, 235 U and 238 U were determined by gamma spectrometry.The radium equivalent activity, the gamma-index, the absorbed gamma dose rate in indoor air and the corresponding annual effective dose have been determined to assess the radiological hazards from these building materials commonly used in Serbia.The results show that there are considerable variations in the measured concentrations of radionuclides in ceramic tiles originating from different areas.This fact is important from the point of view of selecting suitable materials for use in building and construction.Probably, the higher natural activity concentrations found in ceramic tiles is due to the amount of zircon added for glaze.10 samples show gamma indexes lower than the limit for interior use, while 6 samples have gamma index higher than 1 for interior use, but for exterior, these values are lower than 1.The radium equivalent activity is within the limit set by the OECD.Calculated gamma dose rate ranged from 0.16 to 0.29 μGy/h while calculated annual effective dose ranged from 0.80 to 1.43 mSv.The obtained radium equivalent activity, the absorbed gamma dose rate in indoor air and the corresponding annual effective dose calculated based on the activity concentrations of 226 Ra, 232 Th and 40 K given for ceramic tiles from different origin in literature cite in Table 2 (comparison with literature), was 121-290 Bq/kg for Ra eq , 0.14-0.26μGy/h for Ḋ and 0.48-1.26for D E .These values are in good agreement with values for ceramic tiles investigated in this paper.
From radiological point of view, the results indicate that the use of these materials in construction of dwellings could be considered safe for inhabitants.