Comparative Analysis of Natural Radioactivity Content in Tiles made in Nigeria and Imported Tiles from China

In this investigation, natural radioactive contents in tiles manufactured in Nigeria and tiles imported from China were measured using gamma ray spectroscopy. High Purity Germanium detector was used to estimate the concentrations of some radioisotopes present in 17 samples of various tiles from Nigeria and China. The average activity concentrations of 226Ra, 232Th, and 40K for the tiles were found to be 68.2 ± 0.5; 173.9 ± 9.2 and 490 ± 15 Bq/kg and 58.2 ± 0.5, 161.5 ± 9.4 and 455.7 ± 15.1 Bq/kg for the tiles from Nigeria and China respectively. Radiological hazard indices such as absorbed dose rate, radium equivalent activity, external Hazard Index (Hex), internal Hazard Index (Hin), Annual Effective Dose (mSv/y), Gamma activity Index (Iγ) and Alpha Index (Iα) were determined for both kind of tiles from Nigeria and China. The mean values obtained were: 354.56 and 317.16 Bq/kg; 169.22 nGyh−1 and 153.92 nGyh−1; 0.95 and 0.87; 1.14 and 1.08; 1.59 mSv/y and 1.52 mSv/y; 1 and 1.15 and; 0.34 and 0.29 respectively. The mean value of radium equivalent obtained in this study is less than that of the international reference value of 370 Bq/kg for the both kind of tiles.

broken into smaller pieces so as to allow further processing. All the samples were crushed using the Pascall Engineering Lab milling machine. After each tile sample was crushed, the crusher or lab milling machine was thoroughly cleaned with high pressure blower (Wolf from Kango Wolf power tools, made in London, type 8793 and serial no: 978A) before the next sample was crushed. This whole process was repeated until all the samples were completely crushed into powder. The pulverizer used is the disk 'grinder/pulverizer' by Christy & Norris Limited. After each pulverizing process, the machine was cleaned properly and blown with high pressure blower to avoid cross contamination of the samples. The crushed samples were passed through 250 µm sieve mesh size; thereafter1 kg of the sieved sample was weighed out and put in high density polyethylene bottle, well labeled with indelible marker. The samples were transported to Universiti Teknologi Malaysia Nuclear Laboratory for gamma ray analysis. Each sample was weighed again using a digital balance of detection limit of ±0.01 g differently into the Marinelli beakers. The samples were sealed in Marinelli beakers for 4 weeks secular equilibrium for the assumption that the parent nuclide is equal to the daughter. The Marinelli beaker used in this analysis is for both samples and IAEA standard of which they are of the same geometry of the detector size. Gamma ray detection system for the selected samples. The analysis was carried out using the gamma ray spectrometry facilities at the Nuclear Lab. Faculty of Science, Universiti Teknologi Malaysia. A high resolution spectrometer was used for the measurement of the gamma energy spectrum of emitted gamma-rays in the energy range between 50 keV and 2000 keV. The gamma ray spectrometry consists of a high purity germanium (HPGe) detector with a relative efficiency of 20%, a resolution of 1.8 keV for 1332 keV gamma ray emission of 60 Co. The detector used in gamma ray measurements was Canberra GC2018 with Genie-2000 software. The gamma detector was cooled by liquid nitrogen at 77 K for the purpose of reducing leakage current and thermal noise, and its warm-up sensor is coupled to the high voltage detector bias supply. The pre-amplifier was placed inside a lead shield to reduce background radiation 24 . The Minimum Detectable Activity (MDA) for each radionuclide is U (1 Bq/kg), Th (2 Bq/kg) and K (13 Bq/kg) respectively. The decay isotopes, gamma energy and gamma disintegrations are shown in Table 3.
Standard sample preparation for gamma spectrometry. The IAEA standard sample Thorium Ore (S- 14) and Lake Sediment (SL-2) were used as reference materials and mixed with SiO 2 in Marinelli beakers. The uranium and Thorium contents from S-14 are 29 ppm and 610 ppm respectively. A weight of 20.00 g from Sample IAEA S-14 was thoroughly mixed with 100.00 g of SiO 2 in a Marinelli beaker (Coded as S-14). After mixing with SiO 2 , the uranium and thorium concentrations are 4.63 ppm and 97.3 ppm respectively. The IAEA standard sample SL-2 was used to calculate the specific activity of potassium (K). It has a specific activity of 240 Bq kg −1 . Another Marinelli beaker of 74.18 g of SL-2 was mixed with 100.00 g of SiO 2 (Coded as SL-2). This provides background for standard samples. The IAEA standard samples used in this study are presented in Table 4.   Measurement of gamma-ray radioactivity from the tiles samples used in this study. The tiles produced in Nigeria and China of different brands were purchased from different suppliers and prepared according to IAEA TRS-295 25 . The samples were sealed and stored for four weeks to achieve secular equilibrium between radium and its progeny [26][27][28][29] . Under the conditions of secular equilibrium, 232 Th concentration was determined from the average concentration of 208 Tl using the 583 keV peak and 228 Ac by using the 911 keV peak. 238 U was determined from the average concentrations of the 214 Pb by using the 352 keV peak and 214 Bi by using the 609 keV peak [27][28][29] . The 1460 keV peak was used to determine the concentration of K. Each sample was put into a shielded HPGe detector and measured for 21600 s. The background gamma-ray spectrum of the detection system was determined with an empty Marinelli beaker under identical conditions, and was subtracted from the spectra of each sample. The activity concentrations were determined by comparing with IAEA standard samples S-14 (Thorium ore) and SL-2 (Lake Sediment). 238 U activity concentrations were calculated as the arithmetic means of the activities of 214 Pb, 214 Bi isotopes and 208 Tl, 228 Ac isotopes for 232 Th. The concentrations of K in (%) was determined from the value obtained in Bq kg −1 , 232 Th and 238 U (ppm) in measured samples were calculated using conversion factors given by 25 .
The concentration of the 238 U and 232 Th was determined using Eqs (1) and (2). Equations 3 and 4 was used for 40 K where C samp = concentration of sample collected (ppm). C std = concentration of the standard sample (ppm). W std = weight of the standard sample (g). W samp = weight of the sample collected (g). N samp = net counts of the photopeak area of the sample collected. N std = net counts of the photopeak area of the standard sample.
The uncertainty of the sample concentration was calculated by using the accurate approach by [28][29][30] .    The activity concentration of potassium was calculated by using the formula.
where A samp = activity concentration of the sample collected (Bq Kg −1 ).A std = activity concentration of standard sample (Bq Kg −1 ).W std = weight of the standard sample (g).W samp = weight of the sample collected (g).N samp = net counts of the photopeak area of the sample collected.N std = net counts of the photopeak area of the standard sample. The uncertainty of the activity concentration of potassium was calculated by using the following formula [28][29][30] :

Results and Discussions
Determination of activity concentration. The radioactivity concentration evaluated in the tiles made in Nigeria and China is presented in Tables 5 and 6. The observed activities concentration of the radionuclides content in the tiles from Nigeria and China ranged from 27 ± 0.5-241 ± 0.5 and 42.5 ± 0.5-75.0 ± 0.5 Bq/kg for 226 Ra, 41 ± 9.4-461 ± 9.5 and 41.   Considering the absorbed dose rates presented in Tables 7 and 8, it can be observed that the highest value of 352.51 and 305.17nGyh −1 was reported in NISPRO and Virony Glazed tiles whereas the lowest value of 70.61 and 81.60 nGyh −1 was noted in Goodwill super polish and Virony Rustic Glass tile respectively. Comparing the average absorbed dose rate in this present study with the standard value of 80 nGyh −1 recommended by 12 , the highest value obtained in this present study is higher by a factor of 4.4 and 3.7 respectively for both locally produced tile in Nigeria and imported tiles from China.
Radium equivalent (Raeq) determination. The level of radionuclides from 226 Ra, 232 Th and 40 K in the analyzed building materials is non-uniformly distributed. The Raeq activity of the measured radionuclides is used to compare the activity of each of 226 Ra, 232 Th and 40 K contents in the building materials such as tiles. Raeq with unit as BqKg −1 was calculated using Eq. (6).    radium equivalent is 317.16 BqKg −1 and is within the recommended international reference value of 370 BqKg −1 . This is shown in Table 8.
Evaluation of external hazard index. The gamma ray radiation hazards index due to the specified radionuclides were assessed by external radiation hazard and was calculated using Eq. (7) according to 1 .
ex Ra Th K where, C Ra , C Th and C K are the average activity concentrations of 226 Ra, 232 Th and 40 K in Bq kg −1 respectively. For the radiation hazard to be acceptable, it is recommended that the H ex be less than unity. The estimated H ex for all the tile samples produced varies from 0.37 to 2.11 with highest value noted in NISPRO tile whereas the lowest value reported in Goodwill super polish as shown in Table 7. This highest value from the present study is higher than the recommended value of ≤1 according to 1 . The estimated external hazard index for the imported tiles from China varies from 0.42 to 1.82 with highest value observed in Virony Glazed tile of size 40 × 40 mm whereas the lowest value was reported in Virony Rustic tile. The estimated highest value for the H ex of the tiles from Nigeria and China are 2.11 and 1.82 respectively. This is presented in Table 8.

Determination of internal hazard index.
The hazard which is defined in relation to internal hazard is represented by H in respectively and can be found using Eq.  Table 8. Its value ranged between 0.55 and 2.03 with a mean value of 1.08. The result obtained for internal hazard for the IDDRIS Tiles and Virony Glazed are higher than recommended limit of 1 for the China tiles as well as the tiles produced in Nigeria.
The annual effective dose rate. The indoors annual effective dose equivalent received by human is estimated from the indoor internal dose rate (Din), occupancy factor which is defined as the level of human occupancy in an area in proximity with radiation source; is given as 80% of 8760 h in a year, and the conversion factor of 0.7 Sv Gy −1 which is used to convert the absorbed does in air to effective dose 1 . The annual effective dose equivalent is estimated using Eq. (9).
= . + . + . × . × − AEDR (0 49CRa 0 76CRTh 0 048CK) 8 76 10 ( 9) 3 The value of the AEDE for the tiles produced in Nigeria and tiles imported from China ranges from 0.65 to 3.69 mSv y −1 with a mean value of 1.59 mSv y −1 and 0.73 to 3.14 mSv y −1 with an average value of 1.52 respectively. The mean values from the samples (tiles made in Nigeria and imported tiles from China) surpass the world's average value of 0.07 mSv y −1 . Details of annual effective dose rate for all the samples are presented in Tables 9 and 10.
Gamma index determination (Iγ). Gamma index is used to evaluate the γ-radiation hazard related to the natural radionuclide in the particular samples under investigation. The gamma index representation (Iγ) is estimated using Eq. (10) as presented by 33 .  Table 9. The Annual Effective Dose (mSv/y), γ -activity Index (Iγ) and Alpha Index for the tiles made in Nigeria.
SCiENTifiC The estimated results are presented in Tables 9 and 10. The controls on the radioactivity of building materials according to RP122 34 is based on the dose criterion for control and exemption. The dose effective that is above the criterion level of 1 mSvy −1 should be taken into consideration for radiation protection. It recommends that controls of dose range of 0.3 to 1 mSvy −1 , which is the excess gamma dose to that received outdoors. The gamma activity index is used to identify whether a dose criterion is met 34 . This gamma activity index accounts for the ways and amounts in which the materials used in building, with limit value of their indices not exceeding the recommended value and depends on the dose criterion shown in Tables 9 and 10. In this present investigation, the dose has been evaluated excluding the background dose which was shielded by the building materials when used in bulk but does not still exclude when building materials used as a superficial material. This is because the thin layers of superficial material do not reduce significantly the background dose. The gamma activity index ≤1, corresponds to annual effective dose less than or equal to 1 mSvy −1 , while gamma activity index ≤0.5 corresponds to 0.3 mSvy −1 if the materials are used in bulk quantity. At the same time, gamma activity index ≤6 corresponds to annual effective dose of 1 mSvy −1 and gamma activity index ≤2 corresponds to an annual effective dose ≤0.3 mSvy −1 if the bulk materials are used in a superficial way. In this study as shown in Tables 9 and 10  Determination of alpha index (Iα). The assessment of the alpha index is another important aspect of hazard assessment that deals with the estimation of that excess alpha radiation due to radon inhalation originating from building materials. The alpha index calculated using Eq. (11) 35,36 is: Ra 1 where C Ra is the activity concentration of radium Bqkg −1 in building materials. If the radium activity level in building material exceeds the values of 200 Bqkg −1 there is possibility that the radon exhalation from the material could cause indoor radon concentrations exceeding Bqm −3 . Tables 9 and 10 presents the values for alpha index. The International Commission on Radiation protection recommends an action level of 200 Bqm −3 for radon in dwellings 37 . At the same time, if this radium activity level is below 100 Bqkg −1 , it shows that radon exhalation from building materials may not likely cause indoor concentration greater than 200 Bqm −3 36 . It is reported that the recommended exempted value and the recommended upper limit for radon concentrations are 100 Bqkg −1 and 200 Bqkg −1 respectively in building materials 38 . It is noted that the upper limit of radon concentration (Iα) is equal to 1 39 . The results of the present study show that the radon concentration varies from 0.14 to 1.21 respectively with average value of 0.34 for the tile produced in Nigeria while for imported tiles from China; it varies from 0.21 to 0.33 with mean value of 0.29.

Conclusions
The measurement of natural radioactivity concentrations and its associated radiological risks have been evaluated in 17