In this study, mass activity of naturally occurring radioactive materials were measured in twenty-three building material samples, use extensively in the area exposed to a high level of natural background radiation (Mahallat, Iran), to determine the radioactivity index and changes to the level of indoor gamma radiation. The mass activity of 232Th, 226Ra and 40K were within the ranges from 18 ± 3 to 44 ± 10 Bq/kg (average of 27 ± 6 Bq/kg), 22 ± 5 to 53 ± 14 Bq/kg (average of 34 ± 6 Bq/kg) and 82 ± 18 to 428 ± 79 Bq/kg (average of 276 ± 58 Bq/kg), respectively. The gamma dose rates for population were estimated between 48 ± 9 and 111 ± 26 nGy/h with exception of radon exhalation from building materials. Since the air kerma rate in the town varies from 0.8 to 4 µGy/h, the attenuation coefficient was calculated for buildings made of the aforementioned materials. Additionally, the annual gamma radiation doses for inhabitants were calculated based on time spent outdoors and indoors.
Humans are exposed to background radiation from both natural and artificial sources on a daily basis, however, more than 80% of our entire exposure to ionizing radiation is from natural sources (
Two well-known HNBRAs in Iran, Ramsar a famous area in radiation point of view, a northern coastal city in Mazandaran province and Mahallat located in central of Iran. In Mahallat, the igneous bedrocks are rich in uranium, which decays into 226Ra as a soluble in groundwater but in the presence of dissolved calcium carbonate (CaCO3) precipitates out as radium carbonate (RaCO3) houses (
Over recent years, many studies have been carried out in High Natural Background Radiation Areas to assess the radiation (
Mahallat, with a population of approximately 53 thousand and surface area of 37 km2, is one of the oldest towns in Iran. It is located in Markazi Province in north-western Iran (N 32° 54′, E 50° 27′) and is surrounded by mountains. Mahallat is recognised as a HNBRA (
For the investigations, twenty-three different samples of commonly use building materials in Iran, e.g. gypsum, cement, bricks, sand and gravel, ceramics and tiles, were randomly collected directly from local building material suppliers and construction sites in the year 2015. For every building material investigated, between three and five subsamples were collected, stored and labelled at the time of collection. The samples, after being transferred to the laboratory, were stored at room temperature for several days in order to dry and then pulverised, homogenised and sieved (grain size <3 mm) to achieve the same parameters as the reference material (IAEA-375, Soil standard). Afterwards, the samples were dried in a ventilated oven at 105 °C for 24 hours to reach a constant weight. 500 grams of each prepared and homogenized sample was transferred into a leak-proof Marinelli beaker, weighed and sealed for 30 days in order to allow 226Ra to reach secular equilibrium with its daughters.
The concentrations of three naturally occurring radionuclides (40K, 226Ra and 232Th) in the samples of building materials were determined using a High Purity Germanium (HPGe) gamma-ray detector (ORTEC GMX40-76) with a relative efficiency of 40%, and an energy resolution of 1.9 keV at 1332.5 keV. The spectra were recorded by an ORTEC DSPEC LF 8196 MCA (multichannel analyzer) and analysed using Aptec MCA software. The energy calibration was carried out by three sealed sources: 137Cs with a gamma line of 661.6 keV, 60Co with two gamma lines of 1173.2 and 1332.5 keV and 241Am with an X-ray line of 59.5 keV (
The mass activity of each radionuclide was determined using its own specific gamma lines or gamma lines of its decay products: for 40K, the line 1461 keV (11%); in the case of 226Ra, the gamma lines of its decay products were used – the lines of 214Pb and 214Bi with energies of 352 keV (35%) and 609 keV (45%), while to measure the decay products of 232Th, that are, 228Ac and 208Tl, gamma lines of 911 keV (28%) and 2614.5 keV (36%) were applied, respectively (
The detection efficiencies for particular gamma lines and the detection limits (LID) were determined using a reference material provided by the International Atomic Energy Agency (soil standard IAEA-327) for the system applied in our gamma spectrometric investigations. The detection efficiency was estimated separately for 40K, 226Ra and 232Th as 1.2%, 2.4% and 1.4%, respectively. Meanwhile, the LIDs were calculated as 23.0, 0.5 and 0.7 Bq/kg, respectively.
The radium equivalent activity from natural radionuclides was estimated for the assessment of radiological hazards of radioactivity in environmental materials (or building materials) for the population health. The radium equivalent was determined according to Equation [
Where
The absorbed dose rate, caused by naturally occurring radionuclides in building materials, the radioactivity index and the annual effective dose were calculated using Equations [
where
Additionally, the attenuation coefficient of the outdoor gamma dose rate was estimated – which given the wall thickness of approximately 30–40 cm and the average density of roughly 2 g/cm3 for the building materials – to be about 500 for gamma rays of 500 keV in energy. As a result, the increased indoor gamma dose rate, due to the high outdoor gamma dose rate, should not exceed 8–10 nGy/h. Therefore, the following estimation was made – the changes in annual effective dose were calculated by taking into consideration different values of indoor occupancy time. As predicted, the highest annual effective doses were estimated for a scenario in which the person spends more time outdoors than indoors. However, this simulation was executed without taking into account possible differences in outdoor and indoor radon levels.
Measured mass activities and statistical analysis of naturally occurring radionuclides in building materials (Bq/kg).
Building Material | Ra-226 | Th-232 | K-40 | I |
---|---|---|---|---|
Sand and Gravel | 38 ± 8 | 44 ± 10 | 204 ± 46 | 0.4 |
Sand and Gravel | 29 ± 4 | 25 ± 4 | 238 ± 38 | 0.3 |
Sand and Gravel | 33 ± 6 | 34 ± 6 | 337 ± 60 | 0.4 |
Sand and Gravel | 28 ± 5 | 27 ± 5 | 378 ± 71 | 0.4 |
Bricks | 35 ± 5 | 23 ± 4 | 410 ± 78 | 0.4 |
Bricks | 38 ± 7 | 32 ± 6 | 357 ± 66 | 0.4 |
Bricks | 31 ± 8 | 28 ± 5 | 428 ± 79 | 0.4 |
Bricks | 34 ± 6 | 27 ± 5 | 413 ± 80 | 0.4 |
Gypsum | 29 ± 4 | 20 ± 4 | 82 ± 17 | 0.2 |
Gypsum | 22 ± 5 | 18 ± 3 | 94 ± 16 | 0.2 |
Gypsum | 25 ± 4 | 23 ± 5 | 85 ± 19 | 0.2 |
Gypsum | 31 ± 4 | 24 ± 5 | 107 ± 22 | 0.3 |
Cement | 31 ± 4 | 23 ± 5 | 224 ± 48 | 0.3 |
Cement | 42 ± 8 | 28 ± 6 | 239 ± 52 | 0.4 |
Cement | 34 ± 5 | 23 ± 5 | 257 ± 56 | 0.3 |
Cement | 27 ± 4 | 25 ± 5 | 198 ± 40 | 0.3 |
Ceramic | 35 ± 5 | 23 ± 5 | 308 ± 67 | 0.3 |
Ceramic | 33 ± 5 | 28 ± 8 | 258 ± 73 | 0.3 |
Ceramic | 33 ± 4 | 33 ± 7 | 381 ± 81 | 0.4 |
Tile | 43 ± 10 | 29 ± 6 | 314 ± 71 | 0.4 |
Tile | 53 ± 14 | 31 ± 8 | 292 ± 75 | 0.4 |
Tile | 41 ± 9 | 30 ± 8 | 348 ± 92 | 0.4 |
Tile | 46 ± 10 | 33 ± 9 | 401 ± 82 | 0.5 |
Distribution of the 40K mass activity in the various building materials.
Distribution of the 226Ra and 232Th mass activity in the various building materials.
The normal distribution for the specified mean and standard deviation of the measured mass activity of radionuclides in the various building materials is also shown in
According to
The radium equivalent activities, gamma-ray absorbed dose rates and annual effective doses were estimated for the analysed samples based on the mass activity of the naturally occurring radionuclides. The results of the calculations can be seen in
Evaluation of Radium equivalent (Ra(eq)), Gamma-ray absorbed dose (D) and Annual effective dose (ED) for studied building materials.
Building Material | Ra(eq) (Bk/kg) | ± | D (nGy/hr) | ± | ED (μsv/yr) | ± |
---|---|---|---|---|---|---|
Sand and Gravel | 117.4 | 25.8 | 100.3 | 22.0 | 492 | 108 |
Sand and Gravel | 82.9 | 12.6 | 73.1 | 11.1 | 358 | 55 |
Sand and Gravel | 107.7 | 19.2 | 94.9 | 16.9 | 466 | 83 |
Sand and Gravel | 95.3 | 17.6 | 85.4 | 15.8 | 419 | 77 |
Bricks | 100.2 | 16.7 | 90.9 | 15.2 | 446 | 75 |
Bricks | 111.9 | 20.7 | 99.2 | 18.3 | 487 | 90 |
Bricks | 103.6 | 21.2 | 93.3 | 19.2 | 458 | 94 |
Bricks | 104.9 | 19.3 | 94.4 | 17.4 | 463 | 85 |
Gypsum | 62.8 | 11.1 | 54.3 | 9.5 | 266 | 47 |
Gypsum | 55.2 | 10.5 | 47.8 | 9.2 | 234 | 45 |
Gypsum | 63.7 | 12.6 | 54.5 | 10.7 | 267 | 52 |
Gypsum | 74.2 | 12.9 | 64 | 10.9 | 314 | 54 |
Cement | 82.1 | 14.8 | 72.5 | 13.0 | 356 | 64 |
Cement | 100.0 | 20.6 | 88.3 | 18.1 | 433 | 89 |
Cement | 86.4 | 16.5 | 76.9 | 14.6 | 377 | 72 |
Cement | 77.8 | 14.2 | 68.1 | 12.4 | 334 | 61 |
Ceramic | 91.5 | 17.3 | 82 | 15.5 | 402 | 76 |
Ceramic | 93.2 | 22.0 | 82 | 19.2 | 402 | 94 |
Ceramic | 109.8 | 20.2 | 97.4 | 17.8 | 478 | 88 |
Tile | 108.1 | 24.0 | 96.1 | 21.5 | 471 | 105 |
Tile | 120.3 | 31.2 | 106.6 | 27.7 | 523 | 136 |
Tile | 110.6 | 27.5 | 98.4 | 24.5 | 483 | 120 |
Tile | 124.8 | 29.0 | 111.3 | 25.5 | 546 | 125 |
Statistical Values of Samples | ||||||
We correlated the values of 226Ra, 232Th and 40K for all samples, and also separately. The correlation study between 226Ra, 232Th and 40K mass activity concentrations of the studied building materials are shown in
Scatter plots of 232Th vs. 226Ra; 40K vs. 226Ra and 40K vs. 232Th for A) Sand and Gravel; B) Brick and C) Gypsum.
Scatter plots of 232Th vs. 226Ra; 40K vs. 226Ra and 40K vs. 232Th for A) Cement; B) Ceramic and C) Tile.
In most of the studied building materials, there is a positive correlation between 226Ra and 232Th. For instance, as it is revealed that from
To estimate the correlations between the mass activity of 40K, 232Th and 226Ra among all sample as a single component of the building, scatter plots of the mass activity of 232Th vs. 226Ra, 40K vs. 226Ra and 40K vs. 232Th were drawn and are shown in
Scatter plots of 232Th vs. 226Ra for all samples.
Scatter plots of 40K vs. 226Ra for all samples.
Scatter plots of 40K vs. 232Th for all samples.
Since the relative distribution of 226Ra and 232Th is positively correlated with all of the calculated radiological parameters, this result may be due to the rich content of 226Ra and 232Th, which plays an important role in determining the hazardous nature in the building materials; poor correlation was observed between 226Ra and 40K.
The concentrations of radionuclides measured within this study were compared with those from other studies worldwide in
Comparison of mass activity of naturally occurring radionuclides in building materials in this study with results from other studies (Bq/kg).
Country | Building Material | aRa-226(mean) | aTh-232 (mean) | aK-40 (mean) | References |
---|---|---|---|---|---|
Austria | Bricks | 20-71 (38) | 16-112 (45) | 520-880 (635) | ( |
Cement | 11-49 (27) | 10-26 (14) | 89-286 (210) | ||
Concrete | 7-21 (15) | 3-57 (14) | 16-382 (164) | ||
Denmark | Bricks | 8-42 (25) | 8-34 (21) | 280-630 (455) | ( |
Cement | 9-30 (20) | 4-21 (12) | 20-140 (90) | ||
Concrete | 15-670 (152) | 10-53 (27) | 280-1190 (620) | ||
Hungary | Bricks | 8-42(25) | 8-34 (21) | 280-630 (455) | ( |
Cement | 8-61 (30) | 13-53 (22) | 95-402 (218) | ||
Concrete | 13-18 (16) | 11-33 (22) | 204-437 (356) | ||
Europe | Brick | 2-148 (47) | 2-164 (48) | 12-1169 (598) | ( |
Cement | 4-422 (45) | 3-266 (31) | 4-846 (216) | ||
Natural gypsum | 1-70 (15) | 1-100 (9) | 5-279 (91) | ||
Aden, Yemen | Portland cement | 33-45 (40) | 19-31 (25) | 234-502 (428) | ( |
Sand | 14-26 (21) | 20-32 (28) | 859-1267 (1118) | ||
Red bricks | 46-60 (55) | 30-52 (37) | 1209-1343 (1256) | ||
Egypt | Red-brick | (23) | (23) | (448) | ( |
Sand | (17) | (13) | (119) | ||
Cement | (45) | (10) | (51) | ||
Gypsum | (8) | (8) | (85) | ||
Ceramic | (51) | (41) | (683) | ||
Brazil | Sand | (14) | (18) | (807) | ( |
Cement | (62) | (59) | (564) | ||
Gypsum | (6) | - | (18) | ||
Granite | (49) | (288) | (1335) | ||
Weinan, China | Sand | 97-131 (119) | 21-62 (36) | 181-274 (250) | ( |
Red-clay bricks | 119-130 (125) | 28-30 (29) | 377-418 (390) | ||
Gravel | 91-125 (96) | 12-27 (17) | 281-398 (325) | ||
Ceramic | 50-95 (70) | 23-52(39) | 285-644 (417) | ||
Tile | 150-266 (395) | 35-64 (44) | 682-5515 (835) | ||
Sand and Gravel | 28-38 (32) | 25-44 (34) | 204-378 (289) | ||
Regarding the impact of exposure to gamma radiation on one’s health, it was decided not to calculate the excess lifetime cancer risk from gamma dose rates based on the life expectancy of the Iranian population as 74 years based on WHO: Iran national profile in 2012. Such results were published for the studies concerning Jhelum valley (
Due to the aforementioned factors, the external gamma doses originating from background radiation in the area and building materials were estimated for various indoor occupancy. As we assumed the maximum indoor gamma dose rate to be 111 nGy/h (estimated by this study) and the minimum outdoor gamma dose rate about 800 nGy/h (
Assessment of annual effective dose from external gamma radiation for various occupancy times.
Indoor residence time Percentage (%) | 111 nGy/h indoor gamma dose rate (mSv/y) | 800 nGy/h outdoor gamma dose rate (mSv/y) | The annual dose(mSv/y) |
---|---|---|---|
0.55 | 0.98 | 1.53 | |
0.48 | 1.47 | 1.95 | |
0.41 | 1.96 | 2.37 | |
0.34 | 2.45 | 2.79 | |
0.28 | 2.94 | 3.22 | |
0.21 | 3.43 | 3.64 |
It can be seen that for the maximum an indoor occupancy time 0.8 (80% of one’s time is spent in a building), just 37% of the annual effective dose of gamma radiation is caused by radiation produced by building materials. For those who spend more time outdoors (farmers, children), the fraction of the annual effective dose that originates from natural background radiation is greater, while the percentage of the dose that originates from building materials may decrease to 14 % or even less.
Some preliminary measurements of the radon level in dwellings in the investigated area were conducted a few years ago (
Unlike in previous studies, it was decided not to calculate the excess lifetime cancer risk from gamma radiation in dwellings because these building materials are in use in an area exposed to high levels of natural background radiation with elevated values of external gamma radiation dose rates. Therefore, in our opinion, such calculations might yield misleading results.
In the present study, the mass activity of naturally occurring radionuclides in twenty-three samples of building materials available on the market in Mahallat, Iran - a city exposed to a high level of natural background radiation - were determined in order to assess radiation exposure in dwellings. The distribution of results with regard to the measured mass activity of naturally occurring radionuclides found in the samples of building materials was different for 40K compared to 226Ra and 232Th. The average mass activity of 226Ra, 232Th and 40K in all of the samples were determined as 34 ± 6, 27 ± 6 and 276 ± 58 Bq/kg, respectively, and were all below the worldwide average values. The cumulative averages of the gamma absorbed dose rate and annual effective dose rate were estimated to be 84 ± 17 nGy/h and 412 ± 82 µSv/yr, respectively, both of which are higher than the worldwide average value (55 nGy/h) and below the EU BSS recommended annual value (1 mSv/yr). The radioactivity index, calculated according to EU BSS recommendations, was less than 1 for all of the samples. In line with the EU BSS, building materials with a radioactivity index of less than one is exempt from radiological examinations before being placed on the market. However, even for the samples with low radioactivity indexes - due to the absence of national regulations in Iran - to monitor and control the radioactivity of building materials, such measurements are necessary. The legislation of a national standard into the Iranian legal system describing the requirements for the radiological examination of building materials, is necessary before their introduction on the market.
This work was supported by Hungarian National Research OTKA grant No. K128805 and K128818 and GINOP Grant of the Hungarian Government No. 2016-0016.