Radioactivity of building materials in Mahallat, Iran – an area exposed to a high level of natural background radiation – attenuation of external radiation doses

A. Shahrokhi^a, M. Adelikhah^a, S. Chalupnik^b, E. Kocsis^a, E. Toth-Bodrogi^a, T. Kovács^a*

a. Institute of Radiochemistry and Radioecology, University of Pannonia, (Veszprém, Hungary)

b. Silesian Centre for Environmental Radioactivity, Central Mining Institute, (Katowice, Poland)

* kt@almos.uni-pannon.hu

1. IntroductionTOP

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 (1). However, the main contributor of human exposure is radon and its decay products, building materials can be also a source of external exposure to ionizing radiation (1, 2). Clay, sands and other raw materials in addition to recycled by-products, used in the production of the majority of building materials, contain trace amounts of naturally occurring radioactive materials, e.g. radionuclides from the uranium (²³⁸U) and thorium (²³²Th) radioactive decay series as well as potassium (⁴⁰K), another primordial radionuclide (3, 4). The worldwide average mass activity of ²²⁶Ra, ²³² Th and ⁴⁰ K in the soil are 35, 30 and 400 Bq/kg, respectively; but these values can be several times higher in High Natural Background Radiation Areas (HNBRAs), e.g. Ramsar, Mahallat, Yangjiang, Kerala, etc., and people are receiving radiation doses much greater than the worldwide average background dose (1). In accordance with the findings of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), the annual effective doses received by inhabitants in HNBRAs are classified as: low (<5 mSv/yr), medium (5–10 mSv/yr), high (20–50 mSv/yr) and very high (>50 mSv/yr) (1, 5). This classification is based on the recommended dose level prescribed by the International Commission on Radiological Protection (ICRP) and the worldwide average dose level for the general public (2.4 mSv/yr) (1, 2, 5–8).

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 (8). Thus, RaCO₃ may ultimately occur as a component of building materials, utilized to construct local. According to the 1, the absorbed dose rate in this area was estimated to fall within the range of 800 to 4000 nGy/h (including cosmic and terrestrial radiation), which is at least fifteen times higher than the worldwide average of 55 nGy/h (1, 9). Therefore, it should be obligatory to monitor building materials as a secondary potential source of emitting indoor gamma radiation, however, the introduction of additional sources of enhanced radioactivity into such an environment would not be preferable. Moreover, according to the new European Union Basic Safety Standards (Council Directive 2013/59/EURATOM), the radioactivity index of building materials was restricted to less than 1 (to correspond to a reference level of external gamma dose -1 mSv/yr - before being released to the market) (1, 2, 10, 11).

Over recent years, many studies have been carried out in High Natural Background Radiation Areas to assess the radiation (5, 8, 12–21). Our study focused on the assessment of gamma-ray emitters in building materials and the estimated exposure to indoor gamma radiation from these materials in the HNBRA of Mahallat, Iran. Attempts were made to evaluate the distribution of naturally occurring radionuclides in different materials, therefore, measurements of the radioactivity of twenty-three of the most common building materials available on the local market were taken. As an objective of the present study and with respect to radiation protection, the radium equivalent activity index, resultant absorbed dose of gamma radiation and annual effective dose of each sample of building material were estimated. An attempt was not made to estimate the excess lifetime cancer risk from indoor gamma radiation based on the life expectancy of the Iranian population because the use of building materials that emit relatively low levels of radioactivity (compared with outdoor values) would be meaningless. On the other hand, the radioactivity index of samples, according to the European Union Basic Safety Standards (EU BSS), was calculated in order to screen the products on the market and avoid the introduction of materials with enhanced radioactivity to the HNBRA. The annual effective dose in relation to time spent outdoors and indoors was estimated to show the shielding effect of building materials from gamma radiation. Additional investigations of the radon levels in dwellings should be undertaken to provide a comprehensive estimation.

2. Materials and MethodsTOP

2.1. Area of InterestTOP

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 (1). It contains several sources of spring water with high levels of radioactivity known as Abegarm-e-Mahallat, a region to the north-east of Mahallat, but the exposure detailed in this study is derived from other sources (8, 13).

2.2. Sampling and Sample PreparationTOP

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.

2.3. Gamma spectrometry analysisTOP

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 (22).

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 (22). The concentration of radionuclides in the samples, uncertainty and Minimum Detectable Activity were determined using calculations explained in our recently published papers (23, 24).

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.

2.4. Radiation AssessmentTOP

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 [1] (25):

Where a denotes the mas activity of a specific radionuclide (Bq/kg).

The absorbed dose rate, caused by naturally occurring radionuclides in building materials, the radioactivity index and the annual effective dose were calculated using Equations [2, 3, 4], respectively (2, 26–28):

where I denotes the radioactivity index, D represents the gamma dose rate from naturally occurring radionuclides (nGy/h), ED stands for the annual effective dose (µSv/yr), 0.7 is the conversion factor for (Sv/Gy), 0.8 is the indoor occupancy factor, and T is the number of hours in a year (8760 h/yr).

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.

3. Results and DiscussionTOP

Table 1 shows the concentrations of naturally occurring radionuclides recorded in the dry samples of building materials. The average mass activities of ²²⁶Ra, ²³²Th and ⁴⁰K in the samples measured were 34 ± 6, 27 ± 6 and 276 ± 58 Bq/kg, respectively. It can be seen that concentrations measured were, in most cases, below worldwide average values (1). The excel program was used for data analysing using built-in commands. Due to the statistical analysis, it was found that the concentrations of ⁴⁰K and ²²⁶Ra were higher than the worldwide average value in 17% and about 26% of the samples, respectively. The highest mass activity of ²²⁶Ra, ²³²Th and ⁴⁰K were found in various building materials (tiles, sand, gravel and bricks), but did not significantly exceed the worldwide average values (Figures 1 and 2). The activity indexes for all of the samples were calculated and found to be less than 1.0, the value recommended by EU BSS, likewise the statistical analysis of the obtained data with regard to the mass activity of ⁴⁰K, ²²⁶Ra and ²³²Th in the samples of building materials is presented in as shown in Table 1.

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 Figures 1 and 2. The excel Polynomial, 3^rd Order, were used to draw the trendlines between the data.

According to Figures 1 and 2, in most samples, the mass activity of ⁴⁰K fell within the range of ~200 to ~380 Bq/kg, while the corresponding values for ²²⁶Ra and ²³²Th were from ~30 to ~40 and ~25 to ~30 Bq/kg, respectively.

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 Table 2.

Table 2 shows the results of radium equivalent (Ra_(eq)), gamma-ray absorbed dose (D) and annual effective dose (ED) for studied building materials. The calculated values of radium equivalent activities were between 55 and 125 Bq/kg with a cumulative average of 95 Bq/kg. Meanwhile, the gamma-ray absorbed dose rates were measured to be between 48 and 111 nGy/h with a cumulative average of 84 nGy/h, which is higher than the worldwide average value of 55 nGy/h (1). Based on the values of estimated gamma-ray absorbed dose rates, the annual effective doses were calculated to be within the range of 234 to 546 µSv/yr with a cumulative average of 412 µSv/yr.

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 Figure 3 and Figure 4. Since ²²⁶Ra series and ²³²Th series are usually found together in nature, and good correlation between them is indicative of common sources, which, in general, are associated with a mineralogical component; in the other words, mass ratio between Th and U is dependent on the type of the material but is correlated with activity ratio. Having activity ratio a_Th:a_U it can multiply it by a ratio of half-lives of Th (14 billion years) and U (4.5 billion years). That ratio is a constant value roughly 14/4.5 = 3.1(UNSCEAR reports show the average ²³⁸U concentration in soil as 35 Bq/kg and similar value for ²³²Th, therefore, it would give us mass ratio of about 3) (1); However, to estimate the dose from the construction materials, activity concentration is needed instead of the mass concentrations or mass ratio.

In most of the studied building materials, there is a positive correlation between ²²⁶Ra and ²³²Th. For instance, as it is revealed that from figures 3 and 4, the strongest linear relationship is observed between ²²⁶Ra and ²³²Th in sand and gravel, and ceramic with R² = 0.98 and R² = 0.56, respectively; However, the corresponding correlation coefficient is slightly lower for other building materials. Brick and tile have shown the most positive correlation between ²³²Th and ⁴⁰K mass activity concentrations with R² = 0.55 and R² = 0.48 than other building materials. The weakest relationships can be also found between ²³²Th and ⁴⁰K mass activity concentrations in cement. Moreover, the strongest correlation between ²²⁶Ra and ⁴⁰K with R² = 0.86 was found in bricks. The weakest corresponding relationships can be also found in ceramic.

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 Figures 5 – 7. No strong correlations are visible from the scatter plots shown in Figures 5, 6 and 7. On the other hand, it can be seen that the general trend of these plots is that the mass activity of each particular radionuclide increases as the mass activity of other radionuclides increases.

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 Table 3. According to Table 3, the results of this study are consistent with other reported results of similar studies in other countries.

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 (33) or Kirklareli in Turkey (34). The main reason for its absence was that these building materials are used in areas of high natural background radiation with elevated external gamma-ray dose rates (1). Therefore, it is believed that these calculations may yield misleading results. Instead of following the typical approach, the authors decided to calculate the shielding effect of dwellings built from materials with much lower concentrations of radionuclides compared with soils in the area. This approximate estimation shows that the attenuation factor calculated for a wall thickness of 30 to 40 cm with an average density of the building material equal to about 2 g/cm³ may be as high as 500 for a gamma-ray energy of 500 keV. As a result, the additional indoor gamma dose rate would not exceed the previously measured dose in studying area environment as 4 μGy/h (35). If this is compared with the minimum estimated value of gamma-ray dose from building materials in this study (~50 nGy/h), the attenuation effect of the walls of dwellings is confirmed.

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 (9), then the annual effective dose without considering the dose from inhaled radon shows in Table 4.

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 (36). The radon concentration measured was as high as 350 Bq/m³. By applying the recommendations from ICRP Publication 126, the estimated values of annual effective doses from that area were as high as 12 mSv/yr (the new value based on ICRP-137 were measured by this paper’s authors as 13.6 mSv/yr), which is significantly greater than calculated for external gamma radiation. As a result, measurements of radon levels and its decay products in dwellings situated in areas exposed to high levels of background radiation are most important.

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.

4. ConclusionsTOP

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.

AcknowledgementsTOP

This work was supported by Hungarian National Research OTKA grant No. K128805 and K128818 and GINOP Grant of the Hungarian Government No. 2016-0016.

ReferencesTOP