Introduction

All construction materials of natural origin may contain certain concentrations of radionuclides from the series of 238U and 232Th as well as the primordial radionuclide 40K, and such materials are classified into the Naturally Occurring Radioactive Materials (NORM) group [1,2,3,4,5,6]. Since these radionuclides are not evenly distributed in materials, knowledge of their activity concentrations is very important for assessing the impact on human health and radiation protection [7, 8]. The greatest contribution to the exposure of workers and public to the radiation from building materials has activity concentrations of 226Ra, 232Th and 40K whose average values for building materials in the world are 50 Bq kg−1, 50 Bq kg−1 and 500 Bq kg−1, respectively [9]. The increased content of these radionuclides can affect the exposure of workers to radiation when working with such materials (for example, in the ceramic industry), therefore it is very important to carry out tests on the level of exposure to the radiation [5]. The objective of assessing the level of exposure when working with building materials is based on the estimation of the annual effective radiation dose using the appropriate dose criterions. According to the recommendations of the European Commission in 1999 [10], dose optimization should be in the range between 0.3 mSv y−1 and 1 mSv y−1, whereas according to the European Union Directive from 2014, this limit for public is set at 1 mSv y−1 while for workers is 20 mSv y−1 [11].

The typical annual effective dose for workers exposed to zircon minerals is 70–260 μSv y−1 from external exposure and 600–3000 μSv y−1 from inhalation of dust, giving an overall annual effective dose of 700–3100 μSv y−1 [12].

Serbia is one of the leading countries in southeastern Europe in the production of ceramic tiles for floors and walls with a tradition in process of production for over 50 years. Huge quantities of raw materials are imported every year for the production of ceramic tiles, and some of them are zircons (zircon minerals).

According to its composition, zircon minerals are in the form of zirconium silicate (ZrSiO4), or zircon sand. For zircon crystals, various impurities can be related, as some radionuclides from the 238U and 232Th series, and may have an increased level of radioactivity [7, 13, 14]. Based on the measured values of the activity concentrations of 226Ra in the earlier research of raw materials used in the ceramic industry, it can be concluded that the zircon is one of the most radioactive raw materials [4, 13, 15, 16]. The activity concentration of 226Ra measured in some samples of zircon from North Korea reaches values up to 11,000 Bq kg−1 [2]. The zircon is used in the ceramic industry in bulk, and the increased activity concentration of 226Ra in zircon samples can have a significant risk of exposure to gamma radiation as well as radon (222Rn) radioactive gas inhalation and its progenies [17]. In the Directive of 2014, the European Union passed the permitted limit to exposure to radon at a workplace of about 300 Bq m−3 [11], as recommended by the World Health Organization in the 2009 report [18].

Before use in the ceramic industry, zircon goes through the grinding process where fine particles of size ≤ 50 μm (zircon flour) are formed [13, 14]. Inhalation or ingestion of fine aerosol particles in the air, when using zircon minerals with an increased content of 226Ra, poses a risk to the internal exposure of the organism (respiratory organs) to ionizing radiation, which can lead to lung cancer [12, 17, 19, 20]. Proper hygiene management in the industry is enough to minimize the impact of radiation generated by zircon flour [19].

This paper presents gamma spectrometric measurements of activity concentrations of 226Ra, 232Th and 40K for 41 samples of zircon mineral used in the Serbian ceramic industry. On the basis of the measured values of these radionuclides, the assessment of the radiation risk in working with these materials in terms of hazard index [radium equivalent activity index (Raeq), gamma index (Iγ), alpha index (Iα), annual effective dose (E), excessive lifetime cancer risk (ELCR), alpha dose equivalent (Hα) and radon mass exhalation rate (EM)]. The obtained values are compared with the permitted values given in national and international directives, as well with the measured values for zircon minerals and other raw materials used in ceramic industries in the world.

Materials and methods

Samples of zircon minerals were collected when they were imported into the Republic of Serbia while performing a dosimetric inspection at border crossings with Croatia, Batrovci and Sid, in the period September 2018–April 2019. The gamma spectrometric measurements of all samples were carried out at the Department of Physics at the Faculty of Sciences, University of Novi Sad, Serbia. Before gamma spectrometric measurements, all samples were dried at a temperature of 105 °C for about 8 h and afterward ground (some of the samples were already in the powdery state—zircon flour) and packed in a plastic cylinder container with a diameter of 67 mm and height of 62 mm. Analysis of the radionuclides in the samples was carried out 40 days after the preparation of the samples since a secular radioactive equilibrium was established between 226Ra and 222Rn [21]. The mass of the prepared samples was about 400 g.

The gamma spectrometric analysis of samples was performed according to the IAEA TRS 259 standard method [22] using a low-background HPGe gamma spectrometer manufactured by Canberra, with a relative efficiency of about 36% and a resolution of 1.9 keV. The gamma spectrometry system has lead protection thickness of 12 cm and an additional 3 mm-thick copper shield, to prevent penetration of lead K-shell X-rays in the energy range of (75–85) keV.

The measurement time of the individual sample was approximately 72,000 s. The activity concentration of 226Ra was estimated from gamma lines of its decay products: 214Pb at 295.2 keV and 351.9 keV, and 214Bi at 609.3 keV, and 1120.3 keV. The gamma lines emitted from 228Ac at 338.3 and 969.0 keV, from 212Pb at 238.6 keV and the gamma line of 208Tl at 2614.5 keV were used to determine the activity concentration of 232Th. The activity concentration of 40K was determined using gamma line emitted by this radioisotope at 1460.8 keV [21, 23].

The calibration of the detector was carried out using a reference radioactive standard embedded in a silicone resin of cylindrical geometry, a volume of 250 cm3 of the Czech Metrology Institute (Cert. No. 1035-SE-40001-17). By using the ANGLE software, correction to the effect of self-absorption was made due to the different density of the matrix of the analyzed material. This precise calibration is performed to ensure a small measurement uncertainty below 10% necessary when the activity of radioisotopes is determined in the low-energy region (below 100 keV) (e.g. for 234Th, a progeny of 238U) [21].

Assessment of radiation risk for workers

Radium equivalent activity index (Raeq)

Radium equivalent activity index (Raeq) was used to evaluate radiation hazard to the persons working with building materials (occupationally exposed individuals). Radium equivalent activity index was introduced due to the fact that the distribution of 226Ra, 232Th and 40K in building materials is not uniform. Radium equivalent activity index was introduced with the assumption that 370 Bq kg−1 of 226Ra, 259 Bq kg−1 of 232Th, and 4810 Bq kg−1 of 40K produce the same dose of gamma radiation and can be calculated using the Eq. (1) [24]:

$$ {\text{Ra}}_{\text{eq}} \left( {{\text{Bq}}\;{\text{kg}}^{ - 1} } \right) = C_{\text{Ra}} + 1.43C_{\text{Th}} + 0.077C_{\text{K}} $$
(1)

where CRa, CTh, and CK are activity concentrations of 226Ra, 232Th, and 40K in Bq kg−1 for the given building material, respectively. Radiologically safe radiation exposure is limited to the annual effective radiation dose of 1.5 mSv y−1, while the value of radium equivalent activity index must not exceed the limit of 370 Bq kg−1 [25, 26].

Gamma index (I γ)

To evaluate exposure to the gamma radiation, a gamma index (Iγ) has been introduced. In this paper, the gamma index is calculated based on the Eq. (2), proposed by the European Commission in 1999 [10]:

$$ I_{\upgamma} = \frac{{C_{\text{Ra}} }}{{300\,{\text{Bq}}\,{\text{kg}}^{ - 1} }} + \frac{{C_{\text{Th}} }}{{200\,{\text{Bq}}\,{\text{kg}}^{ - 1} }} + \frac{{C_{\text{K}} }}{{3000\,{\text{Bq}}\,{\text{kg}}^{ - 1} }} \le 1 $$
(2)

where CRa, CTh, and CK are activity concentrations of 226Ra, 232Th, and 40K in Bq kg−1 for the given building material, respectively. In the 2014 directive, the European Union introduced a gamma index for screening building materials where the values of the gamma index Iγ ≤ 1 correspond to the annual effective dose of less than 1 mSv y−1 [11], which is also the recommended value by the United Nations Scientific Committee on the Effects of Atomic Radiation [26].

Alpha index (I α)

An alpha index (Iα), which can be calculated using the Eq. (3), was introduced to estimate the exposure to excess alpha radiation generated by the building material [1, 27]:

$$ I_{\alpha } = \frac{{C_{\text{Ra}} }}{{200\,{\text{Bq}}\,{\text{kg}}^{ - 1} }} \le 1 $$
(3)

where CRa is the activity concentration of 226Ra in Bq kg−1 for the given building material. The recommended alpha index value is Iα≤ 1, which corresponds to the activity concentration of 226Ra CRa ≤ 200 Bq kg−1. The activity concentration of 226Ra greater than 200 Bq kg−1 may give a radon concentration greater than 200 Bq m−3, which represents a significant exposure to alpha radiation [27].

Absorbed dose rate (D)

The absorbed dose rate (D) due to the emission of gamma radiation from natural radionuclides present in building materials (gypsum, limestone, zircon minerals, cement, and bricks) can be estimated according to the Eq. (4) [10, 27]:

$$ D\left( {{\text{nGyh}}^{ - 1} } \right) = 0.92 \cdot C_{\text{Ra}} + 1.1 \cdot C_{\text{Th}} + 0.080 \cdot C_{\text{K}} $$
(4)

where CRa, CTh, and CK are activity concentrations of 226Ra, 232Th, and 40K in Bq kg−1 for the given building material, respectively. Values 0.92, 1.1 and 0.080 in the Eq. (4) represent a specific dose rate in nGy per Bq kg−1 [10]. The average absorbed dose rate for building materials in the world is 55 nGy h−1 [28].

Annual effective dose (E)

In order to assess the exposure of workers in the ceramic industry, it is useful to know the annual effective dose derived from gamma radiation from natural radionuclides and can be calculated using the Eq. (5) [10]:

$$ E\left( {{\text{mSv}}\,{\text{y}}^{ - 1} } \right) = D \times 800\,{\text{h}} \times 0.7\,{\text{SvG}}\,{\text{y}}^{ - 1} $$
(5)

where D is the absorbed dose rates given in mGy h−1; 800 h is the annual exposure time when working with zircon minerals in the ceramic industry [5]; 0.7 Sv Gy−1 is the conversion factor of the dose [10]. According to the law in Serbia, the annual effective dose of 20 mSv y−1 is allowed for workers [29]. This policy is in line with the EU directive from 2014 [11]. The average annual effective dose from building materials in the world is 0.460 mSv y−1 [26].

Excess lifetime cancer risk (ELCR)

The excessive lifetime cancer risk (ELCR) can be estimated based on the obtained annual effective dose using the Eq. (6):

$$ {\text{ELCR}} = E \times {\text{DL}} \times {\text{RF}} $$
(6)

where E is the annual effective dose, DL is the average life span (70 years) and RF is a risk factor (Sv−1), fatal cancer risk per Sievert. In case of stochastic effects, the usual assumption is RF = 0.05 [30]. The average value of excess lifetime cancer risk in world (ELCR) is 0.3 × 10−3 [26].

Alpha dose equivalent (H α)

In a European Commission report of 1990, the use of dose criteria is recommended for a radon concentration of 1 Bq m−3 corresponding to an annual effective dose of 0.05 mSv y−1 [31]. According to this criterion, the concentration range 222Rn of 100–300 Bq m−3 [18] corresponds to the effective dose range of 5–15 mSv y−1. The alpha dose level from 222Rn and its decay products from the building material can be calculated using the Eq. (7) [32]:

$$ H_{\alpha } = 0.18 \times \varepsilon \times C_{\text{Ra}} + 0.45 $$
(7)

where Hα is alpha dose equivalent in mSv y−1; ε is the coefficient of emanation 222Rn from given building material and CRa is measured activity concentration of 226Ra in Bq kg−1.

Radon mass exhalation rate (E M)

Inside of grains of building material, the 222Rn is produced by the decay of 226Ra, afterward, it emanates from grains into the pores of the material. The final process is exhalation of 222Rn from the pores of the material to the surrounding air. Radon mass exhalation rate (EM) can be calculated using the Eq. (8):

$$ E_{\text{M}} = \lambda_{\text{Rn}} \times C_{\text{Ra}} \times \varepsilon $$
(8)

where λRn is the radioactive decay constant of 222Rn (2.1 × 10−6 s−1); CRa is the activity concentration of 226Ra in the sample measured after the establishment of a secular radioactive equilibrium of 1 month; ε is the coefficient of radon emanation from given building material. The radon mass exhalation rate (EM) is given in units of Bq kg−1 s−1 [33, 34].

Results and discussion

A list of analyzed samples of zircon minerals with countries of origin used in the Serbian ceramic industry with measured values of activity concentrations of 226Ra, 232Th and 40K are given in Table 1. The measured values of the activity concentration of 226Ra range from 404 ± 24 Bq kg−1 (sample No. 27 from Slovenia) to 4120 ± 40 Bq kg−1 (sample No. 35 from Spain) and the average value for all 41 zircon samples is 2532 ± 117 Bq kg−1 (mean value ± standard deviation). The measured values of the activity concentration of 232Th are in the range from 71 ± 7 Bq kg−1 (sample No. 27 from Slovenia) to 560 ± 40 Bq kg−1 (sample No. 13 from Italy) and the average value for all 41 samples of zircon samples is 360 ± 16 Bq kg−1. Measured values of 40K range from 42 ± 5 Bq kg−1 (sample No. 15 from Italy) to 356 ± 32 Bq kg−1 (sample No. 38 from Spain) and the average value for all 41 samples of zircon samples is 183 ± 12 Bq kg−1. The measured values of 226Ra and 232Th for all samples are above average values for building materials in the world of 50 Bq kg−1, while activity concentrations of 40K are below the average value of 500 Bq kg−1 [9]. The measured activity concentrations of 226Ra, 232Th, and 40K are comparable with the results reported for zircon mineral samples from Belgium, Czech Republic, Germany, Italy and Spain [4, 14, 16, 19, 35], given in Table 2. Compared to the activity concentration of 226Ra, 232Th and 40K for other raw materials used in the ceramic industry (kaolin, quartz, feldspar, dolomite, and stone), the zircon minerals analyzed in this paper have a significantly higher level of these radionuclides, primarily 226Ra and 232Th (see Table 3).

Table 1 List of samples, country of origin and activity concentration of 226Ra, 232Th and 40K for 41 zircon mineral samples used in Serbian ceramic industry
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Table 2 Comparison of activity concentration of 226Ra, 232Th and 40K in zircon samples measured in the world and in this paper
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Table 3 Comparison of activity concentrations of 226Ra, 232Th and 40K for other raw materials in the ceramic industry in the world with values for zircon minerals in this paper
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The relative contribution of 226Ra, 232Th, and 40K in 41 zircon mineral samples is 82%, 12%, and 6%, respectively (see Fig. 1).

Fig. 1
figure 1

Relative contribution of 226Ra, 232Th, and 40K in 41 zircon mineral samples used in Serbian ceramic industry

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Distribution of activity concentrations of 226Ra, 232Th and 40K in samples of zircon minerals is presented in Fig. 2. It can be noticed that the distribution of activity concentrations of 226Ra, 232Th and 40K are in good agreement with Gaussian distribution, as indicated by high correlation factors (R2): 0.93, 0.89 and 0.95, respectively.

Fig. 2
figure 2

Frequency distributions of a226Ra (Bq kg−1), b232Th (Bq kg−1) and c40K (Bq kg−1) activity concentrations in zircon mineral samples from Table 1

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In Fig. 3 correlations between activity concentrations of 232Th and 226Ra were observed with correlation factor (R2) of 0.70, while correlations between activity concentration of 40K and 226Ra and 232Th and 40K are not remarkable, based on the obtained correlation factors (R2) of 0.13 and 0.33, respectively.

Fig. 3
figure 3

Correlation between 232Th and 226Ra, 40K and 226Ra, 232Th and 40K activity concentration of 41 zircon samples from Table 1

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The obtained values of radium equivalent activity indices (Raeq), gamma indices (Iγ), alpha indices (Iα) and radon mass exhalation rate (EM) are given in Table 4.

Table 4 Radium equivalent activity index (Raeq), gamma index (Iγ), alpha index (Iα) and radon mass exhalation rate (EM) for 41 zircon mineral samples used in Serbian ceramic industry
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The radium equivalent activity index (Raeq) ranges from 512 ± 26 Bq kg−1 (sample No. 27 from Slovenia) to 4663 ± 51 Bq kg−1 (sample No. 35 from Spain). The average value of radium equivalent activity index for all 41 samples of zircon minerals is 3062 ± 135 Bq kg−1 (mean value ± standard deviation). The values obtained for all samples are above the recommended value of 370 Bq kg−1 [26].

Figure 4 shows a strong correlation between activity concentration of 226Ra and Raeq with a correlation factor (R2) of 0.99, while weaker correlations were observed between Raeq–232Th and Raeq–40K with correlation factors (R2) of 0.77 and 0.35, respectively.

Fig. 4
figure 4

Correlation between activity concentration of 226Ra, 232Th, 40K, and Raeq for 41 zircon mineral samples from Table 1

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Gamma index values (Iγ) range from 1.73 ± 0.09 (sample No. 27 from Slovenia) to 15.7 ± 0.2 (sample No. 35 from Spain). The average gamma index for all samples is 10.3 ± 0.5 (mean value ± standard deviation). Gamma index values for all samples are above the recommended value of Iγ≤ 1 [26]. The obtained values of the gamma index in this paper are comparable with the values reported in papers [16, 35].

Alpha index (Iα) range from 2.02 ± 0.12 (sample No. 27 from Slovenia) to 20.6 ± 0.2 (sample No. 35 from Spain). The average alpha index for all samples is 12.7 ± 0.56 (mean value ± standard deviation). For all samples, the alpha index is higher than the recommended value of Iα≤ 1 [27].

Calculated values of absorbed dose rates (D), annual effective doses (E), excess lifetime cancer risks (ELCR) and alpha dose equivalents (Hα) are given in Fig. 5.

Fig. 5
figure 5

Absorbed dose rates-D (a), annual effective doses-E (b), excess lifetime cancer risks—ELCR (c), and alpha dose equivalents-Hα (d) for 41 zircon mineral samples used in Serbian ceramic industry

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The obtained absorbed dose rates (D) range from 450 ± 30 nGy h−1 (sample No. 27 from Slovenia) to 4200 ± 60 nGy h−1 (sample No. 35 from Spain). The average value of absorbed dose rate for all samples of zircon minerals is 2730 ± 120 nGy h−1 (mean value ± standard deviation). The values obtained for all 41 samples of zircon minerals are above average values of 55 nGy h−1 for building materials worldwide [28].

The obtained annual effective dose (E), calculated base on the assumption that the worker in the ceramics industry spends 800 h in a year in the work with zircon minerals, ranges from 0.252 ± 0.015 mSv y−1 (sample No. 27 from Slovenia) to 2.35 ± 0.015 mSv y−1 (sample No. 35 from Spain). The average annual effective dose for all 41 samples is 1.53 ± 0.07 mSv y−1 (mean value ± standard deviation). All annual effective dose values are below 20 mSv y−1 as defined by Directive for professional exposure radiation in the Republic of Serbia and the European Union [11, 29]. All obtained values of annual effective doses exceed the average value of 0.460 mSv y−1 for building materials in the world [26].

Calculated values of ELCR range from (0.88 ± 0.05) × 10−3 (sample No. 27 from Slovenia) to (8.0 ± 0.4) × 10−3 (sample No. 35 from Spain). The average value of ELCR for all 41 samples is (5.4 ± 0.3) × 10−3 (mean value ± standard deviation). All obtained values exceed the average value in the world for building materials of 0.3 × 10−3 [26].

To estimate the alpha dose equivalent (Hα) value using the Eq. (7), the value of the emanation coefficient (ε) obtained in earlier research was used [37], which is 0.3%. The obtained alpha dose equivalent values range from 0.668 ± 0.013 mSv y−1 (sample No. 27 from Slovenia) to 2.67 ± 0.02 mSv y−1 (sample No. 35 from Spain), see Fig. 5. The average value of the alpha dose equivalent is 1.67 ± 0.08 mSv y−1 (mean value ± standard deviation). The obtained alpha dose equivalent values are less than 5 mSv y−1 for all samples, which according to the dose criterion given in [31] and the recommended values of the World Health Organization for exposures of radon in range 100–300 Bq m−3 [18], indicate that workers are exposed to a radon concentration of less than 100 Bq m−3. The obtained alpha dose equivalent values are significantly higher than those estimated for cement, gypsum, ceramic, granite, brick, concrete, and other building materials, given in the papers [32, 38].

The obtained values of the radon mass exhalation rate (EM) for all samples are shown in Table 4 and are in the range from 2.5 ± 0.2 μBq kg−1 s−1 (sample No. 27 from Slovenia) to 26.0 ± 0.3 μBq kg−1 s−1 (sample No. 35 from Spain). The average value of the radon mass exhalation rate for all 41 samples of zircon samples is 16.0 ± 0.7 μBq kg−1 s−1 (mean value ± standard deviation). The obtained radon mass exhalation rates are comparable with the values for granites given in the work [34] and for mortar, cement and chalk powder, which were analyzed in the paper [33]. Small values of emanation coefficient and mass exhalation rate can be attributed to the high density of zircon minerals matrix, which is about 4200–4500 kg m−3. The high density of the matrix prevents the exhalation of the radon from the zircon sample compared with other building materials that usually have a lower density.

Conclusion

This paper presents the results of gamma spectrometric measurements for 41 samples of zircon minerals used in the ceramic industry in Serbia. Based on measured values of 226Ra, 232Th and 40K, the radiation risk assessment was carried out when working with these materials in terms of hazard indices, annual effective doses, ELCRs, alpha dose equivalents, and radon mass exhalations. On the basis of the obtained values, it can be concluded that the activity concentration of 226Ra is significantly high, compared to the other raw materials used in the ceramic industry. The obtained hazard indices exceed the recommended values, while the annual effective doses do not exceed the allowed limit of 20 mSv per year, for all samples. Estimated alpha dose equivalent and the radon mass exhalation rate for zircon are comparable with the values for other building materials. From this, it can be concluded that there is no particular danger to the exposure of workers in the ceramic industry to the ionizing radiation originated from zircon minerals.

Implementing a standard precaution while working with zircon minerals such as maintaining personal hygiene after the work are sufficient to reduce the level of radiation risk to an acceptable level.