Radiation Protection Dosimetry (2017), pp. 1–5 doi:10.1093/rpd/ncx128 ASSESSMENT OF RADIUM ACTIVITY CONCENTRATION AND RADON EXHALATION RATES IN IBERIAN PENINSULA BUILDING MATERIALS E. Andrade1,2,*, C. Miró3, M. Reis1,2, M. Santos1,2 and M. J. Madruga1,2 Centro de Ciências e Tecnologias Nucleares, Instituto Superior Técnico, Universidade de Lisboa, Estrada Nacional 10, ao km 139,7, 2695-066 Bobadela LRS, Portugal 2 Laboratório de Proteção e Segurança Radiológica, Instituto Superior Técnico, Universidade de Lisboa, Estrada Nacional 10, ao km 139,7, 2695-066 Bobadela LRS, Portugal 3 Departamento de Física Aplicada, Universidad de Extremadura, Av. de la Universidad, s/n, 10005 Cáceres, Spain 1 *Corresponding author: firstname.lastname@example.org Radium (226Ra) is a natural radioactive element of the uranium decay series, which could also be present in building materials. Radon (222Rn) is continuously produced by the decay of 226Ra and its presence inside buildings can contribute to the increase of the population exposure to ionizing radiation. In this work, the amount of radium activity concentration and radon exhalation rates in several types of building materials that are commonly used in the Iberian Peninsula have been tested. The radium activity concentration was measured by gamma-ray spectrometry, whereas the radon exhalation rates were carried out using a continuous radon monitor (active measuring technique) and a solid state nuclear track detectors (passive measuring technique). The 226Ra mean values range from 5.0 to 123.4 Bq kg−1. As expected, the results show that the radon exhalation rate is higher in granites samples relatively to others building materials analysed. A positive correlation was found between radium activity concentration and radon exhalation rates in both techniques. The emanation fraction and alpha index were also calculated. INTRODUCTION The knowledge of the natural radioactivity of building materials is important for the determination of population exposure to radiations and for the assessment of the possible radiological hazards to human health. Building materials contribute to natural radiation exposure, either by gamma radiation from 40K, 238 U and 232Th and their decay products to an external whole body dose exposure, or by radon exhalation to an internal dose exposure due to deposition of radon decay products in the human lung tissue(1–3). Radium (226Ra) is a natural radioactive element of the uranium decay series, which could also be present in building materials. Radon (222Rn) is continuously produced by the decay of 226Ra and its presence inside dwellings can contribute for the increase of indoor radon levels as well as to a higher risk of lung cancer(4, 5). Therefore, there is a considerable public awareness and a growing concern in relation to radon exhalation from building materials, in particular those that are used for interior decorations(1). Over the last years, the building industry has developed new building materials that may contain signiﬁcant quantities of naturally or technologically enhanced levels of radioactivity. For that reason, it is important to evaluate the amount of natural radioactivity present in the building materials, which are used in the construction of dwellings(4, 6). The main purpose of this study is to evaluate the radium (226Ra) activity concentration in ornamental rocks and composites, commonly used as building materials in the Iberian Peninsula versus radon (222Rn) exhalation rates by using passive and active measuring techniques. For the assessment of the potential radiological hazards, both the alpha index and the emanation fraction are also determined. MATERIALS AND METHODS A total of 23 samples representing different types of building materials were analysed for radium activity concentration and radon exhalation rates. The sample set includes granites, limestone, slate, marbles, ceramic tiles, clay brick, cements and concrete mostly obtained at Extremadura (Spain), and Portugal. The samples were reduced to powder by grinding, afterwards were sieved (fraction grain size <1 mm), dried, homogenised and placed inside different containers according to the measurement techniques. Radium activity concentrations were determined by gamma-ray spectrometry and the radon © The Author 2017. Published by Oxford University Press. All rights reserved. For Permissions, please email: email@example.com E. ANDRADE ET AL. exhalation rates were measured using a continuous radon monitor and a solid state nuclear track detectors. Radon exhalation measurements using intact building materials were also performed. However, for the purpose of this work, those results are not presented, since comparison were made with 226Ra measurements obtained by gamma spectrometry with processed samples. Ci Vλ E= M [T + 1 λ (e−λt − 1)] (1) where, Ci (Bq m−3 h−1) is the integrated radon concentration obtained by the LR115 track detectors, V (m3) is the volume of the container, λ (h−1) is the radon decay constant, T (h) is the detector exposure time and M (kg) is the mass of the sample. Regarding the active technique, the radon exhalation rate was measured by using a continuous radon monitor RTM1688-2, from SARAD, in order to follow the radon activity growth as a function of time and determining the average equilibrium of the gas concentration during the exposure period. The radon emanated from the grains of the sample migrates through the pores and is ﬁnally exhaled from the surface. The equipment has an internal pump working at a ﬂow rate of 0.25 L min−1, and it is connected to the sealed container in a closed loop. The radon concentration inside the cylindrical sealed container was measured for a period of ~8 days at intervals of 2 h. The radon concentration growth as a function of time is given by the following equation: EXPERIMENTAL Gamma-ray spectrometry The 226Ra activity concentrations of the samples were analysed by gamma-ray spectrometry using HPGe detectors. The detectors are shielded from natural radioactive background by lead shields with cooper and tin lining. Genie 2000 software (version 3.0) was used for data acquisition and spectral analysis and GESPECOR software (version 4.2) was used to correct for self-attenuation and coincidence summing effects. The detection efﬁciency was determined using NIST-traceable multi-gamma radioactive standards with an energy range from 46.5 to 1836 keV and customised in a water-equivalent epoxy resin matrix (1.15 g cm3 density) reproducing exactly the geometry of the samples. The samples containers were sealed and stored for a period no lesser than one month in order to achieve the radioactive equilibrium between 222Rn and its short-lived progeny. The 226Ra activity was calculated through the 295.2, 351.9 and 609.3 keV photopeaks, by assuming secular equilibrium. The stability of the system (activity, FWHM, centroid) was checked once a week with a 152Eu certiﬁed point source. External QC was assured through the participation in several international intercomparison exercises (IAEA, EC, etc.). C (t ) = EM [1 − e−(λ + α ) t] + C 0 e−(λ + α ) t (λ + α ) V (2) being C0 (Bq m−3) the radon concentration at t = 0, and α (s−1) the leakage rate from the container. Radon emanation fraction The emanation fraction is deﬁned as the fraction of radon atoms generated that escape the solid phase in which they are formed and become free to migrate through the interstitial space between the grains(7) and was determined through the following equation(2, 5, 8): Radon exhalation rates F= The radon exhalation rates were determined by using two different techniques, the active and passive techniques(1). The passive technique is based on integrated measurements by using solid state nuclear track detectors (SSNTD) placed on the inner upper surface of cylindrical sealed containers. LR115 type II plastic track detectors from Kodak were used for this purpose. After 90 days of exposure, the detectors were chemically etched in 2.5 mol L−1 NaOH solution in a water bath at 60 ± 1°C for 2 h. The resulting alpha tracks were counted using Spark Counter model UFC-2 equipment. After obtaining the radon activity concentration, the radon speciﬁc exhalation rate, E (Bq kg−1 h−1), can be calculated through the following expression: E CRa λRn (3) where E (Bq kg−1 h−1) is the radon exhalation rate, CRa (Bq kg−1) is the radium activity concentration and λRn (h−1) is the decay constant of 222Rn. Alpha index Alpha index is an important index dealing with the assessment of the excess alpha radiation due to radon inhalation originating from building materials. This index is deﬁned as follows(2, 5, 9): Iα = 2 CRa 200 (4) ASSESSMENT OF RADIUM ACTIVITY CONCENTRATION RESULTS AND DISCUSSION since the detection limit for radon (active technique) is ~4 Bq m−3. As presented in Figure 1, the results show that in all samples there are a common pattern between 226 Ra activity concentration and radon exhalation rate, except to ceramic tiles and clay brick. As expected, granites present the highest 226Ra content, as well as higher radon exhalation rate relative to the other building materials and are in the same range of those reported for other countries(2, 9). Nonetheless, it can also be seen that ceramic tiles show high radium activity concentrations, but low or <DL radon exhalation rates. Generally, the main opacifying constituent of glazes, applied to ceramic tiles, is zircon and its presence should justify the high 226Ra activity concentration measured relative to the others composites materials(10). According to the results obtained for the different types of building materials, a positive relationship has been observed between radium activity concentration and radon exhalation rates calculated for passive and active techniques (Figure 2). However the correlation coefﬁcients (r) obtained are not statistically signiﬁcant (p > 0.05). On the other hand, the results obtained by both techniques (passive and active) are strongly correlated (r = 0.95). In order to highlight the results, the alpha index and emanation fraction was also calculated (Table 2). The experimental results of radium activity concentration and radon exhalation rates, for the different types of building materials considered in this study are presented in Table 1. As it can be seen, in the different types of rocks analysed the granites show the highest mean 226Ra values (123.4 ± 5.2 Bq kg−1), whereas marbles present the lowest mean 226Ra values (5.0 ± 0.7 Bq kg−1) measured. In addition, the different types of granites sampled show a wide range of 226Ra values (51.0–239.1 Bq kg−1), which may be related with different amounts of uraniumbearing minerals (e.g. feldspar, biotite and zircon). In turn, marbles show a more restricted range (4.2–5.9 Bq kg−1), and the limestone and the slate display 226Ra values of 40.7 ± 2.0 and 28.6 ± 2.2 Bq kg−1, respectively. Regarding the composites materials, the highest value was found in ceramic tiles (92.9 ± 4.0 Bq kg−1), while the lowest value was found in concrete (10.8 ± 0.9 Bq kg−1). The clay brick presents 226Ra values of 41.8 ± 2.0 Bq kg−1, and the cements show a wide range of 226Ra values (8.3–62.2 Bq kg−1) with a mean of 24.9 ± 1.4 Bq kg−1. The large variation of 226 Ra values in cements may be arisen due to the difference in the origin and composition of the samples. The radon exhalation rate in terms of mass varies from 0.9 ± 0.2 to 16.9 ± 1.8 mBq kg−1 h−1 for the passive technique, and between 10.6 ± 2.6 and 70.3 ± 9.8 mBq kg−1 h−1 for the active technique. In some samples, the radon exhalation rate is below the DL, Table 1. The mean and range (minimum and maximum) of the radium activity concentration and radon exhalation rates (passive and active techniques) measured in different building materials. Samples type No. of samples 226 Ra (Bq kg−1) Mean ± σ (Range) Rocks Granite 9 Limestone Slate Marble 1* 1* 3 Composites Ceramic tile 2 Clay brick Cement 1* 5 Concrete 1* 1 E (mBq kg−1 h−1) Mean ± σ (Range) Passive technique Active technique 123.4 ± 5.2 (51.0–239.1) 40.7 ± 2.0 28.6 ± 2.2 5.0 ± 0.7 (4.2–5.9) 16.9 ± 1.8 (6.0–44.5) 2.9 ± 0.1 8.4 ± 0.5 0.9 ± 0.2 (0.5–1.3) 70.3 ± 9.8 (20.6–221.3) 26.0 ± 9.0 21.5 ± 6.6 <DL1 92.9 ± 4.0 (84.5–101.3) 41.8 ± 2.0 24.9 ± 1.4 (8.3–62.2) 10.8 ± 0.9 1.2 ± 0.4 (0.7–1.8) 4.3 ± 1.3 4.5 ± 0.5 (2.6–8.5) 3.5 ± 1.0 <DL1 DL, detection limit; *Individual values (no. of samples=1), A ± U (k = 2). 3 <DL1 19.0 ± 1.9 (8.6–29.4) 10.6 ± 2.6 E. ANDRADE ET AL. Table 2. Radon emanation fraction (passive and active techniques) and alpha index in the different building materials. Samples type Granite Limestone Slate Marble Ceramic tile Clay brick Cement Concrete Figure 1. Variations of the radium activity concentration and the radon exhalation rates for the different types of building materials using passive and active techniques. Iα (Bq kg−1) F (%) Mean Mean 0.62 0.20 0.14 0.02 0.46 0.21 0.12 0.05 Passive Active 2.1 0.9 3.9 2.4 0.2 1.4 3.0 4.3 8.4 8.5 10.0 — — — 11.8 13.0 recoil range in solids is very small (typically <0.05 μm) and most of the recoiling atoms remain within the mineral grain. This means that if the radium is mainly distributed on the grains surface, radon could be more effectively released and become available to migrate. Therefore, the emanation fraction depends not only on the radium content found in the building materials, but also on the mineralogical aspects such as lattice structure, porosity, grain size and shape, and elemental composition(7). Moreover, UNSCEAR (2000) reported that the typical emanation coefﬁcients for rocks and soils range from 5 to 70%, and established 20% as a representative value. In the present study, all samples analysed are below the representative value. The alpha index was proposed based on the assumption that if 226Ra activity concentration exceeds 200 Bq kg−1, it is possible that the indoor radon concentration will exceed 200 Bq m−3(11). Hence, the recommended upper limit concentration of 226Ra is 200 Bq kg−1 for an Iα equal to 1. The results obtained for the alpha index show that the samples analysed are below than the recommended upper limit. These results ranged from 0.02 to 0.62, which means that the building materials collected do not present dangerous levels of indoor radon, except one type of granite rock where Iα is 1.20. Figure 2. Relationship between radium activity concentration and radon exhalation rates (E) by using passive and active techniques. The emanation fraction ranges from 0.2 to 4.3% for the passive technique, and from 8.4 to 13.0% for the active technique. It is interesting to note that the highest emanation fraction (4.3 and 13.0%, for passive and active techniques, respectively) correspond to the lowest 226Ra activity concentrations (10.8 Bq kg−1). This could probably be explained by the nonuniform distribution of 226Ra on the minerals of the different samples. In fact, alpha recoil plays an important role in the emanation phenomenon, since CONCLUSIONS • • 4 The granites presented the highest 226Ra activity concentration, as well as higher radon exhalation rates, when compared with the other studied building materials. The tiles can be considered good materials to be used for interior decorative purpose since these materials present very low radon exhalation rates. 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