Radiation Measurements 46 (2011) 538e545
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Natural radionuclides content and radiological hazard of commercial ornamental stones: An integrated radiometric and mineralogical-petrographic study Marta Marocchi a, *, Serena Righi b, Giuseppe Maria Bargossi c, Giorgio Gasparotto c a
Institut de Minéralogie et de Physique des Milieux Condensés, Université Pierre et Marie Curie, 4 Place Jussieu, 75005 Paris, France Centro Interdipartimentale di Ricerca per le Scienze Ambientali and Dipartimento di Fisica, Università di Bologna, via dell’Agricoltura 5, 48100 Ravenna, Italy c Dipartimento di Scienze della Terra e Geologico-Ambientali, Università di Bologna, P.za P.ta S. Donato, 1-40126 Bologna, Italy b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 May 2010 Received in revised form 7 February 2011 Accepted 22 March 2011
Twenty samples of natural building materials commonly employed as ornamental stones in the international market have been investigated for natural radioactivity. External (gamma), as defined and used by the European Commission, and internal (alpha) hazard indexes were calculated and radon specific exhalation rate and emanation fraction were measured. The radiological investigation was complemented by an integrated mineralogical-petrographic and rock characterization approach. The most common radioactive accessory minerals occurring in the investigated samples are apatite, zircon and allanite, with minor monazite, thorite, thorianite, REE and Zr-oxides. Significant correlations with total activity concentration have been observed for K2O, Th and Ce concentrations. The emanation fraction is also influenced by both total porosity and porosity distribution. Radon exhalation rate and emanation fraction are very variable ranging from 0.0011 to 0.64 Bq kg1 h1 and from 0.2 to 62%, respectively. Most of the materials have radiological hazard indexes that do not exceed the European Commission limit values when used as ornamental or paving or flooring stones. However, three volcanic (Tufo Giallo Riano, Tufo Grigio Riano and Peperino Viterbese) samples could cause significant exposure both from excess radon indoor concentration (>200 Bq m3) and from gamma radiation (>1 mSv y1) when used as structural materials. This study further indicates that limit values for hazard indexes based on natural activity concentration and Rn emanation should take into account the lithological properties and use of the materials. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Natural radioactivity Ornamental stones External exposure Radon Effective dose
1. Introduction The exposure of human beings to ionizing radiation from natural sources is continuous. The average worldwide annual effective dose to natural radiation sources results in 2.4 mSv y1 (UNSCEAR, 2000). Natural occurring radionuclides of terrestrial origin (primordial radionuclides) are present in various concentrations in environmental matter. In terms of dose, the principal primordial radionuclides are 40K, 232Th and 238U. As known, these thorium and uranium isotopes head series of several radionuclides, named ‘natural decay series’. External exposure from terrestrial origin is caused by direct gamma radiation (worldwide average: 0.48 mSv y1). Internal exposures arise from the intake of terrestrial radionuclides by inhalation (worldwide average: 1.26 mSv y1) and ingestion
* Corresponding author. Tel.: þ33 1 44276247; fax: þ33 1 44273785. E-mail address:
[email protected] (M. Marocchi). 1350-4487/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2011.03.017
(worldwide average: 0.29 mSv y1). The dominant components of inhalation exposure are the short-lived decay products of radon (UNSCEAR, 2000). All building materials derived from rocks contain various amounts of primordial radionuclides. Enhanced or elevated levels of natural radionuclides in building materials may cause additional doses due to external and internal exposure. The internal exposure is mainly caused by the inhalation of radon (222Rn) and their shortlived decay products. Typical excess indoor radon concentration due to building materials is about 10e20 Bq m3, but in some zones and in rare cases it may rise up to greater than 1000 Bq m3. Building materials are also the most important source of indoor thoron (220Rn) that can be a relevant source of exposure when the building materials contain high concentrations of thorium (EC, 1999). The European Commission published two recommendations regarding indoor exposure to members of the public from natural radiation. The first is the guideline on advisory levels for radon in residential dwellings (EC, 1990). The second one states the
M. Marocchi et al. / Radiation Measurements 46 (2011) 538e545
radiological protection principles for natural radioactivity in building materials (EC, 1999). In spite of recent interest on indoor exposure, these recommendations have not yet been fully adopted by most of the Member States of the European Union. In the meantime, recent studies on indoor radon and lung cancer in Europe (Darby et al., 2005, 2006), North America (Krewski et al., 2005, 2006) and Asia (Lubin et al., 2004) reported a substantial evidence of a risk increase even below 200 Bq m3. In view of the latest scientific data, WHO adjusted the reference level of indoor radon to minimize health hazards due to radon exposure (WHO, 2009). The present study reports data concerning the natural radioactivity in relation to the lithological features of 20 natural building materials that are of widespread use as ornamental stones. At first, activity concentrations of natural radionuclides and radon exhalation have been measured and, at the same time, the stones have been characterised by petrographic-geochemicalechemical investigation and total porosity determination. Subsequently, a mathematical model for calculating the annual effective dose due to the external gamma radiation and the excess of indoor radon concentration has been applied in order to assess the radiological risk of these building materials. 2. Materials and methods 2.1. Sampling and sample preparation A range of plutonic, volcanic (lavas and pyroclastites) and metamorphic rocks commonly employed in Italy and worldwide, as ornamental stones for paving and for indoor building have been selected. The rocks have been chosen on the basis of diffusion on the market: some are widespread both in Italy and in the international market, others are important for local use. Sedimentary rocks widely used as ornamental and building stones were not considered in this work because measurements performed on sandstones generally ruled out any radioactive hazard (Gascoyne, 1992). We have also excluded marbles, limestones and serpentinites as these lithotypes have normally negligible content of radioactive minerals (Eisenbud and Gesell, 1997). All the investigated materials were purchased from commercial companies and local suppliers in Italy. Four types of natural ornamental stones were imported: Kashmir White from India, Nero Assoluto Belfast from South Africa, Labrador Chiaro Larvik from Norway and Rosa Porriño from Galicia, Spain. The remaining types were collected from quarries located in regions of northern (Pietra di Luserna, Beola Valdossola, Serizzo Valdossola, Rosa Predazzo, Serizzo Ghiandone, Beola Dorata, Trachite Euganea, Porfido della Valcamonica, Porfido della Val di Cembra), central (Nenfro Tuscania, Rosa Sardo, Peperino Viterbese, Tufo di Riano giallo, Tufo di Riano grigio) and southern (Pietra Lavica Vesuvio, Basalt from Mt. Etna) Italy. Samples were collected in a representative way from polished slabs or blocks available on the market. The samples were prepared as: a) as thin sections for petrographic examinations and SEM/EDX analyses, b) as powder for chemical- and radiochemical-analyses and c) as specimen for porosimetry. For g-spectrometric measurements the samples have been powdered to an average particle size less than 1 mm. Then, the pulverized samples were dried at 105 C in order to eliminate any water content, transferred to 450-mL Marinelli beakers and weighed. Then, they were sealed and stored for at least 30 days to capture 222Rn gas (T1/2 ¼ 3.8 d) and ensure secular equilibrium between 226Ra (T1/2 ¼ 1.60 103 y) and measured daughters 214Pb and 214Bi (T1/2 ¼ 27 and 20 min, respectively). In order to determine the radon specific exhalation rate, samples were cut into small blocks with an average dimension of about 5 5 cm, and a weight of about 150 g.
539
2.2. X-ray fluorescence Bulk-rock compositions were determined by XRF (X-Ray Fluorescence) with a Philips PW 1480/10 Spectrometer. L.O.I. (Loss On Ignition) was determined after heating at 950 C. Major element compositions were corrected according to the procedure by Franzini et al. (1972), trace elements according to Leoni and Saitta (1976) and Leoni et al. (1986). 2.3. SEM-EDX microscopy EDX spectra of radioactive accessory minerals and back-scattered (BSE) electron images were acquired using a Scanning Electron Microscope (SEM) Philips 515B fitted with an EDAX DX4 microanalytical device. The operating conditions were: accelerating voltage of 15 kV, beam current of 2 nA, spot size of 1 mm. 2.4. Porosity Porosity measurements were carried out using a Thermoquest PASCAL 140 mercury porosimeter (operating range 0e4000 Pa, for macroporosity measurement) and PASCAL 240 (operating range 1e200 Pa, for microporosity measurement). This technique allowed the determination of total porosity (%) and distribution of pores in micro (10 mm) porosity classes. 2.5. Natural radioactivity Measurements of radioactivity concentration in building materials were carried out using g-spectrometry by means of an n-type coaxial HPGe detector with an active volume of about 110 cm3 connected to a multichannel analyzer. The energy resolution (fullwidth at half-maximum) of the system was 1.9 keV at 1.33 MeV (60Co) with 22.6% efficiency. The considered energy range was 200e1500 keV. The detection system was calibrated for energy and efficiency using a multi-nuclide source supplied by the CEA (Commissariat à l’Energie Atomique en France) containing 57Co, 60 Co, 85Sr, 88Y, 109Cd, 113Sn, 139Ce, 137Cs, and 241Am. More details are given by Bruzzi et al. (2000), Righi et al. (2000) and Righi and Bruzzi (2006). The short-lived daughters 214Pb (351.9 keV), 214Bi (609.3 keV), 228Ac (911.2 keV) and 212Pb (238.6 keV) were used for proxy determinations of the activity concentrations of their respective parents 226Ra and 232Th. The 1460 keV gamma-ray peak was used to determine 40K concentration. Self-attenuation corrections were calculated as suggested by Debertin and Ren (1989). Coincidence summing corrections were not applied. In order to take into account this systematic error the overall uncertainty of 226Ra and 232Th activity concentrations was increased according to coincidence summing correction factors derived by García-Talavera et al. (2001). Any increase to the total uncertainty was applied to the 1460.8 keV gamma-ray peak because it is the only gamma emission from 40K and therefore not prone to coincidence summing. Radium-equivalent activity is used to obtain the sum of 226Ra 232 Th and 40K activity concentrations and is calculated as:
Ra eq ¼ ARa þ 1:43ATh þ 0:077AK where ARa, ATh and AK are the activities of 226Ra, 232Th and 40K in Bq kg1, respectively (Stranden, 1976; Beretka and Mathew, 1985). 2.6. Radon exhalation Procedure used to determine radon specific exhalation rate involved the E-PERM electret ion chambers and consisted of determining the 222Rn activity accumulated in a vessel after a given
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Table 1 Rock classification, petrographic description of ornamental stones and porosity values. Mineral abbreviations after Kretz (1983), Siivola and Schmidt (2007). Code
Rock name
Rock classification
Porosity Total%
LC NA RPo RS RPr SG PLV BE TE TGiR TGrR NE PV PVCa
Labrador chiaro Nero assoluto Rosa Porrino Rosa sardo Rosa Predazzo Serizzo Ghiandone Val Masino Pietra Lavica Vesuvio Basalto Etneo Trachite Euganea Tufo Giallo Riano Tufo Grigio Riano Nenfro Peperino Viterbese Porfido Val Camonica
Nepheline Syenite Gabbro Norite Granite Granite Granite Tonalite Phonotephrite Hawaiite Trachyte Tephriphonolite Trachyandesite (Latite) Trachyte Trachyandesite (Latite) Trachydacite
PVCe BD BV SeV PL KW
Porfido Val Cembra Beola Dorata Beola Valdossola Serizzo Valdossola Pietra Luserna Kashmir white
Rhyolite Micaschist Gneiss Orthogneiss Gneiss Orthogneiss
Micro-
Meso-
Macro-
0.2 0.7 1.9 0.6 0.2 1.0
35 85 90 77 97 73
35 8 6 14 3 18
30 7 4 9 0 9
Zrn, Ap, REE oxides Ilm, Mag, Ap Ilm, Mag, Zrn, Ap, Aln, thorite Ilm, Mag, Zrn, Ap, thorianite Ilm, Mag, Zrn, Rt, Aln, Tur Ep, Aln, Zrn, Ttn
4.0 0.2 9.7 14.6 11.3 30 2.1 0.9
82 40 94
8 29 5
10 31 1
3 52 82 95
78 47 8 4
19 1 10 1
0.4 0.8 1.7 1.7 1.3 0.4
65 80 88 84 75 74
25 8 5 5 6 12
10 12 7 11 19 14
Ap, Aln Zrn, Ap Zrn, Ap, Aln Zeo, Op, Ap, Aln Zeo, Op, Ap, Aln, Zrn Aln Ilm, Zrn, Ap, Rt Ilm, Zrn, Ap, Rt, REE and Zroxides Ilm, Mag, Ttn, Zrn, Ap, Rt Zrn, Aln Zrn, Ap, Aln, Mnz Ep, Tur, Zrn, Ap Ttn, Ap, Zrn, Tur, Fl, Cal Zrn
build-up time (Collé et al., 1981; Kotrappa and Stieff, 1994, 2008, 2009). The ‘‘accumulator method’’ is a common technique for radon exhalation rate measurements in building materials (Petropoulos et al., 2001). The growth of 222Rn activity concentration CRn (Bq m3) as a function of time t within a closed accumulation vessel may be given in approximate form as:
CRn ¼
E 1 elRn t m V lRn
Accessories Distribution range (%)
0 lRn t þ CRn e
where: E is the radon specific exhalation rate (Bq kg1 h1) from the sample, lRn is the decay constant for 222Rn (h1), V is the volume of the accumulation vessel (m3), m is the mass of the 0 (Bq m3) is the 222Rn activity concentration sample (kg) and CRn in the accumulation vessel at the start of an accumulation time (t ¼ 0).
The accumulation measurements were performed with screwcapped and gasketed glass jars that are a component part of the EPERM radon-in-water measurement test kit (Kotrappa and Jester, 1993). To determine radon concentration, an integrated measurement with a 210 mL E-PERM chamber configured with a short-term electret was applied. The exposure time varied strongly in base on the exhalation rate of stone: from a minimum of 5 days (e.g. for tuffs) to a maximum of 20 days (e.g. for Serizzo Ghiandone and Beola Dorata). Too much long exposure times can run the shortterm electret down; on the contrary too much short exposure times can do the measurement undistinguishable to the blank reading. Adequate exposure time for each sample type was determined through preliminary tests. The detection limit (3s of the background) was estimated to be 0.0015 Bq h1 for an exposure time of 240 h. Since the 222Rn concentration in the accumulator at the start of the experiment cannot be negligible, the blank reading was subtracted from the total 222Rn concentration in order to find the
Fig. 1. Classification of volcanic (left panel) and plutonic (right panel) rocks employed as ornamental stones, according to SiO2 (wt%) vs Na2O þ K2O (wt%) (TAS, Le Bas et al., 1986) and De La Roche et al. (1980) classification diagrams, respectively. Samples abbreviations as in Table 1.
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541
Fig. 2. SEM images and related EDX spectra of radioactive accessory minerals: A) Zoned zircon, sample SG; B) Zoned zircon with tiny thorite inclusion, sample SG; C) Monazite, sample BV; D) Zoned allanite with zircons (light white), sample RPo. Samples abbreviations as in Table 1.
net 222Rn concentration. The blank reading was determined by introducing E-PERM chambers into empty jars equal to those used for the experiments. These E-PERM chambers were exposed for the same time that was used for experimental ones. The reader is referred to Righi and Bruzzi (2006) for further details on the analytical technique. The emanation fraction f, i.e. the fraction of radon that reaches the external atmosphere by means of the diffusion process, was determined through the following equation:
f ¼
E CRa lRn
where E is the specific radon exhalation rate, CRa is the 226Ra activity concentration (Bq kg1) and lRn is the decay constant of 222 Rn (h1). 2.7. External gamma radiation and indoor radon The method developed by Markkanen (1995) has been used to assess the excess in indoor radon concentration and the indoor dose rate due to external exposure. The assessment is done for a rectangular room (12 m 7 m 2.8 m) of uniform density and activity concentration. In this study, thickness value was assumed to be 20 cm and 4 cm for structural materials and cladding/
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Table 2 Natural activity concentration, radon exhalation rate and emanation fraction of investigated ornamental stones. Average activity concentrations observed in worldwide soils (UNSCEAR, 2000) are reported for comparison. Sample abbreviations as in Table 1. Th Bq kg1
Code
232
LC NA RPo RS RPr SG PLV BE TE TGiR TGrR NE PV PVCa PVCe BD BV SeV PL KW Worldwide soil mean
45 5 20 2 80 8 47 5 210 20 39 4 53 6 28 3 43 4 280 30 290 30 490 50 140 15 53 6 61 6 58 6 24 3 40 17 100 10 33 4 30
226
Ra Bq kg1
41 6 12 2 62 9 35 5 160 20 30 4 390 60 43 6 40 6 195 40 137 20 360 50 100 20 41 6 44 6 18 3 14 9 48 7 115 17 352 50 35
flooring materials, respectively. In the case that the building material is used for cladding/flooring, only walls and floor have been considered as radiation sources; ceiling has not been included. In the case that the material is used as structural material, the whole room (walls, floor and ceiling) has been considered as radiation source. The excess in indoor radon concentration is calculated on the basis of volume of the room, ventilation air, 222Rn decay constant, radon emanation fraction, 226Ra activity concentration of the material and total mass of the material. In this study, ventilation rate was assumed to be 0.5 h1. The dose rate due to gamma radiation (mGy h1) is calculated by summing the separately calculated dose rates caused by walls, floor and ceiling. Conversion factor of 0.7 Sv Gy1 and occupation factor of 7.000 h y1 was used to calculate the annual effective dose (mSv). 3. Results and discussion 3.1. Rock characterization The petrographic characters and mineralogical compositions determined by optical microscopy, SEM and the porosity values for investigated ornamental stones are reported in Table 1. Complete details of major and trace element compositions determined by XRF are provided in Appendix Table 4. Volcanic and plutonic rocks were classified according to Le Bas et al. (1986) (Total AlkalieSilica diagram, Fig. 1) and De La Roche et al., 1980 (R1-R2 diagram, Fig. 1). Metamorphic rocks were classified according to textural and mineralogical content. The high level of radioactivity, especially in igneous rocks, is generally attributed to the presence of radioactive minerals. The SEM-EDX analyses outlined that the most common radioactive accessory minerals occurring in the investigated samples are apatite, zircon and allanite, with minor monazite, thorite, thorianite, REE and Zr-oxides (Fig. 2). Apatite, Ca5[(PO4)3(F, OH, Cl)], is typically found as accessory mineral in igneous rocks. Due to the similarity in ionic size between U4þ (0.97 Å) and Ca2þ (0.99 Å), the tetravalent uranium can readily enter the apatite structure (Altschuler et al., 1958). Zircon, ZrSiO4, generally forms when molten rocks cool; during crystal growth, atoms of thorium and uranium became trapped in the crystal lattice (NRPB, 1993) thanks to similar dimensions of atomic radii (U4þ
40
K Bq kg1
Exhalation rate Bq kg1 h1
f (%)
850 25 240 7 1160 33 1030 28 1230 35 720 20 1520 50 350 10 1110 31 1890 50 1770 50 2000 70 1210 36 970 28 1090 31 740 21 490 20 740 30 1050 40 1100 32 400
0.0011 0.0001 0.007 0.001 0.07 0.01 0.011 0.001 0.11 0.01 0.005 0.001 0.031 0.003 0.0021 0.0004 0.030 0.003 0.59 0.04 0.64 0.05 0.45 0.01 0.039 0.004 0.007 0.001 0.036 0.002 0.002 0.001 0.002 0.001 0.0007 0.0002 0.018 0.004 0.13 0.02
0.4 7.3 15 4.3 10 2.0 1.0 0.6 9.8 43 62 17 3.8 2.3 11 1.6 2.1 0.2 2.0 4.9
0.97 Å, Th4þ 0.98 Å and Zr4þ 0.86 Å). Zircon usually contains uranium and thorium concentration ranging from 0.01% to 0.19%, 1% to 2%, respectively (Cuney and Friedrich, 1987). Similarly, uranium and thorium may be incorporated in the structure of allanite, (REE, *Ca)2 (Fe,Al)3 Si3 O12 (OH), because of their similarity to the rare earths in ionic radius. In the granite samples (Rosa Porriño and Rosa Sardo) thorite (ThSiO4) and thorianite (ThO2) have also been observed. Moreover, in Beola Valdossola and in Porfido Val Camonica monazite ((Ce, Y, La, Th) PO4) and baddeleyte ZrO2 occur. These radioactive minerals are always present with low to very low concentrations (always less than 1%) in the investigated rocks. 3.2. Natural activity concentration, radon exhalation and effective dose assessment Table 2 reports activity concentrations of 232Th, 226Ra and 40K, radon specific exhalation rate and emanation fraction determined in the samples. The average activity concentrations observed in worldwide soils (UNSCEAR, 2000) are also reported for comparison. Among all the activity concentrations values reported in the table, the lowest ones were measured in Nero Assoluto and Beola Dorata, whereas the highest values are those of Nenfro Tuscania, Tufo Giallo Riano, Tufo Grigio Riano and Rosa Predazzo. The wide range of concentrations (20e490, 12e390 and 240e2000 Bq kg1 for 232Th, 226 Ra and 40K, respectively) illustrates the great variations encountered (Table 2). However, the results generally indicate natural activity concentrations higher than the mean worldwide soil concentrations. As a matter of fact, it is well known that igneous rocks generally show activity concentrations higher than sedimentary rocks (UNSCEAR, 2000). Whenever the comparison is possible, our values well agree with previous studies (Sciocchetti et al., 1983; Carrera et al., 1997; Tuccimei et al., 2006). Interesting differences were found between acid and basic igneous rocks: the first ones show the highest activities according to other authors (Bellanca, 1969; Rizzo et al., 2001). Moreover, data show that 232Th, 226 Ra and 40K mean activity concentrations of volcanic rocks (160, 150 and 1300 Bq kg1, respectively) are higher than plutonic (74, 64 and 870 Bq kg1, respectively) and metamorphic ones (51, 120 and 820 Bq kg1, respectively). The fifth column of Table 2 shows that specific exhalation rates range over two orders of magnitude, i.e. from 0.0011 Bq kg1 h1 to 0.64 Bq kg1 h1. These data are in good agreement with previous
M. Marocchi et al. / Radiation Measurements 46 (2011) 538e545
543
Fig. 3. Correlation diagrams of radium-equivalent activity (Ra-eq) vs selected major (g/100 g) and trace (g/106 g) analyses by XRF. The correlation coefficients are reported on single diagrams. Symbols: open circle, igneous plutonic rocks; cross, volcanic rocks (lavas); open triangle, volcanic pyroclastic rocks; X, metamorphic rocks.
works (Savidou et al., 1995; Carrera et al., 1997; Hafez et al., 2001; Al-Jarallah, 2001). The highest values were found in Nenfro Tuscania (0.45 Bq kg1 h1), Tufo Giallo Riano (0.59 Bq kg1 h1) and Tufo Grigio Riano (0.64 Bq kg1 h1). Since literature data show maximum values ranging between 0.2 and 0.4 Bq kg1 h1 (cf. Petropoulos et al., 2001; Stoulos et al., 2003), exhalation rates in these three stones result markedly high. Also the emanation fraction varies widely (from 0.2 to 62%) and even this parameter shows the highest values in Nenfro Tuscania (17%), Tufo Giallo Riano (43%) and Tufo Grigio Riano (62%). However, about 50% of the analyzed
samples show values below the typical emanation fractions for rocks and soils that range from 5 to 70% (Nazaroff et al., 1988). Fig. 3 illustrates the correlation of radium-equivalent activity vs selected major and trace elements. The Silica content is a parameter that is employed to define the degree of magmatic differentiation (cf. Bellia et al., 1996) and was also proposed as a selective criterion for materials of low radiological impact in geological areas of prevalent magmatic origin (Rizzo et al., 2001). Nevertheless according to our results no significant correlation between Ra-eq and SiO2 was found. More precisely, there is a good linear positive
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Table 3 Excess of indoor radon concentration and effective dose due to indoor gamma exposure calculated for the investigated stones. Sample abbreviations as in Table 1. Code
LC NA RPo RS RPr SG PLV BE TE TGiR TGrR NE PV PVCa PVCe BD BV SeV PL KW
Excess of 222Rn concentration (Bq/m3)
Effective dose (mSv/y)
Use of material
Use of material
Ornamental
Structural
Ornamental
Structural
0.4 3 24 4.4 20 0.8 4.8 0.7 11 45 50 51 3.9 1.2 2.8 0.4 0.4 0.1 3.4 24
* * * 32 * * * * * 325 360 * 28 * * * * * * *
0.33 0.14 0.51 0.35 0.61 0.09 0.72 0.21 0.37 0.45 0.4 0.72 0.44 0.22 0.13 0.18 0.1 0.19 0.38 0.59
* * * 0.76 * * * * * 3 2.7 * 1.6 * * * * * * *
* Not applicable since materials used only for cladding/flooring use.
correlation in plutonic and metamorphic rocks, but no correlation was found for volcanic ones. The total alkali (Na2O þ K2O) values show a quite poor positive correlation with radium-equivalent (R2 ¼ 0.41). This result agrees with the work by Rizzo et al. (2001), which suggested it is a reliable natural radioactivity content indicator in all building materials. On the other hand, a much significant correlation with K2O (R2 ¼ 0.75) has been observed. Plots of selected trace elements show a good correlation for Th and Ce, whereas correlation for Zr is considered not significant (Fig. 3). All these trace elements are common constituents of radioactive minerals, such as allanite and monazite. It is well known that radon exhalation is influenced by many factors such as 226Ra content of the material, its porosity, density, grain size and shape (UNSCEAR, 2000). The results suggest that the correlation between specific exhalation rate and 226Ra activity concentration is insignificant in the investigated samples (R2 ¼ 0.21) confirming the large amount of factors affecting the radon emanation. Not significant correlations were observed also between emanation fraction and density (R2 ¼ 0.15) and between emanation fraction and porosity (R2 ¼ 0.29). Despite the fact that all the tuff samples exhibit lower (i.e. about 11e14%) porosity than Nenfro Tuscania (30%), on the other hand the emanation fraction of tuffs is much higher than that of the Nenfro Tuscania. This could be likely ascribed to porosity distribution. Tuff sample porosity is mainly distributed into meso- and macro-porosity classes, while the porosity of Nenfro Tuscania shows a distribution ranging from 0.01 to 3.5 mm. Indeed, samples showing highest emanation have variable levels of meso- and macroporosity; on the contrary, all remaining samples exhibit microporosity exclusively, or almost exclusively (cf. Tables 1 and 2). Thus, the results so far outline that not only total porosity, but also the porosity distribution are parameters that greatly influence the rock emanation fraction. Table 3 shows excess of indoor radon concentration and effective dose due to indoor gamma exposure calculated for the investigated stones. Both ornamental (cladding/flooring) and structural use for each stone has been assessed. The European Commission recommend an action level for existing buildings and the planning level for future constructions of 400 Bq m3 and 200 Bq m3,
respectively (EC 1990). More conservatively, the World Health Organization (WHO) has recently set a higher reference level of 100 Bq m3 in order to limit the risk to individuals (WHO, 2009). As it can be appreciated in Table 3, these values are always respected for use as ornamental materials but could be exceeded for use of tuffs as structural materials in new buildings. The European Commission has recommended that a maximum dose of 1 mSv y1 due to external gamma radiation from building materials should be set for the public (EC, 1999). We observe that doses from gamma radiation resulting from cladding and flooring use are well below the recommended level (Table 3). Besides, the assumptions made are rather conservative, since walls are considered completely lined with stone tiles. On the other hand, three (Tufo Giallo Riano, Tufo Grigio Riano and Peperino Viterbese) out of four stones used as structural materials cause doses higher than the recommended level. It is to stress that these stones were employed as structural materials in their Regions of origin (central-southern Italy) where the outdoor average effective dose due to gamma radiation is 1.5e2 mSv per year (Cardinale et al., 1972). Such values are due to bedrock and soils present in these areas, for example in Lazio and Campania Regions where the bedrock mainly consists of volcanic rocks. Out of their regions of origin these materials are employed almost exclusively as both internal and external decorative elements such as claddings, slabs, columns, capitals, borders, step blocks, flowerbeds, etc. 4. Conclusions The following conclusions can be drawn from the present study. Results highlight a wide range of values for all the measured parameters. Samples contain a large variety of radioactive minerals, the range of concentrations of major and trace elements is very wide, the total porosity varies from 0.2 to 30%, the activity concentration of 232Th, 226Ra and 40K ranges from 20 to 490, from 12 to 390 and from 240 to 2000 Bq kg1, respectively. Also radon exhalation rate and emanation fraction are very variable ranging from 0.0011 to 0.64 Bq kg1 h1 and from 0.2 to 62%, respectively. The most common radioactive accessory minerals occurring in the investigated samples are apatite, zircon and allanite, with minor monazite, thorite, thorianite, REE and Zr-oxides. The mineralogical composition does not permit to highlight correlation with the radioactive content. Rock samples show average activity concentration values higher than average worldwide ones. This evidence confirms that igneous rocks, and pyroclastites in particular, have higher levels of natural radionuclides than sedimentary rocks. The lowest values were measured in Nero Assoluto (gabbro norite) and Beola Dorata (micaschist) whereas the highest values occurred in Nenfro Tuscania (trachyte), Tufo Giallo Riano (tephriphonolite) and Tufo Grigio Riano (trachyandesite). No significant correlation was noted between Ra-eq and SiO2 and Zr levels. On the contrary, total alkali (Na2O þ K2O) and much more K2O show better correlation with Ra-eq. Significant correlation with total activity concentration are also observable for Th and Ce concentrations. If these results will be confirmed on a larger number of samples, K2O, Th and Ce, easily measurable by XRF, could be used as potentially good radioactivity concentration indexes in stone materials. The comparison of specific radon exhalation rate to 226Ra activity concentration shows a not significant correlation, confirming that radon exhalation is affected by variations of several parameters. This result emphasizes the crucial importance of building materials characterization not only in terms
M. Marocchi et al. / Radiation Measurements 46 (2011) 538e545
of their 226Ra activity concentrations but also of their emanation properties. The emanation fraction also appears to be affected by several factors. Particularly interesting is the relationship between the emanation factor and porosity distribution of the rock, i.e. radon emanation increases with increasing meso- and macroporosity. Concerning the radiological hazard aspects, we conclude that the employment of stone materials analyzed in this study normally do not involve a health hazard. When these stones are used as ornamental- or flooring- or paving materials no exceeding of recommended limits is predictable. Conversely, three volcanic samples could cause significant exposure both from excess radon indoor concentration (>200 Bq m3) and from gamma radiation (>1 mSv y1) when used as structural materials. Acknowledgements The authors wish to thank Dr. F. Peddis (Laboratory of Geotechnologies and Georesources DICMA, University of Bologna) for assistance during porosimetry measurements, M. Carnevali and C. Orlandi (Health Physics Service of Ravenna Hospital) for their kind technical assistance with gamma-spectrometric measurements, F. Coatti and N. Cardinale for data processing and collaboration. Prof. V. Simoncini is thanked for her valuable comments on a first version of the manuscript. Constructive comments by two anonymous reviewers improved the manuscript. Appendix. Supplementary data Supplementary data related to this article can be found online at doi:10.1016/j.radmeas.2011.03.017. References Al-Jarallah, M., 2001. Radon exhalation from granite used in Saudi Arabia. J. Environ. Radioactiv 53, 91e98. Altschuler, Z.S., Clarke, R.S., Young, E.Y., 1958. Geochemistry Uranium in Apatite and Phosphorite. In: US Geological Survey Professional Paper, 314 D, p. 87. Bellanca, A., 1969. Marmi di Sicilia. IRFIS, Italy. Bellia, S., Brai, M., Hauser, S., Puccio, P., Rizzo, S., 1996. Environmental radioactivity and volcanological features of three islands of the Mediterranean Sea (Pantelleria, Ustica and Vulcano). Chem. Ecol. 12, 297e302. Beretka, J., Mathew, P.J., 1985. Natural radioactivity of Australian building materials, industrial washes and by-products. Health Phys. 48, 87e95. Bruzzi, L., Baroni, M., Mazzotti, G., Mele, R., Righi, S., 2000. Radioactivity in raw materials and end products in Italian ceramic industry. J. Environ. Radioactiv 47, 171e181. Cardinale, A., Cortellessa, G., Gera, F., et al., 1972. Distribution in the Italian population of the absorbed dose due to the natural background radiation. In: Adams, J.A.S., Lowder, W.M., Gesell, T.F. (Eds.), The Natural Radiation Environment II. CONF-720805-P1, pp. 421e440. Carrera, G., Garavaglia, M., Magnoni, S., Valli, G., Vecchi, R., 1997. Natural radioactivity and radon exhalation in stony materials. J. Environ. Radioactiv 34, 149e159. Collé, R., Rubin, R.J., Hutchinson, J.M., 1981. Radon Transport through and Exhalation from Building Materials: A Review and Assessment. In: U.S. National Bureau of Standards, Report NBS TN 1139. Cuney, M., Friedrich, M., 1987. Physicochemical and crystal chemical controls on accessory minerals paragenesis in granitoids: implications on uranium metallogenesis. Bull. Mineral. 110, 235e247. Darby, S., et al., 2005. Radon in homes and risk of lung cancer: collaborative analysis of individual data from 13 European case-control studies. BMJ 330 (7485), 223e227. Darby, S., et al., 2006. Residential radon and lung cancer: detailed results of a collaborative analysis of individual data on 7148 subjects with lung cancer and 14208 subjects without lung cancer from 13 epidemiologic studies in Europe. Scand. J. Work Environ. Health 32 (Suppl. 1), 1e83. De La Roche, H., Leterrier, J., Grandclaude, P., Marchal, M., 1980. A classification of volcanic and plutonic rocks using R1 R2-diagram and major element analyses e its relationships with current nomenclature. Chem. Geol. 29, 183e210. Debertin, K., Ren, J., 1989. Measurements of the activity of radioactive samples in Marinelli beakers. Nucl. Instrum. Methods Phys. Res. Sect. A 278, 541e549.
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