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Natural gamma-ray spectrometry as a tool for radiation dose and radon hazard modelling
ARTICLE IN PRESS Applied Radiation and Isotopes 67 (2009) 964–968
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Natural gamma-ray spectrometry as a tool for radiation dose and radon hazard modelling M. Verdoya a, P. Chiozzi a, P. De Felice b, V. Pasquale a,, M. Bochiolo a, I. Genovesi a a b
` di Genova, Viale Benedetto XV 5, I-16132 Genoa, Italy Dipart. Studio Territorio Risorse, Settore di Geofisica, Universita Instituto Nazionale Metrologia Radiazioni Ionizzanti ENEA, Centro Ricerche Casaccia, Via Anguillarese, 301 S.M. Galeria, I-00100 Rome, Italy
a r t i c l e in f o
a b s t r a c t
Keywords: Spectrometry Natural radioactivity Absorbed dose rate Radon potential
We reviewed the calibration procedures of gamma-ray spectrometry with particular emphasis to factors that affect accuracy, detection limits and background radiation in field measurements for dosimetric and radon potential mapping. Gamma-ray spectra were acquired in western Liguria (Italy). The energy windows investigated are centred on the photopeaks of 214Bi (1.76 MeV), 208Tl (2.62 MeV) and 40K (1.46 MeV). The inferred absorbed dose rate and the radon flux are estimated to be lower than 60 nGy h1 and 22 Bq m2 h1, respectively. & 2009 Elsevier Ltd. All rights reserved.
1. Introduction Radiometric surveys cover many different scientific and practical interests, ranging from basic geophysics to mineral exploration and environmental radiation monitoring. The environmental aspects of natural radioactivity, above all regarding radon effects and exposure to ionising radiation, is a topic of growing interest during the last years (see e.g. A˚kerblom and Linde´n, 1996; Bellotti et al., 2007 and references therein). Natural radioactivity originated from uranium, thorium and potassium, which are hold in variable quantities above all in the rocks forming the Earth’s crust, is the major cause of natural, potentially hazardous, gamma-ray exposure. Albeit aerial gamma-ray spectrometry can provide an overview and mapping of radioelement concentrations over wide regions (e.g. Aydin et al., 2006), only ground, high-resolution measurements yield detailed mapping over relatively small rock outcrops, and allow cheap and fast monitoring of environmental radioactivity even in a rough terrain. In this paper, we present results of natural radioactivity from a ground gamma-ray survey carried out in the Alpine geological units of western Liguria (Italy) (Fig. 1). Gamma-ray spectrometry results were used for assessment of absorbed dose rates. Moreover, uranium concentration was considered as a radon potential indicator in the different geological environments and was used for preparing radon prognosis maps. Particular care was paid to the calibration of the gamma-ray apparatus, with reference to the assessment of the background radiation. Since the surveyed area has a rough
Corresponding author. Tel.: +39 010 3538090; fax: +39 010 352169.
E-mail address: [email protected] (V. Pasquale). 0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.01.066
topography, we evaluated the variation of background radiation with elevation. The interest for the study area is above all based on the presence of several lithotypes, spanning from sedimentary, to metasedimentary and metavolcanic rocks, the latter particularly known in literature for their abundance of radionuclides (see Verdoya et al., 2001; Bochiolo, 2007; Genovesi, 2007 and references therein). The stratigraphy consists of: (1) basement rocks (Late Carboniferous-Permian), embodying polimetamorphic terms (orthogneisses and granitoids), metasediments (phyllites, micaschists and quartzschists) and metavolcanics (metaandesites, metarhyolites and porphyric schists); (2) cover rocks (TriasMiocene), formed by orthoquartzites, arkosic sandstones, and conglomerates, and by a calcareous-dolomitic succession, sometimes alternate to arenaceous and marly terms.
2. Measurement technique 2.1. Instrumentation and calibration The type of spectrometer, detector volume, measurement times, and mode of measurement depend on the radiation environment and the type, size and distribution of radioactive sources. Portable, hand-held gamma-ray spectrometers are widely used in field studies for both regional and detailed mapping surveys aimed to estimate the surface concentrations of the radionuclides. Our portable apparatus for gamma-ray activity measurements (Satisgeo Brno, Czech Republic) consists of a thallium-activated sodium iodide scintillation detector of about 440 cm3 together with a photomultiplier tube, a high-voltage supply and a signal preamplifier. It is thermally insulated and
ARTICLE IN PRESS M. Verdoya et al. / Applied Radiation and Isotopes 67 (2009) 964–968
965
44°16' GV
BP
Genoa Varazze Savona
Pietra Ligure a Se an uri Imperia Lig
44°14' Spotorno 44°12' N
nGy h-1 150-200
44°10' 8°18'
ian
r Ligu Finale Ligure
8°20'
8°22'
Sea
100-150 50-100 <50
8°24'E
8°26'
Fig. 1. Distribution map of the absorbed dose rate and site location (full dot) in western Liguria. BP—Brianc- onnaise and Piedmont zone and GV—Voltri Massif Alpine geological units. Inset: full square shows the area where the detector was calibrated for the local background.
housed in an aluminium cylinder. The probe is connected to a 256-channel analyser. The instrument can record the full gammaray spectrum as well as sum channels over broad energy windows for the in situ estimation of K, U and Th concentrations. The determination of U, Th and K is based on the method of the three energy windows (Chiozzi et al., 1998, 2000a). Each window is centred on the characteristic photopeak of the gamma-ray spectrum from the decay of 214Bi (1.76 MeV) in the 238U decay series and from 208Tl (2.62 MeV) in the 232Th series. The primary decay of 40K (1.46 MeV) is measured directly. The investigated energy spectrum ranges from 0.12 to 3.00 MeV. A gamma-ray emitting reference source of 137Cs (approximate activity of 15 kBq), with a low-energy peak of 0.66 MeV is accommodated within the probe for automatic instrument gain adjustment and spectrum stabilisation. Gamma-ray spectrometer calibration requires the estimation of those constants that relate instrument count rates to either radionuclide concentration or environmental dose rate, namely the sensitivity constants and the background radiation. Portable instruments are usually calibrated by means of standard spectra acquired using three concrete pads enriched in K, U and Th and highly unbalanced K/U, Th/U and K/Th ratios (e.g. Multala, 1981). Ideally, calibration pads should simulate a geological source of radiation and should be kept dry, as variation in moisture content can lead to a change in radiation output. Pads are usually of cylindrical shape, and recommended diameter and thickness are 2–3 and 0.5 m, respectively (IAEA, 1989). A fourth, low-radioactivity calibration pad of lead is customarily used to measure the background. The gamma-ray count rate for each spectrum within each energy window depends primarily on (i) the concentration and interference of the radioactive elements; (ii) Compton scattering and the linear attenuation coefficient appropriate to the rock material and the energies of the transmitted gamma rays; (iii) detector sensitivity and background count rate. Practically, the calibration procedure consists of measuring the net count rate at each window at each pad, which has finite dimensions and may differ from the recommended size. Grasty et al. (1991) showed that smaller transportable pads are also suitable for calibrating portable gamma-ray spectrometers, provided that a geometrical correction factor is applied to the instrument sensitivity derived from these calibration experiments. Furthermore, the detector is set at a few centimetres from the surface. In such conditions the count rates are lower than those expected for a 2p infinite geometry. Therefore, the recorded spectra must be corrected for a geometric factor G ¼ 12h/d, where h is the height of the detector to the pad surface and d is the diameter of the
standard pad. The determination of the sensitivity of our spectrometer (with h ¼ 63 mm) is based on standard pads of 2 m in diameter, located at the Czech Uranium Industry (Prague) (see Chiozzi et al., 2000a for details). Under these conditions, we may claim that the recorded gamma rays are emitted from a rock volume of about 1 m3. The sampling time required for a measurement depends on the radioactivity of the outcrop and the precision required. Since the K, U, and Th spectra overlap, the counting time required for a specified precision in the estimates of these radionuclides is more complicated. Lovborg and Mose (1987) derived equations giving the counting time for assays of each radionuclide with a 10% error at various K, U, and Th ratios. In practise, for NaI(Tl) detectors, IAEA (2003) recommends a sampling time of 120 s for highly radioactive rocks and 360 s for low radioactivity outcrops. Due to the large variability of radionuclide concentrations of our rocks, we adopted a sampling time of 300 s. This implies that the detection limit is estimated to be 0.2 and 0.3 mg kg1 for U and Th, respectively, and 0.03% for K (Chiozzi et al., 2000b). Several factors may affect accuracy in the determinations of these elements. Sources of bias could be inherent to the instrument itself (e.g. drift of energy calibration, calibration of the reference materials, K contamination of the photomultiplier tube) and of geological nature (self-absorption due to the density variation in the analysed rock, outcrop alteration and geometry). Based on counting statistics, in common rocks the relative combined standard uncertainty of the measured concentrations is 3% for K, 5% for Th and 8% for U.
2.2. Background radiation The net count rate and, consequently, the concentration results are dependent on the background, i.e. the gamma radiation that does not originate from the rock. Background radiation is due to the internal radioactivity of the instrument, the cosmic radiation—which changes with the geomagnetic latitude—and the atmospheric radon. Therefore, before starting a field radiometric survey, it is important to assess the local background. Besides on the lead calibration pad, the background can also be estimated by taking measurements from a small boat (preferably fibreglass) over sea or lake, and at a few hundred metres from the shore, which should be possibly flat (IAEA, 2003). Offshore experiments in southern Italy (Chiozzi et al., 2001) argued that in the Mediterranean Sea the potassium concentration is larger than the ocean water (0.051% against 0.039%; Vengosh et al.,
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1998), causing some increase of the background absorbed dose rate. Since artificial sources are not always available and, of course, differ from natural conditions, outcrops with energy distributions of gamma spectra similar to those typically emitted from the standards and as close as possible to 2p geometry can represent an alternative calibration site. Chiozzi et al. (2000a) claimed that serpentinitic rocks are a good background source as their content of K, U and Th is nearly negligible. The correction for the background radiation was thus estimated on serpentinitic outcrops at about 30 km from to the surveyed area (Fig. 1). The elevation of gamma-ray measurement sites ranges from 0 to about 1000 m a.s.l. Thus some variation with elevation is expected in the background count rate. This aspect was evaluated with a set of measurements at different elevations. Table 1 summarises the count rate change for each energy window recorded at nine serpentinite outcrops located at different elevations. The background count rate generally increases with elevation. This is particularly evident in the 40K window. The overall effect is relatively small, but a correction for elevation was incorporated to improve accuracy.
3. Field results and discussion Radioisotope concentrations were measured in static mode, i.e. at discrete points, taking note of any geological, topographic, climatic and environmental features that may help with subsequent data Table 1 Count rate of the background gamma radiation for each energy window versus elevation measured over serpentinite outcrops. Elevation (m)
214
Bi (min1)
Windows Tl (min1)
40
4 5 6 7 8
25 30 34 37 42
K (min1)
208
0–250 250–500 500–750 750–1000 1000–1250
8 11 12 14 17
interpretation. It must be stressed that secular radioactive equilibrium in the decay series is necessary for appropriate measurements. This condition is generally fulfilled in the 40K and 232Th series while problems may arise for uranium, where a minimum age of 0.3 Ma is required for 226Ra being in equilibrium. Since in the analysed area formations are at least 20 Ma old, secular equilibrium can be assumed also for the 238U series. Particular care was also used in choosing measurement sites showing no or scarce alteration to avoid possible disequilibrium problems. Constant source-detector geometry was kept for all observations. A rigid survey grid could be not held due to the uneven topography and exposure of the rock formations. This implies that point sources of small anomaly may have not been detected in our survey. On the other hand, the relatively high sampling density we adopted (about 4 spectra acquisitions per km2) can give good estimates of the radioactivity of broad-scale lithological units (Fig. 1). The spectrometer functioning and variations in background radioactivity were regularly checked at an established reference station. Table 2 summarises the results of U, Th and K concentrations in the investigated area. For the basement rocks, the radioisotope concentrations of the metavolcanic products are related to the magma original composition and track the magmatic differentiation processes. In fact the largest concentrations were found in the more acid (SiO2 rich) terms, i.e. metarhyolites and porphyric schists. In the metasedimentary rocks of the basement, the largest concentrations of U, Th and K, were observed in quartzschists and micaschists. In the cover lithotypes, U, Th and K content is generally lower. Orthoquartzites, arkosic sandstones and conglomerates show relatively larger concentrations, whereas bioclastic limestones are characterised by the lowest level of natural radioactivity. The air-absorbed dose rate D, expressed in Gray per hour (Gy h1) was estimated at each measurement site by means of the relation by Beck et al. (1972). The adopted conversion factor for potassium is 12.0 (nGy h1 per percent). For U and Th factors of 4.8 and 2.2 (both in nGy h1 per mg kg1), respectively, were used as recently proposed by Tzortzis et al. (2003). The average uncertainty of D is 5.9 nGy h1 for the basement and 1.8 nGy h1 for the cover. Table 2 summarises the expected minima, maxima
Table 2 Average content of U, Th and K, minimum, maximum and average absorbed dose rate D and potential radon flux Em for the different rock types. Ea is the specific exhalation rate (in Bq m–2 h–1 of 222Rn per Bq kg–1 of 226Ra). Lithology
n
U (mg kg1)
Basement Orthogneiss and granitoids Phyllites and micaschists Quartz schists and micaschists Granodiorites and metaandesites Metarhyolites and porphyric schists Total
10 26 37 12 79 164
3.8 4.5 5.9 3.8 6.3
Cover Quartzites, arkosic sandstones and conglomerates Dolomitic limestones Calcareous schists and limestones Conglomerates and marly sandstones Bioclastic limestones Total
31 104 21 5 62 223
2.2 (1.5) 2.1 (0.9) 0.9 (0.7) 1.9 (1.5) 0.5 (0.3)
All
387
n ¼ number of sites; standard deviation in brackets.
(1.3) (1.2) (2.3) (1.5) (2.2)
Th (mg kg1)
15.5 16.5 16.4 10.6 18.2
(3.5) (4.5) (6.4) (4.0) (5.3)
6.9 (3.5) 1.6 (1.3) 1.5 (0.9) 2.7 (1.6) 1.5 (0.9)
D (nGy h1)
K (%)
Ea
Min
Max
Av.
(0.4) (0.6) (1.3) (1.1) (1.2)
90.6 66.5 34.9 19.3 72.2 19.3
136.1 146.9 210.2 97.5 199.3 210.2
108.4 (12.2) 101.9 (18.8) 108.4 (35.7) 64.5 (22.7) 120.1 (29.7) 109.8 (28.7)
2.6 (1.5) 0.5 (0.5) 0.4 (0.4) 1.0 (0.6) 0.4 (0.3)
19.7 5.3 3.0 16.4 1.0 1.0
129.0 56.8 60.2 38.5 31.6 129.0
56.3 (29.5) 19.4 (10.0) 12.3 (15.5) 26.8 (8.5) 10.3 (6.6) 21.5 (14.2)
1.0
210.2
58.9 (21.6)
4.8 3.8 3.8 2.0 4.3
Em (Bq m2 h1) Min
Max
Av.
0.4 0.5 0.6 0.4 0.7
9.9 14.3 14.6 6.2 22.9 6.2
30.7 42.8 86.5 32.1 98.8 98.8
18.7 (6.2) 27.7 (7.2) 43.5 (16.8) 18.7 (7.5) 54.2 (18.6) 42.8 (15.7)
0.4 0.3 0.4 0.3 0.3
4.2 1.3 1.1 1.3 0.7 0.7
43.0 16.4 14.4 16.2 4.6 43.0
10.8 (7.6) 7.7 (3.3) 4.4 (4.2) 7.0 (5.6) 1.8 (1.0) 6.2 (4.0)
0.7
98.8
21.7 (10.7)
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967
44°16'
44°14'
Spotorno Bq m-2 h-1 80-100
44°12' N
60- 80 40-60 ian
r Ligu
44°10'
Sea
<20
Finale Ligure 8°18'
8°20'
20-40
8°22'
8°24' E
8°26'
Fig. 2. Distribution map of the potential radon flux.
and average values of absorbed dose rate over the investigated lithotypes. On the whole, the average D is 58.9 nGy h1 that is comparable to the world average (about 57 nGy h1; UNSCEAR, 2000). The dose rate of the basement rocks is five times larger than that of the sedimentary cover. In the former group of rocks, D ranges from 64.5 nGy h1 (granodiorites) to 120.1 nGy h1 (metarhyolites), while in the latter the dose rate is minimum in bioclastic limestones (10.3 nGy h1) and maximum in orthoquartzites, arkosic sandstones and conglomerates (56.3 nGy h1). Generally, the average dose rate of the basement rocks nearly equals or is larger than that of other lithotypes cropping out in other parts of Liguria and investigated in previous studies (Pasquale et al., 2001; Verdoya et al., 2001; Chiozzi et al. 2002). The distribution map of D (Fig. 1) appears strictly related to the geological characteristics. It turns out that the northern sector of the surveyed area (basement) is characterised by values ranging from 50 to 200 nGy h1, whereas the southern part shows a lower level of natural radioactivity with Do50 nGy h1. In the northern sector, maxima are in generally caused by U and K enrichment within quartzschists, micaschists, metarhyolites and porphyric schists. In the south, minima are ascribable to the sedimentary sequences, in particular to the carbonatic ones. In this part of the surveyed area, values of D4100 nGy h1 are related to outcrops of orthoquartzites, arkosic sandstones and conglomerates. The potential flux of 222Rn due to 238U is also shown in Table 2. Radon is the product of the decay of 226Ra in the 238U series. Its transport within rocks is essentially controlled by diffusion. For a given uranium concentration, fractures, porosity and water content within the rock play a decisive role in the radon migration processes. These characteristics can be in a first approximation summarised with an empirical coefficient, the specific exhalation Ea. This parameter is combined with the 226Ra specific activity (1 Bq kg1 of 226Ra is equivalent to 0.0813 mg kg1 of U, when the secular equilibrium is not broken) to obtain the expected radon
flux Em. Based on experimental and theoretical observations, A˚kerblom and Linde´n (1996) report a compilation of Ea for a set of lithotypes and argue that it also increases with the U content of the rock. Taking into account this compilation, we used Ea values ranging from 0.3 to 0.7 Bq m2 h1 of 222Rn per Bq kg1 of 226Ra depending on the degree of fracturing and the uranium content of the investigated lithotypes. For a bias of 20% on Ea, the average uncertainty of Em is 6.2 Bq m2 h1 for the basement and 2.1 Bq m2 h1 for the cover. The potential flux of radon Em in the basement rocks is almost seven times larger than in the cover (42.8 Bq m2 h1 against 6.2 Bq m2 h1). Among the cover lithotypes, only those of detrital origin (quartzschists, arkosic sandstones and conglomerates) show Em comparable to that expected in methamorphic rocks of the basement, whereas among the metavolcanics, the lower average Em (18.7 Bq m2 h1) is estimated for granodiorites, metandesites, orthogneisses and granitoids. On the whole, maxima correspond to volcanic and volcanoclastic products with higher U content (54.2 and 43.5 Bq m2 h1, respectively). Relatively high Em (27.7 Bq m2 h1) is inferred also in phyllites and micaschists. The potential flux of radon is lower in the sedimentary cover and reaches its minimum value over the bioclastic limestones (1.8 Bq m2 h1). Similarly to the dose rate, the map of the potential radon flux (Fig. 2) reflects the spatial distribution of lithology. In the northern sector of the investigated area, the basement presents almost always Em larger than 20 Bq m2 h1. Within this part, there are several maxima exceeding 60 Bq m2 h1. Most of the southern part shows a radon flux o20 Bq m2 h1, due to the large presence of low U content carbonatic successions.
4. Conclusions We presented results of air absorbed dose rate and estimates of radon exhalation of the Alpine geological units of western Liguria
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(Italy) from field gamma-ray spectrometry surveys. The large variety of sedimentary, metasedimentary and metavolcanic rock types cropping out over a relatively narrow area provides an opportunity for testing the radiometric technique. We stress the importance of the implementation of appropriate calibration procedures before running field operations, with particular emphasis to factors affecting accuracy and detection limits and the evaluation of the background radiation in a rough terrain. In areas of rugged topography, like that investigated in this study, attention should be paid to the evaluation of the background radiation and its variation with elevation. Gamma-ray spectra determined at 387 sites yield a corresponding average absorbed dose rate due to the gamma-ray emission of about 60 nGy h1, with maxima of 120 nGy h1 occurring at metavolcanic rocks. The knowledge of the geological distribution of uranium contributes to defining the areas with potentially enhanced 222Rn exhalation. The maximum estimated potential radon flux is about 54 Bq m2 h1 over the metavolcanic outcrops, while the average for all the investigated rock types is about 22 Bq m2 h1. In synthesis, the content of naturally occurring radioactive elements of the rocks of the investigated area is in agreement with the expectations from the geo-litological structure of the region. The results forms a baseline for further surveys, involving direct radon assessment, focussed on the anomalous points, where the contents of U and, consequently, the expected radon flux appear to be significant and potentially relevant to the radio-protection rules. References A˚kerblom, G., Linde´n, A., 1996. Predicting the radon concentration in deep nuclear waste repository. Report of the Swedish Radiation Protection Institute, 15pp. Aydin, I., Aydogan, M.S., Oksum, E., Koc-ak, A., 2006. An attempt to use aerial gamma-ray spectrometry results in petrochemical assessments of the volcanic and plutonic associations of Central Anatolia (Turkey). Geophysical Journal International 67, 1044–1052. Beck, H.L., DeCampo, J.A., Gogolak, C.V., 1972. In-situ Ge (Li) and NaI (Tl) gammaray spectrometry. Report HASL-258, US Atomic Energy Commission, New York.
Bellotti, E., Di Carlo, G., Di Sabatino, D., Ferrari, N., Laubenstein, M., Pandola, L., Tomei, C., 2007. g-Ray spectrometry of soil samples from the Provincia dell’Aquila (Central Italy). Applied Radiation and Isotopes 65, 858–865. Bochiolo, M., 2007. Radioattivita` gamma naturale nei settori interno e intermedio del Brianzonese Ligure. Degree Thesis, University of Genoa, 115pp. Chiozzi, P., Pasquale, V., Verdoya, M., 1998. Ground radiometric survey of U, Th and K in the Lipari Island, Italy. Journal of Applied Geophysics 38, 209–217. Chiozzi, P., DeFelice, P., Pasquale, M., Verdoya, M., 2000a. Practical applicability of field radiometric analyses. Applied Radiation and Isotopes 53, 215–220. Chiozzi, P., De Felice, P., Fazio, A., Pasquale, V., Verdoya, M., 2000b. Laboratory application of NaI(Tl) gamma-ray spectrometry to studies of natural radioactivity in geophysics. Applied Radiation and Isotopes 53, 127–132. Chiozzi, P., Pasquale, V., Verdoya, M., Minato, S., 2001. Natural gamma-radiation in the Aeolian volcanic arc. Applied Radiation and Isotopes 55, 737–744. Chiozzi, P., Pasquale, V., Verdoya, M., 2002. Natural occurring radioactivity at the Alps-Apennines transition. Radiation Measurements 35, 147–154. Genovesi, I., 2007. Proprieta` termofisiche dei litotipi del Brianzonese Ligure. Degree Thesis, University of Genoa, 157pp. Grasty, R.L., Holman, P.B., Blanchard, Y.B., et al., 1991. Transportable calibration pads for ground and airborne gamma-ray spectrometers. Geological Survey of Canada, Paper No. 90-23, 25pp. International Atomic Energy Agency—IAEA, 1989. Construction and use of calibration facilities for radiometric field equipment. Technical Reports Series No. 309, IAEA, Vienna. International Atomic Energy Agency—IAEA, 2003. Guidelines for radioelement mapping using gamma-ray spectrometry data. IAEA-TECDOC-1363. Lovborg, L., Mose, E., 1987. Counting statistics in radioelement assaying with a portable spectrometer. Geophysics 52, 555–563. Multala, J., 1981. The construction of g-ray calibration pads. Geoexploration 19, 33–46. Pasquale, V., Verdoya, M., Chiozzi, P., 2001. Radioactive heat generation and its thermal effects in the Alps-Apennines boundary zone. Tectonophysics 331, 269–283. Tzortzis, M., Tsertos, H., Christofides, S., Christodoulides, G., 2003. Gamma-ray measurements of naturally occurring radioactive samples from Cyprus characteristic geological rocks. Radiation Measurements 37, 221–229. United Nations Scientific Committee on Effects of Atomic Radiation—UNSCEAR, 2000. Sources, effects and risks of ionising radiation. Report to the General Assembly, with Scientific Annexes, United Nations, New York. Vengosh, A., De Lange, G.J., Starinsky, A., 1998. Boron isotope and geochemical evidence for the origin of Urania and Bannock brines at the eastern Mediterranean: effects of water-rock interactions. Geochimica et Cosmochimica Acta 62, 3221–3228. Verdoya, M., Chiozzi, P., Pasquale, V., 2001. Heat-producing radionuclides in metamorphic rocks of the Brianc- onnais-Piedmont Zone (Maritime Alps). Eclogae geologicae Helvetiae 94, 213–219.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Natural gamma-ray spectrometry as a tool for radiation dose and radon hazard modelling M. Verdoya a, P. Chiozzi a, P. De Felice b, V. Pasquale a,, M. Bochiolo a, I. Genovesi a a b
` di Genova, Viale Benedetto XV 5, I-16132 Genoa, Italy Dipart. Studio Territorio Risorse, Settore di Geofisica, Universita Instituto Nazionale Metrologia Radiazioni Ionizzanti ENEA, Centro Ricerche Casaccia, Via Anguillarese, 301 S.M. Galeria, I-00100 Rome, Italy
a r t i c l e in f o
a b s t r a c t
Keywords: Spectrometry Natural radioactivity Absorbed dose rate Radon potential
We reviewed the calibration procedures of gamma-ray spectrometry with particular emphasis to factors that affect accuracy, detection limits and background radiation in field measurements for dosimetric and radon potential mapping. Gamma-ray spectra were acquired in western Liguria (Italy). The energy windows investigated are centred on the photopeaks of 214Bi (1.76 MeV), 208Tl (2.62 MeV) and 40K (1.46 MeV). The inferred absorbed dose rate and the radon flux are estimated to be lower than 60 nGy h1 and 22 Bq m2 h1, respectively. & 2009 Elsevier Ltd. All rights reserved.
1. Introduction Radiometric surveys cover many different scientific and practical interests, ranging from basic geophysics to mineral exploration and environmental radiation monitoring. The environmental aspects of natural radioactivity, above all regarding radon effects and exposure to ionising radiation, is a topic of growing interest during the last years (see e.g. A˚kerblom and Linde´n, 1996; Bellotti et al., 2007 and references therein). Natural radioactivity originated from uranium, thorium and potassium, which are hold in variable quantities above all in the rocks forming the Earth’s crust, is the major cause of natural, potentially hazardous, gamma-ray exposure. Albeit aerial gamma-ray spectrometry can provide an overview and mapping of radioelement concentrations over wide regions (e.g. Aydin et al., 2006), only ground, high-resolution measurements yield detailed mapping over relatively small rock outcrops, and allow cheap and fast monitoring of environmental radioactivity even in a rough terrain. In this paper, we present results of natural radioactivity from a ground gamma-ray survey carried out in the Alpine geological units of western Liguria (Italy) (Fig. 1). Gamma-ray spectrometry results were used for assessment of absorbed dose rates. Moreover, uranium concentration was considered as a radon potential indicator in the different geological environments and was used for preparing radon prognosis maps. Particular care was paid to the calibration of the gamma-ray apparatus, with reference to the assessment of the background radiation. Since the surveyed area has a rough
Corresponding author. Tel.: +39 010 3538090; fax: +39 010 352169.
E-mail address: [email protected] (V. Pasquale). 0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.01.066
topography, we evaluated the variation of background radiation with elevation. The interest for the study area is above all based on the presence of several lithotypes, spanning from sedimentary, to metasedimentary and metavolcanic rocks, the latter particularly known in literature for their abundance of radionuclides (see Verdoya et al., 2001; Bochiolo, 2007; Genovesi, 2007 and references therein). The stratigraphy consists of: (1) basement rocks (Late Carboniferous-Permian), embodying polimetamorphic terms (orthogneisses and granitoids), metasediments (phyllites, micaschists and quartzschists) and metavolcanics (metaandesites, metarhyolites and porphyric schists); (2) cover rocks (TriasMiocene), formed by orthoquartzites, arkosic sandstones, and conglomerates, and by a calcareous-dolomitic succession, sometimes alternate to arenaceous and marly terms.
2. Measurement technique 2.1. Instrumentation and calibration The type of spectrometer, detector volume, measurement times, and mode of measurement depend on the radiation environment and the type, size and distribution of radioactive sources. Portable, hand-held gamma-ray spectrometers are widely used in field studies for both regional and detailed mapping surveys aimed to estimate the surface concentrations of the radionuclides. Our portable apparatus for gamma-ray activity measurements (Satisgeo Brno, Czech Republic) consists of a thallium-activated sodium iodide scintillation detector of about 440 cm3 together with a photomultiplier tube, a high-voltage supply and a signal preamplifier. It is thermally insulated and
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965
44°16' GV
BP
Genoa Varazze Savona
Pietra Ligure a Se an uri Imperia Lig
44°14' Spotorno 44°12' N
nGy h-1 150-200
44°10' 8°18'
ian
r Ligu Finale Ligure
8°20'
8°22'
Sea
100-150 50-100 <50
8°24'E
8°26'
Fig. 1. Distribution map of the absorbed dose rate and site location (full dot) in western Liguria. BP—Brianc- onnaise and Piedmont zone and GV—Voltri Massif Alpine geological units. Inset: full square shows the area where the detector was calibrated for the local background.
housed in an aluminium cylinder. The probe is connected to a 256-channel analyser. The instrument can record the full gammaray spectrum as well as sum channels over broad energy windows for the in situ estimation of K, U and Th concentrations. The determination of U, Th and K is based on the method of the three energy windows (Chiozzi et al., 1998, 2000a). Each window is centred on the characteristic photopeak of the gamma-ray spectrum from the decay of 214Bi (1.76 MeV) in the 238U decay series and from 208Tl (2.62 MeV) in the 232Th series. The primary decay of 40K (1.46 MeV) is measured directly. The investigated energy spectrum ranges from 0.12 to 3.00 MeV. A gamma-ray emitting reference source of 137Cs (approximate activity of 15 kBq), with a low-energy peak of 0.66 MeV is accommodated within the probe for automatic instrument gain adjustment and spectrum stabilisation. Gamma-ray spectrometer calibration requires the estimation of those constants that relate instrument count rates to either radionuclide concentration or environmental dose rate, namely the sensitivity constants and the background radiation. Portable instruments are usually calibrated by means of standard spectra acquired using three concrete pads enriched in K, U and Th and highly unbalanced K/U, Th/U and K/Th ratios (e.g. Multala, 1981). Ideally, calibration pads should simulate a geological source of radiation and should be kept dry, as variation in moisture content can lead to a change in radiation output. Pads are usually of cylindrical shape, and recommended diameter and thickness are 2–3 and 0.5 m, respectively (IAEA, 1989). A fourth, low-radioactivity calibration pad of lead is customarily used to measure the background. The gamma-ray count rate for each spectrum within each energy window depends primarily on (i) the concentration and interference of the radioactive elements; (ii) Compton scattering and the linear attenuation coefficient appropriate to the rock material and the energies of the transmitted gamma rays; (iii) detector sensitivity and background count rate. Practically, the calibration procedure consists of measuring the net count rate at each window at each pad, which has finite dimensions and may differ from the recommended size. Grasty et al. (1991) showed that smaller transportable pads are also suitable for calibrating portable gamma-ray spectrometers, provided that a geometrical correction factor is applied to the instrument sensitivity derived from these calibration experiments. Furthermore, the detector is set at a few centimetres from the surface. In such conditions the count rates are lower than those expected for a 2p infinite geometry. Therefore, the recorded spectra must be corrected for a geometric factor G ¼ 12h/d, where h is the height of the detector to the pad surface and d is the diameter of the
standard pad. The determination of the sensitivity of our spectrometer (with h ¼ 63 mm) is based on standard pads of 2 m in diameter, located at the Czech Uranium Industry (Prague) (see Chiozzi et al., 2000a for details). Under these conditions, we may claim that the recorded gamma rays are emitted from a rock volume of about 1 m3. The sampling time required for a measurement depends on the radioactivity of the outcrop and the precision required. Since the K, U, and Th spectra overlap, the counting time required for a specified precision in the estimates of these radionuclides is more complicated. Lovborg and Mose (1987) derived equations giving the counting time for assays of each radionuclide with a 10% error at various K, U, and Th ratios. In practise, for NaI(Tl) detectors, IAEA (2003) recommends a sampling time of 120 s for highly radioactive rocks and 360 s for low radioactivity outcrops. Due to the large variability of radionuclide concentrations of our rocks, we adopted a sampling time of 300 s. This implies that the detection limit is estimated to be 0.2 and 0.3 mg kg1 for U and Th, respectively, and 0.03% for K (Chiozzi et al., 2000b). Several factors may affect accuracy in the determinations of these elements. Sources of bias could be inherent to the instrument itself (e.g. drift of energy calibration, calibration of the reference materials, K contamination of the photomultiplier tube) and of geological nature (self-absorption due to the density variation in the analysed rock, outcrop alteration and geometry). Based on counting statistics, in common rocks the relative combined standard uncertainty of the measured concentrations is 3% for K, 5% for Th and 8% for U.
2.2. Background radiation The net count rate and, consequently, the concentration results are dependent on the background, i.e. the gamma radiation that does not originate from the rock. Background radiation is due to the internal radioactivity of the instrument, the cosmic radiation—which changes with the geomagnetic latitude—and the atmospheric radon. Therefore, before starting a field radiometric survey, it is important to assess the local background. Besides on the lead calibration pad, the background can also be estimated by taking measurements from a small boat (preferably fibreglass) over sea or lake, and at a few hundred metres from the shore, which should be possibly flat (IAEA, 2003). Offshore experiments in southern Italy (Chiozzi et al., 2001) argued that in the Mediterranean Sea the potassium concentration is larger than the ocean water (0.051% against 0.039%; Vengosh et al.,
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1998), causing some increase of the background absorbed dose rate. Since artificial sources are not always available and, of course, differ from natural conditions, outcrops with energy distributions of gamma spectra similar to those typically emitted from the standards and as close as possible to 2p geometry can represent an alternative calibration site. Chiozzi et al. (2000a) claimed that serpentinitic rocks are a good background source as their content of K, U and Th is nearly negligible. The correction for the background radiation was thus estimated on serpentinitic outcrops at about 30 km from to the surveyed area (Fig. 1). The elevation of gamma-ray measurement sites ranges from 0 to about 1000 m a.s.l. Thus some variation with elevation is expected in the background count rate. This aspect was evaluated with a set of measurements at different elevations. Table 1 summarises the count rate change for each energy window recorded at nine serpentinite outcrops located at different elevations. The background count rate generally increases with elevation. This is particularly evident in the 40K window. The overall effect is relatively small, but a correction for elevation was incorporated to improve accuracy.
3. Field results and discussion Radioisotope concentrations were measured in static mode, i.e. at discrete points, taking note of any geological, topographic, climatic and environmental features that may help with subsequent data Table 1 Count rate of the background gamma radiation for each energy window versus elevation measured over serpentinite outcrops. Elevation (m)
214
Bi (min1)
Windows Tl (min1)
40
4 5 6 7 8
25 30 34 37 42
K (min1)
208
0–250 250–500 500–750 750–1000 1000–1250
8 11 12 14 17
interpretation. It must be stressed that secular radioactive equilibrium in the decay series is necessary for appropriate measurements. This condition is generally fulfilled in the 40K and 232Th series while problems may arise for uranium, where a minimum age of 0.3 Ma is required for 226Ra being in equilibrium. Since in the analysed area formations are at least 20 Ma old, secular equilibrium can be assumed also for the 238U series. Particular care was also used in choosing measurement sites showing no or scarce alteration to avoid possible disequilibrium problems. Constant source-detector geometry was kept for all observations. A rigid survey grid could be not held due to the uneven topography and exposure of the rock formations. This implies that point sources of small anomaly may have not been detected in our survey. On the other hand, the relatively high sampling density we adopted (about 4 spectra acquisitions per km2) can give good estimates of the radioactivity of broad-scale lithological units (Fig. 1). The spectrometer functioning and variations in background radioactivity were regularly checked at an established reference station. Table 2 summarises the results of U, Th and K concentrations in the investigated area. For the basement rocks, the radioisotope concentrations of the metavolcanic products are related to the magma original composition and track the magmatic differentiation processes. In fact the largest concentrations were found in the more acid (SiO2 rich) terms, i.e. metarhyolites and porphyric schists. In the metasedimentary rocks of the basement, the largest concentrations of U, Th and K, were observed in quartzschists and micaschists. In the cover lithotypes, U, Th and K content is generally lower. Orthoquartzites, arkosic sandstones and conglomerates show relatively larger concentrations, whereas bioclastic limestones are characterised by the lowest level of natural radioactivity. The air-absorbed dose rate D, expressed in Gray per hour (Gy h1) was estimated at each measurement site by means of the relation by Beck et al. (1972). The adopted conversion factor for potassium is 12.0 (nGy h1 per percent). For U and Th factors of 4.8 and 2.2 (both in nGy h1 per mg kg1), respectively, were used as recently proposed by Tzortzis et al. (2003). The average uncertainty of D is 5.9 nGy h1 for the basement and 1.8 nGy h1 for the cover. Table 2 summarises the expected minima, maxima
Table 2 Average content of U, Th and K, minimum, maximum and average absorbed dose rate D and potential radon flux Em for the different rock types. Ea is the specific exhalation rate (in Bq m–2 h–1 of 222Rn per Bq kg–1 of 226Ra). Lithology
n
U (mg kg1)
Basement Orthogneiss and granitoids Phyllites and micaschists Quartz schists and micaschists Granodiorites and metaandesites Metarhyolites and porphyric schists Total
10 26 37 12 79 164
3.8 4.5 5.9 3.8 6.3
Cover Quartzites, arkosic sandstones and conglomerates Dolomitic limestones Calcareous schists and limestones Conglomerates and marly sandstones Bioclastic limestones Total
31 104 21 5 62 223
2.2 (1.5) 2.1 (0.9) 0.9 (0.7) 1.9 (1.5) 0.5 (0.3)
All
387
n ¼ number of sites; standard deviation in brackets.
(1.3) (1.2) (2.3) (1.5) (2.2)
Th (mg kg1)
15.5 16.5 16.4 10.6 18.2
(3.5) (4.5) (6.4) (4.0) (5.3)
6.9 (3.5) 1.6 (1.3) 1.5 (0.9) 2.7 (1.6) 1.5 (0.9)
D (nGy h1)
K (%)
Ea
Min
Max
Av.
(0.4) (0.6) (1.3) (1.1) (1.2)
90.6 66.5 34.9 19.3 72.2 19.3
136.1 146.9 210.2 97.5 199.3 210.2
108.4 (12.2) 101.9 (18.8) 108.4 (35.7) 64.5 (22.7) 120.1 (29.7) 109.8 (28.7)
2.6 (1.5) 0.5 (0.5) 0.4 (0.4) 1.0 (0.6) 0.4 (0.3)
19.7 5.3 3.0 16.4 1.0 1.0
129.0 56.8 60.2 38.5 31.6 129.0
56.3 (29.5) 19.4 (10.0) 12.3 (15.5) 26.8 (8.5) 10.3 (6.6) 21.5 (14.2)
1.0
210.2
58.9 (21.6)
4.8 3.8 3.8 2.0 4.3
Em (Bq m2 h1) Min
Max
Av.
0.4 0.5 0.6 0.4 0.7
9.9 14.3 14.6 6.2 22.9 6.2
30.7 42.8 86.5 32.1 98.8 98.8
18.7 (6.2) 27.7 (7.2) 43.5 (16.8) 18.7 (7.5) 54.2 (18.6) 42.8 (15.7)
0.4 0.3 0.4 0.3 0.3
4.2 1.3 1.1 1.3 0.7 0.7
43.0 16.4 14.4 16.2 4.6 43.0
10.8 (7.6) 7.7 (3.3) 4.4 (4.2) 7.0 (5.6) 1.8 (1.0) 6.2 (4.0)
0.7
98.8
21.7 (10.7)
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967
44°16'
44°14'
Spotorno Bq m-2 h-1 80-100
44°12' N
60- 80 40-60 ian
r Ligu
44°10'
Sea
<20
Finale Ligure 8°18'
8°20'
20-40
8°22'
8°24' E
8°26'
Fig. 2. Distribution map of the potential radon flux.
and average values of absorbed dose rate over the investigated lithotypes. On the whole, the average D is 58.9 nGy h1 that is comparable to the world average (about 57 nGy h1; UNSCEAR, 2000). The dose rate of the basement rocks is five times larger than that of the sedimentary cover. In the former group of rocks, D ranges from 64.5 nGy h1 (granodiorites) to 120.1 nGy h1 (metarhyolites), while in the latter the dose rate is minimum in bioclastic limestones (10.3 nGy h1) and maximum in orthoquartzites, arkosic sandstones and conglomerates (56.3 nGy h1). Generally, the average dose rate of the basement rocks nearly equals or is larger than that of other lithotypes cropping out in other parts of Liguria and investigated in previous studies (Pasquale et al., 2001; Verdoya et al., 2001; Chiozzi et al. 2002). The distribution map of D (Fig. 1) appears strictly related to the geological characteristics. It turns out that the northern sector of the surveyed area (basement) is characterised by values ranging from 50 to 200 nGy h1, whereas the southern part shows a lower level of natural radioactivity with Do50 nGy h1. In the northern sector, maxima are in generally caused by U and K enrichment within quartzschists, micaschists, metarhyolites and porphyric schists. In the south, minima are ascribable to the sedimentary sequences, in particular to the carbonatic ones. In this part of the surveyed area, values of D4100 nGy h1 are related to outcrops of orthoquartzites, arkosic sandstones and conglomerates. The potential flux of 222Rn due to 238U is also shown in Table 2. Radon is the product of the decay of 226Ra in the 238U series. Its transport within rocks is essentially controlled by diffusion. For a given uranium concentration, fractures, porosity and water content within the rock play a decisive role in the radon migration processes. These characteristics can be in a first approximation summarised with an empirical coefficient, the specific exhalation Ea. This parameter is combined with the 226Ra specific activity (1 Bq kg1 of 226Ra is equivalent to 0.0813 mg kg1 of U, when the secular equilibrium is not broken) to obtain the expected radon
flux Em. Based on experimental and theoretical observations, A˚kerblom and Linde´n (1996) report a compilation of Ea for a set of lithotypes and argue that it also increases with the U content of the rock. Taking into account this compilation, we used Ea values ranging from 0.3 to 0.7 Bq m2 h1 of 222Rn per Bq kg1 of 226Ra depending on the degree of fracturing and the uranium content of the investigated lithotypes. For a bias of 20% on Ea, the average uncertainty of Em is 6.2 Bq m2 h1 for the basement and 2.1 Bq m2 h1 for the cover. The potential flux of radon Em in the basement rocks is almost seven times larger than in the cover (42.8 Bq m2 h1 against 6.2 Bq m2 h1). Among the cover lithotypes, only those of detrital origin (quartzschists, arkosic sandstones and conglomerates) show Em comparable to that expected in methamorphic rocks of the basement, whereas among the metavolcanics, the lower average Em (18.7 Bq m2 h1) is estimated for granodiorites, metandesites, orthogneisses and granitoids. On the whole, maxima correspond to volcanic and volcanoclastic products with higher U content (54.2 and 43.5 Bq m2 h1, respectively). Relatively high Em (27.7 Bq m2 h1) is inferred also in phyllites and micaschists. The potential flux of radon is lower in the sedimentary cover and reaches its minimum value over the bioclastic limestones (1.8 Bq m2 h1). Similarly to the dose rate, the map of the potential radon flux (Fig. 2) reflects the spatial distribution of lithology. In the northern sector of the investigated area, the basement presents almost always Em larger than 20 Bq m2 h1. Within this part, there are several maxima exceeding 60 Bq m2 h1. Most of the southern part shows a radon flux o20 Bq m2 h1, due to the large presence of low U content carbonatic successions.
4. Conclusions We presented results of air absorbed dose rate and estimates of radon exhalation of the Alpine geological units of western Liguria
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(Italy) from field gamma-ray spectrometry surveys. The large variety of sedimentary, metasedimentary and metavolcanic rock types cropping out over a relatively narrow area provides an opportunity for testing the radiometric technique. We stress the importance of the implementation of appropriate calibration procedures before running field operations, with particular emphasis to factors affecting accuracy and detection limits and the evaluation of the background radiation in a rough terrain. In areas of rugged topography, like that investigated in this study, attention should be paid to the evaluation of the background radiation and its variation with elevation. Gamma-ray spectra determined at 387 sites yield a corresponding average absorbed dose rate due to the gamma-ray emission of about 60 nGy h1, with maxima of 120 nGy h1 occurring at metavolcanic rocks. The knowledge of the geological distribution of uranium contributes to defining the areas with potentially enhanced 222Rn exhalation. The maximum estimated potential radon flux is about 54 Bq m2 h1 over the metavolcanic outcrops, while the average for all the investigated rock types is about 22 Bq m2 h1. In synthesis, the content of naturally occurring radioactive elements of the rocks of the investigated area is in agreement with the expectations from the geo-litological structure of the region. The results forms a baseline for further surveys, involving direct radon assessment, focussed on the anomalous points, where the contents of U and, consequently, the expected radon flux appear to be significant and potentially relevant to the radio-protection rules. References A˚kerblom, G., Linde´n, A., 1996. Predicting the radon concentration in deep nuclear waste repository. Report of the Swedish Radiation Protection Institute, 15pp. Aydin, I., Aydogan, M.S., Oksum, E., Koc-ak, A., 2006. An attempt to use aerial gamma-ray spectrometry results in petrochemical assessments of the volcanic and plutonic associations of Central Anatolia (Turkey). Geophysical Journal International 67, 1044–1052. Beck, H.L., DeCampo, J.A., Gogolak, C.V., 1972. In-situ Ge (Li) and NaI (Tl) gammaray spectrometry. Report HASL-258, US Atomic Energy Commission, New York.
Bellotti, E., Di Carlo, G., Di Sabatino, D., Ferrari, N., Laubenstein, M., Pandola, L., Tomei, C., 2007. g-Ray spectrometry of soil samples from the Provincia dell’Aquila (Central Italy). Applied Radiation and Isotopes 65, 858–865. Bochiolo, M., 2007. Radioattivita` gamma naturale nei settori interno e intermedio del Brianzonese Ligure. Degree Thesis, University of Genoa, 115pp. Chiozzi, P., Pasquale, V., Verdoya, M., 1998. Ground radiometric survey of U, Th and K in the Lipari Island, Italy. Journal of Applied Geophysics 38, 209–217. Chiozzi, P., DeFelice, P., Pasquale, M., Verdoya, M., 2000a. Practical applicability of field radiometric analyses. Applied Radiation and Isotopes 53, 215–220. Chiozzi, P., De Felice, P., Fazio, A., Pasquale, V., Verdoya, M., 2000b. Laboratory application of NaI(Tl) gamma-ray spectrometry to studies of natural radioactivity in geophysics. Applied Radiation and Isotopes 53, 127–132. Chiozzi, P., Pasquale, V., Verdoya, M., Minato, S., 2001. Natural gamma-radiation in the Aeolian volcanic arc. Applied Radiation and Isotopes 55, 737–744. Chiozzi, P., Pasquale, V., Verdoya, M., 2002. Natural occurring radioactivity at the Alps-Apennines transition. Radiation Measurements 35, 147–154. Genovesi, I., 2007. Proprieta` termofisiche dei litotipi del Brianzonese Ligure. Degree Thesis, University of Genoa, 157pp. Grasty, R.L., Holman, P.B., Blanchard, Y.B., et al., 1991. Transportable calibration pads for ground and airborne gamma-ray spectrometers. Geological Survey of Canada, Paper No. 90-23, 25pp. International Atomic Energy Agency—IAEA, 1989. Construction and use of calibration facilities for radiometric field equipment. Technical Reports Series No. 309, IAEA, Vienna. International Atomic Energy Agency—IAEA, 2003. Guidelines for radioelement mapping using gamma-ray spectrometry data. IAEA-TECDOC-1363. Lovborg, L., Mose, E., 1987. Counting statistics in radioelement assaying with a portable spectrometer. Geophysics 52, 555–563. Multala, J., 1981. The construction of g-ray calibration pads. Geoexploration 19, 33–46. Pasquale, V., Verdoya, M., Chiozzi, P., 2001. Radioactive heat generation and its thermal effects in the Alps-Apennines boundary zone. Tectonophysics 331, 269–283. Tzortzis, M., Tsertos, H., Christofides, S., Christodoulides, G., 2003. Gamma-ray measurements of naturally occurring radioactive samples from Cyprus characteristic geological rocks. Radiation Measurements 37, 221–229. United Nations Scientific Committee on Effects of Atomic Radiation—UNSCEAR, 2000. Sources, effects and risks of ionising radiation. Report to the General Assembly, with Scientific Annexes, United Nations, New York. Vengosh, A., De Lange, G.J., Starinsky, A., 1998. Boron isotope and geochemical evidence for the origin of Urania and Bannock brines at the eastern Mediterranean: effects of water-rock interactions. Geochimica et Cosmochimica Acta 62, 3221–3228. Verdoya, M., Chiozzi, P., Pasquale, V., 2001. Heat-producing radionuclides in metamorphic rocks of the Brianc- onnais-Piedmont Zone (Maritime Alps). Eclogae geologicae Helvetiae 94, 213–219.