- Email: [email protected]
In situ gamma spectroscopy in environmental research and monitoring
ARTICLE IN PRESS Applied Radiation and Isotopes 66 (2008) 1615– 1618
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
In situ gamma spectroscopy in environmental research and monitoring Cristina Nuccetelli ` (National Institute of Health), Viale Regina Elena 299, 00161 Rome, Italy Istituto Superiore di Sanita
article info Keywords: Environmental radioactivity Gamma radiation Building materials Indoor exposure
abstract In situ gamma spectroscopy was introduced to determine the outdoor gamma dose rate from soil and to calculate the radionuclide concentration and the relative contribution to the dose rate. This paper reviews the most common and proven applications of in situ gamma spectroscopy—together with the most recent and innovative research outcomes obtained with this technique, particularly for its use indoors. Advantages and limitations of its utilization to assess environmental radioactivity—indoors and outdoors—are also discussed. & 2008 IAEA. Published by Elsevier Ltd. All rights reserved.
1. Introduction In situ gamma spectroscopy is a well-known technique introduced by Beck et al. (1972) to determine the concentration of natural and artificial radionuclides in soil, the relevant outdoor gamma dose rate in the air above, and the relative contribution of the 238U and 232Th series and 40K to the dose rate. This technique soon appeared as a powerful tool to provide rapid and spatially representative estimates of environmental radioactivity. It basically consists of the elaboration of the full absorption peak areas multiplied by ad hoc coefficients calculated according to two fundamental assumptions:
the source—in this case the soil—can be modeled as an infinite half-space;
the vertical distribution of radionuclides can be reasonably assumed (typically uniform distribution for natural radionuclides and exponential distribution for artificial ones). Under these conditions it is possible to use a standard point source calibration performed in the laboratory (Beck et al., 1972; Cutshall and Larsen, 1986). With this calibration and the coefficients elaborated following Beck’s method, the dose rate— produced by the unscattered and scattered fluence of gamma rays—can be estimated, and the radionuclide inventory calculated.
2. Capabilities and limitations of in situ gamma spectroscopy In the view of the above features, in situ gamma spectroscopy—even in its original release—can be a useful support to Fax: +39 649387075.
E-mail address: [email protected]
measure surface flat soils for many research or institutional activities, for example, to characterize sites in terms of natural background radiation, perform surveys to study sites contaminated by artificial radionuclides and/or NORM, or to measure nuclear weapon test fallout contamination. Further applications are the assessment of routine and accidental releases from nuclear facilities, and monitoring of soil contamination level in the different phases of environmental restoration projects. In short, in situ gamma spectroscopy is a very versatile and efficient tool for studying environmental radioactivity, but has some limits mainly associated to the need of a priori assumptions about the distribution of nuclides in the soil, an important source of uncertainties of activity concentration estimates and dose rate evaluations, since dose rate is obtained as a sum of different radionuclide contributions to unscattered and scattered gamma flux at the detector. Moreover, the original method cannot be used for all kinds of source geometry, e.g. indoors or urban outdoors, because it is not feasible to elaborate the build-up factors—taking into account the scattered gamma flux and necessary to determine the gamma dose rate from photopeaks in a recorded spectrum—or produce calibration curves from which to derive the radionuclide activity concentrations in the source.
3. Research activities Since the 1980s many research activities have aimed to improve and simplify the use of this technique. Specifically, a parametric calibration was elaborated, based on the efficiency and geometric dimensions of the detector (Helfer and Miller, 1988). Another improvement was the introduction of a stripping method that subtracts partial absorption and the contribution from cosmic ray events from a collected spectrum. With this procedure, in situ gamma spectroscopy could be applied to indoor gamma
0969-8043/$ - see front matter & 2008 IAEA. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2007.10.019
ARTICLE IN PRESS 1616
C. Nuccetelli / Applied Radiation and Isotopes 66 (2008) 1615–1618
dose rate estimates (Miller, 1984; Clouvas et al., 1998). An important step for the diffusion of this technique was the publication of ICRU Report 53 (ICRU, 1994) which addressed the main aspects of in situ gamma spectroscopy. This report paid special attention to airborne gamma spectrometry (AGS), the aerial technique used to monitor wide areas and map natural radioactivity, extending the use of the method to fission product surveys. In the following years many studies were devoted to further widen the use of this method and overcome the above limitations, rendering in situ gamma spectrometry ‘‘independent’’ of source geometry, in order to estimate the gamma dose rate and the dose contribution from the 238U and 232Th series, 40K and 137Cs in any measurement situation, and ‘‘independent’’ of the vertical distribution of radionuclides. This research task has yielded important outcomes such as indoor and outdoor build-up factors and the estimate of the actual radionuclide distribution in soil. These results have been obtained following two approaches: Monte Carlo simulation and the use of algorithms aimed to the direct elaboration of spectra. With these new methodologies the applications of in situ gamma spectroscopy have been improved and/or extended to indoor environments, and to outdoor environments not easily represented with a model, such as forests (Gering et al., 2002; Plamboeck et al., 2006), urban outdoor areas (Medeiros and Yoshimura, 2005) and large areas (Cresswell et al., 2006; Tyler, 2004; Guillot, 2001; Hendriks et al., 2001). In particular, indoor utilization provides interesting information on building materials, one of the most significant sources of population exposure to natural radionuclides (Risica et al., 2001; Risica and Nuccetelli, 2001). The following paragraphs will describe some research activities in this field.
4. In situ gamma spectroscopy indoors Early investigations regarding the indoor application of this technique started in the 1980s (Miller, 1984; Miller and Beck, 1984) and other methods and developments appeared in the 1990s. At present, with different approaches—e.g. Monte Carlo (Clouvas et al., 2000), elaboration of spectra (Bochicchio et al., 1994), computation plus room model (Markkanen, 1995), called ‘‘integrated method’’ (Nuccetelli and Bolzan, 2001)—the use of in situ gamma spectroscopy indoors allows the evaluation of the gamma dose and the relative contribution of the various nuclides to the total gamma dose rates. In some countries an indoor methodology was also applied to perform surveys in order to get not only information on population exposure from building materials, but also a more detailed description of the sources as well (Clouvas et al., 2001, 2004; Svoukis and Tsertos, 2006). Indoor applications of in situ gamma spectroscopy can also provide interesting information about building materials as sources of radon, thoron and gamma rays (Nuccetelli and Bolzan, 2001; Clouvas et al., 2003, 2006) and supply quantitative estimates about the activity concentrations of radionuclides in building materials. Finally, on the basis of average spectroscopy information on building materials, a methodology was developed to estimate the indoor gamma dose rate from measurements on the external wall of a building. This method, called ‘‘IN–OUT’’, is in a validation phase after very encouraging preliminary results. 4.1. An integrated method to characterize building materials in situ The method, described in detail elsewhere (Nuccetelli and Bolzan, 2001) and sketched in Fig. 1, was tested in two test rooms
of which we had a sample of the material they were built with, i.e. tuff. The agreement between the activity concentration estimates obtained with the ‘‘integrated method’’ and the laboratory gamma spectroscopy was very good. Encouraged by these results, we applied the method to other dwellings. With in situ gamma spectroscopy the ratio between radon decay products (RnDp) and 226 Ra—i.e. the emanation fraction e—in the building materials, and the relevant exhalation rate E could be estimated as well. From these parameters it was also possible to calculate the contribution of building material to 222Rn activity concentration CRn estimated. Table 1 shows the results obtained in eight dwellings in Rome. It is worth noting that only in dwelling no. 4—built with tuff and on the ground floor—the contribution of Rn from the soil appears to be significant and computable by the difference between the estimated values, obtained by accounting for only the building material contribution CRn estimated, and the total measured value CRn total measured. 4.2. IN– OUT method With this method the indoor gamma dose rate is estimated from the gamma dose rate measurements performed on an external wall (Bochicchio et al., 2004; Nuccetelli et al., 2005). The method consists of the evaluation of 226Ra, 222RnDP, 228AcDP and 40 K activity concentrations of the building materials—with an ad hoc algorithm—using the external wall measurements and data from in situ gamma spectroscopy, from which radiation field characteristics are derived. Since spectroscopy parameters are fairly constant for the same kind of building material and some kind of buildings, in this case masonry (Bochicchio et al., 1994, 1996; Clouvas et al., 1998, 2000), on this occasion we used spectroscopy data we already had for some dwellings in Rome. With the radionuclide content data, it was possible to estimate the indoor gamma dose rate by applying a room model (Markkanen, 1995), which needs information on the geometry and structure of the dwelling. Table 2 shows the results of the preliminary study. The power of the ‘‘IN–OUT’’ method to make accurate and precise estimates stems from the ratios between estimated and measured values of gamma dose rate. Obviously, this method can be very useful in epidemiological studies, where lack of exposure data resulting
CHARACTERISTICS OF THE ROOM -geometrical dimension of the room -density of walls, floor and ceiling -thickness of walls, floor and ceiling -detector position
γ DOSE RATE MEASUREMENTS high pressure ionisation chamber or plastic 3” x 3” scintillator
γ SPECTROMETRY in situ HPGe 26 % efficiency and 1.73 keV resolution
% contribution of 40K, 232Th and 226Ra to dose
Activity conc. ratios: CRa/CAc and CRa/CPb
ELABORATION WITH ROOM MODEL DRnDp, DTh, DK
- ACTIVITY CONCENTRATIONS OF 40K, 232Th,232RnDP AND 226Ra - 222Rn EMANATION and EXHALATION Fig. 1. Diagram of procedure and steps of the ‘‘integrated method’’.
ARTICLE IN PRESS C. Nuccetelli / Applied Radiation and Isotopes 66 (2008) 1615–1618
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Table 1 Results of the application of the ‘‘integrated method’’ in eight dwellings in Rome Dwell
Floor
Building material
CRn Dp (Bq kg
1
)
CAc-228 (Bq kg 1)
CK-40 (Bq kg
1
)
CRa-226 (Bq kg 1)
e emanation power
E (Bq m
2
h
1
)
CRn
CRn
estimated 3
measured 3
(Bq m 1 2 3 4 Office 5 6 7
5th 5th 5th Ground 1st 4th 3rd 4th
Tuff Tuff Tuff Tuff Tuff Concrete Concrete Concrete
152 198 219 144 166 51 117 86
327 355 445 313 299 86 210 170
1195 1603 1493 885 1243 543 917 665
219 269 294 204 235 53 137 105
0.31 0.26 0.25 0.29 0.29 0.05 0.15 0.18
179 196 217 254 301 3 33 30
116 89 104 161 158 2 15 21
)
total
(Bq m
)
124 135 140 338 130 40 58 86
Table 2 Preliminary results of the application of the ‘‘IN– OUT’’ method External wall building materials
Tuff/stone Concrete/bricks All
Number of dwellings
45 46 91
Estimated ‘‘IN–OUT’’ g dose rate/ measured g dose rate indoors
Measured g dose rate outdoors/ measured g dose rate indoors
Arith. mean
Min–Max
SD(%)
Arith. mean
Min–Max
SD(%)
1.00 1.05 1.03
0.82–1.24 0.79–1.32 0.79–1.32
9 12 11
1.09 1.07 1.08
0.71–1.97 0.64–2.44 0.64–2.44
26 33 30
from refusing or missing subjects can bias the survey results. In this sense, the estimates obtained from the direct use of the external wall measurements cannot assure an adequate level of accuracy and precision (see the last three columns in Table 2).
5. Conclusions The fundamental characteristic of in situ gamma spectroscopy is its capability of providing rapid and integrated measurements of the investigated environment and dose rate contributions of radionuclides. This makes in situ gamma spectroscopy a powerful tool for widely encompassing environmental monitoring (indoors, NORM, early warning, emergency, etc.), supported by ongoing research in this field. On the other hand, the interest in this technique as a tool for regulatory aims, is demonstrated by the periodic intercomparisons—e.g. EURADOS ‘02 (Wissmann and Saez Vergara, 2006), ECCOMAGS 02 (Sanderson et al., 2004), ISIGAMMA ‘05, in-situ INTERCOMPARISON SCENARIO ‘07 (http://isis2007.healthphysics. at/)—which can extend the use and improve the performance of this technique, and even consolidate its application in the regulatory decision-making process.
Acknowledgment The author is grateful to Ms. Monica Brocco (Istituto Superiore di Sanita`) for the linguistic revision of the manuscript. References Beck, H.L., De Campo, J., Gogolak, C., 1972. In situ Ge (Li) and NaI (Tl) gamma-ray spectrometry. US Atomic Energy Commission Report HASL-258, New York. Bochicchio, F., Campos Venuti, G., Felici, F., Grisanti, A., Grisanti, G., Kalita, S., Moroni, G., Nuccetelli, C., Risica, S., Tancredi, F., 1994. Characterisation of some parameters affecting the radon exposure of the population. Radiat. Prot. Dosim. 56, 137–140. Bochicchio, F., Campos Venuti, G., Nuccetelli, C., Risica, S., Tancredi, F., 1996. Indoor measurements of thoron, radon and their decay products in a mediterranean climate area. Environ. Int. 22 (Suppl. 1), S633–S639.
Bochicchio, F., Nuccetelli, C., SETIL Working Group, 2004. A method to evaluate the contribution of building material to indoor gamma dose rate through outdoor measurements: preliminary results. Radiat. Prot. Dosim. 111, 413–416. Clouvas, A., Xanthos, S., Antonopoulos-Domis, M., 1998. Monte Carlo based method for conversion of in-situ gamma ray spectra obtained with a portable Ge detector to an incident photon flux energy distribution. Health Phys. 74, 216–230. Clouvas, A., Xanthos, S., Antonopoulos-Domis, M., 2000. Derivation of indoor gamma dose rate from high resolution in situ gamma ray spectra. Health Phys. 79, 274–281. Clouvas, A., Xanthos, S., Antonopoulos-Domis, M., 2001. Extended survey of indoor and outdoor terrestrial gamma radiation in Greek urban areas by in situ gamma spectrometry with a portable Ge detector. Radiat. Prot. Dosim. 94, 233–246. Clouvas, A., Xanthos, S., Antonopoulos-Domis, M., 2003. A combination study of indoor radon and in situ gamma spectrometry measurements in Greek dwellings. Radiat. Prot. Dosim. 103, 363–366. Clouvas, A., Xanthos, S., Antonopoulos-Domis, M., 2004. Radiological maps of outdoor and indoor gamma dose rates in Greek urban areas obtained by in situ gamma spectrometry. Radiat. Prot. Dosim. 112, 267–275. Clouvas, A., Xanthos, S., Antonopoulos-Domis, M., 2006. Simultaneous measurements of indoor radon, radon–thoron progeny and high-resolution gamma spectrometry in greek dwellings. Radiat. Prot. Dosim. 118, 482–490. Cresswell, A.J., Sanderson, D.C., White, D.C., 2006. 137Cs measurement uncertainties and detection limits for airborne gamma spectrometry (AGS) data analysed using a spectral windows method. Appl. Radiat. Isot. 64, 247–253. Cutshall, N.H., Larsen, I.L., 1986. Calibration of a portable intrinsic Ge gamma-ray detector using point sources and testing for field applications. Health Phys. 51, 53–59. Gering, F., Kiefer, P., Fesenko, S., Voigt, G., 2002. In situ gamma-ray spectrometry in forests: determination of kerma rate in air from 137Cs. J. Environ. Radioactive 61, 75–89. Guillot, L., 2001. Extraction of full absorption peaks in airborne gamma-spectrometry by filtering techniques coupled with a study of the derivatives. Comparison with the window method. J. Environ. Radioactive. 53, 381–398. Helfer, I.K., Miller, K.M., 1988. Calibration factors for Ge detectors used for field spectrometry. Health Phys. 55, 15–29. Hendriks, P.H., Limburg, J., de Meijer, R.J., 2001. Full-spectrum analysis of natural gamma-ray spectra. J. Environ. Radioactive. 53, 365–380. /http://isis2007.healthphysics.at/S. International Commission on Radiation Units and Measurements (ICRU), 1994. Gamma-ray spectrometry in the environment, ICRU Report 53, ICRU Publications, Bethesda, MD. Markkanen, M., 1995. Radiation dose assessment for materials with elevated natural radioactivity. Report STOK-STO 32, Finnish Centre for Radiation and Nuclear Safety, Helsinki, Finland. Medeiros, F.H., Yoshimura, E.M., 2005. Influence of soil and buildings on outdoor gamma dose rates in Sao Paulo, Brazil. Health Phys. 88, 65–70.
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Miller, K.M., 1984. A spectral stripping method for a Ge spectrometer used for indoor gamma exposure rate measurements. Report EML-419, Environmental Measurement Laboratory, USDOE, New York, NY. Miller, K.M., Beck, H.L., 1984. Indoor gamma and cosmic ray exposure rate measurements using a Ge spectrometer and pressurised ionisation chamber. Radiat. Prot. Dosim. 7, 185–189. Nuccetelli, C., Bolzan, C., 2001. In situ gamma spectroscopy to characterize building materials as radon and thoron sources. Sci. Total. Environ. 272, 355–360. Nuccetelli, C., Bolzan, C., Bochicchio, F., SETIL Working Group, 2005. A tentative method to evaluate the building material contribution to indoor gamma dose rate. In: McLaughlin, J.P., Simopoulos, S.E., Steinha¨usler, F. (Eds.), Radioactivity in the Environment, vol. 7. Elsevier, Oxford, pp. 1123–1127. Plamboeck, A.H., Nylen, T., Agren, G., 2006. Comparative estimations of 137Cs distribution in a boreal forest in northern Sweden using a traditional sampling approach and a portable NaI detector. J. Environ. Radioactiv. 90, 100–109.
Risica, S., Nuccetelli, C., 2001. Building materials as a source of population exposure to ionising radiation. Phys. Med. 17, 3–8. Risica, S., Bolzan, C., Nuccetelli, C., 2001. Radioactivity in building materials: room model analysis and experimental methods. Sci. Total Environ. 272, 119–126. Sanderson, D.C., Cresswell, A.J., Scott, E.M., Lang, J.J., 2004. Demonstrating the European capability for airborne gamma spectrometry: results from the eccomags exercise. Radiat Prot. Dosim. 109, 119–125. Svoukis, E., Tsertos, H., 2006. Indoor and outdoor in situ high-resolution gamma radiation measurements in urban areas of cyprus. Radiat. Prot. Dosim. (Advance access published on October 25, 2006). Tyler, A.N., 2004. High accuracy in situ radiometric mapping. J. Environ. Radioactiv. 72, 195–202. Wissmann, F., Saez Vergara, J.C., 2006. Dosimetry of environmental radiation— a report on the achievements of EURADOS WG3. Radiat. Prot. Dosim. 118, 167–175.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
In situ gamma spectroscopy in environmental research and monitoring Cristina Nuccetelli ` (National Institute of Health), Viale Regina Elena 299, 00161 Rome, Italy Istituto Superiore di Sanita
article info Keywords: Environmental radioactivity Gamma radiation Building materials Indoor exposure
abstract In situ gamma spectroscopy was introduced to determine the outdoor gamma dose rate from soil and to calculate the radionuclide concentration and the relative contribution to the dose rate. This paper reviews the most common and proven applications of in situ gamma spectroscopy—together with the most recent and innovative research outcomes obtained with this technique, particularly for its use indoors. Advantages and limitations of its utilization to assess environmental radioactivity—indoors and outdoors—are also discussed. & 2008 IAEA. Published by Elsevier Ltd. All rights reserved.
1. Introduction In situ gamma spectroscopy is a well-known technique introduced by Beck et al. (1972) to determine the concentration of natural and artificial radionuclides in soil, the relevant outdoor gamma dose rate in the air above, and the relative contribution of the 238U and 232Th series and 40K to the dose rate. This technique soon appeared as a powerful tool to provide rapid and spatially representative estimates of environmental radioactivity. It basically consists of the elaboration of the full absorption peak areas multiplied by ad hoc coefficients calculated according to two fundamental assumptions:
the source—in this case the soil—can be modeled as an infinite half-space;
the vertical distribution of radionuclides can be reasonably assumed (typically uniform distribution for natural radionuclides and exponential distribution for artificial ones). Under these conditions it is possible to use a standard point source calibration performed in the laboratory (Beck et al., 1972; Cutshall and Larsen, 1986). With this calibration and the coefficients elaborated following Beck’s method, the dose rate— produced by the unscattered and scattered fluence of gamma rays—can be estimated, and the radionuclide inventory calculated.
2. Capabilities and limitations of in situ gamma spectroscopy In the view of the above features, in situ gamma spectroscopy—even in its original release—can be a useful support to Fax: +39 649387075.
E-mail address: [email protected]
measure surface flat soils for many research or institutional activities, for example, to characterize sites in terms of natural background radiation, perform surveys to study sites contaminated by artificial radionuclides and/or NORM, or to measure nuclear weapon test fallout contamination. Further applications are the assessment of routine and accidental releases from nuclear facilities, and monitoring of soil contamination level in the different phases of environmental restoration projects. In short, in situ gamma spectroscopy is a very versatile and efficient tool for studying environmental radioactivity, but has some limits mainly associated to the need of a priori assumptions about the distribution of nuclides in the soil, an important source of uncertainties of activity concentration estimates and dose rate evaluations, since dose rate is obtained as a sum of different radionuclide contributions to unscattered and scattered gamma flux at the detector. Moreover, the original method cannot be used for all kinds of source geometry, e.g. indoors or urban outdoors, because it is not feasible to elaborate the build-up factors—taking into account the scattered gamma flux and necessary to determine the gamma dose rate from photopeaks in a recorded spectrum—or produce calibration curves from which to derive the radionuclide activity concentrations in the source.
3. Research activities Since the 1980s many research activities have aimed to improve and simplify the use of this technique. Specifically, a parametric calibration was elaborated, based on the efficiency and geometric dimensions of the detector (Helfer and Miller, 1988). Another improvement was the introduction of a stripping method that subtracts partial absorption and the contribution from cosmic ray events from a collected spectrum. With this procedure, in situ gamma spectroscopy could be applied to indoor gamma
0969-8043/$ - see front matter & 2008 IAEA. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2007.10.019
ARTICLE IN PRESS 1616
C. Nuccetelli / Applied Radiation and Isotopes 66 (2008) 1615–1618
dose rate estimates (Miller, 1984; Clouvas et al., 1998). An important step for the diffusion of this technique was the publication of ICRU Report 53 (ICRU, 1994) which addressed the main aspects of in situ gamma spectroscopy. This report paid special attention to airborne gamma spectrometry (AGS), the aerial technique used to monitor wide areas and map natural radioactivity, extending the use of the method to fission product surveys. In the following years many studies were devoted to further widen the use of this method and overcome the above limitations, rendering in situ gamma spectrometry ‘‘independent’’ of source geometry, in order to estimate the gamma dose rate and the dose contribution from the 238U and 232Th series, 40K and 137Cs in any measurement situation, and ‘‘independent’’ of the vertical distribution of radionuclides. This research task has yielded important outcomes such as indoor and outdoor build-up factors and the estimate of the actual radionuclide distribution in soil. These results have been obtained following two approaches: Monte Carlo simulation and the use of algorithms aimed to the direct elaboration of spectra. With these new methodologies the applications of in situ gamma spectroscopy have been improved and/or extended to indoor environments, and to outdoor environments not easily represented with a model, such as forests (Gering et al., 2002; Plamboeck et al., 2006), urban outdoor areas (Medeiros and Yoshimura, 2005) and large areas (Cresswell et al., 2006; Tyler, 2004; Guillot, 2001; Hendriks et al., 2001). In particular, indoor utilization provides interesting information on building materials, one of the most significant sources of population exposure to natural radionuclides (Risica et al., 2001; Risica and Nuccetelli, 2001). The following paragraphs will describe some research activities in this field.
4. In situ gamma spectroscopy indoors Early investigations regarding the indoor application of this technique started in the 1980s (Miller, 1984; Miller and Beck, 1984) and other methods and developments appeared in the 1990s. At present, with different approaches—e.g. Monte Carlo (Clouvas et al., 2000), elaboration of spectra (Bochicchio et al., 1994), computation plus room model (Markkanen, 1995), called ‘‘integrated method’’ (Nuccetelli and Bolzan, 2001)—the use of in situ gamma spectroscopy indoors allows the evaluation of the gamma dose and the relative contribution of the various nuclides to the total gamma dose rates. In some countries an indoor methodology was also applied to perform surveys in order to get not only information on population exposure from building materials, but also a more detailed description of the sources as well (Clouvas et al., 2001, 2004; Svoukis and Tsertos, 2006). Indoor applications of in situ gamma spectroscopy can also provide interesting information about building materials as sources of radon, thoron and gamma rays (Nuccetelli and Bolzan, 2001; Clouvas et al., 2003, 2006) and supply quantitative estimates about the activity concentrations of radionuclides in building materials. Finally, on the basis of average spectroscopy information on building materials, a methodology was developed to estimate the indoor gamma dose rate from measurements on the external wall of a building. This method, called ‘‘IN–OUT’’, is in a validation phase after very encouraging preliminary results. 4.1. An integrated method to characterize building materials in situ The method, described in detail elsewhere (Nuccetelli and Bolzan, 2001) and sketched in Fig. 1, was tested in two test rooms
of which we had a sample of the material they were built with, i.e. tuff. The agreement between the activity concentration estimates obtained with the ‘‘integrated method’’ and the laboratory gamma spectroscopy was very good. Encouraged by these results, we applied the method to other dwellings. With in situ gamma spectroscopy the ratio between radon decay products (RnDp) and 226 Ra—i.e. the emanation fraction e—in the building materials, and the relevant exhalation rate E could be estimated as well. From these parameters it was also possible to calculate the contribution of building material to 222Rn activity concentration CRn estimated. Table 1 shows the results obtained in eight dwellings in Rome. It is worth noting that only in dwelling no. 4—built with tuff and on the ground floor—the contribution of Rn from the soil appears to be significant and computable by the difference between the estimated values, obtained by accounting for only the building material contribution CRn estimated, and the total measured value CRn total measured. 4.2. IN– OUT method With this method the indoor gamma dose rate is estimated from the gamma dose rate measurements performed on an external wall (Bochicchio et al., 2004; Nuccetelli et al., 2005). The method consists of the evaluation of 226Ra, 222RnDP, 228AcDP and 40 K activity concentrations of the building materials—with an ad hoc algorithm—using the external wall measurements and data from in situ gamma spectroscopy, from which radiation field characteristics are derived. Since spectroscopy parameters are fairly constant for the same kind of building material and some kind of buildings, in this case masonry (Bochicchio et al., 1994, 1996; Clouvas et al., 1998, 2000), on this occasion we used spectroscopy data we already had for some dwellings in Rome. With the radionuclide content data, it was possible to estimate the indoor gamma dose rate by applying a room model (Markkanen, 1995), which needs information on the geometry and structure of the dwelling. Table 2 shows the results of the preliminary study. The power of the ‘‘IN–OUT’’ method to make accurate and precise estimates stems from the ratios between estimated and measured values of gamma dose rate. Obviously, this method can be very useful in epidemiological studies, where lack of exposure data resulting
CHARACTERISTICS OF THE ROOM -geometrical dimension of the room -density of walls, floor and ceiling -thickness of walls, floor and ceiling -detector position
γ DOSE RATE MEASUREMENTS high pressure ionisation chamber or plastic 3” x 3” scintillator
γ SPECTROMETRY in situ HPGe 26 % efficiency and 1.73 keV resolution
% contribution of 40K, 232Th and 226Ra to dose
Activity conc. ratios: CRa/CAc and CRa/CPb
ELABORATION WITH ROOM MODEL DRnDp, DTh, DK
- ACTIVITY CONCENTRATIONS OF 40K, 232Th,232RnDP AND 226Ra - 222Rn EMANATION and EXHALATION Fig. 1. Diagram of procedure and steps of the ‘‘integrated method’’.
ARTICLE IN PRESS C. Nuccetelli / Applied Radiation and Isotopes 66 (2008) 1615–1618
1617
Table 1 Results of the application of the ‘‘integrated method’’ in eight dwellings in Rome Dwell
Floor
Building material
CRn Dp (Bq kg
1
)
CAc-228 (Bq kg 1)
CK-40 (Bq kg
1
)
CRa-226 (Bq kg 1)
e emanation power
E (Bq m
2
h
1
)
CRn
CRn
estimated 3
measured 3
(Bq m 1 2 3 4 Office 5 6 7
5th 5th 5th Ground 1st 4th 3rd 4th
Tuff Tuff Tuff Tuff Tuff Concrete Concrete Concrete
152 198 219 144 166 51 117 86
327 355 445 313 299 86 210 170
1195 1603 1493 885 1243 543 917 665
219 269 294 204 235 53 137 105
0.31 0.26 0.25 0.29 0.29 0.05 0.15 0.18
179 196 217 254 301 3 33 30
116 89 104 161 158 2 15 21
)
total
(Bq m
)
124 135 140 338 130 40 58 86
Table 2 Preliminary results of the application of the ‘‘IN– OUT’’ method External wall building materials
Tuff/stone Concrete/bricks All
Number of dwellings
45 46 91
Estimated ‘‘IN–OUT’’ g dose rate/ measured g dose rate indoors
Measured g dose rate outdoors/ measured g dose rate indoors
Arith. mean
Min–Max
SD(%)
Arith. mean
Min–Max
SD(%)
1.00 1.05 1.03
0.82–1.24 0.79–1.32 0.79–1.32
9 12 11
1.09 1.07 1.08
0.71–1.97 0.64–2.44 0.64–2.44
26 33 30
from refusing or missing subjects can bias the survey results. In this sense, the estimates obtained from the direct use of the external wall measurements cannot assure an adequate level of accuracy and precision (see the last three columns in Table 2).
5. Conclusions The fundamental characteristic of in situ gamma spectroscopy is its capability of providing rapid and integrated measurements of the investigated environment and dose rate contributions of radionuclides. This makes in situ gamma spectroscopy a powerful tool for widely encompassing environmental monitoring (indoors, NORM, early warning, emergency, etc.), supported by ongoing research in this field. On the other hand, the interest in this technique as a tool for regulatory aims, is demonstrated by the periodic intercomparisons—e.g. EURADOS ‘02 (Wissmann and Saez Vergara, 2006), ECCOMAGS 02 (Sanderson et al., 2004), ISIGAMMA ‘05, in-situ INTERCOMPARISON SCENARIO ‘07 (http://isis2007.healthphysics. at/)—which can extend the use and improve the performance of this technique, and even consolidate its application in the regulatory decision-making process.
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