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Time series data from routine in situ gamma spectroscopy measurements
Journal of Environmental Radioactivity 125 (2013) 81e85
Contents lists available at SciVerse ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Time series data from routine in situ gamma spectroscopy measurements Helmut W. Fischer*, Bernd Hettwig University of Bremen, Institute of Environmental Physics, Otto-Hahn-Allee 1, 28359 Bremen, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2013 Accepted 18 January 2013 Available online 6 February 2013
Time series of in situ gamma spectroscopy data from 6 sites, obtained over a period of 13 years as part of a routine surveillance program, have been investigated for variability, reproducibility and occurrence of trends. Natural isotopes (40K, 208Tl, 212Pb and 214Pb) show variability up to a factor of 2, with time patterns varying from site to site. At five (level) sites 137Cs values decreased at a rate higher than given by the physical half-life, consistent with literature data on migration of Cs. At one (downhill) site, an increase of 137 Cs with time was observed. The finding can be explained by erosion processes from uphill territories. The observed variations were larger than the experimental uncertainty, and the equipment long-term stability appeared to be satisfactory. It can be concluded that the obtained routine in situ data provide a valuable data pool with potential usefulness for scientific work. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: In situ gamma spectroscopy Erosion Cs-137 Routine measurements
1. Introduction Like several other countries, Germany conducts a routine surveillance program for environmental radioactivity. This so-called IMIS (“Integriertes Mess- und InformationsSystem”) (Weiss and Leeb, 1993) serves two purposes: first, to provide representative nationwide data on the status of “background” contamination with artificial radioisotopes, and second, to act as a reliable data source in case of a new contamination, i.e. to support decision-making in nuclear emergencies. In addition to laboratory measurements of relevant environmental media ranging from air to food, in situ measurements form an important part of the obtained data sets. Two kinds of measurement devices are used: about 1800 permanently operating detectors produce an almost real-time map of the ambient dose rate, and about 60 portable high resolution gamma spectrometers are used to detect nuclide-specific surface contamination and nuclide-specific ambient dose rates derived from these activity data (Weiler, 2012). Whilst the permanent ambient dose rate network is operated by one central institution (the Federal Office of Radiation Protection, BfS), the portable gamma spectrometers belong either to BfS, to the German Weather Service DWD or to one of the 16 federal German states and are thus being operated under very different conditions either at fixed sites or as mobile devices. Altogether, more than 1000 in situ gamma spectra are recorded per year at more than 200
* Corresponding author. Tel.: þ49 421 218 62761; fax: þ49 421 218 98 6276. E-mail address: hfi[email protected] (H.W. Fischer). 0265-931X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvrad.2013.01.012
locations distributed quite evenly over the territory of the country (Fig. 1). The question may arise whether the data produced with this equipment under variable conditions might be useful for more than emergency preparedness, i.e. whether the data satisfy scientific criteria like precision and reproducibility. A positive result would indicate that the huge data pool might be useful for scientific purposes like the investigation of nuclide migration in soil. This is of particular interest as the regulations for IMIS provide a data storage period of 30 years. In order to gain some more insight into the quality of routine data, time series of in situ spectroscopy results from the Bremen federal state situated in north-west Germany have been reviewed. 1.1. Experimental data pool In situ gamma spectroscopy within IMIS means the semiautomatic recording of uncollimated gamma spectra with a detector positioned at 1 m above ground (Fig. 2). The equipment is designed in a way that it can be operated by trained technical staff, i.e. it is not routinely run by scientists. Spectral data are converted into nuclide-specific dose rate values at 1 m height in mSv h1 and into surface contamination values in Bq m2. The conversions are performed applying predefined and system-specific factors and, for surface contamination, assuming a relaxation length within the soil of either 3 mm (for dry deposition) or 10 mm (for wet deposition). In the present case, measurements have been taken only during dry weather periods, so 3 mm is used for the relaxation length. The detection limit required by the IMIS regulations is 200 Bq m2 for
82
H.W. Fischer, B. Hettwig / Journal of Environmental Radioactivity 125 (2013) 81e85
Fig. 1. Map of Germany with all positions of recordings of in situ gamma spectra for IMIS in the year 2011. Source: IMIS.
60
Co, which serves as reference isotope. A detailed description of the method is given in the technical guidelines for the IMIS system (Messanleitungen, 1992). Whilst spectra are stored locally, data transferred to the IMIS network comprise just the obtained values for dose rate and surface activity for each identified radionuclide plus the relative standard error. Data for natural isotopes like 40K and Th and U decay chain members are included, where available.
Fig. 2. Exemplary setup for the recording of an in situ gamma spectrum. The hpGe detector is mounted on a tripod facing downwards. The associated electronics remain in the transport van.
For many years, the only artificial isotope detected in Germany by in situ gamma spectroscopy is 137Cs, which originates from both atmospheric bomb test fallout and Chernobyl emissions. This did not change after the Fukushima emissions, as the maximal depositions of I and Cs isotopes were in the range of some Bq m2 or below (Pittauerova et al., 2011). The equipment (typical for most IMIS stations) consists of a high purity germanium detector of 10% relative efficiency (15% since a detector replacement) and a spectral resolution of about 2 keV (FWHM) at 1332 keV, housed under an aluminum end cap and attached to a small, portable cryostat (7 L liquid nitrogen capacity, sufficient for 3e4 days of operation without refilling). The electronics (high voltage supply, amplifier, analog-to-digital converter, multichannel analyzer) is highly integrated into one small unit, and spectra are displayed and stored on a notebook computer. Efficiency calibration and quality assurance tests are done using 152 Eu and 133Ba point sources. Data transfer to IMIS’ central servers is performed via the GSM cell phone network. The complete equipment is battery-operated, with the option to obtain 12 V backup battery power from the transport vehicle. During the multi-year time period considered in this study, the germanium detector had to be exchanged twice due to defects, the portable computer was exchanged once (loss due to theft), and the system was operated by 3 different persons due to changes in laboratory personnel. We consider these numbers as not exceptional, but clearly the conditions are not ideal for long-term stability of a series of measurements.
H.W. Fischer, B. Hettwig / Journal of Environmental Radioactivity 125 (2013) 81e85
The data set investigated here comprises recordings over 13 years from 6 field sites which are normally visited twice a year. The sites included lawn, meadow, and uncultivated land and are all publicly accessible. No information is available about treatment of the areas, e.g. frequency of mowing or application of fertilizer. Five of the sites are level and the remaining one lies at the foot of a slope of about 15 m elevation. The time period of 13 years was chosen because the corresponding data was comparably easy to access, being stored on one computer system. Including data from earlier years would have required considerably more effort in data retrieval. Only the numerical data extracted from the spectra, as described above, are considered. 2. Results The data are presented as time series of surface activity for the artificial isotope 137Cs and the natural isotopes 40K, 208Tl, 212Pb (from the 232Th decay chain) and 214Pb (from the 238U decay chain). These isotopes could be readily detected in all spectra. Three data sets were chosen for illustration: 1) a “normal” data set, representative for the majority of the sites, showing quite constant values and 137Cs data decreasing with time, 2) an “abnormal” data set with a general negative trend in all isotope concentrations, and 3) an “interesting” data set, characterized by an increase of 137Cs activity with time. It should be noted that the calibration factor, which involves a relaxation length of 3 mm, is only applicable to recently deposited isotopes. Therefore, the activity data for the encountered “old” 137Cs are underestimated. This “old” Cs has migrated to deeper soil layers (Kirchner et al., 2009), resulting in stronger self-attenuation of the emitted gamma radiation within the soil. By contrast, activity values of the natural isotopes are overestimated when applied to the uppermost soil layer, as photons emitted from deeper soil layers
83
also contribute to the spectrum. Furthermore, the natural radioisotope content of soil cannot be expressed correctly in units of surface contamination, as these are distributed more or less homogeneously with depth. Consequently, the measured activity values in their present form should only be considered as relative, not as absolute numbers. 1) “normal” data set (Fig. 3) The data displayed in Fig. 3 show a typical variability of just under a factor of 2 for the natural isotopes 40K, 208Tl, 212Pb and 214 Pb. The soil at the measurement site was uncultivated, and, as previously mentioned, no additional information on the recent history of the territory was available. The reason for the variability of the data, which is considerably larger than the experimental uncertainty (expressed as one standard deviation), remains thus unexplained and must be regarded as an additional part of the uncertainty on long time scales. Generally, no clear trend with time is observable. The data for 137Cs show, apart from a scatter of similar amplitude, a steady decrease with time. A simple exponential fit yields an effective half-life of about 15 years, considerably shorter than the physical half-life of the isotope. The finding is not uncommon (Pröhl et al., 2006) and is consistent with a combined effect of radioactive decay, weathering to deeper soil layers and uptake by plants. As the physical relocation to deeper layers affects detection efficiency in in situ measurements, no absolute soil concentration values can be derived from the time series. The data set is characteristic for four of the six sites under investigation and similar conclusions could be drawn from all data sets. 2) “abnormal” data set (Fig. 4) The data shown in Fig. 4 are characterized by a general decrease of amplitude with time for all isotopes. Again, the variations within the single time series are larger than the experimental uncertainty
Fig. 3. Time series of nuclide-specific surface deposition derived from in situ gamma spectroscopy data, obtained at an uncultivated soil site. The right y-scale refers to 40K values, the left y-scale to all other isotopes. The dotted line represents a phenomenological exponential fit to the 137Cs data. Error bars represent the measurement uncertainty expressed as one standard deviation. Data are typical for four of the investigated sites.
84
H.W. Fischer, B. Hettwig / Journal of Environmental Radioactivity 125 (2013) 81e85
Fig. 4. Time series of nuclide-specific surface deposition derived from in situ gamma spectroscopy data, obtained at a grassland site.
and exceed a factor of 2 in some cases. The vegetation consists of lawn, with no available information on treatment and application of fertilizer. The most plausible explanation would be a general loss of minerals from the soil, possibly caused by lack of fertilizer.
quantitatively assess this effect (He and Walling, 1997). Again, data variability is close to a factor of 2.
3. Discussion 3) “interesting” data set (Fig. 5) Fig. 5 shows data from another uncultivated soil. Whilst the natural isotopes show similar behavior and variability as in Fig. 3, 137 Cs data appear to increase with time. This finding can probably be explained by the topographical situation of the study plot at the bottom of a descending slope. It is well known that 137Cs in soil can be subject to erosion and that concentration values may be used to
It might be tempting to use the data directly for comparison with literature data, but the obtained activity values are not correct due to calibration issues. Additional knowledge about the depth distribution of the radioisotopes would be needed, e.g. by using a different relaxation length for known old contaminations. For the natural isotopes a uniform distribution with depth might be assumed, which again would require a different calibration factor.
Fig. 5. Time series of nuclide-specific surface deposition derived from in situ gamma spectroscopy data, obtained at an uncultivated soil site situated at the foot of a slope.
H.W. Fischer, B. Hettwig / Journal of Environmental Radioactivity 125 (2013) 81e85
Further factors of uncertainty are introduced by soil water content and the thickness of the plant cover. It might be possible to introduce additional correction factors for these parameters, but this would complicate the routine measurement process. The changes in equipment and laboratory personnel seem have no visible effect on the data, as the obvious changes in deposition data follow different time patterns for different sites. It appears that the overall design of the IMIS in situ spectroscopy system provides long-term stability. As a consequence, time series data should also be reliable, the variations in the data then being attributable to environmental changes. The records for the natural isotopes appear very valuable because they may allow for the recognition of trends and external influences. By taking them into account, data sets which appear very variable at first sight might become plausible, whilst clearly inconsistent data sets might be recognized and excluded from further treatment. The observed decline in 137Cs data with time, with a time constant faster than the physical half-life, is supported by literature values. The concentration increase observed at one site, situated at the foot of a slope, becomes plausible when one considers the well-documented effect of erosion on 137Cs distribution. 4. Conclusions The presented data show that the accumulated in situ data can have some potential for further scientific evaluation. By taking into
85
account the omnipresent natural isotopes it should be possible to recognize stable data sets, or to recognize trends and external influences. This can open a path for the investigation of, for instance, isotope inventory, migration and erosion, be it on local, regional or national scales.
References He, Q., Walling, D.E., 1997. The distribution of fallout Cs-137 and Pb-210 in undisturbed and cultivated soils. Applied Radiation and Isotopes 48, 677e690. Kirchner, G., Strebl, F., Bossew, P., Ehlken, S., Gerzabek, M.H., 2009. Vertical migration of radionuclides in undisturbed grassland soils. Journal of Environmental Radioactivity 100, 716e720. Messanleitungen, 1992. Messanleitungen für die Überwachung der Radioaktivität in der Umwelt und zur Erfassung radioaktiver Emissionen aus kerntechnischen Anlagen. Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit, Bonn (in German). Available online: http://www.bmu.de/files/pdfs/allgemein/ application/pdf/strlsch_messungen_b01.pdf. Pittauerova, D., Hettwig, B., Fischer, H.W., 2011. Fukushima fallout in Northwest German environmental media. Journal of Environmental Radioactivity 102, 877e880. Pröhl, G., Ehlken, S., Fiedler, I., Kirchner, G., Klemt, E., Zibold, G., 2006. Ecological half-lives of 90Sr and 137Cs in terrestrial and aquatic ecosystems. Journal of Environmental Radioactivity 91, 41e72. Weiler, F., 2012. Gamma-Ortsdosisleistung und Radioaktivität auf der Bodenoberfläche. In: Umweltradioaktivität in der Bundesrepublik Deutschland e Bericht der Leitstellen des Bundes und des Bundesamtes für Strahlenschutz. Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit, Bonn, pp. 56e58 (in German). Weiss, W., Leeb, H., 1993. IMIS-the German integrated radioactivity information and decision support system. Radiation Protection Dosimetry 50, 163e170.
Contents lists available at SciVerse ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Time series data from routine in situ gamma spectroscopy measurements Helmut W. Fischer*, Bernd Hettwig University of Bremen, Institute of Environmental Physics, Otto-Hahn-Allee 1, 28359 Bremen, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2013 Accepted 18 January 2013 Available online 6 February 2013
Time series of in situ gamma spectroscopy data from 6 sites, obtained over a period of 13 years as part of a routine surveillance program, have been investigated for variability, reproducibility and occurrence of trends. Natural isotopes (40K, 208Tl, 212Pb and 214Pb) show variability up to a factor of 2, with time patterns varying from site to site. At five (level) sites 137Cs values decreased at a rate higher than given by the physical half-life, consistent with literature data on migration of Cs. At one (downhill) site, an increase of 137 Cs with time was observed. The finding can be explained by erosion processes from uphill territories. The observed variations were larger than the experimental uncertainty, and the equipment long-term stability appeared to be satisfactory. It can be concluded that the obtained routine in situ data provide a valuable data pool with potential usefulness for scientific work. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: In situ gamma spectroscopy Erosion Cs-137 Routine measurements
1. Introduction Like several other countries, Germany conducts a routine surveillance program for environmental radioactivity. This so-called IMIS (“Integriertes Mess- und InformationsSystem”) (Weiss and Leeb, 1993) serves two purposes: first, to provide representative nationwide data on the status of “background” contamination with artificial radioisotopes, and second, to act as a reliable data source in case of a new contamination, i.e. to support decision-making in nuclear emergencies. In addition to laboratory measurements of relevant environmental media ranging from air to food, in situ measurements form an important part of the obtained data sets. Two kinds of measurement devices are used: about 1800 permanently operating detectors produce an almost real-time map of the ambient dose rate, and about 60 portable high resolution gamma spectrometers are used to detect nuclide-specific surface contamination and nuclide-specific ambient dose rates derived from these activity data (Weiler, 2012). Whilst the permanent ambient dose rate network is operated by one central institution (the Federal Office of Radiation Protection, BfS), the portable gamma spectrometers belong either to BfS, to the German Weather Service DWD or to one of the 16 federal German states and are thus being operated under very different conditions either at fixed sites or as mobile devices. Altogether, more than 1000 in situ gamma spectra are recorded per year at more than 200
* Corresponding author. Tel.: þ49 421 218 62761; fax: þ49 421 218 98 6276. E-mail address: hfi[email protected] (H.W. Fischer). 0265-931X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvrad.2013.01.012
locations distributed quite evenly over the territory of the country (Fig. 1). The question may arise whether the data produced with this equipment under variable conditions might be useful for more than emergency preparedness, i.e. whether the data satisfy scientific criteria like precision and reproducibility. A positive result would indicate that the huge data pool might be useful for scientific purposes like the investigation of nuclide migration in soil. This is of particular interest as the regulations for IMIS provide a data storage period of 30 years. In order to gain some more insight into the quality of routine data, time series of in situ spectroscopy results from the Bremen federal state situated in north-west Germany have been reviewed. 1.1. Experimental data pool In situ gamma spectroscopy within IMIS means the semiautomatic recording of uncollimated gamma spectra with a detector positioned at 1 m above ground (Fig. 2). The equipment is designed in a way that it can be operated by trained technical staff, i.e. it is not routinely run by scientists. Spectral data are converted into nuclide-specific dose rate values at 1 m height in mSv h1 and into surface contamination values in Bq m2. The conversions are performed applying predefined and system-specific factors and, for surface contamination, assuming a relaxation length within the soil of either 3 mm (for dry deposition) or 10 mm (for wet deposition). In the present case, measurements have been taken only during dry weather periods, so 3 mm is used for the relaxation length. The detection limit required by the IMIS regulations is 200 Bq m2 for
82
H.W. Fischer, B. Hettwig / Journal of Environmental Radioactivity 125 (2013) 81e85
Fig. 1. Map of Germany with all positions of recordings of in situ gamma spectra for IMIS in the year 2011. Source: IMIS.
60
Co, which serves as reference isotope. A detailed description of the method is given in the technical guidelines for the IMIS system (Messanleitungen, 1992). Whilst spectra are stored locally, data transferred to the IMIS network comprise just the obtained values for dose rate and surface activity for each identified radionuclide plus the relative standard error. Data for natural isotopes like 40K and Th and U decay chain members are included, where available.
Fig. 2. Exemplary setup for the recording of an in situ gamma spectrum. The hpGe detector is mounted on a tripod facing downwards. The associated electronics remain in the transport van.
For many years, the only artificial isotope detected in Germany by in situ gamma spectroscopy is 137Cs, which originates from both atmospheric bomb test fallout and Chernobyl emissions. This did not change after the Fukushima emissions, as the maximal depositions of I and Cs isotopes were in the range of some Bq m2 or below (Pittauerova et al., 2011). The equipment (typical for most IMIS stations) consists of a high purity germanium detector of 10% relative efficiency (15% since a detector replacement) and a spectral resolution of about 2 keV (FWHM) at 1332 keV, housed under an aluminum end cap and attached to a small, portable cryostat (7 L liquid nitrogen capacity, sufficient for 3e4 days of operation without refilling). The electronics (high voltage supply, amplifier, analog-to-digital converter, multichannel analyzer) is highly integrated into one small unit, and spectra are displayed and stored on a notebook computer. Efficiency calibration and quality assurance tests are done using 152 Eu and 133Ba point sources. Data transfer to IMIS’ central servers is performed via the GSM cell phone network. The complete equipment is battery-operated, with the option to obtain 12 V backup battery power from the transport vehicle. During the multi-year time period considered in this study, the germanium detector had to be exchanged twice due to defects, the portable computer was exchanged once (loss due to theft), and the system was operated by 3 different persons due to changes in laboratory personnel. We consider these numbers as not exceptional, but clearly the conditions are not ideal for long-term stability of a series of measurements.
H.W. Fischer, B. Hettwig / Journal of Environmental Radioactivity 125 (2013) 81e85
The data set investigated here comprises recordings over 13 years from 6 field sites which are normally visited twice a year. The sites included lawn, meadow, and uncultivated land and are all publicly accessible. No information is available about treatment of the areas, e.g. frequency of mowing or application of fertilizer. Five of the sites are level and the remaining one lies at the foot of a slope of about 15 m elevation. The time period of 13 years was chosen because the corresponding data was comparably easy to access, being stored on one computer system. Including data from earlier years would have required considerably more effort in data retrieval. Only the numerical data extracted from the spectra, as described above, are considered. 2. Results The data are presented as time series of surface activity for the artificial isotope 137Cs and the natural isotopes 40K, 208Tl, 212Pb (from the 232Th decay chain) and 214Pb (from the 238U decay chain). These isotopes could be readily detected in all spectra. Three data sets were chosen for illustration: 1) a “normal” data set, representative for the majority of the sites, showing quite constant values and 137Cs data decreasing with time, 2) an “abnormal” data set with a general negative trend in all isotope concentrations, and 3) an “interesting” data set, characterized by an increase of 137Cs activity with time. It should be noted that the calibration factor, which involves a relaxation length of 3 mm, is only applicable to recently deposited isotopes. Therefore, the activity data for the encountered “old” 137Cs are underestimated. This “old” Cs has migrated to deeper soil layers (Kirchner et al., 2009), resulting in stronger self-attenuation of the emitted gamma radiation within the soil. By contrast, activity values of the natural isotopes are overestimated when applied to the uppermost soil layer, as photons emitted from deeper soil layers
83
also contribute to the spectrum. Furthermore, the natural radioisotope content of soil cannot be expressed correctly in units of surface contamination, as these are distributed more or less homogeneously with depth. Consequently, the measured activity values in their present form should only be considered as relative, not as absolute numbers. 1) “normal” data set (Fig. 3) The data displayed in Fig. 3 show a typical variability of just under a factor of 2 for the natural isotopes 40K, 208Tl, 212Pb and 214 Pb. The soil at the measurement site was uncultivated, and, as previously mentioned, no additional information on the recent history of the territory was available. The reason for the variability of the data, which is considerably larger than the experimental uncertainty (expressed as one standard deviation), remains thus unexplained and must be regarded as an additional part of the uncertainty on long time scales. Generally, no clear trend with time is observable. The data for 137Cs show, apart from a scatter of similar amplitude, a steady decrease with time. A simple exponential fit yields an effective half-life of about 15 years, considerably shorter than the physical half-life of the isotope. The finding is not uncommon (Pröhl et al., 2006) and is consistent with a combined effect of radioactive decay, weathering to deeper soil layers and uptake by plants. As the physical relocation to deeper layers affects detection efficiency in in situ measurements, no absolute soil concentration values can be derived from the time series. The data set is characteristic for four of the six sites under investigation and similar conclusions could be drawn from all data sets. 2) “abnormal” data set (Fig. 4) The data shown in Fig. 4 are characterized by a general decrease of amplitude with time for all isotopes. Again, the variations within the single time series are larger than the experimental uncertainty
Fig. 3. Time series of nuclide-specific surface deposition derived from in situ gamma spectroscopy data, obtained at an uncultivated soil site. The right y-scale refers to 40K values, the left y-scale to all other isotopes. The dotted line represents a phenomenological exponential fit to the 137Cs data. Error bars represent the measurement uncertainty expressed as one standard deviation. Data are typical for four of the investigated sites.
84
H.W. Fischer, B. Hettwig / Journal of Environmental Radioactivity 125 (2013) 81e85
Fig. 4. Time series of nuclide-specific surface deposition derived from in situ gamma spectroscopy data, obtained at a grassland site.
and exceed a factor of 2 in some cases. The vegetation consists of lawn, with no available information on treatment and application of fertilizer. The most plausible explanation would be a general loss of minerals from the soil, possibly caused by lack of fertilizer.
quantitatively assess this effect (He and Walling, 1997). Again, data variability is close to a factor of 2.
3. Discussion 3) “interesting” data set (Fig. 5) Fig. 5 shows data from another uncultivated soil. Whilst the natural isotopes show similar behavior and variability as in Fig. 3, 137 Cs data appear to increase with time. This finding can probably be explained by the topographical situation of the study plot at the bottom of a descending slope. It is well known that 137Cs in soil can be subject to erosion and that concentration values may be used to
It might be tempting to use the data directly for comparison with literature data, but the obtained activity values are not correct due to calibration issues. Additional knowledge about the depth distribution of the radioisotopes would be needed, e.g. by using a different relaxation length for known old contaminations. For the natural isotopes a uniform distribution with depth might be assumed, which again would require a different calibration factor.
Fig. 5. Time series of nuclide-specific surface deposition derived from in situ gamma spectroscopy data, obtained at an uncultivated soil site situated at the foot of a slope.
H.W. Fischer, B. Hettwig / Journal of Environmental Radioactivity 125 (2013) 81e85
Further factors of uncertainty are introduced by soil water content and the thickness of the plant cover. It might be possible to introduce additional correction factors for these parameters, but this would complicate the routine measurement process. The changes in equipment and laboratory personnel seem have no visible effect on the data, as the obvious changes in deposition data follow different time patterns for different sites. It appears that the overall design of the IMIS in situ spectroscopy system provides long-term stability. As a consequence, time series data should also be reliable, the variations in the data then being attributable to environmental changes. The records for the natural isotopes appear very valuable because they may allow for the recognition of trends and external influences. By taking them into account, data sets which appear very variable at first sight might become plausible, whilst clearly inconsistent data sets might be recognized and excluded from further treatment. The observed decline in 137Cs data with time, with a time constant faster than the physical half-life, is supported by literature values. The concentration increase observed at one site, situated at the foot of a slope, becomes plausible when one considers the well-documented effect of erosion on 137Cs distribution. 4. Conclusions The presented data show that the accumulated in situ data can have some potential for further scientific evaluation. By taking into
85
account the omnipresent natural isotopes it should be possible to recognize stable data sets, or to recognize trends and external influences. This can open a path for the investigation of, for instance, isotope inventory, migration and erosion, be it on local, regional or national scales.
References He, Q., Walling, D.E., 1997. The distribution of fallout Cs-137 and Pb-210 in undisturbed and cultivated soils. Applied Radiation and Isotopes 48, 677e690. Kirchner, G., Strebl, F., Bossew, P., Ehlken, S., Gerzabek, M.H., 2009. Vertical migration of radionuclides in undisturbed grassland soils. Journal of Environmental Radioactivity 100, 716e720. Messanleitungen, 1992. Messanleitungen für die Überwachung der Radioaktivität in der Umwelt und zur Erfassung radioaktiver Emissionen aus kerntechnischen Anlagen. Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit, Bonn (in German). Available online: http://www.bmu.de/files/pdfs/allgemein/ application/pdf/strlsch_messungen_b01.pdf. Pittauerova, D., Hettwig, B., Fischer, H.W., 2011. Fukushima fallout in Northwest German environmental media. Journal of Environmental Radioactivity 102, 877e880. Pröhl, G., Ehlken, S., Fiedler, I., Kirchner, G., Klemt, E., Zibold, G., 2006. Ecological half-lives of 90Sr and 137Cs in terrestrial and aquatic ecosystems. Journal of Environmental Radioactivity 91, 41e72. Weiler, F., 2012. Gamma-Ortsdosisleistung und Radioaktivität auf der Bodenoberfläche. In: Umweltradioaktivität in der Bundesrepublik Deutschland e Bericht der Leitstellen des Bundes und des Bundesamtes für Strahlenschutz. Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit, Bonn, pp. 56e58 (in German). Weiss, W., Leeb, H., 1993. IMIS-the German integrated radioactivity information and decision support system. Radiation Protection Dosimetry 50, 163e170.