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Accelerator mass spectrometry as a powerful tool for the determination of 129I in rainwater
Applied Radiation and Isotopes 53 (2000) 81±85
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Accelerator mass spectrometry as a powerful tool for the determination of 129I in rainwater J.M. LoÂpez-GutieÂrrez a,*, H.-A. Synal b, M. Suter c, Ch. Schnabel d, M. GarcõÂ a-LeoÂn a a Facultad de FõÂsica, Universidad de Sevilla, Apdo. 1965, 41080, Sevilla, Spain Paul Scherrer Institut, C/O Institut fuÈr Teilchenphysik, ETH Honggerberg, CH-8093, ZuÈrich, Switzerland c Institut fuÈr Teilchenphysik, ETH Honggerberg, CH-8093, ZuÈrich, Switzerland d Zentrum fuÈr Strahlenshutz un RadiooÈkologie (ZSR), UniversitaÈt Hannover, Am Kleinen Felde 30, D-30167, Hannover, Germany b
Received 22 October 1999; accepted 31 December 1999
Abstract 129 I is a very long-lived radionuclide T1=2 15:7 106 years) that is present in the environment both because of natural and anthropogenic sources. Its environmental interest, for example, as a tracer of geological processes, makes it the research target of a growing scienti®c community. However, its detection in environmental samples by radiometric methods is very dicult because of its long half-life. In this work, we present the methodology developed for its detection by Accelerator Mass Spectrometry (AMS) in rainwater. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction The detection of long-lived radionuclides in environmental samples is interesting, for example, because it permits to trace geological processes like marine currents mixing, water masses movements, atmospheric gases exchanges, etc. However, they are sometimes very dicult to detect by radiometric methods because of their long half-life. In these cases, it is possible to use mass spectrometry techniques, which are independent of the radioactive properties of the interesting radioisotope. These techniques provide generally very good resolution, but this can be sensibly improved using Accelerator Mass Spectrometry (AMS). Here, an electrostatic accelerator is
* Corresponding author. Fax: +34-9545-52890. E-mail address: [email protected] (J.M. LoÂpez-GutieÂrrez).
used, and this fact permits to take advantage of nuclear properties (like stopping power) for the ®nal discrimination of the ions by their nuclear charge. One of the interesting long-lived radionuclides studied by AMS is 129 I: It is a b emitter Emax 152:4 keV) with T1=2 15:7 106 years. Its presence in nature is mainly due to ®ssion processes of 235 U and 239 Pu and spallation reactions on Xe on the atmosphere, among other minor production ways. In the last years, the amount of 129 I in the environment has been sensitively increased because of human nuclear activities: nuclear weapons tests, nuclear accidents and residues of nuclear power and reprocessing plants. From the environmental point of view, this radionuclide is very important, for example, as a tracer of marine current movements. This and other properties justify the necessity of developing techniques able to detect 129 I in natural samples. In this work, we present some results obtained by AMS in atmospheric samples
0969-8043/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 0 0 ) 0 0 1 1 6 - 0
82
J.M. LoÂpez-GutieÂrrez et al. / Applied Radiation and Isotopes 53 (2000) 81±85
Fig. 1. Schematic layout of the ETH/PSI AMS facility.
(air ®lters and rainwater) taken in Sevilla (Spain). In Section 2, the experimental system is presented. Section 3 will describe the sampling and radiochemical methods used and some results will be given and discussed in Section 4. The ®nal conclusions are summarized in Section 5.
2. Experimental system Measurements have been carried out at the AMS facility of the ETH/PSI Zurich (Fig. 1). Ions from the sample are extracted in a Cs+ sputter ion source. After that, a double (mass and energy) analysis is performed in the low energy side. In the tandem, set at a terminal voltage of 4.7 MV, ions change their charge state (originallyÿ1) by loosing electrons in the stripper, a narrow tube ®lled with Ar gas at low pressure. Molecules are also eliminated in the stripping process because of the electron loss. This means a very important background reduction ®lter. At the exit, the ion beam is composed of a certain distribution of charge states, from which the 5+ one is selected in the electrostatic de¯ector. The last magnetic de¯ector carries out the ®nal mass selection. Because of their mass 5 dierence, 127 I ions go straight to a Faraday Cup, 129 5 while the I are directed to the detection system. Because of the extremely low mass dierence, it is 5 possible that 128 Te ions enter the accelerator as ÿ 128 TeH and, after suering some scattering or charge 5 changing process, follow the same trajectory as 129 I in the high energy side (Kilius et al., 1987). 127 I ions may also suer this eect. This is the reason why the detection system must be able to discriminate 129 I from these molecular fragments. Traditionally, this problem is solved in AMS by
measuring both energy and energy loss in a DE ÿ E gas detector. In the case of 129 I, this is not possible, because the extremely low nuclear charge dierence between I and Te leads to a low dierence in stopping power also, which cannot be distinguished by gas detectors. In our case, the alternative consists of the measurement of the velocity of the ions. As their energies are very similar, the dierence in velocity will give immediate information about the dierence in mass. This measurement is carried out by a time-of-¯ight (TOF) spectrometer (Fig. 2) that has been especially developed for 129 I: This consists of two detectors (start and stop ) separated by 1.09 m that produce an electronic pulse when the ion passes through them. The time between the two pulses is the ¯ying time of the ion. Each of the TOF detectors includes a very thin carbon foil (3 mg/cm2) that emits secondary electrons when it is crossed by the ion. These electrons are accelerated to a pair of Microchannel Plates (MCP) that amplify the pulse. In addition, an ionization chamber with a double anode is coupled to the TOF spectrometer, that per-
Fig. 2. TOF spectrometer and ionization chamber.
J.M. LoÂpez-GutieÂrrez et al. / Applied Radiation and Isotopes 53 (2000) 81±85
mits to obtain information about energy and energy loss. This is necessary in order to eliminate interferences caused by lighter molecular fragments that enter the accelerator as molecules with mass 129 amu and break in the stripper. Some of these fragments get cine5 matic properties similar to 129 I (even the same velocity) and can then arrive to the ®nal detection system. The eciency of the system is determined by the angular straggling of the beam at the start foil and by the MCPs. Approximately 75% of the ions that pass the start detector produce a signal at the stop. The typical eciency of the MCPs is about 80%. The time resolution is about 400 ps, although 350 ps have been reached. This is enough to separate the 129 I and 128 Te peaks, as is shown in Fig. 3. A more detailed description of the detection system can be found in LoÂpez-GutieÂrrez et al. (1999). 3. Sampling and radiochemistry Traditionally, for 129 I, the ®nal AMS sample must be in the form of silver iodide (AgI). At least 1 mg of AgI (including 129 I and stable iodine) is necessary for an AMS measurement. Although this amount is very little, it makes it indispensable to extract and concentrate iodine in the case of environmental samples. So a
Fig. 3. Separation of
129
83
radiochemical process is required. In some cases, as it will be described now, even carrier addition (made of stable iodine) is necessary. 3.1. Atmospheric ®lters Sampling and sample preparation for atmospheric ®lters have been published elsewhere (LoÂpez-GutieÂrrez et al., 1999), so only a short description will be presented here. All the samples have been taken at the roof of the Faculty of Physics in Seville (37.48 N, 68 W). Air is passed by a pump (Mod. Busch, SU1004) through an tryethylenediamine activated charcoal ®lter (F&J Specialty Products). A paper ®lter is placed right before the charcoal ®lter, so only gaseous iodine is trapped in the charcoal ®lter. After extracting the charcoal from the cartridge, it is introduced in double-distilled water, together with the carrier as NaI solution. Two kinds of carrier have been used: Woodward carrier 129 I=127 I < 5 10 ÿ14 ) and commercial NaI (Fluka, ref. 71710, 129 127 I= I 2 10 ÿ13 ). Then, HNO3 (7 M) and NaNO2 are added in order to oxidize Iÿ to I2, so that it can be easily trapped by the charcoal. Adsorption yields are obtained by measuring the residual solution by ion chromatography, being typically around 85%. Once dry, the charcoal is introduced in a solution of
I, 128 Te and
127
I in a TOF spectrum.
84
J.M. LoÂpez-GutieÂrrez et al. / Applied Radiation and Isotopes 53 (2000) 81±85
Table 1 Comparison between dierent measurements of
129
I concentration in the air
Concentration (105 129 I at/m3)
Activity (10ÿ7 Bq/m3)
Place
Method
Year
Reference
9.423.5 (Average value) 27212 (Average value) 10402240 260280 8496 1513
0.01320.005 0.03820.017 1.420.3 0.3620.11 11.9 2.1
Sevilla (Spain) Several background zones in USA Richland, Wash Yavne (Israel) Karlsruhe, 0.7 km away from WAK Karlsruhe, 14 km away from WAK
AMS RNAA RNAA AMS RNAA RNAA
1993±1995 1965±1970 1965±1968 1986 1986 1986
This work Brauer et al. (1974) Brauer et al. (1974) Paul et al. (1987) Wershofen et al. (1991) Wershofen et al. (1991)
NaOH and NaHSO3, so that iodine is extracted and remains in the solution as iodide. After oxidation by NaNO2, iodine is extracted into chloroform. After that, iodine is back-extracted into aqueous solution by NaHSO3 and H2SO4 and precipitated as AgI by AgNO3. In order to introduce it in the ion source, AgI is mixed with Ag powder and pressed into a high purity tantalum holder. 3.2. Rainwater Rainwater samples were taken in the same place as air ®lters. Rainwater falling into a conic stainless steel funnel with a circular surface of 1 m2 is stored in a 25 l polyurethane container. Then, it is clear that both wet and dry deposition of 129 I are included in the analysis. Sampling is carried out monthly, except if the container is already full. In this case, a new sample is taken. If there has been no rain during a certain period, the funnel is washed with double-distilled water to get the dry deposition fraction. The radiochemical method used for the determination of the 129 I concentration in rainwater is very similar to that in atmospheric ®lters. Less than 1 l is necessary for an AMS sample. Approximately 3 mg iodine carrier is added to the sample. Then, NaOH and NaHSO3 are added in order to reduce iodine to iodide. After that, iodine is extracted into chloroform and back-extracted into water and precipitated as AgI by the addition of AgNO3 as in the case of charcoal ®lters.
from the literature. Only some of the results from Brauer et al. (1974) correspond to zones far away from the impact of nuclear facilities, and in fact, they are the only ones we have found in literature for this kind of zones. As could be expected, our results agree quite well with these, showing that Seville can be considered as one of the background zones. The concentrations in background zones are sensibly lower than in zones aected by nuclear facilities. This can be seen, for example, in the measurements carried out by Brauer et al. (1974) near the Hanford reprocessing plant or in those from Wershofen et al. (1991) near the Karlsruhe reprocessing plant (WAK). Also the Chernobyl accident provoked an increase of the background 129 I concentration in the air, as it can be observed in the sample taken by Paul et al. (1987) in Yavne (Israel) some days after the accident. Although the exhaustive analysis of the results is not ®nished, it is already possible to give some information about the time distribution of the gaseous 129 I concentration in the air in background zones. In Fig. 4, the time pro®le of the 129 I concentration is compared to the frequency (expressed as the fraction of the time) in which the wind in Seville comes from the ocean. Both curves clearly seem to be related. It can then be concluded that the oceans seem to behave as the main 129 I
4. Results and discussion 4.1. Atmospheric ®lters Results for the samples taken between March 1993 and March 1994 range from 4.3 105 129 I at/m3 (12.7%) to 15.6 105 129 I at/m3 (5.4%). The average value is (9.423.5) 105 129 I at/m3. In Table 1, our results are compared to other data
Fig. 4. Time variation of the gaseous air of Seville.
129
I concentration in the
J.M. LoÂpez-GutieÂrrez et al. / Applied Radiation and Isotopes 53 (2000) 81±85
85
developed and optimized. This detection system allows, with very good resolution, the discrimination of 129 I against serious interferences as 128 Te, and therefore, the sensitivity of the whole system is increased. Apart from this, radiochemical methods for the determination of 129 I in atmospheric ®lters and rainwater have been developed. These methods lead to ®nal AgI targets that are directly introduced in the ion source. Some applications of these techniques have been presented. The oceanic origin of gaseous 129 I in background zones has been shown. However, 129 I in rainwater is determined by the air-suspended matter.
Fig. 5. Time variation of the in rainwater from Seville.
129
I concentration and deposition
source in absence of anthropogenic sources. This is in good agreement with literature (Whitehead, 1984), in which the ocean is shown to be the most important source of stable iodine to the atmosphere. This fact also suggests a method to trace the movement of gaseous substances from the sea to inland zones. 4.2. Rainwater In Fig. 5, the time variation of the 129 I concentration in rainwater from Seville from January to July 1996 is shown. The mean concentration was 1.36 109 129 I at/l s 1:37 109 129 I at/l), except in one of the samples, which presented a concentration of 6:0020:08 1010129 I at/1. This means an activity of 1.9 mBq/l, which is also too low to be detected by radiometric methods. In Fig 5, the 129 I deposition, the precipitation rate and the suspended particles concentration are also shown. It can be seen that the deposition curve is determined by these two. This means that the 129 I in these samples could have a double origin: marine aerosols and particles resuspended from the land surface. 5. Conclusions In this work, the methodology for the determination of 129 I in atmospheric samples by Accelerator Mass Spectrometry (AMS) has been presented. A detection system consisting of a Time-of-Flight (TOF) spectrometer and a DE ÿ E ionization chamber has been
Acknowledgements J.M. LoÂpez-GutieÂrrez is deeply indebted to the Spanish Ministerio de EducacioÂn y Cultura, the Andalousian ConsejerõÂ a de EducacioÂn y Ciencia and to the Institute of Particle Physics of the ETH-Zurich and the Paul Scherrer Institut for the ®nancial support of this work and his stay at ETH. Thanks are due to Adrian Amman from EAWAG, DuÈbendorf, Switzerland, who helped us with IC measurements and to SoÈnke Szidat, from the ZSR at the University of Hannover, who performed some of these measurements.
References Brauer, F.P., Rieck, H.G., Hooper, R.L., 1974. Particulate and Gaseous Atmospheric Iodine Concentrations. IAEASM-181/6, pp. 351±366. Kilius, L.R., Rucklidge, J.C., Litherland, A.E., 1987. Accelerator mass spectrometry of 129I at isotrace. Nucl. Instrum. Meth. B29, 72±76. LoÂpez-GutieÂrrez, J.M., GarcõÂ a-LeoÂn, M., Schnabel, Ch., Schmidt, A., Michel, R., Synal, H.-A., Suter, M., 1999. Determination of 129 I in atmospheric samples by Accelerator Mass Spectrometry. Appl. Radiat. Isot. 51, 315±322. Paul, M., Fink, D., Hollos, G., Kaufman, A., Kutschera, W., Margaritz, M., 1987. Measurement of 129 I concentrations in the environment after the chernobyl reactor accident. Nucl. Instrum. Meth. B29, 341±345. Wershofen, H., Aumann, D.C., HuÈbschmann, W.G., 1991. Iodine-129 in the environment of a nuclear fuel reprocessing plant. Part VII: Concentrations and chemical forms of 129 I and 127 I in the atmosphere. J. Environ. Radioactivity 13, 93±101. Whitehead, D.C., 1984. The distribution and transformations of iodine in the environment. Env. Int. 10, 321±339.
www.elsevier.com/locate/apradiso
Accelerator mass spectrometry as a powerful tool for the determination of 129I in rainwater J.M. LoÂpez-GutieÂrrez a,*, H.-A. Synal b, M. Suter c, Ch. Schnabel d, M. GarcõÂ a-LeoÂn a a Facultad de FõÂsica, Universidad de Sevilla, Apdo. 1965, 41080, Sevilla, Spain Paul Scherrer Institut, C/O Institut fuÈr Teilchenphysik, ETH Honggerberg, CH-8093, ZuÈrich, Switzerland c Institut fuÈr Teilchenphysik, ETH Honggerberg, CH-8093, ZuÈrich, Switzerland d Zentrum fuÈr Strahlenshutz un RadiooÈkologie (ZSR), UniversitaÈt Hannover, Am Kleinen Felde 30, D-30167, Hannover, Germany b
Received 22 October 1999; accepted 31 December 1999
Abstract 129 I is a very long-lived radionuclide T1=2 15:7 106 years) that is present in the environment both because of natural and anthropogenic sources. Its environmental interest, for example, as a tracer of geological processes, makes it the research target of a growing scienti®c community. However, its detection in environmental samples by radiometric methods is very dicult because of its long half-life. In this work, we present the methodology developed for its detection by Accelerator Mass Spectrometry (AMS) in rainwater. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction The detection of long-lived radionuclides in environmental samples is interesting, for example, because it permits to trace geological processes like marine currents mixing, water masses movements, atmospheric gases exchanges, etc. However, they are sometimes very dicult to detect by radiometric methods because of their long half-life. In these cases, it is possible to use mass spectrometry techniques, which are independent of the radioactive properties of the interesting radioisotope. These techniques provide generally very good resolution, but this can be sensibly improved using Accelerator Mass Spectrometry (AMS). Here, an electrostatic accelerator is
* Corresponding author. Fax: +34-9545-52890. E-mail address: [email protected] (J.M. LoÂpez-GutieÂrrez).
used, and this fact permits to take advantage of nuclear properties (like stopping power) for the ®nal discrimination of the ions by their nuclear charge. One of the interesting long-lived radionuclides studied by AMS is 129 I: It is a b emitter Emax 152:4 keV) with T1=2 15:7 106 years. Its presence in nature is mainly due to ®ssion processes of 235 U and 239 Pu and spallation reactions on Xe on the atmosphere, among other minor production ways. In the last years, the amount of 129 I in the environment has been sensitively increased because of human nuclear activities: nuclear weapons tests, nuclear accidents and residues of nuclear power and reprocessing plants. From the environmental point of view, this radionuclide is very important, for example, as a tracer of marine current movements. This and other properties justify the necessity of developing techniques able to detect 129 I in natural samples. In this work, we present some results obtained by AMS in atmospheric samples
0969-8043/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 0 0 ) 0 0 1 1 6 - 0
82
J.M. LoÂpez-GutieÂrrez et al. / Applied Radiation and Isotopes 53 (2000) 81±85
Fig. 1. Schematic layout of the ETH/PSI AMS facility.
(air ®lters and rainwater) taken in Sevilla (Spain). In Section 2, the experimental system is presented. Section 3 will describe the sampling and radiochemical methods used and some results will be given and discussed in Section 4. The ®nal conclusions are summarized in Section 5.
2. Experimental system Measurements have been carried out at the AMS facility of the ETH/PSI Zurich (Fig. 1). Ions from the sample are extracted in a Cs+ sputter ion source. After that, a double (mass and energy) analysis is performed in the low energy side. In the tandem, set at a terminal voltage of 4.7 MV, ions change their charge state (originallyÿ1) by loosing electrons in the stripper, a narrow tube ®lled with Ar gas at low pressure. Molecules are also eliminated in the stripping process because of the electron loss. This means a very important background reduction ®lter. At the exit, the ion beam is composed of a certain distribution of charge states, from which the 5+ one is selected in the electrostatic de¯ector. The last magnetic de¯ector carries out the ®nal mass selection. Because of their mass 5 dierence, 127 I ions go straight to a Faraday Cup, 129 5 while the I are directed to the detection system. Because of the extremely low mass dierence, it is 5 possible that 128 Te ions enter the accelerator as ÿ 128 TeH and, after suering some scattering or charge 5 changing process, follow the same trajectory as 129 I in the high energy side (Kilius et al., 1987). 127 I ions may also suer this eect. This is the reason why the detection system must be able to discriminate 129 I from these molecular fragments. Traditionally, this problem is solved in AMS by
measuring both energy and energy loss in a DE ÿ E gas detector. In the case of 129 I, this is not possible, because the extremely low nuclear charge dierence between I and Te leads to a low dierence in stopping power also, which cannot be distinguished by gas detectors. In our case, the alternative consists of the measurement of the velocity of the ions. As their energies are very similar, the dierence in velocity will give immediate information about the dierence in mass. This measurement is carried out by a time-of-¯ight (TOF) spectrometer (Fig. 2) that has been especially developed for 129 I: This consists of two detectors (start and stop ) separated by 1.09 m that produce an electronic pulse when the ion passes through them. The time between the two pulses is the ¯ying time of the ion. Each of the TOF detectors includes a very thin carbon foil (3 mg/cm2) that emits secondary electrons when it is crossed by the ion. These electrons are accelerated to a pair of Microchannel Plates (MCP) that amplify the pulse. In addition, an ionization chamber with a double anode is coupled to the TOF spectrometer, that per-
Fig. 2. TOF spectrometer and ionization chamber.
J.M. LoÂpez-GutieÂrrez et al. / Applied Radiation and Isotopes 53 (2000) 81±85
mits to obtain information about energy and energy loss. This is necessary in order to eliminate interferences caused by lighter molecular fragments that enter the accelerator as molecules with mass 129 amu and break in the stripper. Some of these fragments get cine5 matic properties similar to 129 I (even the same velocity) and can then arrive to the ®nal detection system. The eciency of the system is determined by the angular straggling of the beam at the start foil and by the MCPs. Approximately 75% of the ions that pass the start detector produce a signal at the stop. The typical eciency of the MCPs is about 80%. The time resolution is about 400 ps, although 350 ps have been reached. This is enough to separate the 129 I and 128 Te peaks, as is shown in Fig. 3. A more detailed description of the detection system can be found in LoÂpez-GutieÂrrez et al. (1999). 3. Sampling and radiochemistry Traditionally, for 129 I, the ®nal AMS sample must be in the form of silver iodide (AgI). At least 1 mg of AgI (including 129 I and stable iodine) is necessary for an AMS measurement. Although this amount is very little, it makes it indispensable to extract and concentrate iodine in the case of environmental samples. So a
Fig. 3. Separation of
129
83
radiochemical process is required. In some cases, as it will be described now, even carrier addition (made of stable iodine) is necessary. 3.1. Atmospheric ®lters Sampling and sample preparation for atmospheric ®lters have been published elsewhere (LoÂpez-GutieÂrrez et al., 1999), so only a short description will be presented here. All the samples have been taken at the roof of the Faculty of Physics in Seville (37.48 N, 68 W). Air is passed by a pump (Mod. Busch, SU1004) through an tryethylenediamine activated charcoal ®lter (F&J Specialty Products). A paper ®lter is placed right before the charcoal ®lter, so only gaseous iodine is trapped in the charcoal ®lter. After extracting the charcoal from the cartridge, it is introduced in double-distilled water, together with the carrier as NaI solution. Two kinds of carrier have been used: Woodward carrier 129 I=127 I < 5 10 ÿ14 ) and commercial NaI (Fluka, ref. 71710, 129 127 I= I 2 10 ÿ13 ). Then, HNO3 (7 M) and NaNO2 are added in order to oxidize Iÿ to I2, so that it can be easily trapped by the charcoal. Adsorption yields are obtained by measuring the residual solution by ion chromatography, being typically around 85%. Once dry, the charcoal is introduced in a solution of
I, 128 Te and
127
I in a TOF spectrum.
84
J.M. LoÂpez-GutieÂrrez et al. / Applied Radiation and Isotopes 53 (2000) 81±85
Table 1 Comparison between dierent measurements of
129
I concentration in the air
Concentration (105 129 I at/m3)
Activity (10ÿ7 Bq/m3)
Place
Method
Year
Reference
9.423.5 (Average value) 27212 (Average value) 10402240 260280 8496 1513
0.01320.005 0.03820.017 1.420.3 0.3620.11 11.9 2.1
Sevilla (Spain) Several background zones in USA Richland, Wash Yavne (Israel) Karlsruhe, 0.7 km away from WAK Karlsruhe, 14 km away from WAK
AMS RNAA RNAA AMS RNAA RNAA
1993±1995 1965±1970 1965±1968 1986 1986 1986
This work Brauer et al. (1974) Brauer et al. (1974) Paul et al. (1987) Wershofen et al. (1991) Wershofen et al. (1991)
NaOH and NaHSO3, so that iodine is extracted and remains in the solution as iodide. After oxidation by NaNO2, iodine is extracted into chloroform. After that, iodine is back-extracted into aqueous solution by NaHSO3 and H2SO4 and precipitated as AgI by AgNO3. In order to introduce it in the ion source, AgI is mixed with Ag powder and pressed into a high purity tantalum holder. 3.2. Rainwater Rainwater samples were taken in the same place as air ®lters. Rainwater falling into a conic stainless steel funnel with a circular surface of 1 m2 is stored in a 25 l polyurethane container. Then, it is clear that both wet and dry deposition of 129 I are included in the analysis. Sampling is carried out monthly, except if the container is already full. In this case, a new sample is taken. If there has been no rain during a certain period, the funnel is washed with double-distilled water to get the dry deposition fraction. The radiochemical method used for the determination of the 129 I concentration in rainwater is very similar to that in atmospheric ®lters. Less than 1 l is necessary for an AMS sample. Approximately 3 mg iodine carrier is added to the sample. Then, NaOH and NaHSO3 are added in order to reduce iodine to iodide. After that, iodine is extracted into chloroform and back-extracted into water and precipitated as AgI by the addition of AgNO3 as in the case of charcoal ®lters.
from the literature. Only some of the results from Brauer et al. (1974) correspond to zones far away from the impact of nuclear facilities, and in fact, they are the only ones we have found in literature for this kind of zones. As could be expected, our results agree quite well with these, showing that Seville can be considered as one of the background zones. The concentrations in background zones are sensibly lower than in zones aected by nuclear facilities. This can be seen, for example, in the measurements carried out by Brauer et al. (1974) near the Hanford reprocessing plant or in those from Wershofen et al. (1991) near the Karlsruhe reprocessing plant (WAK). Also the Chernobyl accident provoked an increase of the background 129 I concentration in the air, as it can be observed in the sample taken by Paul et al. (1987) in Yavne (Israel) some days after the accident. Although the exhaustive analysis of the results is not ®nished, it is already possible to give some information about the time distribution of the gaseous 129 I concentration in the air in background zones. In Fig. 4, the time pro®le of the 129 I concentration is compared to the frequency (expressed as the fraction of the time) in which the wind in Seville comes from the ocean. Both curves clearly seem to be related. It can then be concluded that the oceans seem to behave as the main 129 I
4. Results and discussion 4.1. Atmospheric ®lters Results for the samples taken between March 1993 and March 1994 range from 4.3 105 129 I at/m3 (12.7%) to 15.6 105 129 I at/m3 (5.4%). The average value is (9.423.5) 105 129 I at/m3. In Table 1, our results are compared to other data
Fig. 4. Time variation of the gaseous air of Seville.
129
I concentration in the
J.M. LoÂpez-GutieÂrrez et al. / Applied Radiation and Isotopes 53 (2000) 81±85
85
developed and optimized. This detection system allows, with very good resolution, the discrimination of 129 I against serious interferences as 128 Te, and therefore, the sensitivity of the whole system is increased. Apart from this, radiochemical methods for the determination of 129 I in atmospheric ®lters and rainwater have been developed. These methods lead to ®nal AgI targets that are directly introduced in the ion source. Some applications of these techniques have been presented. The oceanic origin of gaseous 129 I in background zones has been shown. However, 129 I in rainwater is determined by the air-suspended matter.
Fig. 5. Time variation of the in rainwater from Seville.
129
I concentration and deposition
source in absence of anthropogenic sources. This is in good agreement with literature (Whitehead, 1984), in which the ocean is shown to be the most important source of stable iodine to the atmosphere. This fact also suggests a method to trace the movement of gaseous substances from the sea to inland zones. 4.2. Rainwater In Fig. 5, the time variation of the 129 I concentration in rainwater from Seville from January to July 1996 is shown. The mean concentration was 1.36 109 129 I at/l s 1:37 109 129 I at/l), except in one of the samples, which presented a concentration of 6:0020:08 1010129 I at/1. This means an activity of 1.9 mBq/l, which is also too low to be detected by radiometric methods. In Fig 5, the 129 I deposition, the precipitation rate and the suspended particles concentration are also shown. It can be seen that the deposition curve is determined by these two. This means that the 129 I in these samples could have a double origin: marine aerosols and particles resuspended from the land surface. 5. Conclusions In this work, the methodology for the determination of 129 I in atmospheric samples by Accelerator Mass Spectrometry (AMS) has been presented. A detection system consisting of a Time-of-Flight (TOF) spectrometer and a DE ÿ E ionization chamber has been
Acknowledgements J.M. LoÂpez-GutieÂrrez is deeply indebted to the Spanish Ministerio de EducacioÂn y Cultura, the Andalousian ConsejerõÂ a de EducacioÂn y Ciencia and to the Institute of Particle Physics of the ETH-Zurich and the Paul Scherrer Institut for the ®nancial support of this work and his stay at ETH. Thanks are due to Adrian Amman from EAWAG, DuÈbendorf, Switzerland, who helped us with IC measurements and to SoÈnke Szidat, from the ZSR at the University of Hannover, who performed some of these measurements.
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