Accelerator Mass Spectrometry of 36Cl and 129I Analytical Aspects and Applications

Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1 Accelerator Mass Spectrometry of 36Cl and 129I Analytical Aspects and Applications

BY VASILY ALFIMOV

ACTA UNIVERSITATIS UPSALIENSIS ISSN 1651-6214 UPPSALA ISBN 91-554-6124-7 2005 urn:nbn:se:uu:diva-4725 Dissertation presented at Uppsala University to be publicly examined in Häggsalen, The Ång- ström Laboratory, Uppsala, Friday, January 21, 2005 at 10:15 for the Degree of Doctor of Philosophy. The examination will be conducted in English.

Abstract Alﬁmov, V. 2005. Accelerator Mass Spectrometry of 36Cl and 129I: Analytical Aspects and Applications. Acta Universitatis Upsaliensis. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1. 81 pp. Uppsala. ISBN 91-554-6124-7

36 129 Two long-lived halogen radionuclides ( Cl, T1/2 = 301 kyr, and I, T1/2 = 15.7 Myr) have been studied by means of Accelerator Mass Spectrometry (AMS) at the Uppsala Tandem Labo- ratory. The 36Cl measurements in natural samples using a medium-sized tandem accelerator (∼1 MeV/amu) have been considered. A gas-filled magnetic spectrometer (GFM) was proposed for the separation of 36Cl from its isobar, 36S. Semi-empirical Monte-Carlo ion optical calculations were conducted to define optimal conditions for separating 36Cl and 36S. A 180° GFM was constructed and installed at the dedicated AMS beam line. 129I has been measured in waters from the Arctic and North Atlantic Oceans. Most of the 129I currently present in the Earth’s surface environment can be traced back to liquid and gaseous releases from the nuclear reprocessing facilities at Sellafield (UK) and La Hague (France). The anthropogenic 129I inventory in the central Arctic Ocean was found to increase proportionally to the integrated 129I releases from these reprocessing facilities. The interaction and origin of water masses in the region have been clearly distinguished with the help of 129I labeling. Predictions based on a compartment model calculation showed that the Atlantic Ocean and deep Arctic Ocean are the major sinks for the reprocessed 129I. The variability in 129I concentration measured in seawater along a transect from the Baltic Sea to the North Atlantic suggests strong enrichment in the Skagerrak–Kattegat basin. The 129I inventory in the Baltic and Bothnian Seas is equal to ∼0.3% of the total liquid releases from the reprocessing facilities. A lake sediment core sampled in northeastern Ireland was analyzed for 129I to study the history of the Sellafield releases, in particular the nuclear accident of 1957. High 129I concentration was observed corresponding to 1990 and later, while no indication of the accident was found. The results of this thesis research clearly demonstrate the uniqueness and future potential of 129I as a tracer of processes in both marine and continental archives.

Keywords: Accelerator Mass Spectrometry, iodine-129, chlorine-36, gas-ﬁlled magnet, radioactive tracer, Arctic Ocean, Nordic Seas, North Atlantic, thermohaline circulation, Sellaﬁeld, La Hague

Vasily Alﬁmov, Department of Materials Science, Ion Physics, Box 534, Uppsala University, SE-75121 Uppsala, Sweden

ISSN 1651-6214 ISBN 91-554-6124-7 urn:nbn:se:uu:diva-4725 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-4725) To my family

List of Papers

This thesis is based on the following papers, which are referred to in the text by their Roman numerals.

I V. Alfimov and G. Possnert (2004) Computer simulation of ion- beam optics in a gas-filled magnetic spectrometer. Nucl. Instrum. Methods Phys. Res. Sect. B, 223–224:180–184 © 2004 Elsevier II V. Alfimov, A. Aldahan, G. Possnert, and P. Winsor (2004) An- thropogenic 129I in seawater along a transect from the Norwegian Coastal Current to the North Pole. Mar. Pollut. Bull., 49(11–12):1097–1104 © 2004 Elsevier III V. Alfimov, A. Aldahan, and G. Possnert (2004) Tracing water masses with 129I in the western Nordic Seas in early spring 2002. Geophys. Res. Lett. 31:L19305 © 2004 American Geophysical Union IV V. Alfimov, A. Aldahan, G. Possnert, A. Kekli, and M. Meili (2004) Concentrations of 129I along a transect from the North At- lantic to the Baltic Sea. Nucl. Instrum. Methods Phys. Res. Sect. B, 223–224:446–450 © 2004 Elsevier V V. Alfimov, G. Possnert, and A. Aldahan (2004) 129I in the Arctic Ocean and Nordic Seas: numerical modeling and prognoses. Manuscript VI D. Gallagher, E. J. McGee, P. I. Mitchell, V. Alfimov, A. Alda- han, G. Possnert, (2004) A retrospective search for evidence of the 1957 Windscale fire in NE Ireland using 129I. Environ. Sci. Technol., in press © 2004 American Chemical Society

Reprints were made with permission from the publishers. I participated in the experiment, performed data analysis, and wrote papers 1–3 and 5. I evaluated the experimental data and wrote paper 4. In paper 6 I took part in sample preparation and was responsible for measurement and analysis of 129I part.

v vi Contents

1 Introduction ...... 1 1.1 Radiometric dating ...... 1 1.2 Accelerator mass spectrometry ...... 2 1.3 Objectives ...... 4 1.4 Outline ...... 4 2 Iodine and chlorine in nature ...... 5 2.1 Stable iodine (127I) ...... 5 2.2 Radioactive iodine (129I) and its applications ...... 8 2.3 Stable chlorine (35Cl and 37Cl)...... 13 2.4 Radioactive chlorine (36Cl) and its applications ...... 14 3 Analytical techniques ...... 17 3.1 Sampling and storage of seawater for 129I analysis ...... 17 3.2 Extraction of 129I from water samples ...... 17 3.3 Extraction of 129I from solid samples ...... 18 3.4 Extraction of 36Cl from water samples ...... 19 3.5 A tandem accelerator as a mass spectrometer ...... 20 3.6 Small AMS facilities ...... 23 3.7 The Uppsala AMS system ...... 24 3.7.1 Detectors at the AMS beam line ...... 26 3.7.2 The 129I measuring procedure ...... 30 3.7.3 Background correction ...... 30 4 Design of a gas-ﬁlled magnetic spectrometer for AMS ...... 31 4.1 Introduction ...... 31 4.2 Calculations ...... 32 4.3 Results and discussion ...... 35 4.4 Conclusions ...... 37 4.5 The Uppsala gas-ﬁlled magnetic spectrometer ...... 37 5 129I in marine waters ...... 41 5.1 129I in the Arctic Ocean in 2001 ...... 41 5.2 129I in the western Nordic Seas in 2002 ...... 47 5.3 129I along a transect from North Atlantic to the Baltic Sea in 1999 ...... 52 5.4 Numerical modeling of 129I ...... 54 6 129I in lake sediment ...... 57

vii 7 Conclusions ...... 59 8 Acknowledgements ...... 61 9 Summary in Swedish ...... 63 10 Appendix: Measuring procedure and calculations ...... 67 10.1 Measured isotopic ratios ...... 67 10.2 Normalized isotopic ratios ...... 68 10.3 Background correction ...... 69 10.4 Isotopic abundances ...... 70 10.5 129I concentration ...... 70

viii 1 Introduction

1.1 Radiometric dating In 1905, just 10 years after the discovery of radioactivity by Henri Becquerel and Marie and Pierre Curie, Ernest Rutherford proposed in a lecture at Har- vard University the idea of using U/He or U/Pb (parent/daughter) ratios and the law of radioactive decay to compute the age of rocks. At that time nothing was known of isotopes, and thus his idea concerned chemical dating based on ratios of elements without regard to their isotopes. Subsequent research by Bertram Boltwood and Arthur Holmes had by 1913 produced the ﬁrst chemical U/Pb estimate of the Earth’s age,1 1600 Ma, giving a birth to geochronology. The same year saw the advent of mass spectrometry when Joseph Thomson demonstrated the existence of isotopes with his parabola spectrograph. Further development of mass spectrometric technique by Fran- cis Aston, Arthur Dempster, Alfred Nier, and others, as well as the parallel accumulation of knowledge of uranium decay chains and lead isotopes had made the cumbersome chemical version of radiometric dating obsolete by the end of the 1930s. The development of the atomic bomb during World Word II gave a huge impulse to efforts to improve instrumentation used for identifying and analyzing isotopes. After the war one of the leading mass spectrometry experts of the Manhattan project, Alfred Nier, returned to academia, and by the end of 1940s his workshop had supplied dozens of mass spectrographs to other universities, making radiometric dating available to a great number of researchers. A new and truly revolutionary branch of radiometric dating, radiocarbon dating (14C), was introduced by Willard Libby and his students in 1947. It was based on the cosmic ray research of Serge Korff and others, who showed in 1939 that cosmic rays2 produce secondary neutrons in their initial collisions in the upper layers of the atmosphere [Libby, 1964]. Korff and his collabora- tors proposed that the principal way in which these secondary neutrons could disappear would be to form 14C by nuclear reaction with 14N nuclei. A few years later Libby suggested that this 14C could enter the food chain through photosynthesis, and become a “radioactive clock” for dating organic remains

1Today’s best estimate of the age of the Earth is 4550 ± 20 Ma (U/Pb isotopic age). 2Cosmic rays are high energy particles, mainly protons, that bombard the Earth from anywhere beyond its atmosphere (discovered by Viktor Hess in 1912).

1 after the death of a plant or animal [Libby, 1964]. He and his group developed a gas counting technique, showed that latitudinally uneven production of 14C was compensated for by atmosphere mixing, and dated several arti- facts of known age. The 14C age estimates produced were equal or close to the known ages, thus conﬁrming the usability of the method for the dating of organic materials [Libby, 1964]. As a result, by the end of 1940s, radiometric dating had already been applied to both primordial and cosmogenic radionuclides, and could potentially cover time scales from 200–50,000 yr (organic materials, 14C) to 10–4,500 Myr (minerals, 238U–206Pb, 235U–207Pb). Following decades witnessed a boom in radiometric dating using both cosmogenic and primordial radionuclides. New isotopes, new applications, and new instrumentation reﬁnements were proposed. Anthropogenic radionuclides literally became part of the modern world after the start of nuclear explosions and the reprocessing of nuclear fuel wastes. These radionuclides provided new means of dating and tracing various objects and processes in the environment. Today we have better knowledge of our planet and its history mainly thanks to radiometric dating.

1.2 Accelerator mass spectrometry A remarkable improvement in radiometric instrumentation was proposed in 1977. The major radiometric techniques at that time, decay counting and mass spectrometry, had limited success in dating using long-lived cosmogenic ra- 14 = dionuclides (with half-lives longer than that of C, T1/2 5730 yr). The main disadvantage of decay counting comes from the dependence of decay counting statistics on integrated sample activity, that is, on the amount of a radionuclide in a sample and the half-life of the radionuclide. For the same amount of material a longer half-life of the radionuclide used in dating by decay counting requires longer measurement time, or that a larger sample has to be used. On the other hand, the most established mass spectrometry in geochronology, thermal ionization mass spectrometry, does not offer a solution here, since its maximum ppm isotopic sensitivity (10−6, e.g. Finnigan TRITON), complicated by molecular and isobaric interferences, is unsuitable for determining the isotopic abundances of long-lived cosmogenic radionuclides (> 10−11). The limitations were overcome in 1977, when Richard Muller proposed using an accelerator as a high energy mass spectrometer3 for the general problem of trace element detection and for radioisotope dating

3The ﬁrst use of the technique should be credited to Alvarez and Cornog [1939], who tuned a cyclotron into a mass spectrograph to investigate the stability of 3He. The potential of the technique was not recognized at the time for lack of applications.

2 in particular [Muller, 1977]. He pointed out that this “can greatly increase the maximum age that can be determined while simultaneously reducing the size of sample required,” and considered measuring the isotopic ratios 14C/C, 10Be/Be, and 3H/H using this technique. Muller’s proposal provided a frame- work for the development of accelerator mass spectrometry (AMS). By accel- erating ions to high energies and using the detection techniques of nuclear physics, AMS enables the unique identiﬁcation of single ions at low isotopic abundances (10−16). Additional advantages come from the use of a tandem accelerator. First, differences in the efﬁciency of negative ion formation for various elements provide a means of isobar separation at the ion source, e.g. in the cases of 14C and 129I, a means which is impossible for mass spectrometry itself. Second, the selection of highly charged positive ions at the high energy side removes molecular interferences, because they dissociate through “Coulomb explosion.” More technical details on the method will be given in Chapter 3. AMS has made possible radiometric dating with long-lived radionuclides. For example, in the case of 14C with its “intermediate” half- life, AMS allows a three order of magnitude reduction in sample size (mg vs. g) and shorter measurement times (minutes vs. hours), while still being capable of the determination of 14C/C isotopic ratios in the order of 10−16– 10−10. These criteria (reduction in sample size and measurement time) are even more crucial for radioisotopes with extremely long half-lives for which decay counting is impractical. Shortly after the publication of the original proposal, AMS was successfully proven to be suitable for measurements of isotopic abundances of several long-lived radionuclides: 14C/C[Bennett et al., 1977; Nelson et al., 1977], 10Be/Be [Raisbeck et al., 1978], 26Al/Al [Raisbeck et al., 1979], 36Cl/Cl [Elmore et al., 1979], 129I/I[Elmore et al., 1980], 41Ca/Ca [Raisbeck et al., 1981]. Today over 60 AMS facilities exist around the world. Measurements of 14C are performed at almost all AMS laboratories, while 10Be is measured at 25 laboratories, 129I at 18 laboratories, 26Al at 15, and 36Cl at 12. Other isotopes are less commonly used in AMS. The major applications of AMS are in: • Use of long-lived natural and anthropogenic radionuclides as tracers and chronometers in archeology and the earth sciences. • Labeling and microdosing (µg/kg doses) in biomedicine with long- lived tracers while also reducing radiation exposure. • Stable isotope detection at low concentrations (ppb and less) in the material and environmental sciences.

3 1.3 Objectives The goal of this research is to use the isotopic abundances of long-lived halogen radionuclides (36Cl, 129I) present in natural samples to study the pathways, rates, and time scales of processes in the Earth’s environment. The first objective is to initiate an 129I measuring procedure at the newly installed Uppsala tandem accelerator. The reproducibility and background of measurements have to be investigated. The second objective is to continue the earlier 129I research program with oceanographic and environmental applications: • Continue monitoring of 129I contamination in oceanic waters of the Arctic and North Atlantic, and compare 129I distribution with that of other tracers. The data from the following recent expeditions was studied: Tundra Northwest 1999, Arctic Ocean 2001, and Arc- tic Ocean 2002. • Explore the extent and sources of 129I contamination in the Baltic and Bothnian Seas. • Initiate research into 129I distributions in sediments. Study a lake sediment sampled in northeastern Ireland and trace the history of Sell- afield releases, in particular the nuclear accident of 1957. The third objective is to study the general problem of measuring 36Cl isotopic abundances in natural samples with a medium-sized tandem accelerator (∼1 MeV/amu). Technical constraints have to be defined. Necessary instru- mental improvements (detectors, ion optical dispersive elements, etc) as well as the development of chemical procedures for sample preparation have to be considered.

1.4 Outline Chapter 2 contains the geochemical background of the studied long-lived radionuclides, 36Cl and 129I, and their stable counterparts. Sources, pathways, and typical concentrations are presented. Chapter 3 describes the analytical procedure and instrumentation used to measure 36Cl and 129I concentrations. The newly installed gas-ﬁlled magnet, designed as a part of this thesis research to improve the detection of 36Cl at the Uppsala AMS facility, is presented in Chapter 4. The last section of the thesis, consisting of Chapters 5 and 6, is focused on applications of 129I AMS for oceanographical, environmental, and climatic studies. The major chapters are followed by conclusions and acknowledgements.

4 2 Iodine and chlorine in nature

This chapter contains a brief overview of the geochemical cycles of iodine and chlorine as well as an introduction into applications of 129I and 36Cl. Both iodine and chlorine are members of the halogen group of elements and tend to complete1 their outer electron shell. Of all halogens, iodine has the largest number of electrons in its shells, and therefore exhibits the weakest halogen characteristics, e.g. I− easily loses an acquired electron to become the I2 molecule [CRC, 2004].

2.1 Stable iodine (127I) Iodine has only one stable isotope, 127I. Iodine is a biophilic element and most highly enriched in marine plants (up to 10,000 mg kg−1)[Fuge and Johnson, 1986; Hou et al., 2000b]. Soils are generally rich in iodine (4–8 mg kg−1), and most of it is associated with organic matter. Seawater has a mean iodine content of about 60 µgl−1, while fresh surface waters have lower and variable levels (1–20 µgl−1). Rain typically contains 1–10 µgl−1. The compartment model of the global iodine cycle is presented in Figure 2.1. It is possible to infer from the global iodine cycle (Figure 2.1) that there is a sub-cycle of mobile iodine between the ocean and ocean atmosphere compartments. In addition, this system has side branches that are an order of magnitude weaker: deep ocean compartment ↔ ocean sediments, and ocean atmosphere compartment → land atmosphere → surface soil region → ocean mixed layer. Most of the iodine is found in the sedimentary and igneous rock compartments (6 × 1018 g, not shown in Figure 2.1), but both these reservoirs are virtually barred from interaction with other compartments due to low fluxes from/to them (< 109 gyr−1) and due to the “buffer” effect of the deep ocean. Therefore the main reservoir of mobile iodine (with time constant, 104 yr) is the ocean compartment, which pumps some of its iodine into the atmosphere. A tenth of the ocean ↔ ocean atmosphere flux is trans- ported from the ocean atmosphere to the land atmosphere and then falls on the Earth’s surface soils, waters, and vegetation. Soils comprise the second- largest reservoir of iodine. An iodine flux drained from the soils compartment

1Electron afﬁnities of iodine and chlorine are 3.059 and 3.617 eV, respectively.

5 into the ocean is similar in amount to the flux of iodine from the atmosphere to the soils. Iodine occurs in natural water systems mainly as iodide (I−) and iodate − (IO3 ) with a minor addition of organic iodine [Wong, 1991]. Iodate is more thermodynamically stable than iodide is in seawater,2 but in surface ocean waters up to 50% of iodate becomes reduced to iodide [Wong, 1991]. The exact processes behind this transformation are still unknown, but it seems to be connected with biological activity of bacteria and phytoplankton. Seaweeds and phytoplankton accumulate iodide, modify it into organic iodine, and release it under stress mainly in the form of CH3I[Baker et al., 2001; Whitehead, 1984]. The other contribution to the sea–air flux of iodine might be driven by the oxidation of iodide into elemental iodine or HOI upon reaction with atmospheric ozone in the marine boundary layer [Wong, 1991; Whitehead, 1984]. In the troposphere, after a period lasting from several minutes (CH2I2) to 2–6 days (CH3I), both organic and inorganic iodine species undergo pho- todissociation to form elemental iodine [Roehl et al., 1997]. In turn, elemental iodine reacts with ozone to generate IO radicals. The subsequent reactions of IO establish a reactive iodine pool (IOx, HOI, ION2,I2O2) in the atmosphere. Little is known of the behavior of these species, although it was suggested that some of them interact with O3,HxO, and NOx and lead to, for example, the reduction of atmospheric O3 [Wong, 1991]. Removal of reactive iodine from the atmosphere occurs via dry (aerosols) and wet (rain and snow) deposition. The residence time of iodine species in the atmosphere is about two − − weeks [Whitehead, 1984]. One more process that extracts I and IO3 from the oceans is transport by sea spray in sea salt particles, which is important in coastal regions [Heumann, 1993]. After fallout, part of the iodine enters surface waters, whereas the other part becomes fixed in the organic matter of upper soils. Plants accumulate iodine from rain and drainage waters and enrich soils in iodine when they die [Fuge and Johnson, 1986]. There is a dependence between the iodine − content of soils and soil pH: acidic soils tend to convert I into I2, and lose iodine to atmosphere, whereas alkaline soils retain iodine as non-volatile iodate. Volatilization and, to lesser extent, leaching are the major processes of iodine loss from soils [Whitehead, 1984; Fuge and Johnson, 1986; Fuge, 2002]. While mobile in the hydrosphere and the atmosphere, iodine is immobile in the biosphere [Fuge and Johnson, 1986]. Marine plants accumulate iodine up to 1% or 10 g kg−1 of their dry weight; iodine is supposed to be used in their structural fabric [Fuge and Johnson, 1986]. Fish and mollusks have ∼1mgkg−1, and similar values are observed in terrestrial plants. Iodine is not believed to have any metabolic function in terrestrial plants [Fuge and

2Except for under anoxic conditions, as in the Black Sea or Baltic Sea, where iodide is the predominant form of iodine [Wong, 1991].

6 1.2×1011 1.6×109 OCEAN g/yr LAND g/yr ATMOSPHERE ATMOSPHERE 8.3×1010 g 2.0×1010 5.7×109 g TERRESTRIAL g/yr BIOSPHERE 1.6×1010 3.0×1011 g 1.9×1012 2.0×1012 1.0×1011 g/yr g/yr g/yr g/yr 8.2×1010 1.4×1010 OCEAN g/yr SURFACE SOIL g/yr MIXED LAYER REGION 1.4×1015 g 4.2×1014 g

7.2×1013 7.2×1013 1.5×1010 1.5×1010 g/yr g/yr g/yr g/yr

SHALLOW DEEP SUBSURFACE OCEAN 16 REGION

3725 m8.1×10 g 75 m 1.4×1013 g

2.9×108 1.8×1011 1.8×1011 2.9×108 g/yr g/yr g/yr g/yr

RECENT DEEP OCEAN SUBSURFACE SEDIMENTS REGION 8.9×1017 g 1.1×1013 g

HYDROSPHERE LITHOSPHERE

Figure 2.1: The compartment model of the global iodine cycle [Kocher, 1981].

7 Johnson, 1986]. In the case of mammals, iodine is an essential nutrient due to its presence in thyroid hormones [Whitehead, 1984].

2.2 Radioactive iodine (129I) and its applications 129 = . β There is only one long-lived radionuclide of iodine, I(T1/2 15 7 Ma, - decay to 129Xe, decay energy: 0.194 MeV). 129I and 127I (stable iodine) are expected to have similar geochemical cycles. The main differences between their cycles arise from 129I production (sources) and decay. The half-life of 129I is longer than any characteristic time (∼inventory/ﬂux) of the stable iodine cycle (Figure 2.1) with except for the sedimentary and igneous rock compartments (not shown in the ﬁgure). Therefore radioactive decay can be ex- cluded from consideration for time periods typical of the mobile iodine system (up to 104 years).

a) Liquid releases b) Atmospheric releases 400 10 Total = Sellafield + La Hague La Hague Sellafield 8 300 I (kg) 6 129 200 4 (estimated)

100 Releases of 2 (estimated) 0 0 1950 1960 1970 1980 1990 2000 2010 1950 1960 1970 1980 1990 2000 2010 t (year)

Figure 2.2: Diagram of liquid and atmospheric releases of 129I from nuclear reprocessing facilities in Sellaﬁeld, UK, and La Hague, France. Data is taken from a compilation by López-Gutiérrez et al. [2004], which is based on multiple sources [Gray et al., 1995; GRNC, 1999; Raisbeck and Yiou, 1999; Jackson et al., 2000; BNFL, 1999–2002]. The plot is expanded for 2001–2003 with discharge reports from http://www.bnﬂ.co.uk and http://www.cogemalahague.com.

Sources. 129I is naturally produced by the cosmic-ray-induced spallation of xenon in the atmosphere, and by the spontaneous fission of 238U and the neutron-induced fission of 235U inside the Earth [e.g. Kilius et al., 1992; Geyh and Schleicher, 1991]. Two minor sources of 129I in the terrestrial environment are meteoritic debris and neutron capture by 128Te. It should be noted that our knowledge of the contributions of fission and spallation is uncertain,

8 80°N

WSC 70°N EGC Greenland Nordic Seas

Iceland NWAC NCC Scandinavia

North Sea

30°W Sellafield La Hague 20°W 10°E 10°W 0°

Figure 2.3: Map showing the nuclear reprocessing facilities at Sellafield and La Hague and the main currents in the North Atlantic and Nordic Seas [Hansen and Osterhus, 2000]. Red arrows are the surface input of 129I. WSC, West Spitzbergen Current; EGC, East Greenland Current; NwAC, Norwegian Atlantic Current; NCC, Norwegian Coastal Current. and the contributions could vary by an order of magnitude [Kilius et al., 1992]. In the 1950s civil and military nuclear activities started to contribute anthropogenic 129I to the environment. Since that time, releases of anthropogenic 129I have outweighed all natural sources in the Earth’s surface environment. Today anthropogenic 129I comprises over 99% of the mobile 129I pool. Most of this 129I(>90%) is released into the environment from the nuclear reprocessing facilities located at Sellafield, UK, and La Hague, France, while the rest comes from other nuclear reprocessing facilities, nuclear power plants, and nuclear bomb tests (Table 2.1 and Figures 2.3 and 2.2). The release of anthropogenic 129I has pushed the 129I system out of natural equilibrium, and it will take thousands of years for the mobile iodine pool and millions of years for the whole system to return to a steady state. The compartment model of the global 129I cycle developed by Fabryka- Martin et al. [1985] for pre-anthropogenic conditions is presented in Fig- ure 2.4. Natural production and decay of 129I are not shown in the figure, but were included in calculating the steady state inventories and fluxes of 129I presented in the figure. The largest compartment of both radioactive and stable mobile iodine is the deep ocean, which therefore defines the homogeneous pre-anthropogenic 129I/127I ratio as equal to ∼ 10−12 [Moran et al., 1998; Fehn et al., 1986] throughout the mobile iodine pool and in recent marine

9 3.7×10-3 1.5×10-3 g/yr g/yr 7.7×10-2 1.0×10-3 OCEAN g/yr LAND g/yr ATMOSPHERE ATMOSPHERE 5.3×10-2 g 1.3×10-2 3.7×10-3 g TERRESTRIAL g/yr BIOSPHERE 1.0×10-2 2.0×10-1 g 1.2 1.3 6.5×10-2 g/yr g/yr g/yr g/yr 5.3×10-2 9.1×10-3 OCEAN g/yr SURFACE SOIL g/yr MIXED LAYER REGION 9.0×102 g 2.7×102 g

4.6×101 4.6×101 1.6×10-2 9.8×10-3 1.9×10-4 g/yr g/yr g/yr g/yr g/yr SHALLOW DEEP SUBSURFACE OCEAN 4 REGION

3725 m5.2×10 g 75 m 1.5×101 g

1.2×10-1 8.1×10-2 4.4×10-3 g/yr g/yr g/yr

RECENT DEEP OCEAN SUBSURFACE SEDIMENTS REGION 4.1×105 g 1.7×102 g

1.4×10-2 4.1×10-3 g/yr g/yr 6.2×10-3 SEDIMENTS g/yr 2.2×106 g

5.3×10-4 g/yr 5.2×10-3 Not estimated IGNEOUS g/yr ACTIVITY 5.2×107 g

Figure 2.4: The compartment model of the global 129I cycle before the 1950s [Fabryka-Martin et al., 1985], based on the work of Kocher [1981]. The model was calculated by Fabryka-Martin et al. [1985] using their estimate of the 129I/I ratio in the deep ocean, 6.4 × 10−13. 129I production and decay rates are not shown.

10 Table 2.1: Estimated releases of 129I to the environment from documented anthropogenic sources. Source 129I 129I Type Ref. (×1026 atoms) (kg)

Nuclear weapons testing 2–7 50–150 Air (1,2,3) 1945–1970s Chernobyl accident, 1986 0.28 6 Air (*) Nuclear reprocessing facilities: Hanford, USA 12 260 Air (4) 1944–1972 Sellaﬁeld, UK 8 170 Air (5,6,7,8) 1952–2000 46 980 Liquid (5,6,7) La Hague, France 3 65 Air (5,9) 1966–2000 110 2320 Liquid (5,9) Note: Total liquid releases: 3300 kg until 2000. Pre-anthropogenic era 129I in oceans: 50 kg [Fabryka-Martin et al., 1985].

(1) = Eisenbud and Gesell [1997], (2) = Wagner et al. [1996], (3) = Raisbeck and Yiou [1999], (4) = Hanford [1997], (5) = López-Gutiérrez et al. [2004], (6) = BNFL [1999–2002], (7) = Jackson et al. [2000], (8) = Gray et al. [1995], (9) = GRNC [1999]. (*) The estimate is based on more recent data concerning the 129I/131I ratio [Mironov et al., 2002] of the Chernobyl release and the amount of 131I released [UNSCEAR, 2000] than is the widely cited estimate of Paul et al. [1987]. sediments. Anthropogenic activity has drastically changed the equilibrium of the 129I cycle by adding a huge and well-deﬁned source of 129I to the surface environment. Nowadays, anthropogenic 129I can be found in all compartments of the mobile iodine pool: oceans, atmosphere, soils, and modern organic materials (Figure 2.5). The distribution of 129I is constantly changing on both the temporal and spatial coordinates. The 129I/127I ratio in the surface environment now encompasses a whole range of values starting from its natural ratio, ∼ 10−12 [Fehn et al., 1986; Moran et al., 1998], to as much as 10−5 [Rao and Fehn, 1999]. In terms of concentration, 129I is present in aquatic systems in the range of 106–1011 atoms l−1 [rainwater and seawater; Yiou et al., 1994; Buraglio et al., 1999; Raisbeck and Yiou, 1999; Buraglio et al., 2000b; Kekli et al., 2003] and 105–109 atoms l−1 [surface and ground waters; Buraglio et al., 2000b; Kekli et al., 2003]. Applications involving the use of 129I can be classiﬁed by the origin of the radionuclide: (a) natural 129I or (b) anthropogenic 129I.

11 upper ocean 1000 deep ocean

100 soil 10 biosphere 1 ocean atmosphere land atmosphere I (kg) 0.1 soil water

129 0.01

0.001

0.0001 stratosphere 10 -5 1950 1960 1970 1980 1990 2000 Year

Figure 2.5: 129I inventory in different compartments from the European reprocessing facilities, bomb tests, and Chernobyl accident calculated by Buraglio [2000] using model by Wagner et al. [1996] (based on the models by Fabryka-Martin et al. [1985] and Kocher [1981]). Note that compartment model calculations use an assumption of homogeneity within each compartment.

Natural 129I has been used for dating underground waters and estimating of their sources and formation ages [Fabryka-Martin et al., 1985; Fehn et al., 1994; Moran et al., 1995a; Fehn et al., 2000]. Anthropogenic and natural 129I have been used for tracing water masses and transient time estimations in oceanography [Yiou et al., 1994; Kilius et al., 1995; Raisbeck et al., 1995; Smith et al., 1998, 1999; Buraglio et al., 1999; Raisbeck and Yiou, 1999; Hou et al., 2001; Gascard et al., 2004], and for the retrospective monitoring of nuclear accidents and activities [e.g. Jacobsen et al., 2000; Mironov et al., 2002]. However, exploiting the full potential of this environmental tracer requires considerable preparatory work: time series should be established that can map the distribution and evolution of the tracer concentration, local and regional transport models should be developed and validated, and questions regarding iodine geochemistry should be cleared up. The ﬁrst two items are addressed in our publications (see Chapter 5). Anthropogenic and natural 129I could potentially be used for dating old organic matter [Fabryka-Martin et al., 1985], dating recent organic matter, and assessing iodine deﬁciency disorders [Santschi and Schwehr, 2004; Schink et al., 1995].

12 2.3 Stable chlorine (35Cl and 37Cl) There are two stable isotopes of chlorine, 35C and 37C, which occur in 76% to 24% relative abundance. Therefore the 36Cl/Cl isotopic ratio used in this thesis means the ratio of 36Cl in a sample to the total amount of both stable chlorine isotopes in the sample, i.e. 36Cl/35,37Cl. The compartment model of the global chlorine cycle (Figure 2.6; Graedel and Keene [1996]) is similar to that of the global iodine cycle (Figure 2.1). Both chlorine and iodine are found in relatively great amounts in the oceans, and are involved in ocean–atmosphere sub-cycles. Chlorine is found in all higher life forms and participates in biological processes (e.g. photosynthesis, maintaining osmotic balances in cells, and neuronal signaling). Chlorine exhibits a hydrophilic nature, is conservative in groundwater, and does not react with any minerals in aquifers [Fontes and Andrews, 1994]. Since inorganic chlorine compounds are highly soluble, chlorine is mobile in soils and ulti- mately accumulates in hydrosphere (oceans) [Graedel and Keene, 1996]. An additional input of chlorine to the surface environment is the crust to freshwater ﬂux due to the weathering of Cl-containing minerals. The ﬂux from oceans to the crust (to sediments via diagenesis) does not balance the loss of chlorine from the crust, and this is consistent with the hypothesis that concentrations of oceanic chlorine are slowly increasing in absence of large evaporite pans on the Earth’s surface [Graedel and Keene, 1996].

Stratosphere 4×1011 ∆ = 0 3×1010 3×1010 6×1012 2×1012 Troposphere 12

5.3×10 13 ∆ = 0 6×1015 6×1015 3.4×1013 1.9×10 Cryosphere Oceans Freshwater Pedosphere 5×108 2.6×1022 3.2×1017 2.4×1016 ∆ = 0 12 ∆ = 2×1014 14 ∆ = 0 13 ∆ = -3×1013

6×10 13 1.7×10 2.2×10 4.5×10 Crust 6×1022 14 ∆ =-1.6×1014 1.8×10 Mantle 2.2×1025 ∆ = -2×1012

Figure 2.6: The compartment model of the global chlorine cycle [Graedel and Keene, 1996]. Inventories are presented in grams, ﬂuxes in grams/yr.

13 2.4 Radioactive chlorine (36Cl) and its applications 36 There is only one long-lived radionuclide of chlorine, Cl (T1/2 = 301 ka, mainly β-decay to 36Ar, decay energy: 0.709 MeV). Natural sources of 36Cl include production in the atmosphere and in the lithosphere. In the atmosphere, 36Cl is formed by the cosmic-ray-induced spallation of 40Ar and to a lesser extent by neutron capture 36Ar(n,p)36Cl. Contributions of the stratosphere and troposphere to the atmospheric production break down as 60% and 40%, respectively. In the lithosphere, 36Cl is formed by 40Ca capturing muons at the Earth’s surface, or by 35Cl and 39K capturing neutrons at the Earth’s surface or in the vicinity of uranium ores [e.g. Bentley et al., 1986a; Geyh and Schleicher, 1991]. The major anthropogenic sources of 36Cl are nuclear bomb tests from 1945 to the 1970s [78 kg released to atmosphere, Synal et al., 1990]. The estimated global inventories and ﬂuxes of 36Cl are shown in Figure 2.7 [Bentley et al., 1986a].

STRATOSPHERE 1.5×103 g +8 g/yr

8 g/yr

TROPOSPHERE 102 g 10 g/yr +6 g/yr 4 g/yr

<0.1 g/yr LAND SURFACE OCEANS 10 g/yr Hydro- Litho- 107 g sphere 6 g/yr sphere 3 4 +13 g/yr 5×10 g 10 g <0.1 g/yr +7 g/yr <0.1 g/yr DEEP SUBSURFACE (Lithosphere) <0.1 g/yr 3×108 g +103 g/yr

Figure 2.7: Estimated global inventories and ﬂuxes of 36Cl based on the work of Bentley et al. [1986a].

The most common applications of natural 36Cl include determinating the exposure ages and erosion rates of rocks [Phillips et al., 1986, 1990; Zreda et al., 1993; Shanahan and Zreda, 2000; Mitchell et al., 2001; Briner et al., 2001], dating saline sediments [Phillips et al., 1983], dating old groundwater

14 and estimating groundwater residence times [Andrews et al., 1986; Bentley et al., 1986b; Fabryka-Martin et al., 1987; Fehn et al., 1994; Balderer and Synal, 1997], and reconstructing the geomagnetic dipole record in ice cores [Baumgartner et al., 1998; Beer et al., 2002]. Applications of anthropogenic 36Cl include using the bomb “spike” for evaluating locations for nuclear waste storage facilities [Scanlon et al., 1990; Conrad, 1993; Fabryka-Martin et al., 1998], tracing 36Cl from the reprocessing of nuclear waste in groundwater to determine aquifer dispersivity [Cecil et al., 2000], and evaluating radiological conditions near nuclear test sites or nuclear power plants [Jacobsen et al., 2000].

3 Analytical techniques

This chapter examines the procedure for investigating and analyzing 129I/127I, and to some extent 36Cl/Cl, isotopic ratios, including sampling, storage, sample treatment, and determination of isotopic ratios at the tandem accelerator.

3.1 Sampling and storage of seawater for 129I analysis The required volume of sampled seawater is deduced from the expected 129I concentration in the sample. Seawater from locations under the direct influence of marine releases (surface of the North Sea, Nordic Seas, and Arctic Ocean) can be sampled using 50–100 ml polyethylene bottles, whereas seawater from locations that are remote from the reprocessing plume (the deep Arctic Ocean, deep Nordic Seas, North Atlantic Ocean surface; Figure 2.3) has to be sampled using 1000–2000 ml bottles. The sampling bottles should be filled to the top and closed with tight screw-tops to preclude 129I loss. Seawater samples are often stored for several months before being chemi- cally treated. To reduce biological activity, which may influence 129I concentration in samples, the polyethylene bottles containing samples are placed in a dark and cold (4°C) room. So far as we have determined by testing, storage under such conditions for periods of up to 1.5 years does not introduce significant changes in measured 129I concentrations.

3.2 Extraction of 129I from water samples 129I must be extracted from water and prepared in a form suitable for AMS measurement. The chemical procedure for iodine extraction (Figure 3.1) starts with the addition of iodine carrier1 to increase sample size and make chemical handling easier. The amount of carrier and the volume of the water sample should be chosen so that the ﬁnal 129I/127I ratio in the sample is above the analytical background in our facility (10−13). To ensure isotopic equilibrium − − − between the iodine species of the sample (IO3 +I ) and the carrier (I ), all − − iodate (IO3 ) in solution is reduced to iodide (I , Figure 3.1). Iodide is then

1∼300 Myr old. Extracted from a deep underground brine by Woodward Corporation, Okla- homa.

17 oxidized to molecular iodine, I2, which is rapidly extracted into chloroform, CCl4, due to its ∼100 times higher solubility in this non-polar solvent than in water. Molecular iodine is reduced to iodide and “back-extracted” by its charge from non-polar CCl4 to water. By the addition of AgNO3, iodide is precipitated as silver iodide, AgI. The chemical procedure was developed by Buraglio et al. [2000a] based on the procedure of Moran et al. [1995b].

filtered water 200 ml - - IO3 , I

carrier 2 mg - - - IO3 , I +I

NaHSO3 2 ml, 0.1 M - - reduction IO3 I I5+ I- HNO3 until pH = 2 H O 4 ml 2 2 - oxidation I I2 I- I0 CCl4

I2 (aq) I2 (org) CCl CCl +NH OH·HCl 4 2 4 - +I reduction of

IO3 I2 (org) 2 5+ 0 (org) (possible) I I NaHSO3 10 ml, 0.1 M H SO few drops,conc. 2 4 - reduction I2 (org) I I0 I- AgNO3 1.5 ml, 0.01 M AgI precipitation of AgI

NH3, dist. water AgI washing away Cl and S compounds

o AgI drying (60 C, over night) mixing with Nb binder

Figure 3.1: Flow chart of the chemical procedure for extraction of iodine from water samples. Values are given for a typical sample. We have recently started to use chloroform, CHCl3, instead of tetrachloride, CCl4.

3.3 Extraction of 129I from solid samples The extraction of 129I from solid materials is done by combusting the sample (mixed with iodine carrier) in an oxygen ﬂow, and the subsequent capture of iodine in an alkaline solution (KOH + NaHSO3) through reduction to iodide − I2 → I . Trapped iodine is then precipitated as silver iodide, AgI, by the addition of AgNO3. The procedure is partly based on the work of Marchetti et al. [1997].

18 O Quartz wool 2

Furnace Sample Air + carrier

Trapping solution (KOH+NaHSO3)

Figure 3.2: Sketch of combustion setup.

Our combustion setup is shown in Figure 3.2. Quartz wool inside the furnaces keeps escaping particles inside the combustion area. The temperature of the furnaces should be increased gradually to burn different fractions of the sample separately.

3.4 Extraction of 36Cl from water samples Chlorine is extracted from water samples using a well-established chemical method [Conard et al., 1986; Jiang et al., 1990; Nagashima et al., 2000, 2004]. The main obstacle to 36Cl AMS is the interference of an abundant stable isobar 36S, which has chemical properties similar to those of 36Cl. Even 1 ppm of sulfur impurity would produce 1000 times more 36S than 36Cl counts in the detector for a typical 36Cl/Cl = 10−13 ratio [Elmore et al., 1979]. Therefore any chemical procedure for 36Cl extraction has to reduce the sulfur content to achieve an acceptable 36S rate in the ∆E–E detector, i.e. 103 counts s−1 [Knies and Elmore, 1994]. The traditional procedure uses differences in solubility of two compounds, 2− AgCl and BaSO4, to remove SO4 from the solution. First, the solution is acidiﬁed with HNO3. Then AgCl is precipitated using AgNO3, and the rest of the solution discarded. The precipitate is dissolved in NH4OH solution. 2− Ba(NO3)2 is added to the solution to remove SO4 by precipitating BaSO4. The precipitate is ﬁltered out, and HNO3 added to force AgCl precipitation. The cycle, dissolution of AgCl → precipitation of BaSO4 → precipitation of AgCl, is repeated as many times as necessary. An alternative approach has recently been proposed by Jiang et al. [2004]. 2− They used Ba, H, and Na cation exchange columns to remove SO4 , to ab- sorb metal ions, and to convert Cl− → HCl → NaCl. Then AgCl is precipitated from the NaCl solution by adding AgNO3. While the traditional

19 preparation method can take 1 to 2 days with recovery rates below 80%, this method promises to reduce sample preparation time to 6 hours and to raise the recovery rate to ∼90% while producing a similar or even better analytical background [Jiang et al., 2004].

3.5 A tandem accelerator as a mass spectrometer

A+Q - +q AB Magnet, B Magnet, mE/q2 = const 5 MV tandem tank mE/q2 = const

High-energy Post-acceleration accelerator analyzer Injector

X- Electrostatic Electrostatic deflector, deflector, E/q = const Detector Ion source E/q = const E = const 2 dE1 Z1 2 Z2 Detectors: dx v1 - Time-of-flight - Gas-ionisation chamber - Surface barrier detector

Figure 3.3: Sketch of an accelerator mass spectrometer.

An accelerator mass spectrometer based on a tandem accelerator can be divided into four principle components (Figure 3.3): an ion injector, a high energy accelerator, a post-acceleration analyzer, and a detector. In a typical ion source, Cs+ ions sputter a solid target to produce negative ions, positive ions, and neutral particles of sample material. AMS uses negative ions, which normally have a sputtering yield in the range of 1–10% of total sputtered material. The efficiency of negative ion formation is defined by the electron affinity of the element and is a bottleneck for the AMS of radionuclides of low electron affinity (EA). For example, high yields of negative ions are measured for 14C(EA = 1.28 eV), 36Cl (3.62 eV), and 129I (3.06 eV), while low yields are obtained for 41Ca (0.04 eV) and 10Be (0.20 eV). Therefore 10Be 41 10 − 41 −/41 − and Ca AMS use the molecular ions BeO and CaH3 CaF3 , which have higher electron affinity. On the other hand, the production of negative ions at the ion source can provide a means of isobar separation, for example, in the case of 14N (isobar of 14C) and 129Xe (isobar of 129I), because these elements do not form metastable (> 1µs) negative ions.

20 There are general requirements for the ion source for AMS measurements [Kilius et al., 1997a]: the ion source must produce high and stable currents at high efficiency, have low memory effects, hold enough samples for a night shift, have a short switching time from one sample to another, and, if possible, provide additional isobar separation. The Cs+ sputter ion source can fulfill all these requirements reasonably well [Kilius et al., 1997a]. Together with the ions of interest, interfering negative ions are also produced in the ion source. These comprise ions of neighboring isotopes (e.g. 35Cl−, 37Cl−, 127I−, and 128Te−), molecular ions of the same mass as that of 35 − 127 − 128 − the ion of interest (e.g. ClH , IH2 , and TeH ), and isobaric ions (e.g. 36S−). Moreover, each of these ions has an energy distribution with a major peak surrounded by high and low energy tails [Kilius et al., 1988]. The isotopic interference is removed by magnetic and electrostatic analyzers of the injector and of post-acceleration analyzer, the molecular interference is removed by the stripping process and subsequent “Coulomb explosion” at the terminal of the accelerator, while the isobaric interference is not affected by ion optics and should be separated at a final detector unless special measures are undertaken. After pre-acceleration by several tens of keV in the ion source, the beam of negative ions is focused by Einzel lenses at the focal point of an electrostatic and/or magnetic analyzer (Figure 3.3). The analyzers perform electrostatic and magnetic separation according to the Lorentz force F = q(v×B)+qε.To pass through these elements, the ion energy, E, charge, q, and mass, m,haveto fulfill the following conditions defined by the magnitude of electrostatic and magnetic fields ε and B at radii rε and rB, respectively: m · E = const magnet only, (3.1) q2

  E rε · ε  E = = const  = const q 2 q magnet and ⇒ (3.2) m · E r2 · B2  m  electrostatic deﬂector. = B = const = const q2 2 q

An injector for light element AMS (e.g. 10Be and 14C) can function with a magnetic analyzer only (Equation 3.1). However, in the case of heavier elements, the relative mass difference, ∆m/m, between neighboring masses becomes smaller, and, together with the aforementioned energy tails of sputtered ions, this results in the existence of isotopic or molecular ions which mimic the mass-energy product. Therefore, an injector for heavy element AMS has to be equipped with both electrostatic and magnetic analyzers (Equation 3.2) [Kilius et al., 1988].

21 To summarize, the injector of an accelerator mass spectrometer is itself a conventional mass spectrometer. It removes interferences with masses or energies different from those of the ion of interest. However, isobaric and molecular ions of the same mass and energy as those of the ion of interest still present in the outgoing beam. The beam is pre-accelerated by several further tens of keV to reduce the inﬂuence of the lens effect at the entrance of the accelerator, and is focused towards the terminal of the accelerator (Figure 3.3). Inside the accelerator tank, negative ions are accelerated by the attraction of the positively charged megavolt (MV) terminal. When they reach the terminal, the ions enter a “stripper,” commonly a thin foil or a low pressure gas cell, where several electrons are removed (stripped) from each ion. The stripping removes molecular interferences, because molecules with charge states of q > +2 are not stable and dissociate at the high voltage terminal (“Coulomb explosion”). The formed positive ions are further accelerated (consequently, the name “tandem accelerator”) by repulsion from the terminal down to ground potential, reaching energies E > 1 MeV amu−1: mout mout E = q + eVterm + Einj, (3.3) min min where min and Einj are the mass and the energy of the injected ion, mout and q are the mass and the charge of the ion after stripping, and Vterm is the terminal voltage. The stripping in the accelerator destroys molecules, but the resulting mole- 35 − → 35 X+ 127 − cular fragments are still present in the beam (e.g. ClH Cl , IH2 → 127IX+, and 128TeH− → 128TeX+). Most of the molecular fragments are removed by magnetic and electrostatic analyzers at the high energy side of the accelerator (Figure 3.3). However, the fragments that have M/Q equal to m/q of the ions of interest arrive at the de- 12 − →12 2+ / = 36 6+ tector unsuppressed (e.g. C3 C : M Q 6, interference with Cl ) and must be avoided by choosing a m/q that does not have such interferences or by reduction the unwanted molecules in the beam using an improved chemical treatment of the samples [Kilius et al., 1997b]. The next most problematic fragments are these for which M/Q ≈ m/q [Kilius et al., 1997b]. For example, the following molecular fragments can interfere with 129I measurements [Kilius et al., 1997b]: 129I4+: 97Mo3+, 32S+ 129I5+: 103Rh4+, 26Mg+ 129I7+: 111Cd6+, 92Mo5+, 92Zr5+, 37Cl2+ In the case of a modest m/q and E/q resolution at the high energy side of the accelerator, 129I5+ is preferable for 129I measurements at the level of

22 129I/127I ≥ 10−14, because Rh is naturally rare [Kilius et al., 1997b]. Additional ion interference may arise from interactions of the beam and residual gas atoms: (1) additional charge-exchange collisions in the accelerator tubes, and (2) elastic scattering of stable isotope ions at the high energy side. These problems are solved by the existence of a good vacuum throughout the system, and, most importantly, in the accelerator tubes [Tuniz et al., 1998]. After separation in electrostatic and magnetic analyzers (Figure 3.3), the re- maining ions in the beam are identified in a detector system by using standard nuclear physics detection and identification techniques. These ions comprise the ions of interest, isobaric ions, and, possibly, some molecular fragments mentioned above. The isobar separation in the detector is possible because the ion stopping power, dE/dx, for MeV/amu ions is specific for each element. However, the level of interference should be acceptable for the detector. For example, the most typical detector in AMS, a gas ionization detector, has an upper limit of ∼103 counts s−1, after which the performance of the detector deteriorates due to the recombination of electrons with positive ions (Knies and Elmore [1994]; more on detectors in section 3.7.1). Therefore, the isobaric interference should be reduced as much as possible before the detector by chemical means, by choice of binder and sample holder material, by choice of negative ion, and by use of a gas-filled magnet, or absorbing foil followed by a magnet, or complete stripping, or X-ray emission [Tuniz et al., 1998]. Three major features of AMS should be emphasized: • The use of negative ions at the ion source makes isotopic separation already possible even at the formation of the negative ions, since some isobars do not form metastable (> 1µs) negative ions. • After electron stripping of ions in the stripper, molecular interferences with charge states q > +2 dissociate through “Coulomb explosion.” • Ion identification is achievable, since the ion stopping power, dE/dx at MeV ion energies, is specific to each element.

3.6 Small AMS facilities Until recently, all AMS facilities needed to be based on relatively large and expensive tandem accelerators (2.5 MV and higher) so as to have sufﬁcient stripping yields for charge states of q > +2. The cost of analysis at such AMS facilities is high, limiting application of the technique. This is especially true in the case of biomedical applications, which require the rapid and inexpen- sive analysis of a large number of 14C-labeled samples, but which allow for an order of magnitude lower accuracy (∼3%) than required in radiocarbon dat-

23 ing. The pioneering work on small accelerators had already been presented in 1984 by the IsoTrace group in Toronto [Lee et al., 1984]. They demonstrated that 14C2+ detection at low terminal voltages is possible, because the inter- 12 2+ fering CH2 molecules can be eliminated by sufﬁciently high stripper gas thickness. However, it took then over a decade until growing interest from the biomedical community [Felton et al., 1990; Vogel et al., 1990; Meirav et al., 1990; Barker et al., 1990; Turteltaub et al., 1992; Vogel and Turteltaub, 1994] stimulated the development of concepts and prototypes for small AMS facilities [Purser, 1994; Suter et al., 1997; Mous et al., 1997; Hughey et al., 1997]. Subsequent studies by the ETH/PSI AMS group in collaboration with Na- tional Electrostatic Corporation (NEC), USA [Suter et al., 2000; Synal et al., 2000a,b], found that radiocarbon dating is possible with a small and compact 0.5 MV tandem accelerator. They used 14C+ ions at the high energy side (> 50% stripping yield at 200–600 keV beam energy) and a gas stripper with a sufﬁcient thickness (2 µgcm−2 of argon) to reduce molecular interference by 10 orders of magnitude. Since then, four similar commercial systems have been manufactured by NEC [Schroeder et al., 2004]. At the AMS-9 conference held in Nagoya, Japan, in 2001, a new approach based on low ion beam energies (< 0.5 MV) was demonstrated [Synal et al., 2004]: a 200 kV vacuum-isolated tandem accelerator capable of radiocarbon dating. The latest development of radiocarbon AMS is a 300 kV, open air-isolated, single stage accelerator [Schroeder et al., 2004], which is already commercially available from NEC.

3.7 The Uppsala AMS system The layout of the Uppsala tandem accelerator facility is shown in Figure 3.7. Two injectors, each equipped with a Cs+ sputter negative ion source, are used for AMS, and each of their ion sources can be loaded with up to 20 samples. One injector has a 90° double-focusing sector magnet, whereas the other injector has a 90° double-focusing electrostatic deflector and a 90° double- focusing sector magnet. The vacuum chambers of both injector magnets float at potentials defined by external power supplies that can produce rapid square- type voltage pulses. Therefore both injectors are capable of forming pulsed ion beams, which reduces the loading on the accelerator and improves stability while measuring stable isotope currents. The central component of the laboratory, a 5 MV tandem accelerator (NEC 15SDH-2 Pelletron®), was manufactured by National Electrostatic Corporation, Wisconsin, USA, and installed in 2001. The next elements along the beam line are the analyzing and switching magnets. Their high mass-energy product makes separation of heavy elements feasible. A dedicated AMS beam line is located at +20° after the switching magnet, and has a 20° electrostatic deflector followed by a gas-filled magnetic

24 Injector magnet Analyzing magnet Tandem accelerator 90° double focusing 90° double focusing 5 MeV·amu, r = 350 mm <176 MeV·amu, r = 1270 mm 5 MV Bmax = 1.0 T, gap 52 mm Bmax = 1.5 T, gap 36 mm

Injector magnet Switching magnet 90° double focusing <176 MeV·amu 8.5 MeV·amu for <20° Electrostatic deflector r = 375 mm Bmax = 1.4 T 20°, r = 1200 mm Bmax = 1.1 T gap 41 mm εmax = 65 kV/cm gap 48 mm gap 25 mm Ion source for Ion source for 14C and 10Be 129I and 36Cl Electrostatic deflector 90° double focusing r = 400 mm gap 40 mm

Gas-filled magnet 180° dipole magnet <25 MeV·amu, r = 600 mm 01 10 m Bmax = 1.2 T, gap 36 mm AMS beamline vertical mount

Figure 3.4: Drawing of the Uppsala tandem accelerator. 25 spectrometer. When this spectrometer is in operation, ions are deﬂected in the vertical plane to the lower exit of the spectrometer, where a setup for beam diagnostics (a Faraday cup and a viewer) and a ∆E–E gas ionization detector are located. When this spectrometer is not in operation, ions travel straight through towards other detectors: two energy detectors (silicon barrier), two time detectors (carbon-foil), and a ∆E–E gas ionization detector. Two setups for beam diagnostics consisting of a Faraday cup, a viewer, and a videocamera are located after each time-of-ﬂight detector.

3.7.1 Detectors at the AMS beam line There are several detectors on our AMS beam line: two energy detectors, two time detectors, and two gas ionization detectors (one is located at the lower exit of the gas-filled magnet). They are used for routine measurement of the following long-lived radionuclides: 10Be (the “upper” gas ionization detector), 14C (by the first energy detector), and 129I (by the time-of-flight setup and the second energy detector); 36Cl will be measured by the “lower” gas ionization detector. Our setup for making 129I measurements is shown in Figure 3.5a. Ions pen- etrate a carbon foil (thickness 5 µgcm−2, 20 mm in diameter), knocking out several electrons (Figure 3.5b). Electrons are reflected by an electrostatic mirror towards a package of two microchannel plates (MCP), which multiplies the signal by up to 107 times. The signal is then taken to a constant fraction discriminator (CFD, Figure 3.5a), and when the signal is above the CFD’s threshold, the CFD creates a fast standard logic pulse. The CFD signals from both time detectors are combined by a time-to-amplitude converter (TAC) to form a pulse with a height equal to the time-of-flight of an ion (Figure 3.5a). This time-of-flight pulse is then analyzed by the analog-to-digital converter (ADC) of the data acquisition system. After passing the time-of-flight setup, ions are stopped in a semiconductor charged-particle detector (energy detector). In this detector, ion energy is converted to an electrical discharge by the creation of free carriers in a p–n junction of the energy detector. The current pulse is then amplified by a charge-sensitive preamplifier and a linear pulse- shaping amplifier. One output from the amplifier is connected to the ADC of the data acquisition system, while the other output of the amplifier is first analyzed by a single-channel analyzer (SCA) to select only a narrow band of energies around the 129I energy, and then converted into a gate signal for the both time and energy ADCs. The efficiency of the time-of-flight setup for 129I5+ ions is about 70% of the energy detector efficiency. The resolution of the time-of-flight setup is 1.5– 2 ns (FWHM), and this is sufficient to identify mimic 127I5+ (peak at +4.6 ns), but not mimic 128Te5+ (peak at +2.3 ns). Fortunately, we did not observe

26 a) ToF-E setup b) Carbon foil detector 7 kV Carbon-foil Energy electrostatic detectors detector 10 MΩ Beam mirror 10 MΩ C-foil L = 1.65 m e- 7.5 MΩ Pre-amp MCP1 CFD CFD 7.5 MΩ Amp MCP2 Stop Start 1.5 MΩ Delay TAC collector Ω SCA 50 coaxial connector Delay & Gate to CFD grid ADC1 Gate ADC2

Figure 3.5: (a) Combination of two time detectors (time-of-ﬂight) and one energy detector used in 129I AMS together with the electronics needed for data acquisition. CFD = constant fraction discriminator, TAC = time-to-amplitude converter, SCA = single channel analyzer. (b) Carbon-foil time detector after Busch et al. [1980]. Electrons travel the same time-of-ﬂight to channel plates (isochronism), because electrostatic mirror compensates for a spread in positions from which the electrons are emitted. C-foil = carbon foil, MCP = microchannel plate.

128Te5+ in our measurements. From our practical experience with the new Uppsala accelerator, 129I5+ is always a sole peak in a ToF–E spectra with the present setup. Therefore it would be reasonable to exclude the time-of-flight setup when measuring 129I, but we have so far kept it to reduce an eventual background for samples with low 129I abundance. The measurement setup for 36Cl AMS consists of a gas-filled magnet (GFM) and a gas ionization detector (also known as a ∆E–Eresidual detector) located at the lower exit of the GFM. 36Cl ions after GFM have higher energy than do the isobaric 36S ions, and are located further inside the GFM in the deflection plane (the vertical plane with regard to the laboratory layout, see also the next chapter). Therefore the gas ionization detector for measurements of 36Cl is designed to provide position information together with detailed stopping (a split anode, 4 anodes in total) and total energy of each incoming ion (cathode or upper grid, see Figure 3.6). Two fine grids at intermediate potentials are placed between the anodes and the cathode in such a way that the tracks of all incoming ions are confined in the volume between the upper grid and the cathode. The purpose of the grids is to make the signals on the anodes and cathode independent of the lateral position of an ionizing event. The upper grid is coupled to the cathode through a large capacitance to form a Fara- day cage [Fulbright, 1979]: no signal will appear at the Etotal pre-amplifier connected to the upper grid or to the cathode before electrons have passed

27 the upper grid. After all the electrons have passed the lower grid, the charge collected by the pre-amplifier is proportional to the total energy signal, Etotal. When electrons arrive at anode i, the charge collected by the ∆Ei pre-amplifier is proportional to the energy loss, ∆Ei, of the incoming ion above this plate. Elemental identification in a gas ionization detector is based on the ion stopping power (dE1/dx), which is a function of the atomic numbers of incoming ion Z1 and target material Z2. This dependence can be written for ion energies −1 E1 above 1 MeV amu in the following form (see also Figure 3.7):

2 dE1 ∝ Z1 · 2 Z2, (3.4) dx v1 where v1 is the ion velocity.

E cathode total

ions + - E1 E1 E1 Npairs= = Epair ~30 eV gas prod. Frisch grid 1 Frisch grid 2

anodes ∆ ∆ ∆ E1 E2 E3 Eresidual right left

Figure 3.6: Our multi-anode gas ionization detector for 36Cl measurements. Central electrode is split to provide position information available after GFM. Note the difference in energy deposition of 36Cl and 36S in the gas ionization detector placed after GFM (Figure 3.7b).

In the ordinary case (no GFM) the ion of interest (36Cl) has the same energy as does its isobaric counterpart (36S), and their stopping curves intersect each other in a gas ionization detector (Figure 3.7a), thus deﬁning a separation point for the anodes. This limitation does not hold for a gas ionization detector placed after a GFM, because 36Cl ions have shorter trajectories in a GFM and lose less energy than do 36S ions (e.g. Figure 3.7b). The example in Figure 3.7b has the following background: after 36Cl and 36S ions with initial energy of 38 MeV have passed through 2.5 Torr (333 Pa) of N2 in a GFM, their kinetic energies become 20 MeV (36Cl) and 19 MeV (36S), respectively, while their beams are partly separated by a distance of 47 mm (the 36Cl beam extension of 2σ = 34 mm).

28 a) Energy deposition of 36Cl (40 MeV) and 36S (40 MeV) in ionization detector 0.3 36Cl

1 36S

0.2

0.5 0.1 split here

(MeV) Anode 1 Anode 2

0 0 0 20 40 60 80 100 120 140 160 180 200 in [0;x] 36S

deposited X (mm)

- E b) Energy deposition of 36Cl (20 MeV) and 36S (19 MeV) in ionization detector

0.3 dE/dx (MeV/mm)

in [0;x] 36Cl 36Cl deposited E 1 36S

0.2

0.5 0.1

Positions of anodes are not fixed. All anodes collect useful signals. 0 0 0 20 40 60 80 100 120 X (mm)

Figure 3.7: Calculated energy deposition of 36Cl and 36S ions in a gas ionization detector (80 Torr or 106 mbar Ar) for two cases: (a) E(36Cl) = E(36S)=40MeV (ordinary case), and (b) E(36Cl) = 20 MeV and E(36S) = 19 MeV (after GFM, see text for details). Calculation is based on SRIM2000 stopping tables and the article by Synal et al. [1987].

29 3.7.2 The 129I measuring procedure First, the entire accelerator mass spectrometer is adjusted to achieve maximum transmission of the stable isotope (127I) from the ion source to the last Faraday cup in the AMS beam line. Then slits are set to an acceptance slightly larger that the beam emittance; this reduces amount of unwanted interferences at the detector, while still providing maximum transmission and stable measurement conditions. Finally, the magnetic elements are scaled to the rare isotope (129I), and the measuring procedure can be started. In the measuring procedure a ratio, R, between the amount of rare isotope ions at the detector and the current of stable isotope (∝ 129I/127I ratio) is se- quentially determined in a standard sample of known 129I/127I ratio, in 4 to 6 unknown samples, and then again in the standard, in unknown samples, etc. The ratio, R, of the unknown samples is then normalized to the time-linear- interpolated R in the standards, corrected for the background of the chemical procedure and the accelerator setup (see below), and multiplied by the known value of the 129I/127I ratio in the standard to obtain the 129I/127I ratio of the unknown samples. Background is corrected with the help of blank samples prepared in the same way as the unknown samples. For details see the appendix. The entire measuring procedure is conducted from a computer running AccelNET/LINUX software, and does not require any manual intervention. For data acquisition of a single rare or stable isotope measurement, the Ac- celNET/LINUX server triggers another computer running FastComtech/DOS software and collects data after the measurement.

3.7.3 Background correction Raw 129I/127I ratios measured in the accelerator are affected by a certain background level of 129I that comes from the chemical preparation, and, though to a much less extent, from the accelerator setup itself. To determine the extent of the ﬁrst type of background, chemical blanks are prepared from an iodine carrier at regular time intervals. The accelerator background is determined using a target pressed from a >300 Myr old natural AgI crystal (iodargyrite). Since (a) the standards were not subject to the chemical preparation procedure, (b) the concentration of 127I in seawater samples is negligible compared with the amount of carrier added, and (c) the accelerator background is small (129I/127I ≤ 4 · 10−14), the background-corrected ratios are obtained by sub- tracting the ratios measured for blank samples from the measured ratios of unknown samples prepared using the same chemical preparation sequence. The average background of our chemical procedure for the period between January 2003 and June 2004 was estimated at 129I/127I ∼ 2 × 10−13. More details are given in the appendix.

30 4 Design of a gas-ﬁlled magnetic spectrometer for AMS

This chapter considers the design of a gas-ﬁlled magnet, a type of isobar separation tool, able to mitigate the major limitation of AMS, namely, isobaric interferences. The complete description of this work is given in my Philoso- phy Licentiate thesis [Alﬁmov, 2003] and summarized in paper 1.

4.1 Introduction Accelerator Mass Spectrometry (AMS) is a powerful tool for determining low concentrations of rare radionuclides. However, measurements of heavier isotopes (≥36 amu) in the presence of abundant stable isobars are more complicated than for low-Z elements. The increasing similarity in the energy loss of the ions of interest and their isobars limits the separation possible in a gas ionization detector [Suter, 1990], the most commonly used AMS detection technique. To improve sensitivity it is therefore necessary to use refined and dedicated chemistry and to introduce additional isobar separation techniques, especially at medium-sized AMS facilities (3–5 MV). A gas-filled magnet is an approach to improve isobar separation. This technique was proposed by Cohen and Fulmer [1958] and later used by Arm- bruster [1961] and Sistemich et al. [1975] in separation of fission fragments. The basic principle of the gas-filled magnetic separation technique is as follows (Figure 4.1): when ions traverse a gas in the presence of a magnetic field, the average charge state, qm, in the beam depends on the velocity, v, and 1/2 the atomic number, Z, of the ions, where qm ∝ Z · v according to Sayer’s formula [Sayer, 1977] at energies <0.5 MeV amu−1. For isobars moving in a dipole GFM, the magnetic rigidity then becomes r·B = m·v/q ∝ Z−1/2, which means that the isobar separation depends only on the nuclear charge, Z, and is independent of the velocity, v. Already this simplified approach demonstrates the ability of GFM to separate isobars, although higher-order effects have to be taken into account in quantitatively evaluating isobar separation. The first application of GFM for AMS was conducted at the Argonne Na- tional Laboratory [Henning et al., 1986] for the separation of isobaric 58Ni and 58Fe and for 41Ca and 60Fe AMS. Since then several other installations have been reported [Kubik et al., 1990; Zoppi et al., 1994; Müller et al., 1992;

31 Figure 4.1: Sketch of a gas-ﬁlled magnet. Dependence of mean ion charge state, qm, in the beam on the atomic number, Z, and velocity, v, of the ions allows for the possibility of isobar separation.

Korschinek et al., 1994; Knie et al., 1997; Hatori et al., 2000]. The first instruments were old reoriented Enge split-pole spectrographs, Q3D, and dipole magnets, while later GFMs were specially designed ones (with two 180◦ dipole magnets [Zoppi et al., 1994; Hatori et al., 2000] and one 135◦ dipole magnet [Knie et al., 1997]). It was thus shown that a simple dipole magnet configuration could meet the AMS requirements of isobar separation. In order to understand the gas-filled magnetic spectrometer in detail and to determine the optimal physical conditions, we have performed an ion optical simulation in order to optimize the isobar separation for various parameters, such as gas pressure, bending radius, initial ion energy, and separation angle.

4.2 Calculations The calculations were specifically made for 36Cl and its isobar, 36S, with initial energies of 33–45 MeV covering energies achievable at the new 5 MV tandem accelerator in Uppsala. Nitrogen (1–5 Torr1) was chosen as the filling gas, based on previous experimental studies [Knie et al., 1997; Zoppi et al., 1994]. Our investigation is divided into two parts: 1) the calculation of the mean 36 36 ion charge trajectories (qm-trajectories) for Cl and S, and 2) the calculation of the 36Cl beam profiles along the beam path in the gas-filled magnet. Due to the lack of charge-exchange cross-sections for sulfur in N2, the beam profiles of 36S are estimated from 36Cl beam profiles.

11 Torr = 133.3 Pa = 1.333 mbar

32 Flight path: - Lorentz force - Small-angle multiple scattering B - Energy loss

- Energy straggling values integral gas

m, qi, Z

Ion-atomic collision with charge exchange processes

Figure 4.2: Model of the physical processes of an ion traversing a gas-ﬁlled magnetic region.

36 36 The qm-trajectories of Cl and S are calculated for 45, 38, and 33 MeV kinetic energies and 1–5 Torr N2 gas pressures in the same way as described by Zoppi et al. [1994]. The stopping powers used are interpolated from the stopping power tables for chlorine ions in N2 calculated by SRIM2000 [Ziegler et al., 1985; Ziegler, 1999, and http://www.srim.org]. The radial coordinates of obtained qm-trajectories of isobars provide a measure of the distance, ∆ρ, between isobaric beams. Several models of ion beam optics in a gas-filled magnet have been presented earlier [Paul et al., 1989; Zoppi, 1993; Hatori et al., 2000] and applied to various spectrometer designs. Our calculation has adopted a similar approach as that presented by Paul et al. [1989] and Zoppi [1993], except for some details, such as modified angle scattering and stopping power tables. The 36Cl beam profiles along the trajectory are simulated by a Monte-Carlo approach. The computer code is written in a general form, capable of dealing with non-dipole magnetic fields and various ion species, gases, and incoming beam emittances. The critical parameter in the input is the charge-exchange cross-section, which can be estimated theoretically, but is preferably obtained experimentally. The model of the physical processes of an ion traversing a gas-filled magnetic region used in the calculation is as follows (Figure 4.2): a flight interval, where ions are subjected to the Lorentz force, energy loss, energy straggling and angle scattering, followed by a charge-exchange collision, where ions lose or capture electrons. The charge-exchange cross-sections for 35Cl at 36 and 45 MeV kinetic energy in N2 are taken from Zoppi [1993] and extrapo-

33 2.5 a) R = 60 cm B ~ 0.9 T 2.0

1.5 2.5 Torr 1.0 2 Torr 3 Torr 1.5 Torr FWHM 0.5 R 3.5 Torr 1 Torr

0.0 2.5 b) R = 80 cm B ~ 0.65 T 2.0

1.5

Separation = 1 / res Separation 1.0 2 Torr 1.5 torr 0.5 2.5 torr 1 Torr

0.0 2.5 c) R = 100 cm B ~ 0.5 T 2.0

1.5

1.0 1.5 Torr 0.5 1 Torr 2 Torr

0.0 0 50 100 150 200 Deflection angle (°)

Figure 4.3: Separation diagrams for the isobar pair 36Cl and 36S at 38 MeV initial energy in N2 gas. Separation is deﬁned as the ratio of the distance, ∆ρ, between the 36 36 qm trajectories for isobars Cl– S to the full width at half maximum FWHMr of the 36Cl beam (see insert). Three bending radii are tested: a)R=60cm,b)R=80cm, and c) R = 100 cm.

34 lated as ∝ E−1.65 for other energies E [Zoppi et al., 1994; Zoppi, 1993]. The Schlachter’s electron capture formula [Schlachter et al., 1983] is not used as in Paul et al. [1989], since this formula does not cover our ion energy region. The mean free path is calculated from the charge-exchange cross-sections, and the path length (of the order of mm) between charge-exchange collisions is simulated according to Paul et al. [1989]. The equations of motion in the dipole magnetic field are integrated along each flight interval. Although ion interaction with gas is a continuous process, the net effect of energy loss, energy straggling, and angle scattering in each flight interval is introduced at consecutive collision points for the gas thickness, τ, of the interval. This sim- plification is justified by the short distances between charge-exchange collision events: Since the flight interval length is of the order of mm, the constant stopping power can be used between collisions. Furthermore, a difference of less than 1% in the final results is observed if energy loss, energy straggling, and angle scattering are applied before the flight interval between collisions. The stopping power tables used are the same as for the calculation of the qm-trajectories, and the energy straggling is calculated according to Bohr’s formula [Bohr, 1948]. The angle scattering is obtained using a modified (see below) Sigmund-Winterbon small-angle multiple scattering theory [Sigmund and Winterbon, 1974]. The calculations are compared with the TRIM code (http://www.srim.org) with regard to stopping and angle scattering, and we find that a scaling factor depending on gas thickness (specially for low-τ values) has to be introduced to the Sigmund-Winterbon small-angle scattering table in order to improve the agreement in ion-beam profiles. The program was also tested with measurements from the Zürich GFM [Zoppi et al., 1994], and reasonable agreement was found. Our final calculations are performed assuming a Gaussian beam emittance at the entrance of the magnet (σx × σx = 1.5 mm × 2 mrad). The obtained profiles are fit to a Gaussian shape to simplify analysis. However, the omitted non-Gaussian tails might influence the background of the isobar separation.

4.3 Results and discussion The results of the simulation for various pressures and radii are presented in Figure 4.3, where the isobar separation is deﬁned as the ratio of the distance, ∆ρ, between isobar beams to the full width at half maximum (FWHMR)of the 36Cl beam in the deﬂection plane (see insert in Figure 4.3a). It is generally observed that the separation rises constantly until 150◦, because the distance, ∆ρ, between isobar beams (Figure 4.4) grows faster than does the 36 ◦ Cl FWHMR. For bending angle >150 , both the distance, ∆ρ, and the beam

35 50 1 Torr 3 Torr R = 60 cm 40

(mm) 30 ∆ρ

10 Distance

0 0 50 100 150 200 Deflection angle (°)

Figure 4.4: Distance ∆ρ between 36Cl and 36S beams at 38 MeV initial energy in 1 Torr (upper curve), 2 Torr (middle), and 3 Torr (lower) of N2 gas for the bending radiusR=60cm.

width, FWHMR, are constant due to the focusing properties of the magnet, and therefore the isobar separation has a broad maximum for bending angles in the range of 150–180◦ for all three radii at their optimal pressures. The maximum separation is found to be almost independent of the bending radius. Generally, larger magnets require lower gas pressures and lower magnetic fields. On the other hand, our calculations show that for a larger magnet a wider gap is necessary in order to improve the acceptance for larger scattering, because the beam size at the exit increases for larger bending radii. Thus it is not obvious that a large magnet, which requires more laboratory space and a larger detector, is the best choice; on the contrary, the same performance can be obtained with a smaller magnet. The comparison of the bending angles in the range of 150–180◦ can be done using other data from the simulation: extension of the 36Cl beam in the deflection and non-deflection directions is plotted in Figure 4.5 (supplementary to Figure 4.3a) for the 150–180◦ bending angles. Since separation reaches a maximum at 2–3 Torr and 150–180◦, only these intervals are to be examined. Lower bending angles (≤140◦) produce lower separation, because there is still the potential to gain more from the distance, ∆ρ, between isobar beams (Figure 4.4). For example, the variation in ∆ρ for 140–150◦ is the same as for the consecutive range of 150–180◦. Five examples (Figure 4.5) are provided for the 150◦ and 180◦ bending angles as an illustration. Given the same gas pressure, beam extension in deflection plane Y is the same for 150–180◦, and since the distance, ∆ρ, between isobar beams is almost constant (Figure 4.4), there is a similar separation for these angles (Figure 4.3a). On the other hand, the beam extension in the non-deflection direction, Z, is considerably lower at 150◦, while the ion has about 2 MeV more residual energy at 150◦ than at 180◦ (for example, 2 Torr: 26 MeV and 24 MeV). Hence, the 150◦ bending angle

36 50 45 R = 60 cm Y 40 180° 170° (mm) 35 160° 150° 30 25 Z 20 180° 170° 150°, 2 Torr 15 160° 180°, 2 Torr 150° 150°, 2.5 Torr beam extension 10 180°, 2.5 Torr σ 5 150°, 3 Torr ±2 0 00.511.522.533.54 Pressure (Torr)

Figure 4.5: Extension (±2σ)of36Cl beam in the deflection (Y) and non-deflection ◦ (Z) directions in N2 gas for the bending angles 150–180 . The initial kinetic energy is 38 MeV and the bending radius is R = 60 cm. Straight lines are obtained for the beam extensions in the non-deflection direction. Curves correspond to the deflection direction. Selected examples are discussed in text. has better merits than does the 180◦ bending angle, even though the isobar separation is similar. This result is in accordance with Knie et al. [1997].

4.4 Conclusions A general conclusion from this work is that using the same magnetic field and radius it is possible to construct various dipole gas-filled magnets of similar separation, but with various deflection angles in the 150–180◦ range. How- ever, a deflection angle of approximately 150◦ is preferable due to less beam extension in the non-deflection plane and greater residual energies. In this study we have concentrated on the optimization of a stand-alone gas- filled magnet, although other parameters can improve the overall performance of such a spectrometer. For example, it is possible to make a more complex simulation to optimize the separation of the combined GFM–detector system [K. Knie, pers. comm.]. Another proposed improvement of a GFM system, which has yet to be tested experimentally, is to introduce the detection of the ion’s position and energy along its trajectory inside a GFM [D. Elmore, pers. comm.].

4.5 The Uppsala gas-ﬁlled magnetic spectrometer From our calculations and considering the beam heights in our laboratory (1500 mm), limited laboratory space in the experimental area, and the hor-

37 1 m

Figure 4.6: Drawing of our gas-filled magnet. izontal focusing plane of the electrostatic deflector, the following design was chosen: a vertically mounted GFM, with a 600 mm bending radius, 180◦ deflection angle, and 36 mm free height inside vacuum chamber. An isobar suppression factor can be estimated for 36Cl and 36Sat38MeV ◦ kinetic energy in a 180 dipole GFM of 60 cm radius filled with 2.5 Torr N2. In this case the isobar mean trajectories are separated by 47 mm, whereas the width of the 36Cl beam is 34 mm (±2σ). If the beam profile is assumed to be Gaussian, only a 2 × 10−4 part of the 36S beam will cause interference within ±2σ of the 36Cl beam profile measured from its center. This suppression factor is reasonably close to the 10−3 estimate of Zoppi [1993] for 36Cl and 36 S at 48 MeV, with 3 Torr N2 in a GFM, a 30 mm diameter detector, and a beam profile with non-Gaussian tails caused by the wide-angle Coulomb scattering. This GFM in combination with a gas-ionization detector is expected to further improve the suppression of 36S by a factor of ∼100 compared with a stand-alone gas-ionization detector. Suppression factors for a stand-alone gas-ionization detector were taken from Suter [1990]. Position sensitivity and energy difference between isobars after the GFM, which will additionally improve the isobar suppression, are not taken into account in this estimate. The GFM (Figure 4.6) was constructed by Danfysik2 and installed at the Uppsala Tandem Laboratory in November 2002. Since that time several preliminary tests have been conducted with stable isotopes of chlorine (e.g. Fig- ure 4.7), but no AMS measurements have been made due to limitations on the accelerator side. In summer 2004 the accelerator tubes were finally replaced,

2http://www.danfysik.com

38 and currently the accelerator can reach its nominal terminal voltage (5 MV). 36Cl AMS is at the top of our priority list, and we plan to test our GFM for 36Cl AMS as soon as possible.

qm = 9.1 6.7

2.5 mbar N2

+11 +12 -2 +10 <10 mbar N2

Intensity (arb. units) 120 A 260 A +13 +9 +14 +8

Magnetic field expressed as a current through the coil (A)

−2 Figure 4.7: Magnetic ﬁeld scan of GFM for <10 mbar N2 (lower plot, discrete 37 peaks from the entrance foil) and for 2.5 mbar N2 (upper plot, single peak). Cl beam at initial 32 MeV (= 36 − 4 for energy loss in 150 µgcm−2 entrance foil). In the case of 2.5 mbar N2, chlorine ions have 15 MeV after GFM. Scales for the plots are the same.

5 129I in marine waters

This chapter describes the temporal and spatial distribution of 129I in the marine waters of the Arctic and Northern Atlantic Oceans. As was noted in Chapter 2, the full potential of this tracer can only be achieved through the establishment of time series measurements, calculation of inventory balances, and development of models of 129I transport. In our studies of the marine 129I pool, attention has been paid to all these objectives as well as to the origins and pathways of the water masses and estimations of their transient times.

5.1 129I in the Arctic Ocean in 2001 Among the radioactive contaminants, a relatively large amount of 129I has been observed to accumulate in the ecologically sensitive waters of the Arctic Ocean [Kilius et al., 1993; Ellis et al., 1995; Kilius et al., 1995; Josefsson, 1998; Smith et al., 1998, 1999; Buraglio et al., 1999]. Most of this 129I origi- nates in releases from the nuclear reprocessing facilities at Sellafield, UK, and La Hague, France, (Figure 5.1), and 129I from these releases is abundant in the top 1000 m of the Eurasian basin of the Arctic Ocean. The transit time of 129I from Sellafield to the central Arctic Ocean has been estimated to be 6–10 years [Dahlgaard, 1995] and 10–11 years [Smith et al., 1999; Buraglio et al., 1999]; the transit time for La Hague discharge is two years shorter. The dramatic increase in 129I discharges from the two facilities during the 1990s (see Figure 2.2) consequently reached the Arctic region by the beginning of the twenty-first century. In paper 2 we present the data from one of the most recent expeditions to the region, Arctic Ocean 2001, which aimed to continue monitoring 129I releases and update changes in the inventory and pathways of the isotope. Sampling was accomplished from the Swedish icebreaker Oden during July 2001. Surface seawater was collected along a transect from the Norwegian Sea to the North Pole (Figure 5.1). Three depth profiles were sampled in the Arctic Ocean, namely, from the Nansen, Amundsen, and Makarov Basins. Results of 129I measurements are shown in Figures. 5.2–5.5. The large difference (about 20 times) between the 129I concentration in the surface waters of the Norwegian Coastal Current (NCC) and of the Arctic Ocean (Figure 5.2a) can be related to: (1) delay in the transport of the conta-

41 INSERT 2001: this study 1996: Buraglio et al (1999) North 1996: Josefsson (1998) America Surface sample ? Depth profile Makarov Amundsen Arctic 90˚E Ocean Laptev Sea BSB Nansen

? 90°E FSB 80˚N Makarov Amundsen St Anna Trough Kara Nansen Sea 80°N Svalbard Fram Svalbard Strait 0˚ Barents Sea WSC NC 70°N EGCNordic Seas

60°N NWAC NCC

Surface currents NAC North Atlantic Layer Sea EuropeArctic Outflow Sellafield La Hague 129I pathways 0°

Figure 5.1: Location of surface samples and depth proﬁles collected in 2001. General circulation of major water masses, including NAC, North Atlantic Current; NWAC, Norwegian Atlantic Current; NCC, Norwegian Coastal Current; WSC, West Spitsber- gen Current; NC, North Cape Current; EGC, East Greenland Current [Rudels et al., 1994; Jones et al., 1998; Jones, 2001]. Insert: Location of samples collected in 1996 together with samples of this study in the Arctic Ocean.

42 11 a) 9 b)

10 ) 2.5x10 -1

10 10 2.0x109

) [LOG scale] 9 -1 10 1.5x109

108 NCC FS & BS Arctic Ocean 1.0x109

7 (2) I concentration (atoms l 10 129 5.0x108 106

I concentration (atoms l (1) Nansen-Gakkel Ridge 5 Lomonosov Ridge 129 10 0 60 65 70 75 80 85 90 0 30 60 90 120 150 Latitude (°) Longitude (°)

Figure 5.2: (a) 129I concentration in surface water along the transect from the Norwe- gian coast to the North Pole [log scale]. Dashed lines show: (1) pre-atomic era natural level, and (2) post-atomic bomb tests background level. (b) 129I concentration in the part of the transect covering the central Arctic. NCC, Norwegian Coastal Current; FS, Fram Strait; BS, Barents Sea. minant of about 6 years from the south of Norway to the central Arctic Ocean, (2) dilution by 129I-poor marine and riverine waters, and (3) sinking of most of the 129I-rich Atlantic surface waters (Fram Strait Water Branch, FSB, and Barents Sea Water Branch, BSB; insert in Figure 5.1) into deeper layers in the Arctic Ocean. The 25%–75% division of NCC’s 129I between the West Spits- bergen Current (WSC) and the North Cape Current (NC), as suggested by Gascard et al. [2004], introduces an uncertainty in the estimation of the delay that complicates the exact labeling of the circulation pattern for the 129I signal in the central Arctic region. Such a response can be indicated by the 129I distribution in the surface of the Arctic Ocean, which shows concentrations that vary by a factor of 1.5. Low values are observed inside the basins and high values near the ridges (Figure 5.2b). This variation cannot be explained by a single-handed inclusion of freshwater (ice meltwater or riverine waters) and might be attributed to features of the surface circulation in the region (note the arrow with a question mark, Figure 5.1). Distribution of 129I concentration in the sampled depth profiles (Figure 5.4a) seems to be consistent with the two-branch circulation pattern of the Atlantic waters [Figure 5.1, Rudels et al., 1994]. In the Nansen basin, 129I is found predominantly in the upper part of the Atlantic layer, thus reflecting the Fram Strait water branch. In the Amundsen basin, however, 129I is uniformly distributed at 200–1000 m depths in the Atlantic layer and upper polar deep waters, thus confirming the addition of the denser Barents Sea water branch to the Fram Strait water branch. The uniform distribution of recent Atlantic waters at 200–1000 m depths of the Amundsen Basin is clearly observed in the

43 Nansen basin Amundsen basin Makarov basin Salinity (‰) 30 32 34 36 30 32 34 36 30 32 34 36

1000

2000

Depth (m)

3000

4000 Temperature (°C) -2 -1 0 1 2 -2 -1 0 1 2 -2 -1 0 1 2 0 6.0x108 1.2x109 1.8x109 0 6.0x108 1.2x109 1.8x109 0 6.0x10899 1.2x10 1.8x10 129I Salinity T 129I concentration (atoms l-1)

Figure 5.3: 129I concentration, salinity, and temperature along the depth proﬁles sampled in interiors of the Nansen, Amundsen, and Makarov Basins examined in this study.

129I profile — an observation not traced in the temperature and salinity data (Figure 5.3). Apparently, recent Atlantic inflow has had less influence on the Makarov Basin than on the other basins investigated in this study, and the influence can be found only in the polar mixed layer and upper Atlantic layer (0–300 m, Figure 5.4a), which are similar in terms of their 129I concentration to that of the Amundsen basin. The concentrations of 129I in the investigated waters of the Arctic are much higher than the pre-atomic era natural or even post-bomb background levels (4000 and 60 times, respectively, for the surface waters; assuming natural and post-bomb backgrounds 129I/127I = 1.5 × 10−12 and 129I/127I = 10−10, respectively [Moran et al., 1998; Fehn et al., 1986]). Comparison of data for year 2001 with earlier 129I measurements from the Arctic Ocean 1996 expedition [insert in Figure 5.1; Buraglio et al., 1999; Josefsson, 1998] suggests several interesting observations. First, a two-fold increase in 129I concentrations appears in the data for year 2001, from the surface down to a depth of 1000 m (Figures 5.4b and 5.5). This change happened during the five-year period between the expeditions, and the most strongly in- fluenced part of the central Arctic Ocean is the Amundsen Basin. The Nansen Basin is less affected, while the Makarov Basin below the polar mixed layer does not show any temporal change at all. These temporal variations in 129I concentration (Figure 5.5) are consistent with the circulation pattern of the Atlantic layer described above (Figure 5.1). The 129I inventory in the upper 1000 m of the central Arctic Ocean (“ac-

44 a) b) 129 -1 I concentration (atoms l ) 0 5x1089 1x10 1.5x10 99 2x10

0 PML )

-1 9 H 2.0x10 AL 9 1.5x10 1000 1.6 UPDW 9 1.0x10

2000 DW 8 0.7 5.0x10 I concentration (atoms l Depth (m)

129 0.0 0 30 60 90 120 150 3000 Nansen basin BW Amundsen basin Longitude (°) Makarov basin

4000

Figure 5.4: (a) 129I concentrations along the three depth proﬁles of the central Arc- tic compared with water mass distribution. PML, Polar Mixed Layer; H, halocline; AL, Atlantic Layer; UPDW, Upper Polar Deep Water; DW, Deep Water, BW, Bottom Water. Water mass deﬁnitions taken from Jones [2001]. (b) Comparison between 129I concentration in surface waters of the Arctic Ocean collected during 1996 and 2001. The two dashed lines and numbers denote the average 129I concentrations in 1996 and 2001.

129 I concentration (atoms l -1) 9 9 9 9 9 9 0 1x10 2x10 0 1x10 2x10 0 1x10 2x10 0 Doubling Doubling

1000 Doubling

2001: center of basin 2001: near LRidge 2000 1996: southern boundary 1996: near LRidge (st 5, Josefsson, 1998) 2001: center of basin Depth (m) (St 69, Buraglio et al, 1999) 1996: northern boundary 1996: eastern part of (st 6, Josefsson, 1998) basin (Buraglio et al, 1999) 1996: center of basin 3000 (St 33, Josefsson, 1998)

Nansen basin Amundsen basin Makarov basin 4000 Figure 5.5: Comparison of 129I concentrations in the Nansen, Amundsen, and Makarov Basins for samples collected in 1996 and 2001. LRidge, Lomonosov Ridge.

45 tive” layer) in 1996 and 2001 was estimated at about (1.4±0.1)×1027 atoms (290 kg) and (2.1 ± 0.1) × 1027 atoms (440 kg), respectively. Hence, the amount of 129I in the “active” layer of the central Arctic has increased by 50% in ﬁve years almost solely because of the contribution of the Eurasian basin. Comparing this enhancement with the known history of releases from the nuclear reprocessing facilities in western Europe, it is expected that the 129I inventory in the “active” layer (0–1000 m) of the central Arctic Ocean will increase by 100% within a period of ﬁve years starting in 2001. As the 129I in the polar mixed layer and Atlantic layer of the Arctic Ocean is venti- lated by the East Greenland Current into the Nordic Seas and North Atlantic Ocean, further dispersal and increase of the isotope concentration in these regions will be encountered in the near future.

80°N 23 19 14 11 30 YP FS WSC 70°N EGC 64 GS 44 Greenland69 88 6158

DS IS 85 NwS 83 IC Iceland NWAC NCC Scandinavia

North Sea

30°W Sellafield La Hague 20°W 10°E 10°W 0°

Figure 5.6: Map showing the main currents in the Nordic Seas and the sampled sections in the western Nordic Seas. Red arrows are the surface input of 129I. Numbers denote sampling stations. FS, Fram Strait; DS, Denmark Strait; GS, Greenland Sea; IS, Iceland Sea; NwS, Norwegian Sea; WSC, West Spitzbergen Current; EGC, East Greenland Current; IC, Irminger Current; NWAC, Norwegian Atlantic Current; NCC, Norwegian Coastal Current; YP, Yermak Plateau.

46 5.2 129I in the western Nordic Seas in 2002 The Nordic Seas represent a region where return flow of the thermohaline circulation (“the conveyor belt”) starts. The region offers the potential for se- questering anthropogenic CO2 into the world ocean [Anderson et al., 2000]. The formation of dense waters in the Nordic Seas is caused by open-ocean cooling and is moderated by near-surface air temperature [Bacon, 1998], the North Atlantic Oscillation [Marsh, 2000], and fresh waters [e.g. Rahmstorf , 1995] that are brought from the Arctic Ocean by the East Greenland Current (EGC). The EGC also brings the intermediate and deep waters of the Arctic Ocean, together with derivatives of Atlantic Waters (AW), into the Greenland and Iceland Seas, where they join the deep water formation. There is no clear consensus as to how and where these waters are modified into deeper ones, which contribute to outflows into the North Atlantic [Swift et al., 1980; Strass et al., 1993; Mauritzen, 1996; Rudels et al., 1999; Jonsson and Valdimarsson, 2004]. Hence the natural and anthropogenic influence on deep water formation and thermohaline circulation cannot be accurately estimated. Any model of water mass circulation and transformation in the Nordic Seas would likely be time-dependent, since rapid temporal changes in convective activity in the region have been observed [Dickson et al., 1996; Bönisch et al., 1997]. In paper 3 we present new data about the distribution of radioactive 129Iin water masses of the western Nordic Seas. Sample collection was performed during a relatively short and specific time window (April–May, 2002). This implies both a well-defined temporal span and the first ever opportunity to capture early spring mixing and circulation signals while ice cover still per- sists in most of the region. Sampling was done from the Swedish icebreaker Oden in early spring 2002 (April–May) during the Arctic Ocean 2002 expedition organized by the Swe- dish Polar Secretariat as part of the Nordic Seas 2002 expedition, held jointly with American colleagues based on the US R/V Knorr. Concentrations of 129I were measured along three sections (Figure 5.6); temperature and salinity data were taken from Rudels et al. [in press]. The 129I concentration, temperature, and salinity data for the northern Fram Strait (NFS) and 72°N sections are shown in Figure 5.7. The most dynamic part of the NFS section is found in the upper 1000 m, while deeper waters seem to display less variability, especially in terms of temperature and salinity. The 129I concentration below 1000 m on the western side and in the center shows about a two-fold change every 500 m of depth, and generally larger values than at the same depths on the eastern side of NFS. This might be attributed to a relatively weak but still non-negligible near-slope propagation of the 129I signal of AW by shelf/slope convection upstream on the Arctic shelf. In contrast to NFS, the east side of the 72°N section (Figure 5.7b) exhibits

47 no variability in 129I concentration from the surface down to 1000 m, thus indicating fresh convective activity in the Greenland Sea (GS) gyre and defin- ing the Arctic Intermediate Waters (AIW) of GS. Below the convective zone, 129I concentration abruptly decreases (10 times between 1500 and 2000 m), a feature that has been noted as occurring, though less dramatically so, in other tracers in the 1980s and 1990s [Bönisch et al., 1997]. Bottom waters (>2500 m) in this section and in NFS have 129I concentration of ∼0.2×108 atoms l−1, which is considered to be typical of North Atlantic seawater after nuclear weapons tests [Moran et al., 1998; Edmonds et al., 1998]. This value is already 10 times lower than the relatively high concentration of 129I, 2×108 atoms l−1, observed in the surface of GS in 1981 [Edmonds et al., 1998]. Accordingly, the bottom waters of GS have received a very limited amount of anthropogenic 129I from the nuclear reprocessing facilities. From this near bottom concentration we estimated that the annual overturning rate for depths >2000 m in GS was not more than 0.3–0.7% averaged over the last two decades. This value is close to the 0.5–1.2% estimate of average deep water production for similar depths and a similar time period from CFC-11 and CCl4 [Anderson et al., 2000]. The variability of 129I concentration, temperature, and salinity in the dy- namically active upper 750 m of the NFS, 72°N, and Denmark Strait (DS) sections is plotted in Figure 5.8. Although T-S data clearly discriminate between the warm and saline water mass and the overlying colder and fresher surface waters in the center and east of the NFS section (Figure 5.8a), the 129I distribution in this part of the transect is distinctly different from that of the salinity and temperature data. Both waters, presumably, comprise AW of the West Spitzbergen Current (WSC), which becomes fresher and colder at the surface due to melting ice. The variation in 129I concentrations of AW likely reflects differences in the sources of 129I to AW, which can be explained by, for example, incomplete admixing from the Norwegian Coastal Current (NCC) to the Norwegian Atlantic Current (NWAC, Figure 5.6). The western side of NFS is occupied by relatively fresher and colder waters (Figure 5.8a) that constitute EGC and include Polar Waters (PW, S<34.3‰, intermediate 129I concentration 10–7×108 atoms l−1) on the surface and a deeper layer of presumably modified AW of the central Arctic Ocean (2– 4×108 atoms l−1). A similar range of 129I concentrations was observed in the Amundsen basin in 2001 suggesting the dominant source of these waters (Figure 5.5). In the 72°N section the cold and fresh core of PW of EGC is clearly labeled with a high 129I (10–12×108 atoms l−1, yellow, Figure 5.8b) content and is located above the shelf. Indications of this water are also found at the western- most station (#30) of the NFS section. Therefore the high 129I concentration in the 72°N section may reflect either solely PW of the Arctic Ocean and/or

48 the addition of cold and fresh near-surface AW through recirculation1 in the Fram Strait. The hypothesis of sole Arctic contribution is strengthened by the fact that the measured 129I concentration in PW of EGC falls in the range of concentrations found in PW over the Amundsen Basin and Lomonosov Ridge in 1996 (Figure 5.4b). PW of the 72°N section are embedded in warmer and more saline waters with a lower 129I concentration (7–10×108 atoms l−1, green, Figure 5.8b). The subsurface warm core, a sign of recirculated AW, is not distinctive in terms of 129I, most probably because there are other waters with similar 129I contents that are also part of the core but have experienced heat loss. East of the 72°N section corresponds to the GS gyre and its AIW mentioned above (sky blue 129I concentration, Figure 5.8b). Signature of the cold and fresh PW of EGC with the high 129I concentration (10–12×108 atoms l−1) observed on the Greenland shelf continues to the Den- mark Strait (DS, Figure 5.8c). Warmer and more saline waters with a slightly lower 129I concentration, 8–9×108 atoms l−1, underlie PW and suggest incor- poration of the recirculated AW there. The eastern side of the DS section is occupied by warm and saline waters with an extremely low 129I concentration, 0.1–0.6×108 atoms l−1, which is about or slightly higher than the background of North Atlantic seawater after nuclear weapons tests [Moran et al., 1998; Edmonds et al., 1998]. These clearly 129I-labeled North Atlantic waters form the Irminger Current (IC, Figure 5.6). The transition between EGC and IC occurs abruptly in the upper 150 m in the center of the DS and is underlain by a cavity filled with dense waters, which have a complex pattern of intermediate 129I concentrations, temperature, and salinity. The densest water mass is located in the bottom of the sill, close to the Icelandic slope of the strait. Although its T-S data can link it to GS, its too low 129I concentration (2×108 atoms l−1, blue, Figure 5.8c) makes the connection doubtful. These waters are surrounded by less dense waters, which are warmer and can be identified as AIW of GS by their 129I content (5×108 atoms l−1, sky blue, Figure 5.8c). Adopting the fluxes of seawater through the Nordic Seas presented by Rudels et al. [1999], the input of 129I into the Arctic Ocean in 2002 was 4×1025 atoms y−1 (∼10 kg y−1). This is half of the 129I burden (23 kg y−1)inthe Fram Strait branch of NWAC in 2000 [Gascard et al., 2004]. The output of 129I from the Arctic Ocean (PW+modified AW) in 2002 was (1.3+0.9)×1025 atoms y−1 or only 1% of the Arctic Ocean inventory in the upper 1000 m in 2001 (see section 5.1). Outflow through DS, comprising the 129I burdens carried by surface + intermediate waters, is estimated to be (3 + 3) × 1025 atoms y−1, which is three times greater than at NFS. Both surface and subsurface waters have gained 129I. The former has received either unaccounted

1The recirculation was also reported by Rudels et al. [1999].

49 50

129I concentration (x108 atoms l-1) Salinity (‰) Temperature (°C) a)

NFS (Nothern Fram Strait) Depth (m)

72°N b)

Figure 5.7: 129I concentration, salinity, and temperature in sections (a) northern Fram Strait and (b) 72°N. Note that each section has a different length (see Figure 5.6). 129I concentration (x108 atoms l-1) Salinity (‰) Temperature (°C) a)

NFS (Nothern Fram Strait)

b) Depth (m)

72°N

DS (Denmark Strait) 51

Figure 5.8: 129I concentration, temperature, and salinity in upper 750 m of sections (a) northern Fram Strait, (b) 72°N, and (c) Denmark Strait. Note that each section has a different length (see Figure 5.6). for Arctic inflow over the shelf and/or recirculated AW from WSC, while the latter has been replaced by AIW of GS and some dense, low in 129I, presently unidentified water mass. To summarize, in this study the distribution of 129I was used for the labeling water masses in three sections of the western Nordic Seas. An increase of the tracer in the Polar Waters of the East Greenland Current was observed between the Fram Strait and the 72°N section and attributed to either unaccounted for Polar Waters and/or the recirculation of cold and fresh Atlantic Waters from the West Spitzbergen Current. Recent convection had homogenized 129I in the upper 1000 m of the Greenland Sea and similar concentrations were observed in the dense waters of the Denmark Strait. The densest outflow waters were not found in either the Greenland Sea or the East Greenland Current at 72°N.

5.3 129I along a transect from North Atlantic to the Baltic Sea in 1999

70°Nû Surface Deep profile Norwegian Sea Surface (Hou et al, 2001) 65°Nû

Bothnian3 Sea 60°Nû 13 12 11 10 2 9 7 1 North 8 SK5 NAC Sea 6 Baltic 55°Nû Sea Sellafield 4

50°Nû La Hague

45°Nû 30°W 20°W 10°Wû 0°û 10°E 20°E 30°Eû

Figure 5.9: Location of samples collected in 1999. “SK” denotes the region of Skager- rak and Kattegat. Full arrows correspond to surface ﬂows in the Northern Atlantic, the North Sea, and the Baltic Sea, while the dashed line represents the deep ﬂow into the Baltic Sea.

The Baltic Sea and the basins of Skagerrak and Kattegat have been, and still are, among the sinks for radioactive and other waste from Central and Northern Europe. Among the radioactive waste that has apparently started

52 to accumulate in the Baltic Sea and the basins of Skagerrak and Kattegat is 129I. In this study, we present the results of 129I measurements in surface and deep waters of the Baltic and Bothnian Seas and compare them with results of 129I measurements in the surface waters of Skagerrak–Kattegat, the North Sea, and the North Atlantic. The samples were collected in 1999 and comprise two subsets (Figure 5.9): (1) a continuous transect in surface water extending from the North Atlantic to the Baltic and Bothnian Seas, and (2) three depth profiles, two in the Baltic Sea and one in the Bothnian Sea. Both 129I datasets are unique and therefore potentially represent initial points for time series. The range of 129I concentrations in the transect covers about four orders of magnitude (107–1011 atoms l−1, Figure 5.10a). The lowest and highest values are encountered in the surface water of the western branch of the North Atlantic Current (NAC) and in Skagerrak, respectively. Adopting the general surface water circulation pattern of Figure 5.9, it is apparent that the 129I data have captured part of the Sellafield plume in the eastern (and main) branch of NAC. This feature gives a specific character to this NAC water band, which can be used for the detailed description of water parcel interaction in this part of the North Atlantic. Most of the NAC extends into the Norwegian Sea, while a minor part of the NAC diverts into the North Sea. There, the Sellafield-labeled waters of NAC mix with Atlantic waters that come through the English Channel and carry marine discharges from the La Hague facility. Circulation in the North Sea is highly variable on a local scale, but part of the outflow circulates into the Skagerrak–Kattegat–Baltic Sea and further along the coast of Norway into the Norwegian Sea. Our 129I transect reflects this outflow path, which is most likely the La Hague-dominated plume2. The 129I distribution in the surface water (Figure 5.10) shows a decrease by a factor of 2 from the central Baltic Sea to the Bothnian Sea. There is also an order of magnitude increase in 129I concentration with depth in all three profiles studied here (Figure 5.10b). The increase occurs at depths between 40 and 80 m and seems to agree with the position of the halocline [HELCOM, 1986]. Apparently, this pattern of increasing 129I with depth strongly reflects the dominance of the more saline North Sea water (Figure 5.9) that enters the Baltic Sea and forms a bottom layer [HELCOM, 1986]. An explanation of the lower concentration of 129I in the Bothnian basin could be the smaller amount of overflow water from the main portion of the Baltic Sea [HELCOM, 1986]. A simple inventory calculation shows that a relatively large amount of 129I resided in the Baltic and Bothnian Seas in 1999 (3 × 1025 atoms or 0.3% of the total liquid releases up to that time). The source of this 129I is mainly marine transport from the North Sea, which is highly contaminated by releases

2See the release pattern in Figure 2.2 of Chapter 2. Transient times from La Hague and Sellaﬁeld to Kattegat are 1–2 and 3–4 years [Hou et al., 2000a, and references therein].

53 a) Surface transect b) Depth profiles in the Baltic Sea 1012 129 -1 Skagerrak I (atoms )

) 1089 10 1010 -1 North 0 1010 North Atlantic Sea Current 40 Station 8 Baltic Sea I (atoms 10 80 1 Current marine background 2 129

Depth (m) 120 3 106 Natural concentration 160 -30° -20° -10° 0° 10° 20° 30° Longitude

Figure 5.10: (a) 129I concentrations along a surface water transect from the North Atlantic to the Baltic Sea in 1999 (see Figure 5.9). Estimate of natural concentration is taken from [Moran et al., 1998]. (b) 129I concentrations at three stations of the Baltic Sea in 1999. See Figure 5.9 for locations. from the La Hague and Sellaﬁeld nuclear reprocessing facilities. Contribution from fresh water systems (rivers and precipitation) is found to be negligible [Kekli et al., 2003]. The observed levels of 129I may not represent an environmental hazard, although further research is needed into the distribution of 129I between water, sediment, and biomass to evaluate the situation.

5.4 Numerical modeling of 129I A compartment model [ANWAP, 1997] was used for simulating 129I dispersion in the Arctic region. The model was modified with respect to the geographic location of the input function and corresponding fluxes (Figures 5.11 and 5.12). At time equal to 1970, the 129I inventories in all compartments are 129 set to zero and discharge of I, Q1(t), is introduced in compartment #1. Q1(t) is defined as a sum of known discharges from Sellafield and La Hague facilities with a delay of 2 and 4 years, respectively (Figure 2.2). A single scenario was considered, where the discharge function was extended until 2010 at a constant annual value from 2002. Thereafter zero discharge is assumed mark- ing a fictitious closing of releases from the reprocessing facilities in Sellafield and La Hague in already 2006 and 2008, respectively. The model has successfully simulated the variability of 129I inventory in the Arctic Ocean and the Nordic Seas (Figure 5.13). Results of the simula- tions suggest fast transport rates and large inventory of the anthropogenic 129I in the Arctic and North Atlantic Oceans. In a fictitious case of abrupt clo- sure of the reprocessing facilities, a rapid decline of inventories is observed in all compartments except the North Atlantic Ocean, the deep Nordic Seas and the deep Arctic Ocean. Fifteen years after the end of reprocessing, the

54 23* 20,21 14

13 15 11 12 9 16,17 10

18,19 8 7 6

WSC 5 NC 3,4 1,2 NCC/ 22 NWAC NAC S LH

Figure 5.11: Map of the Arctic Ocean and Nordic Seas showing locations of water compartments used in this study. Boxes are numbered according to ANWAP [1997] with an exception for box #23*, which was created for a separate representation of the Pacific and Indian Oceans in the modified RAIG model. Two numbers in a single area denote upper and lower compartments of a two-layer structure. Hatched gray area of compartments 3 and 4 of the original RAIG model was transferred to compartments 1 and 2 of the modified RAIG model. S = Sellafield, LH = La Hague, NAC = North Atlantic Current, NCC/NWAC = Norwegian Coastal Current and Norwegian Atlantic Current, WSC = West Spitzbergen Current, NC = North Cape Current. Dashed arrows represent the pathway of the 129I discharge from Sellafield and La Hague. Solid arrows represent surface water currents.

55 model prediction indicates that near-equilibrium conditions are reached in all compartments.

0.2 0.2 18 18 6 6 0.8 0.4 1.8 0.8 0.4 1.8 0.9

0.7 19 1 0.8 1

0.4 1.1 19 0.4 0.6 0.8 5 0.6 1.3 5 2.3 0.8 0.6 0.8 311.2 3* 1* 0.7 0.2 0.5 1.1 0.7 0.2 1.1 4 2 4* 2* 1.6 1.6 2.4 0.5 0.7 0.3 0.3 22 1.4 22 1.4 1.4 1.4 Flux below 0.5 Removed in the modified RAIG model

a) Original RAIG model b) Modified RAIG model

Figure 5.12: A part of the original (a) and the modified RAIG model (b) showing differences in fluxes. Values near arrows are fluxes in ×105 km3 y−1 (1 Sv = 0.32 × 105 km3 y−1) taken from [ANWAP, 1997; Mauritzen, 1996] and adjusted for mass balance. The Nordic Seas compartments (1–4) of the modified RAIG model are marked with stars to show that their geographic definition have been changed as shown in Figure 5.11.

56 104 Real releases Constant End of reprocessing Atlantic Ocean 103 r Greenland Sea, lowe

102 Norwegianupper Sea Greenland Sea 1 , upper atoms) 10 Barents Sea, south

25 Barents Sea, north lower 100 Norwegian Sea 1970 1980 1990 2000 2010 2020 2030 2040 2050 2060 2070 104

I inventory (x10 103 Eurasian basin, lower 129 Canadian basin, lower pper 102 Canadian basin, upper

Eurasian basin, u East Siberian Sea, shelf 101 Laptev Sea, shelf East Siberian Sea, coastal 100 1970 1980 1990 2000 2010 2020 2030 2040 2050 2060 2070 Year

Figure 5.13: 129I inventory in the different compartments calculated in the model.

6 129I in lake sediment

This chapter focuses on the measurement of 129I in a lake sediment sampled in northeast Ireland, and on the interpretation of these results. This work was conducted as a part of a retrospective search for evidence of radioactive contamination from the Windscale (Sellafield) accident in 1957 on the northeast coast of Ireland (Paper 6). A lake yielding a high-resolution sedimentary record was identified near the northeast coast of Ireland. This site was used for the reconstruction of the radionuclide input history of the region, based on the analysis of a series of cores extracted from the lake. A chronology of sediment accumulation within the lake was established using radioisotopic dating techniques (including 210Pb). High-resolution gamma and alpha spectrometry techniques were used to quantify concentrations of 137Cs, 239,240Pu, and 241Am, all of which were released during the accident. The primary radioactive component of the 131 release was I(T1/2 = 8 days), but this short-lived isotope has long since decayed. However, 129I was also released during the accident, and in a known ratio to 131I. It appears that no profiles of 129I in lake sediment sequences have been published in the literature to date, although concentrations in lake waters have been measured [Buraglio et al., 2001]. Dendro-dated tree ring sequences have been used in an attempt to reconstruct recent atmospheric concentrations of 129I[Rao et al., 2002]. However, redistribution of the isotope was found to occur throughout the trunk, therefore the reconstruction of absolute ambient atmospheric concentrations using this approach was precluded. Measurements have been carried out on sediment cores from marine and riverine systems [Fehn et al., 1986; Moran et al., 1998; Oktay et al., 2000; López-Gutiérrez et al., 2000, 2004]. The data from the Kattegat, located in the strait between Denmark and Sweden [López-Gutiérrez et al., 2000, 2004], do show trends indicative of reprocessing activities, although the core was sampled in 1984 and so does not include recent (post-1990) increases in releases from the facilities. Results of 129I, 137Cs, and 241Am measurements in the sediment core of Ballywillan Lake (northeast Ireland) are presented on Figure 6.1. The trend of the 129I data in the sediment core closely resembles that of liquid releases from Sellafield (Figure 2.2). Profiles for fallout isotopes show

59 129I concentration 137Cs deposition 241Am deposition (x109 atoms g-1) (Bq m-2) (Bq m-2) 0 2684 0 100 200 300 0 2 486 10 0 2000

5 1990

10 1979 CRS year Depth (cm) 15 1968

20 1956

25 1944

Figure 6.1: Concentration of 129I and deposition of 137Cs and 241Am in the core re- trieved from Ballywillan Lake (northeast Ireland) plotted against depth, and showing a secondary y-axis which gives estimated ages at proﬁle depths based on the constant rate of supply 210Pb model (see paper 6). that the sequence is undisturbed. Sediment portions corresponding to the Windscale accident (1957, expected depth 20 cm) show no discernible enhancement in terms of either 129I concentration or other isotopes (137Cs, 238Pu, 239,240Pu, and 241Am) above the weapons fallout background. In light of these ﬁndings, we must conclude that the Windscale Fire of 1957 has had little or no impact on the northeast coast of Ireland.

60 7 Conclusions

The main ﬁndings presented in the thesis are summarized below:

129I AMS at the newly installed Uppsala tandem accelerator • A procedure for routine 129I measurements has been established at the newly installed Uppsala tandem accelerator. Reproducibility of measurements was conﬁrmed. • A lower background relative to the old Uppsala EN tandem accelerator system was observed. This improvement was attributed to better vacuum and higher dispersion of ion optical elements at the new installation. • Pulsing of stable iodine beam was introduced to avoid terminal voltage instability caused by a high ion current load.

Measurements of 129I in marine waters from the Arctic and North Atlantic • The anthropogenic 129I inventory in the upper 1000 m of the central Arctic increased by 50% from 1996 to 2001, the main contribution being from the Eurasian basin. This inventory is expected to increase by additional 100% within a period of ﬁve years starting from 2001. • The distribution of anthropogenic 129I has been studied in three sections of the western Nordic Seas. The interaction and origin of water masses in the region have been clearly distinguished with the help of 129I labeling. • The variability in 129I concentration measured in seawater along a transect from the Baltic Sea to the North Atlantic suggests strong enrichment in the Skagerrak–Kattegat basin. The 129I inventory in the Baltic and Bothnian Seas was found to be equal to ∼0.3% (3 × 1025 atoms) of the total liquid releases from the La Hague and Sellaﬁeld reprocessing facilities. • Future distribution of 129I tracer in the Arctic and North Atlantic has been simulated with a compartment model. It was shown that the Atlantic Ocean and the deep Arctic Ocean are the major sinks for the reprocessed 129I.

Measurements of 129I in a lake sediment from northeastern Ireland • 129I distribution has been analyzed in a sediment proﬁle sampled

61 from a lake in northeastern Ireland. The trend of the 129I data in the sediment core closely resembled that of liquid releases from Sel- laﬁeld. No indication of the Windscale nuclear accident (1957) was found, conﬁrming that the accident had no impact on northeastern Ireland.

36Cl AMS at a medium size tandem accelerator (∼1 MeV/amu) • The 36Cl measurements in natural samples at a medium-sized tandem accelerator (∼1 MeV/amu) have been considered. A gas-ﬁlled magnetic spectrometer (GFM) was proposed for the separation of 36Cl from its isobar, 36S. Semi-empirical Monte-Carlo ion optical calculations showed that a GFM based on a 150° dipole magnetic sector B(0) is an optimal solution for the separation of 36Cl and 36S. Due to practical considerations, a 180° GFM has been constructed and installed at the AMS beam line for 36Cl AMS. This GFM in combination with a gas-ionization detector is expected to further improve the suppression of 36S by a factor of ∼100 compared with a stand-alone gas-ionization detector.

In view of the experience gained during the course of this thesis research, the following should be mentioned: • The efficiency of 129I AMS at the Uppsala Tandem Laboratory can be improved by removing the time-of-flight setup and normalizing to stable isotope by the rapid sequential injection of isotopes. • Ongoing 129I projects will continue with studies to improve our un- derstanding of the anthropogenic 129I cycle between oceans, atmosphere, inland hydrological system, and sediments. In particular, 129I distributions in precipitation, aerosols, and continental sediments have to be investigated in detail in order to clarify atmospheric transport and the atmosphere → land flux. Further attention should be paid to the ocean–atmosphere interaction, because the major sources of atmospheric and continental 129I are the North Atlantic and Arctic Oceans. • Our 129I results for the North Atlantic and the Arctic have to be con- tinued and extended in space and time with a focus on uncertainties in 129I inventories and on tracing global thermohaline circulation by means of this unique tracer. • 36Cl AMS will be applied to exposure dating in Northern Europe in relation to glacial cycles, as well as to studies of underground water systems and polar ice cores.

62 8 Acknowledgements

My research supervisor, Göran Possnert, deserves my profound gratitude for his unwavering support in all scientific and practical matters and for sharing with me his extensive experience. I have enjoyed learning from his powerful and highly precise analytical mind, and am lucky to have been able to conduct research under his good-humored leadership. My assistant research supervisor, Ala Aldahan, also deserves my gratitude for introducing me to the mysterious and exciting world of the earth sciences, for his always cheerful character, and for his insatiable appetite for scientific debate. He generously shares his enthusiasm, inspiring the same in his students and helping them reach new heights of science. Many thanks go to Per Håkansson, who has kindly introduced me to the field of experimental physics and supervised my exchange student project in the Ion Physics group. This student project would not have been possible without the exchange student program organized by Olov Ågren and Oleg Tsybin. I also wish to acknowledge Yuri Golikov, who helped me choose my profes- sion when I was only 16 years old, and supervised my undergraduate studies. His invariably optimistic nature shone throughout those days, independent of the political, economic, and social upheaval around us. My sincere thanks also go to the accelerator group — Jonas Åström, Bengt Beckman, Rogério Zorro and Gösta Widman — for teaching me about the accelerator. My work would also not have been possible without Yngve Kjell- gren, who helps the accelerator group with high-precision engineering at the workshop. Inger Påhlsson deserves my thanks for supporting my research with the chemical preparation of enormous numbers of samples. Special thanks are deserved by Magnus Palmblad for exciting daily scientific discussion and for thoughtful comments on various matters. In the first year of my studies Nadia Buraglio provided considerable useful advice on 129I and other matters, and for this I am thankful. Many thanks are due to Paula Almqvist for providing highly professional administrative support and for organizing many pleasant social events. The Ion Physics group has allowed me the privilege of meeting many won- derful people: Alenka, Anders, Anne, Bogdan, Christian, César, Curt, Edvard, Gamini, Inge, Jan, Johan, Jos, Judit, Kicki, Lotta, Marit, Margareta, Maud,

63 Mikael, Nadia S., Oleg S., Raad, Ramal, Roman, Thomas and Yanwen. I would like to thank all of them for the friendly and encouraging atmosphere they helped create at the department. Some of my time was spent at GeoCentrum, where I performed the chemical preparation and combustion of the 129I samples. There I met many warm, friendly people whom I would like to thank for sharing their knowledge. I would also like to thank them for the many tremendous times at TGiF (Thank God it’s Friday!) and Ph.D. parties. Bill Smethie, Jim Swift, and the rest of the scientific crew aboard the research vessel Knorr are gratefully acknowledged for introducing me to the world of physical oceanography. I am thankful to the Swedish Institute for their financial support of the exchange student program in which I participated and of the first nine months of my doctoral studies. Travel grants from the Liljewalchs Foundation and the Knut and Alice Wallenberg Foundation are also gratefully acknowledged. I would like to thank my friends for their support, inspiration, and good humor. You know who you are, and I would like to thank you for being who you are — my friends! Last, but not least, I wish to thank my parents, Grigory and Nadezhda, my sister Anya and my brother-in-law Denis for supporting and encouraging me.

64 9 Summary in Swedish

Acceleratormasspektrometrisk analys av 36Cl och 129I: Analytiska frågeställningar och tillämpningar

Acceleratormasspektrometri (AMS) är en ultrakänslig analysteknik för detektion av enskilda atomer i extremt låga isotopkoncentrationer (>10−16). Metoden grundar sig på att man utnyttjar ett partikelacceleratorsystem som mätinstrument, och den höga känsligheten uppnås genom att flera välbestämda fysikaliska fenomen samverkar. Atomerna från ett prov joniseras, accelereras och separeras i elektriska och magnetiska fält, för att slutligen identifieras med hjälp av etablerade kärnfysikaliska detektionssystem. Eftersom många av de naturliga långlivade kosmogena radionukliderna i vår miljö förekommer i isotopkoncentrationer mellan 10−7 och 10−17, öpp- nar AMS-metoden möjligheter att detektera och utnyttja dessa som känsliga spårämnen och dateringsinstrument inom geologi och arkeologi. Den stora fördelen med AMS-metoden jämfört med den konventionella sönderfallsde- tektionsmetoden är att mycket små prover (∼mg) kan analyseras med relativt korta analystider (∼timme). 36 129 Cl (T1/2 = 301 ka) och I(T1/2 = 15.7 Ma) är två långlivade radionuklider som tillhör halogenerna. 36Cl är främst kosmogent producerat i atmos- fären genom spallation av Ar, men produceras till en del även i litosfären genom neutroninfångning i 35Cl, medan det 129I som förekommer i vår miljö idag helt kan spåras till antropogena källor relaterade till kärnenergianvänd- ning. Utsläpp av 129I till atmosfären och världshaven har ägt rum i stor skala de senaste 30 åren och då främst från upparbetningsanläggningarna av kärn- bränsle i Sellafield i England och La Hague i Frankrike. Det övergripande syftet med avhandlingen är att öka förståelsen för pro- duktion och transport av halogena radionuklider i naturen och utveckla mät- tekniska förutsättningar för detektion av dessa med hjälp av AMS-tekniken. Mer specifikt kan tre detaljerade mål urskiljas: • Utveckla det nyligen installerade 5 MV tandemacceleratorsystemet i Uppsala för analys av 129I i naturliga prover. • Fullfölja och utvidga den pågående forskningen om 129I inom ocea- nografi med speciellt fokus på Arktis och Norra Atlanten samt initiera

65 studier av lakustrina och marina sedimentarkiv. • Klarlägga förutsättningarna för analys av 36Cl med en medelstor tandemaccelerator (∼1 MeV/amu), speciellt beträffande isotopsepara- tion med hjälp av en gasfylld magnetisk spektrometer (GFM). Det första målet har uppnåtts och en rutinprocedur för mätning av 129I med den nya tandemacceleratorn har etablerats. God reproducerbarhet och låga bakrundsbidrag har demonstrerats i mätdata. Vid en jämförelse med det äl- dre, avvecklade, acceleratorsystemet har en betydligt förbättrad separation observerats, som främst går att förklara med att vakumförhållandena är två storleksordningar (∼10−8 mbar) bättre och att den magnetiska dispersionen är kraftfullare i den nya installationen. För att undvika strömlasteffekter i maski- nen, som kan påverka terminalspänningsstabiliteten, har en pulserad injektion av den stabila isotopen introducerats. Tidigare undersökningar av 129I i Norra Polarhaven har utökats genom om- fattande studier av marina vatten från såväl Arktis som Norra Atlanten. Analy- sen av prover, som insamlades under 2001 i samband med den svenska is- brytarexpeditionen med fartyget ODEN, visar att den antropogena mängden av 129I i centrala Arktis har ökat proportionellt med det integrerade utsläppet från de europeiska upparbetningsanläggningarna. Material från expeditionen Arctic Ocean 2002, är främst inriktad mot Norska havet och klarlägger med hjälp av 129I-signalen ett tydligt mönster i växelverkan och ursprung för olika vattenmassor i området. På motsvarande sätt har en studie av 129I i havs- vatten längs en transekt från Östersjön till Norra Atlanten påvisat den starka anrikningen av antropogena radionuklider från upparbetningsanläggningarna i Skagerrak-Kattegattområdet. För att öka förståelsen för dynamiken i de studerade oceanbassängerna har flöden och volymer parametriserats i en box- modell. Av beräkningar i denna modell med olika utsläppscenarior, framgår att djupare delar av Arktiska oceanen och till en del även Norra Atlanten är de centrala sänkorna för 129I på en tidskala om 100 år. Undersökningar av 129I i sediment har en stor potential för att förbättra tolkningen av processer i akvatiska system. En inledande studie har därför genomförts av en sedimentprofil från en sjö i Nordöstra Irland. Trenden i 129I data återspeglar på ett tydligt sätt utsläppen från det närbelägna Sellafield. Vi ser däremot ingen signifikant signal från den rapporterade utsläppsolyckan som ägde rum 1957. Den tredje målsättningen att undersöka förutsättningarna för att detektera 36Cl med en 5 MV tandemaccelerator är främst relaterad till möjligheterna att separera isobarerna 36Cl och 36S. Vid sidan av befintlig isobaridentifiering genom detektion i en multielektrodgasdetektor föreslås installation av en gasfylld magnetisk spektrometer. För att optimera isobarseparationen har Monte- Carlo beräkningar av jonoptiken utförts och resultatet har legat till grund för konstruktionen av den 180° GFM som installerats vid tandemanläggningen.

66 10 Appendix: Measuring procedure and calculations

10.1 Measured isotopic ratios (atoms C−1) In the measuring procedure a ratio R between the amount of rare isotope ions at the detector and the current of stable isotope (∝ 129I/127I ratio) is sequen- tially determined in a standard sample with a known 129I/127I ratio, in 4 to 6 unknown samples, and than again in the standard sample, in unknown samples, etc. The ratio R of the unknown samples is then normalized to the time- linear-interpolated R in the standard samples, corrected for background of the chemical procedure and the accelerator setup, and multiplied with the known value of 129I/127I ratio in the standard to obtain 129I/127I isotopic abundance of the unknown samples. Background is corrected with the help of blank samples prepared in the same way as the unknown samples. The measurement of the ratio R in a given sample is performed in several si 127 cycles. First, the pulsed current I0 of the stable isotope I is measured for 5 seconds at the Faraday cup after the analyzing magnet. Then the rare isotope 129 ri I is counted for 300 seconds at the detector (N0 ). These measurements are si repeated n = 1–4 times, and at the end the current of the stable isotope In+1 is 129 127 measured once again. For each cycle i, the ratio Ri (∝ I/ I ratio) in the σ sample and the error Ri of Ri are calculated:

ri Nri = Ni , σ = i . Ri si+ si Ri si+ si (10.1) Ii Ii+1 Ii Ii+1 2 2

σ It is considered that the error Ri is associated solely with the measurements of rare radionuclide. For i > 1 cycle the weighted average R of the ratio Ri and its Poisson and standard errors are calculated:

i ∑ Rk σ 2 = R R = k 1 k , (10.2) i ∑ 1 σ 2 k=1 Rk

i ( − )2 ∑ Rk R σ 2 1 = R σ Poisson = , σ standard = k 1 k . (10.3) R R i i ( − ) 1 1 i 1 ∑ 2 ∑ 2 σ σ k=1 Rk k=1 Rk

The last cycle n is completed when both the standard and Poisson errors are below 8% for an unknown sample and 5% for a standard, or when the maximum amount of cycles is done (3 for an unknown sample and 4 for a standard).

10.2 Normalized isotopic ratios After the sequence consisting of a standard, 4 to 6 unknown samples, and one more standard is completed, a time-linear-interpolated ratio Rstd(t) of the standard is calculated, and a normalized ratio R of the sample is obtained:

R R = smp . (10.4) Rstd(t)

The error σR is found by applying the error propagation rule: σ 2 σ 2 Rsmp Rsmp R σR = + std , (10.5) Rstd Rsmp Rstd σ σ standard σ Poisson σ where R is the maximum of , , and R is the maximum smp Rsmp Rsmp std of σ standard , σ Poisson , σ standard , σ Poisson . Rstd before smp Rstd before smp Rstd after smp Rstd after smp The measurement sequence is repeated several times (m) with different combinations of samples and standards, so at least two independent measurements for each sample are acquired (m ≥ 2). Then the weighted mean R of the normalized ratio R and its Poisson and standards errors are calculated:

m Rk ∑ 2 σR R = k=1 k m , (10.6) 1 ∑ 2 σR k=1 k

m (R −R)2 ∑ k σ 2 1 = R σ Poisson = , σ standard = k 1 k . (10.7) R m R m 1 ( − ) ∑ 1 ∑ 2 m 1 σ 2 σR = R k=1 k k 1 k

68 10.3 Background correction The measured ratios R contain some background level of 129I that came from the chemical preparation and to much less extent from the accelerator setup itself. To determine the ﬁrst type of background, chemical blanks are prepared from iodine carrier in regular intervals. The accelerator background is determined with a target pressed from >300 Myr old natural AgI (iodargyrite) crystal. Since (a) the standards were not subjected to the chemical preparation procedure, (b) concentration of 127I in seawater samples is negligible compared with the amount of carrier added, and (c) the accelerator background is small (129I/127I ≤ 4 · 10−14), then the background-corrected ratios R are obtained by subtraction of the Rbl ratios measured for blank samples from the R ratios of unknown samples prepared in the same chemical preparation sequence: 129 129 129 I Ismp+ Icont 127I 127 R = smp w cont = Ismp = R + R ⇒ 129 bl (10.8) 129I Istd 127 127I I std std

2 2 R = R − Rbl, σR = σ + σ , (10.9) R Rbl standard Poisson where both σR are maximum of σ ,σ . i Ri Ri Between January 2003 and June 2004 almost 100 chemical blanks have been prepared from Woodward iodine and measured at the accelerator. Wood- ward iodine (∼300 Myr old) is an extract from a deep underground brine, which was kindly provided to us by Woodward Corporation, Oklahoma, USA. The average background of our chemical procedure for the period between 129 127 January 2003 and June 2004 was estimated at Rbl = 0.022±0.001 or I/ I ∼ 2 × 10−13. If isotopic standards were prepared in the same way as unknown samples, the background correction would have a different form [e.g. Winkler et al., 1997]:

σ 2 + σ 2 σ 2 R − Rbl R R R R = , σR = R · bl + bl . (10.10) − R 2 2 1 bl R − Rbl 1 − Rbl

69 10.4 Isotopic abundances (atoms atoms−1) By multiplying R with the 129I/127I ratio of the standard, the 129I/127I isotopic abundance in the sample is estimated: 129 129 I = R · I 127 127 , (10.11) I smp I std

  σ 2 129 129I σR 2 127 I  I std  σ 129 = R · · + . (10.12) I 127 R 129 127I I I smp std 127 I std standard Poisson where σR is the maximum of σR ,σR . Our standards are prepared from the sample reference material 4949C.Iodi- ne-129 of the National Institute for Standards and Technology (NIST). The internal isotopic standard TIS1 (tandem internal standard, 1 × 10−11) with 129I/127I ratio (1.056 ± 0.024) × 10−11 has been used in all recent experi- ments. Apart from TIS1 there are two more internal standards and one external standard: TIS0.2 with ratio (0.225 ± 0.023) × 10−11, TIS5 with ratio (5.28 ± 0.12) × 10−11, and the IsoTrace standard with ratio (1.174 ± 0.022) × 10−11. All three internal standards were normalized to the IsoTrace standard.

10.5 129I concentration (atoms l−1 or atoms kg−1) 129 −1 I concentration in a seawater sample Csw (atoms l ) is calculated as: 129 · = I · m NA · 1 Csw 127 , (10.13) I smp Mi V where m (kg) is the mass of added carrier (0.5–2 mg), Mi is the molar mass 127 −1 23 of I (= 126.905 g mol ), NA is the Avogadro constant (= 6.022 × 10 mol−1), and V (l) is the volume of the seawater sample. 129 −1 I concentration in a sediment sample Csed (atoms kg ) is calculated in a similar way assuming that the amount of carrier is much greater than the amount of iodine in the sample (invalid, for example, for seaweeds): 129 · = I · m NA · 1 Csed 127 , (10.14) I smp Mi msed where msed (kg) is the mass of the sediment sample dried at 40°C (1–2 g).

70 Usually the error in the concentration σC is associated solely with the measurement of the isotopic ratio: σ 129I 127 I smp σC = C · . (10.15) 129I 127 I smp

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