Ultratrace Determination of 99Tc in Small Natural Water Samples by

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Ultra-trace determination of Tc in small natural water samples by Accelerator Mass Spectrometry with the Gas-Filled Analyzing Magnet System Francesca Quinto, Christoph Busser, Thomas Faestermann, Karin Hain, Dominik Koll, Gunther Korschinek, Stephanie Kraft, Peter Ludwig, Markus Plaschke, Thorsten Schäfer, and Horst Geckeis Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b05765 • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 10, 2019

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Analytical Chemistry

Ultra-trace determination of 99Tc in small natural water samples by Accelerator Mass Spectrometry with the Gas-Filled Analyzing Magnet System Francesca Quinto*1, Christoph Busser2, Thomas Faestermann2, Karin Hain3, Dominik Koll2,4, Gunther Korschinek2, Stephanie Kraft1, Peter Ludwig2,5, Markus Plaschke1, Thorsten Schäfer1,6, Horst Geckeis1. 1Karlsruhe

Institute of Technology, Institute for Nuclear Waste Disposal, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany. 2Technische Universität München, Physik Department, James-Franck-Straße 1, D-85748 Garching, Germany. 3University of Vienna, VERA Laboratory, Faculty of Physics, Währinger Straße 17, A-1090 Vienna, Austria. 4The Australian National University, Department of Nuclear Physics, Research School of Physics and Engineering, ACT, 2601, Canberra, Australia. 5TÜV

SÜD Industrie Service GmbH, Westendstraße 199, D-80686 München, Germany.

6Friedrich-Schiller-University

Jena, Applied Geology, Institute for Geoscience, Burgweg 11, D-07749 Jena, Germany.

AUTHOR INFORMATION Corresponding Author *Phone: + 49 721 608-22233. Fax: +49 721 608-23927. E-mail: [email protected]

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Ultra-trace determination of 99Tc in small natural water samples by Accelerator Mass Spectrometry with the Gas-Filled Analyzing Magnet System Francesca Quinto*1, Christoph Busser2, Thomas Faestermann2, Karin Hain3, Dominik Koll2,4, Gunther Korschinek2, Stephanie Kraft1, Peter Ludwig2,5, Markus Plaschke1, Thorsten Schäfer1,6, Horst Geckeis1. 1Karlsruhe

Institute of Technology, Institute for Nuclear Waste Disposal, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany. 2Technische Universität München, Physik Department, James-Franck-Straße 1, D-85748 Garching, Germany. 3University of Vienna, VERA Laboratory, Faculty of Physics, Währinger Straße 17, A-1090 Vienna, Austria. 4The Australian National University, Department of Nuclear Physics, Research School of Physics and Engineering, ACT, 2601, Canberra, Australia. 5TÜV

SÜD Industrie Service GmbH, Westendstraße 199, D-80686 München, Germany.

6Friedrich-Schiller-University

Jena, Applied Geology, Institute for Geoscience, Burgweg 11, D-07749 Jena, Germany.

ABSTRACT: In the frame of studies on the safe disposal of nuclear waste, there is a great interest for understanding the migration behavior of 99Tc. 99Tc originating from nuclear energy production and global fallout shows environmental levels down to 107 atoms/g of soil (~2 fg/g). Extremely low concentrations are also expected in groundwater after diffusion of 99Tc through the bentonite constituting the technical barrier for nuclear waste disposal. The main limitation to the sensitivity of the mass spectrometric analysis of 99Tc is the background of its stable isobar 99Ru. For ultra-trace analysis, the Accelerator Mass Spectrometry (AMS) setup of the Technical University of Munich using a Gas-Filled Analyzing Magnet System (GAMS) and a 14 MV Tandem accelerator is greatly effective in suppressing this interference. In the present study, the GAMS setup is used for the analysis of 99Tc in samples of the seawater reference material IAEA-443, a peat bog lake and groundwater from an experiment of in situ diffusion through bentonite in the controlled zone of the Grimsel Test Site (GTS) within the Colloid Formation and Migration (CFM) project. With an adapted chemical preparation procedure, measurements of 99Tc concentrations at the fg/g levels with a sensitivity down to 0.5 fg are accomplished in notably small natural water samples. The access to these low concentration levels allows for the long-term monitoring of in situ tracer tests over several years and for the determination of environmental levels of 99Tc in small samples.

Naturally occurring 99Tc (t1/2 = 2.111 × 105 y)1 is continuously generated in the Earth´s crust, especially in uranium ores, by spontaneous fission of 238U, neutron induced fission of 235U and nuclear reactions of cosmic rays mainly with molybdenum (Mo) and ruthenium (Ru). It can be estimated2 that the 99Tc/238U atom ratio arising from the spontaneous fission in U-bearing minerals is equal to ~1.6 × 10-12. Assuming an average 238U concentration of ~1.8 ppm for common rock types in the Earth´s crust3 and 7% in uranium ore, the corresponding concentrations of 99Tc can be estimated to ~7.2 × 103 and 2.8 × 108 atoms/g, respectively. This latter value is equivalent to ~45.8 × 10-15 g/g and very close to those directly measured in uranium ores by Dixon et al., 1997.4 A further fraction of 99Tc has to be added whose levels depend on the neutron flux and 235U concentration as well as on the cosmic ray flux and Mo and Ru occurrences in the Earth´s crust. The global inventory of 99Tc generated by natural processes has been reported to be ~5.8 × 1029 atoms.5 This value constitutes the major fraction of 99Tc in the environment and it is dispersed in the Earth´s crust.

Anthropogenic 99Tc is produced in nuclear reactors and nuclear weapon detonations by the neutron induced fission of 235U and 239Pu. The main sources of anthropogenic 99Tc to the environment have been listed in Shi et al.,5 and are, in order of the highest to the lowest contribution, the reprocessing plants in Sellafield and La Hague, global fallout, Chernobyl and Fukushima nuclear accidents, medical applications and operational releases from nuclear power plant (NPP). Summing up the contributions of such sources, an inventory of anthropogenic 99Tc in the environment equal to ~2 × 1028 atoms is estimated. This value is ~30 times lower than the naturally occurring inventory of 99Tc, but is released to the surface environment. Levels of anthropogenic 99Tc in the environment as low as ~3.8 × 107 atoms/g (6.2 fg/g) have been measured in 50 to 100 g of incinerated soil affected solely by the global fallout in Japan.6 Significantly higher concentrations up to 2.2 × 1011 atoms/g (36 pg/g) were found in forest soil within the 30 km-zone from the Chernobyl NPP.7 In the reference material IAEA-443, namely an Irish Sea water sample affected by the liquid discharges of the Sellafield reprocessing plant, an information value for the concentration of 99Tc of


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(1.9 ± 0.4) × 109 atoms/ml (312 fg/g) is given.8,9 Sub-tidal sediments of the Sellafield offshore area present significantly higher levels of 99Tc and, in fact, up to ~4.8 × 1011 atoms/g (79 pg/g) of 99Tc were determined in a sediment core collected in 2005.10 Due to its long half-life and high fission yield, 99Tc would be one of the major contributors until ~6 × 105 year to the long term-radiotoxicity of spent nuclear fuel.11 In order to ensure a safe geological disposal of nuclear waste, the environmental behavior of Tc, occurring under oxidizing conditions as the highly mobile pertechnetate,5 must be understood. Such investigations can be carried out by laboratory scale experiments, by the analysis of the contamination of 99Tc in the environment, as well as by in situ radionuclide tracer tests performed in repository relevant conditions. At the Grimsel Test Site (GTS) in Switzerland, the diffusion of several radionuclide tracers and, among them 99Tc, through bentonite is investigated in the frame of the Long Term In situ Test (LIT) within the Colloid Formation and Migration (CFM) project.12 The bentonite compacted in the shape of rings simulate the bentonite buffer of an engineered barrier system towards a water (Grimsel groundwater) conducting natural shear zone situated in the granodiorite rock of the GTS.13 Concentrations at the level of fg/g are expected for the 99Tc tracer in the eluted Grimsel groundwater samples with available volumes of a few ml. Similarly, the environmental concentrations of 99Tc arising from human nuclear activities in regions far away from highly contaminated areas lie mostly close to or below the fg/g levels. For all those studies, the analytical capability of determining 99Tc in natural samples at these ultra-trace levels is, therefore, of great relevance. Analytical Techniques. The determination of the long lived pure ß- emitter 99Tc with LSC presents detection limits (DL) of ~1010 atoms/g, (1.6 pg/g) while lower DL can be achieved with mass spectrometric techniques.5 The main limitation to the sensitivity of the mass spectrometric analysis of 99Tc is the background of the daughter and stable isobar 99Ru as well as several molecular isobars. This is however not valid for resonance ionization mass spectrometry (RIMS) in which the ionization of the sample is selective towards Tc.14 Recent optimization of the RIMS setup for 99Tc measurements allowed DL equal to ~3 × 106 atoms (0.5 fg) for a tracer solution, while higher DL of ~3 × 108 atoms (50 fg) were obtained for a soil sample spiked with 99Tc.15 Chemical separation of technetium from the matrix elements prior to measurements with inductively coupled plasma mass spectrometry (ICPMS) and thermal ionization mass spectrometry (TIMS) is greatly effective in allowing a sensitive determination of 99Tc in environmental samples. Several chemical procedures involving ion exchange or extraction chromatography have been applied in order to reduce the background from 99Ru and the molecular isobar 98Mo1H in ICPMS as well as of the molecular isobar MoFO3 interfering with the detection of TcO4 with TIMS. An average 99Tc blank of 5 ± 4 fg and a corresponding DL of 11 fg were obtained from uranium ore samples with TIMS.4 Using ICPMS and by the sufficient removal of Ru and Mo, a DL of 0.23 pg/g analyzing an aliquot of 10 g seaweed or soil,16 and 0.4 pg/l of 99Tc in river water samples of 1 l volume17 were achieved.

Accelerator mass spectrometry (AMS) is characterized by the acceleration of the ions to MeV energies and the stripping process. This provides the destruction of molecular ions and, in this way, prevents the formation of a molecular isobaric background coming from 98MoH. Regarding the atomic isobaric background from 99Ru, different strategies have been employed up to now. The first reported AMS measurement of 99Tc took advantage of the detection of 99Tc14+ at the energy of 215 MeV in a propane-filled ionization chamber at the Australian National University (ANU, Canberra, Australia).18 This provided an only partial separation of the 99Tc from the 99Ru ions, but allowed the sufficient energy resolution to separate the signal of 99Tc from that of 101Ru. The contribution of the 99Ru isobar may be subtracted by considering the isotopic composition of natural Ru allowing for a sensitivity at the fg levels.18 Further measurements were reported shortly after at the Center for Accelerator Mass Spectrometry (CAMS, Livermore, USA) with the use of a gas ionization detector filled to 110 Torr with P10 (90% Argon and 10% methane). In this detector via the differential energy loss of 99Tc as compared to that of 99Ru measured in 3 anodes, a sensitivity of ~1 × 108 atoms (16 fg) could be achieved.9 Then, again at ANU, using the energy loss signals of a gas filled ionization detector with an 8-fold segmented anode optimized for 99Tc, a DL of ~2.5 × 107 atoms (4 fg) was reported.19 At the IsoTrace Laboratory (Toronto, Canada), the possibility to suppress the interference of 99Ru already in the ion source with a matrix-assisted method was investigated,20 and a relative suppression of the sputtered 99RuF - with respect to 99TcF - by up to two orders of magnitude 4 4 was achieved. Another possibility to suppress the background of 99Ru in the ion source was then explored at the China Institute of Atomic Energy (CIAE, Beijing, China) with the extraction of carbide and the advantageous relative ionization yields of Tc: TcC- = 1 : 25 and Ru- : RuC = 6 : 1; in this way, DLs of ~1 × 109 atoms (164 fg) were obtained.21 At the 14 MV Tandem AMS facility of the Maier-LeibnitzLaboratorium (MLL) of the Technical University of Munich (TUM, Garching, Germany) a Gas-Filled Analyzing Magnet in addition to a 5-fold segmented ionization chamber (GAMS)22-23 is available in order to suppress the interference from the isobar 99Ru.24 In the actual manuscript, we present a novel analytical procedure comprising the use of the GAMS setup for ultra-trace determination of 99Tc in groundwater samples from the longterm diffusion experiment of 99Tc through bentonite at the GTS, a first result on the global fallout level of 99Tc in a peat bog lake sample and the testing of the analytical method with the reference material IAEA-443. EXPERIMENTAL SECTION Tracer for chemical recovery and instrumental efficiency. The quantification of 99Tc with AMS requires the use of a tracer to estimate the chemical recovery of the sample preparation as well the instrumental efficiency of the measurement. A known concentration of a suitable nuclide has to be added to the sample and its count rate is used in order to normalize the count rate of the unknown concentration of 99Tc. In the present study we have tested the use of 93Nb that was previously used at CAMS9 and CIAE21 and also that of 55Mn as reference nuclide for instrumental normalization of the AMS measurements. We

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have then chosen to not use any yield tracer in the chemical preparation procedure but to employ an adapted chemical procedure for the reasons discussed below. The use of the 142.68 keV γ-emission from 99mTc (t1/2 = 6.01 h) as chemical tracer for determination of 99Tc in environmental samples is a well-established procedure.25-26 However, in case the expected level of 99Tc in the sample is in the fg or sub-fg level (~107 – 106 atoms), spiking with e.g. 50 Bq of freshly prepared 99mTc will introduce in the sample at least ~1.6 x 106 atoms of mass 99 u (99mTc and 99Tc). Chemical yield determination via γ-measurements is also possible with 95mTc (t 95mTc 1/2 = 61 d), however, commercially available standard solutions contain too high levels of 99Tc, as already experimentally observed.9, 19, 27 For mass spectrometric analysis of ultra-trace levels of 99Tc, the long-lived 97Tc (4.21 × 106 y) or 98Tc (4.2 × 106 y) constitute an ideal choice,28 since they could present a suitable isotopic purity and could be determined together with 99Tc with AMS. The MLL AMS group has pursued the production of 97Tc tracer solutions via two different nuclear reactions at the research reactor FRMII and at the MLL.24 Such tracer solutions are presently under investigation and could not yet be employed in the present study. We also considered the use of a non-isotopic tracer, namely the nuclide of an element sharing with Tc chemical properties relevant to the experiment. To this purpose, 55Mn is a far too abundant nuclide in nature (1000 μg/g in the earth´s crust) to be used as chemical tracer. The use of 185Re is supported both thanks to its trace level occurrence on the earth´s crust (1 ng/g) and its chemical behavior similar to that of Tc in the chromatographic extraction with TEVA®29 and TRU® (Triskem, Bruz, France).17 This approach is suited for ICPMS analysis, however cannot be chosen for AMS measurements in the present study. In fact, in order to detect two nuclides with a large mass difference between each other, like 185Re and 99Tc, substantial changes in the setting of the AMS setup would be required. This would imply a different beam transport of the two nuclides impeding in this way the normalization of the signal of 99Tc by using that of 185Re. Standard solutions, natural samples and sample preparation. Three kinds of natural water samples together with samples of a standard 99Tc solution and procedural blanks were investigated in the present study. Sample provenience and preparation are described below. All the samples were acidified with conc. HNO3 of ultrapure quality (Roth, Karlsruhe, Germany) to pH ≤ 1 in the container in which they were sampled. After one week and occasional shaking, the samples underwent the following chemical treatments. Standards and sample type 1): groundwater samples. 99Tc was determined in seven Grimsel groundwater samples from the in situ LIT experiment at the GTS. In this experiment, compacted bentonite rings spiked with several radionuclides including 99Tc were placed in a fracture of the granodiorite rock and contacted with Grimsel groundwater. In the constructed geometry of the LIT, the release of radionuclides into the groundwater occurs mainly due to a preceding diffusion through the bentonite.13 The samples investigated in the present study were collected within a time interval of 129 to 877 d from the starting of the experiment and were named LIT 14-35, -44,

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-52, -54, -84, -102, and -141. For the sake of conciseness in the actual manuscript, they are referred to as A, B, C, D, E, F and G, respectively. Due to experimental requirements, the samples had a limited size and aliquots of 1.7 or 8 ml were dedicated to 99Tc analysis, according to the available volume. The Grimsel groundwater contains very low concentration of matrix elements30 favoring in this way the use of a sample preparation without chemical separation of Tc from the matrix. Considering furthermore the already discussed non-availability of a suitable chemical tracer for 99Tc, the adopted sample preparation aimed to prevent as much as possible any loss of 99Tc. Samples were poured into Duran® evaporation vessels to which 3 or 4 mg of 93Nb from a Niobium ICP standard solution (Certified Reference Material, NH4NbF6 in H2O, Certipur) was added. To samples A, B, C and D, as well as to the corresponding blanks and standard samples (see text below), also 0.2 mg of 55Mn from an ICP standard solution (Mn in 5% HNO3, Alfa Aesar, Specpure) was added. The solution obtained in this way was evaporated to dryness on a hot plate at 80 oC. The hot plate temperature was then increased stepwise up to 395 oC and left at such temperature for 1 hour in order to obtain the sample in a matrix of Nb oxide. The whole material was then thoroughly pressed into Cu sample holders suited to be used with the Cssputter negative ion source of the AMS at MLL. Aliquots of a standard 99Tc solution (TCZ44, NH4TcO4 in H2O, Eckert and Ziegler, Nuclitec) containing ~108, 109 and 1010 atoms of 99Tc were prepared following the same procedure, so that they could be used as external reference samples for calibration of the 99Tc count rate in the natural water samples. In order to identify the energy loss signals of 99Ru and calibrate the AMS detector response to 99Ru, a sample containing ~2.4 × 1016 atoms of 99Ru from an ICP standard solution (RuCl3 in 20% HCl, Alfa Aesar, Specpure) was prepared according to the above described procedure. Procedural blanks were prepared and used to define the background of 99Ru introduced into the groundwater samples from the handling and from the employed chemicals. From the procedural blanks the sensitivity for 99Tc was estimated (see Results and Discussion). Similar to the samples, also the various standard solutions and procedural blanks were acidified with conc. HNO3 before their evaporation. Sample type 2): Wildseemoor lake sample (high organic carbon content). In order to investigate the environmental levels of 99Tc from global fallout, a sample of lake water was collected from the Wildseemoor (WSM) peat bog in Southern Germany. This sample contained a high level of organic carbon that had to be fully oxidized. To this purpose, instead of the traditional acid-oxidation stages of sample preparation, like ashing in muffle oven and leaching in concentrated HNO3, a microwave digestion procedure in sealed PTFE digestion vessels was applied. Such procedure has proven to prevent losses of Tc in its fully oxidized state as the highly volatile HTcO4.18 An aliquot of 100 ml of the WSM lake water sample was evaporated on a hot plate at 80 oC almost to dryness in a PTFE beaker. A brownish drop left on the bottom of the beaker was collected with a Pasteur pipette and transferred to a PTFE vessel suited to a microwave digestion oven. The beaker was rinsed twice with 2.5 ml conc. HNO3 that was then added to a microwave PTFE vessel. The employed digestion program was

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composed by the following sequence: 3 min at 140 oC, 3 min at 200 oC and 10 min at 230 oC. The clear sample solution obtained in this way was then submitted to the preparation steps described for sample type 1). Together with this sample, also a procedural blank was prepared according to the same procedure. Sample type 3): Reference Material (RM) IAEA-443 Irish seawater. Currently no reference material (RM) certified for the 99Tc concentration exists, but information values are provided for IAEA-443 with (1.9 ± 0.4) × 109 atoms/g of 99Tc. Four aliquots (1 to 10 ml) of this RM were analyzed in the present study. These samples, containing a significant matrix, cannot be submitted to a simplified sample preparation, but their 99Tc fraction must be chemically separated from the matrix elements. Since this RM is provided from IAEA already acidified to a pH of ~1, no further acidification prior sample preparation was performed, but the aliquots were directly submitted to a chromatographic separation with TEVA® resin. In particular, 0.065 ml H2O2 was added to the sample that was then loaded onto a small chromatographic column containing 1 ml of TEVA® resin previously conditioned with a solution of 0.1 M HNO3. After the sample had passed through, the column was rinsed twice with 10 ml 0.1 M HNO3. Finally, 99Tc was eluted from the column with three loadings of 2 ml 8 M HNO3. The so obtained sample solution was then submitted to the preparation steps described for sample type 1). Table 1: values for the main parameters chosen during the measurement runs, run I and run II. The samples analyzed in each of these runs are also listed (Details are given in the text). Run I Samples Terminal voltage (MV)

A, B, C and D 11.25

Run II E, F, G and WSM 11.5

GAMS, B (mT)

974

960

GAMS, p (mbar)

3.5

3.5

Detector, p (mbar)

39

32.5

99Tc,

13+

12+

156

148

charge state, q

99Tcq+,

E [MeV]

AMS measurement. The natural water samples described in the actual work were analyzed during two measurement runs, in which different values of measurement parameters were chosen, as listed in Table 1. The aliquots of the RM IAEA-443 were analyzed in further two measurement runs, where parameters had the same values as used for measurement run I (Table 1). The employed AMS setup has been presented in details in a previous publication,22 while in the actual paper we describe the main steps of the measurement procedure for 99Tc. From the single-cathode Cs-sputter negative ion source, the mono-oxide 99Tc16O- was selected and pre-accelerated to 178 keV at the low energy (LE) side of the tandem accelerator. After foil stripping at a terminal voltage ≥ 11.25 MV, 99Tc13+ or 99Tc12+ cations

(Table 1) with a final energy of 156 and 148 MeV, respectively, were selected in the high energy (HE) side of the tandem accelerator. In the GAMS magnet filled with nitrogen gas at the pressure of 3.5 mbar, 99Tc (Z = 43) and 99Ru (Z = 44) can be separated spatially in x-direction on the basis of their different nuclear charge. The trajectory of 99Tc in the GAMS has a larger radius than 99Ru that can be already partially stopped by the aperture in front of the detector and, in this way, suppressed by ~4 orders of magnitude before the particles enter the detector. At the same time, also a certain fraction of the 99Tc cations is stopped by the aperture, leading to a corresponding decrease in detection efficiency for 99Tc. The ionization chamber was filled with 2-methylpropane gas, in which the split anode records the energy loss of 99Tc and 99Ru. Taking into account the 5 differential energy loss and the total energy loss signals together with the x- and y- angle and the x- position of the detected events,22 it was possible to identify the 99Tc counts from those of the 99Ru fraction that passes the aperture at the entrance of the ionization chamber after separation in the GAMS magnet. In this way, a nine-dimensional software cut was used in order to identify the region of interest for 99Tc events. The conditions for such software cut were determined per each measurement run by analyzing standard samples containing ~109 to 1010 atoms of 99Tc and were used for the analysis of all the samples belonging to the same measurement run. After that, the remaining background of 99Ru was estimated using the sample containing 93Nb, 55Mn and 99Ru as tracer. Thanks to its high 99Ru content of ~2.4 × 1016 atoms, the fraction of 99Ru events that could still create false positive 99Tc counts could be estimated. The count-rate of 99Tc13+ or 99Tc12+ in the detector was related to the current of 93Nb12+ or 93Nb11+, respectively, at a Faraday cup on the HE side before the entrance of the GAMS. In case of samples A, B, C and D, the count rate of 99Tc was additionally normalized to the current of 55Mn16O- at a Faraday cup on the LE side before the entrance of the tandem accelerator. The normalization to 93Nb12+ or 93Nb11+ includes the transmission of the ions of interest through the LE side, the accelerator and most of the HE beamline. On the other hand, Mn, as a chemical homologue, is expected to behave more similar to Tc in the ion source, but like in the case described for 185Re, 55Mn cannot be measured at the HE side with the setup optimized for 99Tc because of the large difference in mass between them. Therefore, the normalization to 55Mn16Oincludes the transmission of the wanted ions only on the LE side of the AMS instrument.

Figure 1: Energy loss signal at the anode segment 5 of the detector as function of the x- position of the events corresponding to mass 99 u collected after the GAMS magnet. The left side (a) of the figure represents a blank sample,

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containing 93Nb and 55Mn (named Blank-Nb-Mn), and the right side (b) the standard sample, containing 93Nb, 55Mn and ~3.6 × 1010 atoms of 99Tc (named Standard Nb-Mn-Tc-E10) analyzed during run I. The red and black ellipses mark the areas in which 99Tc and 99Ru events are located, respectively.

Figure 2: Area of the spectra of the Blank-Nb-Mn (left side) and the Standard Nb-Mn-Tc-E10 (right side) showing the region of interest for 99Tc events delimited with the black line. Like in Figure 1, the energy loss signal at the anode segment 5 of the detector is depicted as function of the x- position of the events corresponding to mass 99 u collected after the GAMS magnet for the Blank-Nb-Mn (a) and the Standard Nb-Mn-TcE10 (b). RESULTS AND DISCUSSION All the 99Tc concentration values are estimated relative to the concentration and count rate of 99Tc in the standard samples analyzed together with the natural water samples. In Figure 1 and Figure 2, the energy loss signal (segment 5) over x- position spectrum of a standard sample containing ~3.6 × 1010 atoms of 99Tc in the matrix of 93Nb and 55Mn (named Nb-Mn-Tc-E10) is represented at the right side. The left side of the figures represents a blank sample, containing 93Nb and 55Mn (named Blank-Nb-Mn). In Figure 1, on the lower right side of each spectrum, the 99Ru events are distributed in the area marked with the black ellipse with “energy loss at the anode segment 5” between the channels 200 and 1100 and at the channels of the “x-position” starting from 2700. The red ellipse (“energy loss at the anode segment 5” from channel 500 to 1200 and “x-position” from channel 2000 to 2850) marks the area in which 99Tc events are collected. As it can be seen by comparing the spectra of the Blank-Nb-Mn and the Standard Nb-Mn-Tc-E10 in Figure 1, the distribution of the events of 99Tc is clearly recognizable to the upper left of the 99Ru distribution. In Figure 2, the region of the plot marked with the black line represents the region of interest for real 99Tc counts and contains ~3000 events. Such region of interest is obtained after the application of the nine-dimensional software cut described in section “AMS measurements”. The fraction of 99Tc events falling outside of its region of interest varies according to the parameters chosen for the GAMS setup. On average, fractions equal to ~2.5% and ~4.3% of the 99Tc events entering the GAMS were collected inside of the software cut employed in run I and run II, respectively. This resulted in a loss of detection efficiency of ~97.5% and ~95.7%, but allowed for an effective suppression of the 99Ru background and an accurate determination of 99Tc. Like in Figure 1, at the lower right side of Figure 2, there is the region in which counts from 99Ru are

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expected. An example of the 99Ru background level is also given in Figure 2 (left side) with the blank sample (Blank-NbMn), from which no events were collected inside of the region of interest for 99Tc in 1.8 hours of measurement. The fraction of 99Ru events that could still be included in the region of interest of 99Tc and create false positive 99Tc counts was estimated to be equal to ~3 × 10-5 in run I and ~9 × 10-6 in run II. In this way, the number of background events due to the presence of 99Ru can be determined for any sample. From sample A, for instance, 4.7 × 104 counts of 99Ru collected during a measurement time of 1.3 hours, created ~1.4 false 99Tc counts. Such background events that are falsely counted as 99Tc were determined per each sample and subtracted in the results presented in the following discussion. As explained in the previous paragraph, the concentration of 99Tc in the first four analyzed groundwater samples, A, B, C and D, was determined according to two methods of instrumental normalization, as relative to the current of 93Nb12+ and to that of 55Mn16O-.

Figure 3: Number of 99Tc atoms in the groundwater samples A, B, C and D. The bars filled with blue horizontal lines represent the 99Tc concentration obtained by normalizing the 99Tc count rate to the 55MnO- current at the LE side of the accelerator, while the full blue bars indicate the values obtained with the normalization to the 93Nb12+ current at the HE side. In Figure 3, the number of 99Tc atoms per sample estimated by applying the two normalization methods are depicted. It can be seen that the two methods produce results that are consistent to each other within their rather large statistical uncertainties within a 1-sigma confidence interval. These errors consist of the counting uncertainty according to Feldman and Cousins (1998)31 and the variation in the transmission that was determined with the standard samples, whose measurement was repeated several times during each measurement run. The standard deviation for the transmission of 99Tc was ~73% for measurements relative to 55Mn16O-, and ~46% for measurements relative to 93Nb12+. The generally large variation in the transmission could be explained by a different time evolution of the ionization yield of 99Tc16O- compared to that of 55Mn16O- and 93Nb16O-. Furthermore, as previously mentioned, with the normalization to 55Mn16O- the variations in the settings starting from the accelerator until the detection system are not taken into account. Considering the higher uncertainty of the


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normalization to 55Mn16O-, we have decided to normalize the concentration of 99Tc only to the 93Nb12+ or 93Nb11+ current for all the samples. In particular, the atom ratio 99Tc/93Nb measured in the groundwater samples analyzed during the two measurement runs ranges between ~2 × 10-12 and 2 × 10-11. The 1-sigma upper limit of the blank level is ~3 × 106 atoms (0.5 fg) per sample in the first measurement run (samples A, B, C and D) and ~5.7 × 106 atoms (0.9 fg) per sample in the second measurement run (samples E, F, G and WSM), representing an excellent sensitivity. These values were obtained from blank samples prepared according to the chemical procedure used for sample type 1). Figure 4 depicts the concentration of 99Tc (atoms/ml) in all the investigated groundwater samples collected in the course of the LIT experiment (sample type 1) and in the WSM sample (sample type 2). On the horizontal axis, the sampling time (in days) from the start of the LIT experiment is indicated for the groundwater samples as number in parenthesis. It can be seen that, among the LIT groundwater samples, sample F presents the lowest 99Tc level equal to 8 × 106 atoms/ml (~0.8 fg/g) with a positive and negative uncertainty of 9 × 106 and 5 × 106 atoms/ml, respectively. The highest level of 99Tc was determined in correspondence of sample B with 1.2 × 109 atoms/ml (~96 fg/g) with a positive and negative uncertainty of 5 × 108 and 6 × 108 atoms/ml, respectively. It is important to note that the obtained data demonstrate that 99Tc concentrations at fg/g levels can be quantified in groundwater. This qualifies AMS-GAMS as an extremely sensitive analytical method for studying 99Tc migration over a long term period under in situ conditions. The sample WSM presents the lowest measured concentration of 99Tc, equal to 1 × 106 atoms/ml (~0.5 fg/g) with a positive and negative uncertainty of 5 × 105 and 3 × 105 atoms/ml, respectively. The number of 99Tc atoms in the entire investigated volume of the WSM sample (100 ml) is equal to ~9.6 × 107. This value is significantly higher (8.2 times) than the number of 99Tc atoms determined in the corresponding procedural blank, namely ~1.2 × 107. This sample provides a first estimate of the global fallout level of 99Tc in a peat bog lake of Southern Germany.

Figure 4: Concentration of 99Tc (number of atoms/ml) in the 7 groundwater samples from the in situ diffusion test and in the surface lake sample, WSM. The number in parenthesis beside

the sample code represent the sampling time from the start of the LIT experiment. The analytical procedure was validated with the analysis of four aliquots of the RM IAEA-443, as depicted in Figure 5. The aliquots IAEA443_1 and IAEA443_2 were analysed in a separate measurement run different from that used for the aliquots IAEA443_3 and IAEA443_4. It can be seen in Figure 5 that the concentration of 99Tc determined in each of the four aliquots is in agreement with the given information values. The average and standard deviation of the measured values of the 99Tc concentration are equal to (2.2 ± 0.6) × 109 atoms/ml that is consistent with the information value of (1.9 ± 0.4) × 109 atoms/ml within the analytical uncertainties.8 However, due to instable conditions of the AMS accelerator during the measurement of IAEA443_3 and IAEA443_4, these samples present a higher uncertainty than observed in the previous run for samples IAEA443_1 and IAEA443_2.

Figure 5: Concentration of 99Tc (number of atoms/ml) in 4 aliquots of the Reference Material IAEA-443 (blue empty squares). The corresponding nominal value is represented with a full red circle. CONCLUSIONS AND OUTLOOK With the GAMS setup, a suppression of the 99Ru signal of up to 5 or 6 orders of magnitude was achieved, allowing an accurate measurement of 99Tc in small natural water samples (1.7 to 100 ml) at the fg/g level. Such high background suppression allowed for groundwater samples and the WSM lake water sample the use of a simplified chemical procedure in which no chemical separation of Ru from Tc is performed and the sample is quantitatively transferred onto the AMS sample holder, permitting in this way the analysis of those samples even in the case where an isotopic tracer is not available. The procedure was successfully validated towards the information value of the RM IAEA-443. The analytical precision will of course be improved when applying an isotopic spike, such as the 97Tc that is under preparation at MLL AMS department.24 The excellent sensitivity down to 0.9 and 0.5 fg reached in two different measurement runs for the first time has permitted to monitor the long-term (~29 months) transport of 99Tc out of a compacted bentonite at ultra-trace levels under in situ conditions in an underground rock laboratory. For this kind of experiments, ultra-trace methods, like the present one, are indispensable. These results and those to be obtained in future experiments will support our understanding on radionuclide

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behavior in bentonite based geo-engineered barriers in the frame of the geological disposal of high level nuclear waste. In addition, a first determination of the level of global fallout 99Tc in a peat bog lake of Southern Germany has been presented. The capability of AMS to analyse extremely low concentration of ~0.5 fg/g paves the way to further investigations on the behavior of 99Tc in environmental compartments such as sediments and organic rich peat bogs. The presented analytical method is highly beneficial also for other new analytical challenges, like migration studies using global fallout 99Tc in the general environmental and the determination of surface values related to nuclear facilities or in the proximity of a future nuclear waste repository site.

AUTHOR INFORMATION Corresponding Author * Phone: + 49 721 608-22233. Fax: +49 721 608-23927.

E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors would like to acknowledge the support of the CFM Project partners and of the local Grimsel Test Site/NAGRA staff. The work has received funding by the Federal Ministry of Science and Education, Germany, under contract 02NUK030E (TransAqua), and the DFG Cluster of Excellence ’Origin and Structure of the Universe ’ (www.universe-cluster.de), by the Federal Ministry of Economics and Technology (BMWi) under the joint KIT-INE, GRS research project “KOLLORADO-e2” (Grant agreement no. 02E11456A), through the project BEACON (Bentonite Mechanical Evolution) of the Euratom research and training programme 2014-2018 (grant agreement No 745942), and through the HGF PoF-III program “NUSAFE”.

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Ultratrace Determination of 99Tc in Small Natural Water Samples by

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