Design of a prototype thermal ionization cavity source intended for isotope ratio analysis

International Journal of Mass Spectrometry 434 (2018) 70–80

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Design of a prototype thermal ionization cavity source intended for isotope ratio analysis Colin Maden a,∗ , Anne Trinquier a , Anne-Laure Fauré b , Amélie Hubert b , Fabien Pointurier b , Jörg Rickli a , Bernard Bourdon c a

ETH Zürich, Institute of Geochemistry and Petrology, Clausiusstrasse 25, CH-8092, Zürich, Switzerland CEA, DAM, DIF, 91297, Arpajon Cedex, France c Laboratoire de Géologie de Lyon, ENS Lyon and UCBL, UMR 5276, CNRS, France b

a r t i c l e

i n f o

Article history: Received 13 July 2018 Received in revised form 28 August 2018 Accepted 9 September 2018 Available online 12 September 2018 Keywords: Thermal ionization cavity Ion source Ionization efficiency Ion source design Ion optics Electron impact heating

a b s t r a c t The design of a prototype thermal ionization cavity (TIC) source attached to the mass analyser of a MAT262 thermal ionization mass spectrometer is presented. In addition to a detailed calculation and experimental verification of the ion optics, the new design includes several innovative features that set it apart from previously reported TIC sources. The goal is to employ this new ion source for high-efficiency isotope ratio analysis. For uranium adsorbed to a single resin bead, an overall efficiency gain of about a factor of 10 compared to the same sample type analysed by state-of-the-art conventional thermal ionization mass spectrometry (TIMS) on a Triton instrument is reported (ions detected per atom loaded). First attempts at isotope ratio analysis of micrometre sized uranium oxide particles, as is commonly done for nuclear safeguards, seem to confirm the enhancement in overall efficiency and could help with the analysis of the minor uranium isotopes on such samples. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Thermal ionization cavity sources (TIC sources), which have also been called hot cavity sources or high-efficiency cavity sources, have received continuous interest over the past decades because of their experimentally demonstrated high ionization efficiencies [1–9]. They were first developed at radioactive beam facilities [1,2] where they are still being used, e.g. [10,11]. The key component of this type of ion source, the cavity, is a small, almost enclosed volume. It is open to an analysing beam line through a typically circular orifice less than 1 mm in diameter. The material of the cavity walls is a refractory metal, usually W, Re, or Ta. For the applications at isotope separation on-line (ISOL) facilities, the cavities are generally built to larger dimensions of a few millimetres in each direction and heated to high temperatures (>2700 K). Usually, exotic nuclides are studied, which are produced by high-energy nuclear reactions. To reduce the energy of the quite often short-lived nuclides to a few keV, they are implanted in absorber materials positioned within the hot cavity, from where

∗ Corresponding author at: Clausiusstrasse 25, NW C83.2, CH-8092, Switzerland. E-mail address: [email protected] (C. Maden). https://doi.org/10.1016/j.ijms.2018.09.006 1387-3806/© 2018 Elsevier B.V. All rights reserved.

they diffuse out into the volume of the cavity for ionization and subsequent analysis. The physical processes involved within hot cavities of this kind have been summarised by Kirchner [6,7]. They revolve around the formation of a quasi-neutral plasma within the volume of the cavity, but the main ionization process is considered to be thermal ionization on the hot cavity walls. There is also intriguing potential for use of TIC sources within the fields of nuclear forensics and Earth Sciences, where conventional thermal ionization mass spectrometry (TIMS) has established itself as a state-of-the-art analytical technique [12,13]. Here, isotope ratios of a given element are of interest. The sample is conventionally loaded onto a thin metal filament, which, once transferred to the vacuum of the ion source, is then heated by an electrical current flowing through the filament. The sample evaporates and a fraction of the sample atoms thermally ionize on the hot filament surface (or on the surface of another hot filament in the case of double- or triple-filament configurations). Because of the open geometry of the filament assembly, most of the evaporated sample atoms are un-retrievably lost to the vacuum system of the ion source. However, if the sample could be confined within a hot cavity, then – simply speaking – evaporating sample atoms could have more chances at thermally ionizing, because of the multiple surface contacts the atoms would have on average before escaping the

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cavity. A higher ionization efficiency would be the result. For applications involving very small sample sizes and where the accuracy of the measured isotope ratios is limited by the counting statistics of the detected ions, more efficient ionization could result in more accurate isotope ratio measurements. Or, if the ion source background is free of a particular isotope of interest, determination of lower levels of minor isotopes could become possible. This is of great interest in nuclear forensics, where the minor isotopes give precious information on the origin of the nuclear material analysed. Several groups have attempted to employ a TIC source on a mass analyser capable of isotope ratio analysis. These cavities are smaller than those used at ISOL facilities and the relevant physical processes during operation are different, in that no quasi-neutral plasma can be present within the cavity [14]. Cesario et al. operated a TIC source, for which the sample element uranium was electroplated onto the tip of a tantalum rod and then inserted into a rhenium tube heated by electron bombardment [15]. Overall efficiencies (ion detected per atom loaded) of 0.02–0.14% were achieved for sample sizes of 40–300 ng. Duan et al. used tungsten rods with holes drilled into their ends as cavities and attached them to a quadrupole mass spectrometer [16,17]. The average overall efficiencies measured for several elements were comparable to a similar TIC source mounted on an isotope separator system at the same institution, e.g. 79%, 51%, 18%, 8.5%, and 8% for Eu, Sm, Nd, U, and Pu with sample sizes of 1 ␮g, 400 ␮g, 200 ␮g, 1 ␮g, and 0.1 ␮g, respectively. This was done by tuning the quadrupole mass analyser to a “high-mass-passfilter” mode, which does not provide sufficient mass resolution for meaningful isotope ratio measurements [17]. Conversely, the same studies demonstrate precise isotope ratio measurements using the same instrument. However, to achieve this the instrument was tuned to a normal mass resolution of M/M ≈ 150 resulting in a two orders of magnitude reduction in transmission through the quadrupole mass analyser, thus, reducing the overall efficiency by the same amount. Wayne et al. [18,19] were able to build on the experience gained by Duan et al. [16,17] and built a simplified TIC source attached to a time-of-flight mass spectrometer (TOF-MS). An advantage of a TOFMS can be its capability of simultaneously detecting ion species covering a large mass range. This property allowed the authors to study various effects of oxide and/or carbide formation during analysis on top of isotope ratio measurements on Zr, Mg, K, and Th samples. Furthermore, several multiply charged ion species, in particular W2+ and W3+ , were observed. These species cannot originate from thermal ionization on the cavity wall. The authors conclude that electron impact ionization of neutral atoms and ions escaping the cavity is significant. This effect can result in poorer quality mass spectra (increased peak tailing due to increased energy spread in the ion beam) or interferences on mass peaks of interest. The cavities used by Wayne et al. were holes spark eroded into the ends of tungsten wire. Ionization efficiencies of 1–3% for Th were measured (sample sizes <0.1–25 ng). But as the authors point out, it should be kept in mind that the ionization efficiencies reported in these studies are not overall efficiencies of ion detection, because of the pulsed operation mode of the TOF-MS. Its duty cycle (∼1.9% for the Th measurements) reduces the number of actually detected ions by the corresponding amount compared to a mass analyser capable of continuous ion beam detection. This is a big disadvantage for analyses involving low-level signals, because the precision of such measurements is usually limited by the counting statistics of the detected ions. Ingeneri et al. also used tungsten and rhenium rods with holes drilled into their ends as cavities, and mounted them on a Finnigan MAT262 thermal ionization mass spectrometer [20]. An overall efficiency of 2% was measured for both uranium and plutonium adsorbed onto single ion-exchange resin beads (10 ng and 30 pg

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of sample element per resin bead for U and Pu, respectively). In a later study investigating resin beads with different amounts of sample element adsorbed to them, the same authors reported an average overall efficiency of 5% for U (100–500 pg) and 7% for Pu (20–330 pg) [21]. This corresponds to an increase in overall efficiency of one to two orders of magnitude compared to the analysis of the same samples by conventional TIMS on the same instrument. The major isotope ratios of U and Pu were also measured in the second study and reproduced with standard deviations of 1.3% and 0.13%, respectively. Bürger et al. [22] continued the work of Ingeneri et al. [20] and Riciputi et al. [21] by installing a similar TIC source design on a Thermo Fisher Scientific Triton TIMS instrument. For unknown reasons the new configuration did not yield the same overall efficiencies as had been observed previously on a MAT262 instrument. Yet the overall TIC efficiencies of many more elements, including performance of isotope ratio measurements, were determined and summarised in [23]. Zhai et al. designed a TIC source based on resistive heating of a rhenium tube cavity mounted on an in-house built mass spectrometer similar to a MAT261 [24,25]. Their design allows for independent heating of an evaporation zone and a hotter ionization zone within the cavity. The advantage of a resistively heated cavity is that no electron beam is present to produce ion species in higher charge states. However, if a hot-spot develops on a cavity, then there is a positive feedback of cavity material evaporation thinning the cavity, which in turn increases the local temperature for a given heating current, and apparently shortens the lifetime of a cavity. Overall efficiencies were measured for U (0.5–2%) and Pu (4–9%), but the ionization efficiency of the cavity (ions produced per atom loaded) is expected to be higher due to transmission losses through the mass spectrometer [24]. Major isotope ratios of U and Pu were measured in peak-jumping mode yielding reproducibilities better than 1% for both elements [25]. In spite of the promising results of these previous studies, to our knowledge, none of these TIC sources reached routine operation for the analysis of isotope ratios. Possible reasons for this could be the difficulties involved in reproducibly loading small samples at the bottom of a deep and narrow hole [16,18,20–22], or a cavity design requiring a considerable amount of skilled manufacturing labour [24], thus making production of large numbers of cavities less feasible. Here we present a new design of a prototype TIC source installed on a Finnigan MAT262 TIMS instrument. It explores a compromise between the advantages and disadvantages reported by previous studies with the goal of reaching routine analyses of isotope ratios of samples for nuclear forensics or Earth Sciences applications. Modification of the existing instrument from filament based conventional TIMS to operation with a TIC source was aided by the software code Sofie [14] that allows calculation of the ion trajectories leaving the cavity while considering space charge within the cavity. We experimentally verified the detailed simulations of the existing and new ion optics on the MAT262 instrument and report the results of first samples analysed with the new TIC source. 2. Design of a prototype TIC source 2.1. Ion optics of the original MAT262 TIMS instrument Designing an ion source that efficiently ionizes sample atoms is one challenge. However, it is equally important for the produced ions to be transmitted through the mass analyser for detection at the collector array. This requires consideration of the ion optics of the system. To be sure that an ion optical calculation will yield meaningful results it is critical to have proper understanding of the original system before attempting a large modification

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Fig. 1. Illustration of the part of the mass spectrometer simulated with SIMION. The locations of source and alpha slits are indicated. An ion passing both of these slits is considered to be transmitted through the mass analyser and detected.

of instrument hardware. In order to test our concept for ion optical calculations for the new source, the original optics of the MAT262 instrument with samples loaded on conventional TIMS filaments was simulated and compared to the experimental behaviour of the instrument. The concept of the simulations is the following: for ionization from conventional filaments the resulting ion beams are too small for space charge to have any influence on the beams. Hence, the ion optics can be simulated using the code SIMION 8.0 by D. Dahl [26]. Not all the mass spectrometer was included in the simulation, only the ion source region up to the source slit, i.e. the object plane of the mass analyser (Fig. 1). If an ion passes the source slit (0.2 × 3 mm, w × h), its position and velocity vectors at the source slit are used to calculate the position the ion would have at the plane of the alpha slit (10 × 3 mm), which is located 321 mm towards the magnet. The space between the source and alpha slit is field-free on the MAT262 instrument. If the extrapolation indicates that the ion will pass through the alpha slit as well, then the ion is considered to be transmitted through the mass analyser and registered in a detector. The construction of the filament and lens stack was replicated in computer aided design (CAD) software and then exported to SIMION in STL-format to create the boundary conditions for solving Laplace’s equation for the electrostatic potential in the ion source. The lattice spacing chosen for the simulation was 0.1 mm, the smallest possible. It is limited by the largest block of memory this older version of SIMION can compute. Extensive use of SIMION’s “workbench programs” and “batch mode programs” features was made. “Workbench programs” are evaluated for each ion propagated through the simulation and were used to evalu-

ate if an individual ion would be transmitted to the detector. “Batch mode programs” were used to simulate large numbers of ions (typically 8000 per simulation), change electrode voltages, and output to results files, in order to automate determination of the transmission for a large number of electrode voltages. Fig. 2a shows a cut-away illustration of the simulation, including ion trajectories for 88 Sr+ ions emitted from a single filament at a temperature of 1600 K. The initial conditions of the Sr ions (position and velocity) were defined with a random number generator. Their positions were uniformly distributed over a 1 mm length of the 0.8 mm wide filament, whereas their initial velocities were sampled from a Maxwell velocity distribution modified to account for particles crossing a unit area per unit of time at the temperature of the filament (also referred to as cosine or Lambert distribution) [27,28]. The theoretical transmission of the ions as a function of the shield and lens voltages, which are two electrodes in the extraction lens stack of the MAT262, is shown by the contour plot in Fig. 2b. Superimposed on the figure are experimentally determined data, which were acquired by heating a Sr sample to provide a relatively constant 88 Sr+ ion current of about 20 pA (=2 V over a 1011 Ohm resistor) into the central Faraday cup. The shield voltage was set to pre-defined values before recording the lens voltages yielding maximal momentary signal (circles) and 90% of that momentary maximum (squares to either side). All electrode voltages were measured with a high-voltage probe at the corresponding power supply outputs. This experiment provides a qualitative map of the transmission as a function of the electrode voltages without determining it quantitatively. The good agreement between data and simulations demonstrates that the simulations capture the real behaviour of the MAT262 well and that this approach to simulating the ion optics is valid. It should be mentioned, that the simulation results are sensitive to, e.g., the length of the filament area chosen for Sr ion emission or the distance between filament surface and the first electron shield, but simulations generally agree to the experimental data to within 100–200 volts, which is sufficient for our purposes. 2.2. Choice of heating and ion optics of the new TIC source While designing our TIC source we attempted to learn from previous TIC source projects, and in addition, our prototype TIC source has been equipped with a few features that give it a unique character, all of which is discussed in the following sections: In spite of some disadvantages, electron impact heating is possibly still the best choice for heating the cavity. Zhai et al. have mentioned problems occurring from hot spots on resistively heated cavities [24]. Laser beam induced heating also did not seem to be

Fig. 2. a) Visualisation of a SIMION simulation of the original MAT262 ion optics. 88 Sr+ ions are uniformly emitted from a central 1 mm × 0.8 mm area of the 1600 K hot filament (at the origin of the xyz-coordinate system and set at a potential of 10 kV). In accordance with the MAT262 manual, the numbered electrodes are 1: shield lens, 2: split-lens, 3: R-focus, 4: electrodes at ground potential, 5: z-focus, and 6: source slit. b) Simulation results vs. experimental data: grey-scale contour plot shows the simulated transmission (the fraction of the total number of virtual ions leaving the filament) as a function of shield and split-lens voltages for a R-focus voltage of 4300 V and a z-focus voltage of 1000 V. Experimental data is superimposed with circles indicating the lens voltage providing maximal momentary 88 Sr+ current for a given shield voltage. The squares indicate the two lens voltages required to reach 90% of the corresponding maximal momentary beam current.

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simple either because of the following three reasons reported by Colle et al. [29]: Firstly, laser heating of the cavity would require significant investment in a continuous wave laser with an output power of several kilowatts. Secondly, simple and direct heating of the outside of the cavity with the laser beam would lead to a very inhomogeneous temperature distribution throughout the cavity, and a significant fraction of the laser beam power could be reflected off it and lost to the surroundings. Therefore, achieving a uniform temperature distribution throughout the cavity would be technically more challenging and probably require the use of a “susceptor” surrounding the cavity as described in [29] and into which the laser beam could be directed. This would make the design of the cavity heating system more complex, especially, because there is also a requirement for high electric field strengths at the exit orifice of the cavity (see discussion below). And thirdly, it is possible that optical elements such as viewports and/or mirrors could be coated by material evaporated from the hot cavity, which would be problematic for routine operation of a source. Because our laboratory is experienced with in-house designed and built electron impact heating systems we chose this heating method. However, electron impact heating leads to the problem of multiply charged ions and peak tailing in the mass spectrum [19]. To mitigate this problem, the electron beam heating the cavity needs to be spatially separated from the space in front of the cavity orifice. This is achieved here by pushing the end of the cavity up against a graphite plate with a hole in it, such that the end of the cavity is flush with the side of the plate facing towards the extraction lens stack (Fig. 3). Thus, a large fraction of the electron beam should be confined behind the plate, and ions leaving the cavity in direction of the extraction lens stack should be protected from the beam. Thereby, the chosen design reduces the probability of electron impact ionization and the resulting tailing effects in the mass spectrum. The second purpose of the plate is to homogenise the electric field around the orifice of the cavity. Previous simulations of the ionization processes within the cavity have shown that maximising the extracting electric field strength at the orifice of a cavity enhances its ionization efficiency [14]. Therefore, the new source design tries to maximise the field strength to a reasonably operable technical limit (1–1.5 kV/mm). In order to reduce the risk of electrical discharges of the high voltage applied to the cavity, but also to reduce ion beam aberrations due to slight geometrical imperfections in the construction of the source, a design with a flat surface around the cavity orifice is better than the tip of a hot rod. The necessity of the high extracting field strength requires a complete re-design of the extraction lens stack for best transmission of the ion beam through the mass analyser. The immersion lens optics commonly used on conventional TIMS instruments can no longer be used, because the initial accelerating field of this configuration is typically weaker (∼100 V/mm). If a distance of about 5 mm is chosen between the orifice of the cavity and the first extraction electrode then the voltage on the first electrode should be at most +5 kV if the cavity is at +10 kV. Because the ion energy at the extraction electrode is already high (∼5 keV), any focussing of the beam after passing the first electrode, therefore, has to be done with optical elements other than with an immersion lens. Hence, a different approach is taken. The present design features two einzel lenses after the extraction electrode and before the source slit – one single electrode with an oval opening, and one split electrode for additional horizontal steering (Fig. 4). The ion optics for the new TIC source were simulated with the same methodology as the original extraction lens stack of the MAT262 (Section 2.1, Figs. 1 and 2). An exception is how the initial conditions of the ions used for the SIMION simulations were defined: first, a Sofie simulation of the cavity to be built into the MAT262 was performed. This includes the initial acceleration that the ions experience in the electric field between the graphite plate and the extraction electrode. The lat-

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tice spacing of the Sofie simulation was 0.01 mm, which is ten times smaller than the one used in SIMION. To reduce systematic errors due to differences in the electrostatic potentials of the Sofie and SIMION simulations at the same nominal location, it was considered best to record position and velocities of the ions in Sofie after they had left the cavity and had been accelerated to 2–2.5 keV. Thus, the ions had reached a relatively homogeneous region of the accelerating field, and transferring their positions between codes resulted in a smaller systematic error than if the same transition were done near the cavity orifice. This process is the reason why the ion trajectories originate half way between the cavity and the extraction lens in Fig. 4. The geometries and spacing of the lenses and other electrodes were iteratively modified, with the goal of maximising transmission from the cavity through the mass analyser to the detector array. An illustration of the final design as programmed into SIMION is shown in Fig. 4 together with experimental verification of the simulation of the ion optics after construction of the TIC source (almost identical to the procedures used for Fig. 2). During the simulation process described above it became clear that the best cavity geometry theoretically found by Maden et al. [14], which could potentially ionize 100 times more efficiently than a theoretical conventional filament, produced an ion beam with too large emittance to be transmitted efficiently to the detector array. Even by widening the source slit from its original 0.2 mm to 0.8 mm resulted in a transmission of ∼11% at best (this is discussed further in section 3.2). In comparison, a cavity with the regular tube geometry, which typically enhances ionization efficiency by about a factor of 10 compared to the theoretical conventional filament, could reach a transmission of up to 95% if the source slit is widened to 0.8 mm (depending on tube geometry and temperature distribution over the cavity surface). Hence, the increased ionization efficiency of the better (but more complicated) cavity geometry would be reduced by the significantly lower transmission, and in the end would provide about the same overall efficiency as a cavity with a regular tube geometry, i.e. about 10 times enhanced overall efficiency. For technical reasons it is easier to build a tube geometry cavity, therefore, we pursued this route during construction of the prototype source. 2.3. Cavity assembly and sample loading Another unique feature of our source design is the sectioned construction of the cavity that makes it easier to load a sample to its bottom. Fig. 3 shows a technical drawing of the cross section through the centre of an assembled cavity. The rhenium cavity is made up out of two sections, termed “sample holder” and “cavity tube”, which are pushed up against the graphite plate by an 85 mm long rhenium rod. There is a stainless steel screw cap at the end of the Re rod, with which the pressure of the cavity assembly against the graphite plate can be adjusted. The pieces of the cavity are aligned against each other by matching conical surfaces and the cavity tube fits into a hole in the plate. The cavity sections are made (in-house) out of rhenium, which is chosen for its high work function, its ability to withstand temperatures above 2000 ◦ C, and its availability as raw material. Rhenium carbide would have been a better choice due to its even higher work function that theoretically enhances ionization efficiency according to the Saha-Langmuir relation [30], but its availability in pure form is limited. Instead, graphite was chosen as the material of the plate in contact with the cavity tube. This was mainly due to the lack of reactivity of rhenium and graphite even at high temperatures, such that the cavity tube could easily be separated from the plate to change cavities. However, it was hoped that with this configuration a fraction of carbon from the plate would migrate over the cavity surfaces or diffuse through the rhenium cavity material to

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Fig. 3. Illustration of the sectioned design of the cavity. a) Technical schematic of the cross section through the centre of the cavity along the ion optical axis indicating the locations of graphite plate, cavity tube, sample holder, and Re rod. b) Photograph of the tip of a sample holder during deposition of a uranium oxide particle (enclosed in collodion) on the tip of a tungsten carbide needle held by a micro-manipulator.

Fig. 4. a) Visualisation of a SIMION simulation of the final TIC ion optics. 238 U+ ions from a Sofie simulation of a 1 mm diameter tube cavity are used and transferred to SIMION at the appropriate location (see text for details). The numbered items are 1: orifice of the tube cavity (set at 10 kV), 2: extraction electrode (5 kV), 3: einzel lens, 4: electrodes at ground potential, 5: x-lens, 6: source slit. b) Simulation results vs. experimental data: grey-scale contour plot shows the simulated transmission (the fraction of the total number of virtual ions leaving the cavity) as a function of (einzel) lens and x-lens voltages. Experimental data is superimposed with circles indicating the lens voltage providing maximal momentary 238 U+ current for a given x-lens voltage. The squares indicate the two lens voltages required to reach 80% of the corresponding maximal momentary beam current.

form rhenium carbide at the inner cavity walls, thereby, enhancing the ionization efficiency of the cavity. Preliminary test results suggest that this effect could be visible (see discussion further down in Section 3.2). The tip of the cone of the sample holder has got a 1 mm diameter dimple in it (Fig. 3b), into which the sample can be loaded. This can be done by pipetting microliter amounts of sample solution – similar to conventional TIMS loading techniques – or depositing a small solid sample adhesing to the tip of a micro-manipulator (as shown in Fig. 3b). With the tests performed so far, there is no indication that the presence of the crack separating the sample holder from the cavity tube is a disadvantage. Loading and removal of the stack of cavity sections from the source is done manually with the aid of a special tool after venting the source. 2.4. Electron impact heating Theoretically, it is necessary that the hottest region of a tube cavity should be located around its opening end [14]. Unfortunately, the presence of the graphite plate by the orifice of the cavity prevents electrons from being accelerated naturally towards the cavity tube. It is impractical to position the filament providing the electron beam very close to the cavity tube. Consequently, the electric field around the filament of the electron impact heating needs to be shaped such that the electrons are accelerated towards the cavity tube, even if the filament is at a distance from it. This requires an additional repelling shield electrode. Again, the code Sofie was used to simulate the electron impact heating, in order to determine

the best filament position and repelling shield electrode geometry before construction. An illustration of the final design viewed in Sofie (projection of the rotationally symmetrical geometry into the (r,z)-plane) and a picture of the source in operation are shown in Fig. 5. Notably, the thermal conductivity between individual parts in vacuum is very poor. Hence, it is even more important to concentrate heating power onto the cavity tube by focussing the electron beam onto it. The filament is a 0.3 mm diameter tantalum wire mounted in a circle with a radius of 20 mm at a distance of about 24 mm from the graphite plate. Its current (5–6 A) is provided by a CPX400 power supply from Thurlby Thandar Instruments. The tantalum shield used to direct the electrons onto the cavity is typically set to about -500 V and is powered by a HCP 14–3500 supply from FuG Elektronik GmbH. The cavity is charged to +10 kV, which is the original acceleration voltage of the MAT262 instrument. Due to the additional current provided by the electron impact heating (currently not operated above 40 mA max, corresponding to a heating power of 400 W), the original 10 kV supply of the instrument was not powerful enough. It was replaced with an external, high-stability, low-ripple HCP 1400–12500 supply (12.5 kV/100 mA) from FuG Elektronik GmbH, which also powers the voltage divider units of the MAT262 (BKC- and BQ-units). These units were modified to provide the necessary lens electrode voltages. The emission current of the electron impact heating is measured by converting the difference of the filament current flowing to the filament and the current returning from the filament with a LEM CT0.1-P current transducer. Heating power is kept constant by adjusting the fila-

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Fig. 5. Design of the electron impact heating. a) Sofie simulation of the rotationally symmetrical electron impact heating. The symmetry axis is the left hand edge of the picture. Equipotential lines are drawn at 5000 V, 2500 V, 2000 V, 1000 V, 500 V, 250 V, 100 V, 50 V, 40 V, 30 V, 20 V, 10 V, 5 V, 1 V, 0.5 V, 0.1 V, −125 V, −250 V, and −375 V. b) Photograph of the ion source in operation showing the hot cavity tube (2) and cooler sample holder (3) pushed against the graphite plate (1). The repelling shield electrode (4) is hiding the filament from view.

Fig. 6. Illustration of the final design of the prototype TIC source. a) CAD design of the TIC and extraction lens stack. b) and c) Photographs of the TIC source installed in the source head of the MAT262. The numbered elements are 1: graphite plate, 2: cavity tube and sample holder, 3: repelling shield (hiding the filament from view), 4: Re rod with screw cap, which pushes the cavity up against the graphite plate, 5: extraction electrode, 6: einzel lens, 7: x-lens, 8: source slit, and 9: xyz-table, from which the cavity assembly is suspended.

ment current to provide a given emission current via an in-house written LabView program run on a second control PC. The program can also control slow ramping of the heating power. An additional electronic modification to the instrument allows the output pulses of the (amplifier output) voltage-frequency converters (UFC) to be recorded in parallel by LabView, such that peak scanning is possible during manual tuning, i.e. scanning the externally supplied 10 kV acceleration voltage to centre on mass spectrum peaks. The rest of the instrument’s functionalities, especially, the routines for measuring isotope ratios, are controlled by the original MAT262 control software. 2.5. Mounting and operation of the TIC source The TIC source is mounted into the original ion source vacuum chamber of the MAT262. The original extraction lens stack and sample turret motor drive have been removed. The new in-house built

lens stack is mounted using the same self-centring baseplate the original lens stack was mounted on. The original adjustable source slit mount, which provides the possibility to switch between three different source slit widths, is left installed. For reasons discussed in Section 3.2, one of the positions has been widened to 0.8 mm slit width. The cavity, graphite plate, electron impact heating filament, and repeller shield are fixed in position relative to each other, but together are suspended from a xyz-table mounted on the flange originally used for the liquid nitrogen cold trap. This allows the cavity to be moved with a reproducibility of 0.01 mm relative to the ion optical axis, such that thermal expansion of the source components can be compensated during operation. Fig. 6 shows pictures of the CAD design of the TIC source and the real source installed in the MAT262. The base pressure of the source is in the high 10−8 mbar range. Operation of the TIC source causes the source vacuum chamber to heat up and start to degas. In spite of moderate baking of the

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source every time it is vented, after about an hour of operation at 400 W heating power, the pressure in the source usually has risen to ∼1 × 10−6 mbar, a value above which high-voltage discharges are to be expected, and operation of the source has to be suspended until the system has cooled down. Given that the ion source has to be vented to atmosphere to change samples, thus, allowing air molecules to re-adsorb to the vacuum chamber walls, this is one of the bigger disadvantages of the current system and would need to be improved on in a future version of the source, e.g. by introducing water-cooling of the vacuum chamber, increasing vacuum pumping speed, and a vacuum lock for sample changing. Furthermore it was noticed that the filament moves unpredictably during operation due to thermal expansion. Filament movement will slightly change the location of electron impact on the cavity, which in turn has an effect on the signal stability of the ion current produced by the cavity. These spontaneous changes in ion beam currents can correlate with slight pressure changes in the source. They are assumed to be caused by a change in the temperature of the graphite plate, and consequently, a change in the rate it is degassing adsorbed air. Thus, in a future version of the source, the design of the heating filament will be improved. 3. Results of first measurements 3.1. Doubly charged ions Other groups have reported the observation of multiply charged ions in the mass spectra of cavity sources [6,19,24]. To estimate how well the newly introduced graphite plate shields the electron impact heating beam from the region just in front of the cavity orifice, we have measured the 187 Re2+ /187 Re+ ratio on our system at a heating power of 370 W. It is about 10−3 , which is roughly two orders of magnitude worse than the value found by Zhai et al. (∼10−5 ) [24], who used resistive heating to heat their cavity, but still about three orders of magnitude better than the W2+ /W+ ≈ 0.5 measured by Wayne et al. [19], who employed an electron impact heating. While comparing these values it should be kept in mind that electron energies varied between the mentioned experiments. They were up to 2.5 keV for Wayne et al. [19] and up to 10 keV for Zhai et al. [24] and this study, in each case depending on the unknown potentials of the origins of the ionizing electrons, which for electron impact heating does not necessarily need to be the filament. Also, there will be differences in the electron impact ionization cross sections for W and Re. A quantitative comparison of the doubly charged to singly charged ion species ratios to evaluate the effectivity of the graphite plate is, therefore, not possible. Nevertheless, we can conclude that the graphite plate significantly reduces electron impact ionization of ions and neutrals escaping the cavity, thus helping to reduce tailing effects and interferences in the mass spectrum. 3.2. Analysis of uranium oxide particles The rhenium rod that is used to make the cavity sections is available in a 99.98% purity grade. This is not as pure as the zone-refined Re ribbon (99.999%) that is commonly used for conventional TIMS analysis. Therefore, it is not surprising that a freshly made cavity shows a uranium background signal. Generally, the 238 U+ background signals from freshly machined rhenium cavities start at levels of a couple of hundred counts per second at 400 W heating power and then successively increase over a few heating sessions to 10’000 – 20’000 cps, for the same cavity and without loading a sample or other material into it. As mentioned in Section 2.2, this increase in background signal could be due to carbon diffusing into or over the cavity material, thereby, forming rhenium carbide that

ionizes impurities more efficiently due to its higher work function. Assuming a homogeneous distribution of the uranium impurity in the rhenium and the absence of any effect other than diffusion, one would expect the signal to drop due to diffusive depletion of U in the surface layer of the cavity material. This is not the case, therefore, supporting the hypothesis that the ionization efficiency of the cavity is increasing by repeated heating of the cavity. However, there could be other reasons than formation of rhenium carbide that cause an increase in 238 U+ signal with time. For example, an effect related to the growth of the crystal zones within the polycrystalline rhenium, which visibly grow from sub-mm to millimetre-size over the lifetime of a cavity. The study of such effects is beyond the scope of this work and require further investigation. In particular, the contribution from carbon diffusing off the graphite plate to enhance the ionization efficiency could be negligible compared to loading samples in a carbon-rich matrix, which could evaporate and coat the inner cavity walls to form a rhenium carbide layer much more efficiently. In order to verify if isotope ratios of 10 pg-sized uranium samples can be reliably measured above the background of the prototype source, samples with a not-natural isotope signature were analysed. By use of a micro-manipulator with a sharp tungsten carbide tip, uranium oxide particles of 1–3 ␮m diameter encased in a thin collodion film were loaded onto the tips of sample holders. This corresponds to uranium amounts of 3.7–120 pg assuming the particles are solid U3 O8 , which is not necessarily the case as these particles are known to be porous. A drop of glucose was used to help the particle stick to the sample holder (Fig. 3b). Three uranium particles were from the New Brunswick Laboratory U100 certified reference material with 235 U/238 U = 0.1136 ± 0.00011 and 234 U/238 U = (7.536 ± 0.022) × 10−4 [31]. Two other particles contained uranium from standard reference material SRM 960 with 235 U/238 U = (7.248 ± 0.032) × 10−3 and 234 U/238 U = (6.1 ± 2.6) × 10−5 . All oxide particle samples gave a clear signal well above cavity background. In most cases a maximal 238 U+ current of 0.5–1 pA were measured in a Faraday cup ( = 50–100 mV over a 1011  resistor), and signal intensities larger than 0.1 pA ( = 10 mV) lasted for 30 min at least, which is long enough to measure meaningful isotope ratios. Beyond the ion current instabilities mentioned above (section 2.5), which are presumably caused by thermal expansion of the electron impact heating filament, ion currents could be equally stable as could be expected during conventional TIMS analysis. For these small samples this means that the ion currents are not constant but increase initially and then start to decrease as the sample is consumed and maximal heating power of the cavity source is reached. However, at such signal intensities and with the isotope ratio of NBS U100, a multi-collection measurement with 235 U and 238 U measured in Faraday cups and 234 U in an ion counter is possible. Furthermore, two other measurement methods were tested: 1) peak-jumping between 238 U, 235 U, and 234 U with all isotopes detected in the same ion counter and 2) the same peak-jumping method, but with 238 U measured in a Faraday cup instead of the ion counter. Three to four analyses using one of the three above methods could be performed on each of the uranium oxide particles before the pressure in the ion source had risen so high that its operation had to be interrupted. No mass fractionation, background or any other correction was applied to the data, therefore, all isotope ratios reported here are raw ratios. For each of the five uranium oxide particles Table 1 shows the average of the 3–4 analyses together with their respective relative standard deviation, which should represent the scatter of an individual analysis around the average of the analyses of that sample. Additionally, and as discussed also in the following section, four NBS U100 solution samples, which were pipetted directly into the cavities, were analysed for comparison. The maximal ion beam cur-

0.20 0.04 1.1 0.08 1.9 40 13 12 75.36 75.36 75.36 75.36

0.6 – 19 1.2 – 38 −12.8 4.9 6.1 6.1

2 – 65 1.2 – 38 3 – 110 −4.6 6.3 0.8 75.36 75.36 75.36

Overall efficiency [%] Rel. Diff. [%] Certified x(10−5 )

U/238 U

3.9 5.0 1.3 5.6 73.9 44.9 65.5 66.1 113.6 113.6 113.6 113.6

−0.2 41 9.2 13.6

8.3 9.6 6.9 5.8 0.8 1.6 7.248 7.248

16 1.6 2.0 78.8 70.6 74.7 −6.2 −0.41 0.6 113.6 113.6 113.6

Measured x(10−5 ) Certified x(10−3 )

Rel. Diff. [%]

1 Rel. Std. Dev. [%]

234

U/238 U

1.4 2.3 0.7 2.5 113.8 66.8 103.2 98.1 10.8 10.5 10.6 10.7 NBS U100 solution NBS U100 solution NBS U100 solution NBS U100 solution Re3 Re5 Re6 Re20

1 1 1 1

1.0 3.2 7.18 7.2 3.7 – 120 3.7 – 120 SRM960 particle SRM960 particle Re1 Re2

3 4

8.0 0.26 0.5 120.7 114.1 113.0 3.7 – 120 3.7 – 120 3.7 – 120 NBS U100 particle NBS U100 particle NBS U100 particle Re6 Re8 Re9

4 3 3

Measured x(10−3 )

1 Rel. Std. Dev. [%]

235

Number of analyses per sample Mass of U [pg] Sample Cavity

Table 1 Summary of isotope ratios measured on uranium samples together with the measured overall efficiencies for the uranium solution samples. If several isotope ratio measurements could be performed on the same sample, then “Rel. Std. Dev.” refers to the relative standard deviation of the analyses. Otherwise, “Rel. Std. Dev.” refers to the internal relative standard error of the mean of the single measurement that could be performed. For an individual analysis on a uranium oxide particle sample, the internal relative standard errors of the mean on the 235 U/238 U ratio were about 0.1–0.5% for the multi-collection method and 1–2% for the peak jumping methods. For the uranium oxide particle samples the overall efficiencies are unknown. The quoted ranges are very rough estimations and assume the particles contained between 120 and 3.7 pg of U while ignoring potential porosity of the particles.

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rents during analysis from these samples were significantly smaller and only one isotope ratio measurement per sample could be performed. The results are summarised in Table 1 as well, but for these samples the error reported is the internal relative standard error of the mean of the single measurement done on the sample. While looking at the data in Table 1 it should be kept in mind that these are the very first analyses performed on this prototype TIC source and that there is still a lot of potential to improve on the analytical methods used. In some cases, such as for the 235 U/238 U ratios of the NBS U100 particles analysed in cavities Re8 and Re9 as well as for the SRM960 particle measured in Re1, the measured ratios scatter around the certified values by <1%. However, in other cases, such as for the 235 U/238 U and 234 U/238 U ratios of the NBS U100 particle in Re6 and of the NBS U100 solution samples in Re5, Re6, and Re20, the measured ratios are characterised by large errors, and for the solution samples they additionally differ from the certified values beyond statistical agreement. There are several reasons for this: Firstly, all measurements were performed with the source slit width set to 0.8 mm (four times wider than the 0.2 mm width in the original configuration of the MAT262). The reason for this originates in Liouville’s theorem, which is a conservation law in classical Hamiltonian mechanics. Its first order approximation applied to the ion optics of the MAT262 states that once a group of ions leaving the ion source has been accelerated to a specific mean energy, the volume of the four-dimensional phase space (defined by position and momenta coordinates x, y, px , py ) occupied by the ions is conserved along the central path through the mass spectrometer (the ion optical axis, or z-coordinate) [32]. Consequently, this means that an ion beam cannot be focussed to a small spot without appropriately increasing the divergence of the beam, i.e. the angle at which individual ions fly away from the ion optical axis, such that the four-dimensional phase space is conserved. Our simulations of the ion optics revealed that within the limitations presented to the ion beam by the fixed dimensions of the alpha slit (Fig. 1), the beam spot size at the source slit cannot be reduced enough for all of the beam to pass through the original slit. Therefore, in order to maximise transmission of the ion beam, the source slit was widened from 0.2 to 0.8 mm to allow a larger fraction of the beam to pass it. However, increasing the source slit width also increases the beam width size at the collector array. This leads to mass spectrum peak shapes without flat tops, as shown in Fig. 7. For comparison, two peak scans with source slit widths of 0.2 mm and 0.8 mm, respectively, are shown in Fig. 7b. The increase of signal intensities at the detector by widening the slit width is a factor of 3.3–4, as is to be expected by changing the open area of the slit by a factor of four. This gain factor is independent of the momentary beam intensity, but it depends on how uniformly the source slit is illuminated by the ion beam. The example shown in Fig. 7b was recorded using 187 Re+ ions, which are ionized cavity material and will also originate from the front face of the cavity as well, thereby, creating a larger beam spot at the source slit than sample ions would, resulting in the factor of 4 signal increase when switching to the larger slit width. For uranium ions a factor of ∼3.3 was measured. The missing peak top flat would require perfect peak alignment for multi-collection analysis if the raw isotope ratio measured at slightly different positions on the mass peak should not vary. For peak-jumping methods each ion current integration would have to be performed at the exact same relative position on each of the peaks in order to avoid additional scatter in the measured ratios. Experimentally, and without being able to centre the beams properly during analysis, these conditions are hardly achievable, therefore, resulting in larger scatter in the data and sometimes also systematic offsets between measured and certified ratios. This disadvantage of the current design, which originates in the physics of ion optics, can be remedied by a re-design of the source to include a

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Fig. 7. Peak scans of uranium ion beams showing the effect that the widening of the source slit has on the peak shape in the mass spectrum. a) Normalized ion currents as a function of acceleration voltage for a source slit width of 0.8 mm. 238 U+ (red), 235 U+ (green), and 234 U+ (white) beams. b) Comparisons of peak scans with the source slit set to widths of 0.2 mm and 0.8 mm. The data was recorded with a 187 Re+ ion beam that will have a more uniform current density distribution over the source slit and results in a four times higher maximal signal for the wider slit setting (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

mass analyser with an appropriate acceptance for the cavity geometry in question. A second reason for larger errors on peak-jumping methods is ion current instability, as mentioned above (section 2.5). Peak jumping methods rely on the relative current intensities remaining constant between integrations on the different peaks of the mass spectrum. If there is instability in the signal intensities, then more scatter in the data is to be expected. Thirdly, for the NBS U100 liquid samples, a reason for the systematic difference between measured and certified ratios lies in the weak ion currents recorded from these samples. Generally, the 238 U+ ion currents of these samples were significantly lower than for the oxide particles (100’000 – 200’000 cps compared to 3.1–6.2 × 106 cps, respectively). Therefore, the contribution of background uranium from the cavity material, which has got a different isotope ratio to the sample, starts to get significant and bias the measured raw isotope ratios away from the certified values. In spite of these short-comings, our data shows that if multicollection analyses can be performed, then the internal precision of an individual measurement can be <0.1%. Pure counting statistics of the numbers of detected ions suggest that the physical limit for the uranium samples analysed here is even lower (<0.01%). Fig. 8 shows the stability of the raw 238 U/235 U ratio as a function of time during a multi-collection analysis of a NBS U100 particle, and after which the sample was only partly consumed. The resulting precision of the analysis mean is 0.08% (1 RSE) with a relative difference from the certified ratio of 0.6%. This indicates the potential that the prototype TIC source has got with respect to more accurate isotope ratio measurements. However, before routine analysis of samples can be attempted an improved and more reliable measurement method needs to be developed. 3.3. Overall efficiency of the TIC source for uranium samples The magnitude of uranium ion beam signals from the uranium oxide particle samples is very promising for isotope ratio analysis. They appear to be about ten times larger than for the same samples analysed by conventional TIMS [33]. Ignoring possible porosity of the oxide particles and assuming the amounts of U in them lie within 3.7–120 pg, one can calculate limits for each particle, within which the overall efficiency for uranium oxide particle analysis should lie. The calculation is based on the integration of the 238 U+ current over the duration, for which the sample provided sufficient

current for a meaningful isotope ratio measurement. The overall efficiency ranges are shown in Table 1. They should be regarded as very rough preliminary estimations and primarily reflect the magnitude of the integrated 238 U+ current. Based on the ranges of the 5 analysed particles, the overall efficiency for uranium oxide particle analysis should lie between 3 and 19%. Should these values be confirmed, then TIC source analysis of uranium oxide particles would be very competitive compared to other established analytical methods, e.g. secondary ion mass spectrometry (SIMS, overall efficiency for uranium oxide particle analysis: 0.7–1.2% [34]) or multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS, overall efficiency for U solution samples: 1.5–2.5 % [35]), but also compared to more exotic applications of TIMS, such as the resin bead sample loading technique (∼0.5% overall efficiency for U [36]) or porous ion emitters (1–2% for U [37]). For a better estimation of the overall efficiency, known amounts of NBS U100 uranium nitrate solution with a known U concentration were pipetted into cavities and allowed to dry (without any other so-called activators). The amounts of U in these samples were about 11 pg and the results of the subsequent analyses are also shown in Table 1. The overall efficiencies of three of the four samples are surprisingly low, yielding values of 0.04%–0.2%. An exception is the measurement on cavity “Re6”, yielding 1.1%. But this measurement could have been influenced by the analysis of a uranium oxide particle in the same cavity shortly beforehand, thus, yielding an apparently higher efficiency due to higher background memory from the particle analysed in cavity “Re6”. Generally, the 238 U+ ion current intensities of the uranium solution samples were significantly lower than for the oxide particles (100’000 – 200’000 cps compared to 3.1–6.2 × 106 cps, respectively). No mentionable 238 U16 O+ or 238 U16 O2 + currents were detected during any of the measurements. Hence, ionization of uranium did not preferentially occur via a different molecular uranium ion species. This suggests that the analysis of uranium from dried down solutions in a TIC source is chemically and/or physically different to the analysis of the uranium oxide particles. Considering the significant effort that has gone into enhancing the overall efficiency for uranium analysis by conventional TIMS (e.g [38]. and references therein), this conclusion is not surprising. Two possible explanations for our observations are as follows: 1) the thin layer of uranium nitrate spread over the cavity walls from the dried down solution evaporated and escaped the cavity at lower temperatures, before the sample could be efficiently ionized, com-

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Fig. 8. Raw 235 U/238 U ratios of individual integrations (4 s) as a function of time during a multi-collection measurement of a NBS U100 uranium oxide particle. Both beams are collected by Faraday cups attached to amplifiers with 1011  feedback resistors. The nominal ratio (235 U/238 U = 0.1136) is indicated by the horizontal line.

pared to the uranium oxide particles enclosed in the collodion films. 2) The carbon-rich collodion around the oxide particles decomposed during heating and distributed carbon over the inner cavity walls, thereby forming some degree of a rhenium carbide layer. The work function of rhenium carbide is higher than for a pure rhenium surface [30]. According to the Saha-Langmuir relation this would result in more efficient ionization at the same temperature. Judging by the overall efficiencies of Sr and Nd from solution and resinbead-load samples analysed in a TIC source reported by Bürger et al. [22], this group made a similar observation. Further investigation of these effects is required. For a better quantification of the overall yield of uranium oxide particles, therefore, a type of sample with a known uranium amount is required, but it should also imitate the properties of the oxide particles, i.e. a particle-like source of uranium sitting in a carbonrich matrix. Uranium adsorbed to a single resin bead, as already reported by, e.g., Smith et al. [36], fulfils these conditions. The first such resin bead analysed in our TIC source gave an overall efficiency of 1.6%, which is 6–13 times more efficient than the same sample type analysed on a Thermo Fisher Scientific Triton TIMS instrument at ETH Zurich. This is in very good agreement with predictions made by the code Sofie, which would predict an 8-fold enhancement of ionization efficiency for the cavity geometry used in the prototype TIC source (assuming the filament and cavity temperatures are the same for both methods). We conclude, that the prototype TIC source provides about 10 times higher overall efficiency compared to conventional TIMS, but a more detailed future study needs to confirm this figure. 4. Conclusions A prototype TIC source has been designed and built with the help of the simulation codes SIMION and Sofie. It has been mounted onto a MAT262 TIMS instrument and it includes some unique features that set it apart from previously reported TIC sources. Most notable are the sectioned construction of the cavity for easier loading of samples, and a graphite plate that demonstrably reduces the formation of doubly charged ions and, consequently, also peak tailing effects. The detailed ion optical calculations have been experimentally verified. The current analytical capabilities of the new source have been explored with uranium samples. It seems to be able to analyse 1–3 ␮m diameter uranium oxide particles well and to provide a 10 fold

overall efficiency enhancement compared to the same samples analysed by conventional TIMS. The measured 1.6% overall efficiency for uranium adsorbed to a single resin bead indicates that the performance of the prototype source falls within the range of other reported efficiencies, but further experimental confirmation of the results is required. The data at hand suggests that, currently, 238 U/235 U ratios from uranium oxide particle samples can be measured with a reproducibility of 1–2% (1␴). But the potential for an order of magnitude better precision, especially, if multi-collection methods can be employed is evident. The main limitations of the prototype system are the small ion optical acceptance of the mass analyser of the MAT262, ion beam current instability, probably due to thermal expansion of the components of the electron impact heating, and background from the cavity material. These issues would need to be addressed in a future version of the source, for which a better vacuum chamber needs to be designed to reduce outgassing of the chamber and lower the pressure during operation of the cavity source. Acknowledgments This work was performed as part of the ETH ZurichCommissariat à l’énergie atomique et aux énergies alternatives collaboration agreement 188-C-SACO-Av2 and with the additional support of ETH internal funds. Maria Schönbächler’s support of the project has been very important. Urs Menet and Donat Niederer each deserve a big thank-you for mechanically building the new TIC source and modifying the electronics of the MAT262, respectively. Morten Andersen provided help to quantify the amount of uranium adsorbed to a single resin bead. Heinrich Baur has always been available to discuss the physics of ion sources and mass spectrometers in general. References [1] G.J. Beyer, E. Herrmann, A. Piotrowski, V.J. Raiko, H. Tyrroff, A new method for rare-earth isotope separation, Nucl. Instr. Meth. 96 (1971) 437–439, http://dx. doi.org/10.1016/0029-554x(71)90613-6. [2] P.G. Johnson, A. Bolson, C.M. Henderson, A high temperature ion source for isotope separators, Nucl. Instr. Meth. 106 (1973) 83–87, http://dx.doi.org/10. 1016/0029-554x(73)90049-9. [3] V.P. Afanas’ev, V.A. Obukhov, V.I. Raiko, Thermoionization efficiency in the ion source cavity, Nucl. Instr. Meth. 145 (1977) 533–536, http://dx.doi.org/10. 1016/0029-554x(77)90584-5.

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