A direct-AMS multi-isotope survey of uranium ore concentrates

Nuclear Inst. and Methods in Physics Research B 459 (2019) 98–114

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Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb

A direct-AMS multi-isotope survey of uranium ore concentrates a,⁎

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X.-L. Zhao , B.B.A. Francisco , W.E. Kieser , A. El-Jaby , C. Cochrane , I.D. Clark a b c

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Department of Physics, A. E. Lalonde AMS Laboratory, University of Ottawa, 25 Templeton St., Ottawa, ON K1N 6N5, Canada Department of Earth and Environmental Sciences, A. E. Lalonde AMS Laboratory, University of Ottawa, 25 Templeton St., Ottawa, ON K1N 6N5, Canada Directorate of Security and Safeguards, Canadian Nuclear Safety Commission (CNSC), 280 Slater St., P.O. Box 1046 – Station B, Ottawa, ON K1P 5S9, Canada

ARTICLE INFO

ABSTRACT

Keywords: Direct-AMS Multi-isotope survey Uranium ore concentrate (UOC) Nuclear forensics 236 U/238U 187 Os/188Os

Uranium ore concentrate (UOC) is an important nuclear material of interest for Canada. A large scale analytical program is being led by the Directorate of Security and Safeguards (DSS) of the Canadian Nuclear Safety Commission (CNSC) to establish a reference dataset of UOCs that have passed through and/or are currently under Canadian regulatory control. Isotopic ratios are among the signatures being captured under the reference dataset. Accelerator Mass Spectrometry (AMS) has been used for the measurement and assessment of 236U/238U and 187Os/188Os, respectively. Furthermore, since UOCs have uranium typically concentrated to ≥70% by weight, a direct-AMS assay method is possible wherein the samples can be measured without time consuming chemical digestion and processing. Using this direct-AMS approach, several related ratios (231Pa/238U, 230 Th/238U, 226Ra/238U) were also assessed within the data acquisition sequence used for measuring the 236 U/238U ratios, and 185Re, 187Re and 187Os, 188Os, 191Ir and 193Ir in the sequence for the 187Os/188Os ratios. The sum of these results can be shown together in a “bar-code” pattern to strengthen the capability for UOC source identification. Unexpectedly large 236U/238U ratios (up to ≥1 × 10−7) have been found in several UOC samples. The 187Os/188Os ratio has also been shown for the first time to be a viable supplementary signature of UOCs. This work shows that the direct-AMS method has the potential to become an effective tool for nuclear forensics provenance assessment applications with UOCs. The implications of the results, and the need for further refinement of the sputter target preparation, as well as the Cs+ sputter ion source itself, are also discussed.

1. Introduction The Directorate of Security and Safeguards (DSS) of the Canadian Nuclear Safety Commission (CNSC) is leading several whole-of-government initiatives aimed at further developing and enhancing Canada’s National Nuclear Forensics Capability. Among these initiatives is included the development of a National Nuclear Forensics Library (NNFL). CNSC is the lead federal government agency responsible for the development, maintenance and operation of Canada’s NNFL. Among the data types that are catalogued in the NNFL are characteristic analytical signature data for nuclear and radioactive materials. The CNSC currently maintains a signature reference dataset for uranium ore concentrate (UOC) material, and is interested in expanding the number of analytes of this reference dataset by acquiring measurements that can be done using a variety of analytical instruments. Accelerator mass spectrometry (AMS) is a highly sensitive mass spectrometry technique that has the built-in capability of eliminating molecular isobar interferences when detecting an atomic ion of selected

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mass and charge. This tandem accelerator based method for molecular isobar elimination was patented by Purser [1] at the time of the invention of AMS for radiocarbon dating, to which it contributed much. This field has since grown significantly in both technology and application, as has been well illustrated in a recent review [2]. An area of AMS which is relatively less explored is its direct use as highly sensitive secondary ion mass spectrometry (SIMS), to analyze suitable samples. UOC is one such suitable material type, in which uranium is already concentrated to typically no less than 70% by weight. The direct-AMS measurement of the very low 236U/238U ratios (from low-10−12 to mid10−10 as previously known) in UOCs is possible by the ultra-high isotopic abundance sensitivity of AMS. If the method can be demonstrated, it could become an expeditious and highly effective tool for nuclear forensics, as time consuming sample chemistry is not needed in the preparation for analysis. For similar considerations, there could also be several other potential direct UOC analyses using AMS. Among the numerous possibilities examined, the direct assessment of 187Os/188Os ratio, as once shown by Fehn et al. [3], is selected to be included in this study, in addition to 236U/238U.

Corresponding authors. E-mail addresses: [email protected] (X.-L. Zhao), [email protected] (W.E. Kieser).

https://doi.org/10.1016/j.nimb.2019.08.012 Received 20 July 2019; Received in revised form 18 August 2019; Accepted 19 August 2019 Available online 13 September 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.

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The University of Ottawa currently hosts the only AMS facility within Canada [4], and was thus selected to participate in this nationwide UOC signature study program. As part of this initiative, this facility performed the direct measurement of 236U/238U and 187 Os/188Os ratios, in 307 samples (covering 20 UOC producers) selected from the CNSC’s inventory of UOCs under Canadian regulatory control, as well as 3 additional UOC materials which are under development as UOC references by the National Research Council (NRC) Canada. The 3 quality-control samples were provided as part of a multiinstitution assessment; and are referred to as Ref-A, -B and -C. The source identities of all these samples were deliberately withheld for the entire two-year duration of this task to avoid potential interference from bias. This paper describes the considerations used to determine the scope of analysis selected (Section 2), the experimental and data analysis procedures, the batch measurements and the compiled range results for the U-group (Section 3) and the Os-group (Section 4). Finally, the main findings and the multiple prospects of the rapid UOC assay by direct-AMS are discussed (Section 5).

among the least abundant metal elements in the Earth’s crust, along with Iridium. Because of the elemental partitioning during the differentiation of the Earth’s crust, Os is even more depleted than Re by 1–3 orders of magnitude in most crust materials, such as old crustal rocks, seawater, and organic-rich sediments (except that in ultramafic rocks their relative concentrations can be reversed in the occurrence of mantle up-mixing). In the Earth’s surface environment, there are several different [Re, Os] reservoirs where the 187Os/188Os ratios may be elevated considerably because of the relatively higher concentration of [Re] over [Os] and the radioactive decay of 187Re to 187Os with a very long 42 Ga half-life. The 187Os/188Os ratio could reach ≥1 in old crustal rocks, seawater and organic-rich sediments. The Re-Os systematics are described well in recent textbooks (e.g. Dickin [12]). The redox properties of Re are also known to be similar to that of uranium. Thus, in the formation of many types of uranium ores, particularly the filtration types, the sandstone types, and those associated with rich organic activities that concentrate Re, Re can be expected to be concentrated along with uranium, resulting in a uranium ore deposit with an initial additionally higher [Re/Os] enrichment over the surrounding geological environment. Depending on the geological setting of the uranium ore for the initial [Re/Os] enrichment and the age of the ore for the 187Os ingrowth, the 187Os/188Os ratio measured today in most UOC materials, expected to be ≥1 or much higher, is a reflection of the geological setting of the uranium ores leading to the UOC, and this ratio cannot be altered significantly by the manufacturing process because of the expected ultra-low presence of [Re, Os] in the chemicals applied to the ore rocks. However, the 187Os/188Os ratio can be variable from one part to another throughout an ore body, and more than one ore can be supplied to a producer. Therefore, again, UOC from one producer must carry variable 187Os/188Os ratios over the volume of their product, and also through the date-stamped batches of production. The knowledge on the 187Os/188Os ratio and the range of its variation in UOCs is currently lacking; it is thus the objective of this work to find a preliminary answer.

2. Scope of analysis Uranium-236 is a long-lived radioactive isotope with a half-life of 23.42(3) Ma. 236U is produced with high probability through thermal neutron absorption by 235U. Since the first detection in uranium ore [5], it has now been estimated that ∼30 kg of naturally occurring 236U is present in the upper layers of land surface and <0.5 kg in the oceans [6]. In the environment today, however, the majority of 236U, on the order of 106 kg, are from anthropogenic sources related to nuclear activities. The pre-anthropogenic 236U/238U ratios are mostly found so far in the range from low-10−12 to mid-10−10 [7], whereas in the spent fuel of nuclear reactors this could reach over 10−4. The present knowledge on the environmental 236U inventory is well described by Steier et al. [6] and Eigl et al. [8], and the 236U/238U ratio within geological uranium deposits is modeled in a great detail by Keith [9]. The 236U/238U ratio in a UOC material is theoretically pre-anthropogenic, because UOC is produced directly from mined uranium ore. This ratio is the result of numerous factors as shown in the comprehensive modeling by Keith [9]. The source of neutrons is a primary factor, which, for UOCs produced from deep underground deposits, is mainly from 238U α-decay induced (α,n) reactions with light elements. The presence of ground water for the fast neutron thermalization is also a significant factor, as is the presence of neutron absorbers such as Gd and Sm. The 236U/238U ratio of a UOC reflects the geological settings of the uranium ores supplied to the producer; this ratio cannot be expected to be altered significantly by the UOC manufacturing process. On the other hand, the 236U/238U ratio can clearly vary from one part to another throughout an ore body, and more than one ore can be supplied to a producer to manufacture their UOC product over different time periods. Thus, UOCs from one producer must carry variable 236U/238U ratios over the volume of their product, and also through the datestamped batches of production. It is not clear whether the range of variation could remain sufficiently narrow so that this ratio is still useful to differentiate UOCs among different producers. Despite earlier measurement campaigns (e.g. Srncik et al. [10]), there is no statistically firm conclusion because large bodies of measurement data are still lacking. CNSC’s provision of the 307 UOC samples covering 20 producers has created an opportunity for answering this question. Using the direct-AMS approach it is also possible to assess a number of other isotope groups at trace levels in UOCs. Among the possibilities, the 187Os/188Os ratio is selected as a complementary component to this work. This is primarily motivated by the fact that, up to now, the Os isotopes had never been considered for UOC signature studies [11]; it is thus a subject of fundamental interest. Rhenium and Osmium are rare siderophile elements and are mostly concentrated in the Earth’s core, where the isotopic ratio 187Os/188Os is in theory the same as the Solar System average ∼0.13. Thus, both are

3. Analysis of the U-group isotopes (236U, 238U, 231Pa, 230Th, 226Ra) 3.1. Basic consideration and sample preparation In an earlier phase of this work, 1–60 (in CNSC outgoing order) of the 307 UOC samples had been measured in our facility using the method commonly adapted for AMS analysis. That is: several milligrams of a sample were first dissolved in concentrated nitric acid, uranium was purified and co-precipitated with Fe to form (U)Fe(OH)3, which was then calcined into (U)Fe2O3. This fully prepared sample was mixed with Nb power and pressed into a small (∼1 mm ∅) pellet ready for AMS analysis, known as a target. UO− ions were extracted from a Cs+ sputter ion source, then converted to U+3 ions in a tandem accelerator. The multiple MeV 236U+3 ions were counted in an ionization chamber, and the 238U+3 ion beam current was measured in an off-axis Faraday cup. The measurement of each isotope was alternated sequentially; the 236U/238U ratio of a sample was determined averaging over a sufficient number of measurement cycles. In a subsequent phase, 61–200 of the 307 UOC samples had also been measured using an alternative method, still with sample processing chemistry: several milligrams of a UOC sample were dissolved in concentrated nitric acid, uranium was purified and co-precipitated with Nd to form (U)NdF3, which was then dried and mixed with an excess amount of PbF2 powder to be pressed into a target. UF5− ions were extracted for AMS measurement in similar procedures as with UO−. Unfortunately, both analyses were interrupted by the passing of the lead author, with incomplete documentation and some extremely large 236 U/238U ratios (up to ∼1 × 10−7) measured but unexplained. It was therefore decided that the entire set of the 307 UOC samples must be reanalyzed anew. This time, a simplified approach was adapted with the following consideration: 99

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Fig. 1. Layout of the Lalonde AMS system, the parameters to be changed for detecting a selected isotope, and the typical final ion energy spectra in this work.

1. Although the standard UO− method is reasonably well established for AMS [13,14], it requires a Cs+ sputter ion source to be operated steadily with a relatively large 1–1.5 mA Cs+ flux in order to produce a steady ≤300nA 238U16O− beam current from a uraniumconcentrated (U)Fe2O3 + Nb target [15,16]. When a less intense ion source had to be operated, as in our case in the earlier work for producing UO−, not only could the 238U16O− beam current be rarely maintained above 10 nA, but also the ionizer became seriously “poisoned” by the deposition of uranium containing materials [17]. 2. The need of chemical processing by the (U)Fe2O3 method, and also by the (U)NdF3 method, would imply again a major undertaking for the sample preparation, which is quite impractical given the remaining time frame and cost recovery allocated to the complete set of the >300 samples in this work. Realizing the fact that UOC is a material in which uranium is already concentrated, a more efficient analytical approach may exist based on our experiences in producing fluoride anions from PbF2-based matrixes [18]. That is, when UOC is mixed with an excess amount of PbF2 powder to form a target, the fluorinating reactions of most metal elements, triggered by the multiple keV Cs+ ion bombardment, could take place most rapidly. The outcome is usually that the metal elements are converted into specific fluorides in both neutral and ionic form, with the metal atoms probably ‘combusted’ into their highest oxidation states. The fluoride products from such surface reactions are desorbed into vacuum, and often quite large poly-atomic fluoride negative ion beams can be produced from a Cs+ sputter ion source, requiring little flux of the primary sputter beam. For uranium, UF5− is produced as the most prolific molecular negative ion, and the 238 UF5− beam current from a UOC + PbF2 (1:10 by volume) target could reach as high as 1 μA at the peak, using a very low Cs+ flux (estimated ≤0.02 mA based on the ≤3 μA 12C− currents from graphite targets). Even after ∼30 min of output decline, the 238 UF5− beam current could still persist in the range of 10–100 nA for hours. 3. The use of UF5− for 236U AMS instead of UO− could enhance the final 236U/238U abundance sensitivity with AMS systems having just

one large analyzing magnet in the high energy side. For the AMS system in our facility we have shown a potential abundance sensitivity of 236U/238U ∼4 × 10−14 [19] using UF5−, possibly an order of magnitude better than that using UO− with the same system, likely due to the lack of 235UHF5− ions and the rarity of 17O in forming 238U17OF4− ions. This abundance sensitivity, even if it could be sacrificed somewhat by the oxygen content in UOCs, should still be well suited to the UOC analysis because the 236U/238U ratios are expected to be above 1 × 10−12. By the new approach, the sample preparation needed is considerably simplified: it only requires taking about 10 mg of each UOC sample with a micro-hematocrit capillary tube (Fisherbrand, 22–362566), mixing it with 10 times the volume of a fine PbF2 power (Alfa Aesar, Purotronic). A 2.0 mL round bottom cryogenic vial (Corning, 430661) and half of an 8″ round end plastic stirring rod (Bel-Art – SP Scienceware, 37766008) are used for the mixing manually. About 15 mg of the mixture is loaded into an AMS-ready copper target holder, pressed from behind by a copper pin, leaving a ∅1.3 mm sample volume in the front to be exposed to the Cs+ sputter beam. 3.2. Direct-AMS and the opportunity for multi-isotope assessment An AMS system is an analytical instrument that uses particle accelerator paired with usually two mass spectrometers to detect ultratrace concentrations of rare isotopes. A simple mass spectrometer by itself cannot distinguish between isobars–atomic and molecular ions of the same mass and charge as the analyte ion. In particular, the use of a tandem accelerator requires negative ions, allowing their selection by mass and charge through a low energy mass spectrometer. The selected negative ions are accelerated to a positive high voltage terminal, where they are charge changed to positive ions in collision with a gas or foil electron stripper. The positively charged ions are further accelerated, and the selected rare ions of interest are analyzed by the second, high energy mass spectrometer, and finally counted in an ionization chamber. It is due to the charge changing process in the tandem accelerator that the interfering molecular isobars can be all disintegrated 100

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by Coulomb repulsion forces. This method [1] increases the sensitivity of rare atom detection in all cases whenever atomic isobars are either absent or can be eliminated by other means. It also offers the potential, though less emphasized to date, to relax the stringent sample preparation needs of mass spectrometry in rapid survey applications, which lead to the direct UOC measurement/assessment of this work. The AMS situated at the University of Ottawa in the A. E. Lalonde Laboratory ([4], also see Fig. 1) is equipped with a 200-sample carousel where the targets are held in vacuum. For measurement the target is inserted into the ion source one at a time. When inserted, the sample surface area is exposed to the sputtering of 7 keV Cs+ ions. Negative ions are extracted out of the ion source with 35 keV energy. For measuring the uranium isotopes, the main pilot beam 238UF5− → 238U+3 is always first well tuned at the terminal voltage 2.5 MV, using Ar as the electron stripping gas for the charge change. The other common AMS system conditions are chosen, based on experience, to balance the need for (a) low-background from scattering of abundant isotopes, (b) adequate mass/energy resolution for rare isotope selection, (c) optimal charge changing yield for the selected positive ions and (d) sufficient reduction of all interfering molecular ions. An important step at the end of AMS tuning is a determination of the beam transmissions through the system (see Appendix A for the results of one such measurement). The direct-AMS approach enables additional findings in the UOC signature studies. That is, it is possible to also assess multiple other isotopes along with the measurement of 236U/238U ratio. An initial pilot study (see Appendix B) indicated that 231Pa, 230Th and 226Ra are the best ones to be included along with the measurement of 236U/238U, to form the U-group isotopes within a batch measurement sequence. If a uranium ore is sufficiently old and environmentally isolated, which is expected to be so for most cases, the long-lived daughters would be present in the ore at secular equilibrium with uranium so that ratios of 231Pa/238U = 3.37 × 10−7, 230Th/238U = 1.69 × 10−5 and 226 Ra/238U = 3.62 × 10−7. From ore to UOC, however, the manufacturing process washes out other metals, causing these ratios to drop orders of magnitude below the equilibrium levels. The degree of reduction is therefore the result of the processing method applied, so the ratios of 231Pa/238U, 230Th/238U and 226Ra/238U are also potential UOC signatures. Due to the differences in oxidation states of the different elements, these bi-elemental ratios cannot be accurately determined like 236 U/238U; but they can be estimated. However, even an estimation can be very much complicated by the fact that there are both oxygen and fluorine in the targets made of mixed UOC + PbF2. To provide some reasonable degree of normalization for the estimation of 231Pa/238U, 230 Th/238U and 226Ra/238U, the relative probabilities for U, Pa, Th and Ra to form general MOaFb− (Metal-Oxygena-Fluorineb) molecular anions must be surveyed (see Appendix A for the results of one such survey). Under the assumption that the probability of a neutral atom to form Σ(MOaFb−) is the same for all these elements, the results in Table A2 provide the necessary normalization multiplication factors for estimating the bi-elemental ratios, using the most suitably-chosen ions for the detection of the two elements. For U and Th, MF5− is the most prolific ion for their detection; for Ra, it is MF3−. For Pa, however, MF5− is clearly not the most prolific, but is still selected for better consistency. The above selection works for most samples. However, the 230 Th+3 count rates using 230ThF5− from some samples are too excessive to be measured by the ionization chamber. For these samples, 230 Th/238U is estimated using 230ThO2F− for 230Th detection instead. The survey results in Appendix A show that the addition of an excess amount of PbF2 into UOCs boosts the element-specific MOaFb− ion yields for all the selected elements, especially U and Ra, as compared to the element-specific MOa− ion yields when there is only oxygen available. This is despite the fact that the amount of the UOC material packed in a UOC + PbF2 target is less than that packed in a UOC-only target. The direct-AMS measurement of 236U/238U ratio using UO− from calcined UOCs, without the full-length sample processing

chemistry, is also possible, as has already been demonstrated by Srncik et al. [10]. However, the concurrent assay of other isotopes from calcined UOCs, especially 226Ra, may be more difficult without the assistance of PbF2. Using the normalization parameters given in Table A2, a single batch measurement run, using the Slow Sequential Injection (SSI) mode of the AMS operation, can be performed to determine the 236U/238U ratio and to estimate the ratios of 231Pa/238U, 230Th/238U and 226 Ra/238U at the same time. The procedure is rather straightforward: Once the main pilot beam 238UF5− → 238U+3 is well tuned using any UOC + PbF2 target (of low 236U and 230Th content avoiding contamination), the AMS parameters can be accurately calculated for the SSI measurement of each of the U-group isotopes, either as beam current (for 238U+3) in an off-axis Faraday cup in the high energy system, or as count rate of ions (for the others) in the ionization chamber, with energy spectra similar to that of 236U+3 ions (see Fig. 1): (1) (2) (3) (4) (5)

236

UF5− → 236U+3 (20 s) UF5− → 238U+3 (2 s) 231 PaF5− → 231Pa+3 (10 s) 230 ThF5− or 230ThO2F− → 230Th+3 (2 s) 226 RaF3− → 226Ra+3 (10 s) 238

In the SSI mode of the AMS operation, these beams are measured one at a time for the specified length of time. To switch from one beam to another, only the bias-voltage on the insulated low energy magnet box (the “bouncer”), the terminal voltage of the accelerator, and the voltage applied to the high energy electrostatic energy analyzer, are changed (see Fig. 1). This measurement cycle (called “a block of measurement”) is usually repeated 10 times on one target, before changing to the next. The change on the terminal voltage requires a minimum 1 s for settling. Unfortunately, the techniques adapted to implement the SSI data acquisition also add delays caused by several hardware and software operations. As a result, a “pass” on one target for the usual 10 blocks of measurement takes about 15 min (about twice the time used for the actual data acquisition). When all targets included in a batch are analyzed for one pass, this grand cycle is repeated. The batch run is stopped manually when a sufficient total number of measurement blocks are accumulated for each target. 3.3. Raw data inspection and result analysis with a trial batch run Due to the slow and inefficient nature of the SSI mode, a reliable block measurement requires the beam output from the target to remain unchanged. For the entire SSI measurement to be reliable, the beam output variations from all targets must remain slow and steady throughout. Meeting these requirements with the PbF2-based targets is very challenging, which is why such analysis work is rare and remains under development [20–22]. Because of the many potential problems that might be caused by the instabilities of the ion source, the block sequences of the raw data must be plotted and inspected. Blocks of measurement affected by sudden instabilities, especially during the first pass of target sputtering, must be excluded before calculating the final results. To minimize and to soften the severities of the source instability induced problems that must be corrected later, the most important step in AMS tuning has been to determine patiently the right ion source conditions of the Cs reservoir heating and even the ionizer heater current. Unfortunately, this has been rather difficult at times, and rules of thumbs based on experience often have to be relied upon. Shown in Fig. 2 are the raw data along the block measurement sequence, taken from the initial trial batch run that included Ref-A, -B and -C plus the PbF2-only blank, with 3 replicate targets for each. In this example, 230Th+3 ions are measured using 230ThF5−. This particular set of raw data appears very promising, where the instability-affected blocks of measurement are not numerous, and after excluding the few blocks at the beginning, the beam variations of most targets appear 101

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Fig. 2. MFn− → M+3 raw data from a trial batch run of the U-group isotopes for Ref-A, -B and -C.

steady enough for up to 150 blocks of measurement. To analyze the measurement results, as summarized in Table 1, the following steps are taken:

considered the 1-σ uncertainty of the concerned ratio from that target. (4) The average count rate for each isotope from the PbF2-only blanks included in the batch run, is considered in ideal cases the upper limit of the measurement background for that isotope. The results obtained in Step (3) are then corrected with the average count rate of each isotope subtracted by its corresponding background from the PbF2-only blanks. The errors involved in this correction are propagated for the background corrected results. However, this correction by the PbF2-only blanks is applicable only when it is relatively insignificant (as shown in Fig. 2 for example). (5) Based on the results obtained in Step (4), the final results of 236 U/238U, 231Pa/238U, 230Th/238U and 226Ra/238U for each target can be calculated after applying the relevant multiplication factors (Table A2). Finally, the weighted averages among the replicate targets are calculated for each sample (Table 1 for example).

(1) All saved raw AMS data are first assembled in an Excel workbook with macros written for the data analysis of SSI batch runs. Data belonging to the same target are collected in one sheet, in format of one row per measurement block. The block sequences of counts or current of all isotopes are plotted for visual inspection (Fig. 2 for example). (2) All raw data are inspected carefully for any periods of beam output instabilities. The affected data are usually seen as down-ticks in the block sequence plots, especially when measured with high count rates or as beam current. The first few blocks are usually flagged to be discarded as the initial exposure of a new target to Cs+ bombardment often causes ion source instabilities. The final period of measurement may also be discarded if a target (from the beam output) is seen to be exhausted. (3) Based on the accepted raw data of a target, typically more than 50 blocks of measurement, a set of ratios of each isotope to the first (236U in this case) is calculated by the macros developed here for the Excel workbook. The results of all targets are listed in a summary sheet. These ratios are the weighted averages of all the accepted blocks, the uncertainties are calculated for both the counting statistics of the two isotopes involved and the 1-σ scatter of ratios in the block series. The greater of these is taken as the error, and is

3.4. Batch measurement runs and discussions of the U-group results The ranges of the U-group results for all samples measured in this work are compiled in Appendix C. Included are the weighted averages of the ≥ 2 replicate targets for each sample: 236U/238U, ∼231Pa/238U, ∼230Th/238U and ∼226Ra/238U. The measurement and data analysis steps described earlier with the short trial batch run were both straightforward and encouraging, but they became considerably more complicated when it came to a major 102

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batch measurement run that, for practical reasons, must include a large number of targets, usually > 100. There are two major reasons for this:

(2.091 ± 0.067) × 10−11 (±3.2%) (1.362 ± 0.046) × 10−11 (±3.4%) (2.171 ± 0.071) × 10−11 (±3.3%) (1.60 ± 0.37) × 10−11 (±23%) (1.390 ± 0.019) × 10−9 (±1.4%) (1.138 ± 0.016) × 10−9 (±1.4%) (1.324 ± 0.019) × 10−9 (±1.4%) (1.216 ± 0.092) × 10−9 (±7.6%) (1.114 ± 0.024) × 10−10 (±2.2%) (1.020 ± 0.024) × 10−10 (±2.4%) (1.098 ± 0.025) × 10−10 (±2.3%) (1.057 ± 0.039) × 10−10 (±3.7%) (1.981 ± 0.027) × 10−11 (±1.4%) (1.947 ± 0.027) × 10−11 (±1.4%) (2.049 ± 0.028) × 10−11 (±1.4%) (1.996 ± 0.051) × 10−11 (±2.6%) Ref-C

≤1 × 10−14 ≤1 × 10−14 ≤1 × 10−14 ≤1 × 10−14 (2.966 ± 0.072) × 10−10 (±2.4%) (3.293 ± 0.073) × 10−10 (±2.2%) (2.660 ± 0.051) × 10−10 (±1.9%) (2.87 ± 0.30) × 10−10 (±11%) (3.722 ± 0.084) × 10−8 (±2.3%) (4.446 ± 0.092) × 10−8 (±2.1%) (3.729 ± 0.066) × 10−8 (±1.8%) (3.97 ± 0.34) × 10−8 (±8.6%) (8.58 ± 0.19) × 10−12 (±2.2%) (8.95 ± 0.29) × 10−12 (±3.2%) (7.33 ± 0.13) × 10−12 (±1.8%) (7.60 ± 0.60) × 10−12 (±7.9%) Ref-B

(5.95 ± 1.5) × 10−14 (±25%) (3.5 ± 1.2) × 10−14 (±34%) (3.50 ± 0.97) × 10−14 (±28%) (3.50 ± 0.75) × 10−14 (±21%) (4.233 ± 0.044) × 10−10 (±1.0%) (4.061 ± 0.041) × 10−10 (±1.0%) (3.926 ± 0.033) × 10−10 (±0.8%) (3.980 ± 0.066) × 10−10 (±1.7%) (7.776 ± 0.074) × 10−9 (±1.0%) (8.083 ± 0.062) × 10−9 (±0.8%) (7.930 ± 0.085) × 10−9 (±1.1%) (8.024 ± 0.073) × 10−9 (±0.9%) (1.941 ± 0.009) × 10−10 (±0.5%) (1.927 ± 0.008) × 10−10 (±0.4%) (1.934 ± 0.007) × 10−10 (±0.4%) (1.930 ± 0.005) × 10−10 (±0.3%) Ref-A

3.62 × 10−7 1.69 × 10−5 3.37 × 10−7 Not applicable Maximum at natural equilibrium

Ra/238U 226

Th/238U 230

Pa/238U 231 236

U/238U

Estimation Measurement UOC Sample

Table 1 U-group results of each target (top to bottom corresponding to blue, red and green in Fig. 2) and the weighted average of the three replicates obtained in the trial batch run for Ref-A, -B and -C.

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(1) Contents of 236U, 231Pa, 230Th and 226Ra in all these samples are initially unknown. To prevent severe ion source cross contamination, all targets must be briefly surveyed first. So far it has been shown that only 236U and 230Th need to be particularly monitored. Those having exceptionally high 236U contents must be kept and measured later in a special high 236U batch. Most targets have high 230 Th content, so ion source cross contamination is not a concern in this case, but the exceptionally high count rates can cause severe detector processing dead-time. Those that yield exceptionally high count rates when using ThF5− are kept, and measured later using ThO2F− instead in special high 230Th batches. In these cases the 230 Th normalization could require adjustment by multiplying a factor of 1.5–2 in addition to that shown in Table A2. The reason could be that the oxygen contained in a target exhausts faster than the more abundantly added fluorine, causing the count rate of 230 Th+3 ions from 230ThO2F− to decline 1.5–2 times faster than the 236 +3 U ions from UF5−. (2) The brief pre-screening is often enough to raise the ion source cross contamination to unusable levels. These difficulties are anticipated due to the unknown, wildly variable content of the U-group isotopes to be measured. Unless it is so extremely severe that it is irreversible without an open-source cleaning, such contamination does decline as a batch measurement run progresses. Or, it can be reduced sufficiently by operating the ion source on a blank target for a prolonged (several hours) idle time, and by reducing both the Cs reservoir temperature and the ionizer heater current. Batch runs could thus still be managed by adequate pre-characterization of each target and careful control of the ion source output. To enhance the chances of success, the 238U+3 current is best kept to a maximum of 100 nA, despite the possibility that much greater currents can be readily obtained using UF5−. In order to obtain sufficient usable blocks of measurement, a batch run usually takes a very long time to complete; five or more days depending on the number of samples included. In addition to the two basic problems described above, considerable further difficulties, mainly associated with the sever volatility of some of the UOCs (rather than PbF2 being the base matrix), can also be encountered. For these reasons, the reproducibility of the U-group results reported in this work, must be further discussed. It should be first pointed out here that, in principle, only relative measurements of AMS can be expected to be truly accurate and reproducible. This is due to the unique characteristics of AMS–the dynamic nature of the sputtering and charge changing processes, the need to work with generally large beam currents compared to those used in other types of mass spectrometers, and the relatively large dimensions and complexities of the AMS beam lines. These features can result in considerable variations in the phase space occupied by the ion beams throughout the (often lengthy) course of measurement, and therefore the vulnerability of maintaining constant transmissions from the ion source to the detector. Usually a well characterized reference must be measured together with the unknowns, and also the reference and unknowns must all be chemically processed to form a suitable matrix for the target, in order to obtain both identical and optimal sputtering properties, including (a) electrically and thermally conductive, and (b) thermally stable and non-volatile upon sputtering and heating. Only then the ideal measurement conditions can be created so that “apples are really compared to apples, and steadily so”. In the direct measurement approach, however, in which AMS is basically used as a Super-SIMS on samples in their original chemical forms, these desirable conditions are expected to be much less fulfilled. The degree of fulfillment with UOCs, or the lack of it, are commented below: 103

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(1) Without sample chemistry, clearly the sample materials packed in target holders cannot be expected to be identical. However, UOCs have at least one thing in common, that they typically contain over 70% uranium by weight. By mixing one volume unit of UOC with 10 volume units of PbF2, it is expected that the final matrices in the targets are sufficiently similar. Perhaps the more PbF2 is used the better, so long that the output of ion beams for the U-group isotopes is still adequate. This mixing parameter could be further optimized in future. (2) The electric conductivity in this case is based on the ionic conductivity of PbF2, which cannot be as good as those provided by the use of metal binders. The same is true for the thermal conductivity. Thus, for the direct measurement of the UOC + PbF2 targets, some random ion source instabilities are more difficult to avoid, especially when some of the UOCs are not moisture-free (which is a must to avoid destroying ionic conductivity). In future work, it would be worth experimenting with the addition of a metal binder, that has weak fluorine bonds to avoid competing for and reducing the availability of fluorine (such as Cu), into the UOC + PbF2 mix, to guarantee and improve both electric and thermal conductivities. (3) The mixing of PbF2 is the factor that is mainly responsible for creating large ion beam currents of poly-atomic metal fluorides from the UOC + PbF2 targets [18]. It is due to the unique property of PbF2 that undergoes a phase transition when heated. This phase change collapses its lattice structure and therefore frees up copious quantities of fluorine for combustion with the metals in a target surface. Unfortunately, this beneficial factor is at the same time also the most difficult one to control for the direct-AMS measurement, as the entire process is volatile by nature, which is made worse by the variable stabilities of UOCs. The volatility is further amplified by the location, in present day Cs+ sputter sources, of the hot (usually ∼1100 °C) Cs ionizer directly in front of the target. It can cause the vapors released from the target to reach the ionizer surface, some of which become ionized and thus form unspecified sputter beams to bombard the same and subsequent targets, along with the originally provided primary Cs+ sputter beam. Such an undesirable process can elevate cross contaminations significantly and also exhaust a target very rapidly if not controlled. We have made some seminal studies on this recycled sputtering phenomenon [23], and have therefore gained some limited experience working with the PbF2based targets for controlling the magnitude of the recycled sputtering, through adjustment of the Cs+ sputter ion flux and the ionizer surface temperature. Despite such experience, much further development is required to determine sufficient parameters for this PbF2 matrix-assisted method to create and maintain the desired ion source working conditions. (4) To compensate for the lack of ideal target properties and precise ion source controls, two extra steps were taken for the direct measurements in this work: (a) For each sample, at least one replicate target was measured; in cases where both failed, additional targets were analyzed. In the few cases when only the result of one target could be accepted, its error was doubled to compensate for the additional uncertainty. (b) In most batch runs, replicates of Ref-A, -B and -C were included. Their 236U/238U ratios spanned a range from 1 × 10−11 to 2 × 10−10, which was the range of half the UOC samples. A required small amount of blank correction was applied to each batch to result in overall reproduced results for Ref-A, -B and -C.

between this work and the previous analyses (Fig. 4). These lead to the following general assessment: 1. As expected, the 236U/238U result of a sample is shown to be generally more reproducible than the others, but the performance clearly also depends on the homogeneity and stability of the UOC sample material (as can be seen in Fig. 2 for example). The excellent reproducibility of 236U/238U by Ref-A makes it a good normalizing standard for all batch measurement runs. The results for 236U/238U relative to that of Ref-A within the same batch could be considered more accurate. When the 236U/238U ratio for Ref-A is well determined in future, more accurate results for all the UOC samples in this work can then be re-calculated. For now, the 236U/238U ratios determined by the direct transmission measurements, with values ≥5 × 10−11, could also be considered to be reasonably reliable with the deduced 1-σ uncertainties as described earlier. 2. The comparison of the 236U/238U results between this work and the earlier analyses which included sample chemistry shows consistency across several orders of magnitude, supporting the general validity of the direct-AMS measured results, and confirming the large 236 U/238U ratios measured in previous analyses. However, the general agreement is accompanied with large scatters, especially for 236 U/238U with values <5 × 10−11. This could be partly caused by sample material inhomogeneity, but it is also probably due to the large effects of ion source cross contamination to samples with low 236 U/238U ratios. However, the possibly contamination-infected results are seen in both the direct-AMS measurement and the earlier measurement of chemically processed samples. In any case, it appears that 236U/238U with values <5 × 10−11 were measured less accurately, for which a minimum 30% uncertainty must be considered to override any smaller deduced uncertainties (except for Ref-A, -B and -C as they were measured in multiple batches). 3. For the results of 231Pa/238U, 230Th/238U and 226Ra/238U, they must be viewed as neither precise nor accurate. The main reason is due to the unknown degree of elemental fractionations resulting from the formation of the ions used for the measurement, and also the varying output of these ions throughout the course of measurement. The survey presented in Appendix A is a minimal effort, which is only from one target of one of the 307 UOC samples. Other samples may yield variable (but hopefully similar) results. The uncertainty is further increased due to the lack of measurement of the neutral molecules through which the elements are also depleted from the target. Overall, a minimum 50% overriding uncertainty must be associated with these bi-elemental results, and therefore, they are best expressed as ∼231Pa/238U, ∼230Th/238U and ∼226Ra/238U. 4. Despite the large analysis uncertainties discussed above, the still larger variations of the results for UOCs from each producer, and between producers, are clearly revealed as compiled in Appendix C. 4. Analysis of the Os-group isotopes (185Re, Ir, 193Ir)

191

187

(Re + Os),

188

Os,

4.1. Basic consideration, sample preparation, and direct-AMS for multiisotope assessment A direct mass spectrometry measurement of 187Os can be difficult, because in this case there is an interference from the atomic isobar 187 Re. However, because Os has a relatively large electron affinity of 1.1 eV whereas Re has a small electron affinity of 0.3 eV, mass spectrometry of 187Os using the atomic negative ion is assisted by a significant suppression of the interfering 187Re intensity. AMS uses negative ions naturally, and is further assisted by its ability to eliminate all molecular isobar interferences through the charge changing process. It is therefore viable to use AMS to determine 187Os/188Os directly without the need for the prerequisite chemical separation of Os and Re. This has been demonstrated in the early days of AMS development by

Despite the precautions undertaken, the deduced uncertainties for the weighted averages of the ≥2 replicate targets still do not always represent the 1-σ statistical reproducibility. The realistic uncertainties for these results can be evaluated further based on two additional sets of data: (a) the performance of Ref-A, -B and -C for their multiple number of measurements from multiple batch runs (Fig. 3), and (b) the comparison of the 236U/238U results of the first 1–200 UOC samples 104

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Fig. 3. Realistic analysis uncertainties for the U-group isotopes shown by Ref-A, -B and -C, where the weighted averages of all measurements (excluding outliers) are given.

Fehn et al. [3]. Thus, for the second part of this work, about 10 mg of each UOC sample is directly loaded into a copper target holder, pressed from behind by a copper pin, leaving ∅1.3 mm sample volume in the front to be exposed to the 7 keV Cs+ sputter beam. Similar to the assessment of U-group isotopes, multiple Os-group isotopes can be assessed at the same time. By measuring 185Re−, 187 (Re + Os)−, 188Os−, 191Ir− and 193Ir−, the 187Os/188Os can be determined based on the assumption that the Re isotopic abundances in UOCs are at the natural level, i.e., 37.40% 185Re and 62.60% 187Re. (Given 187Re’s 42 Ga long half-life, this assumption could be off by up to 3.3% for UOCs produced from uranium ore deposits with ages up to ∼ 2 Ga.) In addition, the assessment of the elemental concentration ratio of [Re/Os], [Re/Ir] and [Re/U] (see Appendix B for the selection) could also be attempted. Since the production of UOC involves chemical treatment for concentrating uranium, the elemental concentrations of Re, Os, Ir and U must have been altered. Thus, these additional quantities could be difficult to interpret, but nevertheless they could still be useful for forensic applications for being the results of both the geological settings of the uranium ores and the UOC manufacturing process. For similar reasons as for the U-group measurements using UOC + PbF2 targets, the use of UOC-only targets must be surveyed for all the oxide negative ions of the chosen elements (see Appendix A for the results of one such survey). The results show that, in addition to the relative suppression of Re− by its low electron affinity, most Re atoms are consumed in the formation of oxide negative ions (and unknown numbers of neutral molecules). Thus, the use of Os− for 187Os detection from the UOC-only targets is assisted by a total of two orders of magnitude reduction of the interfering 187Re. The AMS operating conditions and setup steps for the Os-group isotopes are similar to those for the U-group isotopes, and are simplified in many ways because of the use of atomic negative ions. The system was once well tuned using a pilot beam 197Au− → 197Au+3 (see Appendix A), from which the parameters for transmitting the Os-group

ions were readily determined. The beam transmission through the AMS system is better than that for the U-group isotopes using the polyatomic molecular ions. Also, because all the Os-group ions are counted in the final ionization chamber, the transmission measurement for each Os-group batch run is not needed for the normalization of the final results. For each batch run, with the convenient count rates from a Ref-B target, the pilot beam 187(Os + Re)− → 187(Os + Re)+3 can be first well tuned. Then, the AMS parameters can be accurately calculated for the SSI measurement of the rest of the Os-group isotopes: (1) (2) (3) (4) (5)

185

Re− → 185Re+3 (10 s) (Os + Re)− → 187(Os + Re)+3 (10 s) 188 Os− → 188Os+3 (10 s) 191 − Ir → 191Ir+3 (5 s) 193 − Ir → 193Ir+3 (5 s) 187

Again, both the SSI measurement procedure and the data acquisition time are similar to those for the U-group isotopes. 4.2. Raw data inspection and result analysis with a trial batch run In Fig. 5, examples of raw data are shown taken from an initial trial batch run that included Ref-A, -B and -C plus the blank made of the bare Cu-target and Cu-pin, all with 3 replicates. To analyze the measurement results, the following steps are taken: (1) Similar to the U-group analysis, all AMS saved raw data are first assembled in the Excel workbook, the block sequences of the collected counts of each isotope are plotted for visual inspection (Fig. 5 for example), and the bad measurement blocks are flagged to be discarded. (2) Based the remaining usable raw data, typically more than 50 blocks 105

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Fig. 4. Comparison with the earlier chemistry.

236

U/238U measurements. The AMS measurement and result normalization are similar for these data, with and without sample

of measurement, the total counts and total counting time of each isotope are summed together by the macros included in the Excel work book. The results for all the isotopes and all the targets included in a batch run, are listed in a summary sheet. (3) The average count rate of each isotope in a UOC target is then corrected for background by subtracting the corresponding average of the Cu/Cu-blanks included in the batch. The errors involved in this correction are propagated; but all errors considered here are only for counting statistics based on the total counts accumulated for an isotope. (4) For each target, the count rate of 187Os is then calculated as that of [187Re+187Os] subtracting (62.60/37.40) × [185Re]. (5) Based on the count rate results obtained thus far, the final results of 187 Os/188Os, 191Ir/193Ir, [Re/Os], [Re/Ir], [Os/Ir], and the content of Re relative to that of Ref-B, can be calculated for each target after applying the relevant multiplication factors shown in Table A4. Then, the weighted averages of the replicate targets are calculated for each sample (Table 2 for example).

targets. Again, the latter three quantities are only quoted with rough assessment qualities. As has been discussed for the results of the U-group isotopes, a reliable AMS measurement would require the reference material to be prepared as a target with ideal and identical properties as the unknowns. In the direct measurement of the UOC-only targets, most such requirements are poorly met, but surprisingly the targets support stable electric potential under Cs+ sputtering without a metal binder. The reason why the UOC-only targets were chosen for this initial Os-group assessment, was mainly to avoid the possibility of adding contaminations to the expected extremely low concentrations of Re, Os and Ir that might present in the UOC samples. This concern was given priority because limited tests indicated that their results could be affected by the materials used for the target holders and press-pins. For example, stainless steel target holders and press-pins were found to carry considerably higher Os and Ir contents, but similar Re content, in comparison with the Cu target holders and press-pins. The latter were thus used in this work. A major consequence of using the UOC-only targets is that, the validity of isotopic ratio measurement by SIMS using atomic cations or anions could be fundamentally hindered by the large and unknown isotopic fractionations caused by the rapid formation of relatively large and variable quantities of oxide species of the element under investigation, especially when the sample materials are oxides to begin with. Unfortunately, such effects were not realized earlier because of the apparent agreement with the known natural abundances of the determined 191Ir/193Ir ratio in the trial batch run (Fig. 5 and Table 2). The inclusion of the 191Ir/193Ir ratio in the Os-group measurement was primarily intended for serving as a validity assurance for the

4.3. Batch measurement runs and discussions of the Os-group results In the compilation of the range results in Appendix C for the Osgroup isotopes, only 187Os/188Os, ∼[Re/Os], ∼[Re/Ir] and the “relative 185Re+3 count rate to that of Ref-B in same batch”, are listed as independent quantities. The last quantity can be associated with ∼[Re/ U] relative to that of Ref-B, which is quoted to be ∼3 atom ppb by the assessment of the total ion yields of Re and U from Ref-B targets. This association assumes that all UOC-only targets endure Cs+ sputtering similarly, so the ion flux yields are proportional to their contents in the 106

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Fig. 5. M− → M+3 raw data from a trial batch run of the Os-group isotopes for Ref-A, -B and -C. The Chondrite 187Os/188Os and 191Ir/193Ir isotopic abundance ratios are indicated. 187

Os/188Os ratio determination; thus the trial batch run resulted a false sense of satisfaction. When it later came to the measurement of large batches, it turned out that when one target became overheated and triggered the recycled sputtering phenomenon, the adverse effects tended to last a long time, so that one target overheating exposed many

other targets to overheat along with it. Before this problem eventually subsided, after a sufficient degree of out-gassing from all the targets that had been sputtered, some of the targets could have become nearly exhausted. In these cases the assessment of the Os-group isotopes could be very inaccurate, as indicated by the erratic 191Ir/193Ir ratios

Table 2 Os-group results of each target (top to bottom corresponding to blue, red and green in Fig. 5) and the weighted average of the three replicates obtained in the trial batch run for Ref-A, -B and -C. UOC Sample

Intended Measurement Os/188Os

Estimation

187

191

Ir/193Ir

Chondrites Abundance Ratio

0.127

0.595

Ref-A

37.2 ± 3.8 65.9 ± 7.9 94 ± 15 72 ± 12

Ref-B

Ref-C

Perception

[Re/Os]

[Re/Ir]

[Os/Ir]

Re concentration relative to that of Ref-B

0.62 ± 0.02 0.56 ± 0.02 0.61 ± 0.02 0.58 ± 0.02

44 16 27 29

5.5 1.5 1.6 2.9

0.13 0.09 0.06 0.09

0.1–0.2

279 ± 17 272 ± 16 320 ± 21 290 ± 23

0.59 ± 0.01 0.58 ± 0.01 0.59 ± 0.01 0.59 ± 0.01

79 80 81 80

8.7 9.6 7.9 8.7

0.11 0.12 0.10 0.11

1 (∼3 ppb atom [Re/U])

12.17 ± 0.08 12.12 ± 0.13 12.30 ± 0.28 12.15 ± 0.11

0.59 ± 0.01 0.57 ± 0.01 0.60 ± 0.03 0.59 ± 0.01

43 41 29 38

333 289 204 280

7.8 7.1 7.1 7.3

3–60

107

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Fig. 6. An overview of all U-group and Os-group range results from 20 UOC producers (color coded as they appear). 187

Os/188Os must also have to be at least 50%. The uncertainties on the other bi-elemental ratios are much greater; they are quoted only for the magnitude significance. Therefore, the Os-group results from this work can only be used for very rough assessment purposes, and for the main purpose that this has been the first of such assessment. That said, however, the variable enhancement of the 187Os/188Os ratio over ∼1, and the apparent grouping of the 187Os/188Os ratios due to the average geological settings of uranium ores, are still clearly shown for a significant population of the UOC samples. Much further development is needed for the reliable direct-AMS measurement of the 187Os/188Os and 191Ir/193Ir ratios in the UOC samples. Extra sample preparation steps are clearly necessary to create stable and lasting target properties for SIMS, such as pre-sintering, mixing UOCs with an excess amount of certain metal powder (e.g. ultrapure Al) to reduce volatility and rate of oxidation reactions of the analytes. The sample material thickness and its area that is exposed to Cs+ sputtering must also be increased, to prevent the sputter beam from hitting the target holder material. Ultimately, the Cs+ sputter ion source itself must also be further developed to eliminate the recycled sputtering phenomena, and to be configured more suitably for the SIMS-type applications by direct-AMS.

measured (up to 50% higher than the expected 0.595). The assessment of the bi-elemental ratios could be more seriously hindered because the needed normalization parameters might have become very different from those given in Table A4. Another probable cause for the inaccuracy could be that, the target surface work function varied rapidly along with significant out-gassing and surface composition change. The atomic negative ion yield is determined by the difference of the element’s electron affinity and the surface work function, in an exponential term. It was a frequent observation that the count rates from the Re, Os and Ir atomic negative ions could be suddenly increasing and decreasing out of proportion. The slow execution of our present SSI mode of data acquisition also made the already unsatisfying situation more difficult to cope with. Finally, for those UOCs with very high Re content relative to Os, the estimation of the underlying 187Os was inherently difficult. Because of these numerous and rather fundamental problems, several rules of thumbs had to be relied upon to carry through the Osgroup measurement and data analysis, including: (a) the first 10–20 blocks of measurement were always discarded for their rapid changes in general; (b) blocks showing any one isotope experiencing rapid increase or decrease of count rate were discarded; (c) blocks showing signs of early target material exhaustion were discarded; (d) background correction by the Cu/Cu blanks was not applied to avoid frequent overdeduction; (e) for samples with high [Re/Os] and low 187Os/188Os that repeatedly led to negative 187Os/188Os values to be deduced, their 187 Os/188Os ratios were quoted by the range of the other samples from the same producer. Although most of the 187Os/188Os results could be deduced with uncertainties representing the scatter of ≥2 replicate targets for a sample, or with the error of a single successful target doubled, they cannot be considered to reflect well the true reproducibility. Statistically meaningful uncertainties of the 187Os/188Os results have so far not been determined, but judging by the up to 50% inaccuracy of the measured 191Ir/193Ir for most samples, the inaccuracy of the measured

5. Summary and discussions As part of the CNSC-led Canadian NNFL development program, 307 uranium ore concentrate (UOC) samples from 20 different producers and 3 UOC references under development in Canada were analyzed by direct-AMS at the A. E. Lalonde AMS Laboratory, University of Ottawa. Two direct measurements were performed, the first for the U-group isotopes (236U, 238U, 231Pa, 230Th, 226Ra) using sputter targets made of UOC + PbF2 (1:10 by volume) with fluoride molecular negative ions, and the second for the Os-group isotopes (185Re, 187(Re + Os), 188Os, 191 Ir, 193Ir) using UOC-only targets with atomic negative ions. These measurements resulted in the determination of the 236U/238U and 108

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Os/188Os ratios in these samples, which are theoretically the sole outcome of the geological settings of the uranium ores from which the UOCs are produced. The direct-AMS approach offers two advantageous opportunities: to bypass the time-consuming chemical sample processing and to concurrently assess multiple other isotopes that reflect the outcome of the UOC manufacturing process. Major technical issues were also experienced during the execution of the analytical work reported in this paper. These are mainly associated with the volatilities of the UOC present in the sputter targets for direct-AMS. Despite the many pitfalls of the analysis discussed in the text above, the entire set of the U-group and Os-group results obtained in this work (compiled in Appendix C as ranges and outlier individuals), when presented together in Fig. 6 as color-coded “bar-codes”, reveals some rather significant features of UOCs. The detailed analysis will be presented in future when these UOC samples can be traced back to their mineral origins. For now, the following aspects and implications are briefly discussed:

production of UOCs. Yet, there is a significant population of UOC samples with 236U/238U ratios well above 5 × 10−10 (see Figs. 4 and 6), which is thought to be the upper limit for naturally occurring 236 U by the production mechanisms considered so far [9]. This unexpected finding invites further forensic investigations to first prove (or disprove) the notion that UOCs ought to be the sole products of natural uranium ores, and if indeed so, to conduct scientific studies to confirm the high 236U/238U ratios directly in their source ore materials. At present, we could only do a limited investigation as follows: Using the procedures as described in Appendix B, a brief survey of 239,240,242,244Pu, 241,243Am and 247Cm was done for T12, the sample that yielded the highest 236U/238U ratio of ∼2 × 10−7. Only 239Pu was detected and estimated at 239Pu/238U ∼1 × 10−12; none of the others was detected and could be quoted at X/238U <1 × 10−13. This additional fact appears more consistent with the high 236U/238U ratio being natural, although further studies are clearly needed. 3. If both the 236U/238U and 187Os/188Os ratios measured from UOCs are indeed the outcome of naturally occurring processes, then the results reported in this work, in addition to being useful to nuclear forensics, would also have two further implications for the geophysical sciences. First, the >1 × 10−7 236U/238U ratios in UOCs from the producer Q and T suggest that there could be unidentified natural production processes at work. Long-overlooked hypotheses of episodic nuclear reactors in planetary interiors (e.g. Herndon [26]) might be worth revisiting, with the corroboration prospects by the measurement of 236U/238U in mantle materials up-mixed into the crust. Second, if the UOC manufacturing process had not caused [Re/Os] reductions by orders of magnitude, then the ranges of the estimated [Re/Os] in UOCs from the producer A and D, as well as Ref-B, could not explain their measured large 187Os/188Os ratios by the 187Os ingrowth from 187Re decay with a 42 Ga half-life, even if the original uranium deposits have the oldest possible ages up to ∼2.5 Ga. However, this could be naturally explained if the half-life of 187Re had been shortened in the radiation environment of uranium ores. Such a possibility has long been pointed out by Bosch et al. [27], but a natural example has not been reported. Both implications described here could stimulate some significant scientific investigations.

1. Because of the inhomogeneity of uranium ore materials, the ratios of 236 U/238U and 187Os/188Os do vary within ranges which are wider than the analytical uncertainties, despite the presently quite large uncertainties of the direct-AMS approach. Differences among different UOC producers are in some cases clearly indicated by the 236 U/238U and 187Os/188Os ratios, with the latter being shown for the very first time. However, neither the 236U/238U nor the 187 Os/188Os ratio alone, nor their combination, is unique enough to differentiate a UOC producer from others in all cases. The multiisotope “bar-code” pattern of a UOC producer appears to increase the chance of uniqueness significantly, although still not entirely. The large spread of the bars of the extra quantities may indeed reflect the inhomogeneity of the UOC materials, but still, the directAMS methodology must be improved to reduce the analysis uncertainties. Considering that no sample chemistry is required, the direct-AMS measurement/assessment of both the U-group and Osgroup isotopes does appear to be quite useful, and should be further developed to become an efficient and effective tool for UOC forensic studies. 2. A startling finding of the 236U/238U ratios is that for some producers this isotopic ratio bar-width can be clearly seen as having more than one distinct segments. The most distinct case is with the producer Q and T, where two segments with extremely large 236U/238U ratios are found. These ratios are dramatically greater than all the rest, and the ≥10−7 magnitude has never been reported or anticipated for environmental samples where 236U is known to be naturally occurring [6–8,24]. Thus, it appears probable that the UOCs from those two producers were fed with ore materials that were the outcome of unknown natural 236U-production processes that have occurred in the recent past (on the order of the 236U half-life). It is not unusual to find high 236U/238U ratios in chemical products from commercial sources, possibly due to complex recycling of spent fuel and irradiated reactor material. We have had several such encounters, including with a PbF2 powder product (which we must avoid its use). Such encounters were also experienced by others (e.g. Berkovits et al. [25]). However, the case found in this work is rather perplexing as UOCs are theoretically products of material which has not been exposed to contamination from the uranium cycle within the nuclear industry. It certainly does not make economic sense to recycle spent fuel into directly mined uranium ores for the

Declaration of Competing Interest None. Acknowledgment This work was supported by the Defense Research and Development Canada (DRDC) Centre for Security Science (CSS), under the leadership of the CNSC. Financial support from CFI, ORF, CSS and NSERC for the establishment and operation of A. E. Lalonde AMS Laboratory are acknowledged. We are indebted to late Prof. Jack Cornett who passed away shortly after having initiated our involvement in the nationwide program for the development of the Canadian NNFL. We would like to express our gratitude to Dr. Zoltan Mester of the National Research Council (NRC) Canada for providing us the three developing UOC reference materials used in this work. XLZ would like to acknowledge Prof. Ted Litherland for his continuing and highly stimulating discussions on AMS and science topics in general.

Appendix A:. Relevant AMS beam transmission and relative MOaFb− ion yield survey Table A1 shows the results of one beam transmission measurement. The most critical beam line section needed for results normalization is the transmission from the high energy system off-axis Faraday cup, used for the 238U+3 beam current measurement, to the very last Faraday cup just before the gas ionization chamber, used for the rare ion counting. This transmission is found unexpectedly low at ∼48%, due to the excessive transmission loss through the high energy analyzing magnet image slits by the fragmentation of the large UF5− molecular ion. Fortunately, it has remained constant for all batch measurement runs in this work. This is likely due to the non-optimized slit-width used to emphasize resolution, 109

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Table A1 Transmission of the main pilot beams under AMS conditions used for the batch measurement runs in this work. Beam 238

−

238

FC (Faraday Cup) location

Current

Accelerator entrance FC with LE magnet image slits in front, but no aperture Off-axis-FC after HE magnet without slits/aperture FC after HE magnet with image slits in front, but no aperture FC in front of gas ionization chamber (GIC) with image slits in front, but no aperture

43.0 nA 25.5 nA 13.1 nA 12.3 nA

Accelerator entrance with LE magnet image slits in front, but no aperture FC after HE magnet with image slits in front, but no aperture FC in front of GIC with image slits in front, but no aperture

770 nA 281 nA 271nA

Transmission

+3

UF5 → U UF5− in ACC-FC +3 U in Off-axis-FC 238 +3 U in RI-FC 238 +3 U in GIC-FC 238

238

Au− → 197Au+3 Au− in ACC-FC 197 Au+3 in RI-FC 197 Au+3 in GIC-FC

From From From From

ACC to Off-axis-FC for U+3: ∼20% ACC to RI for U+3: ∼10% RI to GIC for U+3: ∼94% Off-axis-FC to GIC for U+3: ∼48%

197 197

From ACC to RI for Au+3: ∼12% From RI to GIC for Au+3: ∼96%

which must be further investigated for better balanced needs in future. For the time being, similar to the earlier phases of analysis, this low transmission value is used for calculating the transmission-calibrated results of 236U/238U, bearing in mind that a set of more accurate results can be calculated in future based on the measured relative ratios to that of Ref-A included in same batch, once Ref-A itself is fully characterized someday. In order to provide some degree of normalization for the estimation of the bi-elemental ratios of 231Pa/238U, 230Th/238U and 226Ra/238U, the relative probabilities for U, Pa, Th and Ra to form general MOaFb− molecular anions must be surveyed. Such a survey can be done in the following steps: After AMS is first well tuned for detecting the main pilot beam 238UF5− → 238U+3, all other beams of MOaFb− → M+3 can be measured briefly, in either the ionization chamber or the Faraday cup in front, by adjusting key AMS analyzer parameters, so that all these molecular anions are made to undergo charge exchange at the accelerator terminal (around 2.5 MV in this case) at close-by energies. That way, a similar yield for the resulting +3 ions is assumed for all. The UOC sample P1 was used for this and other methodology related studies because of the relatively large quantity (∼5 g) provided to us. Using a new P1 + PbF2 target, the beam intensities of MOaFb− → M+3 were surveyed (Table A2). Likewise, using a new P1-only target, the beam intensities of MOa− → M+3 were also surveyed (Table A3). These surveys were done with the same modest Cs+ sputter beam intensity used in this Table A2 Survey of MOaFb− → M+3 beam intensities and the relative MOaFb− ion yields using a P1 + PbF2 (1:10 by volume) target for the U-group ions. (The normalized rate for M+3 is in “counts/second per µA of total I− measured in BI-FC”.) Negative Ion

238

U+3

231

Pa+3

230

Th+3

226

Ra+3

Norm. Rate (c/s)

[% of SUM]

Norm. Rate (c/s)

[% of SUM]

Norm. Rate (c/s)

[% of SUM]

Norm. Rate (c/s)

[% of SUM]

MO1− MO2− MO3− MO4− MO5− MF1− MF2− MF3− MF4− MF5− MF6− MO1F1− MO1F2− MO1F3− MO1F4− MO1F5− MO2F1− MO2F2− MO2F3− MO2F4− MO3F1− MO3F2− MO3F3− SUM

2.1E + 06 3.3E + 05 7.0E + 06 5.0E + 06 3.3E + 05 3.3E + 05 3.3E + 05 2.5E + 06 1.4E + 09 8.4E + 09 5.5E + 08 4.5E + 06 2.0E + 07 1.5E + 09 1.2E + 09 2.3E + 07 9.5E + 06 1.5E + 09 1.2E + 09 3.3E + 05 3.8E + 08 1.1E + 07 3.3E + 05 1.6E + 10

0.01 0.00 0.04 0.03 0.00 0.00 0.00 0.02 8.74 51.94 3.39 0.03 0.12 8.95 7.41 0.14 0.06 9.46 7.20 0.00 2.37 0.07 0.00 100%

0.0031 0.0079 0.0433 0.0017 0.0002 0.0002 0.0002 0.0017 0.0002 0.4386 0.8000 0.0032 0.0017 0.0386 1.4620 0.0091 0.0203 1.0877 0.0071 0.0002 0.0259 0.0018 0.0002 3.9649

0.08 0.20 1.09 0.04 0.01 0.01 0.01 0.04 0.01 11.09 20.23 0.08 0.04 0.98 36.97 0.23 0.51 27.50 0.18 0.01 0.65 0.05 0.01 100%

0.0029 0.0582 0.0312 0.0002 0.0002 0.0029 0.0015 0.0344 0.0034 25.8621 0.0035 0.1212 0.2281 5.3672 0.0983 0.0002 0.6000 0.1540 0.0002 0.0018 0.0032 0.0002 0.0002 32.5751

0.01 0.18 0.10 0.00 0.00 0.01 0.00 0.11 0.01 79.39 0.01 0.37 0.70 16.48 0.30 0.00 1.84 0.47 0.00 0.01 0.01 0.00 0.00 100%

0.0034 0.0001 0.0001 0.0001 0.0001 0.0035 0.0054 0.7091 0.0001 0.0001 0.0001 0.0001 0.0018 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0019 0.0001 0.0001 0.0001 0.7268

0.47 0.01 0.01 0.01 0.01 0.48 0.74 97.56 0.01 0.01 0.01 0.01 0.25 0.01 0.01 0.01 0.01 0.01 0.01 0.26 0.01 0.01 0.01 100%

Rate relationship between the chosen beam and SUM

SUM ∼ 1.9 × MF5−

SUM ∼ 9.0 × MF5−

SUM ∼ 1.3 × MF5− Or SUM ∼ 54 × MO2F−

SUM ∼ 1.0 × MF3−

Normalization from the measured ion rates to the targeted isotopic ratios

[236U/238U] ≈ 2.1 × [236UF5−/238UF5−]m (This “2.1 factor” is based on the transmission measurement shown in Table A1.)

[231Pa/238U] ≈ 2.1 × 4.7 × [231PaF5−/238UF5−] m (This is also based on relative ion yields in this table.)

[230Th/238U] ≈ 2.1 × 0.65 × [230ThF5−/238UF5−] m Or [230Th/238U] ≈ 2.1 × 28 × [230ThO2F−/238UF5−] m (This is also based on relative ion yields in this table.)

[226Ra/238U] ≈ 2.1 × 0.53 × [226RaF3−/238UF5−] m (This is also based on relative ion yields in this table.)

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Table A3 Survey of MOa− → M+3 beam intensities and the relative MOa− ion yields using a P1-only target for the U-group ions. (The normalized rate for M+3 is in “counts/ second per µA of total I− measured in BI-FC”.) Negative Ion

MO1− MO2− MO3− MO4− MO5− SUM

238

U+3

231

Pa+3

230

Th+3

226

Ra+3

Norm. Rate (c/s)

[% of SUM]

Norm. Rate (c/s)

[% of SUM]

Norm. Rate (c/s)

[% of SUM]

Norm. Rate (c/s)

[% of SUM]

2.0E + 08 3.4E + 08 8.1E + 08 1.3E + 08 1.0E + 06 1.5E + 09

13.5 23.0 54.7 8.8 0.1 100%

0.0349 0.9055 2.2986 0.0093 0.0047 3.2530

1.1 27.8 70.7 0.3 0.1 100%

1.0000 11.9617 1.5625 0.0291 0.0784 14.6317

6.83 81.75 10.68 0.20 0.54 100%

0.0050 0.0001 0.0001 0.0001 0.0001 0.0054

92.6 1.9 1.9 1.9 1.9 100%

Table A4 Survey of MOa− → M+3 beam intensities and the relative MOa− ion yields using a P1-only target for the Os-group ions. (The normalized rate for M+3 is in in “counts/ second per µA of total I− measured in BI-FC”.) Negative Ion

191

Ir+3

188

Os+3

185

Re+3

Norm. Rate (c/s)

[% of SUM]

Norm. Rate (c/s)

[% of SUM]

Norm. Rate (c/s)

[% of SUM]

M− MO1− MO2− MO3− MO4− MO5− SUM

0.3600 0.0810 0.0076 0.0008 0.0004 0.0002 0.4500

80.0 18.0 1.7 0.2 0.1 0.0 100%

0.2850 0.1460 0.0165 0.0008 0.0060 0.0008 0.4551

62.6 32.1 3.6 0.2 1.3 0.2 100%

0.2722 11.7500 6.7500 7.5210 5.6970 0.0574 32.0476

0.85 36.66 21.06 23.47 17.78 0.18 100%

Rate relationship between the chosen beam and SUM

SUM ∼ 1.3 × M− (having considered ∼ 10% extra molecular ion stripping loss)

SUM ∼ 1.7 × M− having considered ∼ 10% extra molecular ion stripping loss)

SUM ∼ 140 × M− (having considered ∼ 20% extra molecular ion stripping loss)

work. The sputter beam intensity is worth mentioning here because it may be an important factor for the measured relative intensities of UO1,2,3,4−. If a much greater Cs+ intensity is used, oxygen may be exhausted very rapidly, resulting in UO− to show the highest yield instead of UO3−. To account for the slow target output decline during the longish course of such survey, the final results are always normalized to the average of the total negative ion beam current measured in BI-FC, the Faraday cup located at the focal plane of the low energy electrostatic analyzer (Fig. 1), at the beginning and end of each M+3 beam measurement. For similar reasons, the Os-group assessment also requires the use of UOC-only targets to be surveyed for all the oxide negative ions of the chosen elements: Re, Os and Ir. Using a new P1-only target as an example, the beam intensities of MOa− → M+3 were surveyed for the Os-group elements (Table A4). Appendix B:. Selection of other isotopes with

236

U/238U and

187

Os/188Os measurements

The AMS system used in this work permits 5 isotopes to be detected in the SSI mode of data acquisition. In order to make an informed selection of the most suitable extra isotopes to be included in the two SSI measurements of 236U/238U and 187Os/188Os, three UOC samples (which had the most material available) of different colors were chosen for a brief survey of the ion count rates of all the potentially interested isotopes in the two mass groups. The AMS conditions used were the same as described in the corresponding sections of the text; the experiments were carried out manually. The results were summarized in Tables B1 and B2 for the 236U/238U and 187Os/188Os mass groups, respectively.

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Table B1 Brief measurement of intensities (c/s) of potential isotopes to be included with

236

U/238U.

Beam

PbF2 Blank

I9 (black) + PbF2

L7 (yellow) + PbF2

P1 (orange) + PbF2

Comments

210

<0.002 <0.002 <0.002 <0.002 <0.002 0.113 <0.002 162 <0.002 – – <0.002 <0.002 – <0.002 <0.002 <0.002 <0.002

<0.002 <0.002 0.005 <0.002 <0.002 52.8 1.29 2960 <0.002 – – 0.067 0.027 13.0nA 0.015 <0.002 <0.002 <0.002

<0.002 <0.002 0.016 <0.002 <0.002 2770 0.220 >10,000 <0.002 – – 0.053 0.012 13.2nA <0.002 <0.002 <0.002 <0.002

<0.002 <0.002 0.412 <0.002 <0.002 88.8 1.81 3000 <0.002 – – 0.220 0.012 22.2nA <0.002 <0.002 <0.002 <0.002

Too weak to consider. Too weak to consider. Could be a useful UOC signature. (Selected) Too weak to consider. Too weak to consider. Could be a useful UOC signature. (Selected) Could be a useful UOC signature. (Selected) Could be useful UOC signature, but may encounter extremely intense count rates. This cannot be selected. Too weak to consider. These are too intense to count by an ionization chamber, and too weak to measure as currents in a Faraday cup. Must measure. (Selected) Could be a useful UOC signature; worth considering in future. Must measure in an off-axis Faraday cup in the AMS high energy system. (Selected) Too weak to consider. Too weak to consider. Too weak to consider. Too weak to consider.

PbF3− → 210Pb+3 BiF4− → 210Bi+3 226 RaF3− → 226Ra+3 228 ThF5− → 228Th+3 229 ThF5− → 229Th+3 230 ThF5− → 230Th+3 231 PaF5− → 231Pa+3 232 ThF5− → 232Th+3 233 UF5− → 233U+3 234 UF5− → 234U+3 235 UF5− → 235U+3 236 UF5− → 236U+3 237 NpF4− → 237Np+3 238 UF5− → 238U+3 239 PuF4− → 239Pu+3 243 AmF4− → 243Am+3 244 PuF4− → 244Pu+3 247 CmF4− → 247Cm+3 210

Table B2 Brief measurement of M− → M+3 intensities (c/s) of potential isotopes to be included with

187

Os/188Os.

Isotope

Empty Cu/Cu Blank

I9 (black)

L7 (yellow)

P1 (orange)

Comments

185

Re (37.40%) Re (62.60%) 187 Os (1.96%) 187 Os after 187Re correction 188 Os (13.24%) 189 Os (16.19%)

<0.003 <0.003

5.77 17.1

1.03 4.42

15.7 56.6

Must measure. (Selected) Must measure. (Selected)

– <0.001 –

7.44 0.405 –

2.70 0.095 –

30.3 2.77 –

190

0.004

0.993

0.294

4.22

Corrected based on known 187Re and 185Re abundances. Must measure. (Selected) Could be a useful UOC signature. Unfortunately it cannot be measured with 189Os+3 due to intense 63 Cu+1 ion fluxes. Could be a useful UOC signature; worth considering in future.

– 0.020 0.030 13.3 8.94

0.837 6.57 12.0 368 241

0.258 22.2 38.1 83.6 53.0

4.20 1.34 1.99 46.8 18.8

Corrected based on known 190Pt and 194Pt abundances. Could be a useful UOC signature. (Selected) Could be a useful UOC signature. (Selected) Blanks too high to consider. All signals are 196Pt. Blanks too high to consider.

551 2.46

6750 61.4

515 14.2

5220 7.16

Blanks too high to consider. All signals are 198Pt. Blanks too high to consider.

<0.001 <0.001 0.023 – 0.076 – – – 1200 <0.002

<0.001 <0.001 0.103 – 0.243 – – – 4810 <0.003

<0.001 <0.001 0.059 – 0.173 – – – 2570 <0.003

<0.001 <0.001 0.961 – 2.75 – – – 14,000 <0.003

Hg does not make negative ions, so these results are exactly as expected.

187

Os (26.26%) 190 Pt (0.014%) 190 Os after 190Pt correction 191 Ir (37.3%) 193 Ir (62.7%) 194 Pt (32.967%) 196 Pt (25.242%) 196 Hg (0.15%) 197 Au (100%) 198 Pt (7.163%) 198 Hg (9.97%) 200 Hg (23.10%) 202 Hg (29.86%) 203 Tl (29.524%) 204 Pb (1.4%) 205 Tl (70.476%) 206 Pb (24.1%) 207 Pb (22.1%) 208 Pb (52.4%) 209 Bi (100%) 210 Po

Could be a useful UOC signature; worth considering in future. Not measured. Could be a useful UOC signature; worth considering in future. Not measured. Blanks too high to consider. Too weak to consider.

112

(1.65–4.91) e-11 (3.5–19.7) e-12 5.12 (0.68) e-11 (1.6–38.4) e-12

(5.1–19.7) e-12 (6.43–9.33) e-11 (4.8–42.8) e-11 (1.09–1.96) e-9 (1.59–4.42) e-10

(1.1–21.3) e-11 (7.8–36.6) e-12 (4.1–22.5) e-12

(8.6–46.9) e-12

(5.2–23.5) e-12

(6.8–29.3) e-12

(3.6–28.9) e-12 (2.8–15.0) e-12 (5.7–25.7) e-12 8.91 (0.44) e-11 (1.54–5.04) e-11 1.026 (0.058) e-10 1.27 (0.20) e-9 (2.0–53.4) e-10 (8.6–12.6) e-8

(9.5–52.7) e-12 (3.9–20.6) e-12

(2.29–7.80) e-10 (2.2–17.3) e-8

A1 − 17 B1 − 15

D1 − 17

G1 − 13 H1 − 13 I1 − 15

J1 − 17

K1 − 17

5L1 − 17

M1 − 15 N1 − 15 O1 − 12

113

R1 − 15 S1 − 17

T1 − 15

Q1 − 15

P1 − 17

F1 − 17

E1 − 17

C1 − 11

1.940 (0.011) e-10 1.153 (0.092) e-11 2.25 (0.10) e-11

Ref-A (n = 18 U, 9 Os) Ref-B (n = 11 U, 13 Os) Ref-C (n = 11 U, 9 Os)

U/238U

236

UniqueUOC Code by [Producer] [Number]

(9.5–217) e-10

(1.2–33.2) e-11 (8.6–87.9) e-11

(2.0–23.8) e-9

(5.2–37.8) e-11

6.2 (3.3) e-12 (7.4–25.9) e-11 6.17 (0.21) e-10 (2.51–8.34) e-9 (7.0–20.0) e-10 (9.5–23.8) e-10

(2.10–7.43) e-10

(3.2–12.8) e-10

(2.86–5.62) e-9 (8.4–22.3) e-9 (1.20–3.99) e-10 (1.25–11.34) e-10 (9.6–37.7) e-11

(2.62–10.22) e-10

3.90 (0.13) e-10 (1.61–3.01) e-9 (1.28–7.83) e-8

(6.0–69.1) e-11 (3.7–44.9) e-10

7.38 (0.30) e-9 3.21 (0.10) e-8 1.20 (0.11) e-10

∼231Pa/238U

(2.9–25.9) e-9 6.720 (0.047) e-8 (1.2–3.0) e-7 (9.1–32.6) e-9 (1.9–8.48) e-9 (3.12–5.45) e-8 (3.3–18.6) e-9

(1.10–3.16) e-9 (1.14–3.41) e-8

(1.7–16.9) e-9 (1.5–11.9) e-8 (1.31–4.69) e-9

(1.50–8.39) e-8 (1.2–72.0) e-9 (5.0–25.7) e-10 8.99 (0.26) e-9 7.1 (2.5) e-9 (1.3–14.5) e-8 (9.3–49.3) e-10 (8.2–305) e-9 (2.9–35.9) e-8

(2.1–15.3) e-9 5.10 (0.29) e-8 (5.3–45.1) e-10 (7.8–45.7) e-9 5.25 (0.29) e-9 (2.15–18.13) e-8 (4.1–67.1) e-9

(6.8–125) e-9 (2.2–278) e-8

3.37 (0.45) e-10 4.27 (0.89) e-10 8.14 (0.64) e-10

∼230Th/238U

(≤1 − 10.8) e-13

(2.8–13.7) e-12 (5.9–153) e-13

6.6 (2.8) e-14 (1.2–44.9) e-13

(9.7–36.2) e-12

(≤1 − 44.4) e-14 (5.8–10.6) e-11 (≤1 − 42.5) e-14

(8.2–50.9) e-13

(3.5–523) e-14

(4.8–46.3) e-13

(8.5–18.0) e-13 (2.5–73.9) e-14 (≤1 − 83.7) e-14

1.11 (0.08) e-14 (3.1–14.8) e-13 (≤1 − 221) e-14

(≤1 − 52.6) e-14

(≤1 − 57.1) e-14

(1.8–26.5) e-13 (5.9–35.1) e-12

3.20 (0.79) e-14 2.3 (1.6) e-14 1.386 (0.049) e-11

∼226Ra/238U

Os/188Os

3.2–49.5

0.4–10 0.3–5.5

4.4–60.3

10.0–14.7

0.18–2.12 0.17–3.7 0.07–0.79

2.9–24.2

0.5–13

2.1–7.5

0.1–11 0.8–6.6 0.5–10

12.5–106.6

6–140

139–341

0.2–1.4

174–458 0.2–3.1

61 (14) 329.6 (8.7) 12.43 (0.32)

187

[Format “a (b) or (a − b) e-c” stands for (a ± b) or (a − b) × 10−c. See section 3.4&4.3 for discussion of realistic uncertainties.]

Appendix C:. Ranges of results

50–2000

600–5000 200–30000

50–3000

30–200

5–90 40–2000 5–200

200–3000

50–3000

60–600

30–2000 80–20000 400–7000

90–1000

100–6000

50–400

20–200

30–2000 10–3000

800 100 50

∼[Re/Os]

3–1000

1000–7000 60–10000

7–600

200–2000

0.09–30 6–2000 0.5–20

30–2000

100–1000

300–3000

8–700 8–30000 200–2000

4–60

20–3000

3–50

60–2000

5–400 3–1000

100 20 500

∼[Re/Ir]

0.1–10

0.9–20 0.8–200

0.6–30

0.3–20

0.3–2 0.07–2 0.03–0.8

0.3–10

2–50

2–20

0.03–2 0.6–100 5–30

0.2–2

0.06–1

0.3–3

0.5–30

0.4–10 0.04–7

2 1 9

Relative 185Re+3 count rate to that of Ref-B in same batch

X.-L. Zhao, et al.

Nuclear Inst. and Methods in Physics Research B 459 (2019) 98–114

Nuclear Inst. and Methods in Physics Research B 459 (2019) 98–114

X.-L. Zhao, et al.

References

[15] D.P. Child, M.A.C. Hotchkis, K. Whittle, B. Zorko, Ionisation efficiency improvements for AMS measurement of actinides, Nucl. Inst. Methods Phys. Res. B 268 (2010) 820–823. [16] R. Buompane, M. De Cesare, N. De Cesare, A. Di Leva, A. D’Onofrio, L.K. Fifield, M. Frohlich, L. Gialanella, F. Marzaioli, C. Sabbarese, F. Terrasi, S. Tims, A. Wallner, Background reduction in 236U/238U measurements, Nucl. Inst. Methods Phys. Res. B 361 (2015) 454–457. [17] Z.H. Kazi, C.R.J. Charles, X.-L. Zhao, R.J. Cornett, W.E. Kieser, Ion beam enhancement in 236UO− measurement by AMS using Si powder as the binder, Nucl. Inst. Methods Phys. Res. B 456 (2019) 218–221. [18] X.-L. Zhao, A.E. Litherland, J. Eliades, W.E. Kieser, Q. Liu, Studies of anions from sputtering I: Survey of MFn−, Nucl. Inst. Methods Phys. Res. B 268 (2010) 807–811. [19] X.-L. Zhao, A.E. Litherland, W.E. Kieser, R.J. Cornett, Yb− and 236UF5−—two case studies of E/q and EM/q2 interferences in AMS, Nucl. Inst. Methods Phys. Res. B 455 (2019) 224–229. [20] X.-L. Zhao, W.E. Kieser, X. Dai, N.D. Priest, S. Kramer-Tremblay, J. Eliades, A.E. Litherland, Preliminary studies of Pu measurement by AMS using PuF4−, Nucl. Inst. Methods Phys. Res. B 294 (2013) 356–360. [21] R.J. Cornett, Z. Kazi, X.-L. Zhao, M.G. Chartrand, R.J. Charles, W.E. Kieser, Actinide measurements by AMS using fluoride matrices, Nucl. Inst. Methods Phys. Res. B 361 (2015) 317–321. [22] R.J. Cornett, X.-L. Zhao, X.-L. Hou, W.E. Kieser, A preliminary study of 99Tc measurement using matrix-assisted low energy AMS, Nucl. Inst. Methods Phys. Res. B 455 (2019) 181–189. [23] X.-L. Zhao, C.R.J. Charles, R.J. Cornett, W.E. Kieser, C. MacDonald, Z. Kazi, N. StJean, An exploratory study of recycled sputtering and CsF2− current enhancement for AMS, Nucl. Inst. Methods Phys. Res. B 366 (2016) 96–103. [24] M.J. Murphy, M.B. Froehlich, L.K. Fifield, S.P. Turner, B.F. Schaefer, In-situ production of natural 236U in ground waters and ores in high-grade uranium deposits, Chem. Geol. 410 (2015) 213–222. [25] D. Berkovits, H. Feldstein, S. Ghelberg, A. Hershkowitz, E. Navon, M. Paul, 236U in uranium minerals and standards, Nucl. Inst. Methods Phys. Res. B 172 (2000) 372–376. [26] J.M. Herndon, Feasibility of a nuclear fission reactor at the center of the Earth as the energy source for the geomagnetic field, J. Geomag. Geoelectr. 45 (1993) 423–437. [27] F. Bosch, T. Faestermann, J. Friese, F. Heine, P. Kienle, E. Wefers, K. Zeitelhack, K. Beckert, B. Franzke, O. Klepper, C. Kozhuharov, G. Menzel, R. Moshammer, F. Nolden, H. Reich, B. Schlitt, M. Steck, T. Stöhlker, T. Winkler, K. Takahashi, Observation of bound-state β− decay of fully ionized 187Re: 187Re-187Os cosmochronometry, Phys. Rev. Lett. 77 (26) (1996) 5190–5193.

[1] K.H. Purser, (1977). Ultra-sensitive spectrometer for making mass and elemental analyses. US Patent No. 4,037,100 July 19th 1977. Applied for on March 1st 1976. [2] W. Kutschera, Accelerator mass spectrometry: state of the art and perspectives, Adv. Phys.: X 1 (4) (2016) 570–595. [3] U. Fehn, R. Teng, D. Elmore, P.W. Kubik, Isotopic composition of osmium in terrestrial samples determined by accelerator mass spectrometry, Nature 323 (1986) 707–710. [4] W.E. Kieser, X.-L. Zhao, I.D. Clark, R.J. Cornett, A.E. Litherland, M. Klein, D.J.W. Mous, J.F. Alary, The André E. Lalonde AMS Laboratory – The new accelerator mass spectrometry facility at the University of Ottawa, Nucl. Inst. Methods Phys. Res. B 361 (2015) 110–114. [5] X.-L. Zhao, M.-J. Nadeau, L.R. Kilius, A.E. Litherland, Detection of naturally occurring 236U in uranium ore, Earth Planet. Sci. Lett. 124 (1994) 241–244. [6] P. Steier, M. Bichler, L.K. Fifield, R. Golser, W. Kutschera, A. Priller, F. Quinto, S. Richter, M. Srncik, P. Terrasi, L. Wacker, A. Wallner, G. Wallner, K.M. Wilcken, E.M. Wild, Natural and anthropogenic 236U in environmental samples, Nucl. Inst. Methods Phys. Res. B 266 (2008) 2246–2250. [7] K.M. Wilcken, L.K. Fifield, T.T. Barrows, S.G. Tims, L.G. Gladkis, Nucleogenic 36Cl, 236 U and 239Pu in uranium ores, Nucl. Inst. Methods Phys. Res. B 266 (2008) 3614–3624. [8] R. Eigl, M. Srncik, P. Steier, G. Wallner, 236U/238U and 240Pu/239Pu isotopic ratios in small (2L) sea and river water samples, J. Environ. Radioact. 116 (2013) 54–58. [9] C.C. Keith (2016). Investigation of trace U-236 content variation in worldwide geological deposits. Ph.D. Thesis, Texas A&M University. [10] M. Srncik, K. Mayer, E. Hrnecek, M. Wallenius, Z. Varga, P. Steier, G. Wallner, Investigation of the 236U/238U isotope abundance ratio in uranium ores and yellow cake samples, Radiochim. Acta 99 (2011) 335–339. [11] J. Davydov (2017). Current status of IAEA nuclear forensics coordinated research projects and future needs for R&D. International Symposium on Technology Development for Nuclear Forensics, Rokyo, Japan, 05 June 2017. [12] A.P. Dickin, Radiogenic Isotope Geology (Second Edition), Cambridge University Press, 2005. [13] P. Steier, F. Dellinger, O. Forstner, R. Golser, K. Knie, W. Kutschera, A. Priller, F. Quinto, M. Srnick, F. Terrasi, C. Vockenhuber, A. Wallner, G. Wallner, E.M. Wild, Analysis and application of heavy isotopes in the environment, Nucl. Inst. Methods Phys. Res. B 268 (2010) 1045–1049. [14] S.R. Winkler, P. Steier, J. Buchriegler, J. Lachner, J. Pitters, A. Priller, R. Golser, He stripping for AMS of 236U and other actinides using a 3 MV tandem accelerator, Nucl. Inst. Methods Phys. Res. B 361 (2015) 458–464.

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