Accelerator Mass Spectrometry Targets Research Articles

Hydrogen isotope ratio measurement by AMS at CIAE

Knowledge of the ratio of hydrogen isotopes contributes to many research fields. We have developed an accelerator mass spectrometry (AMS) method allowing simultaneous measurement of D/H and T/H ratios. We present a novel TiH2 sample preparation method suitable for the preparation of AMS targets and we describe the measurement technique developed with our in-house 300 kV AMS system at CIAE. We achieved a H+ transmission efficiency of 65 % and T/H measurement background of 2 × 10−15. Measurement of isotopic standard samples gave linear relationships between the measured and nominal values for both T/H and D/H.

Scavenger hunt: Searching for the optimal target material for low-level 210Pb accelerator mass spectrometry

The Centre of Excellence for Dark Matter Particle Physics aims to measure 210Pb (half-life of 22.2 years) in 1 kg of NaI via accelerator mass spectrometry (AMS) for the SABRE (Sodium iodide with Active Background REjection) South dark matter experiment. The first step is to find the optimal target material (chemical compound and binder) to produce the highest and most stable negative ion output. We initially studied the outputs from Pb3O4, PbO and PbF2 mixed 1:2 with Ag. The 208PbO2− and 208PbF3− currents were 0.5–1.2 μA with the procedural PbF2 compound performing slightly better. Based on these results, we explored the performance of PbF2 mixed with fluorinating agents such as AgF, AgF2 and SbF3 at different ratios. The 1:1 mixture was the best for all additives. The possibility of using either Fe (oxide/fluoride) or NaF as bulk material for the AMS target was also studied, however, none proved suitable.

VOLUMES OF WORTH—DELIMITING THE SAMPLE SIZE FOR RADIOCARBON DATING OF PARCHMENT

ABSTRACTMedieval manuscripts are invaluable archives of the written history of our past. Manuscripts can be dated and localized paleographically, but this method has its limitations. The Fragmenta membranea manuscript collection at the National Library of Finland has proved difficult to date using paleographic methods. Radiocarbon dating has been applied to manuscripts of parchment before, but a systematic protocol for radiocarbon dating of parchment has not been established with a minimally destructive sampling strategy. In this work, we have established a radiocarbon dating procedure for parchments combining a clean-room based chemical pretreatment process, elemental analyzer combustion, automatic graphitization and accelerator mass spectrometry (AMS) measurements to reduce the AMS target size from a typical 1 mg of carbon. Prolonged acid treatment resulted in improved dating accuracy, since this is consistent with the manufacturing process of medieval parchment involving a lime bath. Two different combustion processes were compared. The traditional closed tube combustion (CTC) method provided a well-established though labor-intensive way to produce 1 mg AMS targets. The Elemental Analyzer-based process (EA-HASE, Elemental Analyzer Helsinki Adaptive Sample prEparation line), is designed for fast combustion and smaller sample sizes. The EA-HASE process was capable of reproducing the simulated radiocarbon ages of known-age samples with AMS graphite target sizes of 0.3 mg of carbon, corresponding to a 3 mm2 area of a typical medieval parchment. The full potential of the process to go down to as little as 50 μg will be further explored in the future in parallel to studies of sample-specific contamination issues.

BaCO3 targets produced from dissolved carbonate in groundwater for direct AMS measurement

This paper describes a technique of making carbonate by rapid CO2 capture from groundwater for direct measurement of the 14C content by accelerator mass spectrometry (AMS). The DIC in groundwater was extracted from acidified water in the form of carbon dioxide (CO2), and transferred into saturated barium hydroxide solution to form barium carbonate (BaCO3). After the BaCO3 precipitate was freeze-dried, it was mixed with a metal powder and pressed into an AMS target for measurement. The behaviors of Si, Al, Fe and Ta powder as the binder with carbonate were evaluated. The C3+ currents from carbonate targets mixed with Fe or Ta were found generally larger than those with Si or Al by a factor of two. The baked Ta was shown to produce fewer contaminating 14C counts than all the others, with or without baking. The CaCO3/Ta and BaCO3/Ta mass mixing ratios in the range from 1:2 to 1:4 and in 1:1.5, respectively, produced optimized currents. The precision and accuracy of the measurement by direct CaCO3 or BaCO3 sputtering using Ta as the binder, were evaluated in comparison to a selected reference material. The agreement was reasonably good between the direct carbonate measurement and the high precision measurement through combustion and graphitization. These findings support the further development of a rapid assessment method directly from field work to measurement.

Determination of 41Ca with LSC and AMS: method development, modifications and applications

Despite the emission of only low energy Auger electrons (ca. 3.6 keV) and the difficulty of obtaining a certified standard, Liquid scintillation counting (LSC) determinations are still reasonable options for a radioanalytical laboratory involved in nuclear installation decommission. Besides, accelerator mass spectrometry (AMS), being the most sensitive analytical technique not only for 41Ca, is gaining increasingly broader accessibility and applicability. Herein, we present a radiochemical separation procedure developed for 41Ca determination with LSC and AMS in varying materials (i.e. water, concrete, sediment, soil, and biota). The radioanalytical isolation consists of anion exchange and extraction chromatography as well as carbonate precipitation and recrystallization from organic solvents. Thereby, interfering radionuclides as 55Fe, 60Co, 152Eu, U or actinides are removed with decontamination factors of 102–104. Quench curves for determining the measurement efficiency is generated with a 41Ca solution gained from the 41Ca/40Ca certified reference material ERM-AE701. In routine application the procedure is characterized by chemical yields of 67–86 %, measurement efficiencies of 1–10 % and detection limits of 0.05 Bq g−1 and 0.3 Bq L−1. Aliquots of the digestion solutions of LSC can be easily converted into CaF2–AMS targets by successive oxsalate and fluoride precipitation. Pros and cons for both measurement techniques are addressed based on 41Ca results from LSC and AMS for the same material.

Reprocessing of 10B-contaminated 10Be AMS targets

10Be accelerator mass spectrometry (AMS) is an increasingly important tool in studies ranging from exposure age dating and palaeo-geomagnetism to the impact of solar variability on the Earth’s climate. High levels of boron in BeO AMS targets can adversely impact the quality of 10Be measurements through interference from the isobar 10B. Numerous methods in chemical sample preparation and AMS measurement have been employed in order to reduce the impact of excessive boron rates. We present details of a method developed to chemically reprocess a set of forty boron-contaminated BeO targets derived from modern Antarctic ice. Previously, the excessive boron levels in these samples, as measured in an argon-filled absorber cell preceding the ionisation detector, had precluded routine AMS measurement. The procedure involved removing the BeO+Nb mixture from the target holders and dissolving the BeO in hot concentrated H2SO4. The solution was then heated with HF to remove the boron as volatile BF3 before re-precipitating as Be(OH)2 and calcining to BeO. This was again mixed with niobium and pressed into fresh target holders. Following reprocessing, the samples gave boron rates reduced by 10–100×, which were sufficiently low and similar to previous successful batches of ice core, snow and associated blank samples, thus allowing a successful 10Be measurement in the absence of any boron correction. Overall recovery of the BeO for this process averaged 40%. Extensive testing of relevant processing equipment and reagents failed to determine the source of the boron. As a precautionary measure, a similar H2SO4+HF step has been subsequently added to the standard ice processing method.

A sensitive method for the determination of chlorine-36 in foods using accelerator mass spectrometry

A method using accelerator mass spectrometry (AMS) has been developed to offer a more sensitive alternative to scintillation techniques for the determination of chlorine-36 (36Cl) in foods. The main problem in method development was the potential interference of the sulfur-36 (36S) isobar. This was overcome by reducing the sulfur level of acid digests of food by precipitation of chloride as silver chloride, then purification by washing, dissolution and reprecipitation to present silver chloride as the AMS target. The limit of detection was around 0.1 Bq kg−1 and the limit of quantitation was around 0.2 Bq kg−1. The AMS method was only semi-quantitative at the lowest levels of interest. To test the method a few samples of milk (five) and blackberries (three) collected from near two nuclear power stations as potential sources of contamination were analysed. Blackberries spiked at 0.2 Bq kg−1 and milk spiked at 0.1 Bq kg−1 could be distinguished from method blanks. There was no 36Cl detectable in the unspiked samples.

Biological/Biomedical Accelerator Mass Spectrometry Targets. 1. Optimizing the CO2 Reduction Step Using Zinc Dust

Biological and biomedical applications of accelerator mass spectrometry (AMS) use isotope ratio mass spectrometry to quantify minute amounts of long-lived radioisotopes such as 14C. AMS target preparation involves first the oxidation of carbon (in sample of interest) to CO2 and second the reduction of CO2 to filamentous, fluffy, fuzzy, or firm graphite-like substances that coat a −400-mesh spherical iron powder (−400MSIP) catalyst. Until now, the quality of AMS targets has been variable; consequently, they often failed to produce robust ion currents that are required for reliable, accurate, precise, and high-throughput AMS for biological/biomedical applications. Therefore, we described our optimized method for reduction of CO2 to high-quality uniform AMS targets whose morphology we visualized using scanning electron microscope pictures. Key features of our optimized method were to reduce CO2 (from a sample of interest that provided 1 mg of C) using 100 ± 1.3 mg of Zn dust, 5 ± 0.4 mg of −400MSIP, and a reduction temperature of 500 °C for 3 h. The thermodynamics of our optimized method were more favorable for production of graphite-coated iron powders (GCIP) than those of previous methods. All AMS targets from our optimized method were of 100% GCIP, the graphitization yield exceeded 90%, and δ13C was −17.9 ± 0.3‰. The GCIP reliably produced strong 12C− currents and accurate and precise Fm values. The observed Fm value for oxalic acid II NIST SRM deviated from its accepted Fm value of 1.3407 by only 0.0003 ± 0.0027 (mean ± SE, n = 32), limit of detection of 14C was 0.04 amol, and limit of quantification was 0.07 amol, and a skilled analyst can prepare as many as 270 AMS targets per day. More information on the physical (hardness/color), morphological (SEMs), and structural (FT-IR, Raman, XRD spectra) characteristics of our AMS targets that determine accurate, precise, and high-hroughput AMS measurement are in the companion paper.

Biological/Biomedical Accelerator Mass Spectrometry Targets. 2. Physical, Morphological, and Structural Characteristics

The number of biological/biomedical applications that require AMS to achieve their goals is increasing, and so is the need for a better understanding of the physical, morphological, and structural traits of high quality of AMS targets. The metrics of quality included color, hardness/texture, and appearance (photo and SEM), along with FT-IR, Raman, and powder X-ray diffraction spectra that correlate positively with reliable and intense ion currents and accuracy, precision, and sensitivity of fraction modern (Fm). Our previous method produced AMS targets of gray-colored iron−carbon materials (ICM) 20% of the time and of graphite-coated iron (GCI) 80% of the time. The ICM was hard, its FT-IR spectra lacked the sp2 bond, its Raman spectra had no detectable G′ band at 2700 cm−1, and it had more iron carbide (Fe3C) crystal than nanocrystalline graphite or graphitizable carbon (g-C). ICM produced low and variable ion current whereas the opposite was true for the graphitic GCI. Our optimized method produced AMS targets of graphite-coated iron powder (GCIP) 100% of the time. The GCIP shared some of the same properties as GCI in that both were black in color, both produced robust ion current consistently, their FT-IR spectra had the sp2 bond, their Raman spectra had matching D, G, G′, D+G, and D′′ bands, and their XRD spectra showed matching crystal size. GCIP was a powder that was easy to tamp into AMS target holders that also facilitated high throughput. We concluded that AMS targets of GCIP were a mix of graphitizable carbon and Fe3C crystal, because none of their spectra, FT-IR, Raman, or XRD, matched exactly those of the graphite standard. Nevertheless, AMS targets of GCIP consistently produced the strong, reliable, and reproducible ion currents for high-throughput AMS analysis (270 targets per skilled analyst/day) along with accurate and precise Fm values.

Microgram level radiocarbon (14C) determination on carbonaceous particles in ice

In climate research the interest on carbonaceous particles has increased over the last years because of their influence on the radiation balance of the earth. Nevertheless, there is a paucity of available data regarding their concentrations and sources in the past. Such data would be important for a better understanding of their effects and for estimating their influence on future climate. Here, a technique is described to extract carbonaceous particles from ice core samples with subsequent separation of the two main constituents into organic carbon (OC) and elemental carbon (EC) for analysis of their concentrations in the past. This is combined with further analysis of OC and EC 14C/12C ratios by accelerator mass spectrometry (AMS), what can be used for source apportionment studies of past emissions. We further present how 14C analysis of the OC fraction could be used in the future to date any ice core extracted from a high-elevation glacier. Described sample preparation steps to final analysis include the combustion of micrograms of water–insoluble carbonaceous particles, primary collected by filtration of melted ice samples, the graphitisation of the obtained CO2 to solid AMS target material and final AMS measurements. Possible fractionation processes were investigated for quality assurance. Procedural blanks were reproducible and resulted in carbon masses of 1.3±0.6μg OC and 0.3±0.1μg EC per filter. The determined fraction of modern carbon (fM) for the OC blank was 0.61±0.13. The analysis of processed IAEA-C6 and IAEA-C7 reference material resulted in fM=1.521±0.011 and δ13C=−10.85±0.19‰, and fM=0.505±0.011 and δ13C=−14.21±0.19‰, respectively, in agreement with consensus values. Initial carbon contents were thereby recovered with an average yield of 93%.

Progress in AMS Target Production of Sub-Milligram Samples at the NERC Radiocarbon Laboratory

Recent progress in graphite target production for sub-milligram environmental samples in our facility is presented. We describe an optimized hydrolysis procedure now routinely used for the preparation of CO2 from inorganic samples, a new high-vacuum line dedicated to small sample processing (combining sample distillation and graphitization units), as well as a modified graphitization procedure. Although measurements of graphite targets as small as 35 μg C have been achieved, system background and measurement uncertainties increase significantly below 150 μg C. As target lifetime can become critically short for targets <150 μg C, the facility currently only processes inorganic samples down to 150 μg C. All radiocarbon measurements are made at the Scottish Universities Environmental Research Centre (SUERC) accelerator mass spectrometry (AMS) facility. Sample processing and analysis are labor-intensive, taking approximately 3 times longer than samples ≥ 500 μg C. The technical details of the new system, graphitization yield, fractionation introduced during the process, and the system blank are discussed in detail.

Low-level (submicromole) environmental 14C metrology

Accelerator mass spectrometry (AMS) measurements of environmental 14C have been employed during the past decade at the several micromole level (tens of μg carbon), but advanced research in the atmospheric and marine sciences demands still higher (μg) sensitivity, an extreme example being the determination of 14C in elemental or “black” carbon (BC) at levels of 2–10 μg per kg of Greenland snow and ice (Currie et al., 1998). A fundamental limitation for 14C AMS is Poisson counting statistics, which sets in at about 1 μg modern-C. Using the small sample (25 μg) AMS target preparation facility at NOSAMS (Pearson et al., 1998), and the microsample combustion–dilution facility at NIST, we have demonstrated an intrinsic modern-C quantification limit ( m Q) of ca. 0.9 μg, based on a 1-parameter fit to the empirical AMS variance function. (For environmental 14C, the modern carbon quantification limit is defined as that mass ( m Q) corresponding to 10% relative standard deviation (rsd) for the fraction of modern carbon, σ( f M)/ f M.) Stringent control, required for quantitative dilution factors (DL), is achieved with the NIST on-line manometric/mass spectrometry facility that compensates also for unsuspected trace impurities from vigorous chemical processing (e.g., acid digestion). Our current combustion blank is trivial (mean: 0.16 ± 0.02 μg C, n=13) but lognormally distributed (dispersion [σ]: 0.07 ± 0.01 μg). An iterative numerical expression is introduced to assess the quantitative impacts of fossil and modern carbon blank components on m Q; and a new “clean chemistry” BC processing system is described for the minimization of such blanks. For the assay of soot carbon in Greenland snow/ice, the overall processing blank has been reduced from nearly 7 μg total carbon to less than 1 μg, and is undetectable for BC.

Ultra-separation of nickel from copper metal for the measurement of 63Ni by AMS

Measurements of 63Ni (t12 = 100 yr) produced by the reaction 63Cu(n,p)63Ni could be used in the assessment of fast-neutron fluence from the Hiroshima atomic bomb. Such measurements would add new information to help resolve the current discrepancy between measured thermal neutron activation values and those calculated with the DS86 dosimetry system. It has been estimated that the 63Ni production at 5 m from the hypocenter was (1.4 ± 0.1) × 107 atoms/g Cu. Because of its sensitivity, accelerator mass spectrometry (AMS) is ideal for measurements at this low level. However, 63Ni has to be separated from large amounts of stable atomic isobar 63Cu (69% of pure Cu). In this study, a procedure is presented for the electrochemical separation of ultra-low amounts of Ni from Cu. The method was developed using samples of electrical Cu wire that were irradiated with fission neutrons from a 252Cf source. The wire samples were electrochemically dissolved in a solution containing 1 mg of Ni carrier. The Cu was selectively deposited on a cathode at controlled potential. Measurements of total Ni after electroseparation indicate ∼ 100% carrier recovery. To prevent Cu contamination, AMS targets were prepared by nickel carbonyl generation. The AMS results show a successful quantitative separation of ∼ 107 atoms of 63Ni from 2–20 g samples of Cu.

Examination of Background Contamination Levels for Gas Counting and AMS Target Preparation in Trondheim

The Radiological Dating Laboratory in Trondheim relatively often dates samples with ages >30 ka BP. Contaminated background materials are known to affect the accuracy of very old dates. We have found, by measurements of different materials, that such contamination is small when using our conventional gas proportional counting (GPC) system. We have also studied contamination levels of our target preparation for14C accelerator mass spectrometry (AMS) dating in Uppsala. A significant lower background is obtained for Icelandic double spar than for marbles, probably due to a crystal structure of the double spar that is more insensitive to contaminating processes. The background for combusted samples is at the same level as for samples of double spar, indicating that additional14C contamination due to combustion is negligible. Dates obtained on interstadial samples (T >30 ka bp) by both GPC and AMS agree well.

Radiocarbon targets for AMS: A review of perceptions, aims and achievements

Sample target preparation techniques for radiocarbon dating by accelerator mass spectrometry (AMS) are reviewed. The procedures leading to AMS targets are internally reproducible yet often result in targets of varying stability, duration and ion beam yield. Sample size requirements, factors affecting background and ion beam stability are some of the variables that are considered.