Browsing by Author "Wallner, A"
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- ItemAchieving the ultimate sensitivity in Accelerator Mass Spectrometry of high mass isotopes(Australian National University, 2019-09-09) Hotchkis, MAC; Child, DP; Williams, ML; Wallner, A; Froehlich, MB; Koll, DThe VEGA AMS system at ANSTO, based on a 1MV tandem accelerator, was custom-designed to achieve the highest possible sensitivity for high mass isotopes. It incorporates multiple medium-resolving power analysing elements: one magnetic element for the injected negative ions, followed by magnetic, electrostatic and second magnetic elements for positive ions after acceleration. This design, with mass and energy resolving powers in the range 500 to 1000, separates isotopes and suppresses backgrounds that may originate from a variety of ion species. The gas stripper in the high-voltage terminal is key both to system efficiency and to background suppression. Helium gas stripping is used, providing around 40% ion yield to the most abundant charge state (3+). The stripper pressure must be sufficient to break up all molecules while minimising the scattering angle of the ions as they undergo charge-changing collisions. Our recent work has demonstrated that the need for production of negative molecular ions in AMS of actinides is not such a barrier to high efficiency: the VEGA sputter ion source can achieve greater than 1% efficiency for production of plutonium oxide negative ions and so overall sensitivity to a few hundred atoms in a sample is possible. We are involved in a number of projects requiring high sensitivity and low backgrounds. Examples include the detection of 244Pu of extraterrestrial origin in deep oceanic ferromanganese crusts; radioecology of plutonium in the environment of former nuclear test sites; detection of nuclear signatures for nuclear safeguards and forensics; use of Pu in global fallout as a chrono-marker in environmental studies; measurement of platinum-group-element isotope ratios in meteorites; evaluation of the radio-purity of materials for use in dark matter searches. Each of these projects presents their own particular challenges. In some cases, sensitivity is limited by background from scattered ions of species other than the one of interest. In other situations, cross-contamination between samples, in the sample prep lab or ion source, limits sensitivity. Other projects or previous uses of laboratories may leave residual contamination. For stable and very long-lived species, such as PGEs and major uranium isotopes, the ubiquity of those species at low levels in almost all materials sets limits. © The Authors.
- ItemActinides AMS on the VEGA accelerator(Elsevier B. V., 2019-01-01) Hotchkis, MAC; Child, DP; Froehlich, MB; Wallner, A; Wilcken, KM; Williams, MLThe VEGA 1MV accelerator at ANSTO is designed to be a highly versatile AMS instrument. In this paper we focus on describing those aspects of the system that are designed to optimise its performance for actinides isotopic analysis, in particular the implementation of fast isotope cycling and multiple isotope detection methods to enable isotope detection across a wide range of rates and currents. Charge state yields are reported in the energy range from 0.3 to 1.0 MeV with helium gas stripping, showing that the highest yield for the 3+ charge state occurs around 1 MeV and exceeds 40%. Accuracy and precision for uranium isotope ratios are shown to approach 1% over a wide range of concentrations and isotope ratios. The ionisation efficiency for plutonium is shown to exceed 3%, leading to overall detection efficiency over 1%. In the absence of background, this leads to sub-attogram detection limits for several Pu isotopes including 244Pu. Crown Copyright © 2018 Published by Elsevier B.V.
- ItemCosmogenic radionuclides as signatures of past Solar storm events(Australian National University, 2019-09-09) Smith, AM; Wilcken, KM; Simon, KJ; Dee, MW; Kuitems, M; Scifo, A; Moy, A; Curran, MAJ; Wallner, A; Fink, D; Fujioka, TThis collaborative project examines the relationship between the ‘Carrington Event’ (CE), the largest solar storm of modern times, and two recently discovered cosmic radiation events of greater magnitude, the ‘Miyake Events’ (ME). The intention is to construct cosmogenic isotope (14C, 10Be and 36Cl) profiles across the CE, so they can be compared with similar data that have already been obtained for the ME. We will use ice cores from Law Dome, East Antarctica, collected under Australian Antarctic Science awards, for the 10Be and 36Cl analyses. The large diameter DSS0506 ice core will permit high-resolution measurements at ANSTO of 10Be and 36Cl across the CE. Furthermore, we also intend to measure 10Be and 36Cl in the main DSS ice core across the ME. These measurements will complement existing data as both isotopes will be measured in the same ice core for each event for the first time and at high temporal resolution. New tree rings spanning the CE and ME, sourced from the Oxford Dendrochronology Laboratory, have been measured for 14C at the University of Groningen at mostly annual resolution. The ultimate goal of this study is to determine whether or not all three events are manifestations of the same phenomena. A secondary goal is to provide a check on the independent DSS-main ice core chronology. The CE of 1859 is known from geomagnetic data and contemporary records of the aurorae, which were observed as far south as the tropics. The event predated ground-based neutron detectors and routine cosmogenic isotope measurement, so the intensity of the incident particle radiation is still a matter of conjecture. Indeed, this question has been thrown into sharp focus recently by new discoveries in palaeoastronomy. Analyses of natural archives (tree-rings and ice-cores) have revealed that production of the cosmogenic isotopes 14C, 10Be and 36Cl spiked dramatically in the years 774-775 AD and 993-994 AD. Such anomalies could only have been generated by sudden bursts of cosmic radiation. Several sources were initially proposed for the radiation, however, the consensus now is that they were driven by solar activity. Here we discuss progress with the measurement of the cosmogenic radioisotopes and consider how the relative production rates of the cosmogenic radioisotopes may be used to substantiate a solar cause for the historical radiation events and to infer the spectral hardness of the initiating solar protons. © The Authors.
- ItemEvidence for recent interstellar 60Fe on Earth(Australian National University, 2019-09-09) Koll, D; Faestermann, T; Feige, J; Fifield, LK; Froehlich, MB; Hotchkis, MAC; Korschinek, G; Merchel, S; Panjkov, S; Pavetich, S; Tims, SG; Wallner, AOver the last 20 years the long-lived radionuclide 60Fe with a half-life of 2.6 Myr was shown to be an expedient astrophysical tracer to detect freshly synthesized stardust on Earth. The unprecedented sensitivity of Accelerator Mass Spectrometry for 60Fe at The Australian National University (ANU) and Technical University of Munich (TUM) allowed us to detect minute amounts of 60Fe in deep-sea crusts, nodules, sediments and on the Moon [1-5]. These signals, around 2-3 Myr and 6.5-9 Myr before present, were interpreted as a signature from nearby Supernovae which synthesized and ejected 60Fe into the local interstellar medium. Triggered by these findings, ANU and TUM independently analyzed recent surface material for 60Fe, deep-sea sediments and for the first time Antarctic snow, respectively [6, 7]. We find in both terrestrial archives corresponding amounts of recent 60Fe. We will present these discoveries, evaluate the origin of this recent influx and bring it into line with previously reported ancient 60Fe findings.
- ItemHigh-sensitivity isobar-free AMS measurements and reference materials for 55Fe, 68Ge and 202gPb(Elsevier B.V., 2013-01-01) Wallner, A; Bichler, M; Buczak, K; Fink, D; Forstner, O; Golser, R; Hotchkis, MAC; Klix, A; Krása, A; Kutschera, W; Lederer, C; Plompen, AJM; Priller, A; Schumann, D; Semkova, VM; Steier, PIsobaric interference represents one of the major limitations in mass spectrometry. For a few cases in AMS with tandem accelerators, isobaric interference is completely excluded like the well-known major isotopes 14C, 26Al, 129I. Additional isotopes are 55Fe (t1/2=2.74years), 68Ge (t1/2=270.9days) and 202Pb (t1/2=52.5kyr), with 68Ge and 202Pb never been used in AMS so far. Their respective stable isobars, 55Mn, 68Zn and 202Hg do not form stable negative ions. The exceptional sensitivity of AMS for 55Fe, 68Ge and 202gPb offers important insights into such different fields like nuclear astrophysics, fundamental nuclear physics and technological applications. VERA, a dedicated AMS facility is well suited for developing procedures for new and non-standard isotopes. AMS measurements at the VERA facility established low backgrounds for these radionuclides in natural samples. Limits for isotope ratios of <10−15, <10−16 and ⩽2×10−14 were measured for 55Fe/56Fe, 68Ge/70Ge and 202Pb/Pb, respectively. In order to generate accurate isotope ratios of sample materials, AMS relies on the parallel measurement of reference materials with well-known ratios. A new and highly accurate reference material for 55Fe measurements with an uncertainty of ±1.6% was produced from a certified reference solution. In case of 68Ge dedicated neutron activations produced a sufficiently large number of 68Ge atoms that allowed quantifying them through the activity of its decay product 68Ga. Finally, for 202Pb, the short-lived isobar 202Tl was produced via neutron activation and served as a proxy for 202Pb AMS measurements. © 2012 Elsevier B.V.
- ItemInvestigating the lead-210 background in lead materials and chemical reagents(South Pacific Environmental Radioactivity Association, 2022-11-29) Froehlich, MB; Hotchkis, MAC; Dastgiri, F; Fifield, LK; Koll, D; Merchel, S; Pavetich, S; Slavkovská, Z; Tims, SG; Wallner, ASABRE (Sodium iodide with Active Background REjection) is a direct detection dark matter experiment based on ultra-pure NaI(Tl) crystals. This experiment is well-shielded against external radiation and thus its background rate is driven by radioactive contaminants in the detector material and in the materials used for the construction of the experimental setup. Such radioactive contamination may come from long-lived, naturally occurring radionuclides or from cosmogenic activation. Therefore, a careful selection and development of ultra-pure materials and equipment is required, as well as a detailed knowledge of the residual radioactivity. Here, we focus on exploring the extraction of the radioisotope lead-210 (210Pb) in analytical grade NaI prior to examining Astro-grade NaI(Tl), which will eventually serve in the SABRE-South experiment as a scintillator detector for dark matter studies based in the Southern Hemisphere. We aim to measure 210Pb in NaI by accelerator mass spectrometry (a single atom counting technique), however this is challenging owing to the anticipated large mass of 1 kg. We will discuss two methods to extract Pb using different resins such as the Anion Exchange Resin (1-X8, 100-200 mesh Chloride form) and Sr® resin (100-150 mm). Furthermore, it is essential that any material and reagents in use should contain as little 210Pb as possible. For the chemical extraction of 210Pb from NaI, a stable Pb carrier is being used, which may contain traces of 210Pb as well. As 210Pb has a half-life of 22.2 years, the “older” the material (i.e., age of manufacturing and processing) the better, as most, if not all, of the 210Pb has decayed. However, 210Pb is a decay product of U, which is omnipresent in the environment. Therefore, if uranium has not been completely removed from the Pb material during processing, 210Pb will be continuously produced. Here, we will present results for a series of Pb materials together with various reagents which were measured using the 1 MV Vega accelerator at ANSTO. Their 210Pb/208Pb isotopic ratios vary between (3-30)´10-14 for the Pb carriers (0.38-173 mBq 210Pb/g) and range from 1´10-14 to 3´10-11 for the reagents (4-194 mBq 210Pb/g), respectively.
- ItemInvestigation of gas stripping at 4.1MeV for high mass negative ions(Elsevier Science BV, 2013-01-01) Hotchkis, MAC; Child, DP; Fink, D; Levchenko, VA; Wallner, AWe have used a tandem accelerator to measure charge state distributions at 4.1 MeV for negative ions ranging from carbon to uranium oxide passing through the gases helium, argon and xenon. The gas density was varied over a wide range to observe the onset of equilibrium stripping conditions. Using a 12 degrees electrostatic deflector after the accelerator, charge states from 1+ to 8+ have been measured. For carbon, the charge state is seen to strip progressively from a low charge of around 1+ towards equilibrium with increasing gas thickness. However, for all the other ions, there is a high probability of multiple electron loss in single collisions with the gas, leading to mean charge states greater than 2 even at very low gas stripper pressure. This effect is observed to be gas-independent. © 2013, Elsevier Ltd.
- ItemInvestigation of gas stripping at 4MV for high mass negative ions(GNS Science, 2011-03-21) Hotchkis, MAC; Child, DP; Fink, D; Wallner, AFor AMS analysis of actinides using the ANTARES tandem accelerator, we currently achieve an accelerator transmission of 4-5%, when injecting UO– ions and analysing the 5+ charge state at a terminal voltage of 4MV with argon stripper gas. The present study was undertaken to better understand the stripping process for such heavy ions and, if possible, find a means to achieve higher transmission and therefore improved overall efficiency. We have measured charge state distributions at 4MV terminal voltage using negative ions ranging from carbon to uranium oxide. The stripper gas density was varied over a wide range to observe the onset of equilibrium stripping conditions. Using a 12º electrostatic deflector after the accelerator, charge states from 1+ to 8+ have been measured. The results show several interesting features: (i) for heavy ions such as iodine, uranium and thorium, there is a high probability of multiple electron loss in a single collision with the argon gas, leading to the mean charge state for I, Th and U ions being greater than 2.5 even at very low gas stripper pressure; (ii) with increased stripper pressure, scattering losses dominate before equilibrium stripping conditions are achieved. Further experiments are underway using helium and xenon stripper gases. Helium is of particular interest, as in earlier studies [1] performed externally to an accelerator, helium was shown to give the highest equilibrium charge state for uranium at low energy. (c) 2011 AMS12
- ItemLead-210: a contaminant in particle detectors for dark matter studies(Australian Nuclear Science and Technology Organisation, 2021-11-17) Froehlich, MB; Merchel, S; Slavkovská, Z; Dastgiri, F; Fifield, LK; Hotchkis, MAC; Koll, D; Pavetich, S; Tims, SG; Wallner, AThe DAMA/LIBRA (DArk Matter/Large sodium Iodide Block for RAre processes) is a very low background NaI(Tl) detector array that has been running for two decades in the Gran Sasso underground laboratory in Italy. It gives a robust annual modulation signal in the 2 to 6 keV region that may be due to dark matter [1]. In order to verify this result with higher sensitivity, the SABRE (Sodium iodide with Active Background REjection) experiment [2] is being developed. Radioimpurities such as ⁴ ⁰ K, ²³⁸ U, ²¹⁰ Pb and ²³²Th, either intrinsic to the detector material or surface contamination, provide a fundamental limit to the sensitivity of SABRE. Therefore, it is crucial to characterise this background for improved identification of any additional signal above it. Here, we focus on ²¹⁰ Pb (half-life of 22.2 years) as its beta decay to ²¹⁰ Bi contributes to the low-energy “dark matter” spectra [3]. Lead-210 measurements are usually performed using alpha -, beta - or gamma counting depending on the sample size and concentration [4]. However, in recent years, the interest and therefore developments to measure ²¹⁰ Pb using accelerator mass spectrometry (AMS) has increased [5], [6]. From a chemical point of view, we need to optimise the Pb extraction of ~1 mg of stable Pb carrier through precipitations and ion exchange chromatography using about a kilogram of NaI. This is not trivial and methods using two different resins, i.e., 1x8 anion exchange resin and Sr® resin, have been tested. It is also essential that the stable Pb carrier and any material and chemical product in use should contain as little ²¹⁰ Pb as possible. Hence, several materials have been investigated including a piece from a 16th century roof and radiation shielding blocks as a source of Pb carrier. Furthermore, we studied PbO and PbF₂ samples to identify the optimal negative-ion beam and the suitability of using either Fe₂ O₃ or NaF as bulk material for the AMS target to reduce the stable Pb content. AMS measurements related to this work have been made using the 14UD pelletron accelerator at the Australian National University and the 1 MV VEGA accelerator at the Australian Nuclear Science and Technology Organisation.
- ItemNatural and anthropogenic 236U in environmental samples(Elsevier, 2008-03-06) Steier, P; Bichler, M; Fifield, LK; Golser, R; Kutschera, W; Priller, A; Quinto, F; Richter, S; Srncik, M; Terrasi, P; Wacker, L; Wallner, A; Wilcken, KM; Wild, EMThe interaction of thermal neutrons with 235U results in fission with a probability of ∼85% and in the formation of 236U (t1/2 = 2.3 × 107 yr) with a probability of ∼15%. While anthropogenic 236U is, therefore, present in spent nuclear fuel at levels of 236U/U up to 10−2, the expected natural ratios in the pre-anthropogenic environment range from 10−14 to 10−10. At VERA, systematic investigations suggest a detection limit below 236U/U = 5 × 10−12 for samples of 0.5 mg U, while chemistry blanks of ∼2 × 107 atoms 236U per sample limit the sensitivity for smaller samples. We have found natural isotopic ratios in uranium reagents separated before the onset of human nuclear activities, in uranium ores from various origins and in water from a subsurface well in Bad Gastein, Austria. Anthropogenic contamination was clearly visible in soil and rivulet samples from Salzburg, Austria, whereas river sediments from Garigliano river (Southern Italy) were close to the detection limit. Finally, our natural in-house standard Vienna-KkU was calibrated against a certified reference material (IRMM REIMEP-18 A). © 2008 Elsevier B.V.
- ItemNegative ionisation efficiencies for 10Be, 26Al and Pu with MCSNICS at ANSTO(Australian Nuclear Science and Technology Organisation, 2021-11-17) Wilcken, KM; Child, DP; Hotchkis, MAC; Mann, M; Simon, KJ; Koll, D; Wallner, A; Hauser, T; Kitchen, RLow overall detection efficiency for actinides and cosmogenic isotopes (Al, Be) is the limiting factor affecting precision and sensitivity for applications where the amount of available sample material is small and/or rare isotope concentration is low. Due to low ionisation efficiencies for these isotopes it is not uncommon that more than 99% of the rare isotopes in the sample do not contribute to the statistical precision of the measurement. Optimising ion transmission and detection efficiency in the AMS measurement offers some room for improvement but these avenues are already close to their theoretical limits. On the other hand, optimising the performance and operation of the negative ion Cs-sputter sources has significant scope for improvement but is challenging. One often needs to compromise between competing requirements, for example, maintaining high sputtering rate to allow expedient consumption of the sample material but at the same time keeping the source insulators clean for longevity. The lack of a well-understood theoretical model for the negative ionisation process adds to the engineering challenges. Negative ionisation efficiencies above 30% have been demonstrated for radiocarbon [1] but remain often more than an order of magnitude lower for Be, Al and actinides. This is sometimes taken to be an inherent limitation of the technique, rather than a challenge to be addressed. Here we present details of the modified MC-SNICS sources at ANSTO, including engineering modifications that have improved longevity and stability. With attention to a combination of ion source running conditions, sample masses and sample binders the total efficiency for Pu measurements was increased up to 1.5%, corresponding to a negative ionisation yield of 4%. For Aland BeO- negative ion source yields are 0.2% and 3%, respectively.
- ItemNew exotic and non-standard radionuclides in AMS(GNS Science, 2011-03-21) Wallner, A; Buczak, K; Plompen, A; Schumann, D; Semkova, VIsobaric interference represents one of the major limitations in mass spectrometry. In the very few cases in AMS where nature allows isobaric-free measurements, lowest background levels are accessible. Such conditions are given, if the isobar does not form stable negative ions either as atomic ion, or by choosing a suitable molecular species; well-known examples are e.g. 14C−, 26Al−, 129I−, or 41CaH3 −, respectively, and also for the unstable isotopes in the mass range above Bi. In this work, we will present a few additional cases where isobaric interference is completely excluded, among them 55Fe, 68Ge and 202Pb. Applications will be discussed where the exceptional sensitivity of AMS offers important insights to such different fields like nuclear astrophysics, nuclear physics and general physics issues. VERA, a dedicated AMS facility, based on a 3-MV tandem, featuring high mass resolution in combination with efficient background suppression and an automated measurement procedure, allows to transport all nuclides from hydrogen to the actinides through the system up to the detector stations. Such a facility is well suited for developing the tuning and measurement procedures for new and non-standard isotopes. We will demonstrate the actual measurement limits of such radionuclides for the VERA facility and results for selected applications. In order to generate final values AMS usually relies on the parallel measurement of reference materials. We will discuss the production of such materials for these non-standard AMS nuclides.Copyright (c) 2011 AMS12.
- ItemA novel approach for neutron-capture studies of 235U and 238U(GNS Science, 2011-03-25) Wallner, A; Belgya, T; Bichler, M; Buczak, K; Dillmann, I; Käppeler, F; Mengoni, A; Quinto, F; Steier, P; Szentmiklósi, LImproved and highly accurate nuclear data are urgently required for the design of advanced reactor concepts. This demand holds for minor actinides but also for the main fuel materials. Existing data were measured by detection of the prompt capture γ-rays. A major difficulty in these experiments is the safe discrimination against the strong γ-background from the competing fission channel. Recent studies exhibit critical discrepancies at keV energies for both, 235U(n,γ) and 238U(n,γ) with great impact on the keff-value of fission reactors. Neutron activation with subsequent accelerator mass spectrometry (AMS) measurement of the reaction product represents an independent technique, where interference from fission is completely excluded. Within the European EFNUDAT project, new measurements were performed with the goal to determine the neutron capture cross sections of 235U and 238U via neutron irradiations at thermal (cold) and keV energies: Activations were performed with cold neutrons (Budapest Research Reactor), thermal (Atominstitut, Vienna) and with neutrons of 25 and 450 keV (Karlsruhe Institute of Technology). The produced long-lived 236U and the decay product of 239U, 239Pu were subsequently counted by AMS at the Vienna Environmental Research Accelerator (VERA). This method for measuring the neutron capture cross section has the advantages that the involved systematic uncertainties are in no way correlated with those inherent to previous techniques. Therefore, this experiment provides important and independent information for these key reactions of reactor physics with uncertainties expected below 5%. The high sensitivity of AMS requires only very small samples. New results for 235U(n,γ)236U and 238U(n,γ)239U in the energy range from thermal to 500 keV will be presented. The challenges of measuring 236U/238U isotope ratios at the 10-12 level and to quantify 239Pu with high precision will be highlighted. Finally, the potential for extending that method to other isotopes in that mass range will be discussed. Copyright (c) 2011 AMS12
- ItemRadio-impurity measurements for a dark matter dodium Iidide detector(Australian Nuclear Science and Technology Organisation, 2021-11-17) Dastgiri, F; Slavkovska, Z; Froehlich, MB; Hotchkis, MAC; Koll, D; Merchel, S; Pavetich, S; Sims, SG; Fifield, LK; Wallner, AThe first dark matter detector is being built in the Stawell gold mine in south-eastern Australia, as the southern hemisphere arm of an international collaboration SABRE (Sodium Iodide with Active Background Rejection). This experiment employs ultra-low background sodium iodide (NaI) detectors placed in highly shielded vessels across both hemispheres. The aim is to confirm or refute annual modulation claims attributed to dark matter particles by the DAMA/LIBRA collaboration at the Laboratori Nazionali del Gran Sasso in Italy. This requires the lowest possible concentration of radio-contaminants that can be achieved, to minimise the potential for radiation signals that can mimic dark matter particles signals. We report on the techniques employed for the detection of potentially problematic contaminants in the NaI material from which the crystals will be grown. We focus on the establishment of the measurement techniques of ⁴ ⁰ K and ²¹⁰ Pb at the Australian National University and ANSTO. For the measurement of ⁴ ⁰ K, Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was used to measure the concentration of ³⁹ K, and from the well-known natural abundance ratios of ³⁹ K/⁴ ⁰ K, the concentration of ⁴ ⁰ K was inferred. The challenges associated with measuring ultraprecise levels of ³⁹ K, and the techniques of minimising the introduction of potassium in the sample preparation will be discussed. 210-Lead was measured using AMS. The ²¹⁰ Pb concentration in the NaI powder is very low, which necessitates that large amounts (~ 1kg) of the powder need to be processed to result in sufficient atoms for an AMS measurement. This low concentration requires the additions of a Pb-carrier (~ 1mg), which itself needs to contain minimal ²¹⁰ Pb. Several lead materials have been investigated and will be reported. In addition, we will discuss the different lead compounds and cathode materials used to optimise the beam current and minimise the background. Other contaminants of potential interest such as ³H, ²³²Th and ²³⁸ U; especially those identified in DAMA/LIBRA and other NaI detectors will be presented.
- ItemReproducibility and accuracy of actinide AMS – lessons learned from precision studies for nuclear data(Australian Nuclear Science and Technology Organisation, 2021-11-17) Wallner, A; Christl, M; Hotchkis, MAC; Lippold, J; Froehlich, MB; Fifield, LK; Steier, P; Tims, SG; Winkler, SRActinide detection has grown into an important discipline for environmental and geological sciences, for oceanography, e.g. as monitors of anthropogenic activities, but also in nuclear (astro)physics. Consequently, AMS measurements of actinides have become routine at many facilities. In particular, applications in nuclear (astro)physics continue to challenge the present limits in accuracy and abundance sensitivity of actinide detection. Presently, there is a major ongoing effort in experiment and theory to better understand cross sections at thermal and higher neutron energies. These activities are motivated by the urgent need for improved and highly accurate nuclear data for optimised designs of advanced reactor concepts, nuclear fusion reactors, or next generation nuclear power plants (Gen IV) and accelerator driven systems (ADS). One example is the cross-section value for 235U neutron-capture at thermal energies: serving as a so-called thermal constant, this quantity is believed to be known to better than 1%. Despite its importance, direct measurements are rare (only two older data exist for thermal energies) and exhibit large uncertainties, thus its knowledge is based on indirect information. For these applications, accurate actinide data are required, e.g. with uncertainties better than 2-3% for capture reactions. The combination of activation and subsequent AMS detection offers a powerful and complementary tool to measure these cross sections. However, this method had been applied only very recently for measurements on actinides. Importantly, adding an independent technique to established methods helps also to identify unrecognized systematic uncertainties in the existing nuclear database. Several uranium and thorium samples had been irradiated with neutrons of energies between sub-thermal and 22 MeV at seven different neutron-producing facilities. These samples were then analysed at different AMS facilities: at the Vienna Environmental Research Accelerator (VERA), at ANSTO’s ANTARES, at ETH’s TANDY and at HIAF (ANU). These facilities cover terminal voltages for actinide AMS between 0.3 and 4 MV. We present systematic investigations of nuclear data from a series of neutron-irradiated samples that were obtained by AMS. Long-lived reaction products that were measured include Th-229, Pa-231,233, U-233,236 and various Pu isotopes. Some irradiated samples were directly pressed into sample holders. Some samples were dissolved and spiked with well-known amounts of one or more reference isotopes, relative to which the radionuclides were quantified. To achieve the highest accuracy, we compared the results from repeated measurements at the different facilities. We also had to take into account the measurement reproducibility of the individual facilities; an uncertainty component that represents unknown uncertainties beyond counting statistics and other known systematic uncertainties. A comparison of these data provides the present limits in the measurement accuracy of heavy-ion AMS. © The Authors
- ItemSample preparation for AMS astrophysics projects – size does (not) matter(Australian National University, 2019-09-09) Merchel, S; Child, DP; Faestermann, T; Fröhlich, M; Gosler, R; Hotchkis, MAC; Koll, D; Korschinek, G; Pavetich, S; Wallner, AThe determination of long-lived radionuclides by means of accelerator mass spectrometry (AMS) is usually outstandingly successful when an interdisciplinary team comes together. The “heart” of AMS research is of course an accelerator equipped with sophisticated ion sources, analytical tools and detectors run by experienced and ambitious physicists. Setting-up and further developing AMS systems is one of the most interesting and challenging topics. The reputation to be reached here is the greatest uniqueness of analysis possible, lowest detection levels, and/or most reliable data world-wide. For sure, another primary pillar of AMS research is based on the questions addressed within fundamental and applied research. “How have supernovae explosions influenced Earth, our solar system and beyond?” or “How does the Earth’s surface and environment respond to earthquakes, climate change and anthropogenic influences?” are just two examples of high-quality studies. However, somehow in-between there are groups of hidden figures like people developing software for data analysis or performing the required chemical sample preparation for AMS. These often unacknowledged individuals do crucial work for the overall outcome of the studies. Chemists can spend weeks and months trying (and failing) on sample preparation before they find a “safe way” and start the actual work on the most valuable sample material, repeat all over again the same “recipe” for hundreds of samples, or train non-chemists the secrets of their successful recipes. Nevertheless, interdisciplinary AMS work can also be very exciting for a chemist: touching (and destroying) samples from outer space, the deep ocean or (currently) frozen places like Antarctica is quite thrilling. But at the end of the day, the whole AMS chemist’s work can be described as “reducing the sample matrix, other impurities and especially isobars to a level the AMS machine can handle while enriching the radionuclide of interest”. Starting materials for applications such as astrophysical research can be “orders of magnitude” different: a neutron-irradiated sample of 1 g tungsten powder, over 40 g of clay-rich material from the Cretaceous–Tertiary (K-T) boundary, 100 g of ultra-pure sodium iodide, or 500 kg of snow from Antarctica can cause totally different and sometimes unexpected problems in the chemistry lab. In general, smaller samples are not always easier to handle for example if they are chemically rather resistant or reactive. The cream of the crop of failure and success in a few AMS chemistry labs will be presented.
- ItemSm-146 – feasibility studies to re-date the chronology of the early solar system(Australian Nuclear Science and Technology Organisation, 2021-11-17) Pavetich, S; Fifield, LK; Froehlich, MB; Koll, D; Slavkovská, Z; Stopic, A; Tims, SG; Wallner, AAMS measurements of long-lived radionuclides can make significant contributions to the understanding of the temporal evolution of our early solar system. Samarium-146 has a half-life in the order of 100 Myr and decays via emission of α-particles into stable ¹⁴ ²Nd. Due to different geochemical behaviour and the radioactive decay of ¹⁴ ⁶ Sm, the Sm-Nd isotopic system can serve as a chronometer for the early solar system and planetary formation processes. The half-life of ¹⁴ ⁶ Sm, which provides the time scale for this clock, is in dispute. The most recent and notably precise measurements for the half-life are (103±5) Myr (adopted from [1,2]) and (68±7) Myr [3] and differ by more than 5 standard deviations. In addition to potentially resolving this discrepancy, developing AMS for ¹⁴ ⁶ Sm might provide the means to study stellar nucleosynthesis on the proton rich side of the chart of nuclei and serve as radiometric tracer for geosciences. Due to the extremely challenging task of separating ¹⁴ ⁶ Sm from its stable isobar ¹⁴ ⁶ Nd, to date the only AMS measurement of ¹⁴ ⁶ Sm was performed at Argonne National Laboratory with energies in the order of ~880 MeV. At the Heavy Ion Accelerator Facility at ANU, the possibility to measure ¹⁴ ⁶ Sm at energies of 200-250 MeV is being explored. Different sample materials, molecular negative ion beams and detector setups are investigated. So far, the lowest Nd backgrounds, from commercially available sample material without additional Nd separation were achieved using SmO₂ - beams extracted from Sm₂ O₃ samples. In order to explore the limits of the Sm detection capabilities, Sm₂ O₃ samples were irradiated with thermal neutrons in the reactor at ANSTO to produce the shorter lived ¹⁴ ⁵ Sm (t1/2 = (340±3) d [4]) via ¹⁴ ⁴ Sm(n,γ)¹⁴ ⁵ Sm. The production of ¹⁴ ⁵ Sm is easier and faster and the challenges in measuring ¹⁴ ⁵ Sm via AMS are very similar to those measuring ¹⁴ ⁶ Sm. In addition, ¹⁴ ⁵ Sm has the potential to serve as a tracer for future reference materials for AMS measurements of Sm.
- ItemStable platinum isotope measurements in presolar nanodiamonds by TEAMS(Elsevier, 2013-01-01) Wallner, A; Melber, K; Merchel, S; Otte, U; Forstner, O; Gosler, R; Kutschera, W; Priller, A; Steier, PNanodiamonds are stardust grains commonly found in primitive meteorites. They survived the formation of the solar system and kept their own individuality. Measurements of trace-element isotopic signatures in these grains will help understanding heavy element nucleosynthesis in massive stars and dust formation from their ejecta. We have continued previous attempts to search for stable Pt isotope anomalies in nanodiamonds via trace element accelerator mass spectrometry (TEAMS). The installation of a new injector beam line at the VERA facility allowed studying low traces of stable elements in different materials. Moreover, recent experiments showed that VERA provides the required measurement precision together with a low Pt machine background. Here, we observed for the first time an indication for enhancements of 198Pt/195Pt isotope ratios in two diamond residues prepared by different chemical separation techniques from the Allende meteorite. Variations in other isotopic ratios were within analytical uncertainty, and no anomaly was identified in a third diamond fraction. © 2012, Elsevier B.V.
- ItemTime-resolved interstellar Pu-244 and Fe-60 Ppofiles in a Be- 10 dated ferromanganese crust(Australian Nuclear Science and Technology Organisation, 2021-11) Koll, D; Wallner, A; Hotchkis, MAC; Child, DP; Fifield, LK; Froehlich, MB; Harnett, M; Lachner, J; Merchel, S; Pavetich, S; Rugel, G; Slavkovska, Z; Tims, SGMore than 20 years have passed since the first attempts to find live supernova Fe-60 (t1/2 = 2.6 Myr) in a deep-sea ferromanganese crust [1]. Within these 20 years, strong evidence was presented for a global influx of supernova dust into several geological samples around 2 Myr ago. Recently, a much younger continuous influx was found in Antarctic snow and in deep-sea sediments [2-4] and an older peak around 7 Myr in deep-sea crusts [5,6]. The long-lived isotope Pu-244 (t1/2 = 80 Myr) is produced in the astrophysical r-process similarly to most of the heaviest elements. Although the production mechanism is believed to be understood, the astrophysical site is heavily disputed. Most likely scenarios involve a combination of rare supernovae and neutron star mergers. The search for Pu-244 signatures in samples with known Fe-60 signatures allows to test for either common influx patterns or independent Pu-244 influxes disentangled from stellar Fe-60. Accordingly, this information provides a unique and direct experimental approach for identifying the production site of the heavy elements. Very recently and first reported in the AMS-14 conference, the first detection of interstellar Pu-244 was published [6]. This was only feasible by achieving the highest detection efficiencies for plutonium in AMS ever reported [7]. The achieved time resolution of 4.5 Myr integrates over the supernova influxes and is therefore not high enough to unequivocally show a correlated influx pattern of Fe-60 and Pu-244. Based on this progress, we are now aiming to measure highly time-resolved profiles of Fe-60 and Pu-244 in the largest ferromanganese crust used so far. Results on the characterization of the crust including cosmogenic Be-10 (t1/2 = 1.4 Myr) dating and a 10 Myr profile of interstellar Fe-60 including the confirmation of the 7 Myr influx will be presented along with first data on interstellar Pu-244.