Browsing by Author "Koll, D"

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    Achieving 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, D
    The 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.
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    Evidence 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, A
    Over 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.
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    Investigating 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, A
    SABRE (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.
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    Lead-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, A
    The 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.
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    Negative 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, R
    Low 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.
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    Radio-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, A
    The 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.
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    Sample 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, A
    The 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.
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    Sm-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, A
    AMS 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.
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    Time-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, SG
    More 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.