Browsing by Author "Tims, SG"
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- 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.
- 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.
- 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.
- 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
- 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.
- 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.