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Abstract Three state-of-the-art ionization chambers have recently been designed at the Australian National University (ANU) to further enhance the isotopic and isobaric separation capabilities of the AMS setup at the Heavy Ion Accelerator Facility (HIAF). Two compact split-anode ionization chambers with low noise preamplifiers allow for measurements of actinides and other radioisotopes that do not have atomic isobar interferences. Both assemblies sit inside Dependex-100-Tees and are constructed such that they are retractable from the ion beam under vacuum. A high energy resolution version will be used for Pu, 129I and 55Fe. The second detector serves as the final energy detector in a time-of-flight setup and is employed for 236U and 210Pb detection. The new Flexible-AntiScattering Multi-Anode (FASMA) detector is a 7-anode ionization chamber optimized for the discrimination of 53Mn from 53Cr in the gas-filled magnet setup with the ENGE-split-pole spectrograph. The entire setup was simulated with Raytrace and SRIM. Finally, an existing 8-anode ionization chamber has been upgraded and now provides a 93Zr-93Nb isobar suppression of 5 × 104 resulting in the best limit of detection for 93Zr of any AMS-facility. This article details the various features of these detectors and compares first experimental results to performance values expected from modelling. [ABSTRACT FROM AUTHOR]

Abstract: Sediment core chronologies of optical dates on single-grains/very small aliquots of sand-sized quartz are compared with Accelerator Mass Spectrometry (AMS) radiocarbon (14C) chronologies from ostracod carbonate, mixed carbonates, sedimentary organic matter and charcoal in order to establish the age of laminated Holocene sediments in maar crater lakes Keilambete and Gnotuk, Victoria, Australia. Samples for optical and AMS 14C dating were taken from the same Mackereth cores, allowing a direct comparison of the two techniques from two laminated sedimentary sequences. Additional AMS 14C samples were taken from water in Lake Keilambete and from groundwater discharging into Lake Keilambete from the crater wall, with equivalent reservoir ages of 150 ± 30 and 1940 ± 30 years respectively. AMS 14C dating of modern ostracod carbonate in Lake Keilambete demonstrates a reservoir age of 670 ± 175 years. Optical dating of ‘single-grain/very small aliquots’ of sand-sized-quartz indicate the presence of a radiocarbon reservoir in Lake Keilambete that is consistent with that measured on modern ostracods, and also demonstrate that there is no 14C reservoir in Lake Gnotuk during the Holocene. The chronology presented here supports the premise that previously published bulk conventional 14C dates from Lake Keilambete were affected by old carbon, meaning that past chronologies require revision. Limitations on the use of optical dating of single-grain/very small aliquots include the relative paucity of sand-sized quartz, which decreases the precision of the sample equivalent dose (D e), and is further confounded by low environmental dose rates and resultant large uncertainties on the final age assessment. Nevertheless, evidence for partial bleaching confirms that single-grain quartz dating is the most appropriate luminescence technique, and may prove a useful alternative in situations where 14C dating is unsuitable or an alternative chronometer is required. [Copyright &y& Elsevier]

Abstract: Silicon-32, with a half-life of approximately 140 years, has the potential to fill the dating gap that lies between those chronologies based on the shorter-lived isotopes of 3H and 210Pb, and those based on the longer-lived 14C. Silicon-32 is produced in the atmosphere by cosmic ray bombardment of argon, and falls out on the Earth''s surface in precipitation. Silicon-32 methods may be used to date siliceous sediments and sponges, groundwater and glacier ice. Measurement of 32Si concentrations in these archives is, however, not straightforward. Two methods are available: radioactive-decay counting of the activity of the daughter nucleus, 32P, and accelerator mass spectrometry, but in both cases the detection of 32Si pushes the boundaries of the technique. Even the half-life of 32Si is not known to a precision of better than ±10%. In this paper, we review efforts to determine the isotope''s half-life, survey the detection methods and discuss the applications of 32Si chronology. We show that at least some of the chronometric potential of this radioisotope is close to being realised as a result of recent improvements in methods of measurement. [Copyright &y& Elsevier]

Abstract: It is critical that a comprehensive terrestrial radiocarbon (14C) calibration curve is developed beyond 26ka for high-precision calibration and correlation of climatic, environmental and archaeological records. Abundant sub-fossil New Zealand kauri (Agathis australis) wood, preserved in Oxygen Isotope Stage-2 and 3 peat swamps, provides an important resource for 14C calibration; nowhere else in the world does such an extensive collection of ancient wood exist. Although finite 14C ages beyond 50ka are becoming routinely reported, few attempts have been made to demonstrate their accuracy or precision. Finite ages beyond 50ka require optimization of all elements involved in sample preparation and 14C analysis. Here we discuss the methodology employed for optimizing the 14C dating of near-background wood samples by both benzene synthesis for liquid scintillation counting (LSC) or graphite synthesis for accelerator mass spectrometry (AMS). We report the mean background blank activities for both methods and present a statistical model for assigning blank standard errors when blank activity variation is over and above counting statistics. We also present duplicate analyses (using LSC and AMS) of nine successive samples of wood obtained from a sub-fossil kauri log near-background in age to investigate the significance of the measured blank levels and variability. [Copyright &y& Elsevier]

Background: Soluble silica, a ubiquitous component of the diet, may be the natural ligand for dietary aluminum and may prevent its accumulation and toxicity in animals. However, previous studies on the inhibition of aluminum absorption and toxicity by soluble silica have produced conflicting results. We recently identified a soluble silica polymer, oligomeric silica, that has a much higher affinity for aluminum than does monomeric silica and that may be involved in the sequestration of aluminum. Objective: By using 26Al as a tracer, we investigated the effects of oligomeric and monomeric silica on the bioavailability of aluminum (study 1) and compared the availability of silicon from oligomeric and monomeric silica in the human gastrointestinal tract (study 2). Design: In study 1, three healthy volunteers each ingested aluminum alone (control), aluminum with oligomeric silica (17 mg), and aluminum with monomeric silica (17 mg). In study 2, five healthy volunteers ingested both the oligomeric and monomeric forms of silica (34 mg). Serum and urine samples were analyzed for aluminum and silicon. Results: Oligomeric silica reduced the availability of aluminum by 67% (P = 0.01) compared with the control, whereas monomeric silica had no effect (P = 0.40). Monomeric silica was readily taken up from the gastrointestinal tract and then excreted in urine (53%), whereas oligomeric silica was not detectably absorbed or excreted. Conclusions: The oligomeric, high-aluminum-affinity form of soluble silica reduces aluminum availability from the human gastrointestinal tract. Its potential role in the amelioration of aluminum toxicity in other biological systems requires attention. [ABSTRACT FROM AUTHOR]

The University of Wollongong (UOW) cosmogenic 10Be and 26Al sample preparation laboratory has been in operation since the start of 2017. As primarily a feeder laboratory to ANSTO's Centre for Accelerator Science, our sample preparation procedures have been optimised with consideration to the setup of ANSTO's 6MV SIRIUS accelerator, and aim to achieve a balance between sample throughput as well as Be and Al target purity. A comparatively small number of samples (n = 68) have also been prepared for measurement at the Australian National University 14UD accelerator. 10Be/9Be ratios of procedural blanks measured on SIRIUS have fluctuated over time with the median for the year 2017 being 5.16 × 10−16 (IQR = 4.11 × 10−16 to 7.16 × 10−16, n = 18), increasing to 1.62 × 10−15 (IQR = 1.05 × 10−15 to 2.17 × 10−15, n = 31) for 2018 and 2019, a period coinciding with elevated boron levels in our samples, and finally decreasing to 1.15 × 10−15 (IQR = 8.63 × 10−16 to 1.60 × 10−15, n = 34) for 2020 and 2021. In contrast, 26Al/27Al ratios of procedural blanks measured on SIRIUS have shown a slight but continuous improvement over time with the median for 2017 of 1.70 × 10−15 (IQR = 8.19 × 10−16 to 5.25 × 10−15, n = 12) decreasing to 1.07 × 10−15 (IQR = 8.5 × 10−16 to 1.53 × 10−15, n = 13) for 2021. Median 10Be/9Be relative uncertainty of procedural blanks analysed on SIRIUS is 18 % (IQR = 15 % to 22 %; n = 86) whereas the median 26Al/27Al relative uncertainty of procedural blanks is higher at 60 % (IQR = 41 % to 100 %; n = 56), statistic resulting from most blanks yielding low 26Al counts (median = 2; IQR = 1–4.5). Average 9BeO− output relative to standard is between ∼ 70 % – 80 % for samples analysed on SIRIUS (n = 895) and ∼ 130 % for samples analysed at ANU (n = 68). 27Al− output relative to standard is lower for samples pressed into cathodes at UOW (∼60 %, n = 432) and analysed on SIRIUS than for those pressed at ANSTO (∼90 %, n = 119). Average 27Al− output relative to standard for samples analysed at ANU is ∼ 80 % (n = 62). 10Be and 26Al measurements of various laboratory intercomparison materials prepared at UOW between 2017 and 2022 yield results in agreement with consensus values confirming that our chemistry procedures are robust and in line with those elsewhere. [ABSTRACT FROM AUTHOR]

In nature, primordial 236 U has long since decayed to concentrations below detection. However, measurement of 236 U produced in-situ by neutron capture on 235 U in high-grade uranium deposits is made possible by recent advances in accelerator mass spectrometry (AMS). The detection of appreciable quantities of 236 U in groundwaters may reflect local uranium mineralisation, and thus prove useful in uranium exploration and potential age and ore grade estimations. Nine mineralised sediments from the South Australian Beverley North sandstone-hosted uranium deposits have 236 U/ 238 U ratios ranging from (1.57 ± 0.43) × 10 − 12 to (9.09 ± 0.55) × 10 − 12 , and U concentrations that vary by almost three orders of magnitude, ranging from 78.9 to 24,200 μg/g. Overall, the samples with the highest [U] have higher 236 U/ 238 U ratios, consistent with the generation of higher neutron fluxes with one notable exception with anomalously high [U] and a relatively low 236 U/ 238 U ratio. The observed variability in the 236 U/ 238 U ratio both within the deposits themselves, and between deposits may reflect heterogeneous mineralogy, elemental composition and water contents, which can affect the neutron flux generated within the samples. A single groundwater sampled within mineralisation from the Pepegoona West deposit yielded a 236 U/ 238 U ratio of (6.57 ± 2.97) × 10 − 12 . This is the first published data detecting natural, non-anthropogenic 236 U in groundwater in contact with a uranium deposit. The 236 U/ 238 U isotopic composition of the single groundwater sample is indistinguishable from that of the mineralised sediments from the same deposit. This is interpreted to reflect isotopic equilibration between the mineralisation and groundwater, rather than the in-situ production of 236 U by neutron capture on dissolved 235 U in the waters due to the low [U] typical of these highly reducing groundwaters. 236 U appears to have limited mobility in the Pepegoona West groundwater system, as evidenced by the lack of signature in groundwaters sampled from nearby wells in low-grade and un-mineralised portions of the deposit. This suggests that the detection of 236 U in the highly reducing groundwaters prevalent in this area may not be applicable as a proxy for uranium mineralisation. However, use of this technique as a potential exploration tool may have greater success in other areas with different hydrogeological conditions, specifically where the groundwaters are oxidising and uranium has a greater solubility as U(VI) complexes. [ABSTRACT FROM AUTHOR]

Zirconium-93 is a long-lived radionuclide with a half-life of (1.61 ± 0.05) × 106 yr. Production cross sections for 93Zr by neutron capture on stable 92Zr in the keV and thermal energy ranges are important input parameters for astrophysical network calculations and nuclear industry, respectively. Despite their importance, existing experimental data suffer from large uncertainties. Here, the combination of neutron activation and accelerator mass spectrometry (AMS) is presented as an alternative method to online time-of-flight measurements for the determination of these cross sections. The main challenges for AMS of 93Zr are the interference from the stable isobar 93Nb and the production of suitable reference material. At the Heavy Ion Accelerator Facility (HIAF) the first challenge is tackled with the available high particle energies and by investigating different Zr compounds and extracted molecular beams. Using ZrF 5 − beams extracted from ZrF 4 sample material, it is shown that the Nb background can be reduced by up to two orders of magnitude relative to the extraction of ZrO− beams from ZrO 2. Using the 13+ charge state and particle energies of ∼190 MeV, 93Zr/Zr background levels in the 10−12 range are regularly achieved at HIAF. The ZrF 5 − output from ZrF 4 samples may be highly variable but can be stabilised using an intimate mixture of ZrF 4 and PbF 2 at a ratio of 1:10 that has been dried down from a concentrated hydrofluoric acid solution. An option to produce a well-characterised 93Zr reference material by utilising the well-known 235U-fission yield of 93Zr is discussed. Once this remaining challenge is resolved, accurate measurements of the astrophysically and technologically relevant neutron-capture cross sections of 92Zr using AMS will be possible at HIAF. [ABSTRACT FROM AUTHOR]

Generally low levels of plutonium in environmental samples, often combined with limited sample sizes, necessitate reliable low-level techniques for determination of Pu isotopes. Accelerator mass spectrometry (AMS) has proved to be a powerful method for measuring low-level Pu activity concentrations and Pu isotope ratios. Based on procedural blanks, detection limits for AMS were below 1 fg Pu (equivalent to ca. 2 μBq 139Pu), which can compete with both TIMS, high sensitivity ICP-MS, and certainly alpha-spectrometry, while showing less interference, memory and matrix effects as compared to routine ICP-MS techniques. In addition to low detection limits, the technique offers the advantage of giving information on Pu isotope ratios. Measurements of sediments collected from dumping sites at Novaya Zemlya showed deviation from global fallout 240Pu/239Pu ratios. [Copyright &y& Elsevier]

We searched for the presence of 26Al in deep-sea sediments as a signature of supernova influx. Our data show an exponential dependence of 26Al with the sample age that is fully compatible with radioactive decay of terrigenic 26Al. The same set of samples demonstrated a clear supernova 60Fe signal between 1.7 and 3.2 Myr ago. Combining our 26Al data with the recently reported 60Fe data results in a lower limit of 0.18+0.15-0.08 for the local interstellar 60Fe/26Al isotope ratio. It compares to most of the ratios deduced from nucleosynthesis models and is within the range of the observed average galactic 60Fe/26Al flux ratio of (0.15±0.05). [ABSTRACT FROM AUTHOR]