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The new multi-purpose 1-MV AMS facility HAMSTER (Helmholtz Accelerator Mass Spectrometer for Tracing Environmental Radionuclides) is built within the HZDR research campus in Dresden- Rossendorf starting in 2022. The new machine is especially dedicated to the analysis of ultra-trace levels of actinides in environmental samples. Therefore, eventual contamination of the site where the new accelerator building is being constructed should be avoided and clarified. Hence, several soil samples close to the construction site of the new accelerator building have been analyzed to assess the content and isotopic ratios of the actinides U, Np and Pu. The samples have been processed in the existing chemistry labs of HZDR’s 6-MV DREAMS facility showing low background levels. Overall, the samples show expected signatures of global fallout in Pu concentrations and APu/239Pu ratios. However, in some samples increased 236U concentrations and relatively low 233U/236U atomic ratios have been detected pointing to an additional source of 236U. Additional analysis is currently ongoing.

Accelerator Mass Spectrometry (AMS) is an ultra-trace analytical technique capable of measuring radionuclides down to ppq levels (i. e. fg/g) and even below. Thus, AMS is especially competitive to classical radioanalytical techniques, such as α, or γ-spectrometry, for nuclides with half-lives > 10 years. Amongst these radionuclides, actinides and their fission products are of major concern, as they possess a serious health risk to humans due to their radioactivity in combination with relatively long half-lives. These nuclides have been released to the environment by humanity’s extensive use of actinides for civil (e.g. nuclear power plants) and military purposes. Obviously, monitoring these radionuclides is crucial for safety assessments and for understanding their behaviour in the environment. The new AMS facility HAMSTER (Helmholtz Accelerator Mass Spectrometer Tracing Environmental Radionuclides) at HZDR will be especially dedicated to the analysis of actinides in ultra-trace levels. Due to the long lasting history of the research campus Dresden-Rossendorf as the major site for nuclear research in the former GDR a contamination of the construction site cannot be excluded at least on the ultra-trace level relevant for various AMS applications. Thus, soil samples on HZDR‘s research campus have been collected and analysed regarding their 233/236U, 239/240/241/244Pu and 241Am content and isotopic ratios in order to assess possible contamination of the new accelerator building. Special attention is paid to isotopic signatures deviating from the expected global fallout signal, which may point to additional anthropogenic sources. Eight surface soil samples next to the future HAMSTER facility have been taken prior to the beginning of the construction. The samples have been processed in the existing chemistry labs on HZDR’s campus and measured at two different AMS facilities (VEGA at ANSTO, and VERA at University of Vienna). The Pu concentrations and 24x/239Pu atomic ratios in the samples agree with values reported for global fallout due to atmospheric weapons tests in the 1950s and 60s.[1,2] The 241Am/241Pu ratios in the samples are also pointing to this origin. Besides the transuranium elements, different uranium isotopes (i.e. 233U and 236U) have been analysed in the soil samples. Recently, it has been shown that the 233U/236U ratio is extremely sensitive to distinguish between releases from civil nuclear industry and global fallout.[3] interestingly, the measured isotopic ratios are lower than expected for a pure global fallout signature. This may point to an additional source of reactor fuel whose origin has to be analysed with future investigations including fission products like 90Sr, 99Tc, 129I and 135/137Cs, which will also be measurable at HAMSTER. In summary, the actinide background next to the upcoming AMS facility on HZDR’s research campus mainly relates to global fallout signature with some hints for additional 236U background. Proper blank correction is anyway necessary to get reliable results for ultra-trace analysis of actinides and fission products with the new HAMSTER facility at HZDR.

Half of the chemical elements heavier than iron are produced by the rapid neutron capture process (r-process). The sites and yields of this process are disputed, with candidates including some types of supernovae (SNe) and mergers of neutron stars. We search for two isotopic signatures in a sample of Pacific Ocean crust: 60Fe (half-life 2.6 million years, Myr), predominantly produced in massive stars and ejected in SN explosions; and 244Pu, (half-life 80.6 Myr) produced solely in r-process events. We detect two distinct influxes of 60Fe to Earth in the last 10 Myr and accompanying lower quantities of 244Pu. The 244Pu/60Fe influx ratios are similar for both events. The 244Pu influx is lower than expected if SNe dominate r-process nucleosynthesis, implying some contribution from other sources.

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 a 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 characterisation 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. [1] Knie et. al., Phys. Rev. Lett. 83 (1999). [2] Koll et al., Phys. Rev. Lett. 123 (2019). [3] Koll et al., EPJ 232 (2020). [4] Wallner et al., PNAS 117 (2020). [5] Wallner et al., Nature 532 (2016) [6] Wallner et al., Science 372 (2021) [7] Hotchkis et al., NIMB 438 (2019)

Stars are the major element factories in the universe. In 1999, live supernova Fe-60 (t1/2 =2.6 Myr) was detected in a deep-sea ferromanganese crust (1 ) indicating the accumulation of supernova dust on Earth about 2 million years ago. This was followed by several projects reinforcing the initial evidence for a global influx of supernova Fe-60. Recently, a much younger continuous influx was found in Antarctic snow and in deep-sea sediments (2 –4 ) and an older peak around 6 - 8 Myr in deep-sea crusts (5 , 6 ). In contrast to the well-known production mechanism and synthesis site of Fe-60, the long-lived plutonium isotope Pu-244 (t1/2 =80 Myr) is a pure r-process nucleus. The nucleosynthesis site for the astrophysical r-process is still debated in the astrophysics community. Potential candidates involve rare supernovae and neutron star mergers. To date no evidence was presented that would point to an exclusive r-process site and combinations of different sites are considered. Experimentally, we can search for Pu-244 signatures in samples with known Fe-60 signatures 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. Based on the recent publication of the first detection of interstellar Pu-244 in a ferromanganese crust with a time resolution of 4.5 Myr (integrating over much shorter Fe-60 influxes) (6 ), we are now working on a highly time-resolved profile of Fe-60 and Pu-244 in the large ferromanganese crust VA13/237KD. This direct experimental input will further constrain models for r-process nucleosynthesis in the galaxy. The recently determined profile of Fe-60 clearly shows two influxes, one at 2 Myr, the other at 7 Myr, confirming and refining previous results. Preliminary data on Pu-244 and an outlook for future measurement campaigns will be given. References 1. K. Knie et al., Phys. Rev. Lett. 83, 18–21 (1999). 2. D. Koll et al., Phys. Rev. Lett. 123, 072701 (2019). 3. A. Wallner et al., Proceedings of the National Academy of Sciences 117, 21873–21879 (2020). 4. D. Koll et al., EPJ Web Conf. 232, 02001 (2020). 5. A. Wallner et al., Nature 532, 69–72 (2016). 6. A. Wallner et al., Science 372, 742–745 (2021).

Nuclides synthesized in massive stars are ejected into space via their stellar winds and in supernova explosions. The Solar System moves through the interstellar medium and collects some of these nucleosynthesis products. One such product is ⁶⁰Fe, a radionuclide with 2.6 million years half-life, that is predominantly produced in massive stars and ejected in supernova explosions. Extraterrestrial ⁶⁰Fe has been found on Earth, suggesting close-by supernova explosions ~2–3 and ~6 million years ago. Here, we report on the detection of a continuous interstellar ⁶⁰Fe-influx on Earth over the past ~33,000 years. This time period coincides with passage of our Solar System through such interstellar clouds, which have a significantly larger particle density compared to the local average interstellar medium embedding our Solar System for the past few million years. The interstellar ⁶⁰Fe was extracted from five deep-sea sediment samples and accelerator mass spectrometry was used for single atom counting. Despite the low number of 19 detected atoms, owing to a low influx, the ⁶⁰Fe-deposition rate does not indicate large variations over the 33,000 years. The measured approximately constant ⁶⁰Fe-time profile does not seem to reflect any large changes in the interstellar particle density during Earth’s passage through local interstellar clouds, that could be expected if the local cloud represented an isolated remnant of the most recent Supernova ejecta that traversed the Earth ~2–3 million years ago. The identified ⁶⁰Fe influx may signal a late echo of some million-year old supernovae with the ⁶⁰Fe-bearing dust particles still permeating the interstellar medium.

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. [1] K. Knie et. al. “Indication for supernova produced ⁶⁰Fe activity on Earth”, Phys. Rev. Lett. 83 (1999) 18. [2] K. Knie et. al. “⁶⁰Fe anomaly in a deep-sea manganese crust and implications for a nearby supernova source”, Phys. Rev. Lett. 93 (2004) 171103. [3] P. Ludwig et. al. “Time-resolved 2-million-year-old super-nova activity discovered in Earth's microfossil record”, PNAS 113 (2016) 9232. [4] A. Wallner et. al. “Recent near-Earth supernovae probed by global deposition of interstellar radioactive ⁶⁰Fe”, Nature 532 (2016) 69. [5] L. Fimiani et. al. “Interstellar ⁶⁰Fe on the surface of the Moon”, Phys. Rev. Lett. 116 (2016) 151104. [6] D. Koll et. al. “Interstellar ⁶⁰Fe in Antarctica”, Phys. Rev. Lett., submitted [7] A. Wallner et al. in preparation

The radiological residues at the former British nuclear weapons testing sites in Australia at Maralinga, Emu and the Montebello Islands are of ongoing interest in terms of environmental fate, transport, and uptake of radioactive contamination into the biosphere. The physical and chemical characteristics of these residues govern their mobility in the various environmental zones in which they reside (surface soil, ocean sediment, active beach zones) and availability of the radiological components for uptake into living organisms. At the Taranaki site, Maralinga, substantial body burdens of Pu were observed in mammals and the pattern of organ uptake was found to match that reported for exposure to respirable radioactive particles bearing refractory Pu. In order to fully deconvolute the disposition and radio-ecological impact of the contamination at these sites it was necessary to study in detail the radioactive particle composition. Plutonium often occurs in particulate forms at nuclear accident and test sites. Such particles require advanced techniques for characterisation including Accelerator Mass Spectrometry (AMS), Scanning electron microscopy and synchrotron X-ray fluorescence microscopy (XFM). Many such particles have core-shell structures where the surface is dominated by lighter elements sourced from local soils and the Pu concentrated in the interior. Particles formed during nuclear detonations and accidents can have complex structures and compositions which may be susceptible to variable leaching and weathering rates. Modelling results suggest that for respirable-sized Pu-bearing particles (that can be inhaled and lodged in the lung), most of the alpha emissions escape the particle and are deposited in the surrounding tissue. We are currently using advanced techniques to compare the radionuclide forms from the inland sites (Maralinga and Emu) with the marine site (Montebello Islands) to compare leaching and dose implications. The characteristics of the particles will largely determine the overall potential implications for long term fate of radiological contamination at the sites.

The Interstellar Medium (ISM) is continuously fed with new nucleosynthetic products. The solar system moves through the ISM and collects dust particles. Therefore, direct detection of freshly produced radionuclides on Earth, i.e. before decaying, provide insight into recent and nearby nucleosynthetic activities [1,2]. Indeed, a pioneering work at TU Munich [3,4], which applied the ultra-sensitive single atom counting technique of accelerator mass spectrometry (AMS) to an ocean crust-sample, showed an enhanced 60Fe signal possibly of extraterrestrial origin. Within an international collaboration [5-7] we have continued to search for ISM radionuclides incorporated in terrestrial archives. We have analyzed several deep-sea sediments, crusts and nodules for extraterrestrial 60Fe (t1/2=2.6 Myr), 26Al (t1/2=0.7 Myr) and 244Pu (t1/2=81 Myr) [5-8] which are complemented by independent work at TU Munich [9-11]. All the data demonstrate a clear global 60Fe influx that is interpreted as exposure of Earth to recent (≤10 Myr) supernova explosions. Furthermore, the low concentrations measured for 244Pu suggest an unexpectedly low abundance of interstellar 244Pu [5]. This finding signals a rarity of actinide r–process nucleosynthesis which is incompatible with the rate and expected yield of standard core collapse supernovae as the predominant actinide-producing sites. In this talk I will also present additional new results for 60Fe and 244Pu measured with unprecedented sensitivity. These data provide new insights into their concomitant influx and their ISM concentrations over a time period of the last 11 million years. [1] J. Ellis et al., ApJ. 470, 1227 (1996). [2] G. Korschinek et al., Radiocarbon 38, 68 (1996); abstract. [3] K. Knie et al., Phys. Rev. Lett. 83, 18 (1999). [4] K. Knie et al., Phys. Rev. Lett. 93, 171103 (2004). [5] A. Wallner et al., Nature Comm. 6, 5956 (2015). [6] J. Feige et al., EPJ Web of Conf. 63, 3003 (2013). [7] A. Wallner et al., Nature 532, 69 (2016). [8] M. Paul M. et al. Astrophys. J. Lett. 558, L133L135 (2001). [9] C. Wallner et al. New Astron. Rev. 48, 145150 (2004). [10] L. Fimiani et al., Phys. Rev. Lett. 116, 151104 (2016). [11] P. Ludwig et al., PNAS 113, 9232 (2016).

The radiological residues at the former weapons testing sites in Australia at Maralinga, Emu and the Montebello Islands are of ongoing interest in terms of environmental fate, transport, and uptake into the biosphere1. The physical and chemical characteristics of these residues affect their mobility and availability for uptake into living organisms2. At the Taranaki site, Maralinga, substantial body burdens of Pu were observed in mammals, likely due to the presence of respirable particles. Actinides often occur in particulate forms that, for characterisation, require advanced techniques including Accelerator Mass Spectrometry (AMS)3, Scanning electron microscopy and synchrotron X-ray fluorescence microscopy (XFM). Many nuclear test site particles have core-shell, or inhomogenous structures where the surface is dominated by lighter elements sourced from local soils and the Pu concentrated in the interior4. Modelling results suggest that for respirable-sized Pu-containing particles (that can be inhaled and lodged in the lung), most of the alpha emissions escape the particle and are deposited in the surrounding tissue.4 For larger particles, (e.g. >7 µm), which typically do not lodge in the lung but could be ingested, most of the alpha emissions do not escape the particle, but are instead captured within the particle itself (self-shielding) therefore decreasing the effective dose. We are currently using advanced techniques to compare the radionuclide forms from the inland sites (Maralinga and Emu) with the marine site (Montebello Islands).

Accelerator Mass Spectrometry (AMS) provides a high-sensitivity method for detection of long-lived radioisotopes. New facilities at ANSTO’s Centre for Accelerator Science are enabling us to detect plutonium by AMS with unprecedented level of sensitivity. We can now detect traces of the isotope 244Pu (half-life 80 million years) which arrive on earth on interstellar dust. However, the predominant source of plutonium on earth’s surface is from human activities, in particular from atmospheric nuclear testing of the 1950-1960’s. In Australia, the radiological residues originating from the British tests at the Montebello Islands, WA, occur in distinct isotopic and morphologic forms. The three tests had slightly different Pu isotopic signatures. Today, aided by the high sensitivity of AMS, their distinct 240/239Pu atom ratios can be differentiated in biological samples, such as failed sea turtles eggs gathered from beaches. Local fish tend to reflect a mixture of all three tests due to the movement of the fish and transport of Pu by water currents. On a larger scale, the 240/239Pu atom ratios in all samples (median ratio 0.04) are distinct from worldwide fallout (0.17-0.18) and can be used as a tracer for migrating species. The Pu exists in the environment in the form of ‘hot’ particles; the mobility of these particles and their availability for uptake into living organisms depends on their physical and chemical characteristics, which we are currently studying using a range of methods including synchrotron-based X-ray fluorescence microscopy (XFM).

The radiological residues at the former British weapons testing sites at Maralinga, Emu and the Monte Bello Islands often occur in particulate form (so called hot particles). Large numbers of these particles were emitted from nuclear and non-nuclear tests. For example each square meter in a plume that extends for tens of kilometres at the Taranaki site (Maralinga) can contain more than 3000 readily identifiable particles. The physical and chemical characteristics of these particles affect their mobility and availability for uptake into living organisms. When they contain long-lived radionuclides (e.g. 239Pu) these particles may slowly weather, and thus provide a persistent source of ionic forms, or smaller particles, for many thousands of years. Here we present a status on a range of methods being used at ANSTO to evaluate the physical and chemical characteristics of particles gathered from Australian sites. Methods include gamma spectrometry, autoradiography, high sensitivity Accelerator Mass Spectrometry analysis (AMS), leaching studies, and synchrotron-based X-ray fluorescence microscopy/spectroscopy. We focus on some of the practical issues involved when gathering and working with hot particles, as well as challenges in determining speciation and its influence on radioecological outcomes. We discuss data gaps and recommendations for current and future use of analysis methods in radioecological studies in Australia and the wider international community.

We examined the distribution of plutonium (Pu) in the tissues of mammalian wildlife inhabiting the relatively undisturbed, semi-arid former Taranaki weapons test site, Maralinga, Australia. The accumulation of absorbed Pu was highest in the skeleton, followed by muscle, liver, kidneys, and blood. Pu activity concentrations in lung tissues were elevated relative to the body average. Foetal transfer was higher in the wildlife data than in previous laboratory studies. The amount of Pu in the gastrointestinal tract was highly elevated relative to that absorbed within the body, potentially increasing transfer of Pu to wildlife and human consumers that may ingest gastrointestinal tract organs. The Pu distribution in the Maralinga mammalian wildlife generally aligns with previous studies related to environmental exposure (e.g. Pu in humans from worldwide fallout), but contrasts with the partitioning models that have traditionally been used for human worker-protection purposes (approximately equal deposition in bone and liver) which appear to under-predict the skeletal accumulation in environmental exposure conditions.

The radiological residues at the former weapons testing sites at Maralinga, Emu and the Monte Bello Islands often occur in particulate form (so called hot particles). Large numbers of these particles were emitted from nuclear and non-nuclear tests. For example each square meter in a plume which extends for tens of kilometres at the Taranaki site (Maralinga) can contain more than 3000 readily identifiable particles. The physical and chemical characteristics of these particles affect their mobility and availability for uptake into living organisms. When they contain long-lived radionuclides (e.g. 239Pu) these particles may slowly weather, and thus provide a persistent source of ionic forms, or smaller particles, for many thousands of years. From these Australian sites, we have gathered a series of particles that have weathered and interacted with the environment for 50+ years since their initial formation and release events. The particles are being evaluated using a range of methods including gamma spectrometry, autoradiography, high sensitivity Accelerator Mass Spectrometry analysis (AMS), leaching studies, and synchrotron X-ray fluorescence microscopy. Significant findings include the clustering of Cs on the exterior of a glassy fission fragment, with Sr occurring in the nearby interior, suggesting the 137Cs may be more available for weathering processes, and the beta emissions from the 90Sr may be largely self-shielded within the particle. In contrast, a different particle from a nearby site lacked any fission products, but contained Pu(IV) oxyhydroxides consistent with weathering in a semi-arid environment. Although the 239Pu is very active, detailed dose modelling suggests most of the alpha emissions from particles > 5µm are shelf-shielded within the particles themselves, and therefore impart lower dose than the equivalent Pu dissolved and distributed throughout an organ. However, when Pu exists on exterior surfaces, a hot particle that has been internalised (e.g. lodged in a mammalian lung) may produce relatively highly concentrated dose rates to adjacent tissues.

The Dresden Accelerator Mass Spectrometry (DREAMS) facility is designed for the measurement of 10Be, 26Al, 36Cl, 41Ca and 129I [1]. The actual goal is to extend the measurement capabilities to actinides. For this purpose, a time-of-flight system was designed and is currently under construction. The system is based on a 1.5 m long flight path and thin carbon foils with Micro Channel Plates as start and stop detectors. For an optimal tuning of the system with low currents, special beam diagnostic elements are planned. In order to characterize the existing system, first measurements of actinide samples have been performed in collaboration with the ANU and ANSTO, using an ionization chamber as detector. Measurements of Pu-isotopes in the 3+ and the 5+ charge state have been conducted. [1] S. Akhmadaliev et al., NIMB 294 (2013) 5.

The soil particles collected at a former British nuclear test site in Australia were investigated by synchrotron-based X-ray fluorescence microscopy (XFM), in order to determine the chemical speciation of radioactive nuclides retained in the particles. The results demonstrate that the particles contain a high concentration of Pu which derives from the original nuclear bomb material. The outcomes of this study would have a potential impact on the safety and environmental assessment associated with the former nuclear test sites.

In the last decade, compact AMS systems have demonstrated their potential to measure actinides (236U, 239,240,244Pu, 237Np). With an appropriate detection system, kinematic filters with enough mass resolution, and a simple chemical procedure, the determination of plutonium isotopes and 237Np at environmental levels is currently possible with this new generation of facilities with even better performance than with conventional AMS systems. However, the measurement of 236U (T1/2=23.4 My), produced by neutron capture on 235U, is still a challenge, due to the interference caused by 235U and 238U. In this work, we will explore the possibilities that the 1 MV AMS system at the CNA offers for the measurement of uranium isotopes at environmental levels, in terms of detection limit, efficiency, and precision. Considering the very promising 239Pu/238U mass suppression factor achieved with our system, of about 10-9, a limiting 236U/238U atomic ratio of about 10-11 was expected, approaching the levels expected in natural uranium. However, to date, only the 10-9 level has been obtained, possibly due to the lack of an appropriate uranium material. Currently, different natural uranium materials are being studied, in order to elucidate the origin of the interference. On the other hand, we will discuss the status of the plutonium measurements at our facility, based on the experience we have accumulated in recent years from the analysis of different matrixes over a wide range of plutonium concentrations.