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Oceanic archives are contemporary witnesses of Earth's recent astrophysical history by incorporating extraterrestrial radionuclides. VA13/2 - 237KD is one of the most studied ferromanganese crusts and it has been shown that the crust contains live interstellar 60Fe. Here, we have characterized a large piece of this crust with a 3D optical scan, a micro-CT scan and 3D modeling, followed by the chemical extraction of highly purified, element-specific fractions for accelerator mass spectrometry. High-accuracy cosmogenic 10Be dating of two independent drill-holes showed a time-dependent variability in growth rate across the surface of the crust. This well-characterized crust is used to search for interstellar radionuclides, such as supernova-produced 60Fe and the r-process nuclide 244Pu. Other extraterrestrial radionuclides including 26Al, 53Mn, 129I, 182Hf or 247Cm could be investigated in the future.

Typical neutron energies for the astrophysical s-process follow the Maxwell-Boltzmann distribution in the keV energy range. Neutron capture cross sections highly relevant for modelling the s-process can be experimentally determined by using the Time-of-Flight (ToF) method [1] or by the activation technique. If the reaction product is a long-lived radionuclide (t1/2 ~ yr -100 Myr), the cross section can be determined by activation with a quasi-stellar neutron distribution (typically kT = 25 keV) and a subsequent accelerator mass spectrometry (AMS) measurement of the reaction product [2]. Comparison of a number of such neutron capture cross sections shows a systematic bias, i.e. AMS data being lower than the ToF data [3, 4]. To investigate this discrepancy, we repeated experiments for two reactions that allow for highly precise AMS data: Maxwellian-averaged cross sections for the reactions 54Fe(n,γ)55Fe and 35Cl(n,γ)36Cl were investigated with dedicated activations at the Frankfurt Neutron Source (FRANZ) in Germany [5] and AMS measurements at two independent facilities. Analogously to previous activations, a quasi-stellar neutron spectrum of kT = 25 keV was produced via the 7Li(p,n) reaction, but at a different neutron-producing facility. Furthermore, to complement existing ToF and AMS data, an additional neutron activation of 54Fe and 35Cl at a proton energy of 2 MeV was performed, yielding data in the not-yet explored kT = 90 keV region. The irradiated metallic Fe foil and NaCl pellet (both of natural isotopic composition) were chemically processed and converted to AMS targets (Fe2O3 and AgCl) together with non-irradiated blanks. The subsequent AMS measurements of both radionuclides, 36Cl and 55Fe, were performed at two complementary AMS facilities, the Heavy Ion Accelerator Facility (HIAF) at the Australian National University [6] and at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Germany [7]. AMS allows a direct measurement of the 55Fe/54Fe and 36Cl/35Cl conversion ratios that result from the irradiation. The cross section is then deduced from the isotope ratio and the neutron fluence, which is determined using Au monitor foils. The new experiment was designed to produce highly accurate data and, owing to the two independent AMS measurements, it minimizes unrecognized sources of uncertainties in the AMS technique. The new preliminary data obtained in this work seem to confirm the previous AMS results. Consequently, the systematic discrepancy between AMS and ToF data remains unresolved. [1] Guber, K.H., et al., Phys. Rev. C 65, 058801 (2002). [2] Györky, Gy., et al., Eur. Phys. J. A 55, 41 (2019). [3] Capote, R., et al., Nucl. Data Sheets 163 (2020): 191. [4] Slavkovská, Z., et al., EPJ Web Conf. Vol. 232, p.02005, EDP Sciences, 2020. [5] Reifarth, R., et al., Publ. Astron. Soc. Aust. 26.3 (2009): 255. [6] Fifield, L.K., et al. Nucl. Instr. Meth. B: 268 (2010): 858. [7] Rugel, G., et al., Nucl. Instr. and Meth. in Phys. Res. B 370 (2016) 94.

The AMS program at ANU’s Department of Nuclear Physics is based on a 14UD tandem accelerator which runs regularly above 14 MV with stable measurement conditions. This setup provides particle energies between ~24 MeV (actinides) and >200 MeV (e.g. 53Mn, 93Zr). The facility is equipped with a dedicated SNICS ion source, provides typically 155 keV beam injection energy and is capable of a simultaneous use of both gas and foil stripper. Dedicated beamlines feature multi-anode ionisation chambers, an ENGE gas-filled magnet and a 6m TOF flight path. Further, a new fast cycling system is now being implemented (see contribution by L.K. Fifield et al.) that will replace our slow cycling method. The ANU has a strong focus in projects in environmental, safeguards and geological research with several nuclear astrophysics projects added recently as an additional major research topic. New isotopes recently introduced include 60Fe, 93Zr, 210Pb, 210mBi and 231Pa. We will give an overview of recent research activities and will summarize the performance of the different AMS setups. The potential of the ENGE setup for isobaric suppression will be exemplified for the two isotopes 60Fe and 53Mn. Measurement reproducibility and absolute detection efficiency will be compared.

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.

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

We searched for presence of 26Al (t1/2=0.7 Myr) in deep-sea sediments as a signature for extraterrestrial 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 extraterrestrial 60Fe signal between 1.7 and 3.2 Myr ago. Combining our 26Al data with the recently reported 60Fe data [1] gives a lower limit for the local interstellar 60Fe/26Al isotope ratio. Our Limit of 0.24 is higher than the observed average galactic 60Fe/26Al flux ratio of (0.15 + 0.05).It favours the higher ratios deduced from nucleosynthesis models.

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 first international round-robin exercise for the measurement of the long-lived radionuclide 10Be has been conducted. Ten participating accelerator mass spectrometry (AMS) facilities have each measured three samples at the 10-12-10-14 10Be/9Be level. All results have been made traceable to the NIST SRM 4325 standard to avoid additional discrepancies that arise when different facilities use different calibration materials. Hence, the data concentrates on pure measurement distinctions. Multivariate statistical investigations have been performed to reveal a bias between facilities, i.e. two distinguished groups could be identified. Maximum discrepancies between two single facilities are in the range of 6-31% depending on the absolute 10Be/9Be value. These findings should be considered when comparing 10Be data produced at one AMS facility with that produced at another facility, which is e.g. often the case for in-situ 10Be dating studies. Round-robin exercises are a very helpful tool as part of an overall quality assurance scheme to improve the accuracy, and not only the precision, of AMS data.

As a first step towards using world-wide a common 36Cl standard-type material for the normalization of 36Cl AMS results, we have initiated an interlaboratory comparison, but with differences in the execution of the project compared to other round-robin exercises: a) participants have given up anonymity to better optimize discussion and conclusions, b) preliminary results have been shown at various scientific meetings and workshops to discuss first trends and to increase the number of participants, c) participants had the possibility to repeat their AMS measurements and/or change measurement setups or data evaluation to improve individual data [e.g. 1]. We are aware and acknowledge that these later changes by some of the participants, which led to different overall data compared with the initial results, are not fully consistent with the idea of a normal proficiency test layout. However, these data changes were accepted to better meet the main objectives of the intercomparison: to identify and eliminate differences between AMS laboratories, thus, improving the 36Cl data quality world-wide as soon as possible. The evaluation of the final results of the eight participating AMS laboratories for three synthetic AgCl samples with 36Cl/Cl ratios at the 10-11 (SM-Cl-11), 10-12 (SM-Cl-12), and 10-13 (SM-Cl-13) level shows no difference in the sense of simple statistical significance [2]. However, more detailed statistical analyses indicate a certain interlaboratory bias and an under-estimation of uncertainties by some of the laboratories. The round-robin data demonstrate that 36Cl/Cl results from two individual AMS laboratories can differ by up to 17% (Fig. 1). Thus, the necessity for further work on harmonizing the 36Cl business on a world-wide scale and on improving the measurements is obvious. Fig. 1: Regression of normalized results for the three investigated samples in the order of ascending average. [1] V. Alfimov et al., Ion Beam Physics, ETH Zurich, Annual Report (2009) 13. [2] S. Merchel et al., Anal. Bioanal. Chem., submitted.

Driven by the progress in AMS and its spreading application within geosciences, measurements of increasing numbers of samples with low isotopic ratios will be required in the future. Therefore, we have examined the linearity of 10Be/9Be as a function of isotope ratio by distributing 3 secondary standards (dilutions of NIST4325: 10-12-10-14) to 9 AMS labs. The problem of low ratio samples is even more crucial for 36Cl mainly due to the high volatility of chlorine. Thus, we have prepared large quantities of 3 36Cl/Cl solutions from a certified 36Cl activity (NIST4943) by dilution with NaCl. AgCl precipitated from these solutions (10-11-10-13) has been distributed to 9 AMS labs. Some measurements are still ongoing. First results from 6 labs for each nuclide show that these interlaboratory exercises are very valuable.

Accelerator mass spectrometry (AMS) is presently the most sensitive technique for the measurement of long-lived actinides, e.g. 236U and 239Pu. A new actinide line is in operation at the Center for Isotopic Research on Cultural and Environmental heritage (CIRCE) in Caserta, Italy. Using the actinide line a uranium mass sensitivity of around 4 μg has been reached measuring with a 16-strip silicon detector, and a 239Pu background level of below 0.1 fg has been obtained. In this work we also discuss preliminary results for environmental and structural samples from in and around the Garigliano nuclear power plant (GNPP), presently in the decommissioning phase. Measurements on environmental samples from the vicinity of the plant allow the assessment of contamination, if any, over the years. Measurements of structural samples from the plant are relevant to the optimization of the decommissioning program for the GNPP.