Search

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)

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 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).