⁶⁰Fe and ²⁴⁴Pu on Earth – Access to the Solar Neighbourhood, Stars and the Past of Earth

Long-lived radionuclides such as ⁶⁰Fe (t_1/2=2.6 Myr) or ²⁴⁴Pu (t_1/2=81 Myr) are synthesised in significant quantities in stellar environments by the capture of free neutrons. ⁶⁰Fe is in addition also produced, by cosmic ray interactions with interplanetary bodies, albeit at much lower quantities. Importantly, on Earth natural production of both isotopes is negligible, making them a valuable
tracer of extraterrestrial origin. Since these two isotopes are synthesised by the slow-neutron capture process (s-process) and predominantly ejected in supernova explosions, and the rapid neutron capture process (r-process), respectively, the potential detection of both isotopes opens the possibility to connect both processes in one astrophysical production site. The only measurement technique at this time which is sensitive enough to measure lowest concentrations of both isotopes is Accelerator Mass Spectrometry (AMS).
[1] History of detecting extraterrestrial ⁶⁰Fe
Extraterrestrial ⁶⁰Fe was detected first on Earth in a ferromanganese crust by the Munich AMS group in 2004 [1,2]. These samples allowed to analyse the past ~10 Myr for its ⁶⁰Fe content. Time-profile and absolute influx indicates that these ⁶⁰Fe atoms in the deep-sea crust were produced and ejected by one or more supernovae about 2 to 3 Myr ago and subsequently incorporated in this geological archive.
This discovery triggered several other projects to confirm this finding and to look for the same signal in other reservoirs like deep-sea sediments [3,4].
Furthermore, ⁶⁰Fe was also discovered on the Moon in lunar regolith [5]. These measurements all point towards an interstellar ⁶⁰Fe entry about 2-3 Myr ago, but the signal weakens and approaches measurement background for recent times.
However, such ⁶⁰Fe measurements are extremely difficult and only two AMS facilities (TU Munich and ANU) are sensitive enough for such measurements. For these reasons, also no significantly enhanced extraterrestrial influx of contemporary ⁶⁰Fe (i.e. within the last few 10
kyr) on Earth could be reported.
[2] ⁶⁰Fe in Antarctic snow
AMS is a relative measurement for isotope ratios, here extraterrestrial ⁶⁰Fe relative to stable terrestrial Fe. One major problem in the detection of modern ⁶⁰Fe influx from space by AMS, is the presence of the highly abundant stable terrestrial iron. Combined with the short ⁶⁰Fe accumulation periods, detection of a recent extraterrestrial signal becomes extremely challenging. To overcome this limiting factors, 500 kg of pure Antarctic surface snow (i.e. with lowest terrestrial Fe content) were recovered from the Kohnen Station in Antarctica and chemically
processed for an AMS measurement. Indeed, ⁶⁰Fe was discovered in Antarctic snow and by comparison with other isotopes such as ⁵³Mn, which is dominantly produced by cosmic ray interactions with solar system objects, the origin of these ⁶⁰Fe atoms could be
deduced [6].
[3] Search for concomitant ⁶⁰Fe and ²⁴⁴Pu influx onto Earth
Recently, we have started a project to extend previous measurements of ⁶⁰Fe and ²⁴⁴Pu in several geological reservoirs. The search for the coincident influx of ⁶⁰Fe and ²⁴⁴Pu into the same terrestrial archive opens the possibility to investigate a connection between Supernova-signatures (⁶⁰Fe production) and r-process nucleosynthesis (²⁴⁴Pu is a pure r-process nuclide). For this purpose, compared to
previous studies [2,7], a substantially larger sample of the same ferromanganese crust is available for ⁶⁰Fe [2] and ²⁴⁴Pu [7]. In this multi-isotope approach, we aim for a detailed time-profile for both isotopes in the crust. Such a project has become feasible also due to a substantially improved detection efficiency in ²⁴⁴Pu measurements. In addition, we plan to extend the time-period further into the past.
[4] References
[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”, Proceedings of the National Academy of Sciences 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 “Search for recent ⁶⁰Fe deposition in Antarctica with AMS” Master's Thesis TUM (2018).
[7] A. Wallner et al., “Abundance of live ²⁴⁴Pu in deep-sea reservoirs on Earth points to rarity of actinide nucleosynthesis”, Nat. Comm. 6 (2015) 5956.