Interstellar ⁶⁰Fe detected on Earth - but where is the r-process nuclide ²⁴⁴Pu?

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, before decaying, provides insight into recent and nearby nucleosynthesis [1,2]. Indeed, a pioneering work at Munich [3], using AMS for ocean crust-samples, showed an enhanced ⁶⁰Fe signal of extraterrestrial origin.

Within an international collaboration we have continued to search for ISM radionuclides trapped in deep oceanarchives. We have analyzed sediments, crusts and nodules for extraterrestrial ⁶⁰Fe (t_1/2=2.6 Myr), ²⁶Al (0.7 Myr) and ²⁴⁴Pu (81 Myr) [4-7] complemented by independent work at Munich [8-10]. We demonstrated that multiple events happened in our galactic neighbourhood and left their fingerprint on Earth. A global ⁶⁰Fe influx is evidence for exposure to recent (<10 Myr) supernova explosions.

The site where the heaviest elements are made in nature is, however, still unknown. The low concentrations measured for ²⁴⁴Pu suggest an unexpectedly low abundance of interstellar ²⁴⁴Pu [5]. It signals a rarity of actinide r-process nucleosynthesis, which is incompatible with the rate and expected yield of supernovae as the predominant actinide-producing sites.

We will present new results for ⁶⁰Fe measured at the ANU and ²⁴⁴Pu at ANSTO with unprecedented sensitivity.
These data provide new insights into their concomitant influx and their ISM concentrations over a time period of the last 11 Myr.

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