Scientists made a rare find in a sample of oceanic crustal material: the rock contained around a dozen atoms of the plutonium isotope Pu-244. This element does not occur naturally on earth, and nuclear weapon reactors also breed a different isotope. As a result, the only conclusion that remains is that it came to earth from space, where it can still be detected today at the bottom of the deep sea.
The team around Anton Wallner from the Helmholtz-Zentrum Dresden-Rossendorf reported on his find in the specialist magazine >>Science<<
All elements with the exception of hydrogen and helium can be traced back to atoms that were formed in stars. The heavier an element, the more extreme conditions must usually exist for it to be able to form. In the case of plutonium, two extreme cosmic events come into consideration: certain types of supernovae and the collision of two neutron stars. In both cases, the newly formed atoms are thrown far into interstellar space, where they occasionally trickle onto the planets of passing stars. In a supernova, a dying star goes through a complex pattern of contractions and expansions in which star material repeatedly collides. In a neutron star merger, the tiny but ultra-dense remnants of such supernovae come so close that they will probably merge into a black hole in an unimaginably high-energy collision. With the help of gravitational waves, physicists were able to observe such a process in 2017.
With their search for the rare atoms, Wallner and his team would like to find out more about which of these two processes produces how much plutonium. Conversely, this provides information on how the two events take place and which forces are at work in each case.
Another extraterrestrial element in the rock – the iron isotope Fe-60 – allows the conclusion that traces of two separate supernova showers have been preserved in this sample. The isotope is also only produced in dying stars. According to their dating of the sample layers, it came down to earth once in the period four to a million years ago and again about seven million years ago. Plutonium could also have trickled down in each case.
Where does the plutonium come from?
In Wallner’s view, however, there are some indications that plutonium emerged from the much rarer, but much more extreme merger of two neutron stars. Because if the fairly common supernovae (they happen once or twice a century in the Milky Way) would produce the plutonium, the team would either have had to find more of it, or astrophysics would have to revise its supernovae models. Pu-244 decays with a half-life of 81 million years and is therefore also preserved over long periods of time, so that the plutonium in the rock sample from the ocean floor may have come to earth much earlier than the Fe-60. Theoretical considerations also speak in favor of this scenario, says Wallner in a press release.
The find was made possible by highly specialized detectors such as those operated at the HIAF (Heavy Ion Accelerator Facility) of the Australian National University (ANU) in Canberra. They not only allowed the team to fish out the individual plutonium atoms from the sample, but also to distinguish them from their terrestrial variants: In the upper layers of the rock sample, the experts came across plutonium-239, which was presumably released during atomic bomb tests and then in the Pacific had reached.
This article was originally published by spectrum.de
Article source: spektrum.de
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