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Traces (elements) of cosmic explosions



13.05.2021 20:00

Traces (elements) of cosmic explosions

An international research team led by Prof. Anton Wallner from HZDR has found new evidence for a series of supernova events that have occurred relatively close to Earth over the past 10 million years. Instead of scanning the sky for traces of these star explosions, they searched the ocean floor – and found what they were looking for. In the journal Science they report on their meticulous search for two different isotopes, both of which do not occur naturally on Earth and which have their origin in the explosive end of massive stars. In this way, the scientists are adding a previously missing building block to our understanding of the creation of elements.

The team from Australia, Israel, Japan, Switzerland and Germany first examined 10 million year old deep-sea crust material from the Pacific for iron-60. The radioactive isotope is a good indicator of nearby supernova events, that is, massive star explosions that occur when they run out of fuel and then collapse. In the process, heavier chemical elements such as silver and platinum are formed and thrown into the cosmic neighborhood.

“While these star remnants migrate through interstellar space, small amounts of them also go down to earth and accumulate in the sea floor, including iron-60,” explains Anton Wallner, who is responsible for research at the HIAF facility (Heavy Ion Accelerator Facility ) at the Australian National University (ANU) in Canberra. The researcher from the HZDR Institute for Ion Beam Physics and Materials Research and the TU Dresden continues: “The proportion of iron-60 from space in the crust is only one millionth of a billionth of the amount of normal iron that occurs naturally on earth.”

In order to identify the tiny amounts of interstellar iron 60 atoms, Wallner and his team used extremely sensitive detection methods. For this purpose, the researchers separated the iron-60 traces from other, terrestrial isotopes after the chemical processing of the oceanic soil sample, using the heavy ion accelerator of the ANU, the only facility in the world that is sensitive enough for this type of research.

Independently of this, they determined the age of the individual layers of the sample from the sea floor using a different radioactive isotope, beryllium-10, which can also be found in the samples. The combined dating also revealed that the iron-60 input had clearly occurred in two bursts, once in the range four to a million years ago, and another time around seven million years ago. “Iron-60 decays with a half-life of 2.6 million years and is practically no longer detectable after about ten million years. We therefore know that our iron 60 sample must have been created within this time window, ”Wallner sums up.

Advances in analytical technology bring researchers more discoveries

The story does not end here. Surprisingly, the research team also discovered traces of plutonium-244, which experts assume is produced in supernova explosions or when neutron stars merge. “We were thrilled to discover interstellar plutonium in our sample material. It is the first time that traces of its presence in geological archives on earth – such as our crustal material – have been found so clearly, ”says Wallner happily. Similar to iron-60, plutonium-244 does not occur naturally on earth. But with a half-life of 81 million years, it decays much more slowly than iron-60.

In a sophisticated chemical sample preparation, the scientists were able to separate tiny traces of interstellar plutonium-244 and detect them with the VEGA accelerator of the Australian Nuclear Science and Technology Organization (ANSTO) in Sydney. This success was only made possible by the most recent technical improvements in the process, because the proven concentrations of plutonium-244 were again 10,000 times lower than those of the extremely rare iron-60.

However, the researchers had already expected to find another form of plutonium. When analyzing the crust material, the team came across man-made plutonium in the top, youngest layers, which was also installed there: a contemporary witness of the Cold War, when nuclear weapons tests ensured the global distribution of plutonium.

Why is there so little plutonium-244?

On average, our galaxy experiences one or two supernova explosions every hundred years. That is why the scientists had expected higher amounts of plutonium-244 in the sample from the Pacific floor – if the element was created during such events. “There is a lot of evidence that this plutonium-244 came from the same supernova explosions as iron-60. However, it could also have been left over from a much older but even more spectacular event such as a neutron star merger, ”Wallner outlines his train of thought. The first variant would contradict more recent work, which suggests that plutonium is only produced during such rare events that end with the detonation of the colliding neutron stars. But more data is needed to clear up this speculation. This also applies to the underpinning of ideas that supernova influences on the climate and the evolution of the earth are conceivable.

The team now hopes to be able to examine a significantly larger sample of the crustal material in order to gain insights into a period even further back – up to the late Oligocene, around 25 million years ago. The search is to be extended to other interstellar isotopes. The researchers want to find out more about the origins of the heavy elements of the periodic table.

Publication:
A. Wallner, M. B. Froehlich, M. A. C. Hotchkis, N. Kinoshita, M. Paul, M. Martschini, S. Pavetich, S.G. Tims, N. Kivel, D. Schumann, M. Honda, H. Matsuzaki, T. Yamagata, 60Fe and 244Pu deposited on Earth constrain the r-process yields of recent nearby supernovae, in Science, 2021.

Additional Information:

Prof. Anton Wallner
Institute for Ion Beam Physics and Materials Research at the HZDR
Tel .: +49 351 260 3274 | Email: [email protected]

Media contact:

Simon Schmitt | Management and press officer
Communication and Media Department at HZDR
Tel .: +49 351 260 3400 | Email: [email protected]

The Helmholtz-Zentrum Dresden-Rossendorf (HZDR) conducts research in the fields of energy, health and matter. The focus here is on the following questions:
• How do you use energy and resources efficiently, safely and sustainably?
• How can cancer diseases be better visualized, characterized and effectively treated?
• How do matter and materials behave under the influence of high fields and in the smallest of dimensions?
The HZDR develops and operates large infrastructures that are also used by external measurement guests: the ion beam center, the Dresden high-field magnet laboratory and the ELBE center for high-power radiation sources. It is a member of the Helmholtz Association, has six locations (Dresden, Freiberg, Görlitz, Grenoble, Leipzig, Schenefeld near Hamburg) and employs almost 1,400 people – including around 500 scientists including 170 doctoral students.


Scientific contact:

Prof. Anton Wallner
Institute for Ion Beam Physics and Materials Research at the HZDR
Tel .: +49 351 260 3274 | Email: [email protected]


Originalpublikation:

A. Wallner, M. B. Froehlich, M. A. C. Hotchkis, N. Kinoshita, M. Paul, M. Martschini, S. Pavetich, S.G. Tims, N. Kivel, D. Schumann, M. Honda, H. Matsuzaki, T. Yamagata, 60Fe and 244Pu deposited on Earth constrain the r-process yields of recent nearby supernovae, in Science, 2021.


Features of this press release:

Journalists
Chemistry, physics / astronomy
supraregional
Research results, scientific publications
Deutsch


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