Rare mergers of binary neutron stars found as the source of radioactive plutonium-244 in nature

December 8, 2015 by Dov Smith
Neutron star. Credit: NASA

In a letter published in the prestigious journal Nature Physics, a team of scientists from The Hebrew University of Jerusalem suggests a solution to the Galactic radioactive plutonium puzzle.

All the Plutonium used on Earth is artificially produced in nuclear reactors. Still, it turns out that it is also produced in nature.

"The origin of produced in nature through rapid neutron capture ('r-process') by seed nuclei is one of the current nucleosynthesis mysteries," Dr. Kenta Hotokezaka, Prof. Tsvi Piran and Prof. Michael Paul from the Racah Institute of Physics at the Hebrew University of Jerusalem said in their letter.

Plutonium is a radioactive element. Its longest-lived isotope is -244 with a lifetime of 120 million years.

Detection of plutonium-244 in nature would imply that the element was synthesized in astrophysical phenomena not so long ago (at least in Galactic time scales) and hence its origin cannot be too far from us.

Several years ago it was discovered that the early Solar system contained a significant amount of plutonium-244. Considering its short-lived cycle, plutonium-244 that existed over four billion years ago when Earth formed has long since decayed but its daughter elements have been detected.

But recent measurements of the deposition of plutonium-244, including analysis of Galactic debris that fell to Earth and settled in deep sea, suggest that only very small amount of plutonium has reached Earth from outer space over the recent 100 million years. This is in striking contradiction to its presence at the time when the Solar system was formed, and that is why the Galactic radioactive plutonium remained a puzzle.

The Hebrew University team of scientists have shown that these contradicting observations can be reconciled if the source of radioactive plutonium (as well as other rare elements, such as gold and uranium) is in mergers of binary neutron stars. These mergers are extremely rare events but are expected to produce large amounts of heavy elements.

The model implies that such a merger took place accidentally in the vicinity of our Solar System within less than a hundred million years before it was born. This has led to the relatively large amount of plutonium-244 observed in the early Solar system.

On the other hand, the relatively small amount of plutonium-244 reaching Earth from interstellar space today is simply accounted for by the rarity of these events. Such an event hasn't occurred in the last 100 million years in the vicinity of our Solar system.

Explore further: Ocean floor dust gives new insight into supernovae

More information: Kenta Hotokezaka et al. Short-lived 244Pu points to compact binary mergers as sites for heavy r-process nucleosynthesis, Nature Physics (2015). DOI: 10.1038/nphys3574

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5 comments

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Shootist
2 / 5 (4) Dec 08, 2015
Naturally occurring fission reactor in Gabon?
OdinsAcolyte
2.3 / 5 (3) Dec 08, 2015
Stardust, indeed!
gwrede
5 / 5 (2) Dec 09, 2015
Its longest-lived isotope is plutonium-244 with a lifetime of 120 million years.
Sigh. When I went to school, radioactive elements had half-lives.
Mr Som-o
3.7 / 5 (3) Dec 09, 2015
Well, Pu-244 has a half life of 80 million years (+/- an hour or two), so I'm not quite sure what to make of the author's statement of a "lifetime of 120 million years." Maybe the purpose is to just generate discussion. That's it! A discussion of how much of the article is fact and how much is fiction.
AGreatWhopper
1 / 5 (1) Dec 12, 2015
Spot on.

The money taken from conservative groups to molly-coddle the trolls obviously isn't going to the staff. Attracting trolls seems to be a bigger requirement than knowing anything about science writing.

Why no discussion of whether the hypothesized event would have caused the collapse of the proto-solar nebula?

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