The Riddle of Plutonium’s Magnetism Has Been Cracked
plutonium used on curiosity rover

Plutonium was first produced in 1940. Its uses span the entire moral spectrum from weapons of mass destruction to fuel sources for deep space spacecraft. Both NASA’s Curiosity rover and the New Horizons probe flying by Pluto use plutonium as a fuel source.

But, one aspect of plutonium has always puzzled scientists. Plutonium’s magnetism. Scientists have theorized it for decades but have never been able to observe it.

Conventional theories surrounding plutonium were able to explain its intricate structure properties. These same theories also predicted plutonium’s magnetism.

Now, scientists from two Department of Energy national laboratories have solved plutonium’s magnetism riddle. Plutonium is indeed magnetic, but its magnetism is extremely difficult to detect because it’s always changing.

“Plutonium sort of exists between two extremes in its electronic configuration—in what we call a quantum mechanical superposition,” said Marc Janoschek, a scientist at Los Alamos. “Think of the one extreme where the electrons are completely localized around the plutonium ion, which leads to a magnetic moment. But then the electrons go to the other extreme where they become delocalized and are no longer associated with the same ion anymore.”

The short video below illustrates the constantly fluctuating magnetism.

How did the scientists figure this out? Neutron measurements were key. Using the ARCS instrument at ORNL’s Spallation Neutron Source, Janoschek and his team observed fluctuations have different numbers of electrons in the plutonium’s outer valence shell. This also explains the changes in plutonium’s volume during different phases seen in the video above.

Plutonium ARCS instrument

Doug Abernathy and Marc Janoschek preparing sample for experiments at Spallation Neutron Source. Credit: Genevieve Martin, ORNL

Janoschek explains why neutrons were vital to this discovery. “The fluctuations in plutonium happen on a specific time scale that no other method is sensitive to,” said Janoschek.

He added, “This is a big step forward, not only in terms of experiment but in theory as well. We successfully showed that dynamical mean field theory more or less predicted what we observed,” Janoschek said. “It provides a natural explanation for plutonium’s complex properties and in particular the large sensitivity of its volume to small changes in temperature or pressure.”

Janoschek and his team’s work is being praised throughout the scientific community.

Siegfried Hecker, a former director of Los Alamos, called the paper a “tour de force.”

“More than one person has stated this is the most significant measurement on plutonium in a generation,” said Lawrence Livermore National Laboratory’s Program Chair for Plutonium Futures Scott McCall.

The paper was published in the journal Science Advances.

Featured image credit: Idaho National Laboratory/Flickr. Picture of NASA’s Curiosity Rover power system

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