Physicists at the DoE’s Los Alamos and Oak Ridge national laboratories have made the first direct measurements of a unique characteristic of plutonium’s fluctuating magnetism.

Plutonium was first produced in 1940 and its unstable nucleus allows it to undergo fission, making it useful for nuclear fuels as well as for nuclear weapons. Much less known, however, is that the electronic cloud surrounding the plutonium nucleus is equally unstable and makes plutonium the most electronically complex element in the periodic table.

While conventional theories have successfully explained plutonium’s complex structural properties, they also predict that plutonium should order magnetically. This is in stark contrast with experiments, which had found no evidence for magnetic order in plutonium. Finally, this mystery has been resolved.

According to Dr Marc Janoschek of Los Alamos National Laboratory and co-authors, plutonium is not devoid of magnetism, but in fact its magnetism is just in a constant state of flux, making it nearly impossible to detect.

“Plutonium sort of exists between two extremes in its electronic configuration – in what we call a quantum mechanical superposition,” Dr Janoschek said.

“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 physicists determined that the fluctuations have different numbers of electrons in plutonium’s outer valence shell – an observation that also explains abnormal changes in plutonium’s volume in its different phases.

“The fluctuations in plutonium happen on a specific time scale that no other method is sensitive to,” said Dr Janoschek, lead author of a paper about the results available online in the journal Science Advances.

“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.”

He added: “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.”

“The article by M. Janoschek, et al., is a tour de force,” said Dr Siegfried Hecker, former director of Los Alamos National Laboratory.

“Through a great combination of dynamical mean field theory and experiment, neutron spectroscopy, it demonstrates that the magnetic moment in delta-plutonium is dynamic, driven by valence fluctuations, rather than missing.”

“It also provides the best explanation to date as to why plutonium is so sensitive to all external perturbations – something that I have struggled to understand for 50 years now.”

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Marc Janoschek et al. 2015. The valence-fluctuating ground state of plutonium. Science Advances, vol. 1, no. 6, e1500188; doi: 10.1126/sciadv.1500188