Neutron Scattering Helps to Determine Unique Property of Plutonium’s Magnetism

Doug Abernathy, left, ARCS instrument scientist at Oak Ridge National Laboratory, and Marc Janoschek, Los Alamos National Laboratory, prepare their sample for experiments at the Spallation Neutron Source. Credit: Genevieve Martin/ORNL.

Researchers at the Department of Energy’s Oak Ridge (ORNL) and Los Alamos National Laboratories have directly measured the unique property of plutonium’s variable magnetism using the neutron scattering technique.

This breakthrough study has confirmed the magnetism of plutonium which researchers had hypothesized for a long time but were never able to observe at the experimental level. This latest discovery offers a huge potential for energy, materials and computing applications.

Initially created in 1940, plutonium is a radioactive element containing an unstable nucleus. This nucleus causes it to undergo fission and makes it useful for both nuclear weapons and nuclear fuels.

However, the lesser known fact is that the electronic cloud enclosing the plutonium nucleus is also unstable, making plutonium the most multifaceted element in the periodic table, with complex characteristics for a basic elemental metal.

Traditional concepts have effectively elucidated the complicated structural characteristics of plutonium and also predicted the magnetic order in plutonium. This theory is in complete contrast with experiments which till date have found no proof for the magnetic order in plutonium. Now, after years of research, this scientific anonymity on plutonium’s “missing” magnetism has been finally resolved.

Marc Janoschek from Los Alamos, the lead scientist of the paper published in the journal Science Advances, explained that plutonium’s magnetism remains in a continuous state, which makes it almost impossible to detect.

Plutonium sort of exists between two extremes in its electronic configuration - in what we call a quantum mechanical superposition. 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.

Marc Janoschek - Los Alamos National Laboratories

Janoschek and his colleagues, leveraging the neutron measurements carried out on the ARCS instrument at ORNL’s Spallation Neutron Source, determined that the variations exhibit different numbers of electrons in the outer valence shell of the plutonium, an observation which also elucidates the unusual changes in different phases of plutonium’s volume. Neutrons are specifically suitable to this study since they are capable of detecting magnetic fluctuations.

“The fluctuations in plutonium happen on a specific time scale that no other method is sensitive to,” said Janoschek.

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. 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.

Marc Janoschek - Los Alamos National Laboratories

The latest study was the culmination of a wider attempt to examine plutonium but was hindered with several difficulties along the way. Since plutonium is a radioactive element and need to be handled carefully, the approval procedure for this experiment took a couple of years before the project was eventually accepted.

Although the researchers were aware that neutron spectroscopy measurements were integral to making advancements on plutonium’s “missing” magnetism, the study of prior neutron attempted by other researchers revealed that the present sample had to be enhanced in two distinct ways.

First, plutonium mainly includes the isotope plutonium-239 that effectively absorbs neutrons and would block the weak signal required. Instead, the researchers utilized plutonium-242, an isotope that absorbs only a fewer number of neutrons. Plutonium also adsorbs hydrogen, resulting in intense false signals at precisely the same place where the magnetic signals were believed to exist.

We used a special method developed at Los Alamos to remove the hydrogen from our sample. Many people across our laboratory and the complex helped solve these problems, but I’m especially grateful to Eric Bauer, Capability Leader for Materials Synthesis and Characterization in the Condensed Matter and Magnet Science group at Los Alamos, for helping me design a successful experiment.

Marc Janoschek - Los Alamos National Laboratories

Siegfried Hecker, former director of Los Alamos and one of the international authorities on plutonium science, said, "The article by M. Janoschek, et al., is a tour de force. 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,” Hecker said.

More than one person has stated this is the most significant measurement on plutonium in a generation.

Lawrence Livermore - National Laboratory’s Program Chair for Plutonium Futures Scott McCall

Such observations provide a reason on why plutonium is structurally not stable, and on a wider scale, proposes a better understanding of intricate, functional materials characterized by identical electronic dichotomies. The dynamical mean field theory calculations applied in this study has certainly reached a new standard.

According to Janoschek, the techniques developed in this work paves the way for future studies on other complex materials which are believed to be equally important for energy and computing applications in the future.

The dynamical mean field theory calculations were carried out on the Titan supercomputer based at ORNL’s Oak Ridge Leadership Computing Facility (OLCF). Janoschek informed that the researchers utilized as much as 10 million core hours for their calculations.

Co-authors of the study include Mark Lumsden and Doug Abernathy from ORNL, J.M. Lawrence, Pinaki Das, J. D. Thompson, S. Richmond, J.N. Mitchell, F. Trouw, M. Ramos, Eric Bauer, and J.-X. Zhu from Los Alamos; G. Kotliar, K. Haule, and B. Chakrabarti from Rutgers University, and G.H. Lander from the European Commission.

Alexander Chilton

Written by

Alexander Chilton

Alexander has a BSc in Physics from the University of Sheffield. After graduating, he spent two years working in Sheffield for a large UK-based law firm, before relocating back to the North West and joining the editorial team at AZoNetwork. Alexander is particularly interested in the history and philosophy of science, as well as science communication. Outside of work, Alexander can often be found at gigs, record shopping or watching Crewe Alexandra trying to avoid relegation to League Two.

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