Livermore scientists achieve first full mapping of phonons in plutonium

Making a landmark event in the history of the experimental investigation of plutonium, scientists from the Lawrence Livermore National Laboratory for the first time have fully mapped the phonons in gallium-stabilized delta plutonium.

The experiment promises to reveal much about the physics and material properties of plutonium and its alloys.

The research, conducted in collaboration with researchers at the European Synchrotron Radiation Facility in Grenoble, France, and the University of Illinois at Champaign- Urbana- and led by Livermore physical chemist Joe Wong will be published in the Aug. 22 edition of Science.

Other Livermore researchers include Daniel Farber, Florent Occelli, Adam Schwartz, Mark Wall and Carl Boro.

Wong’s team took the first measurements of the complete phonon dispersions in a delta plutonium-gallium alloy, using a unique high-resolution inelastic X-ray scattering technique developed at the European Synchrotron Radiation Facility.

Since its discovery in 1941, many technical and safety issues have made experimental observations of plutonium extremely difficult. Measuring the phonon dispersion curves is key to understanding the properties of plutonium materials such as force constants, sound velocities, elasticity, phase stability and thermodynamics.

But for years, scientists have been plagued trying to measure these phonon dispersion curves in plutonium because they were unable to grow the large single crystals necessary for inelastic neutron scattering.

Instead, Wong and his colleagues used an inelastic X-ray scattering technique to impinge a micro-beam from a highly brilliant X-ray synchrotron source on a single grain in a polycrystalline plutonium alloy to make their measurements.

“The phonon dispersions are very fundamental to the understanding of the properties and behavior of plutonium and its alloys,” Wong said. “The new phonon data will greatly enhance scientists’ understanding of the transformations and phases plutonium undergoes in different environments and over time. Basic knowledge of this sort is much needed and contributes greatly to the Laboratory’s science-based stockpile stewardship mission to ensure the safety and reliability of the nation’s aging nuclear weapons without testing.”

Founded in 1952, Lawrence Livermore National Laboratory is a national security laboratory, with a mission to ensure national security and apply science and technology to the important issues of our time. Lawrence Livermore National Laboratory is managed by the University of California for the U.S. Department of Energy’s National Nuclear Security Administration.

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