First Full Mapping of Phonons in Pu

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A team of scientists from Lawrence Livermore National Laboratory, the European Synchrotron Radiation Facility in Grenoble, France, and the University of Illinois at Urbana-Champaign reported in the August 22 issue of Science that they have, for the first time, fully mapped the phonons in fcc d-plutonium-gallium by inelastic x-ray scattering. Measuring the phonon dispersion curves is critical to understanding Pu's material properties, such as force constants, sound velocities, elasticity, phase stability, and thermodynamics. In the figure reproduced above, phonon dispersions are shown along high-symmetry directions in d-Pu—0.6 weight % Ga alloy. The longitudinal and transverse modes are denoted L and T, respectively. The experimental data are shown as circles.

Plutonium has attracted enormous scientific interest since its discovery in 1941, primarily because of its radiotoxicity and use in nuclear weapons. But those very properties have made experimental observation of Pu extremely difficult. For years, scientists have been unable to measure phonon dispersion curves in Pu because they were unable to grow the large single crystals necessary for inelastic neutron scattering. In the new experiment, the team 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 Pu alloy to make their measurements.

The new phonon data will greatly contribute to determining how Pu properties change in different environments and over time. Such understanding is essential to science-based stockpile stewardship, the assurance of the safety and reliability of the nation's aging nuclear weapons in the absence of actual testing.

The Illinois team was led by Professor of Physics Tai-Chang Chiang.

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