Development of the EFX Diagnostic at the Laboratory for Laser Energetics
To understand the evolution of Earth and Earth-like planets, researchers at the Laboratory for Laser Energetics are using x-ray absorption spectroscopy to study Earth core materials under extreme conditions. X-ray absorption fine structure (XAFS) spectroscopy is sensitive to temperature and the complex chemistry that occurs at these extreme energy densities. The goal of the XAFS platform is to characterize the temperature and complex chemistry of compressed materials to above 500 GPa and 5000 K, a regime in which it is intrinsically difficult to measure temperature. Because XAFS measures the modulations above an absorption edge requiring a high spectral resolution and signal-to-noise ratio, LLE developed the diagnostic EFX (extended x-ray absorption fine structure flat crystal), a high-resolution flat crystal x-ray spectrometer, to perform the XAFS measurements on the 60-beam OMEGA Laser System.
Planetary interiors can reach pressures millions of times higher than standard atmospheric pressure. Under these conditions, it has been historically difficult to characterize the temperature of such materials, which is critical for measuring their equations of state. Furthermore, at these extreme pressures, the chemical rules that the material follows can change, which causes a distortion of interior electron orbitals and core electrons to participate in bonding.
XAFS spectroscopy, the study of the modulations in the absorption coefficient, is influenced by the chemical and physical state of the atoms. These modulations are produced by the interactions of an excited photoelectron with the surrounding material. The density of the absorbing material can be extracted from the period of the XAFS modulations. Furthermore, the amplitude of the modulations yields information about the thermal behavior of the compressed material. With high enough spectral resolution, the near-edge region of the absorption spectrum can be used to infer solid–liquid melting, the oxidation state, and electron orbital hybridization.
EFX was developed for XAFS spectroscopy measurements of compressed metals ranging from iron to copper, as well as metal oxides on the OMEGA Laser System. The EFX crystal is a silicon 111 crystal with an energy range of 6.3 to 11.3 keV. The external tungsten shielding and internal fluorescence shielding reduce noise and allow it to make lower signal measurements. EFX has an adjustable front filter pack to provide on-shot energy calibration lines to characterize the energy dispersion relation. Lastly, it has approximately double the spectral resolution of the Rowland (Yaakobi) x-ray spectrometer (XRS) with a germanium crystal, a previous x-ray spectrometer used for XAFS measurements on OMEGA.
XAFS measurements with EFX have already been performed on iron and iron oxides at conditions above 500 GPa and 5000 K. Future plans include studying nickel and other metal alloys at similar conditions. The XAFS work at LLE involves a strong collaborative effort, including researchers at the Lawrence Livermore National Laboratory; the Sorbonne Université in Paris, France; and the European Synchrotron Radiation Facility located in Grenoble, France. Other applications for this spectrometer beyond XAFS are being investigated. For example, LLE researchers have used EFX to measure the copper emission in spherical implosions in order to characterize the electronic structure of compressed materials.