X-ray spectra of shock compression

A research group at Lawrence Livermore National Laboratory recently reported an interesting application of ultrafast X-ray spectrometry to studies on the compression and heating of shocked matter. Here, the sample is 300 μm thick LiH, which is heated by a 450 J nsec laser, and the X-ray used is Ti Kα X-ray fluorescence (4.51 keV) from Ti foil heated by another pulse laser of 5 psec. X-ray photons produced at the Ti foil are estimated as a 2 × 10¹³/pulse. The energy spectra of X-ray scattering by the LiH sample during compression were taken by a spectrometer consisting of a large curved graphite (HOPG) crystal in van Hamos geometry and an Imaging Plate (IP) detector. It was found that the X-ray scattering spectrum from shocked LiH shows elastic Rayleigh scattering and inelastic plasmon scattering features. Whereas earlier in time only elastic scattering was observed, at 7 nsec, a plasmon energy shift of 24 eV was detected. This indicates the transition to metallic free electron plasma in the solid phase. For more information, see the paper, "Ultrafast X-ray Thomson Scattering of Shock-Compressed Matter", A. L. Kritcher et al., Science, 322, 69 -71 (2008).