X-ray echo spectroscopy, a counterpart of neutron spin-echo,
was recently introduced [1] to overcome limitations in spectral resolution
and weak signals of the traditional inelastic x-ray scattering (IXS)
probes. An image of a point-like x-ray source is defocused by a
dispersing system comprised of asymmetrically cut specially arranged
Bragg diffracting crystals. The defocused image is refocused into a
point (echo) in a time-reversal dispersing system. If the defocused
beam is inelastically scattered from a sample, the echo signal
acquires a spatial distribution, which is a map of the inelastic
scattering spectrum. The spectral resolution of the echo
spectroscopy does not rely on the monochromaticity of the x-rays,
ensuring strong signals along with a very high spectral resolution.
Particular schemes of x-ray echo spectrometers for 0.1-meV and 0.02-meV
ultra-high-resolution IXS applications (resolving power > 10^8)
with broadband ~5-13 meV dispersing systems will be presented
featuring more than 1000-fold signal enhancement. The technique is
general, applicable in different photon frequency domains.\\
[1.] Yu. Shvyd’ko, Phys. Rev. Lett. 116, accepted (2016), arXiv:1511.01526.
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