Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: X-ray echo spectroscopy   X-ray echo spectroscopy, a counterpart of neutron spin-echo, is being
introduced here 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--0.02-meV ultra-high-resolution IXS applications (resolving power $> 10^8$)
with broadband $\simeq$~5--13~meV dispersing systems are introduced featuring
more than $10^3$ signal enhancement. The technique is general, applicable in
different photon frequency domains.

37
The categories of this paper are: supr-con
15
The categories of this paper are: nucl-ex
19
The categories of this paper are: eess
20
The categories of this paper are: acc-phys
26
The categories of this paper are: bayes-an
32
The categories of this paper are: atom-ph
14
The categories of this paper are: math-ph
28
The categories of this paper are: alg-geom
17
The categories of this paper are: q-fin
36
The categories of this paper are: cmp-lg
34
The categories of this paper are: chem-ph
7
The categories of this paper are: hep-th
13
The categories of this paper are: quant-ph
4
The categories of this paper are: cond-mat
35
The categories of this paper are: mtrl-th
16
The categories of this paper are: stat