Hard-x-ray spectrographs with resolution beyond 100μeV

Abstract

Spectrographs take snapshots of photon spectra with array detectors by dispersing photons of different energies into distinct directions and spacial locations. Spectrographs require optics with a large angular dispersion rate as the key component. In visible light optics diffraction gratings are used for this purpose. In the hard x-ray regime, achieving large dispersion rates is a challenge. Here we show that multi-crystal, multi-Bragg-reflection arrangements feature cumulative angular dispersion rates almost two orders of magnitude larger than those attainable with a single Bragg reflection. As a result, the multi-crystal arrangements become potential dispersing elements of hard x-ray spectrographs. The hard x-ray spectrograph principles are demonstrated by imaging a spectrum of photons with a record high resolution of ∆E 90 µeV in hard x-ray regime, using multi-crystal optics as dispersing element. The spectrographs can boost research using inelastic ultra-high-resolution x-ray spectroscopies with synchrotrons and seeded XFELs.A dream x-ray spectrometer is actually a spectrograph that images x-ray spectra in one shot, and with an ultimate spectral resolution. State of the art single shot x-ray spectrometers [1][2][3][4] are imaging spectra with array detectors, using Bragg's law dispersion (BD). BD links the angle of incidence θ to the energy E of photons Bragg reflected from the crystal atomic planes. However, the spectral resolution of the BD-spectrometers is always limited by the Bragg reflection (Darwin) bandwidth.Angular dispersion (AD) is one way how to overcome the Darwin width limitation and substantially improved spectral resolution of x-ray optics [5,6]. AD is a variation of the photon angle of reflection θ , for a fixed incidence angle θ, with the photon energy E. AD takes place in Bragg diffraction, albeit only if the diffracting atomic planes are at a nonzero angle η = 0 (asymmetry angle) to the entrance crystal surface [5,7,8], see Fig. 1Unlike BD, AD links θ to E for a fixed θ. AD is independent of the Darwin width, and can be therefore used to resolve much narrower spectral features. Using angular-dispersive monochromators, x-rays were already monochromatized to bandwidths (0.45 meV) almost two orders of magnitude smaller than the width of the Bragg reflections (27 meV) involved [9]. New concepts are required, however, to realize single shot angular-dispersive spectrographs.We show here that multi-Bragg-reflection arrangements feature, in theory and in experiment, cumulative angular dispersion rates almost two orders of magnitude greater than those attainable in a single Bragg reflection. An angular-dispersive x-ray spectrograph of a Czerny-Turner-type [10] is introduced with the enhanced angular-dispersive optics as a "diffraction grating". A record high spectral resolution of ∆E 90 µrad is demonstrated in the hard x-ray regime.Czerny-Turner grating spectrographs are nowadays standard in infrared, visible, and ultraviolet spectro-scopies [11,12]. In its classical arrangement, the spectrographs ...

X-ray Echo Spectroscopy

“…Such components have been experimentally demonstrated recently [8,14]. Implementation of x-ray echo spectrometers is, therefore, realistic.…”

“…Such large dispersion rates D ∪ , unfortunately, tend to decrease the transmission bandwidths ∆E ∪ of the dispersing elements [8,9]. Achieving strong signals in IXS experiments, however, requires ∆E ∪ ∆ε.…”

“…This is achieved, first, by using strongly asymmetric Bragg reflections close to backscattering [6,7], and, second, by enhancing the single-reflection dispersion rate considerably by subsequent asymmetric Bragg reflections from crystals in special arrangements [8] exemplified below. In the following two steps, we will show how the principle scheme of a generic echo spectrometer presented above, can be realized in the hard x-ray regime by using multi-crystal arrangements as dispersing elements.…”

“…Typically, in a single Bragg reflection, a maximum dispersion rate is D 6−8 µrad/meV for photons with energy E 10 keV [7,15]. As mentioned before, multi-crystal arrangements can be used to enhance the dispersion rate [8].…”

“…Diffraction gratings are not practical in the hard x-ray regime. However, as was demonstrated in [6,7], the angular dispersion in the hard x-ray regime can be achieved by Bragg diffraction from asymmetrically cut crystals or from special arrangements of asymmetrically cut crystals [8,9], which are a hard x-ray analog of the * Electronic address: shvydko@aps.anl.gov optical diffraction gratings and optical prisms. In the space-domain echo-spectrometer proposed here, an image of a point-like x-ray source is defocused by a dispersing system comprised of asymmetrically cut Bragg diffracting crystals.…”