Hard-x-ray spectrographs with resolution beyond 100 $\mu$eV

Spectrographs take snapshots of photon spectra with array detectors by dispersing photons of different energies into distinct directions and spatial 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 multicrystal, 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 multicrystal 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 $\Delta E\simeq 90$ $\mu$eV in the hard-x-ray regime, using multicrystal optics as the dispersing element. The spectrographs can boost research using inelastic ultrahigh-resolution x-ray spectroscopies with synchrotrons and seeded x-ray free electron lasers.

Figure 1

Scheme of the Czerny-Turner-type [10] spectrograph with a diffraction grating (a), or a crystal in asymmetric x-ray Bragg diffraction (b) as a dispersing element, DE. Other components include radiation source S, collimating and focusing mirrors M${}_{\mathrm{C}}$ and M${}_{\mathrm{F}}$, and position-sensitive detector Det. (c) Multicrystal multireflection CDFDW optics [9] is an example of a hard-x-ray “diffraction grating” (DE element) with an enhanced dispersion rate, suitable for hard-x-ray spectrographs.

Figure 2

Enhanced angular dispersion rate of the CDFDW optics. Schematic of the experiment showing the CDFDW$+$W monochromator (a) probing the angular dispersion rate of the CDFDW optics (b) with the $+$W channel-cut crystal as an angular analyzer. (c) Spectral dependencies of x-ray transmission through the CDFDW optics for fixed angular positions ${\Theta }_{+\mathrm{W}}$ of $+$W. (d) Same as (c) presented as a two-dimensional (2D) plot. (e) Transmission peak position as a function of ${\Theta }_{+\mathrm{W}}$.

Figure 3

Image of the CDFDW spectral function on the spatial $x$ scale (b) measured in the prototype spectrograph setup (a).

Figure 4

Prototype hard-x-ray spectrograph with a $\simeq 90$-$\mu$eV resolution using CDFDW optics as dispersing element (DE). (a) Schematic of the experiment. (b) Images of the CDFDW$+$W spectral function on the spatial $x$ scale for fixed angular positions ${\Theta }_{+\mathrm{W}}$ of $+$W. The gray profile in the background is the image of the CDFDW spectral function from Fig. 3b. (c) Same as (b) presented as a 2D plot. (d) Spatial peak position as a function of ${\Theta }_{+\mathrm{W}}$.