X-ray focal properties of spherically bent crystals for synchrotron and XFEL spectroscopy

Brilliant synchrotron radiation and x-ray free-electron laser facilities have promoted the development of x-ray spectroscopy, especially the resonant inelastic x-ray scattering and femtosecond time-resolved x-ray spectroscopy. Comparing with the flat crystals, the spherically bent crystals (SBCs) are widely used in the relevant x-ray spectrometers for focusing, which will dramatically improve the detection efficiency. Therefore, focal properties of the SBCs should be experimentally well-investigated. Here, we build a device based on the Rowland circle geometry to study the focusing performance of Si(444) SBCs at different Bragg angles from 60° to 86° with the area detector on(off) the Rowland circle. As a result, the SBCs shows different focal properties in the Rowland plane (sagittal direction) and out of the Rowland plane (meridional direction) due to the unequal focal length. These two foci are separated, while the sagittal focus is on the Rowland circle and the meridional focus is off the Rowland circle. The focused spot on the detector is arcuate, exhibiting the best meridional focal quality near backscatter, and spreading rapidly when the Bragg angle downs to 60°. The sagittal focal quality stays as good as 1.5 pixels on the detector regardless of the Bragg angles.