Design of soft x-ray high diffraction efficiency and spectral flux Au/Ni bilayer coated laminar-type diffraction grating for objective soft x-ray flat-field spectrograph in a region of 250–550 eV.

The article focuses on the design and numerical simulation of a soft x-ray flat-field spectrograph employing a laminar-type, varied-line-spacing (VLS), spherical grating coated with an Au/Ni bilayer to enhance diffraction efficiency and spectral flux in the 250–550 eV energy range. By optimizing parameters such as incidence angle, groove density, groove depth, and Ni layer thickness, the newly designed grating (denoted G3) achieves 2 to 18 times higher spectral flux and improved spectral resolution compared to previously designed single-layer coated gratings (G1 and G2). The study introduces "spectral flux" (SF) as a key design criterion, defined as the product of collection solid angle and diffraction efficiency, to better correlate with detection sensitivity. These advancements aim to improve the sensitivity and practicality of soft x-ray emission spectroscopy (SXES) for trace element analysis in fragile biological and polymer samples.