Soft x-ray laminar-type varied-line-spacing multilayer spherical diffraction grating designed for a flat-field grating spectrograph with enhanced spectral flux in the 1-2 keV region.

A soft x-ray diffraction grating with an aperiodic multilayer coating was designed to enhance spectral flux in a photon energy range of 1-2 keV by means of numerical calculation. Optimized groove depth and angle of incidence were 2.65 nm and 87.50°, respectively, for the grating with a groove density of 2400 lines/mm. The optimum number of W/B4C layer pairs was 6; the thicknesses of four bottom B4C layers were constant, and those of two top layers were unequal, while all W layer thicknesses were the same. An additional top layer of Au was crucial to provide uniform diffraction efficiency across the spectral range. In the region of 1-2 keV, the calculated diffraction efficiency of the designed gratings was ∼4.4% on average, and the calculated spectral flux (which correlates with analytical sensitivity) was 2.8 times larger than that of a previously designed Au-coated grating with an angle of incidence of 88.65°. The designed grating was manufactured; its measured diffraction efficiency was close to the calculated one in the 0.6-1.4 keV and >2 keV spectral ranges but somewhat lower in the 1.6-1.8 keV range. The spectrograph equipped with the new grating exhibited resolving power exceeding 400, which approached the pixel-size limit.