Flat Field Soft X-ray Spectrometry with Reflection Zone Plates on a Curved Substrate

We report on the first experimental results obtained with a newly designed instrument for high-resolution soft X-ray spectroscopy, using reflection zone plates (RZPs) on a spherical substrate. The spectrometer was tested with a fluorescence source. High-resolution flat field spectra within +/- 50% around the design energies were measured at an interval of 150-750 eV, using only two RZPs: the first RZP, with its design energy of 277 eV, covered the band of 150-370 eV, and the second RZP, with a design energy of 459 eV, covered the band of 350-750 eV, where the upper boundary of this energy range was defined by the Ni coating of the RZPs. The absolute quantum efficiency of the spectrometer, including the optical element and the detector, was, on average, above 10%, and reached 20% at the designed energies of the RZPs. The resolving power E/ increment E exceeded 600 for energies E inside the core range of 200-550 eV.