# Flat Field Soft X-ray Spectrometry with Reflection Zone Plates on a Curved Substrate
## 5 Experimental Results  
The spectrometer was tested using standard samples, applied for the calibration of the EBMA instrumentation. Figure 7 shows the spectrum of standard stainless steel, which contains several metals, and is also contaminated by carbon and oxygen. The spectrum in Figure 7 was obtained as follows: in the focal plane of the RZP, the X-ray CCD camera [9] recorded a two-dimensional (2D) digital image of the diffracted beam. The photon counts of this intensity distribution were integrated along the sagittal direction, i.e., perpendicular to the  
Figure 6: (a) Diffraction efficiency of RZP 1 and RZP 2, measured at the reflectometer of the synchrotron facility BESSY II [8]. (b) Quantum efficiency of the CCD “GE-VAC 2048 512 BI UV1” (greateyes GmbH) sensor [9]. Figure 7: Measured sample spectrum of stainless steel. The data (red) are not corrected for the total quantum efficiency. X-ray emission lines are identified (blue vertical lines), and the observed diffraction order is noted in brackets. dispersive direction of the RZP. This procedure resulted in a raw spectrum, consisting of integrated count rates over the dispersive direction of the RZP. By calibration, with the pixel size of 13.5 \(\upmu\)m and the geometrical parameters as given in Table 1, the pixel axis \(x\) was then transformed into the energy axis. For the grazing angles \(\alpha\), \(\beta\), the arm length \(R_{2}\), and \(d\) as the central groove period of the RZP in dispersive direction, the grating equation led to  
\[\mathrm{E(eV)}=\frac{1239.8}{d(\mathrm{nm})\big{[}\cos(\alpha)-\cos\big{(} \tan^{-1}(x/R_{2})-\beta\big{)}\big{]}}\]  
Afterward, the calibration could be continued using the known chemical composition of the sample, as in, e.g., Figure 7. The spectra were recorded in the energy range of 150-750 eV. The upper boundary of the energy range was due to the Ni absorption edge of the RZP coating. The entire spectrum was measured for three positions of the detector, at \"0 mm\" and at \"\(\pm\)20 mm\", where 0 mm corresponded to the camera position where the designed energy was focused to the center of the detector. Displacements of \(\pm\)20 mm referred to respective lateral translations of the detector (see Section 3). It should be mentioned that a camera with an aperture of 60 mm could cover the entire spectral range and would not require a detector movement. The strong influence of higher-order diffraction on the spectral measurements is notable. For example, in Figure 7 we can identify the first, second, and third diffraction order of Fe L\({}_{\alpha,\beta}\) emission lines, as well as Cr L\({}_{\mathrm{I}}\), etc. A disadvantage of this diffraction phenomenon was that more complicated spectra were produced. As an advantage, we gained the possibility of measuring higher energies out of the first order diffraction range, e.g., L lines of metals with a higher atomic weight, like Cu. In this case, the limiting factor was the critical angle of total external reflection. The Ti/TiN spectra, as shown in Figure 8, demonstrated a high resolution and flexibility of the spectrometer. The Ti spectra were recorded from a pure Ti metal sample and a thin TiN film. The entire spectra of these samples were recorded in parallel using the same RZP 2. No detector or RZP movement was required. The data were not corrected for the total quantum efficiency. In Figure 9, the spectrum of a metallic boron target is shown. Obviously, the spectrometer could be used as a sensitive detector of boron concentration with simultaneous control of the carbon-oxygen contamination of the sample. In Figure 9, O fluorescence is evidently seen in the second order, beside the first order spectral line of C.  
Figure 8: (**a**) Measured L\({}_{2}\) and L\({}_{3}\) spectra of Ti and TiN. (**b**) Measured L\({}_{\mathrm{n}}\), L\({}_{\mathrm{I}}\), and N K\({}_{\alpha}\) spectra of Ti and TiN. Figure 10 depicts the total quantum efficiency of the entire spectrometer, switching from RZP 1 to RZP 2 near 360 eV (see Figure 6a,b).