Performance of a tuneable secondary x-ray spectrometer

An efficient, high-resolution secondary x-ray spectrometer is critical to studies of x-ray-excited fluorescent x-ray emission spectra. Even with the highest available incident flux of x rays, the signal count rate can become unacceptably low when dispersed by an analyzing crystal. This problem is most serious in studies of gas targets, or at low energies where fluorescence yields are low. We have characterized the performance of a spectrometer based on the Rowland circle geometry with a variable-radius curved (Johann) analyzing crystal. A position-sensitive detector was used so that counts at a range of points on the Rowland circle corresponding to different wavelengths can be recorded in parallel. The efficiency of the spectrometer permits the observation of weak processes, such as subthreshold elastic and elastic x-ray scattering from gases. Energy resolution at low energies is sufficient to allow observations of spectral peak widths which are narrower than lifetime broadening widths. Polarization dependence of the fluorescence can also be studied.