ComIXS: A compact inelastic x-ray spectrometer

Soft x-ray emission spectroscopy is a well established technique, which has seen increased interest in recent years due to the development of high brilliance synchrotron radiation facilities. Emission spectra are obtained when a hole is created by impinging photons, and an electron from the valence band decays into the core hole emitting its excess energy radiatively. The fluorescence yield is intrinsically low in the range of soft x-rays, usually less than about 0.5%, since the most probable decay channels are radiationless channels like Coster Kronig and Auger processes. Emission is over all space, but only a restricted solid angle can be collected and this further reduces the efficiency of the instrumentation. Moreover, the emitted photons must be energy selected, and this involves the use of soft x-ray monochromators with good resolving power. We designed a photon spectrometer to be located in the experimental chamber of one of ELETTRA's bean-dines (BACH) to perform inelastic X-ray scattering measurement in the energy range 20-1200 eV with relatively high resolving power (above 2000 in most of the range). The spectrometer is based on two interchangeable mechanically ruled spherical VLS gratings with blaze profile of the grooves. The diffracted photons are collected by a back illuminated CCD of 1300x1340 pixels, each of 20x20 mum(2) dimension. The two interchangeable gratings are inserted in or removed from the beam by manual translation and are kept fixed during the measurement. The energy scan (or selection) is made by a mono-dimensional translation of the CCD. To cover the whole energy range, including the zero order light, only a small movement (about 7 cm of linear translation) of the detector is necessary. No detector rotation or bi-dimensional displacement is needed. Since the size of the CCD is quite large, it is possible to collect, in a single shot, a very large photon energy window, about 20-30% of the full energy range of each grating. The optical layout and the expected performances are reported.