Instrument for single-shot X-ray emission-spectroscopy experiments

We present the design and the characteristics of a portable and compact photon spectrometer to be installed in free-electron-laser (FEL) beamlines for photon in - photon out experiments, in particular single-shot X-ray emission spectroscopy. The instrument is operated in the 30 - 800 eV energy range with two channels and is designed to be initially used in the LDM (Low-Density Matter) and TIMEX (TIme-resolved studies of Matter under EXtreme conditions) beamlines of the FERMI@ELETTRA FEL, covering the whole spectral range of FERMI-1 and FERMI-2 emissions. The design of the instrument is tailored to achieve high spectral resolution in the whole interval of operation, high acceptance angle and high dynamic range. These characteristics are achieved in a compact environment to give a portable instrument that may be easily installed in different experimental chambers. The design consists of an entrance slit, a grazing-incidence diffraction grating and a detector. The number of elements within the optical path is kept to a single component, to minimize the losses due to reflectivity. The grating is spherical with variable line spacing along its surface, to provide an almost flat spectral focal plane that is perpendicular to the direction of the diffracted light. The detector is a back-illuminated CCD. The spectral resolution is better than 0.2% in the 30 - 800 eV region and the acceptance angles are 10 x 17 mrad in the 30-250 eV and 5 x 17 mrad in the at 250-800 eV.