High-efficiency high-energy-resolution spectrometer for inelastic X-ray scattering

A nine-element analyzer system for inelastic X-ray scattering has been designed and constructed. Each individual analyzer crystal is carefully aligned with an inverse joystick goniometer. For the analyzers silicon wafers with 100 mm diameter are spherically bent to 1 or 0.85 m radius, respectively. Additionally, an analyzer with an extra small radius of 0.182 m and diameter of 100 mm was constructed for X-ray absorption spectroscopy in fluorescence mode. All analyzer crystals with large radius have highly uniform focusing property. The total energy resolution is approximately 0.5 eV at backscattering for the 1 m radius Si(440) analyzer array and approximately 4 eV for the 0.182 m radius Si(440) analyzer at 6493 eV. q 2005 Elsevier Ltd. All rights reserved. 1. Background and spectrometer design Over the last 20 years, high-energy resolution inelastic X-ray scattering and valence band resonant emission spectroscopy have gained interest in the scientific community. The high flux from synchrotron radiation sources has significantly benefited this research area. Significant experimental work was carried out in a large variety of fields, such as in physics, chemistry, material science and biology [1–6]. Due to the low cross-section in inelastic scattering, one needs to consider increasing the incidence flux or the efficiency of the spectrometer. Since the flux from most synchrotron radiation sources is close to its theoretical limit, the only alternative remains to improve the spectrometer to maximize detection efficiency. We chose to increase the solid angle covered by the analyzer system to collect more scattered photons. A few systems have already been built and are in use [7,8] for highand lowresolution inelastic X-ray scattering. These systems use linear arrays of analyzer crystals, and either separate detectors for each analyzer to measure the signal at several momentum transfer values at the same time [7], or just to cover a larger solid angle by using one detector [8]. In our case, the multiple crystal analyzers are mounted in a two-dimensional compact array. Each analyzer is mounted on 0022-3697/$ see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2005.09.069 * Corresponding author. Tel.: C1 973 642 7991. E-mail address: [email protected] (Q. Qian). its individual “inverted-joystick” goniometer [9] for perfect alignment of all nine analyzer crystals. Furthermore, translation of the analyzer system and of the detector keeps sample, analyzer crystals and detector on the Rowland circle, which improves the resolution. Finally, the system is modular and hence it is possible to upgrade the system with a specially designed nine-element detector, and to measure, by slightly detuning each analyzer crystal, the spectrum at nine different momentum transfer values at the same time. The spectrometer is based on a two-circle goniometer. Its accuracy is better than 25 arcsec and its repeatability is less than 4 arcsec. Analyzer energy-scans are simple q–2q scans. Fig. 1 shows a schematic drawing of our nine-crystal analyzer array spectrometer. The crystal analyzers are made from flat crystal wafers, which are bent to a spherically curved shape to fit on curved substrate mirror blank. Several aspects such as energy resolution, solid angle covered by the analyzer, and the manufacturing process of the analyzers influence each other. The bending radius influences the energy resolution, and it is limited by the strain tensor that causes the crystal to break if the bending radius is too small compared to the thickness of the crystal. The bending radius also determines the distance of the analyzer crystal to the sample, and thus the covered solid angle. The strain of bending the crystal broadens the rocking curve, thus increasing the band-width and decreasing the peak-reflectivity. We chose to use a bending radius of 1 m with a diameter of 100 mm and a thickness of 300 mm. The silicon wafers (FZ type with resistivity larger than 1000 O cm, TTV!5 um, both surfaces polished) are glued onto glass substrates, which gives reasonable energy resolution of about 0.5 eV for Si(440) close Journal of Physics and Chemistry of Solids 66 (2005) 2295–2298 www.elsevier.com/locate/jpcs