Beamline description

Optical scheme

The XAFS spectrometer is built in the direction of the tangential fan of the bending magnet 11.1 of the Elettra synchrotron light source.

A mask is placed at 13.3 m downstream the bending magnet source. It consists of a copper support with two 20 mm wide rectangular slots defined by tungsten alloy blades of 0.2 and 2 mm height. The two slots are used respectively to align and define the correct shape of the x-ray beam before the next optical element. The second optical element is a mirror placed 16.5 m from the source. The mirror provides vertical collimation and consists of a silicon ingot with platinum coating. It has an optically active length of 1000 mm and width of 60 mm respectively and it is positioned at 3 mrad respect to the direct beam. The mirror is water cooled and is operated under ultra high vacuum conditions. Although the contribution from L edges of Pt (11.5 - 14 keV) may have an impact on the x-ray absorption spectra, systematic tests have shown no noticeable features on the XAFS signal by the Pt coating when the detectors of the x-rays intensity before and
after the sample operate in optimal conditions of linearity. Two pairs of entrance W alloy slits define the shape of the beam impinging on the monochromator. The monochromator, placed at 21.7 m from the source is a double flat crystal double cam Kohzu apparatus. The energy range 2.4 - 27 keV can be covered using interchangeable under vacuum pairs of Si(111) and Si(311) crystals. Two successive Bragg reflections, with an inherent energy resolution given by the Darwin angular width directs photons of the desired energy parallel to the incoming beam direction, but offset upward by 25 mm. The fixed exit is achieved by the rotation and translation of the first crystal along the two cams. The detuning of the second crystal provides harmonic rejection at working energies below 9 keV. Downstream the monochromator a second set of W alloy slits are used to define the beam on the sample.