Beamline description

Elements accessible at the XAFS beamline

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.

Experimental station

  • “Exit slits” to define the beam before the first ionization chamber I0.
  • Three ionization chambers by Oxford Instruments for measurements in transmission mode. These are filled with optimal He, N2, Ar, Kr gas mixture at a total pressure of 2 bars and are operated at a field of 2 kV per 30 cm of length. The ionization chambers signals are amplified by Keithley picoamperomers and the digitalized by a voltage to frequency converters and finally read by the counters of the data collection PC.
  • A multi-sample holder with movement on vertical axis
  • A removable motorized table (1.5 x 1 m) with horizontal and vertical movements is placed between the first two ion chambers. The top of this table is about 80 cm below the beam and all this space is available for users' experimental apparatus.
  • A silicon drift detector for measurements in fluorescence mode is available

Sample environments

  • Liquid-N2 cryostat. This device allows measurements at fixed temperature of 80 K, both in transmission and fluorescence mode.
  • L’Aquila-Camerino furnace for high temperature measurements. The furnace operates under high vacuum conditions in a wide range of temperatures 300-2500 K, measured by means of a thermocouple or by a pyrometric probe. The furnace can be used only for transmission mode measurements and is controlled remotely by the beamline control system allowing for temperature cycles with continuous collection of the temperature.
  • SPECAC cell for liquid samples. Aqueous and non aggressive solutions can be measured in transmission mode. The maximum loading volume of solution is 7 ml and optical tunable pathlength ranges from 0.1 to 6 mm.

Acquisition modes

  • Transmission
  • Fluorescence
Last Updated on Thursday, 28 March 2024 10:28