From undulator to end station

By changing the gap value of the undulator, the energy position of the undulator peaks can be varied over a wide energy range. This enables maximizing the photon intensity output at any energy. The new undulator magnets (since summer 2010) provide us with light from 42.5 eV and upwards (period = 54.4 mm, 45 periods, K_eff = 4.31).The characteristics of the undulator are summarized in the two following tables (using a 1220 lines/mm monochromator):

• gap vs photon energy (.pdf)
• OLD gap vs photon energy (.pdf) - in case you need it
• Intensity vs photon energy - old and new undulator (.pdf)
• NEW!! Beamcurrents of the past ( >Aug 2009)

Beamline layout

The light from the undulator is focussed horizontally by means of a cylindrical pre-mirror (M1). The plane mirror (M2) moves synchronously with the monochromator grating (G), and is followed by a spherical mirror (M3). This arrangement keeps the image of the virtual source (S') fixed on the exit slit (S1) position for each value of the fixed focus constant C_ff.

The re-focusing system consists of two spherical mirrors mounted in a Kirkpatrick–Baez arrangement. The vertically deflecting mirror (M4) focuses the exit slit onto the sample while the horizontally deflecting mirror (M5) images the focus of M1 onto the sample position. With a fixed radius of M4 the vertical focusing is optimal only for a specific exit slit position which was chosen to correspond to a C_ff value of 4.5. The spot size on the sample is then about 0.5 mm horizontally and 0.1 mm vertically.

Monochromator

The modified plane grating Zeiss SX-700 monochromator houses 3 gratings - first, there is the new 1220 lines/mm grating, which was installed in Dec 2007. The old 1220 lines/mm grating is also still there, and has been thoroughly cleaned. As a result, the intensity on the beamline towards higher photon energies has increased significantly, providing a factor of 10 more photon intensity around 1000 eV than before the re-shuffle and cleaning. Both of these gratings can be used for the spectroscopy station (with the new 1220 being the default).

The 1220 lines/mm monochromators can be operated at different C_ff values, with a corresponding position of the moveable exit slit. Increasing the C value gives better resolution with worse 2nd order suppression, whilst lowering the C value does the opposite. C = 2.25 is the most commonly used and gives the best compromise of flux, resolution and 2nd order rejection.

• 1220 lines/mm grating resolution vs exit slit size at C_ff = 2.25 (.jpg)

The third grating has 300 lines/mm, and provides the intense light needed for the PEEM station.

Exit slit, photodiode and mesh

• exit slit size vs micrometer reading (.txt)
• graph of diode sensitivity vs photon energy (.pdf)
• diode sensitivity vs photon energy (.txt)

Photon Resolution tests

The ultimate performance of the 1220 lines/mm monochromator grating has been tested using X-ray absorption spectroscopy on gases. Examples can be found here. The curve below illustrates the energy resolution of the 1220 lines/mm monochromators over the entire photon energy region.