ALOISA Beamline

The Grating-Crystal Monochromator

A peculiar monochromator has been developed which covers a very wide energy range from 130 eV to 8000 eV. The light is produced by a wiggler/undulator insertion device, selected in angle by a pinhole and collimated by a paraboloidal mirror in sagittal configuration. The parallel light beam impinges on the energy dispersing device which can be selected to be a plane mirror - plane grating for the low energy range (130-1500 eV) or Si channel-cut for the high energy range (2800 to 8000 eV).

The light is focused onto the exit slits by a second paraboloidal mirror and refocused at the centre of the experimental chamber by a toroidal mirror. In the low energy range, thanks to the sagittal configuration of the mirrors and the small emittance of the source, the monochromator reaches high resolutions (see table) with a resolving power higher than 10000 in the energy region around 400 eV. In the high energy region, covered by the Si(111) crystal, the resolving power is about 7500. The spot size at the centre of the experimental chamber is 20-30x150-200 µm. The flux at the sample is above 10¹¹ photon/s for a resolving power exceeding 5000 in the low energy range. A flux of around 5x10¹⁰ photon/s is typical in the high energy range.