3.3 GEM Beam-Scraper

The sixth and final aperture on the GEM beamline is known as the 'beam-scraper'. This is a computer-controlled motorised set of slits located inside the sample tank, 45.6cm before the sample. It is very important for keeping the penumbra of the beam from becoming excessively large. The beam-scraper uses four blocks of sintered boron carbide to define the beam.

The beam-scraper's aperture is set using the LabView PC. Usually it is best to set the other 5 apertures using the PLC, and then to set the beam-scraper to settings given on the PLC for aperture number 6.

The maximum beam size at the sample which can be set is 20mm wide and 40mm high.

The maximum area of the moderator which can be viewed is 90mm wide and 84mm high.

Click here for a schematic of the GEM beamline.

The flange for the beam-scraper is positioned behind the sample position flange: Overview    Close-up

The beam-scraper can also be removed from the beam if necessary. The motorised beam-scraper can be replaced by the old hand-adjusted beam-scraper if necessary. In order to change the size of the old beam-scraper's aperture, it is necessary to unbolt the beam-scraper flange, and lift the device out of the instrument. A 3mm Allen key may then be used to loosen and re-tighten the four beam-defining pieces. 

Last updated on 04 Nov 2004 by Alex Hannon (a.c.hannon@rl.ac.uk)