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EXMAD Résumé de rapport

Project ID: HPRI-CT-1999-50015
Financé au titre de: FP5-HUMAN POTENTIAL
Pays: United Kingdom

Adjustable beamstop to minimise X-ray background

It is pivotal to minimise every source of background scatter in diffraction experiments at long wavelengths. In particular, a large benefit can be gained by careful design of the beamstop. The main X-ray beam scatters from the air between the collimator and the crystal, and between the crystal and the beamstop. Thus a beamstop which can be positioned as close as possible to the crystal, but which is small enough not to cast a large central shadow on the detector, is highly beneficial in increasing the signal/noise ratio of counts on the detector, thereby making the small MAD and SAD signals easier to measure. In addition, a beamstop, which can be easily centred and is robust to knocks by experimenters is highly desirable, since through proper centering, asymmetric scatter of X-rays from the beamstop can be eliminated. Thus a new beamstop was designed, fabricated and extensively tested.

The new beamstop has the following features, and is innovative for macromolecular crystallography equipment in combining all these:
- It is adjustable on x and y translations on easy to turn knurled screws (no tool required).

- It is cut away at one side so that it can be positioned under the goniometer, allowing it to be placed much nearer the crystal, so cutting down the air path.

- It has a 1.5mm diameter collection cup for the beam as opposed to the 2mm or 2.5 mm diameter cups used previously, so minimising the size of the shadow cast on the detector and thus allowing much lower resolution data to be collected.

- A thin metal strip with its plane parallel to the beam direction attaches the cup to one side of the beamstop support. This arrangement is much more robust than the strips of mylar foil stretched across the support which were used previously, making the whole beamstop less prone to being knocked and misaligned by experimenters.

- It attaches to the existing MAR beamstop carriage with no modifications being necessary.

Informations connexes


Elspeth GARMAN, (Reader)
Tél.: +44-1865-275398
Fax: +44-1865-275182