Minimising background radiation in synchrotron crystallography
When using synchrotron radiation to probe the structure of macromolecules through crystallography, the X-ray beam scatters as it passes through the air. The crystal is positioned between two devices, called the collimator and the beamstop, that are used to limit the radiation to a specific region. Scattering occurs within this space and can affect the detection of small signals when using, for example, multiple-wavelength anomalous dispersion (MAD) techniques. The EXMAD project team has developed an adjustable beamstop in order to be positioned much nearer to the crystal. The beamstop is movable in two perpendicular directions by turning screws, and cut away on one side This cuts down the length of the path through the air that the beam must follow, reducing scatter and improving the signal-to-noise ratio for the detector. Furthermore, the size of the collection cup for the X-ray beam has been reduced from the 2 to 2.5mm diameter typical of previous models to a much smaller 1.5mm size. This reduces the size of the shadow cast on the beam detector, improving the resolution of the data detected. The newly developed beamstop cup attaches to an existing carriage, without modification, by using a thin metal strip rather than mylar foil. This arrangement is stronger and more resistant to knocks by synchrotron users, allowing a more accurate centring of the beamstop and prevention of asymmetric scattering of the beam. The new design is available for further exploitation by crystallographic labs, educational institutions and industry.
