Highlights
MAD Crystallography at F2

Over the last few years, the crystallographic technique known as MAD, or multiwavelength anomalous diffraction, has emerged as one of the more powerful methodologies available to the macromolecular crystallographer. Obviously, a tunable beam line with the usual high flux and low divergence is necessary to carry out such an experiment.

At CHESS, such a facility exists at the F2 doubly-focused wiggler beam line. Station F2 accepts 2 mrad of radiation which is monochromatized by a fixed offset, sagitally-focusing monochromator. A rhodium coated Si mirror located downstream of the monochromator is used to focus the beam vertically and to suppress unwanted harmonics. This doubly focused wiggler beam has a focal size of 3.6 mm x 0.32 mm in the hutch. At 13 keV, the flux is 1010 ph/s after a 0.3 mm collimator (at 100 mA).

Provisions have been built into the MAD setup which permit a rapid periodic check of the calibration of the monochromator to insure that the anomalous signal is being properly recorded. With one computer command, the experimenter rotates a reference foil into the beam at the upstream end of the hutch and initiates a quick scan of the monochromator through the edge energy. After the absorption spectrum is displayed on the terminal, the reference foil and the monochromator return to their starting positions. If the spectrum shows a shift in the apparent edge position, the monochromator adjusted to restore proper calibration. This process is typically carried out at the beginning of each fill, consuming less than 1 minute of beam time for each check.

Schematic of the F2 hutch for MAD experiments. Note that the foil is moved into the beam to perform quick energy calibration scans.

More details on performing MAD experiments at F2 are available.

Last Update: 2006-09-21