AN INCIDENT-BEAM MONITOR FOR USE IN PROTEIN CRYSTALLOGRAPHY AT A SYNCHROTRON SOURCE

A compact incident-beam monitor has been developed for use in synchrot ron protein crystallography at the National Synchrotron Light Source b eamline X8C. Incident-beam intensity is monitored by measurement of th e radiation scattered from a thin polymer film into a PIN diode. For i mproved statistical accuracy, the scattering film can be replaced by a thin metal foil. Fluorescent radiation emitted by the metal foil incr eases the beam-monitoring signal up to 50 times relative to that produ ced by scattering alone. With this design, the forward-scattered radia tion is restricted to an area not larger than the beam stop, minimizin g excessive background radiation. Support materials have been optimize d so that no unwanted edge effects are present between 6 and 19.5 keV, making the detector useful for monitoring incident-beam intensities o ver the wide range of absorption edges often associated with multiwave length anomalous diffraction (MAD) experiments. Accurate incident-beam monitoring also simplifies optimization of the X-ray beam through the diffractometer collimation after each new electron orbit is establish ed in the synchrotron.